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

Age‐Related Effects on the Biological Clock and its Behavioral Output in a Primate

In humans, activity rhythms become fragmented and attenuated in the elderly. This suggests an alteration of the circadian system per se that could in turn affect the expression of biological rhythms. In primates, very few studies have analyzed the effect of aging on the circadian system. The mouse lemur provides a unique model of aging in non-human primates. To assess the effect of aging on the circadian system of this primate, we recorded the circadian and daily rhythms of locomotor activity of mouse lemurs of various ages. We also examined age-related changes in the daily rhythm of immunoreactivities for vasoactive intestinal polypeptide (VIP) and arginine-vasopressin (AVP) in suprachiasmatic nucleus neurons (SCN), two major peptides of the biological clock. Compared to adult animals, aged mouse lemurs showed a significant increase in daytime activity and an advanced activity onset. Moreover, when maintained in constant dim red light, aged animals exhibited a shortening of the free-running period compared to adult animals. In adults, AVP immunoreactivity (ir) peaked during the second part of the day, and VIP ir peaked during the night. In aged mouse lemurs, the peaks of AVP ir and VIP ir were significantly shifted with no change in amplitude. AVP ir was most intense at the beginning of the night; whereas, VIP ir peaked at the beginning of the daytime. A weakened oscillator could account for the rhythmic disorders often observed in the elderly. Changes in the daily rhythms of AVP ir and VIP ir may affect the ability of the SCN to transmit rhythmic information to other neural target sites, and thereby modify the expression of some biological rhythms.

The role of the brain in female reproductive aging

In middle-aged women, follicular depletion is a critical factor mediating the menopausal transition; however, all levels of the hypothalamic-pituitary-gonadal (HPG) axis contribute to the age-related decline in reproductive function. To help elucidate the complex interactions between the ovary and brain during middle-age that lead to the onset of the menopause, we utilize animal models which share striking similarities in reproductive physiology. Our results show that during middle-age, prior to any overt irregularities in estrous cyclicity, the ability of 17beta-estradiol (E(2)) to modulate the cascade of neurochemical events required for preovulatory gonadotropin-releasing hormone (GnRH) release and a luteinizing hormone (LH) surge is diminished. Middle-aged female rats experience a delay in and an attenuation of LH release in response to E(2). Additionally, although we do not observe a decrease in GnRH neuron number until a very advanced age, E(2)-mediated GnRH neuronal activation declines during the earliest stages of age-related reproductive decline. Numerous hypothalamic neuropeptides and neurochemical stimulatory inputs (i.e., glutamate, norepinephrine (NE), and vasoactive intestinal peptide (VIP)) that drive the E(2)-mediated GnRH/LH surge appear to dampen with age or lack the precise temporal coordination required for a specific pattern of GnRH secretion, while inhibitory signals such as gamma-aminobutyric acid (GABA) and opioid peptides remain unchanged or elevated during the afternoon of proestrus. These changes, occurring at the level of the hypothalamus, lead to irregular estrous cycles and, ultimately, the cessation of reproductive function. Taken together, our studies indicate that the hypothalamus is an important contributor to age-related female reproductive decline.

Aging in the circadian system: considerations for health, disease prevention and longevity

The circadian system orchestrates internal physiology on a daily schedule to promote optimal health and maximize disease prevention. Chronic disruptions in circadian function are associated with an increase in a variety of disease states including, heart disease, ulcers and diabetes. With advanced age, the genes regulating circadian function at the cellular level become disorganized and the ability of the brain clock to entrain to local time diminishes. As a result, aged individuals exhibit a loss of temporal coordination among bodily systems, leading to deficits in homeostasis and sub-optimal functioning. Such disruptions in the circadian system appear to accelerate the aging process and contribute to senescence, with some systems being more vulnerable than others. This review explores aging-associated changes in circadian function and examines evidence linking such alterations to adverse health consequences in late life and promotion of the aging process.

Calbindin D28K protein cells in a primate suprachiasmatic nucleus: localization, daily rhythm and age-related changes

In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian pacemaker. The SCN controls daily rhythms and synchronizes the organism to its environment and especially to photic signals. Photic signals via the retinohypothalamic tract reach the ventral part of the SCN, where the majority of calbindin-containing neurons are located. Calbindin cells seem important for the control of circadian rhythmicity. As ageing leads to marked changes in the expression of circadian rhythms, we investigated in the mouse lemur, a nocturnal primate, age-related changes in the oscillation of calbindin protein expression in SCN neurons. We used immunohistochemistry and quantitative analysis of calbindin expression in the SCN of adult and aged mouse lemurs. In this primate, a dense cluster of calbindin-positive neurons was found in the ventral part of the SCN. In adult animals, calbindin-positive SCN neurons did not exhibit daily rhythms in their number or intensity, but exhibited significant daily variations in the percentage of cells with a calbindin-positive nucleus, characterized by high values during the daytime and low values during the night. Immunoreactive intensity peaked in the middle of the daytime. Calbindin expression in the nuclei of calbindin cells in the SCN tends to be modified by ageing. The amplitude of daily variation in calbindin expression was damped, with a lower immunointensity during the daytime and a delayed decrease during the night. These changes may affect the ability of the SCN to transmit rhythmic information to other SCN cells and thereby modify the synchronization of the different cell populations in the SCN.

