Bioactive Peptides at the Neuromuscular Junction of Insects
In a symposium on neurosecretion it may seem inappropriate to discuss the physiology and pharmacology of the skeletal neuromuscular junction. It is becoming clear, however, that in both vertebrates and invertebrates active substances other than the primary neuromuscular transmitters can exert profound effects at the neuromuscular junction. Such substances may be secreted by the motoneurons themselves thereby acting as cotransmitters. They may also be secreted by specialized neuromodulatory neurons which innervate the muscle to regulate the actions of the classical motoneurons. Finally they may be humoral substances secreted at a distance and carried to the target muscle in the circulation. Thus the neuromuscular junction appears in some organisms, especially invertebrates, to be a site for the integration of various secreted chemical signals. Since it is also relatively accessible for physiological and biochemical studies, it can provide a model for understanding how multiple secreted products contribute to the control of physiological processes. In this brief essay I will describe how secreted peptides are involved in neuromuscular transmission in insects. There seem to be at least two known types of action, first as peptidergic cotransmitters and second as humoral peptides which may be released rather close to the target muscle or at a greater distance. Since more is known about peptidergic cotransmission, most of this paper will be devoted to this type of action.
KeywordsNeuromuscular Junction Neuromuscular Transmission Neurosecretory Cell Thoracic Ganglion Inhibitory Motoneuron
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