Impact of estrogen receptor agonists and model of menopause on enzymes involved in brain metabolism, acetyl-CoA production and cholinergic function

Our goal is to understand how loss of circulating estrogens and estrogen replacement affect brain physiology and function, particularly in brain regions involved in cognitive processes. We recently conducted a large metabolomics study characterizing the effects of rodent models of menopause and treatment with estrogen receptor (ER) agonists on neurochemical targets in hippocampus, frontal cortex, and striatum. Here we characterize effects on levels of several key enzymes involved in glucose utilization and energy production, specifically phosphofructokinase, glyceraldehyde 3-phosphate dehydrogenase, and pyruvate dehydrogenase. We also evaluated effects on levels of β-actin and α-tubulin, choline acetyltransferase (ChAT) activity, and levels of ATP citrate lyase. All experiments were conducted in young adult rats. Experiment 1 compared the effects of ovariectomy (OVX), a model of surgical menopause, and 4-vinylcyclohexene diepoxide (VCD)-treatments, a model of transitional menopause, with tissues collected at proestrus and at diestrus. Experiment 2 used a separate cohort of rats to evaluate the same targets in OVX and VCD-treated rats treated with estradiol or with selective ER agonists. Differences in the expression of metabolic enzymes between cycling animals and models of surgical and transitional menopause were detected. These differences were model-, region- and time- dependent, and were modulated by selective ER agonists. Collectively, the findings demonstrate that loss of ovarian function and ER agonist treatments have differing effects in OVX vs. VCD-treated rats. Differences may help to explain differences in the effects of estrogen treatments on brain function and cognition in women who have experienced surgical vs. transitional menopause.

Use of the REVERT® total protein stain as a loading control demonstrates significant benefits over the use of housekeeping proteins when analyzing brain homogenates by Western blot: An analysis of samples representing different gonadal hormone states

Western blot is routinely used to quantify differences in the levels of target proteins in tissues. Standard methods typically use measurements of housekeeping proteins to control for variations in loading and protein transfer. This is problematic, however, when housekeeping proteins also are affected by experimental conditions such as injury, disease, and/or gonadal hormone manipulations. Our goal was to evaluate an alternative and perhaps superior method for conducting Western blot analysis of brain tissue homogenates from rats with distinct physiologically relevant gonadal hormone states. Tissues were collected from the hippocampus, frontal cortex, and striatum of young adult female rats that either were ovariectomized to model surgical menopause, or were treated with the ovatotoxin 4-vinylcyclohexene diepoxide (VCD) to model transitional menopause. Tissues also were collected from rats with a normal estrous cycle killed at proestrus when estradiol levels are high, and at diestrus when estradiol levels are low. Western blot detection of α-tubulin, β-actin, and GAPDH was performed and were compared for sensitivity and reliability with a fluorescent total protein stain (REVERT^®). Results show that the total protein stain was much less variable across samples and had a greater linear range than α-tubulin, β-actin, or GAPDH. The stain was stable and easy to use, and did not interfere with the immunodetection or multiplexed detection of the housekeeping proteins. In addition, we show that normalization of our data to total protein, but not to GAPDH, revealed significant differences in α-tubulin expression in the hippocampus as a function of treatment, and that gel-to-gel consistency in measuring differences between paired samples run on multiple gels was significantly better when data were normalized to total protein than when normalized to GAPDH. These results demonstrate that the REVERT^® total protein stain can be used in Western blot analysis of brain tissue homogenates to control for variations in loading and protein transfer, and provides significant advantages over the use of housekeeping proteins for quantifying changes in the levels of multiple target proteins.

Role of GPR30 in mediating estradiol effects on acetylcholine release in the hippocampus

We have hypothesized that estradiol enhances basal forebrain cholinergic function and cognitive performance, at least in part, via activation of the novel estrogen receptor GPR30. Here we evaluated the effects of estradiol, G-1 (a selective GPR30 agonist), and tamoxifen (TAM; an ERα/ERβ antagonist that also acts as a GPR30 agonist), on acetylcholine (ACh) release in the hippocampus, as well as the ability to block the effects of 17β-estradiol (E) or TAM with the GPR30 antagonist G-15. Note that G-1 was included to evaluate the effects of selectively activating GPR30, whereas TAM was included to differentiate effects of E associated with activation of GPR30 vs. ERα or ERβ. The study was designed to test effects on potassium-stimulated release, as well as on ACh release stimulated by feeding. Effects of feeding were included because the tasks we used previously to demonstrate beneficial effects of E on cognitive performance were motivated by food reward, and we hypothesized that E may enhance performance by increasing ACh release in association with that reward. Ovariectomized rats were treated for 1week, and ACh release was evaluated using in vivo microdialysis. In addition, rats were fed at the same time daily for several days and were fasted overnight prior to microdialysis. For each rat, ACh release was evaluated under basal conditions, in response to feeding, and in response to elevated potassium. Both feeding and elevated potassium increased ACh release in the hippocampus. In response to feeding, E, G-1, and TAM all significantly increased the percent change in release. The effects of E and TAM were blocked by G-15, and the effects of combining E+TAM did not differ significantly from the effects of E or TAM alone. In response to elevated potassium, E, and TAM significantly increased the percent change in ACh release. G-1 produced a slightly lesser effect. The effect of TAM was reduced by G-15, but the effect of E was not. These findings suggest that activation of GPR30 is both necessary and sufficient to account for the effects of E on ACh release associated with feeding. In contrast, activation of GPR30 appears to be sufficient, but may not be necessary for increased release associated with elevated potassium. The changes associated with feeding are consistent with the effects of E, G-1 and G-15 on acquisition of a spatial learning task previously described. These data confirm and extend previous reports, and support a hypothesis wherein E treatment can improve learning on specific tasks by activating GPR30 and enhancing ACh release in association with food reward.

The effect of the steroid sulfatase inhibitor (p-O-sulfamoyl)-tetradecanoyl tyramine (DU-14) on learning and memory in rats with selective lesion of septal-hippocampal cholinergic tract

Dehydroepiandrosterone sulfate (DHEAS), is an excitatory neurosteroid synthesized within the CNS that modulates brain function. Effects associated with augmented DHEAS include learning and memory enhancement. Inhibitors of the steroid sulfatase enzyme increase brain DHEAS levels and can also facilitate learning and memory. This study investigated the effect of steroid sulfatase inhibition on learning and memory in rats with selective cholinergic lesion of the septo-hippocampal tract using passive avoidance and delayed matching to position T-maze (DMP) paradigms. The selective cholinergic immunotoxin 192 IgG-saporin (SAP) was infused into the medial septum of animals and then tested using a step-through passive avoidance paradigm or DMP paradigm. Peripheral administration of the steroid sulfatase inhibitor, DU-14, increased step-through latency following footshock in rats with SAP lesion compared to both vehicle treated control and lesioned animals (p<0.05). However, in the DMP task, steroid sulfatase inhibition impaired acquisition in lesioned rats while having no effect on intact animals. These results suggest that steroid sulfatase inhibition facilitates memory associated with contextual fear, but impairs acquisition of spatial memory tasks in rats with selective lesion of the septo-hippocampal tract.

Chronic treatment with a GPR30 antagonist impairs acquisition of a spatial learning task in young female rats

We hypothesize that the beneficial effects of estradiol on cognitive performance may be mediated through GPR30, a putative membrane target of estrogens. Recently we showed that administration of a selective GPR30 agonist (G-1) to ovariectomized rats enhanced acquisition of a delayed matching-to-position (DMP) T-maze task and increased potassium-stimulated acetylcholine release in the hippocampus, similar to estradiol (E2) (Hammond et al., 2009). The present study tested whether treating with a selective GPR30 antagonist (G-15) would impair spatial learning in gonadally intact rats and in ovariectomized (OVX) rats treated with E2. As predicted, G-15 dose-dependently impaired DMP acquisition both in gonadally intact rats and in OVX rats treated with E2. G-15 specifically reduced the rate of acquisition, and this effect was associated with an increased predisposition to adopt a persistent turn. In contrast, G-15 alone at the highest dose had no significant effect on DMP acquisition in OVX controls. The effects were task dependent, as similar effects of G-15 were not observed in gonadally intact rats tested on an operant discrimination/reversal learning task motivated by the same food reward. This suggests that the effects on DMP acquisition were not due to effects on motivation for food. Effects of G-15 on DMP acquisition were similar to previously published work showing significant impairment produced by selective cholinergic denervation of the hippocampus. These data suggest that GPR30 can play an important role in mediating the effects of estradiol on spatial learning, possibly by mediating estradiol effects on basal forebrain cholinergic function.

Galanthamine plus estradiol treatment enhances cognitive performance in aged ovariectomized rats

We hypothesize that beneficial effects of estradiol on cognitive performance diminish with age and time following menopause due to a progressive decline in basal forebrain cholinergic function. This study tested whether galanthamine, a cholinesterase inhibitor used to treat memory impairment associated with Alzheimer's disease, could enhance or restore estradiol effects on cognitive performance in aged rats that had been ovariectomized in middle-age. Rats were ovariectomized at 16-17 months of age. At 21-22 months of age rats began receiving daily injections of galanthamine (5mg/day) or vehicle. After one week, half of each group also received 17ß-estradiol administered subcutaneously. Rats were then trained on a delayed matching to position (DMP) T-maze task, followed by an operant stimulus discrimination/reversal learning task. Treatment with galanthamine+estradiol significantly enhanced the rate of DMP acquisition and improved short-term delay-dependent spatial memory performance. Treatment with galanthamine or estradiol alone was without significant effect. Effects were task-specific in that galanthamine+estradiol treatment did not significantly improve performance on the stimulus discrimination/reversal learning task. In fact, estradiol was associated with a significant increase in incorrect responses on this task after reversal of the stimulus contingency. In addition, treatments did not significantly affect hippocampal choline acetyltransferase activity or acetylcholine release. This may be an effect of age, or possibly is related to compensatory changes associated with long-term cholinesterase inhibitor treatment. The data suggest that treating with a cholinesterase inhibitor can enhance the effects of estradiol on acquisition of a DMP task by old rats following a long period of hormone deprivation. This could be of particular benefit to older women who have not used hormone therapy for many years and are beginning to show signs of mild cognitive impairment. Potential mechanisms for these effects are discussed.

