Developmental attenuation of Manduca pre-ecdysis behavior involves neural changes upstream of motoneurons and relay interneurons

Each molt in the hawkmoth, Manduca sexta, culminates in the shedding of the old cuticle at ecdysis. Prior to each larval ecdysis, the old cuticle is loosened by pre-ecdysis behavior, which includes rhythmic, synchronous compressions in all abdominal segments. Prior to ecdysis to the pupal stage, pre-ecdysis behavior and its underlying motor pattern are markedly attenuated. A single pair of interneurons located in the terminal abdominal ganglion, the IN-402s, drives compression motoneuron activity during the pre-ecdysis motor pattern via monosynaptic excitatory connections. The present study tested the hypotheses that (1) changes in intrinsic properties (resting membrane potential, spike threshold, input resistance and excitability) of compression motoneurons, or (2) changes in the strength of synaptic connections from IN-402s to compression motoneurons, underlie the developmental attenuation of the pre-ecdysis motor pattern. Membrane potential was slightly more hyperpolarized in prepupal as compared to larval motoneurons, but no other findings supported the tested hypotheses. These results suggest that developmental weakening of the pre-ecdysis motor pattern results from changes upstream of the compression motoneurons and their synaptic connections from IN-402s. Accepted: 29 September 1999