Pharmacologic characteristics of excitatory gamma-amino-butyric acid (GABA) receptors in a snail neuron

1. The pharmacologic characteristics of excitatory gamma-aminobutyric acid (GABA) receptors, termed muscimol II type GABA receptors, found in a giant neuron type, v-LCDN (ventral-left cerebral distinct neuron), of an African giant snail (Achatina fulica Ferussac), were studied using the mammalian GABA receptor agonists, antagonists and synergists and GABA uptake inhibitor using the voltage clamp technique. 2. GABA and its agonists, ejected by brief pressure, produced an inward current (I-in) of the following order of potency: trans-t-aminocrotonic acid (TACA)>GABA>muscimol>isoguvacine>5-aminopentanoic acid and cis-4-aminocrotonic acid (CACA). (+/-)-Baclofen and 3-aminopropylphosphonic acid (APPA) were ineffective. The I-in values produced by GABA, TACA, isoguvacine and CACA were stable for at least 60 min, whereas the I-in induced by muscimol was not. 3. According to the dose-response curves of GABA, TACA, isoguvacine and CACA, measured by the varied pressure duration method, the ED(50) value of CACA was larger than those of the other compounds, and E(max) of TACh was larger than that of GABA, whereas E(max) values of isoguvacine and CACA were smaller. 4. The perfusion of beta-alanine, pentobarbital and 5-aminopentanoic acid inhibited the I-in induced by GABA, whereas (-)-bicuculline, pitrazepin, diazepam and 2-hydroxysaclofen had no effect. 5. From the effects of beta-alanine on the dose-response curves of GABA, measured by the varied pressure duration method, beta-alanine competitively inhibited the I-in caused by GABA. According to the effects of pentobarbital on the dose-response curves of GABA, this drug noncompetitively inhibited the I-in using the varied pressure duration method, and partly competitively and partly noncompetitively using the Y-tube method. The effects of 5-aminopentanoic acid on the dose-response curves of GABA indicated that this drug noncompetitively inhibited the I-in using the varied pressure duration method, and partly noncompetitively and partly uncompetitively using the Y-tube method. 6. The pharmacologic features of the Achatina muscimol II type GABA receptors were similar to those of mammalian GABA(c) (GABA(p1)) receptors, except for the effects of pentobarbital. Copyright (C) 1997 Elsevier Science Inc.