Antidepressant drugs can slow or dissociate circadian rhythms

Les rythmes circadiens dans la dépression

Les auteurs font une présentation générale des connaissances sur les troubles des rythmes circadiens dans la dépression et en proposent une synthèse. Ils soulignent d’abord les difficultés méthodologiques de ces éludes: difficultés liées à la nécessité du recueil d’un nombre considérable de données et difficultés pour analyser ces données et vérifier que les différences observées correspondent bien é des cycles circadiens.

Ils envisagent ensuite les troubles des phases, des taux moyens et des amplitudes. Les phases des cycles circadiens ont été beaucoup étudiées surtout après la proposition du concept d’avance de phase comme caractéristique propre de la dépression, qui avait été faite par Wehr en 1980. II semble actuellement que, plus qu’une avance de phase, ce qui caractérise la dépression c’est une désorganisation des phases. Les taux moyens sont souvent modifiés. Parlois diminués (mélatonine), parfois augmentés (cortisol). Les amplitudes des oscillations sont le plus souvent diminuées. Cette constatation, plus récente que les précédentes, semble être de première importance. Moins un rythme est ample, moins il est stable. Il est possible que désynchronisation et diminution d’amplitude soient deux manifestations d’une «faiblesse» des rythmes circadiens qui serait leur caractère principal dans la dépression.

Ces anomalies n’existent pas que dans la dépression, mais c’est dans son cas qu’elles sont le plus manifestes. En général, elles disparaissent en période de rémission clinique. Pour expliquer ces perturbations, il semble qu’il faille envisager à la fois un trouble de la sensibilité aux synchroniseurs externes et un trouble des systèmes de contrôle interne des rythmes.

Le sommeil de sujets dépressifs endogenes avant, pendant et apres un décalage des horaires de sommeil

Cette étude décrit la structure et l'organisation temporelle du sommeil de 5 sujets dépressifs endogènes qui participèrent à un processus de décalage en avance de 5 heures des horaires de sommeil maintenu pendant deux semaines.

Comparée à ces témoins du même âage, I'architecture du sommeil est, chez les dépressifs avant décalage, très perturbée avec, notamment, fragmentation et inefficacité du sommeil, réduction du taux de sommeil lent et raccourcissement de la latence du sommeil paradoxal. L'organisation temporelle semble traduire, elle, une lutte entre tendance au sommeil lent et tendance au sommeil paradoxal.

Le sommeil des mêmes sujets s'est pratiquement normalisé pendant et surtout après le decalage des horaires de sommeil, exceptée la persistance de la réduction importante de la latence du sommeil paradoxal.

Il serait ainsi possible d'associer la dépression à une désynchronisation entre rythmicités contrôlant sommeil lent, sommeil paradoxal et d'autres rythmes circadiens. Une resynchronisation forcée par décalage des horaires de sommeil pourrait alors rendre compte des améliorations cliniques et biologiques qui ont été observées.

Rythmes endocriniens en période de dépression et de rémission

La participation des rythmes biologiques circadiens à la pathogénie de la dépression repose sur une série d’arguments cliniques, thérapeutiques et biologiques. La recrudescence matinale des symptômes dépressifs oriente vers une anomalie de l’organisation temporelle. Les aspects épidémiologiques de la dépression ou des suicides évoquent, quant à eux, l’existence de recrudescence saisonnière dans laquelle les facteurs naturels de synchronisation pourraient jouer un role. De plus, les thérapeutiques de la dépression agissent directement sur les horloges biologiques, qu’il s’agisse des manipulations du cycle veille/sommeil, de la photothérapie ou des antidépresseurs.

Notre étude longitudinale a permis de comparer les rythmes circadiens de cortisol, de TSH et de mélatonine plasmatique chez des sujets déprimés (n = 16), chez des sujets en rémission clinique (n = 15) ainsi que chez des sujets sains (n = 16). Nos résultats montrent de profondes perturbations du système circadien endocrinien en période dépressive. L’anomalie essentielle semble porter sur l’amplitude des rythmes, les sécrétions nocturnes de TSH et de mélatonine étant effondrées chez les sujets dépressifs. Ces perturbations disparaissent avec l’amélioration clinique des sujets.

