Serotonin and the mammalian circadian system: II. Phase-shifting rat behavioral rhythms with serotonergic agonists

The neural addiction of cancer

The recently uncovered key role of the peripheral and central nervous systems in controlling tumorigenesis and metastasis has opened a new area of research to identify innovative approaches against cancer. Although the 'neural addiction' of cancer is only partially understood, in this Perspective we discuss the current knowledge and perspectives on peripheral and central nerve circuitries and brain areas that can support tumorigenesis and metastasis and the possible reciprocal influence that the brain and peripheral tumours exert on one another. Tumours can build up local autonomic and sensory nerve networks and are able to develop a long-distance relationship with the brain through circulating adipokines, inflammatory cytokines, neurotrophic factors or afferent nerve inputs, to promote cancer initiation, growth and dissemination. In turn, the central nervous system can affect tumour development and metastasis through the activation or dysregulation of specific central neural areas or circuits, as well as neuroendocrine, neuroimmune or neurovascular systems. Studying neural circuitries in the brain and tumours, as well as understanding how the brain communicates with the tumour or how intratumour nerves interplay with the tumour microenvironment, can reveal unrecognized mechanisms that promote cancer development and progression and open up opportunities for the development of novel therapeutic strategies. Targeting the dysregulated peripheral and central nervous systems might represent a novel strategy for next-generation cancer treatment that could, in part, be achieved through the repurposing of neuropsychiatric drugs in oncology.

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.

The impact of glucocorticoids and statins on sleep quality

Glucocorticoids and statins are the foundation of lifelong therapies and as such, may generate a variety of side effects. Among these, sleep impairments are one of the least explored and, simultaneously, majorly underestimated in clinical practice. Based on the available evidence, we have concluded that glucocorticoid action on the suprachiasmatic nucleus (SCN) that drives sleep disturbances is dual in nature. It involves both serotonin depletion and reduced arginine vasopressin signalling in the SCN. The former seems to involve activation of glucocorticoid receptors in the dorsal raphe, whereas the latter likely results from changes in glucose serum levels affecting the SCN, among other blood-borne factors which are yet to be discovered. Literature remains inconclusive when it comes to statins. Their diverse potential to cross the blood-brain barrier is considered the key factor determining statins' capability to evoke sleep impairments. Concurrently, an effect similar to that produced by steroids occurs - alteration in serum levels of blood-borne factors, such as glucose, which is a likely cause of statin-induced sleep disturbances.

Clock control of mammalian reproductive cycles: Looking beyond the pre-ovulatory surge of gonadotropins

Several aspects of the physiology and behavior of organisms are expressed rhythmically with a 24-h periodicity and hence called circadian rhythms. Such rhythms are thought to be an adaptive response that allows to anticipate cyclic events in the environment. In mammals, the circadian system is a hierarchically organized net of endogenous oscillators driven by the hypothalamic suprachiasmatic nucleus (SCN). This system is synchronized by the environment throughout afferent pathways and in turn it organizes the activity of tissues by means of humoral secretions and neuronal projections. It has been shown that reproductive cycles are regulated by the circadian system. In rodents, the lesion of the SCN results on alterations of the estrous cycle, sexual behavior, tonic and phasic secretion of gonadotropin releasing hormone (GnRH)/gonadotropins and in the failure of ovulation. Most of the studies regarding the circadian control of reproduction, in particular of ovulation, have only focused on the participation of the SCN in the triggering of the proestrus surge of gonadotropins. Here we review aspects of the evolution and organization of the circadian system with particular focus on its relationship with the reproductive cycle of laboratory rodents. Experimental evidence of circadian control of neuroendocrine events indispensable for ovulation that occur prior to proestrus are discussed. In order to offer a working model of the circadian regulation of reproduction, its participation on aspects ranging from gamete production, neuroendocrine regulation, sexual behavior, mating coordination, pregnancy and deliver of the product should be assessed experimentally.

Serotonin Prevents Differentiation of Brown Adipocytes by Interfering with Their Clock

OBJECTIVE

Serotonin was shown to interfere with the differentiation of brown adipocytes. In addition, clock components inhibit brown adipogenesis through direct transcriptional control of key components of the transforming growth factor β pathway. The aim of this study was to investigate whether serotonin abrogates brown adipogenesis by affecting clock functionality.

METHODS

Nondifferentiated and differentiated HIB1B brown adipocytes were treated with serotonin, and their clock expression and functionality and differentiation state were examined.

RESULTS

Nondifferentiated HIB1B brown adipocytes treated with serotonin showed increased brown adipocyte markers alongside increased brain-muscle Arnt-like protein 1 (Bmal1) and RAR related orphan receptor A (Rora) but decreased nuclear receptor Rev-erbα mRNA levels. BMAL1 overexpression together with serotonin led to significantly lower brown adipocyte markers. Serotonin in the differentiation cocktail led to reduced brown adipocyte markers as well as clock gene expression. After differentiation, serotonin treatment significantly decreased brown adipocyte markers and reduced BMAL1 and RORα but increased REV-ERBα protein levels. Addition of serotonin to the differentiation medium or addition after differentiation reduced activity of calcium/calmodulin-dependent protein kinase type II subunit gamma, which interferes with circadian locomoter output cycles protein kaput (CLOCK):BMAL1 dimerization and transactivation.

CONCLUSIONS

Clock expression is required at the early stages of differentiation to brown adipocytes, and serotonin interferes with this process by modulating clock functionality. Serotonin interferes with clock functionality by reducing the levels of the active form of calcium/calmodulin-dependent protein kinase type II subunit gamma.

Circadian Vision in Zebrafish: From Molecule to Cell and from Neural Network to Behavior

Most visual system functions, such as opsin gene expression, retinal neural transmission, light perception, and visual sensitivity, display robust day-night rhythms. The rhythms persist in constant lighting conditions, suggesting the involvement of endogenous circadian clocks. While the circadian pacemakers that control the rhythms of animal behaviors are mostly found in the forebrain and midbrain, self-sustained circadian oscillators are also present in the neural retina, where they play important roles in the regulation of circadian vision. This review highlights some of the correlative studies of the circadian control of visual system functions in zebrafish. Because zebrafish maintain a high evolutionary proximity to mammals, the findings from zebrafish research may provide insights for a better understanding of the mechanisms of circadian vision in other vertebrate species including humans.

