Serotonergic regulation of circadian rhythms in Syrian hamsters

Genesis of the Master Circadian Pacemaker in Mice

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central circadian clock of mammals. It is responsible for communicating temporal information to peripheral oscillators via humoral and endocrine signaling, ultimately controlling overt rhythms such as sleep-wake cycles, body temperature, and locomotor activity. Given the heterogeneity and complexity of the SCN, its genesis is tightly regulated by countless intrinsic and extrinsic factors. Here, we provide a brief overview of the development of the SCN, with special emphasis on the murine system.

Time-of-day influences on respiratory sequelae following maximal electroshock-induced seizures in mice

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in refractory epilepsy patients. Although specific mechanisms underlying SUDEP are not well understood, evidence suggests most SUDEP occurs due to seizure-induced respiratory arrest. SUDEP also tends to happen at night. Although this may be due to circumstances in which humans find themselves at night, such as being alone without supervision or sleeping prone, or to independent influences of sleep state, there are a number of reasons why the night (i.e., circadian influences) could be an independent risk factor for SUDEP. We explored this possibility. Adult male WT mice were instrumented for EEG, EMG, and EKG recording and subjected to maximal electroshock (MES) seizures during wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep during the nighttime/dark phase. These data were compared with data collected following seizures induced during the daytime/light phase. Seizures induced during the nighttime were similar in severity and duration to those induced during the daytime; however, seizures induced during the nighttime were associated with a lesser degree of respiratory dysregulation and postictal EEG suppression. Seizures induced during REM sleep during the nighttime were universally fatal, as is seen when seizures are induced during REM during the daytime. Taken together, these data implicate a role for time of day in influencing the physiological consequences of seizures that may contribute to seizure-induced death.NEW & NOTEWORTHY Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. SUDEP frequently occurs during the night, which has been attributed to an effect of sleep. We have shown that sleep state does indeed influence survival following a seizure. That SUDEP occurs during the night could also implicate a circadian influence. In this study we found that time of day independently affects the physiological consequences of seizures.

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 dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

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.

Selective pharmacological blockade of the 5-HT7 receptor attenuates light and 8-OH-DPAT induced phase shifts of mouse circadian wheel running activity

Recent reports have illustrated a reciprocal relationship between circadian rhythm disruption and mood disorders. The 5-HT7 receptor may provide a crucial link between the two sides of this equation since the receptor plays a critical role in sleep, depression, and circadian rhythm regulation. To further define the role of the 5-HT7 receptor as a potential pharmacotherapy to correct circadian rhythm disruptions, the current study utilized the selective 5-HT7 antagonist JNJ-18038683 (10 mg/kg) in three different circadian paradigms. While JNJ-18038683 was ineffective at phase shifting the onset of wheel running activity in mice when administered at different circadian time (CT) points across the circadian cycle, pretreatment with JNJ-18038683 blocked non-photic phase advance (CT6) induced by the 5-HT1A/7 receptor agonist 8-OH-DPAT (3 mg/kg). Since light induced phase shifts in mammals are partially mediated via the modulation of the serotonergic system, we determined if JNJ-18038683 altered phase shifts induced by a light pulse at times known to phase delay (CT15) or advance (CT22) wheel running activity in free running mice. Light exposure resulted in a robust shift in the onset of activity in vehicle treated animals at both times tested. Administration of JNJ-18038683 significantly attenuated the light induced phase delay and completely blocked the phase advance. The current study demonstrates that pharmacological blockade of the 5-HT7 receptor by JNJ-18038683 blunts both non-photic and photic phase shifts of circadian wheel running activity in mice. These findings highlight the importance of the 5-HT7 receptor in modulating circadian rhythms. Due to the opposite modulating effects of light resetting between diurnal and nocturnal species, pharmacotherapy targeting the 5-HT7 receptor in conjunction with bright light therapy may prove therapeutically beneficial by correcting the desynchronization of internal rhythms observed in depressed individuals.

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.

Timing, sleep, and respiration in health and disease

Breathing is perhaps the physiological function that is most vital to human survival. Without breathing and adequate oxygenation of tissues, life ceases. As would be expected for such a vital function, breathing occurs automatically, without the requirement of conscious input. Breathing is subject to regulation by a variety of factors including circadian rhythms and vigilance state. Given the need for breathing to occur continuously with little tolerance for interruption, it is not surprising that breathing is subject to both circadian phase-dependent and vigilance-state-dependent regulation. Similarly, the information regarding respiratory state, including blood-gas concentrations, can affect circadian timing and sleep-wake state. The exact nature of the interactions between breathing, circadian phase, and vigilance state can vary depending upon the species studied and the methodologies employed. These interactions between breathing, circadian phase, and vigilance state may have important implications for a variety of human diseases, including sleep apnea, asthma, sudden unexpected death in epilepsy, and sudden infant death syndrome.