Nos résultats confirment, dans une certaine mesure, les interrelations temporelles et fonctionnelles qui existent entre les sécrétions de cortisol, de TSH et de mélatonine. Par ailleurs, l’absence de décalage horaire de la position des rythmes étudiés incite à nuancer l’hypothèse d’une désynchronisation biologique dans la dépression pour privilégier une défaillance des mécanismes de couplage entre les facteurs synchronisants de l’environnement et les oscillateurs centraux. Dans cette perspective, les traitements de la dépression tels que les manipulations du cycle veille/sommeil, la photothérapie ainsi que les tricycliques semblent agir sur ces mécanismes de transmission de l’information temporelle soit en augmentant artificiellement leur intensité, soit en abaissant le seuil de perception. Cette hypothèse pourrait aboutir à concevoir de nouveaux outils thérapeutiques des troubles de l’humeur.

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.

Effects of N-acetylcysteine and imipramine in a model of acute rhythm disruption in BALB/c mice

Circadian rhythm disturbances are among the risk factors for depression, but specific animal models are lacking. This study aimed to characterize the effects of acute rhythm disruption in mice and investigate the effects of imipramine and N-acetylcysteine (NAC) on rhythm disruption-induced changes. Mice were exposed to 12:12-hour followed by 10:10-hour light:dark cycles (LD); under the latter, mice were treated with saline, imipramine or NAC. Rhythms of rest/activity and temperature were assessed with actigraphs and iButtons, respectively. Hole-board and social preference tests were performed at the beginning of the experiment and again at the 8th 10:10 LD, when plasma corticosterone and IL-6 levels were also assessed. Actograms showed that the 10:10 LD schedule prevents the entrainment of temperature and activity rhythms for at least 13 cycles. Subsequent light regimen change activity and temperature amplitudes showed similar patterns of decline followed by recovery attempts. During the 10:10 LD schedule, activity and temperature amplitudes were significantly decreased (paired t test), an effect exacerbated by imipramine (ANOVA/SNK). The 10:10 LD schedule increased anxiety (paired t test), an effect prevented by NAC (30 mg/kg). This study identified mild but significant behavioral changes at specific time points after light regimen change. We suggest that if repeated overtime, these subtle changes may contribute to lasting behavioral disturbancess relevant to anxiety and mood disorders. Data suggest that imipramine may contribute to sustained rhythm disturbances, while NAC appears to prevent rhythm disruption-induced anxiety. Associations between sleep/circadian disturbances and the recurrence of depressive episodes underscore the relevance of potential drug-induced maintenance of disturbed rhythms.

Chronobiology and mood disorders

The clinical observations of diurnal variation of mood and early morning awakening in depression have been incorporated into established diagnostic systems, as has the seasonal modifier defining winter depression (seasonal affective disorder, SAD). Many circadian rhythms measured in depressive patients are abnormal: earlier in timing, diminished in amplitude, or of greater variability. Whether these disturbances are of etiological significance for the role of circadian rhythms in mood disorders, or a consequence of altered behavior can only be dissected out with stringent protocols (eg, constant routine or forced desynchrony). These protocols quantify contributions of the circadian pacemaker and a homeostatic sleep process impacting on mood, energy, appetite, and sleep. Future studies will elucidate any allelic mutations in “circadian clock” –related or “sleep”-related genes in depression. With respect to treatment, antidepressants and mood stabilizers have no consistent effect on circadian rhythmicity. The most rapid antidepressant modality known so far is nonpharmacological: total or partial sleep deprivation in the second half of the night. The disadvantage of sleep deprivation, that most patients relapse after recovery sleep, can be prevented by coadministration of lithium, pindolol, serotonin (5-HT) reuptake inhibitors, bright light, or a subsequent phase-advance procedure. Phase advance of the sleep-wake cycle alone also has rapid effects on depressed mood, which lasts longer than sleep deprivation. Light is the treatment of choice for SAD and may prove to be useful for nonseasonal depression, alone or as an adjunct to medication. Chronobiological concepts emphasize the important role of zeitgebers to stabilize phase, light being the most important, but dark (and rest) periods, regularity of social schedules and meal times, and use of melatonin or its analogues should also be considered. Advances in chronobiology continue to contribute novel treatments for affective disorders.

