Serotonergic stimulation and nonphotic phase-shifting in hamsters

Bimodal serotonin synthesis in the diurnal rodent, Arvicanthis ansorgei

In mammals, behavioral activity is regulated both by the circadian system, orchestrated by the suprachiasmatic nucleus (SCN), and by arousal structures, including the serotonergic system. While the SCN is active at the same astronomical time in diurnal and nocturnal species, little data are available concerning the serotonergic (5HT) system in diurnal mammals. In this study, we investigated the functioning of the 5HT system, which is involved both in regulating the sleep/wake cycle and in synchronizing the SCN, in a diurnal rodent, Arvicanthis ansorgei. Using in situ hybridization, we characterized the anatomical extension of the raphe nuclei and we investigated 24 h mRNA levels of the serotonin rate-limiting enzyme, tryptophan hydroxylase 2 (tph2). Under both 12 h:12 h light/dark (LD) and constant darkness (DD) conditions, tph2 mRNA expression varies significantly over 24 h, displaying a bimodal profile with higher values around the (projected) light transitions. Furthermore, we considered several SCN outputs, namely melatonin, corticosterone, and locomotor activity. In both LD and DD, melatonin profiles display peak levels during the biological night. Corticosterone plasma levels show a bimodal rhythmic profile in both conditions, with higher levels preceding the two peaks of Arvicanthis locomotor activity, occurring at dawn and dusk. These data demonstrate that serotonin synthesis in Arvicanthis is rhythmic and reflects its bimodal behavioral phenotype, but differs from what has been previously described in nocturnal species.

Distinct feedback actions of behavioural arousal to the master circadian clock in nocturnal and diurnal mammals

The master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus provides a temporal pattern of sleep and wake that - like many other behavioural and physiological rhythms - is oppositely phased in nocturnal and diurnal animals. The SCN primarily uses environmental light, perceived through the retina, to synchronize its endogenous circadian rhythms with the exact 24 h light/dark cycle of the outside world. The light responsiveness of the SCN is maximal during the night in both nocturnal and diurnal species. Behavioural arousal during the resting period not only perturbs sleep homeostasis, but also acts as a potent non-photic synchronizing cue. The feedback action of arousal on the SCN is mediated by processes involving several brain nuclei and neurotransmitters, which ultimately change the molecular functions of SCN pacemaker cells. Arousing stimuli during the sleeping period differentially affect the circadian system of nocturnal and diurnal species, as evidenced by the different circadian windows of sensitivity to behavioural arousal. In addition, arousing stimuli reduce and increase light resetting in nocturnal and diurnal species, respectively. It is important to address further question of circadian impairments associated with shift work and trans-meridian travel not only in the standard nocturnal laboratory animals but also in diurnal animal models.

Circadian disruption and human health: A bidirectional relationship

Circadian rhythm disorders have been classically associated with disorders of abnormal timing of the sleep-wake cycle, however circadian dysfunction can play a role in a wide range of pathology, ranging from the increased risk for cardiometabolic disease and malignancy in shift workers, prompting the need for a new field focused on the larger concept of circadian medicine. The relationship between circadian disruption and human health is bidirectional, with changes in circadian amplitude often preceding the classical symptoms of neurodegenerative disorders. As our understanding of the importance of circadian dysfunction in disease grows, we need to develop better clinical techniques for identifying circadian rhythms and also develop circadian based strategies for disease management. Overall this review highlights the need to bring the concept of time to all aspects of medicine, emphasizing circadian medicine as a prime example of both personalized and precision medicine.

Serotonin, a possible intermediate between disturbed circadian rhythms and metabolic disease

It is evident that eating in misalignment with the biological clock (such as in shift work, eating late at night and skipping breakfast) is associated with increased risk for obesity and diabetes. The biological clock located in the suprachiasmatic nucleus dictates energy balance including feeding behavior and glucose metabolism. Besides eating and sleeping patterns, glucose metabolism also exhibits clear diurnal variations with higher blood glucose concentrations, glucose tolerance and insulin sensitivity prior to waking up. The daily variation in plasma glucose concentrations in rats, is independent of the rhythm in feeding behavior. On the other hand, feeding itself has profound effects on glucose metabolism, but differential effects occur depending on the time of the day. We here review data showing that a disturbed diurnal eating pattern results in alterations in glucose metabolism induced by a disrupted circadian clock. We first describe the role of central serotonin on feeding behavior and glucose metabolism and subsequently describe the effects of central serotonin on the circadian system. We next explore the interaction between the serotonergic system and the circadian clock in conditions of disrupted diurnal rhythms in feeding and how this might be involved in the metabolic dysregulation that occurs with chronodisruption.

