Effects of aging on light-induced phase-shifting of circadian behavioral rhythms, fos expression and CREB phosphorylation in the hamster suprachiasmatic nucleus

Stability of circadian timing with age in Syrian hamsters

The causes of age-related disruptions in the timing of human sleep and wakefulness are not known but may include changes in both the homeostatic and circadian regulation of sleep. In Syrian hamsters the free running period of the circadian activity/rest rhythm has been reported to shorten with age. Although this has been observed under a variety of experimental conditions, the changes have been small and their consistency uncertain. In the present study, the wheel running activity/rest rhythm was continuously measured in male Syrian hamsters (Mesocricetus auratus) in dim constant light (<1 lx) from 8 wk of age until death. Fifteen hamsters survived to at least 90 wk (28%). The average free running period of these hamsters did not change with age. In 18 hamsters that died between 50 and 88 wk, free running period also did not change before death. In contrast to free running period, other measures related to activity level changed significantly with age and before death. Despite changes in the expression of the activity/rest rhythm, the free running period of the hamster circadian pacemaker remained remarkably stable with age.

Circadian behavior and plasticity of light-induced c-fos expression in SCN of tau mutant hamsters

In hamsters homozygous for the circadian clock mutation tau, the photic history dramatically affects the magnitude of light-induced circadian phase shifts. The maximum amplitude of phase shifts produced by 1-h light pulses presented at CT 14 was less than 2 h in animals that had been in DD for 2 days, whereas animals that had been kept in DD for 49 days could be shifted by more than 8 h. In this study, the authors compared the effect of previous light history on the amplitude of circadian phase shifts and on c-fos expression in the SCN of tau mutant hamsters. Although the maximum amplitude of behavioral phase shifts was drastically different between animals that had been held for either 2 or 49 days in DD, maximal fos induction was not significantly different in these two groups. However, photic thresholds for light-induced behavioral phase shifts, c-fos mRNA, and Fos immunoreactivity were closely correlated within both groups, and these thresholds were lower (more sensitive to light) after 49 than after 2 days in DD. The correlation between phase shifting and Fos induction thresholds, under conditions where both responses are dramatically altered by the previous light history, demonstrates an association between changes in circadian behavioral phase-shifting responses of tau mutant hamsters and plasticity of light-induced c-fos expression in SCN. However, because the maximum amplitudes of Fos induction and phase shifting were not correlated in animals that had been in DD for 2 days, we speculate that the level of c-fos expression does not directly determine phase shift amplitude.

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.

Diurnal rhythms in ornithine decarboxylase activity and norepinephrine and acetylcholine synthesis in submaxillary lymph nodes and spleen of young and aged rats during Freund's adjuvant-induced arthritis

Aging has been associated with attenuation of amplitude and changes in period of many circadian rhythms. The present study was carried out to examine, in young (50 days old) and old (18 months old) rats, whether 24-h rhythms of cell proliferation (as assessed by measuring ornithine decarboxylase activity) and of presynaptic adrenergic and cholinergic markers change in lymph nodes and spleen during Freund's adjuvant-induced arthritis. Groups of young and old Sprague-Dawley rats were studied the day before, and on days 6, 12 and 18 after Freund's adjuvant injection. On day 16 after adjuvant injection, inflammation of hind paws, mainly in the ankle joints, was less marked in old than in young rats. Lymph node and splenic ornithine decarboxylase activity exhibited significant 24-h variations with maximal activity during daily hours. Before treatment, enzyme activity values were significantly lower in old rats in both tissues examined. During the immune reaction, lymph node and splenic ornithine decarboxylase augmented 8-10-fold, with progressively smaller amplitude of daily variations as arthritis developed. In every case, mesor and amplitude of ornithine decarboxylase activity were lowest in old rats. Submaxillary lymph node and splenic tyrosine hydroxylase activity attained maximal values at night. At every time interval after mycobacterium adjuvant injection, amplitude and mesor of tyrosine hydroxylase activity rhythm were lowest in old rats. A maximum in submaxillary lymph node 3H-acetylcholine synthesis occurred at the afternoon. On day 6 and 12 after Freund's adjuvant injection, lymph node 3H-acetylcholine synthesis was significantly smaller in old rats. Day-night differences in submaxillary lymph node or splenic ornithine decarboxylase and tyrosine hydroxylase activities, or in submaxillary lymph node 3H-acetylcholine synthesis, of rats treated with the adjuvant's vehicle, did not differ significantly from those seen in untreated controls. The results are compatible with an age-dependent decline of immune-mediated inflammatory responses. The activity of the central circadian oscillator, driven to the organs in part via the autonomic nervous system, seems also to deteriorate during aging.