Chronic agomelatine and fluoxetine induce antidepressant-like effects in H/Rouen mice, a genetic mouse model of depression

The novel antidepressant agomelatine behaves as an agonist at melatonergic MT(1) and MT(2) receptors and as an antagonist at serotonin 5-HT(2C) receptors. This study investigated the effects of agomelatine and fluoxetine in a genetic model of depression called H/Rouen mice Male and female H/Rouen (helpless line) and NH/Rouen (nonhelpless line) mice, received once daily for 3 weeks agomelatine (10 and 50 mg/kgi.p.), fluoxetine (10 mg/kgi.p.) or vehicle. Immobility duration in the tail suspension test (TST) was assessed on day 1 (D1), day 8 (D8), day 15 (D15) and day 22 (D22). Locomotor activity in a novel environment was assessed on day 18 (D18) and anhedonia (2-bottle sucrose preference test) was considered after the end of chronic treatment, from days 22 to 25. Agomelatine (50 mg/kg) significantly reduced immobility at D15 (p<0.01), and D22 (p<0.001) in treated H/Rouen mice whereas agomelatine at 10 mg/kg did not induce a statistically significant change. Fluoxetine reduced immobility at D8 (p<0.01), D15 (p<0.001) and D22 (p<0.001). Locomotor activity was unchanged in all treated groups as compared to vehicle groups. In the sucrose test, there was a significant decrease in sucrose preference in H/Rouen mice compared with NH/Rouen mice receiving vehicle. Both agomelatine doses (10 mg/kg (p=0.05) and 50 mg/kg (p<0.001) as well as fluoxetine (p<0.001) significantly increased the sucrose preference in H/Rouen mice as compared with H/Rouen mice that had received vehicle. These data indicate that the novel antidepressant agomelatine has antidepressant-like properties in H/Rouen mice, a genetic model of depression.

Biological clocks and the practice of psychiatry

Endogenous biological clocks enable living species to acquire some independence in relation to time. They improve the efficiency of biological systems, by allowing them to anticipate future constraints on major physyological systems and cell energy metabolism. The temporal organization of a giwen biological function can be impaired in its coordination with astronomical time or with other biological function. There are also external conditions that influence biological clocks. This temporal organization is complex, and it is possible that a series of psychiatric disorders and syndromes involve primary or secondary changes in biological clocks: seasonal and other mood disorders, premenstrual syndromes, social jet lag, free-running rhythms, and several sleep disorders are among them. In this review, we describe the main concepts relevant to chronobiology and explore the relevance of knowledge about biological clocks to the clinical practice of psychiatry

Neuroendocrine and other studies of the mechanism of antidepressant action of desipramine

It is not known whether in depressed patients antidepressant treatment increases or reduces monoaminergic neurotransmission. Clinical studies are therefore reviewed that investigate adaptive changes at adrenoceptors in depressed patients treated with desipramine, and the net effect of these changes upon neurotransmission. Although in animals chronic desipramine treatment enhances the responsiveness of alpha 1-adrenoceptors to phenylephrine, no such effect could be demonstrated in patients upon the responsiveness of pupil diameter to phenylephrine. However, in keeping with animal studies, clinical evidence of altered responsiveness of alpha 2-adrenoceptors could be demonstrated after chronic desipramine treatment. The alpha 2-mediated growth hormone response to clonidine was increased after one week's treatment with desipramine and then reduced during the second and third weeks of treatment. No clinical measure of the responsiveness of central beta-adrenoceptors is available. However, the secretion of melatonin is a measure of neurotransmission at noradrenergic terminals in the pineal with alpha 1-, alpha 2- and beta 1-adrenoceptors. In normal volunteers the secretion of melatonin was increased by the noradrenaline uptake inhibitors desipramine and (+)-oxaprotiline; (-)-oxaprotiline had no effect. In depressed patients melatonin secretion was increased after three weeks' treatment with desipramine. These and other clinical studies suggest that antidepressant treatments increase noradrenergic neurotransmission in depressed patients.

