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

Adaptation to photoperiod via dynamic neurotransmitter segregation

Changes in the amount of daylight (photoperiod) alter physiology and behaviour1,2. Adaptive responses to seasonal photoperiods are vital to all organisms-dysregulation associates with disease, including affective disorders3 and metabolic syndromes4. The circadian rhythm circuitry is implicated in such responses5,6, yet little is known about the precise cellular substrates that underlie phase synchronization to photoperiod change. Here we identify a brain circuit and system of axon branch-specific and reversible neurotransmitter deployment that are critical for behavioural and sleep adaptation to photoperiod. A type of neuron called mrEn1-Pet17 in the mouse brainstem median raphe nucleus segregates serotonin from VGLUT3 (also known as SLC17A8, a proxy for glutamate) to different axonal branches that innervate specific brain regions involved in circadian rhythm and sleep-wake timing8,9. This branch-specific neurotransmitter deployment did not distinguish between daylight and dark phase; however, it reorganized with change in photoperiod. Axonal boutons, but not cell soma, changed neurochemical phenotype upon a shift away from equinox light/dark conditions, and these changes were reversed upon return to equinox conditions. When we genetically disabled Vglut3 in mrEn1-Pet1 neurons, sleep-wake periods, voluntary activity and clock gene expression did not synchronize to the new photoperiod or were delayed. Combining intersectional rabies virus tracing and projection-specific neuronal silencing, we delineated a preoptic area-to-mrEn1Pet1 connection that was responsible for decoding the photoperiodic inputs, driving the neurotransmitter reorganization and promoting behavioural synchronization. Our results reveal a brain circuit and periodic, branch-specific neurotransmitter deployment that regulates organismal adaptation to photoperiod change.

A brain circuit and neuronal mechanism for decoding and adapting to change in daylength

Changes in daylight amount (photoperiod) drive pronounced alterations in physiology and behaviour1,2. Adaptive responses to seasonal photoperiods are vital to all organisms - dysregulation is associated with disease, from affective disorders3 to metabolic syndromes4. Circadian rhythm circuitry has been implicated5,6 yet little is known about the precise neural and cellular substrates that underlie phase synchronization to photoperiod change. Here we present a previously unknown brain circuit and novel system of axon branch-specific and reversible neurotransmitter deployment that together prove critical for behavioural and sleep adaptation to photoperiod change. We found that the recently defined neuron type called mrEn1-Pet17 located in the mouse brainstem Median Raphe Nucleus (MRN) segregates serotonin versus VGLUT3 (here proxy for the neurotransmitter glutamate) to different axonal branches innervating specific brain regions involved in circadian rhythm and sleep/wake timing8,9. We found that whether measured during the light or dark phase of the day this branch-specific neurotransmitter deployment in mrEn1-Pet1 neurons was indistinguishable; however, it strikingly reorganizes on photoperiod change. Specifically, axonal boutons but not cell soma show a shift in neurochemical phenotype upon change away from equinox light/dark conditions that reverses upon return to equinox. When we genetically disabled the deployment of VGLUT3 in mrEn1-Pet1 neurons, we found that sleep/wake periods and voluntary activity failed to synchronize to the new photoperiod or was significantly delayed. Combining intersectional rabies virus tracing and projection-specific neuronal silencing in vivo, we delineated a Preoptic Area-to-mrEn1Pet1 connection responsible for decoding the photoperiodic inputs, driving the neurochemical shift and promoting behavioural synchronization. Our results reveal a previously unrecognized brain circuit along with a novel form of periodic, branch-specific neurotransmitter deployment that together regulate organismal adaptation to photoperiod changes.

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

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

Biological Timing and Neurodevelopmental Disorders: A Role for Circadian Dysfunction in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) are a spectrum of neurodevelopmental disorders characterized by impaired social interaction and communication, as well as stereotyped and repetitive behaviors. ASDs affect nearly 2% of the United States child population and the worldwide prevalence has dramatically increased in recent years. The etiology is not clear but ASD is thought to be caused by a combination of intrinsic and extrinsic factors. Circadian rhythms are the ∼24 h rhythms driven by the endogenous biological clock, and they are found in a variety of physiological processes. Growing evidence from basic and clinical studies suggest that the dysfunction of the circadian timing system may be associated with ASD and its pathogenesis. Here we review the findings that link circadian dysfunctions to ASD in both experimental and clinical studies. We first introduce the organization of the circadian system and ASD. Next, we review physiological indicators of circadian rhythms that are found disrupted in ASD individuals, including sleep-wake cycles, melatonin, cortisol, and serotonin. Finally, we review evidence in epidemiology, human genetics, and biochemistry that indicates underlying associations between circadian regulation and the pathogenesis of ASD. In conclusion, we propose that understanding the functional importance of the circadian clock in normal and aberrant neurodevelopmental processes may provide a novel perspective to tackle ASD, and clinical treatments for ASD individuals should comprise an integrative approach considering the dynamics of daily rhythms in physical, mental, and social processes.

