Chronic clorgyline treatment of Syrian hamsters: an analysis of effects on the circadian pacemaker

Bipolar mood cycles associated with lunar entrainment of a circadian rhythm

In bipolar disorder, episodes of depression and mania are associated with dramatic disturbances in sleep, which experiments show are likely to contribute to the pathogenesis of the episodes. A recent finding that 18 patients' manic-depressive cycles oscillated in synchrony with biweekly surges in amplitude of the moon's tides provided a clue to the cause of the sleep-disturbances. Analyses of one of the patients' sleep-wake cycles suggest that his mood cycles arose when a circadian rhythm that normally is entrained to dawn and controls the daily onset of wakefulness became entrained instead to 24.8-h recurrences of every second 12.4-h tidal cycle. The finding provides the basis for a comprehensive description of the pathogenesis and pathophysiology of the mood cycle.

Bipolar mood cycles and lunar tidal cycles

In 17 patients with rapid cycling bipolar disorder, time-series analyses detected synchronies between mood cycles and three lunar cycles that modulate the amplitude of the moon's semi-diurnal gravimetric tides: the 14.8-day spring-neap cycle, the 13.7-day declination cycle and the 206-day cycle of perigee-syzygies ('supermoons'). The analyses also revealed shifts among 1:2, 1:3, 2:3 and other modes of coupling of mood cycles to the two bi-weekly lunar cycles. These shifts appear to be responses to the conflicting demands of the mood cycles' being entrained simultaneously to two different bi-weekly lunar cycles with slightly different periods. Measurements of circadian rhythms in body temperature suggest a biological mechanism through which transits of one of the moon's semi-diurnal gravimetric tides might have driven the patients' bipolar cycles, by periodically entraining the circadian pacemaker to its 24.84-h rhythm and altering the pacemaker's phase-relationship to sleep in a manner that is known to cause switches from depression to mania.

Monoaminergic control of vasopressin and VIP expression in the mouse suprachiasmatic nucleus

We studied the effects of serotonin and noradrenaline on the expression of arginine-vasopressin (AVP) and vasoactive intestinal peptide (VIP) in the suprachiasmatic nucleus (SCN). We used transgenic Tg8 mice knockout for the MAO-A (monoamine oxidase A) gene, which are characterized by increased amounts of serotonin and noradrenaline in brain compared to wild-type mice (C3H). The MAO-A deficiency caused an increase in AVP and VIP expression (determined by immunohistochemistry, enzyme immunoassay, and in situ hybridization) compared to C3H mice. The number of peptidergic neurons was also increased. Inhibiting serotonin or noradrenaline synthesis in Tg8 mice by the administration of parachlorophenylalanine or alpha-methylparatyrosine, respectively, the amounts of AVP, VIP and their mRNAs were decreased, but not the number of peptidergic neurons. This study indicates that serotonin and noradrenaline stimulate AVP and VIP expression, and could participate in the differentiation of the neurochemical phenotype in the mouse SCN.

Serotonin and the regulation of mammalian circadian rhythmicity

The suprachiasmatic nucleus (SCN), the site of the primary mammalian circadian clock, contains one of the densest serotonergic terminal plexes in the brain. Although this fact has been appreciated for some time, only in the last decade has there been substantial approach toward the understanding of the function of serotonin in the circadian rhythm system. The intergeniculate leaflet, which projects to the SCN via the geniculohypothalamic tract, receives serotonergic innervation from the dorsal raphe nucleus, and the SCN receives its serotonergic input from the median raphe nucleus. This separation of serotonergic origins provides the opportunity to investigate the function of the two projections. Loss of serotonergic neurones of the median raphe yields earlier onset and later offset of the nocturnal activity phase, longer duration of the activity phase, and increased sensitivity of circadian rhythm response to light. Despite the simplicity of the origins of serotonergic anatomy with respect to the circadian rhythm system, the actual involvement of serotonin in rhythm modulation is not so obvious. A variety of pharmacological studies have clearly implicated serotonin as a direct regulator of circadian rhythm phase, but others employing different methods suggest that simple elevation of SCN serotonin concentrations does not modify rhythm phase. The most convincing role of serotonin is its apparent ability to modulate sensitivity of the circadian rhythm to light. The putative method for such modulation is via a presynaptic 5-HT1B receptor on the retinohypothalamic tract, the activation of which attenuates photic input to the SCN thereby reducing phase response to light. Serotonin may modulate phase response to benzodiazepines, but does not appear to modify such response to environmentally induced locomotor activity. Current interest in serotonergic modulation of circadian rhythmicity is strong and the research is vigorous. There is an abundance of information about serotonin and circadian rhythm function that lacks a satisfactory framework for its interpretation. The next decade is likely to see the gradual evolution of this framework as the role of serotonin in circadian rhythm regulation is further elucidated.

5-HT agonist-induced phase-advances of the circadian pacemaker are diminished by chronic antidepressant drug treatment

Serotonin (5-HT) and its agonists alter the timing of the circadian pacemaker. Previous research has shown that when they are injected 4 h before or after the onset of wheel-running, they phase-advance or delay, respectively, the timing of the pacemaker. Because serotonergic interventions alter 5-HT receptor number in the hypothalamus, we asked whether chronic treatment with an antidepressant drug (AD) that modifies serotonergic function could alter the phase-shifting effects of the 5-HT agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT). Hamsters were treated chronically with the monoamine oxidase inhibitor (MAOI), clorgyline, and then injected with 8-OH-DPAT or vehicle (VEH) either 4 h before or after the onset of wheel-running. MAOI treatment decreased the magnitude of both 8-OH-DPAT- and VEH-induced phase advances, but not the magnitude of 8-OH-DPAT-induced phase-delays. The results indicate that 8-OH-DPAT-induced phase-advances and delays are functionally distinct with regard to adaptive changes during chronic AD treatment.