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.

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.

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.

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.

Chronic administration of imipramine and lithium changes the phase-angle relationship between the activity and core body temperature circadian rhythms in rats

Evidence suggests that there is an association between the pathophysiology of depression and a disturbance of circadian rhythms. Accordingly, attention has focused on the possible effects of antidepressants on circadian rhythms. In the present study, we examined the effects of chronic administration of two clinically effective antidepressant agents, imipramine and lithium, on several circadian rhythms in the rat. Activity, core body temperature, and drinking rhythms were assessed in constant darkness (DD) and light-dark (LD) conditions. In DD, lithium significantly lengthened the circadian period of the activity, temperature, and drinking rhythms, while imipramine had no effect. In LD, both drugs significantly delayed the phase of the activity rhythm, but did not change that of the other two rhythms. As a result, the phase-angle differences between the activity and temperature rhythms significantly increased. Neither lithium nor imipramine produced any effect on the resynchronization of these rhythms after an 8-h delay in the LD cycle. These results indicate that although both drugs produced different effects on the circadian period of individual rhythms, both caused a relative phase advance of the temperature rhythm as compared to the activity rhythm, and this effect may be related to the similarity in their antidepressant effects.

Effect of chronic mild stress on circadian rhythms in the locomotor activity in rats

The purpose of this study was to assess whether the chronic mild stress (CMS) procedure, as a realistic animal model of depression, affects the rhythms of the locomotor activity in rats. Rhythm parameters (period, mesor, amplitude, acrophase, and percent rhythm) were estimated from the best-fitted cosine function curves. Period is the length, mesor is the mean level, amplitude (A) is the extent, acrophase is the timing of the rhythm; percent rhythm represents the variability estimated by the cosine regression and expressed as a percentage of the total variability of raw data. The animals were kept on the 12 L : 12 D cycle during 13 weeks of the experiment and subjected to CMS for first 4 weeks. In week 5 the rats were under the constant light for 24 h a day (LL), and in week 9, under the constant darkness (DD). In LD 12:12 CMS decreased the activity in the dark phase by approximately 50% (p < 0.01) and did not change the activity in the light phase, resulting in a drop of the 24 h activity by about 40% in comparison to controls. The amplitude of diurnal variations of the activity was highly statistically different from zero at p(A = 0) < 0.0001, and the percent rhythm was in range of 40-75% in both the CMS and control groups. The mesor and the amplitude of the diurnal rhythm (with a period of 24 h) in the CMS rats were significantly (p < 0.001) lower than those in the control. In LL, the activity of both groups was diminished about 50% during the subjective dark phase. On the other hand, in the subjective light phase the activity of CMS rats only was diminished. The percent rhythm for the CMS and control rats was 30 and 58%, respectively, and values of mesor, amplitude, and acrophase for both groups were highly statistically different. In DD, the activity in the CMS group was statistically significantly lower in both the subjective dark and light phases. In contrast to the results from LL, the cosine curves from DD were similarly shifted in relation to the subjective light-dark cycle. After a restoration of the LD cycle the levels of the 24-h activity of both groups became equal in the 13th week, but the light and dark phase differences between the groups were still statistically significant (p < 0.05). The present results indicate that CMS exerts distinct and prolonged disturbances of the diurnal and circadian rhythms of the locomotor activity in the rats.

Recent developments in anxiety, stress, and depression

Recent research in the development, analysis, and pharmacology of animal tests of state anxiety is discussed, including the use of responses to predator odours, the role of learning in modifying the anxiety measured in the plus-maze, and the roles of cholinergic, NMDA, and dopaminergic systems. Developmental and genetic factors are considered with particular reference to the development of tests of trait anxiety. The roles of 5-HT1A receptors in anxiety, depression, impulsivitity, and agonistic behaviours are discussed. Recent studies on the impacts of stress on neurotransmitter, endocrine, and immune systems and the interactions between these systems are discussed, with particular emphasis on their contributions to the development of pathologic states relevant to anxiety and depression.