Orexinergic modulation of serotonin neurons in the dorsal raphe of a diurnal rodent, Arvicanthis niloticus

The hypothalamic neuropeptide, orexin (or hypocretin), is implicated in numerous physiology and behavioral functions, including affective states such as depression and anxiety. The underlying mechanisms and neural circuits through which orexin modulates affective responses remain unclear. The objective of the present study was to test the hypothesis that the serotonin (5-HT) system of the dorsal raphe nucleus (DRN) is a downstream target through which orexin potentially manifests its role in affective states. Using a diurnal rodent, the Nile grass rat (Arvicanthis niloticus), we first characterized the expression of the orexin receptors OX1R and OX2R in the DRN using in situ hybridization. The results revealed distinct distributions of OX1R and OX2R mRNAs, with OX1R predominantly expressed in the dorsal and lateral wings of the DRN that are involved in affective processes, while OX2R was mostly found in the ventral DRN that is more involved in sensory-motor function. We next examined how the orexin-OX1R pathway regulates 5-HT in the DRN and some of its projection sites using a selective OX1R antagonist SB-334867 (10 mg/kg, i.p.). A single injection of SB-334867 decreased 5-HT-ir fibers within the anterior cingulate cortex (aCgC); five once-daily administrations of SB-334867 decreased 5-HT-ir not only in the aCgC but also in the DRN, oval bed nucleus of the stria terminalis (ovBNST), nucleus accumbens shell (NAcSh), and periaqueductal gray (PAG). HPLC analysis revealed that five once-daily administrations of SB-334867 did not affect 5-HT turnover to any of the five sites, although it increased the levels of both 5-HT and 5-HIAA in the NAcSh. These results together suggest that orexinergic modulation of DRN 5-HT neurons via OX1Rs may be one pathway through which orexin regulates mood and anxiety, as well as perhaps other neurobiological processes.

Early weaning disrupts feeding patterns in female juvenile rats through 5HT-system modulations

Convergent evidence in literature shows that rapid disruption of maternal care and breastfeeding due to an early weaning protocol changes the development of several neurobehavioral patterns in rodents, including the circadian pattern of feeding. The serotoninergic system has been associated with the control of feeding patterns. Therefore, we aim to evaluate the patterns of feeding, the mRNA expression of 5 H T-1b, 5 H T-2c, and SERT on the hypothalamus, brainstem, and the body weight of female juvenile Wistar rats, submitted to early (PND15) or regular (PND30) weaning. The results demonstrate that early weaning promotes an increase in food intake in a 24 -h period, in the dark phase of the circadian cycle and in the four-hour time intervals at the beginning of the dark and light phases. Also, early weaning decreases the mRNA expression of 5 H T-1b, 5 H T-2c, and SERT on the hypothalamus, but increases it on the brainstem. Additionally, early weaning promotes an increase in body weight. Therefore, the present data demonstrate that early weaning changes the patterns of feeding in juvenile female rats and suggests that this behavioral modification is due to the modulations promoted in the 5 H T-system.

Circadian Rhythm Disturbances in Mood Disorders: Insights into the Role of the Suprachiasmatic Nucleus

Circadian rhythm disturbances are a common symptom among individuals with mood disorders. The suprachiasmatic nucleus (SCN), in the ventral part of the anterior hypothalamus, orchestrates physiological and behavioral circadian rhythms. The SCN consists of self-sustaining oscillators and receives photic and nonphotic cues, which entrain the SCN to the external environment. In turn, through synaptic and hormonal mechanisms, the SCN can drive and synchronize circadian rhythms in extra-SCN brain regions and peripheral tissues. Thus, genetic or environmental perturbations of SCN rhythms could disrupt brain regions more closely related to mood regulation and cause mood disturbances. Here, we review clinical and preclinical studies that provide evidence both for and against a causal role for the SCN in mood disorders.

