Effects of maintained depolarization of presynaptic neurons on inhibitory transmission in lobster neuropil

Abstract

1. Intracellular microeleotrode recordings were obtained from somata of the pre- and postsynaptic neurons of each of four neuron pairs in the stomatogastric ganglion ofPanulirus argus. The microelectrodes were incorporated into a bridge circuit, permitting simultaneous recording and current passing. 2. The following cell pairs were investigated: I. Pyloric Dilator—Pyloric Neuron (PD-PY); II. Anterior Median—Gastric Neuron (AM—GM); III. Large BPSP—Lateral Cardiac or Posterior Gastro-pyloric (EX—LC/GP; IV. Large EPSP—Gastric Mill (EX—GM). The postsynaptic effects of current-induced variations in presynaptic membrane potential were studied in each neuron pair under a variety of experimental conditions. 2. In two cell pairs, the PD—PY and the AM—GM, action potentials initiated antidromically in the presynaptic element did not evoke postsynaptic potentials. If the postsynaptic neuron was simultaneously depolarized through the soma electrode postsynaptic potentials were observed. It is suggested that in the normal functioning of the ganglion local presynaptic depolarizations as well as spikes contribute to transmitter release. 2. In the EX—GM and the EX—LC/GM cell pairs, presynaptic depolarization that did not initiate action potentials nevertheless evoked postsynaptic inhibition and hyperpolarization for as long as the presynaptic current was maintained. Control experiments showed that presynaptic hyperpolarization had no postsynaptic effect. 2. The hyperpolarizing effect of presynaptic depolarization could be blocked by picrotoxin in parallel with blockade of other IPSPs in the ganglion. 2. The experimentally induced voltage changes in the presynaptic terminals are probably comparable to those resulting from synaptic input to the neuron. These experiments support the hypothesis that, in life, depolarization of presynaptic terminals that are subthreshold for action potentials may facilitate or evoke transmitter release. Such presynaptic, nonspike potentials may play a significant role in the modulation of synaptic transmission in neuropil.