Stroke injury in rats causes an increase in activin A gene expression which is unaffected by oestradiol treatment

Activins are members of the transforming growth factor-beta superfamily that exert neurotrophic and neuroprotective effects on various neuronal populations. To determine the possible function of activin in stroke injury, we assessed which components of the activin signalling pathway were modulated in response to middle cerebral artery occlusion (MCAO). Furthermore, because oestradiol replacement protects against MCAO-induced cell death, we explored whether oestradiol replacement influences activin gene expression. Female Sprague-Dawley rats underwent permanent MCAO and the expression of activins and their corresponding receptors was determined by semiquantitative reverse transcriptase-polymerase chain reaction at 24 h after onset of ischaemia. We observed up-regulation of activin betaA and activin type I receptor A mRNA in response to injury. Dual-label immunocytochemistry followed by confocal z-stack analysis showed that the activin A expressing cells comprised neurones. Next, we monitored the time course of activin betaA mRNA expression in oestradiol- or vehicle-treated rats at 4, 8, 16 and 24 h after MCAO via in situ hybridisation. Starting at 4 h after injury, activin betaA mRNA was up-regulated in cortical and striatal areas in the ipsilateral hemisphere. Activin betaA mRNA levels in the cortex increased dramatically with time and were highest at 24 h after the insult, and oestradiol replacement did not influence this increase.

Estrogen therapy: does it help or hurt the adult and aging brain? Insights derived from animal models

Hormone therapy and estrogen therapy in postmenopausal women have been thought to ameliorate cognitive dysfunction and decrease the risk and/or progress of neurodegenerative conditions such as Alzheimer's disease and stroke. Furthermore, estrogens have been shown to exert neuroprotective actions in a variety of in vitro and in vivo models of brain injury. However, the findings of the Women's Health Initiative have made us re-evaluate these assumptions. Our laboratory has shown that physiological levels of estradiol attenuate ischemic brain injury in young and middle-aged female rats. We have begun to probe the cellular and molecular mechanisms that underlie these novel non-reproductive actions of this steroid. Our findings demonstrate that in both young and aging rats, treatment with physiological concentrations of estradiol decreases ischemic injury by almost 50%, compared with oil-treated controls. Additionally, our data suggest that estradiol acts by altering the expression of genes that suppress apoptosis and enhance survival in the penumbral region of the infarct. These observations demonstrate that estrogen therapy protects against stroke-related injury in young and aging female rats and strongly suggest that middle-aged animals remain responsive to the protective actions of estradiol. Furthermore, they suggest that estrogen therapy protects against cell death by influencing the expression of genes that suppress apoptotic cell death pathways.

Glutamic acid decarboxylase 67 (GAD67) gene expression in discrete regions of the rostral preoptic area change during the oestrous cycle and with age

Gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, is important to the timing and amplitude of the gonadotrophin-releasing hormone (GnRH)-induced luteinizing hormone (LH) surge on pro-oestrus. Data suggest that GABA input in the preoptic area must decrease for a normal LH surge to occur in young rats. We have previously found that ageing alters the timing and amplitude of the LH surge. Therefore, this study focused on changes in GAD(67) gene expression, a reflection of GABA synthesis, in two regions of the rostral preoptic area, the organum vasculosum of the lamina terminalis (OVLT) and the anteroventral periventricular nucleus (AVPV) during the oestrous cycle and with age. We assessed the possibility that the expression of GAD(67) mRNA in these regions displays time-related and age-dependent changes on pro-oestrus. Our results demonstrate that, with age, overall expression of GAD(67) mRNA decreases in the area surrounding the OVLT and in the AVPV. Young rats display a diurnal rhythm in GAD(67) mRNA in both regions. GAD(67) mRNA expression is high during the early morning hours of pro-oestrus and then declines around the time of the GnRH-induced LH surge. In addition, the diurnal rhythm disappears in the AVPV and is attenuated in the area surrounding the OVLT of middle-aged proestrous rats. These findings suggest that a loss of rhythmicity in GAD(67) gene expression and maintenance of inhibitory tone on proestrous afternoon may alter the timing and amplitude of the LH surge, as previously observed in middle-aged rats.

The 'menopause' and the aging brain: causes and repercussions of hypoestrogenicity

We are beginning to recognize that the regulation of reproductive senescence in females and its consequences are more complex than originally thought. We now realize that the brain plays an important role in the transition to infertility in rodents and perhaps contributions to the perimenopausal transition in women. Furthermore, the absence of estrogens leads to changes in many physiological systems. Thus, it is becoming clear that it is important to understand the broad impact of the prolonged hypoestrogenic state that characterizes the menopause.

Estradiol is a protective factor in the adult and aging brain: understanding of mechanisms derived from in vivo and in vitro studies

We have shown that 17beta-estradiol exerts profound protective effects against stroke-like ischemic injury in female rats. These effects are evident using physiological levels of estradiol replacement in ovariectomized rats and require hormone treatment prior to the time of injury. The protective actions of estradiol appear to be most prominent in the cerebral cortex, where cell death is not apparent until at least 4 h after the initiation of ischemic injury and where cell death is thought to be apoptotic in nature. Middle-aged rats remain equally responsive to the protective actions of estradiol. The maintenance of responsiveness of the cerebral cortex to the neuroprotective actions of estradiol was unexpected since responsiveness of the hypothalamus to estradiol decreases dramatically by the time animals are middle-aged. We believe that the protective actions of estradiol require the estrogen receptor-alpha, since estradiol does not protect in estrogen receptor-alpha knockout mice. We have also implemented a method of culturing cerebral cortical explants to assess the protective effects of estradiol in vitro. This model exhibits remarkable parallelisms with our in vivo model of brain injury. We have found that 17beta-estradiol decreases the extent of cell death and that this protective effect requires hormone pretreatment. Finally, 17alpha-estradiol, which does not interact effectively with the estrogen receptor, does not protect; and addition of ICI 182,780, an estrogen receptor antagonist, blocks the protective actions of estradiol. We have begun to explore the molecular and cellular mechanisms of estradiol-mediated protection. In summary, our findings demonstrate that estradiol exerts powerful protective effects both in vivo and in vitro and suggest that these actions are mediated by estrogen receptors.