Chronic BMY7378 treatment alters behavioral circadian rhythms

The mammalian circadian clock is synchronized to the day : night cycle by light. Serotonin modulates the circadian effects of light, with agonists inhibiting response to light and antagonists enhancing responses to light. A special class of serotonergic compounds, the mixed 5-HT_1A agonist/antagonists, potentiates light-induced phase advances by up to 400% when administered acutely. In this study, we examine the effects of one of these mixed 5-HT_1A agonist/antagonists, BMY7378, when administered chronically. Thirty adult male hamsters were administered either vehicle or BMY7378 via surgically implanted osmotic mini pumps over a period of 28 days. In a light : dark cycle, chronic BMY7378 advanced the phase angle of entrainment, prolonged the duration of the active phase and attenuated the amplitude of the wheel-running rhythm during the early night. In constant darkness, chronic treatment with BMY7378 significantly attenuated light-induced phase advances, but had no significant effect on light-induced phase delays. Non-photic phase shifts to daytime administration of a 5-HT_1A/7 agonist were also attenuated by chronic BMY7378 treatment. qRT-PCR analysis revealed that chronic BMY7378 treatment upregulated mRNA for 5-HT_1A and 5-HT_1B receptors in the hypothalamus and downregulated mRNA for 5-HT_1A and monoamine oxidase-A in the brainstem. These results highlight adaptive changes of serotonin receptors in the brain to chronic treatment with BMY7378 and link such up- and downregulation to changes in important circadian parameters. Such long-term changes to the circadian system should be considered when patients are treated chronically with drugs that alter serotonergic function.

The flexible clock: predictive and reactive homeostasis, energy balance and the circadian regulation of sleep–wake timing

The Darwinian fitness of mammals living in a rhythmic environment depends on endogenous daily (circadian) rhythms in behavior and physiology. Here, we discuss the mechanisms underlying the circadian regulation of physiology and behavior in mammals. We also review recent efforts to understand circadian flexibility, such as how the phase of activity and rest is altered depending on the encountered environment. We explain why shifting activity to the day is an adaptive strategy to cope with energetic challenges and show how this can reduce thermoregulatory costs. A framework is provided to make predictions about the optimal timing of activity and rest of non-model species for a wide range of habitats. This Review illustrates how the timing of daily rhythms is reciprocally linked to energy homeostasis, and it highlights the importance of this link in understanding daily rhythms in physiology and behavior.

Circuit-based interrogation of sleep control

Sleep is a fundamental biological process observed widely in the animal kingdom, but the neural circuits generating sleep remain poorly understood. Understanding the brain mechanisms controlling sleep requires the identification of key neurons in the control circuits and mapping of their synaptic connections. Technical innovations over the past decade have greatly facilitated dissection of the sleep circuits. This has set the stage for understanding how a variety of environmental and physiological factors influence sleep. The ability to initiate and terminate sleep on command will also help us to elucidate its functions within and beyond the brain.

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.

A lack of functional NK1 receptors explains most, but not all, abnormal behaviours of NK1R-/- mice(1)

Mice lacking functional neurokinin-1 receptors (NK1R-/-) display abnormal behaviours seen in Attention Deficit Hyperactivity Disorder (hyperactivity, impulsivity and inattentiveness). These abnormalities were evident when comparing the behaviour of separate (inbred: 'Hom') wildtype and NK1R-/- mouse strains. Here, we investigated whether the inbreeding protocol could influence their phenotype by comparing the behaviour of these mice with that of wildtype (NK1R+/+) and NK1R-/- progeny of heterozygous parents ('Het', derived from the same inbred strains). First, we recorded the spontaneous motor activity of the two colonies/genotypes, over 7 days. This continuous monitoring also enabled us to investigate whether the diurnal rhythm in motor activity differs in the two colonies/genotypes. NK1R-/- mice from both colonies were hyperactive compared with their wildtypes and their diurnal rhythm was also disrupted. Next, we evaluated the performance of the four groups of mice in the 5-Choice Serial Reaction-Time Task (5-CSRTT). During training, NK1R-/- mice from both colonies expressed more impulsive and perseverative behaviour than their wildtypes. During testing, only NK1R-/- mice from the Hom colony were more impulsive than their wildtypes, but NK1R-/- mice from both colonies were more perseverative. There were no colony differences in inattentiveness. Moreover, a genotype difference in this measure depended on time of day. We conclude that the hyperactivity, perseveration and, possibly, inattentiveness of NK1R-/- mice is a direct consequence of a lack of functional NK1R. However, the greater impulsivity of NK1R-/- mice depended on an interaction between a functional deficit of NK1R and other (possibly environmental and/or epigenetic) factors.

