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

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.

Destruction of serotonergic neurons in the median raphe nucleus blocks circadian rhythm phase shifts to triazolam but not to novel wheel access

Systematic treatment of hamsters with triazolam (TRZ) or novel wheel (NW) access will yield PRCs similar to those for neuropeptide Y. Both TRZ and NW access require an intact intergeniculate leaflet (IGL) to modulate circadian rhythm phase. It is commonly suggested that both stimulus types influence rhythm phase response via a mechanism associated with drug-induced or wheel access-associated locomotion. Furthermore, there have been suggestions that one or both of these stimulus conditions require an intact serotonergic system for modulation of rhythm phase. The present study investigated these issues by making serotonin neuron-specific neurotoxic lesions of the median or dorsal raphe nuclei and evaluating phase response of the hamster circadian locomotor rhythm to TRZ treatment or NW access. The expected effect of TRZ injected at CT 6 h on the average phase advance was virtually eliminated by destruction of serotonin neurons in the median, but not the dorsal, raphe nucleus. No control or lesioned animal engaged in substantial wheel running in response to TRZ. By contrast, all median raphe-lesioned hamsters that engaged in substantial amounts of running when given access to a NW had phase shifts comparable to control or dorsal raphe-lesioned animals. The results demonstrate that serotonergic neurons in the median raphe nucleus contribute to the regulation of rhythm phase response to TRZ and that it is unlikely that these neurons are necessary for phase response to NW access. The data further suggest the presence of separate pathways mediating phase response to the two stimulus conditions. These pathways converge on the IGL, a nucleus afferent to the circadian clock, that is necessary for the expression of phase response to each stimulus type.

Serotonin antagonists do not attenuate activity-induced phase shifts of circadian rhythms in the Syrian hamster

A variety of observations from several rodent species suggest that a serotonin (5-HT) input to the suprachiasmatic nucleus (SCN) circadian pacemaker may play a role in resetting or entrainment of circadian rhythms by non-photic stimuli such as scheduled wheel running. If 5-HT activity within the SCN is necessary for activity-induced phase shifting, then it should be possible to block or attenuate these phase shifts by reducing 5-HT release or by blocking post-synaptic 5-HT receptors. Animals received one of four serotonergic drugs and were then locked in a novel wheel for 3 h during the mid-rest phase, when novelty-induced activity produces maximal phase advance shifts. Drugs tested at several doses were metergoline (5-HT1/2 antagonist; i.p.), (+)-WAY100135 (5-HT1A postsynaptic antagonist, which may also reduce 5-HT release by an agonist effect at 5-HT1A raphe autoreceptors; i.p.), NAN-190 (5-HT1A postsynaptic antagonist, which also reduces 5-HT release via an agonist effect at 5-HT1A raphe autoreceptors; i.p.) and ritanserin (5-HT2/7 antagonist; i.p. and i.c.v.). Mean and maximal phase shifts to running in novel wheels were not significantly affected by any drug at any dose. These results do not support a hypothesis that 5-HT release or activity at 5HT1, 2 and 7 receptors in the SCN is necessary for the production of activity-induced phase shifts in hamsters.

Serotonin and feedback effects of behavioral activity on circadian rhythms in mice

Wheel running activity can shorten the period (tau) of circadian rhythms in rats and mice. The role of serotonin (5HT), in this effect of behavior on circadian pacemaker function, was assessed by measuring tau during wheel-open and wheel-locked conditions in mice sustaining neurotoxic 5HT lesions directed at the suprachiasmatic nucleus (SCN). Intact mice exhibited a significant lengthening of tau (approximately 10 min) within 3 weeks when running wheels were locked. Mice with immunocytochemically confirmed 5HT depletion showed significantly longer tau than intact mice during wheel access, and did not show a significant change in tau up to 6 weeks after wheels were locked. In these mice, variability of tau across wheel access conditions was similar in magnitude to tau variability in intact mice at two time points without wheel access (+/- 3 min). 5HT-depleted mice also exhibited significantly longer activity periods (alpha), and a significantly delayed peak of activity within alpha. Previous studies show that a delayed peak of activity within alpha is associated with longer tau. Group differences in tau, and apparent failure of wheel-locking to lengthen tau in mice with 5HT lesions, may thus be due to loss of a serotonergic behavioral input pathway to the SCN, or to a lesion-induced change in the waveform of the activity rhythm.

Expression of Fos in the circadian system following nonphotic stimulation

Syrian hamsters, Mesocricetus auratus, were confined to novel running wheels for a 3-h period, starting at approximately circadian time (CT) 4.5 (i.e., approaching the middle of their subjective day). It can be reliably predicted from the amount of running in this situation whether or not there will be a subsequent phase-shift. Expression of the immediate early genes c-fos and fosB was examined by immunocytochemistry in the suprachiasmatic nucleus (SCN), the intergeniculate leaflet (IGL) of the thalamus, and the medial pretectal area of hamsters that ran vigorously in the novel wheel and would have phase-shifted. c-Fos was increased, compared to levels in a control group left in their home cages, in the IGL, and the pretectum (PT), but decreased in the SCN. No significant changes in FosB were detected in any region examined. An additional experiment argued against the possibility that the changes in c-Fos could be attributed to a rapid advance of the pacemaker to a different phase in the circadian cycle. Counts of c-Fos-positive cells in the IGL were similar in animals given pulses of running starting at CT 4.5 and starting at CT 12.5-16 (i.e., in the subjective night when they would have been active anyway). Altogether the results support the view that activation of the IGL is important in nonphotic clock resetting, and raise the possibility that the PT may also be involved in nonphotic resetting. However, the results also indicate that novelty-induced running does not alter c-Fos induction in a phase-specific manner in the IGL. The inhibition of c-Fos in the SCN by nonphotic phase-shifting events contrasts with the well-known inducing effects of light pulses. These different effects might underlie some of the interactions between nonphotic and photic zeitgebers when both act together on the circadian system.

Roles of suprachiasmatic nuclei and intergeniculate leaflets in mediating the phase-shifting effects of a serotonergic agonist and their photic modulation during subjective day

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

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.

Entrainment of the circadian system of mammals by nonphotic cues

Although light is the principal zeitgeber to the mammalian circadian system, other cues can be shown to have a potent resetting effect on the clock of both adult and perinatal mammals. Nonphotic entrainment may have both biological and therapeutic significance. This review focuses on the effect of behavioral arousal as a nonphotic cue and the neurochemical circuitry that mediates arousal-induced entrainment in the adult rodent. In addition, it considers the role of nonphotic entrainment of the developing circadian system in perinatal life prior to the establishment of retinal input to the clock.