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.

Circadian rhythm connections to oxidative stress: implications for human health

SIGNIFICANCE

Oxygen and circadian rhythmicity are essential in a myriad of physiological processes to maintain homeostasis, from blood pressure and sleep/wake cycles, down to cellular signaling pathways that play critical roles in health and disease. If the human body or cells experience significant stress, their ability to regulate internal systems, including redox levels and circadian rhythms, may become impaired. At cellular as well as organismal levels, impairment in redox regulation and circadian rhythms may lead to a number of adverse effects, including the manifestation of a variety of diseases such as heart diseases, neurodegenerative conditions, and cancer.

RECENT ADVANCES

Researchers have come to an understanding as to the basics of the circadian rhythm mechanism, as well as the importance of the numerous species of oxidative stress components. The effects of oxidative stress and dysregulated circadian rhythms have been a subject of intense investigations since they were first discovered, and recent investigations into the molecular mechanisms linking the two have started to elucidate the bases of their connection.

CRITICAL ISSUES

While much is known about the mechanics and importance of oxidative stress systems and circadian rhythms, the front where they interact has had very little research focused on it. This review discusses the idea that these two systems are together intricately involved in the healthy body, as well as in disease.

FUTURE DIRECTIONS

We believe that for a more efficacious management of diseases that have both circadian rhythm and oxidative stress components in their pathogenesis, targeting both systems in tandem would be far more successful.

The medial prefrontal cortex is both necessary and sufficient for the acquisition of conditioned defeat

We have previously demonstrated that the basolateral amygdala (BLA) is a key component of a neural circuit mediating memory formation for emotionally relevant stimuli in an ethologically-based model of conditioned fear, termed conditioned defeat (CD). In this model, subjects are socially defeated by a larger, more aggressive hamster. Upon subsequent exposure to a smaller, non-aggressive intruder, the defeated animal will show high levels of submissive behaviors and fail to defend its territory. Here we examined whether the medial prefrontal cortex (mPFC), an area with extensive connections with the amygdala, is also a component of this circuit. Temporary inactivation of the mPFC using muscimol, a GABA(A) receptor agonist, significantly enhanced the acquisition but not expression of CD, while blockade of GABA(A) receptors in the mPFC using bicuculline, a GABA(A) antagonist, impaired acquisition of CD. Given these findings, we next sought to test whether plasticity related to the defeat experience occurs in the mPFC. We infused anisomycin, a protein synthesis inhibitor, in the mPFC but this treatment did not alter the acquisition of CD. In our final experiment, we demonstrated that bicuculline failed to alter the acquisition of CD. Together, these results demonstrate for the first time that while the mPFC is both necessary and sufficient for the acquisition of CD, it does not appear to mediate plasticity related to the defeat experience. In contrast, while plasticity underlying CD does appear to occur in the BLA, GABAergic receptor inhibition in the BLA is not sufficient to enhance CD. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Neural mechanisms mediating circadian phase resetting by activation of 5-HT(7) receptors in the dorsal raphe: roles of GABAergic and glutamatergic neurotransmission

5-HT(7) receptors in the dorsal raphe nucleus (DRN) influence circadian rhythms, sleep, and serotonin release. Because interactions between 5-HT(7) receptors and glutamatergic and GABAergic neurons have been demonstrated previously, the current studies tested the hypothesis that GABAergic and/or glutamatergic neurons mediate phase shifts induced by activation of DRN 5-HT(7) receptors. Hamsters were fitted with guide cannulae aimed at the DRN, housed in cages with running wheels, and exposed to 14h light (L):10h dark (D). In Experiment 1, hamsters received DRN pretreatment with muscimol (87.6 pmol) or vehicle before DRN 8-OH-DPAT (6 pmol) microinjections at ZT6. After exposure to constant darkness (10 days), phase shifts were calculated and animals were re-exposed to 14L:10D. The procedure was repeated to give each animal the alternate pretreatment. In Experiment 2, hamsters received DRN pretreatment with NMDA (20 pmol) or vehicle before 8-OH-DPAT at ZT 6. Other experiments tested the effects of single DRN microinjections of muscimol, bicuculline (136 pmol), NMDA, MK-801 (10 pmol) or vehicle. Phase shifts (mean ± S.E.M., h) in muscimol/8-OH-DPAT-microinjected hamsters (1.02 ± 0.30) were not different (P=0.11) from those in vehicle/8-OH-DPAT-microinjected hamsters (1.34 ± 0.30), while those in NMDA/8-OH-DPAT-microinjected hamsters (0.67 ± 0.17) were smaller (P<0.05) than those in vehicle/8-OH-DPAT-microinjected hamsters (0.97 ± 0.10). DRN single microinjections of bicuculline, but not muscimol, NMDA, or MK-801 induced phase advances. Bicuculline also potentiated 8-OH-DPAT-induced phase advances (P<0.05). These finding suggest that the mechanism mediating DRN 5-HT(7) receptor induction of phase advances involves decreased glutamatergic neurotransmission, and furthermore, that inhibition of DRN GABAergic neurotransmission causes a phase advance.