Parallel declines in Fos activation of the medial anteroventral periventricular nucleus and LHRH neurons in middle-aged rats

The middle-age decline in reproductive function is manifested by reduced LHRH release, resulting in a decreased magnitude and delay of onset of the LH surge. Earlier studies suggested that the reductions in LHRH neural activation in middle-aged rats resulted from deficits in the afferent drive to the LHRH neurons. One critical afferent to the LHRH neurons lies in the anteroventral periventricular preoptic area (AVPv) nucleus. The neurons of the medial AVPv are synchronously activated to express Fos with LHRH neurons at the time of an LH surge in young adult animals. The present study examined whether, in middle age, reductions in the activation of AVPv neurons accompany the reduction in Fos activation in LHRH neurons. Young (3- to 4-month-old) and middle-aged (10- to 12-month-old) spontaneously cycling and ovariectomized steroid-replaced rats were killed during peak and early descending phase of the LH surge, and their brains were examined for Fos in LHRH and AVPv neurons. Young animals had a characteristic increase in Fos expression in both LHRH and AVPv neurons. In middle-aged rats, the proportion of LHRH neurons expressing Fos at the time of an LH surge was reduced by approximately 50%, irrespective of whether surges were spontaneous or induced by exogenous steroids. A similar reduction in the number of Fos+ cells (by approximately 50%) was noted in the medial AVPv. Linear regression analysis of the relationship between the extent of Fos activation in LHRH and AVPv neurons revealed a strong positive correlation (r(2) = 0.66; P < 0.01), suggesting that changes in the AVPv's drive to LHRH neurons underlie the decrease in LHRH activity in middle age. A second series of experiments examined whether decreased input from the AVPv could account for reduced Fos activation in LHRH neurons seen in middle-aged animals. When the medial AVPv was lesioned, LHRH neurons failed to express Fos on the side ipsilateral to the lesion. Animals with lesioned medial AVPv also had significantly lower LH values than animals with an intact medial AVPv. Taken together, these data suggest that a principal deficit in middle-aged rats is the ability of the medial AVPv to stimulate LHRH neurons.

Orbital response indicates nasal pungency: analysis of biomechanical strain on the skin

Stimulation of the human nasal passage with pungent vapor elicits motor responses in a zone around the eye. This investigation addressed whether quantification of such responses, particularly activity of the orbicularis oculi muscle, could yield a sensitive index of nasal pungency. We placed an array of small, high-contrast targets just beneath the lower eyelid and videotaped their movement to capture deformation of the skin atop the orbicularis oculi during 3 s stimulation with pungent concentrations of ethyl acetate. Eleven subjects participated. Analysis of the movements served to determine mechanical strain, which yielded a single index that we termed 'maximum strain'. This increased with concentration of the vapor and with time during and just after stimulation. Comparison with psychophysical data showed that the strain became evident at concentrations just detectable as pungent. Maximum strain measured on the skin shows promise as an objective index of pungency.

Chronic smoking enhances tachykinin synthesis and airway responsiveness in guinea pigs

This study tests the hypothesis that the bronchial hyperreactivity induced by chronic cigarette smoke (CS) exposure involves the increased expression and release of tachykinins and calcitonin gene-related peptide (CGRP) from afferent nerve fibers innervating the airways. In guinea pigs chronically exposed to CS (20 min twice daily for 14-17 d), peak response in total lung resistance to capsaicin (1.68 microg/kg, intravenously) was significantly greater than that evoked by the same dose of capsaicin in control (air-exposed) animals. This augmented response in CS-exposed animals was abolished after treatment with CP-99994 and SR-48968, the neurokinin (NK)-1 and NK-2 receptor antagonists, suggesting the involvement of tachykinins in chronic CS-induced airway hyperresponsiveness (AHR). Further, substance P (SP)-like immunoreactivity (LI) and CGRP-LI in the airway tissue were significantly greater in the CS animals than in the control animals. Finally, beta-preprotachykinin (PPT, a splice variant from the PPT A gene encoding tachykinins including SP and NKA) messenger RNA levels as measured by in situ hybridization histochemistry displayed a significant increase in jugular ganglion neurons but not in dorsal root or nodose ganglion neurons. These data suggest that chronic CS-induced AHR is related to an increase in SP synthesis and release in jugular ganglion neurons innervating the lungs and airways.

Neurotensin gene expression increases during proestrus in the rostral medial preoptic nucleus: potential for direct communication with gonadotropin-releasing hormone neurons

Neurotensin (NT)-containing neurons in the rostral portion of the medial preoptic nucleus (rMPN) of the brain may play a key role in regulating the pattern of secretion of GnRH, thereby influencing the reproductive cycle in females. The major goals of this study were to determine whether NT messenger RNA (mRNA) levels in the rMPN exhibit a unique pattern of expression in temporal association with the preovulatory LH surge and to assess whether NT neurons may communicate directly with GnRH neurons. We analyzed NT gene expression in rats using in situ hybridization over the day of proestrus and compared this with diestrous day 1. We also determined whether the high-affinity NT receptor (NT1) is expressed in GnRH neurons using dual-label in situ hybridization and whether this expression varies over the estrous cycle. We found that NT mRNA levels in the rMPN increase significantly on the day of proestrus, rising before the LH surge. No such change was detected on diestrous day 1, when the LH surge does not occur. Furthermore, we observed that a significant number of GnRH neurons coexpress NT1 mRNA and that the number of GnRH neurons expressing NT1 mRNA peaks on proestrus. Together with previous findings, our results suggest that increased expression of NT in the rMPN may directly stimulate GnRH neurons on proestrus, contributing to the LH surge. In addition, our results suggest that responsiveness of GnRH neurons to NT stimulation is enhanced on proestrus due to increased expression of NT receptors within GnRH neurons.

Localization of kappa opioid receptors in oxytocin magnocellular neurons in the paraventricular and supraoptic nuclei

It has been demonstrated previously that kappa opioid receptor agonists, such as dynorphin, inhibit oxytocin secretion in the rat. To determine whether kappa agonists act directly on oxytocin-containing magnocellular neurons to inhibit hormone secretion, we utilized immunofluorescence to examine the cellular localization of kappa opioid receptors in the rat paraventricular and supraoptic nuclei. kappa Opioid receptor immunoreactivity co-localized with oxytocin-containing cell bodies, their axons and axon terminals. Thus, our results suggest that kappa opioid receptor agonists can exert direct inhibitory actions on oxytocin magnocellular neurons.

Fos-induction in gonadotropin-releasing hormone neurons receiving vasoactive intestinal polypeptide innervation is reduced in middle-aged female rats

A hallmark of reproductive aging in rats is a delay in the initiation and peak, and a decrease in the amplitude, of both proestrous and steroid-induced surges of LH and a decrease in the number of GnRH neurons that express Fos during the surge. The altered timing of the LH surge and the decline in Fos expression in GnRH neurons may be due to changes in the rhythmic expression of vasoactive intestinal polypeptide (VIP), a neuropeptide that carries time-of-day information from the circadian pacemaker, located in the suprachiasmatic nuclei (SCN), to GnRH neurons. The goals of our study were to determine if aging alters 1) the innervation of GnRH neurons by VIP and 2) the ability of VIP to activate GnRH neurons by examining the effects of aging on the number of GnRH neurons apposed by VIP fibers and the number of GnRH neurons that receive VIP input that express Fos. Immunocytochemistry for GnRH and VIP; or GnRH, VIP, and Fos was performed on tissue sections collected from young (2-4 mo), regularly cycling females and middle-aged (10-12 mo) females in constant estrus. The number of GnRH neurons, GnRH neurons apposed by VIP fibers, and GnRH neurons that express Fos and apposed by VIP fibers were counted in both age groups. Our results clearly demonstrate that aging does not alter the number of GnRH neurons that receive VIP innervation. However, the number of GnRH neurons that receive VIP innervation and coexpress Fos decreases significantly. We conclude that the age-related delay in the timing of the LH surge is not due to a change in VIP innervation of GnRH neurons, but instead may result from a decreased sensitivity of GnRH neurons to VIP input.

