INTRINSIC PROPERTIES OF LARVAL DROSOPHILA MOTONEURONS AND THEIR CONTRIBUTION TO MOTONEURON RECRUITMENT AND FIRING BEHAVIOR DURING FICTIVE LOCOMOTION

Abstract

Locomotion is controlled in large part by neural circuits (CPGs) that generate rhythmic stereotyped outputs in the absence of descending or sensory inputs. The output of a neural circuit is determined by the configuration of the circuit, synapse properties, and the intrinsic properties of component neurons. In order to understand how a neural circuit functions component neurons, their connections, and their intrinsic properties must be characterized. Motoneurons are a useful cell in which to begin investigation of CPG function because they are accessible and provide a measure of the cumulative activity of the circuit. Drosophila is a potentially useful model system for the study of motoneuron intrinsic properties, their contribution to locomotion, and of locomotor CPGs because the genetic and molecular techniques available in Drosophila are surpassed in no other organism and because the Drosophila nervous system is small in comparison to vertebrate nervous systems. Further, whole-cell in situ patch clamp recordings from adult and larval motoneurons in relatively intact preparations are possible. Therefore, the first goal of this work was to investigate whether the firing behavior and recruitment of identified Drosophila 1b and 1s motoneurons is analogous to the recruitment of high-threshold, phasic and low-threshold, tonic motoneurons in other organisms. The second goal was to determine whether active conductances influence motoneuron recruitment in response to synaptic input. The final aim was to investigate how these factors influence CPG output to muscles. Findings from current clamp studies indicate that1b motoneurons are more easily recruited than 1s motoneurons, in agreement with the hypothesis that 1b motoneurons are analogous to low-threshold motoneurons described in other organisms. Further, orderly recruitment of Drosophila 1b motoneurons before 1s motoneurons is not a result of passive properties. Instead, the Shal channel that encodes a large portion of IA in motoneuron somatodendritic regions is a critical determinant of delay-to-spike in larval Drosophila motoneurons. These findings are behaviorally-relevant because the same recruitment order is seen in simultaneous recordings from motoneuron pairs recruited by synaptic input.