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.

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.

Effects of chronic imipramine on exploration, locomotion, and food/water intake in rats

Three groups of rats were subjected to 15 daily injections of imipramine (10 or 20 mg/kg) or vehicle control injections, respectively. During the treatment period, both imipramine groups failed to grow while the control group gained weight normally. Both dosages of imipramine suppressed food intake significantly, while water intake was only reduced by 20 mg/kg of imipramine and only during the first 5 days of treatment. Twenty-four hours after the last imipramine injection, the animals were subjected to a test battery designed to demonstrate potential changes in locomotion and/or exploration. While locomotion appeared unaffected by both dosages of imipramine, the group receiving 20 mg/kg of imipramine demonstrated a significantly reduced exploration. The exploration of the group receiving imipramine in the concentration of 10 mg/kg was only marginally changed. The temporal pattern of exploration of the animals receiving 20 mg/kg of imipramine revealed that chronic imipramine treatment was associated with an initial "hyperexploration" followed by an "overhabituation," resulting in an overall reduction of exploration during a 15-min period.

Geomagnetic storms: association with incidence of depression as measured by hospital admission

The hypothesis that geomagnetic storms may partly account for the seasonal variation in the incidence of depression, by acting as a precipitant of depressive illness in susceptible individuals, is supported by a statistically significant 36.2% increase in male hospital admissions with a diagnosis of depressed phase, manic-depressive illness in the second week following such storms compared with geomagnetically quiet control periods. There is a smaller but not statistically significant increase in female psychotic depression and non-psychotic depression admissions following storms. There was no correlation between geomagnetic storm levels and number of male admissions with psychotic depression, which is consistent with a threshold event affecting predisposed individuals. Phase advance in pineal circadian rhythms of melatonin synthesis may be a possible mechanism of causation or be present as a consequence of 5-hydroxytryptamine and adrenergic system dysfunction associated with geomagnetic disturbance. Effects on cell membrane permeability, calcium channel activity and retinal magneto-receptors are suggested as possible underlying biochemical mechanisms.

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.

Effects of the antidepressant drugs clomipramine and mianserin on tryptophan hydroxylase activity in the rat brain, measured at two points in the light/dark cycle

Tryptophan hydroxylase activity has been measured in synaptosomal preparations from rat brain. A significant circadian variation in enzyme activity was apparent in corpus striatum, (p < 0.001), brainstem and cortex (p < 0.01). A single dose of clomipramine or mianserin (20 mg/kg) had no effect on brainstem enzyme activity measured at mid-light or mid-dark. Chronic administration (7.5 mg/kg i.p. twice daily for 2 weeks) of either clomipramine or mianserin significantly increased enzyme activity. There was no significant difference between the drug effects measured at mid-light or mid-dark.

[Sleep and biological rhythms in depression. Changes caused by antidepressants]

Sleep in depression is characterized by an increase in the number and duration of awakenings, sleep instability, and SWS decrease. REM sleep occurs earlier. REMs density during the 1st REM period is higher than in normal controls matched in age. Accordingly, sleep in depression is similar to sleep in normal aging. Endogenous depression cannot be distinguished from other types of depression by means of polygraphic criteria. Sleep recordings at the beginning of tricyclic compound treatment could be predictive of clinical response to treatment. Sleep modifications induced by antidepressive drugs are reviewed. Sleep recordings enabled us to formulate several physiopathological hypotheses of depression mechanisms: cholinergic-aminergic hypothesis, phase advance, deficiency of process S. Other hypotheses are reviewed: flattening of a hypothetical circadian rhythm of arousal, depressogenic property of sleep in itself (or only of SWS) or timing delay for the start of sleep. A significant phase advance of biological rhythms (temperature, cortisol) is rarely found. A reduction in the amplitude of rhythms (temperature, TSH, melatonine) is more frequent.

