Distribution of dihydropyridine and omega-conotoxin-sensitive calcium currents in acutely isolated rat and frog sensory neuron somata: diameter-dependent L channel expression in frog

Calcium channel subtypes in adult rat and frog sensory neuron somata, acutely isolated from dorsal root ganglia (DRG neurons), were studied using Bay K 8644, nimodipine, and omega-conotoxin GVIA (omega-CgTx) as specific probes. The DRG neurons varied in diameter 15-60 microns (rat) and 20-80 microns (frog). Bay K 8644 produced a large increase in calcium currents of small-diameter rat DRG neurons and shifted channel activation and the peak of the I-V curve in the hyperpolarizing direction. At a physiological holding potential (HP) of -60 mV, nimodipine blocked 50% of the peak calcium current in small-diameter frog and rat DRG neurons, indicating a large L channel component. At HP -80 mV, nimodipine blocked a lower percentage of peak current in small-diameter rat and frog DRG neurons than expected (based on experiments at HP -60 mV) probably due to nimodipine's voltage dependence. At HP -60 mV, omega-CgTx blocked 25% and 50% of peak current in small-diameter rat and frog DRG neurons, respectively. Omega-CgTx blocked a larger percentage of current at HP -80 mV than at -60 mV, probably because of the repriming of N channels. Observation of nimodipine- and Bay K 8644-sensitive calcium current in small-diameter rat and frog DRG neurons after omega-CgTx treatment, suggests that omega-CgTx is not a potent L channel blocker. The combination of omega-CgTx and nimodipine blocked all current in small-diameter frog DRG neurons but left a small portion of current unblocked in small-diameter rat DRG neurons at HP -60 mV, suggesting the possibility of omega-CgTx- and nimodipine-insensitive calcium channels in rat DRG neurons. Calcium current in most large-diameter frog DRG neurons was insensitive to nimodipine, but was completely blocked by omega-CgTx. This indicates significant variation in the expression of calcium channel subtypes in small- and large-diameter frog DRG neurons, which may subserve different sensory modalities.