Age-related decline of acetylcholine release evoked by depolarizing stimulation

Effect of bite-raised condition on the hippocampal cholinergic system of aged SAMP8 mice

Occlusal disharmony induces chronic stress, which results in learning deficits in association with the morphologic changes in the hippocampus, e.g., neuronal degeneration and increased hypertrophied glial fibrillary acidic protein-positive cells. To investigate the mechanisms underlying impaired hippocampal function resulting from occlusal disharmony, we examined the effects of the bite-raised condition on the septohippocampal cholinergic system by assessing acetylcholine release in the hippocampus and choline acetyltransferase immunoreactivity in the medial septal nucleus in aged SAMP8 mice that underwent the bite raising procedure. Aged bite-raised mice showed decreased acetylcholine release in the hippocampus and a reduced number of choline acetyltransferase-immunopositive neurons in the medial septal nucleus compared to age-matched control mice. These findings suggest that the bite-raised condition in aged SAMP8 mice enhances the age-related decline in the septohippocampal cholinergic system, leading to impaired learning.

Neuroanatomical changes associated with cognitive aging

The literature on the neuroanatomical changes that occur during normal, non-demented aging is reviewed here with an emphasis on the improved accuracy of studies that use stereological techniques. Loss of neural tissue involved in cognition occurs during aging of humans as well as the other mammals that have been examined. There is considerable regional specificity within the cerebral cortex and the hippocampus in both the degree and cellular basis for loss. The anatomy of the prefrontal cortex is especially vulnerable to the effects of aging while the major subfields of the hippocampus are not. A loss of neurons, dendrites and synapses has been documented, as well as changes in neurotransmitter systems, in some regions of the cortex and hippocampus but not others. Species differences are also apparent in the cortical white matter and the corpus callosum where there are indications of loss of myelin in humans, but most evidence favors preservation in rats. The examination of whether the course of neuroanatomical aging is altered by hormone replacement in females is just beginning. When hormone replacement is started close to the time of cycle cessation, there are indications in humans and rats that replacement can preserve neural tissue but there is some variability due to the type of hormones and regimen of administration.

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.

Neurokinin3-R agonism in aged rats has anxiolytic-, antidepressant-, and promnestic-like effects and stimulates ACh release in frontal cortex, amygdala and hippocampus

Neurokinin-3 receptors (NK(3)-R) are localized in brain regions which have been implicated in processes governing learning and memory as well as emotionality. The effects of acute subcutaneous (s.c.) senktide (0.2 and 0.4 mg/kg), a NK(3)-R agonist, were tested in aged (23-25 month old) Wistar rats: (a) in an episodic-like memory test, using an object discrimination task (this is the first study to test for deficits in episodic-like memory in aged rats, since appropriate tests have only recently became available); (b) on parameters of anxiety in an open field test, (c) on indices of depression in the forced swimming test and (d) on the activity of cholinergic neurons of the basal forebrain, using in vivo microdialysis and HPLC. Neither the saline-, nor senktide-treated aged animals, exhibited episodic-like memory. However, the senktide-, but not the vehicle-treated group, exhibited object memory for spatial displacement, a component of episodic memory. Senktide injection also had anxiolytic- and antidepressant-like effects. Furthermore, the active doses of senktide on behavior increased ACh levels in the frontal cortex, amygdala and hippocampus, suggesting a relationship between its cholinergic and behavioral actions. The results indicate cholinergic modulation by the NK(3)-R in conjunction with a role in the processing of memory and emotional responses in the aged rat.