Age-related disruptions of circadian rhythm and memory in the senescence-accelerated mouse (SAMP8)

Common complaints of the elderly involve impaired cognitive abilities, such as loss of memory and inability to attend. Although much research has been devoted to these cognitive impairments, other factors such as disrupted sleep patterns and increased daytime drowsiness may contribute indirectly to impaired cognitive abilities. Disrupted sleep-wake cycles may be the result of age-related changes to the internal (circadian) clock. In this article, we review recent research on aging and circadian rhythms with a focus on the senescence-accelerated mouse (SAM) as a model of aging. We explore some of the neurobiological mechanisms that appear to be responsible for our aging clock, and consider implications of this work for age-related changes in cognition.

Vasoactive intestinal polypeptide regulates dynamic changes in astrocyte morphometry: impact on gonadotropin-releasing hormone neurons

Recent studies suggest that astrocytes modulate the GnRH-induced LH surge. In particular, we have shown that the surface area of astrocytes that ensheath GnRH neurons exhibits diurnal rhythms. Vasoactive intestinal polypeptide (VIP) influences numerous aspects of astrocyte function in multiple brain regions and is a neurotransmitter in the suprachiasmatic nucleus (SCN) that affects GnRH neurons. The goals of this study were to: 1) assess whether astrocytes that surround GnRH neurons express VIP receptors, 2) determine the effects VIP suppression in the SCN on the morphometry of astrocytes surrounding GnRH neurons, and 3) assess whether this effect mimics aging-like changes in surface area of astrocytes. Young rats were ovariectomized (d 0), implanted with cannulae into the SCN (d 5), injected with VIP antisense (antioligo) or random sequence oligonucleotides, implanted with capsules containing 17beta-estradiol dissolved in oil (d 7), and perfused at 0300, 1400, and 1800 h (d 9). Brains were processed for immunocytochemistry. Our results demonstrate that astrocytes in close apposition to GnRH neurons express VIP receptors. Antioligo treatment blocked diurnal rhythms in surface area of astrocytes ensheathing GnRH neurons. The absence of diurnal rhythms resembles observations in middle-aged rats. Together these findings suggest that the ability of the VIP-containing neurons in the SCN to relay diurnal information to GnRH neurons may be by influencing dynamic changes in the morphometry of astrocytes that surround GnRH neurons. Furthermore, the absence of a VIP rhythm in aging animals may lead to altered GnRH activity via astrocyte-dependent mechanisms.

Neuroendocrine control of reproductive aging: roles of GnRH neurons

The process of reproductive senescence in many female mammals, including humans, is characterized by a gradual transition from regular reproductive cycles to irregular cycles to eventual acyclicity, and ultimately a loss of fertility. In the present review, the role of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons is considered in this context. GnRH neurons provide the primary driving force upon the other levels of the reproductive axis. With respect to aging, GnRH cells undergo changes in biosynthesis, processing and release of the GnRH decapeptide. GnRH neurons also exhibit morphologic and ultrastructural alterations that appear to underlie these biosynthetic properties. Thus, functional and morphologic changes in the GnRH neurosecretory system may play causal roles in the transition to acyclicity. In addition, GnRH neurons are regulated by numerous inputs from neurotransmitters, neuromodulators and glia. The relationship among GnRH cells and their inputs at the cell body (thereby affecting GnRH biosynthesis) and the neuroterminal (thereby affecting GnRH neurosecretion) is crucial to the function of the GnRH system, with age-related changes in these relationships contributing to the reproductive senescent process. Therefore, the aging hypothalamus is characterized by changes intrinsic to the GnRH cell, as well as its regulatory inputs, which summate to contribute to a loss of reproductive competence in aging females.

Increased Fos immunoreactivity in suprachiasmatic nucleus before luteinizing hormone surge in estrogen-treated ovariectomized female rats

BACKGROUND/AIMS

The suprachiasmatic nucleus (SCN) is thought to control the timing of luteinizing hormone (LH) surges. The present study was designed to examine temporal patterns of Fos expression in the dorsomedial and ventrolateral parts of the SCN (SCNdm and SCNvl) of female rats during an LH surge. It also included examination of temporal changes in plasma LH levels and temporal changes in Fos levels in the anteroventral periventricular nucleus (AVPV) and gonadotropin-releasing hormone (GnRH) neurons.

METHODS

Ovariectomized rats injected with 20 microg estradiol benzoate (EB) or vehicle were sacrificed at various times from Zeitgeber time (ZT) 8:00 to 16:30 h (ZT8-16.5; ZT0 = lights on; ZT12 = lights off) on the 2nd day after the injection. Immunohistochemical analyses for Fos and GnRH and enzyme-linked immunosorbent assays for LH were then performed.

RESULTS

In both the SCNdm and SCNvl of EB rats, the number of Fos-immunoreactive cells significantly increased between ZT9.5-10.5 and ZT11-12. On the other hand, in EB rats there were significant peaks of LH levels and Fos levels in GnRH neurons and the AVPV between ZT11-12 and ZT13-14. There was no significant difference in the number of Fos-immunoreactive cells between EB and control rats in either the SCNdm or SCNvl at ZT9.5-10.5, or in the SCNdm at ZT11-12, whereas the SCNvl of EB rats contained more Fos-immunoreactive cells than that of control rats at ZT11-12.

