Pharmacological characteristics of GABAA responses in postnatal suprachiasmatic neurons in culture

GABAA receptor subunit composition regulates circadian rhythms in rest-wake and synchrony among cells in the suprachiasmatic nucleus

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) produces robust daily rhythms including rest-wake. SCN neurons synthesize and respond to γ-aminobutyric acid (GABA), but its role remains unresolved. We tested the hypothesis that γ2- and δ-subunits of the GABAA receptor in the SCN differ in their regulation of synchrony among circadian cells. We used two approaches: 1) shRNA to knock-down (KD) the expression of either γ2 or δ subunits in the SCN or 2) knock-in mice harboring a point mutation in the M2 domains of the endogenous GABAA γ2 or δ subunits. KD of either γ2 or δ subunits in the SCN increased daytime running and reduced nocturnal running by reducing their circadian amplitude by a third. Similarly, δ subunit knock-in mice showed decreased circadian amplitude, increased duration of daily activity, and decreased total daily activity. Reduction, or mutation of either γ2 or δ subunits halved the synchrony among, and amplitude of, circadian SCN cells as measured by firing rate or expression of the PERIOD2 protein, in vitro. Surprisingly, overexpression of the γ2 subunit rescued these phenotypes following KD or mutation of the δ subunit, and overexpression of the δ subunit rescued deficiencies due to γ2 subunit KD or mutation. We conclude that γ2 and δ GABAA receptor subunits play similar roles in maintaining circadian synchrony in the SCN and amplitude of daily rest-wake rhythms, but that modulation of their relative densities can change the duration and amplitude of daily activities.

Diurnal properties of tonic and synaptic GABAA receptor-mediated currents in suprachiasmatic nucleus neurons

Synaptic and extrasynaptic GABA_A receptor (GABA_AR)-mediated neurotransmission is a critical component of the suprachiasmatic nucleus (SCN) neuronal network. However, the properties of the GABA_A tonic current (I_tonic) and its origin remain unexplored. Spontaneous GABA_A postsynaptic currents (sGPSCs) and I_tonic were recorded from SCN neurons with the whole cell voltage-clamp technique at different times of the day. GABA_AR antagonists (bicuculline, gabazine, and picrotoxin) inhibited sGPSC and induced an outward shift of the holding current, which defined the I_tonic amplitude. The sGPSC frequency, synaptic charge transfer, and I_tonic amplitude all demonstrated significant diurnal rhythms, with peaks in the middle of the day [zeitgeber time (ZT)7-8] and nadirs at night (ZT19-20). The I_tonic amplitude increased proportionally with the sGPSC frequency and synaptic charge transfer during the day and required action potential-mediated GABA release, which was confirmed by TTX application. The activation of presynaptic GABA_B receptors by baclofen did not significantly alter the I_tonic of neurons with low-frequency sGPSC. The equilibrium potential (Eq) for I_tonic was similar to the Eq for chloride and GABA_A receptor-activated currents. I_tonic showed outward rectification at membrane potentials over the range of -70 to -10 mV and then was linear at voltages greater than -10 mV. GABA_AR containing α4-, α5-, and δ-subunits were expressed in SCN, and their contribution to I_tonic was confirmed by application of the GABA_AR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and the GABA_AR inverse agonist 11,12,13,13a-tetrahydro-7-methoxy-9-oxo-9H-imidazo[1,5-a]pyrrolo[2,1-c][1,4]benzodiazepine-1-carboxylic acid ethyl ester (L655,708). Thus, the I_tonic was mediated by extrasynaptic GABA_ARs activated predominantly by GABA diffusing out of GABAergic synapses.NEW & NOTEWORTHY A tonic current (I_tonic) mediated by GABA_A receptors (GABA_ARs) containing α4-, α5- and δ-subunits was observed in the suprachiasmatic nucleus. The I_tonic amplitude strongly depended on the action potential-mediated synaptic release of GABA. The equilibrium potential for I_tonic corresponds to that for GABA_A currents. The frequency of GABA_A postsynaptic currents and I_tonic amplitude increased during the day, with peak in the middle of the day, and then gradually declined with a nadir at night, thus showing a diurnal rhythm.

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.

Examination of Zinc in the Circadian System

Zinc is a trace element that is essential for a large number of biological and biochemical processes in the body. In the nervous system zinc is packaged into synaptic vesicles by the ZnT3 transporter, and synaptic release of zinc can influence the activity of postsynaptic cells, either directly through its own cognate receptors, or indirectly by modulating activation of receptors for other neurotransmitters. Here, we explore the anatomical and functional aspects of zinc in the circadian system. Melanopsin-containing retinal ganglion cells in the mouse retina were found to colocalize ZnT3, indicating that they can release zinc at their synaptic targets. While the master circadian clock in the hamster suprachiasmatic nucleus (SCN) was found to contain, at best, sparse zincergic input, the intergeniculate leaflet (IGL) of hamsters and mice were found to have prominent zincergic input. Levels of zinc in these areas were not affected by time of day. Additionally, IGL zinc staining persisted following enucleation, indicating other prominent sources of zinc instead of, or in addition to, the retina. Neither enhancement nor chelation of free zinc at either the SCN or IGL altered circadian responses to phase-shifting light in hamsters. Finally, entrainment, free-running, and circadian responses to light were explored in mice lacking the ZnT3 gene. In every aspect explored, the ZnT3 knockout mice were not significantly different from their wildtype counterparts. These findings highlight the presence of zinc in areas critical for circadian functioning but have yet to identify a role for zinc in these areas.