Donepezil plus estradiol treatment enhances learning and delay-dependent memory performance by young ovariectomized rats with partial loss of septal cholinergic neurons

Effects of estrogen therapy on cognitive performance appear to diminish with age and time following the loss of ovarian function. We hypothesize that this is due to a reduction in basal forebrain cholinergic function and that treatment with a cholinergic enhancer can reverse the effect. This study tested whether combining the cholinesterase inhibitor donepezil with estradiol treatment can enhance/restore estradiol effects on cognitive performance in young ovariectomized rats with selective lesions of septal cholinergic neurons. 192IgG-saporin was injected directly into the medial septum to produce selective cholinergic lesions. Rats were then treated with donepezil (Don, daily injections of 3mg/kg/day, i.p.) or vehicle, and then with 17β-estradiol (E2, administered by silastic capsule implanted s.c.) or an empty capsule. Rats were trained on a delayed matching-to-position (DMP) T-maze task which previous studies have shown is sensitive to ovariectomy and estrogen replacement. Results show that neither estradiol nor donepezil alone significantly enhanced acquisition of the DMP task in rats with cholinergic lesions. Combination therapy was effective, however, depending on the severity of the lesion. Don+E2 significantly enhanced acquisition of the task in rats with partial lesions (<50% loss of cholinergic neurons), but not in rats with severe lesions. This effect was due largely to a reduction in perseverative behavior. Don+E2 also improved working memory in rats with partial lesions, as evidenced by significantly better performance than controls during increased intertrial delays. These findings suggest that even partial loss of septal cholinergic neurons can reduce effects of estrogen therapy on cognitive performance, and demonstrate that combining a cholinesterase inhibitor with estrogen therapy can help to restore beneficial effects on performance. We propose that combination therapy may have similar beneficial effects in women, particularly in older women who have not used estrogen therapy for many years and are beginning to show signs of cognitive impairment or early Alzheimer's disease.

GPR30 is positioned to mediate estrogen effects on basal forebrain cholinergic neurons and cognitive performance

Beneficial effects of estrogen therapy on cognitive performance diminish with age and time following the loss of ovarian function. This has led to the 'Window of Opportunity' hypothesis, which states that estrogen therapy must be administered within a limited period of time following menopause in order to be effective. Effects of estrogen therapy on cognitive performance are due, at least in part, to the effects on cholinergic afferents innervating the hippocampus and cortex, and it has been suggested that the loss of estrogen effect with age and time following menopause is due to a substantial reduction in the function of these projections. The mechanisms that underlie the effects are not clear. GPR30 is a novel G-protein coupled estrogen receptor that is expressed in the brain and other tissues. Our recent studies show that GPR30 is expressed in areas of the brain important for spatial learning, memory, and attention. In addition, GPR30 in expressed by the vast majority of cholinergic neurons in the basal forebrain, and appears to be an important regulator of basal forebrain cholinergic function. We hypothesize that GPR30 plays an important role in mediating direct effects of estradiol on basal forebrain cholinergic neurons, with corresponding effects on cognitive performance. Hence, GPR30 may be an important target for developing new therapies that can enhance or restore estrogen effects on cognitive performance in older women. Here we briefly review the cholinergic hypothesis and summarize our findings to date showing effects of a GPR30 agonist and antagonist on basal forebrain cholinergic function and cognitive performance.

Studies of the localisation of kisspeptin within the pituitary of the rhesus monkey (Macaca mulatta) and the effect of kisspeptin on the release of non-gonadotropic pituitary hormones

Kisspeptin neurones in the arcuate nucleus play a pivotal role in the regulation of hypothalamic gonadotrophin-releasing hormone (GnRH) secretion in higher primates. To examine whether kisspeptin also influences the function of the primate pituitary directly, two experiments were performed in adult male rhesus monkeys. First, the distribution of kisspeptin-containing cells in the pituitary was described using fluorescence immunohistochemistry. Second, the secretion of non-gonadotrophin adenohypophysial hormones [growth hormone (GH), prolactin and thyroid-stimulating hormone (TSH)] and cortisol in response to i.v. kisspeptin administration was examined. Eight animals were deeply anaesthetised and transcardially perfused with 4% paraformaldehyde. Fluorescence immunohistochemistry was performed on 25-microm thick free-floating pituitary sections to localise immunopositive kisspeptin cells and to examine their relationship with immunostaining for luteinising hormone (LH), follicle-stimulating hormone, GH, prolactin, alpha-melanocyte-stimulating hormone (MSH), adrenocorticotrophic hormone (ACTH) and GnRH. Kisspeptin cells were found in the intermediate lobe of all animals and, in four monkeys, this neuropeptide was also observed in cells scattered in the periphery of the anterior lobe. Kisspeptin colocalised with alpha-MSH-immunopositive cells in the intermediate lobe and, in 50% of the monkeys, with ACTH-immuunopositive cells in the periphery of the adenohypophysis. There was no evidence for colocalisation of kisspeptin with gonadotrophs, somatotrophs or lactotrophs. Beaded kisspeptin axons were observed in the neural lobe. In addition, assay of plasma samples that had been collected for a previous study documenting kisspeptin-10-induced LH release in male monkeys revealed that kisspeptin administration failed to influence circulating concentrations of GH, prolactin, TSH and cortisol. Release of all four of these non-gonadotrophic hormones, however, was stimulated markedly by NMDA, which is considered to act centrally. Although the morphological findings obtained in the present study are consistent with the notion that kisspeptin may act directly at the level of the pituitary, the nature of such an action remains to be defined.

Chronic treatment with estrogen receptor agonists restores acquisition of a spatial learning task in young ovariectomized rats

Previous work has shown that continuous estradiol replacement in young ovariectomized rats enhances acquisition of a delayed matching-to-position (DMP) T-maze task over that of ovariectomized controls. The mechanism by which estradiol confers this benefit has not been fully elucidated. This study examined the role of selective estrogen receptor agonists of ERalpha, ERbeta, and GPR30 in the enhancement of spatial learning on a DMP task by comparing continuous estradiol replacement with continuous administration of PPT (an agonist of ERalpha), DPN (an agonist of ERbeta), or G-1 (an agonist of GPR30) relative to gonadally intact and ovariectomized vehicle-treated controls. It was found that ovariectomy impaired acquisition on this task, whereas all ER selective agonists restored the rate of acquisition to that of gonadally intact controls. These data suggest that estradiol can work through any of several estrogen receptors to enhance the rate of acquisition on this task.

Effects of long-term hormone treatment and of tibolone on monoamines and monoamine metabolites in the brains of ovariectomised, Cynomologous monkeys

The effects of long-term hormone treatment on monoamines and monoamine metabolites in different regions of the primate brain were examined and compared. Ovariectomised Cynomologous monkeys received daily oral administration of either conjugated equine oestrogens (CEE), CEE + medroxyprogesterone acetate (MPA), or a low or high dose of tibolone, for a period of 2 years. Tissue punches collected from frozen sections through various regions of the forebrain, midbrain, and hindbrain were assayed for levels of dopamine, dihydroxyphenylacetic acid (DOPAC), serotonin, 5-hydroxyindole acetic acid (5-HIAA), and norepinephrine by high-performance liquid chromatography. Few differences between hormone-treated animals and ovariectomised controls were observed. No statistically significant effects of CEE relative to controls were detected in any of the seven brain regions analysed. Animals treated with CEE + MPA showed significant reductions in 5-HIAA in the dorsal raphe nucleus, a significant reduction in dopamine in the hypothalamus, and a significant reduction in serotonin (5-HT) levels in area 8AD of the frontal cortex. Similar to CEE, no significant effects of tibolone relative to controls were detected; however, animals treated with high-dose tibolone showed a decrease in 5-HT levels in the frontal cortex that approached significance and was similar to the effect of CEE + MPA. Collectively, the findings suggest that long-term oral administration of these compounds has relatively few effects on the levels of dopamine, serotonin, and their primary metabolites in the primate brain. This differs from the significant effects on serotonergic and dopaminergic systems detected following parenteral treatment with oestradiol and progesterone, and likely reflects differences between the effects of treating with CEE + MPA versus oestradiol and progesterone on brain monoaminergic systems.

Estrogen-mediated regulation of cholinergic expression in basal forebrain neurons requires extracellular-signal-regulated kinase activity

Beyond the role estrogen plays in neuroendocrine feedback regulation involving hypothalamic neurons, other roles for estrogen in maintaining the function of CNS neurons remains poorly understood. Primary cultures of embryonic rat neurons together with radiometric assays were used to demonstrate how estrogen alters the cholinergic phenotype in basal forebrain by differentially regulating sodium-coupled high-affinity choline uptake and choline acetyltransferase activity. High-affinity choline uptake was significantly increased 37% in basal forebrain cholinergic neurons grown in the presence of a physiological dose of estrogen (5 nM) from 4 to 10 days in vitro whereas choline acetyltransferase activity was not significantly changed in the presence of 5 or 50 nM estrogen from 4 to 10 or 10 to 16 days in vitro. Newly-synthesized acetylcholine was significantly increased 35% following 6 days of estrogen treatment (10 days in vitro). These effects are in direct contrast to those found for nerve growth factor; that is, nerve growth factor can enhance the cholinergic phenotype through changes in choline acetyltransferase activity alone. This is most surprising given that mitogen-activated protein kinase and extracellular-signal-regulated kinase1/2, kinases also activated in the signaling pathway of nerve growth factor, were found to participate in the estrogen-mediated changes in the cholinergic phenotype. Likewise, general improvement in the viability of the cultures treated with estrogen does not account for the effects of estrogen as determined by lactate dehydrogenase release and nerve growth factor-responsiveness. These findings provide evidence that estrogen enhances the differentiated phenotype in basal forebrain cholinergic neurons through second messenger signaling in a manner distinct from nerve growth factor and independent of improved survival.

