Heterogeneous expression of gamma-aminobutyric acid and gamma-aminobutyric acid-associated receptors and transporters in the rat suprachiasmatic nucleus

Neural function of Bmal1: an overview

Bmal1 (Brain and muscle arnt-like, or Arntl) is a bHLH/PAS domain transcription factor central to the transcription/translation feedback loop of the biologic clock. Although Bmal1 is well-established as a major regulator of circadian rhythm, a growing number of studies in recent years have shown that dysfunction of Bmal1 underlies a variety of psychiatric, neurodegenerative-like, and endocrine metabolism-related disorders, as well as potential oncogenic roles. In this review, we systematically summarized Bmal1 expression in different brain regions, its neurological functions related or not to circadian rhythm and biological clock, and pathological phenotypes arising from Bmal1 knockout. This review also discusses oscillation and rhythmicity, especially in the suprachiasmatic nucleus, and provides perspective on future progress in Bmal1 research.

The opposing chloride cotransporters KCC and NKCC control locomotor activity in constant light and during long days

Cation chloride cotransporters (CCCs) regulate intracellular chloride ion concentration ([Cl-]i) within neurons, which can reverse the direction of the neuronal response to the neurotransmitter GABA.1 Na+ K+ Cl- (NKCC) and K+ Cl- (KCC) cotransporters transport Cl- into or out of the cell, respectively. When NKCC activity dominates, the resulting high [Cl-]i can lead to an excitatory and depolarizing response of the neuron upon GABAA receptor opening, while KCC dominance has the opposite effect.1 This inhibitory-to-excitatory GABA switch has been linked to seasonal adaption of circadian clock function to changing day length,2-4 and its dysregulation is associated with neurodevelopmental disorders such as epilepsy.5-8 In Drosophila melanogaster, constant light normally disrupts circadian clock function and leads to arrhythmic behavior.9 Here, we demonstrate a function for CCCs in regulating Drosophila locomotor activity and GABA responses in circadian clock neurons because alteration of CCC expression in circadian clock neurons elicits rhythmic behavior in constant light. We observed the same effects after downregulation of the Wnk and Fray kinases, which modulate CCC activity in a [Cl-]i-dependent manner. Patch-clamp recordings from the large LNv clock neurons show that downregulation of KCC results in a more positive GABA reversal potential, while KCC overexpression has the opposite effect. Finally, KCC and NKCC downregulation reduces or increases morning behavioral activity during long photoperiods, respectively. In summary, our results support a model in which the regulation of [Cl-]i by a KCC/NKCC/Wnk/Fray feedback loop determines the response of clock neurons to GABA, which is important for adjusting behavioral activity to constant light and long-day conditions.

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

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

Long-Term Imaging Reveals a Circadian Rhythm of Intracellular Chloride in Neurons of the Suprachiasmatic Nucleus

Both inhibitory and excitatory GABA transmission exist in the mature suprachiasmatic nucleus (SCN), the master pacemaker of circadian physiology. Whether GABA is inhibitory or excitatory depends on the intracellular chloride concentration ([Cl-]i). Here, using the genetically encoded ratiometric probe Cl-Sensor, we investigated [Cl-]i in AVP and VIP-expressing SCN neurons for several days in culture. The chloride ratio (RCl) demonstrated circadian rhythmicity in AVP + neurons and VIP + neurons, but was not detected in GFAP + astrocytes. RCl peaked between ZT 7 and ZT 8 in both AVP + and VIP + neurons. RCl rhythmicity was not dependent on the activity of several transmembrane chloride carriers, action potential generation, or the L-type voltage-gated calcium channels, but was sensitive to GABA antagonists. We conclude that [Cl-]i is under circadian regulation in both AVP + and VIP + neurons.

The Suprachiasmatic Nucleus Regulates Anxiety-Like Behavior in Mice

Individuals suffering from mood and anxiety disorders often show significant disturbances in sleep and circadian rhythms. Animal studies indicate that circadian rhythm disruption can cause increased depressive- and anxiety-like behavior, but the underlying mechanisms are unclear. One potential mechanism to explain how circadian rhythms are contributing to mood and anxiety disorders is through dysregulation of the suprachiasmatic nucleus (SCN) of the hypothalamus, known as the "central pacemaker." To investigate the role of the SCN in regulating depressive- and anxiety-like behavior in mice, we chronically manipulated the neural activity of the SCN using two optogenetic stimulation paradigms. As expected, chronic stimulation of the SCN late in the active phase (circadian time 21, CT21) resulted in a shortened period and dampened amplitude of homecage activity rhythms. We also repeatedly stimulated the SCN at unpredictable times during the active phase of mice when SCN firing rates are normally low. This resulted in dampened, fragmented, and unstable homecage activity rhythms. In both chronic SCN optogenetic stimulation paradigms, dampened homecage activity rhythms (decreased amplitude) were directly correlated with increased measures of anxiety-like behavior. In contrast, we only observed a correlation between behavioral despair and homecage activity amplitude in mice stimulated at CT21. Surprisingly, the change in period of homecage activity rhythms was not directly associated with anxiety- or depressive-like behavior. Finally, to determine if anxiety-like behavior is affected during a single SCN stimulation session, we acutely stimulated the SCN in the active phase (zeitgeber time 14-16, ZT14-16) during behavioral testing. Unexpectedly this also resulted in increased anxiety-like behavior. Taken together, these results indicate that SCN-mediated dampening of rhythms is directly correlated with increased anxiety-like behavior. This work is an important step in understanding how specific SCN neural activity disruptions affect depressive- and anxiety-related behavior.

Role of the cation-chloride-cotransporters in the circadian system

The circadian system plays an immense role in controlling physiological processes in our body. The suprachiasmatic nucleus (SCN) supervises this system, regulating and harmonising the circadian rhythms in our body. Most neurons present in the SCN are GABAergic neurons. Although GABA is considered the main inhibitory neurotransmitter of the CNS, recent studies have shown that excitatory responses were recorded in this area. These responses are enabled by an increase in intracellular chloride ions [Cl^-]_i levels. The chloride (Cl^-) levels in GABAergic neurons are controlled by two solute carrier 12 (SLC12) cation-chloride-cotransporters (CCCs): Na⁺/K⁺/Cl^- co-transporter (NKCC1) and K⁺/Cl^- co-transporter (KCC2), that respectively cause an influx and efflux of Cl^-. Recent works have found altered expression and/or activity of either of these co-transporters in SCN neurons and have been associated with circadian rhythms. In this review, we summarize and discuss the role of CCCs in circadian rhythms, and highlight these recent advances which attest to CCC's growing potential as strong research and therapeutic targets.

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.