Effects of bright light on age-related changes in the locomotor activity of Syrian hamsters

Syrian hamsters display age-related changes in the expression of circadian rhythms and in responsiveness of the circadian system to photic and non-photic stimuli. This study characterized the effects of age on the locomotor activity rhythm of middle-aged and old hamsters and evaluated the effects of strengthening the entraining light signal. Compared with young (4.5 mo) animals, middle-aged (11.25 mo) and old (16 mo) animals displayed increased daily bouts of activity (P < 0.001) and reduced total daily activity and activity rhythm amplitude (P < 0.05) in 14:10-h light-dark cycles. After the light intensity was increased from 300 to 1,500 lx during the light cycle, middle-aged hamsters demonstrated decreased daily activity bouts (P < 0.05) and increased total daily activity (P < or = 0.01) and activity rhythm amplitude (P < or = 0.001) compared with controls maintained in 300 lx. The pattern of changes in the activity rhythm of old experimental animals was similar to trends observed in middle-aged experimental hamsters, although not as robust. Thus age-related changes in the activity rhythm are occurring by middle age in hamsters, and the provision of stronger entraining signals may lead to more stable circadian organization.

Melatonin: marvel or marker?

Melatonin blanches the skin of frogs, whitens the fur of hamsters, and sometimes makes the gonads atrophy. It is remarkable that such a hormone would be put forward as a defense against ageing. We have been examining excretion of the urinary metabolite of melatonin, 6-sulphatoxymelatonin (6-SMT), in 150 postmenopausal women, in 72 volunteers over the age of 60 years who complained of insomnia or depression, and in 20 healthy younger adult controls, aged 18-40 years. The acrophase or fitted peak of 6-SMT excretion was computed as a marker of the timing of the circadian system. Total daily excretion of 6-SMT was not significantly related to total sleep time, wake-within-sleep or sleep complaints. Nevertheless, whereas the 20 controls displayed a normal range of 6-SMT acrophases from 01.32 to 05.44 h, 42% of the postmenopausal women and 48% of the symptomatic elders had acrophases outside this normal range. Those volunteers with more deviant acrophases displayed more disturbed sleep and more sleep complaints. These data suggest that melatonin is a useful marker of circadian rhythm phase disorders, but suggest a need for more caution in melatonin administration.

The aged suprachiasmatic nucleus is phase-shifted by cAMP in vitro

The cyclic adenosine monophosphate (cAMP) analog, 8-bromo-cAMP, phase advanced circadian neuronal rhythms in both aged and adult rat suprachiasmatic nuclei (SCN) by approximately 2 h in vitro. Rhythm amplitude was 20% lower in aged compared to adult SCN. The diminished efficacy of serotonergic agonists to phase shift behavioral rhythms of aged animals may be due to decrements in signal transduction mechanisms proximal to cAMP.

Responsiveness to melatonin and its receptor expression in the aging circadian clock of mice

This study determined the effect of age on the efficacy of melatonin treatment to phase shift circadian activity rhythms and on melatonin receptor expression in the suprachiasmatic nucleus (SCN) and paraventricular nucleus of the thalamus (PVNT) of C3H/HeN mice. The circadian rhythm of 2-[125I]iodomelatonin binding, assessed at three times of the day [circadian times (CT) 2, 10, and 18], showed a modest age-related decrease in the SCN but not the PVNT of old C3H/HeN mice (24 mo). There was a tendency for age to reduce Mel1a melatonin receptor mRNA expression in the suprachiasmatic nucleus during the day, but not during the night. The magnitude of phase shifts of circadian activity rhythms (advances or delays) induced by administration of melatonin at CT 10 or CT 2 was identical in young and old C3H/HeN mice. Together, these results suggest that the decrease in melatonin receptor expression in the SCN had little effect on melatonin-induced phase shifts of circadian activity rhythms. We conclude that the responsiveness of the circadian timing system to melatonin administration does not decrease with age.

Effects of aging on the circadian rhythm of wheel-running activity in C57BL/6 mice

