Neuropeptide Y, GABA and circadian phase shifts to photic stimuli

Gpr19 is a circadian clock-controlled orphan GPCR with a role in modulating free-running period and light resetting capacity of the circadian clock

Gpr19 encodes an evolutionarily conserved orphan G-protein-coupled receptor (GPCR) with currently no established physiological role in vivo. We characterized Gpr19 expression in the suprachiasmatic nucleus (SCN), the locus of the master circadian clock in the brain, and determined its role in the context of the circadian rhythm regulation. We found that Gpr19 is mainly expressed in the dorsal part of the SCN, with its expression fluctuating in a circadian fashion. A conserved cAMP-responsive element in the Gpr19 promoter was able to produce circadian transcription in the SCN. Gpr19^−/− mice exhibited a prolonged circadian period and a delayed initiation of daily locomotor activity. Gpr19 deficiency caused the downregulation of several genes that normally peak during the night, including Bmal1 and Gpr176. In response to light exposure at night, Gpr19^−/− mice had a reduced capacity for light-induced phase-delays, but not for phase-advances. This defect was accompanied by reduced response of c-Fos expression in the dorsal region of the SCN, while apparently normal in the ventral area of the SCN, in Gpr19^−/− mice. Thus, our data demonstrate that Gpr19 is an SCN-enriched orphan GPCR with a distinct role in circadian regulation and may provide a potential target option for modulating the circadian clock.

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.

Phase shifts to light are altered by antagonists to neuropeptide receptors

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is a heterogeneous structure. Two key populations of cells that receive retinal input and are believed to participate in circadian responses to light are cells that contain vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP). VIP acts primarily through the VPAC2 receptor, while GRP works primarily through the BB2 receptor. Both VIP and GRP phase shift the circadian clock in a manner similar to light when applied to the SCN, both in vivo and in vitro, indicating that they are sufficient to elicit photic-like phase shifts. However, it is not known if they are necessary signals for light to elicit phase shifts. Here we test the hypothesis that GRP and VIP are necessary signaling components for the photic phase shifting of the hamster circadian clock by examining two antagonists for each of these neuropeptides. The BB2 antagonist PD176252 had no effect on light-induced delays on its own, while the BB2 antagonist RC-3095 had the unexpected effect of significantly potentiating both phase delays and advances. Neither of the VIP antagonists ([d-p-Cl-Phe6, Leu17]-VIP, or PG99-465) altered phase shifting responses to light on their own. When the BB2 antagonist PD176252 and the VPAC2 antagonist PG99-465 were delivered together to the SCN, phase delays were significantly attenuated. These results indicate that photic phase shifting requires participation of either VIP or GRP; phase shifts to light are only impaired when signalling in both pathways are inhibited. Additionally, the unexpected potentiation of light-induced phase shifts by RC-3095 should be investigated further for potential chronobiotic applications.

Nocturnal Light and Nocturnal Rodents

Investigators typically study one function of the circadian visual system at a time, be it photoreception, transmission of photic information to the suprachiasmatic nucleus (SCN), light control of rhythm phase, locomotor activity, or gene expression. There are good reasons for such a focused approach, but sometimes it is advantageous to look at the broader picture, asking how all the parts and functions complete the whole. Here, several seemingly disparate functions of the circadian visual system are examined. They share common characteristics with respect to regulation by light and, to the extent known, share a common input neuroanatomy. The argument presented is that the 3 hypothalamically mediated effects of light for which there are the most data, circadian clock phase shifts, suppression of nocturnal locomotion (“negative masking”), and suppression of nocturnal pineal function, are regulated by a common photic input pathway terminating in the SCN. For each, light triggers a relatively fixed interval response that is irradiance-dependent, the effective stimulus can be very brief light exposure, and the response continues to completion in the absence of additional light. The presence of a triggered, fixed-length response interval is of particular importance to the understanding of the circuitry and mechanisms regulating circadian rhythm phase shifts because it implies that the SCN clock response to light is not instantaneous. It also may explain why certain stimuli (neuropeptide Y or novel wheel running) administered many minutes after light exposure are able to block light-induced phase shifts. The understanding of negative masking is complicated by the fact that it can be represented as a positive change, that is, light-induced sleep, not just as a reduction in locomotion. Acute nocturnal light exposure also induces adrenal hormone secretion and a rapid drop in body temperature, physiological responses that appear to be regulated similarly to the other light effects. The likelihood of a common regulatory basis for the several responses suggests that additional light-induced responses will be forthcoming and raises questions about the relationships between light, SCN cellular anatomy, the molecular clockworks of SCN neurons, and SCN throughput mechanisms for regulating disparate downstream activities.

