TOXIN-INSENSITIVE CA CURRENT IN DORSAL RAPHE NEURONS

About 54% of the whole-cell Ca current recorded in dorsal raphe neuron s cannot be categorized as N-, L-, or P-type Ca current. This current, I-Ca-Raphe, was not blocked by a combination of nimodipine, omega-CgT x-GVIA, and omega-AGA-IVA. Differences in toxin sensitivity and voltag e dependence suggest that I-Ca-Raphe is distinct from Q- or R-type Ca currents. In raphe neurons activation of 5-HT1A receptors by 5-HT inhi bits similar to 50% of the Ca current and slows activation. 5-HT inhib its both N-type Ca channels and I-Ca-Raphe channels by similar to 50% and slows the activation of both currents to a similar extent. Other s imilarities between I-Ca-Raphe and N-type Ca current were observed; th ey are both blocked to a similar extent by Ni2+, their activation prop erties, their current kinetics and channel availability as a function of holding potential are almost identical. Thus, I-Ca-Raphe represents a current that is not sensitive to known antagonists, but which is si milar to N-type Ca current. Although it is possible that I-Ca-Raphe be longs to a heretofore undiscovered family of Ca channels it is also po ssible that it represents an omega-CgTx GVIA-insensitive isoform of th e N-type Ca channel family.