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.

GABA transporters regulate tonic and synaptic GABAA receptor-mediated currents in the suprachiasmatic nucleus neurons

GABA is a principal neurotransmitter in the hypothalamic suprachiasmatic nucleus (SCN) that contributes to intercellular communication between individual circadian oscillators within the SCN network and the stability and precision of the circadian rhythms. GABA transporters (GAT) regulate the extracellular GABA concentration and modulate GABAA receptor (GABAAR)-mediated currents. GABA transport inhibitors were applied to study how GABAAR-mediated currents depend on the expression and function of GAT. Nipecotic acid inhibits GABA transport and induced an inward tonic current in concentration-dependent manner during whole cell patch-clamp recordings from SCN neurons. Application of either the selective GABA transporter 1 (GAT1) inhibitors NNC-711 or SKF-89976A, or the GABA transporter 3 (GAT3) inhibitor SNAP-5114, produced only small changes of the baseline current. Coapplication of GAT1 and GAT3 inhibitors induced a significant GABAAR-mediated tonic current that was blocked by gabazine. GAT inhibitors decreased the amplitude and decay time constant and increased the rise time of spontaneous GABAAR-mediated postsynaptic currents. However, inhibition of GAT did not alter the expression of either GAT1 or GAT3 in the hypothalamus. Thus GAT1 and GAT3 functionally complement each other to regulate the extracellular GABA concentration and GABAAR-mediated synaptic and tonic currents in the SCN. Coapplication of SKF-89976A and SNAP-5114 (50 µM each) significantly reduced the circadian period of Per1 expression in the SCN by 1.4 h. Our studies demonstrate that GAT are important regulators of GABAAR-mediated currents and the circadian clock in the SCN.NEW & NOTEWORTHY In the suprachiasmatic nucleus (SCN), the GABA transporters GAT1 and GAT3 are expressed in astrocytes. Inhibition of these GABA transporters increased a tonic GABA current and reduced the circadian period of Per1 expression in SCN neurons. GAT1 and GAT3 showed functional cooperativity: inhibition of one GAT increased the activity but not the expression of the other. Our data demonstrate that GABA transporters are important regulators of GABAA receptor-mediated currents and 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.

Ethanol modulates mammalian circadian clock phase resetting through extrasynaptic GABA receptor activation

Ethanol modulates the actions of multiple neurotransmitter systems, including GABA. However, its enhancing effects on GABA signaling typically are seen only at high concentrations. In contrast, although GABA is a prominent neurotransmitter in the circadian clock of the suprachiasmatic nucleus (SCN), we see ethanol modulation of clock phase resetting at low concentrations (<50 mM). A possible explanation is that ethanol enhances GABAergic signaling in the SCN through activating GABA(A) receptors that contain the delta subunit (GABA(Adelta) receptors), which are sensitive to low ethanol concentrations. Therefore, we investigated whether ethanol acts on GABA(Adelta) receptors in the SCN. Here we show that acute application of the GABA(Adelta) receptor antagonist, RO15-4513, to mouse hypothalamic slices containing the SCN prevents ethanol inhibition of nighttime glutamate-induced (photic-like) phase delays of the circadian clock. Diazepam, which enhances activity of GABA(A) receptors containing the gamma subunit (GABA(Agamma) receptors), does not modulate these phase shifts. Moreover, we find that RO15-4513 prevents ethanol enhancement of daytime serotonergic (non-photic) phase advances of the circadian clock. Furthermore, diazepam phase-advances the SCN circadian clock when applied alone in the daytime, while ethanol has no effect by itself at that time. These data support the hypothesis that ethanol acts on GABA(Adelta) receptors in the SCN to modulate photic and non-photic circadian clock phase resetting. They also reveal distinct modulatory roles of different GABA(A) receptor subtypes in circadian clock phase regulation.

Acute ethanol modulates glutamatergic and serotonergic phase shifts of the mouse circadian clock in vitro

Alcohol abuse is associated with sleep problems, which are often linked to circadian rhythm disturbances. However, there is no information on the direct effects of ethanol on the mammalian circadian clock. Acute ethanol inhibits glutamate signaling, which is the primary mechanism through which light resets the mammalian clock in the suprachiasmatic nucleus (SCN). Glutamate and light also inhibit circadian clock resetting induced by nonphotic signals, including 5-HT. Thus, we investigated the effects of acute ethanol on both glutamatergic and serotoninergic resetting of the mouse SCN clock in vitro. We show that ethanol dose-dependently inhibits glutamate-induced phase shifts and enhances serotonergic phase shifts. The inhibition of glutamate-induced phase shifts is not affected by excess glutamate, glycine or d-serine, but is prevented by excess brain-derived neurotrophic factor (BDNF). BDNF is known to augment glutamate signaling in the SCN and to be necessary for glutamate/light-induced phase shifts. Thus, ethanol may inhibit glutamate-induced clock resetting at least in part by blocking BDNF enhancement of glutamate signaling. Ethanol enhancement of serotonergic phase shifts is mimicked by treatments that suppress glutamate signaling in the SCN, including antagonists of glutamate receptors, BDNF signaling and nitric oxide synthase. The combined effect of ethanol with these treatments is not additive, suggesting they act through a common pathway. Our data indicate further that the interaction between 5-HT and glutamate in the SCN may occur downstream from nitric oxide synthase activation. Thus, acute ethanol disrupts normal circadian clock phase regulation, which could contribute to the physiological and psychological problems associated with alcohol abuse.

