GABA: circadian fluctuations in rat hypothalamus

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 (R_Cl) demonstrated circadian rhythmicity in AVP + neurons and VIP + neurons, but was not detected in GFAP + astrocytes. R_Cl peaked between ZT 7 and ZT 8 in both AVP + and VIP + neurons. R_Cl 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.

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 GABA_A receptor (GABA_AR)-mediated currents. GABA transport inhibitors were applied to study how GABA_AR-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 GABA_AR-mediated tonic current that was blocked by gabazine. GAT inhibitors decreased the amplitude and decay time constant and increased the rise time of spontaneous GABA_AR-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 GABA_AR-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 GABA_AR-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 GABA_A receptor-mediated currents and the circadian clock.

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.

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.

Diurnal stability of gamma-aminobutyric acid concentration in visual and sensorimotor cortex

PURPOSE

To establish the diurnal stability of edited magnetic resonance spectroscopy measurements of gamma-aminobutyric acid (GABA) in visual and sensorimotor regions of the brain.

MATERIALS AND METHODS

GABA measurements were made in two regions of the brain (an occipital, "visual" region and a "sensorimotor" region centered on the precentral gyrus) using the MEGA-PRESS editing method, scanning eight healthy adults at five timepoints during a single day. GABA concentration was quantified from the ratio of the GABA integral to the unsuppressed water signal.

RESULTS

No significant effect of time on GABA concentration was seen (P = 0.35). GABA was shown to be significantly more concentrated in visual regions than in sensorimotor regions (1.10 i.u. and 1.03 i.u., respectively; P = 0.050). Coefficients of variability (CVs) across all subjects of 9.1% and 12% (visual and sensorimotor) were significantly higher than mean within-subjects CVs of 6.5% and 8.8.

CONCLUSION

This study demonstrates the excellent reproducibility of MEGA-PRESS detection of GABA, demonstrating that the method is sufficiently sensitive to detect inter-subject variability, and suggests that (within the sensitivity limits of current measurements) time of day can be ignored in the design of MRS studies of visual and sensorimotor regions.

An increase in in vivo release of LHRH and precocious puberty by posterior hypothalamic lesions in female rhesus monkeys (Macaca mulatta)

We have previously shown that a decrease in gamma-aminobutyric acid (GABA) tone and a subsequent increase in glutamatergic tone occur in association with the pubertal increase in luteinizing hormone releasing hormone (LHRH) release in primates. To further determine the causal relationship between developmental changes in GABA and glutamate levels and the pubertal increase in LHRH release, we examined monkeys with precocious puberty induced by lesions in the posterior hypothalamus (PH). Six prepubertal female rhesus monkeys (17.4 +/- 0.1 mo of age) received lesions in the PH, three prepubertal females (17.5 +/- 0.1 mo) received sham lesions, and two females received no treatments. LHRH, GABA, and glutamate levels in the stalk-median eminence before and after lesions were assessed over two 6-h periods (0600-1200 and 1800-2400) using push-pull perfusion. Monkeys with PH lesions exhibited external signs of precocious puberty, including significantly earlier menarche in PH lesion animals (18.8 +/- 0.2 mo) than in sham/controls (25.5 +/- 0.9 mo, P<0.001). Moreover, PH lesion animals had elevated LHRH levels and higher evening glutamate levels after lesions, whereas LHRH changes did not occur in sham/controls until later. Changes in GABA release were not discernible, since evening GABA levels already deceased at 18-20 mo of age in both groups and morning levels remained at the prepubertal levels. The age of first ovulation in both groups did not differ. Collectively, PH lesions may not be a good tool to investigate the mechanism of puberty, and, taking into account the recent findings on the role of kisspeptins, the mechanism of the puberty onset in primates is more complex than we initially anticipated.

Daily changes of GABA and taurine concentrations in various hypothalamic areas are affected by chronic hyperprolactinemia