Photic entrainment of circadian rhythms in rodents

Photic entrainment of circadian rhythms occurs as a consequence of daily, light-induced adjustments in the phase and period of the suprachiasmatic nuclei (SCN) circadian clock. Photic information is acquired by a unique population of retinal photoreceptors, processed by a distinct subset of retinal ganglion cells, and conveyed to the SCN through the retinohypothalamic tract (RHT). RHT neurotransmission is mediated by the release of the excitatory amino acid glutamate and appears to require the activation of both NMDA- and non-NMDA-type glutamate receptors, the expression of immediate early genes (IEGs), and the synthesis and release of nitric oxide. In addition, serotonin appears to regulate the response of the SCN circadian clock to light through postsynaptic 5-HT1A or 5-ht7 receptors, as well as presynaptic 5-HT1B heteroreceptors on RHT terminals.

Decreased sensitivity to light of the photic entrainment pathway during chronic clorgyline and lithium treatments

Certain antidepressant drugs (ADs) cause disturbances in sleep that could result from their capacity to alter the timing of circadian rhythms. Effects on the timing of rhythms could be due to the drugs' known capacity to alter the frequency of the intrinsic rhythm of the circadian pacemaker, or to a capacity to modify the pacemaker's response to external stimuli that serve as time cues (Zeitgebers) that regulate the timing (phase) of its rhythm. To examine the possibility that ADs alter the sensitivity of the system that mediates the phase-shifting effects of light, hamsters were treated chronically with the MAOI, clorgyline, and lithium. Each hamster was then exposed to a single 5-min light pulse (intensity range = 0.00137 to 137 microW/cm2) at circadian phases known to elicit phase advances (CT18) and phase delays (CT13.5) in the daily onset of wheel running. The half-saturation constant (sigma), photic sensitivity (1/sigma), and maximum phase-shifting response to light were estimated from the best-fit stimulus response curves. In addition, threshold sensitivity, the light intensity required to produce a threshold phase-shifting response, was determined. Clorgyline decreased the magnitude of light-induced phase advances at each of the light intensities tested. Clorgyline also decreased the magnitude of light-induced phase delays at low light intensities, but increased the magnitude of phase delays at higher light intensities. Clorgyline decreased the sensitivity of the photic phase-shifting system, as indicated both by the threshold sensitivities at CT13.5 and CT18, and by 1/sigma at CT13.5. Lithium decreased the threshold sensitivity at CT18, and 1/sigma at CT13.5. Lithium decreased the magnitude of phase delays, but not phase advances. Clorgyline's effects on the photic entrainment pathway may be mediated by its effects on serotonin, which has been shown to modulate the pacemaker's response to morning and evening light, and by tolerance to this effect of serotonin. The fact that both clorgyline and lithium decrease the photic sensitivity of the entrainment pathway suggests that other psychoactive drugs might also share this property. It is possible that the decreased sensitivity to light of the entrainment pathway affects the clinical response to these and other psychoactive medications.

Endogenous regulation of serotonin release in the hamster suprachiasmatic nucleus

Serotonin (5-HT) has been strongly implicated in the regulation of the mammalian circadian clock located in the suprachiasmatic nuclei (SCN). However, little is known of the pattern of neuronal 5-HT release in the SCN or of the factors involved in regulating its release. Using in vivo microdialysis, we demonstrated the existence of a daily rhythm in the output of 5-HT in the SCN of freely behaving hamsters. This rhythm was characterized by a sharp increase in release from a nadir during late midday to peak levels at the light/dark transition. Output declined to basal levels throughout the remainder of the night. A similar pattern also was evident under constant darkness, with increased 5-HT output occurring at the onset of subjective night. Locomotor activity induced by exposure to a novel running wheel had a pronounced phase-dependent effect on 5-HT release in the SCN, with stimulation during the light phase and suppression during the late dark phase. Systemic application of the somatodendritic 5-HT1A agonist BMY 7378 had a significantly greater suppressive effect on 5-HT release in the SCN during the late dark phase compared with mid light phase, indicating that a variation in raphe autoreceptor response may underlie the time-dependent effects of wheel running on 5-HT release. Collectively, these results show that the daily rhythm in output of 5-HT in the SCN is generated endogenously, and that behavioral state can strongly influence serotonergic activity in the circadian clock in a phase-dependent manner.

Neonatal clomipramine treatment of Syrian hamsters: effect on the circadian system

The circadian behavior of male Syrian hamsters injected with the serotonin/norepinephrine reuptake inhibitor clomipramine (15 mg/kg from postnatal days 8 to 21) was examined. Clomipramine treatment significantly augmented mean activity values of wheel running rhythm, as well as delayed its acrophase. After a 6-h phase advance of the light-dark cycle, reentrainment of clomipramine-treated hamsters took significantly longer than controls. Clomipramine-treated hamsters exhibited a shorter circadian period than controls in constant light conditions, but no differences were found in constant darkness. Light pulses applied at late subjective night to clomipramine-treated hamsters caused significantly reduced phase advances as compared to controls, while no differences were found in phase delay magnitudes when light pulses were applied during early subjective night. Administration of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) at circadian time 8 significantly advanced the onset of activity to a greater extent in clomipramine-treated hamsters than in controls. The results indicate that neonatal clomipramine treatment of hamsters causes long-lasting changes in the circadian system, by increasing activity levels and by partially inhibiting light-evoked responses. An enhancement of a non-photic, serotonergic-induced response was also unveiled.

Disruption of the activity-rest cycle by MAOI treatment: dependence on light and a secondary visual pathway to the circadian pacemaker

The disruptive effects on the activity-rest cycle of the monoamine oxidase inhibitor (MAOI) clorgyline and of continuous light were examined in Syrian hamsters. When administered in dim and moderate light intensities, clorgyline delayed the daily onset of wheel-running. When administered in bright light, it dissociated the circadian rhythm of wheel-running. This dissociation was prevented by lesions of the intergeniculate leaflet of the ventral lateral geniculate nucleus. Constant darkness restored the circadian rhythm of wheel-running in hamsters with disrupted circadian rhythms. The phase of the restored rhythm of wheel-running was shifted 6-12 h later than the phase of wheel-running prior to dissociation. Our results suggest that MAOI treatment weakens the coupling between oscillators that comprise the circadian pacemaker, and augments the disruptive effects of continuous light acting via the intergeniculate leaflet region of the ventral lateral geniculate nucleus. These effects on the circadian pacemaker may be responsible for disruptions of the sleep-wake cycle that occur as side effects when MAOIs are used clinically to treat depression and might play a role in the induction of mania and rapid cycling by antidepressants.

