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

Systematic review of drugs that modify the circadian system's phase-shifting responses to light exposure

We searched PubMed for primary research quantifying drug modification of light-induced circadian phase-shifting in rodents. This search, conducted for work published between 1960 and 2018, yielded a total of 146 papers reporting results from 901 studies. Relevant articles were those with any extractable data on phase resetting in wildtype (non-trait selected) rodents administered a drug, alongside a vehicle/control group, near or at the time of exposure. Most circadian pharmacology experiments were done using drugs thought to act directly on either the brain's central pacemaker, the suprachiasmatic nucleus (SCN), the SCN's primary relay, the retinohypothalamic tract, secondary pathways originating from the medial/dorsal raphe nuclei and intergeniculate leaflet, or the brain's sleep-arousal centers. While the neurotransmitter systems underlying these circuits were of particular interest, including those involving glutamate, gamma-aminobutyric acid, serotonin, and acetylcholine, other signaling modalities have also been assessed, including agonists and antagonists of receptors linked to dopamine, histamine, endocannabinoids, adenosine, opioids, and second-messenger pathways downstream of glutamate receptor activation. In an effort to identify drugs that unduly influence circadian responses to light, we quantified the net effects of each drug class by ratioing the size of the phase-shift observed after administration to that observed with vehicle in a given experiment. This allowed us to organize data across the literature, compare the relative efficacy of one mechanism versus another, and clarify which drugs might best suppress or potentiate phase resetting. Aggregation of the available data in this manner suggested that several candidates might be clinically relevant as auxiliary treatments to suppress ectopic light responses during shiftwork or amplify the circadian effects of timed bright light therapy. Future empirical research will be necessary to validate these possibilities.

Bright-light therapy in the treatment of mood disorders

Bright-light therapy (BLT) is established as the treatment of choice for seasonal affective disorder/winter type (SAD). In the last two decades, the use of BLT has expanded beyond SAD: there is evidence for efficacy in chronic depression, antepartum depression, premenstrual depression, bipolar depression and disturbances of the sleep-wake cycle. Data on the usefulness of BLT in non-seasonal depression are promising; however, further systematic studies are still warranted. In this review, the authors present a comprehensive overview of the literature on BLT in mood disorders. The first part elucidates the neurobiology of circadian and seasonal adaptive mechanisms focusing on the suprachiasmatic nucleus (SCN), the indolamines melatonin and serotonin, and the chronobiology of mood disorders. The SCN is the primary oscillator in humans. Indolamines are known to transduce light signals into cells and organisms since early in evolution, and their role in signalling change of season is still preserved in humans: melatonin is synthesized primarily in the pineal gland and is the central hormone for internal clock circuitries. The melatonin precursor serotonin is known to modulate many behaviours that vary with season. The second part discusses the pathophysiology and clinical specifiers of SAD, which can be seen as a model disorder for chronobiological disturbances and the mechanism of action of BLT. In the third part, the mode of action, application, efficacy, tolerability and safety of BLT in SAD and other mood disorders are explored.

New light on the serotonergic paradox in the rat circadian system

The main mammalian circadian clock, localized in the suprachiasmatic nuclei can be synchronized not only with light, but also with serotonergic activation. Serotonergic agonists and serotonin reuptake inhibitors (e.g., fluoxetine) have a non-photic influence (shifting effects during daytime and attenuation of photic resetting during nighttime) on hamsters' and mice' main clock. Surprisingly, in rats serotonergic modulation of the clock shows essentially photic-like features in vivo (shifting effects during nighttime). To delineate this apparent paradox, we analyzed the effects of fluoxetine and serotonin agonists on rats' clock. First, fluoxetine induced behavioral phase-advances associated with down-regulated expression of the clock genes Per1 and Rorbeta and up-regulated expression of Rev-erbalpha during daytime. Moreover, fluoxetine produced an attenuation of light-induced phase-advances in association with altered expression of Per1, Per2 and Rorbeta during nighttime. Second, we showed that 5-HT(1A) receptors -maybe with co-activation of 5-HT(7) receptors- were implicated in non-photic effects on the main clock. By contrast, 5-HT(3) and 5-HT(2C) receptors were involved in photic-like effects and, for 5-HT(2C) subtype only, in potentiation of photic resetting. Thus this study demonstrates that as for other nocturnal rodents, a global activation of the serotonergic system induces non-photic effects in the rats' clock during daytime and nighttime.