Estrogen therapy and cognition: a review of the cholinergic hypothesis

The pros and cons of estrogen therapy for use in postmenopausal women continue to be a major topic of debate in women's health. Much of this debate focuses on the potential benefits vs. harm of estrogen therapy on the brain and the risks for cognitive impairment associated with aging and Alzheimer's disease. Many animal and human studies suggest that estrogens can have significant beneficial effects on brain aging and cognition and reduce the risk of Alzheimer's-related dementia; however, others disagree. Important discoveries have been made, and hypotheses have emerged that may explain some of the inconsistencies. This review focuses on the cholinergic hypothesis, specifically on evidence that beneficial effects of estrogens on brain aging and cognition are related to interactions with cholinergic projections emanating from the basal forebrain. These cholinergic projections play an important role in learning and attentional processes, and their function is known to decline with advanced age and in association with Alzheimer's disease. Evidence suggests that many of the effects of estrogens on neuronal plasticity and function and cognitive performance are related to or rely upon interactions with these cholinergic projections; however, studies also suggest that the effectiveness of estrogen therapy decreases with age and time after loss of ovarian function. We propose a model in which deficits in basal forebrain cholinergic function contribute to age-related changes in the response to estrogen therapy. Based on this model, we propose that cholinergic-enhancing drugs, used in combination with an appropriate estrogen-containing drug regimen, may be a viable therapeutic strategy for use in older postmenopausal women with early evidence of mild cognitive decline.

Donepezil treatment restores the ability of estradiol to enhance cognitive performance in aged rats: evidence for the cholinergic basis of the critical period hypothesis

Recent studies suggest that the ability of estradiol to enhance cognitive performance diminishes with age and/or time following loss of ovarian function. We hypothesize that this is due, in part, to a decrease in basal forebrain cholinergic function. This study tested whether donepezil, a cholinesterase inhibitor, could restore estradiol effects on cognitive performance in aged rats that had been ovariectomized as young adults. Rats were ovariectomized at 3 months of age, and then trained on a delayed matching to position (DMP) T-maze task, followed by a configural association (CA) operant condition task, beginning at 12-17 or 22-27 months of age. Three weeks prior to testing, rats started to receive either donepezil or vehicle. After one week, half of each group also began receiving estradiol. Acclimation and testing began seven days later and treatment continued throughout testing. Estradiol alone significantly enhanced DMP acquisition in middle-aged rats, but not in aged rats. Donepezil alone had no effect on DMP acquisition in either age group; however, donepezil treatment restored the ability of estradiol to enhance DMP acquisition in aged rats. This effect was due largely to a reduction in the predisposition to adopt a persistent turn strategy during acquisition. These same treatments did not affect acquisition of the CA task in middle-aged rats, but did have small but significant effects on response time in aged rats. The data are consistent with the idea that estrogen effects on cognitive performance are task specific, and that deficits in basal forebrain cholinergic function are responsible for the loss of estradiol effect on DMP acquisition in aged ovariectomized rats. In addition, the data suggest that enhancing cholinergic function pharmacologically can restore the ability of estradiol to enhance acquisition of the DMP task in very old rats following long periods of hormone deprivation. Whether donepezil has similar restorative effects on other estrogen-sensitive tasks needs to be explored.

Heavy metal uranium affects the brain cholinergic system in rat following sub-chronic and chronic exposure

Uranium is a heavy metal naturally present in the environment that may be chronically ingested by the population. Previous studies have shown that uranium is present in the brain and alters behaviour, notably locomotor activity, sensorimotor ability, sleep/wake cycle and the memory process, but also metabolism of neurotransmitters. The cholinergic system mediates many cognitive systems, including those disturbed after chronic exposure to uranium i.e., spatial memory, sleep/wake cycle and locomotor activity. The objective of this study was to assess whether these disorders follow uranium-induced alteration of the cholinergic system. In comparison with 40 control rats, 40 rats drank 40 mg/L uranyl nitrate for 1.5 or 9 months. Cortex and hippocampus were removed and gene expression and protein level were analysed to determine potential changes in cholinergic receptors and acetylcholine levels. The expression of genes showed various alterations in the two brain areas after short- and long-term exposure. Nevertheless, protein levels of the choline acetyltransferase enzyme (ChAT), the vesicular transporter of acetylcholine (VAChT) and the nicotinic receptor beta2 sub-unit (nAChRbeta2) were unmodified in all cases of the experiment and muscarinic receptor type 1 (m1AChR) protein level was disturbed only after 9 months of exposure in the cortex (-30%). Acetylcholine levels were unchanged in the hippocampus after 1.5 and 9 months, but were decreased in the cortex after 1.5 months only (-22%). Acetylcholinesterase (AChE) activity was also unchanged in the hippocampus but decreased in the cortex after 1.5 and 9 months (-16% and -18%, respectively). Taken together, these data indicate that the cholinergic system is a target of uranium exposure in a structure-dependent and time-dependent manner. These cholinergic alterations could participate in behavioural impairments.

