Inhibition of GABA-gated chloride channels in brain by the arachidonic acid metabolite, thromboxane A2

Previously, we showed that arachidonic acid and prostaglandin metabolites inhibited GABAA responses in rat cerebral cortex. Thromboxane A2 (TXA2), a metabolite of arachidonic acid, has potent actions on blood vessels and platelets, but its actions on neurons are not known. Here, we examined the effects of several TXA2 analogs on the functional and binding characteristics of GABAA receptors in rat brain. The stable analogs of TXA2, pinane and carbocyclic TXA2, and the TXA2 agonist, U-46619, inhibited muscimol-induced 36Cl- uptake in cerebral cortical synaptoneurosomes. Carbocyclic TXA2 decreased the maximal response to muscimol, consistent with a non-competitive interaction. The TXA2 antagonist, SQ 25,548, did not block the effects of either arachidonic acid or carbocyclic TXA2. Neither the biologically inactive metabolite of TXA2, TXB2, nor carbacyclin, a stable analog of prostacyclin (prostaglandin I2) had an effect on GABAA responses. Thus the pharmacology differs from that in vascular smooth muscle and platelets. To determine if GABAA receptors were sensitive to the thromboxanes, the effect of pinane TXA2 on the binding of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) to GABA-gated Cl- channels was measured using receptor autoradiography. Pinane TXA2 inhibited [35S]TBPS binding in a regionally selective and non-competitive manner; the greatest inhibition was in the cerebral cortex, hippocampus and striatum, areas which are selectively vulnerable to cerebral ischemia. We conclude that TXA2 can interact with neuronal membranes to inhibit GABA receptor function, independent of its actions on the cerebrovasculature and on glial cells. This may be important during pathologic states such as ischemia, when TXA2 accumulates in extracellular spaces.

The mediodorsal nucleus of the thalamus in rats--II. Behavioral and neurochemical effects of GABA agonists

The aim of this study was to determine how GABA receptors in the mediodorsal nucleus of the thalamus in rats might contribute to the regulation of locomotor behavior. Microinjections of the GABAB and GABAA agonists, baclofen and muscimol, into the mediodorsal nucleus produced dose-dependent increases in locomotion that were blocked by co-administration of the GABAB antagonist, 2-hydroxysaclofen. Microinjection of baclofen along the midline, lateral into the ventrolateral thalamus or into the lateral ventricles produced significantly smaller dose-dependent increases in locomotion, indicating that the anatomical locus for baclofen-induced locomotion resides in the mediodorsal nucleus. The motor response elicited by microinjected baclofen was associated with a reduction in dopamine metabolism in the prefrontal cortex and an increase in metabolism in the core of the nucleus accumbens, but not in the accumbal shell or the dorsolateral striatum. These results suggest that GABAergic afferents to the mediodorsal nucleus may oppose a tonic inhibitory tone on locomotor activity. The data also suggest that the motor response produced by baclofen in the mediodorsal thalamus may arise by inhibiting the projections to the prefrontal cortex which modulate mesocorticolimbic dopamine transmission.

Antisense oligonucleotide to GABAA receptor gamma 2 subunit induces loss of neurones in rat hippocampus

The binding site for 1,4-benzodiazepines in the brain is part of the hetero-oligomeric gamma-aminobutyric acid (GABA)A receptor complex which regulates a chloride ion channel. The presence of the gamma 2 subunit in the complex is necessary for the binding of benzodiazepines to their binding site. This study demonstrates a reduction of benzodiazepine receptor radioligand binding by 43% compared to control following infusion of phosphorothioate antisense oligodeoxynucleotide to gamma 2 subunit into rat hippocampus. Reduction of benzodiazepine binding sites was paralleled by a decrease in [35S]tert-butyl-bicyclo-phosphorothionate ([35S]TBPS) binding (51%) and [3H]muscimol binding (37%), indicating a reduction in the number of GABAA receptors. Changed macroscopic appearance, reduced protein content and severe loss of neurones in antisense-treated hippocampi suggests that the reduced formation of GABAA receptors leads to neuronal cell death.

