Role of GABAA and GABAC receptors in the biphasic GABA responses in neurons of the rat major pelvic ganglia

The role of gamma-aminobutyric acid-a (GABA(A)) and GABA(C) receptors in the GABA-induced biphasic response in neurons of the rat major pelvic ganglia (MPG) were examined in vitro. Application of GABA (100 mu M) to MPG neurons produced a biphasic response, an initial depolarization (GABA(d)) followed by a hyperpolarization (GABA(h)). The input resistance of the MPG neurons was decreased during the GABA(d), whereas it was increased during the GABA(h). The GABA(d) could be further separated into the early component (early GABA(d)) with a duration of 27 +/- 5 s (mean +/- SE; n = 11) and the late component (late GABA(d)) with a duration of 109 +/- 11 s (n = 11). The duration of the GABA(h) was 516 +/- 64 s (n = 11). The effects of GABA (5-500 mu M) in producing the depolarization and the hyperpolarization were concentration-dependent. GABA (5-30 mu M) induced only late depolarizations. The early component of the depolarization appeared when the concentration of GABA was >50 mu M. Muscimol produced only early depolarizing responses. Baclofen (100 mu M) had no effect on the membrane potential and input resistance of MPG neurons. Bicuculline (60 mu M) blocked the early GABA(d) but not the late GABA(d)and the GABA(h). Application of picrotoxin (100 mu M) with bicuculline (60 mu M) blocked both the late GABA(d) and the GABA(h). CGP55845A (3 mu M), a selective GABA(B) receptor antagonist, did not affect the GABA-induced responses, cis-4-Aminocrotonic acid (CACA, 1 mM) and trans-4-aminocrotonic acid (TACA, 1 mM), selective GABA(C) receptor agonists, produced late biphasic responses in the MPG neurons. The duration of the CACA responses was almost the same as those of the late GABA(d) and GABA(h) obtained in die presence of bicuculline. Imidazole-4-acetic acid (I4AA, 100 mu M), a GABA(h) receptor antagonist, depressed the late GABA(d) and the GABA(h) but not the early GABA(d). I4AA (100 mu M) and picrotoxin (100 mu M) also suppressed the biphasic response to CACA. The early GABA, and the late GABA, were reversed in polarity at -32 +/- 3 mV (n = 7) and -38 +/- 2 mV (n = 4), respectively, in the Krebs solution. The reversal potential of the GABA(h) was -34 +/-: 2 mV (n = 4) in the Krebs solution. The reversal potentials of the late GABA(d) and the GABA(h) shifted to -20 +/- 3 mV (n = 5) and -22 +/- 3 mV (n = 5), respectively, in 85 mM Cl- solution. These results indicate that the late GABA(d) and the GABA(h), are mediated predominantly by bicuculline-insensitive, picrotoxin-sensitive GABA receptors, GABA(C) (or GABA(AOr)) receptors, in neurons of the rat MPG.