The effect of curcumin on ethanol induced changes in suprachiasmatic nucleus (SCN) and pineal

(1) Circadian clocks have been localized to discrete sites within the nervous system of several organisms and in mammals to the suprachiasmatic nucleus (SCN) in the anterior hypothalamus. The SCN controls and regulates the production and discharge of melatonin (hormonal message of darkness) from the pineal gland via a multisynaptic efferent pathway. The nocturnal rise in melatonin production from serotonin results due to an increased activity of serotonin N-acetyl transferase (NAT). (2) The complex interaction between alcohol and biological clock need to be understood as alcoholism results in various clock linked neuronal disorders especially loss of memory and amnesia like state of consciousness, sleep disorders, insomnia, dementia etc. (3) Serotonin, 5-Hydroxy-tryptamine (5-HT) plays an important role in mediating alcohol's effects on the brain. Understanding the impact of alcohol consumption on circadian system is a pre-requisite to help in treatment of alcohol induced neurological disorders. We, therefore, studied the effect of ethanol drinking and ethanol withdrawal on daily rhythms of serotonin and its metabolite, 5-hydroxy-indole acetic acid (5-HIAA) in SCN and Pineal of adult male Wistar rats maintained under light-dark (LD, 12:12) conditions. (4) Curcumin is well known for its protective properties such as antioxidant, anti-carcinogenic, anti-viral and anti-infectious etc. Hence, we studied the effect of curcumin on ethanol induced changes on 5-HT and 5-HIAA levels and rhythms in SCN and Pineal. (5) Ethanol withdrawal could not restore either rhythmicity or phases or levels of 5-HT and 5-HIAA. Curcumin administration resulted in partial restoration of daily 5-HT/5-HIAA ratio, with phase shifts in SCN and in Pineal. Understanding the impact of alcohol consumption on circadian system and the role of herbal medication on alcohol withdrawal will help in treatment of alcohol induced neurological disorders.

Circadian Rhythmicity in AVP Secretion and GABAergic Synaptic Transmission in the Rat Suprachiasmatic Nucleus

A variety of physiological and behavioral functions exhibit circadian changes and these circadian rhythms are driven by oscillatory expression of clock genes in the suprachiasmatic nuclei (SCN). It is still unknown how this molecular clockwork is controlled by extracellular neurohormones and neurotransmitters and which membrane receptors undergo circadian modulation. Circadian rhythm can be measured as a secretion of arginine vasopressin (AVP) in organotypic SCN culture for several weeks. Melatonin applied directly to the SCN late in the day induces a phase advance, when applied late at night or at the beginning of the day melatonin causes a phase delay. The time window for phase advance corresponds with the highest level of melatonin receptors in the SCN but the mechanism of melatonin-induced phase delay is unknown. The principal neurotransmitter on SCN synapses is gamma-aminobutyric acid (GABA), which acts at postsynaptic GABA(A) receptors. Spontaneous release of GABA from presynaptic nerve terminals, recorded as miniature inhibitory postsynaptic currents in the presence of TTX, does not change, but zinc sensitivity of exogenous GABA-induced currents varies during the day and night, possibly due to changes in subunit composition of GABA(A) receptors. We conclude that there is daily variation in the postsynaptic, but not presynaptic, function in the SCN.

Blockade of GABAA receptors disrupts isoperiodic neuronal oscillations in the intergeniculate leaflet of the rat

The intergeniculate leaflet of the thalamus is, besides the suprachiasmatic nucleus of the hypothalamus, the other important neuronal element of the mammalian biological clock. The extracellularly recorded activity of neurons constituting the intergeniculate leaflet, recorded in vivo, is characterized by distinct, very regular ultradian oscillations. The majority of neurons in the circadian timing system are GABAergic. Many, if not all, neurons of the suprachiasmatic nucleus and intergeniculate leaflet contain GABA. In the present study we examined the effects of the GABA(A) receptor antagonist bicuculline and the chloride channel blocker picrotoxin on isoperiodic neuronal oscillations in the intergeniculate leaflet of rats. We recorded extracellular multiple-unit neuronal activity from the intergeniculate leaflet of anesthetized rats. During the recording of isoperiodic oscillations, bicuculline or picrotoxin were stereotaxically injected at different concentrations into the lateral ventricle of rat brain. In all the experiments, injection of GABA(A) receptor antagonists transiently disrupted the isoperiodic phasic discharge recorded from the intergeniculate leaflet. These data suggest that GABA(A) receptors are involved in the generation of ultradian rhythmical neuronal oscillations in rat intergeniculate leaflet.