The Therapeutic Potential of Neuronal K-Cl Co-Transporter KCC2 in Huntington's Disease and Its Comorbidities

Intracellular chloride levels in the brain are regulated primarily through the opposing effects of two cation-chloride co-transporters (CCCs), namely K+-Cl- co-transporter-2 (KCC2) and Na+-K+-Cl- co-transporter-1 (NKCC1). These CCCs are differentially expressed throughout the course of development, thereby determining the excitatory-to-inhibitory γ-aminobutyric acid (GABA) switch. GABAergic excitation (depolarisation) is important in controlling the healthy development of the nervous system; as the brain matures, GABAergic inhibition (hyperpolarisation) prevails. This developmental switch in excitability is important, as uncontrolled regulation of neuronal excitability can have implications for health. Huntington's disease (HD) is an example of a genetic disorder whereby the expression levels of KCC2 are abnormal due to mutant protein interactions. Although HD is primarily considered a motor disease, many other clinical manifestations exist; these often present in advance of any movement abnormalities. Cognitive change, in addition to sleep disorders, is prevalent in the HD population; the effect of uncontrolled KCC2 function on cognition and sleep has also been explored. Several mechanisms by which KCC2 expression is reduced have been proposed recently, thereby suggesting extensive investigation of KCC2 as a possible therapeutic target for the development of pharmacological compounds that can effectively treat HD co-morbidities. Hence, this review summarizes the role of KCC2 in the healthy and HD brain, and highlights recent advances that attest to KCC2 as a strong research and therapeutic target candidate.

GABAergic mechanisms in the suprachiasmatic nucleus that influence circadian rhythm

The mammalian central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN contains multiple circadian oscillators which synchronize with each other via several neurotransmitters. Importantly, an inhibitory neurotransmitter, γ-amino butyric acid (GABA), is expressed in almost all SCN neurons. In this review, we discuss how GABA influences circadian rhythms in the SCN. Excitatory and inhibitory effects of GABA may depend on intracellular Cl^- concentration, in which several factors such as day-length, time of day, development, and region in the SCN may be involved. GABA also mediates oscillatory coupling of the circadian rhythms in the SCN. Recent genetic approaches reveal that GABA refines circadian output rhythms, but not circadian oscillations in the SCN. Since several efferent projections of the SCN have been suggested, GABA might work downstream of neuronal pathways from the SCN which regulate the temporal order of physiology and behavior.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

The Concept of Coupling in the Mammalian Circadian Clock Network

The circadian clock network regulates daily rhythms in mammalian physiology and behavior to optimally adapt the organism to the 24-h day/night cycle. A central pacemaker, the hypothalamic suprachiasmatic nucleus (SCN), coordinates subordinate cellular oscillators in the brain, as well as in peripheral organs to align with each other and external time. Stability and coordination of this vast network of cellular oscillators is achieved through different levels of coupling. Although coupling at the molecular level and across the SCN is well established and believed to define its function as pacemaker structure, the notion of coupling in other tissues and across the whole system is less well understood. In this review, we describe the different levels of coupling in the mammalian circadian clock system - from molecules to the whole organism. We highlight recent advances in gaining knowledge of the complex organization and function of circadian network regulation and its significance for the generation of stable but plastic intrinsic 24-h rhythms.

Functional Interactions Between Sleep and Circadian Rhythms in Learning and Learning Disabilities

The propensity for sleep is timed by the circadian system. Many studies have shown that learning and memory performance is affected by circadian phase. And, of course it is well established that critical processes of memory consolidation occur during and depend on sleep. This chapter presents evidence that sleep and circadian rhythms do not just have separate influences on learning and memory that happen to coincide because of the circadian timing of sleep, but rather sleep and circadian systems have a critical functional interaction in the processes of memory consolidation. The evidence comes primarily from research on two models of learning disability: Down's syndrome model mice and Siberian hamsters. The Down's syndrome model mouse (Ts65Dn) has severe learning disability that has been shown to be due to GABAergic over-inhibition. Short-term, chronic therapies with GABA_A antagonists restore learning ability in these mice long-term, but only if the antagonist treatments are given during the dark or sleep phase of the daily rhythm. The Siberian hamster is a model circadian animal except for the fact that a light treatment that gives the animal a phase advance on one day and a phase delay on the next day can result in total circadian arrhythmia for life. Once arrhythmic, the hamsters cannot learn. Learning, but not rhythmicity, is restored by short-term chronic treatment with GABA antagonists. Like many other species, if these hamsters are made arrhythmic by SCN lesion, their learning is unaffected. However, if made arrhythmic and learning disabled by the light treatment, subsequent lesions of their SCNs restore learning. SCN lesions also appear to restore learning in the Ts65Dn mice. The collective work on these two animal models of learning disability suggests that the circadian system modulates neuroplasticity. Our hypothesis is that a previously unrecognized function of the circadian system is to dampen neuroplasticity during the sleep phase to stabilize memory transcripts during their transfer to long-term memory. Thus, sleep and circadian systems have integrated roles to play in memory consolidation and do not just have separate but coincident influences on that process.

Enteropathogenic Escherichia coli remodels host endosomes to promote endocytic turnover and breakdown of surface polarity

Enteropathogenic E. coli (EPEC) is an extracellular diarrheagenic human pathogen which infects the apical plasma membrane of the small intestinal enterocytes. EPEC utilizes a type III secretion system to translocate bacterial effector proteins into its epithelial hosts. This activity, which subverts numerous signaling and membrane trafficking pathways in the infected cells, is thought to contribute to pathogen virulence. The molecular and cellular mechanisms underlying these events are not well understood. We investigated the mode by which EPEC effectors hijack endosomes to modulate endocytosis, recycling and transcytosis in epithelial host cells. To this end, we developed a flow cytometry-based assay and imaging techniques to track endosomal dynamics and membrane cargo trafficking in the infected cells. We show that type-III secreted components prompt the recruitment of clathrin (clathrin and AP2), early (Rab5a and EEA1) and recycling (Rab4a, Rab11a, Rab11b, FIP2, Myo5b) endocytic machineries to peripheral plasma membrane infection sites. Protein cargoes, e.g. transferrin receptors, β1 integrins and aquaporins, which exploit the endocytic pathways mediated by these machineries, were also found to be recruited to these sites. Moreover, the endosomes and cargo recruitment to infection sites correlated with an increase in cargo endocytic turnover (i.e. endocytosis and recycling) and transcytosis to the infected plasma membrane. The hijacking of endosomes and associated endocytic activities depended on the translocated EspF and Map effectors in non-polarized epithelial cells, and mostly on EspF in polarized epithelial cells. These data suggest a model whereby EPEC effectors hijack endosomal recycling mechanisms to mislocalize and concentrate host plasma membrane proteins in endosomes and in the apically infected plasma membrane. We hypothesize that these activities contribute to bacterial colonization and virulence.