Assessing ethanol's actions in the suprachiasmatic circadian clock using in vivo and in vitro approaches

Research over the past decade has demonstrated substantial interactions between the circadian system and the processes through which alcohol affects behavior and physiology. Here we summarize the results of our collaborative efforts focused on this intersection. Using a combination of in vivo and in vitro approaches, we have shown that ethanol affects many aspects of the mammalian circadian system, both acutely as well as after chronic administration. Conversely, we have shown circadian influences on ethanol consumption. Importantly, we are beginning to delve into the cellular mechanisms associated with these effects. We are also starting to form a picture of the neuroanatomical bases for many of these actions. Finally, we put our current findings into perspective by suggesting new avenues of inquiry for our future efforts.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Chronic unpredictable stress induces a reversible change of PER2 rhythm in the suprachiasmatic nucleus

Many clinical studies have shown that circadian rhythm abnormalities are strongly associated with major depression. The master clock of the circadian system in mammals is located in the suprachiasmatic nucleus (SCN) within the anterior hypothalamus, where Per1 and Per2 are essential core components of circadian rhythm oscillation. Chronic unpredictable stress (CUS) is a reliable animal model of depression with good face, predictive, and constructive validity. In the present study, we investigated the effects of CUS on the circadian expression of PER1 and PER2 in the SCN. We found that CUS led to depressive-like behavior and reduced the amplitude of PER2 oscillation in the SCN, which were blocked by 3 weeks of desipramine (DMI) treatment. 2 weeks after termination of CUS, the decreased peak of PER2 expression returned to control levels, whereas depressive-like behavior remained unchanged. Our findings suggest that the dampened amplitude of PER2 expression in the SCN may participate in the development of depressive-like behavior induced by CUS but is unlikely involved in the long-lasting effects of CUS on depressive-like behavior.

Circadian clock resetting in the mouse changes with age

The most widely recognised consequence of normal age-related changes in biological timing is the sleep disruption that appears in old age and diminishes the quality of life. These sleep disorders are part of the normal ageing process and consist primarily of increased amounts of wakefulness and reduced amounts of deep sleep. Changes in the amplitude and timing of the sleep-wake cycle appear to represent, at least in part, a loss of effective circadian regulation of sleep. Understanding alterations in the characteristics of stimuli that help to consolidate internal rhythms will lead to recommendations to improve synchronisation in old age. Converging evidence from both human and animal studies indicate that senescence is associated with alterations in the neural structure thought to be primarily responsible for the generation of the circadian oscillation, the suprachiasmatic nuclei (SCN). Work has shown that there are changes in the anatomy, physiology and ability of the clock to reset in response to stimuli with age. Therefore it is possible that at least some of the observed age-related changes in sleep and circadian timing could be mediated at the level of the SCN. The SCN contain a circadian clock whose activity can be recorded in vitro for several days. We have tested the response of the circadian clock to a number of neurochemicals that reset the clock in a manner similar to light, including glutamate, N-methyl-D-aspartate (NMDA), gastrin-releasing peptide (GRP) and histamine (HA). In addition, we have also tested agents which phase shift in a pattern similar to behavioural 'non-photic' signals, including neuropeptide Y (NPY), serotonin (5HT) and gamma-aminobutyric acid (GABA). These were tested on the circadian clock in young and older mice (approximately 4 and 15 months old). We found deficits in the response to specific neurochemicals but not to others in our older mice. These results indicate that some changes seen in the responsiveness of the circadian clock to light with age may be mediated at the level of the SCN. Further, the responsiveness of the circadian clock with age is attenuated to some, but not all stimuli. This suggests that not all clock stimuli lose their effectiveness with age, and that it may be possible to compensate for deficits in clock performance by enhancing the strength of those stimulus pathways which are intact.

Acute ethanol impairs photic and nonphotic circadian phase resetting in the Syrian hamster

Disrupted circadian rhythmicity is associated with ethanol (EtOH) abuse, yet little is known about how EtOH affects the mammalian circadian clock of the suprachiasmatic nucleus (SCN). Clock timing is regulated by photic and nonphotic inputs to the SCN involving glutamate release from the retinohypothalamic tract and serotonin (5-HT) from the midbrain raphe, respectively. Our recent in vitro studies in the SCN slice revealed that EtOH blocks photic phase-resetting action of glutamate and enhances the nonphotic phase-resetting action of the 5-HT1A,7 agonist, 8-OH-DPAT. To explore the basis of these effects in the whole animal, we used microdialysis to characterize the pharmacokinetics of intraperitoneal injection of EtOH in the hamster SCN extracellular fluid compartment and then studied the effects of such EtOH treatment on photic and serotonergic phase resetting of the circadian locomotor activity rhythm. Peak EtOH levels (approximately 50 mM) from a 2 g/kg injection occurred within 20-40 min with a half-life of approximately 3 h. EtOH treatment dose-dependently attenuated photic phase advances but had no effect on phase delays and, contrary to in vitro findings, markedly attenuated 8-OH-DPAT-induced phase advances. In a complementary experiment using reverse microdialysis to deliver a timed SCN perfusion of EtOH during a phase-advancing light pulse, the phase advances were blocked, similar to systemic EtOH treatment. These results are evidence that acute EtOH significantly affects photic and nonphotic phase-resetting responses critical to circadian clock regulation. Notably, EtOH inhibition of photic signaling is manifest through direct action in the SCN. Such actions could underlie the disruption of circadian rhythmicity associated with alcohol abuse.