The effects of age on the circadian clock system have been extensively studied, mainly in two rodent species, the laboratory rat and the golden hamster. However, less information is available on how aging alters circadian rhythmicity in a commonly studied rodent animal model, the mouse. Therefore, in the present study we compared the rhythm of wheel-running activity in adult (6-9 mo) and old (19-22 mo) C57BL/6J mice maintained under different lighting conditions for a period of 4 mo. During this period, mice were subjected to phase advances and phase delays of the light-dark (LD) cycle and eventually to constant darkness (DD). In LD (12 h light, 12 h dark), old mice exhibited delayed activity onset relative to light offset and an increase in the variability of activity onset compared with adult mice. After a 4-h phase advance of the LD cycle, old mice took significantly longer to reentrain their activity rhythm when compared with adult animals. Old mice also demonstrated a decline in the number of wheel revolutions per day and a tendency toward a decrease in the length of the active phase. An increase in fragmentation of activity across the 24-h day was obvious in aging animals, with bouts of activity being shorter and longer rest periods intervening between them. No age difference was detected in the maximum intensity of wheel-running activity. In DD, the free-running period was significantly longer in old mice compared with adults. In view of the rapidly expanding importance of the laboratory mouse for molecular and genetic studies of the mammalian nervous system, the present results provide a basis at the phenotypic level to begin to apply genetic methods to the analysis of circadian rhythms and aging in mammals.

Chronic alcohol consumption and withdrawal do not induce cell death in the suprachiasmatic nucleus, but lead to irreversible depression of peptide immunoreactivity and mRNA levels

There is evidence that chronic ethanol treatment (CET) disrupts the biological rhythms of various brain functions and behaviors. Because the suprachiasmatic nucleus (SCN) is widely recognized as the dominant pacemaker of the circadian system, we have examined the effects of CET and withdrawal on the main morphological features and chemoarchitecture of this hypothalamic nucleus. Groups of rats ethanol-treated for 6 and 12 months were compared with withdrawn rats (ethanol-treated for 6 months and then switched to a normal diet for an additional 6 months) and with groups of age-matched control and pair-fed control rats. The volume and the total number of neurons of the SCN were estimated from conventionally stained material, whereas the total number of astrocytes and of neurons containing vasopressin (AVP), vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP), and somatostatin (SS) were estimated from immunostained sections. The estimates were obtained using unbiased stereological methods, based on Cavalieri's principle and the optical fractionator. The volume of the SCN and the total number of SCN neurons and astrocytes did not vary among groups. We found, however, that CET induced a significant reduction in the total number of AVP-, VIP-, GRP-, and SS-containing neurons. Withdrawal from alcohol did not reduce but rather augmented the loss of VIP- and GRP-immunoreactive neurons. The CET-induced neurochemical alterations seem to result from a decrease in neuropeptide synthesis, as revealed by the reduction in AVP and VIP mRNA levels demonstrated by in situ hybridization with radioactively labeled 48-mer AVP and 30-mer VIP probes. It is thus possible to conclude that the irreversible CET-induced changes in the neurochemistry of the SCN might underpin the disturbances in circadian rhythms observed after long-term alcohol consumption.

Light-induced c-fos mRNA expression in the suprachiasmatic nucleus and the retina of C3H/HeN mice

Light-induced expression of c-fos mRNA was studied over a circadian period (approximately 24 h) in C3H/HeN mice maintained in constant dark. This mouse strain expresses an rd mutation (retinal degeneration) which does not affect light-induced phase shifts of circadian rhythms. c-fos mRNA expression in the retina and the suprachiasmatic nucleus (SCN) after a light pulse (300 lux) was determined by in-situ hybridization autoradiography using a 35S-labeled c-fos riboprobe. Light induced the expression of c-fos mRNA in retino-recipient areas of the SCN. This response was dependent on the circadian time (CT) and was observed only during the subjective night (CT14-CT22) and early subjective day (CT2). However, the period of photosensitivity for c-fos induction extended 1 h over the period of photosensitivity for phase shifts in circadian behavior. In the retina of C3H/HeN mice, light-induced c-fos mRNA expression was observed in a small number of cells in the ganglion cell layer (approximately 0.2%) which may represent ganglion cells projecting to the SCN. A dependence of c-fos expression with the circadian time was observed in retinal ganglion cells, suggesting that retinal photosensitivity may also be controlled by a circadian oscillator. In conclusion, we demonstrated light-induced expression of the immediate early gene c-fos mRNA in both the retina and SCN of C3H/HeN mice expressing the rd mutation.

Light, immediate-early genes, and circadian rhythms

Many diverse behaviors exhibit clear circadian rhythms in their expression. In mammals, these rhythms originate from a neural circadian clock located in the suprachiasmatic nuclei (SCN). Recently, signaling pathways activated by light in the SCN have begun to be identified. A specific set of immediate-early genes is induced by light in the SCN, and their expression is correlated with the resetting of circadian behavioral rhythms. These light-regulated immediate-early genes offer multiple inroads into the biology of the SCN: first, they are functional markers for the activation of SCN neurons by light; second, they can direct us to the upstream light-activated (and clock-regulated) signal transduction pathways which mediate their induction; and finally, they encode transcription factor proteins which may play a role in the molecular mechanism of resetting the circadian clock.