Enteropathogenic Escherichia coli (EPEC) are pathogenic bacteria that cause infantile diarrhea. Upon ingestion, EPEC reaches the small intestine, where an injection device termed the type III secretion system is utilized to inject a set of effector proteins from the bacteria into the host cell. These proteins manipulate the localization and functions of host proteins, lipids and organelles and contribute to the emergence of the EPEC disease. The molecular mechanisms underlying the functions of the EPEC effector proteins are not completely understood. Here we show that early upon infection, two such effector proteins, EspF and Map, hijack host endosomes at bacterial adherence sites to facilitate endocytosis and recycling of plasma membrane proteins at these sites. The consequence of this event is the enrichment and mislocalization of host plasma membrane proteins at infection sites. One such protein is the transferrin receptor, which is a carrier for transferrin, whose function is to mediate cellular uptake of iron. Iron is a critical nutrient for bacterial growth and survival. We postulate that the unique manipulation of transferrin receptor endocytic membrane trafficking by EPEC plays an important role in its survival on the luminal surface of the intestinal epithelium.

Functionally Complete Excision of Conditional Alleles in the Mouse Suprachiasmatic Nucleus by Vgat-ires-Cre

Mice with targeted gene disruption have provided important information about the molecular mechanisms of circadian clock function. A full understanding of the roles of circadian-relevant genes requires manipulation of their expression in a tissue-specific manner, ideally including manipulation with high efficiency within the suprachiasmatic nuclei (SCN). To date, conditional manipulation of genes within the SCN has been difficult. In a previously developed mouse line, Cre recombinase was inserted into the vesicular GABA transporter (Vgat) locus. Since virtually all SCN neurons are GABAergic, this Vgat-Cre line seemed likely to have high efficiency at disrupting conditional alleles in SCN. To test this premise, the efficacy of Vgat-Cre in excising conditional (fl, for flanked by LoxP) alleles in the SCN was examined. Vgat-Cre-mediated excision of conditional alleles of Clock or Bmal1 led to loss of immunostaining for products of the targeted genes in the SCN. Vgat-Cre⁺; Clock^fl/fl; Npas2^m/m mice and Vgat-Cre⁺; Bmal1^fl/fl mice became arrhythmic immediately upon exposure to constant darkness, as expected based on the phenotype of mice in which these genes are disrupted throughout the body. The phenotype of mice with other combinations of Vgat-Cre⁺, conditional Clock, and mutant Npas2 alleles also resembled the corresponding whole-body knockout mice. These data indicate that the Vgat-Cre line is useful for Cre-mediated recombination within the SCN, making it useful for Cre-enabled technologies including gene disruption, gene replacement, and opto- and chemogenetic manipulation of the SCN circadian clock.

Circuit development in the master clock network of mammals

Daily rhythms are generated by the circadian timekeeping system, which is orchestrated by the master circadian clock in the suprachiasmatic nucleus (SCN) of mammals. Circadian timekeeping is endogenous and does not require exposure to external cues during development. Nevertheless, the circadian system is not fully formed at birth in many mammalian species and it is important to understand how SCN development can affect the function of the circadian system in adulthood. The purpose of the current review is to discuss the ontogeny of cellular and circuit function in the SCN, with a focus on work performed in model rodent species (i.e., mouse, rat, and hamster). Particular emphasis is placed on the spatial and temporal patterns of SCN development that may contribute to the function of the master clock during adulthood. Additional work aimed at decoding the mechanisms that guide circadian development is expected to provide a solid foundation upon which to better understand the sources and factors contributing to aberrant maturation of clock function.

Oxytocin and Vasopressin, and the GABA Developmental Shift During Labor and Birth: Friends or Foes?

Oxytocin (OT) and vasopressin (AVP) are usually associated with sociability and reduced stress for the former and antidiuretic agent associated with severe stress and pathological conditions for the latter. Both OT and AVP play major roles during labor and birth. Recent contradictory studies suggest that they might exert different roles on the GABA excitatory/inhibitory developmental shift. We reported (Tyzio et al., 2006) that at birth, OT exerts a neuro-protective action mediated by an abrupt reduction of intracellular chloride levels ([Cl-]i) that are high in utero, reinforcing GABAergic inhibition and modulating the generation of the first synchronized patterns of cortical networks. This reduction of [Cl-]i levels is abolished in rodent models of Fragile X Syndrome and Autism Spectrum Disorders, and its restoration attenuates the severity of the pathological sequels, stressing the importance of the shift at birth (Tyzio et al., 2014). In contrast, Kaila and co-workers (Spoljaric et al., 2017) reported excitatory GABA actions before and after birth that are modulated by AVP but not by OT, challenging both the developmental shift and the roles of OT. Here, I analyze the differences between these studies and suggest that the ratio AVP/OT like that of excitatory/inhibitory GABA depend on stress and pathological conditions.

Role of GABA in the regulation of the central circadian clock of the suprachiasmatic nucleus

In mammals, circadian rhythms, such as sleep/wake cycles, are regulated by the central circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN consists of thousands of individual neurons, which exhibit circadian rhythms. They synchronize with each other and produce robust and stable oscillations. Although several neurotransmitters are expressed in the SCN, almost all SCN neurons are γ-amino butyric acid (GABA)-ergic. Several studies have attempted to understand the roles of GABA in the SCN; however, precise mechanisms of the action of GABA in the SCN are still unclear. GABA exhibits excitatory and/or inhibitory characteristics depending on the circadian phase or region in the SCN. It can both synchronize and destabilize cellular circadian rhythms in individual SCN cells. Differing environmental light conditions, such as a long photoperiod, result in the decoupling of circadian oscillators of the dorsal and ventral SCN. This is due to high intracellular chloride concentrations in the dorsal SCN. Because mice with functional GABA deficiency, such as vesicular GABA transporter- and glutamate decarboxylase-deficient mice, are neonatal lethal, research has been limited to pharmacological approaches. Furthermore, different recording methods have been used to understand the roles of GABA in the SCN. The excitability of GABAergic neurons also changes during the postnatal period. Although there are technical difficulties in understanding the functions of GABA in the SCN, technical developments may help uncover new roles of GABA in circadian physiology and behavior.

Species Differences in the Organization of the Ventral Cochlear Nucleus

The mammalian cochlear nuclei (CN) consist of two major subdivisions, the dorsal (DCN) and ventral (VCN) nuclei. We previously reported differences in the structural and neurochemical organization of the human DCN from that in several other species. Here we extend this analysis to the VCN, considering both the organization of subdivisions and the types and distributions of neurons. Classically, the VCN in mammals is composed of two subdivisions, the anteroventral (VCA) and posteroventral cochlear nuclei (VCP). Anatomical and electrophysiological data in several species have defined distinct neuronal types with different distributions in the VCA and VCP. We asked if VCN subdivisions and anatomically defined neuronal types might be distinguished by patterns of protein expression in humans. We also asked if the neurochemical characteristics of the VCN are the same in humans as in other mammalian species, analyzing data from chimpanzees, macaque monkeys, cats, rats and chinchillas. We examined Nissl- and immunostained sections, using antibodies that had labeled neurons in other brainstem nuclei in humans. Nissl-stained sections supported the presence of both VCP and VCA in humans and chimpanzees. However, patterns of protein expression did not differentiate classes of neurons in humans; neurons of different soma shapes and dendritic configurations all expressed the same proteins. The patterns of immunostaining in macaque monkey, cat, rat, and chinchilla were different from those in humans and chimpanzees and from each other. The results may correlate with species differences in auditory function and plasticity. Anat Rec, 301:862-886, 2018. © 2017 Wiley Periodicals, Inc.

