Fictive locomotion induced by octopamine in the earthworm

Abstract

SUMMARY We investigated the function of octopamine (OA) as a motor pattern inducer in the earthworm Eisenia fetida. We used semi-intact preparations, consisting of 20 sequential segmental ganglia of the ventral nerve cord (VNC) together with the body wall from the middle of the animal. Bath-application of 10–4 mol l–1 OA to the semi-intact preparation induced phasic muscle contractions, which are consistent with crawling. In the isolated VNC, OA induced bursts of motor neuron activity in the first lateral nerves. Burst frequency increased with OA concentration, with a large increase in activity observed in the range 10–6–10–4 mol l–1. At 10–4 mol l–1, the coefficient of variation of burst periods (BPs) was smaller than that seen upon application of OA at other concentrations, which is indicative of rhythmic bursts. These rhythmic bursts propagated along the VNC from the anterior to posterior, with a propagation velocity ranging from 60 to 110 mm s–1. This velocity is consistent with the propagation velocity of muscle contraction during crawling behavior in the intact earthworm. From these results, we conclude that fictive crawling motor patterns are observed at 10–4 mol l–1 OA, and that OA can induce rhythmic bursts in the isolated VNC of the earthworm.