Neuromuscular mechanisms in the insect foregut

Bioelectric potentials were recorded intracellularly from the foregut muscle fibres of the larva of the waxmoth Gelleria mellonella. Myogenic spikes were generated spontaneously or by direct stimulation of the fibres of the crop, anterior and posterior proventriculus. The action potentials from these three regions were distinguishable from each other by their characteristic shapes and duration. Pacemaker potentials were recorded from fibres of the crop and the posterior proventriculus but not recorded from the anterior proventriculus, where only follower type potential changes could be observed. Spontaneous and evoked EJPs were obtained from the crop and the posterior proventriculus but were not recorded from the anterior proventriculus. The reversal potential for evoked EJPs was estimated at about 0 mV by a method of extrapolation. Chemical sensitivity of evoked EJPs was examined. Seven putative transmitters including acetylcholine, L‐glutamic acid, L‐aspertic acid, norepinephrine, octopamine, dopamine, and 5‐hydroxytryptamine were tested. It was found that L‐glutamic acid depressed EJP at about 10⁻⁴ M. Specific agonists and antagonists were also administered on EJPs. Effects of atropine, phentolamine, tyramine, and BOL‐148 were negligible and eserine and propranolol blocked transmission at 5 × 10⁻⁴ M. D‐tubocurarine (5 × 10⁻⁴ M) enhanced the EJP, with occasional exceptions. The actions of the last three drugs were, however, inconsistent, and were considered to be nonspecific. In contrast, kainic acid had consistent effects; potentiated EJPs at 2.5‐,5 × 10⁻⁵ M and blocked at 1 × 10⁻³ M. From these observations it was suspected that among seven compounds tested, L‐glutamic acid is the most probable candidate for a transmitter. The firing rhythm of myogenic spikes generating spontaneously could be reset by neurally evoked spikes.