GABAergic inhibitory inputs to subfornical organ neurons in rat slice preparations

To investigate GABAergic inhibitory inputs to neurons of the subfornical organ (SFO), intracellular recordings were made in rat brain slice preparations. Inhibitory postsynaptic potentials, which occurred spontaneously or were evoked by focal electric stimulation, had reversal potentials of approximately -60 mV, and were almost totally abolished by the GABAA antagonists bicuculline at 3-100 microM or picrotoxin at 50 microM. Following the application of bicuculline or picrotoxin, the resting membrane potentials were decreased by 4-8 mV. GABA at 10-100 microM and the GABAA agonist muscimol at 1-100 microM decreased the membrane resistance and the firing rate in all neurons tested. The reversal potential of the response to muscimol was similar to that for inhibitory postsynaptic potentials. The actions of muscimol persisted in the presence of 1 microM tetrodotoxin, implying that muscimol must act directly on the recorded neurons. These results suggest that there is a tonic inhibitory GABAergic input to SFO neurons which are mainly mediated through GABAA receptors.

Startle and sensorimotor correlates of ventral thalamic dopamine and GABA in rodents

Recent studies have implicated the thalamus as a possible site for neuroanatomical and neurochemical changes in schizophrenia. In the present study, we investigated thalamic neurochemical correlates of behaviors potentially linked to schizophrenia. Whole thalamic DOPAC levels were elevated in rats that had poor extinction of the acoustic startle response. The dopamine agonist apomorphine microinjected into the ventromedial thalamus (VmT) disrupted prepulse inhibition of startle. Catalepsy was induced by VmT microinjections of the GABA-A agonist muscimol. A previous study revealed attentional disturbances and suppression of frontal cortical metabolic activity after muscimol microinjections into the mediodorsal thalamic nucleus. Together with recent findings of neuron cell loss and elevated DA levels in the thalamus of schizophrenics, these data suggest the involvement of disturbances of thalamic neurotransmission in schizophrenia.

The enhancement of muscimol-stimulated 36C1 influx by the antispastic 5-aryl-3-(alkylsulfonyl)-4H-1,2,4-triazole (MDL 27,531) in rat brain membrane vesicles

The antispastic triazole, 4-methyl-3-methylsulphonyl-5-phenyl-4H-1,2,4-triazole (MDL 27,531) was tested in glycine- and muscimol-stimulated 36Cl- influx into brain membrane preparations. MDL 27,531 (100 nM) had no effect on glycine- (100 nM-400 microM) stimulated 36Cl- influx in brain stem tissue; on the other hand, MDL 27,531 (10 nM-10 microM) enhanced muscimol- (1 microM) stimulated 36Cl- influx in cerebellar but not cortical membranes. In the presence of the benzodiazepine (BZD) antagonist, flumazenil (10 microM), MDL 27,531 inhibited muscimol-stimulated flux. These data suggest a novel interaction of this triazole with subtypes of the gamma-aminobutyric acid (GABA)A receptor complex and the BZD receptor.

Effects of GABAergic drugs on physostigmine-induced yawning in rats

In the present work the effects of GABA agonists and antagonists on yawning induced by physostigmine have been studied. Intraperitoneally (IP) injection of physostigmine (0.05-0.3 mg/kg) induced dose-related yawning in rats. The maximum yawning response was observed with 0.2 mg/kg of the drug. The GABA agonists muscimol (1-4 mg/kg) and baclofen (0.125-1 mg/kg) decreased yawning induced by physostigmine (0.2 mg/kg) dose dependently. Combination of both GABA agonists elicited greater inhibition of yawning. The GABA-A antagonists bicuculline or picrotoxin but not the GABA-B antagonist phaclofen reduced the inhibitory response induced by muscimol, whereas phaclofen but not bicuculline or picrotoxin reduced baclofen's inhibitory effect. Administration of bicuculline, picrotoxin or phaclofen also decreased the yawning induced by physostigmine. However, when the GABA-A and GABA-B antagonists were employed in combination, the inhibitory responses of both drugs were lost. It is concluded that GABA-A and/or GABA-B receptor stimulation may inhibit physostigmine-induced yawning.