Neuronal oscillations on an ultra-slow timescale: daily rhythms in electrical activity and gene expression in the mammalian master circadian clockwork

Neuronal oscillations of the brain, such as those observed in the cortices and hippocampi of behaving animals and humans, span across wide frequency bands, from slow delta waves (0.1 Hz) to ultra-fast ripples (600 Hz). Here, we focus on ultra-slow neuronal oscillators in the hypothalamic suprachiasmatic nuclei (SCN), the master daily clock that operates on interlocking transcription-translation feedback loops to produce circadian rhythms in clock gene expression with a period of near 24 h (< 0.001 Hz). This intracellular molecular clock interacts with the cell's membrane through poorly understood mechanisms to drive the daily pattern in the electrical excitability of SCN neurons, exhibiting an up-state during the day and a down-state at night. In turn, the membrane activity feeds back to regulate the oscillatory activity of clock gene programs. In this review, we emphasise the circadian processes that drive daily electrical oscillations in SCN neurons, and highlight how mathematical modelling contributes to our increasing understanding of circadian rhythm generation, synchronisation and communication within this hypothalamic region and across other brain circuits.

Cognate Ligand Chaperoning: a Novel Mechanism for the Post-translational Regulation of Neurotransmitter Receptor Biogenesis

The functional unit for inter-neuronal communication in the central nervous system is the neuronal synapse. The number of postsynaptic neurotransmitter receptors at the cell surface is an important determinant of synaptic efficacy and plasticity. A diverse array of post-translational processes regulate postsynaptic receptor number, including receptor exocytosis, lateral diffusion, surface stabilization, endocytosis, and recycling, thus highlighting the importance of mechanisms that control postsynaptic receptor levels. Another putative post-translational mechanism for regulating receptor surface expression is cognate ligand chaperoning. It has been proposed that neurotransmitters function as cognate ligand chaperones by binding, within the endoplasmic reticulum (ER) lumen, to their nascent neurotransmitter receptors and facilitating receptor biogenesis. Here we discuss proof-of-concept evidence that small molecules can selectively facilitate the biogenesis of their targets and examine the specific evidence in support of cognate ligand chaperoning of neurotransmitter receptor biogenesis.

Intracellular Chloride Regulation in AVP+ and VIP+ Neurons of the Suprachiasmatic Nucleus

Several reports have described excitatory GABA transmission in the suprachiasmatic nucleus (SCN), the master pacemaker of circadian physiology. However, there is disagreement regarding the prevalence, timing, and neuronal location of excitatory GABA transmission in the SCN. Whether GABA is inhibitory or excitatory depends, in part, on the intracellular concentration of chloride ([Cl-]i). Here, using ratiometric Cl- imaging, we have investigated intracellular chloride regulation in AVP and VIP-expressing SCN neurons and found evidence suggesting that [Cl-]i is higher during the day than during the night in both AVP+ and VIP+ neurons. We then investigated the contribution of the cation chloride cotransporters to setting [Cl-]i in these SCN neurons and found that the chloride uptake transporter NKCC1 contributes to [Cl-]i regulation in SCN neurons, but that the KCCs are the primary regulators of [Cl-]i in SCN neurons. Interestingly, we observed that [Cl-]i is differentially regulated between AVP+ and VIP+ neurons-a low concentration of the loop diuretic bumetanide had differential effects on AVP+ and VIP+ neurons, while blocking the KCCs with VU0240551 had a larger effect on VIP+ neurons compared to AVP+ neurons.

Vasopressin casts light on the suprachiasmatic nucleus

KEY POINTS

A subpopulation of retinal ganglion cells expresses the neuropeptide vasopressin. These retinal ganglion cells project predominately to our biological clock, the suprachiasmatic nucleus (SCN). Light-induced vasopressin release enhances the responses of SCN neurons to light. It also enhances expression of genes involved in photo-entrainment of biological rhythms.

ABSTRACT

In all animals, the transition between night and day engages a host of physiological and behavioural rhythms. These rhythms depend not on the rods and cones of the retina, but on retinal ganglion cells (RGCs) that detect the ambient light level in the environment. These project to the suprachiasmatic nucleus (SCN) of the hypothalamus to entrain circadian rhythms that are generated within the SCN. The neuropeptide vasopressin has an important role in this entrainment. Many SCN neurons express vasopressin, and it has been assumed that the role of vasopressin in the SCN reflects the activity of these cells. Here we show that vasopressin is also expressed in many retinal cells that project to the SCN. Light-evoked vasopressin release contributes to the responses of SCN neurons to light, and enhances expression of the immediate early gene c-fos in the SCN, which is involved in photic entrainment of circadian rhythms.

Geniculohypothalamic GABAergic projections gate suprachiasmatic nucleus responses to retinal input

KEY POINTS

Visual input to the suprachiasmatic nucleus circadian clock is critical for animals to adapt their physiology and behaviour in line with the solar day. In addition to direct retinal projections, the clock receives input from the visual thalamus, although the role of this geniculohypothalamic pathway in circadian photoreception is poorly understood. In the present study, we develop a novel brain slice preparation that preserves the geniculohypothalamic pathway to show that GABAergic thalamic neurons inhibit retinally-driven activity in the central clock in a circadian time-dependent manner. We also show that in vivo manipulation of thalamic signalling adjusts specific features of the hypothalamic light response, indicating that the geniculohypothalamic pathway is primarily activated by crossed retinal inputs. Our data provide a mechanism by which geniculohypothalamic signals can adjust the magnitude of circadian and more acute hypothalamic light responses according to time-of-day and establish an important new model for future investigations of the circadian visual system.

ABSTRACT

Sensory input to the master mammalian circadian clock, the suprachiasmatic nucleus (SCN), is vital in allowing animals to optimize physiology and behaviour alongside daily changes in the environment. Retinal inputs encoding changes in external illumination provide the principle source of such information. The SCN also receives input from other retinorecipient brain regions, primarily via the geniculohypothalamic tract (GHT), although the contribution of these indirect projections to circadian photoreception is currently poorly understood. To address this deficit, in the present study, we established an in vitro mouse brain slice preparation that retains connectivity across the extended circadian system. Using multi-electrode recordings, we first confirm that this preparation retains intact optic projections to the SCN, thalamus and pretectum and a functional GHT. We next show that optogenetic activation of GHT neurons selectively suppresses SCN responses to retinal input, and also that this effect exhibits a pronounced day/night variation and involves a GABAergic mechanism. This inhibitory action was not associated with overt circadian rhythmicity in GHT output, indicating modulation at the SCN level. Finally, we use in vivo electrophysiological recordings alongside pharmacological inactivation or optogenetic excitation to show that GHT signalling actively modulates specific features of the SCN light response, indicating that GHT cells are primarily activated by crossed retinal projections. Taken together, our data establish a new model for studying network communication in the extended circadian system and provide novel insight into the roles of GHT-signalling, revealing a mechanism by which thalamic activity can help gate retinal input to the SCN according to time of day.