This study was designed to characterize, in anterior, mediobasal, and posterior hypothalamic and median eminence, the 24h changes of gamma aminobutyric acid (GABA) and taurine (TAU) contents in adult male rats and to analyze whether chronic hyperprolactinemia may affect these patterns. Rats were turned hyperprolactinemic by a pituitary graft. Plasma prolactin (PRL) levels increased after pituitary grafting at all time points examined. A disruption of the circadian rhythm was observed in pituitary-grafted rats, whereas GABA and TAU content followed daily rhythms in all areas studied in controls. In the mediobasal hypothalamus, two peaks for each amino acid were found at midnight and midday. In the anterior hypothalamus, GABA and TAU showed only one peak of concentration at midnight. In the posterior hypothalamus, the values of both GABA and TAU were higher during the light as compared to the dark phase of the photoperiod. In the median eminence GABA content peaked at 20:00h, the time when TAU exhibited the lowest values. Hyperprolactinemia abolished the 24h changes of GABA in the mediobasal hypothalamus and reduced its content as compared to controls. Hyperprolactinemia advanced the diurnal peak of TAU to 12:00h in the mediobasal hypothalamus and did not modify the 24:00h peak. In the anterior hypothalamus, hyperprolactinemia increased GABA and TAU contents during the light phase while it decreased them during the dark phase of the photoperiod. In the posterior hypothalamus hyperprolactinemia did not modify GABA or TAU patterns as compared to controls. In the median eminence hyperprolactinemia increased the 20:00h peak of GABA and shift advanced the decrease in TAU content at 20:00h and its maximum at 24:00h as compared to controls. These data show that GABA and TAU content exhibit specific daily patterns in each hypothalamic region studied. PRL differentially affects the daily pattern of these amino acids in each hypothalamic region analyzed.

Circadian activity of the GABAergic system in the golden hamster retina

Daily changes in gamma-aminobutyric acid (GABA) turnover rate were studied in the golden hamster retina. This parameter showed significant variations throughout the light-dark cycle, with minimal values during the day. Retinal glutamic acid decarboxylase (GAD) activity was higher at midnight than at noon. Moreover, [3H]GABA binding significantly varied throughout the 24-h cycle, with maximal values during the day. Saturation studies performed at 12:00 and 24:00 h indicated that the maximal concentration of [3H]GABA binding sites (Bmax) was significantly higher at noon, whereas the dissociation constant (Kd) remained unchanged. High K+-induced GABA release was significantly higher at midnight than at midday. Daily variations in retinal GABA turnover rate, GABA release, and in its specific binding persisted in golden hamsters exposed to constant darkness. In summary, these results support the idea of a circadian clock-controlled GABAergic activity in the hamster retina.

GABA release from suprachiasmatic nucleus terminals is necessary for the light-induced inhibition of nocturnal melatonin release in the rat

The daily rhythm of melatonin production in the mammalian pineal is driven by the endogenous circadian pacemaker in the suprachiasmatic nuclei. The major release period of melatonin is closely linked to the dark phase of the 24-h day/night cycle. Environmental light will affect melatonin release in two ways: (i) it entrains the rhythm of the circadian oscillator; and (ii) it causes an acute suppression of nocturnal melatonin release. These two effects of light are both mediated by the suprachiasmatic nucleus and enable the pineal gland to convey information about day length to the reproductive system through changes in melatonin levels. Glutamate is currently believed to be the major transmitter in the retinal ganglion cell fibers reaching the suprachiasmatic nucleus. At present no information is available, however, about the transmitter(s) implicated in the further propagation, i.e. from the suprachiasmatic nucleus onwards, of the light information. In the present study we provide evidence that the endogenous release of GABA from suprachiasmatic nucleus terminals is implicated in the further transmission of light information to the pineal gland. Bilateral administration of the GABA-antagonist bicuculline to hypothalamic target areas of the suprachiasmatic nucleus completely prevents the inhibitory effect of nocturnal light on melatonin secretion and the present study thus documents that retina-mediated photic activation of suprachiasmatic nucleus neurons induces the release of GABA from efferent suprachiasmatic nucleus nerve terminals, resulting in an inhibition of melatonin release by the pineal gland. Together with our previous (electro)physiological data these results identify GABA as an important mediator of rapid synaptic transmission of suprachiasmatic nucleus output to its target areas.

Daily variations of amino acid concentration in mediobasal hypothalamus, in rats injected with Freund's adjuvant. Effect of cyclosporine