Effects of lighting condition on circadian behavior in 5-HT1A receptor knockout mice

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light-dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6 hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

The serotonergic anxiolytic buspirone attenuates circadian responses to light

Serotonergic drugs modify circadian responses to light, with agonists attenuating and some partial agonists or antagonists potentiating photic phase shifts. The anxiolytic buspirone is a 5-HT1A receptor partial agonist. Given that buspirone is used therapeutically to manage generalised anxiety disorder, it would be useful to understand if and how this drug may modify circadian responses to light, not only to help manage side effects, but also to examine its potential use as a chronobiotic. Here we examined behavioral and molecular responses to phase-shifting light in mice and hamsters treated with buspirone. Phase advances to late subjective night light pulses in hamsters and wildtype mice were significantly attenuated by buspirone. 5-HT1A receptor knockout mice exhibited potentiated photic phase shifts when pretreated with buspirone. In wildtype mice, the attenuated phase shifts were accompanied by increased cFos expression in the suprachiasmatic nucleus, whereas potentiated phase shifts in knockouts were accompanied by increased phosphorylation of extracellular signal-regulated kinase (ERK) and cyclic AMP response element-binding protein (CREB), and decreased cFos expression. Attenuated photic phase shifts in buspirone-treated hamsters were accompanied by decreased phosphorylation of ERK and CREB. Chronic buspirone treatment decreased the amplitude of wheel-running rhythms, lengthened the duration of the active phase and advanced the phase angle of entrainment. Buspirone administration at midday produced non-photic phase advances in wildtype but not 5-HT1A receptor knockout mice. These findings suggest that buspirone affected the circadian system in a manner similar to the 5-HT1A/7 agonist (±)-8-Hydroxy-2-dipropylaminotetralin hydrobromide, primarily through the 5-HT1A receptor, and suggest that therapeutic use of buspirone to manage anxiety may impact circadian function.

Cocaine modulates mammalian circadian clock timing by decreasing serotonin transport in the SCN

Cocaine abuse disrupts reward and homeostatic processes through diverse processes, including those involved in circadian clock regulation. Recently we showed that cocaine administration to mice disrupts nocturnal photic phase resetting of the suprachiasmatic (SCN) circadian clock, whereas administration during the day induces non-photic phase shifts. Importantly, the same effects are seen when cocaine is applied to the SCN in vitro, where it blocks photic-like (glutamate-induced) phase shifts at night and induces phase advances during the day. Furthermore, our previous data suggest that cocaine acts in the SCN by enhancing 5-HT signaling. For example, the in vitro actions of cocaine mimic those of 5-HT and are blocked by the 5-HT antagonist, metergoline, but not the dopamine receptor antagonist, fluphenazine. Although our data are consistent with cocaine acting through enhanced 5-HT signaling, the nonselective actions of cocaine as an antagonist of monoamine transporters raises the question of whether inhibition of the 5-HT transporter (SERT) is key to its circadian effects. Here we investigate this issue using transgenic mice expressing a SERT that exhibits normal 5-HT recognition and transport but significantly reduced cocaine potency (SERT Met172). Circadian patterns of SCN behavioral and neuronal activity did not differ between wild-type (WT) and SERT Met172 mice, nor did they differ in the ability of the 5-HT1A,2,7 receptor agonist, 8-OH-DPAT to reset SCN clock phase, consistent with the normal SERT expression and activity in the transgenic mice. However, (1) cocaine administration does not induce phase advances when administered in vivo or in vitro in SERT Met172 mice; (2) cocaine does not block photic or glutamate-induced phase shifts in SERT Met172 mice; and (3) cocaine does not induce long-term changes in free-running period in SERT Met172 mice. We conclude that SERT antagonism is required for the phase shifting of the SCN circadian clock induced by cocaine.

Photic and nonphotic responses of the circadian clock in serotonin-deficient Pet-1 knockout mice

The neurotransmitter serotonin plays an important role in the regulation of the circadian clock. To gain further insight into the mechanisms by which serotonin regulates rhythmicity, the authors investigated photic and nonphotic effects on the circadian clock in Pet-1 knockout mice. In these mice, the serotonergic system suffers a developmental loss of 70% of serotonin neurons, with the remaining neurons being deficient in serotonergic function as well. Pet-1 knockout mice show significantly decreased phase delays of the circadian clock in response to light pulses in the early night; however, this difference was not reflected in a difference in the expression of Fos protein in the suprachiasmatic nucleus. There were no genotypic differences detected in the phase-shifting response to injection of the 5-HT1A/7 (serotonin 1A and 7) agonist 8-OH-DPAT ((±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide); however, there were small but significant differences in the phase-shifting responses to cages between genotypes and sexes. Several different patterns of wheel-running activity were observed in knockout mice that differed from those in wild-type mice, suggesting that normal serotonergic function is necessary for the proper consolidation of nocturnal activity. Overall, these data are consistent with other pharmacological and genetic studies demonstrating a significant role for serotonin in circadian clock function.

