Wandering Behavior in Manduca Sexta: Investigating Steroid Hormone Effects on Neural Circuits For Locomotor Behavior

Abstract

Steroid hormones alter the excitability of neural circuits for motor behavior in vertebrates and invertebrates. The insect Manduca sexta, with its well-characterized developmental and endocrinological history, is a useful model system to study these effects. The wandering behavior is a stage-specific locomotor behavior triggered by the steroid hormone 20-hydroxyecdysone (20E) and consists of crawling and burrowing movements as the animal searches for a pupation site.The results of this dissertation show that 20E acts on the isolated larval nervous system to induce wandering activity. The mechanisms underlying the generation of this activity share features similar to other invertebrate systems, including the presence of segmental central pattern generating circuits. The time course for the nervous system response to 20E is long, suggestive of a genomic mechanism of action, and there are no earlier rapid effects of 20E on the intrinsic membrane properties of the abdominal motoneurons. The site of 20E action in inducing wandering locomotion is unlikely to be the abdominal motoneurons, but interneurons presynaptic to these motoneurons. One possible site of 20E action is the brain, which shows stage-dependent expression of ecdysteroid receptors in certain populations of neurons.Descending regulation by the brain and subesophageal ganglion (SEG) is exerted over the segmental motor circuits for crawling and burrowing and reflects stage-dependent differences. Prior to wandering, the brain exerts inhibition over the segmental motor circuits for crawling, but this inhibition is not present during wandering. Removal of the brain, SEG, and thoracic ganglia during on-going fictive locomotion alters the phase relationships between abdominal segments. Further alterations of fictive crawling motor output are observed in more reduced preparations, indicating the importance of intact connections between abdominal ganglia in the production of a reliable motor program. The SEG drives the fictive burrowing motor program. The burrowing motor program is more robustly expressed in nerve cords from wandering larvae, suggesting a stage-dependent difference due to 20E exposure. Subsequent future experiments will use electrophysiological methods and genetic manipulations in Manduca sexta and Drosophila melanogaster, respectively, to explore target sites for hormone action in the brain and the characterization of brain neurons that drive wandering behavior.