CONCLUSION

These results suggest that in female rats during an LH surge, a peak in the Fos level in the SCN precedes peaks in Fos levels in the AVPV and GnRH neurons.

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.

Arginine-vasopressin and vasointestinal polypeptide rhythms in the suprachiasmatic nucleus of the mouse lemur reveal aging-related alterations of circadian pacemaker neurons in a non-human primate

The suprachiasmatic nucleus (SCN) of the hypothalamus, the mammalian circadian pacemaker, is entrained by external cues and especially by photic information. Light is transmitted primarily via the retinohypothalamic tract, which terminates in the ventral part (or core) of the SCN, where vasoactive intestinal polypeptide (VIP)-containing neurons are located. VIP cells are mainly intrinsic and project to the dorsal part (or shell) of the SCN, where neurons containing arginine-vasopressin (AVP) reside. As aging leads to marked changes in the expression of circadian rhythms, we examined in primates whether age-related decay in biological rhythmicity is associated with changes in the oscillation of peptide expression in SCN neurons. We used double immunohistochemistry and quantitative analysis in the SCN of mouse lemurs, which provide a unique model of aging in non-human primates. In adult animals, VIP-positive and AVP-positive SCN neurons exhibited daily rhythms of their number and immunostaining intensity: AVP immunoreactivity peaked during the second part of the day, and VIP peaked during the night. In aged mouse lemurs, the peaks of AVP and VIP immunopositivity were significantly shifted, so that AVP was most intense at the beginning of the night, whereas VIP peaked at the beginning of daytime. The results show that the circadian rhythm of neuropeptides in the SCN is modified by aging in primates, with a differential regulation of the two main peptidergic cell populations. These changes may affect the ability of the SCN to transmit rhythmic information to other neural target sites, and thereby to modify the expression of some biological rhythms.

Nerve growth factor restores the expression of vasopressin and vasoactive intestinal polypeptide in the suprachiasmatic nucleus of aged rats

Aging leads to a decrease in the number of neurons expressing vasopressin (VP) and vasoactive intestinal polypeptide (VIP) in the suprachiasmatic nucleus (SCN) of the rat. Similar results were observed following prolonged alcohol consumption and withdrawal. In the latter circumstances, the administration of nerve growth factor (NGF) restored the synthesis and expression of those neuropeptides despite the absence of TrkA receptors in SCN neurons. Thus, we decided to test whether the administration of NGF would improve the expression of neuropeptides in the SCN of aged rats. For this purpose, NGF was delivered intraventricularly to aged rats over a period of 14 days. The somatic volume and the total number of VP- and VIP-immunostained SCN neurons were estimated by applying stereological methods. No age-related variations were found regarding the volume of the neuronal cell bodies. Yet, a striking reduction in the number of VP- and VIP-immunoreactive neurons was detected in aged animals and found to be completely retrieved by NGF. This finding shows that exogenous NGF administered to aged rats restores the neurochemical phenotype of the SCN. This might occur either through direct signaling of SCN neurons via p75NTR or through enhancement of the cholinergic input to the SCN.

Age-related changes in electrophysiological properties of the mouse suprachiasmatic nucleus in vitro

Endogenous biological rhythms are altered at several functional levels during aging. The major pacemaker driving biological rhythms in mammals is the suprachiasmatic nucleus of the hypothalamus. In the present study we used tissue slices from young and old mice to analyze the electrophysiological properties of the retinorecipient ventrolateral part of the suprachiasmatic nucleus. Loose patch and whole-cell recordings were performed during day and night. Both young and old mice displayed a significant variation between day and night in the mean firing rate of suprachiasmatic nucleus neurons. The proportion of cells not firing spontaneous action potentials showed a clear day/night rhythm in young but not in old animals, that had an elevated number of such silent cells during the day compared to young animals. Analysis of firing patterns revealed a more regular spontaneous firing during the day than during the night in the old mice, while there was no difference between day and night in young animals. The frequency of spontaneous inhibitory postsynaptic currents was reduced in ventrolateral suprachiasmatic nucleus neurons in the old animals. Since the inhibitory input to these neurons is mainly derived from within the suprachiasmatic nucleus, this reduction most likely reflects the greater proportion of silent cells found in old animals. The results show that the suprachiasmatic nucleus of old mice is subject to marked electrophysiological changes, which may contribute to physiological and behavioral changes associated with aging.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Neuroendocrinology of female aging

BACKGROUND

Because the average human life span has increased, a greater part of more women's lives will be lived in a hypoestrogenic state.

OBJECTIVE

This article provides an overview of our current knowledge of the neuroendocrine processes in the aging female brain.

METHODS

Using the search terms cardiovascular disease, cognition, dementia, depression, estrogens, female aging, gonadotropins, immune function, mood, neuroendocrinology, neurotransmitters, osteoporosis, and ovarian steroids, a review of English-language literature on the MEDLINE database was conducted from 1970 through June 2004.