Although the existence of central responses to inflammatory injuries was already reported, the existence of hypothalamic amino acid responses has been less explored. The present study was designed to characterize the 24-h changes in mediobasal hypothalamic excitatory and inhibitory amino acid neurotransmitter contents and to analyze the effect of Freund's complete adjuvant administration on these patterns. Also the effects of the immunosuppressant drug Cyclosporine was studied. The content of aspartate, glutamate, glutamine, GABA and taurine was measured by HPLC with fluorimetric detection. The results show the existence of specific daily rhythms of aspartate, glutamate, glutamine, GABA and taurine contents in the mediobasal hypothalamus of control rats. Maxima for these amino acids was found at midnight, although another peak of lesser magnitude, occurred during the light phase of the photoperiod, except for TAU in which both peaks were of similar magnitude. Freund's complete adjuvant administration did not modify the 24-h pattern of any amino acid studied. It reduced the midnight peak of glutamate, glutamine and GABA and increased that of taurine. Moreover, it increased and extended the midday peak of glutamate. Besides, Freund's adjuvant did not modify aspartate content at any time point studied. Cyclosporine pretreatment did not prevent the inhibitory effects of Freund's complete adjuvant on glutamate, glutamine and GABA midnight peaks. However, the drug blocked the increase in the content of taurine at midnight and increased its midday peak. Moreover, cyclosporine administration abolished the variations of ASP during the scotophase, as compared to control animals and shift delayed both peaks of glutamate. The results indicate the existence of a significant effect of immune-mediated inflammatory response of the mediobasal hypothalamic amino acids studied, at an early phase after Freund's adjuvant administration, and that these changes were partially sensitive to the immunosuppression induced by cyclosporine.

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.

GABAergic control of Arg-vasopressin release from suprachiasmatic nucleus slice culture

gamma-Aminobutyric acid (GABA) is contained in many neurons in the suprachiasmatic nucleus (SCN), and is considered to be a circadian entraining factor. Arg-vasopressin (AVP)-containing neurons represent one of the output paths from the SCN to other brain areas. We examined the effects of GABA, muscimol (GABA-A agonist), bicuculline (GABA-A antagonist), baclofen (GABA-B agonist) and phaclofen (GABA-B antagonist) on AVP release using SCN slice preparations in culture. SCN slices were prepared from coronally sliced brain tissue and cultured in organic tissue culture dishes with DMEM/N2 medium in a CO2 (5%) incubator. The culture medium was changed at 3-h intervals until 9 h after 3 h application of each drug. Concentrations of AVP in 1 ml aspirates of the medium were analyzed by EIA. Muscimol (1, 10 microM) increased and bicuculline (1, 10, 100 microM) decreased the AVP release 3-6 h after application. However, baclofen and phaclofen had no apparent effects on AVP release. Riluzole (0.1 mM) and nipecotic acid (1 mM), GABA uptake inhibitors, increased AVP release 3-6 h after application. These results indicate that GABA promotes AVP release mediated by GABA-A receptors in the SCN.

Glutamic acid decarboxylase in rat olfactory bulb: effect of ovarian steroids or male pheromones

The effect of ovarian steroids and pheromones on the activity of glutamic acid decarboxylase, the enzyme that synthesizes gamma-aminobutyric acid (GABA), was studied in the rat olfactory bulbs. The enzyme activity was measured in the main and accessory olfactory bulbs at 11:00 h and 17:00 h in ovariectomized rats, and in rats treated with ovarian steroids or exposed to male pheromones. The enzyme activity in both bulbs showed a diurnal fluctuation that was not affected in the accessory bulbs by the exposure to pheromones while the rhythm disappeared in the main bulbs. Estrogen and estrogen-progesterone treatments decreased the enzyme activity in both bulbs either in the morning or in the afternoon. The exposure of ovariectomized estrogen-primed rats to male pheromones reversed the effect of estrogen on the enzyme activity in the morning but not in the afternoon. Ovarian hormones plus pheromones prevented the steroid effect only in the morning. These results support the view that in olfactory bulbs, the GABAergic system can be modulated by endocrine and pheromonal factors.

GABAergic neuronal activity and mRNA levels for both forms of glutamic acid decarboxylase (GAD65 and GAD67) are reduced in the diagonal band of Broca during the afternoon of proestrus