Locking and unlocking of running wheel affects circadian period stability differently in three inbred strains of rats

Running-wheel access has been shown to shorten the circadian period length (tau) of various mammalian species. Due to the close correlation between tau and the level of activity, running wheel-induced changes of the activity level are thought to be responsible for the observed changes in tau. In the present study, the influence of the running wheel on tau and the activity level was examined in three inbred strains of rats (ACI, BH, LEW). Four animals of each strain had free access to their running wheels, while the wheels of the other 4 animals of each strain were mechanically locked. These conditions were changed twice, so that each animal encountered both kinds of changes, that is, from a locked to an unlocked running wheel and vice versa. During the whole study, overall activity was measured by infrared detectors. Running-wheel access resulted in a significant increase of overall activity in strains LEW and ACI. However, significant changes of tau were observed only in LEW rats. These rats showed a significant shortening of tau after the second change of the housing conditions regardless of whether the wheel was locked or unlocked. Consequently, no causal relationship was found between changes of tau and running wheel-induced changes of overall activity. Instead, the results suggest that subtle environmental influences like locking or unlocking the running wheel affect tau in a strain-dependent manner, whereas changes in the activity level are neither necessary nor sufficient to induce changes of tau.

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.

Retinal projections in mice with inherited retinal degeneration: implications for circadian photoentrainment

The availability of naturally occurring and transgenic retinal mutants has made the mouse an attractive experimental model to address questions regarding photoentrainment of circadian rhythms. However, very little is known about the retinal cells and the retinal projections to the nuclei of the murine circadian timing system. Furthermore, the effect of inherited retinal degeneration on these projections is not understood. In this report, we have used pseudorabies virus as a neuroanatomical tract tracer in mice to address a series of questions: Which retinal cells mediate circadian responses to light? What is the nature of the retinohypothalamic projection? What is the impact of the inherited retinal disorder, retinal degenerate (rd/rd), on the structures of the photoentrainment pathway? Our results show that a class ofretinal ganglion cell, morphologically similar to the type III ganglion cells of the rat, appears to project to central circadian structures of the mouse. They are few in number and sparsely distributed throughout the retina. The low number and broad distribution of these specialized retinal ganglion cells may be an adaptive mechanism to integrate environmental irradiance without compromising the spatial resolution required for vision. In addition, viral infection of conelike and rodlike photoreceptors and amacrinelike cells suggest that these cells may mediate or contribute to circadian responses to light. Inherited retinal degeneration has no obvious effect on the anatomy of the retinal cells or their projections to the circadian axis. These anatomical findings are consistent with our previous findings showing that aged rd/rd mice are capable of regulating their circadian rhythms by light with unattenuated sensitivity.

A role for serotonin in the circadian system revealed by the distribution of serotonin transporter and light-induced Fos immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet

Components of the circadian system, the suprachiasmatic nucleus and the intergeniculate leaflet receive serotonin input from the raphe nuclei. Manipulations of serotonin neurotransmission disrupt cellular, electrophysiological, and behavioural responses of the circadian system to light, suggesting that serotonin plays a modulatory role in photic regulation of circadian rhythms. To study the relation between serotonin afferents and light-activated cells in the suprachiasmatic nucleus and intergeniculate leaflet, we used immunostaining for the serotonin transporter and for the transcription factor, Fos. Serotonin transporter, a plasma membrane protein located on serotonin neurons, regulates the amount of serotonin available for neurotransmission by re-accumulating released serotonin into presynaptic neurons; expression of Fos in the suprachiasmatic nucleus identifies light-activated cells involved in photic resetting of circadian clock phase. In the suprachiasmatic nucleus, immunostaining for serotonin transporter revealed a dense plexus of fibres concentrated primarily in the ventrolateral region. In the intergeniculate leaflet, serotonin transporter immunostaining identified vertically-oriented columns of fibres. Serotonin transporter immunostaining was abolished by pretreatment with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Exposure to light for 30 min during the dark phase of the light cycle induced Fos expression in the ventrolateral suprachiasmatic nucleus and intergeniculate leaflet regions. In both structures the Fos-expressing cells were encircled by serotonin transporter-immunoreactive fibres often in close apposition to these cells. These results support the idea that serotonin activity plays a modulatory role in processing of photic information within the circadian system.

The influence of different wavelengths of light on human biological rhythms

The purpose of this review was to discuss the influence of light on humans in the environment, focusing the relation between the quality of light and human biological rhythms, and also to apply the results on lighting planning to a living space which takes into account human health and comfort. The main discussions were as follows: 1) The effects of light on the behavior of core temperature and melatonin vary depending on its wavelength. Light with long wavelengths, such as light with a low color temperature and red light, had little influence on the human biological rhythms. On the other hand, green and blue light--light of mid-short wavelength such as light with a high color temperature--had a greater influence. 2) From the relation between the stimulus received by each photoreceptor and the inhibition of core temperature and melatonin, it might be concluded that the photoreceptor responsible for transmitting light information that affects biological rhythms is M-cones. 3) A higher light intensity was required in the morning than in the evening to induce the inhibition of melatonin secretion. This result suggests the possibility of existence of a diurnal change of sensitivity of the photoreceptors (M-cones). 4) From all these results, it is proposed in the field of living environment and living engineering that light with a low color temperature should be used for low-level lighting at night, and high-level light with a high color temperature in the morning.

Aggressive and sexual social stimuli do not phase shift the circadian temperature rhythm in rats

The objective of the present study was to determine whether the rat circadian system is sensitive to social stimuli. Male rats were subjected to a sociosexual interaction with an estrous female or to an aggressive interaction with a dominant male conspecific. The interactions lasted for 1 h and took place in the middle of the circadian resting phase. Control animals were picked up and handled for a few minutes, but were otherwise left undisturbed. Animals were housed under constant dim red light during the whole period of the experiment. To assess the effects of the interactions on free-running circadian rhythmicity, body temperature was measured by means of radio telemetry. neither the sociosexual interaction with a female nor the aggressive interaction with another male induced phase shifts or changes in the free-running period. The rat circadian system does not seem to be sensitive to social stimuli directly. Moreover, the finding that aggressive interactions do not phase shift circadian rhythms indicates that the endogenous pacemaker in rats is not sensitive to stressors.