Acute activation and inactivation of macaque frontal eye field with GABA-related drugs

1. This project tests the behavioral effects of reversible activation and inactivation of sites within the frontal eye field of rhesus monkeys with microinjections of the gamma-aminobutyric acid (GABA)-related drugs bicuculline and muscimol. 2. Muscimol injections impaired the monkeys' ability to make both visually and memory-guided saccades to targets at the center of the area represented by the injection site. The latencies of saccades to targets in regions flanking the injection were increased. For memory-guided saccades, saccades in the direction opposite to that represented by the injection site, were made with shorter latency than controls and often occurred before the movement cue. 3. Bicuculline injections produced irrepressible saccades equivalent to the saccade vector represented by the injection site, often in a staircase of several closely spaced movements. 4. Both substances decreased the accuracy of fixation of a central light. The distribution of points of fixation on different trials was diffuse, and the angle of gaze tended to deviate towards the side of the injection. 5. The results of these acute injections are similiar to those observed in the superior colliculus and are much more substantial than the effects observed in the long term after surgical removal of the frontal eye field. The results of this study promote a central role for the frontal eye field in the generation of all voluntary saccades and in the control of fixation.

Identification and functional characterization of a dual GABA/taurine transporter in the bullfrog retinal pigment epithelium

Intracellular microelectrodes, fluorescence imaging, and radiotracer flux techniques were used to investigate the physiological response of the retinal pigment epithelium (RPE) to the major retinal inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). GABA is released tonically in the dark by amphibian horizontal cells, but is not taken up by the nearby Müller cells. Addition of GABA to the apical bath produced voltage responses in the bullfrog RPE that were not blocked nor mimicked by any of the major GABA-receptor antagonists or agonists. Nipecotic acid, a substrate for GABA transport, inhibited the voltage effects of GABA. GABA and nipecotic acid also inhibited the voltage effects of taurine, suggesting that the previously characterized beta-alanine sensitive taurine carrier also takes up GABA. The voltage responses of GABA, taurine, nipecotic acid, and beta-alanine all showed first-order saturable kinetics with the following Km's: GABA (Km = 160 microM), beta-alanine (Km = 250 microM), nipecotic acid (Km = 420 microM), and taurine (Km = 850 microM). This low affinity GABA transporter is dependent on external Na, partially dependent on external Cl, and is stimulated in low [K]o, which approximates subretinal space [K]o during light onset. Apical GABA also produced a significant conductance increase at the basolateral membrane. These GABA-induced conductance changes were blocked by basal Ba2+, suggesting that GABA decreased basolateral membrane K conductance. In addition, the apical membrane Na/K ATPase was stimulated in the presence of GABA. A model for the interaction between the GABA transporter, the Na/K ATPase, and the basolateral membrane K conductance accounts for the electrical effects of GABA. Net apical-to-basal flux of [3H]-GABA was also observed in radioactive flux experiments. The present study shows that a high capacity GABA uptake mechanism with unique pharmacological properties is located at the RPE apical membrane and could play an important role in the removal of GABA from the subretinal space (SRS). This transporter could also coordinate the activities of GABA and taurine in the SRS after transitions between light and dark.

Central GABAergic mechanisms are defective in salt-induced hypertension in borderline hypertensive rats

We examined the role of central GABAergic mechanisms in salt-induced hypertension and exaggerated responses to stress in borderline hypertensive rats (BHR), the first offspring of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). The studies were done in conscious BHR and WKY on high (H) (8% NaCl) or normal (N) (0.3% NaCl) salt diets for 5 weeks. A high-salt diet elevated arterial pressure (AP) (p < 0.01) and augmented pressor responses to shaker stress (p < 0.05) in BHR but not in WKY. Intravenous hexamethonium caused a greater decrease in AP in BHR-H than in BHR-N at rest. Muscimol (a GABA agonist) injected into the central ventricle (i.c.v.) caused a greater decrease in resting AP (p < 0.01) and heart rate (HR) (p < 0.05) and BHR-H than in BHR-N. Renal sympathetic nerve activity (RSNA) did not change in BHR-H, but increased (p < 0.05) in BHR-N during muscimol-induced hypotension, although the magnitudes of muscimol-induced hypotension were greater in BHR-N than in BHR-N. The increases in RSNA in response to intravenous nitroglycerin were similar in BHR-N and BHR-N. Muscimol attenuated pressor and tachycardic responses to stress more in BHR-N than in BHR-N (p < 0.01). Muscimol did not alter AP and HR at rest or their responses to stress in the two groups of WKY. The magnitudes of pressor response to bicuculline (a GABA antagonist) did not differ between the two groups of BHR. These results suggest that a high salt diet may alter the central GABAergic system in BHR, which contributes to salt-induced hypertension and augmented pressor and tachycardic responses to stress.