RESULTS

It is thought that the temporal patterns of neural signals are altered during middle age, leading to cessation of reproductive cycles, and that the complex interplay of ovarian and hypothalamic/pituitary pacemakers becomes increasingly dysfunctional with aging, ultimately resulting in menopause. Estrogen deficiency is associated with low mood, whereas estrogen therapy tends to be linked with improvements in measures of well-being and a decline in depression scores. It is likely that these effects of estrogens are mediated through changes in the metabolism of serotonin and nor epinephrine. Evidence exists to support the role of estrogens in specific effects on cognitive functioning in women, enhancing aspects of verbal memory, abstract reasoning, and information processing. Significant gender dimorphism is evident in both humoral and cell-mediated immune responses. The effects of estrogens on the cardiovascular system are complex; recent evidence suggests a negative role for oral estrogen in primary and secondary prevention of cardiovascular events. Additionally, estrogens increase the risk of stroke, and estrogen deficiency influences the pathogenesis of osteoporosis in both men and women.

CONCLUSIONS

Changes in the neuroendocrine system due to the loss of ovarian function at menopause have an important biological role in the control of reproductive and nonreproductive functions, and regulate mood, memory, cognition, behavior, immune function, the locomotor system, and cardiovascular functions. More detailed insights are needed into the complex mechanisms of neuroendocrine alterations with aging.

Age-related change and its sex differences in histoarchitecture of the hypothalamic suprachiasmatic nucleus of F344/N rats

The present study examined the effects of aging and sex differences on the suprachiasmatic nucleus (SCN) of F344/N rats. In juveniles (1.6-1.9 months of age), adults (11.7-16.3 months of age), and old (29.2-34 months of age) rats, the volume, size of neuronal nucleus and neuronal cell number of the SCN were determined with cresyl fast violet-stained sections. In addition, immunohistochemical analysis was performed for glial fibrillary acidic protein (GFAP). There was no significant effect of aging and sex differences on the SCN volume. The number of neurons in the SCN gradually decreased from juvenile to old age in females. However, in males, the number was significantly decreased in adult and old age rats. The size of neuronal nuclei in the SCN was significantly decreased by increasing age in both sexes, except for the ventrolateral part of the SCN of males. In the dorsomedial part of the SCN of females, the density of GFAP-immunoreactive components was significantly higher in adult age rats than in rats of other ages. However, there was no significant increase in the density of the SCN in adult males. These results suggest that morphological changes in neuronal and astroglial cells occur in the SCN with aging in a sex-specific manner.

Nuclear Hormone Receptors, Metabolism, and Aging: What Goes Around Comes Around

Previous studies have linked the mysterious and inevitable process of aging to essential processes such as metabolism, maturation, and fecundity. Each of these processes is controlled to a large extent by nuclear hormone receptors (NHRs). NHRs also play important roles in the control of periodical processes, the most recently implicated being circadian rhythm. This Review stresses the mounting evidence for tight relationships between each of these NHR-regulated processes and the processes of aging.

Food restriction during lactation results in prolonged hyposensitivity to the positive-feedback effects of oestradiol

Food restriction prolongs lactational infertility in rats. Here, we investigated whether an attenuated response to the positive-feedback effects of oestrogen on luteinizing hormone release contributed to this effect. The ability of oestrogen to induce surges in luteinizing hormone in ad libitum fed and food-restricted dams at different times of lactation was compared. The results showed that, on day 20 postpartum, ad libitum fed dams showed luteinizing hormone surges after oestrogen treatment, but food-restricted dams did not. Ovariectomy or RU486 treatment restored the ability of oestrogen to induce luteinizing hormone surges in food-restricted dams, and chronic progesterone exposure reduced oestrogen-induced surges of luteinizing hormone in ad libitum fed ovariectomized dams. Food restriction also resulted in a reduced ability of oestrogen to induce progesterone receptor immunoreactivity, but did not reduce the number of oestrogen receptors (ERalpha) in the anteroventralperiventricular area. As with the surge in luteinizing hormone, the effects of food restriction on oestrogen induction of progesterone receptors were mediated by progesterone. Together, these results suggest that the ability of food restriction to extend the length of lactational diestrus is mediated, in part, by a decrease in sensitivity to the positive-feedback effects of oestrogen. This results from high circulating concentrations of progesterone which apparently reduce the ability of oestrogen to induce progesterone receptor expression.

Aging alters circadian and light-induced expression of clock genes in golden hamsters

Aging alters numerous aspects of circadian biology, including the amplitude of rhythms generated by the suprachiasmatic nuclei (SCN) of the hypothalamus, the site of the central circadian pacemaker in mammals, and the response of the pacemaker to environmental stimuli such as light. Although previous studies have described molecular correlates of these behavioral changes, to date only 1 study in rats has attempted to determine if there are age-related changes in the expression of genes that comprise the circadian clock itself. We used in situ hybridization to examine the effects of age on the circadian pattern of expression of a subset of the genes that comprise the molecular machinery of the circadian clock in golden hamsters. Here we report that age alters the 24-h expression profile of Clock and its binding partner Bmal1 in the hamster SCN. There is no effect of age on the 24-h profile of either Per1 or Per2 when hamsters are housed in constant darkness. We also found that light pulses, which induce smaller phase shifts in old animals than in young, lead to decreased induction of Per1, but not of Per2, in the SCN of old hamsters.