Acute ethanol disrupts photic and serotonergic circadian clock phase-resetting in the mouse

BACKGROUND

Alcohol dependence is associated with impaired circadian rhythms and sleep. Ethanol administration disrupts circadian clock phase-resetting, suggesting a mode for the disruptive effect of alcohol dependence on the circadian timing system. In this study, we extend previous work in C57BL/6J mice to: (i) characterize the suprachiasmatic nucleus (SCN) pharmacokinetics of acute systemic ethanol administration, (ii) explore the effects of acute ethanol on photic and nonphotic phase-resetting, and (iii) determine if the SCN is a direct target for photic effects.

METHODS

First, microdialysis was used to characterize the pharmacokinetics of acute intraperitoneal (i.p.) injections of 3 doses of ethanol (0.5, 1.0, and 2.0 g/kg) in the mouse SCN circadian clock. Second, the effects of acute i.p. ethanol administration on photic phase delays and serotonergic ([+]8-OH-DPAT-induced) phase advances of the circadian activity rhythm were assessed. Third, the effects of reverse-microdialysis ethanol perfusion of the SCN on photic phase-resetting were characterized.

RESULTS

Peak ethanol levels from the 3 doses of ethanol in the SCN occurred within 20 to 40 minutes postinjection with half-lives for clearance ranging from 0.6 to 1.8 hours. Systemic ethanol treatment dose-dependently attenuated photic and serotonergic phase-resetting. This treatment also did not affect basal SCN neuronal activity as assessed by Fos expression. Intra-SCN perfusion with ethanol markedly reduced photic phase delays.

CONCLUSIONS

These results confirm that acute ethanol attenuates photic phase-delay shifts and serotonergic phase-advance shifts in the mouse. This dual effect could disrupt photic and nonphotic entrainment mechanisms governing circadian clock timing. It is also significant that the SCN clock is a direct target for disruptive effects of ethanol on photic shifting. Such actions by ethanol could underlie the disruptive effects of alcohol abuse on behavioral, physiological, and endocrine rhythms associated with alcoholism.

Phenotype and function of raphe projections to the suprachiasmatic nucleus

The circadian clock, located in the suprachiasmatic nucleus (SCN), receives a major afferent from the median raphe nucleus (MRN). In the Syrian hamster, only about 50% of the cells giving rise to this afferent contain serotonin. There is mixed evidence as to whether the serotonergic portion of this projection is involved in non-photic phase shifting of circadian locomotor rhythms. In order to better characterize the non-serotonergic projections, we conducted retrograde tract tracing using the beta subunit of cholera toxin combined with multi-label immunohistochemistry. Similar to previous findings, almost half of the retrogradely labeled cells contained serotonin. Additionally, approximately 30% of the retrogradely labeled cells contained vesicular glutamate transporter 3 (VGLUT3), but not serotonin. Surprisingly, some dorsal raphe cholera toxin labeling was also noted, particularly in animals with central-SCN injections. To determine if the non-serotonergic projections were important for non-photic phase shifts elicited by MRN stimulation, the MRN was electrically stimulated in animals pretreated with SCN injection of either the serotonin neurotoxin 5,7-dihydroxytryptamine or vehicle control. Intact animals phase advanced to midday electrical stimulation of the raphe while lesioned animals did not. Together, these results show that although some of the non-serotonergic raphe projections to the SCN contain VGLUT3, it is the serotonergic raphe innervation of the SCN that is critical for non-photic phase shifting elicited by MRN stimulation.