Day–night variations in zinc sensitivity of GABAA receptor-channels in rat suprachiasmatic nucleus

In the suprachiasmatic nucleus (SCN), electrical activity, secretion, and other cellular functions undergo profound rhythm during day-night cycle due to oscillatory expression of clock gene constituents. Although SCN is enriched with gamma-aminobutyric acid (GABA)-ergic neurons, it is unknown whether there are circadian changes in the GABAA receptor expression and/or function. Here we investigated the possible daily variations in zinc sensitivity of GABAA channels in rat SCN neurons maintained in brain slices. Extracellular zinc inhibited GABA-induced currents in all ventrolateral (VL) and dorsomedial (DM) SCN neurons studied, as well as in neurons of non-SCN regions. In SCN neurons, the currents evoked by 30 microM GABA were inhibited by Zn2+ with an IC50 of 50.3+/-3.2 microM, whereas currents evoked by 100 microM GABA were inhibited with an IC50 of 181.6+/-32.0 microM. The antagonist action of zinc saturated at 97.4+/-0.7% for 30 microM GABA and 91.6+/-2.7% for 100 microM GABA. These observations indicate that Zn2+ inhibits SCN GABAA receptor competitively and in part non-competitively. In SCN neurons, but not in other neurons, the zinc sensitivity varied with daily time. During the day, the calculated IC50 for zinc was significantly lower than during the night (43.9+/-4.7 microM vs. 58.6+/-3.8, respectively). These results indicate that native GABAA receptors in SCN neurons display pharmacological properties of receptors having and not having gamma subunit and that the proportionality of these receptors could change during the day and night.

Presynaptic and postsynaptic GABAA receptors in rat suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus (SCN), the brain's circadian clock, is composed mainly of GABAergic neurons, that are interconnected via synapses with GABA(A) receptors. Here we report on the subcellular localization of these receptors in the SCN, as revealed by an extensively characterized antibody to the alpha 3 subunit of GABA(A) receptors in conjunction with pre- and postembedding electron microscopic immunocytochemistry. GABA(A) receptor immunoreactivity was observed in neuronal perikarya, dendritic processes and axonal terminals. In perikarya and proximal dendrites, GABA(A) receptor immunoreactivity was expressed mainly in endoplasmic reticulum and Golgi complexes, while in the distal part of dendrites, immunoreaction product was associated with postsynaptic plasma membrane. Many GABAergic axonal terminals, as revealed by postembedding immunogold labeling, displayed GABA(A) receptor immunoreactivity, associated mainly with the extrasynaptic portion of their plasma membrane. The function of these receptors was studied in hypothalamic slices using whole-cell patch-clamp recording of the responses to minimal stimulation of an area dorsal to the SCN. Analysis of the evoked inhibitory postsynaptic currents showed that either bath or local application of 100 microM of GABA decreased GABAergic transmission, manifested as a two-fold increase in failure rate. This presynaptic effect, which was detected in the presence of the glutamate receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione and the selective GABA(B) receptor blocker CGP55845A, appears to be mediated via activation of GABA(A) receptors. Our results thus show that GABA(A) receptors are widely distributed in the SCN and may subserve both pre- and postsynaptic roles in controlling the mammalian circadian clock.

Functional characterization of GABA(A) receptors in neonatal hypothalamic brain slice

The hypothalamus influences a number of autonomic functions. The activity of hypothalamic neurons is modulated in part by release of the inhibitory neurotransmitter GABA onto these neurons. GABA(A) receptors are formed from a number of distinct subunits, designated alpha, beta, gamma, delta, epsilon, and theta, many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA(A) receptors present on hypothalamic neurons. To gain insight into which GABA(A) receptor subunits are functionally expressed in the hypothalamus, we used an array of pharmacologic assessments. Whole cell recordings were made from thin hypothalamic slices obtained from 1- to 14-day-old rats. GABA(A) receptor-mediated currents were detected in all neurons tested and had an average EC(50) of 20 +/- 1.6 microM. Hypothalamic GABA(A) receptors were modulated by diazepam (EC(50) = 0.060 microM), zolpidem (EC(50) = 0.19 microM), loreclezole (EC(50) = 4.4 microM), methyl-6,7-dimethoxy-4-ethyl-beta-carboline (EC(50) = 7.7 microM), and 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). Conversely, these receptors were inhibited by Zn(2+) (IC(50) = 70.5 microM), dehydroepiandrosterone sulfate (IC(50) = 16.7 microM), and picrotoxin (IC(50) = 2.6 microM). The alpha4/6-selective antagonist furosemide (10-1,000 microM) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA(A) receptor subtypes that incorporate alpha1 and/or alpha2 subunits, beta2 and/or beta3 subunits, and the gamma2 subunit. Our results suggest receptors expressing alpha3-alpha6, beta1, gamma1, and delta, if present, represent a minor component of functional hypothalamic GABA(A) receptors.

