Plamen D. Penev

Chronic circadian desynchronization decreases the survival of animals with cardiomyopathic heart disease

Shift work is associated with increased cardiovascular morbidity and mortality. Whereas it has been suggested that continuous shifting of the circadian clock/sleep-wake cycle may have negative effects on health, there is very little experimental evidence to support such a hypothesis. Cardiomyopathic Syrian hamsters were either maintained on a fixed light-dark (LD) cycle (n = 31) or were subjected to a 12-h phase shift in the LD cycle on a weekly basis (n = 32). The duration of the life span was recorded for each animal. Chronic reversal of the external LD cycle at weekly intervals resulted in a significant decrease in the survival time in cardiomyopathic hamsters with the median life span being reduced by 11%. Disrupting normal circadian rhythmicity in an animal susceptible to early mortality due to cardiac disease results in a further decrease in longevity. The deleterious effects of the chronic phase shifts in the LD cycle in cardiomyopathic hamsters may be related to reports of increased cardiovascular morbidity and mortality in humans engaged in shift work.Shift work is associated with increased cardiovascular morbidity and mortality. Whereas it has been suggested that continuous shifting of the circadian clock/sleep-wake cycle may have negative effects on health, there is very little experimental evidence to support such a hypothesis. Cardiomyopathic Syrian hamsters were either maintained on a fixed light-dark (LD) cycle ( n = 31) or were subjected to a 12-h phase shift in the LD cycle on a weekly basis ( n = 32). The duration of the life span was recorded for each animal. Chronic reversal of the external LD cycle at weekly intervals resulted in a significant decrease in the survival time in cardiomyopathic hamsters with the median life span being reduced by 11%. Disrupting normal circadian rhythmicity in an animal susceptible to early mortality due to cardiac disease results in a further decrease in longevity. The deleterious effects of the chronic phase shifts in the LD cycle in cardiomyopathic hamsters may be related to reports of increased cardiovascular morbidity and mortality in humans engaged in shift work.

Effects of age on the circadian system

While aging has been associated with changes in the period and amplitude of circadian rhythms, little is known about how aging influences the response of the circadian clock to environmental stimuli. In this paper, we report on recent studies designed to determine the effects of advanced age on the response of the circadian clock to both photic and nonphotic stimuli in old hamsters (e.g., over 16 mo of age). Among the most pronounced age-related changes in the circadian rhythm of locomotor activity are: (a) alterations in the phase-angle of entrainment to the light-dark cycle; (b) an increase in the magnitude of phase shifts induced by pulses of light presented at specific circadian times; and (c) a loss of responsiveness to the phase shifting or entraining effects of stimuli which induce an acute increase of activity. Depletion of brain monoamine levels in young animals can induce changes in the responsiveness of the circadian clock to environmental stimuli which are similar to those which occur spontaneously in old animals, suggesting that aging alters monoaminergic inputs to the clock. Some of the age-related changes in the response of the clock to an activity-inducing stimulus can be reversed by implanting old animals with fetal SCN tissue. Determining the physiological basis for age related changes in the responsiveness of the clock to both internal and external stimuli, and the mechanisms by which normal circadian function can be restored, should lead to new insight into the functioning of the circadian clock and may lead to new approaches for normalizing disturbed circadian rhythms.

Roles of Suprachiasmatic Nuclei and Intergeniculate Leaflets in Mediating the Phase-Shifting Effects of a Serotonergic Agonist and Their Photic Modulation during Subjective Day

