Toxin-insensitive Ca current in dorsal raphe neurons.

About 54% of the whole-cell Ca current recorded in dorsal raphe neurons cannot be categorized as N-, L-, or P-type Ca current. This current, ICa-Raphe, was not blocked by a combination of nimodipine, omega-CgTx-GVIA, and omega-AGA-IVA. Differences in toxin sensitivity and voltage dependence suggest that ICa-Raphe is distinct from Q- or R-type Ca currents. In raphe neurons activation of 5-HT1A receptors by 5-HT inhibits approximately 50% of the Ca current and slows activation. 5-HT inhibits both N-type Ca channels and ICa-Raphe channels by approximately 50% and slows the activation of both currents to a similar extent. Other similarities between ICa-Raphe and N-type Ca current were observed; they are both blocked to a similar extent by Ni2+, their activation properties, their current kinetics and channel availability as a function of holding potential are almost identical. Thus, ICa-Raphe represents a current that is not sensitive to known antagonists, but which is similar to N-type Ca current. Although it is possible that ICa-Raphe belongs to a heretofore undiscovered family of Ca channels it is also possible that it represents an omega-CgTx GVIA-insensitive isoform of the N-type Ca channel family.