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.

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.

Circadian clock resetting by behavioral arousal: neural correlates in the midbrain raphe nuclei and locus coeruleus

Some procedures for stimulating arousal in the usual daily rest period (e.g., gentle handling, novel wheel-induced running) can phase shift circadian rhythms in Syrian hamsters, while other arousal procedures are ineffective (inescapable stress, caffeine, modafinil). The dorsal and median raphe nuclei (DRN, MnR) have been implicated in clock resetting by arousal and, in rats and mice, exhibit strong regionally specific responses to inescapable stress and anxiogenic drugs. To examine a possible role for the midbrain raphe nuclei in the differential effects of arousal procedures on circadian rhythms, hamsters were aroused for 3 h in the mid-rest period by confinement to a novel running wheel, gentle handling (with minimal activity) or physical restraint (with intermittent, loud compressed air stimulation) and sacrificed immediately thereafter. Regional expression of c-fos and tryptophan hydroxylase (TrpOH) were quantified immunocytochemically in the DRN, MnR and locus coeruleus (LC). Neither gentle handling nor wheel running had a large impact on c-fos expression in these areas, although the manipulations were associated with a small increase in c-Fos in TrpOH-like and TrpOH-negative cells, respectively, in the caudal interfascicular DRN region. By contrast, restraint stress significantly increased c-Fos in both TrpOH-like and TrpOH-negative cells in the rostral DRN and LC. c-Fos-positive cells in the DRN did not express tyrosine hydroxylase. These results reveal regionally specific monoaminergic correlates of arousal-induced circadian clock resetting, and suggest a hypothesis that strong activation of some DRN and LC neurons by inescapable stress may oppose clock resetting in response to arousal during the daily sleep period. More generally, these results complement evidence from other rodent species for functional topographic organization of the DRN.

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.

Brief constant light accelerates serotonergic re-entrainment to large shifts of the daily light/dark cycle

Brief ( approximately 2 day) constant light exposure (LL(b)) in hamsters dramatically enhances circadian phase-resetting induced by the 5-HT receptor agonist, (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) and other nonphotic stimuli. The present study was undertaken to determine if LL(b) can also amplify phase-resetting responses to endogenous 5-HT and accelerate re-entrainment to large-magnitude advance and delay shifts of the light/dark (LD) cycle. First, central serotonergic activity was increased by i.p. injection of L-tryptophan+/-the 5-HT reuptake inhibitor fluoxetine. Hamsters under LD or exposed to LL(b) received vehicle or drugs during the early morning, and phase-shifts of the locomotor activity rhythm were measured after release to constant darkness. Neither drug phase-shifted animals not exposed to LL(b) (P>0.5 vs. vehicle); however in animals receiving LL(b,)L-tryptophan with and without fluoxetine produced large phase-advance shifts (means=2.5+/-0.4 h and 2.6+/-0.2 h, respectively; both P<0.035 vs. vehicle). Next, the effects of LL(b) combined with 8-OH-DPAT or L-tryptophan+fluoxetine on serotonergic re-entrainment to 10 h phase-advance and phase-delay shifts of the LD cycle were assessed. In groups not exposed to LL(b), vehicle controls re-entrained slowly to the advance and delay shifts (means=16+/-1 and 24+/-4 days, respectively), but those treated with 8-OH-DPAT re-entrained faster (means=11+/-2 and 9+/-2 days, respectively; both P<0.05 vs. vehicle). In groups exposed to LL(b), vehicle controls re-entrained slowly to the advance and delay shifts (means=15+/-2 and 25+/-3 days, respectively); however those receiving 8-OH-DPAT rapidly re-entrained to the delay and advance shifts, with the majority (75%) requiring only 1-2 days (means=2+/-1 and 4+/-2 days, respectively; both P<0.05 vs. vehicle). Animals exposed to LL(b) and treated with L-tryptophan+fluoxetine also exhibited accelerated re-entrainment to a 10 h advance shift (mean=5+/-2 days; P<0.05 vs. vehicle). Thus through enhancing serotonergic phase-resetting, LL(b) facilitates rapid re-entrainment to large shifts of the LD cycle which offers a potential approach for treating circadian-related desynchronies.

