Functional Consequences of Peptide Cotransmission in Arthropod Muscle

Abstract

While the widespread occurrence of peptidergic neurons has been amply demonstrated, their physiological significance, particularly with regard to co-release of peptides and conventional transmitters, remains a topic of considerable interest. The innervation of arthropod muscle by proctolinergic neurons provides favorable opportunities for analysis of cotransmitter actions and their physiological consequences. Three uniquely identified neurons are described, each of which releases the neuropeptide proctolin (Arg-Tyr- Leu-Pro-Thr) as a cotransmitter at a well-defined muscle target. Activity in the neurosecretory LW neuron increases the force and frequency of insect heartbeat in a manner similar to that obtained by bath application of the peptide. This neuron may release peptide hormones together with multiple cardioacceleratory peptides at the heart to achieve, through increased blood flow, rapid changes in hormonal state. Two motoneurons co-release proctolin with a conventional neurotransmitter to produce both direct and joint actions on muscle targets. The cockroach Ds motoneuron induces slow, sustained tension in the coxal depressor muscle in the absence of depolarization, and slows the relaxation of fast twitch events caused by L-glutamate. In the crayfish tonic flexor muscle, proctolin release from the f 1 motoneuron amplifies depolarization-dependent tension, but does not affect the time course of relaxation. Proctolinergic cotransmission may be an adaptation for amplification of synaptic input and/or maintenance of long-term tension, both of which increase the efficiency of muscle activation by motoneurons.