Effects of ageing and long-term hormone replacement on cholinergic neurones in the medial septum and nucleus basalis magnocellularis of ovariectomized rats

Ovariectomized aged rats, some of which received long-term hormone replacement with oestrogen or oestrogen plus progesterone, were evaluated for the number and size of basal forebrain cholinergic neurones, as well as relative levels of choline acetyltransferase (ChAT) and trkA mRNA, in order to determine whether effects on basal forebrain cholinergic cell survival and function correspond with differences in cognitive performance previously described. The results show that ageing combined with long-term loss of ovarian function produced substantial reductions in the levels of ChAT and trkA mRNA in the medial septum and nucleus basalis magnocellularis, relative to much younger ovariectomized controls. In contrast, no significant effects on the number or size of the cholinergic cells were detected, indicating that loss of ovarian function does not cause a loss of cholinergic neurones with age. Long-term hormone replacement had no apparent effect on the number of ChAT-positive neurones detected, and did not prevent the reductions in ChAT and trkA mRNA associated with ovariectomy and ageing. Collectively, the data suggest that ageing combined with long-term loss of ovarian function has a severe negative impact on basal forebrain cholinergic function, but not on cholinergic cell survival per se.

Effects of long-term hormone replacement and of tibolone on choline acetyltransferase and acetylcholinesterase activities in the brains of ovariectomized, cynomologus monkeys

We examined long-term effects of low and high doses of tibolone, conjugated equine estrogens, and conjugated equine estrogens plus medroxyprogesterone acetate on choline acetyltransferase and acetylcholinesterase activities within different regions of the brain in cynomologus monkeys. All treatments were administered for 2 years. None of the treatments produced significant increases in either choline acetyltransferase or acetylcholinesterase in any of eight brain regions analyzed. In contrast, treatment with conjugated equine estrogens plus medroxyprogesterone acetate, but not conjugated equine estrogens alone, produced significant reductions in both choline acetyltransferase and acetylcholinesterase in the medial septum/diagonal band of Broca compared with untreated controls. Treatment with tibolone also resulted in significant reductions in both choline acetyltransferase and acetylcholinesterase in the medial septum/diagonal band of Broca, and this effect was dose-related. These findings are the first to report the effects of long-term therapies used by postmenopausal women on cholinergic measures in the primate brain. The findings are consistent with recent reports in rats, and suggest that any positive effects of long-term estrogen or hormone replacement therapy on cognitive processes are probably not due to significant effects on choline acetyltransferase or acetylcholinesterase activities.

Effects of gonadal hormone replacement on measures of basal forebrain cholinergic function

The effects of different hormone replacement regimens on basal forebrain cholinergic function were examined by measuring changes in choline acetyltransferase activity and high affinity choline uptake in adult, ovariectomized, rats. Increases in choline acetyltransferase activity were detected in the frontal cortex (20. 1%) and olfactory bulbs (30.4%) following two weeks, but not four weeks, of repeated treatment with estrogen plus progesterone. Increases in high affinity choline uptake were detected in the frontal cortex (39.5-55.1%), hippocampus (34.9-48.9%), and olfactory bulbs (29.9%) after two weeks, but not four weeks, of either continuous estrogen administration, repeated progesterone administration, or repeated treatment with estrogen plus progesterone. Repeated administration of estradiol (2-25 microg/250 g body weight) for two or four weeks, and continuous estrogen administration for four weeks and six months, produced no significant changes in choline acetyltransferase activity or high affinity choline uptake in the hippocampus, frontal cortex or olfactory bulbs. Continuous estrogen administration for 13 months produced a significant decrease in high affinity choline uptake across all regions with the largest effect (-28.1%) detected in the hippocampus. The findings demonstrate that short-term treatment with estrogen and/or progesterone can significantly enhance cholinergic function within specific targets of the basal forebrain cholinergic projections. Most important is the fact that the effects varied considerably according to the manner and regimen of hormone replacement and did not persist with prolonged treatment. These findings could have important implications for the effective use of hormone replacement strategies in the prevention and treatment of Alzheimer's disease and age-related cognitive decline in women.

Oestrogen and the cholinergic hypothesis: implications for oestrogen replacement therapy in postmenopausal women

Cholinergic deficits in the basal forebrain, hippocampus and cortex are thought to contribute to the risk and severity of cognitive decline associated with ageing and Alzheimer's disease. Work in our laboratory has demonstrated that in rats, basal forebrain cholinergic neurons are affected by physiological fluctuations in circulating oestrogen and progesterone, and that long-term loss of ovarian function produces decreases in cholinergic parameters and nerve growth factor receptor (trkA) mRNA beyond the effects of normal ageing. Conversely, short-term treatment with oestrogen or oestrogen plus progesterone produces increases in cholinergic parameters and trkA, as well as increases in potassium-stimulated acetylcholine release, that are consistent with an increase in basal forebrain cholinergic function. These findings are consistent with recent studies showing the ability of oestrogen and progesterone replacement to enhance spatial memory and reduce performance deficits associated with hippocampal cholinergic impairment. We hypothesize that similar effects of the ovarian hormones on basal forebrain cholinergic neurons in humans may contribute to the effects of hormone replacement on cognitive processes that have recently been described, and to the ability of oestrogen replacement to reduce the risk and severity of Alzheimer's-related dementia in postmenopausal women.

Estrogen replacement does not prevent the loss of choline acetyltransferase-positive cells in the basal forebrain following either neurochemical or mechanical lesions

Recent studies have shown that estrogen replacement can enhance the functional status of basal forebrain cholinergic neurons. Studies have also shown that estrogen has neuroprotective effects both in vitro and in vivo on a variety of cells and against a variety of insults. The present study examined the ability of estrogen replacement to protect basal forebrain cholinergic neurons from the effects of neurochemical and mechanical injury. Ovariectomized Sprague-Dawley rats received either estrogen replacement or sham surgery, and then received either a unilateral injection of ibotenic acid into the nucleus basalis magnocellularis, or unilateral transection of the fimbria fornix. Cholinergic neurons in the medial septum and nucleus basalis were detected and quantified using immunohistochemical techniques. The data show that neither 3 weeks nor 13 weeks of continuous estrogen replacement prevented the loss of choline acetyltransferase (ChAT)-containing cells in the nucleus basalis following a unilateral injection of ibotenic acid. Likewise, estrogen replacement did not prevent a decrease in ChAT-positive cells detected in the medial septum following unilateral transection of the fimbria fornix. Notably, increased numbers of ChAT-positive cells were detected in the contralateral nucleus basalis, and in the ipsilateral and contralateral medial septum, at 2 weeks following a unilateral injection of ibotenic acid into the nucleus basalis; however, these effects were not related to hormone treatment. These data suggest that estrogen replacement does not protect cholinergic neurons in the medial septum and nucleus basalis from the effects of excitotoxic or mechanical injury.

Long-term treatment with estrogen and progesterone enhances acquisition of a spatial memory task by ovariectomized aged rats

Female Sprague-Dawley rats were ovariectomized at 13 months of age. Four groups received different regimens of estrogen or estrogen plus progesterone replacement beginning either immediately, 3 months, or 10 months after ovariectomy and were compared with non-hormone-treated controls. Eight to twelve months after ovariectomy, animals were trained on a delayed matching-to-position (DMP) spatial memory task. Long-term treatment with estrogen or estrogen plus progesterone significantly enhanced acquisition of the DMP task by aged animals after long-term loss of ovarian function. Weekly administration of estrogen and progesterone was at least as effective as, if not more effective than, continuous treatment with estrogen alone. In addition, treatment initiated 3 months, but not 10 months, after ovariectomy was as effective at enhancing DMP acquisition as continuous estrogen treatment initiated immediately after ovariectomy, suggesting a window of opportunity after the loss of ovarian function during which hormone replacement can effectively prevent the effects of aging and hormone deprivation on cognitive function. These findings suggest that repeated treatment with estrogen and progesterone initiated within a specific period of time after the loss of ovarian function may be effective at preventing specific negative effects of hormone deprivation on brain aging and cognitive decline.

Estrogen replacement enhances acquisition of a spatial memory task and reduces deficits associated with hippocampal muscarinic receptor inhibition

A delayed matching-to-position (DMP) T-maze task was used to examine the effects of estrogen replacement on spatial learning and memory, as well as the ability of estrogen replacement to reduce performance deficits produced by acute systemic and intrahippocampal muscarinic cholinergic inhibition. Two experiments were performed. In Experiment 1, ovariectomized animals were trained to criterion on the DMP task and then tested with increased intertrial delays and following systemic scopolamine administration. The animals then received either continuous estrogen replacement or sham surgery and were retested beginning 10 days later. In Experiment 2, ovariectomized animals received guide cannulae implanted bilaterally into the hippocampus. Half of these animals also began receiving continuous estrogen replacement. Two months later, the animals were trained on the DMP task and then tested with increased intertrial delays and following systemic as well as intrahippocampal scopolamine administration. Animals received the same test battery 8 months later and were then immediately trained on a reversal task. The results indicate that estrogen-treated animals acquired the DMP task at a significantly faster rate than the ovariectomized, non-estrogen-treated controls. In addition, estrogen replacement significantly reduced deficits in DMP performance produced by intrahippocampal, but not systemic, scopolamine administration. This occurred when animals were tested after 3.5 months, as well as after 12 months, of continuous estrogen replacement. No evidence for an effect of estrogen replacement on spatial working memory or reversal learning was detected. These findings demonstrate that estrogen replacement can enhance acquisition of a spatial memory task and reduce performance deficits associated with hippocampal cholinergic impairment.

