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

Cardiac Sympathetic Nerve Biology and Brain Monoamine Turnover in Panic Disorder

Panic disorder serves as a clinical model for testing whether mental stress can cause heart disease. Our own cardiologic management of panic disorder provides case material of recurrent emergency room attendances with angina and electrocardiogram ischemia, triggered arrhythmias (atrial fibrillation, ventricular fibrillation), and documented coronary artery spasm, in some cases with coronary spasm being complicated by coronary thrombosis. Application of radiotracer catecholamine kinetics and clinical microneurography methodology suggests there is a genetic predisposition to panic disorder that involves faulty neuronal norepinephrine uptake, possibly sensitizing the heart to symptom generation. During panic attacks there are large sympathetic bursts, recorded by clinical microneurography in the muscle sympathetic nerve neurogram, and large increases in cardiac norepinephrine spillover, accompanied by surges of adrenal medullary epinephrine secretion. In other conditions such as heart failure and presumably here also, a high level of sympathetic nervous activation can mediate increased cardiac risk. The sympathetic nerve cotransmitter, neuropeptide Y (NPY), is released from the cardiac sympathetics during panic attacks, an intriguing finding given that NPY can cause coronary artery spasm. There is ongoing, continuous release of epinephrine from the heart in panic sufferers, perhaps attributable to epinephrine loading of cardiac sympathetic nerves by uptake from plasma during panic attacks, or possibly to in situ synthesis of epinephrine through the action of intracardiac phenylethanolamine-N-methytransferase (PNMT) activated by repeated cortisol responses. We have used internal jugular venous sampling and measurement of overflowing lipophilic brain monoamine metabolites to quantify brain norepinephrine and serotonin turnover in untreated patients with panic disorder. We find normal norepinephrine turnover but a marked increase in brain serotonin turnover in patients with panic disorder, in the absence of a panic attack, which presumably represents an underlying neurotransmitter substrate for the condition.

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.

The novel melatonin agonist agomelatine (S20098) is an antagonist at 5-hydroxytryptamine2C receptors, blockade of which enhances the activity of frontocortical dopaminergic and adrenergic pathways

Agomelatine (S20098) displayed pKi values of 6.4 and 6.2 at native (porcine) and cloned, human (h)5-hydroxytryptamine (5-HT)2C receptors, respectively. It also interacted with h5-HT2B receptors (6.6), whereas it showed low affinity at native (rat)/cloned, human 5-HT2A (<5.0/5.3) and 5-HT1A (<5.0/5.2) receptors, and negligible (<5.0) affinity for other 5-HT receptors. In antibody capture/scintillation proximity assays, agomelatine concentration dependently and competitively abolished h5-HT2C receptor-mediated activation of Gq/11 and Gi3 (pA2 values of 6.0 and 6.1). As measured by [3H]phosphatidylinositol depletion, agomelatine abolished activation of phospholipase C by h5-HT2C (pKB value of 6.1) and h5-HT2B (pKB value of 6.6) receptors. In vivo, it dose dependently blocked induction of penile erections by the 5-HT2C agonists (S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (Ro60,0175) and 1-methyl-2-(5,8,8-trimethyl-8H-3-aza-cyclopenta[a]inden-3-yl) ethylamine (Ro60,0332). Furthermore, agomelatine dose dependently enhanced dialysis levels of dopamine in frontal cortex of freely moving rats, whereas they were unaffected in nucleus accumbens and striatum. Although the electrical activity of ventrotegmental dopaminergic neurons was unaffected agomelatine, it abolished their inhibition by Ro60,0175. Extracellular levels of noradrenaline in frontal cortex were also dose dependently enhanced by agomelatine in parallel with an acceleration in the firing rate of adrenergic cell bodies in the locus coeruleus. These increases in noradrenaline and dopamine levels were unaffected by the selective melatonin antagonist N-[2-(5-ethyl-benzo[b]thien-3-yl)ethyl] acetamide (S22153) and likely flect blockade of 5-HT2C receptors inhibitory to frontocortical dopaminergic and adrenergic pathways. Correspondingly, distinction to agomelatine, melatonin showed negligible activity 5-HT2C receptors and failed to modify the activity of adrenergic and dopaminergic pathways. In conclusion, in contrast to melatonin, agomelatine behaves as an antagonist at 5-HT2B and 5-HT2C receptors: blockade of the latter reinforces frontocortical adrenergic and dopaminergic transmission.

