Inhibitory postsynaptic currents and the effects of GABA on visually identified sacral parasympathetic preganglionic...

1. The actions of gamma-aminobutyric acid (GABA) on sacral parasympathetic preganglionic (SPP) neurons were examined in slice preparations using the whole cell patch-clamp technique. 2. Inhibitory postsynaptic currents (IPSCs), which were evoked by focal electrical stimulation, were recorded from SPP neurons in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a glutamate receptor antagonist. The IPSCs were substantially reduced by strychnine (1 microM), a glycine receptor antagonist. The remaining IPSCs were completely blocked by bicuculline (20 microM), a GABAA receptor antagonist. The mean peak amplitude of bicuculline-sensitive, GABAergic currents recorded at -60 mV was 53.6 +/- 10.9%, mean +/- SD (n = 8), of that of the total IPSCs. The GABAergic currents were reversed in polarity at about -30 mV, near the Cl- equilibrium potential. 3. GABA (5-50 microM) induced inward currents in SPP neurons with symmetrical internal and external Cl- concentrations. This response was completely blocked by 100 microM bicuculline. Muscimol (2-8 microM), a GABAA agonist, mimicked the GABA-induced responses, whereas a GABAB receptor agonist, baclofen (20-200 microM), produced responses in only a few cells. The GABA-induced currents reversed their polarity at approximately 0 mV, near the Cl- equilibrium potential. When the internal Cl- concentration was reduced, the reversal potential was shifted according to the Nernst equation for Cl-. 4. GABA-induced currents exhibited an outward "hump" between -35 and 15 mV. This voltage range coincided with that at which a depolarization-induced inward whole cell current was elicited.(ABSTRACT TRUNCATED AT 250 WORDS)