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.

Baseline and 8-OH-DPAT-induced release of acetylcholine in the hippocampus of aged rats with different levels of cognitive dysfunction

During aging, neurotransmission systems such as the cholinergic and serotonergic ones are altered. Using rats aged 3 or 24-26 months, this study investigated whether the well-described 8-OH-DPAT-induced increase of hippocampal acetylcholine release was altered in aged rats and whether it may vary according to the magnitude of age-related cognitive deficits. Long-Evans female rats aged 24-26 months were classified as good or bad performers on the basis of their reference-memory performance in a Morris water-maze task. Subsequently, the efficiency of 5-HT(1A) receptor agonist 8-OH-DPAT (0.5 mg/kg, s.c.) in triggering hippocampal acetylcholine release was evaluated by in vivo microdialysis and high performance liquid chromatography analysis. Besides a reduced baseline release in aged rats and a correlation between the baseline release and probe-trial performance in all rats, the results demonstrated that 8-OH-DPAT produced a significant increase of hippocampal acetylcholine release (peak value) in all rats, whether aged or young. While significant in bad performers (+56%), this increase did not reach significance in good performers (+32%). The results suggest that (i) some aspects of cognitive alterations related to aging might be linked to the baseline release of acetylcholine in the hippocampus, and (ii) the cholinergic innervation of the hippocampus of aged rats responds almost normally to systemic activation of 5-HT(1A) receptors, and (iii) differential alterations of cholinergic/serotonergic interactions assessed by determination of the 8-OH-DPAT-induced release of acetylcholine in the hippocampus could not be linked with clarity to the cognitive status of aged rats.

DHA-enriched phospholipid diets modulate age-related alterations in rat hippocampus

Our previous work on rat hippocampus showed that a loss of docosahexaenoic acid (DHA) occurs in the fatty acid composition of phosphatidylethanolamine (PE), plasmenylethanolamine (PmE) and phosphatidylserine (PS) with increasing age. The present study investigated whether a DHA-enriched phospholipid dietary supplement could restore DHA levels and cholinergic activity. Male rats were fed a balanced diet containing both linoleic and alpha-linolenic acids until the age of 2, 18 and 21 months. From 18 to 21 months, one subgroup received a diet supplemented with DHA-enriched phospholipids from egg yolk (E-PL), and another a diet with DHA-enriched phospholipids from pig brain (B-PL). Compared to the control diet, the E-PL diet restored the proportion of polyunsaturated fatty acids (PUFAs: 22:6n-3 and 20:4n-6) in PE and PmE, while enhancing spontaneous and evoked-acetylcholine (Ach) release. The B-PL diet had no effect on PUFAs, but increased basal extracellular levels of Ach in 21-month-old rats as compared to the age-matched control. Our results show that supplementation with DHA-enriched egg PL can enhance Ach release and correct PUFA composition.

Changes in the septohippocampal cholinergic system following removal of molar teeth in the aged SAMP8 mouse

We investigated the effect of dysfunctional teeth on age-related changes in the septohippocampal cholinergic system by assessing acetylcholine (ACh) release and choline acetyltransferase (ChAT) activity in the hippocampus and ChAT immunohistochemistry in the medial septal nucleus and the vertical limb of the diagonal band in young-adult and aged SAMP8 mice after removal of their upper molar teeth (molarless condition). Aged molarless mice showed decreased ACh release and ChAT activity in the hippocampus and a reduced number of ChAT-immunopositive neurons in the medial septal nucleus compared to age-matched control mice, whereas these effects were not seen in young-adult mice. The results suggest that the molarless condition in aged SAMP8 mice may enhance an age-related decline in the septohippocampal cholinergic system.