Sleep homeostasis and the circadian clock: Do the circadian pacemaker and the sleep homeostat influence each other's functioning?

Sleep is regulated by a homeostatic and a circadian process. Together these two processes determine most aspects of sleep and related variables like sleepiness and alertness. The two processes are known to be able to work independently, but also to both influence sleep and sleep related variables in an additive or more complex manner. The question remains whether the two processes are directly influencing each other. The present review summarizes evidence from behavioural and electroencephalographic determined sleep, electrophysiology, gene knock out mouse models, and mathematical modelling to explore whether sleep homeostasis can influence circadian clock functioning and vice versa. There is a multitude of data available showing parallel action or influence of sleep homeostatic mechanisms and the circadian clock on several objective and subjective variables related to sleep and alertness. However, the evidence of a direct influence of the circadian clock on sleep homeostatic mechanisms is sparse and more research is needed, particularly applying longer sleep deprivations that include a second night. The strongest evidence of an influence of sleep homeostatic mechanisms on clock functioning comes from sleep deprivation experiments, demonstrating an attenuation of phase shifts of the circadian rhythm to light pulses when sleep homeostatic pressure is increased. The data suggest that the circadian clock is less susceptible to light when sleep pressure is high. The available data indicate that a strong central clock will induce periods of deep sleep, which in turn will strengthen clock function. Both are therefore important for health and wellbeing. Weakening of one will also hamper functioning of the other. Shift work and jet lag are situations where one tries to adapt to zeitgebers in a condition where sleep is compromised. Adaptation to zeitgebers may be improved by introducing nap schedules to reduce sleep pressure, and through that increasing clock susceptibility to light.

Circadian behavior of adult mice exposed to stress and fluoxetine during development

INTRODUCTION

Women of child-bearing age are the population at greatest risk for depression. The stress experienced during pregnancy and the associated antidepressant treatments can both affect fetal development. Fluoxetine (FLX) is among the most common antidepressants used by pregnant women. We have previously demonstrated that perinatal exposure to FLX can alter expression of circadian rhythms in adulthood. Here, we examine the combined effects of maternal stress during pregnancy and perinatal exposure to the antidepressant FLX on the circadian behavior of mice as adults.

METHODS

Mouse dams were exposed to chronic unpredictable stress (embryonic (E) day 7 to E18), FLX (E15 to postnatal day 12), a combination of both stress and FLX, or were left untreated. At 2 months of age, male offspring were placed in recording chambers and circadian organization of wheel running rhythms and phase shifts to photic and non-photic stimuli were assessed.

RESULTS

Mice exposed to prenatal stress (PS) had smaller light-induced phase delays. Mice exposed to perinatal FLX required more days to re-entrainment to an 8-h phase advance of their light-dark cycle. Mice subjected to either perinatal FLX or to PS had larger light-induced phase advances and smaller phase advances to 8-OH-DPAT. FLX treatment partially reversed the effect of PS on phase shifts to late-night light exposure and to 8-OH-DPAT.

CONCLUSIONS

Our results suggest that, in mice, perinatal exposure to either FLX, or PS, or their combination, leads to discernible, persistent changes in their circadian systems as adults.

Metabolic effect of fluvoxamine in mouse peripheral tissues

Serotonin leads to reduced food intake and satiety. Disrupted circadian rhythms lead to hyperphagia and obesity. The serotonergic and circadian systems are intertwined, as the central brain clock receives direct serotonergic innervation and, in turn, makes polysynaptic output back to serotonergic nuclei. Our objective was to test the hypothesis that peripherally serotonin alters circadian rhythms leading to a shift towards fat synthesis and weight gain. We studied the effect of serotonin and fluvoxamine, a selective serotonin reuptake inhibitor (SSRI), on the circadian clock and metabolic gene and protein expression in mouse liver, muscle and white adipose tissue (WAT) and cell culture. We found that serotonin and/or the SSRI fluvoxamine led to fat accumulation in mouse liver and hepatocytes by shifting metabolism towards fatty acid synthesis mainly through low average levels of phosphorylated acetyl CoA carboxylase (pACC) and phosphorylated protein phosphatase 2A (pPP2A). This shift towards fat synthesis was also observed in adipose tissue. Muscle cells were only slightly affected metabolically by serotonin or fluvoxamine. In conclusion, although centrally it leads to increased satiety, in peripheral tissues, such as the liver and WAT, serotonin induces fat accumulation.

Circadian rhythms of clock gene expression in the cerebellum of serotonin-deficient Pet-1 knockout mice

Serotonin plays an important role in the central regulation of circadian clock function. Serotonin levels are generally higher in the brain during periods of high activity, and these periods are in turn heavily regulated by the circadian clock located in the suprachiasmatic nucleus. However, the role of serotonin as a regulator of circadian rhythms elsewhere in the brain has not been extensively examined. In this study, we examined circadian rhythms of clock gene expression in the cerebellum in mice lacking the Pet-1 transcription factor, which results in a developed brain that is deficient in serotonin neurons. If serotonin helps to synchronize rhythms in brain regions other than the suprachiasmatic nucleus, we would expect to see differences in clock gene expression in these serotonin deficient mice. We found minor differences in the expression of Per1 and Per2 in the knockout mice as compared to wild type, but these differences were small and of questionable functional importance. We also measured the response of cerebellar clocks to injections of the serotonin agonist 8-OH-DPAT during the early part of the night. No effect on clock genes was observed, though the immediate-early gene Fos showed increased expression in wild type mice but not the knockouts. These results suggest that serotonin is not an important mediator of circadian rhythms in the cerebellum in a way that parallels its regulation of the circadian clock in the suprachiasmatic nucleus.