Clock genes and clock-controlled genes in the regulation of metabolic rhythms

Daily rotation of the Earth on its axis and yearly revolution around the Sun impose to living organisms adaptation to nyctohemeral and seasonal periodicity. Terrestrial life forms have developed endogenous molecular circadian clocks to synchronize their behavioral, biological, and metabolic rhythms to environmental cues, with the aim to perform at their best over a 24-h span. The coordinated circadian regulation of sleep/wake, rest/activity, fasting/feeding, and catabolic/anabolic cycles is crucial for optimal health. Circadian rhythms in gene expression synchronize biochemical processes and metabolic fluxes with the external environment, allowing the organism to function effectively in response to predictable physiological challenges. In mammals, this daily timekeeping is driven by the biological clocks of the circadian timing system, composed of master molecular oscillators within the suprachiasmatic nuclei of the hypothalamus, pacing self-sustained and cell-autonomous molecular oscillators in peripheral tissues through neural and humoral signals. Nutritional status is sensed by nuclear receptors and coreceptors, transcriptional regulatory proteins, and protein kinases, which synchronize metabolic gene expression and epigenetic modification, as well as energy production and expenditure, with behavioral and light-dark alternance. Physiological rhythmicity characterizes these biological processes and body functions, and multiple rhythms coexist presenting different phases, which may determine different ways of coordination among the circadian patterns, at both the cellular and whole-body levels. A complete loss of rhythmicity or a change of phase may alter the physiological array of rhythms, with the onset of chronodisruption or internal desynchronization, leading to metabolic derangement and disease, i.e., chronopathology.

Localization of relaxin-3 in brain of Macaca fascicularis: identification of a nucleus incertus in primate

Relaxin-3 (RLN3) is a highly conserved, ancestral member of the insulin/relaxin peptide family. RLN3 mRNA is highly expressed in rat, mouse, and human brain and molecular genetic and pharmacological studies suggest that RLN3 is the cognate ligand for the relaxin family peptide-3 receptor (RXFP3). The distribution of RLN3/RXFP3 networks has been determined in rat and mouse brain, but not in higher species. In this study we describe the distribution of RLN3 neurons in the brain of macaque (Macaca fascicularis) using in situ hybridization histochemistry and immunohistochemistry. RLN3 mRNA and high levels of RLN3-like immunoreactivity (-LI) were observed in neurons within a ventromedial region of the central gray of the pons and medulla that appears to represent the primate analog of the nucleus incertus (NI) described in lower species. Nerve fibers and terminals containing RLN3-LI were observed throughout brain regions identical to those known to receive afferents from the NI in the rat, including the septum, hippocampus, entorhinal cortex, lateral, dorsomedial and ventromedial hypothalamus, supramammillary and interpeduncular nuclei, anterodorsal, paraventricular and reuniens thalamic nuclei, lateral habenula, central gray, and dorsal raphe, solitary tract, and ambiguus nuclei. Experimental studies in the rat strongly implicate a role of this neuropeptide-receptor system in arousal, feeding, and metabolism, learning and memory, and central responses to psychological stressors. These new anatomical findings support the proposition that the RLN3 system is similarly involved in the integration and modulation of behavioral activation and arousal and responses to stress in nonhuman primates and humans.