Effects of estrogen deficiency on brain function. Implications for the treatment of postmenopausal women

A growing body of evidence suggests that postmenopausal estrogen deficiency accelerates brain aging and increases the risk of various neurodegenerative processes, including Alzheimer's disease. Recent preclinical and clinical studies have indicated that estrogen has positive effects on brain homeostasis by preserving neural plasticity and the neurotransmitter pathways involved in learning, memory, and balance. In this article, Dr Birge and his coauthors address the effects of estrogen on brain function and discuss their implications for the use of selective estrogen receptor modulators, particularly tamoxifen and raloxifene, in postmenopausal women.

Minireview: neuroprotective effects of estrogen-new insights into mechanisms of action

An accumulating body of evidence clearly establishes that estradiol is a potent neuroprotective and neurotrophic factor in the adult: it influences memory and cognition, decreases the risk and delays the onset of neurological diseases such as Alzheimer's disease, and attenuates the extent of cell death that results from brain injuries such as cerebrovascular stroke and neurotrauma. Thus, estradiol appears to act at two levels: 1) it decreases the risk of disease or injury; and/or 2) it decreases the extent of injury incurred by suppressing the neurotoxic stimulus itself or increasing the resilience of the brain to a given injury. During the past century, the average life span of women has increased dramatically, whereas the time of the menopause has remained essentially constant. Thus, more women will live a larger fraction of their lives in a postmenopausal, hypoestrogenic state than ever before. Clearly, it is critical for us understand the circumstances under which estradiol exerts protective actions and the cellular and molecular mechanisms that underlie these novel, nonreproductive actions.

Estrogen receptor , not , is a critical link in estradiol-mediated protection against brain injury

Estradiol protects against brain injury, neurodegeneration, and cognitive decline. Our previous work demonstrates that physiological levels of estradiol protect against stroke injury and that this protection may be mediated through receptor-dependent alterations of gene expression. In this report, we tested the hypothesis that estrogen receptors play a pivotal role in mediating neuroprotective actions of estradiol and dissected the potential biological roles of each estrogen receptor (ER) subtype, ER alpha and ER beta, in the injured brain. To investigate and delineate these mechanisms, we used ER alpha-knockout (ER alpha KO) and ER beta-knockout (ER beta KO) mice in an animal model of stroke. We performed our studies by using a controlled endocrine paradigm, because endogenous levels of estradiol differ dramatically among ER alpha KO, ER beta KO, and wild-type mice. We ovariectomized ER alpha KO, ER beta KO, and the respective wild-type mice and implanted them with capsules filled with oil (vehicle) or a dose of 17 beta-estradiol that produces physiological hormone levels in serum. One week later, mice underwent ischemia. Our results demonstrate that deletion of ER alpha completely abolishes the protective actions of estradiol in all regions of the brain; whereas the ability of estradiol to protect against brain injury is totally preserved in the absence of ER beta. Thus, our results clearly establish that the ER alpha subtype is a critical mechanistic link in mediating the protective effects of physiological levels of estradiol in brain injury. Our discovery that ER alpha mediates protection of the brain carries far-reaching implications for the selective targeting of ERs in the treatment and prevention of neural dysfunction associated with normal aging or brain injury.

Neuroprotective effects of estradiol in middle-aged female rats

Estrogen replacement therapy in postmenopausal women ameliorates cognitive dysfunction and decreases the risk and/or severity of neurodegenerative conditions such as Alzheimer's disease and stroke. Furthermore, estradiol exerts neuroprotective effects in a variety of in vitro and in vivo models of brain injury. We have previously shown that physiological levels of estradiol attenuate ischemic brain injury in young female rats. However, neurodegenerative events occur more frequently in elderly women who are chronically hypoestrogenic. Therefore, we investigated whether aging rats remain responsive to the neuroprotective actions of estradiol. Young (3-4 months) and middle-aged (9-12 months) rats were ovariectomized and treated for 1 week with estradiol before middle cerebral artery occlusion (MCAO). Regional cerebral blood flow was monitored in some animals at the time of injury. Brains were collected 24 h after MCAO and infarct volume was analyzed. Our data demonstrate that in both young and aging rats, low and high physiological doses of estradiol decrease ischemic injury by almost 50%, compared with oil-treated controls. Additionally, our data suggest that estradiol acts in both age groups via blood flow-independent mechanisms, as basal and postinjury blood flow was equivalent between estradiol- and oil-treated young and aging rats. These data demonstrate that replacement with physiological levels of estradiol protects against stroke-related injury in young and aging female rats and strongly suggest that older animals remain responsive to the protective actions of estradiol.

Estrogens: trophic and protective factors in the adult brain

Our appreciation that estrogens are important neurotrophic and neuroprotective factors has grown rapidly. Although a thorough understanding of the molecular and cellular mechanisms that underlie this effect requires further investigation, significant progress has been made due to the availability of animal models in which we can test potential candidates. It appears that estradiol can act via mechanisms that require classical intracellular receptors (estrogen receptor alpha or beta) that affect transcription, via mechanisms that include cross-talk between estrogen receptors and second messenger pathways, and/or via mechanisms that may involve membrane receptors or channels. This area of research demands attention since estradiol may be an important therapeutic agent in the maintenance of normal neural function during aging and after injury.

Localization of the VIP2 receptor protein on GnRH neurons in the female rat

In mammals, the timing and occurrence of the preovulatory LH surge critically depends on the proper functioning of the suprachiasmatic nucleus (SCN). Recent evidence suggests that vasoactive intestinal polypeptide (VIP) conveys time of day information from the SCN to GnRH neurons. However, it is not completely clear whether this action is exerted directly at the level of the GnRH neuron. To determine if GnRH neurons are direct targets for VIP, triple-label immunofluorescence was utilized to simultaneously localize GnRH, VIP and VIP2 receptor protein. The present results demonstrate that about 40% of all GnRH neurons analyzed contain VIP2 receptor immunoreactivity and that VIP-containing processes were seen in close apposition to a significant number of VIP2 receptor-positive GnRH neurons. GnRH neurons that exhibit immunoreactivity for the VIP2 receptor are located predominantly in the OVLT region and the rostral preoptic area. In the median eminence, where the majority of GnRH neurons terminate, VIP2 receptor immunoreactivity was absent. In summary, these findings indicate that VIP can communicate directly with GnRH neurons.