KATP Channels Mediate Differential Metabolic Responses to Glucose Shortage of the Dorsomedial and Ventrolateral Oscillators in the Central Clock

The suprachiasmatic nucleus (SCN) central clock comprises two coupled oscillators, with light entraining the retinorecipient vasoactive intestinal peptide (VIP)-positive ventrolateral oscillator, which then entrains the arginine vasopressin (AVP)-positive dorsomedial oscillator. While glucose availability is known to alter photic entrainment, it is unclear how the SCN neurones respond to metabolic regulation and whether the two oscillators respond differently. Here we show that the ATP-sensitive K⁺ (K_ATP) channel mediates differential responses to glucose shortage of the two oscillators. RT-PCR and electrophysiological results suggested the presence of Kir6.2/SUR1 K_ATP channels in the SCN neurones. Immunostaining revealed preferential distribution of Kir6.2 in the dorsomedial subregion and selective colocalization with AVP. Whole cell recordings with ATP-free pipette solution indicated larger tolbutamide-induced depolarisation and tolbutamide-sensitive conductance in dorsal SCN (dSCN) than ventral SCN (vSCN) neurones. Tolbutamide-sensitive conductance was low under perforated patch conditions but markedly enhanced by cyanide inhibition of mitochondrial respiration. Glucoprivation produced a larger steady-state inhibition in dSCN than vSCN neurones, and importantly hypoglycemia via opening K_ATP channels selectively inhibited the K_ATP-expressing neurones. Our results suggest that the AVP-SCN oscillator may act as a glucose sensor to respond to glucose shortage while sparing the VIP-SCN oscillator to remain in synch with external light-dark cycle.

Circadian Plasticity of Mammalian Inhibitory Interneurons

Inhibitory interneurons participate in all neuronal circuits in the mammalian brain, including the circadian clock system, and are indispensable for their effective function. Although the clock neurons have different molecular and electrical properties, their main function is the generation of circadian oscillations. Here we review the circadian plasticity of GABAergic interneurons in several areas of the mammalian brain, suprachiasmatic nucleus, neocortex, hippocampus, olfactory bulb, cerebellum, striatum, and in the retina.

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.

Collective timekeeping among cells of the master circadian clock

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the master circadian clock that coordinates daily rhythms in behavior and physiology in mammals. Like other hypothalamic nuclei, the SCN displays an impressive array of distinct cell types characterized by differences in neurotransmitter and neuropeptide expression. Individual SCN neurons and glia are able to display self-sustained circadian rhythms in cellular function that are regulated at the molecular level by a 24h transcriptional-translational feedback loop. Remarkably, SCN cells are able to harmonize with one another to sustain coherent rhythms at the tissue level. Mechanisms of cellular communication in the SCN network are not completely understood, but recent progress has provided insight into the functional roles of several SCN signaling factors. This review discusses SCN organization, how intercellular communication is critical for maintaining network function, and the signaling mechanisms that play a role in this process. Despite recent progress, our understanding of SCN circuitry and coupling is far from complete. Further work is needed to map SCN circuitry fully and define the signaling mechanisms that allow for collective timekeeping in the SCN network.

The rat suprachiasmatic nucleus: the master clock ticks at 30 Hz

Key points

Light‐responsive neurones in the rat suprachiasmatic nucleus discharge with a harmonic distribution of interspike intervals, whereas unresponsive neurones seldom do.

This harmonic patterning has a fundamental frequency of close to 30 Hz, and is the same in light‐on cells as in light‐off cells, and is unaffected by exposure to light.

Light‐on cells are more active than light‐off cells in both subjective day and subjective night, and both light‐on cells and light‐off cells respond more strongly to changes in light intensity during the subjective night than during the subjective day.

Paired recordings indicate that the discharge of adjacent light‐responsive cells is very tightly synchronized.

The gap junction inhibitor carbenoxolone increases the spontaneous activity of suprachiasmatic nucleus neurones but does not block the harmonic discharge patterning.

Abstract

The suprachiasmatic nucleus (SCN) of the hypothalamus has an essential role in orchestrating circadian rhythms of behaviour and physiology. In the present study, we recorded from single SCN neurons in urethane‐anaesthetized rats, categorized them by the statistical features of their electrical activity and by their responses to light, and examined how activity in the light phase differs from activity in the dark phase. We classified cells as light‐on cells or light‐off cells according to how their firing rate changed in acute response to light, or as non‐responsive cells. In both sets of light‐responsive neurons, responses to light were stronger at subjective night than in subjective day. Neuronal firing patterns were analysed by constructing hazard functions from interspike interval data. For most light‐responsive cells, the hazard functions showed a multimodal distribution, with a harmonic sequence of modes, indicating that spike activity was driven by an oscillatory input with a fundamental frequency of close to 30 Hz; this harmonic pattern was rarely seen in non‐responsive SCN cells. The frequency of the rhythm was the same in light‐on cells as in light‐off cells, was the same in subjective day as at subjective night, and was unaffected by exposure to light. Paired recordings indicated that the discharge of adjacent light‐responsive neurons was very tightly synchronized, consistent with electrical coupling.

Light‐responsive neurones in the rat suprachiasmatic nucleus discharge with a harmonic distribution of interspike intervals, whereas unresponsive neurones seldom do.

This harmonic patterning has a fundamental frequency of close to 30 Hz, and is the same in light‐on cells as in light‐off cells, and is unaffected by exposure to light.

Light‐on cells are more active than light‐off cells in both subjective day and subjective night, and both light‐on cells and light‐off cells respond more strongly to changes in light intensity during the subjective night than during the subjective day.

Paired recordings indicate that the discharge of adjacent light‐responsive cells is very tightly synchronized.

The gap junction inhibitor carbenoxolone increases the spontaneous activity of suprachiasmatic nucleus neurones but does not block the harmonic discharge patterning.

Suprachiasmatic Nucleus Neuropeptides and Their Control of Endogenous Glucose Production

Defective control of endogenous glucose production is an important factor responsible for hyperglycaemia in the diabetic individual. During the past decade, progressively more evidence has appeared indicating a strong and potentially causal relationship between disturbances of the circadian system and defects of metabolic regulation, including glucose metabolism. The detrimental effects of disturbed circadian rhythms may have their origin in disturbances of the molecular clock mechanisms in peripheral organs, such as the pancreas and liver, or in the central brain clock in the hypothalamic suprachiasmatic nuclei (SCN). To assess the role of SCN output per se on glucose metabolism, we investigated (i) the effect of several SCN neurotransmitters on endogenous glucose production and (ii) the effect of SCN neuronal activity on hepatic and systemic insulin sensitivity. We show that silencing of SCN neuronal activity results in decreased hepatic insulin sensitivity and increased peripheral insulin sensitivity. Furthermore, both oxytocin neurones in the paraventricular nucleus of the hypothalamus (PVN) and orexin neurones in the lateral hypothalamus may be important targets for the SCN control of glucose metabolism. These data further highlight the role of the central clock in the pathophysiology of insulin resistance.