Altered photic and non-photic phase shifts in 5-HT(1A) receptor knockout mice

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) is thought to be modulated by 5-HT. 5-HT is though to inhibit photic phase shifts by inhibiting the release of glutamate from retinal terminals, as well as by decreasing the responsiveness of retinorecipient cells in the SCN. Furthermore, there is also evidence that 5-HT may underlie, in part, non-photic phase shifts of the circadian system. Understanding the mechanism by which 5-HT accomplishes these goals is complicated by the wide variety of 5-HT receptors found in the SCN, the heterogeneous organization of both the circadian clock and the location of 5-HT receptors, and by a lack of sufficiently selective pharmacological agents for the 5-HT receptors of interest. Genetically modified animals engineered to lack a specific 5-HT receptor present an alternative avenue of investigation to understand how 5-HT regulates the circadian system. Here we examine behavioral and molecular responses to both photic and non-photic stimuli in mice lacking the 5-HT(1A) receptor. When compared with wild-type controls, these mice exhibit larger phase advances to a short late-night light pulse and larger delays to long 12 h light pulses that span the whole subjective night. Fos and mPer1 expression in the retinorecipient SCN is significantly attenuated following late-night light pulses in the 5-HT(1A) knockout animals. Finally, non-photic phase shifts to (+/-)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) are lost in the knockout animals, while attenuation of the phase shift to the long light pulse due to rebound activity following a wheel lock is unaffected. These findings suggest that the 5-HT(1A) receptor plays an inhibitory role in behavioral phase shifts, a facilitatory role in light-induced gene expression, a necessary role in phase shifts to 8-OH-DPAT, and is not necessary for activity-induced phase advances that oppose photic phase shifts to long light pulses.

Asymmetries of vestibular dysfunction in major depression

Depression is characterized by alterations in the circadian secretion of hormones, sleep and motor activity, all of which are regulated by suprachiasmatic nuclei (SCN). The vestibular system in the inner ear registers the amount of motor activity. To test the integrity of this motion sensitive system in depression, we studied the vestibulo-ocular reflex (VOR) in depressive patients who were not taking medication and healthy control subjects, which allowed us to investigate each ear and its corresponding nerve centers. Ocular reflex movement depends on vestibular nuclei activity, and we found that at 30 degrees C stimulation the right vestibular system in depressive patients has approximately half the activity of the left side. Significant asymmetry was not detected in control subjects. We also found a significant decrease in the slow phase (16.92+/-9.13 degrees/s) of the reflex in the depressed group as compared with the control group (43.77+/-16.04 degrees/s). The vestibular nuclei of the right and left sides are hypoactive. Specifically, the right vestibular nucleus is hypoactive in depressed people and can easily be measured using VOR. These results support the abnormal asymmetries hypothesis of depression and suggest that these asymmetries also exist at the level of the brain stem or neuronal centers that are afferents to right vestibular nuclei, like SCN or raphe nuclei.

Serotonergic pre-treatments block in vitro serotonergic phase shifts of the mouse suprachiasmatic nucleus circadian clock

The suprachiasmatic nucleus (SCN) contains a circadian clock that maintains its time-generating and phase-modulating capacities in vitro. Previous studies report clear differences in the ability of serotonergic stimuli to phase-shift the SCN clock when applied directly to the SCN either in vivo or in vitro: while mice and rat circadian clocks are readily phase-shifted by serotonin (5-HT) or 5-HT agonists applied in vitro, hamster and mice circadian clocks respond inconsistently to 5-HT agonists injected directly into the SCN in vivo. Here we have investigated one possible explanation for these differences: that the SCN isolated in vitro experiences reduced endogenous 5-HT signaling, which increases clock sensitivity to subsequent 5-HT stimulation. For these experiments we treated mouse SCN brain slices with low concentrations of compounds that increase serotonin signaling: 5-HT, a 5-HT agonist (8-OH-DPAT), the 5-HT precursor, l-tryptophan, or the 5-HT re-uptake inhibitor, fluoxetine. Pretreatment with each of these substances completely blocked subsequent phase-shifts induced by mid-subjective day treatment with either 5-HT or 8-OH-DPAT, while they did not block phase-shifts induced by the adenylate cyclase activator, forskolin. Time-course data on l-tryptophan-induced inhibition are consistent with this treatment inducing receptor internalization, while timing of the recovery from inhibition is consistent with receptor reinsertion. Together these data support the hypothesis that SCN clock sensitivity to serotonergic phase modulation is affected by the amount of prior serotonin signaling present in the SCN, and that this signaling alters the density of surface 5-HT receptors on SCN clock neurons.

Modulation of photic response by the metabotropic glutamate receptor agonist t-ACPD

Glutamate is the primary excitatory transmitter in the hypothalamus. It conveys photic information to the suprachiasmatic nucleus of the hypothalamus, thereby entraining the circadian clock to environmental light cycles. While ionotropic glutamate receptors have been implicated in the transduction of photic information in suprachiasmatic nucleus cells, there is evidence that metabotropic glutamate receptors play a significant modulatory role. We investigated the effects of the metabotropic glutamate agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) on light-evoked phase responses in Syrian hamsters at three phase points: circadian time 6, a time when light has no effect on the circadian timing system; circadian time 13.5, when light evokes the maximum phase delay; circadian time 19, the maximum phase advance. We found that ACPD significantly increased the light-evoked phase shift at circadian time 13.5, and had no effect at other phase points tested. These data support a role for metabotropic glutamate receptors in the circadian photic signal transduction system.