Thyroid and thyroxine effects on adrenoreceptors in relation to circadian activity

Experiments were conducted to ascertain if changes in central adrenergic receptors could be associated with altered circadian activity patterns induced by thyroparathyroidectomy (TPX) and thyroxine. An initial experiment used TPX and sham-operated rats that had been exposed to dim red light for 7 months. The alpha and beta receptor densities were compared in the suprachiasmatic nuclei (SCN), preoptic (PO), septum, and caudate-putamen. TPX animals showed significant reductions in beta 1 and beta 2 receptor densities in SCN and PO, and alpha 1 densities in SCN, but no other changes. A second experiment, lasting 4 months, examined the effects of thyroxine, which has been shown to reverse the period-shortening effects of TPX surgery. Thyroxine significantly increased beta 1 receptors in both the SCN and ventromedial hypothalamus (VMH), the only regions that displayed significant reductions in TPXs during the second experiment. Increases of sevenfold and threefold were observed in the SCNs of TPXs and shams, respectively, but thyroxine's action in the VMH was limited to TPX animals, an effect that mimics thyroxine's action on circadian activity rhythms.

Tau changes after single nonphotic events

Changes in the free-running period of the circadian rhythms of hamsters occur after single nonphotic events such as a 3-h pulse of running induced by being put in a novel wheel. These changes are mostly in the direction of longer periods, and can exceed 0.2 h; the magnitude of the effect depends on the circadian phase of the pulse. The phase response curves for period changes do not match up with those for phase shifts of the rhythms. Data on free-running rhythms after anisomycin injections and after novelty-induced wheel running in tau mutant hamsters support the view that period changes and phase shifts can occur independently of one another.

Serotonin receptor gene expression in the rat suprachiasmatic nuclei

Serotonin (5HT) is thought to reset the biological clock in the suprachiasmatic nuclei (SCN) in vitro through a postsynaptic 5HT-1a receptor. Thus we examined 5HT receptor gene expression in the SCN by in situ hybridization. On film autoradiograms, 5HT-1c receptor mRNA showed intense SCN hybridization, while 5HT-1b receptor mRNA displayed a weaker signal. Emulsion autoradiograms additionally revealed expression of 5HT-1a and 5HT-2 receptor mRNAs by a few scattered SCN cells. 5HT-3 receptor mRNA was not detected in the SCN, although the transcript was detected elsewhere in the brain. 5HT-1d and -1e receptor mRNAs were not detected in the SCN or elsewhere in brain within the sections examined. The results do not support a major role for postsynaptic 5HT-1a receptors in resetting SCN rhythms.

Serotonin and the mammalian circadian system: I. In vitro phase shifts by serotonergic agonists and antagonists

The primary mammalian circadian clock, located in the suprachiasmatic nuclei (SCN), receives a major input from the raphe nuclei. The role of this input is largely unknown, and is the focus of this research. The SCN clock survives in vitro, where it produces a 24-hr rhythm in spontaneous neuronal activity that is sustained for at least three cycles. The sensitivity of the SCN clock to drugs can therefore be tested in vitro by determining whether various compounds alter the phase of this rhythm. We have previously shown that the nonspecific serotonin (5-HT) agonist quipazine resets the SCN clock in vitro, inducing phase advances in the daytime and phase delays at night. These results suggest that the 5-HT-ergic input from the raphe nuclei can modulate the phase of the SCN circadian clock. In this study we began by using autoradiography to determine that the SCN contain abundant 5-HT1A and 5-HT1B receptors, very few 5-HT1C and 5-HT2 receptors, and no 5-HT3 receptors. Next we investigated the ability of 5-HT-ergic agonists and antagonists to reset the clock in vitro, in order to determine what type or types of 5-HT receptor(s) are functionally linked to the SCN clock. We began by providing further evidence of 5-HT-ergic effects in the SCN. We found that 5-HT mimicked the effects of quipazine, whereas the nonspecific 5-HT antagonist metergoline blocked these effects, in both the day and night. Next we found that the 5-HT1A agonist 8-OH-DPAT, and to a lesser extent the 5-HT1A-1B agonist RU 24969, mimicked the effects of quipazine during the subjective daytime, whereas the 5-HT1A antagonist NAN-190 blocked quipazine's effects. None of the other specific agonists or antagonists we tried induced similar effects. This suggests that quipazine acts on 5-HT1A receptors in the daytime to advance the SCN clock. None of the specific agents we tried were able either to mimic or to block the actions of 5-HT or quipazine at circadian time 15. Thus, we were unable to determine the type of 5-HT receptor involved in nighttime phase delays by quipazine or 5-HT. However, since the dose-response curves for quipazine during the day and night are virtually identical, we hypothesize that the nighttime 5-HT receptor is a 5-HT1-like receptor.