Increase of 5HT and VIP immunoreactivity within the hamster (Mesocricetus auratus) SCN during chronic MAOI treatment

The effects of chronic treatment with the monoamine oxidase inhibitor (MAOI), clorgyline (CLG; 2 mg/kg per day) on serotonin (5HT) and vasoactive intestinal peptide (VIP) immunoreactivity (IR) within the hamster suprachiasmatic nucleus (SCN) were examined. Optical densities of 5HT IR and VIP IR were increased by MAOI treatment. VIP IR was increased in both the ventrolateral and dorsal regions of the SCN, suggesting that VIP content was increased within both perikarya and terminals of VIP neurons. The results suggest that previously described effects of MAOIs on the mammalian circadian system may be mediated in part, by their effects on serotonergic input to VIP neurons within the SCN.

Effects of bilateral olfactory bulbectomy on circadian rhythms of ACTH, corticosterone, motor activity and body temperature in male rats

Bilateral olfactory bulbectomy (BOX) has major biochemical and behavioral effects, and is one of the most widely investigated of animal models of depression. We studied the consequences of BOX in male rats, on the organization of endogenous circadian rhythms for ACTH, corticosterone (Cort), motor activity (MA) and body temperature (BT). Mean levels were increased for Cort and MA, whereas no significant changes were observed for ACTH and BT. Significantly higher plasma Cort morning values were evidenced in BOX than sham-operated animals. In addition, compared with the single prominent power spectrum for the 24 hours period of control rats, the BOX animals displayed substantially lower 24 hours spectral power for the MA and BT circadian rhythms. These alterations suggest that olfactory bulbectomy, by disruption of the afferences and efferences, induced drastic changes in the function of the endogenous clock or of its regulating systems. From this point of view, bulbectomized rats may therefore be a valuable model to studying the etiology of psychiatric disorders with rhythm disturbance.

Serotonin-containing fibers in the suprachiasmatic hypothalamus attenuate light-induced phase delays in mice

Photic and non-photic stimuli phase shift and entrain circadian rhythms through distinct but interacting mechanisms which impinge on the suprachiasmatic nucleus (SCN), the circadian pacemaker. Our understanding of this mechanism is incomplete. Serotonin (5-HT) injected locally at the SCN reduces light-induced glutamate release and decreases the expression of c-fos, a marker of photic transduction. Furthermore, in SCN slices, 5-HT application reduces field potentials after optic nerve stimulation. We therefore predicted that 5-HT-terminal destruction restricted to the SCN would augment phase shifts of circadian rhythms induced by light exposure. To investigate this possibility, we compared photic phase delays and Fos-like immunoreactivity in mice which had previously received bilateral infusions directed at the SCN containing either the selective 5-HT neurotoxin 5,7-dihydroxytryptamine (DHT, n = 16) or vehicle (VEH, n = 12). Phase delays after a light pulse given during the mid-subjective night (30 lux, 30 min starting at circadian time (CT) 12-20) in DHT-mice were 50% greater than in VEH-mice (P = 0.017). DHT mice (n = 5) had 76% larger Fos responses to a mid-subjective night light pulse than VEH-mice (n = 5) (P = 0.029). We conclude that 5-HT at or near the SCN in mice reduces photic phase shifts and modulates the magnitude of the photic phase response in the mouse.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

Diazepam modulates the period of locomotor rhythm in mice (Mus booduga) and attenuates light-induced phase advances

Experiments were carried out on the continuous action of diazepam (benzodiazepine) offered through drinking water in 2% ethanol on psi and tau of the activity rhythms under LD (12:12) and DD conditions. Under entraining conditions diazepam failed to evoke striking changes in psi. On the other hand, under free-running conditions period-lengthening and period-shortening effects were observed. Further experiments conducted on the continuous effect of diazepam on light pulse evoked phase shifts revealed that phase advances were attenuated significantly in diazepam-treated animals at CT 20 and 24. These results were discussed with regard to the action of diazepam on the light sensitivity of the circadian pacemaker.

Circadian rhythms and the pharmacology of affective illness

The chronic effects of antidepressant drugs (ADs) on circadian rhythms of behavior, physiology and endocrinology are reviewed. The timekeeping properties of several classes of ADs, including tricyclic antidepressants, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, serotonin agonists and antagonists, benzodiazepines, and melatonin are reviewed. Pharmacological effects on the circadian amplitude and phase, as well as effects on day-night measurements of motor activity, sleep-wake, body temperature (Tb), 3-methoxy-4-hydroxyphenylglycol, cortisol, thyroid hormone, prolactin, growth hormone and melatonin are examined. ADs often lower nocturnal Tb and affect the homeostatic regulation of sleep. ADs often advance the timing and decrease the amount of slow wave sleep, reduce rapid eye movement sleep and increase or decrease arousal. Together, AD effects on nocturnal Tb and sleep may be related to their therapeutic properties. ADs sometimes delay nocturnal cortisol timing and increase nocturnal melatonin, thyroid hormone and prolactin levels; these effects often vary with diagnosis, and clinical state. The effects of ADs on the coupling of the central circadian pacemaker to photic and nonphotic zeitgebers are discussed.

Chronobiotics--drugs that shift rhythms

A chronobiotic is defined and levels of action within the mammalian circadian pacemaker system, such as the retina, retinohypothalamic tract, geniculohypothalamic tract, suprachiasmatic nuclei, output and feedback systems are identified. Classes of drug that include the indoleamines, cholinergic agents, peptides, and benzodiazepines, which might act as chronobiotics within these levels, are evaluated. Particular emphasis is placed on the indole, melatonin (MLT). The clinical circumstances for use of chronobiotics in sleep disturbances of the circadian kind, such as jet lag, shift work, delayed sleep-phase syndrome, advanced sleep-phase syndrome, irregular and non-24-hr sleep-wake cycles, are described under reorganized headings of disorders of entrainment, partial entrainment, and desynchronization. Specific attention is given to the blind and the aged. Both human and animal studies suggest that MLT has powerful chronobiotic properties. MLT shows considerable promise as a prophylactic and therapeutic alternative or supplement to the use of natural and artificial bright light for resetting the circadian pacemaker. Throughout this discussion, the hypnotic and hypothermic versus the chronobiotic actions of MLT are raised. Finally, problems in the design of delivery systems for MLT are discussed.