Pharmacologic restoration of suppressed temperature rhythms in rats by melatonin, melatonin receptor agonist, S20242, or 8-OH-DPAT

Endogenous circadian rhythms in body temperature and locomotor activity rhythms are suppressed in Sprague-Dawley rats exposed to prolonged continuous light, possibly as a result of a profound alteration of the melatonin secretion rhythm. The ability to restore circadian system function with either exogenous melatonin, or melatonin receptor agonist S20242 (N-[2-(7-methoxy napth-1-yl)ethyl] propionamide), or 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), was investigated under these conditions. Seven rats received a daily 6-h intravenous infusion of melatonin (0.01 mg kg(-1)) for 10 days, which generates a nearly physiological circadian rhythm of urinary 6-sulfatoxy-melatonin, the main urinary metabolite of melatonin. Nevertheless, there was no effect on body temperature or locomotor activity rhythms. Then, 49 rats received daily subcutaneous melatonin (0.01, 1 or 5 mg kg(-1) day(-1)), S20242 (1 or 5 mg kg(-1) day(-1)) or 8-OH-DPAT (5 mg kg(-1) day(-1)) for 30 days. The circadian rhythm in body temperature was restored by subcutaneous melatonin or by S20242 as a function of the dose or by 8-OH-DPAT. The effect started within the first 10 days of treatment and persisted for I to 3 weeks following the end of treatment in 8 of 10 rats receiving melatonin, in 9 of 11 rats treated with S20242 and in 1 of 4 rats treated with 8-OH-DPAT. Activity was less susceptible to entrainment than temperature with these drugs, since circadian rhythmicity was restored in only 2 of 6 rats treated with melatonin and in 1 of 4 rats treated with 8-OH-DPAT. These data demonstrate a specific action of subcutaneous melatonin, S20242 or 8-OH-DPAT on temperature rather than on activity rhythms. This differential effect on two major outputs of the suprachiasmatic nucleus further supports the existence of two independent oscillators in this hypothalamic circadian clock, which may be considered as separate pharmacological targets in the circadian system.

Actual problems of the cerebrospinal fluid-contacting neurons

Cerebrospinal fluid (CSF)-contacting neurons form a part of the circumventricular organs of the central nervous system. Represented by different cytologic types and located in different regions, they constitute a CSF-contacting neuronal system, the most central periventricular ring of neurons in the brain organized concentrically according to our concept. Because the central nervous system of deuterostomian echinoderm starfishes and the prochordate lancelet is composed mainly of CSF-contacting-like neurons, we hypothesize that this cell type represents ancient cells, or protoneurons, in the vertebrate brain. Neurons may contact the ventricular CSF via their dendrites, axons, or perikarya. Most of the CSF-contacting nerve cells send their dendritic processes into the ventricular cavity, where they form ciliated terminals. These ciliated endings resemble those of known sensory cells. By means of axons, the CSF-contacting neurons also may contact the external CSF space, where the axons form terminals of neurohormonal type similar to those known in the neurohemal areas. The most simple CSF-contacting neurons of vertebrates are present in the terminal filum, spinal cord, and oblongate medulla. The dendritic pole of these medullospinal CSF-contacting neurons terminates with an enlargement bearing many stereocilia in the central canal. These cells are also supplied with a 9 x 2 + 2 kinocilium that may contact Reissner's fiber, the condensed secretory material of the subcommissural organ. The Reissner's fiber floating freely in the CSF leaves the central canal at the caudal open end of the terminal filum in lower vertebrates, and open communication is thus established between internal CSF and the surrounding tissue spaces. Resembling mechanoreceptors cytologically, the spinal CSF-contacting neurons send their axons to the outer surface of the spinal cord to form neurosecretory-type terminals. They also send collaterals to local neurons and to higher spinal segments. In the hypothalamic part of the diencephalon, neurons of two circumventricular organs, the paraventricular organ and the vascular sac, of the magnocellular neurosecretory nuclei and several parvocellular nuclei, form CSF-contacting dendritic terminals. A CSF-contacting neuronal area also was found in the telencephalon. The CSF-contacting dendrites of all these areas bear solitary 9 x 2 + 0 cilia and resemble chemoreceptors and developing photoreceptors cytologically. In electrophysiological experiments, the neurons of the paraventricular organ are highly sensitive to the composition of the ventricular CSF. The axons of the CSF-contacting neurons of the paraventricular organ and hypothalamic nuclei terminate in hypothalamic synaptic zones, and those of magno- and parvocellular neurosecretory nuclei also form neurohormonal terminals in the median eminence and neurohypophysis. The axons of the CSF-contacting neurons of the vascular sac run in the nervus and tractus sacci vasculosi to the nucleus (ganglion) sacci vasculosi. Some hypothalamic CSF-contacting neurons contain immunoreactive opsin and are candidates to represent the "deep encephalic photoreceptors." In the newt, cells derived from the subependymal layer develop photoreceptor outer segments protruding to the lumen of the infundibular lobe under experimental conditions. Retinal and pineal photoreceptors and some of their secondary neurons possess common cytologic features with CSF-contacting neurons. They contact the retinal photoreceptor space and pineal recess, respectively, both cavities being derived from the third ventricle. In addition to ciliated dendritic terminals, there are intraventricular axons and neuronal perikarya contacting the CSF. Part of the CSF-contacting axons are serotoninergic; their perikarya are situated in the raphe nuclei. Intraventricular axons innervate the CSF-contacting dendrites, intraventricular nerve cells, and/or the ventricular surface of the ependyma. (ABSTRACT TRUNCATED)

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.

Nicotine and nicotinic receptors in the circadian system

Considerable data support a role for cholinergic influences on the circadian system. The extent to which these influences are mediated by nicotinic acetylcholine receptors (nAChRs) has been controversial, as have the specific actions of nicotine and acetylcholine in the suprachiasmatic nucleus (SCN) of the hypothalamus. In this article we review the existing literature and present new data supporting an important role for nAChRs in both the developing and adult SCN. Specifically, we present data showing that nicotine is capable of causing phase shifts in the circadian rhythms of rats. Like light and carbachol, nicotine appears to cause phase delays in the early subjective night and phase advances in the late subjective night. In the isolated SCN slice, however, only phase advances are seen, and, surprisingly, nicotine appears to cause the inhibition rather than the excitation of neurons. Among nAChR subunit mRNAs, alpha 7 appears to be the most abundant subunit in the adult SCN, whereas in the perinatal period, the more typical nAChRs with higher affinity for nicotine predominate in the SCN. This developmental change in subunit expression may explain the dramatic sensitivity of the perinatal SCN to nicotine that we have previously observed. The effects of nicotine on the SCN may contribute to alterations caused by nicotine in other physiological systems. These effects might also contribute to the dependence properties of nicotine through influences on arousal.

Restricted daytime feeding modifies suprachiasmatic nucleus vasopressin release in rats

The authors have shown previously that vasopressin (VP) release from suprachiasmatic nucleus (SCN) efferents in rats is important for the timing of the circadian activity of the hypothalamo-pituitary-adrenal (HPA) axis, resulting in a circadian rise in corticosterone at dusk. When meals are supplied at a fixed time during the light period, however, this normal circadian activity of the HPA axis is strongly modified. Under such a restricted feeding regimen, a corticosterone peak appears just before the daily meal in addition to the circadian corticosterone peak at dusk. This feeding-associated rise in corticosterone is regarded as an SCN-independent circadian rhythm because it is sustained in SCN-lesioned animals. Despite these previous results, the authors investigated a putative involvement of SCN-derived VP in the control of the prefeeding corticosterone peak by measuring the intranuclear release of VP in the SCN and plasma corticosterone levels in rats in ad libitum feeding conditions as well as in animals that were obliged to feed during a 2-h period in the middle of the light period. Restricted daytime feeding caused clear changes in the daily release pattern of VP from SCN terminals. Both a delayed onset of the diurnal rise and a premature decline of the elevated daytime levels were observed, but the acrophase of the VP rhythm was not phase shifted. Concerning the circadian corticosterone peak, no phase shift of its acrophase was observed either. It is concluded that (1) restricted daytime feeding does affect SCN activity, (2) intranuclear release of VP within the SCN is an important mechanism to amplify and synchronize the circadian rhythms as dictated by the light/dark-entrained circadian pacemaker, and (3) VP release observed in animals on restricted feeding is completely compatible with the previously proposed inhibitory action of SCN-derived VP on the HPA axis.

