HOLLIS, D.H.*; BOYD, S.K.: Characterization and distribution of [3H]muscimol binding to a GABA-A-like receptor in the bullfrog brain.

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) has multiple receptor subtypes. The GABA_A receptor is widespread in the mammalian brain, but little is known about GABA receptors in the amphibian brain. Binding of the GABA_A receptor agonist, [³H]muscimol, to membrane preparations from the brain of the bullfrog, Rana catesbeiana, was investigated in kinetic and competition experiments. Binding of 20nM [³H]muscimol to membranes was specific and could be displaced by 10uM GABA. Association binding curves showed that equilibrium occurred rapidly, within 10 minutes, and 76% dissociation occurred within 10 minutes as well. Binding of [³H]muscimol could be inhibited by GABA (IC₅₀ around 100nM) and by the GABA_A receptor antagonist, bicuculline (IC₅₀ around 10uM). In situ autoradiography with [³H]muscimol showed putative GABA_A receptors broadly distributed in the bullfrog brain. Specific binding of [³H]muscimol was especially dense in the olfactory bulb, striatum, and hypothalamus. In the mammalian brain, muscimol is specific for the GABA_A subtype of receptor. The affinity of the bullfrog binding sites for muscimol, GABA, and bicuculline supports the hypothesis that the bullfrog brain contains a GABA_A receptor-like protein. Supported by NSF# IBN95-14305.