There is considerable evidence that GABAergic neurons play an important role in the regulation of gonadotropin-releasing hormone (GnRH) secretion, and that these neurons may mediate the feedback actions of gonadal steroids on GnRH neurons. The aim of the present study was to investigate whether endogenous changes in ovarian steroid secretion during the estrous cycle influenced GABAergic neuronal activity in the preoptic region of the hypothalamus, and in other steroid-sensitive brain regions. Intact, adult female rats were sacrificed at various times during the days of metestrus or proestrus. GABAergic neuronal activity was estimated by measuring the rate of accumulation of GABA in microdissected brain regions after pharmacological inhibition of GABA degradation. Concentrations of mRNA for both forms of glutamic acid decarboxylase (GAD65 and GAD67) were quantified in microdissected brain regions by a microlysate ribonuclease protection assay. In the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis (DBB(ovlt)), GABAergic neuronal activity was significantly reduced during the afternoon of proestrus compared with the morning of either proestrus or metestrus. In the lateral septal nucleus, GABAergic neuronal activity was significantly increased in the afternoon of proestrus compared with the morning. There were no significant effects of time of day or day of estrous cycle in the medial preoptic nucleus, median eminence, ventromedial nucleus, suprachiasmatic nucleus, medial septal nucleus, hippocampus (CA1 region), or cingulate cortex. In the DBB(ovlt), mRNA levels for both GAD65 and GAD67 were significantly reduced in the afternoon of proestrus compared with the afternoon of metestrus. By contrast, there was no change in GAD65 and GAD67 mRNA levels in the cingulate cortex at any of the times examined. These results demonstrate that GABAergic neuronal activity, and mRNA levels for both GAD65 and GAD67, are reduced in the DBB(ovlt) during the afternoon of proestrus. These results support the hypothesis that decreased GABAergic neuronal activity in this region plays a major permissive role in the generation and maintenance of the estrogen-induced LH surge.

Chronobiotics--drugs that shift rhythms

A chronobiotic is defined and levels of action within the mammalian circadian pacemaker system, such as the retina, retinohypothalamic tract, geniculohypothalamic tract, suprachiasmatic nuclei, output and feedback systems are identified. Classes of drug that include the indoleamines, cholinergic agents, peptides, and benzodiazepines, which might act as chronobiotics within these levels, are evaluated. Particular emphasis is placed on the indole, melatonin (MLT). The clinical circumstances for use of chronobiotics in sleep disturbances of the circadian kind, such as jet lag, shift work, delayed sleep-phase syndrome, advanced sleep-phase syndrome, irregular and non-24-hr sleep-wake cycles, are described under reorganized headings of disorders of entrainment, partial entrainment, and desynchronization. Specific attention is given to the blind and the aged. Both human and animal studies suggest that MLT has powerful chronobiotic properties. MLT shows considerable promise as a prophylactic and therapeutic alternative or supplement to the use of natural and artificial bright light for resetting the circadian pacemaker. Throughout this discussion, the hypnotic and hypothermic versus the chronobiotic actions of MLT are raised. Finally, problems in the design of delivery systems for MLT are discussed.

The lateral hypothalamic area revisited: ingestive behavior

This article discusses the role of the lateral hypothalamic area (LHA) in feeding and drinking and draws on data obtained from lesion and stimulation studies and neurochemical and electrophysiological manipulations of the area. The LHA is involved in catecholaminergic and serotonergic feeding systems and plays a role in circadian feeding, sex differences in feeding and spontaneous activity. This article discusses the LHA regarding dietary self-selection, responses to high-protein diets, amino acid imbalances, liquid and cafeteria diets, placentophagia, "stress eating," finickiness, diet texture, consistency and taste, aversion learning, olfaction and the effects of post-operative period manipulations by hormonal and other means. Glucose-sensitive neurons have been identified in the LHA and their manipulation by insulin and 2-deoxy-D-glucose is discussed. The effects on feeding of numerous transmitters, hormones and appetite depressants are described, as is the role of the LHA in salivation, lacrimation, gastric motility and secretion, and sensorimotor deficits. The LHA is also illuminated as regards temperature and feeding, circumventricular organs and thirst and electrolyte dynamics. A discussion of its role in the ischymetric hypothesis as an integrative Gestalt concept concludes the review.

Daily variations in GABA receptor function in Syrian hamster cerebral cortex

The existence of diurnal changes in postsynaptic expression of gamma-aminobutyric acid (GABA) type A receptors was assessed in cerebral cortex of Syrian hamsters by measuring [3H]GABA binding and the influx of 36Cl- in synaptoneurosomes. A diurnal variation in dissociation constant of [3H]GABA binding to cerebral cortex membranes, and the absence of diurnal differences in maximal number of sites, were found. When the nycthemeral changes in muscimol-stimulated 36Cl- uptake by cortical synaptoneurosomes were assessed, a maximum occurred late at night (i.e. 0400 h). At 1600 h, micromolar concentrations of flunitrazepam potentiated significantly the influx of chloride induced by muscimol, while at 0400 h flunitrazepam did not exert any significant effect on 36Cl- uptake. The results indicate that postsynaptic type A GABAergic activity peaked at nocturnal hours in the cerebral cortex of Syrian hamsters.

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.