Estradiol protects against ischemic brain injury in middle-aged rats

Several clinical studies suggest that estradiol acts as a potent growth and protective factor in the adult brain. Postmenopausal women experience permanent hypoestrogenicity and suffer from increased risk of brain injury associated with neurodegenerative diseases such as stroke and Alzheimer's disease. Estrogen replacement therapy appears to decrease the risk and severity of these neurodegenerative conditions. Studies using animal models have shown that estradiol exerts similar effects in rodents and can enhance cell survival and induce synaptic plasticity. Therefore, we undertook studies to assess whether estradiol treatment can decrease brain injury and cell death induced by an experimental model of ischemia and whether aging animals remain responsive to the protective effects of estradiol. We will review results from recent studies that demonstrate that 1) in young animals, estrogens exert profound protective effects against ischemic brain injury induced by cerebral artery occlusion and 2) the response of aging animals has been tested with varying results. We will discuss and compare our experimental findings that utilize a permanent cerebral artery occlusion model and physiological levels of estradiol replacement therapy in young and middle-aged rats with those of previous studies. These observations provide important insights into the potential protective actions of estrogen replacement therapy on age- and disease-related processes in the brain.

Estradiol protects against injury-induced cell death in cortical explant cultures: a role for estrogen receptors

Estradiol has been shown to exert trophic and protective actions in the brain. Our laboratory has shown that in vivo, low physiological levels of estradiol protect the female rat brain against ischemic injury. In the present study, we used organotypic cortical explant cultures to begin to decipher the mechanisms of estradiol's actions. Injury was induced by exposure to kainic acid or potassium cyanide/2-deoxyglucose (KCN/2-DG) for varying lengths of time, and cell death was monitored by LDH release at 2, 6, 12, 24, 48, 72 and 96 h after injury. We found that exposure to 1 mM KCN/2 mM 2-DG for 2 h produced consistent delayed cell death that was detectable by 24 h. The presence of 17beta-estradiol (E2) during the 7 days prior to injury significantly reduced the extent of cell death; whereas, administration of E2 at the time of injury did not protect. The protective effects of estradiol were dose dependent. Low doses of E2 (1, 10, and 30 nM) significantly reduced cell death; however, higher concentrations of E2 (>60 nM) had no protective effect. The observations that low levels of E2 protect against cell death, and that pretreatment is required suggest that the protective actions of estradiol may involve estrogen receptors. Therefore, we examined the ability of 17alpha-estradiol, which does not efficiently activate the estrogen receptor, and the addition of the estrogen receptor antagonist, ICI 182,780, to influence the extent of cell death induced by KCN/2-DG. 17alpha-Estradiol failed to protect, and ICI 182,780 prevented E2 from protecting against cell death. Furthermore, E2 pretreatment is required for more than 24 h to be neuroprotective. Our results clearly show that in cortical explant cultures, estradiol protects cells against ischemic injury, and suggest that these protective actions involve estrogen receptors.

Quantification of odor quality

The relationship between odor quality and molecular properties is arguably the most important issue in olfaction. Despite sophistication in the chemical characterization of molecules, accompanying perceptual characterization has had little quantitative usefulness, relying mostly on enumerative description. As a result of weak interest in the topic outside industry and little agreement regarding how to measure quality, the field of olfactory psychophysics has failed to develop a substantial database for odor quality and has offered little help to other researchers, e.g. neurobiologists, in choice of stimuli, interpretation of outcome or testable hypotheses. This review scrutinizes how psychophysicists and others have measured quality and offers criteria for useful techniques. Most measures have had a subjective component that makes them anachronistic with modern methodology in experimental behavioral science, indeterminate regarding the extent of individual differences, unusable with infrahumans and of unproved ability to discern small differences. Techniques based upon performance, rather than on the more common reporting of mental content, offer firmer possibilities for growth. These techniques inevitably tap the discriminative basis of perception. The nonsubjective techniques have high sensitivity, can have counterparts in infrahuman research, are suitable to examine individual differences and yield non-negotiable answers with potential archival value. Discriminative techniques have their limitations, too-principally excess sensitivity that abridges their use to comparisons between similar-smelling stimuli. Research has begun to extend that range and may overcome the limitation. Application of discriminative methods may have the side-effect of shifting focus in structure-activity research from searches for molecular least common denominators that underlie often vague similarity to the search for molecular properties of importance in discrimination of small differences.

Latency and accuracy of discriminations of odor quality between binary mixtures and their components

Subjects made timed, same-different discriminations of odor quality, with the following principal findings: (i) latency reflected accuracy, with difficult discriminations, i.e. those between 50-50 mixtures and their components, requiring more time than less difficult discriminations, i.e. those between unmixed chemicals. This finding demonstrated the face validity of latency as a measure of qualitative similarity. (ii) Latency provided better resolution among pairs of odors than did errors of discrimination. This finding demonstrated the utility of collecting response times. (iii) Latency-based similarities among odors tested previously predicted similarities among pairs not yet tested. This finding demonstrated internal/predictive validity. (iv) A signal detection model assuming a differencing strategy best described the pattern of errors. Subjects appeared to make relative judgements regarding quality. (v) Finally, latency-based similarities between mixtures and their components demonstrated additivity. This finding suggested that binary mixtures fall on straight lines connecting their components in 'odor-space'.

Estradiol is a neuroprotective factor in in vivo and in vitro models of brain injury

Many clinical studies suggest that estrogen enhances memory and cognition and protects against neurodegenerative diseases and injury associated with stroke or stress. These results are strongly supported by experiments performed in animal models using both in vivo and in vitro methods. We present here data from our lab that establishes that physiological levels of estradiol exert profound protective actions against ischemic injury. Further we will present evidence that these effects may be mediated through estrogen receptors that may influence the bcl-2 family of genes.

Estradiol: a protective factor in the adult brain

Clinical studies suggest that estradiol acts as a protective factor in the adult brain. Postmenopausal women suffer from an increased risk of brain injury associated with neurodegenerative diseases such as Alzheimer's disease, and estrogen replacement therapy appears to decrease the risk and severity of this neurodegenerative condition. Studies using animal models show that estradiol exerts similar effects in rodents and can enhance cell survival. Therefore, we designed experiments to determine whether estradiol treatment can decrease brain injury induced by an experimental model of ischemia. Our experiments used a permanent middle cerebral artery occlusion model and physiological levels of estradiol replacement therapy. The results demonstrate that estradiol exerts profound protective effects against ischemic brain injury induced by cerebral artery occlusion and that this protective action correlates with changes in the level of gene expression of estradiol receptors and members of the Bcl-2 family. These data suggest that estrogen replacement therapy may provide important protection against age- and disease-related degenerative processes in the brain.

Neuroendocrine modulation of the "menopause": insights into the aging brain

The menopause marks the permanent end of fertility in women. It was once thought that this dramatic physiological change could be explained simply by the exhaustion of the reservoir of ovarian follicles. New data from studies performed in women and animal models make us reassess this assumption. An increasing body of evidence suggests that there are multiple pacemakers that contribute to the transition to irregular cycles, decreasing fertility, and the timing of the menopause. We will present evidence that lends credence to the possibility that a dampening and desynchronization of the precisely orchestrated neural signals lead to miscommunication between the brain and the pituitary-ovarian axis, and that this constellation of hypothalamic-pituitary-ovarian events leads to the deterioration of regular cyclicity and heralds menopausal transition.