Aging alters light- and PACAP-induced cAMP accumulation in the suprachiasmatic nucleus of female rats

Light-induced release of pituitary adenylate cyclase activating peptide (PACAP) from retinal ganglion cells can modulate the phase-shifting effects of light though a cAMP-mediated mechanism in neurons of the suprachiasmatic nucleus (SCN). Since older animals (12 months or older) show a reduced behavioral and cellular response to light presented during the early portion of the dark phase of the cycle, we hypothesized that aging may alter the ability of PACAP and cAMP to modulate the phase shifting effects of light. In Expt. 1, we examined basal and light-induced cAMP accumulation at zeitgeber time 14 (ZT14 where ZT0 is the time of lights on). Light exposure resulted in a significant increase in cAMP accumulation in SCN tissue collected from young, but not middle-aged animals. The failure to see an increase in cAMP accumulation in the SCN of middle-aged animals may be related to the fact that basal levels of cAMP were elevated in the SCN of these animals at ZT14. In Expt. 2, we used an in vitro slice preparation of the SCN to determine if aging altered the ability of PACAP to stimulate cAMP accumulation in the SCN at ZT14. PACAP stimulated cAMP in the SCN of both young and middle-aged animals. However, PACAP-induced cAMP accumulation was lower in the SCN of middle-aged animals. Based on these results, we conclude that age-related changes in the responsiveness of the SCN to light input are due to: (1) changes in other input pathways capable of modulating cAMP, and (2) decreases in PACAP receptors in SCN neurons.

Effects of aging on central and peripheral mammalian clocks

Circadian organization changes with age, but we do not know the extent to which age-related changes are the result of alterations in the central pacemakers, the peripheral oscillators, or the coupling mechanisms that hold the system together. By using transgenic rats with a luciferase (luc) reporter, we assessed the effects of aging on the rhythm of expression of the Period 1 (Per1) gene in the suprachiasmatic nucleus (SCN) and in peripheral tissues. Young (2 months) and aged (24-26 months) Per1-luc transgenic rats, entrained to light-dark cycles, were killed, and tissues were removed and cultured. Per1-luc expression was measured from 10 tissues. In the SCN, the central mammalian pacemaker, Per1-luc expression was robustly rhythmic for more than 7 weeks in culture. The only difference between SCN rhythmicity in young and old rats was a small but significant age-related shortening of the free-running period. Circadian rhythmicity in some peripheral tissues was unaffected by aging, whereas rhythmicity in other tissues was either phase advanced relative to the light cycle or absent. Those tissues that were arrhythmic could be induced to oscillate by application of forskolin, suggesting that they retained the capacity to oscillate but were not being appropriately driven in vivo. Overall, the results provide new insights into the effects of aging on the mammalian circadian system. Aging seems to affect rhythms in some but not in all tissues and may act primarily on interactions among circadian oscillators, perhaps attenuating the ability of the SCN to drive damped oscillators in the periphery.

Age-related disruptions in circadian timing: evidence for "split" activity rhythms in the SAMP8

In the senescence-accelerated mouse of the P8 line (SAMP8), age-related changes in circadian timing include a decrease in amplitude of the rhythm, a slower rate of re-entrainment following a phase advance, and a longer free-running period (tau). The present study extends previous research by investigating possible split activity rhythms in the SAMP8. Running wheel activity was examined in 2-, 7-, and 12-month SAMP8. Consistent with previous research, rhythms of older SAMP8 were decreased in amplitude and showed high levels of activity during the light phase of the light-dark cycle. Contrary to previous reports, lengthening of tau in constant darkness was not observed. Additionally, activity rhythms were "split" in older SAMP8, demonstrated by 1) the appearance of a secondary peak in the periodogram at approximately half the value of tau and 2) the perceptual classification of actograms by naïve observers. This result differs from previous studies in hamsters demonstrating an age-related decrease in the incidence of rhythm splitting. Overall, the present findings provide further evidence for age-related disruption of circadian timing in SAMP8.

Senescence, sleep, and circadian rhythms

The goal of this review article is to summarize our knowledge and understanding of the overlapping (interdisciplinary) areas of senescence, sleep, and circadian rhythms. Our overview comprehensively (and visually wherever possible), emphasizes the organizational, dynamic, and plastic nature of both sleep and circadian timing system (CTS) during senescent processes in animals and in humans. In this review, we focus on the studies that deal with sleep and circadian rhythms in aged animals and how these studies have closely correlated to and advanced our understanding of similar processes in ageing humans. Our comprehensive summary of various aspects of the existing research on animal and human ageing, both normal and pathological, presented in this review underscores the invaluable advantage of close collaboration between clinicians and basic research scientists and the future challenges inherent in this collaboration. First, our review addresses the common age-related changes that occur in sleep and temporal organization of both animals and humans. Second, we examine the specific modifications that often accompany sleep and CTS during aging. Third, we discuss the clinical epidemiology of sleep dysfunctions during ageing and their current clinical management, both pharmacological and non-pharmacological. Finally, we predict the possible future promises for complementary and alternative medicine (CAM) that pave the way to the emergence of a "Holistic Sleep Medicine" approach to the treatment of sleep disorders in the ageing population. Further studies will provide additional valuable insights into the understanding of both sleep and circadian rhythms during senescence.