Changes in winter depression phenotype correlate with white blood cell gene expression profiles: a combined metagene and gene ontology approach

In the present study we evaluate the feasibility of gene expression in white blood cells as a peripheral marker for winter depression. Sixteen patients with winter type seasonal affective disorder were included in the study. Blood was taken by venous puncture at three time points; in winter prior and following bright light therapy and in summer. RNA was isolated, converted into cRNA, amplified and hybridized on Illumina® gene expression arrays. The raw optical array data were quantile normalized and thereafter analyzed using a metagene approach, based on previously published Affymetrix gene array data. The raw data were also subjected to a secondary analysis focusing on circadian genes and genes involved in serotonergic neurotransmission. Differences between the conditions were analyzed, using analysis of variance on the principal components of the metagene score matrix. After correction for multiple testing no statistically significant differences were found. Another approach uses the correlation between metagene factor weights and the actual expression values, averaged over conditions. When comparing the correlations of winter vs. summer and bright light therapy vs. summer significant changes for several metagenes were found. Subsequent gene ontology analyses (DAVID and GeneTrail) of 5 major metagenes suggest an interaction between brain and white blood cells. The hypothesis driven analysis with a smaller group of genes failed to demonstrate any significant effects. The results from the combined metagene and gene ontology analyses support the idea of communication between brain and white blood cells. Future studies will need a much larger sample size to obtain information at the level of single genes.

Pet-1 deficiency alters the circadian clock and its temporal organization of behavior

The serotonin and circadian systems are two important interactive regulatory networks in the mammalian brain that regulate behavior and physiology in ways that are known to impact human mental health. Previous work on the interaction between these two systems suggests that serotonin modulates photic input to the central circadian clock (the suprachiasmatic nuclei; SCN) from the retina and serves as a signal for locomotor activity, novelty, and arousal to shift the SCN clock, but effects of disruption of serotonergic signaling from the raphe nuclei on circadian behavior and on SCN function are not fully characterized. In this study, we examined the effects on diurnal and circadian behavior, and on ex vivo molecular rhythms of the SCN, of genetic deficiency in Pet-1, an ETS transcription factor that is necessary to establish and maintain the serotonergic phenotype of raphe neurons. Pet-1^−/− mice exhibit loss of rhythmic behavioral coherence and an extended daily activity duration, as well as changes in the molecular rhythms expressed by the clock, such that ex vivo SCN from Pet-1^−/− mice exhibit period lengthening and sex-dependent changes in rhythmic amplitude. Together, our results indicate that Pet-1 regulation of raphe neuron serotonin phenotype contributes to the period, precision and light/dark partitioning of locomotor behavioral rhythms by the circadian clock through direct actions on the SCN clock itself, as well as through non-clock effects.

The trouble with circadian clock dysfunction: multiple deleterious effects on the brain and body

This review consolidates research employing human correlational and experimental work across brain and body with experimental animal models to provide a more complete representation of how circadian rhythms influence almost all aspects of life. In doing so, we will cover the morphological and biochemical pathways responsible for rhythm generation as well as interactions between these systems and others (e.g., stress, feeding, reproduction). The effects of circadian disruption on the health of humans, including time of day effects, cognitive sequelae, dementia, Alzheimer's disease, diet, obesity, food preferences, mood disorders, and cancer will also be discussed. Subsequently, experimental support for these largely correlational human studies conducted in non-human animal models will be described.

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.

The melatonergic system in mood and anxiety disorders and the role of agomelatine: implications for clinical practice

Melatonin exerts its actions through membrane MT1/MT2 melatonin receptors, which belong to the super family of G-protein-coupled receptors consisting of the typical seven transmembrane domains. MT1 and MT2 receptors are expressed in various tissues of the body either as single ones or together. A growing literature suggests that the melatonergic system may be involved in the pathophysiology of mood and anxiety disorders. In fact, some core symptoms of depression show disturbance of the circadian rhythm in their clinical expression, such as diurnal mood and other symptomatic variation, or are closely linked to circadian system functioning, such as sleep-wake cycle alterations. In addition, alterations have been described in the circadian rhythms of several biological markers in depressed patients. Therefore, there is interest in developing antidepressants that have a chronobiotic effect (i.e., treatment of circadian rhythm disorders). As melatonin produces chronobiotic effects, efforts have been aimed at developing agomelatine, an antidepressant with melatonin agonist activity. The present paper reviews the role of the melatonergic system in the pathophysiology of mood and anxiety disorders and the clinical characteristics of agomelatine. Implications of agomelatine in "real world" clinical practice will be also discussed.

The effects of combining serotonin reuptake inhibition and 5-HT7 receptor blockade on circadian rhythm regulation in rodents

Disruption of circadian rhythms may lead to mood disorders. The present study investigated the potential therapeutic utility of combining a 5-HT7 antagonist with a selective serotonin (5-HT) reuptake inhibitor (SSRI), the standard of care in depression, on circadian rhythm regulation. In tissue explants of the suprachiasmatic nucleus (SCN) from PER2::LUC mice genetically modified to report changes in the expression of a key clock protein, the period length of PER2 bioluminescence was shortened in the presence of AS19, a 5-HT7 partial agonist. This reduction was blocked by SB269970, a selective 5-HT7 antagonist. The SSRI, escitalopram, had no effect alone on period length, but a combination with SB269970, yielded significant increases. Dosed in vivo, escitalopram had little impact on the occurrence of activity onsets in rats given access to running wheels, whether the drug was given acutely or sub-chronically. However, preceding the escitalopram treatment with a single acute dose of SB269970 produced robust phase delays, in keeping with the in vitro explant data. Taken together, these findings suggest that the combination of an SSRI and a 5-HT7 receptor antagonist has a greater impact on circadian rhythms than that observed with either agent alone, and that such a multimodal approach may be of therapeutic value in treating patients with poor clock function.

Circadian rhythmicity in serotonin transporter knockout mice

AIMS

Serotonin transporter knockout (5-HTT KO) mice exhibit elevated basal extracellular serotonin, increased depressive-like behaviors and increased rapid eye movement sleep. Because abnormalities of circadian rhythms are associated with mood disorders, we tested the hypothesis that 5-HTT KO mice would have altered circadian rhythmicity.

