Neuronal Excitatory Action of GABA on the Pelvic Ganglia

OBJECTIVE: In the central nervous system, gamma-aminobutyric acid (GABA) is well known to act as an inhibitory neurotransmitter by hyperpolarizing postsynaptic neurons through gating GABA-activated Cl- channels. To date, however, the functional roles of GABA remain unclear in the autonomic nervous system. In the present study, we characterize GABA-activated Cl- currents in the neurons of major pelvic ganglia (MPG). METHODS: MPG neurons, located on the lateral surfaces of the prostate gland, from male rats were enzymatically dissociated. Ionic currents were recorded using whole-cell variant patch-clamp technique. Membrane potential was recorded under current clamp mode. Current traces were filterd at 2kHz by using 4-pole Bassel filter in the amplifier. RESULTS: Application of GABA (100micrometer) induced inward currents in the neurons, with holding potentials being maintained below the Cl- equilibrium potential (ECl). The GABA response was concentration-dependent and its reversal potential was close to the theoretical ECl. The GABA-induced Cl- currents were largely blocked by bicuculline (10micrometer, n=5), a GABAA receptor antagonist, but were not affected by 9-AC and niflumic acid, chloride channel blockers. GABA also produced significant membrane depolarization (19mV, n=28). As in the case of the Cl- currents, the GABA-induced depolarizations were largely blocked by bicuculline(10micrometer, n=6), but not by DIDS(50micrometer, n=4), another chloride channel blocker. CONCLUSION: The data suggest that GABAergic roles may be due to it's activation of excitatory GABAA receptors, which are expressed in MPG neurons.

Identification of ATP-sensitive K+ Conductances in Male Rat Major Pelvic Ganglion Neurons

Major pelvic ganglia (MPG) neurons are classified into sympathetic and parasympathetic neurons according to the electrophysiological properties; membrane capacitance (Cm), expression of T-type Ca2+ channels, and the firing patterns during depolarization. In the present study, function and molecular expression of ATP-sensitive K+ (K(ATP)) channels was investigated in MPG neurons of male rats. Only in parasympathetic MPG neurons showing phasic firing patterns, hyperpolarizing changes were elicited by the application of diazoxide, an activator of K(ATP) channels. Glibenclamide (10microM), a K(ATP) channel blocker, completely abolished the diazoxide-induced hyperpolarization. Diazoxide increased inward currents at high K+ (90 mM) external solution, which was also blocked by glibenclamide. The metabolic inhibition by the treatment with mitochondrial respiratory chain inhibitors (rotenone and antimycin) hyperpolarized the resting membrane potential of parasympathetic neurons, which was not observed in sympathetic neurons. The hyperpolarizing response to metabolic inhibition was partially blocked by glibenclamide. RT-PCR analysis revealed that MPG neurons mainly expressed the K(ATP) channel subunits of Kir6.2 and SUR1. Our results suggest that MPG neurons have K(ATP) channels, mainly formed by Kir6.2 and SUR1, with phenotype-specificity, and that the conductance through this channel in parasympathetic neurons may contribute to the changes in excitability during hypoxia and/or metabolic inhibition.