A blocker-resistant, fast-decaying, intermediate-threshold calcium current in palaeocortical pyramidal neurons

Authors

  • Jacopo Magistretti,

    1. Laboratorio di Biofisica e Neurofisiologia dei Sistemi Corticali, Dipartimento di Neurofisiologia Sperimentale, Istituto Nazionale Neurologico ‘Carlo Besta’, via Celoria 11, 20133 Milano, Italy
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  • Sara Brevi,

    1. Laboratorio di Biofisica e Neurofisiologia dei Sistemi Corticali, Dipartimento di Neurofisiologia Sperimentale, Istituto Nazionale Neurologico ‘Carlo Besta’, via Celoria 11, 20133 Milano, Italy
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  • Marco De Curtis

    1. Laboratorio di Biofisica e Neurofisiologia dei Sistemi Corticali, Dipartimento di Neurofisiologia Sperimentale, Istituto Nazionale Neurologico ‘Carlo Besta’, via Celoria 11, 20133 Milano, Italy
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: Dr Marco de Curtis, as above.
E-mail: decurtis@istituto-besta.it

Abstract

The whole-cell patch-clamp technique was used to record Ca2+ currents in acutely dissociated neurons from layer II of guinea-pig piriform cortex (PC). Ba2+ (5 mm) was used as charge carrier. In a subpopulation of layer II cells (≈ 22%) total Ba2+ currents (IBas) displayed a high degree (> 70%) of inactivation after 300 ms of steady depolarization. The application of L-, N- and P/Q-type Ca2+-channel blockers to these high-decay IBas left their fast inactivating component largely unaffected. The inactivation phase of the blocker-resistant, fast-decaying IBa thus isolated had a bi-exponential time course, with a fast time constant of ≈ 20 ms and a slower time constant of ≈ 100 ms at voltage levels positive to −10 mV. The voltage dependence of activation of the blocker-resistant, fast-decaying IBa was shifted by ≈ 7–9 mV in the negative direction in comparison with those of other pharmacologically and/or kinetically different high-voltage-activated Ca2+ currents. We named this blocker-resistant, fast-decaying, intermediate-threshold current IRfi. The amplitude of IRfi decreased only slightly (by ≈ 9%) when extracellular Ca2+ was substituted for Ba2+, in contrast with that of slowly decaying, high-voltage-activated currents, which was reduced by ≈ 41% on average. Moreover, IRfi was substantially inhibited by low concentrations of Ni2+ (50 μm). We conclude that IRfi, because of its fast inactivation kinetics, intermediate threshold of activation and resistance to organic blockers, represents a definite, identifiable Ca2+ current different from classical high-voltage-activated currents and clearly distinguishable from classical IT. The striking similarity found between IRfi and Ca2+ currents resulting from heterologous expression of α1E-type channel subunits is discussed.

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