Tryptophan loading modulates light-induced responses in the mammalian circadian system

Enhanced endogenous serotonergic activity, stimulated by L-tryptophan (TRYPT) loading, was found to have a substantial impact on neurochemical and behavioral aspects of the circadian response to light in the male Syrian hamster. An intraperitoneal (i.p.) injection of 150 mg/kg TRYPT significantly stimulated serotonin (5-HT) release in the suprachiasmatic nuclear (SCN) region, as reflected by a 205 +/- 30% maximal increase in the extracellular concentration of 5-HT assessed using microdialysis. Administration of TRYPT 1 h before exposure to a light pulse (30 min, 40 lux) delivered during late subjective night dose-dependently suppressed the number of SCN cells expressing light-induced Fos-like immunoreactivity (Fos-LI; maximal suppression @200 mg/kg was 77 +/- 4%, p < 0.001). This action of TRYPT was attenuated by pretreatment with the 5-HT1a antagonist, NAN-190, and was abolished by the 5-HT2/5-HT7 antagonist, ritanserin, or the nonselective 5-HT antagonist, metergoline (all 10 mg/kg). These antagonists alone had no effect on light-induced Fos. In a second experiment, pretreatment of free-running hamsters housed under constant darkness with 150 mg/kg TRYPT 45-60 min prior to light exposure (10 min, 20 lux) during late subjective night (CT 19) significantly attenuated the light-induced phase advances of the circadian activity rhythm (66 +/- 7 min vs. 100 +/- 6 min for vehicle controls; p < 0.001). The same dose of TRYPT given 1 h before lights-on for 5 consecutive days in hamsters maintained under 14L:10D altered the phase angle of entrainment such that activity onsets were delayed by 36 +/- 8 min relative to controls (p < 0.05). The same dose of TRYPT administered during late subjective night also suppressed the extracellular concentration of glutamate in the SCN region assessed using microdialysis (55 +/- 8% suppression; p < 0.05 vs. baseline). These results support the hypothesis that the ascending serotonergic projection to the SCN modulates photic entrainment processes within the circadian oscillator.

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.

Effect of lithium carbonate on activity level and circadian period in different strains of rats

Lithium, an important pharmacological agent for the treatment of manic-depressive illness in humans, is known to lengthen the circadian period in a number of different species. Recent experiments, on the other hand, suggest that pharmacological agents may affect the circadian system indirectly through an increase or decrease of activity. To explore the interaction between pharmacological and activity effects on the circadian system, lithium was administered chronically to three different strains of rats (ACI, BH, and LEW) while wheel-running activity was studied quantitatively. Two of these inbred strains (BH and LEW) show profound abnormalities in their circadian activity rhythms, namely, a reduced overall level of activity and bimodal or multimodal activity patterns. Wheel-running activity was monitored for 4 weeks under baseline conditions, followed by 3 weeks with lithium treatment (0.3% Li2CO3 administered with food) and 4 weeks with normal food. Treatment with lithium (average intake per day = 3.6 +/- 0.2 mg) consistently decreased both the overall level and the circadian amplitude of the activity rhythm. The free-running period tau was slightly lengthened during lithium treatment, while the most dramatic effect on period was observed after lithium withdrawal. Correlation analysis, however, revealed only a small negative correlation between activity level and period length, which proved significantly only for animals of the ACI strain. Our data support the traditional interpretation that lithium lengthens circadian period by a direct pharmacological effect on the circadian pacemaker rather than through indirect effects of activity feedback.

Neurochemical organization of circadian rhythm in the suprachiasmatic nucleus

The circadian rhythm in mammals is under control of the pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. This tiny nucleus contains a number of neurochemicals, including peptides, amines and amino acids. Heterogeneous distribution of these neurochemicals defines the substructures of the SCN. In the present review, functional significance of such neurochemical heterogeneity in the SCN is discussed in the light of circadian patterns of the concentrations of these neurochemicals in the SCN and their effects on SCN neurons in in vitro slice preparation. In particular, the hypothesis that the dorsomedial SCN is involved in maintaining the circadian rhythm, while the ventrolateral SCN is involved in adjusting the phase of the rhythm, is critically discussed. These considerations suggest that distinct sub-components of the SCN as marked by neurochemicals, interact with each other and this organizational architecture could be the basis of the proper operation of the circadian time keeping system in this nucleus.

Antidepressant and depressogenic drugs lack consistent effects on hamster circadian rhythms

The circadian wheel-running rhythm of golden hamsters was monitored during chronic oral treatment with four antidepressants and two potentially depressogenic agents. Desmethylimipramine shortened the circadian period (tau) by 0.1 hour. In contrast, clorgyline lengthened tau by 0.1 hour and delayed light-synchronized wheel-running rhythms by 1.4 hour. Phenelzine, fluoxetine, clonidine, and propranolol did not significantly alter light-entrained phase or free-running period over a range of doses. Other rhythm parameters were also unaffected by antidepressant or depressogenic drugs. These data suggest that mood-altering drugs do not consistently influence circadian rhythms in the hamster.

Effect of chronic tryptophan depletion on the circadian rhythm of wheel-running activity in rats

The effect of chronic treatment with a tryptophan (TRP)-free diet on the free-running circadian wheel-running rhythm and the central serotonergic system was investigated in blinded male rats. The long-term TRP-free diet did not change periods of activity, but disordered their patterns. This seemed to be due to masking, entrainment, enhancement of the morning activity, and obscuring of the activity onset as well as appearance of some periodic activities within the subjective night. A long-term TRP-fre diet decreased the concentration of TRP, 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in all brain regions tested: frontal cortex, hippocampus, thalamus, hypothalamus, midbrain, and pons. Density of 5-HT1A receptor binding was significantly decreased in the frontal cortex and hypothalamus, whereas no significant change was observed in the density of 5-HT2 receptor binding in all regions. These results suggest that the period of primary circadian pacemaker is not affected, but its oscillation, as well as the coupling strength between the primary and secondary pacemakers, is weakened by the dysfunction of the serotonergic system caused by chronic TRP depletion.