Altered photic and non-photic phase shifts in 5-HT(1A) receptor knockout mice

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) is thought to be modulated by 5-HT. 5-HT is though to inhibit photic phase shifts by inhibiting the release of glutamate from retinal terminals, as well as by decreasing the responsiveness of retinorecipient cells in the SCN. Furthermore, there is also evidence that 5-HT may underlie, in part, non-photic phase shifts of the circadian system. Understanding the mechanism by which 5-HT accomplishes these goals is complicated by the wide variety of 5-HT receptors found in the SCN, the heterogeneous organization of both the circadian clock and the location of 5-HT receptors, and by a lack of sufficiently selective pharmacological agents for the 5-HT receptors of interest. Genetically modified animals engineered to lack a specific 5-HT receptor present an alternative avenue of investigation to understand how 5-HT regulates the circadian system. Here we examine behavioral and molecular responses to both photic and non-photic stimuli in mice lacking the 5-HT(1A) receptor. When compared with wild-type controls, these mice exhibit larger phase advances to a short late-night light pulse and larger delays to long 12 h light pulses that span the whole subjective night. Fos and mPer1 expression in the retinorecipient SCN is significantly attenuated following late-night light pulses in the 5-HT(1A) knockout animals. Finally, non-photic phase shifts to (+/-)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) are lost in the knockout animals, while attenuation of the phase shift to the long light pulse due to rebound activity following a wheel lock is unaffected. These findings suggest that the 5-HT(1A) receptor plays an inhibitory role in behavioral phase shifts, a facilitatory role in light-induced gene expression, a necessary role in phase shifts to 8-OH-DPAT, and is not necessary for activity-induced phase advances that oppose photic phase shifts to long light pulses.

5-HT3 receptor-mediated photic-like responses of the circadian clock in the rat

Serotonin (5-HT) and 5-HT agonists have various resetting effects on the master clock, located in the suprachiasmatic nucleus (SCN), depending on the species. In rats, they induce photic-like effects on both locomotor activity rhythms and gene expression in the SCN. The 5-HT receptor(s) mediating these effects at circadian time 22 are localized in the SCN, most likely at a presynaptic level, on the retinohypothalamic terminals (RHT) known to convey photic information by releasing glutamate. Indeed, RHT degeneration blocks photic-like effects of a non-specific 5-HT agonist, quipazine. However, the 5-HT receptor subtype(s) involved is still unknown, although 5-HT(3) receptor activation is known to induce glutamate release. We thus analyzed the effects of selective 5-HT(3) agonist and antagonist, as well as a specific NMDA receptor antagonist, on different parameters of the clock. This study shows that the 5-HT(3) receptor mediates the resetting effects of quipazine on locomotor activity rhythms. The 5-HT(3) receptor is only partially implicated in quipazine-induced expression of c-FOS, while NMDA receptor inhibition blocks quipazine photic-like effects on both parameters. Taken together, photic-like responses produced by 5-HT stimulation in rats are likely mediated by (presynaptic?) 5-HT(3) receptor activation followed by NMDA receptor activation.

Involvement of the retinohypothalamic tract in the photic-like effects of the serotonin agonist quipazine in the rat