Regulation of anxiety by GABAA receptors in the rat amygdala

Blockade of GABAergic inhibition in the region of the anterior basolateral amygdala (BLA) of rats elicits physiologic changes associated with a defense reaction. The present study was undertaken to determine whether GABA receptors in the BLA might be involved in regulating experimental anxiety using the social interaction (SI) and conflict test. Guide cannulae were stereotaxically implanted bilaterally in the BLA of rats for intracerebral microinjections. In the BLA, injection of the GABAA receptor antagonists bicuculline methiodide (BMI) and picrotoxin (PIC) produced anxiogenic-like effects in the SI paradigm, as did BMI injection using the conflict paradigm. Injection of the GABAA agonist muscimol (MUS) into the central nucleus of the amygdala (Ce) produced anxiolytic-like effects in the SI test. Microinjection of MUS, baclofen (GABAB agonist), 2OH-saclofen (GABAB antagonist) or strychnine (glycine antagonist) into the BLA or BMI into the Ce elicited no change in experimental anxiety as measured by the SI test. These results suggest that endogenous GABA acts tonically at GABAA receptors in the BLA to inhibit anxiety responses.

GABAergic transmission in the nucleus accumbens is involved in the termination of ethanol self-administration in rats

Long-Evans rats (n = 12) were trained to lever-press on a fixed-ratio 4 schedule of reinforcement with ethanol (10% v/v) presented as the reinforcer. After implantation of bilateral stainless-steel guide cannulae aimed at the nucleus accumbens, site-specific microinjections of muscimol (1-30 ng) and bicuculline (1-10 ng) were tested for effects on ethanol-reinforced responding. Baseline response patterns were characterized by initial high rates that terminated abruptly after approximately 20 min. Muscimol administration in the nucleus accumbens decreased the total number of ethanol-reinforced responses and obtained reinforcers. Bicuculline also decreased ethanol-reinforced responses and reinforcers at the highest dose tested. When a dose of bicuculline (1 ng) that was ineffective by itself was coadministered with an effective dose of muscimol (10 ng), the muscimol-induced decreases in responding were blocked. Analysis of response patterns showed that muscimol decreased ethanol self-administration by terminating responding, normally lasting 20 min, after approximately 10 min with no changes in local response rate. Bicuculline decreased total responding by producing parallel, but nonsignificant, changes in time course and response rate. These data suggest that GABAergic transmission in the nucleus accumbens is involved in the termination, but not the onset or maintenance of ethanol self-administration. The specificity of this effect gives emphasis to the importance of measuring behavioral parameters, as well as products of behavior (such as intake volume) in the study of ethanol self-administration.

Differential modulation by muscimol and baclofen on antinociception induced by morphine, beta-endorphin, D-Pen2,5-enkephalin and U50,488H administered intracerebroventricularly in the mouse

The present study was designed to investigate the modulatory effects of stimulation of GABAA and GABAB receptors at supraspinal sites on antinociception induced by supraspinally administered mu-, epsilon-, delta-, and kappa-opioid receptor agonists. The effects of the GABAA and GABAB receptor agonists, muscimol and baclofen respectively, on the antinociception induced by morphine (a mu-receptor agonist), beta-endorphin (an epsilon-receptor agonist), D-Pen2,5-enkephalin (DPDPE, a delta-receptor agonist) and U50,488H ([trans-3,4-dichloroN-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeocetamide]; a kappa-receptor agonist) injected intracerebroventricularly (i.c.v.) were studied. The antinociception was assayed using the tail-flick and hot-plate tests. Muscimol at doses of 25-200 ng, administered i.c.v. alone did not affect the latencies of tail-flick and hot-plate thresholds, but attenuated dose-dependently the inhibition of the tail-flick and hot-plate responses induced by i.c.v. administered morphine (2 micrograms), beta-endorphin (1 microgram), DPDPE (10 micrograms), and U50,488H (60 micrograms). Baclofen (1.25-10 ng) administered i.c.v. alone did not affect the latencies of the tail-flick and hot-plate responses, but attenuated dose-dependently the inhibition of the tail-flick and hot-plate responses induced by beta-endorphin and U50,488H, without affecting morphine- or DPDPE-induced responses. Our results indicate that activation of GABAA receptors at the supraspinal sites by i.c.v. injection of muscimol antagonizes antinociception induced by supraspinally administered mu-, epsilon-, delta-, and kappa-opioid receptor agonists. On the other hand, activation of GABAB receptors at supraspinal sites by i.c.v. baclofen antagonizes antinociception induced by i.c.v. administered epsilon- and kappa-opioid agonists, but not mu- or delta-opioid agonists.