NAN-190 potentiates the circadian response to light and speeds re-entrainment to advanced light cycles

Health problems can arise from de-synchrony between the external environment and the endogenous circadian rhythm, yet the circadian system is not able to quickly adjust to large, abrupt changes in the external daily cycle. In this study, we investigated the ability of NAN-190 to potentiate the circadian rhythm response to light as measured by phase of behavioral activity rhythms. NAN-190 (5 mg/kg, i.p.) was able to significantly potentiate the response to light both in dark-adapted and entrained hamsters. Furthermore, NAN-190 was effective even when administered up to 6 h after light onset. Response to a light pulse was both greater in magnitude and involved fewer unstable transient cycles. Finally, NAN-190 was able to speed re-entrainment to a 6 h advance of the light/dark cycle by an average of 6 days when compared with vehicle-treated animals. This work suggests that compounds like NAN-190 may hold great potential as a pharmaceutical treatment for jetlag, shift work, and other circadian disorders.

Minireview: Entrainment of the suprachiasmatic clockwork in diurnal and nocturnal mammals

Daily rhythmicity, including timing of wakefulness and hormone secretion, is mainly controlled by a master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN clockwork involves various clock genes, with specific temporal patterns of expression that are similar in nocturnal and diurnal species (e.g. the clock gene Per1 in the SCN peaks at midday in both categories). Timing of sensitivity to light is roughly similar, during nighttime, in diurnal and nocturnal species. Molecular mechanisms of photic resetting are also comparable in both species categories. By contrast, in animals housed in constant light, exposure to darkness can reset the SCN clock, mostly during the resting period, i.e. at opposite circadian times between diurnal and nocturnal species. Nonphotic stimuli, such as scheduled voluntary exercise, food shortage, exogenous melatonin, or serotonergic receptor activation, are also capable of shifting the master clock and/or modulating photic synchronization. Comparison between day- and night-active species allows classifications of nonphotic cues in two, arousal-independent and arousal-dependent, families of factors. Arousal-independent factors, such as melatonin (always secreted during nighttime, independently of daily activity pattern) or gamma-aminobutyric acid (GABA), have shifting effects at the same circadian times in both nocturnal and diurnal rodents. By contrast, arousal-dependent factors, such as serotonin (its cerebral levels follow activity pattern), induce phase shifts only during resting and have opposite modulating effects on photic resetting between diurnal and nocturnal species. Contrary to light and arousal-independent nonphotic cues, arousal-dependent nonphotic stimuli provide synchronizing feedback signals to the SCN clock in circadian antiphase between nocturnal and diurnal animals.

Serotonergic mediation of constant light-potentiated nonphotic phase shifting of the circadian locomotor activity rhythm in Syrian hamsters

Short-term (1–3 days) constant light exposure (brief LL) potentiates nonphotic phase shifting induced by sleep deprivation and serotonin (5-HT) agonist stimulation. The present assessments reveal that exposure to brief LL markedly alters the magnitude and shape of the 5-HT1A,7 receptor agonist, 8-(+)2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahyronapthalene (8-OH-DPAT) phase-response curve, facilitating (∼12 h) phase-advance shifts during the early morning when serotonergics have no phase-shifting effect. Brief LL also reduces the threshold for 8-OH-DPAT shifting at midday, evidenced by 5- to 6-h phase-advance shifts elicited by dosages that have no effect without the LL treatment. The brief LL-potentiated phase advances to intraperitoneal 8-OH-DPAT at zeitgeber time 0 (ZT 0) were blocked by the 5-HT1A antagonists, pindolol and WAY 100635, indicating that this shifting is mediated by 5-HT1A receptors. Antagonists with action at 5-HT7 receptors, including ritanserin and metergoline, were without effect. Although autoradiographic analyses of [3H]8-OH-DPAT binding indicate that brief LL does not upregulate suprachiasmatic nucleus (SCN) 5-HT1A receptor binding, intra-SCN microinjection of 8-OH-DPAT at ZT 0 in brief LL-exposed hamsters induced shifts similar to those produced by intraperitoneal injection, suggesting that SCN 5-HT1A receptors mediate potentiated 8-OH-DPAT-induced shifts during the early morning. Lack of shifting by intra-SCN 8-OH-DPAT at ZT 6 or 18 (when intraperitoneal 8-OH-DPAT induces large shifts), further indicates that brief LL-potentiated shifts at these time points are mediated by 5-HT target(s) outside the SCN. Significantly, sleep deprivation-induced phase-advance shifts potentiated by brief LL (∼9 h) at ZT 0 were blocked by pindolol, suggesting that these behavioral shifts could be mediated by the same SCN 5-HT1A receptor phase-resetting pathway as that activated by 8-OH-DPAT treatment.

Short-term constant light potentiation of large-magnitude circadian phase shifts induced by 8-OH-DPAT: effects on serotonin receptors and gene expression in the hamster suprachiasmatic nucleus