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

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

Tetrodotoxin blocks NPY-induced but not muscimol-induced phase advances of wheel-running activity in Syrian hamsters

During the middle of the subjective day, circadian activity rhythms in Syrian hamsters can be phase advanced by a variety of stimuli including microinjection of neuropeptide Y (NPY) or muscimol into the suprachiasmatic nucleus (SCN). It is not known, however, if these treatments shift activity rhythms by acting directly on pacemaker cells within the SCN. In the present study NPY and muscimol were microinjected with either tetrodotoxin or saline in order to determine whether classical synaptic transmission within the SCN is necessary for the phase advances produced by NPY or muscimol. Blockade of sodium-dependent action potentials within the SCN prevented NPY- but not muscimol-induced phase advances. These data, along with our previous finding that bicuculline blocks NPY-induced phase advances, suggest that NPY requires sodium-dependent action potentials within GABAergic neurons in order to phase-shift the circadian pacemaker.

Both neuropeptide Y and serotonin are necessary for entrainment of circadian rhythms in mice by daily treadmill running schedules

This study investigated the role of the suprachiasmatic nucleus (SCN) circadian pacemaker and its neuropeptide Y (NPY) and serotonin (5-HT) afferents in entrainment (synchronization) of mouse circadian rhythms by treadmill running. Blind C57BL/6j mice were run in treadmills for 3 hr/d for 3-10 weeks after receiving radio-frequency lesions of the SCN or the intergeniculate leaflet (IGL, the source of SCN NPY) or infusions of the 5-HT neurotoxin 5,7-DHT into the SCN area. Of 25 intact mice, 22 entrained and three showed period (tau, the mean duration of the circadian cycle) modulations to scheduled running. Arrhythmic SCN-ablated mice did not synchronize to scheduled running in a way suggestive of circadian pacemaker mediation. Of 15 mice with IGL lesions, only two with partial lesions entrained. Mice with complete IGL lesions (five), confirmed by immunocytochemistry, showed no entrainment or tau changes. Of 19 mice with 5-HT lesions, only two with partial lesions entrained. All but two mice with complete (10) or nearly complete (4) 5-HT denervation, confirmed by immunocytochemistry, showed tau modulations during the treadmill schedule. Failure to entrain was not explained by group differences in tau before the treadmill schedules. The results indicate that the SCN and both NPY and 5-HT are necessary for entrainment to 24 hr schedules of forced running but that complete loss of 5-HT does not prevent modulations of pacemaker motion by behavioral stimuli. Treadmill entrainment in mice may involve synergistic interactions between 5-HT and NPY afferents at some site within the circadian system.

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.

Quipazine and light have similar effects on c-fos induction in the rat suprachiasmatic nucleus

The effects of the serotonin agonist, quipazine, on the induction of c-fos in the suprachiasmatic nucleus of the rat was examined at different times of the 24 h cycle. Quipazine administered at night induced Fos production in a dose dependent manner (1, 3, 10, 30 mumol/kg) in the ventrolateral portion of the suprachiasmatic nucleus at ZT18. Administration of the highest dose at other times resulted in c-fos induction at ZT15 but not at other times of the day or subjective day examined (CT6 and ZT12). When compared to the effects of light pulses (2 lux/1 min), quipazine only caused c-fos induction at times when light caused induction. Our results support a role of serotonergic pathways in the transmission or modulation of photic information from the retina to the suprachiasmatic nucleus of the rat.

A thalamic contribution to arousal-induced, non-photic entrainment of the circadian clock of the Syrian hamster

It is well established that the circadian clock of the suprachiasmatic nuclei (SCN) is entrained by light. More recently, the potent effects of arousing, non-photic cues on the clock have been recognized. The neural mediators of non-photic entrainment are yet to be identified. To examine the contribution of the thalamic intergeniculate leaflet (IGL) and its NPY-immunopositive projection, the geniculo-hypothalamic tract to non-photic entrainment by arousal, male Syrian hamsters received lesions of the IGL (IGLX) which ablated NPY-immunoreactivity in the SCN. Their circadian responses to both photic and non-photic cues were then tested. Lesions resulted in a delay in the timing of activity onset following lights out, but had no effect on the behavioural or cellular circadian responses to phase-advancing light pulses presented at circadian time (CT) CT19 (where CT12 represents the time of activity onset). Injection with a benzodiazepine (chlordiazepoxide, 100 mg/kg) at CT6 suppressed wheel-running, increased general locomotion of intact controls and induced large phase advances of the circadian rhythm of wheel-running. Chlordiazepoxide also inhibited wheel-running in lesioned animals, but there was no significant increase in general locomotion and the lesioned animals did not phase advance. Serial arousal by injection of saline at intervals of 23.5 h for 6 days entrained the circadian rhythm of wheel-running of intact hamsters and was associated with an increase in general locomotor activity. Entrainment by serial arousal was abolished by IGLX. However, the lesioned animals did show a clear behavioural response to every presentation of the non-photic cue. These results show that the IGL is a necessary component of the neural pathways mediating both arousal- and benzodiazepine-induced non-photic entrainment.

Serotonergic regulation of circadian rhythms in Syrian hamsters

This study investigated the effects of (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthaline hydrobromide (8-OH-DPAT) on circadian rhythms in Syrian hamsters. Systemic administration of 8-OH-DPAT (0.75 mg in 150 microl saline) at circadian time 7 produced phase advances in the circadian activity rhythm. These 8-OH-DPAT-induced phase advances were blocked by microinjection of bicuculline (166 ng, 200 nl) into the suprachiasmatic nucleus, suggesting that GABAergic activity in the suprachiasmatic nucleus mediates the phase shifts produced by systemic injections of 8-OH-DPAT. Microinjection of 8-OH-DPAT (1 microg, 200 nl) or serotonin (0.7 microg, 200 nl) directly into the suprachiasmatic nucleus did not induce phase shifts at circadian time 7, suggesting that the phase shifting effects of systemic injection of 8-OH-DPAT are mediated outside the suprachiasmatic nucleus. To examine possible sites of action of 8-OH-DPAT, 8-OH-DPAT (0.5 microg (100 nl) or 1.0 microg (200 nl)) was microinjected into the intergeniculate leaflet, dorsal raphe nuclei, and the median raphe nucleus at circadian time 7. Significant phase advances were observed after microinjection into the dorsal raphe and median raphe but not the intergeniculate leaflet. These results support the hypothesis that systemic injection of serotonergic agonists can alter circadian rhythms via action in the midbrain raphe nucleus, and that the phase shifts induced by microinjection of 8-OH-DPAT into the raphe nuclei are mediated by a neurotransmitter other than serotonin within the suprachiasmatic nucleus.

