Characterization of pharmacologically identified voltage-gated calcium channel currents in acutely isolated rat neocortical neurons. I. Adult neurons.

1. Whole cell recordings were obtained from pyramidal neurons acutely dissociated from the sensorimotor cortex of adult rats. 2. Whole cell calcium channel currents were similar in appearance when elicited from holding potentials of -90 or -40 mV. With 5 mM Ba2+ as the charge carrier, currents began to activate at approximately -45 mV, peaked at approximately -10 mV, and had an apparent reversal potential of approximately +45 mV. Current amplitude and voltage dependence varied with the concentration and identity of the charge carrier (Ca2+ vs. Ba2+). Calcium channel currents were blocked completely by > 200 microM Cd2+ (IC50 approximately 3.5 microM). 3. We determined saturating doses for blockade of currents by nifedipine (Nif), omega-conotoxin GVIA (CgTx), and omega-agatoxin IVA (AgTx) in adult cells. We also tested the selectivity of these compounds by applying them in combination and in different orders. We found the three compounds to be highly, but not perfectly, specific. 4. L-type current was operationally defined as that blocked by 5 microM Nif, N-type current as that blocked by 1 microM CgTx, and P-type current as that blocked by 100 nM AgTx. In adult cells, each of these compounds blocked 30-35% of the current. When all three blockers were applied concurrently, approximately 80% of the current was blocked (20% of current was resistant to the 3 blockers). 5. Few biophysical differences were found between the pharmacologically defined current components in adult cells. The resistant current had a more rapid time-to-peak, inactivated more rapidly and completely, and activated at more negative potentials than the other three types.