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.

Assessment of behavioral effects of tetrachloroethylene using a set of time-series analyses

A systematic approach to the analysis of time-series data for spontaneous locomotor activity in the rat was developed to evaluate the behavioral effects of chemical substances. Chronogram, spectral analysis, analysis of the slope of fluctuation, and the cosinor method were used to analyze data obtained by continuously recording spontaneous locomotor activity in the rat. Under synchronized conditions, a circadian period of 24 h was observed and the 1/f fluctuation, in which the power spectral density is inversely proportional to frequency, was confirmed. The 1/f fluctuation was considered to reflect the fractal-like structure of ultradian components of spontaneous locomotor activity. IP administration of 1.0 g/kg body weight of tetrachloroethylene at 0900 h produced a remarkable phase-shift (4.0 +/- 0.9 h) in the activity rhythm and an increase in the slope of fluctuation (0.4 +/- 0.1) in contrast to a relatively smaller decrease in the total amount of spontaneous locomotor activity. These behavioral effects of tetrachloroethylene were dose-dependently lower at lower doses. The present study demonstrated the usefulness of our systematic approach in evaluating the behavioral effects of chemical substances.

Influence of clorgyline treatment on chronopharmacology of furosemide in rats

Circadian variations in the adrenergic nervous system have been reported to be altered by chronic treatment with clorgyline, a monoamine-oxidase inhibitor. In the present study, the influence of clorgyline on the chronopharmacology of furosemide, a loop diuretic agent, was examined in rats maintained under conditions of light from 7 am to 7 pm and dark from 7 pm to 7 am. Clorgyline (4 mg/kg/day) or its vehicle alone was infused subcutaneously by osmotic minipumps for 14 days. Furosemide (30 mg/kg) was given orally at 12 am [noon (N)] or 12 pm [midnight (M)]. Urine was collected for 8 hours after the agent, and urinary excretions of sodium and furosemide were determined. Urine volume and urinary excretions of sodium and furosemide were significantly greater at 12 N than at 12 M in the vehicle-infused group of rats. However these administration time-dependent changes in the effects of furosemide and its urinary excretion disappeared in the clorgyline-infused animals. These results suggest that the mode of the diurnal variation in the effects of furosemide is altered by chronic treatment with clorgyline. As chronic clorgyline is considered to disturb the adrenergic nervous system, the present findings are compatible with the hypothesis that this system is involved in the mechanism responsible for the time-dependent change in the effects of furosemide.

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.

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.

Dietary calcium blocks lithium toxicity in hamsters without affecting circadian rhythms

Lithium can be toxic to rodents at plasma concentrations (0.6-1.2 mmol/L) that also phase delay circadian rhythms. In hamsters, raising the concentration of calcium in the diet from 0.1%-3% reduced weight loss and polydipsia caused by 0.4% lithium carbonate. Calcium ingestion did not affect plasma lithium concentration or the phase of the circadian wheel-running rhythm in lithium-treated animals. Calcium ingestion did not alter weight gain, salt or water intake, or circadian rhythms in hamsters not receiving lithium. Dietary calcium supplementation may prevent some toxic effects of lithium without interfering with other central nervous system actions.

Circadian variation in total plasma tryptophan. Antidepressant treatment: drugs and phase advance

The authors investigated hourly total plasma tryptophan concentrations over a 24 hour period in 3 patients with bipolar depression during 2 types of treatment: a phase advance process and conventional antidepressant therapy. Both treatment modalities increased the 24 hour mean tryptophan levels and the amplitude of circadian tryptophan concentrations. By contrast, in a previous study, moderately decreased tryptophan levels and a blunted amplitude in the circadian rhythm were observed in these same patients in a depressive phase.