Daily oscillation of phospholipase C beta4 in the mouse suprachiasmatic nucleus

An endogenous biological clock located in the hypothalamic suprachiasmatic nucleus (SCN) regulates the timing of an organism's physiology and behavior. A variety of receptors are found on SCN pacemaker cells which permit the clock mechanism to respond to extra- and intra-SCN chemical messengers. A subset of these receptors is coupled to G-proteins, which when bound, lead to the activation of a variety of intracellular signaling cascades. One common signaling pathway employs the phosphotidylinositol-specific phospholipase C enzyme to increase intracellular calcium levels. A specific isoform of this enzyme, phospholipase C beta4, is of particular interest to circadian biologists because in its absence, mice display a circadian phenotype. Moreover, it has been shown to be associated with receptor types that are involved in clock resetting. Despite compelling data that this enzyme could be a critical component of an intracellular signaling pathway in the SCN, no study to date has investigated the possible oscillation of phospholipase C in any mammalian tissue. In the present study, we analyzed the temporal variation in the number of phospholipase C beta4 immunoreactive cells in the SCN. Herein, we show that PLCbeta4 levels oscillate in the SCN of mice housed in a light:dark photoperiod. Protein levels reached a significant peak during the early night and a trough during the day. The oscillation was considerably damped in the SCN of mice housed in constant dark conditions indicating the cycle is photoperiod-dependent. These data are critical to understanding the temporal regulation of a variety of inputs to the mammalian central circadian clock.

Influences of octopamine and juvenile hormone on locomotor behavior and period gene expression in the honeybee, Apis mellifera

Octopamine (OA) and juvenile hormone (JH) are implicated in the regulation of age-based division of labor in the honeybee, Apis mellifera. We tested the hypothesis that these two neuroendocrine signals influence task-associated plasticity in circadian and diurnal rhythms, and in brain expression of the clock gene period (per). Treatment with OA, OA antagonist (epinastine), or both, did not affect the age at onset of circadian rhythmicity or the free running period in constant darkness (DD). Young bees orally treated with OA in light-dark (LD) illumination regime for 6 days followed by DD showed reduced alpha (the period between the daily onset and offset of activity) during the first 4 days in LD and the first 4 days in DD. Oral treatment with OA, epinastine, or both, but not manipulations of JH levels, caused increased average daily levels and aberrant patterns of brain per mRNA oscillation in young bees. These results suggest that OA and JH do not influence the development or function of the central pacemaker but rather that OA influences the brain expression of a clock gene and characteristics of locomotor behavior that are not thought to be under direct control of the circadian pacemaker.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

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.

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.

Functional analysis of the role of the median raphe as a regulator of hamster circadian system sensitivity to light

The retinohypothalamic tract, a monosynaptic retinal projection to the suprachiasmatic nucleus (SCN), is the path by which light entrains the circadian system to the external photoperiod. Serotonergic neurons in the mesencephalic median raphe nucleus (MnR) also give rise to a major SCN afferent projection. The present study was designed to determine the extent to which MnR serotonergic projections regulate sensitivity of the circadian rhythm system to light. Serotonergic neurons in the MnR were destroyed by the direct application of the neurotoxin, 5,7-dihydroxytryptamine. Animals in constant darkness were given 5-min white light pulses at circadian time 19. Light intensity varied from 0.0011 to 70 microW/cm2. Assessment of rhythm phase response to light by lesioned and control animals revealed that animals lacking the MnR serotonergic projection are considerably more sensitive to light at high irradiances. The results are consistent with behavioral and physiological evidence implicating serotonin as an inhibitory modulator of the effects of light on circadian rhythmicity.

Midbrain raphe modulation of nonphotic circadian clock resetting and 5-HT release in the mammalian suprachiasmatic nucleus

Serotonin (5-HT) is an important regulator of the mammalian circadian clock of the suprachiasmatic nucleus (SCN); however, critical questions remain concerning the control of serotonergic activity in the SCN and how this relates to the putative clock-resetting actions of 5-HT. Previously, we reported that electrical stimulation of the dorsal raphe nucleus (DRN) or median raphe nucleus (MRN) in hamsters evoked 5-HT release in the SCN. This DRN-stimulated 5-HT release was blocked by systemic injection of 5-HT antagonists, indicating a 5-HT receptor-mediated pathway from the DRN to the SCN. In the present study, targeted injections of the 5-HT1,2,7 antagonist metergoline or the selective 5-HT7 antagonist DR4004 into the DRN or MRN attenuated DRN-electrically stimulated SCN 5-HT release, supporting a multisynaptic DRN-->MRN-->SCN route. Intra-DRN and intra-MRN injections of the GABA(A) antagonist bicuculline significantly stimulated SCN 5-HT release, whereas intra-DRN or intra-MRN injections of the GABAA agonist muscimol suppressed this release. The 5-HT release induced by intra-DRN bicuculline was also blocked by co-injection of DR4004. In complementary behavioral trials, SCN 5-HT release associated with a phase-advancing sleep deprivation stimulus at midday was prevented by intra-DRN injection of metergoline. Also, phase-advance shifts induced by novel wheel access at midday were suppressed, but not blocked, by intra-DRN injection of DR4004 or muscimol. These results indicate that 5-HT7 and GABAergic receptors of the DRN and MRN regulate behaviorally induced 5-HT release in the SCN, and that DRN output modulates nonphotic phase-resetting responses.

