A SWITCH BETWEEN 2 MODES OF SYNAPTIC TRANSMISSION MEDIATED BY PRESYNAPTIC INHIBITION

PRESYNAPTIC inhibition reduces chemical synaptic transmission in the c entral nervous system between pairs of neurons(1-4), but its role(s) i n shaping the multisynaptic interactions underlying neural network act ivity are not well studied. We therefore used the crustacean stomatoga stric nervous system to study how presynaptic inhibition of the identi fied projection neuron, modulatory commissural neuron 1 (MCN1), influe nces the MCN1 synaptic effects on the gastric mill neural network. Ton ic MCN1 discharge excites gastric mill network neurons and activates t he gastric mill rhythm(5,6). One network neuron, the lateral gastric ( LG) neuron, presynaptically inhibits MCN1 and is electrically coupled to its terminals(5,6). We show here that this presynaptic inhibition s electively reduces or eliminates transmitter-mediated excitation from MCN1 without reducing its electrically mediated excitatory effects, th ereby switching the network neurons excited by MCN1. By switching the type of synaptic output from MCN1 and, hence, the activated network ne urons, this presynaptic inhibition is pivotal to motor pattern generat ion.