5-Hydroxytryptamine7 (5-HT7) receptor immunoreactivity-positive 'stigmoid body'-like structure in developing rat brains

We examined the expression of 5-hydroxytryptamine(7) (5-HT(7)) receptor protein in developing and adult rats with immunohistochemical technique. In adult male rats, 5-HT(7) receptor immunoreactivity was detected in the septum, striatum, indusium griseum, tenia tecta, thalamus, hippocampus and hypothalamus in the forebrain as well as the pons and cerebellum. In brains of 1, 7, 15 and 21 days old male rats but not of adult ones, 5-HT(7) receptor immunoreactivity-positive dot-like structures were detected. The dot-like structures were visualized in hypothalamus, hippocampus, frontal cortex, brainstem and cerebellum at 1 day old male rats. In 7 days old male rats, the dot-like structures were found in the hypothalamus, medial preoptic area (MPA), bed nucleus of the stria terminalis (BST), amygdaloid nucleus and brainstem reticular formation. In 15 and 21 days old male and female rats, 5-HT(7) receptor immunoreactive dots were most clearly detected in MPA, hypothalamus, raphe pallidus, raphe magnus and brainstem reticular formation. The 5-HT(7) receptor immunoreactivity-positive dot-like structures were shown in the cytoplasm and they were less than 1 microm in diameter in 1 and 7 days old rats and became larger to 1-3 microm in 15 and 21 days old rats. From the distribution and morphologic features, the 5-HT(7) receptor immunoreactivity-positive dot-like structure found in developing rat brains is considered to be identical to a cytoplasmic inclusion named 'stigmoid body'.

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.

Serotonin inhibits glutamate- but not PACAP-induced per gene expression in the rat suprachiasmatic nucleus at night

Circadian rhythms of physiology and behaviour generated by the brain's biological clock located in the suprachiasmatic nucleus are entrained by light via the retinohypothalamic tract. Two neurotransmitters, glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP), found in this monosynaptic pathway mediate the effects of light to the clock. It is well known that not only light entrains the clock. Nonphotic cues mediated by neurotransmitters such as serotonin reaching the suprachiasmatic nucleus from the midbrain raphe nucleus modulate light-induced phase shifts at night. Two clock genes, per1 and per2, have been attributed a role in light-induced phase shift. In the present study, using an in vitro brain slice model and quantitative in situ hybridization for per1 and per2, we have shown that serotonin induces per1 gene expression at late subjective night but not at early night. Furthermore, serotonin application before glutamate or PACAP blocked glutamate-induced per1 expression at early night and per2 gene expression at late night. In contrast, serotonin did not influence PACAP-induced per gene expression at late night. Triple antigen immunohistochemistry and confocal microscopy supported both a pre- and post-synaptic interaction of retinohypothalamic tract (PACAP-immunoreactive) and serotonin projections on vasoactive intestinal peptide- and gastrin-releasing peptide-containing cell bodies in the ventro-lateral suprachiasmatic nucleus. Our findings suggest that the per genes could be the molecular target for the modulatory effects of serotonin on light signalling to the clock.