Autonomic and behavioral thermoregulation in the golden hamster during subchronic administration of clorgyline

Chronic administration of clorgyline, a type-A monoamine oxidase inhibitor, leads to a decrease in peritoneal (i.e., core) temperature of golden hamsters. To better understand the mechanisms of clorgyline's thermoregulatory effects, autonomic and behavioral thermoregulatory effectors were measured in Syrian hamsters following chronic infusion of clorgyline via a minipump (2 mg/kg/day). Metabolic rate, evaporative water loss, motor activity, and core temperature were measured after 60 min of exposure to ambient temperatures (Ta) of 5, 20, 30, and 35 degrees C. Behavioral thermoregulatory responses were assessed by measuring selected Ta and motor activity of the same animals in a temperature gradient over the course of 23 h. Metabolic rate and motor activity were significantly elevated in clorgyline-treated hamsters exposed to a Ta of 5 degrees C. There were no effects of clorgyline on evaporative water loss. In the temperature gradient the mean selected Ta of clorgyline-treated hamsters was nearly equal to that of the saline-treated hamsters, 30.7 and 31.2 degrees C, respectively. On the other hand, the mode of selected Ta in the clorgyline group was 2.8 degrees C higher than that of the saline group. Motor activity in the gradient was significantly elevated and food consumption was depressed by clorgyline treatment. Overall, these findings indicate that chronic clorgyline treatment in the golden hamster results in novel autonomic and behavioral modification; it stimulates metabolic thermogenesis during cold exposure, but appears to increase the behavioral zone of thermoneutrality. This latter effect may mean an improvement in heat tolerance, suggesting that this drug might assist in the adaptation to warm temperatures.

Rubidium chloride fuses split circadian activity rhythms in hamsters housed in bright constant light

Chronotypic effects of rubidium (Rb) were examined in hamsters whose circadian activity rhythms had split into two components while they were housed in bright constant light. Seven of 12 hamsters receiving RbCl in drinking water for 10 weeks showed fusing of the components into an intact rhythm compared with none of 7 control hamsters (p = 0.016). Rb may modify coupling between circadian oscillators via reduced photic input to the suprachiasmatic nuclei. Alternative mechanisms include changes in potassium metabolism or endocrine function or behavioral changes that in turn alter circadian function. This normalization of a circadian anomaly by a putative antidepressant suggests that Rb may be valuable in strengthening coupling between oscillators in cases of human chronopathology, including those implicated in the etiology of some affective disorders.

The interaction of thyroid state, MAOI drug treatment, and light on the level and circadian pattern of wheel-running in rats

In order to examine the relationship between thyroid status, the circadian system, and antidepressant drug response, the antidepressant drug clorgyline, a monoamine oxidase inhibitor (MAOI), was administered chronically to sham-operated or thyroparathyroidectomized rats. Wheel-running was monitored continuously in a light-dark (LD) cycle, and then in constant dim light. In LD, MAOI treatment increased levels of running. This effect was delayed in hypothyroid rats relative to euthyroid rats. In constant light, the MAOI-induced increase in running was diminished in euthyroid but not hypothyroid animals. Hypothyroid animals were less responsive to the change in lighting than were euthyroid animals, and this was more apparent in hypothyroid rats given MAOI. The daily pattern of running differed with lighting condition as well as with treatment group. MAOI-treatment of hypothyroid animals phase-advanced the pattern of wheel-running. MAOI-treatment of control animals increased the amplitude of wheel-running particularly in the LD cycle. These results indicate that thyroid status, lighting, and MAOI treatment interact to alter the behavioral response to chronic drug treatment.

Modulation of light-induced C-Fos expression in the suprachiasmatic nuclei by 5-HT1A receptor agonists

In previous studies, we showed that light-induced Fos protein expression in the ventrolateral SCN is markedly inhibited by the nonselective serotonergic, quipazine. The present experiments were undertaken to characterize the effects of various serotonin (5-HT) receptor ligands on photic signalling in the SCN. The extent of expression of light-induced Fos-like immunoreactivity (Fos-LI) in the SCN was used as a marker for this response. Exposure of hamsters to a light pulse delivered during the latter part of the dark phase (7 h after lights-off; LD 14:10) elicited an intense expression of Fos-LI in nuclei of cells situated principally in the ventrolateral region of the SCN. Pretreatment with an i.p. injection of the 5-HT1A receptor agonists, 8-OH-DPAT or buspirone, 30 min before the light pulse significantly inhibited the photic expression of Fos-LI (maximal suppression 45.7 +/- 8.1 and 43.0 +/- 1.3%, respectively, both P < 0.01 vs. vehicle controls). Treatment with the 5-HT1A receptor antagonist, NAN-190, administered 15 min before 8-OH-DPAT injection prevented the inhibitory effect of 8-OH-DPAT (100.9 +/- 6.0% vs. controls, P > 0.9). Pretreatment with the 5-HT1B receptor agonist, TFMPP, caused a small but significant suppression of Fos-LI (14.8 +/- 3.5% vs. controls, P < 0.05). In contrast to the significant 5-HT1 receptor agonist effects, pretreatment with 5-HT2 or 5-HT3 receptor agonists, alpha-methyl-5-HT and 1-phenylbiguanide had little suppressive effect on Fos-LI (7.9 +/- 2.1 and 13.0 +/- 5.0% suppression, respectively, both P > 0.1 vs. controls).(ABSTRACT TRUNCATED AT 250 WORDS)

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.

