The effect of a transient outward current (IA) on synaptic potentials in sympathetic ganglion cells of the guinea-pig.

The responses to stimulation of preganglionic fibres have been studied in sympathetic neurones in ganglia of the caudal lumbar sympathetic chain (l.s.c.) and in the distal lobes of inferior mesenteric ganglia (i.m.g.) isolated from guinea-pigs. Most l.s.c. neurones were classified as 'phasic' and i.m.g. neurones as 'tonic' (see Cassell, Clark & McLachlan, 1986). The types of preganglionic inputs received by l.s.c. and i.m.g. neurones differed: l.s.c. cells almost invariably received at least one suprathreshold ('strong') input, in addition to several subthreshold ones; i.m.g. neurones more commonly received only subthreshold inputs via the lumbar splanchnic nerves. Prolonged discharges were evoked in some i.m.g. cells by stimulation of lumbar splanchnic nerves at strengths just supramaximal for the conventional fast synaptic responses. These appeared to arise from repetitive discharges evoked in other neurones intrinsic to the i.m.g. The time constants of decay of subthreshold synaptic currents recorded under voltage clamp in l.s.c. neurones (4.9 +/- 0.2 ms) were significantly shorter on average than those recorded in tonic i.m.g. cells (7.1 +/- 0.3 ms), although the values of time constant for the two populations overlapped. In phasic neurones, excitatory synaptic potentials (e.s.p.s) evoked at resting membrane potential by stimulation of preganglionic axons decayed with the same exponential time course as an electrotonic potential. In tonic neurones, the time course of decay of the e.s.p. was briefer, but always followed an exponential with the same time constant as the cell input time constant over the final part of the response. If tonic neurones were hyperpolarized by the passage of current through the recording micro-electrode, the time course of decay of the e.s.p. was prolonged and became the same as that of the electrotonic potential. The shape of e.s.p.s in phasic and tonic neurons could be mimicked in a computer model of the neurones incorporating the different activation/inactivation characteristics of the A current (IA) (Cassell et al. 1986) for each neurone type. It is concluded that, in addition to the contribution of IA to the rhythmic firing properties of tonic sympathetic neurones, this current also markedly inhibits the effects of excitatory synaptic conductance changes in this type of ganglion cell.