Projections from the raphe nuclei to the suprachiasmatic nucleus of the rat

The presence of serotonergic afferents in the hypothalamic suprachiasmatic nucleus (SCN) is well documented and several functional roles of serotonin (5-HT) in circadian function are well established. However, there is some controversy about the precise location of the serotonergic neurones from where this input arises. Discrete injection of the tracer Cholera toxin, subunit B, (ChB) was centred in the rat SCN, and a few retrograde labelled neurones were distributed in the dorsal and median raphe nuclei (MnR) and in the rostral part of the raphe magnus (RMg), but no neurones were found in the raphe pallidus or raphe obscurus. In addition, a group of neurones was consistently found in the medial part of the pontine supra lemniscal nucleus but not including the serotonergic B(9) region. A combination of retrograde tracing with Fluoro-Gold together with 5-HT-immunolabelling, showed few double-labelled neurones in the dorsal, MnR and B(9). However, the majority of projecting neurones were not co-storing 5-HT immunoreactivity. Phaseolus vulgaris-leucoagglutinin (PHA-L) injections in the dorsal raphe resulted in faint labelling, whereas the MnR gave rise to several labelled fibres in the SCN. Individual delicate PHA-L nerve fibres were found in all compartments of the SCN both in terms of rostrocaudal, ventromedial and dorsomedial extent, without any sign of a topographical organisation of the MnR input to the SCN. PHA-L injections into RMg gave rise to labelling of a few processes within the SCN. In conclusion, the main serotonergic input to the rat SCN originates from a few neurones in the MnR.

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.

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.

Serotonin-stimulated glutamate release from an SCN explant culture was higher during light period

Serotonin (5-HT) and glutamate are concerned with the input pathways to the suprachiasmatic nucleus (SCN), a center of biological rhythms in mammals. Arg-vasopressin (AVP) is one of the output paths from the SCN to other brain areas. Functional relations of 5-HT on glutamate release, which stimulates AVP release, were therefore analyzed in an SCN slice culture using a glutamate biosensor. Spontaneous release of glutamate from the SCN culture was higher during the light period than the dark period. The 5-HT-stimulated increase in glutamate release was also larger at 0900 h than at 2000 h. In addition, glutamate, but not 5-HT, increased the AVP release. These findings suggest the possibility that the 5-HT has no direct facilitatory functions in AVP release via liberation of glutamate from the SCN.

Interaction of the retina with suprachiasmatic pacemakers in the control of circadian behavior

The suprachiasmatic nucleus (SCN) is the central circadian pacemaker governing the circadian rhythm of locomotor activity in mammals. The mammalian retina also contains circadian oscillators, but their roles are unknown. To test whether the retina influences circadian rhythms of locomotor behavior, the authors compared the activity of bilaterally enucleated hamsters with the activity of intact controls held in constant darkness (DD). Enucleated hamsters showed a broader range of free-running periods (tau) than did intact hamsters held for the same length of time in DD. This effect was independent of the age at enucleation (on postnatal days 1, 7, or 28). The average tau of intact animals kept in DD from days 7 or 28 was significantly longer than that of intact animals kept in DD from day 1 or any of the enucleated groups. This indicates that early exposure to light-dark cycles lengthens the tau and that the eye is required to maintain this effect even in DD. These data suggest that hypothalamic circadian pacemakers may interact continuously with the retina to determine the tau of locomotor activity. Enucleation caused a large decrease in glial fibrillary acidic protein in the SCN but has no (or slight) effects on calbindin, neuropeptide Y, vasopressin, or vasoactive intestinal polypeptide, which suggests that enucleation does not produce major damage to the SCN, an interpretation that is supported by the fact that enucleated animals retain robust circadian rhythmicity. The presence of an intact retina appears to contribute to system-level circadian organization in mammals perhaps as a consequence of interaction between its circadian oscillators and those in the SCN.

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.