MAIN METHODS

Homecage locomotor activity was recorded in wild-type (WT) and KO mice under a standard 12:12 light-dark cycle. After 4weeks of recording, mice received a one-hour light pulse at circadian time (CT) 14 and then were kept under constant darkness for 3weeks.

KEY FINDINGS

There were no significant differences in amplitude, period, acrophase or total home cage locomotor activity between WT and KO mice during the 12:12 light-dark cycle or during constant darkness. The mean phase delay to a CT 14 light pulse was significantly attenuated in KO compared to WT mice.

SIGNIFICANCE

Acute increases in serotonin have been reported to attenuate photic phase shifts. The current study demonstrates that this effect is maintained in the face of a lifelong absence of 5-HTT.

MDMA induces Per1, Per2 and c-fos gene expression in rat suprachiasmatic nuclei

RATIONALE

±3,4-Methylenedioxymethamphetamine (MDMA, 'ecstasy') is a psychoactive drug that has marked effects on the serotonergic system. Serotonergic agonists are known to interact with the circadian pacemaker located in the suprachiasmatic nuclei (SCN).

OBJECTIVES

Given changes reported in the behavioral activity rhythm following MDMA treatment, the effects of MDMA on core clock gene (Per1, Per2) and c-fos expression were evaluated.

METHODS

Male Long-Evans rats (n = 72) were injected once with MDMA (5 mg/kg i.p.) or saline either at the middle of their 'rest' phase (Zeitgeber Time: ZT6) or the middle of their 'active' phase (Zeitgeber Time: ZT16) and killed at 30, 60, or 120 min posttreatment for gene expression analysis in the SCN using PCR. Behavioral rhythms of a separate group of rats (n = 20) were measured following treatment at ZT16 while they were held in constant darkness for 10 days posttreatment.

RESULTS

At ZT6, c-fos mRNA was significantly induced 120 min post-MDMA treatment but there were no significant changes in Per1 or Per2 mRNA expression. At ZT16, there were significant inductions of c-fos mRNA (30 and 60 min) and Per1 and Per2 mRNA (both 60 min) post-MDMA treatment. However, no differences in behavioral activity patterns were noted following MDMA treatment at ZT16.

CONCLUSIONS

These data provide evidence that MDMA has time of day dependent actions on SCN functioning, as evident from its induction of core clock genes that are important for generating and maintaining circadian rhythmicity.

Cocaine modulates pathways for photic and nonphotic entrainment of the mammalian SCN circadian clock

Cocaine abuse is highly disruptive to circadian physiological and behavioral rhythms. The present study was undertaken to determine whether such effects are manifest through actions on critical photic and nonphotic regulatory pathways in the master circadian clock of the mouse suprachiasmatic nucleus (SCN). Impairment of SCN photic signaling by systemic (intraperitoneal) cocaine injection was evidenced by strong (60%) attenuation of light-induced phase-delay shifts of circadian locomotor activity during the early night. A nonphotic action of cocaine was apparent from its induction of 1-h circadian phase-advance shifts at midday. The serotonin receptor antagonist, metergoline, blocked shifting by 80%, implicating a serotonergic mechanism. Reverse microdialysis perfusion of the SCN with cocaine at midday induced 3.7 h phase-advance shifts. Control perfusions with lidocaine and artificial cerebrospinal fluid had little shifting effect. In complementary in vitro experiments, photic-like phase-delay shifts of the SCN circadian neuronal activity rhythm induced by glutamate application to the SCN were completely blocked by cocaine. Cocaine treatment of SCN slices alone at subjective midday, but not the subjective night, induced 3-h phase-advance shifts. Lidocaine had no shifting effect. Cocaine-induced phase shifts were completely blocked by metergoline, but not by the dopamine receptor antagonist, fluphenazine. Finally, pretreatment of SCN slices for 2 h with a low concentration of serotonin agonist (to block subsequent serotonergic phase resetting) abolished cocaine-induced phase shifts at subjective midday. These results reveal multiple effects of cocaine on adult circadian clock regulation that are registered within the SCN and involve enhanced serotonergic transmission.

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.

Interactions of the serotonin and circadian systems: nature and nurture in rhythms and blues

The serotonin and circadian systems are principal regulatory networks of the brain. Each consists of a unique set of neurons that make widespread neural connections and a defined gene network of transcriptional regulators and signaling genes that subserve serotonergic and circadian function at the genetic level. These master regulatory networks of the brain are extensively intertwined, with reciprocal circuit connections, expression of key genetic elements for serotonin signaling in clock neurons and expression of key clock genes in serotonergic neurons. The reciprocal connections of the serotonin and circadian systems likely have importance for neurobehavioral disorders, as suggested by their convergent contribution to a similar range of mood disorders including seasonal affective disorder (SAD), bipolar disorder, and major depression, and as suggested by their overlapping relationship with the developmental disorder, autism spectrum disorder. Here we review the neuroanatomical and genetic basis for serotonin-circadian interactions in the brain, their potential relationship with neurobehavioral disorders, and recent work examining the effects on the circadian system of genetic perturbation of the serotonergic system as well as the molecular and behavioral effects of developmental imprinting of the circadian system with perinatal seasonal light cycles.

The effects of aging and chronic fluoxetine treatment on circadian rhythms and suprachiasmatic nucleus expression of neuropeptide genes and 5-HT1B receptors

Age-related changes in circadian rhythms, including attenuation of photic phase shifts, are associated with changes in the central pacemaker in the suprachiasmatic nucleus (SCN). Aging decreases expression of mRNA for vasoactive intestinal peptide (VIP), a key neuropeptide for rhythm generation and photic phase shifts, and increases expression of serotonin transporters and 5-HT(1B) receptors, whose activation inhibits these phase shifts. Here we describe studies in hamsters showing that aging decreases SCN expression of mRNA for gastrin-releasing peptide, which also modulates photic phase resetting. Because serotonin innervation trophically supports SCN VIP mRNA expression, and serotonin transporters decrease extracellular serotonin, we predicted that chronic administration of the serotonin-selective reuptake inhibitor, fluoxetine, would attenuate the age-related changes in SCN VIP mRNA expression and 5-HT(1B) receptors. In situ hybridization studies showed that fluoxetine treatment does not alter SCN VIP mRNA expression, in either age group, at zeitgeber time (ZT)6 or 13 (ZT12 corresponds to lights off). However, receptor autoradiographic studies showed that fluoxetine prevents the age-related increase in SCN 5-HT(1B) receptors at ZT6, and decreases SCN 5-HT(1B) receptors in both ages at ZT13. Therefore, aging effects on SCN VIP mRNA and SCN 5-HT(1B) receptors are differentially regulated; the age-related increase in serotonin transporter sites mediates the latter but not the former. The studies also showed that aging and chronic fluoxetine treatment decrease total daily wheel running without altering the phase of the circadian wheel running rhythm, in contrast to previous reports of phase resetting by acute fluoxetine treatment.