Bezafibrate, a peroxisome proliferator-activated receptors agonist, decreases body temperature and enhances electroencephalogram delta-oscillation during sleep in mice

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear receptor family. PPARs play a critical role in lipid and glucose metabolism. We examined whether chronic treatment with bezafibrate, a PPAR agonist, would alter sleep and body temperature (BT). Mice fed with a control diet were monitored for BT, electroencephalogram (EEG), and electromyogram for 48 h under light-dark conditions. After obtaining the baseline recording, the mice were provided with bezafibrate-supplemented food for 2 wk, after which the same recordings were performed. Two-week feeding of bezafibrate decreased BT, especially during the latter half of the dark period. BT rhythm and sleep/wake rhythm were phase advanced about 2-3 h by bezafibrate treatment. Bezafibrate treatment also increased the EEG delta-power in nonrapid eye movement sleep compared with the control diet attenuating its daily amplitude. Furthermore, bezafibrate-treated mice showed no rebound of EEG delta-power in nonrapid eye movement sleep after 6 h sleep deprivation, whereas values in control mice largely increased relative to baseline. DNA microarray, and real-time RT-PCR analysis showed that bezafibrate treatment increased levels of Neuropeptide Y mRNA in the hypothalamus at both Zeitgeber time (ZT) 10 and ZT22, and decreased proopiomelanocortin-alpha mRNA in the hypothalamus at ZT10. These findings demonstrate that PPARs participate in the control of both BT and sleep regulation, which accompanied changes in gene expression in the hypothalamus. Activation of PPARs may enhance deep sleep and improve resistance to sleep loss.

Protein kinase C modulates the phase-delaying effects of light in the mammalian circadian clock

The mammalian circadian pacemaker located in the suprachiasmatic nuclei (SCN) drives a vast array of biochemical and physiological processes with 24-h periodicity. The phasing of SCN pacemaker activity is tightly regulated by photic input from the retina. Recent work has implicated protein kinase C (PKC) as a regulator of photic input, although stimulus-induced PKC activity has not been examined. Here we used a combination of biochemical, immunohistochemical and behavioral techniques to examine both the regulation and role of PKC in light-induced clock entrainment in mice. We report that photic stimulation during the subjective night, but not during the subjective day, stimulates PKC activity within the SCN. To assess the role of PKC in clock entrainment, we employed an in-vivo infusion approach to deliver the PKC inhibitor bisindolylmaleimide I to the SCN. The disruption of PKC activity significantly enhanced the phase-shifting effects of light, indicating that PKC functions as a negative regulator of light entrainment. Importantly, bisindolylmaleimide I infusion in the absence of light treatment did not phase shift the clock, demonstrating that transient disruption of basal PKC activity does not affect inherent pacemaker activity. The capacity of light to stimulate immediate early gene expression in the SCN was not substantively altered by PKC inhibition, suggesting that PKC does not couple light to rapid transcriptional activation. Rather, a combination of in-vivo and cell culture assays indicates that PKC functions as an inhibitor of PERIOD1 degradation. Thus, PKC may influence clock entrainment via a post-translational mechanism that influences clock protein stability.

Gastrin releasing peptide and neuropeptide Y exert opposing actions on circadian phase

Microinjection of gastrin releasing peptide (GRP) into the third ventricle or the suprachiasmatic nucleus (SCN) induces circadian phase shifts similar to those produced by light. Administration of GRP during the day does not alter circadian phase. In contrast, neuropeptide Y (NPY) induces phase shifts of circadian rhythms during the day but has little effect when administered at night, similar to the effects of most non-photic stimuli. NPY inhibits the phase shifting effects of light, and GRP is thought to be part of the photic signaling system within the SCN. This experiment was designed to test whether GRP and NPY inhibit each other's effects on circadian phase. Adult male Syrian hamsters equipped with guide cannulas aimed at the SCN were housed in constant darkness until stable free-running rhythms of wheel running activity were apparent. Microinjection of GRP during the early subjective night induced phase delays that were blocked by simultaneous administration of NPY. During the middle of the subjective day, microinjection of NPY caused phase advances that were blocked by simultaneous administration of GRP. These data suggest that GRP and NPY oppose each other's effects on the circadian clock, and that the actions of NPY on the photic phase shifting mechanism in the SCN occur at least in part downstream from retinorecipient cells.