Inhibitory and excitatory networks balance cell coupling in the suprachiasmatic nucleus: A modeling approach

Neuronal coupling contributes to circadian rhythms formation in the suprachiasmatic nucleus (SCN). While the neurotransmitter vasoactive intestinal polypeptide (VIP) is considered essential for synchronizing the oscillations of individual neurons, γ-aminobutyric acid (GABA) does not have a clear functional role despite being highly concentrated in the SCN. While most studies have examined the role of either GABA or VIP, our mathematical modeling approach explored their interplay on networks of SCN neurons. Tuning the parameters that control the release of GABA and VIP enabled us to optimize network synchrony, which was achieved at a peak firing rate during the subjective day of about 7Hz. Furthermore, VIP and GABA modulation could adjust network rhythm amplitude and period without sacrificing synchrony. We also performed simulations of SCN networks to phase shifts during 12h:12h light-dark cycles and showed that GABA networks reduced the average time for the SCN model to re-synchronize. We hypothesized that VIP and GABA balance cell coupling in the SCN to promote synchronization of heterogeneous oscillators while allowing flexibility for adjustment to environmental changes.

Localization and expression of GABA transporters in the suprachiasmatic nucleus

GABA is a principal neurotransmitter in the suprachiasmatic hypothalamic nucleus (SCN), the master circadian clock. Despite the importance of GABA and GABA uptake for functioning of the circadian pacemaker, the localization and expression of GABA transporters (GATs) in the SCN has not been investigated. The present studies used Western blot analysis, immunohistochemistry and electron microscopy to demonstrate the presence of GABA transporter 1 (GAT1) and GAT3 in the SCN. By using light microscopy, GAT1 and GAT3 were co-localized throughout the SCN, but were not expressed in the perikarya of arginine vasopressin- or vasoactive intestinal peptide-immunoreactive (-ir) neurons of adult rats, nor in the neuronal processes labelled with the neurofilament heavy chain. Using electron microscopy, GAT1- and GAT3-ir was found in glial processes surrounding unlabelled neuronal perikarya, axons, dendrites, and enveloped symmetric and asymmetric axo-dendritic synapses. Glial fibrillary acidic protein-ir astrocytes grown in cell culture were immunopositive for GAT1 and GAT3 and both GATs could be observed in the same glial cell. These data demonstrate that synapses in the SCN function as 'tripartite' synapses consisting of presynaptic axon terminals, postsynaptic membranes and astrocytes that contain GABA transporters. This model suggests that astrocytes expressing both GATs may regulate the extracellular GABA, and thereby modulate the activity of neuronal networks in the SCN.

The Neurobiology of Circadian Rhythms

There is a growing recognition that the coordinated timing of behavioral, physiologic, and metabolic circadian rhythms is a requirement for a healthy body and mind. In mammals, the primary circadian oscillator is the hypothalamic suprachiasmatic nucleus (SCN), which is responsible for circadian coordination throughout the organism. Temporal homeostasis is recognized as a complex interplay between rhythmic clock gene expression in brain regions outside the SCN and in peripheral organs. Abnormalities in this intricate circadian orchestration may alter sleep patterns and contribute to the pathophysiology of affective disorders.

Differential expression patterns of K(+) /Cl(-) cotransporter 2 in neurons within the superficial spinal dorsal horn of rats

γ-Aminobutyric acid (GABA)- and glycine-mediated hyperpolarizing inhibition is associated with a chloride influx that depends on the inwardly directed chloride electrochemical gradient. In neurons, the extrusion of chloride from the cytosol primarily depends on the expression of an isoform of potassium-chloride cotransporters (KCC2s). KCC2 is crucial in the regulation of the inhibitory tone of neural circuits, including pain processing neural assemblies. Thus we investigated the cellular distribution of KCC2 in neurons underlying pain processing in the superficial spinal dorsal horn of rats by using high-resolution immunocytochemical methods. We demonstrated that perikarya and dendrites widely expressed KCC2, but axon terminals proved to be negative for KCC2. In single ultrathin sections, silver deposits labeling KCC2 molecules showed different densities on the surface of dendritic profiles, some of which were negative for KCC2. In freeze fracture replicas and tissue sections double stained for the β3-subunit of GABAA receptors and KCC2, GABAA receptors were revealed on dendritic segments with high and also with low KCC2 densities. By measuring the distances between spots immunoreactive for gephyrin (a scaffolding protein of GABAA and glycine receptors) and KCC2 on the surface of neurokinin 1 (NK1) receptor-immunoreactive dendrites, we found that gephyrin-immunoreactive spots were located at various distances from KCC2 cotransporters; 5.7 % of them were recovered in the middle of 4-10-µm-long dendritic segments that were free of KCC2 immunostaining. The variable local densities of KCC2 may result in variable postsynaptic potentials evoked by the activation of GABAA and glycine receptors along the dendrites of spinal neurons.

Projections from the subparaventricular zone define four channels of output from the circadian timing system

The subparaventricular zone of the hypothalamus (SPZ) is the main efferent target of neural projections from the suprachiasmatic nucleus (SCN) and an important relay for the circadian timing system. Although the SPZ is fairly homogeneous cytoarchitecturally and neurochemically, it has been divided into distinct functional and connectional subdivisions. The dorsal subdivision of the SPZ (dSPZ) plays an important role in relaying signals from the SCN controlling body temperature rhythms, while the ventral subdivision (vSPZ) is critical for rhythms of sleep and locomotor activity (Lu et al. [] J Neurosci 21:4864-4874). On the other hand, the medial part of the SPZ receives input mainly from the dorsomedial SCN, whereas the lateral SPZ receives input from the ventrolateral SCN and the retinohypothalamic tract (Leak and Moore [] J Comp Neurol 433:312-334). We therefore investigated whether there are corresponding differences in efferent outputs from these four quadrants of the SPZ (dorsolateral, ventrolateral, dorsomedial, and ventromedial) by a combination of anterograde and retrograde tracing. We found that, while all four subdivisions of the SPZ share a similar backbone of major projection pathways to the septal region, thalamus, hypothalamus, and brainstem, each segment of the SPZ has a specific set of targets where its projections dominate. Furthermore, we observed intra-SPZ projections of varying densities between the four subdivisions. Taken together, this pattern of organization suggests that the circadian timing system may have several parallel neural outflow pathways that provide a road map for understanding how they subserve different functions.