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

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

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

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

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.

5-HT1B Receptor-Mediated Presynaptic Inhibition of GABA Release in the Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) receives a dense serotonergic innervation that modulates photic input to the SCN via serotonin 1B (5-HT1B) presynaptic receptors on retinal glutamatergic terminals. However, the majority of 5-HT1B binding sites in the SCN are located on nonretinal terminals and most axonal terminals in the SCN are GABAergic. We therefore tested the hypothesis that 5-HT1B receptors might also be located on SCN GABAergic terminals by examining the effects of the highly selective 5-HT1B receptor agonist CP-93,129 on SCN miniature inhibitory postsynaptic currents (mIPSCs). Whole cell patch-clamp recordings of mIPSCs were obtained from rat and mouse SCN neurons in hypothalamic slices. Using CsCl-containing microelectrodes with QX314, we isolated mPSCs that were sensitive to the GABAA receptor antagonist, bicuculline. Bath application of CP-93,129 (1 μM) decreased the frequency of mIPSCs by an average of 22% ( n = 7) in rat SCN neurons and by an average of 30% ( n = 8) in mouse SCN neurons with no clear effect on mIPSC amplitude. In mice lacking functional 5-HT1B receptors, CP-93,129 (1 μM) had no clear effect on the frequency or the amplitude of mIPSCs recorded in any of the cells tested ( n = 4). The decrease in the frequency of mIPSCs of SCN neurons produced by the selective 5-HT1B receptor agonist CP-93,129 is consistent with the interpretation that 5-HT1B receptors are located on GABA terminals in the SCN and that 5-HT inhibits GABA release via a 5-HT1B presynaptic receptor-mediated mechanism.

Pharmacological modulation of circadian rhythms: a new drug target in psychotherapeutics

Daily variation in an organism's physiology and behaviour is regulated by the synchrony that is achieved between the internal timing mechanisms - the circadian rhythms of the biological clock - and the prevailing environmental cues. Proper synchrony constitutes an adaptive response; improper or lost synchrony may well yield maladaptation and, in the case of humans, a psychiatric disorder. On a basic level, the circadian system is comprised of three parts: a central oscillator, its various neuronal inputs and its outputs. For all three of these parts, the dissemination of new information is moving at an unprecedented pace, and the number of molecular targets for the opportunistic pharmacologist is growing in step. Monoamines, neuropeptides, kinases - sorting through all these, much less developing one into a drug discovery programme, may be the biggest challenge. However, the potential benefits in targeting a basic flaw in a fundamental biological system may be enormous.

Effect of Haloperidol on mPer1 Gene Expression in Mouse Suprachiasmatic Nuclei

The effect of a typical neuroleptic haloperidol (Hal) on mPer1 gene expression was investigated in mouse suprachiasmatic nuclei (SCN). Hal induced mPer1 mRNA levels both in vivo and in cultured SCN cells. For mechanisms underlying Hal-induced mPer1 expression, N-methyl-d-aspartate (NMDA) glutamate receptor subtype, the phosphorylation form of the transcription factor, and the Ser-133 phosphorylation form of cAMP-responsive element-binding protein (CREB) played an important role, because the induction of mPer1 mRNA significantly decreased after pretreatment with a non-competitive NMDA receptor antagonist, such as MK-801 or CREB antisense. These results suggest that Hal may increase CREB phosphorylation and mPer1 expression according to the activation of the NMDA receptor through the dopaminergic pathways. Although the injection of Hal during the light period increased the amplitude of mPer1 mRNA rhythmicity in a nondrug state, the injection of the drug during the dark period disturbed the rhythmic pattern of mPer1 mRNA. These results suggest that the rhythmicity of clock genes in SCN may be disturbed depending on the dosing time of Hal. On the other hand, because the induction of mPer1 mRNA by Hal seems to be at least partly caused by the NMDA receptor, showing a phase shift or resetting effect of the circadian clock, Hal may also cause such phase shift effects.