Light is the major synchronizer of the mammalian circadian pacemaker located in the suprachiasmatic nucleus. Photic information is perceived by the retina and conveyed to the suprachiasmatic nucleus either directly by the retinohypothalamic tract or indirectly by the intergeniculate leaflet and the geniculohypothalamic tract. In addition, serotonin has been shown to affect the suprachiasmatic nucleus by both direct and indirect serotonin projections from the raphe nuclei. Indeed, systemic as well as local administrations of the serotonin agonist quipazine in the region of the suprachiasmatic nucleus mimic the effects of light on the circadian system of rats, i.e. they induce phase-advances of the locomotor activity rhythm as well as c-FOS expression in the suprachiasmatic nucleus during late subjective night. The aim of this study was to localize the site(s) of action mediating those effects. Phase shifts of the locomotor activity rhythm as well as c-FOS expression in the suprachiasmatic nucleus after s.c. injection of quipazine (10 mg/kg) were assessed in Lewis rats, which had received either radio-frequency lesions of the intergeniculate leaflet or infusions of the serotonin neurotoxin 5,7-dihydroxytryptamine into the suprachiasmatic nucleus (25 microg) or bilateral enucleation. Lesions of intergeniculate leaflet and serotonin afferents to the suprachiasmatic nucleus did not reduce the photic-like effects of quipazine, whereas bilateral enucleation and the subsequent degeneration of the retinohypothalamic tract abolished both the phase-shifting and the FOS-inducing effects of quipazine. The results indicate that photic-like effects of quipazine are mediated via the retinohypothalamic tract.

Activation of 5-HT2C receptors acutely induces Per gene expression in the rat suprachiasmatic nucleus at night

The suprachiasmatic nucleus (SCN) of the hypothalamus receives dense serotonergic projections from the raphe nuclei and this input has been implicated in the modulation of circadian rhythms. In the present study, we investigated the effect of 5-HT2C receptor activation on various clock genes within the suprachiasmatic nucleus, including Per1 and Per2, which have previously been demonstrated as necessary for phase shifts. Rats were exposed to light (400 lx, 15 min), administered 5-HT2C receptor agonists (+/-)-1-(4-iodo-2,5-dimethoxy-phenyl)-2-aminopropane (DOI) (2 mg/kg) or RO 60-0175 (10 mg/kg) or vehicle 4 or 10 h after dark onset (ZT16 and ZT22). The expression of Per1, Per2, Cry1, Clock, Bmal1, Dec1, Dec2 and c-fos was determined 30 and 120 min after treatment in suprachiasmatic nucleus punches by real time reverse transcription-polymerase chain reaction (RT-PCR). Light exposure induced a 7-fold increase in c-fos expression within 30 min of treatment at both ZT16 and ZT22. Per1 expression was increased 2-fold following light exposure at ZT22, whereas treatment at ZT16 had no significant effect. Per2 expression was significantly induced following light at ZT16, but was not affected at ZT22. RO 60-0175 or DOI administration induced a 5-fold change in c-fos expression at ZT16 and a 3-fold change at ZT22 within 30 min of treatment. The drug increased both Per1 and Per2 expression at ZT16, but had no effect at ZT22. These results provide evidence for 5-HT2C receptors being involved in the modulation of circadian rhythms during early night.

Response of the mouse circadian system to serotonin 1A/2/7 agonists in vivo: surprisingly little

Serotonin (5-HT) is thought to play a role in regulating nonphotic phase shifts and modulating photic phase shifts of the mammalian circadian system, but results with different species (rats vs. hamsters) and techniques (in vivo vs. in vitro; systemic vs. intracerebral drug delivery) have been discordant. Here we examined the effects of the 5-HT1A/7 agonist 8-OH-DPAT and the 5-HT1/2 agonist quipazine on the circadian system in mice, with some parallel experiments conducted with hamsters for comparative purposes. In mice, neither drug, delivered systemically at a range of circadian phases and doses, induced phase shifts significantly different from vehicle injections. In hamsters, quipazine intraperitoneally (i.p.) did not induce phase shifts, whereas 8-OH-DPAT induced phase shifts after i.p. but not intra-SCN injections. In mice, quipazine modestly increased c-Fos expression in the SCN (site of the circadian pacemaker) during the subjective day, whereas 8-OH-DPAT did not affect SCN c-Fos. In hamsters, both drugs suppressed SCN c-Fos in the subjective day. In both species, both drugs strongly induced c-Fos in the paraventricular nucleus (within-subject positive control). 8-OH-DPAT did not significantly attenuate light-induced phase shifts in mice but did in hamsters (between-species positive control). These results indicate that in the intact mouse in vivo, acute activation of 5-HT1A/2/7 receptors in the circadian system is not sufficient to reset the SCN pacemaker or to oppose phase-shifting effects of light. There appear to be significant species differences in the susceptibility of the circadian system to modulation by systemically delivered serotonergics.