Serotonin (5-HT) has been implicated in the phase adjustment of the circadian system during the subjective day in response to nonphotic stimuli. Two components of the circadian system, the suprachiasmatic nucleus (SCN) (site of the circadian clock) and the intergeniculate leaflet (IGL), receive serotonergic projections from the median raphe nucleus and the dorsal raphe nucleus, respectively. Experiment 1, performed in golden hamsters housed in constant darkness, compared the effects of bilateral microinjections of the 5-HT1A/7 receptor agonist, 8-hydroxydipropylaminotetralin (8-OH-DPAT; 0.5 μg in 0.2 μL saline per side), into the IGL or the SCN during the mid-subjective day. Bilateral 8-OH-DPAT injections into either the SCN or the IGL led to significant phase advances of the circadian rhythm of wheel-running activity (p < .001). The phase advances following 8-OH-DPAT injections in the IGL were dose dependent (p < .001). Because a light pulse administered during the middle of the subjective day can attenuate the phase-resetting effect of a systemic injection of 8-OH-DPAT, Experiment 2 was designed to determine whether light could modulate 5-HT agonist activity at the level of the SCN and/or the IGL. Serotonergic receptor activation within the SCN, followed by a pulse of light (300 lux of white light lasting 30 min), still induced phase advances. In contrast, the effect of serotonergic stimulation within the IGL was blocked by a light pulse. These results indicate that the respective 5-HT projections to the SCN and IGL subserve different functions in the circadian responses to photic and nonphotic stimuli.

A serotonin neurotoxin attenuates the phase-shifting effects of triazolam on the circadian clock in hamsters

Several lines of evidence suggest the potential involvement of serotonergic pathways in mediating the effects of activity-inducing stimuli on the circadian clock in rodents. The aim of the present 3 experiments was to examine the effects of the serotonergic neurotoxin, p-chloroamphetamine (PCA, 10 mg/kg) on: (1) the monoamine levels of the hypothalamus, frontal cortex and hippocampus in the hamster; (2) the phase shifts in the circadian rhythm of locomotor activity of hamsters in response to treatment with the short-acting benzodiazepine, triazolam (7.5 mg/kg); and (3) the magnitude of the acute increase in locomotor activity associated with triazolam administration in this species. The administration of PCA to hamsters caused changes of specific monoaminergic systems in the hypothalamus, that were limited to a selective decrease in serotonin levels 7 days post-treatment. The phase shifts of the circadian clock in response to triazolam treatment at CT 6 were considerably attenuated following the administration of the 5-HT neurotoxin. The total amount and the profiles of triazolam-induced wheel-running and general cage activity between CT 6 and CT 12 were not significantly affected by the PCA treatment. The finding that a 5-HT neurotoxin can attenuate the phase-shifting effects of triazolam in hamsters, without interfering with its activity-inducing properties, suggests that serotonergic afferents might be involved in the mechanism for non-photic phase-shifting of the circadian system.

Monoamine depletion alters the entrainment and the response to light of the circadian activity rhythm in hamsters

Reduced amplitude, shorter free-running periods and desynchronization among a number of circadian rhythms are associated with advanced age in rodents. The response of the hamster circadian system to photic stimuli is also altered during senescence. Decreased monoamine levels, receptor sites and neuronal populations are commonly observed in the aging brain. The objective of the present study was to determine if monoamine depletion with reserpine in young hamsters induces changes in the circadian rhythm of locomotor activity similar to those that occur spontaneously with aging. Wheel-running activity of 12 young hamsters under a 14 h-light/10 h-dark cycle was continuously monitored. The total activity level, the times of activity onset, peak and offset and the duration of activity were determined during a 1-week period after vehicle treatment and for three 1-week periods after reserpine treatment (4 mg/kg). A second group of eight reserpine-treated and six vehicle-treated animals was kept in constant darkness (DD). The period of the circadian activity rhythm in DD and the phase-shifts after short light pulses at circadian time 19 (CT19) were determined in the control and reserpine-treated groups. Brain monoamines in the hypothalamus, striatum and pons/medulla after reserpine and vehicle treatment were determined by high-pressure liquid chromatography. The data were analyzed with x2 periodogram and one-way ANOVA followed by Duncans post hoc test. Reserpine treatment significantly reduced total wheel-running activity and the monoamine levels in the hypothalamus, striatum and pons/medulla.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoamine depletion blocks triazolam-induced phase advances of the circadian clock in hamsters

