BOTH GABA(A) AND GABA(B) RECEPTORS PARTICIPATE IN SUPPRESSION OF [CA2+](I) PULSING IN TOAD MELANOTROPHS

The receptor mechanisms involved in the inhibitory effect of gamma-ami nobutyric acid (GABA) in suppressing spontaneous [Ca2+](i) pulsing in melanotrophs of Xenopus laevis were investigated. The selective GABA(B ) receptor agonist, baclofen reversibly arrested [Ca2+](i) pulsing. Th is inhibition was unaffected by the selective GABA(A) receptor antagon ist, bicuculline methiodide, but was blocked by the selective GABA(B) receptor antagonist, CGP 35348 (3-aminopropyl diethyoxymethyl phosphin ic acid). The selective GABA(A) receptor agonist, muscimol, also arres ted [Ca2+](i) pulsing after causing a transient rise in [Ca2+](i). Thi s biphasic response to muscimol was unaffected by CGP 35348, but was b locked by bicuculline. The inhibitory effect of GABA was unaffected by either CGP 35348 or bicuculline when given alone, but was blocked by both antagonists given together. In cells pretreated with pertussis to xin, the response to baclofen was completely lost, whereas responses t o GABA and muscimol persisted; the response to GABA was blocked by bic uculline alone. Thus, both GABA(A) and GABA(B) receptors are involved in the inhibitory effect of GABA in suppressing spontaneous [Ca2+](i) pulsing in Xenopus melanotrophs.