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

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.

Serotonin1A autoreceptor activation by S 15535 enhances circadian activity rhythms in hamsters: evaluation of potential interactions with serotonin2A and serotonin2C receptors

Mammalian circadian activity rhythms are generated by pacemaker cells in the suprachiasmatic nucleus (SCN). As revealed by the actions of diverse agonists, serotonergic input from raphe nuclei generally inhibits photic signaling in the suprachiasmatic nucleus. In contrast, the serotonin (5HT)1A partial agonist, 4-(benzodioxan-5-yl)1-(indan2-yl)piperazine (S 15535), was found to enhance the phase-shifting influence of light on hamster circadian rhythms [Gannon, Neuroscience 119 (2003) 567]. Herein, we extend this observation in showing that S 15535 (5.0 mg/kg, i.p.) markedly (275%) enhanced the light-induced phase shift in circadian activity rhythms: further, this action was dose-dependently abolished by the highly-selective 5HT1A receptor antagonist, WAY 100,635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-2-pyridinyl-cyclohexane-carboxamide maleate) (0.1-0.5 mg/kg, i.p.). WAY 100,635, which was inactive alone, shares the antagonist actions of S 15535 at postsynaptic 5HT1A sites, yet blocks its effects at their presynaptic counterparts. Thus, 5HT1A autoreceptor activation must be involved in this effect of S 15535 which contrasts with the opposite, inhibitory influence upon phase shifts of the "full" agonist, 8-OH-DPAT, which acts by stimulation of postsynaptic 5HT1A receptors [Rea et al., J Neurosci 14 (1994) 3635]. Despite the occurrence of 5HT2A and 5HT2C receptors in the (rat) suprachiasmatic nucleus, their influence on circadian rhythms is unknown since actions of selective ligands have never been evaluated. This issue was investigated with the most selective agents currently available. However, the 5HT2A agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.25 and 0.5 mg/kg), and the 5HT2C agonist, alphaS-6-chloro-5-fluoro-a-methyl-1H-indole-1-ethanamine fumarate (Ro-60-0175) (1.0 and 5.0 mg/kg), failed to affect light-induced phase shifts in hamsters. Moreover, even over broad dose-ranges, the 5HT2A antagonist, (+)-(2,3-dimethoxy-phenyl)-[1-[2-(4-fluoro-phenyl)-ethyl]-piperidin-4-yl]methanol (MDL 100,907) (0.1-1.0 mg/kg), and the 5HT2C antagonist, 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl carbamoyl]indoline (SB 242,084) (1.0-10.0 mg/kg), were likewise inactive. In view of evidence that 5HT2A and 5HT2C sites functionally interact with 5HT1A receptors, we also examined the influence of these agents upon the actions of S 15535, but no significant alteration was seen in its enhancement of rhythms. In conclusion, S 15535 elicits a striking enhancement of light-induced phase shifts in circadian rhythms by specifically recruiting 5HT1A autoreceptors, which leads to suppression of serotonergic input to the suprachiasmatic nucleus. Surprisingly, no evidence for a role of 5HT2A or 5HT2C sites was found, though comparable functional studies remain to be undertaken in rats. Indeed, the present work underlines the importance of comparative studies of circadian rhythms in various species, as well as the need for further study of potential interactions among 5HT receptor subtypes in their control.

Estrogen enhances light-induced activation of dorsal raphe serotonergic neurons

The serotonergic system has been implicated in the modulation of physiological processes including circadian rhythms, learning, memory, mood and food intake. In females, cessation of ovarian function produces deleterious changes in all of these processes and estrogen treatment often ameliorates these conditions. Estrogen may produce these effects by acting on the midbrain raphe, an estrogen-sensitive region that receives direct projections from sensory systems. Here we examined the ability of estradiol to modulate neuronal responses of neurons within raphe nuclei to photic stimulation. Ovariectomized rats treated with estradiol or cholesterol were killed 1 h after the normal onset of light (Zeitgeber time 0) or after a 2-h phase advance (Zeitgeber time 22). In a second study, estradiol-treated ovariectomized rats under constant dark conditions were exposed to light 2 h before the subjective onset of circadian time [(CT)22] and killed 1 h later (CT23). The brains from all animals were processed for Fos and/or serotonin (5-HT) immunocytochemistry. Comparisons showed that the phase shift increased Fos immunoreactivity in all dorsal raphe nucleus (DRN) regions. Although estradiol did not alter the overall number of Fos-positive nuclei, it significantly increased the number of Fos/5-HT double-labelled cells in the medial and lateral DRN. In contrast, neither a phase shift nor estradiol altered the number of Fos-immunoreactive cells or the proportion of Fos-positive 5-HT cells in the median raphe nucleus. Results reveal that the DRN 5-HT system responds to changes in the light : dark cycle and that these responses are modulated by estrogen.

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.

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.

Direct projections from serotonergic neurons in the dorsal and median raphe nuclei of midbrain to the suprachiasmatic nucleus in Tupaia belangeri chinensis

This study investigated the direct serotonergic projections to the suprachiasmatic nucleus (SCN) from the dorsal and median raphe nuclei (DR/MR) of the midbrain in Tupaia belangeri chinensis (TBC) by combined application of retrograde horseradish peroxidase (HRP) tract tracing, immunohistochemistry, and electron microscope techniques. The results provide evidence for the direct projections to the SCN from serotonergic neurons distributed predominantly in the MR (mainly in its lateral portion) and to a lesser degree in the DR (in its ventrolateral portion) more caudally in the midbrain, and the existence of abundant symmetrical and asymmetrical synaptic connections between the serotonergic terminals and the postsynaptic elements in the SCN TBC. The results also revealed that almost all DR neurons projecting to the SCN contained serotonin, whereas about one-half of MR neurons projecting to the SCN were immunoreactive for serotonin.

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.

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.