Hypothalamic and medullary GABAA and GABAB-ergic systems differently regulate sympathetic and cardiovascular systems

1. To determine whether hypothalamic and medullary GABA (gamma-aminobutyric acid)B stimulation would affect the sympathetic and cardiovascular activities, and to determine whether these effects would be altered in hypertension, baclofen (a GABAB agonist) was injected into a hypothalamic pressor area (ventromedial hypothalamus, VMH), a depressor area (anterior hypothalamus, AH), or a nucleus tractus solitarius (NTS) in normotensive and spontaneously hypertensive rats (SHR). 2. Intracerebroventricular (ICV) injections of a GABAA agonist (muscimol, 1 mu g) decreased blood pressure (BP) and heart rate (HR). ICV injections of baclofen (2 mu g) elicited biphasic depressor and pressor effects, and these effects were abolished by a pretreatment with saclofen (GABAB antagonist, 100 mu g, icv). 3. Muscimol (400 ng) and baclofen (800 ng) injected into VMH decreased sympathetic nerve activity (SNA), BP and HR to almost similar levels, while saclofen injected into VMH increased HR without affecting BP levels. 4. The same dose of baclofen injected into AH increased BP, but muscimol (AH) did not alter BP. 5. Both muscimol and baclofen injected into NTS increased BP, but its magnitude was larger in baclofen injections. 6. Depressor and sympatho-inhibitory effects of baclofen (VMH) in SHR were larger than those in normotensive Wistar-Kyoto (WKY) rats, while pressor responses elicited by baclofen (AH) did not differ between SHR and WKY. 7. In summary, GABA reduces SNA, BP and HR through both GABAA and GABAB receptors in VMH. In addition, the GABAB system acts on AH and NTS to further regulate the cardiovascular activities. In SHR, GABAB-ergic dysfunction in VMH but not in AH might contribute to the development of hypertension.

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.

Optical imaging of intracellular chloride in living brain slices

We developed an optical imaging technique to measure changes in intracellular levels of Cl- in neurons within the living brain slice. After rat brain slices were incubated with the permeant form of the Cl(-)-sensitive dye, 6-methoxy-N-ethylquinolinium chloride (MEQ), neurons could be imaged within the hippocampus, cerebral cortex and cerebellum using fluorescence microscopy. Both soma and dendrites were clearly visible in pyramidal neurons, interneurons, Purkinje cells and cerebellar granule cells. Increased intracellular levels of Cl- were produced by bath application of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). Within hippocampal pyramidal neurons and interneurons, GABA produced a concentration-dependent decrease in fluorescence (EC50 = 200 microM). The GABA response was mediated via the GABA receptor since it was blocked by picrotoxin and mimicked by the agonist, muscimol. Muscimol, which is not transported by the GABA re-uptake pump, was approximately 20-fold more potent than GABA. The method developed was also used to image intracellular Cl- levels with UV laser scanning confocal microscopy. Even greater resolution was obtained and deeper structures could be imaged in cerebral cortex and hippocampus. This is the first demonstration of optical imaging to measure intracellular Cl- dynamics in living brain slices using fluorescence microscopy and laser scanning confocal microscopy.

Role of amino acids in salivation and the localization of their receptors in the rat salivary gland

