Fast excitatory postsynaptic currents in neurons of the rabbit pelvic plexus

Fast excitatory postsynaptic currents have been recorded at 23-27 degrees C from rabbit pelvic plexus neurons by a two-electrode voltage-clamp technique. The synaptic current decay was bi-exponential with the fast and slow components characterized at -50 mV by mean time constants of 4.0 +/- 0.3 and 21.9 +/- 2.8 ms (n = 11), respectively. Both components contributed to the synaptic current approximately equally and reversed at -5 mV. Hexamethonium (10 microM) decreased the amplitude and decay time constant of both synaptic current components; this effect increased with hyperpolarization and is consistent with a channel-blocking action. At - 50 mV, mean rate constants of hexamethonium association with open ion channels of nicotinic acetylcholine receptors presumably mediating the fast and slow synaptic current components were (18.4 +/- 2.3) x 10(6) and (6.1 +/- 1.2) x 10(6) M(-1) s(-1) (n = 4), respectively. These data suggest that the fast excitatory postsynaptic current in rabbit pelvic plexus neurons is probably mediated by at least two different subtypes of nicotinic acetylcholine receptors. Hexamethonium blocks open ion channels of both subtypes with efficiency allowing to exclude an appreciable presence of homomeric alpha7 nicotinic acetylcholine receptors on the subsynaptic membrane.