Circadian profile of Per gene mRNA expression in the suprachiasmatic nucleus, paraventricular nucleus, and pineal body of aged rats

Aging alters circadian components such as the free-running period, the day-to-night activity ratio and photic entrainment in behavioral rhythms, and 2-deoxyglucose uptakes and neuronal firing in the suprachiasmatic nucleus (SCN). A core clock mechanism in the mouse SCN appears to involve a transcriptional feedback loop in which Period (Per) and Cryptochrome (Cry) genes play a role in negative feedback. The circadian rhythm systems include photic entrainment, clock oscillation, and outputs of clock information such as melatonin production. In this experiment, we examined clock gene expression to determine whether circadian input, oscillation, and output are disrupted with aging. Circadian expression profiles of rPer1, rPer2, or rCry1 mRNA were very similar in the SCN, the paraventricular nucleus of the hypothalamus (PVN), and the pineal body of young and aged (22-26 months) rats. On the other hand, the photic stimulation-induced rapid expression of Per1 and Per2 in the SCN was reduced with aging. The present results suggest that the molecular mechanism of clock oscillation in the SCN, PVN, and pineal body is preserved against aging, whereas the impairment of Per1 induction in the SCN after light stimulation may result in impaired behavioral photic entrainment in aged rats.

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.

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.

Aging selectively suppresses vasoactive intestinal peptide messenger RNA expression in the suprachiasmatic nucleus of the Syrian hamster

Aging leads to many changes in the expression of circadian rhythms, including reduced amplitude, altered relationship to the environmental illumination cycle, and reduced sensitivity to phase resetting signals. Neuropeptide synthesizing neurons in the suprachiasmatic nucleus (SCN), the principal circadian pacemaker in mammals, play a role in regulating pacemaker function and in coupling the pacemaker to overt circadian rhythms. Aging may alter the activity of neuropeptide neurons in the SCN, which could be reflected in changes in mRNA expression. Therefore, this study investigated whether aging alters the level or rhythm of expression of neuropeptide mRNAs in the SCN of male Syrian hamsters, a well established model for the study of age-related changes in circadian rhythms. Three age groups of hamsters (young [3--5 months old], middle-aged [12--15 months old] and old [19--22 months old] were sacrificed at five times of day. Their brains were dissected and sections through the suprachiasmatic nucleus were prepared and used for in situ hybridization for mRNAs for vasoactive intestinal peptide (VIP), arginine vasopressin (AVP) and somatostatin (SS). Aging selectively decreased the SCN expression of VIP mRNA without affecting AVP mRNA or SS mRNA. Also, only AVP mRNA expression exhibited a robust 24-h rhythm, in contrast to previous findings in other species that VIP mRNA and SS mRNA, as well as AVP mRNA, exhibit 24-h rhythms in the SCN. The present findings suggest that age-related reductions in VIP mRNA expression may contribute to the alterations in entrainment and attenuated sensitivity to phase resetting signals that are characteristic of aging. Furthermore, the results demonstrate that neuropeptide gene expression in the SCN is differentially regulated by aging and varies among species.

Developmental alcohol and circadian clock function

Studies in rats found that alcohol exposure during the early postnatal period, particularly during the brain-growth-spurt period, can result in cell loss in various brain regions and persistent behavioral impairments. Some investigators have speculated that the body's internal clock, which is located in the suprachiasmatic nuclei (SCN) in the brain, may also be affected by developmental alcohol exposure. For example, alcohol-induced damage to the SCN cells and their function could result in disturbances of the circadian timekeeping function, and these disturbances might contribute to the behavioral impairments and affective disorders observed in people prenatally exposed to alcohol. Preliminary findings of studies conducted in rats suggest that developmental alcohol exposure may indeed interfere with circadian clock function as evidenced by a shortened circadian sleep-wake cycle and changes in the release of certain brain chemicals (i.e., neuropeptides) by SCN cells.

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.

Hypothalamic alterations and reproductive aging in female rats: evidence of altered luteinizing hormone-releasing hormone neuronal function

Prior to the age-related loss of regular estrous cycles, female rats exhibit an attenuated preovulatory LH surge, a sign that reproductive decline is imminent. Numerous studies have revealed an important role for the hypothalamus in aging of the reproductive axis in this species. Because LHRH represents the primary hypothalamic signal that regulates gonadotropin release, assessments of LHRH neuronal activity can provide a window into hypothalamic function during reproductive aging. Studies of the dynamic activity of LHRH neurons during times of enhanced secretion have revealed deficits in middle-aged females. Available data are consistent with a decline in LHRH synthesis, transport, and secretion in middle-aged females during times of increased demand for LHRH output. Moreover, the alterations noted in LHRH neuronal function could account, in part, for the attenuation and eventual loss of the preovulatory LH surge with age. Elements extrinsic to LHRH neurons undoubtedly contribute to the decline in the parameters of LHRH neuronal function observed in middle-aged females. Whether alterations intrinsic to LHRH neurons also play a role in the age-associated reduction in LHRH synthesis and secretion remains to be determined. Recent examinations of hormone profiles during the perimenopausal period suggest that a potential hypothalamic contribution to aging of the reproductive axis in women warrants further examination.