Circadian rhythmicity in the GABAergic system in the suprachiasmatic nuclei of the rat

The participation of GABAergic mechanisms in the regulation of circadian rhythmicity by the suprachiasmatic nuclei (SCN) has been suggested from different lines of evidence. Little is known, however, whether GABA synthesis, release, uptake or content within the SCN may show a circadian pattern. The present results show that the activity of the GABAergic system within the SCN region of the rat exhibits circadian rhythmicity, which is manifested by correlative changes of the GABA content and the glutamic acid decarboxylase activity under the light/dark cycle, and by changes in the GABA content in animals kept under constant darkness.

Diurnal changes of GABA turnover rate in brain and pineal gland of Syrian hamsters

Circadian rhythms of GABA turnover rate in cerebral cortex, preoptic area-medial basal hypothalamus (PMBH), cerebellum, and pineal gland were examined in Syrian hamsters kept for 3 months under either long (14 h of light/day) or short days (10 h of light/day). In vivo GABA turnover rate was measured by the increase of GABA levels following inhibition of GABA-transaminase by gamma-acetylenic GABA. Under long photoperiods, a significant rhythm of GABA turnover was detected in the four areas studied (cerebral cortex, PMBH, cerebellum, and pineal gland), with maxima at night. A Cosinor analysis indicated acrophases which varied from 2300 to 0400 h (3rd to 8th h of darkness). Under short photoperiods, there were no significant circadian variations in GABA turnover in the cerebral cortex, and the synchronization in turnover rate among the remaining regions was lost, with acrophases being detectable either during the light phase of daily photoperiod (PMBH) or at night (cerebellum, pineal gland). Steady state levels of GABA also changed periodically in the same brain regions under both lighting environments, although phase relationships of circadian rhythms in GABA content and turnover rate varied significantly among tissues, as well as on photoperiodic conditions.

Chronopharmacology of psychotropic drugs: circadian rhythms in drug effects and its implications to rhythms in the brain

The effects of many kinds of psychotropic drugs have been shown in animal studies to follow a circadian rhythm. Trials for the clinical application of this circadian rhythm have already been undertaken. Although the mechanisms underlying this phenomenon are still unclear, chronological changes in the levels of drugs in the blood and brain suggest that it is primarily due to rhythms in the brain's susceptibility to drugs. Rhythms are present in the level of intracerebral neurotransmitters, receptors and second messengers. Each of these rhythms may cause other rhythms within each system of neurotransmitters, which in turn induces a rhythm in the susceptibility to drugs.

Glutamic acid decarboxylase activity of the preoptic area and hypothalamus is influenced by the serotonergic system

The effect of the serotonergic system on glutamic acid decarboxylase (GAD) activity of the preoptic area and the hypothalamus was studied in female rats on the day of proestrus. A circadian rhythm of GAD activity was observed with higher values in rats killed at 1130 h than in rats killed at 1500 h. In rats bearing lesions of the median raphe nucleus (MRn), a nucleus that sends 5-hydroxytryptamine nerve terminals to the areas under study decreased GAD activity. On the contrary, electrochemical stimulation of the MRn enhanced GAD activity in intact rats killed at 1500 h, but not in those killed at 1130 h, an effect that was prevented by the injection of the 5-hydroxytryptamine antagonist, methysergide. Furthermore, the injection of 5-hydroxytryptamine into the third ventricle, either in intact rats in the afternoon or in MRn-lesioned rats in the morning, also increased GAD activity. The results of the present study suggest that activation of the serotonergic system increases GAD activity in the preoptic area and hypothalamus.

Alterations in synaptosomal neurotransmitter amino acids in "petit-mal" rats at a daytime and a nighttime

The synaptosomal fractions of 6 brain areas-olfactory tubercles (OT), frontal cortex (FC), striatum (Sr), amygdala (A), thalamus (Th), hypothalamus (Hy) - have been analyzed for their neurotransmitter amino acids (AA) content in Wistar rats exhibiting "petit-mal" epilepsy (PM-E) and in controls (C). The analysis was carried out at 11 p.m. (nighttime corresponding to the acrophase for the hourly number of spike-wave complexes) and at 11 a.m. (daytime). A day versus night rhythmicity is recorded for synaptosomal inhibitory AA in control and in PM-E rats. However, day versus night variations are more frequent and more prominent in C rats than in PM-E rats. Two day versus night variations exist only in PM-E rats: increases of GABA level in Sr and of Asp in Hy. Differences between PME-and C in synaptosomal AA content are more likely to be present during the nighttime. During this period lower AA values for PM-E rats are found for one or several inhibitory AA in OT, Th, and FC. It seems that the differences between PM-E and C concerning the inhibitory AA correlate with the number of spike-wave discharges. Only in one brain area is there a similar difference for PM-E and C during daytime and nighttime: a decreased GABA content for PM-E rats in OT. The decrease is larger in nighttime than in daytime. This difference may serve as a marker for this epileptic disorder. Moreover, it is in OT that the greatest number of PM-E versus C differences in synaptosomal neurotransmitter AA are observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain 3-hydroxybutyrate, glutamate, and GABA in a rat model of dietary obesity

