Peptidergic and Aminergic Modulation of Insect Skeletal Muscle

Insect skeletal muscles are frequently innervated by small numbers of motor neurones, all of which can be uniquely identified physiologically. They therefore present excellent model systems in which to study the basic principles of neuromuscular transmission and the modulation of these effects by biogenic amines and peptides. The extensor-tibiae muscle of the hind leg of the locust is a much studied, large muscle that is innervated by three identified motor neurones and one identified modulatory neurone. Much attention has recently been focused on the modulation of neuromuscular transmission and muscular contraction in this muscle by biogenic amines and peptides. One proximal bundle of muscle fibres in the extensor-tibiae muscle exhibits a myogenic rhythm of contraction and relaxation. The rhythm is stimulated by a variety of peptides including proctolin, the AKH-related peptides Mi and M2, and by small cardioactive peptide (SCPB). In addition, it is activated by 5-hydroxytryptamine and by one class of adenosine analogues. The rhythm is inhibited by octopamine and by a second class of adenosine analogues. The actions of these various modulatory compounds will be discussed in terms of the likely numbers of pharmacologically distinct receptors in this preparation and their modes of action. Neuromuscular transmission and muscular contraction in the extensor-tibiae muscle is modulated by the biogenic amine octopamine and by the peptides, proctolin and FMRFamide. The actions of these modulators are discussed in relation to differences in the responsiveness of various regions of the muscle, to the frequency dependence of their effects on motor neurone activity and to their modes of action. The cellular locations and mode of transmission to the muscle of some of these modulators will be considered. Octopamine and proctolin are contained within neurones which innervate the muscle, whilst FMRFamideand SCPe-like peptides appear to be released into the locust haemolymph as neurohormones.