Diurnal variations of serotonin and dopamine levels in discrete brain regions of Syrian hamsters and their modification by chronic clorgyline treatment

In Syrian hamsters, chronic administration of the type A monoamine oxidase inhibitor, clorgyline (CLG), alters the intrinsic period and daily pattern of the circadian rhythm of wheel running, and changes the intensity-response curve for phase-shifting of the rhythm by light pulses. Chronic treatment with CLG also decreases hypothalamic and peritoneal temperatures, particularly during the rest phase of the activity-rest cycle. To help identify monoamines that may mediate CLG's effects on circadian rhythms, we measured levels of dopamine (DA) and serotonin (5-HT) at nine time points over a 24-h period in micro-dissected brain regions in chronic CLG-treated or saline-treated hamsters. For 5-HT, a diurnal variation was detected in all regions in saline-treated animals; for DA, no diurnal variation was detected in any region. In all regions, 5-HT levels and, to a lesser extent, DA levels were higher after CLG treatment. The acrophase of the 5-HT rhythm in the suprachiasmatic nucleus (SCN) was delayed by CLG-treatment, while the acrophase in the dorsal raphe nucleus was unchanged. The diurnal variation of 5-HT in the paraventricular nucleus of the hypothalamus, medial preoptic area, and median raphe nuclei was no longer detectable after chronic CLG-treatment. The phase-delay induced by CLG treatment in the daily rhythm of serotonin levels in the SCN, which functions as a circadian pacemaker, may be an important mechanism underlying the drug's capacity to slow the intrinsic rhythm of the pacemaker and to phase-delay behavioral rhythms that are under its control.

Serotonergic inhibition of light-induced fos protein expression and extracellular glutamate in the suprachiasmatic nuclei

The present experiments were undertaken to explore a role for serotonin (5-HT) in modulating photic signal transduction and extracellular glutamate (Glu) concentration in the suprachiasmatic nuclei (SCN) of the Syrian hamster. Pretreatment with an i.p. injection of the serotonergic, quipazine, caused a marked decrease in the number of SCN cells expressing Fos protein-like immunoreactivity (Fos-LI) induced by a light pulse delivered during the latter part of the dark phase (7 h after lights-off; 55.6 +/- 7.5% of vehicle controls, P < 0.004). This effect of quipazine was dose-dependent and was limited principally to the ventrolateral region of the SCN. In a likewise manner, intra-SCN microinjection of quipazine inhibited light-induced Fos-LI in the ventrolateral SCN, indicating that the suppressive action of quipazine is centered in the SCN. In a separate experiment, localized perfusion of the SCN region with 5-HT using the microdialysis technique caused a significant reduction in the extracellular concentration of Glu. The effect was greater during the dark phase, compared to the light phase of the day-night cycle (60.7 +/- 6.8% vs. 39.3 +/- 6.8% maximal suppression, respectively; P < 0.05). Similar localized application of quipazine also decreased extracellular Glu (48.0 +/- 6.1% maximal suppression; P < 0.05). Collectively, these results are evidence for a serotonergic modulation of retinohypothalamic input in the SCN, which could involve a presynaptic inhibition of Glu release.

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 Duncan's 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)

Effects of imipramine on circadian rhythms in the golden hamster

The effects of the antidepressant imipramine on circadian organization were studied in wild-type and tau-mutant golden hamsters. Chronic imipramine treatment in doses ranging from 0-50 mg kg-1.day-1 depressed general activity and body temperature and caused a reduction in body weight but had no significant effect on circadian organization. Imipramine treatment did not affect the rate of reentrainment after a 6-h advance in the light-dark cycle, did not alter the advanced-phase angle of entrainment of tau-mutant hamsters, did not affect the free-running period of wild type hamsters, and did not alter the phase-response curve to light pulses. Because imipramine, a clinically effective antidepressant, did not have any measurable effect on the circadian system in these experiments, our results do not provide support for the hypothesis that the antidepressant action of imipramine is mediated by alterations in the circadian system.

Serotonin and the mammalian circadian system: II. Phase-shifting rat behavioral rhythms with serotonergic agonists

The suprachiasmatic nuclei (SCN) receive primary afferents from the median and dorsal raphe, but the role of these projections in circadian timekeeping is poorly understood. Studies of the SCN in vitro suggest that quipazine, a general serotonin (5-HT) receptor agonist, can produce circadian time-dependent phase advances and phase delays in circadian rhythms of neuronal activity. The present study addresses whether quipazine and the selective 5-HT1A receptor agonist 8-OH-DPAT are similarly effective in vivo. Drinking and wheel-running patterns of male Wistar rats individually housed in constant darkness were monitored before and after subcutaneous administration of quipazine (5-10 mg/kg) at either circadian time (CT) 6 or CT 18, with and without running wheels available. Dose-dependent phase advances (20-180 min) were produced at CT 6. Significant phase shifts were not observed at CT 18. CT 6 quipazine-treated animals also showed a sustained and significant shortening of rhythm period (tau) following treatment (-0.28 hr; p < 0.002). tau shortening was inconsistently observed in CT 18 quipazine-treated rats. Neither quipazine-induced phase shifts nor tau effects were dependent on wheel-running activity per se. 8-OH-DPAT delivered via intracerebral ventricular treatment into the third ventricle (5 microliters at 100 microM in saline) produced slightly smaller phase advances (20-90 min) at CT 6, but did not produce phase delays at CT 18 or changes in tau. These findings support in vitro evidence that 5-HT-ergic agonists can phase-shift the circadian pacemaker.

Circadian activity rhythms in SHR and WKY rats: strain differences and effects of clonidine

The spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) inbred rat strains have been subjected to extensive behavioral and neurochemical characterization. The present study examined free-running circadian activity rhythms in these two strains. Because previous studies indicated that free-running rhythms are altered during chronic clonidine administration, and that SHRs and WKYs may respond differentially to clonidine, the effects of this agent on rhythmicity were compared in the two strains. SHRs were hyperactive and showed shorter free-running periods than did WKYs. Clonidine administration altered free-running rhythms similarly in the two strains, but reduced activity levels only in the relatively hyperactive SHRs. These results are consistent with the hypothesis that central noradrenergic systems influence circadian locomotor activity rhythms.

A functional model of the mammalian circadian pacemaker

This paper describes a general model of the mammalian circadian pacemaker with specific parameters for golden hamsters. The model successfully simulates free-runs in constant darkness and constant light, phase shifts induced by light pulses and entrainment and relative coordination under light-dark cycles. It also accounts for the recent findings that the amplitude of the phase response curve to light pulses is dependent on the free-running period of the animal and on the previous history of photic stimulation. Effects of non-photic stimuli on the circadian pacemaker are discussed.

