Presynaptic modulation by octopamine at a single neuromuscular junction in the mealworm (Tenebrio molitor)

The effect of octopamine on the neuromuscular junction of the mealworm (Tenebrio molitor) was examined. Octopamine potentiated excitatory junctional potentials (EJPs) recorded intracellularly and extracellularly from ventral longitudinal muscle fibers. The potentiating action of octopamine was blocked in the presence of the alpha-adrenergic blocking agent, phentolamine, but not in the presence of another alpha-blocker, phenoxybenzamine, or the beta-blockers propranolol and dichloroisoproterenol. The resting membrane potential, membrane input resistance, reversal potential of EJP, glutamate potentials, and spontaneous miniature EJPs were found to be unaffected by octopamine. In contrast, quantal contents estimated by the extracellularly recorded EJP failures were greatly increased by octopamine. These results suggest that octopamine acted on the presynaptic terminals via alpha-adrenoceptor-like receptors (octopamine receptors) at the Tenebrio neuromuscular junctions to enhance the transmitter release associated with the motor nerve impulses.