Investigating activity-dependence of exogenous proctolin’s ability to modulate muscle contraction in <i>Drosophila</i>

Proctolin, an arthropod neuropeptide, is present in synaptic terminals innervating body wall muscles of <i>Drosophila</i> larvae. It acts as a cotransmitter with the excitatory neurotransmitter, l-glutamate (Glu). Previous work showed that exogenous proctolin increases nerve-evoked contractions in larval muscles, and increasing neural activity decreases threshold and EC₅₀ values for the effect. We investigated the possibility that the decrease in threshold is related to an increase in Glu release as neural activity increases. We directly applied exogenous Glu to muscles to mimic Glu release in the absence of nerve activity to avoid synaptic release of proctolin and other cotransmitters. Applying Glu to larval muscles elicited contraction, and increasing Glu concentration increased contraction amplitude in a dose-dependent manner. Increasing Glu from a low to a moderate concentration decreased the threshold for exogenous proctolin to enhance contraction. At a higher Glu concentration, however, the threshold increased to the same level observed at the lower concentration. These results do not agree with the predicted effects of exogenous proctolin. Applying proctolin also increased caffeine-induced contractions, but increasing caffeine concentration did not alter the threshold for this effect. Exogenous proctolin did not alter Glu-induced depolarization. These results suggest that the activity-dependent increase in exogenous proctolin's effectiveness is not solely due to increased Glu release, increased muscle depolarization, or increased cytosolic Ca²⁺ in muscle. Exogenous proctolin and Glu both induced contraction. Their effects were supra-additive, but proctolin did not decrease the threshold for Glu to elicit contractions. Thus, the two substances do not exhibit synergism.<b>NEW & NOTEWORTHY</b> Application of proctolin, a cotransmitter, was reported to increase nerve-evoked contractions in <i>Drosophila</i>; increasing nerve activity reduced the threshold for this effect. Here, applying the excitatory transmitter, Glutamate, to postsynaptic cells at increasing concentrations does not fully mimic the effect of applied proctolin on nerve activity, nor does increasing Ca²⁺ release with caffeine. The activity-dependence might involve the release of proctolin or other cotransmitters.