Electrophysiological study of SR 42641, a novel aminopyridazine derivative of GABA: antagonist properties and receptor selectivity of GABAA versus GABAB responses

Contributions of GABAA receptors and GABAC receptors to acetylcholine release and directional selectivity in the rabbit retina

GABA is a major inhibitory neurotransmitter in the mammalian retina and it acts at many different sites via a variety of postsynaptic receptors. These include GABAA receptors and bicuculline-resistant GABAC receptors. The release of acetylcholine (ACh) is inhibited by GABA and strongly potentiated by GABA antagonists. In addition, GABA appears to mediate the null inhibition which is responsible for the mechanism of directional selectivity in certain ganglion cells. We have used these two well-known examples of GABA inhibition to compare three GABA antagonists and assess the contributions of GABAA and GABAC receptors. All three GABA antagonists stimulated ACh release by as much as ten-fold. By this measure, the ED50s for SR-95531, bicuculline, and picrotoxin were 0.8, 7.0, and 14 microM, respectively. Muscimol, a potent GABAA agonist, blocked the effects of SR-95531 and bicuculline, but not picrotoxin. This indicates that SR-95531 and bicuculline are competitive antagonists at the GABAA receptor, while picrotoxin blocks GABAA responses by acting at a different, nonreceptor site such as the chloride channel. In the presence of a saturating dose of SR-95531 to completely block GABAA receptors, picrotoxin caused a further increase in the release of ACh. This indicates that picrotoxin potentiates ACh release by a mechanism in addition to the block of GABAA responses, possibly by also blocking GABAC receptors, which have been associated with bipolar cells. All three GABA antagonists abolished directionally selective responses from ON/OFF directional-selective (DS) ganglion cells. In this system, the ED50S for SR-95531, bicuculline, and picrotoxin were 0.7 microM, 8 microM, and 94.6 microM, respectively. The results with SR-95531 and bicuculline indicate that GABAA receptors mediate the inhibition responsible for directional selectivity. The addition of picrotoxin to a high dose of SR-95531 caused no further increase in firing rate. The comparatively high dose required for picrotoxin also suggests that GABAC receptors do not contribute to directional selectivity. This in turn suggests that feedforward GABAA inhibition, as opposed to feedback at bipolar terminals, is responsible for the null inhibition underlying directional selectivity.

Antinociception produced by microinjection of L-glutamate into the ventromedial medulla of the rat: mediation by spinal GABAA receptors

This study examined whether the antinociception produced by glutamatergic stimulation of neurons in the nucleus raphe magnus (NRM) or nucleus reticularis gigantocellularis pars alpha (NGCp alpha) is mediated by activation of GABAA receptors in the spinal cord. Two approaches were used. The first approach determined the ability of intrathecally (i.t.) administered bicuculline, a competitive GABAA receptor antagonist, to attenuate the antinociception produced by microinjection of L-glutamate in the NRM or NGCp alpha. Bicuculline was selected on the basis of an initial study that determined that 0.3 micrograms i.t. bicuculline caused a 3.4-fold rightward shift in the dose-effect relationship of the i.t.-administered GABAA agonist, isoguvacine, without producing allodynia or alterations in nociceptive threshold. The GABAA antagonist SR 95531 was judged unsuitable because it caused only a modest 1.7-fold rightward shift in the dose-effect relationship of isoguvacine at doses that did not produce allodynia. The second approach determined the ability of i.t. administered diazepam, a benzodiazepine receptor agonist, to enhance the antinociception produced by microinjection of L-glutamate in these nuclei. Diazepam was selected because of its ability to enhance the actions of GABA at the GABAA receptor. Rats were pretreated with i.t. administration of 0.3 micrograms bicuculline, 40 micrograms diazepam, or vehicle, after which 30 nmol L-glutamate was microinjected into the NRM or NGCp alpha. Microinjection of L-glutamate into the NRM or NGCp alpha in vehicle-pretreated rats significantly increased tail flick latency. The antinociception produced by microinjection of L-glutamate in the NGCp alpha was antagonized by bicuculline and enhanced by diazepam. In contrast, the antinociception evoked from sites in the NRM was only partially attenuated by bicuculline and was not enhanced by diazepam. In an ancillary experiment, i.t. administration of 0.3 micrograms bicuculline or 40 micrograms diazepam did not alter tail skin temperature or nociceptive threshold, suggesting that their effects on glutamate-induced antinociception were not secondary to alterations in tail skin temperature. Taken together, these results support the hypothesis that the antinociception produced by activation of neurons in the NGCp alpha, but not the NRM, is mediated in part by an action of GABA at GABAA receptors in the spinal cord.