The role of Clock in the developmental expression of neuropeptides in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the dominant circadian pacemaker in mammals. To understand better the ontogeny of mouse SCN and the role of the pacemaker in peptide expression, the authors examined the distribution of cells that were immunoreactive for vasopressin (AVP) or vasoactive intestinal polypeptide (VIP) in wild type and Clock mutant mice at two developmental stages. Clock homozygous mice failed to show the dramatic increase in the number of VIP-immunoreactive (VIP-ir) neurons from postnatal day 6 (P6) to P30 that was found in the SCN of wild type mice. The number of AVP-ir neurons was relatively constant in the postnatal SCN but was significantly reduced in Clock/Clock mice. The effects of the Clock mutation varied with position in the SCN for both peptides. Densitometry of immunolabeled brains indicated that the Clock mutation reduced AVP expression specifically in the SCN and not in other brain areas. The SCN did not significantly change shape or size with age or Clock genotype. Taken together, these results indicate that the neonatal mouse SCN has its full complement of cells, some of which are not yet mature in their neuropeptide content. Furthermore, the observation that the Clock mutation appears to act on a subset of AVP and VIP cells suggests heterogeneity within these cell classes in the SCN.

Estrogen increases arginine-vasopressin V1a receptor mRNA in the preoptic area of young but not of middle-aged female rats

We determined whether estrogen regulates the expression of arginine-vasopressin (AVP) receptor mRNA in the preoptic area (POA) of female rats. By reverse transcription-polymerase chain reaction (PCR), we found that all three types of the AVP receptor mRNA, V1a, V1b and V2, were expressed in the POA, though the amount of PCR products was apparently different among them. In situ hybridization indicated that AVP V1a receptor mRNA was densely expressed in the POA, especially in the anteroventral periventricular nucleus of the POA; in contrast, AVP V1b and V2 receptor mRNAs were not abundant in this area. Finally, we demonstrated by Northern blot that estrogen significantly increased the expression of AVP V1a receptor mRNA in the POA of young ovariectomized rats. However, this regulation by estrogen was lost in middle-aged rats, indicating an age-related impairment in the regulation of AVP V1a receptor mRNA by estrogen in the POA.

Age-dependent changes of the circadian system

This review summarizes the current knowledge on changes of the circadian system in advanced age, mainly for rodents. The first part is dedicated to changes of the overt rhythms. Possible causes are discussed, as are methods to treat the disturbances. In aging animals and humans, all rhythm characters change. The most prominent changes are the decrease of the amplitude and the diminished ability to synchronize with a periodic environment. The susceptibility to photic and nonphotic cues is decreased. As a consequence, both internal and external temporal order are disturbed under steady-state conditions and, even more, following changes in the periodic environment. Due to the high complexity of the circadian system, which includes oscillator(s), mechanisms of external synchronization and of internal coupling, the changes may arise for several reasons. Many of the changes seem to occur within the SCN itself. The number of functioning neurons decreases with advancing age and, probably, so does the coupling between them. As a result, the SCN is unable, or at least less able, to produce stable rhythms and to transmit timing information to target sites. Initially, only the ability to synchronize with the periodic environment is diminished, whereas the rhythms themselves continue to be well pronounced. Therefore, the possibility exists to treat age-dependent disturbances. This can be done pharmacologically or by increasing the zeitgeber strength. So, some of the rhythm disturbances can be reversed, increasing the magnitude of the light-dark (LD) zeitgeber. Another possibility is to strengthen feedback effects, for example, by increasing the daily amount of activity. By this means, the stability and synchronization of the circadian activity rhythm of old mice and men were improved.

Aging regulates 5-HT(1B) receptors and serotonin reuptake sites in the SCN

Middle age is associated with changes in circadian rhythms (e.g., alterations in the timing of the circadian wheel running rhythm) which resemble changes induced by selective destruction of the serotonergic input to the suprachiasmatic nucleus (SCN), the principal mammalian circadian pacemaker. We hypothesized that serotonergic neurotransmission in the SCN is decreased in middle-aged hamsters, as compared to young adults. This hypothesis was tested indirectly by investigating the effect of aging on two markers of serotonin neurotransmission, 5-HT(1B) receptors and serotonin reuptake sites, which are regulated by serotonin. Previous studies have shown that experimentally induced decreases in serotonergic neurotransmission increase 5-HT(1B) receptors but decrease serotonin reuptake sites. Quantitative autoradiography was conducted using [125I]iodocyanopindolol ([125I]ICYP) and [3H]paroxetine, selective radioligands for the 5-HT(1B) receptors and the serotonin reuptake sites, respectively. Consistent with the hypothesis, specific ([125I]ICYP binding was significantly elevated in the SCN of middle-aged hamsters, as compared to young hamsters. The results also showed that serotonin reuptake sites in the SCN were significantly increased in both middle-aged and old hamsters, as compared to young controls. This result could not have been caused by decreased serotonin release. Alternatively, increased serotonin reuptake, which would reduce serotonin levels in the synaptic cleft, may cause or contribute to the increase in 5-HT(1B) receptor binding in the SCN in middle aged animals. These results show that the SCN exhibits changes in serotonergic function during middle age, which has been characterized by changes in the expression of circadian rhythms. Because these changes occur during middle age, they probably reflect the aging process, rather than senescence or disease.