Treatment with estrogen and progesterone affects relative levels of brain-derived neurotrophic factor mRNA and protein in different regions of the adult rat brain

Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) were used to examine the effects of acute estrogen and progesterone replacement on relative levels of brain-derived neurotrophic factor (BDNF) mRNA and protein in different regions of the adult rat brain. Adult ovariectomized animals were killed 53 h after receiving estrogen (E53), 53 h after receiving estrogen and 5 h after receiving progesterone (E53P), or 72 h after receiving estrogen and 24 h after receiving progesterone (E72P). Ovariectomized controls were killed 53 and 72 h after receiving vehicle. Tissues from the hippocampus, pyriform cortex, olfactory bulbs, septum, and nucleus basalis/ventral pallidum were dissected. Tissues from the right hemisphere were processed for quantitative RT-PCR analysis of BDNF mRNA, and tissues from the left hemisphere were processed for the detection and quantification of BDNF protein by ELISA. The results demonstrate significant increases in BDNF mRNA in the pyriform cortex of E53- and E53P-treated animals, as well as an increase in BDNF protein in the pyriform cortex of E72P-treated animals, relative to controls. Significant increases in BDNF mRNA were likewise detected in the hippocampus of E53- and E72P-treated animals, but were accompanied by a significant decrease in BDNF protein in the hippocampus of E53P- and E72P-treated animals relative to controls. No significant changes in BDNF mRNA or protein were detected in the olfactory bulbs, frontal cortex, or nucleus basalis/ventral pallidum following hormone treatment; however, an increase in BDNF protein was detected in the septum of E53-treated animals. This may indicate an increase in the retrograde transport of BDNF from the hippocampus to the septum, which could help account for the decrease in BDNF protein detected in the hippocampus following hormone treatment. These findings demonstrate that hormone replacement significantly affects relative levels of BDNF mRNA and protein within specific regions of the brain. These effects may, in turn, contribute to the effects of estrogen replacement on hippocampal connectivity and cognitive processes that have recently been reported.

Estrogen replacement attenuates effects of scopolamine and lorazepam on memory acquisition and retention

A multiple-trial passive avoidance paradigm was used to examine and compare the ability for estrogen replacement to attenuate learning and memory deficits produced by the muscarinic antagonist scopolamine and the benzodiazepine lorazepam. The multiple-trial paradigm was used in order to distinguish effects on acquisition from effects on retention. Estrogen replacement significantly attenuated a scopolamine-induced deficit on passive avoidance acquisition, but not retention. The ability for estrogen to attenuate the effect of scopolamine on acquisition was observed only when the analysis was limited to animals with serum levels of estradiol <200 pg/ml, suggesting that higher levels of estradiol were ineffective. This observation is consistent with at least one recent study showing dose-related effects of estrogen on ChAT-like immunoreactivity in the basal forebrain and supports the hypothesis that effects of estrogen on basal forebrain cholinergic neurons can help to reduce cognitive deficits associated with cholinergic impairment. Estrogen replacement was also observed to protect against a lorazepam-induced impairment on passive avoidance retention. This effect was observed specifically in animals that received estrogen prior to and during training and was not due to any effect of estrogen on serum levels of lorazepam following acute lorazepam administration. Collectively, these data demonstrate the ability for estrogen replacement to attenuate specific pharmacologically induced impairments in learning and retention and provide additional clues as to potential mechanisms by which estrogen replacement may help to reduce cognitive deficits associated with aging and Alzheimer's disease in postmenopausal women.

Estrogen and basal forebrain cholinergic neurons: implications for brain aging and Alzheimer's disease-related cognitive decline

Recent studies suggest that estrogen replacement therapy can reduce the risk and severity of Alzheimer's disease (AD)-related dementia in postmenopausal women. Many different mechanisms by which estrogen therapy may help to reduce the risk and severity of AD-related pathophysiology have been proposed. Recent animal studies suggest that one way in which estrogen replacement may help to reduce cognitive deficits associated with aging and AD is by enhancing the functional status of cholinergic projections to the hippocampus and cortex. Here we review the evidence that estrogen is important in the maintenance of cholinergic neurons projecting to the hippocampus and cortex and that estrogen replacement can enhance the functional status of these neurons, as well as reduce cognitive deficits associated with muscarinic cholinergic impairment. Based on these studies, we conclude that, in animals, short-term treatment with physiological levels of estrogen, or estrogen and progesterone, has significant positive effects on cholinergic neurons in the medial septum and nucleus basalis magnocellularis and on their projections to the hippocampus and cortex. We hypothesize that similar effects in humans may help delay the decline in basal forebrain cholinergic function associated with aging and AD and thereby reduce the risk and severity of AD-related dementia in postmenopausal women.

Impairment of basal forebrain cholinergic neurons associated with aging and long-term loss of ovarian function

Recent studies suggest that women are at greater risk for Alzheimer's disease than men and that estrogen replacement can help to reduce the risk and severity of Alzheimer's-related dementia in postmenopausal women. We have hypothesized that the increased risk for Alzheimer's-related dementia is due, in part, to the loss of ovarian function in postmenopausal women and to the effects that decreased levels of ovarian hormones have on basal forebrain cholinergic function. In the present study, the effects of aging and ovariectomy on cholinergic neurons in the rat basal forebrain were examined to determine (1) whether aging differentially affects cholinergic neurons in the basal forebrain of males vs females, and (2) whether long-term loss of ovarian function produces deficits in basal forebrain cholinergic function beyond those associated with aging and sex. In part I of the study, gonadally intact male and female rats were sacrificed at 13, 19, and 25 months of age and the effects of aging on cholinergic neurons in the medial septum (MS) and nucleus basalis magnocellularis (NBM) were compared. In part II of the study, female rats were ovariectomized at 13 months of age and then sacrificed 3 and 6 months later along with gonadally intact, age-matched controls. Adjacent sections through the MS and NBM were processed for either immunocytochemical detection of choline acetyltransferase (ChAT) and p75NTR-like immunoreactivity or for in situ hybridization detection and quantification of ChAT and trkA mRNA. Results from part I revealed no significant effects of age on the relative size or density of cholinergic neurons in the MS and NBM of gonadally intact animals. Likewise, no significant effects on the relative numbers of cholinergic neurons expressing p75NTR protein were detected. However, a significant decrease in trkA mRNA was detected in the MS of gonadally intact females, but not males, between 13 and 25 months of age. No significant effects of aging on ChAT mRNA were detected. Results from part II revealed significant decreases in both ChAT and trkA mRNA in the MS and NBM of female rats sacrificed 6 months, but not 3 months, following ovariectomy relative to age-matched, gonadally intact controls. Short-term estrogen replacement initiated 6 months following ovariectomy and administered for 3 days prior to sacrifice partially restored ChAT mRNA levels in the MS and trkA mRNA levels in the NBM. These findings suggest that ovarian hormones play a role in maintaining normal levels of ChAT and trkA expression in the MS and NBM. The fact that ChAT mRNA was decreased in the MS and NBM at 6 months following ovariectomy suggests that long-term loss of ovarian function produces a decrease in the functional status of basal forebrain cholinergic neurons projecting to the hippocampus and cortex. In addition, we hypothesize that the decreases in trkA mRNA detected both in the MS as a function of aging, and in the MS and NBM in response to ovariectomy, reflect decreases in the production of high affinity nerve growth factor (NGF) receptors, and a decrease in the responsiveness of the cholinergic neurons to endogenous NGF. This, in turn, may increase the susceptibility of the cholinergic neurons to the effects of aging and disease and thereby contribute to basal forebrain cholinergic decline. We conclude that long-term loss of ovarian function combined with aging has a negative impact on basal forebrain cholinergic neurons. These effects may contribute to the risk and severity of cognitive decline associated with aging and Alzheimer's disease in postmenopausal women.

Levels of trkA and BDNF mRNA, but not NGF mRNA, fluctuate across the estrous cycle and increase in response to acute hormone replacement

Recent studies suggest that hormone replacement therapy can help to reduce the risk and severity of Alzheimer's-related dementia in postmenopausal women. We have hypothesized that these effects are due, in part, to the ability for estrogen and progesterone to enhance hippocampal function, as well as the functional status of cholinergic projections to the hippocampus and cortex, by influencing the expression of specific neurotrophins and neurotrophin receptors. In the present study, quantitative in situ hybridization techniques were used to determine whether the levels of trkA mRNA in the basal forebrain, and nerve growth factor (NGF) mRNA and brain-derived neurotrophic factor (BDNF) mRNA in the hippocampus, are significantly affected by physiological changes in circulating gonadal steroids. Gonadally intact animals were sacrificed at different stages of the estrous cycle and ovariectomized animals were sacrificed at different times following the administration of either estrogen or estrogen plus progesterone. In gonadally intact animals, significant fluctuations in the levels of trkA mRNA in the medial septum (MS), and BDNF mRNA in regions CA1 and CA3/4 of the hippocampus, were detected across the estrous cycle. In animals that received hormone replacement, a significant increase (30.4%) in trkA mRNA was detected in the MS of animals sacrificed 24 h following estrogen administration. Levels of trkA mRNA in the MS declined to control levels over the next 48 h; however, a single injection of progesterone administered 48 h after estradiol appeared to prevent any further decline in trkA mRNA over the next 24 h. In addition, significant increases in BDNF mRNA were detected in the dentate granule cell layer (73.4%), region CA1 (28. 1%), and region CA3/4 (76.9%) of animals sacrificed 53 h after receiving estrogen and 5 h after receiving progesterone. No significant changes in trkA mRNA were detected in the nucleus basalis magnocellularis, and no significant changes in NGF mRNA were detected in the hippocampus. These data demonstrate that levels of trkA mRNA in the MS, and BDNF mRNA in the hippocampus, are affected by physiological changes in the levels of circulating gonadal steroids and are elevated in response to acute hormone replacement. The relevance of these effects to the ability for estrogen replacement to enhance cholinergic activity and hippocampal function, and thereby reduce the risk and severity of Alzheimer's-related dementia in postmenopausal women, is discussed.