Modulation of neuronal and recombinant GABAA receptors by redox reagents

1. The functional role played by the postulated disulphide bridge in gamma-aminobutyric acid type A (GABAA) receptors and its susceptibility to oxidation and reduction were studied using recombinant (murine receptor subunits expressed in human embryonic kidney cells) and rat neuronal GABAA receptors in conjunction with whole-cell and single channel patch-clamp techniques. 2. The reducing agent dithiothreitol (DTT) reversibly potentiated GABA-activated responses (IGABA) of alpha1beta1 or alpha1beta2 receptors while the oxidizing reagent 5, 5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) caused inhibition. Redox modulation of IGABA was independent of GABA concentration, membrane potential and the receptor agonist and did not affect the GABA EC50 or Hill coefficient. The endogenous antioxidant reduced glutathione (GSH) also potentiated IGABA in alpha1beta2 receptors, while both the oxidized form of DTT and glutathione (GSSG) caused small inhibitory effects. 3. Recombinant receptors composed of alpha1beta1gamma2S or alpha1beta2gamma2S were considerably less sensitive to DTT and DTNB. 4. For neuronal GABAA receptors, IGABA was enhanced by flurazepam and relatively unaffected by redox reagents. However, in cultured sympathetic neurones, nicotinic acetylcholine-activated responses were inhibited by DTT whilst in cerebellar granule neurones, NMDA-activated currents were potentiated by DTT and inhibited by DTNB. 5. Single GABA-activated ion channel currents exhibited a conductance of 16 pS for alpha1beta1 constructs. DTT did not affect the conductance or individual open time constants determined from dwell time histograms, but increased the mean open time by affecting the channel open probability without increasing the number of cell surface receptors. 6. A kinetic model of the effects of DTT and DTNB suggested that the receptor existed in equilibrium between oxidized and reduced forms. DTT increased the rate of entry into reduced receptor forms and also into desensitized states. DTNB reversed these kinetic effects. 7. Our results indicate that GABAA receptors formed by alpha and beta subunits are susceptible to regulation by redox agents. Inclusion of the gamma2 subunit in the receptor, or recording from some neuronal GABAA receptors, resulted in reduced sensitivity to DTT and DTNB. Given the suggested existence of alphabeta subunit complexes in some areas of the central nervous system together with the generation and release of endogenous redox compounds, native GABAA receptors may be subject to regulation by redox mechanisms.

Zinc and flunitrazepam modulation of GABA-mediated currents in rat suprachiasmatic neurons

The suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for generating circadian rhythms in mammals, and GABA is the predominant neurotransmitter in the SCN. Properties of gamma-aminobutyric acid-A (GABAA) responses in SCN neurons were examined in acutely prepared hypothalamic slices from 3- to 8-wk-old rats with the use of whole cell voltage-clamp techniques. Zn2+ reduced the amplitude of GABAA-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in a concentration-dependent manner ranging from a reduction of control amplitude to 88% at 10 microM to 27% at 1,000 microM. Zn2+ reduced IPSC amplitude to a similar degree in the presence of tetrodotoxin and also significantly reduced the amplitude of currents evoked by application of exogenous GABA (100 microM, pressure applied). Zn2+ increased the frequency of IPSCs at lower concentrations and decreased it at higher ones. Flunitrazepam (100 nM) usually failed to potentiate the amplitude of sIPSCs, but prolonged sIPSC kinetics. Two exponential components were normally resolved in the sIPSC decay constants, and flunitrazepam significantly increased those two components. Thus flunitrazepam increased the duration of sIPSCs and potentiated the amplitude of currents evoked by pressure application of GABA. Zn2+ and benzodiazepine each modulated the effect of GABA in nearly all cells, suggesting that most SCN neurons have a similar GABAA receptor subunit composition in this respect. Zn2+ also affected sIPSC frequency, which suggests that Zn2+ increased neuronal firing rate at lower concentrations. These results begin to define the cellular roles that these GABAA receptor modulators might play in circadian regulation.

The rhythmic GABAergic system

GABA is the major inhibitory neurotransmitter in the mammalian brain, and has been implicated in the regulation of a variety of behavioral functions, including biological rhythms. The focus of this minireview is the rhythmic variation of the central GABAergic system, comprising fluctuations of GABA levels and turnover, GABA receptor affinity and postsynaptic activity on the chloride ionophore in rodent's brain. Neurochemical rhythms correlated with diurnal and circadian changes in several behaviors associated with the GABA(A) receptor, e.g., anxiolysis-related behavior. GABA is considered to be the principal neurotransmitter of the mammalian circadian system, being present in the suprachiasmatic nuclei and the intergeniculate leaflet. Pharmacological manipulations of GABA(A) receptors phase shift circadian rhythms and alter circadian responses to light. Administration of putative modulators of GABA function, like melatonin or neuroactive steroids, affects the timing of biological rhythms. Therefore, not only does the GABAergic system exhibit strong diurnal and circadian variations, but it also serves as one of the key modulators of the circadian apparatus.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Whole-cell NMDA-evoked current in suprachiasmatic neurones of the rat: modulation by extracellular calcium ions

