Biophysical and pharmacological diversity of high-voltage-activated calcium currents in layer II neurones of guinea-pig piriform cortex.

1. High-voltage-activated calcium currents were studied with the whole-cell, patch-clamp technique in acutely dissociated pyramidal neurones from guinea-pig piriform cortex layer II. Barium ions were used as charge carriers. 2. Barium currents (IBa) displayed a remarkable kinetic diversity in different neurones. The ratio between the current amplitude at the end of the test pulses and the peak amplitude (Re/p) showed two frequency-distribution peaks at approximately 0.4 and 0.8. The index of current activation speed (rise time 10-90 %) directly correlated with the index of current persistence, Re/p. 3. The half-activation potential (V ) of total IBas positively correlated with the Re/p of the corresponding currents. This implied that the high-decay IBas also had a more negative voltage range of activation than the more persistent ones. 4. The L- and N-type channel blockers nifedipine (10 microM) and omega-conotoxin GVIA (omega-CTx GVIA, 0.5-1 microM) additively blocked 20 and 25 % of the total IBa, respectively. The P/Q-type calcium channel blockers omega-agatoxin IVA (100 nM), omega-conotoxin MVIIC (1 microM) and 3.3 funnel toxin (1 microM), had little effect on IBa. 5. The nifedipine- and omega-CTx GVIA-sensitive current had a Re/p > 0.55 and their voltage dependence of activation was of the high-voltage-activated type (V approximately 0 mV). 6. High-, intermediate- and low-decay blocker-resistant currents were observed in different neurones. Their Re/p values highly correlated with those of the corresponding total IBas and with the voltage dependence of activation of the underlying conductances. Exponential fittings of the inactivation phase of blocker-resistant currents returned very fast time constants (lower than 30 ms) for high-decay currents (Re/p < 0.25). The intermediate-decay currents (Re/p approximately 0.55) could not derive from variable combinations of high- and low-decay current components. 7. Our data demonstrate a remarkable variety in voltage-activated calcium currents expressed by piriform cortex neurones, that include currents resistant to high-voltage-activated calcium-channel blockers.