Circadian integration of glutamatergic signals by little SAAS in novel suprachiasmatic circuits

Background

Neuropeptides are critical integrative elements within the central circadian clock in the suprachiasmatic nucleus (SCN), where they mediate both cell-to-cell synchronization and phase adjustments that cause light entrainment. Forward peptidomics identified little SAAS, derived from the proSAAS prohormone, among novel SCN peptides, but its role in the SCN is poorly understood.

Methodology/Principal Findings

Little SAAS localization and co-expression with established SCN neuropeptides were evaluated by immunohistochemistry using highly specific antisera and stereological analysis. Functional context was assessed relative to c-FOS induction in light-stimulated animals and on neuronal circadian rhythms in glutamate-stimulated brain slices. We found that little SAAS-expressing neurons comprise the third most abundant neuropeptidergic class (16.4%) with unusual functional circuit contexts. Little SAAS is localized within the densely retinorecipient central SCN of both rat and mouse, but not the retinohypothalamic tract (RHT). Some little SAAS colocalizes with vasoactive intestinal polypeptide (VIP) or gastrin-releasing peptide (GRP), known mediators of light signals, but not arginine vasopressin (AVP). Nearly 50% of little SAAS neurons express c-FOS in response to light exposure in early night. Blockade of signals that relay light information, via NMDA receptors or VIP- and GRP-cognate receptors, has no effect on phase delays of circadian rhythms induced by little SAAS.

Conclusions/Significance

Little SAAS relays signals downstream of light/glutamatergic signaling from eye to SCN, and independent of VIP and GRP action. These findings suggest that little SAAS forms a third SCN neuropeptidergic system, processing light information and activating phase-shifts within novel circuits of the central circadian clock.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Alcohol usage and abrupt cessation modulate diurnal activity

Alcohol has many effects throughout the body. The effect on circadian rhythms and the correlation of these effects to withdrawal effects of alcohol present interesting findings. By measuring 3 planes of activity of female Sprague-Dawley rats during alcohol usage and continuing study through the first 2 days following withdrawal of alcohol allow for the observation of a drastic modulation of the circadian pattern of activity.

Interactions between cognition and circadian rhythms: attentional demands modify circadian entrainment

Animals and humans are able to predict and synchronize their daily activity to signals present in their environments. Environmental cues are most often associated with signaling the beginning or the end of a daily activity cycle, but they can also be used to time the presentation or availability of scarce resources. If the signal occurs consistently, animals can begin to anticipate its arrival and ultimately become entrained to its presence. While many stimuli can produce anticipation for a daily event, these events rarely lead to changes in activity patterns during the rest of the circadian cycle. Here the authors demonstrate that performance of a task requiring sustained attention not only produces entrainment, but produces a robust modification in the animals' activity throughout the entire circadian cycle. In particular, normally nocturnal rats, when trained during the light phase (ZT 4) adopted a significant and reversible diurnal activity pattern. Of importance, control experiments demonstrated that this entrainment could not be attributed to the noncognitive components of task performance, such as handling, water deprivation, access to water used as a reward, or animal activity associated with operant training. These findings additionally indicate that levels of cognitive performance are modulated by the circadian cycle and that such activity can act as a highly effective entrainment signal. These results form the basis for future research on the role of neuronal systems mediating interactions between cognitive activity and circadian rhythms.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

Regulation of Melanopsin Expression

Circadian rhythms in mammals are adjusted daily to the environmental day/night cycle by photic input via the retinohypothalamic tract (RHT). Retinal ganglion cells (RGCs) of the RHT constitute a separate light-detecting system in the mammalian retina used for irradiance detection and for transmission to the circadian system and other non-imaging forming processes in the brain. The RGCs of the RHT are intrinsically photosensitive due to the expression of melanopsin, an opsin-like photopigment. This notion is based on anatomical and functional data and on studies of mice lacking melanopsin. Furthermore, heterologous expression of melanopsin in non-neuronal mammalian cell lines was found sufficient to render these cells photosensitive. Even though solid evidence regarding the function of melanopsin exists, little is known about the regulation of melanopsin gene expression. Studies in albino Wistar rats showed that the expression of melanopsin is diurnal at both the mRNA and protein levels. The diurnal changes in melanopsin expression seem, however, to be overridden by prolonged exposure to light or darkness. Significant increase in melanopsin expression was observed from the first day in constant darkness and the expression continued to increase during prolonged exposure in constant darkness. Prolonged exposure to constant light, on the other hand, decreased melanopsin expression to an almost undetectable level after 5 days of constant light. The induction of melanopsin by darkness was even more pronounced if darkness was preceded by light suppression for 5 days. These observations show that dual mechanisms regulate melanopsin gene expression and that the intrinsic light-responsive RGCs in the albino Wistar rat adapt their expression of melanopsin to environmental light and darkness.

New light on the serotonergic paradox in the rat circadian system

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

Relationship between circadian typology and big five personality domains

We explored the relationship between personality, based on the five-factor model, and circadian preference. To this end, 503 participants (280 females, 223 males) were administered the Morningness-Eveningness Questionnaire (MEQ) and the self-report version of the Big Five Observer (BFO) to determine circadian preference and personality features, respectively. Morning types scored significantly higher than evening and intermediate types on the conscientiousness factor. Evening types were found to be more neurotic than morning types. With reference to the big five personality model, our data, together with those of all the previous studies, indicate that the conscientiousness domain is the one that best discriminates among the three circadian types. Results are discussed with reference to neurobiological models of personality.

