Functional Consequences of Peptide Cotransmission in Arthropod Muscle

While the widespread occurrence of peptidergic neurons has beenamply demonstrated, their physiological significance, particularlywith regard to co-release of peptides and conventional transmitters,remains a topic of considerable interest. The innervation ofarthropod muscle by proctolinergic neurons provides favorableopportunities for analysis of cotransmitter actions and theirphysiological consequences. Three uniquely identified neuronsare described, each of which releases the neuropeptide proctolin(Arg-Tyr- Leu-Pro-Thr) as a cotransmitter at a well-definedmuscle target. Activity in the neurosecretory LW neuron increasesthe force and frequency of insect heartbeat in a manner similarto that obtained by bath application of the peptide. This neuronmay release peptide hormones together with multiple cardioacceleratorypeptides at the heart to achieve, through increased blood flow,rapid changes in hormonal state. Two motoneurons co-releaseproctolin with a conventional neurotransmitter to produce bothdirect and joint actions on muscle targets. The cockroach Dsmotoneuron induces slow, sustained tension in the coxal depressormuscle in the absence of depolarization, and slows the relaxationof fast twitch events caused by L-glutamate. In the crayfishtonic flexor muscle, proctolin release from the f 1 motoneuronamplifies depolarization-dependent tension, but does not affectthe time course of relaxation. Proctolinergic cotransmissionmay be an adaptation for amplification of synaptic input and/ormaintenance of long-term tension, both of which increase theefficiency of muscle activation by motoneurons.