Sustained GABA-induced regulation of the L-type Ca²⁺ conductance in crustacean muscle fibers

The sustained effects of γ-aminobutyric acid (GABA) on voltage-gated conductances, and excitatory and inhibitory postsynaptic currents (EPSC and IPSC, respectively) in crayfish opener muscle fibers were analyzed using the two-electrode voltage-clamp technique. GABA (1.0 mM) was applied for 1-2 min and measurements were performed 30 min after restoring control Ringer solution. The L-type Ca²⁺ current (I(Ca)) was reduced by > 33%. The I(Ca) conductance (g(Ca)) was reduced and the activation and inactivation were slowed down by GABA. The I(Ca) regulation outlasted GABA superfusion (150 min). A small decrease (< 19%) of the Ca²⁺-activated K⁺ current (I(Kca)), due to the I(Ca) reduction, was also recorded. The leak (I(L)), the delayed-rectifier (I(K)) and the hyperpolarization-activated (I(AB)) currents were not af fected. Picrotoxin (0.5 mM) and bicuculline (0.2 mM) blocked the I(Ca) reduction. Neither the GABA(B) antagonist saclofen (1.0 mM) nor the agonist baclofen (1.0 mM) had any effect. Therefore, the I(Ca) regulation was probably mediated through GABA(A) receptors. EPSCs, but not IPSCs, were reduced (30%) for prolonged periods (> 100 min.) after GABA application. We describe a new, potentially functional, role for GABA receptors in the mediation of a sustained reduction of presynaptic and postsynaptic excitability in crustacean muscle.