Evidence of slow IPSP in mammalian prevertebral ganglia.

A short train of nerve stimulation evoked in a portion of neurons of the guinea pig inferior mesenteric ganglia a slow hyperpolarization (slow IPSP) which persisted when the preceding orthodromic spikes were reduced to subthreshold EPSPs by curare, was reversibly abolished in a low Ca solution, and was not blocked by atropine; furthermore, the post-tetanic hyperpolarization induced by direct intracellular stimulation was generally smaller than the slow IPSP evoked by nerve stimulation. The slow IPSP was often associated with a fall in membrane resistance and its amplitude decreased with membrane hyperpolarization. The results indicate that the slow hyperpolarization is a synaptic potential, and differs from the slow IPSP observed in mammalian and amphibian paravertebral ganglionic neurons in that it is atropine insensitive.