Current responses mediated by endogenous GABA_B and GABA_C receptors in Xenopus oocytes

The properties of GABA-activated current in Xenopus oocytes and its underlying mechanism were studied using the two-electrode voltage-clamp technique. External application of GABA (10^-10∼10^-3 mol/L) induced a concentration-dependent outward current in a proportion of oocytes (35.5%, 55/155). Selective GABA_A receptor antagonist bicuculline (10^-5 mol/L) did not block the GABA-activated current (n = 6). However, 2-hydroxysaclofen (10^-4 mol/L), a GABA_B receptor antagonist, reversed the GABA-activated outward current to an inward current (n = 9), which was abolished completely by application of I4AA (10^-5 mol/L), a GABA_C receptor selective antagonist (n = 6). In 20% (12/60) of oocytes, application of baclofen (10^-10∼10^-4 mol/L), a GABA_B receptor agonist, also induced a concentration-dependent outward current 2-Hydroxysaclofen at the concentrations of 3 × 10^-6, 3 × 10^-5 and 3 × 10^-4 mol/L blocked the baclofen (10^-5 mol/L)-activated outward current by (6.3 ± 3.2)%, (44.1 ± 2.2)%, and (86.0 ± 1.6)%, respectively (n = 6). The reversal potential for baclofen-activated current was around -96.8 ± 7.2 mV (n = 6), and the baclofen-activated current could be blocked by TEA (n = 5) and Ba²⁺ (n = 5). These results suggest that there exist endogenous GABA receptors, GABA_B receptors mediating a slow and sustained outward current and GABA_C receptors mediating an inward current in follicular Xenopus oocytes.