Whole brain concentrations of 3-hydroxybutyrate, glutamate and gamma-aminobutyric acid (GABA) have been measured in two strains of rats with differing susceptibility to obesity. S 5B/Pl rats are resistant to developing obesity when eating a high-fat diet, whereas Osborne-Mendel rats readily develop obesity when eating the same diet. We tested the hypotheses that brain 3-hydroxybutyrate, glutamate and GABA differ between S 5B/Pl rats and Osborne-Mendel rats, and that these substrates/neuroregulators are altered when eating a high-fat diet primarily in S 5B/Pl (resistant) rats. Blood and brain 3-hydroxybutyrate concentrations were higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.05) but diet effects were not significant. Brain glutamate concentration, like 3-hydroxybutyrate, was higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.01) and was not affected by adding fat to the diet. Brain GABA differed only slightly between strains but increased after adding fat to the diet (p less than 0.05) in both strains with a greater increase occurring in S 5B/Pl rats. The brains of S 5B/Pl rats are chronically exposed to higher levels of 3-hydroxybutyrate and glutamate than are those of Osborne-Mendel rats. Thus, 3-hydroxybutyrate is a potential signal in the regulation of body weight. Brain GABA increases with fat feeding, especially in S 5B/Pl rats, suggesting that the ability to adjust to an energy dense diet may be through suppression of food intake by elevated brain GABA.

Interaction between GABAergic neurotransmission and rat hypothalamic corticotropin-releasing hormone secretion in vitro

Corticotropin-releasing hormone (CRH) has been considered a major coordinator of the overall physical and behavioral response to stress. Moreover, prolonged hypersecretion of CRH has been implicated in the pathogenesis of disorders characterized by anxiety and/or depression. Drugs acting through the gamma-aminobutyric acid/benzodiazepine (GABA/BZD) receptor system have anxiolytic and/or antidepressant properties whereas benzodiazepine inverse agonists cause anxiety and stimulate the pituitary-adrenal axis in vivo. To examine the involvement of the GABA/BZD system in the regulation of hypothalamic CRH secretion, we studied the effects of various agonists and antagonists of GABAA and GABAB receptors using a sensitive rat hypothalamic organ culture with radioimmunoassayable CRH (IR-rCRH) as endpoint. The GABAA and GABAB receptor agonist GABA inhibited serotonin (5-HT)-induced IR-rCRH secretion from 10(-9) to 10(-6) M, but failed to do so at 10(-5) M. The GABAA receptor agonist muscimol was a weak inhibitor of 5-HT-induced IR-rCRH secretion, being effective only at the concentration of 10(-6) M. In contrast, the specific GABAB receptor agonist baclofen was able to inhibit 5-HT-induced IR-rCRH secretion from 10(-7) to 10(-5) M. The rank of potency was thus, GABA much greater than baclofen greater than muscimol. Bicuculline, a GABAA receptor antagonist, partially reversed the inhibitory effects of GABA. Diazepam, a classic benzodiazepine which interacts with the benzodiazepine-site of the GABAA receptor complex, inhibited 5-HT-induced IR-rCRH secretion from 3.3 X 10(-9) to 10(-5) M, an effect that could be reversed by the BZD inactive ligand Ro15-1788.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian rhythm of histamine metabolism in the rabbit central nervous system (CNS): analysis of brain and ocular structures

The circadian rhythm of the level of histamine (HI) and histidine decarboxylase (HD) and histamine-methyltransferase (HMT) activity in 6 brain and 5 ocular structures of the rabbit was studied. Clear circadian variations of the histaminergic parameters in two brain (hypothalamus and lateral geniculate body) and two ocular (retina and iris-ciliary body) tissues were found. It is suggested that HI in some brain and ocular structures may be functionally involved in activity that depends on circadian rhythm.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

Effect of GABAergic treatment on resting and stress-induced adrenocortical activities in the thalamic pigeon