Neuronal gamma-aminobutyric acid (GABA) type A receptors undergo cognate ligand chaperoning in the endoplasmic reticulum by endogenous GABA

GABA_A receptors mediate fast inhibitory neurotransmission in the brain. Dysfunction of these receptors is associated with various psychiatric/neurological disorders and drugs targeting this receptor are widely used therapeutic agents. Both the efficacy and plasticity of GABA_A receptor-mediated neurotransmission depends on the number of surface GABA_A receptors. An understudied aspect of receptor cell surface expression is the post-translational regulation of receptor biogenesis within the endoplasmic reticulum (ER). We have previously shown that exogenous GABA can act as a ligand chaperone of recombinant GABA_A receptors in the early secretory pathway leading us to now investigate whether endogenous GABA facilitates the biogenesis of GABA_A receptors in primary cerebral cortical cultures. In immunofluorescence labeling experiments, we have determined that neurons expressing surface GABA_A receptors contain both GABA and its degradative enzyme GABA transaminase (GABA-T). Treatment of neurons with GABA-T inhibitors, a treatment known to increase intracellular GABA levels, decreases the interaction of the receptor with the ER quality control protein calnexin, concomittantly increasing receptor forward-trafficking and plasma membrane insertion. The effect of GABA-T inhibition on the receptor/calnexin interaction is not due to the activation of surface GABA_A or GABA_B receptors. Consistent with our hypothesis that GABA acts as a cognate ligand chaperone in the ER, immunogold-labeling of rodent brain slices reveals the presence of GABA within the rough ER. The density of this labeling is similar to that present in mitochondria, the organelle in which GABA is degraded. Lastly, the effect of GABA-T inhibition on the receptor/calnexin interaction was prevented by pretreatment with a GABA transporter inhibitor. Together, these data indicate that endogenous GABA acts in the rough ER as a cognate ligand chaperone to facilitate the biogenesis of neuronal GABA_A receptors.

Vasoactive intestinal polypeptide (VIP)-expressing neurons in the suprachiasmatic nucleus provide sparse GABAergic outputs to local neurons with circadian regulation occurring distal to the opening of postsynaptic GABAA ionotropic receptors

GABAergic synaptic transmission plays an important role in resetting and synchronizing circadian rhythms in the suprachiasmatic nucleus (SCN). Although the circadian modulation of intrinsic membrane currents and biochemical signaling have been examined in the SCN, the modulation of specific synaptic pathways within the SCN is unexplored. In addition, little is known about the functional properties of these pathways, including which ones involve GABAA receptors (GABAA-Rs). In brain slices obtained from mice, we examined synaptic responses originating from the SCN neurons expressing vasoactive intestinal peptide (VIP+ neurons). Focusing on the local projection within the ventromedial SCN, we found that VIP+ afferents provided input onto 49% of neurons with a preference for VIP-negative (VIP-) neurons. Responses were mediated by GABAA-Rs. The projection was sparsely connected and preferentially targeted a subset of SCN neurons unrelated to postsynaptic VIP expression. For most aspects of VIP+ network output, there was no circadian regulation. Excitability and spontaneous firing of the presynaptic VIP+ neurons were unchanged between day and night, and their network connectivity and synaptic function up through the evoked synaptic conductance were also unchanged. On the other hand, VIP+ input onto VIP- neurons became less inhibitory at night suggesting a postsynaptic alteration in the coupling of GABAA-R conductances to action potential firing. These data suggest that components of the VIP network and its synaptic output up through GABAA-R opening are invariant during the circadian cycle, but the effect on action potential firing is modulated by postsynaptic processes occurring after GABAA-R channel opening.

Etiology of distinct membrane excitability in pre- and posthearing auditory neurons relies on activity of Cl- channel TMEM16A

The developmental rehearsal for the debut of hearing is marked by massive changes in the membrane properties of hair cells (HCs) and spiral ganglion neurons (SGNs). Whereas the underlying mechanisms for the developing HC transition to mature stage are understood in detail, the maturation of SGNs from hyperexcitable prehearing to quiescent posthearing neurons with broad dynamic range is unknown. Here, we demonstrated using pharmacological approaches, caged-Ca(2+) photolysis, and gramicidin patch recordings that the prehearing SGN uses Ca(2+)-activated Cl(-) conductance to depolarize the resting membrane potential and to prime the neurons in a hyperexcitable state. Immunostaining of the cochlea preparation revealed the identity and expression of the Ca(2+)-activated Cl(-) channel transmembrane member 16A (TMEM16A) in SGNs. Moreover, null deletion of TMEM16A reduced the Ca(2+)-activated Cl(-) currents and action potential firing in SGNs. To determine whether Cl(-) ions and TMEM16A are involved in the transition between pre- and posthearing features of SGNs we measured the intracellular Cl(-) concentration [Cl(-)]i in SGNs. Surprisingly, [Cl(-)]i in SGNs from prehearing mice was ∼90 mM, which was significantly higher than posthearing neurons, ∼20 mM, demonstrating discernible altered roles of Cl(-) channels in the developing neuron. The switch in [Cl(-)]i stems from delayed expression of the development of intracellular Cl(-) regulating mechanisms. Because the Cl(-) channel is the only active ion-selective conductance with a reversal potential that lies within the dynamic range of SGN action potentials, developmental alteration of [Cl(-)]i, and hence the equilibrium potential for Cl(-) (ECl), transforms pre- to posthearing phenotype.

Laminar and neurochemical organization of the dorsal cochlear nucleus of the human, monkey, cat, and rodents

The dorsal cochlear nucleus (DCN) is a brainstem structure that receives input from the auditory nerve. Many studies in a diversity of species have shown that the DCN has a laminar organization and identifiable neuron types with predictable synaptic relations to each other. In contrast, studies on the human DCN have found a less distinct laminar organization and fewer cell types, although there has been disagreement among studies in how to characterize laminar organization and which of the cell types identified in other animals are also present in humans. We have reexamined DCN organization in the human using immunohistochemistry to analyze the expression of several proteins that have been useful in delineating the neurochemical organization of other brainstem structures in humans: nonphosphorylated neurofilament protein (NPNFP), nitric oxide synthase (nNOS), and three calcium-binding proteins. The results for humans suggest a laminar organization with only two layers, and the presence of large projection neurons that are enriched in NPNFP. We did not observe evidence in humans of the inhibitory interneurons that have been described in the cat and rodent DCN. To compare humans and other animals directly we used immunohistochemistry to examine the DCN in the macaque monkey, the cat, and three rodents. We found similarities between macaque monkey and human in the expression of NPNFP and nNOS, and unexpected differences among species in the patterns of expression of the calcium-binding proteins.

Circadian rhythms and addiction: mechanistic insights and future directions

Circadian rhythms are prominent in many physiological and behavioral functions. Circadian disruptions either by environmental or molecular perturbation can have profound health consequences, including the development and progression of addiction. Both animal and humans studies indicate extensive bidirectional relationships between the circadian system and drugs of abuse. Addicted individuals display disrupted rhythms, and chronic disruption or particular chronotypes may increase the risk for substance abuse and relapse. Moreover, polymorphisms in circadian genes and an evening chronotype have been linked to mood and addiction disorders, and recent efforts suggest an association with the function of reward neurocircuitry. Animal studies are beginning to determine how altered circadian gene function results in drug-induced neuroplasticity and behaviors. Many studies suggest a critical role for circadian rhythms in reward-related pathways in the brain and indicate that drugs of abuse directly affect the central circadian pacemaker. In this review, we highlight key findings demonstrating the importance of circadian rhythms in addiction and how future studies will reveal important mechanistic insights into the involvement of circadian rhythms in drug addiction.

