Classification of GABA receptors in snail neurones

The present study aimed to elucidate the pharmacological features of GABA receptors on identifiable neurones of Achatina fulica Férussac by testing the effects of GABA analogues, muscimol, (+/-)-baclofen, (-)-beta-hydroxy GABA and those conformationally fixed in either the extended or folded form of carbon chain, such as trans- and cis-isomers of (+/-)-2-(aminomethyl)cyclopropane-1-carboxylic acid [+/-)-cyclo-GABA-extended and (+/-)-cyclo-GABA-folded) and trans-4-amino-crotonic acid (GABA-extended). The giant neurones used were TAN, d-LPeLN, v-VNAN, v-LCDN and RPeNLN. The minimum effective concentrations (MEC) of these compounds to produce hyper- or depolarization of the membrane potentials of the neurones were determined, and the effective potency quotient (EPQ) of each compound vis-à-vis that of GABA was calculated for each neurone. The GABA receptors in these neurones were classified into the muscimol I, muscimol II and baclofen types. The muscimol I (TAN and d-LPeLN) and muscimol II (v-VNAN and v-LCDN) receptors were respectively hyperpolarized and depolarized by GABA and muscimol but were insensitive to (+/-)-baclofen. These muscimol receptors are inferred to accept GABA in an extended form of its carbon chain, since muscimol, conformationally fixed in this form from C-1 to C-4, was quite effective. Muscimol was more potent on the muscimol II receptors (MEC: 3 X 10(-7)-3 X 10(-6) M; EPQ: 30-10) than on the muscimol I type (MEC: 3 X 10(-5)-10(-4) M; EPQ: 1-0.3).(ABSTRACT TRUNCATED AT 250 WORDS)