Serotonergic potentiation of dark pulse-induced phase-shifting effects at midday in hamsters

In mammals, resetting of the suprachiasmatic clock (SCN) by behavioral activation or serotonin (5-HT) agonists is mimicked by dark pulses, presented during subjective day in constant light (LL). Because behavioral resetting may be mediated in part by 5-HT inputs to the SCN, here we determined whether 5-HT system can modulate dark-induced phase-shifts in Syrian hamsters housed in LL. Two hours of darkness at mid-subjective day (circadian time 6; CT-6) resulted in increased concentrations of 5-HT in the SCN tissue and induction of c-FOS expression in the raphe nuclei. Injections of the 5-HT(1A/7) agonist +8-OH-DPAT or dark pulses at CT-6 induced phase-advances of the wheel-running activity rhythm and down-regulated the expression of the clock genes Per1-2 and c-FOS in the SCN in a similar way. The combination of both treatments [+8-OH-DPAT + dark pulses], however, resulted in larger phase-advances, while associated molecular changes were not significantly modified, except for the gene Dbp, in comparison to +8-OH-DPAT or dark pulses alone. Dark resetting was blocked by pre-treatment with a 5-HT(7) antagonist, but not with a 5-HT(1A) antagonist. The additive phase-shifts of two different cues to reset the SCN clock open wide the gateway for non-photic shifting, leading to new strategies in chronotherapy.

Intrinsic neuronal rhythms in the suprachiasmatic nuclei and their adjustment

The central role of the suprachiasmatic nuclei in regulating mammalian circadian rhythms is well established. We study the temporal organization of neuronal properties in the suprachiasmatic nucleus (SCN) using a rat hypothalamic brain slice preparation. Electrical properties of single neurons are monitored by extra-cellular and whole-cell patch recording techniques. The ensemble of neurons in the SCN undergoes circadian changes in spontaneous activity, membrane properties and sensitivity to phase adjustment. At any point in this cycle, diversity is observed in individual neurons' electrical properties, including firing rate, firing pattern and response to injected current. Nevertheless, the SCN generate stable, near 24 h oscillations in ensemble neuronal firing rate for at least three days in vitro. The rhythm is sinusoidal, with peak activity, a marker of phase, appearing near midday. In addition to these electrophysiological changes, the SCN undergoes sequential changes in vitro in sensitivities to adjustment. During subjective day, the SCN progresses through periods of sensitivity to cyclic AMP, serotonin, neuropeptide Y, and then to melatonin at dusk. During the subjective night, sensitivities to glutamate, cyclic GMP and then neuropeptide Y are followed by a second period of sensitivity to melatonin at dawn. Because the SCN, when maintained in vitro, is under constant conditions and isolated from afferents, these changes must be generated within the clock in the SCN. The changing sensitivities reflect underlying temporal domains that are characterized by specific sets of biochemical and molecular relationships which occur in an ordered sequence over the circadian cycle.

A non-photic gateway to the circadian clock of hamsters

This paper considers the neural mechanisms underlying a particular kind of non-photic phase shifting, that produced by novelty-induced wheel running in the hamster. The projection from the intergeniculate leaflet (IGL) to the suprachiasmatic nucleus (SCN) appears to be an important part of the mechanism mediating such phase shifts. A number of experiments support this view. First, expression of immediate-early genes in the IGL is induced by non-photic phase-shifting stimuli. Second, Fos-like immunoreactivity in the IGL co-localizes with neuropeptide Y (NPY) immunoreactivity. Third, direct application of NPY to the SCN produces phase shifts which do not depend on the hamsters becoming active following the injections. Fourth, blocking the normal actions of NPY at the SCN blocks or greatly attenuates the phase shifting that is normally produced by novelty-induced wheel running. Progress on the physiological basis of phase shifts associated with activity, or a correlate, depends on understanding the behavioural aspects of this phenomenon. The activity-shift response curve is especially useful.

Chicken lateral septal organ and other circumventricular organs form in a striatal subdomain abutting the molecular striatopallidal border

The avian lateral septal organ (LSO) is a telencephalic circumventricular specialization with liquor-contacting neurons (Kuenzel and van Tienhoven [1982] J. Comp. Neurol. 206:293-313). We studied the topological position of the chicken LSO relative to molecular borders defined previously within the telencephalic subpallium (Puelles et al. [2000] J. Comp. Neurol. 424:409-438). Differential expression of Dlx5 and Nkx2.1 homeobox genes, or the Shh gene encoding a secreted morphogen, allows distinction of striatal, pallidal, and preoptic subpallial sectors. The chicken LSO complex was characterized chemoarchitectonically from embryonic to posthatching stages, by using immunohistochemistry for calbindin, tyrosine hydroxylase, NKX2.1, and BEN proteins and in situ hybridization for Nkx2.1, Nkx2.2, Nkx6.1, Shh, and Dlx5 mRNA. Medial and lateral parts of LSO appear, respectively, at the striatal part of the septum and adjacent bottom of the lateral ventricle (accumbens), in lateral continuity with another circumventricular organ that forms along a thin subregion of the entire striatum, abutting the molecular striatopallidal boundary; we called this the "striatopallidal organ" (SPO). The SPO displays associated distal periventricular cells, which are lacking in the LSO. Moreover, the SPO is continuous caudomedially with a thin, linear ependymal specialization found around the extended amygdala and preoptic areas. This differs from SPO and LSO in some molecular aspects. We tentatively identified this structure as being composed of an "extended amygdala organ" (EAO) and a "preoptohypothalamic organ" (PHO). The position of LSO, SPO, EAO, and PHO within a linear Dlx5-expressing ventricular domain that surrounds the Nkx2.1-expressing pallidopreoptic domain provides an unexpected insight into possible common and differential causal mechanisms underlying their formation.