The distribution of gamma-aminobutyric acid (GABA) receptor subunits such as GABAAR-gamma 1 and GABAAR-gamma 2, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type receptor subunits such as GluR-1, GluR-2/3 and GluR-4, and N-methyl-D-aspartic acid (NMDA) type subunits such as NR1 were investigated by immunocytochemistry. Furthermore, the roles of these amino acids, GABA and glutamate, on salivation were analyzed in the rat submandibular and sublingual glands. Some similarities were observed in the distribution patterns of GABAA type receptors and AMPA receptors. In the submandibular ganglion cells, collecting ducts and striated ducts, these subunits were expressed strongly; however, there were some differences in their expression patterns between the submandibular and sublingual gland acinar cells. Since these receptor subunits were expressed in the acinar cell bodies of the submandibular gland, they were not expressed in the acinar cells but were expressed in the myoepithelial cells in the sublingual gland. On the other hand, no NR1 expression was observed. To examine the roles of GABA and glutamate in salivation, the submandibular and sublingual glands were perfused partially with Ringer's solution via a facial artery to avoid systemic influence, and substrates were infused into the perfusion solution. No salivary secretion was evoked by GABA or glutamate infusion in the absence of electrical stimulation (2-3 V, 5 ms, 20 Hz). Salivary flow evoked by electrical stimulation of the chorda-lingual nerve caused significant inhibition by GABA (10(-6), 10(-5), 10(-4) and 10(-3) M) and the GABAAR agonist muscimol 10(-3) and 10(-6) M) (n = 6, P < 0.05). Such GABA-induced inhibition was antagonized by the GABAAR antagonists bicuculline (BCC; 10(-6) and 10(-3) M) and picrotoxin (PTX; 10(-6) and 10(-3) M). On the other hand, salivary flow evoked by electrical stimulation (8-10 V, 5 ms, 20 Hz) of the superior cervical ganglion (SCG) was not affected by GABA. While high doses of glutamate (10(-1) M) and NMDA (10(-1) M) showed no effects on salivary flow despite application of electrical stimulation, AMPA at a high concentration (10(-1) M) significantly inhibited salivary secretion (n = 6, P < 0.05). These studies revealed that inhibitory and excitatory amino acid receptors such as GABAA and AMPA type receptors are coexpressed in the rat salivary glands, and that GABA inhibits salivary secretion via GABAA receptors which may act with acetylcholine. However, the role of glutamate in salivation remains unclear despite the presence of AMPA type receptors. The present findings suggest that glutamate does not act alone but with other substances such as peptides and/or other amino acids.

Modification by muscimol of naloxone-precipitated withdrawal symptoms in morphine-dependent mice

1. In morphine-dependent mice, muscimol, a GABAA agonist when given i.p. 30 min before naloxone, attenuated the naloxone-precipitated withdrawal symptoms of jumping, "wet dog" shakes and burrowing, but not body weight loss. 2. Muscimol produced a hypothermic effect which was further aggravated by naloxone.

GABAergic modulation of optic nerve-evoked field potentials in the rat suprachiasmatic nucleus

The suprachiasmatic nuclei (SCN) at the base of the hypothalamus are known to be the site of the endogenous circadian pacemaker in mammals. The SCN are innervated by the retinohypothalamic tract, which conveys photic information to the SCN. GABA is one of the most abundant neurotransmitters in the SCN, and has been implicated in the modulation of photic responses of the SCN circadian pacemaker. This study sought to examine the effect of GABAergic compounds on optic nerve-evoked SCN field potentials recorded in rat horizontal hypothalamic slices. The GABAA agonist muscimol (10 microM) potentiated SCN field potentials by 23%, while application of the GABAA antagonist bicuculline (10 microM) inhibited SCN field potentials by a similar amount, (22%). Conversely, the GABA, agonist baclofen (1.0 microM) inhibited SCN field potentials by 48%, while the GABAB antagonist phaclofen (0.5 mM) augmented SCN field potentials by 62%. Recordings performed at both day and night times indicate that there were no qualitative day-night differences in GABAergic activity on SCN field potentials. This study concludes that, in general, GABAA activity tends to increase, and GABAB activity tends to decrease the response of SCN neurons to optic nerve stimulation.

Intrathecal baclofen and muscimol, but not midazolam, are antinociceptive using the rat-formalin model

Both gamma-aminobutyric acid (GABA)A and GABAB receptor subtypes have been implicated in spinally mediated antinociception in acute pain models. In the current study, the formalin test was used as a model of protracted nociception to examine the effect of intrathecally (i.t.) administered baclofen (GABAB agonist), muscimol (GABAA agonist) or midazolam (a benzodiazepine) on antinociception. At doses that did not affect motor function, baclofen (0.3 and 1.0 micrograms, i.t.) decreased the flinch response in a dose-dependent manner during Phase 1 and Phase 2. This effect was reversible by the GABAB-specific antagonist, CGP35348 ([P-(3-aminopropyl)-P-diethoxymethyl-phosphinic acid]). Muscimol (0.3 and 1.0 microgram i.t.) evoked a dose-dependent, bicuculline-reversible decrease in flinching during Phase 1 and Phase 2, but midazolam had no effect on either phase. No attenuation of the quiescent period between Phase 1 and Phase 2 was seen upon administration of baclofen, muscimol or midazolam. Additionally, no increase in nocifensive behavior was observed upon administration of either GABAA or GABAB antagonists alone. Therefore, our conclusions are that both GABAA and GABAB agonists are antinociceptive at the spinal cord level and that endogenous spinal GABA levels are insufficient for a GABA potentiator to act alone in an antinociceptive manner.