The action of N-methyl-D-aspartic acid (NMDA) on suprachiasmatic neurones was studied using whole-cell recordings in coronal hypothalamic slices of the rat. The location of the recorded neurones within the suprachiasmatic nucleus was ascertained by intracellular labelling with biocytin, followed by histological processing of the slice. Suprachiasmatic neurones had an input resistance of 780 +/- 20 M omega (mean +/- S.E.M.; n = 106). They were voltage-clamped at or near their resting membrane potential and their responsiveness to NMDA was tested by adding this compound to the perfusion solution. NMDA generated an inward current in about 85% of the neurones. At 50 microM, the average induced peak current was 30 +/- 10 pA (n = 32); at 100 microM, it was 50 +/- 10 pA (n = 12). The NMDA-induced current was reduced by D-2-amino-5-phosphopentanoic acid (D-AP5), and NMDA receptor antagonist, and was suppressed by MK-801, and NMDA channel blocker. Reducing the extracellular magnesium concentration from 1 to 0.01 mM caused a 2- to 3-fold increase in the amplitude of this current. Thus, suprachiasmatic neurones are endowed with functional NMDA receptor-channels, which may play a role in glutaminergic transmission in this nucleus. Decreasing the extracellular calcium concentration from 2 to 0.01 mM caused a 1.3- to 4.5-fold enhancement in the whole-cell NMDA current. This effect was probably not mediated by a change in the intracellular free calcium concentration. Indeed, loading suprachiasmatic neurones with 11 or 20 mM of the calcium chelator, 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetracetic acid (BAPTA) suppressed a calcium-dependent slowly decaying outward aftercurrent but did not affect the low-calcium-induced facilitation of the NMDA response. NMDA current-voltage relations were established in normal and low-calcium perfusion solutions. In the normal solution, the net current generated by NMDA contained a region of negative slope conductance and reversed in polarity at 7 +/- 2 mV. In the low-calcium solution, this current increased in amplitude in the region of negative slope conductance, whereas at more depolarized potentials it was not altered. The NMDA-induced current was fitted using the Boltzmann equation. The effect of a low-calcium solution could be modelled by shifting the activation of the NMDA-sensitive conductance in the negative direction, by about 17 mV. We conjecture that lowering external calcium can unmask negative surface charges located on or near the NMDA channel and that this, in turn, weakens the voltage-dependent block of the channel by magnesium. A voltage-dependent blockade of the NMDA channel by calcium, however, may be also contribute to this effect. This low-calcium-induced facilitation of the NMDA response could play a regulatory role by enhancing calcium influx through the NMDA channel in case of calcium depletion in its vicinity.

Allosteric modulation of GABAA receptors in acutely dissociated neurons of the suprachiasmatic nucleus

The gamma-aminobutyric acid (GABA)-induced response was investigated in acutely dissociated suprachiasmatic nucleus (SCN) neurons of 11- to 14-day-old rats, under the voltage-clamp condition of nystatin-perforated patch recording. At a holding potential of -40 mV, application of GABA induced inward currents in a concentration-dependent manner. Pentobarbital and 5 beta-pregnan-3 alpha-ol-20-one (pregnanolone) similarly induced inward currents. GABA-induced inward currents were suppressed in a concentration-dependent manner by pretreating neurons with a GABAA receptor antagonist, bicuculline. Bicuculline (3 x 10(-6) M) shifted the concentration-response curve of GABA to the left in a competitive manner. Reversal potential of the GABA response (EGABA) was -3.4 +/- 0.7 mV, close to the theoretical Cl- equilibrium potential of -4.1 mV. Pretreating SCN neurons with diazepam, pentobarbital, and pregnanolone enhanced the 3 x 10(-6) M GABA response. Diazepam (3 x 10(-8) M), pentobarbital (3 x 10(-5) M), and pregnanolone (10(-7) M) shifted the concentration-response curve of GABA to the left without changing the maximal amplitude of GABA responses. EGABA in the presence of diazepam, pentobarbital, or pregnanolone was the same as that in their absence. These results show that the GABA response in acutely dissociated SCN neurons is mediated by the GABAA receptor. Because the GABAA receptor of SCN neurons is allosterically augmented by diazepam, pentobarbital, and pregnanolone, similarly as in other regions of the central nervous system, the present study opens up ways to functionally modulate the GABAA receptors in SCN.

Lanthanum and zinc sensitivity of GABAA-activated currents in adult medial septum/diagonal band neurons from ethanol dependent rats

