VOLTAGE-GATED CALCIUM CURRENTS IN THE MAGNOCELLULAR NEUROSECRETORY-CELLS OF THE RAT SUPRAOPTIC NUCLEUS

1. Whole-cell patch-clamp techniques were used to analyse voltage-depe ndent calcium currents in acutely isolated somata of magnocellular neu rosecretory cells (MNCs) from the supraoptic nucleus of the hypothalam us of adult rats. Currents mere characterized on the basis of their ra tes of inactivation and their sensitivity to a series of calcium chann el blocking agents. 2. Curve fitting analysis of series of long lastin g depolarizing voltage steps from a holding potential of -80 mV reveal ed three current components with different voltage dependences and rat es of inactivation (n = 36). These include a low threshold (-80 mV), r apidly inactivating (tau = 42 +/- 3 ms at -10 mV) component, a high th reshold (-30 mV), slowly inactivating (tau = 1790 +/- 70 ms) component and a component with an intermediate threshold (-50 mV) and rate of i nactivation (tau = 187 +/- 15 ms). There is also a noninactivating por tion of evoked calcium current with a threshold of -50 mV. 3. Based on its voltage dependence, rate of inactivation, greater sensitivity to the divalent cation nickel than to cadmium and insensitivity to omega- conotoxin GVIA (omega-CgTX), the low threshold current appears to be a T-type calcium current. The rate of inactivation, voltage dependence, and sensitivity to omega-CgTX of the slowly inactivating component su ggests that it is an N-type current. The characteristics of the interm ediate component do not correspond to any identified calcium current t ype. 4. Portions of the non-inactivating calcium current are sensitive to nifedipine (23 +/- 2% of the total non-inactivating current at -10 mV; n = 10), suggesting the presence of L-type currents, omega-agatox in-IVA (omega-Aga-IVA; 20 +/- 6 % of total; n = 11), suggesting the pr esence of P-type channels, and omega-CgTX (39 +/- 3% of total; n = 19) , suggesting the presence of a non-inactivating N-type current. The no n-inactivating component at low potentials (greater than or equal to - 50 mV) was selectively blocked by nifedipine, suggesting the presence of a novel, low threshold L-type current. 5. We conclude that MNC soma express T-, N-, L-, and P-type calcium currents, as well as a novel l ow threshold nifedipine-sensitive current and an unidentified inactiva ting component. This complement of currents is different from that see n in the terminals of these cells, suggesting a spatial and functional segregation of calcium current types in MNCs.