Hormonally mediated modifications of neuronal structure, synaptic connectivity, and behavior during metamorphosis of the tobacco hornworm, Manduca sexta

Abstract

During insect metamorphosis, each life stage (larva, pupa, adult) exhibits a largely distinct behavioral repertoire. This turnover in behaviors is accompanied by extensive remodelling of the nervous system, which is controlled hormonally. A fundamental question raised by metamorphosis is how hormonal effects on the structure and function of individual neurons ultimately produce changes in behavior. We have examined this question in the tobacco hornworm moth, Manduca sexta. During the larval-pupal transformation of Manduca, the motoneurons innervating muscles of the abdominal prolegs (larval locomotory appendages) undergo an extensive dendritic regression that is triggered by ecdysteroid hormones. Over the same time period, behaviors involving the prolegs weaken and disappear. In this review we describe experiments examining the relationship between hormonally mediated proleg motoneuron regression, and the elimination of the proleg behaviors. In the case of a simple monosynaptic sensory-to-motoneuron reflex, and a peptide-activated rhythmic ecdysis motor pattern, we suggest that the ecdysteroid-mediated regression of the motoneuron arbors contributes to the developmental loss of the behaviors. In a third example, involving the activity of another class of motoneurons during peptide-activated pre-ecdysis motor patterns, the behavioral changes apparently originate at the interneuronal rather than motoneuronal level. We conclude that the hormonally mediated dendritic regression of neurons, and resultant modifications of their synaptic connections, is one of the mechanisms that contributes to the elimination of outmodedbehaviors during metamorphosis. © 1989 by the American Society of Zoologists.