Nonphotic phase-shifting of mammalian circadian rhythms is thought to be mediated in part by serotonin (5-HT) acting in the suprachiasmatic nucleus (SCN) circadian clock. Previously we showed that brief (1-3 days) exposure to constant light (LL) greatly potentiates nonphotic phase-shifting induced by the 5-HT agonist, (+/-)2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT). Here we investigated potential mechanisms for this action of LL, including 5-HT receptor upregulation and SCN clock gene and neuropeptide gene expression. Autoradiographic analysis of ritanserin inhibition of [3H]8-OH-DPAT binding indicated that LL (approximately 2 days) did not affect 5-HT7 receptor binding in the SCN or dorsal raphe. Measurement of 5-HT1A autoreceptors in the median raphe and 5-HT1B receptors in the SCN also showed no effect of LL. In experiment 2, hamsters held under a 14-h light : 10-h dark photocycle (LD) or exposed to LL for approximately 2 days received an intraperitoneal injection of 8-OH-DPAT or vehicle at zeitgeber time (ZT) 6 or 0 and were killed after 2 h of dark exposure. 8-OH-DPAT suppressed SCN Per1 and Per2 mRNAs at both ZTs, as assessed by in situ hybridization. Per1 mRNA was also suppressed by LL alone. In addition, in situ hybridization of arginine vasopressin (AVP) mRNA and vasoactive intestinal polypeptide mRNA showed that LL significantly suppressed the former but not the latter. The LL-induced suppression of SCN Per1 mRNA and AVP mRNA may be involved in LL-induced potentiation of pacemaker resetting, especially as these data provide additional evidence that LL suppresses circadian pacemaker amplitude, thus rendering the clock more susceptible to phase-shifting stimuli.

MDMA alters the response of the mammalian circadian clock in hamsters: effects on re-entrainment and triazolam-induced phase shifts

Serotonin (5-hydroxytryptamine or 5-HT) is a neurotransmitter that is involved in a wide range of behavioural and physiological processes. Previous work has indicated that serotonin is important in the regulation of the circadian clock, which is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy'), which is widely used as a recreational drug of abuse, is a serotonin neurotoxin in animals and non-human primates. Previous work has shown that MDMA exposure can alter circadian clock function both in vitro and in vivo. Evidence shows that 5-HT may have a modulatory role in the regulation of the circadian clock by non-photic stimuli, such as the benzodiazepine triazolam (TRZ). Triazolam is a short-acting benzodiazepine that results in phase advances of the wheel running activity in hamsters when administered during the mid-subjective day. In the present study, male Syrian hamsters treated with TRZ (5 mg/kg) at ZT6 significantly phase advanced their clock. Treatment with MDMA significantly diminished the TRZ induced phase shift in hamsters. Previous evidence shows the involvement of 5-HT in the re-synchronisation of the endogenous clock to a new shifted light-dark cycle. Untreated animals were successfully entrained to a new, 6 h advanced light-dark cycle within an average of 4.5 +/- 0.1 days. Following treatment with MDMA, these animals took an average of 8.3 +/- 0.1 days to re-entrain to a shifted environmental cycle. Immunohistochemical analysis revealed that animals treated with MDMA showed reduced serotonin staining, as evidenced by a decrease in innervation density in the SCN. No significant differences were found in cell counts within the raphe nuclei. These results demonstrate the importance of the serotonergic system in the modulation of photic and non-photic responses of the circadian pacemaker.

Circadian control during the day and night: Role of neuropeptide Y Y5 receptors in the suprachiasmatic nucleus

Circadian rhythms are reset by light during the night or by nonphotic stimuli during the day. Neuropeptide Y (NPY), which appears to mediate at least some nonphotic phase shifts by its actions in the suprachiasmatic nucleus (SCN), induces phase advances during the day and inhibits light-induced phase advances during the night. In this study, we used a highly selective Y5-like agonist to test whether activation of NPY Y5 receptors is sufficient to mimic NPY during the day and late night in Syrian hamsters. We also tested whether NPY in the early night reduces light-induced phase delays in a dose-dependent manner. Microinjection of a selective Y5 receptor agonist, (Ala(31), Aib(32))-NPY, into the SCN significantly inhibited light-induced phase advances during the late night, but did not induce phase advances during the day. In addition, concentrations of NPY ranging from 0.23 to 23 mM did not attenuate light-induced phase delays in the early night. These results suggest that activation of Y5-like receptors is sufficient to inhibit light-induced phase advances during the late night but is not sufficient to induce phase advances during the day. Furthermore, this study provided no evidence that NPY can inhibit light-induced phase shifts early in the night.

Short-term exposure to constant light promotes strong circadian phase-resetting responses to nonphotic stimuli in Syrian hamsters

Behavioral (nonphotic) stimuli can shift circadian rhythms by serotonin (5-HT) and/or neuropeptide Y (NPY) inputs to the suprachiasmatic nucleus (SCN) circadian clock. Based on the idea that behavioral phase resetting is modulated by endogenous changes in postsynaptic sensitivity to such transmitters, hamsters were exposed to constant light (LL; approximately 250 lx) for 1-3 days, which suppresses locomotor activity and eliminates the daily rhythm of SCN 5-HT release measured by microdialysis. Groups subjected to brief LL or maintained under a light/dark cycle (LD) received phase-resetting treatments with the 5-HT(1A,7) agonist (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) or sleep deprivation (SD). Animals were released to constant darkness at the start of the treatments. Phase advances to 8-OH-DPAT and SD during the day were 11 and 3 h for LL vs. 2 and 1 h for LD, respectively. Phase delays during the night were -12 and -5 h for LL vs. no responses for LD, respectively. Phase-transition curves for both LL treatments had slopes approximating 0, indicative of Type 0 phase resetting. For all treatments, the degree of locomotor suppression by LL was not correlated with the phase shift magnitude. Re-establishing locomotor activity by overnight food deprivation did not prevent potentiated shifting to SD. However, re-establishing peak extracellular 5-HT levels by intra-SCN 5-HT reverse microdialysis perfusion in LL did significantly reduce potentiated 8-OH-DPAT phase advances. Constant light also enhanced intra-SCN NPY-induced phase advances during the day (6 vs. 2 h for LD). These results suggest that LL promotes Type 0 phase resetting by supersensitizing 5-HT and/or NPY postsynaptic responses and possibly by attenuating the amplitude of the circadian pacemaker, thus enhancing circadian clock resetting nonspecifically.