Pharmacological subtypes of the gamma-aminobutyric acidA receptors defined by a gamma-aminobutyric acid analogue 4,5,6,7-tetrahydroisoxazolo[5,4-c] pyridin-3-ol and allosteric coupling: characterization using subunit-specific antibodies

Various alpha and beta 3 subunit-specific antibodies were used to characterize some of the heterogeneous ligand-binding properties of gamma-aminobutyric acidA receptors. Polyclonal antibodies that were raised against the cytoplasmic amino acid sequence (380-392) of the rat beta 3 subunit recognized a single polypeptide of molecular mass of 58 kDa in Western blots with Ro7-1986 affinity-purified GABAA receptors from the rat brain, and a doublet of molecular mass of 54 kDa and 56 kDa in receptors from the bovine cortex, hippocampus, and cerebellum. Deglycosylation of purified receptors from the bovine cortex with N-glycanase resulted in a single band immunostained at molecular mass of 52 kDa. These anti-beta 3 subunit antibodies immunoprecipitated approximately 50% of [3H]flunitrazepam binding sites from soluble extracts of bovine cortex, whereas beta cyto antibodies, which probably recognize all beta subunit isoforms, precipitated almost 100% of benzodiazepine binding sites. These results indicate heterogeneity of GABAA receptor subunit composition with respect to the nature of beta subunits. The GABA analogue 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), like GABA, shows heterogeneous binding affinities in brain homogenates. The higher affinity sites were previously suggested as corresponding to a 58-kDa polypeptide in rat that is photoaffinity-labeled with [3H]muscimol, a band that comigrates with the one stained by anti-beta 3 antibodies. However, THIP affinity was not significantly different between receptors containing beta 3 subunits and those lacking beta 3, as demonstrated by similar affinities in receptors that ere immunoprecipitated by anti-beta 3 antibodies and those that were not. Also, THIP displaced [3H]muscimol binding with similar multiple affinities across brain regions where different beta subunit variants are expressed with varying abundances. These observations suggest that the property of high affinity THIP binding cannot be explained solely by beta 3 subunits. The coupling efficiency between GABA and benzodiazepine binding sites appears to be determined by the nature of alpha subunits rather than of beta subunits. GABA enhanced [3H]flunitrazepam binding with different efficacies and potencies in receptors immunoprecipitated by anti-alpha 1, -alpha 2, and -alpha 3 subunit antibodies. In contrast, beta 3 subunit-enriched and disenriched receptors did not differ in this property. [3H]Flunitrazepam binding in GABAA receptors containing alpha 2 and alpha 3 subunits was enhanced to a significantly greater extent than were those with alpha 1. In addition, receptors containing alpha 1 and alpha 3 subunits had higher potencies of enhancement than did those with alpha 2 subunits.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAA receptor function and binding in stably transfected cells: chronic ethanol treatment

Effects of chronic ethanol exposure on GABAA receptors may contribute to tolerance and dependence to alcohol. Ethanol treatment of mice and rats can produce alterations of GABAA receptor binding, function, and subunit mRNA and protein levels. We treated a cell line (PA3 cells) that stably expresses GABAA receptors chronically with ethanol. Expression of bovine alpha 1, beta 1, and gamma 2L GABAA receptor subunits genes in these cells is controlled by a dexamethasone-sensitive promoter, and this provides an excellent system to study the regulation by chronic ethanol treatment of receptors with a defined subunit composition. The actions of the GABA agonist muscimol on receptor function (36Cl- uptake) were not affected by 100 mM ethanol treatment for 4 days, but the actions of flunitrazepam (1 microM) were decreased in cells treated with ethanol. The functional coupling between benzodiazepine and GABA sites on the receptors was affected by chronic ethanol treatment in a manner consistent with results from mice. Ethanol treatments (50 or 100 mM) for 4 days did not affect the affinity (Kd) or receptor density (Bmax) of [3H]flunitrazepam binding, or the levels of alpha 1 subunit mRNA, or alpha 1 or beta 1 subunit proteins. These results demonstrate that the regulation of the stably expressed GABAA receptors by chronic ethanol, in the absence of neuronal receptor gene promoters, is posttranscriptional and likely posttranslational.