Lesion of the serotonergic terminals in the suprachiasmatic nuclei limits the phase advance of body temperature rhythm in food-restricted rats fed during daytime

The daily rhythm of body temperature was recorded in control rats fed ad libitum and subsequently fed during daytime 50% of ad libitum food intake. Aside from the expression of a feeding-associated component, body temperature rhythm was phase advanced (7 h) by a timed caloric restriction; the new plateau of the acrophase of the nocturnal peak was close to the light-dark transition. A lesion of serotonergic (5-HTergic) terminals in the suprachiasmatic nuclei (SCN)-the endogenous circadian clock(s)-was performed by microinjection of the 5-HT neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). During the ad libitum-fed state, the acrophase of body temperature rhythm was not modified by the 5,7-DHT treatment. In response to a timed caloric restriction, however, the phase advance of the nocturnal peak of body temperature rhythm was reduced by 2 h in rats with 5,7-DHT lesions as compared to that of sham-operated rats. Magnitude and day-night pattern of wheel-running activity between the two groups of rats also were analyzed. No intergroup difference was found in the amount of wheel-running activity prior to the time of feeding. Moreover, the phase advance of nocturnal component of locomotor activity rhythm observed toward the time of feeding in sham-operated rats was limited by 5,7-DHT treatment. It is concluded that the photic synchronization of body temperature rhythm does not depend on the 5-HTergic projection to SCN under ad libitum conditions. By contrast, the phase-advancing property of a timed caloric restriction on the daily rhythm of body temperature is mediated by a neuronal circuit involving the 5-HTergic projection to SCN. That the phase advance was not fully eliminated by 5,7-DHT treatment suggests that other pathways participate in this mediation.

5-HT7 receptors mediate serotonergic effects on light-sensitive suprachiasmatic nucleus neurons

Serotonin (5-HT) has been shown to phase shift circadian rhythms in mammals and to affect responses of the circadian system to light, but it is not clear which receptors are involved in these actions. We found that drugs which act as 5-HT1A receptor agonists suppressed photic responses of hamster SCN cells, but these drugs also exhibit high affinity for the recently cloned 5-HT7 receptor. We therefore studied the effects of 5-HT agonists and antagonists with differential affinities for 5-HT7 and 5-HT1A receptors on responses of hamster SCN cells to retinal illumination. We confirmed that the 5-HT receptor agonists 5-HT, 8-OH-DPAT and 5-CT, dose-dependently reduced photic activation of SCN cells. These effects could be blocked by co-application of antagonists with high affinities for 5-HT7 receptors: ritanserin or clozapine. The 5-HT1A/B/D antagonist, cyanopindolol, which is inactive at 5-HT7 receptors, did not antagonize the actions of 8-OH-DPAT. Selective 5-HT1A antagonists, WAY100635 and p-MPPI, had weak or no antagonist effects on the responses to 8-OH-DPAT in the SCN, but they effectively antagonized the actions of 8-OH-DPAT in the hippocampus. In the cerebellar cortex where few 5-HT7 receptors are present, ritanserin failed to antagonize the effects of 8-OH-DPAT. Our results indicate that the 5-HT7 receptor subtype plays a major role in mediating the effects of 5-HT on photic responses of SCN cells in the hamster.

The serotonergic projection from the median raphe nucleus to the suprachiasmatic nucleus modulates activity phase onset, but not other circadian rhythm parameters

The suprachiasmatic nucleus (SCN) is densely innervated by serotonergic fibers originating in the median raphe nucleus (MR). Serotonin (5-HT) specific lesions of the MR alter entrainment and eliminate 5-HT fibers in the SCN, as well as in all other MR-recipient areas. The present study used 5-HT specific lesions of the SCN or the MR to determine the role of 5-HT in the SCN as a regulator of entrainment. Neurotoxic lesions of the MR significantly reduced 5-HT cell bodies in that nucleus and eliminated essentially all 5-HT innervation of the SCN. As previously demonstrated, these anatomical changes were associated with an advance in activity onset, delay in offset and expansion of the activity phase (alpha). Neurotoxin directly applied to the SCN caused an advance in the average activity onset, but had no effect on offset or alpha. About half of the SCN lesion animals had onsets equivalent to the MR lesion group, whereas onsets of the remaining animals were normal. Loss of SCN 5-HT innervation was severe for all SCN lesion animals, but significantly greater for those with advanced activity onsets. These results suggest that although the 5-HT projection to the SCN is likely to be responsible for modulating activity onset, the timing of activity offset appears to be regulated by a MR projection to an area outside the SCN. Furthermore, surprisingly few 5-HT fibers in the SCN are sufficient to maintain the normal phase angle of entrainment.

Long-term fitness training improves the circadian rest-activity rhythm in healthy elderly males

In old age, the circadian timing system loses optimal functioning. This process is even accelerated in Alzheimer's disease. Because pharmacological treatment of day-night rhythm disturbances usually is not very effective and may have considerable side effects, nonpharmacological treatments deserve attention. Bright light therapy has been shown to be effective. It is known from animal studies that increased activity, or an associated process, also strongly affects the circadian timing system, and the present study addresses the question of whether an increased level of physical activity may improve circadian rhythms in elderly. In the study, 10 healthy elderly males were admitted to a fitness training program for 3 months. The circadian rest-activity rhythm was assessed by means of actigraphy before and after the training period and again 1 year after discontinuation. As a control for possible seasonal effects, repeated actigraphic recordings were performed during the same times of the year as were the pre and post measurements in a control group of 8 healthy elderly males. Fitness training induced a significant reduction in the fragmentation of the rest-activity rhythm. Moreover, the fragmentation of the rhythm was negatively correlated with the level of fitness achieved after the training. No seasonal effect was found. Previous findings in human and animal studies are reviewed, and several possible mechanisms involved in the effect of fitness training on circadian rhythms are discussed. The results suggest that fitness training may be helpful in elderly people suffering from sleep problems related to circadian rhythm disturbances.