Influence of maternal grooming, sex and age on Fos immunoreactivity in the preoptic area of neonatal rats: implications for sexual differentiation

The medial preoptic area (mPOA) of the hypothalamus contains a sexually dimorphic nucleus (SDN-POA) that is 5-7 times larger in males than females and which contributes to the development and expression of male-specific sex behaviors in adulthood. Aside from a critical role for estrogen, the mechanisms that establish and maintain this sex difference are largely unknown. Differences in the size of the SDN-POA are thought to be related to estrogen-associated effects on programmed cell death (apoptosis) during early neonatal development. The expression of male sex behavior is also influenced by maternal behavior during development. During the postnatal period, the dam grooms the anogenital region of the pups to stimulate urination and defecation; however, male pups are groomed significantly more often than females and this maternal attention influences the expression of normal male sexual behavior in adulthood. Based on these observations, we hypothesized that different amounts of anogenital sensory stimulation might contribute to the sexually dimorphic development of the SDN-POA, specifically by providing for different levels of neuronal activation in the SDN-POA resulting in different degrees of cell death. Two experiments were conducted to test this hypothesis. In the first experiment, male and female rat pups on postnatal day 3 (PN 3) received simulated anogenital grooming with a stiff bristle paint brush. One hour later, the brains were removed and sections through the POA were cut and processed for the immunocytochemical detection of Fos-like immunoreactivity (IR) as an indicator of neuronal activation. In the second experiment, male and female littermates were killed on PN 3, 5, 7 and 12 and the number of Fos-immunoreactive cells and pyknotic cells detected in the SDN-POA were counted and compared. Our data demonstrate that anogenital stimulation on PN 3 results in a rapid induction of Fos-immunoreactive in the POA of both males and females. However, the majority of Fos-immunoreactive cells were located in the ventral POA and were distinctly lacking in the SDN-POA. In experiment 2, again no Fos-immunoreactive cells were detected in the SDN-POA of animals examined on PN 5-12. However, there was an increase in the number of pyknotic cells in the area surrounding and including the SDN-POA of females relative to males at PN 5, 7 and 12. Collectively, the data suggest that (1) anogenital grooming during early postnatal development induces a rapid activation of cells in the ventral mPOA, but not in the SDN-POA of rats, (2) there is a greater incidence of cell death in and around the SDN-POA of females vs. males during neonatal development, particularly toward the end of the hormone-sensitive critical period, and (3) Fos expression does not appear to be correlated with the sexually dimorphic development of, and/or programmed cell death within, the developing SDN-POA.

Effects of estrogen on basal forebrain cholinergic neurons vary as a function of dose and duration of treatment

Studies suggest that estrogen replacement can influence learning and memory processes via effects on cholinergic neurons located in specific regions of the basal forebrain. In the present study, immunocytochemical techniques were used to examine the effects of estrogen on basal forebrain cholinergic neurons as a function of the dose and duration of estrogen treatment. Ovariectomized rats received 2, 10, 25, or 100 microg estradiol every other day for a period of 1, 2, or 4 weeks. Sections through the basal forebrain were then processed for the detection of choline acetyltransferase (ChAT) or the low-affinity nerve growth factor receptor (p75NGFR), and the number of immunoreactive cells in the medial septum (MS), the horizontal limb of the diagonal band of Broca (HDB) and the nucleus basalis magnocellularis (NBM) were counted. The effects of dose and duration of estrogen treatment were evaluated by analysis of variance and individual group means were compared with ovariectomized controls using a two-tailed Dunnets test. Administration of 2, 10, or 25 microg estradiol for 1 week produced a dose-related increase in the number of ChAT-like immunoreactive (IR) cells detected in the MS. Likewise treatment with 10 microg estradiol for 1 week, or with 2 microg estradiol for 2 weeks resulted in a significant increase in the number of ChAT-IR cells detected in the NBM. These effects were not observed following treatment with higher doses of estradiol. Nor were they maintained following repeated administration of estradiol for longer periods of time. In contrast, repeated administration of estradiol for 2 or 4 weeks resulted in significant decreases in the number of p75NGFR-IR cells detected in the MS, with the greatest effects observed following treatment with the higher doses of estradiol for longer periods of time. These findings demonstrate that (1) estrogen replacement produces regionally selective effects on basal forebrain cholinergic neurons which vary as a function of both the dose and duration of estrogen treatment, and (2) estrogen has both short-term and longer-term effects on basal forebrain cholinergic neurons, each of which may contribute to the effects of estrogen on learning and memory process and the development of age- and disease-related cognitive decline.

Nerve growth factor induces Fos-like immunoreactivity within identified cholinergic neurons in the adult rat basal forebrain

Immunocytochemical techniques were used to examine and compare the effects of intracerebroventricular administration of nerve growth factor (NGF) on Fos expression within identified cholinergic and non-cholinergic neurons located in different regions of the adult rat basal forebrain. Animals were killed 1, 3, 6, and 12 h after receiving NGF (0.5 or 5.0 microg) or vehicle into the left lateral ventricle and sections through the medial septum, diagonal band of Broca, nucleus basalis magnocellularis, and striatum were processed for the combined immunocytochemical detection of Fos and choline acetyltransferase (a marker for cholinergic neurons), or Fos and parvalbumin (a marker for gamma aminobutyric acid (GABA)-containing neurons). NGF produced a significant increase in the percentage of cholinergic neurons containing Fos-like immunoreactivity within all four regions examined. The largest increases were detected in the medial septum (47.8%) and the horizontal limb of the diagonal band of Broca (67.7%). In these areas, NGF-mediated induction of Fos-like immunoreactivity was detected as early as 3 h, peaked at 6 h, and was reduced by 12 h, postinfusion. Small but significant increases in the percentage of cholinergic neurons containing Fos-like immunoreactivity were also detected in the striatum (4.2%) and in the nucleus basalis magnocellularis (19.2%) 3-12 h following administration of the higher dose of NGF. No evidence for an NGF-mediated induction of Fos within parvalbumin-containing neurons was detected in any of the four regions at any of the time-points examined; however, evidence for an NGF-mediated induction of Fos within epithelial cells lining the lateral ventricle was observed. These data demonstrate that NGF induces Fos expression within cholinergic, and not parvalbumin-containing (GABAergic), neurons in the basal forebrain, and furthermore that intracerebroventricular administration of NGF influences the different subgroups of basal forebrain cholinergic neurons to different degrees.

Effects of estrogen on potassium-stimulated acetylcholine release in the hippocampus and overlying cortex of adult rats

In vivo microdialysis techniques were used to examine the effects of estrogen on potassium-stimulated acetylcholine release in the hippocampus and overlying cortex of adult, ovariectomized rats. Estrogen treatment resulted in a significant increase in the percent change in acetylcholine release induced by potassium relative to controls, particularly after prolonged (90 min) exposure to high potassium. The data suggest that estrogen may help to maintain cholinergic function under conditions where cholinergic afferents to the hippocampal formation and cortex are challenged or impaired.

Expression of estrogen receptor-like immunoreactivity by different subgroups of basal forebrain cholinergic neurons in gonadectomized male and female rats

Recent studies have demonstrated that estrogen administration can produce significant increases in relative levels of choline acetyltransferase (ChAT) mRNA and protein in specific regions of the female, but not the male, rat basal forebrain. In the present study immunocytochemical techniques were used to identify and compare relative numbers of cholinergic neurons containing estrogen receptors within the medial septum, horizontal limb of the diagonal band of Broca, nucleus basalis magnocellularis, and striatum of gonadectomized male and female rats to determine whether there are differences in the percentage of cholinergic neurons expressing estrogen receptors which might contribute to the different regional- and sex-specific effects of estrogen which have been described. Counts of choline acetyltransferase-immunoreactive cells revealed significant regional differences in the average number of cholinergic neurons/section; however, no difference between males and females in the numbers of cholinergic neurons in each of the four regions analyzed was observed. Fifty to eighty percent of the cholinergic neurons detected in both males and females contained estrogen receptor-like immunoreactivity. A small but significant difference between males and females was detected with females having slightly more (10.5%) double-labeled cells than males overall. Individual comparisons revealed that significantly more (18-33%) double-labeled cells were detected in the horizontal limb of the diagonal band, but not in the medial septum, nucleus basalis, or striatum of females vs. males. There was also a small but significant regional difference in the percentage of double-labeled cells with the highest percentage (74.2%) detected in the striatum and the lowest percentage (63.4%) detected in the horizontal limb. None of these differences appear to account for the regional- and sex-specific effects of estrogen on cholinergic neurons which have been observed. We conclude that differences in the effects of estrogen on cholinergic neurons in males vs. females and in different subregions of the female basal forebrain are not due to differences in the percentage of cholinergic neurons expressing estrogen receptors.

Fluctuations in relative levels of choline acetyltransferase mRNA in different regions of the rat basal forebrain across the estrous cycle: effects of estrogen and progesterone

Quantitative in situ hybridization techniques were used to compare relative cellular levels of choline acetyltransferase (ChAT) mRNA in different regions of the female rat basal forebrain at different stages of the estrous cycle and at different time points after the administration of physiological levels of estrogen and progesterone. Significant fluctuations in relative levels of ChAT mRNA were detected during the course of the estrous cycle. In the medial septum (MS) and striatum, the highest levels of ChAT mRNA were detected on diestrus 1. Fluctuations in the nucleus basalis magnocellularis (NBM) were highly variable, with the highest levels detected on diestrus 2. In ovariectomized animals, significant increases in ChAT mRNA were detected in the MS, NBM, and striatum within 1-3 d after a single administration of estradiol. In addition, the effects of estradiol on ChAT mRNA expression in the NBM and striatum were significantly enhanced by the subsequent administration of progesterone. The magnitude and timing of the effects of steroid replacement were consistent with the magnitude and time course of the fluctuations detected during the course of the estrous cycle. These data demonstrate that estrogen and progesterone can increase basal forebrain levels of ChAT mRNA significantly in specific regions of the rat basal forebrain, that the magnitude and time course of the effects vary between different subpopulations of cholinergic neurons, and that the effects are associated with changes in the functioning of specific basal forebrain cholinergic neurons across the estrous cycle.