Serotonergic serotonin (1A) mixed agonists/antagonists elicit large-magnitude phase shifts in hamster circadian wheel-running rhythms

The biological clock that generates circadian rhythms in mammals is located within the suprachiasmatic nuclei at the base of the hypothalamus. The circadian clock is entrained to the daily light/dark cycle by photic information from the retina. The retinal input to the clock is inhibited by exogenously applied serotonin agonists, perhaps mimicking an endogenous inhibitory serotonergic input to the clock arriving from the midbrain raphe. In the present study, a unique class of serotonergic compounds was tested for its ability to modulate retinal input to the circadian clock. The serotonergic ligands 8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-8-azaspiro(4.5)decane-7,9-dione dihydrochloride (BMY 7378), S 15535, and 8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro[4.5]decane-7,9-dione hydrochloride (MDL 73005 EF) can all be classified as mixed agonists/antagonists at type 1A serotonin receptors. Circadian wheel-running activity rhythms were monitored in Syrian hamsters maintained in constant darkness. Dim white-light pulses administered to the hamsters at circadian time 19 advanced the phase of their running rhythms by 1-2 h. Injection of BMY 7378, S 15535, and to a lesser degree MDL 73005 EF, prior to the light pulses resulted in phase advances from 5 to 6 h, and by as much as 8 h. Neither BMY 7378 nor S 15535 had any effect on light-induced phase delays in hamster activity rhythms at circadian time 14. Further, BMY 7378 is able to phase advance circadian rhythms by approximately 1 h at night even without light exposure. Finally, the effects of BMY 7378 on circadian rhythms is opposite to that observed with the prototypical serotonin 1A agonist (+/-)-8-hydroxy-2-(DI-n-propyl-amino)tetralin hydrobromide (8-OH-DPAT) (8-OH-DPAT elicits non-photic phase advances in the day and inhibits photic-induced phase advances at night). These results suggest that pharmacologically blocking raphe input to the suprachiasmatic circadian clock results in substantially larger photically induced phase advances in wheel-running rhythms. This is further evidence that raphe input to the circadian clock is probably acting to dampen the clock's response to light under certain conditions. The large-magnitude phase shifts, and temporal-activity profile seen with BMY 7378 and S 15535, suggest that compounds with this unique pharmacological profile may be beneficial in the treatment of circadian phase delays recently reported to be a complication resulting from Alzheimer's disease.

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.

MDMA alters the response of the circadian clock to a photic and non-photic stimulus

3,4-Methylenedioxymethamphetamine (MDMA or 'Ecstasy') is a widely used recreational drug that damages serotonin 5-HT neurons in animals and possibly humans. Published literature has shown that the serotonergic system is involved in photic and non-photic phase shifting of the circadian clock, which is located in the suprachiasmatic nuclei. Despite the dense innervation of the circadian system by 5-HT and the known selective neurotoxicity of MDMA, little is known about the effects of MDMA on the circadian oscillator. This study investigated whether repeated exposure to the serotonin neurotoxin MDMA alters the behavioural response of the Syrian hamster to phase shift to the serotonin 5-HT1A/7 receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT). This agonist was administered under an Aschoff Type I (CT8) and Aschoff Type II (ZT8) paradigm (5 mg/kg) and was given before and after treatment with MDMA (10, 15 and 20 mg/kg administered on successive days). Pre-treatment with MDMA significantly attenuated phase shifts to 8-OH-DPAT. We also tested the ability of the clock to phase shift to a photic stimulus after treatment with MDMA. A 15-min light pulse (mean lux 125 at CT14 or ZT14) was administered before and after treatment with MDMA. Phase shifts to a photic stimulus were significantly attenuated by pre-treatment with MDMA. Our study demonstrates that repeated exposure to MDMA may alter the ability of the circadian clock to phase shift to a photic and non-photic stimulus in the hamster. Disruption of circadian function has been linked with a variety of clinical conditions such as sleep disorders, mood, concentration difficulties and depression, consequently outlining the potential dangers of long-term ecstasy use.

Anterior paraventricular thalamus modulates light-induced phase shifts in circadian rhythmicity in rats

The reciprocal connections between the paraventricular thalamic nucleus (PVT) and the suprachiasmatic nuclei suggest that PVT may participate in the regulation of circadian rhythms. We studied in rats the effect of lesions of the anterior and midposterior regions of the PVT on phase shifts of drinking circadian rhythm induced by light pulses at circadian times 6, 12, and 23, as well as the phase shifts produced by electrical or glutamatergic stimulation of the anterior PVT at the same circadian times. Lesion of the anterior PVT abolishes the advances induced by light during late subjective night, whereas midposterior PVT lesions did not affect the phase shifts. Electrical stimulation or glutamate injections in the anterior PVT mimic the phase-shifting effects of light pulses. These results indicate the participation of the anterior PVT as a modulator of entrainment of circadian rhythms to light.