Muscimol was injected (0.5 mg kg-1) intravenously to either intact or thalamic-lesioned animals through a chronic catheter. Plasma samples were obtained at 7-min intervals and exhibited a moderate and transient increase in corticosterone levels in both groups, together with some symptoms of light discomfort (e.g., ptiloerection or panting). All these signs disappeared within 45 to 60 min. Intraventricular injection of bicuculline (3.5 micrograms) provoked a corticosterone profile quite similar to a stress-induced polyphasic pattern. Acute stress (electrical footshocks for 30 sec) was applied 2 hr after drug treatment. In controls, the stress-induced adrenocortical response appeared to be markedly modified after muscimol injection. The usual polyphasic rebounding profile was replaced by a monophasic one. This effect of muscimol did not occur when bicuculline had been injected in the 3rd ventricle. Bicuculline alone did not alter the stress-induced profile. GABAergic treatment had no effect on the monophasic response of thalamic-lesioned birds.

Diurnal rhythm of alpha 2-noradrenergic receptors in the paraventricular nucleus and other brain areas: relation to circulating corticosterone and feeding behavior

The paraventricular nucleus alpha 2-noradrenergic system and the glucocorticoid hormone, corticosterone, are known to modulate feeding behavior and exhibit a circadian pattern which may be related to the natural periodicity of feeding in the rat. The results of the present study indicate that the binding of [3H]p-aminoclonidine to alpha 2-noradrenergic receptors specifically in the paraventricular nucleus varies concomitantly with plasma corticosterone levels, as well as spontaneous feeding. A monophasic peak of paraventricular noradrenergic receptor binding is detected at the onset of the dark period, when corticosterone levels are highest and feeding is initiated. On the other hand, the supraoptic nucleus exhibits the reverse diurnal pattern, i.e., a significant decline of [3H]p-aminoclonidine binding at the onset of the dark period. Other hypothalamic and extra-hypothalamic areas fail to show significant changes in alpha 2-noradrenergic receptors as a function of the diurnal cycle. This study supports other evidence indicating a close interaction between circulating corticosterone and alpha 2-noradrenergic receptors in specific hypothalamic areas. It also reveals a potential importance for this interaction in control of the natural feeding rhythm.

Acute and delayed effects of picrotoxin on the adrenocorticotropic system of rats

To investigate the possible effect of GABA on the corticotropic system, the potent GABA antagonist picrotoxin was injected into two groups of 14 female rats at 07.00 h and 19.00 h, respectively. A single subconvulsive I.p. injection dramatically raised plasma ACTH and corticosterone, and thereafter suppressed the circadian rhythm of ACTH, but not of corticosterone, for 24 h in the group injected at 07.00 h and for 48 h in the one injected at 19.00 h and increased mean hormonal levels. Results are discussed in the light of the possibility that inhibition by the GABAergic system and stimulation by the serotoninergic system might be components of the mechanism controlling the circadian rhythm of ACTH.

Inhibition of acute feeding responses to systemic 2-deoxyglucose or insulin in rats pretreated with the GABA-transaminase blocker ethanolamine-O-sulfate (EOS)

Acute feeding responses to 2-deoxyglucose (750 mg/kg) or insulin (12 U/kg) were examined 24 hr after intracisternal injection of the GABA-transaminase inhibitor ethanolamine-O-sulfate (EOS, 400 micrograms) in female rats. EOS pretreatment completely abolished acute feeding responses to both challenges. These findings complement recent research showing that central EOS can reverse chronic overeating in several experimental preparations. The present results are consistent with previous indications that EOS treatment may induce a metabolic shift away from brain glucose utilization, thus making glucoprivation irrelevant as a metabolic challenge. An alternative possibility is that EOS-induced increases of brain GABA may offset specific neural mechanisms through which these glucoprivic agents normally induce feeding.

Effects of gamma-vinyl GABA on food intake of rats

Gamma-vinyl GABA, an inhibitor of GABA-transaminase, produced a dose-related reduction of food intake in rats, after both single, (125-1000 mg/kg IP or 500 mg/kg PO) and repeated (250 mg/kg/day IP) administration. No tolerance was observed to the effect of repeated injections. Catecholamine and indoleamine systems in the CNS do not appear to be implicated in this anorexic effect. Combination of gamma-vinyl GABA with amphetamine (2.5 mg/kg) enhanced the anorexic effects of the latter compound whilst attenuating its stimulant effects. The data suggest an important role for GABA in the control of food intake.