Dorsal raphe nucleus modulates neuronal activity in rat intergeniculate leaflet

Serotonergic input from midbrain raphe nuclei is believed to have a significant effect on mammalian circadian timing system. The suprachiasmatic nucleus (SCN) receives its serotonergic input from the median raphe nucleus, while the intergeniculate leaflet (IGL) receives serotonergic innervation from the dorsal raphe nucleus (DRN). The present paper was aimed at determining whether projection from the DRN affected rhythmic neuronal oscillations in the IGL of rats. We investigated the impact of electrolytic lesions and electric stimulation of the DRN on spontaneous isoperiodic (i.e. burst firing with a constant interburst interval) neuronal activity recorded in the IGL. In all our experiments a complete lesion of the DRN always caused a significant increase (ca. 100%) of spontaneous activity of IGL neurons, their oscillatory character having been maintained, though. On the other hand, electric stimulation of the DRN produced a transient decrease in firing rate oscillations of the IGL neurons. The obtained results indicate that the neuronal projection from the DRN has a substantial modulating effect on IGL activity-an important element of the mechanism of the circadian time-keeping system that mediates the transfer of non-photic information to the SCN by modulating its activity. The observed increase of isoperiodic activity in the IGL after DRN lesion and a transient decrease in this activity after electric stimulation indicate an inhibitory character of this effect. The present findings corroborate the hypothesis that the DRN is a one of the major and extremely important source of the modulatory inputs to the mammalian circadian time-keeping system.

Serotonin, excitatory amino acids and the photic control of melatonin rhythms and SCN c-FOS in the rat

There is a growing acceptance that serotonergic pathways to the suprachiasmatic nucleus play an important role in the mediation and modulation of light entrainment of rhythms. In this study administration of the 5-HT(2A/2C) agonist (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI, 0.5 mg/kg) at mid dark caused a phase shift in the onset of the urinary excretion of 6-sulphatoxymelatonin in rats that was sustained for at least 8 days and was blocked by the specific 5-HT(2C) antagonist SB-242084. Administration of DOI (2 mg/kg) across the night resulted in the appearance of c-FOS in the nucleus of cells in the suprachiasmatic nucleus during subjective darkness, but did not cause induction at the time of expected lights on (CT0). By contrast light exposure induced c-fos throughout the night including CT0. Administration of the NMDA receptor antagonist MK-801 (3 mg/kg) prior to light pulses had no effect on c-fos in the first part of the night, but towards the expected time of lights on, became progressively more potent, such that by CT0, light induction of c-fos was almost completely inhibited. These results provide further evidence that serotonin plays a role in the mediation of light effects on rhythms in the rat.

Serotonergic modulation of retinal input to the mouse suprachiasmatic nucleus mediated by 5-HT1B and 5-HT7 receptors

Serotonin (5-HT) and 5-HT receptor agonists can modify the response of the mammalian suprachiasmatic nucleus (SCN) to light. It remains uncertain which 5-HT receptor subtypes mediate these effects. The effects of 5-HT receptor activation on optic nerve-mediated input to SCN neurons were examined using whole-cell patch-clamp recordings in horizontal slices of ventral hypothalamus from the male mouse. The hypothesis that 5-HT reduces the effect of retinohypothalamic tract (RHT) input to the SCN by acting at 5-HT1B receptors was tested first. As previously described in the hamster, a mixed 5-HT(1A/1B) receptor agonist, 1-[3-(trifluoromethyl)phenyl]-piperazine hydrochloride (TFMPP), reduced the amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked by selectively stimulating the optic nerve of wild-type mice. The agonist was negligibly effective in a 5-HT1B receptor knockout mouse, suggesting minimal contribution of 5-HT1A receptors to the TFMPP-induced reduction in the amplitude of the optic nerve-evoked EPSC. We next tested the hypothesis that 5-HT also reduces RHT input to the SCN via activation of 5-HT7 receptors. The mixed 5-HT(1A/7) receptor agonist, R(+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT), reduced the evoked EPSC amplitude in both wild-type and 5-HT1B receptor knockout mice. This effect of 8-OH-DPAT was minimally attenuated by the selective 5-HT1A receptor antagonist WAY 100635 but was reversibly and significantly reduced in the presence of ritanserin, a mixed 5-HT(2/7) receptor antagonist. Taken together with the authors' previous ultrastructural studies of 5-HT1B receptors in the mouse SCN, these results indicate that in the mouse, 5-HT reduces RHT input to the SCN by acting at 5-HT1B receptors located on RHT terminals. Moreover, activation of 5-HT7 receptors in the mouse SCN, but not 5-HT1A receptors, also results in a reduction in the amplitude of the optic nerve-evoked EPSC. The findings indicate that 5-HT may modulate RHT glutamatergic input to the SCN through 2 or more 5-HT receptors. The likely mechanism of altered RHT glutamatergic input to SCN neurons is an alteration of photic effects on the SCN circadian oscillator.