Age-related modifications of effects of ketamine and propofol on rat hippocampal acetylcholine release studied by in vivo brain microdialysis

BACKGROUND

We sometimes encounter impairment of learning and memory after general anesthesia in elderly patients. The aim of this study was to examine age-related modifications of the effects of ketamine and propofol on rat hippocampal acetylcholine (ACh) release because hippocampal cholinergic neurons are supposed to be involved in learning and memory.

METHODS

The experiments were performed on male Wistar young rats (2 months old) and old rats (18 months old), using in vivo brain microdialysis technique under freely moving condition. After initial sampling of three collections, test drugs were administered. The ACh release was determined by the HPLC-ECD method.

RESULTS

In old rats, the hippocampal basal ACh release was significantly lower than in young rats. Ketamine (25 and 50 mg kg(-1) i.p.) increased and propofol (25 and 50 mg kg(-1) i.p.) decreased the hippocampal ACh release in both young and old rats. Furthermore, ketamine 50 mg kg(-1) i.p. (anesthetic dose) produced facilitatory effects on the hippocampal ACh release in young rats (193% of the basal release), while in old rats the same dose of ketamine i.p. produced more pronounced facilitatory effects on the hippocampal ACh release (317% of the basal release). On the other hand, propofol 50 mg kg(-1) i.p. (anesthetic dose) produced inhibitory effects on the hippocampal ACh release in young rats (56% of control) and in old rats (77% of control). Although the maximal inhibitory peak effects of propofol 50 mg kg(-1) i.p. did not differ significantly between young rats and old rats, decrease of the hippocampal ACh release in old rats persisted longer than in young rats.

CONCLUSION

Ketamine produced more pronounced facilitatory effects on the hippocampal ACh release in old rats, as compared with young rats. On the other hand, propofol has inhibitory effects on the hippocampal ACh release in young and old rats. The aging process may suppress the ability to recover from the inhibitory anesthetic state induced by propofol.

Facilitation of memory performance by a novel muscarinic agonist in young and old rats

AF150(S), a partial M1 muscarinic receptor agonist, was tested for its ability to improve performance in rats in a delayed matching-to-position task. Young and old rats received intraperitoneal injections of 0, 1, and 4 mg/kg of AF150(S). AF150(S) significantly enhanced matching accuracy for both young and old rats. Fits of exponential functions to discriminability measures showed that the enhancement was manifest as a reduction in the rate of forgetting. These results indicate that AF150(S) may be a useful therapeutic agent for improving cognitive function.

Age-related attenuation of stimulated cortical acetylcholine release in basal forebrain-lesioned rats

In vivo microdialysis was used to measure the effects of partial deafferentation of cortical cholinergic inputs on acetylcholine efflux in young (four to seven months) and aged (24-28 months) male F344/BNNIA rats. Partial deafferentation was produced by bilateral infusions of the immunotoxin 192 immunoglobulin G-saporin (0.56 microg/1.0 microl) or its vehicle solution into the ventral pallidum/substantia innominata region of the basal forebrain. The lesion produced comparable (65%) decreases in basal cortical acetylcholine efflux in young and aged rats. Presentation of a complex environmental stimulus (exposure to darkness/palatable food), in conjunction with the systemic administration of the benzodiazepine receptor weak inverse agonist ZK 93 426, increased cortical acetylcholine efflux in young shams, aged shams and young lesioned rats, but not in aged lesioned rats. Administration of the benzodiazepine receptor partial inverse agonist FG 7142, in the absence of the environmental stimulus, comparably stimulated cortical acetylcholine efflux in young and aged sham rats. FG 7142-induced increases in acetylcholine efflux were attenuated by approximately 50% following partial deafferentation in both young and aged rats. These results suggests that, under certain conditions, ageing potently interacts with the integrity of the cortical cholinergic afferent system. The effects of ageing on cortical cholinergic function may be most potently revealed by experiments assessing age-related limitations in the responsiveness of a partially deafferented cholinergic system to certain behavioral and/or pharmacological stimuli.

