Proctolin in the innervation of the locust mandibular closer muscle modulates contractions through the elevation of inositol trisphosphate.

Extracts of the locust (Locusta migratoria) mandibular closer muscle separated on reverse-phase HPLC and tested for bio-activity on the locust oviduct contain a bio-active substance that coelutes with authentic proctolin. Furthermore, the effect on oviduct contractions of this compound is indistinguishable from that of authentic proctolin. Antiserum to proctolin stains numerous axons with beaded endings that run along the fibres of the closer muscles and, in addition, the antiserum stains a number of cell bodies in the suboesophageal ganglion, some of which have axons in the mandibular nerve that innervates the mandibular musculature. The function of proctolin appears to be modulatory as its presence significantly increases the amplitude of neurally evoked contractions of the closer muscle. This effect can be mimicked by the addition of inositol 1,4,5-trisphosphate (IP3) to preparations in which the muscles have been permeabilized with dimethyl sulfoxide. The involvement of this second messenger is further implicated as we also show that proctolin produces a large, significant increase in the IP3 content of homogenized muscle.