The impact of chronic ethanol treatment, sufficient to induce tolerance and physical dependence, on GABAA receptor function was studied in acutely isolated neurons from the medial septum/nucleus diagonal band (MS/nDB) of adult rats using whole cell, patch-clamp recordings. In ethanol-naive Controls, GABA (0.3-300 microM) induced concentration-dependent increases in Cl- current with a threshold of 0.3-1 microM, a mean maximal current of 7645 +/- 2148 pA at 100-300 microM, an EC50 of 11.3 +/- 1.3 microM and a slope of 1.53 +/- 0.07. GABA-activated currents in neurons from animals receiving two weeks of ethanol liquid diet treatment did not differ significantly on any of these measures. The rate of GABAA receptor desensitization (t1/2 = 6.49 +/- 1.19 s) estimated as the time required for loss of 50% of peak current during sustained application of 10 microM GABA, as well as the residual steady state current remaining following complete desensitization for controls was unchanged by chronic ethanol. The impact of chronic ethanol treatment on the GABAA receptor modulation by lanthanum and zinc which act as positive and negative allosteric modulators, respectively, was also evaluated. Test pulses of 3 microM GABA in control neurons showed maximal potentiation by 141 +/- 30% at approximately 1000 microM lanthanum with an EC50 of 107 +/- 34 microM and a slope of approximately 1. Lanthanum potentiation remained the same following chronic ethanol treatment. Initial estimates based on fitted concentration response curves suggested that maximal inhibition of 3 microM GABA responses by zinc at the level of 70.2 +/- 8.5% in control cells was significantly increased by chronic ethanol treatment to 95.3 +/- 2.5%, although the IC50 of 60.2 +/- 25 microM was not changed. However, this difference was not supported by direct tests of maximal 3-10 mM zinc concentrations. These results suggest that chronic ethanol treatment, sufficient to induce tolerance and physical dependence, probably does not lead to readily detectible changes in GABAA receptor function in MS/nDB neurons.

Zinc modulation of GABAA receptor-mediated chloride flux in rat hippocampal slices

We studied the effect of ZnCl2 application on GABAA receptor-mediated 36CI- flux in microsacs prepared from whole rat hippocampus and in region-specific hippocampal slices. Slices were obtained from the dentate gyrus (DG), which contains the zinc-enriched hilar region, and from the CA1 region which contains lower levels of endogenous zinc. Muscimol (10 microM)-evoked 36Cl- flux was significantly reduced by ZnCl2 (100 microM) in hippocampal microsacs. In hippocampal slices, muscimol (50 microM)-evoked 36Cl- efflux was higher in CA1 (112.5 +/- 27.9% above basal efflux rate) than in DG slices (29.7 +/- 5.6%). In the presence of ZnCl2, the muscimol effect on efflux rate in CA1 and DG regions was decreased to 10.6 +/- 5.4% and 6.9 +/- 4.9%, respectively. Preincubation with the zinc chelator, tetrakis(2-pyridylmethyl)ethylenediamine (TPEN, 20 microM), caused a significant increase in muscimol-evoked 36Cl- efflux only in DG slices (57.2 +/- 7.0%), suggesting that GABAA receptors in the DG of rat hippocampus under physiological conditions may function under the inhibitory influence of endogenous chelatable zinc. In intracellular recordings, ZnCl2 (100 microM) application had no effect on the responses to GABA applied perisomatically or in the dendritic region of CA1 neurons. The lack of Zn2+ effect on the postsynaptic GABAA receptor-mediated responses suggests that the decreases of the 36Cl- efflux observed in the biochemical assays may be due to zinc action on neurons other than the principal pyramidal CA1 cells, and possibly the non-neuronal cell populations.

Stable high expression of human gamma-aminobutyric acidA receptors composed of alpha and beta subunits

Multiple classes of pharmacological agents including benzodiazepines, cage convulsants like t-butylbicyclophosphorothionate (TBPS), barbiturates and neuroactive steroids allosterically modulate the gamma-aminobutyric acidA receptor-chloride ionophore complex (GRC). The function of benzodiazepines requires a GRC comprised of alpha, beta and gamma subunits, while TBPS, barbiturates and neuroactive steroids will allosterically modulate GRCs comprised of only alpha and beta subunits. Binary alpha beta complexes are still hypothesized to be expressed in the mammalian brain particularly during development and could contribute to the pharmacological action of neuroactive steroids and barbiturates. In order to examine binary alpha beta complexes we report here the establishment of stable cell lines that express high levels of human GABAA receptors comprised of alpha 1 beta 1, alpha 2 beta 1 and alpha 3 beta 1 subunit combinations. The apparent potencies for allosteric modulation of [35S]TBPS for most naturally occurring neuroactive steroids for the binary subunit combinations was similar to that of the gamma-containing subunit combinations. Also discussed is the usefulness of these cell lines for the biophysical analysis of the GABAA receptor stoichiometry.

GABAA, GABAC, and NMDA receptor subunit expression in the suprachiasmatic nucleus and other brain regions

Identification of the neurotransmitter receptor subtypes within the suprachiasmatic nuclei (SCN) will further understanding of the mechanism of the biological clock and may provide targets to manipulate circadian rhythms pharmacologically. We have focused on the ionotropic GABA and glutamate receptors because these appear to account for the majority of synaptic communication in the SCN. Of the 15 genes known to code for GABA receptor subunits in mammals we have examined the expression of 12 in the SCN, neglecting only the alpha 6, gamma 3, and rho 2 subunits. Among glutamate receptors, we have focused on the five known genes coding for the NMDA receptor subunits, and two subunits which help comprise the kainate-selective receptors. Expression was characterized by Northern analysis with RNA purified from a large number of mouse SCN and compared to expression in the remaining hypothalamus, cortex and cerebellum. This approach provided a uniform source of RNA to generate many replicate blots, each of which was probed repeatedly. The most abundant GABA receptor subunit mRNAs in the SCN were alpha 2, alpha 5, beta 1, beta 3, gamma 1 and gamma 2. The rho 1 (rho 1) subunit, which produces GABAC pharmacology, was expressed primarily in the retina in three different species and was not detectable in the mouse SCN despite a common embryological origin with the retina. For several GABA subunits we detected additional mRNA species not previously described. High expression of both genes coding for glutamic acid decarboxylase (GAD65 and GAD67) was also found in the SCN. Among the NMDA receptor subunits, NR1 was most highly expressed in the SCN followed in order of abundance by NR2B, NR2A, NR2C and NR2D. In addition, both GluR5 and GluR6 show clear expression in the SCN, with GluR5 being the most SCN specific. This approach provides a simple measure of receptor subtype expression, complements in situ hybridization studies, and may suggest novel isoforms of known subunits.