Age-related differences in the effects of GABAA agonists microinjected into rat substantia nigra: pro- and anticonvulsant actions

GABAergic transmission in the substantia nigra pars reticulata (SNR) has an important role in the control of experimental seizures. In the flurothyl seizure model, SNR microinjection of the selective GABAA receptor agonist muscimol results in a biphasic dose-response curve in adults: Intermediate doses are anticonvulsant, but high doses have proconvulsant effects. Another GABAA agonist, THIP (4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3-ol), also produces anticonvulsant effects at lower doses, whereas higher doses tend to produce a proconvulsant effect. In 16-day-old rat pups, no anticonvulsant but only proconvulsant effects of muscimol occur, and at lower doses than in adults. These data suggest that the immature SNR is significantly more sensitive to the proconvulsant effects of GABAA receptor agonists than is the SNR of adults. We hypothesize that the age-related differences in nigral GABAergic response may be due to ontogenic changes in GABAA-sensitive neuronal circuits in the SNR.

Cardiovascular neurons in cat caudal ventrolateral medulla: location and characterization of GABAergic input

The purposes of the present study were to: (1) characterize the GABAergic input to vasodepressor neurons in the caudal ventrolateral medulla of the cat, and (2) define more precisely the anatomical localization of these neurons in this species. This was done by microinjecting GABA receptor antagonists and agonists, and a negative allosteric modulator of the GABA receptor, namely, ethyl-beta-carboline-3-carboxylate, into the caudal ventrolateral medulla of alpha-chloralose-anesthetized animals while monitoring arterial blood pressure and heart rate. Localization studies where performed relating injection sites in the caudal ventrolateral medulla where cardiovascular responses were elicited, to neurons exhibiting immunoreactivity to tyrosine hydroxylase (TH) and phenethyl-N-methyl-transferase (PNMT). Microinjection of 1 and 10 ng of bicuculline into the caudal ventrolateral medulla produced decreases in mean blood pressure and heart rate of -34 +/- 6.4 and -49 +/- 9.2 mmHg, and -22 +/- 4.3 and -35 +/- 8.2 beats/min, respectively. Hypotension and bradycardia were also observed with picrotoxin microinjection (120 ng). Microinjection of muscimol (100-200 ng) and GABA (12 microgram) had no effect on mean blood pressure and heart rate. Microinjection of ethyl-beta-carboline-3-carboxylate also decreased mean blood pressure (-39 +/- 7.0 mmHg). The location of the micropipette tip after bicuculline microinjection in relation to TH and PNMT immunoreactive cells was as follows: (1) TH-immunoreactive cells of the A1 cell group were visible in the same relative location as the micropipette tip, and (2) no PNMT-positive cells were noted at the sites where bicuculline elicited hypotension. These results indicate that there is a tonic GABAergic input to neurons in the caudal ventrolateral medulla. The location of these neurons overlaps with the A1 cells.

Effect of muscimol on haloperidol-induced alteration of neurotensin gene expression in the striatum and nucleus accumbens in the rat

Acute neuroleptic administration increases the expression of neurotensin/neuromedin (NT/N) gene in rat dorsolateral striatum and shell sector of the nucleus accumbens. The purpose of this study was to examine modulation of neuroleptic induction of NT/N and the proto-oncogene c-fos expression by the GABAA agonist muscimol. Adult male Sprague-Dawley rats were treated with saline, haloperidol (1 mg/kg); muscimol (3.2 mg/kg); or haloperidol (1 mg/kg) plus muscimol (3.2 mg/kg). Animals were sacrificed 1 h after drug administration. Expression of NT/N and c-fos mRNA was examined by in situ hybridization using 35S-antisense probes. Muscimol alone had no measurable effect on basal levels of NT/N or c-fos mRNA in either the dorsolateral striatum or the nucleus accumbens. However, co-administration of muscimol with haloperidol reduced haloperidol-induced increases in NT/N as well as c-fos mRNA in the dorsolateral striatum. In contrast, NT/N mRNA expression in accumbal shell induced by haloperidol was not modulated by co-administration of muscimol. These data suggest that GABAA receptors may be involved in regulation of NT/N gene expression in the DLSt, but not in the nucleus accumbens.