Investigating Postsynaptic Effects of a Drosophila Neuropeptide on Muscle Contraction

Abstract

Neuropeptides play key physiological roles by acting as transmitters or hormones that modulate chemical synapses, but distinguishing presynaptic and postsynaptic effects can be challenging. The Drosophila neuropeptide, DPKQDFMRFamide, was previously shown to enhance excitatory junctional potentials (EJPs) and muscle contraction by presynaptic and postsynaptic actions. Here, postsynaptic actions of the peptide were investigated by examining how it modulates effects of the excitatory neurotransmitter L-glutamate on muscles of 3rd instar larvae after removing the central nervous system. DPKQDFMRFamide enhanced glutamate-evoked contractions with at low concentrations (EC50 1.3 nM), consistent with its role as a neurohormone, and the combined effect of both substances was more than additive. Glutamate-evoked contractions were also enhanced when transmitter release was blocked in temperature-sensitive mutants. The peptide increased membrane depolarization in muscle cells when co-applied with glutamate, and its effects were blocked by nifedipine, an L-type channel blocker, indicating actions at the plasma membrane and involving calcium influx. DPKQDFMRFamide also enhanced contractions induced by caffeine in the absence of extracellular calcium, suggesting increased calcium release from the sarcoplasmic reticulum (SR) or effects downstream of calcium release from the SR. The peptide’s effects do not appear to involve calcium/calmodulin-dependent protein kinase II (CaMKII), previously shown to mediate presynaptic effects. The approach used here might be useful for examining postsynaptic effects of neurohormones and cotransmitters in other systems.