Expression of EGFR-, p75NGFR-, and PSTAIR (cdc2)-like immunoreactivity by proliferating cells in the adult rat hippocampal formation and forebrain

Immunocytochemical and autoradiographic techniques were used to examine proliferating cells in the adult rat brain along with the expression of specific growth factor receptors and cell cycle proteins. Two hours following an injection of [3H]thymidine ([3H]Thy), dividing cells were detected in the subgranular region of the dentate gyrus and in the subependymal region (SER) extending into the olfactory bulb. Many cells continued to divide over the next 24 h as demonstrated by the ability for thymidine-labeled cells to incorporate bromodeoxyuridine (BrdU); however, the results of BrdU, PSTAIR, and vimentin staining suggest that the majority of the progeny cells detectable by [3H]Thy autoradiography at 3 days and 1 week after injection are postmitotic and at least partially differentiated. Significant numbers of thymidine-labeled cells detected 2 h following thymidine injection in the subgranular region of the dentate gyrus and in the SER of the lateral ventricle stained positively for epidermal growth factor receptor-, vimentin-, and PSTAIR-like immunoreactivity. Significant numbers of thymidine-labeled cells in the SER also stained positively for the low-affinity neurotrophin receptor p75NGFR. No [3H]Thy/p75NGFR-labeled cells were detected in the dentate gyrus. In addition, very few [3H]Thy/PSTAIR- or [3H]Thy/ vimentin-labeled cells were detected in region CA4. These data suggest that proliferating cells located in different regions of the adult brain may not be homogeneous and may be subject to different growth factor regulation.

Estrogen and nerve growth factor-related systems in brain. Effects on basal forebrain cholinergic neurons and implications for learning and memory processes and aging

Estrogen replacement can significantly affect the expression of ChAT and NGF receptors in specific basal forebrain cholinergic neurons. The time-course of the effects is consistent with a direct up-regulation of ChAT followed by either direct or indirect down-regulation of p75NGFR and trkA NGF receptors, possibly due to increased cholinergic activity in the hippocampal formation and cortex and a decrease in hippocampal levels of NGF. Current evidence suggests ChAT, p75NGFR, trkA, and NGF all play a role in regulating cholinergic function in the hippocampal formation and cortex. In addition, all have been implicated in the maintenance of normal learning and memory processes as well as in changes in cognitive function associated with aging and with neurodegenerative disease. It is possible that estrogen may affect cognitive function via effects on NGF-related systems and basal forebrain cholinergic neurons. Effects of estrogen on cognitive function have been reported, as has some preliminary evidence for beneficial effects of estrogen in decreasing the prevalence of and reducing some cognitive deficits associated with Alzheimer's disease. Whether these effects are related to effects on NGF-related systems or basal forebrain cholinergic neurons is currently unknown. Indirect evidence suggests that estrogen interacts with NGF-related systems and that changes in circulating levels of estrogen can contribute to age-related changes in hippocampal levels of NGF. These findings have important implications for consideration of estrogen replacement therapy in pre- and post-menopausal women. Further studies examining effects of different regimens of estrogen replacement as well as estrogen combined with progesterone on NGF and basal forebrain cholinergic neurons in young and aged animals are required. Prospective studies correlating aging and estrogen replacement with numbers of basal forebrain cholinergic neurons and hippocampal and cortical levels of NGF also need to be performed to better assess the potential benefits of estrogen replacement in reducing age- and disease-related cognitive decline.

Sexual stimulation induces Fos immunoreactivity within GnRH neurons of the female rat preoptic area: interaction with steroid hormones

We have shown previously that sexual stimulation (copulation with intromission or vaginocervical stimulation) induces c-fos mRNA and Fos-like immunoreactivity (IR) within estrogen-concentrating and nonconcentrating regions of the female rat forebrain, including regions that contain gonadotropin-releasing hormone (GnRH) neurons in septum and anterior preoptic area. The overall induction of Fos-like IR within these regions was specific to afferent sensory stimulation and did not require treatment with estrogen and progesterone. Because vaginocervical stimulation facilitates lordosis and increases the release of luteinizing hormone, the present study examined whether hormone treatment that promotes sexual receptivity, with or without sexual stimulation, increases Fos-like IR specifically within GnRH-containing neurons. Sexually experienced ovariectomized rats were administered estradiol benzoate (10 micrograms) 48 h and progesterone (500 micrograms) 4 h before either 1 h of paced copulation with a sexually vigorous male, 50 vaginocervical stimulations with a glass rod distributed over 1 h, or no stimulation. Control rats received injections of the oil vehicle. Fos-like IR was found within a significant number of GnRH-positive neurons in the anterior preoptic area caudal to the organum vasculosum following copulation with intromission or vaginocervical stimulation as compared with no stimulation. Although few GnRH cells coexpressed Fos following hormone treatment alone, this treatment enhanced the number of GnRH neurons that coexpressed Fos following vaginocervical stimulation as compared with the effect of vaginocervical stimulation in oil-treated rats. Together, these data indicate that estrogen and progesterone can augment the responsiveness of certain GnRH neurons to vaginocervical stimulation, consistent with the effects of sexual activity on GnRH release.

Effects of estrogen replacement on the relative levels of choline acetyltransferase, trkA, and nerve growth factor messenger RNAs in the basal forebrain and hippocampal formation of adult rats

Previous studies have shown that estrogen replacement can significantly affect the expression of choline acetyltransferase immunoreactivity (ChAT-IR) and low-affinity (p75NGFR) nerve growth factor receptors within cholinergic neurons located in specific regions of the basal forebrain. To extend this work, we have examined the effects of estrogen replacement on relative levels of choline acetyltransferase (ChAT), trkA, and nerve growth factor (NGF) mRNAs within different regions of the basal forebrain and hippocampal formation using quantitative in situ hybridization techniques. Ovariectomized Sprague-Dawley rats received continuous estrogen replacement for 2 days, 1 week, or 2 weeks. The data show that estrogen replacement results in significant increases in relative cellular levels of ChAT mRNA in the medial septum (MS) and nucleus basalis magnocellularis (nBM), but not in the horizontal limb of the diagonal band of Broca (HDB) or the striatum. In contrast, estrogen replacement resulted in significant decreases in relative levels of NGF mRNA in the hippocampus and of trkA mRNA in the MS and nBM (but not in the HDB or the striatum). The time-course of these effects is consistent with estrogen having a direct effect on ChAT expression which is followed by indirect effects on trkA. The data are also consistent with previous findings in which estrogen replacement resulted in significant increases in ChAT-IR which were followed by significant decreases in p75NGFR mRNA and protein and then a reduction in ChAT-IR back to control levels. Together, these data indicate that estrogen replacement can have significant effects on basal forebrain cholinergic function, and that some of these effects may be mediated by effects of estrogen replacement on the expression of NGF and NGF receptors.

In situ hybridization detection of trkA mRNA in brain: distribution, colocalization with p75NGFR and up-regulation by nerve growth factor

In situ hybridization techniques were used to examine the distribution and the nerve growth factor (NGF) regulation of trkA mRNA in the adult rat brain in order to identify neurons in discrete regions of the brain that may be NGF responsive. In agreement with previous studies, trkA mRNA was detected within cells located in the medial septum (MS), diagonal band of Broca (DBB), and caudate. trkA mRNA was also detected in many other regions of the brain, including the nucleus basalis of Meynert, substantia innominata, paraventricular nucleus of the thalamus, interpeduncular nucleus, prepositus hypoglossal nucleus, vestibular nuclei, raphe obscuris, cochlear nucleus, sensory trigeminal nuclei, and gigantocellular as well as perigigantocellular neurons in the medullary reticular formation. By combining in situ hybridization detection of trkA mRNA with immunocytochemical detection of p75NGFR, it was determined that the vast majority (> 90%) of the trkA mRNA-containing cells detected in the MS and DBB also express p75NGFR. Likewise, the vast majority of p75NGFR-IR cells detected in the MS and DBB expressed trkA mRNA. Intracerebroventricular infusions of NGF into the third ventricle adjacent to the preoptic area resulted in a 58% increase in relative cellular levels of trkA mRNA in the horizontal limb of the DBB. These data provide evidence that both p75NGFR and trkA are expressed by NGF-responsive neurons in the MS and DBB. In addition, we note that areas that contained trkA mRNA and that also have been reported to contain p75NGFR are areas where high-affinity NGF binding sites have been observed autoradiographically, whereas areas that contain either trkA or p75NGFR alone are areas where no high-affinity NGF binding has been reported. Together, these findings suggest that both trkA and p75NGFR play an important role in the formation of high-affinity NGF receptors in brain and, furthermore, suggest that NGF may have physiological effects within many regions of the brain outside of the basal forebrain.

Increase in c-erbA alpha 2 mRNA in the parvocellular region of the paraventricular nucleus of the hypothalamus following thyroidectomy in the adult male rat

To examine thyroid hormone regulation of c-erbA alpha 2 mRNA expression in the parvo-cellular region of the paraventricular nucleus of the rat hypothalamus (pPVN), quantitative in situ hybridization was performed using 3H-labeled probe complementary to c-erbA alpha 2 mRNA. Thyroidectomy induced a significant increase in the number of silver grains overlying the cytoplasm in the pPVN relative to sham-operated controls. This effect was prevented by daily injection of thyroxine. These results indicate that hypothyroidism induced an increase in cellular c-erbA alpha 2 mRNA level in the pPVN.