The ontogeny of induction of c-fos in the rat SCN by a 5-HT(2A/2C) agonist

The induction of c-fos in the suprachiasmatic nucleus (SCN) by the 5-HT(2A/2C) agonist, DOI was studied at mid-dark in neonatal rats. The number of cells expressing c-FOS immunoreactivity following DOI was low 3 days after birth, but increased rapidly over the next 3 days. By contrast, light exposure stimulated cells throughout this period. These results are consistent with the arrival of serotonergic afferents at the SCN and their role in the entrainment of rhythms.

Prenatal exposure to the dopamine agonist SKF-38393 disrupts the timing of the initial response of the suprachiasmatic nucleus to light

The abuse of social drugs such as cocaine during pregnancy represents enormous risks to the offspring. Recent studies showed that drugs administered to the pregnant rat can activate cell populations in the fetal brain, possibly altering the timing of key neuronal developmental events. The current study examined the ontogeny of light-responsiveness of the neonatal rat suprachiasmatic nucleus using c-FOS protein in SCN nuclei as a marker. The effect of acute administration of the dopamine D1 agonist, SKF-38393, on the development of light responsiveness was also examined. Pregnant dams received either SKF-38393 (10 mg/kg) or vehicle 7 h after dawn on gestational day 20. Litters were then assigned to one of seven experimental time points from 4 h after subjective dark onset on the day of birth (P0-CT16) at 4-h intervals until CT16 on the day after birth (P1-CT16). Half of the pups in each litter were exposed to a 200 lux/2 h light pulse and the other half remained in darkness. Three time points (P1-CT0, P1-CT8 and P1-CT16) were used to examine the prenatal drug effects on light-responsiveness. Light exposure at the time of subjective lights on, the day after birth (P1-CT0), resulted in a significant increase in c-FOS-positive cells. The number of positive cells recorded in the SCN after a light pulse at P1-CT0 and P1-CT8 was significantly less in SKF-38393 pretreated pups compared to vehicle treated animals. The exposure to dopaminergic stimulation during gestation may have altered the timing of development of afferent connections to the fetal SCN, resulting in alteration of the initial response of the circadian timing system to light.

MK-801 administration blocks the effects of a 5-HT(2A/2C) agonist on melatonin rhythmicity and c-fos induction in the suprachiasmatic nucleus

Both excitatory amino acids and serotonin have been implicated in the photic control of rhythms, but they have rarely been considered to interact. This study investigated the effects of the NMDA receptor antagonist, MK-801 on the phase shift of the melatonin rhythm and the induction of c-fos in the rat suprachiasmatic nucleus (SCN) provoked by the administration of the serotonin agonist DOI ((+/-)-1-(4-Iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride). The urinary excretion rate rhythm of the melatonin metabolite, 6-sulphatoxymelatonin was delayed by administration of DOI (0.5 mg/kg) at CT18 (6 h after subjective darkness onset) as previously reported by our group. Administration of MK-801 (3 mg/kg) 30 min before DOI blocked the shift in the onset of excretion of the melatonin metabolite on the following nights. Pre-treatment with MK-801 also inhibited by approximately 90% the induction of c-fos in the SCN by DOI at ZT18 (6 h after actual darkness onset) as determined by immunohistochemistry. These results provide evidence for a role of excitatory amino acids in the photomimetic effects of serotonin 5-HT(2C) agonists in the rat.