Estradiol: a protective and trophic factor in the brain

In recent years our appreciation that estradiol is truly a pleiotropic hormone has grown dramatically. We will review the findings that suggest that estrogens may exert important non-reproductive actions on the brain. These studies provide important insights into the clinical effects of estrogen replacement therapy on age- and disease-related processes in the brain. We will also discuss the multiple cellular and molecular mechanisms that may underlie estradiol's neurotrophic and neuroprotective effects.

Estradiol modulates bcl-2 in cerebral ischemia: a potential role for estrogen receptors

We have shown that physiological levels of estradiol exert profound protective effects on the cerebral cortex in ischemia induced by permanent middle cerebral artery occlusion. The major goal of this study was to begin to elucidate potential mechanisms of estradiol action in injury. Bcl-2 is a proto-oncogene that promotes cell survival in a variety of tissues including the brain. Because estradiol is known to promote cell survival via Bcl-2 in non-neural tissues, we tested the hypothesis that estradiol decreases cell death by influencing bcl-2 expression in ischemic brain injury. Furthermore, because estradiol may protect the brain through estrogen receptor-mediated mechanisms, we examined expression of both receptor subtypes ERalpha and ERbeta in the normal and injured brain. We analyzed gene expression by RT-PCR in microdissected regions of the cerebral cortex obtained from injured and sham female rats treated with estradiol or oil. We found that estradiol prevented the injury-induced downregulation of bcl-2 expression. This effect was specific to bcl-2, as expression of other members of the bcl-2 family (bax, bcl-x(L), bcl-x(S), and bad) was unaffected by estradiol treatment. We also found that estrogen receptors were differentially modulated in injury, with ERbeta expression paralleling bcl-2 expression. Finally, we provide the first evidence of functional ERbeta protein that is capable of binding ligand within the region of the cortex where estradiol-mediated neuroprotection was observed in cerebral ischemia. These findings indicate that estradiol modulates the expression of bcl-2 in ischemic injury. Furthermore, our data suggest that estrogen receptors may be involved in hormone-mediated neuroprotection.

Neuroendocrine influences and repercussions of the menopause

In summary, the evidence that both the ovary and the brain are key pacemakers in the menopause is compelling. Our appreciation that estrogens are important neurotrophic and neuroprotective factors has grown rapidly. Future studies will allow us to better understand the ensemble of factors that interact to maintain regular reproductive cyclicity and how this precise dynamic balance changes with age. Furthermore, understanding how estrogen exerts trophic and protective actions should lead to its use as an important therapeutic agent in the maintenance of normal neural function during aging and after injury.

Effects of age on GABA turnover rates in specific hypothalamic areas in female rats

During aging in female rats the estrous cycle ceases and the animals develop phases of constant estrous (CE) or constant diestrous (CD) prior to the irreversible transition into anestrous. In young rats, gamma-aminobutyric acid (GABA) is of pivotal importance for the release of GnRH. In the medial-preoptic area (MPO) where the majority of the GnRH perikarya are located in the rat, GABA release decreases at the time of the preovulatory LH surge. The suprachiasmatic nucleus (SCN) contains numerous GABA neurons. Neurochemical signals from this hypothalamic nucleus provide temporal information to GnRH neurons and thereby influence the preovulatory LH surge and the length of estrous cycles. To investigate aging-related changes of the activity of hypothalamic GABAergic neurons, we determined GABA turnover rates in various hypothalamic nuclei of CE and CD rats and compared them to those determined in young estrous (E) or diestrous rats (D1). In old female rats, GABA activity in the MPO was significantly decreased compared to E and D1 rats. A selective increase of GABA turnover rates was observed in the SCN of CE animals. No age-related changes were observed in the other examined brain areas. These data provide the first evidence for alterations in GABAergic activity in specific hypothalamic areas that depend on age and reproductive status. These may cause changes in ability to induce preovulatory LH surges and to maintain regular estrous cyclicity.

Effects of lifelong moderate caloric restriction on levels of neuropeptide Y, proopiomelanocortin, and galanin mRNA

We are interested in how neuropeptides that regulate both food intake and reproductive function change with age and how life-prolonging moderate caloric restriction may influence the expression of these neuropeptides. We measured neuropeptide Y (NPY) and proopiomelanocortin (POMC) mRNA in the arcuate nucleus (AN), and galanin (GAL) mRNA in the AN, paraventricular nucleus (PVN) and medial septum-diagonal band of Broca in young, middle-aged and old female rats on a controlled feeding regimen. Female Sprague-Dawley rats (7 weeks old) were placed on caloric restriction (CR; n = 70) which was 60% of ad libitum (AL) intake measured in control rats (n = 70). Rats were rapidly decapitated 2.5 weeks following ovariectomy, when they were 4, 12, or 18 months old. Brains were frozen and coronal sections (12 microns) were cut at -20 degrees C using a cryostat. Relative levels of NPY, POMC and GAL mRNA were measured using in situ hybridization histochemistry. cDNA clones complementary to rat NPY, mouse POMC and rat GAL were used to synthesize 35S-UTP-labeled cRNA probes. Slides were dipped in photographic emulsion. Silver grains were quantified using computer-assisted image analysis. Caloric restriction increased NPY mRNA/cell, decreased POMC mRNA/cell in the AN, and did not influence GAL mRNA/cell. Age and caloric restriction did not influence the number of cells with detectable NPY mRNA, POMC mRNA or GAL mRNA. Reproductive status at the time of decapitation influenced both the level and the number of cells expressing GAL mRNA in the PVN. In summary, CR differentially affects levels of NPY, POMC and GAL mRNA in the AN. In this animal model, there was no statistically significant effect of age nor any interaction between age and diet on expression of any of these genes between 4 and 18 months of age.

Estradiol protects against ischemic injury

Clinical studies demonstrate that estrogen replacement therapy in postmenopausal women may enhance cognitive function and reduce neurodegeneration associated with Alzheimer's disease and stroke. This study assesses whether physiologic levels of estradiol prevent brain injury in an in vivo model of permanent focal ischemia. Sprague-Dawley rats were ovariectomized; they then were implanted, immediately or at the onset of ischemia, with capsules that produced physiologically low or physiologically high 17beta-estradiol levels in serum (10 or 60 pg/mL, respectively). One week after ovariectomy, ischemia was induced. Estradiol pretreatment significantly reduced overall infarct volume compared with oil-pretreated controls (mean+/-SD: oil = 241+/-88; low = 139+/-91; high = 132+/-88 mm3); this protective effect was regionally specific to the cortex, since no protection was observed in the striatum. Baseline and ischemic regional CBF did not differ between oil and estradiol pretreated rats, as measured by laser Doppler flowmetry. Acute estradiol treatment did not protect against ischemic injury. Our finding that estradiol pretreatment reduces injury demonstrates that physiologic levels of estradiol can protect against neurodegeneration.