Effects of chronic administration and withdrawal of antidepressant agents on circadian activity rhythms in rats

Experimental and clinical studies indicate that clinical depression may be associated with disturbances of circadian rhythms. To explore the interaction between circadian rhythmicity, behavioral state, and monoaminergic systems, the present study investigated the effects of chronic administration and withdrawal of the following antidepressant agents on circadian wheel-running rhythms of laboratory rats: a) moclobemide, a reversible and selective monoamine oxidase (MAO) type A inhibitor; b) Ro 19-6327, a selective MAO type B inhibitor; c) desipramine, a preferential norepinephrine reuptake inhibitor; d) clomipramine and e) fluoxetine, both serotonin reuptake inhibitors; and f) levoprotiline, an atypical antidepressant whose biochemical mechanism is still unknown. Wheel-running activity rhythms were studied in three inbred strains of laboratory rats (ACI, BH, LEW) under constant darkness (DD). Two of these inbred strains (BH and LEW) show profound abnormalities in their circadian activity rhythms, namely, a reduced overall level of activity and bimodal or multimodal activity patterns. Chronic treatment with moclobemide and desipramine consistently increased the overall level, as well as the circadian amplitude, of the activity rhythm. Furthermore, the abnormal activity pattern of the LEW strain was changed into a unimodal activity pattern like that of other laboratory rats. The free-running period tau was slightly shortened by moclobemide and dramatically shortened by desipramine. Effects of moclobemide and desipramine treatment on overall activity level and duration were reversed shortly after termination of treatment, whereas long aftereffects were observed for the free-running period. All other substances tested had no systematic effects on the activity rhythms of any of the strains. The fact that moclobemide and desipramine altered the period, amplitude, and pattern of circadian activity rhythms is consistent with the hypothesis that monoaminergic transmitters play a significant role in the neuronal control of behavioral state and circadian rhythmicity. Although the present study found that some antidepressives affect parameters of circadian rhythmicity, it could not demonstrate a common effect of all classes of antidepressives.

Effects of 5-HT1A receptor agonists on the circadian rhythm of wheel-running activity in hamsters

The effects of 5-HT1A receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), buspirone and ipsapirone on wheel-running activity in hamsters were investigated in comparison with those of GABAA receptor agonist muscimol and benzodiazepine triazolam. Intraperitoneal administration of 8-OH-DPAT, buspirone, ipsapirone, muscimol and triazolam at circadian time (CT) 8 (CT 12; onset of activity) induced a significant phase advance of wheel-running activity under constant light conditions. However, administration of these drugs at other CT points did not induce phase changes. The administration of trifluoromethylphenylpiperazine (TFMPP), a 5-HT1B receptor agonist, at CT8 produced a small phase advance. The phase advance induced by 8-OH-DPAT was blocked by pretreatment with (-)-pindolol, a 5-HT1A receptor antagonist. In addition, 8-OH-DPAT, buspirone and SM3997 accelerated the rate of re-entrainment to an 8-h phase advance in the light-dark cycle. These observations suggest that 5-HT1A receptors in the brain participate in the regulation of the circadian rhythm of wheel-running activity in hamsters.

Fluoxetine shortens circadian period for wheel running activity in mice

Fluoxetine is a potent and specific serotonin re-uptake inhibitor and an effective antidepressant drug. Male mice were treated with either fluoxetine (8 mg/kg body weight per day) or saline. Wheel running activity was monitored for 2 weeks in a 12:12 LD cycle followed by 2 weeks in constant darkness (DD). Fluoxetine significantly shortened free-running circadian period for wheel running activity (23.93 +/- 0.08 h for fluoxetine treated mice versus 24.17 +/- 0.07 h for saline treated mice; p less than 0.03). These results are consistent with a role for serotonin in the regulation of circadian period in mice.

Depletion of brain serotonin by 5,7-DHT modifies hamster circadian rhythm response to light

The midbrain raphe complex innervates the circadian rhythm regulating system by direct projections to the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL). The present experiments examined the changes in circadian rhythm regulation consequent to the depletion of brain serotonin by central 5,7-dihydroxytryptamine (DHT) application. Adult male hamsters with access to running wheels were entrained to a light-dark cycle 14:10 (LD) of photoperiod, pre-treated with desmethylimipramine and given bilateral lateral ventricle infusions of 75 micrograms DHT/2.5 microliters 0.5% ascorbic acid in saline or vehicle only. Two separate experiments were performed. Four weeks after surgery, animals were transferred to either constant light (LL; Experiment 1) or constant dark (DD; Experiment 2). Animals remained in LL for 85 days, then were transferred to DD for 50 days, followed by a return to LD 14:10 for 14 days. Animals in Expt. 2 remained in DD for 55 days, were given 3 days food deprivation, then, beginning 35 days later, were periodically exposed to 30 min light pulses as a phase response curve (PRC) to light was generated. DHT treatment induced rapid appearance of advanced activity onset, delayed offset and longer duration of the nocturnal activity phase. DHT animals in LL had circadian locomotor rhythms much longer than control animals (24.43 +/- 0.04 vs 24.19 +/- 0.05 h) and normal circadian rhythmicity was rapidly lost by DHT animals in LL. There was no effect of DHT on circadian period in DD, but the DHT treated animals in DD had a larger phase delay region of the PRC than did controls and this was associated with an overall change in the temporal properties of the PRC. Serotonin immunohistochemistry showed an approximate 90% loss of cells from the dorsal raphe nucleus and decreased density of the serotonergic terminal field in the SCN and IGL. The results support the view that the serotonergic system modulates the phasic actions of light on the hamster circadian rhythm system. The data also indicate that hamsters can have a Type 0 PRC.