Sexual stimulation activates c-fos within estrogen-concentrating regions of the female rat forebrain

Regions of the brain that concentrate estrogen and progesterone are thought to regulate female sexual behavior by altering gene expression and neural sensitivity to afferent stimulation. We used immunocytochemistry and in situ hybridization to examine c-fos gene expression within estrogen-concentrating regions of the forebrain following various types of sexual stimulation with or without hormone treatment. Ovariectomized rats received injections of estradiol benzoate 48 h and progesterone 4 h before testing. Control rats that had been ovariectomized at least 5 months before testing did not receive hormone treatment. Rats were then either placed into bilevel testing chambers with sexually vigorous males, received manual stimulation of the flanks, received vaginocervical stimulation with a glass rod, or were left in their home cages. Copulation with intromission and ejaculation in hormone-treated rats, or stimulation of the vaginal cervix in both hormone-treated and control rats, produced a dramatic induction of c-fos mRNA and Fos-like immunoreactivity in estrogen-concentrating regions, such as the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, ventromedial hypothalamus, lateral habenula, and medial amygdala, in addition to regions that do not readily concentrate estrogen, such as the neocortex, thalamus, and striatum. Mechanical stimulation of the flanks produced a smaller induction of Fos in these rats, whereas hormone treatment alone had no effect. These data demonstrate that afferent sensory stimulation, but not estrogen or progesterone, regulates c-fos gene expression within different estrogen-concentrating and non-concentrating regions of the female rat forebrain.

RB and Cdc2 expression in brain: correlations with 3H-thymidine incorporation and neurogenesis

Expression of the cell cycle regulatory proteins RB and p34cdc2 was examined in the adult rat brain, with special emphasis on proliferation and neuronal differentiation in the hippocampal formation and olfactory bulb. RB-like immunoreactivity (RB-IR) was detected throughout the brain, with particularly intense staining observed in hippocampal pyramidal cells, pyriform cortex, and cerebellar Purkinje cells. Intense RB-IR and cdc2-IR were also detected in proliferating neuronal precursor cells in the subgranular region of the dentate gyrus and in the subependymal region extending from the anterior lateral ventricle into the olfactory bulb. Many of these cells developed into neurons as assessed by the expression of neuron-specific enolase (NSE) and, in the hippocampal formation, the expression of Fos-IR following pentylenetetrazol-induced seizure activity. A good correlation was observed between the number of proliferating cells expressing intense nuclear RB-IR staining and the number of thymidine-labeled cells that had differentiated into functional hippocampal neurons. A substantial decrease in RB-IR during differentiation was also observed and occurred prior to the expression of NSE. The possibility that the loss of RB may be necessary for neuronal differentiation to proceed is discussed.

Nerve growth factor affects defense-related behaviors, but not lordosis, in ovariectomized, estrogen-treated rats

Effects of NGF and anti-NGF on estrogen-sensitive behaviors were examined in ovariectomized, estrogen-treated rats. Intracerebroventricular (i.c.v.) administration of NGF resulted in a significant decrease in body weight. Daily treatment with low levels of estradiol resulted in a steady increase in lordosis behavior as reflected by average lordosis quotient and lordosis score. No effects of NGF or anti-NGF on lordosis behavior were detected. Estrogen treatment also resulted in a significant increase in the number of vocalizations elicited from female controls by male contact during sex behavior. NGF-treatment enhanced this effect, resulting in significantly more vocalizations elicited earlier in the course of estrogen treatment than were elicited from non-NGF-treated controls. These effects were blocked by progesterone. An increase in the number of rejections elicited by male contact during sex behavior was also observed in NGF-treated animals relative to controls. In addition, i.c.v. infusions of anti-NGF prevented the estrogen-mediated increase in elicited vocalizations, suggesting that NGF may have a physiological role in regulating this behavior. These data implicate NGF in the regulation of specific defense-related behaviors in estrogen-treated rats. Effects of NGF and anti-NGF on immunocytochemical staining for p75NGFR-, and ChAT-like immunoreactivity were also analyzed and are discussed.

Role of local environmental factors in determining tissue-specific effects of estrogen: examination of uterine tissues transplanted to brain

Estrogen stimulates uterine epithelial cells to divide, but not estrogen-concentrating neurons in the adult brain. This effect correlates with recent evidence that estrogen can induce the expression of certain growth-related genes in uterus which are not directly induced by estrogen in the adult brain. The possibility that local diffusible factors play a major role in determining tissue-specific effects of estrogen was examined by transplanting uterine tissues into the brain, muscle and kidney of adult rats and then comparing the effects of estrogen on the incorporation of [3H]thymidine and the expression of Fos-, cdc2- and Rb-like immunoreactivity (IR) on native and transplanted uterine tissues, as well as in estrogen-concentrating regions of the brain adjacent to the uterine grafts. In native uteri, estrogen treatment stimulated Fos-, cdc2-, and Rb-like IR, as well as [3H]thymidine incorporation, within lumenal and glandular epithelial cells. All of these effects were estrogen responsive--no immunoreactive staining within uterine epithelial cells and no signs of epithelial cell proliferation were observed in the native uteri of non-estrogen-treated animals. When uterine tissues were transplanted to brain, Fos-, cdc2-, and Rb-like IR epithelial cells, as well as many [3H]thymidine-incorporating uterine epithelial cells, were observed in all estrogen-treated animals and in some non-estrogen-treated animals as well. Identical results were obtained when uterine tissues were transplanted to skeletal muscle, but not to kidney (in the kidney, transplanted epithelial cells expressed all four parameters but only in estrogen-treated animals, comparable to the native uterus). In contrast, estrogen did not stimulate cell division and did not induce Fos-, cdc2-, or Rb-like IR within estrogen-concentrating neuronal regions of the ventromedial hypothalamus. In addition, the presence of uterine tissue in the brain did not confer the ability of estrogen to stimulate any of these parameters within nearby, estrogen-concentrating regions. These data suggest that there are factors in brain and muscle which can allow uterine epithelial cells to divide in the absence of estrogen. There was no evidence of a diffusible factor in brain which inhibits uterine epithelial cell division, nor of a diffusible factor in uterus which can confer estrogenic stimulation of growth-related genes and cell division to central nervous system neurons. In addition, the data provide the first evidence for estrogen regulation of cdc2 and Rb expression in normal uterus.

Effects of estrogen and fimbria/fornix transection on p75NGFR and ChAT expression in the medial septum and diagonal band of Broca

NGF receptor-expressing cells located in the basal forebrain have recently been shown to contain estrogen (E) receptors (Toran-Allerand and MacLusky. 1989. Soc. Neurosci. Abstr. 15: 954). In the present study, we have examined the effects of E-treatment on p75NGFR and choline acetyltransferase (ChAT) expression by neurons in the medial septum (MS) and the vertical (VDB) and horizontal (HDB) limbs of the diagonal band of Broca using immunocytochemical and in situ hybridization techniques. First, since E-treatment has been shown to affect neuronal survival and to stimulate synaptic reorganization and growth within various regions of the brain, we hypothesized that E-treatment might attenuate the loss of p75NGFR immunoreactivity (IR) which occurs in the MS and VDB following transection of the fimbria/fornix. Contrary to our hypothesis, E-treatment did not attenuate the effects of fimbria/fornix transection. In fact, E-treatment alone produced a significant decrease in the number of p75NGFR-IR cells detected in the MS. Subsequent experiments confirmed that chronic E-treatment produces a down-regulation of both p75NGFR-IR and p75NGFR mRNA in the MS and VDB. In the MS, estrogen appeared to affect a subpopulation of p75NGFR-expressing neurons which were also affected by fimbria/fornix transection since the effects of these two treatments were not additive. In addition, effects of E-treatment on p75NGFR-IR were sex-specific (observed in females but not in males) and were reversible in the MS after 2 weeks, but not after 4 weeks (allowing 2 weeks recovery), of E-treatment. A time-course analysis revealed that effects of E-treatment on p75NGFR-IR were not observed until after 16 days (MS) or 30 days (VDB) of E-treatment and were preceded by a significant and transient increase in ChAT expression in both the MS and VDB. The data are consistent with the possibility that continuous, long-term exposure to gonadal steroids may contribute to a loss of p75NGFR-expressing neurons with age. In addition, the data suggest that p75NGFR expression may play a role in regulating the functioning of specific basal forebrain cholinergic neurons. Different mechanisms by which E-treatment might influence ChAT and p75NGFR expression in brain are discussed.

Thyroidectomy induces Fos-like immunoreactivity within thyrotropin-releasing hormone-expressing neurons located in the paraventricular nucleus of the adult rat hypothalamus

Effects of thyroidectomy on Fos-like immunoreactivity (IR) in the rat brain were examined using single and double-label immunocytochemical techniques. In particular, the possibility that Fos might be involved in thyroid hormone regulation of thyrotropin releasing hormone (TRH)-containing neurons located in the parvocellular region of the paraventricular nucleus of the hypothalamus (pPVN) was examined. Adult, male, Sprague-Dawley rats were used and all animals received either surgical removal of the thyroid gland or sham surgery. Two experiments were performed. In the first experiment, animals were killed 1, 3, or 6 days after surgery and numbers of Fos-like IR cells in the parvocellular (pPVN) and magnocellular (mPVN) regions of the PVN, the anterior hypothalamic nucleus (AH), the lateral hypothalamic nucleus (LH), and the pyriform cortex were determined. In the second experiment, animals received an intracerebroventricular injection of colchicine 5 days after surgery. The next day, animals were killed and numbers of Fos-like IR cells double-labeled for either TRH, corticotropin releasing factor (CRF), or methionine-enkephalin (met-Enk) were determined. Six days after thyroidectomy there was a significant increase in the number of Fos-like IR cells detected in the pPVN. No induction in the pPVN was observed 1 and 3 days after thyroidectomy, and no effects attributable specifically to thyroidectomy (as opposed to stress) on Fos expression in the mPVN, AH, LH, or pyriform cortex were observed. In addition, a rapid, stress-related, induction of Fos-like IR was detected in the mPVN, AH, and LH and was easily distinguished from Fos expression induced in the pPVN as a function of thyroidectomy. The time course for the effect of thyroidectomy on Fos expression in the pPVN paralleled increased plasma TSH concentration. A significant correlation between numbers of Fos-like IR cells in the pPVN and plasma TSH concentration following thyroidectomy was also observed, suggesting that plasma levels of TSH correlate directly with the number of activated TRH-containing neurons located in the pPVN. Double staining for Fos and TRH, CRF, or met-Enk revealed that thyroidectomy induced Fos-like IR specifically within TRH-, but not within CRF-, or met-Enk, expressing neurons in the pPVN. Taken together, the data suggest that Fos-like IR is induced within TRH-expressing neurons in the pPVN as a consequence of decreasing levels of circulating thyroid hormone (TH). Whether this reflects a direct effect of decreasing TH on Fos expression is not yet known; however, the data are consistent with the hypothesis that Fos is involved in TH-associated regulation of TRH production and release.