Let there be “more” light: enhancement of light actions on the circadian system through non-photic pathways

Circadian rhythms are internally generated circa 24 h rhythms. The phase of the circadian pacemaker in mammals can be adjusted by external stimuli such as the daily cycle of light, as well as by internal stimuli such as information related to the physiological and behavioral status of the organism, collectively called "non-photic stimuli". We review a large number of studies regarding photic-non-photic interactions on the circadian system, with special focus on two widely described neurotransmitters associated with non-photic input pathways: neuropeptide Y (NPY) and serotonin 5-HT. Both neurotransmitters are capable of phase advancing the master pacemaker oscillation when applied during the subjective day, as do several behavioral manipulations. Also, both are capable of inhibiting light-induced phase shifts during the subjective night, suggesting a dynamic interaction between photic and non-photic stimuli in the fine-tuning of the pacemaker function. Suppression of the NPYergic and/or serotonergic non-photic input pathways can in turn potentiate the phase-shifting effects of light. These findings pose new questions about the possibility of a physiological role for the dynamic interaction between photic and non-photic inputs. This might be particularly important in the case of circadian system adjustments under certain conditions, such as depression, shift work or jet lag.

Aging and SB-269970-A, a selective 5-HT7 receptor antagonist, attenuate circadian phase advances induced by microinjections of serotonergic drugs in the hamster dorsal raphe nucleus

Aging leads to many changes in the circadian timekeeping system, including reduced sensitivity to phase-resetting signals such as systemic administration of the serotonergic agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). In previous studies, we observed an age-related decrease in 5-HT7 receptor binding sites, one of the receptor subtypes that is activated by 8-OH-DPAT, in the dorsal raphe nucleus. In this study, we tested the hypotheses that (1) aging reduces circadian phase shifts induced by local administration of 8-OH-DPAT (30 microM, i.e., 1.97 ng) or 5-carboxamidotryptamine (5-CT, 100 nM, i.e., 6.39 pg), another serotonin agonist, into the dorsal raphe and (2) 5-HT7 receptors mediate the phase shifts induced by administration of 5-CT and 8-OH-DPAT into the dorsal raphe. Young (3-5 months), middle-aged (12-13 months) and old hamsters (17-19 months) were surgically implanted with chronic guide cannulae aimed at the dorsal raphe, and were housed in cages equipped with running wheels. Aging significantly inhibited (P<0.01) the phase advances in running-wheel rhythms induced by 8-OH-DPAT microinjected during the midsubjective day. 5-CT induced phase advances tended to decrease with aging, but this effect was not significant (P<0.12). Microinjection of the selective 5-HT7 receptor antagonist, SB-269970-A (50-5000 nM, i.e., 0.39-390 pg), 15 min before microinjection of 5-CT or 8-OH-DPAT into the dorsal raphe of young hamsters, significantly inhibited phase shifts. In conjunction with our previous study, these findings indicate that an age-related reduction in 5-HT7 receptors in the dorsal raphe nucleus is an important neurochemical mechanism leading to aging deficits in the circadian timekeeping system.

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

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

8-OH-DPAT as a 5-HT7 agonist: phase shifts of the circadian biological clock through increases in cAMP production

Neurons in the suprachiasmatic nucleus (SCN), the site of the endogenous biological clock in mammals, fire spontaneously, peaking in firing rate near ZT6 or at the midpoint of the light phase in a 12:12 light-dark cycle. In rat hypothalamic slices, tissue incubations with drugs can produce a shift in this daily rhythm, the magnitude of which is dependent upon dose and the time of treatment. Previous work with 8-OH-DPAT had noted its ability to produce a phase advance, an earlier occurrence of the peak in neuronal firing, when applied at ZT6. Activation of 5-HT7 receptors was thought to be responsible for the shift, despite the clear preference of 8-OH-DPAT for 5-HT1A sites in terms of receptor binding affinity. In the present work, the actions of 8-OH-DPAT in SCN slices were confirmed and expanded to include additional dose-response and antagonist treatments. By itself, 8-OH-DPAT produced a concentration-dependent phase advance that was sensitive to co-application with 5-HT7 antagonists (ritanserin, mesulergine, SB-269970), but not to 5-HT1A antagonists (WAY-100,635, UH-301). Assignment of the receptor mechanisms for the antagonists employed was accomplished in experiments measuring binding affinities and the generation of cAMP, the latter monitored in a HEK-293 cell line expressing the r5-HT7 receptor and in tissue derived from rat SCN. The results indicate that the increases observed in cAMP levels are small but appear to be sufficient to produce a pharmacological resetting of the clock pacemaker. By aiding in the identification of the 5-HT receptor subtype responsible for the observed phase shifts and cAMP changes, 8-OH-DPAT represents an important pharmacological tool for 5-HT7 receptor activation, essentially broadening its role as the prototypical 5-HT1A agonist to one combining these two receptor activities.