Phase shifts to refeeding in the Syrian hamster mediated by running activity

Circadian rhythms in hamsters can be entrained by restricted daily feeding schedules. Phase control may be exerted by feeding per se, or by wheel running in anticipation of food access. Phase modulation by feeding was examined here by depriving hamsters of food for 9-24 h and refeeding at 1 of 7 different zeitgeber times on the first day of constant dim light. Significant group mean phase-advance shifts were observed only following 24 h and 17 h deprivations ending in the mid-subjective day, 7 h before the usual time of lights off (mean shifts 28 min and 66 min, respectively). The largest phase shifts were associated with wheel running during the first 6 h of refeeding. When running wheels were locked during this time in an additional group, no phase shifts were observed. A trend for small phase delays was evident for 14 h deprivations ending at the beginning of the subjective night, but no significant group mean or individual shifts were observed at other refeeding times. Refeeding after food deprivation, thus, appears to have minimal effects on circadian phase in hamsters; wheel running associated with refeeding may account for occasional shifts observed.

Serotonin and gender-specific psychiatric disorders

The serotonergic system has been linked to the etiology of several, albeit disparate, psychiatric disorders. The accumulation of many lines of evidence support the view that there are gender differences in the serotonergic system in humans. It is further proposed that a gender differentiated serotonergic system acts as the nidus for the development of gender-specific psychiatric disorders. Depression, anxiety and eating disorders are largely seen in females, whereas alcoholism, aggressivity and suicide predominate in males. Evidence from both animal and human studies suggesting that the serotonergic system mediates between social-environmental experience and biological states is presented and reviewed. A reconceptualization of the serotonergic system as a gender-specific psychobiological interface is proposed. (Int J Psych Clin Prac 1997; 1: 3-13).

Persistence of nonphotic phase shifts in hamsters after serotonin depletion in the suprachiasmatic nucleus

Serotonin-containing fibres (5-HT) project from the raphe complex to the suprachiasmatic nucleus (SCN). Previous studies have suggested that this pathway may be involved in nonphotic resetting of the circadian clock. For example, 5-HT agonists are capable of phase shifting the biological clock both in vivo and in vitro, producing phase response curves (PRCs) similar in shape to those of other nonphotic stimuli. Therefore we studied the role of the serotonergic projection to the SCN in nonphotic phase shifts by bilateral injection of the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) onto the SCN of hamsters. About 50 days after the administration of the neurotoxin, the 5-HT and 5-HIAA (5-hydroxyindole acetic acid) levels were severely depleted in the SCN, as revealed by high performance liquid chromatography (HPLC), and immunocytochemistry (ICC). The average level of 5-HT depletion was 88% in Experiment 1 and 95% in Experiment 2. This treatment had no effect on the magnitude of phase shifts produced by 3 h of novelty-induced wheel-running starting at circadian time (CT) 4, the peak of the advance region of the PRC to this stimulus. The effect of 5-HT depletion on shifts produced by running at CT 22 were inconclusive because of changes in the behavior of control animals. No changes in the phase angle of entrainment of animals in a 14:10 light:dark (LD) cycle were detected in depleted animals. The results suggest that the 5-HT projection from the raphe to the SCN is not essential for activity-induced phase shifts in hamsters.

Involvement of 5-HT1A receptor mechanisms in the inhibitory effects of methamphetamine on photic responses in the rodent suprachiasmatic nucleus

We examined the role of serotonin 1A (5-HT1A) receptors in the inhibitory effects of methamphetamine (MA) on photic entrainment to the circadian pacemaker in the suprachiasmatic nucleus (SCN) of rodents. MA inhibited optic nerve stimulation-evoked field potential in the SCN, light-induced Fos expression in the SCN and light-induced phase shift of hamster wheel-running rhythm. NAN-190, a 5-HT1A receptor antagonist, eliminated the inhibitory effects of MA. NAN-190 has also been reported to antagonize alpha 1 adrenergic receptors. However, prazosin, which selectively antagonizes alpha 1 adrenergic receptors, did not affect the inhibitory action of MA on light-induced Fos expression. In addition, parachloroamphetamine, which is known to be a 5-HT releaser, dose-dependently inhibited light-induced phase shift of wheel-running rhythm. These findings suggest that elevation of endogenous 5-HT levels by MA inhibits the photic entraining responses of the circadian pacemaker in the SCN via 5-HT1A receptor stimulation of the 5-HT released by MA.

Serotonin agonists mimic the phase shifting effects of light on the melatonin rhythm in rats

The effect of serotonin agonists on the rhythmic excretion of the melatonin metabolite 6-sulphatoxymelatonin was examined in rats. The animals were maintained in 12L:12D and administered saline, quipazine (10 mg/kg), (+/-)-2-propylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT, 5 mg/kg) or buspirone (10 mg/kg), 4 h after dark (ZT16). All three drugs caused an acute, transient suppression of 6-sulphatoxymelatonin excretion and a significant delay (P < 0.01) in the onset of the nocturnal rise on the following night of 2.1 +/- 0.6, 1.4 +/- 0.7 and 1.5 +/- 0.3 h respectively while saline administration had no effect (0.4 +/- 0.2 h delay, P > 0.01). To examine the effects of the time of day of agonist administration, groups of rats were treated with quipazine (10 mg/kg) or 8-OH-DPAT (5 mg/kg) 18, 24 or 30 h after the initiation of continuous darkness (CT6, CT12 or CT18) and monitored for a further two nights. Quipazine but not 8-OH-DPAT injection at CT6 resulted in a small but significant delay in the onset of 6-sulphatoxymelatonin excretion on the following night (1.0 +/- 0.2 h and 0.3 +/- 0.2 h) while treatment with both agonists at CT12 failed to affect the onset of excretion (0.8 +/- 0.2 and 0.1 +/- 0.2 h). When quipazine (10 mg/kg) was administered at CT18, 6-sulphatoxymelatonin excretion was acutely suppressed for the rest of the night and there was a large significant delay in the onset of 6-sulphatoxymelatonin excretion (1.2 +/- 0.2 h) while a smaller delay was observed following 8-OH-DPAT administration (0.8 +/- 0.2 h). The acute suppression of 6-sulphatoxymelatonin excretion and subsequent phase delay following quipazine treatment at CT18 was also evident at doses of 1 mg/kg (1.6 +/- 0.4 h) and 3 mg/kg (1.5 +/- 0.6 h). These results show that peripheral administration of serotonin agonists active at 5HT1a/5HT7 receptors mimic the dual effects of light on melatonin production in the rat and raise the possibility that serotonin pathways are more important in mediating the effects of retinally perceived light in the rat than previously believed.

Citalopram: differential sleep/wake and EEG power spectrum effects after single dose and chronic administration

The sleep/wake effects of the selective serotonin re-uptake inhibitor citalopram were studied in both a single-dose study with three dose levels (0.5, 2.0 and 5.0 mg/kg), and a 5-week chronic administration study (15 mg/kg/24 h). Single doses of citalopram resulted in a dose-dependent inhibition of rapid eye movement (REM) sleep. After chronic citalopram treatment there was a sustained REM sleep inhibition. Single doses of citalopram resulted in only minor changes in non-REM (NREM) sleep as well as in NREM EEG power spectral density. Chronic administration resulted in a major shift from SWS-2 to SWS-1. The observed corresponding changes in EEG power density were regional. A 30 to 40 percent reduction of power density in the 0.5-15 Hz range in the fronto-parietal EEG derivation was seen for the whole 8-h registration period. In the fronto-frontal EEG derivation only minor changes were seen. A decreasing trend in NREM sleep power density between 0.5 and 7 Hz, usually seen during the course of the light period, was not observed in the chronic condition, but was seen in control and single-dose condition, suggesting altered diurnal distribution of slow wave activity in the chronic condition. The data indicate that acute and chronic administration of citalopram shows clear differences in sleep effect, which may be caused by alteration of serotonergic transmission, and may be related to the antidepressant effect.