Presynaptic inhibition by baclofen of retinohypothalamic excitatory synaptic transmission in rat suprachiasmatic nucleus

Optic nerve stimulation evoked monosynaptic excitatory postsynaptic currents in suprachiasmatic nucleus neurons maintained in vitro. These currents were completely blocked by a combination of glutamate receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione and 4-aminophosphonovaleric acid. Stimulation of the ipsilateral or contralateral suprachiasmatic nucleus produced a biphasic response consisting of an excitatory postsynaptic current followed by an bicuculline-sensitive inhibitory postsynaptic current. Most suprachiasmatic nucleus neurons had spontaneous inhibitory and excitatory synaptic currents produced by action potential-independent and, less frequently, action potential-dependent release of GABA and glutamate. Baclofen reversibly reduced the amplitude of excitatory postsynaptic currents evoked by optic nerve stimulation and the effect was antagonized by 2-hydroxysaclofen. In addition, baclofen reduced the frequency but not the amplitude of the spontaneous miniature excitatory postsynaptic currents. In a subset of suprachiasmatic nucleus neurons, baclofen induced an outward current, probably by increasing a potassium conductance. Baclofen had no effect on either evoked or spontaneous inhibitory postsynaptic currents or on currents activated by pulse application of glutamate. These data indicate that activation of GABAB receptors can inhibit suprachiasmatic nucleus neurons by two mechanisms. The first is to inhibit the release of glutamate from terminals of the retinohypothalamic tract. The second is the postsynaptic activation of a potassium conductance in a portion of these neurons.

Primary culture of postnatal rat suprachiasmatic neurons in serum-free supplemented medium

We have previously reported that postnatal hypothalamic neurons can be maintained in low density culture using astrocyte conditioned medium. The present study was designed to establish a method for the culture of postnatal hypothalamic neurons in a chemically defined medium. Neurons were dissociated from the suprachiasmatic nucleus (SCN) of the hypothalamus of 21-day-old rats and plated on plastic dishes. First, the effects of several factors which have been known to exert trophic effects on neuronal cells were examined in culture medium containing 10% fetal bovine serum. We have found that platelet-derived growth factor, interleukin-1 beta and vitronectin in combination markedly increased the number of surviving neurons bearing processes. Next we tested such effects in serum-free minimum essential medium. When these factors were added together the SCN neurons could be maintained in culture for up to 3 weeks without medium change. In this supplemented medium, SCN neurons gradually extended processes from 3-5 days after plating, and the cell number with processes reached maximal at days 8-11. The cells were identified as SCN neurons by the immunocytochemical staining for microtubule-associated protein 2 (MAP2) and vasoactive intestinal polypeptide. This culture method may be valuable for investigating the electrophysiological properties and the mechanisms of regeneration of mature central neurons.

Inhibition by 5-HT7 receptor stimulation of GABAA receptor-activated current in cultured rat suprachiasmatic neurones

1. Whole-cell voltage-clamp recordings were made from postnatal rat suprachiasmatic (SCN) neurones to investigate possible modulation by 5-hydroxytryptamine (5-HT) of gamma-aminobutyric acid (GABA)-activated current (IGABA). 2. 5-HT reversibly inhibited IGABA in a concentration-dependent manner (10(-10) to 10(-6) M). (+/-)-8-Hydroxy-2-N,N-dipropylaminotetralin (8-OH-DPAT, 10(-10) to 10(-5) M) and 5-carboxamidotryptamine (10(-6) M) also inhibited IGABA, whereas 1-(2,5-dimethyl-4-iodophenyl)-2-aminopropane (DOI, 10(-6) M) had no significant effect. 3. The effect of 8-OH-DPAT (10(-7) M) was blocked by ritanserin (10(-7) M), but not by pindolol (10(-7) M). The effect of 5-HT was also suppressed by ritanserin, but not by pindolol, ketanserin (10(-7) M) or ICS 205-930 (10(-6) M). 4. 8-Bromo-cAMP (10(-3) M) or forskolin (5 x 10(-5) M) suppressed IGABA. The effects of forskolin and 5-HT were not additive. Furthermore, the effect of 5-HT (10(-7) M) was significantly reduced by N-[2-(methylamino)ethyl]-5-isoquinoline sulphonamide (H-8, 10(-6) M). 5. It is concluded that 5-HT inhibits IGABA in the SCN neurones, which involves the activation of 5-HT7 receptors and cAMP-coupled systems.