Sex differences in the daily rhythm of vasoactive intestinal polypeptide but not arginine vasopressin messenger ribonucleic acid in the suprachiasmatic nuclei

The timing of the preovulatory surge of LH in female rodents is tightly coupled to the environmental light/dark cycle. This coupling is mediated by the circadian pacemaker located in the suprachiasmatic nuclei (SCN). Studies indicate that vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP), which are synthesized in the SCN, transmit circadian information from the SCN to GnRH neurons, thereby regulating the timing of the LH surge. However, to date, the rhythmic expression of these two peptides in the SCN has only been examined in males. The pattern of VIP expression in males is difficult to reconcile with its role in the LH surge. The purpose of the present study was to assess the rhythm of VIP messenger RNA (mRNA) levels in the SCN of female rats under several endocrine conditions. We compared this rhythm to that in males and to AVP mRNA rhythms in all experimental groups. In all groups of females, VIP mRNA levels were rhythmic, with peak expression occurring during the light phase and a nadir occurring during the dark phase. The rhythm was approximately 12 h out of phase compared with that in males. The rhythmic expression of AVP mRNA in the SCN was virtually identical in all groups of animals. Based on these results, we conclude that 1) the rhythm of VIP seen in the SCN of females during the day may serve as a facilitory signal from the SCN to GnRH neurons; 2) the sex-specific pattern of VIP mRNA does not depend on estradiol; and 3) AVP gene expression within the SCN is not sexually differentiated or altered by estradiol.

Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats

Our laboratory has shown that the ability of the suprachiasmatic nuclei (SCN) to regulate a number of rhythmic processes may be compromised by the time females reach middle age. Therefore, we examined the effects of aging on the rhythmic expression of two neuropeptides synthesized in the SCN, vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP), using in situ hybridization. Because both VIP and AVP are outputs of the SCN, we hypothesized that age-related changes in rhythmicity are associated with alterations in the patterns of expression of these peptides. We found that VIP mRNA levels exhibited a 24 hr rhythm in young females, but by the time animals were middle-aged, this rhythm was gone. The attenuation of rhythmicity was associated with a decline in the level of mRNA per cell and in the number of cells in the SCN producing detectable VIP mRNA. AVP mRNA also showed a robust 24 hr rhythm in young females. However, in contrast to VIP, the AVP rhythm was not altered in the aging animals. The amount of mRNA per cell and the number of cells expressing AVP mRNA also was not affected with age. Based on these results we conclude that (1) various components of the SCN are differentially affected by aging; and (2) age-related changes in various rhythms may be attributable to changes in the ability of the SCN to transmit timing information to target sites. This may explain why the deterioration of various rhythmic processes occurs at different rates and at different times during the aging process.

Basic fibroblast growth factor-2 and interleukin-1 beta regulate S100 beta expression in cultured astrocytes

Basic fibroblast growth factor and interleukin-1 beta are known to regulate the expression of other trophic factors and to stimulate reactive gliosis in vivo. S100 beta is a glial-specific putative neurotrophic factor and has been considered a marker of the reactive status of astrocytes. Therefore, we tested the hypothesis that basic fibroblast growth factor-2 and interleukin-1 beta achieve their effects by altering S100 beta gene expression in cultured rat astrocytes using an RNase protection assay. Short-term treatment with basic fibroblast growth factor-2 produced a transient decrease in S100 beta messenger RNA which was followed by an increase after longer term treatment. In contrast, both short- and long-term treatment with interleukin-1 beta suppressed S100 beta messenger RNA. We measured levels of S100 beta nuclear primary transcript to assess whether alterations in transcriptional rate explain the changes in messenger RNA. Our results indicate that changes in transcription account for changes in steady state levels of messenger RNA since basic fibroblast growth factor-2-induced changes in S100 beta primary transcript temporally preceded changes in messenger RNA. We further measured intracellular S100 beta protein levels by enzyme-linked immunosorbent assay to determine whether changes in gene expression were translated into parallel changes in protein. Our results clearly demonstrate that basic fibroblast growth factor-2 and interleukin-1 beta influence the expression of the S100 beta gene, that this regulation appears to occur at the level of transcription, and that alterations in messenger RNA are sometimes, but not always, reflected in changes at the level of protein. These observations suggest that basic fibroblast growth factor-2 may amplify its trophic effects, in part, by influencing the expression of another trophic factor.

Fetal grafts containing suprachiasmatic nuclei restore the diurnal rhythm of CRH and POMC mRNA in aging rats

We assessed whether fetal tissue containing the suprachiasmatic nuclei (SCN) can restore age-related changes in the diurnal rhythm of hypothalamic corticotropin-releasing hormone (CRH) and anterior pituitary proopiomelanocortin (POMC) mRNA. Young, middle-aged, and middle-aged SCN-transplanted rats were killed at seven times of day. In young rats, CRH mRNA exhibited a diurnal rhythm in the dorsomedial paraventricular nuclei but not in other subdivisions of the nuclei. No rhythm was detected in aging rats. SCN transplants restored a rhythm in CRH mRNA, but the timing was not precisely the same as in young animals. POMC mRNA exhibited a daily rhythm in young rats. Aging abolished the rhythm and decreased the average mRNA level; fetal transplants restored the rhythm, but the amplitude remained attenuated. These data are the first demonstration that fetal tissue can restore the diurnal rhythm of a neuroendocrine axis that is driven by the SCN. We conclude that the neuroendocrine substrate from the aging host remains capable of responding to diurnal cues to express diurnal rhythmicity in CRH/POMC mRNA when fetal SCN transplants confer the appropriate signals.

Dexamethasone regulates basic fibroblast growth factor, nerve growth factor and S100beta expression in cultured hippocampal astrocytes

Glucocorticoids regulate hippocampal neuron survival during fetal development, in the adult, and during aging; however, the mechanisms underlying the effects are unclear. Since astrocytes contain adrenocortical receptors and synthesize and release a wide variety of growth factors, we hypothesized that glucocorticoids may alter neuron-astrocyte interactions by regulating the expression of growth factors in hippocampal astrocytes. In this study, three growth factors, which are important for hippocampal neuron development and survival, were investigated: basic fibroblast growth factor (bFGF), nerve growth factor (NGF), and S100beta. Enriched type I astrocyte cultures were treated with 1 microM dexamethasone (DEX), a synthetic glucocorticoid, for up to 120 h. Cells and culture medium were collected and total RNA and protein were measured at 6, 12, 24, 48, 72, 96 and 120 h after the initiation of hormone treatment. Growth factor mRNA levels were measured and quantified using solution hybridization-RNase protection assays and protein levels were quantified using ELISA methods. We report that DEX stimulates the bFGF mRNA levels over the 120-h treatment. In contrast, DEX suppresses NGF mRNA continuously over the same period of treatment. DEX induces a biphasic response in S100beta mRNA levels. In addition, some of the changes in gene expression are translated into parallel changes in protein levels of these growth factors. Our results demonstrate that dexamethasone can differentially regulate the expression of growth factors in hippocampal astrocytes in vitro. This suggests that one of the mechanisms through which glucocorticoids affect hippocampal functions may be by regulating the expression of astrocyte-derived growth factors.