Novel phase-shifting effects of GABAA receptor activation in the suprachiasmatic nucleus of a diurnal rodent

The vast majority of neurons in the suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals, contain the inhibitory neurotransmitter GABA. Most studies investigating the role of GABA in the SCN have been performed using nocturnal rodents. Activation of GABA(A) receptors by microinjection of muscimol into the SCN phase advances the circadian activity rhythm of nocturnal rodents, but only during the subjective day. Nonphotic stimuli that reset the circadian pacemaker of nocturnal rodents also produce phase advances during the subjective day. The role of GABA in the SCN of diurnal animals and how it may differ from nocturnal animals is not known. In the studies described here, the GABA(A) agonist muscimol was microinjected directly into the SCN region of diurnal unstriped Nile grass rats (Arvicanthis niloticus) at various times in their circadian cycle. The results demonstrate that GABA(A) receptor activation produces large phase delays during the subjective day in grass rats. Treatment with TTX did not affect the ability of muscimol to induce phase delays, suggesting that muscimol acts directly on pacemaker cells within the SCN. These data suggest that the circadian pacemakers of nocturnal and diurnal animals respond to the most abundant neurochemical signal found in SCN neurons in opposite ways. These findings are the first to demonstrate a fundamental difference in the functioning of circadian pacemaker cells in diurnal and nocturnal animals.

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.

Reciprocal serotonergic connections between the hamster median and dorsal raphe nuclei

The median (MnR), but not the dorsal (DR) raphe, sends a serotonergic projection to the suprachiasmatic (SCN) nucleus. Stimulation of either nucleus by electrode or serotonin agonist yields equivalent effects on circadian rhythmicity. This and other evidence suggests the existence of a functional serotonergic pathway from the DR to the MnR that may participate in circadian rhythm regulation. The present investigation was designed to identify such a connection. Tract tracer studies revealed cells in the DR that project to the MnR, as well as cells in the MnR that project to the DR. Double label immunofluorescence methods demonstrated that some of the cells projecting from either nucleus to the other contain serotonin immunoreactivity. The results support the existence of a reciprocal pathway between the DR and MnR that is at least partially serotonergic.

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

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

Response of the mouse circadian system to serotonin 1A/2/7 agonists in vivo: surprisingly little

Serotonin (5-HT) is thought to play a role in regulating nonphotic phase shifts and modulating photic phase shifts of the mammalian circadian system, but results with different species (rats vs. hamsters) and techniques (in vivo vs. in vitro; systemic vs. intracerebral drug delivery) have been discordant. Here we examined the effects of the 5-HT1A/7 agonist 8-OH-DPAT and the 5-HT1/2 agonist quipazine on the circadian system in mice, with some parallel experiments conducted with hamsters for comparative purposes. In mice, neither drug, delivered systemically at a range of circadian phases and doses, induced phase shifts significantly different from vehicle injections. In hamsters, quipazine intraperitoneally (i.p.) did not induce phase shifts, whereas 8-OH-DPAT induced phase shifts after i.p. but not intra-SCN injections. In mice, quipazine modestly increased c-Fos expression in the SCN (site of the circadian pacemaker) during the subjective day, whereas 8-OH-DPAT did not affect SCN c-Fos. In hamsters, both drugs suppressed SCN c-Fos in the subjective day. In both species, both drugs strongly induced c-Fos in the paraventricular nucleus (within-subject positive control). 8-OH-DPAT did not significantly attenuate light-induced phase shifts in mice but did in hamsters (between-species positive control). These results indicate that in the intact mouse in vivo, acute activation of 5-HT1A/2/7 receptors in the circadian system is not sufficient to reset the SCN pacemaker or to oppose phase-shifting effects of light. There appear to be significant species differences in the susceptibility of the circadian system to modulation by systemically delivered serotonergics.

Dorsal raphe nucleus modulates neuronal activity in rat intergeniculate leaflet

Serotonergic input from midbrain raphe nuclei is believed to have a significant effect on mammalian circadian timing system. The suprachiasmatic nucleus (SCN) receives its serotonergic input from the median raphe nucleus, while the intergeniculate leaflet (IGL) receives serotonergic innervation from the dorsal raphe nucleus (DRN). The present paper was aimed at determining whether projection from the DRN affected rhythmic neuronal oscillations in the IGL of rats. We investigated the impact of electrolytic lesions and electric stimulation of the DRN on spontaneous isoperiodic (i.e. burst firing with a constant interburst interval) neuronal activity recorded in the IGL. In all our experiments a complete lesion of the DRN always caused a significant increase (ca. 100%) of spontaneous activity of IGL neurons, their oscillatory character having been maintained, though. On the other hand, electric stimulation of the DRN produced a transient decrease in firing rate oscillations of the IGL neurons. The obtained results indicate that the neuronal projection from the DRN has a substantial modulating effect on IGL activity-an important element of the mechanism of the circadian time-keeping system that mediates the transfer of non-photic information to the SCN by modulating its activity. The observed increase of isoperiodic activity in the IGL after DRN lesion and a transient decrease in this activity after electric stimulation indicate an inhibitory character of this effect. The present findings corroborate the hypothesis that the DRN is a one of the major and extremely important source of the modulatory inputs to the mammalian circadian time-keeping system.