Relaxin-3 in GABA projection neurons of nucleus incertus suggests widespread influence on forebrain circuits via G-protein-coupled receptor-135 in the rat

Relaxin-3 (RLX3) is a newly identified member of the relaxin/insulin peptide family that is highly conserved across a range of species from fish to mammals and is highly expressed in rat, mouse and human brain. Extensive pharmacological studies have demonstrated that RLX3 is a high affinity, selective ligand for G-protein-coupled receptor-135 (GPCR135, now classified as relaxin family peptide-3 receptor; RXFP3). In ongoing studies to understand the physiological functions of RLX3, the distribution of RLX3-containing neuronal elements in rat brain was determined by immunohistochemistry, using an affinity-purified polyclonal antiserum raised against a conserved segment of the RLX3 C-peptide (AS-R3(85-101)). Consistent with the distribution of RLX3 mRNA, neurons containing RLX3-like immunoreactivity (LI) were observed in the pontine nucleus incertus and the majority of these cells, which are known to express corticotropin-releasing factor receptor-1, were shown to express glutamic acid decarboxylase-65-immunoreactivity, suggesting a GABA phenotype. Nerve fibers and terminals containing RLX3-LI were observed adjacent to cells in the nucleus incertus and in various forebrain regions known to receive afferents from the nucleus incertus, including cortex, septum, hippocampus, thalamus, hypothalamus and midbrain. Regions that contained highest densities of RLX3-positive fibers included the medial septum, lateral preoptic area, lateral hypothalamus/medial forebrain bundle and ventral hippocampus; and additional fibers were observed in olfactory bulb and olfactory and frontal/cingulate cortices, bed nucleus of the stria terminalis, dorsal endopiriform, intergeniculate, and supramammillary nuclei, and the periaqueductal gray and dorsal raphe. The RLX3-positive network overlapped the regional distribution of GPCR135 mRNA and specific binding sites for an [125I]-GPCR135-selective, chimeric peptide. These anatomical findings further support the proposition that RLX3 is the endogenous ligand for GPCR135 in rat brain and provide evidence for broad modulatory activity of RLX3 in behavioral activation relating to autonomic and neuroendocrine control of metabolism and reproduction and higher-order processes such as stress and cognition.

The circadian visual system, 2005

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

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

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.

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

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

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.

Dexras1 Potentiates Photic and Suppresses Nonphotic Responses of the Circadian Clock

Circadian rhythms of physiology and behavior are generated by biological clocks that are synchronized to the cyclic environment by photic or nonphotic cues. The interactions and integration of various entrainment pathways to the clock are poorly understood. Here, we show that the Ras-like G protein Dexras1 is a critical modulator of the responsiveness of the master clock to photic and nonphotic inputs. Genetic deletion of Dexras1 reduces photic entrainment by eliminating a pertussis-sensitive circadian response to NMDA. Mechanistically, Dexras1 couples NMDA and light input to Gi/o and ERK activation. In addition, the mutation greatly potentiates nonphotic responses to neuropeptide Y and unmasks a nonphotic response to arousal. Thus, Dexras1 modulates the responses of the master clock to photic and nonphotic stimuli in opposite directions. These results identify a signaling molecule that serves as a differential modulator of the gated photic and nonphotic input pathways to the circadian timekeeping system.

Neuropeptide y and n-methyl-d-aspartic acid interact within the suprachiasmatic nuclei to alter circadian phase

Circadian rhythms are reset by exposure to photic stimuli and nonphotic stimuli. Glutamate appears to be the primary neurotransmitter that communicates photic stimuli to the circadian clock located in the suprachiasmatic nucleus. There is also substantial evidence that neuropeptide Y (NPY) mediates the effects of at least some nonphotic stimuli on the circadian clock. The purpose of this study was to investigate how NPY and glutamate receptor activation interact to reset the phase of the circadian clock. Microinjection of the glutamate agonist N-methyl-D-aspartic acid (NMDA) during the subjective day significantly decreased NPY-induced phase advances. During the late subjective night, NMDA induced light-like phase advances, which were significantly reduced by microinjection of NPY. Microinjection of NPY inhibited NMDA-induced phase advances during the late subjective night, even when sodium-dependent action potentials were inhibited by tetrodotoxin. These data support the hypothesis that, during the subjective night, NPY and NMDA act on the same clock cells or on cells that communicate with clock cells by mechanisms not requiring action potentials. Although NPY and NMDA appear to be mutually inhibitory during both the day and the night, the mechanisms of this inhibition appear to be different during the day versus the night.