Caffeine-induced Ca2+ release inhibits GABAA responsiveness in rat identified native primary afferents

It has been shown previously that an increase in cytoplasmic Ca2+ concentration depresses the GABA-A response. However, little attention has been paid to the Ca2+ source involved. In the present study, we show that the Ca2+ increase triggered by caffeine-induced Ca2+ release from the intracellular pool inhibits the GABA-A response, whereas Ca2+ influx through voltage-activated Ca2+ channels has no effect on this response.

Central-type benzodiazepines and the octadecaneuropeptide modulate the effects of GABA on the release of alpha-melanocyte-stimulating hormone from frog neurointermediate lobe in vitro

The involvement of the GABA-benzodiazepine receptor complex in the regulation of melanotropin secretion has been investigated using perfused frog neurointermediate lobes. The GABAA agonist 3-amino-1 propane sulfonic acid mimicked the biphasic effect of GABA on alpha-melanocyte-stimulating hormone secretion: a brief stimulation followed by an inhibition of melanotropin secretion. The GABAA antagonist SR 95531 (10(-4) M) inhibited both stimulation and inhibition of alpha-melanocyte-stimulating hormone release induced by GABA (10(-4) M). Since the inhibitory effect of baclofen (10(-4) M) was partially antagonized by SR 95531 (10(-4) M), it appears that the GABAergic control of alpha-melanocyte-stimulating hormone release is mainly achieved through activation of GABAA receptors. GABA-induced stimulation of alpha-melanocyte-stimulating hormone release was inhibited by tetrodotoxin (10(-5) M), an Na+ -channel blocker, or nifedipine (10(-5) M), a voltage-dependent Ca2+ -channel blocker, suggesting that Na+ and Ca2+ ions are involved in the stimulatory phase of GABA action. Only central-type benzodiazepine binding site agonists such as clonazepam (10(-4) M) modified alpha-melanocyte-stimulating hormone release. In fact, clonazepam (10(-7) to 10(-5) M) led to a dose-dependent potentiation of both GABA-induced stimulation and inhibition of alpha-melanocyte-stimulating hormone release. This potentiating effect was antagonized by the GABAA antagonist SR 95531 (10(-4) M) or by the central-type benzodiazepine binding site antagonist flumazenil (10(-4) M), whereas picrotoxin (10(-4) M) abolished only the stimulatory phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative evaluation of the properties of a pyridazinyl GABA derivative (SR 95531) as a GABAA competitive antagonist. An electrophysiological approach

We have investigated the effects of an aryl-aminopyridazine derivative of GABA (SR 95531) on dose-response curves of GABA-induced depolarizations from dorsal root ganglion neurones recorded intracellularly. The reversible shift to the right of the dose-response curves in a parallel fashion and the dissociation constant (KB) value of 0.13 +/- 0.02 microM (n = 15) indicate that this compound is a potent competitive GABAA antagonist. The competitive nature of SR 95531-induced antagonism was confirmed by single channel analysis. In excised membrane patches from bovine chromaffin cells (outside out configuration), 0.2-0.5 microM SR 95531 did not alter the mean open time of GABA-activated channels and did not introduce further short closing gaps within bursts. Whole cell recordings from cultured nodose ganglion neurones indicated that SR 95531 (10 microM) did not modify significantly any of the 3 types of calcium currents already reported in sensory neurones. This result might be of importance for further studies of presynaptic GABA actions on transmitter release.

Behavioural evidence for a selective GABA-A antagonistic activity of SR 95103 and SR 42641 after intrastriatal injection in mice

Two pyridazinyl GABA derivatives, SR 95103 and SR 42641 have recently been described as selective GABAA receptor antagonists. We have now investigated the behavioural effects of SR 95103 and SR 42641 after intrastriatal injection in mice. When injected into the right striatum, SR 95103 (0.01-0.5 microgram), SR 42641 (0.0001-0.01 microgram) and bicuculline methiodide (0.005-0.05 microgram) induced contralateral rotations which were antagonized by intraperitoneal injection of muscimol. In contrast, the intrastriatal injection of the GABAA receptor agonist muscimol induced ipsilateral rotations. Muscimol-induced turning was antagonized by SR 95103 (10-30 mg/kg), SR 42641 (1-10 mg/kg) and (+)-bicuculline (0.125-0.5 mg/kg) injected intraperitoneally, but not by strychnine. Intrastriatal glycine also induced ipsilateral rotations which were antagonized by strychnine (0.01-0.3 mg/kg i.p.) but not by (+)-bicuculline, SR 95103 or SR 42641. These results suggest that SR 95103 and SR 42641 induce turning through a selective blockade of GABAA receptors within the striatum.