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.

Motor activity correlates negatively with free-running period, while positively with serotonin contents in SCN in free-running rats

Free-running period of blinded rats kept in a cage with a running wheel varied markedly, while it varied little in rats kept in a cage without a running wheel. The mean free-running period of the former group is significantly shorter than that of the latter. In the former, the free-running period correlated negatively with motor activity, indicating that activity affects the free-running period. In both groups, essentially similar diurnal patterns of biogenic amines and their metabolites were observed in various discrete areas in the brain examined. However, there was a significant difference between the two groups in several areas. In the SCN, 5-HT content correlated positively with motor activity, consequently correlated negatively to the free-running period at 3 out of 4 sampling times over 24 h but no such correlation was observed in other monoamines and their metabolites examined. These facts suggest that 5-HT may be associated with modification of the free-running period.

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.

Relationship between free-running period and motor activity in blinded rats

The free-running rhythms of motor activity in blinded rats were measured by two different devices, an Automex or a running wheel. The period of free-running rhythm measured by a running wheel was likely to be shorter than that measured by an Automex, indicating that subtle environmental difference, such as whether a cage is equipped with a wheel or not, can affect the free-running period. In addition, we found a negative correlation not only between the free-running period measured by a running wheel and that measured by an Automex, but between the free-running period and the number of wheel revolutions per day. This is the first evidence that motor activity, other than the external factors such as light intensity and temperature, may be related to change in the free-running period.

The response of suprachiasmatic neurons of the rat hypothalamus to photic and serotonergic stimulation

Single-unit activity of the rat suprachiasmatic nucleus and surrounding regions was recorded in vivo. Photically responsive neurons were typically inhibited by low intravenous doses of the serotonin (5-HT) agonist quipazine. The 5-HT antagonist, metergoline, administered in a low intravenous dose, reversed the effects of quipazine. The subsequent response to photic stimulation was essentially identical to the initial photic response. Thus the photic response of neurons in the suprachiasmatic nucleus (SCN) and adjacent regions is relatively uninfluenced by 5-HT blockade.

Daily changes in pentylenetetrazol-induced convulsions and open-field behavior in rats

Susceptibility to pentylenetetrazol (PTZ)-induced clonic convulsions was measured in rats over the 24-hr light-dark cycle at four-hour intervals. The results showed a higher sensitivity to PTZ around 2200 hr. Other groups of rats were exposed to a four-min open-field session. In the first two min of the session the animals were submitted to open-field environmental stimuli. The next two-min observation occurred with sound and light presented to the animals. Differences through the 24-hr period of the day for both sessions were seen. A reduction in rearing at 2200 hr and increase in defecation at 2200 hr and 0200 hr was observed.

Temperature dependence of circadian rhythms in golden-mantled ground squirrels

We tested the hypothesis that the free-running period and phase angles of entrainment of circadian locomotor activity rhythms of golden-mantled ground squirrels (Spermophilus lateralis) vary as a function of body temperature (Tb). Animals were maintained in a light-dark cycle (LD 14:10), and at 6-week intervals ambient temperature (Ta) was changed from 20 degrees to 30 degrees C, or vice versa. Data were collected during the animals' homeothermic and heterothermic phases. Subsequently, squirrels were housed in dim constant illumination with the same alternating temperature sequence. In heterothermic ground squirrels (those capable of Tb less than 34 degrees C), a decrease in Ta from 30 degrees to 20 degrees C caused phase delays in activity onset, phase advances in activity termination, and a decrease in duration of the active phase; increases in Ta from 20 degrees to 30 degrees C produced the opposite effect on each of these parameters. The free-running period of the activity rhythm of heterothermic squirrels increased and decreased in response to 10 degrees C decreases and increases in Ta, respectively. Changes in Ta did not affect circadian parameters in homeothermic squirrels. Seasonal variations in circadian organization previously observed in this species appear to reflect the influence of endogenous fluctuations in body temperature. Tissue temperature has a major influence on circadian rhythms of golden-mantled ground squirrels.