Behavioral characterization and modulation of circadian rhythms by light and melatonin in C3H/HeN mice homozygous for the RORbeta knockout

This study reports for the first time the effects of retinoid-related orphan receptors [RORbeta; receptor gene deletion RORbeta(C3H)(-/-)] in C3H/HeN mice on behavioral and circadian phenotypes. Pineal melatonin levels showed a robust diurnal rhythm with high levels at night in wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The RORbeta(C3H)(-/-) mice displayed motor ("duck gait," hind paw clasping reflex) and olfactory deficits, and reduced anxiety and learned helplessness-related behaviors. Circadian rhythms of wheel-running activity in all genotypes showed entrainment to the light-dark (LD) cycle, and free running in constant dark, with RORbeta(C3H)(-/-) mice showing a significant increase in circadian period (tau). Melatonin administration (90 microg/mouse sc for 3 days) at circadian time (CT) 10 induced phase advances, while exposure to a light pulse (300 lux) at CT 14 induced phase delays of circadian activity rhythms of the same magnitude in all genotypes. In RORbeta(C3H)(-/-) mice a light pulse at CT 22 elicited a larger phase advance in activity rhythms and a slower rate of reentrainment after a 6-h advance in the LD cycle compared with (+/+) mice. Yet, the rate of reentrainment was significantly advanced by melatonin administration at the new dark onset in both (+/+) and (-/-) mice. We conclude that the RORbeta nuclear receptor is not involved in either the rhythmic production of pineal melatonin or in mediating phase shifts of circadian rhythms by melatonin, but it may regulate clock responses to photic stimuli at certain time domains.

Serotonin-caused phase shift of circadian rhythmicity in a photosensitive neuron

In the sixth abdominal ganglion (sixth AG) of the crayfish, two photosensitive neurons are located and have been identified as caudal photoreceptors (CPRs). We have expanded our investigation on the role of 5-Hydroxytryptamine (5-HT) as a modulator of the spontaneous and light-induced activity of the CPR. We located, by using immunocytochemistry, neurons in the sixth AG that contain the 5HT1A receptor. The expression of these receptors was examined by binding assays with [3H] 8-hydroxy-2 (di-n-propylamino) tetralin ([3H(8-OH-DPAT). We examined the exogenous action of both 5HT and its agonist 8-OH-DPAT on the phase of circadian rhythms of the spontaneous electrical activity and the photoresponse of the CPR in the isolated sixth AG by conventional extracellular recording methods. Experiments were made on the adult crayfish Procambarus clarkii and Cherax quadricarinatus. Thirteen immunopositive neurons were located, principally near the ventral and dorsal surface of the sixth AG, with the mean diameter of their somata 20+/-3 microm. The specific binding data showed the presence of 5-HT1A receptors with a mean level of 22.4+/-6.6 fmol/mg of wet tissue. Spontaneous and light-induced electrical activity of the CPR showed circadian variations with their activity more intense at night than in the day. Exogenous application of 5-HT or 8-OH-DPAT causes a circadian phase-shift in electrical activity of the CPR. Taken together, these results lead us to believe the 5-HT acts as a modulator of circadian electrical activity of the CPR in the isolated sixth AG of crayfish. Moreover, it suggests that the 5-HT1A receptor participates in this modulation.

Light-evoked FOS induction within the suprachiasmatic nuclei (SCN) of melanopsin knockout (Opn4-/-) mice: a developmental study

The aims of this study were to address three related questions: (1) Do the photosensitive ganglion cells of the mouse convey light information to the suprachiasmatic nuclei (SCN) at P0? (2) Do the differentiating rods and cones contribute to light-evoked FOS induction within the murine SCN at P4? (3) How does light-evoked FOS induction within the SCN of melanopsin knockout (Opn4-/-) mice differ at P4 and P14? Our approaches took advantage of the published descriptions of murine ocular development, melanopsin knockout (Opn4-/-) mouse, and light-induced expression of FOS (the phosphoprotein product of immediate early gene c-fos) within the SCN as a marker of retinohypothalamic tract competence. Collectively, our results show that photosensitive melanopsin-dependent retinal ganglion cells provide light information to the murine SCN on the day of birth, and possibly beforehand, and that developing rods and cones fail to provide light information to the SCN during early postnatal life. On the basis of previous publications and data presented here, we suggest that at ages around P14 the rods and cones might be capable of fully compensating for the loss of melanopsin-photosensitive ganglion cells if exposure to light is of sufficiently long duration. These results are related to the broader context of recent findings and the potential role(s) of a neonatal photoreceptor.

Inner retinal photoreception independent of the visual retinoid cycle

Mice lacking the visual cycle enzymes RPE65 or lecithin-retinol acyl transferase (Lrat) have pupillary light responses (PLR) that are less sensitive than those of mice with outer retinal degeneration (rd/rd or rdta). Inner retinal photoresponses are mediated by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs), suggesting that the melanopsin-dependent photocycle utilizes RPE65 and Lrat. To test this hypothesis, we generated rpe65(-/-); rdta and lrat(-/-); rd/rd mutant mice. Unexpectedly, both rpe65(-/-); rdta and lrat(-/-); rd/rd mice demonstrate paradoxically increased PLR photosensitivity compared with mice mutant in visual cycle enzymes alone. Acute pharmacologic inhibition of the visual cycle of melanopsin-deficient mice with all-trans-retinylamine results in a near-total loss of PLR sensitivity, whereas treatment of rd/rd mice has no effect, demonstrating that the inner retina does not require the visual cycle. Treatment of rpe65(-/-); rdta with 9-cis-retinal partially restores PLR sensitivity. Photic sensitivity in P8 rpe65(-/-) and lrat(-/-) ipRGCs is intact as measured by ex vivo multielectrode array recording. These results demonstrate that the melanopsin-dependent ipRGC photocycle is independent of the visual retinoid cycle.

