[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 x 10(-6), 3 x 10(-5) and 3 x 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 Ba(2+) (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.