Abstract
Previous investigations of locust antennal muscles suggested the involvement of a peptidergic cotransmitter in the excitation-contraction coupling. The presence and action of proctolin in the antennal motor system were therefore investigated with physiological and immunohistochemical techniques. In experiments with the scape muscles, application of synthetic proctolin enhanced electrically-induced single contractions as well as tetanic contractures. In the presence of proctolin, electrical nerve stimulation additionally evoked an underlying slow contraction with a refractory period of several seconds. Neither the membrane potential of scape muscle fibres nor the amplitude of intracellularly recorded junction potentials were altered by proctolin indicating a predominantly postsynaptic action of the pentapeptide. Even in the absence of motoneuronal activity, proctolin was found to induce concentration-dependent increases in muscle tension of the scape and the tentorio-scapal muscles. Proctolin-like immunoreactivity was found to be present on all four antennal muscles and in the nerves containing antennal motoneurones. Up to 12 proctolin-like immunoreactive somata were found on each side of the deutocerebrum within the same region where the somata of antennal motoneurones are located. These results suggest that proctolin is an endogenous modulator within the locust antennal motor system.
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