Neurotransmitters of the suprachiasmatic nuclei

There has been extensive research in the recent past looking into the molecular basis and mechanisms of the biological clock, situated in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Neurotransmitters are a very important component of SCN function. Thorough knowledge of neurotransmitters is not only essential for the understanding of the clock but also for the successful manipulation of the clock with experimental chemicals and therapeutical drugs. This article reviews the current knowledge about neurotransmitters in the SCN, including neurotransmitters that have been identified only recently. An attempt was made to describe the neurotransmitters and hormonal/diffusible signals of the SCN efference, which are necessary for the master clock to exert its overt function. The expression of robust circadian rhythms depends on the integrity of the biological clock and on the integration of thousands of individual cellular clocks found in the clock. Neurotransmitters are required at all levels, at the input, in the clock itself, and in its efferent output for the normal function of the clock. The relationship between neurotransmitter function and gene expression is also discussed because clock gene transcription forms the molecular basis of the clock and its working.

Role of nociceptin and opioid receptor like 1 on entrainment function in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus of the hypothalamus is the master circadian clock in mammals. Phase shifts in circadian locomotor activity occur when an animal is exposed to light during the subjective night. An endogenous ligand of opioid receptor like 1, nociceptin is reported to inhibit light-induced phase shifts in locomotor activity rhythm. However, little is known about the role of opioid receptor like 1 receptors in the entrainment. Therefore, we investigated the involvement opioid receptor like 1 and its endogenous ligand, intnociceptin, in the suprachiasmatic nucleus and in the entrainment of circadian rhythms in rats. In an in vitro experiment, glutamate (1 microM) -induced phase delay of suprachiasmatic nucleus neuronal activity rhythms was inhibited by nociceptin during the early subjective night. An opioid receptor like 1 antagonist, compound B (10 microM), induced a phase delay, and this effect was blocked by nociceptin (10 microM). Moreover, compound B (10 microM) potentiated the glutamate (1 microM) -induced phase delay. Fos expression in the suprachiasmatic nucleus of rats induced by photic stimulation (50 lux, 30 min) during the early subjective night was inhibited by treatment with nociceptin (0.5-10 nM, i.c.v.). The effect of nociceptin (10nM, i.c.v.) was blocked by pretreatment with compound B (30 mg/kg, i.p). In an in vivo experiment, nociceptin significantly inhibited a light-induced (300 lux, 1 h) phase delay of locomotor activity rhythms, and this effect was inhibited by Compound B. Compound B (30 mg/kg, i.p.) significantly potentiated the light-induced phase delay. Nociceptin induced a neuronal firing phase advance (in vitro) and locomotor activity rhythms (in vivo) in the daytime and this effect was blocked by Compound B. These results suggest that opioid receptor like 1 receptors have an inhibitory effect at night, and a facilitative effect in the day, on phase changes.

Overeating after restraint stress in cholecystokinin-a receptor-deficient mice

In mammals, including humans, a brain-gut hormone, cholecystokinin (CCK) mediates the satiety effect via CCK-A receptor (R). We generated CCK-AR gene-deficient (-/-) mice and found that the daily food intake, energy expenditure, and gastric emptying of a liquid meal did not change compared with those of wild-type mice. Because CCK-AR(-/-) mice show anxiolytic status, we examined the effects of restraint stress. Seven hours of restraint stress was found to significantly decrease both body weight and food intake during the subsequent 3 days in all tested animals. On the fourth day after restraint stress, the CCK-AR(-/-) mice showed a significantly higher level of daily food intake than prior to stress, and food intake recovered to prestress levels in the wild-type mice. Since peripheral CCK-AR has been known to mediate gastric emptying, both gastric emptying and gastric acid secretion were determined to examine the mechanism of overeating in CCK-AR(-/-) mice. Neither gastric emptying nor gastric acid secretion differed between CCK-AR(-/-) and wild-type mice on the fourth day after stress. In contrast, however, the contents of dopamine and its metabolites in the cerebral cortex of CCK-AR(-/-) mice were increased by stress, but were rather decreased in wild-type mice. Changes in 5-hydroxytryptamine (5-HT) and its metabolite 5HIAA did not differ between the genotypes. In conclusion, CCK-AR(-/-) mice showed overeating after restraint stress, and dopaminergic hyperfunction in the brain of these mice was observed. The present evidence suggests that the CCK-AR function, possibly via altering the dopaminergic function, might be involved in overeating after stress.

Cyclic AMP mediates circadian phase shifts induced by microinjection of serotonergic drugs in the hamster dorsal raphe nucleus

We have previously shown that pretreatment with a 5-HT(7) receptor antagonist, SB-269970-A, attenuated phase shifts induced by microinjections of serotonergic agonists in the hamster dorsal raphe (Duncan, M.J., Grear, K.E., Hoskins, M.A.; Brain Research 1008:40-48, 2004). Although SB-269970-A is highly selective for the 5-HT(7) receptors, it has moderate affinity for the 5-HT(5A) receptors, which are present in the hamster dorsal raphe. To further test whether the 5-HT(7) receptors mediate the phase shifting effect of serotonergic agonists in the dorsal raphe, we investigated the role of cAMP because this second messenger is increased by activation of the 5-HT(7) receptors but inhibited by activation of the 5-HT(5A) or 5-HT(1A) receptors. As an additional control experiment, the effect of WAY-100,635, an antagonist to the 5-HT(1A) receptors, was tested. The results showed that local administration of Rp-cAMPS (1 microM), a cAMP antagonist, significantly reduced the phase shift induced by the 5-HT(1A/5A/7) agonist, (R)-(+)8-hydroxy-2-(di-n-propylamino)tetralin (10 microM), microinjected into the dorsal raphe 6 h before lights off. Furthermore, microinjection of 8-bromo-cAMP (50 microM) induced significantly larger phase shifts than vehicle. In the last experiment, microinjection of the dorsal raphe with WAY-100,635 (50 nM) before the 5-HT(1A/5A/7) agonist, 5-carboxyamidotryptamine (100 nM), did not significantly affect the phase shift. These results show that activation of cAMP-dependent kinase by cAMP is necessary and sufficient for induction of phase shifts by serotonergic drugs in the hamster dorsal raphe. Furthermore, these findings are consistent with the hypothesis that the 5-HT(7) but not the 5-HT(5A) or 5-HT(1A) receptors mediate serotonergic phase shifts.