A novel inhibitor of gamma-aminobutyrate aminotransferase with anorectic activity

1-(n-decyl)-3-Pyrazolidinone (BW357U) is a potent, selective inhibitor of gamma-aminobutyrate aminotransferase (GABA-T) in vitro and in vivo. After acute or chronic, oral or intraperitoneal administration of BW357U to rats, brain GABA levels were elevated in a dose-dependent manner. When inhibition of brain GABA-T exceeded 50%, whole brain GABA levels were elevated approximately threefold, and an anorectic effect was observed in the absence of other symptoms. This compound, because of its potency and selectivity, may be useful in studies relating to the function of GABA-containing neurons in appetite regulation.

Metabolism and brain uptake of gamma-aminobutyric acid in galactosamine-induced hepatic encephalopathy in rats

Kinetic studies of [3H]gamma-aminobutyric acid ([3H]GABA) after an intravenous injection were performed in normal rats and in rats with severe degree of hepatic encephalopathy due to fulminant hepatic failure induced by galactosamine. Moreover, plasma and brain GABA levels, and GABA and glutamic acid decarboxylase activity were studied in some brain areas. After intravenous injection, [3H]GABA disappeared very rapidly in the blood of normal rats, with a prompt increase of 3H metabolites. In comatose rats, a delayed disappearance of [3H]GABA was parallelled by a lower amount of metabolites, indirectly indicating a peripheral decrease of GABA-transaminase activity. The amount of [3H]GABA in brain was lightly but constantly lower in comatose rats than in controls, indicating that the change in permeability of the blood-brain barrier in hepatic encephalopathy does not affect the [3H]GABA uptake of the brain. Furthermore, the assay of endogenous GABA in blood, whole brain, and brain areas did not show any significant difference in any of the two groups. The finding that glutamic acid decarboxylase activity in brain was reduced, together with the indirect evidence of a reduction in GABA-transaminase, may account for the steady state of GABA in hepatic encephalopathy. However, the reduction in glutamic acid decarboxylase activity is in favor of a functional derangement at the GABA-ergic nerve terminals in this pathological condition.

Evidence of a role for GABA in benzodiazepine effects on food preference in rats

It has previously been shown that chronic treatment with the GABA-transaminase inhibitor ethanolamine-O-sulphate (EOS), which elevates brain GABA levels by around 200%, selectivity enhances novel food consumption in rats treated with chlordiazepoxide (CDP) and given a food preference test. To replicate and extend these findings, the effects of two doses of CDP with and without EOS pretreatment were compared with those of EOS or saline alone. EOS alone had no significant effects except to decrease eating rate but, in combination with 2.5 mg/kg CDP, it antagonised the increase in weight of familiar food eaten found with CDP alone and marginally increased weight eaten and duration of novel foot eating episodes. EOS magnified the effects of 5.0 mg/kg CDP to increase markedly the weight eaten and duration of episodes for novel chocolate drops. As no additive effects of EOS and CDP on rate of eating were found, the results are consistent with a facilitation of novel food consumption by an anxiolytic action of the two drugs, rather than by a rate-retarding action which might bias animals toward novel food. Finally, that EOS alone did not mimic the effects of CDP suggests that the role of GABA in the anxiolytic action of CDP may be indirect.

The diurnal variations of glutamic acid decarboxylase activity in some discrete nuclei of rat brain

The diurnal rhythmicity of glutamic acid decarboxylase (GAD) activity in the nucleus level of the rat brain was examined by the radiochemical micromethod in a light-dark condition. GAD activity showed a biphasic variation in globus pallidus and substantia nigra (zona reticulata and compacta), but a monophasic variation, i.e. high in the dark, in colliculus superior (stratum griseum superficiale and stratum opticum). As for hypothalamus GAD activity of some nuclei (N. arcuatus, N. periventricularis, N. preopticus, etc.) it indicated a monophasic variation, while that of some nuclei (N. suprachiasmaticus, N. ventromedialis, A. lateralis, etc.) it showed no significant variations despite their proved functional activity with rhythmicity.

The diurnal variations of monoamine oxidase activity in discrete nuclei of rat brain

The diurnal variations of MAO activities (type A and type B) in the individual hypothalamic nuclei, in addition to some areas of the rat brain, were studied under two different lighting schedules, D-L condition and D-D condition. Type A MAO activities in some nuclei increased significantly in the light period in D-L condition and the change in N. suprachiasmaticus (SCN) and N. supraopticus (SON) was also maintained in D-D condition. Type B MAO activities in some nuclei showed preferably the opposite phase to that of type A MAO activity. These diurnal changes of each MAO type activity may reflect the biological actions of monoamines in relation to a circadian rhythm of the neuroendocrinological system in the brain to some extent.