Steroid hormone activation of wandering in the isolated nervous system of Manduca sexta

Steroid hormones modulate motor circuits in both vertebrates and invertebrates. The insect Manduca sexta, with its well-characterized developmental and endocrinological history, is a useful model system in which to study these effects. Wandering is a stage-specific locomotor behavior triggered by the steroid hormone 20-hydroxyecdysone (20E), consisting of crawling and burrowing movements as the animal searches for a pupation site. This study was undertaken to determine whether the wandering motor pattern is activated by direct action of 20E on the CNS. 20E acts on the isolated larval nervous system to induce a fictive motor pattern showing features of crawling and burrowing. The latency of the response to 20E is long, suggestive of a genomic mechanism of action. The abdominal motoneurons or segmental pattern generating circuits are unlikely to be the primary targets of 20E action in inducing fictive wandering. Exposure of the segmental ganglia alone to hormone did not evoke fictive wandering. Therefore, as suggested by an earlier study, the likely site of 20E action is within the brain.