Lidocaine and carbamazepine inhibit while phenytoin and lamotrigine paradoxically enhance the insect neuromuscular transmission.

Primary mechanism of action of local anesthetics and various anticonvulsants is the voltage-gated sodium channel block. Many of these small molecules also have other targets in nervous system of vertebrates. However, little is known about their action on invertebrate nervous system. Nevertheless, insect-based models are suggested for high-throughput screening of antiepileptic drugs. In the present work, we characterized action of lidocaine, carbamazepine, lamotrigine, and phenytoin on the neuromuscular transition of Calliphora vicina fly larvae using conventional voltage-clamp approach. Carbamazepine and lidocaine caused inhibition of synaptic transmission, which has presynaptic origin. This action is in agreement with inhibition of voltage-gated sodium channels that reduces depolarization of nerve terminals and, thus, calcium entry. Surprisingly, phenytoin and lamotrigine produced a prominent increase in the evoked postsynaptic currents without any effect on frequency or amplitude of spontaneous miniature currents. Potassium channel blocker 4-aminopyridine affects synaptic transmission in similar way. Elevation of synaptic quantal content via increase in calcium concentration or via application of 1 mM 4-aminopyridine eliminates the enhancement effect or even turns it to modest inhibition. We propose that lamotrigine and phenytoin act as inhibitors of insect potassium channels that cause the membrane depolarization and thus facilitates calcium entry into the nerve terminal.