Luminance coding properties of intergeniculate leaflet neurons in the golden hamster and the effects of chronic clorgyline

Cells in the intergeniculate leaflet (IGL) project to the suprachiasmatic nuclei, a mammalian circadian pacemaker. Chronic treatment with clorgyline alters hamster circadian rhythms in ways similar to alterations seen after ablation of the IGL. Chronic clorgyline also alters the light intensity dependence of phase-shifting. In this study luminance coding properties of IGL cells were measured in control hamsters and in hamsters chronically treated with clorgyline. In control animals three patterns of response to increasing and decreasing luminance were observed. Type I cells showed a monotonic pattern. Type II cells were similar to Type I with additional increases in firing rate at several specific luminance levels. Type III cells only coded increases in luminance. Cells from clorgyline-treated animals did not differ from those from control animals in the pattern of luminance response but IGL cells from these animals showed decreased firing rate in both light and dark conditions. These results suggest that the effects of clorgyline on the photic sensitivity of circadian rhythms may be related to a clorgyline-induced decrease in firing rate of IGL cells. They also indicate that some IGL cells show complex patterns of response to luminance changes in addition to those showing simple monotonic responses.

The visual input stage of the mammalian circadian pacemaking system: II. The effect of light and drugs on retinal function

Acute light pulses as well as long-term light exposure may not only modulate photoreceptive properties, but also induce reversible or irreversible damage to the retina, depending on exposure conditions. Illuminance levels in laboratory animal colonies and manipulations of lighting regimens in circadian rhythm research can threaten retinal structure and physiology, and may therefore modify zeitgeber input to the central circadian system. Given the opportunity to escape light at any time, the nocturnal rat self-selects a seasonally varying "naturalistic skeleton photoperiod" that protects the eyes from potential damage by nonphysiological light exposures. Both rod rod-segment disk shedding and behavioral circadian phase shifts are elicited by low levels of twilight stimulation. From this vantage point, we hypothesize that certain basic properties of circadian rhythms (e.g., Aschoff's rule and splitting) may reflect modulation of retinal physiology by light. Pharmacological manipulations with or without the addition of lighting strategies have been used to analyze the neurochemistry of circadian timekeeping. Drug modulation of light input at the level of the retina may add to or interact with direct drug modulation of the central circadian pacemaking system.

Circadian rhythmicity and behavioral depression: I. Effects of stress

Rats were exposed to repeated sessions of inescapable footshock, and behavioral depression was subsequently assessed by measuring escape performance during exposure to escapable shock in a different testing environment. Free-running circadian activity rhythms were assessed using running wheels for approximately three weeks before and after administration of inescapable shock. Several animals showed lengthening of free-running period and decreases in activity level following shock. Similar effects were also seen in rats that were removed from their running wheels, placed within the shock apparatus, and not given shock, but not in nonhandled control animals. Furthermore, period lengthening in shocked and handled rats was positively correlated with escape performance, suggesting that circadian rhythm alterations occurred in those animals that were best able to cope with shock or handling-related stressors. In contrast, individual differences in circadian period and activity level during baseline conditions were not predictive of either escape performance or circadian rhythm alterations. These results suggest that successful behavioral adaptation to stress may be associated with alterations of circadian rhythmicity.

Circadian rhythmicity and behavioral depression: II. Effects of lighting schedules

Two studies explore the relationship between rhythmicity and behavioral depression. Behavioral depression was induced using inescapable footshock, and assessed by measuring subsequent responses to escapable shock, in rats housed under different light-dark conditions. Experiment 1 compared escape performance in free-running and entrained animals following inescapable shock. Free-running and entrained animals did not exhibit differential vulnerability to the effects of inescapable shock. In addition, there were no systematic effects on phase following shock. However, several free-running animals showed increased circadian period following shock, and lengthening of period was significantly correlated with escape performance. Individual differences in baseline period or phase were not predictive of escape performance. In Experiment 2, "aftereffects" of entrainment to long or short light-dark cycles were utilized to create groups of animals with long or short free-running periods. After the administration of inescapable shock, escape performance was tested. There were no significant differences among experimental groups in escape performance. These results suggest that plasticity of circadian period, but not baseline period per se, may be associated with the ability to adapt to environmental challenges.

Destruction of the hamster serotonergic system by 5,7-DHT: effects on circadian rhythm phase, entrainment and response to triazolam

The role of the serotonergic system in the regulation of hamster circadian rhythms was analyzed using intraventricular injection of the selective neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). Sixty days after 5,7-DHT administration, immunoreactive serotonin in the forebrain, particularly the suprachiasmatic nuclei and intergeniculate leaflets, was severely depleted in 16 animals, moderately depleted in four and only slightly affected in four. 5,7-DHT produced an immediate and sustained advance of the onset of running wheel activity relative to the 24 h light-dark (LD) cycle. Activity onset occurred 0.7 +/- 0.07 h before lights out among 5,7-DHT-treated animals compared with 0.18 +/- 0.04 h after lights out for vehicle-infused controls. This new, advanced phase angle of entrainment was maintained throughout the 60-day period of the study while the animals remained in a LD cycle, including after an 8-h phase advance of the light cycle. 5,7-DHT treatment also delayed the offset of wheelrunning in 16 of 24 animals and reduced the likelihood of a smooth pattern of reentrainment to the shifted LD cycle. The drug treatment did not affect circadian period in constant darkness, the rate of reentrainment to an 8-h phase advance or the amount of wheelrunning activity per day. In addition, 5,7-DHT treatment had no effect on the ability of triazolam, a short-acting benzodiazepine, to accelerate the rate of reentrainment to an 8-h phase advance. These observations show that ascending projections of midbrain raphe serotonin neurons participate in the regulation of the circadian activity phase but are not required for triazolam-induced acceleration of reentrainment to a phase-advanced LD cycle.

Free-running circadian activity rhythms during long-term clonidine administration in rats

Experimental and clinical studies indicate that the alpha-adrenergic agonist clonidine can alter mood and activity. However, the behavioral effects of this agent are complex and appear to depend on duration of treatment. Recent work from this laboratory demonstrated that clonidine systematically alters the period, amplitude, and level of free-running circadian activity rhythms in rats. The present study confirms and extends previous observations by employing a longer duration of clonidine treatment. The results show that chronic clonidine administration reversibly shortens the free-running period and reduces the amplitude of the free-running rhythm in constant light. Furthermore, clonidine treatment can increase or decrease the level of activity, depending on baseline activity level, and these effects are not consistently reversed following the termination of treatment. These observations support the hypothesis that noradrenergic systems influence both the circadian periodicity and the level of spontaneous activity, and that clonidine may influence these two parameters by acting at different neural or neuronal loci.