A single pair of interneurons controls motor neuron activity during pre-ecdysis compression behavior in larval Manduca sexta.

Manduca sexta molts several times as a larva (caterpillar) before becoming a pupa and then an adult moth. Each molt culminates in ecdysis behavior, during which the old cuticle is shed. Prior to each larval ecdysis, the old cuticle is loosened by pre-ecdysis behavior, which includes rhythmic, synchronous compressions of the abdomen. A previous study indicated that motor neuron activity during pre-ecdysis compression behavior is driven by an ascending neural pathway from the terminal abdominal ganglion. The present study describes a pair of interneurons, designated IN-402, that are located in the terminal ganglion and belong to the ascending pathway. Each IN-402 is synchronously active with pre-ecdysis compression motor bursts, and bilaterally excites compression motor neurons throughout the abdominal nerve cord via apparently monosynaptic connections. The pair of IN-402s appears to be the sole source of rhythmic synaptic drive to the motor neurons during the pre-ecdysis compression motor pattern. These interneurons play a key role in the production of larval pre-ecdysis behavior, and are candidates for contributing to the developmental weakening of pre-ecdysis behavior at pupation.