Emergence of altered circadian timing in a cholinergically supersensitive rat line

Mammalian circadian rhythms are controlled by the suprachiasmatic nuclei (SCN) in concert with light information. Several neurotransmitters and neural pathways modulate light effects on SCN timing. This study used a line of rat with an upregulated cholinergic system to investigate the role of acetylcholine in rhythmicity. With the use of a selective breeding program based on the thermic response to a cholinergic agonist, we developed a supersensitive (S(ox)) and subsensitive (R(ox)) rat line. The S(ox) rats showed an earlier onset time of melatonin rhythm under a 12:12-h light-dark photoperiod from generation 3 (3 +/- 0.5 h after dark) compared with R(ox) rats (4.5 +/- 0.1 h) and an earlier morning decline in temperature (0.9 +/- 0.3 h before lights on) compared with R(ox) animals (0.1 +/- 0.1 h). Furthermore, the S(ox) animals displayed a significantly shorter free-running period of temperature rhythm than R(ox) rats (23.9 +/- 0.04 and 24.3 +/- 0.1 h, respectively, P < 0.05). The results suggest that the altered circadian timing of the S(ox) rats may be related to the cholinergic supersensitivity, intimating a role for acetylcholine in the circadian timing system.

Immunohistochemical localization of serotonin receptors in the rat suprachiasmatic nucleus

Serotonin (5-HT) has been implicated in the regulation of circadian rhythms through its actions on the suprachiasmatic nucleus (SCN). Recent data suggests that, along with excitatory amino acids, serotonin may be important in the neural pathway that mediates the transmission of photic information to the circadian system. The present study uses immunohistochemistry to examine the presence of three different 5-HT receptor subtypes in the suprachiasmatic nucleus (5-HT2a, 5-HT2c and 5-HT7) in male albino Wistar rats. In the SCN, there was a considerable amount of 5-HT2c-receptor-like immunoreactivity, a lesser amount of 5-HT2a positive fibres and no staining with antiserum against the 5-HT7 receptor subtype. These results are compatible with previous pharmacological evidence obtained in our laboratory showing that serotonin acting through the 5-HT2c receptor subtype may be important in the phase shifting effects of light on the circadian system.

Serotonin agonist quipazine induces photic-like phase shifts of the circadian activity rhythm and c-Fos expression in the rat suprachiasmatic nucleus

Nonphotic stimuli can reset and entrain circadian activity rhythms in hamsters and mice, and serotonin is thought to be involved in the phase-resetting effects of these stimuli. In the present study, the authors examined the effect of the serotonin agonist quipazine on circadian activity rhythms in three inbred strains of rats (ACI, BH, and LEW). Furthermore, they investigated the effect of quipazine on the expression of c-Fos in the mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN). Quipazine reduced the amount of running wheel activity for 3 h after treatment, however, no long-term changes in tau and in the activity level were observed. More important, quipazine induced significant phase advances of the activity rhythm and c-Fos production in the SCN at the end of the subjective night (Circadian Time [CT] 22), whereas neither phase shifts nor c-Fos induction were observed during the subjective day. Quipazine injections also resulted in moderate phase delays at the beginning of the subjective night (CT 14). A similar phase-response characteristic typically can be observed for photic stimuli. By contrast, nonphotic stimuli normally produce phase advances during the subjective day. The present results suggest species differences between the hamster and the rat with respect to the serotonergic action on circadian timekeeping and indicate that serotonergic pathways play a role in the transmission of photic information to the SCN of rats.