In vivo resetting of the hamster circadian clock by 5-HT7 receptors in the suprachiasmatic nucleus

Serotonin (5-HT) has been strongly implicated in the regulation of the mammalian circadian clock located in the suprachiasmatic nuclei (SCN); however, its role in behavioral (nonphotic) circadian phase resetting remains elusive. Central to this issue are divergent lines of evidence that the SCN may, or may not, be a target for the phase-resetting effects of 5-HT. We have addressed this question using a novel reverse-microdialysis approach for timed perfusions of serotonergic and other agents to the Syrian hamster SCN with durations equivalent to the increases in in vivo 5-HT release during phase-resetting behavioral manipulations. We found that 3 hr perfusions of the SCN with either 5-HT or the 5-HT(1A,7) receptor agonist 2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydro-naphthalene (8-OH-DPAT) at midday advanced the phase of the free-running circadian rhythm of wheel-running assessed using an Aschoff type II procedure. Phase shifts induced by 8-OH-DPAT were enhanced more than threefold by pretreatment with the 5-HT synthesis inhibitor para-chlorophenylalanine. Phase advances induced by SCN 8-OH-DPAT perfusion were significantly inhibited by the 5-HT(2,7) receptor antagonist ritanserin and by the more selective 5-HT(7) receptor antagonist DR4004, implicating the 5-HT(7) receptor in mediating this phase resetting. Concurrent exposure to light during the 8-OH-DPAT perfusion abolished the phase advances. Furthermore, coperfusion of the SCN with TTX, which blocked in vivo 5-HT release, did not suppress intra-SCN 8-OH-DPAT-induced phase advances. These results indicate that 5-HT(7) receptor-mediated phase resetting in the SCN is markedly influenced by the degree of postsynaptic responsiveness to 5-HT and by photic stimulation. Finally, 5-HT may act directly on SCN clock cells to induce in vivo nonphotic phase resetting.

Melatonin or a melatonin agonist corrects age-related changes in circadian response to environmental stimulus

The effects of a melatonin agonist, S-20098, included in the diet were tested on a specific effect of aging in hamsters: the marked decline in the phase shifting effects of a 6-h pulse of darkness on a background of constant light. In contrast to young hamsters, old hamsters fed with the control diet showed little or no phase shifts in response to a dark pulse presented in the middle of their inactive or active period. Old hamsters fed with S-20098 showed phase shifts that were ~70% of the ones in young animals and significantly greater than those in old controls. The phase advancing response to a dark pulse presented during the inactive period was dose dependent and reversed after S-20098 discontinuation. Melatonin included in the diet showed comparable restorative effects on the phase shifting response to a dark pulse in old hamsters. Replacement therapy with melatonin or melatonin-related compounds could prove useful in treating, preventing, or delaying disturbances of circadian rhythmicity and/or sleep in older people.

Localization of 5-HT(7) receptors in rat brain by immunocytochemistry, in situ hybridization, and agonist stimulated cFos expression

5-HT(7) receptors are recently identified members of the serotonin receptor family that have moderate to high affinity for several important psychotropic drugs. However, the lack of selective ligands has impeded the study of the brain distribution of these receptors. In this report, we describe the localization of 5-HT(7) receptor in rat forebrain by immunocytochemistry, in situ hybridization of 5-HT(7) mRNA, and functional stimulation of cFOS expression by 5-HT(7) receptor activation. The anatomical localization of 5-HT(7) mRNA in situ hybridization signal. Prominent immunostaining was apparent in numerous sites within the cerebral cortex, hippocampal formation, tenia tecta, thalamus and hypothalamus. 5-HT(7) receptors were detected in suprachiasmatic nucleus by both immunocytochemistry and in situ hybridization. At a microscopic level, both cell bodies and proximal fibers were strongly stained in these regions, suggesting a somatodendritic subcellular distribution. 5-HT(7) receptor-like immunoreactivity was further compared with 5-HT(7) mediated biological function by administering 8-OH-DPAT intracerebroventricular injection (icv)with WAY 100135 (to block 5-HT(1A) receptors) followed by double immunostaining localization of cFos activation and 5-HT(7) receptors. In all regions examined, cFos stimulation and 5-HT(7)-like immunoreactivity colocalized to the same neurons. Furthermore, cFos activation by 8-OH-DPAT was blocked by pimozide--a 5-HT(7) antagonist. Therefore, by using multiple strategies, we were able to localize 5-HT(7) receptors in rat brain unequivocally. The distribution of these receptors is consistent with their involvement in the control of circadian activity and the action of anti-depressants and atypical neuroleptics.

Changes in 5-HT(7) serotonin receptor mRNA expression with aging in rat brain

We examined 5-HT(7) receptor mRNA expression with in situ hybridization histochemistry in the brains of young (3 months), middle-aged (12 months) and old rats (24 months). In the ventral CA3 area of the hippocampus 5-HT(7) mRNA expression is reduced by approximately 30% between young and middle age without further decline between middle and old age. In other brain areas 5-HT(7) mRNA expression is unaffected by age.

