The pentapeptide proctolin has multiple effects on the locust oviposition digging system. At the neuromuscular junction of the ventral opener muscle, it has a concentration-dependent range of modulatory effects. At low concentrations (10–10 mol l-1), proctolin causes an increase in the frequency of miniature excitatory junctional potentials, but has no apparent effects on the muscle membrane or contractile properties. In the middle range of concentrations (approximately 10–9 mol l- 1) proctolin increases neurally evoked twitch tension three- to fourfold with little change in the basal tension. At high concentrations (>10–8 mol l-1), proctolin causes a large increase in basal tension, upon which is occasionally superimposed a slow (approximately 0.3–0.5 Hz) myogenic rhythm. Stimulation of the ventral opener nerve at 30 Hz for 5 min releases approximately 8 % of the proctolin store of the muscle. In vitro ganglion-muscle preparations which are expressing the oviposition digging rhythm produced in the terminal abdominal ganglion release about 25 % of the store of endogenous proctolin during 5 min of superfusion. This declines to below the level of detectability over about 20 min of superfusion. Muscle contractions decline and then cease over the same period, although the patterned neural input and muscle electromyogram responses are still present. Superfusion of 10–9 mol l-1 proctolin restores the muscle contractions to their original magnitude. Superfusion of 10–8 mol l-1 proctolin over preparations in which the oviposition digging pattern has slowed results in the frequency of the rhythm being restored to its original levels. We suggest that, rather than having a facultative modulatory role in this neuromuscular system, proctolin is required for it to function normally. Furthermore, proctolin may maintain the functional integrity of the central systems driving oviposition digging.

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