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.

Is novel wheel inhibition of per1 and per2 expression linked to phase shift occurrence?

We studied whether access to a novel running wheel in vivo could reset the suprachiasmatic nuclei (SCN) in vitro. Golden hamsters were transferred to dim red light at Zeitgeber time (ZT) 4, given their first exposure to a running wheel for 3 h, and killed at either ZT7 or ZT9. Using a brain slice preparation, the SCN firing rate rhythm in vitro was advanced relative to controls only in the slices prepared at ZT9 (phase shift: 2.36+/-0.06 h, n=4) but not ZT7 (-0.26+/-0.16 h, n=4). Transitions to dim red light or brain slice preparation at ZT7 or ZT9 alone do not shift the rhythm. Hamsters with wheels had significantly lower levels of SCN per1 mRNA compared with controls at ZT7, and lower per2 mRNA when examined at ZT9. We conclude that 3 h of novel wheel access appears to require some extended time in vivo in order for the SCN to be reset, even beyond the time when per1 mRNA levels have been altered.

Aging alters in a region-specific manner serotonin transporter sites and 5-HT(1A) receptor-G protein interactions in hamster brain

Key proteins regulating serotonergic activity, specifically the serotonin transporter and 5-HT(1A) receptor, were examined in the midbrain raphe nuclei of young (3-4 months) and old (17-19 months) hamsters (N=7-10/group). An age-related decrease in the maximal density of serotonin transporter sites labelled with [(3)H]paroxetine (fmol/mg protein, Old: 396+/-13; Young: 487+/-27) was observed in the dorsal raphe nucleus (DRN) but not the median raphe nucleus (MRN), without affecting the affinity of [(3)H]paroxetine. In the DRN and MRN, the stimulation of [(35)S]GTP gamma S binding by the 5-HT(1A) receptor agonist 8-OH-DPAT, or the number of 5-HT(1A) receptor sites labeled with [(3)H] MPPF, was not different in old versus young animals. Thus in the DRN, aging decreased serotonin transporter sites without changing 5-HT(1A) receptor activation of G proteins or 5-HT(1A) receptor density. In the CA(1) region of hippocampus, 8-OH-DPAT-stimulated [(35)S]GTP gamma S binding was increased in the older animals (% above basal, Old: 141+/-21; Young: 81+/-17) without changing specific [(3)H] MPPF binding sites, suggesting that the capacity of 5-HT(1A) receptors to activate G proteins is enhanced. Aging also appears to enhance this capacity in the dentate gyrus, because this region exhibited a constant level of 8-OH-DPAT-stimulated [(35)S]GTP gamma S binding in spite of an age-related decrease in the number of [(3)H] MPPF binding sites (fmol/mg protein, Old: 203+/-21; Young: 429+/-51).

Extended action of MKC-242, a selective 5-HT(1A) receptor agonist, on light-induced Per gene expression in the suprachiasmatic nucleus in mice

We reported previously that (S)-5-[3-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole hydrochloride (MKC-242) (3 mg kg(-1), i.p.), a selective 5-HT(1A) receptor agonist, accelerated the re-entrainment of hamster wheel-running rhythms to a new 8 hr delayed or advanced light-dark cycle, and also potentiated the phase advance of the wheel-running rhythm produced by light pulses. The molecular mechanism underlying MKC-242-induced potentiation of this phase shift, however, has not yet been elucidated. We examined the effects of MKC-242 on light-induced mPer1 and mPer2 mRNA expression in the suprachiasmatic nucleus (SCN) of mice. MKC-242 (5 mg kg(-1), i.p.) potentiated light-induced mPer1 and mPer2 expression in the SCN of mice housed in constant darkness for 2 days, when mRNA levels were observed 3 hr after light-exposure. More potentiating action of MKC-242 on mPer2 expression in the SCN was observed in mice housed in constant darkness for 9-10 days. This facilitatory action of MKC-242 on mPer1 expression was antagonized by WAY100635, a selective 5-HT(1A) receptor blocker, indicating that MKC-242 activated 5-HT(1A) receptors. Other drugs such as 8-hydroxy-dipropylaminotetralin (10 mg kg(-1), i.p.), paroxetine (10 mg kg(-1), i.p.), buspirone (10 mg kg(-1), i.p.), and diazepam (10 mg kg(-1), i.p.) did not display a potentiating action on light-induced mPer1 and mPer2 expression in the SCN. In the behavioral experiments, we found that MKC-242 (5 mg kg(-1), i.p.) potentiated light-induced phase delays of free-running rhythm in mice. The present results suggest that prolonged increase of mPer1 or mPer2 expression in the SCN by MKC-242 may be involved in the potentiation of photic entrainment by MKC-242 in mice.