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.