Dorsal raphe nuclear stimulation of SCN serotonin release and circadian phase-resetting

Serotonin (5-HT) is strongly implicated in the regulation of mammalian circadian rhythms. However, little is known of the functional relationship between the circadian clock located in the suprachiasmatic nucleus (SCN) and its source of serotonergic innervation, the midbrain raphe nuclei. In previous studies, we reported that electrical stimulation of the dorsal or median raphe nuclei (DRN and MRN, respectively) induced 5-HT release in the SCN. Notably, DRN- but not MRN-stimulated 5-HT release was blocked by the 5-HT(1,2,7) antagonist, metergoline, suggesting that the DRN signals to the SCN indirectly via the activation of a 5-HT-responsive multisynaptic pathway. In the present study, pretreatment with the 5-HT(2,7) antagonist, ritanserin, also significantly inhibited DRN-electrically stimulated SCN 5-HT release. However, pretreatment with the 5-HT(1A) antagonist, NAN-190, or the 5-HT(2) antagonists ketanserin and cinanserin had little suppressive effect on this DRN-stimulated 5-HT release. In complementary behavioral trials, electrical stimulation of the DRN during subjective midday caused a 1.3-h advance in the free-running circadian activity rhythm under constant darkness, which was inhibited by metergoline. Collectively, these results are evidence that: (1) DRN-stimulated 5-HT release in the SCN requires the activation of an intermediate target with receptors having 5-HT(7) pharmacological characteristics; (2) electrical stimulation of the DRN induces phase-resetting of the circadian activity rhythm; and (3) activation of 5-HT receptors is necessary for this DRN-stimulated circadian phase-resetting. In view of the dynamic changes in DRN neuronal activity incumbent with the daily sleep-activity cycle, and its functional linkages to the SCN and intergeniculate leaflet, the DRN could serve to provide behavioral/arousal state information to various sites comprising the brain circadian system.

Serotonergic modulation of astrocytic activity in the hamster suprachiasmatic nucleus

The present study was undertaken to explore the effects of central serotonin receptor activation on the expression of glial fibrillary acidic protein in the suprachiasmatic nucleus of Syrian hamsters. Immunoblot and immunohistochemical procedures were used to examine the effects of systemic application of the serotonin-1A and serotonin-7 receptor agonist, (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT; 3.75 mg/kg) on the contents and distribution of glial fibrillary acidic protein in the suprachiasmatic nucleus. Administration of 8-OH-DPAT at midday caused a significant reduction in immunoreactive glial fibrillary acidic protein content within 1 h of injection, compared to vehicle controls. This effect was not evident 3 h after drug injection. Treatment with 8-OH-DPAT during the late dark phase had little effect on glial fibrillary acidic protein content. The 8-OH-DPAT-induced reduction in glial fibrillary acidic protein content seen at midday was blocked partially by pretreatment with the serotonin-2 and serotonin-7 receptor antagonist, ritanserin, and more substantially by pretreatment with the serotonin-1A receptor antagonist, NAN-190. Treatment with 8-OH-DPAT also caused a significant redistribution of immunoreactive glial fibrillary acidic protein, such that the dense mesh-like appearance seen in vehicle controls was significantly reduced. The 8-OH-DPAT treatment also significantly decreased expression of polysialic acid, a cell-surface molecule associated with neural plasticity. Immunoblot assessments of glial fibrillary acidic protein contents 2 h before and 1 h after lights off revealed a significant time-of-day difference in glial fibrillary acidic protein expression, with lowest levels occurring at the latter time-point, associated with maximal endogenous serotonin release in the suprachiasmatic nucleus. Collectively, these results indicate that acute plastic changes in glial fibrillary acidic protein-related astrocytic activity in the suprachiasmatic nucleus can be induced in response to serotonin-7 or serotonin-1A receptor activation in a phase-dependent manner. It is interesting to speculate that circadian reorganizations in astrocytic activity could be regulated by the daily rhythm in serotonin release in the suprachiasmatic nucleus.

Suprachiasmatic nucleus and intergeniculate leaflet in the diurnal rodent Octodon degus: retinal projections and immunocytochemical characterization

The neural connections and neurotransmitter content of the suprachiasmatic nucleus and intergeniculate leaflet have been characterized thoroughly in only a few mammalian species, primarily nocturnal rodents. Few data are available about the neural circadian timing system in diurnal mammals, particularly those for which the formal characteristics of circadian rhythms have been investigated. This paper describes the circadian timing system in the diurnal rodent Octodon degus, a species that manifests robust circadian responses to photic and non-photic (social) zeitgebers. Specifically, this report details: (i) the distribution of six neurotransmitters commonly found in the suprachiasmatic nucleus and intergeniculate leaflet; (ii) the retinohypothalamic tract; (iii) the geniculohypothalamic tract; and (iv) retinogeniculate projections in O. degus. Using immunocytochemistry, neuropeptide Y-immunoreactive, serotonin-immunoreactive and [Met]enkephalin-immunoreactive fibers and terminals were detected in and around the suprachiasmatic nucleus; vasopressin-immunoreactive cell bodies were found in the dorsomedial and ventral suprachiasmatic nucleus; vasoactive intestinal polypeptide-immunoreactive cell bodies were located in the ventral suprachiasmatic nucleus; [Met]enkephalin-immunoreactive cells were located sparsely throughout the suprachiasmatic nucleus; and substance P-immunoreactive fibers and terminals were detected in the rostral suprachiasmatic nucleus and surrounding the nucleus throughout its rostrocaudal dimension. Neuropeptide Y-immunoreactive and [Met]enkephalin-immunoreactive cells were identified in the intergeniculate leaflet and ventral lateral geniculate nucleus, as were neuropeptide Y-immunoreactive, [Met]enkephalin-immunoreactive, serotonin-immunoreactive and substance P-immunoreactive fibers and terminals. The retinohypothalamic tract innervated both suprachiasmatic nuclei equally; in contrast, retinal innervation to the lateral geniculate nucleus, including the intergeniculate leaflet, was almost exclusively contralateral. Bilateral electrolytic lesions that destroyed the intergeniculate leaflet depleted the suprachiasmatic nucleus of virtually all neuropeptide Y- and [Met]enkephalin-stained fibers and terminals, whereas unilateral lesions reduced fiber and terminal staining by approximately half. Thus, [Met]enkephalin-immunoreactive and neuropeptide Y-immunoreactive cells project equally and bilaterally from the intergeniculate leaflet to the suprachiasmatic nucleus via the geniculohypothalamic tract in degus. This is the first report examining the neural circadian system in a diurnal rodent for which formal circadian properties have been described. The data indicate that the neural organization of the circadian timing system in degus resembles that of the most commonly studied nocturnal rodents, golden hamsters and rats. Armed with such data, one can ascertain differences in the functional organization of the circadian system between diurnal and nocturnal mammals.