Attenuated stress response of hippocampal acetylcholine release and adrenocortical secretion in aged rats

The effects of aging on the stress response of the septo-hippocampal cholinergic neurons was assessed by monitoring hippocampal acetylcholine (ACh) release. Young (3-4 month old) and aged (23-24 month old) male rats were subjected to restraint stress for 1 h. ACh was elevated within 15 min of the onset of restraint stress (177.5% of basal level) in young rats but not in aged rats. Corticosterone concentration was significantly elevated by restraint stress in young rats but not in aged rats. The present results suggest that stress response of the septo-hippocampal cholinergic neurons and the hypothalamic-pituitary-adrenocortical axis is attenuated during aging.

Potassium, but not atropine-stimulated cortical acetylcholine efflux, is reduced in aged rats

Using in vivo microdialysis, cortical acetylcholine (ACh) efflux was measured in freely moving Brown Norway/Fischer 344 F1 rats, aged 4 or 22 months. The effects of local, intracortical perfusion of atropine (1.0 or 100.0 microM) via the dialysis probe were compared to local K+ (100.0 mM) stimulation in the presence of elevated extracellular Ca2+ (2.5 mM). Basal cortical ACh efflux in aged rats was similar to that of young animals. Administration of atropine (1.0 or 100.0 microM) via the cortical dialysis probe substantially increased cortical ACh efflux, but did not differentially stimulate ACh efflux in young and aged rats. In contrast, ACh efflux stimulated locally with K+ and Ca2+ was significantly reduced in aged rats relative to young adults. The implications of the dissociable effects of K(+)-depolarization and muscarinic blockade for local regulation of cortical ACh efflux in aged animals are discussed.

Age-related changes in diurnal acetylcholine release in the prefrontal cortex of male rats as measured by microdialysis

Extracellular levels of acetylcholine in the prefrontal cortex were measured using the micro-dialysis method in freely moving young (three to four months old) and old (23 to 24 months old) male rats over a period of 24 h to examine the effect of aging on prefrontal acetylcholine release. Prefrontal acetylcholine release during a 24 h period exhibited a diurnal variation with higher levels during the dark cycle than during the light cycle in young rats but not in old rats. In addition, prefrontal acetylcholine release was closely associated with spontaneous activity in young rats but not in old rats. The present study suggests that aging reduces diurnal changes in the prefrontal acetylcholine release and that there is a cross-correlation between the prefrontal acetylcholine release and spontaneous locomotor activity in male rats.

Modulation of hippocampal ACh release by chronic nicergoline treatment in freely moving young and aged rats

The effects of nicergoline on basal and K(+)-stimulated release of ACh in the hippocampus of 3- and 19-month old rats has been studied by microdialysis. A significant decrease of basal ACh release (59%) was found in aged vehicle treated rats in comparison to young rats. High-K+ (100 mM) in the perfusate strongly increased the release of ACh by up to 6-fold over the baseline of both young and aged rats. Chronic oral administration of nicergoline to aged rats (5 mg/kg b.i.d. for 6 weeks) significantly reversed (93%) the age-related decrease of basal release of ACh, leaving the increase due to K+ depolarization unchanged. In young animals, nicergoline did not affect the basal output of ACh, but enhanced the K(+)-evoked release of ACh by 39%. Results from this study demonstrate that nicergoline treatment increases the ability of hippocampal cholinergic terminals to release ACh, and suggest that this drug can reset the cholinergic impairement associated with aging.

Importance of forebrain cholinergic and GABAergic systems to the age-related deficits in water maze performance of rats

The present study investigated the performance of rats at 3-4 months and 21 months of age in the Morris water maze and correlated age-related cognitive deficits with changes in both cholinergic and GABAergic systems in the frontal cortex. The older rats were divided into two groups, unimpaired old and impaired old according to their ability to find a hidden submerged platform in the water maze, for electrophysiological, neurochemical, and morphological studies. The firing rate of frontal cortical neurones was recorded from the motor area of the frontal cortex under urethane anaesthesia and was found to be significantly slower in the two aged groups of rats compared to the young rats, but there were no differences between the two aged groups. The sensitivity of frontal cortex neurones of the impaired and unimpaired old age groups to ACh and to carbachol was significantly lower than that of the young group, but there were no differences between the two old age groups. In contrast, sensitivity of frontal cortex neurones to bicuculline was significantly higher in the aged rats compared with the young rats and was significantly greater in the impaired old rats than in the unimpaired old rats. The sensitivity of cortical neurones to glutamate was unaffected by age. There were also significant correlations between the percentages of cortical neurones responding to ACh and bicuculline and different parameters of water maze acquisition during days 7-8, but not during days 2-3, when spatial learning had not begun, and days 13-14, when spatial learning was complete. Biochemical and morphological analyses did not show any significant differences in ChAT activity and AChE-positive fibre density in the frontoparietal cortices of the three groups of rats. The results demonstrate that the learning deficit observed in old age rats cannot be adequately explained solely by a reduction in cholinergic receptor sensitivity and that an age-related increase in GABAergic tone may be a more important determinant of cognitive impairment.