Melatonin and jet lag syndrome: experimental model and clinical implications

What is the effect of melatonin on jet lag syndrome? Jet lag desynchronizes the internal sleep-wakefulness cycle with the environmental light/dark cycle. Advance (but not delay) of light onset is known to abolish pineal N-acetyltransferase activity and urine excretion of 6-sulphatoxymelatonin. Measurements of pineal serotonin, the substrate of melatonin biosynthesis; N-acetylserotonin (NAS), the immediate melatonin precursor; and melatonin (high-performance liquid chromatography-fluorimetric method) in the animal (rat) model of jet lag revealed that prolonged delay of dark-phase onset disrupted the rhytms in comparable ways as the advance of light-phase onset. Advance of dark phase onset resulted in less severe disturbances of rhythms as compared with the advance of light phase onset. Melatonin, but not NAS, injections at the beginning of a new dark period accelerated recovery of NAS and melatonin, but not serotonin, rhythms. Spontaneously hypertensive rats were more sensitive to advance of light onset and less responsive to melatonin injections than normotensive rats. NAS and methylene blue, an inhibitor of monoamine oxidase A, attenuated light-induced disruption of NAS but not melatonin rhythms. We draw the following conclusions from our data: the beginning of the dark period may be preferable to the beginning of light period as the arrival time on eastward flights; the efficacy of melatonin in alleviating jet lag may be enhanced by administering it before, during and after rapid transition through time zones; and hypertension may exaggerate jet lag syndrome.

Case study of circadian rhythm sleep disorder following haloperidol treatment: reversal by risperidone and melatonin

A patient with Gilles de la Tourette syndrome treated with haloperidol, ingested once daily after awakening from sleep, exhibited an irregular sleep-wake pattern with a free-running component of approximately 48 h. Transfer to risperidone, ingested once daily after awakening from sleep, was beneficial resulting in a sleep-wake cycle more synchronized at the appropriate phase to the external zeitgebers, and fewer nocturnal disturbances. The circadian sleep-wake schedule was fully synchronized when the patient had been subsequently treated with melatonin at 21:00h, before intended nocturnal sleep, in addition to risperidone in the morning. Restoration of the sleep-wake circadian pattern was accompanied by the patient's subjective report of significant improvement in his quality of life, social interactions, and occupational status. This observation suggests that circadian rhythm sleep disorders can be related to the typical neuroleptic haloperidol and restored by the atypical neuroleptic risperidone. Similar findings reported in patients suffering from other disorders support the hypothesis that the described disruption of the sleep-wake schedule is medication rather than illness-related. Therefore, it is very important to realize that circadian rhythm sleep disorders may be a side effect of neuroleptics.

Phase response relationships between light pulses and the melatonin rhythm in rats

There is some controversy whether phase response curves constructed from studies conducted after acute release into constant darkness (Type II) or after prolonged constant darkness are comparable. This study investigated the effects of brief low-intensity light pulses on the onset of 6-sulphatoxymelatonin excretion in rats 48 to 60 h after lights-off and after 14 days of continuous darkness. In the former condition, maximum phase delays occurred between 4 and 6 h after expected lights-off, but no phase advances were observed within 2 days of the presentation of the stimulus. When the times of the pulses were plotted in relation to the individual onsets, peak light-induced phase delays occurred 0 to 2 h after melatonin onset. After 14 days in continuous darkness, the peak phase delays also occurred 0 to 2 h after melatonin onset and were slightly but significantly smaller. No significant phase advances were observed. In a separate small series of experiments, the temperature rhythm of rats was shown to be delayed by a comparable degree to that of melatonin by light pulses 2 and 4 h after expected lights-off under the Type II conditions. It is concluded that phase response curves conducted under Type I and Type II conditions are comparable.

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.