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

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

Entrainment of circadian rhythms by S-20098, a melatonin agonist, is dose and plasma concentration dependent

The present study determined first the dose-response (0.5 to 10 mg.kg-1) to daily oral administration of S-20098, a melatonin agonist, in entraining circadian rhythms of rats free-running in constant darkness; second, the relation between entrainment and the plasma concentration of S-20098. Finally, responses to 8 mg.kg-1 of S-20098 were compared with those obtained with the same dose of melatonin and ipsapirone. Responses were classified as negative, transient, or true entrainment. The data indicated a clear dose-dependent response from 2.5 to 10 mg.kg-1 of S-20098 with an ED50 of 5.7 mg.kg-1 for true entrainment and a clear relation between entrainment and the plasma concentration of S-20098. S-20098 was as effective as melatonin to entrain free-running rhythms. Ipsapirone was ineffective in our experimental conditions.

Locomotor activity and non-photic influences on circadian clocks

Some of the main themes in this review are as follows. 1. The notion that non-photic zeitgebers are weak needs re-examining. Phase-shifts to some non-photic manipulations can be as large as those to light pulses. 2. As well as being able to phase-shift and entrain free-running rhythms, non-photic events have a number of other effects: these include after-effects of entrainment, period changes, and promotion of splitting. 3. The critical variable for non-photic shifting is unknown. Locomotor activity is more likely to be an index of some other necessary state rather than being causal itself. This index may be better when tendencies to move are channelled into easily measured behaviours like wheel-running. 4. Given ignorance about the critical variable, quantification of activity may be the best presently available measure of zeitgeber intensity. Therefore, the behaviour during non-photic manipulations must be examined as carefully as the shifts themselves. When no phase-shifting follows manipulations such as IGL lesions or serotonin depletion, if the animals are inactive, then little can be inferred. 5. Lack of information on the critical variable(s) for non-photic shifting makes it problematic to compare data from studies using different non-photic manipulations. However, the presence of locomotor activity (or its correlate) does appear to be necessary for triazolam to produce shifts. 6. Novelty-induced wheel-running in hamsters depends on the NPY projection from the IGL to SCN. It remains to be determined how important NPY is in other species or in clock-resetting by other manipulations, but methods are now available to study this. 7. Interactions between photic and non-photic zeitgebers remain virtually unexplored, but it is evident that photic and non-photic stimuli can attenuate the phase-shifting effects of each other. Interactions are not purely additive or predictable from PRCs. 8. The circadian system does more than synchronize free-running rhythms to the solar day. Its non-photic functions and their interactions with photic inputs probably account for some of the anatomical complexity of circadian circuitry.

Bicuculline increases and muscimol reduces the phase-delaying effects of light and VIP/PHI/GRP in the suprachiasmatic region

The present study investigated the effects of gamma-amino butyric acid (GABA)A-active drugs on the ability of light or coadministration of vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and gastrin-releasing peptide (GRP) to phase delay hamster activity rhythms. Microinjection of the GABAA agonist, muscimol, significantly (p < .01) reduced the phase-delaying effect of light administered at circadian time (CT) 13.5. By contrast, microinjection of the GABAA antagonist, bicuculline, significantly (p < .01) increased the phase-delaying effect of light administered at CT 13.5. Microinjection of muscimol or bicuculline into the suprachiasmatic nucleus (SCN) produced little or no effect on circadian phase when no light pulses were provided. Coadministration of muscimol with VIP/PHI/GRP significantly (p < .01) reduced the phase-delaying effect of VIP/PHI/GRP, whereas administration of bicuculline with VIP/PHI/GRP significantly (p < .05) increased the phase-delaying effect of these peptides. These data indicate that changes in GABAA activity within the SCN can modulate the phase-delaying effects of light and VIP/PHI/GRP during the early portion of subjective night.

Regulation of the phase and period of circadian rhythms restored by suprachiasmatic transplants

The influence of exogenous signals on circadian rhythms restored by transplants of the suprachiasmatic nucleus (SCN) of the hypothalamus has received little study. The authors tested the responsiveness of hamsters bearing SCN transplants to photic and pharmacological treatments. Light intensities as high as 6,500 lux were insufficient to produce entrainment, although masking was observed frequently. Triazolam failed to produce statistically significant phase shifts when administered during the subjective day, but 2 animals bearing functional SCN grafts responded to this benzodiazapine during the subjective night. The authors next tested the hypothesis that the host can retain circadian aftereffects that influence the period of the circadian system reconstituted by the graft. Intact hamsters were entrained to light:dark cycles of short (23.25-h) and long (25-h) period (T) for at least 3 months. Control hamsters released into constant darkness exhibited profound and long-lasting aftereffects of entrainment to T cycles. Hamsters that received SCN lesions after exposure to these T cycles and SCN grafts 3 weeks later exhibited marginal but statistically significant aftereffects that disappeared within 3 months. On subsequent transfer to constant light, tau lengthened by 0.25 +/- 0.6 h in hamsters with intact SCN (p < .05). Animals bearing SCN grafts continued to free run in constant light but differed from intact animals in that circadian period did not lengthen. Functional SCN grafts contained vasoactive intestinal polypeptide (VIP), neurophysin (NP), and cholecystokinin (CCK) immunoreactive (ir) cells. Inputs of neuropeptide Y-and serotonin-ir fibers from the host brain to grafted SCN peptide cell clusters were variable. Limited observations using retrograde and anterograde tracers do not support the existence of extensive input to the graft. Retinal input overlapped only rarely with clusters of VIP-ir, CCK-ir, or NP-ir cells. The authors conclude that the circadian system reinstated by SCN transplants is relatively impervious to photic influences that exert parametric and nonparametric influences in intact hamsters. The transient expression of aftereffects induced in the host before transplantation indicates that extra-SCN systems of the host can influence the period of the reconstituted circadian system to at least a limited degree.