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.

Spontaneous acetylcholine release in the hippocampus exhibits a diurnal variation in both young and old rats

Extracellular levels of acetylcholine (ACh) in the hippocampus were measured by the microdialysis method in freely moving young (3-4 months old) and old (18-24 months old) female rats over a period of 24 h to examine the effect of aging on hippocampal ACh release. Hippocampal ACh release during a 24-h period exhibited a diurnal variation with higher levels during the dark cycle than during the light cycle in old rats as well as young rats. The present study suggests that a diurnal variation in ACh release is maintained fairly well until the rats are at least 24 months old.

Impairment of acquisition but not retention of a simple operant discrimination performance in aged Fischer 344 rats

Although deterioration of learning and memory in aged rodents has been reported using a variety of tasks, little information is available on the effects of aging on acquisition and retention of reference memory in food-motivated tasks. In the present study, to examine reference memory, a simple operant discrimination task was used. Aged (24 month) and young (5 month) Fischer 344 rats were trained to discriminate brightness for food reward in an operant chamber. Aged rats showed significantly impaired learning compared to young rats, and the level of discrimination of aged rats was significantly lower than that of young rats. The aged rats also exhibited impaired learning in discriminating an auditory cue. Aged rats as well as young rats had good retention of brightness discrimination performance 30 days after the last training session. Scopolamine (0.3 mg/kg) did not impair discrimination learning nor affect well-trained discrimination performance in young rats. These data suggest that acquisition but not retention of reference memory is impaired in aged rats, and that the impaired acquisition observed in aged rats may not be due to dysfunction of the central cholinergic systems.

Propentofylline (HWA285) inhibits the release of dopamine during transient ischemia and modulates its metabolism in rat striatum

Propentofylline (HWA285) has been reported to protect neuronal cells through the inhibition of glutamate release during transient ischemia. We studied whether HWA285 inhibits dopamine (DA) release, and how HWA285 modulates DA metabolism in the rat model. HWA285 was perfused through a microdialysis probe placed in the rat striatum during 20 min transient ischemia. In rats perfused by HWA285, ischemic DA release was significantly inhibited, and DA metabolism showed better recovery in contrast with unperfused rats.

5-HT3 receptor ligands lack modulatory influence on acetylcholine release in rat entorhinal cortex

The objective of this study was to explore the role of 5-HT3 receptors in modulating potassium (K+)-evoked release of [3H]-acetylcholine ([3H]-ACh) from superfused slices of rat entorhinal cortex previously loaded with [3H]-choline. Rat entorhinal cortices were cross-chopped into 300 microns slices, superfused with oxygenated Krebs buffer containing 2.5 mmol/l Ca2+ and stimulated with two consecutive exposures of 20 mmol/l K+ for 4 min (S1 and S2, respectively). Compounds were added 20 min before S2 stimulation and remained in the superfusion buffer for the duration of the experiment. The S2/S1 ratio was then calculated. Stimulated release of [3H]-ACh was dependent on extracellular Ca2+ and K+ concentration. In Sprague Dawley rats, 2-methyl-5-HT (10(-9)-10(-6) mol/l), in the presence of 1 mumol/l ritanserin or 1 mumol/l ondansetron, had no influence on K(+)-evoked release of [3H]-ACh. In slices prepared from Hooded Lister rats, 2 mumol/l 5-HT but not 2-Me-5-HT significantly (P < 0.05) inhibited K(+)-evoked [3H]-ACh release only 17% in the presence of 1 mumol/l ritanserin. However, 2 mumol/l 2-Me-5-HT plus 1 nmol/l ondansetron had no effect. High performance liquid chromatography coupled to electrochemical detection (HPLC-ECD) was used to monitor endogenous release of ACh in the above conditions to confirm data from the radiolabelled experiments. No significant inhibition or increase in K(+)-evoked ACh release was observed with either 5-HT3 receptor agonists or antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependent modulation of in vivo cortical acetylcholine release by benzodiazepine receptor ligands