A new concept in the pharmacology of neuroprotection

Vasoactive intestinal peptide (VIP), originally discovered in the intestine as a peptide of 28 amino acids, was later found to be a major brain peptide having neuroprotective activities. To exert neuroprotective activity, VIP requires glial cells secreting neuroprotective proteins. Activity-dependent neurotrophic factor (ADNF) is a recently isolated factor secreted by glial cells under the action of VIP. This protein, isolated by sequential chromatographic methods, was named activity-dependent neurotrophic factor since it protected neurons from death associated with blockade of electrical activity. A fourteen-amino-acid fragment of ADNF (ADNF-14) and the more potent, nine-amino-acid derivative (ADNF-9), exhibit activity that surpasses that of the parent protein with regard to potency and a broader range of effective concentration. Furthermore, the peptides exhibit protective activity in Alzheimer's disease-related systems (e.g., beta-amyloid toxicity and apolipoprotein E deficiencies, genes that have been associated with Alzheimer's disease onset and progression). ADNP is another glial mediator of VIP-associated neuroprotection. NAP, an eight-amino-acid peptide derived from ADNP (sharing structural and functional similarities with ADNF-9), was identified as the most potent neuroprotectant described to-date in an animal model of apolipoprotein E-deficiency (knock-out mice). These femtomolar-acting peptides form a basis for a new concept in pharmacology: femtomolar neuroprotection.

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.

Vasopressin induces a luteinizing hormone surge in ovariectomized, estradiol-treated rats with lesions of the suprachiasmatic nucleus

The luteinizing hormone surge in the female rat is the result of the integration of multiple signals within the medial preoptic area. The medial preoptic area contains gonadotropin-releasing hormone neurons that are responsible for the release of luteinizing hormone, neurons containing estrogen receptors and terminals originating from the suprachiasmatic nucleus with, for example, vasopressin as neurotransmitter. Both the medial preoptic area and suprachiasmatic nucleus are crucial for the occurrence of luteinizing hormone surges, since lesioning of either nucleus prevents pre-ovulatory and steroid-induced luteinizing hormone surges. In this study, we investigated whether vasopressin in the medial preoptic area could be the daily neuronal signal from the suprachiasmatic nucleus responsible for the timing of the luteinizing hormone surge. Vasopressin (50 ng/microl) or Ringer solution was administered by reverse microdialysis from Zeitgeber times 7.5 to 12.5 into the medial preoptic area of ovariectomized, estradiol-treated rats. The suprachiasmatic nucleus was lesioned to remove all cyclic luteinizing hormone secretion. This was evaluated by monitoring behavioral activity; animals that were arrhythmic were included in the experiments. Hourly blood samples were taken to measure plasma luteinizing hormone levels. Preoptic vasopressin administration induced a surge-like luteinizing hormone pattern in suprachiasmatic nucleus-lesioned animals, whereas constant, basal luteinizing hormone levels were found in the control animals. These data show that vasopressin, by itself, is able to trigger the luteinizing hormone surge in suprachiasmatic nucleus-lesioned rats. We propose that vasopressin is a timing signal from the suprachiasmatic nucleus responsible for the activation of the hypothalamo-pituitary-gonadal axis in the female rat.

Aging and photoperiod regulate glutamic acid decarboxylase(67) messenger RNA expression

Aging and short photoperiod exposure both induce similar long-term changes in circadian rhythms, including alterations in the timing and the amplitude of rhythms. Furthermore, these chronic conditions affect the function of the circadian pacemaker in the suprachiasmatic nuclei (SCN) by altering rhythmic expression of neuropeptide messenger RNAs (mRNAs). Because GABA modulates SCN neuronal activity, and GABAergic neurons innervate peptidergic neurons in the SCN, the present study investigated whether photoperiod or aging affect the expression of mRNA for GAD(67), the enzyme responsible for regulating the tonic levels of GABA. As a control for regional specificity, the reticular thalamic nucleus (RTN) was also examined. In situ hybridization for GAD(67) mRNA was performed on brain sections derived from Siberian hamsters exposed to a long day or a short day photoperiod for 15 days, and on brain sections from young (3-4 months old) and old (12-17 months old) Syrian hamsters exposed to a long photoperiod. The results showed that photoperiod and aging have different effects on GAD(67) mRNA expression. Exposure to short day photoperiod significantly increased GAD(67) mRNA expression in both the SCN and RTN of Siberian hamsters, while aging significantly decreased GAD(67) mRNA expression in the RTN of Syrian hamsters but had no effect on GAD(67) mRNA expression in the SCN. These findings suggest that modulation of GAD(67) mRNA expression in the SCN is associated with photoperiodic regulation of neuropeptide mRNA expression, but is not a common mechanism for chronic regulation of circadian rhythms. Also, GAD(67) mRNA expression in the RTN is differentially affected by photoperiod and aging.

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.