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 (GABAA) and GABAC receptors in the GABA-induced biphasic response in neurons of the rat major pelvic ganglia (MPG) were examined in vitro. Application of GABA (100 microM) to MPG neurons produced a biphasic response, an initial depolarization (GABAd) followed by a hyperpolarization (GABAh). The input resistance of the MPG neurons was decreased during the GABAd, whereas it was increased during the GABAh. The GABAd could be further separated into the early component (early GABAd) with a duration of 27 +/- 5 s (mean +/- SE; n = 11) and the late component (late GABAd) with a duration of 109 +/- 11 s (n = 11). The duration of the GABAh was 516 +/- 64 s (n = 11). The effects of GABA (5-500 microM) in producing the depolarization and the hyperpolarization were concentration-dependent. GABA (5-30 microM) induced only late depolarizations. The early component of the depolarization appeared when the concentration of GABA was >50 microM. Muscimol produced only early depolarizing responses. Baclofen (100 microM) had no effect on the membrane potential and input resistance of MPG neurons. Bicuculline (60 microM) blocked the early GABAd but not the late GABAd and the GABAh. Application of picrotoxin (100 microM) with bicuculline (60 microM) blocked both the late GABAd and the GABAh. CGP55845A (3 microM), a selective GABAB 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 GABAC 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 GABAd and GABAh obtained in the presence of bicuculline. Imidazole-4-acetic acid (I4AA, 100 microM), a GABAC receptor antagonist, depressed the late GABAd and the GABAh but not the early GABAd. I4AA (100 microM) and picrotoxin (100 microM) also suppressed the biphasic response to CACA. The early GABAd and the late GABAd 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 GABAh was -34 +/- 2 mV (n = 4) in the Krebs solution. The reversal potentials of the late GABAd and the GABAh 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 GABAh are mediated predominantly by bicuculline-insensitive, picrotoxin-sensitive GABA receptors, GABAC (or GABAAOr) receptors, in neurons of the rat MPG.