In vivo microdialysis was utilized to determine the effects of benzodiazepine receptor (BZR) ligands on cortical acetylcholine (ACh) release in awake young and aged rats. There were no significant differences in baseline cortical ACh release as a function of age. While administration of the BZR selective inverse agonist ZK 93 426 increased ACh release in both groups of animals, the aged rats exhibited a greater stimulation. Unexpectedly, under the present testing conditions, the BZR agonist chlordiazepoxide (CDP) had no systematic effect on ACh release in either group. The presence or absence of these drug effects or drug-age interactions was not secondary to the impact of these compounds on behavioral activity. Cortical ACh release could also be stimulated by turning off the lights in the observation room or by the systemic administration of scopolamine. Aged rats were at least as able as their younger counterparts to respond to these manipulations with increased release. These results suggest that basal and stimulated release of cortical ACh is not impaired at the ages studied. Moreover, selective inverse BZR agonists may be a potent way of trans-synaptically stimulating cortical cholinergic transmission.

Decreased [3H]hemicholinium binding to high-affinity choline uptake sites in aged rat brain

The binding of [3H]hemicholinium ([3H]HCh-3) to sodium-dependent high-affinity choline uptake sites provides a useful neuroanatomical and functional marker of the cholinergic system. We examined the autoradiographic distribution of [3H]HCh-3 binding sites in the forebrain of young (4-6 months) and old (32 months) rats. There was a widespread reduction of [3H]HCh-3 binding site density in the aged rat brain. This loss presented regional differences with maximal reduction in the medial and posterior striatum (55%) and in the dentate gyrus (47%), in limbic areas such as basolateral amygdala, tubercle olfactorium and piriform cortex the autoradiographic signal was about 25-30% lower. In aged hippocampus and cerebral cortex the density of [3H]HCh-3 binding sites was about 40% lower, the difference between young and senescent animals being less evident in the medial septum and basal nucleus. No significant alterations were observed in interpeduncular nucleus from old rats. These data are in agreement with the functional results obtained by measuring other cholinergic parameters in the aged rat and confirm the vulnerability of cholinergic system during aging.

Determination of acetylcholine release in the striatum of anesthetized rats using in vivo microdialysis and a radioimmunoassay

A vertical-type in vivo microdialysis probe and a sensitive, specific radioimmunoassay (RIA) were used to study the mechanism of acetylcholine (ACh) release in the striatum of anesthetized rats. Without the use of physostigmine, a cholinesterase inhibitor, our RIA could still detect the amount of ACh present in the perfusate (5.6 +/- 0.6 fmol/min, n = 16). Tetrodotoxin (1 microM) produced a significant decrease in the amount of ACh collected in the perfusate, suggesting that basal ACh determined under the present experimental conditions was related to cholinergic neural activity. Atropine (0.1-1 microM) applied topically via the dialysis probe did not affect the amount of ACh recovered in the perfusate in the absence of physostigmine. Addition of physostigmine (10 microM) to the perfusion fluid produced about a 100-fold increase in the amount of ACh collected. In the presence of physostigmine, topical administration of atropine and pirenzepine (0.01-1 microM) through a dialysis probe produced a further three- to fourfold increase in ACh output, whereas a slight increase was produced by AF-DX 116 at the highest concentration (1 microM). These results indicate that presynaptic modulation of ACh release in the striatum does not occur under basal conditions, and that presynaptic M1 muscarinic receptors are involved in the modulation of ACh release when the ACh concentration is raised under certain conditions.