Effects of daytime and nighttime stress on Fos-like immunoreactivity in the paraventricular nucleus of the hypothalamus, the habenula, and the posterior paraventricular nucleus of the thalamus

Circadian effects on basal and stress-induced Fos-like immunoreactivity (IR) in the paraventricular nucleus of the hypothalamus (PVN), the habenula (Hab) and the posterior paraventricular nucleus of the thalamus (PVN-Thal) were examined. Stress induced a significant increase in the number of Fos-like IR cells within all 3 brain regions. In the Hab, expression was localized specifically to the medial region of the lateral Hab. No differences between the effects of daytime vs nighttime stress on numbers of Fos-like IR cells in the PVN and PVP-Thal were observed. Significantly fewer Fos-like IR cells were observed, however, in the lateral habenula of nighttime vs daytime non-stressed controls, resulting in a significantly greater percentage increase in Fos-like IR in the lateral habenula following nighttime vs daytime stress.

Effects of fimbria-fornix and angular bundle transection on expression of the p75NGFR mRNA by cells in the medial septum and diagonal band of Broca: correlations with cell survival, synaptic reorganization and sprouting

Quantitative in situ hybridization techniques were used to examine the effects of lesions which sever hippocampal cholinergic and cortical afferents on p75NGFR mRNA-expressing cells located in the medial septum (MS) and the vertical (VDB) and horizontal (HDB) limbs of the diagonal band of Broca. Animals received either bilateral transection of the fimbria/fornix, unilateral transection of the angular bundle, or sham surgery. Four days later, animals were sacrificed and sections through the MS, VDB and HDB were processed for detection of the p75NGFR mRNA using in situ hybridization techniques previously described (Mol. Brain Res., 6 (1989) 275-287). Transection of the fimbria/fornix and angular bundle differentially affected p75NGFR-expressing cells in the MS, VDB and HDB within 4 days after injury, in ways which were consistent and correlate with subsequent effects on cell survival, synaptic reorganization and growth. In particular, in the MS and VDB, transection of the fimbria/fornix resulted in a significant decrease in the size of p75NGFR-expressing cells (reductions of 25.9% and 15.1% respectively) which was accompanied by a significant reduction (37.9% and 12.7% fewer grains/cell) in relative levels of p75NGFR mRNA. In contrast, in the HDB, transection of the fimbria/fornix had no significant effect on the average size of p75NGFR-expressing cells; however, a significant increase (49%) in the mean relative level of p75NGFR mRNA was observed which may, in turn, reflect a large increase (as much as 2-3 fold) in the levels of p75NGFR mRNA expressed by a subpopulation of hippocampally projecting cholinergic neurons located in the HDB. Finally, transection of the angular bundle resulted in small, but significant increases (9.4% and 10.9%) in relative levels of p75NGFR mRNA in the MS and VDB, as well as an increase (19.6%) in the number of p75NGFR mRNA-expressing cells in the HDB, on the injured side. No increases in p75NGFR expression in the MS, VDB or HDB contralateral to the lesion were observed; however, a decrease in the size (6.9%) and message content (19.4%) of p75NGFR-expressing cells was detected in the MS contralateral to the lesion. Most importantly, all of these effects are consistent with the subsequent effects of these lesions on the survival of basal forebrain cholinergic cells, and the reorganization and growth of cholinergic afferents to the hippocampal formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term survival of neural transplants to senescence in rats

A critical issue for clinical and research applications of transplant techniques is the long-term survival of transplanted tissue and its effect on the host brain. In this study, entorhinal cortices from donor embryos were transplanted into the lesioned angular bundle of juvenile male Sprague-Dawley rats. Animals were maintained for 2 years and then sacrificed for histological and histochemical examinations. The results indicate that entorhinal transplants survive to old age and that both the host and transplant tissues maintain morphological features consistent with those of short-term neural grafts. An unexpected finding of this experiment was the persistence in the transplanted tissue and adjacent host cortex of a pattern of AChE staining which is typical of early postnatal development.

Expression of NGF receptor in the rat forebrain detected with in situ hybridization and immunohistochemistry

The expression of nerve growth factor (NGF) receptor mRNA and NGF receptor protein was examined in the adult rat basal forebrain using in situ hybridization and immunohistochemical techniques. NGF receptor mRNA and protein were detected within cells in the medial septum, diagonal band of Broca, and nucleus basalis of Meynert. Controls showed that the hybridization signal was not due to nonspecific binding of the probe to heterologous RNAs or other molecules. As expected, the distribution of NGF receptor mRNA-containing cells correlated nicely with the distribution of NGF receptor immunoreactive cells in each of these areas. These data extend previous work which suggests that neurons in these areas express the NGF receptor mRNA and manufacture functional NGF receptors. NGF receptor immunoreactivity was also detected in the arcuate nucleus of the hypothalamus, in the leptomeninges at the base of the brain and overlying the tectum, and within ependymal regions along the lateral walls of the cerebral ventricles. A few weakly stained neurons in the lateral hypothalamus and ventrolateral striatum were also consistently observed. In contrast, NGF receptor mRNA was not detected within any meningial, ependymal, or hypothalamic tissues using in situ hybridization. A cross-linking/immunoprecipitation assay demonstrated normal, membrane-bound NGF receptors within extracts of dorsal superior colliculus, ventromedial hypothalamic, and overlying meningial tissues, proving that the staining observed in these areas was not a non-specific artifact associated with the immunohistochemistry. The lack of hybridization in these areas may reflect levels of NGF receptor mRNA which are too low to be detected by the in situ hybridization methods being used. Alternatively, the staining may represent innervation of these areas by afferents whose cell bodies are located elsewhere, and whose terminals contain the NGF receptor protein.

Regulation of muscarinic receptors in hippocampus following cholinergic denervation and reinnervation by septal and striatal transplants

The regulation of hippocampal muscarinic M1 and M2 receptors was studied by autoradiographic methods following cholinergic denervation and reinnervation from embryonic septal transplant. In young adult male rats the density of M1 sites, labeled either with 3H-pirenzepine (PZ) or 3H-N-methylscopolamine (NMS, in the presence of excess carbachol), exceeded by 4- to 5-fold the density of M2 sites, labeled with 3H-NMS in the presence of excess PZ. Both receptors appeared to be densest in hippocampal regions lowest in acetylcholinesterase or 3H-hemicholinium-3 binding. The distribution of M1 receptors did differ from the distribution of M2 receptors within subregions of the hippocampus. Along the mediolateral axis from the subiculum to the lateral CA 1, the density of M1 receptors is uniform, but the density of M2 receptors decreases. Also apparent is the relatively small difference in density between the CA1 and dentate gyrus for M1 receptors but a significantly greater difference for M2 receptors. However, the response of M1 and M2 receptors to long-term cholinergic denervation following fimbriafornix transection of the septal cholinergic input and to cholinergic innervation by embryonic septal transplants was similar. Long-term denervation (40-60 d) resulted in a 30-60% increase in both M1 and M2 receptors within regions of the hippocampal formation. Receptor levels were reduced to normal in regions innervated by septal transplants. For both receptors, the changes in the density of sites were due to alterations in the Bmax and not the Kd for the radioligands. The specificity of this regulation is supported by the evidence that (1) the degree and topography of the normalization of muscarinic receptor density was entirely dependent on the degree and pattern of cholinergic reinnervation by the fibers of the septal transplant, (2) cholinergic fiber reinnervation by embryonic striatal grafts also down-regulated the density of M1 and M2 receptors, and (3) successfully surviving transplants (e.g., cerebellar and striatal) that did not provide innervation to the hippocampus did not induce down-regulation of muscarinic receptors. Changes in the density of sites were not related to changes in the width of the hippocampus following denervation and reinnervation. The data support the view that the majority of M1 and M2 receptors are located postsynaptically on neurons within the hippocampus and not presynaptically on cholinergic fibers.

Transmitter phenotype is a major determinant in the specificity of synapses formed by cholinergic neurons transplanted to the hippocampus

Embryonic habenular or striatal cholinergic tissues were transplanted to the hippocampal formation of adult rats. The connectivity of these grafts with the host hippocampal formation was analysed using acetylcholinesterase histochemistry and immunocytochemistry with a monoclonal antibody to choline acetyltransferase. Both graft types produced laminar arrangements of acetylcholinesterase-positive fibers in the hippocampal formation that closely resembled the native pattern of cholinergic innervation. In addition, graft-derived choline acetyltransferase-immunoreactive synapses were found in the host hippocampal formation. These synapses were formed on non-immunoreactive dendritic structures and were similar to the types of cholinergic synapses found in the hippocampal formation of normal animals. These data indicate that the cholinergic transmitter phenotype is a major determinant of whether a neuron will form typical cholinergic synapses with hippocampal targets.