An ionotropic GABA receptor in cultured mushroom body Kenyon cells of the honeybee and its modulation by intracellular calcium

GABAergic inhibitory transmission is very abundant within the insect brain. We, therefore, studied the functional properties of the ionotropic GABA receptor of honeybee mushroom body Kenyon cells in vitro. GABA applications elicit rapidly activating and desensitizing currents, which are concentration-dependent between 10 and 500 mu M. The mean peak amplitude induced by 500 mu M GABA at a holding potential of -110 mV is -1.55 +/- 0.23 nA (SEM, n = 29). The GABA-induced current is mediated by Cl- ions because (1) the reversal potential of the GABA-induced current of -40.6 mV is very close to the calculated Nernst potential of chloride (-44.8 mV). (2) With equimolar chloride concentrations the reversal potential shifted to about 0 mV. GABA or muscimol are equally efficient channel agonists, whereas CACA is a partial agonist. Picrotoxin or philanthotoxin (100 mu M) completely and reversibly block the GABA-induced current, bicuculline (100 mu M) has no effect. Elevating the intracellular Ca2+ concentration increases the GABA current amplitude. This modualtory effect is blocked by the kinase blocker K 252a, but not by blockers of CaMkinaseII (KN-93), PKC (bisindolylmaleimide) or PKA (KT 5720). We conclude that Kenyon cells express functional GABA receptors whose properties support an inhibitory role of GABAergic transmission.