Dynamic interactions mediated by nonredundant signaling mechanisms couple circadian clock neurons

Interactions among suprachiasmatic nucleus (SCN) neurons are required for robust circadian rhythms entrained to local time. To investigate these signaling mechanisms, we developed a functional coupling assay that uniquely captures the dynamic process by which SCN neurons interact. As a population, SCN neurons typically display synchronized rhythms with similar peak times, but will peak 6-12 hr apart after in vivo exposure to long days. Once they are removed from these conditions, SCN neurons resynchronize through a phase-dependent coupling process mediated by both vasoactive intestinal polypeptide (VIP) and GABAA signaling. Notably, GABAA signaling contributes to coupling when the SCN network is in an antiphase configuration, but opposes synchrony under steady-state conditions. Further, VIP acts together with GABAA signaling to couple the network in an antiphase configuration, but promotes synchrony under steady-state conditions by counteracting the actions of GABAA signaling. Thus, SCN neurons interact through nonredundant coupling mechanisms influenced by the state of the network.

Activity-dependent regulation of the K/Cl transporter KCC2 membrane diffusion, clustering, and function in hippocampal neurons

The neuronal K/Cl transporter KCC2 exports chloride ions and thereby influences the efficacy and polarity of GABA signaling in the brain. KCC2 is also critical for dendritic spine morphogenesis and the maintenance of glutamatergic transmission in cortical neurons. Because KCC2 plays a pivotal role in the function of central synapses, it is of particular importance to understand the cellular and molecular mechanisms underlying its regulation. Here, we studied the impact of membrane diffusion and clustering on KCC2 function. KCC2 forms clusters in the vicinity of both excitatory and inhibitory synapses. Using quantum-dot-based single-particle tracking on rat primary hippocampal neurons, we show that KCC2 is slowed down and confined at excitatory and inhibitory synapses compared with extrasynaptic regions. However, KCC2 escapes inhibitory synapses faster than excitatory synapses, reflecting stronger molecular constraints at the latter. Interfering with KCC2-actin interactions or inhibiting F-actin polymerization releases diffusion constraints on KCC2 at excitatory but not inhibitory synapses. Thus, F-actin constrains KCC2 diffusion at excitatory synapses, whereas KCC2 is confined at inhibitory synapses by a distinct mechanism. Finally, increased neuronal activity rapidly increases the diffusion coefficient and decreases the dwell time of KCC2 at excitatory synapses. This effect involves NMDAR activation, Ca(2+) influx, KCC2 S940 dephosphorylation and calpain protease cleavage of KCC2 and is accompanied by reduced KCC2 clustering and ion transport function. Thus, activity-dependent regulation of KCC2 lateral diffusion and clustering allows for a rapid regulation of chloride homeostasis in neurons.

GABA networks destabilize genetic oscillations in the circadian pacemaker

Systems of coupled oscillators abound in nature. How they establish stable phase relationships under diverse conditions is fundamentally important. The mammalian suprachiasmatic nucleus (SCN) is a self-sustained, synchronized network of circadian oscillators that coordinates daily rhythms in physiology and behavior. To elucidate the underlying topology and signaling mechanisms that modulate circadian synchrony, we discriminated the firing of hundreds of SCN neurons continuously over days. Using an analysis method to identify functional interactions between neurons based on changes in their firing, we characterized a GABAergic network comprised of fast, excitatory, and inhibitory connections that is both stable over days and changes in strength with time of day. By monitoring PERIOD2 protein expression, we provide the first evidence that these millisecond-level interactions actively oppose circadian synchrony and inject jitter into daily rhythms. These results provide a mechanism by which circadian oscillators can tune their phase relationships under different environmental conditions.

GABAB receptor-mediated frequency-dependent and circadian changes in synaptic plasticity modulate retinal input to the suprachiasmatic nucleus

Light is the most important environmental signal that entrains the circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN). The retinohypothalamic tract (RHT) was stimulated to simulate the light intensity-dependent discharges of intrinsically photosensitive retinal ganglion cells projecting axons to the hypothalamus. EPSCs were evoked by paired-pulse stimulation or by application of stimulus trains, and recorded from SCN neurons in rat brain slices. Initial release probability (Pr) and synaptic plasticity changes depended on the strength of GABAB receptor (GABABR)-mediated presynaptic inhibition and could be different at the same GABABR agonist concentration. Facilitation caused by frequency-dependent relief of GABABR-mediated inhibition was observed when the initial Pr was decreased to less than 15% of control during strong activation of presynaptic GABAB receptors by (±)baclofen (10 μm), GABA (2 mm) or by GABA uptake inhibitor nipecotic acid (5 mm). In contrast, short-term synaptic depression appeared during baclofen (10 μm) application when initial Pr was greater than 30% of control. Block of 4-aminopyridine-sensitive K(+) currents increased the amplitude and time constant of decay of evoked EPSCs (eEPSCs), and decreased the GABABR-mediated presynaptic inhibition. The GABAB receptor antagonist CGP55845 (3 μm) increased the eEPSCs amplitude 30% throughout the light-dark cycle. During light and dark phases the RHT inputs to 55% and 33% of recorded neurons, respectively, were under GABAB inhibitory control indicating that the tonic inhibition induced by local changes of endogenous GABA concentration contributes to the circadian variation of RHT transmitter release. We conclude that GABABR-mediated presynaptic inhibition decreased with increasing frequency and broadening of presynaptic action potentials, and depended on the sensitivity of RHT terminals to GABABR agonists, and diurnal changes of the extracellular GABA concentration around RHT axon terminals in the SCN.

The clock in the brain: neurons, glia, and networks in daily rhythms

The master coordinator of daily schedules in mammals, located in the ventral hypothalamus, is the suprachiasmatic nucleus (SCN). This relatively small population of neurons and glia generates circadian rhythms in physiology and behavior and synchronizes them to local time. Recent advances have begun to define the roles of specific cells and signals (e.g., peptides, amino acids, and purine derivatives) within this network that generate and synchronize daily rhythms. Here we focus on the best-studied signals between neurons and between glia in the mammalian circadian system with an emphasis on time-of-day pharmacology. Where possible, we highlight how commonly used drugs affect the circadian system.

Recent progress in GABAergic excitation from mature brain

The excitatory effect of γ-Aminobutyric acid (GABA) has been recognized in very young animals and in seizure generation, but not so much in animals after weaning age or in adults. The existence of this phenomenon in mature brain is still controversial. In the course of debate, creative studies have identified and characterized the phenomenon in suprachiasmatic nucleus, cortex, hippocampus and basolateral amygdala, albeit mostly in single neurons. In neural circuit activity, presumed GABAergic excitation was observed in basolateral amygdala during the study of a neuropeptide, cholecystokinin. Though the functional meaning of this phenomenon in vivo remains to be uncovered, it may be implicated in epilepsy or anxiety in the adult brain.