PHARMACOLOGICALLY AND FUNCTIONALLY DISTINCT CALCIUM CURRENTS OF STOMATOGASTRIC NEURONS

Previous studies have suggested the presence of different types of cal cium channels in different regions of stomatogastric neurons. We sough t to pharmacologically separate these calcium channel types. We used t wo different preparations from different regions of stomatogastric neu rons to screen a range of selective calcium channel blockers. The two preparations were isolated cell bodies in culture, in which calcium cu rrent was measured directly, and isolated neuromuscular junction, in w hich synaptic transmission was the indirect assay for presynaptic calc ium influx. The selective blockers were two different dihydropyridines , omega-Agatoxin IVA, and omega-Conotoxin GVIA. Cultured cell bodies p ossessed both high-threshold calcium current and calcium-activated out ward current, similar to intact neurons. The calcium current had trans ient and maintained components, but both components had the same volta ge dependence of activation and inactivation. Dihydropyridines at grea ter than or equal to 10 mu M blocked both high-threshold calcium curre nt and calcium-activated outward current. Nanomolar doses of omega-Aga toxin IVA did not block calcium current, but micromolar doses did. ome ga-Conotoxin GVIA did not block either current. In contrast, at the ne uromuscular junction, dihydropyridines reduced the amplitude of postsy naptic potentials by only a modest amount, whereas omega-Agatoxin TVA at doses as low as 64 nM reduced the amplitude of postsynaptic potenti als almost entirely. These effects were presynaptic. omega-Conotoxin G VIA did not change the amplitude of postsynaptic potentials. The diffe rent pharmacological profiles of the two isolated preparations suggest that there are at least two different types of calcium channel in sto matogastric neurons and that omega-Agatoxin IVA and dihydropridines ca n be used to pharmacologically distinguish them.