Retinal afferents to the dorsal raphe nucleus in rats and Mongolian gerbils

A direct pathway from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in both albino rats and Mongolian gerbils. Following intraocular injection of cholera toxin subunit B (CTB), a diffuse stream of CTB-positive, fine-caliber optic axons emerged from the optic tract at the level of the pretectum/anterior mesencephalon. In gerbils, CTB-positive axons descended ventromedially into the periaqueductal gray, moving caudally and arborizing extensively throughout the DRN. In rats, the retinal-DRN projection comprised fewer, but larger caliber, axons, which arborized in a relatively restricted region of the lateral and ventral DRN. Following injection of CTB into the lateral DRN, retrogradely labeled ganglion cells (GCs) were observed in whole-mount retinas of both species. In gerbils, CTB-positive GCs were distributed over the entire retina, and a nearest-neighbor analysis of CTB-positive GCs showed significant regularity (nonrandomness) in their distribution. The overall distribution of gerbil GC soma diameters ranged from 8 to 22 micrometer and was skewed slightly towards the larger soma diameters. Based on an adaptive mixtures model statistical analysis, two Gaussian distributions appeared to comprise the total GC distribution, with mean soma diameters of 13 (SEM +/-1.7) micrometer, and 17 (SEM +/-1.5) micrometer, respectively. In rats, many fewer CTB-positive GCs were labeled following CTB injections into the lateral DRN, and nearly all occurred in the inferior retina. The total distribution of rat GC soma diameters was similar to that in gerbils and also was skewed towards the larger soma diameters. Major differences observed in the extent and configuration of the retinal-DRN pathway may be related to the diurnal/crepuscular vs. nocturnal habits of these two species.

Serotonin modulation of calcium transients in cells in the suprachiasmatic nucleus

Information about environmental lighting conditions is conveyed to the suprachiasmatic nucleus (SCN), at least in part, via a glutamatergic fiber pathway originating in the retina, known as the retinohypothalamic tract (RHT). Previous work indicates that serotonin (5HT) can inhibit this pathway, although the underlying mechanisms are unknown. The authors became interested in the possibility that 5HT can inhibit the glutamatergic regulation of Ca2+ in SCN neurons and, by this mechanism, modulate light-induced phase shifts of the circadian system. To start to examine this hypothesis, optical techniques were used to measure Ca2+ levels in SCN cells in a brain slice preparation. First, it was found that 5HT produced a reversible and significant inhibition of Ca2+ transients evoked by synaptic stimulation. Next, it was found that 5HT did not alter the magnitude or duration of Ca2+ transients evoked by the bath application of glutamate or N-methyl-D-aspartate acid (NMDA) in the presence of tetrodotoxin (TTX). The authors feel that the simplest explanation for these results is that 5HT can act presynaptically at the RHT/SCN synaptic connection to inhibit the release of glutamate. The demonstration that 5HT can have a dramatic modulatory action on synaptic-evoked Ca2+ transients measured in SCN neurons adds support to the notion that the serotonergic innervation of the SCN may function to regulate environmental input to the circadian system. In addition, it was found that the administration of higher concentrations of 5HT can increase Ca2+ in at least a subpopulation of SCN neurons. This effect of 5HT was concentration dependent and blocked by a broad-spectrum 5HT antagonist (metergoline). In addition, both TTX and the gamma-amino-N-butyric acid (GABA) receptor blocker bicuculline inhibited the 5HT-induced Ca2+ transients. Therefore, the interpretation of this data is that 5HT can act within the SCN to alter GABAergic activity and, by this mechanism, cause changes in intracellular Ca2+. It is also suggested that this 5HT-induced Ca2+ increase might play a role in 5HT-induced phase shifts of the SCN circadian oscillator.

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 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.

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

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

Photic entrainment of circadian rhythms in rodents

Photic entrainment of circadian rhythms occurs as a consequence of daily, light-induced adjustments in the phase and period of the suprachiasmatic nuclei (SCN) circadian clock. Photic information is acquired by a unique population of retinal photoreceptors, processed by a distinct subset of retinal ganglion cells, and conveyed to the SCN through the retinohypothalamic tract (RHT). RHT neurotransmission is mediated by the release of the excitatory amino acid glutamate and appears to require the activation of both NMDA- and non-NMDA-type glutamate receptors, the expression of immediate early genes (IEGs), and the synthesis and release of nitric oxide. In addition, serotonin appears to regulate the response of the SCN circadian clock to light through postsynaptic 5-HT1A or 5-ht7 receptors, as well as presynaptic 5-HT1B heteroreceptors on RHT terminals.

Cellular and biochemical mechanisms underlying circadian rhythms in vertebrates

Circadian clocks organize neural processes, such as motor activities, into near 24-hour oscillations and adaptively synchronize these rhythms to the solar cycle. Recently, the first mammalian clock genes have been found. Unpredicted diversity in signaling pathways and clock-controlled gating of signals that modulate timekeeping has been discovered. A diffusible clock output has been found to control some behavioral rhythms. Consensus is emerging that circadian mechanisms are conserved across phylogeny, but that mammals have developed a great complexity of controls.