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

Jacopo Magistretti, Sara Brevi and Marco de Curtis

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.

Barium currents (I_Ba) 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 (R_e/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, R_e/p.

The half-activation potential (V_½) of total I_Bas positively correlated with the R_e/p of the corresponding currents. This implied that the high-decay I_Bas also had a more negative voltage range of activation than the more persistent ones.

The L- and N-type channel blockers nifedipine (10 µM) and -conotoxin GVIA (-CTx GVIA, 0·5-1 µM) additively blocked 20 and 25 % of the total I_Ba, respectively. The P/Q-type calcium channel blockers -agatoxin IVA (100 nM), -conotoxin MVIIC (1 µM) and 3.3 funnel toxin (1 µM), had little effect on I_Ba.

The nifedipine- and -CTx GVIA-sensitive current had a R_e/p > 0·55 and their voltage dependence of activation was of the high-voltage-activated type (V_½ 0 mV).

High-, intermediate- and low-decay blocker-resistant currents were observed in different neurones. Their R_e/p values highly correlated with those of the corresponding total I_Bas 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 (R_e/p < 0·25). The intermediate-decay currents (R_e/p 0·55) could not derive from variable combinations of high- and low-decay current components.

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.

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				S. Brevi, M. de Curtis, and J. Magistretti Pharmacological and Biophysical Characterization of Voltage-Gated Calcium Currents in the Endopiriform Nucleus of the Guinea Pig J Neurophysiol, May 1, 2001; 85(5): 2076 - 2087. [Abstract] [Full Text] [PDF]