SINGLE-CHANNEL PROPERTIES OF 4 CALCIUM-CHANNEL TYPES IN RAT MOTONEURONS

Previous studies have demonstrated multiple components of whole-cell c alcium currents in hypoglossal motoneurons (HMs); HMs possess a low-vo ltage-activated (LVA) current and three types of high-voltage-activate d (HVA) calcium currents based on sensitivity to omega-Aga IVA, omega- Conotoxin GVIA (omega-CgTx) and dihydropyridine analogs (DHPs). In the present study, we recorded single-calcium channel activities from HMs using a cell-attached patch-clamp method and found four types of chan nels that could be discriminated based on kinetics, voltage dependency , DHP sensitivity, and single-channel conductances. The average single -channel conductances with 110 mM barium as a charge carrier were 7, 1 4, 20, and 28 pS, T-type channels had a single-channel conductance of 7 pS, activated at the most negative potentials for the calcium channe ls and inactivated during depolarization. L-type channels (DHP-sensiti ve channels) did not inactivate during depolarization and had a 28-pS single-channel conductance. Based on kinetics and sensitivity to holdi ng potential, it is likely that the channels with conductances of 14 p S and 20 pS represent N-type and P-type channels, respectively. The N- type channel (14 pS) was sensitive to holding potential, showed modal gating, and inactivated during maintained depolarizations, whereas the P-type channel (20 pS) was rather insensitive to holding potential an d did not inactivate during depolarization.