In vivo acetylcholine release as measured by microdialysis is unaltered in the hippocampus of cognitively impaired aged rats with degenerative changes in the basal forebrain

Acetylcholine (ACh) release was studied in awake, freely moving animals using in vivo microdialysis in the hippocampus of young (3-month-old) and aged (24-month-old) female Sprague-Dawley rats. Two groups of aged rats were selected on basis of their spatial learning performance in the Morris water maze: non-impaired aged rats which performed as well as the young control animals, and impaired aged rats which learnt the task very poorly. Baseline ACh overflow (in the presence of 5 microM neostigmine) was 1.9 +/- 0.3 +/- pmol/15 min in the young animals and 1.6 +/- 0.4 pmol/15 min in both the impaired and the non-impaired aged rats; these levels did not differ from each other. Depolarization by KCl (100 mM) or muscarinic receptor blockade by atropine (3 microM) added to the perfusion fluid produced dramatic, 4-6-fold, increases in ACh overflow that was similar in magnitude in both the young and the aged impaired and non-impaired rats. Behavioral activation by either handling or electrical stimulation of the lateral habenula produced 2-3-fold increases in extracellular ACh-levels in the hippocampus similarly in all three groups. The results indicate that hippocampal ACh release is maintained in aged rats that exhibit severe spatial learning and memory impairments and that the septo-hippocampal cholinergic system retains its capacity to increase its ACh release in response to both K(+)-induced depolarization and behavioral activation in the aged rat.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo probing of the brain cholinergic system in the aged rat. Effects of long-term treatment with acetyl-L-carnitine

The regulation of acetylcholine (ACh) release by the different subtypes of muscarinic (M) receptors in the hippocampus of freely-moving Fischer and Sprague-Dawley rats, was investigated. Atropine (10 mumol/kg i.p.) induced a pronounced increase of ACh release (+400% over basal values) in the hippocampus of young rats (3 months) while the effect was drastically reduced (+100% over basal values) in old rats (24 months). The preferential M2 antagonist AF-DX 116 (50 mumol/kg i.p.) showed similar effects in young and old rats being, furthermore, 10 times less potent than atropine. The preferential M1 antagonist pirenzepine (50 mumol/kg i.p.) was even less potent than AF-DX 116 in enhancing ACh release in young rats, while the effect was more pronounced in the old ones. Therefore, the effect of the preferential M3 antagonist 4-DAMP was studied. 4-DAMP 10(-6) M, dissolved in the Ringer solution perfusing the hippocampus, induced an enhancement of ACh release (+200% and +70% over basal values, in young and old rats, respectively) which was comparable to that obtained after atropine at the same concentration. AF-DX 116 and pirenzepine, on the other hand, were by far less potent. Six months' pretreatment with acetyl-l-carnitine (ALCAR) reduced the significant differences between young and old rats in the release response after M1 and M3 receptor antagonists. Taken all together, these findings indicate that the regulation of ACh release, at least in the hippocampus, is mainly through the M3 receptors subtype of muscarinic receptors and that this subtype is the most involved in the aging process. Moreover, the ability of ALCAR to preserve the receptor-mediated functional ACh release response with respect to old animals suggests that ALCAR could be utilized in the amelioration of receptor functionality in the aging brain.

Monitoring of acetylcholine released from postnatal rat basal forebrain cholinergic neurons cultured on membrane filter by cell bed perfusion system and HPLC-ECD

A method for the determination of release of endogenous neurotransmitter from cultured neurons has been developed. Basal forebrain cholinergic neurons from postnatal rats were cultured on polyethyleneimine-coated membrane filter with nerve growth factor. The membrane filter, on which the neurons lived and extended neurites, was packed in a cell bed chamber for a closed perfusion. The perfusate was fractionated and the content of acetylcholine released from cultured neurons was measured by high-performance liquid chromatography-electrochemical detection (HPLC-ECD). Acetylcholine release evoked by glutamate and high K+ was 1.5- and 3-fold greater than the spontaneous level. This method for the determination of the neurotransmitter released from cultured neurons is very advantageous for investigating regulation of transmitter release in central neurons.