Activation of protein kinase C promotes glutamate-mediated transmission at the neuromuscular junction of the mealworm.

1. Actions of protein kinase C activators, 1,2-oleoylacetylglycerol (OAG) and 12-O-tetradecanoylphorbol-13-acetate (TPA), on the glutamate-mediated neuromuscular transmission in the mealworm, Tenebrio molitor, were studied by the microelectrode current-clamp and voltage-clamp techniques. 2. The activators OAG and TPA stimulate the evoked and spontaneous transmitter releases from the presynaptic terminal, as evidenced by an increase in the quantum content estimated by the number of failures of extracellular excitatory postsynaptic potentials (EPSPs), and in the frequency of miniature EPSPs. 3. Both OAG and TPA act on the postsynaptic membrane to enhance responses to the transmitter L-glutamate. Protein kinase C activators increased the apparent maximum of the ionophoretic dose-response curve for glutamate-induced depolarization, without affecting the reversal potential and the voltage-dependent decay rate for the excitatory postsynaptic current (EPSC) under voltage-clamp conditions. 4. The postsynaptic effect of OAG and TPA is distinctly different from that of activators of cyclic nucleotide-dependent protein kinases, such as octopamine, forskolin, CPT-cyclic AMP (8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate), and 8-bromo-cyclic GMP (8-bromoguanosine 3',5'-cyclic monophosphate) which decreased the postsynaptic sensitivity to L-glutamate. 5. I suggest that the responsiveness of the receptor to L-glutamate is under the control of these counteracting enzyme systems in the insect neuromuscular junction.