Electrical stimulation of the median or dorsal raphe nuclei reduces light-induced FOS protein in the suprachiasmatic nucleus and causes circadian activity rhythm phase shifts

Several pharmacological studies have suggested that the large median raphe serotonergic projection to the circadian clock in the suprachiasmatic nucleus may modulate circadian rhythm phase. The present experiments studied the role of dorsal and median raphe nuclei as regulators of circadian rhythmicity by evaluating the ability of electrical stimulation to shift rhythm phase or to alter photic induction of FOS protein synthesis. Male hamsters implanted with bipolar electrodes in either the median or dorsal raphe nucleus were stimulated during the early subjective night coincident with exposure to a saturating light pulse. About 90 min later, animals were anesthetized, perfused and the brains processed for FOS protein immunoreactivity. As previously demonstrated, light alone induces FOS immunoreactivity in nuclei of suprachiasmatic nucleus neurons. This was significantly attenuated by stimulation of either the median or dorsal raphe nucleus, with the extent of attenuation proportional to the intensity of stimulation. Electrical stimulation without light exposure had no effect on FOS expression. The effect of light on FOS expression in the suprachiasmatic nucleus was not modified by pre-treatment with the 5-HT1/2 serotonin receptor antagonist, metergoline, although it greatly reduced electrical stimulation-induced FOS expression in the hippocampus. In a second experiment, hamsters housed with running wheels in constant light were electrically stimulated in the median or dorsal raphe nucleus 6 h prior to (CT6) or 2 h after (CT14) expected activity onset. Regardless of which raphe nucleus was electrically stimulated, approximately 22 min phase advances were elicited at CT6 and 36 min phase delays were elicited at CT14. Despite the fact that the sole direct projection from the raphe complex to the suprachiasmatic nucleus is from the median nucleus, the present data do not distinguish between the median and dorsal raphe with respect to their impact on circadian rhythm regulation. Instead, two possible roles for each raphe nucleus are demonstrated. One main effect is that both raphe nuclei modulate rhythm phase. The second is an interaction between raphe efferent activity and light which, in the present studies, is demonstrated by the ability of raphe stimulation to modulate the action of light on the circadian system. While serotonin is a likely neurotransmitter mediating one or both effects, alternatives such as GABA, must be considered.

In vivo assessment of the midbrain raphe nuclear regulation of serotonin release in the hamster suprachiasmatic nucleus

Serotonin (5-HT) plays important regulatory roles in mammalian circadian timekeeping; however, little is known concerning the regulation of serotonergic activity in the circadian clock located in the suprachiasmatic nuclei (SCN). By using in vivo microdialysis to measure 5-HT release we demonstrated that electrical or pharmacological stimulations of the dorsal or median raphe nuclei (DRN and MRN, respectively) can alter basal release of 5-HT in the hamster SCN. There were similar increases in SCN 5-HT release after electrical stimulation of either the MRN or DRN, indicating that both could contribute to the serotonergic activity in the SCN. Systemic pretreatment with the 5-HT antagonist metergoline abolished DRN-induced SCN 5-HT release but had little effect on MRN-induced SCN 5-HT release, suggesting different pathways for these nuclei in regulating 5-HT output in the SCN. Microinjections of the 5-HT1A autoreceptor agonist 8-OH-DPAT or antagonist WAY 100635 into the MRN caused significant inhibition and stimulation of SCN 5-HT release, respectively. Both drugs had substantially less effect in the DRN. These differential drug actions indicate that somatodendritic 5-HT1A autoreceptors on MRN neurons provide the prominent raphe autoregulation of 5-HT output in the SCN. Collectively the current results are evidence that DRN as well as MRN neurons can contribute to the regulation of 5-HT release in the hamster SCN. On the basis of the current observations and those from recent anatomic tracing studies of serotonergic projections to SCN it is hypothesized that DRN input to the SCN could be mediated by a DRN --> MRN --> SCN pathway involving a 5-HT-sensitive multisynaptic interaction between the DRN and MRN neurons.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.