GFAP and S100beta expression in the cortex and hippocampus in response to mild cortical contusion

We studied the acute response of glial fibrillary acidic protein (GFAP) and S100beta gene expression in the cerebral cortex and hippocampus to mild unilateral cortical contusion. Our goal was to evaluate and compare the expression patterns of each gene in the early stages of the astrocytic response to brain injury. RNA was extracted from the cerebral cortex and hippocampus of male rats at 0, 3, 12, 24, or 96 h after lesion or sham-operation, then quantified using an RNase protection assay. Contusion produced a robust elevation in GFAP mRNA by 12 h in both brain regions on the ipsilateral side to the contusion. In the cortex, but not the hippocampus, this elevation was sustained at 96 h. S100beta mRNA levels were elevated bilaterally in lesioned animals at 24 h in both brain regions. However, these data are difficult to interpret because sham mRNA levels decreased with time, making it unclear whether contusion stimulates S100beta gene expression or whether it mitigates the inhibitory effect of sham. We further analyzed the effect of contusion on GFAP and S100beta immunoreactive astrocyte density at 96 h postlesion or postsham by double-label immunocytochemistry. All detectable astrocytes under all conditions were S100beta immunoreactive in both brain regions. Furthermore, all S100beta immunoreactive astrocytes in the lesioned ipsilateral cortex were also GFAP immunoreactive, whereas only about 11% of S100beta positive cells were also GFAP labeled in the contralateral lesioned or the ipsilateral sham cortex. In the hippocampus, all S100beta immunoreactive cells were also GFAP immunoreactive under all conditions. These data correlate with the gene expression data at 96 h, and suggest that, at least in the cortex, resident S100beta-expressing astrocytes produce GFAP at levels that are undetectable by immunocytochemistry until they are activated in response to injury.

Aging of the female reproductive system: a window into brain aging

The menopause marks the permanent end of fertility in women. It was once thought that the exhaustion of ovarian follicles was the single, most important explanation for the transition to the menopause. Over the past decade, this perception has gradually changed with the realization that there are multiple pacemakers of reproductive senescence. We will present evidence that lends credence to the hypothesis that the central nervous system is a critical pacemaker of reproductive aging and that changes at this level contribute to the timing of the menopause. Studies demonstrate that an increasing de-synchronization of the temporal order of neuroendocrine signals may contribute to the accelerated rate of follicular loss that occurs during middle age. We suggest that the dampening and destabilization of the precisely orchestrated ultradian, circadian, and infradian neural signals lead to miscommunication between the brain and the pituitary-ovarian axis. This constellation of hypothalamic-pituitary-ovarian events leads to the inexorable decline of regular cyclicity and heralds menopausal transition.

Transplantation of fetal suprachiasmatic nuclei into middle-aged rats restores diurnal Fos expression in host

In young animals, the suprachiasmatic nuclei (SCN) of the hypothalamus, which are critical circadian pacemakers, exhibit a light-induced diurnal rhythm in Fos expression. The expression of this immediate-early gene has been used as an index of the activity of the SCN and their ability to respond to external cues that entrain them, such as light. In the present study, we show that by the time rats reach middle age baseline Fos expression increases prematurely during the dark and that light-induced Fos expression is blunted and delayed. We also demonstrate that transplantation of fetal tissue containing the SCN into the third cerebral ventricle of middle-aged rats enables aged hosts to regain the ability to exhibit diurnal patterns of Fos expression that are strikingly similar to those observed in young animals. Our findings lead to the following conclusions: 1) the diurnal pattern of activity of SCN cells is blunted in middle-aged rats, and 2) SCN transplants provide unique signals that enable the cellular systems of the host to regain rhythmic functional capabilities. These results provide new insights into the critical active role that the host plays in restoration of function evoked by the presence of a transplant.

Age-related changes in light-induced Jun-B and Jun-D expression: effects of transplantation of fetal tissue containing the suprachiasmatic nucleus

Fos and Jun mRNA and peptide exhibit a daily light-induced rhythm in the suprachiasmatic nucleus (SCN). The authors previously have reported that Fos expression in the SCN is elevated prematurely during the dark, light-induced Fos expression is attenuated in middle-aged rats, and transplantation of fetal SCN tissue into the third ventricle of rats of this age restores the daily pattern of Fos expression to that of the young. Using immunocytochemistry, the authors performed the present study to determine whether Jun-B and Jun-D expression in the SCN is altered at the same stage during aging and, if so, whether transplantation of fetal tissue containing the SCN can restore the light-induced rhythms of these two immediate early genes. All groups of rats were transcardially perfused 90 min prior to and after light onset. In young rats, light induced a robust increase in the number of Jun-B positive cells in the SCN, whereas very few cells were labeled before light onset. In middle-aged rats, the light-induced increase in the number of Jun-B positive cells was significantly attenuated. Transplantation of fetal SCN tissue into the middle-aged rats successfully restored light-induced Jun-B expression to the levels of young rats. By contrast, Jun-D exhibited a constitutively high level of expression in the SCN both before and after light onset, and light induced only a slight but significant increase. No age-related changes were detected in the expression of Jun-D either before or after light onset. Transplantation of fetal SCN tissue did not alter the daily pattern of Jun-D expression in the middle-aged rats. These data suggest that (1) light-induced activation of SCN neural activity is blunted during aging, (2) fetal SCN tissue can provide the critical support to allow the host to respond properly to light cues, and (3) the age-related change in Jun-B expression in the middle-aged host SCN can be rescued by fetal SCN transplants.

In vivo antisense antagonism of vasoactive intestinal peptide in the suprachiasmatic nuclei causes aging-like changes in the estradiol-induced luteinizing hormone and prolactin surges

