1. Whole cell recordings were obtained from pyramidal neurons acutely dissociated from the sensorimotor cortex of adult (from Lorenzon and Foehring, companion paper) and immature rats postnatal day 1 (P1) to adult. 2. Whole cell calcium channel currents were similar in appearance at all ages. Current amplitudes and estimated densities were initially low (approximately 16 pA/pF at ages < P6) and increased gradually, attaining adult values at approximately 4-5 wk postnatally (approximately 100 pA/pF). 3. L-type current was operationally defined as that blocked by 5 microM nifedipine, N-type current as that blocked by 1 microM omega-conotoxin GVIA, and P-type current as that blocked by 100 nM omega-agatoxin IVA. A resistant current remained in the presence of the combination of these three blockers. The proportions of these four current types did not change during ontogeny. 4. Few biophysical differences were found between the pharmacologically defined current components in adult or 1-wk-old cells. At both ages the resistant current had a more rapid time-to-peak and inactivated more completely and rapidly than the other three types. Resistant currents also activated at more negative potentials. N-, L-, and P-type currents activated at more positive potentials in 1-wk-old cells than in adult cells. For the resistant current, the voltage dependence of activation was not significantly different between the two ages.
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