Nicotine phase shifts the 6-sulphatoxymelatonin rhythm and induces c-Fos in the SCN of rats

The neurotransmitter acetylcholine is not found in the major suprachiasmatic nuclei afferents reported to mediate light effects on entrainment and phase shifts in mammals; however it clearly has some role in the control of circadian rhythmicity. This study examined the effect of the cholinergic agonists nicotine and oxotremorine on (1) the rhythmic production of melatonin using the metabolite, 6-sulphatoxymelatonin as a marker, and (2) the expression of c-Fos protein in the suprachiasmatic nuclei (SCN) of the rat. Nicotine administration (1 mg/kg, s.c.) caused phase delays in the timing of the onset of 6-sulphatoxymelatonin excretion (compared to the pre-treatment night), when administered at circadian time (CT)16 (1.7+/-0.3 h delay) and CT18 (1.7+/-0.2 h delay) but not at CT14 (0.8+/-0.3 h delay), whereas oxotremorine and saline administration had no effect on the timing of the melatonin rhythm. Nicotine administration also caused the induction of c-Fos-like immunoreactivity in the SCN in a dose- and time-dependent manner. Further, pre-treatment with the nicotinic antagonist mecamylamine reduced the number of nicotine-induced c-Fos-positive cells in the SCN by 65%. These data indicate that cholinergic neurons may alter the timing of the onset of melatonin excretion by a direct or indirect effect on the SCN possibly mediated by the nicotinic receptor.

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.

Generation and entrainment of circadian rhythms

1. The present brief review examines some of the new developments in the area of circadian rhythm research. 2. The discovery of the mouse clock and m-per genes and their similarity to other clock genes like per and tim has provided new insight into the control of rhythms in vertebrates. In mice, these genes are expressed in the site of the biological clock, the suprachiasmatic nucleus (SCN), and so will now become a focus of research into the generation of rhythmicity. 3. Because SCN cells expressing endogenous rhythms have a periodicity different from 24 h, there must be mechanisms in place to reset the rhythms on a daily basis. This is achieved in mammals by retinal light perception and neural transmission through several discrete pathways to the SCN. 4. The nature of the neurotransmitters involved in this transfer of environmental information to the timing system is controversial and may even very between similar species but, in the rat, there is compelling evidence that a serotonergic pathway is pre-eminent in mediating the effects of light. How the re-setting is achieved at the cellular level is not known.

Serotonin 5-HT2c agonists mimic the effect of light pulses on circadian rhythms

The serotonin agonist quipazine has been shown to cause phase shifts in melatonin and activity rhythms and to induce c-fos in the suprachiasmatic nucleus of rats. In this study, in vivo pharmacological characterisation of the phase shifting properties of serotonin agonists has been performed, with a view to determining the receptor sub-types involved. Agonists for the 5-HT2a/2c receptors, (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI, 0.1 mg/k), 1-(3-chlorophenyl)-piperazine HCl (mCPP, 2 mg/kg) and N-(3-trifluoromethylphenyl)-piperazine HCl (TFMPP, 2 mg/kg) injected at CT18 resulted in acute transient inhibition of melatonin production and delays in the onset of production on the following nights of 1.2+/-0.2, 1.7+/-0.3 and 1. 4+/-0.8 h respectively. Drugs specific for 5-HT1a/7 and 5-HT3 receptors failed to affect melatonin production. At a dose of 0.07 micromole/kg, the serotonin antagonist, ritanserin inhibited the DOI induced phase delay whereas ketanserin was ineffective at this dose, providing strong evidence that DOI was acting through 5-HT2c receptors. DOI (0.5 mg/kg) at CT18 provoked a phase delay in the core body temperature rhythm of similar magnitude to that following a light pulse. Administration of DOI but not agonists active at other receptor sites resulted in the appearance of c-Fos in the ventrolateral division of the suprachiasmatic nucleus (SCN) at CT18 but not at CT6. Ritanserin was more potent than ketanserin at inhibiting the DOI induced increase in c-Fos labelled cells in the SCN. When rats were pre-treated with metergoline (15 mg/kg), ritanserin (3 mg/kg) or LY 53,857 (3 mg/kg) prior to a 2 lx/ 1 min light pulse, none of the drugs significantly inhibited the responses to light. The results of these experiments indicate that serotonergic agonists active at the 5-HT2c receptor mimic the effects of light on 2 independent rhythms and activate SCN neurones in the rat.