In mammals, the suprachiasmatic nuclei (SCN) regulate the timing of LH surges. Recent evidence suggests that vasoactive intestinal peptide (VIP), an abundantly expressed neuropeptide of the SCN, communicates time of day information from the SCN to GnRH neurons. VIP levels in the SCN decrease with age and may be responsible for alterations in LH surges that become apparent in middle-aged rats. We wished to determine whether suppression of VIP synthesis, through antisense oligonucleotides (oligos) directed at the SCN, results in 1) selective suppression of VIP levels in the SCN and 2) aging-like changes in the secretion of LH and PRL. To test the specificity of antisense oligo treatment, rats were ovariectomized and treated with estradiol. Antisense or control random oligos were infused into the peri-SCN region through stereotaxically placed bilateral cannulas. Beginning at lights off, rats were maintained in constant dim red illumination throughout the remainder of the experiment. They were killed at specific times, brains were microdissected, and VIP concentrations in the SCN, paraventricular nuclei, and cortex were assayed. As a control for the specificity of antisense VIP treatment, we monitored the levels of arginine vasopressin in the SCN. To test the effects of antisense treatment on the pattern of plasma LH and PRL secretion, blood samples were collected from atrial catheters from 1200-2000 h, and plasma samples were assayed for LH and PRL. The results indicate that the effects of antisense treatment were discrete, as they suppressed VIP concentrations in the SCN, but had no effect on VIP concentrations in the paraventricular nuclei or cortex or on arginine vasopressin concentrations in the SCN. Peak LH levels during the surge were delayed and attenuated in antisense-treated animals compared to random oligo-treated control rats in a manner strikingly similar to that observed previously in middle-aged rats. Likewise, PRL, which was unaffected in middle-aged rats, was also unaffected by targeted suppression of VIP. In summary, our findings clearly demonstrate that antisense VIP oligos suppress VIP levels in the SCN and do not affect peptide concentrations in other regions of the brain or other neuropeptides in the SCN. Further, we show that suppression of a single neuropeptide in the SCN can mimic the effects of age on the estradiol-induced surges of LH and PRL. These data support a central role for suprachiasmatic VIP in the regulation of the LH surge and suggest that age-related perturbations in the integrity of this axis may account for alterations in the pattern of LH secretion observed during middle age.

Menopause: the aging of multiple pacemakers

Menopause signals the permanent end of menstrual cyclicity in a woman's life. Its impact reaches far beyond just the reproductive system. An understanding of the factors that interact and govern the process of aging in the reproductive system will help us to develop strategies for alleviating the negative aspects of menopause and may help us to better comprehend the process of biological aging.

Prolactin receptor gene expression in specific hypothalamic nuclei increases with age

Increasing prolactin levels or increasing responsiveness to prolactin may contribute to reproductive aging by influencing the secretory patterns of hypothalamic GnRH, pituitary gonadotropins, and/or ovarian steroids. Some studies have documented changes in the levels of prolactin in peripheral plasma. The goal of this study was to determine whether prolactin receptor mRNA levels in the brain change with aging, which may lead to increasing responsiveness to prolactin. Young (2-4 months) and middle-aged (9-11 months), demonstrating 3 consecutive estrous cycles, and old (16-19 months) and very old (20-21 months) rats, exhibiting repeated pseudopregnancies, were bilaterally ovariectomized. They were implanted with Silastic capsules containing estradiol-17 beta one week later, and killed 2 days after capsule implantation. Changes in prolactin receptor gene expression were assessed using in situ hybridization. The level of prolactin receptor mRNA in choroid plexus, periventricular area of the preoptic nucleus, and arcuate nucleus increased significantly by the time the animals were old. In the lateral ventromedial nucleus, prolactin receptor gene expression did not change significantly during aging, even in the oldest group of rats. These findings suggest that changes in the prolactin receptor gene may influence the ability of prolactin to exert effects and may allow older animals to be more responsive to prolactin than young rats.

Age-related changes in the diurnal rhythm of CRH gene expression in the paraventricular nuclei

A circadian rhythm secretion of corticotropin-releasing hormone (CRH) is thought to regulate the circadian pattern of secretion of adrenocorticotropic hormone and corticosterone. We have previously reported that the amplitude of the diurnal rhythm of serum corticosterone concentrations decreases in 17- to 20-mo-old rats. In the present experiment, we tested whether an age-related alteration in the daily rhythm and/or level of CRH mRNA in the paraventricular nuclei (PVN) occurs during middle age. Groups of young and middle-aged animals were killed at several times of day. We assessed the level of CRH mRNA in the PVN and dorsal medial subdivision of the PVN using in situ hybridization. In young rats, CRH mRNA expression exhibited a diurnal rhythm in the dorsal medial PVN. The same trend was observed in the entire medial PVN. In middle-aged rats, no rhythm was detected in either region. The overall average level of CRH mRNA was not different between these two age groups. These findings suggest that changes in the suprachiasmatic nucleus or in its ability to entrain neuroendocrine outputs occur relatively early during the aging process.

Acute effects of antisense antagonism of a single peptide neurotransmitter in the circadian clock

The circadian clock that resides in the suprachiasmatic nucleus (SCN) of the hypothalamus is the major neural pacemaker driving most 24-h rhythms in mammals. Several neurotransmitter peptides are synthesized within this nucleus and communicate rhythmically with other cells in the SCN as well as with cells in other regions of the brain. At the present time, little is known about their role in regulating outputs of the clock. We demonstrate that antisense oligodeoxynucleotides corresponding to the NH2-terminus and the translation start site of vasoactive intestinal peptide (VIP) mRNA infused into the suprachiasmatic region of rats temporarily abolishes the circadian rhythm of corticosterone secretion without influencing stress-related corticosterone secretion in the same animals. Levels of VIP peptide are suppressed 30% on the second day after antisense treatment. These results indicate that a single neuropeptide transmitter in the circadian clock may serve a distinct role in the control of specific circadian rhythms.

Life-long moderate caloric restriction prolongs reproductive life span in rats without interrupting estrous cyclicity: effects on the gonadotropin-releasing hormone/luteinizing hormone axis

Restricting food intake to 60% that of ad libitum-fed rats results in an extended life span, reduced incidence of age-related diseases, and delayed reproductive senescence. We used this animal model to further elucidate the mechanisms whereby reproductive senescence is delayed. Female Sprague-Dawley rats (7 wk old) were calorically restricted (CR; n = 70) to 60% of the ad libitum(AL) intake measured in control rats (n = 70). Rats were individually housed under a 14L:10D cycle and fed daily within 1.5 h of lights-off. Body weights were monitored every 2 wk, and vaginal lavage was performed until rats were ovariectomized (OVX). Two weeks after OVX, when rats were 4, 12, or 18 mo of age, blood samples were taken via jugular cannulae every 6 min for 3 h, and the plasma was assayed for rat LH. The resulting profiles were examined through use of Cluster analysis for mean LH concentrations, LH pulse amplitude, and interval between LH pulses. CR rats grew at a slower rate, and then maintained body weights at approximately 76% that of AL controls between 4 and 17.5 mo of age. The onset of persistent estrus was delayed by 4 mo in CR rats. Average cycle length was longer (p < 0.01) by less than 0.5 days in CR compared with AL rats between 3.5 and 5.5 mo of age but not different between 6.5 and 11.5 mo. Mean levels of LH in OVX rats decreased with age (p < 0.01), increased with caloric restriction (p < 0.05), and decreased with declining cycling status of the animal prior to OVX (regular [reg] vs. irregular [ir] vs. persistent estrus [pe]; p < 0.05). The increased mean LH due to caloric restriction was attributed to an increase in mean pulse amplitude and not to a decrease in time interval between LH pulses. From these data we conclude that the beneficial effects of caloric restriction on reproductive longevity may be acting at the level of the hypothalamus and/or pituitary to enhance LH secretion and do not require a delay in puberty or a period of acyclicity.