Phase-shifting effect of 8-OH-DPAT, a 5-HT1A/5-HT7 receptor agonist, on locomotor activity in golden hamster in constant darkness

The present results show that under constant darkness the endogenous circadian pacemaker located in the suprachiasmatic nuclei can be affected by administration of 8-hydroxy-2-[di-n-propylamino] tetralin (8-OH-DPAT), a well known 5-HT1A/5-HT7 receptor agonist. A single i.p. injection (0.1 ml) with 8-OH-DPAT (5 mg/kg) induced significant phase-advances of hamster locomotor activity at circadian time (CT) 6 and 8 and a significant phase-delay at CT11. Saline injections by themselves induced a significant phase-advance at CT10-11. The dose-response curve for 8-OH-DPAT showed a maximal phase-shifting effect for doses of at least 2.5 mg/kg at CT8. Thus, in golden hamsters. (1) 8-OH-DPAT has a chronobiological effect with sensitivity depending upon the circadian time of injection, and (2) a single saline injection is able to induce regular phase-advances at the end of the subjective day (CT10-11).

Methamphetamine modifies the photic entraining responses in the rodent suprachiasmatic nucleus via serotonin release

We examined whether methamphetamine modifies the photic entraining responses in the rat suprachiasmatic nucleus. Optic nerve stimulation increased vasoactive intestinal polypeptide release from rat suprachiasmatic nucleus slices, and methamphetamine inhibited this increase in a concentration-dependent manner. Optic nerve stimulation has been reported to evoke field potentials in rat suprachiasmatic nucleus slices. Methamphetamine attenuated this field potential, and maximal inhibition (75.5%) was achieved at a concentration of 100 microM. Systemic administration of methamphetamine (1-5 mg/kg) inhibited light (300 lux, 1h)-induced Fos expression in the suprachiasmatic nucleus; methamphetamine at a dose of 5 mg/kg, i.p. caused 40% inhibition of light-induced Fos expression. We examined whether the inhibitory effect of methamphetamine on photic entraining responses mediates serotonin release from the suprachiasmatic nucleus. High-performance liquid chromatographic analysis revealed that methamphetamine application increased serotonin release from rat suprachiasmatic nucleus slices in a concentration-dependent manner, but did not affect noradrenaline release. In addition, reduction of serotonin content attenuated the effect of methamphetamine on field potential induced by optic nerve stimulation in vitro and also light-induced phase advances of wheel running activity rhythm in vivo. The present results support the idea that methamphetamine produces an inhibitory effect on photic entrainment in the suprachiasmatic nucleus via serotonin release.

Effects of fluoxetine and olfactory bulbectomy on mouse circadian activity rhythms

Olfactory bulbectomy (OBX) in SWR outbred male mice lengthened the free-running period and delayed the phase of a circadian rhythm for wheel-running activity. OBX also increased mean levels of activity. Two weeks of daily intraperitoneal injections of Fluoxetine (8 mg/kg), a serotonin re-uptake inhibitor, reversed the effects of bulbectomy on the mean level of activity and significantly shortened the free-running period of the activity rhythm. The phase of the activity rhythm was not significantly affected by the Fluoxetine treatment. These results are consistent with a hyposerotonergic mediation of the effects of OBX on circadian period and activity level.

Serotonergic antagonists impair arousal-induced phase shifts of the circadian system of the syrian hamster

Single episodes of arousal of Syrian hamsters 2 h before projected activity onset (i.e., CT 10) phase-advanced their free-running circadian rhythm of wheel-running. Serial arousal once every 23 h or once every 23.5 h for 7 days caused large composite phase-advances to the wheel-running rhythm, the latter period being more effective in supporting an interval of stable entrainment. Pre-treatment of hamsters at CT 6 with the serotonergic antagonist ritanserin (1-5 mg/kg, which acts at both 5-HT2 and the putative 5-HT7 receptor, impaired the phase-advancing response to arousal at CT 10 but the drug was without effect on phase advances induced by exposure to light. Pre-treatment with a second serotonergic antagonist, ketanserin (1-5 mg/kg), which is without effect at 5-HT7 but has high affinity for 5-HT2 receptors, was also effective in attenuating the phase advancing effect of arousal at CT 10. However, neither agent was able to achieve complete blockade of the phase advances. These results are discussed in relation to in vitro and in vivo studies in the rat which have identified a role for 5-HT7 receptors in serotonin-mediated circadian entrainment.

Sleep and EEG power spectrum effects of the 5-HT1A antagonist NAN-190 alone and in combination with citalopram

The sleep and waking and EEG power spectrum effects of the putative 5-HT1A antagonist NAN-190 (0.5 mg/kg, i.p.) were studied alone and in co-administration with the selective serotonin re-uptake inhibitor citalopram (5.0 mg/kg, i.p.) in the rat. Citalopram, as in a prior dose-response study, reduced REM sleep. In addition, a slight increase in NREM sleep was observed. Citalopram reduced NREM fronto-parietal (FP) EEG power density in the 5-20 Hz range. When administered alone, NAN-190 suppressed REM sleep in the first 2 h, and reduced SWS-2 in the first 4 after administration. NAN-190 also suppressed selectively NREM sleep slow-wave activity in both fronto-frontal (FF) and FP EEG power spectrum. When administered in combination with citalopram, an attenuation of the power density reduction in the 7-15 Hz range in the FF EEG of citalopram alone, was observed. However, the EEG power spectral density and REM sleep suppressive effects of NAN-190 were both augmented. The results are compatible with the notion that serotonin is involved in the modulation of the slow wave activity in the EEG during NREM sleep. The results are cordant with other data suggesting that postsynaptic 5-HT1A stimulation might increase slow wave activity in the NREM EEG, and that serotonergic stimulation of other receptor subtypes (possibly 5-HT2) may decrease slow wave activity in the NREM EEG.

Serotonergic stimulation and nonphotic phase-shifting in hamsters

Stimuli that make hamsters active, such as dark pulses or triazolam administration, also phase shift their circadian clocks, producing phase advances during the subjective day and phase delays during the subjective night. Activity or its correlate appears to be important in producing the shifts because preventing locomotion blocks the phase shifts associated with these stimuli. The physiological basis of clock resetting induced by activity is not fully understood. The serotonergic (5-HT) projection from the raphe to the suprachiasmatic nucleus (SCN) is a possible route by which nonphotic information could reach the pacemaker. Administration of 8-HYDROXY-2-(DI-N-PROPYLAMINO) TETRALIN HYDROBROMIDE (8-OH-DPAT), a 5-HT1A and 5-HT7 receptor agonist, at circadian time (CT) 8 produces phase advances in the circadian rhythms of hamsters. Before concluding that 5-HT mediates the effect of activity on the pacemaker, it must be shown that 5-HT agonist do not produce shifts simply because they make animals more active. Therefore, we investigated the contribution of activity to 8-OH-DPAT-produced shifts. Preventing hamsters from moving around after administering 8-OH-DPAT did not abolish phase shifts. Moreover, higher doses of 8-OH-DPAT diminished activity on the day of injection but did not affect the amplitude of phase shifts. Suprisingly, quipazine (a non specific 5-HT agonist), when injected in the middle of subjective day did not phase shift the activity rhythm of hamsters, as it has been reported to do in rats.