Injections with the short-acting benzodiazepine, triazolam (Tz), 6 h before activity onset (CT6) produce large phase advances of the circadian pacemaker in hamsters. An increase in locomotor activity and/or the state of arousal is considered essential for the effects of Tz, suggesting the potential involvement of central monoaminergic systems in this process. The present study examines the effect of reserpine-induced monoamine depletion on the phase-shifting effects of Tz in hamsters. Wheel running activity of 16 male golden hamsters (14 weeks old) was continuously monitored in constant darkness. After a stable free-running circadian rhythm was established half of the animals received reserpine (2.5 mg/kg, s.c.) and the other half vehicle treatment. Ten days later all animals were given Tz injections (10 mg/kg i.p.) at CT6 and the circadian activity rhythm was monitored for 2 more weeks. An additional 10 animals were used to determine the effect of reserpine on the central monamine levels using high pressure liquid chromatography. A circadian rhythm of locomotor activity with reduced amplitude and longer free-running period persisted after reserpine treatment, despite the significant monoamine depletion. Triazolam injections at CT6 induced large phase-advances (93.1 +/- 14.9) in the control group that were markedly attenuated in 7 out of the 8 reserpine-treated animals (3.12 +/- 17.7 min, P < 0.01). Our results suggest that monoaminergic systems are essential for the phase-shifting effect of Tz upon the circadian system in hamsters.

Quantitative analysis of the age-related fragmentation of hamster 24-h activity rhythms

The continuous monitoring of spontaneous locomotor activity has emerged as one of the most widely used metrics in rodent circadian research. This behavioral measure is also extremely useful for the description of the effects of aging on circadian rhythms. The present study describes the successful use of a log-survivorship approach to identify discrete bouts of hamster wheel-running activity and provides a detailed description of the age-related fragmentation in the 24-h profile of this behavioral variable. In addition, stepwise discriminant analysis identified the most important quantitative measures for distinguishing between the individual patterns of wheel-running activity of young (3 mo) and old (17-18 mo) golden hamsters. The results suggest that this method of bout analysis can be a valuable tool for the study of genetic, developmental, neurochemical, physiological, and environmental factors involved in the temporal control of rodent locomotor behavior.

Pharmacological and Genetic Approaches for the Study of Circadian Rhythms in Mammals

Two different approaches have been utilized to study the controlling mechanisms that underlie the generation and entrainment of circadian rhythms in mammals. The use of specific drugs to alter the period and/or the phase of circadian rhythms has provided new insights into both the pathways by which environmental information reaches the mammalian circadian pacemaker in the suprachiasmatic nuclei (SCN) and the cellular and neurochemical events within the SCN itself which are involved in circadian rhythmicity. A second approach, which seeks to exploit genetic differences in the properties of the circadian system, holds the promise of eventually defining the cellular and molecular events that are part of the clock itself, the events that underlie the entrainment of the circadian clock by environmental factors, and the expression of overt rhythms driven by the clock. It is anticipated that the pharmacological and genetic approaches to the study of circadian rhythms will complement each other as the underlying physiological mechanisms of the circadian clock system become defined.

Phase-shifting effects of a serotonin agonist in tau mutant hamsters.

Previous studies indicate that the advance region of the tau mutant hamsters phase-response curve (PRC) to non-photic stimuli, such as NPY and wheel pulses, is characterized by earlier timing and increased amplitude in comparison with that of wild-type animals. Since, recent evidence suggests that serotonergic pathways may play an important role for the non-photic phase resetting of the rodent circadian pacemaker, PRCs to the serotonin (5-HT) agonist, 8-OH-DPAT (5 mg/kg i.p), were generated in both wild-type and tau mutant hamsters kept in constant darkness. The results indicate that the tau mutation is associated with changes in the timing, but not the amplitude of the advance region of the PRC to 8-OH-DPAT and suggest that serotonergic agents and other non-photic or activity-inducing stimuli may share some common mechanisms for resetting the phase of the rodent circadian pacemaker.