INTERCIRCUIT CONTROL OF MOTOR PATTERN MODULATION BY PRESYNAPTIC INHIBITION

Rhythmically active neural networks can control the modulatory input t hat they receive via their synaptic effects onto modulatory neurons. T his synaptic control of network modulation can occur presynaptically, at the axon terminals of the modulatory neuron. For example, in the cr ab stomatogastric ganglion (STG), a gastric mill network neuron presyn aptically inhibits transmitter release from a modulatory projection ne uron called modulatory commissural neuron 1. We showed previously that the gastric mill rhythm-timed presynaptic inhibition of the STG termi nals of MCN1 is pivotal for enabling MCN1 to activate this rhythm. We also showed that MCN1 excites the pyloric rhythm within the STG. Here we show that, because MCN1 stimulation conjointly excites the gastric mill and pyloric rhythms, the gastric mill rhythm-timed presynaptic in hibition of MCN1 causes a rhythmic interruption in the MCN1-mediated e xcitation of the pyloric rhythm. Consequently, during each protraction phase of the gastric mill rhythm, presynaptic inhibition suppresses M CN1 excitation of the pyloric rhythm, thereby weakening the pyloric rh ythm. During the retraction phase, presynaptic inhibition is absent an d MCN1 elicits a faster, stronger, and modified pyloric rhythm. Thus, in addition to its role in enabling a neural circuit to regulate the m odulatory transmission that it receives, presynaptic inhibition is als o used effectively to rhythmically control the activity level of a dis tinct, but behaviorally related, neural circuit.