5HT7 receptors in the rodent suprachiasmatic nucleus

Serotonergic modulation of circadian rhythms in rodent model preparations has received considerable attention over the past decade. Investigators have also been trying to determine which of the many serotonin receptor subtypes may be mediating the effects of serotonin in the suprachiasmatic nucleus, the location of the biological clock that generates the circadian rhythms. A single study in 1993 using the in vitro rat hypothalamic slice preparation suggested that serotonergic modulation of circadian rhythms at the level of the suprachiasmatic nucleus was acting via the newly discovered 5HT7 receptor subtype. Since that initial claim, serotonin modulation of circadian rhythms at the level of the suprachiasmatic nucleus has generally been attributed to 5HT7 receptor activation. However, when trying to cite relevant literature in support of 5HT7 involvement, it becomes evident that attributing rhythm-related serotonin activity in the suprachiasmatic nucleus to 5HT7 receptors may be somewhat premature. There are issues related to pharmacological specificity, species-specific results, and significant knowledge gaps that necessitate a careful review of the literature to make a judgment as to whether 5HT7 receptors are responsible for serotonergic activity in the rodent suprachiasmatic nucleus. In addition, there is sufficient data available at present to make an initial determination as to the degree of 5HT7 receptor involvement at any level in the generation or modulation of circadian rhythms in rodent species.

Melatonin and development: physiology and pharmacology

This review discusses the development of melatonin rhythmicity in humans and the factors that may alter the appearance of melatonin rhythms. The literature on the possible consequences of disordered melatonin production in relation to Sudden Infant Death Syndrome, fetal origins of adult disease, and scoliosis is critically reviewed. Finally, the emerging use of melatonin to correct sleep disorders in infants and children is reviewed.

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.

Serotonin depletion decreases light induced c-fos in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the locus of the biological clock in mammals. Daily light cycles entrain the endogenous circadian rhythms in mammals through direct and indirect neural pathways from the retinae to the suprachiasmatic nucleus. We have studied the effect of serotonin depletion on the photic induction of the early response gene c-fas in the SCN of rats. Serotonin depletion, verified by immunohistochemistry, produced a significant decrease (42%) in the number of c-FOS positive cells in the ventrolateral portion of the SCN. These results support the involvement of serotonin as a mediator of photic information to the SCN through the retinal projection to the dorsal raphe nucleus.

Biarylcarbamoylindolines are novel and selective 5-HT(2C) receptor inverse agonists: identification of 5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]- 5-pyridyl]carbamoyl]-6-trifluoromethylindoline (SB-243213) as a potential antidepressant/anxiolytic agent

The evolution, synthesis, and biological activity of a novel series of 5-HT(2C) receptor inverse agonists are reported. Biarylcarbamoylindolines have been identified with excellent 5-HT(2C) affinity and selectivity over 5-HT(2A) receptors. In addition, (pyridyloxypyridyl)carbamoylindolines have been discovered with additional selectivity over the closely related 5-HT(2B) receptor. Compounds from this series are inverse agonists at the human cloned 5-HT(2C) receptor, completely abolishing basal activity in a functional assay. The new series have reduced P450 inhibitory liability compared to a previously described series of 1-(3-pyridylcarbamoyl)indolines (Bromidge et al. J. Med. Chem. 1998, 41, 1598) from which they evolved. Compounds from this series showed excellent oral activity in a rat mCPP hypolocomotion model and in animal models of anxiety. On the basis of their favorable biological profile, 32 (SB-228357) and 40 (SB-243213) have been selected for further evaluation to determine their therapeutic potential for the treatment of CNS disorders such as depression and anxiety.

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.

Local effects of the serotonin agonist quipazine on the suprachiasmatic nucleus of rats

In mammals, circadian rhythms of locomotor activity and many other behavioral and physiological functions are controlled by an endogenous pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). One of the SCN's afferents is a dense serotonergic input from the mesencephalic raphe complex. Previous work from this laboratory demonstrated that systemic administrations of the serotonin agonist quipazine mimic the effects of light on the circadian system of rats, i.e. they induce photic-like phase shifts of the circadian activity rhythm as well as c-Fos expression in the SCN. In contrast, no such effect has been demonstrated so far in the isolated rat SCN slice preparation. In this study we demonstrate that local injections of quipazine (0.5 microg/kg) into the region of the SCN induce photic-like effects similar to those induced by systemic injections. These findings suggest a role for 5-HT in the transmission of photic information to the rat circadian system through a direct action at the level of the SCN.

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.

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.