Ionic mechanisms underlying the depolarizing and hyperpolarizing afterpotentials of single spike in guinea-pig cingulate cortical neurons

Abstract

Intraceliular recordings and hybrid single-microelectrode voltage-clamp techniques were used to study the ionic mechanisms underlying the afterdepolarization and the subsequent slow afterhyperpolarization that followed a single action potential in layers V/VI neurons of the guinea-pig anterior cingulate cortex in in vitro slices. Both the afterdepolarization and afterhyperpolarization were markedly suppressed in size by addition of Co 2+ or Cd 2+, reduction in extracellular Ca 2+, and intracellular EGTA injection. On the other hand, elevation of extracellular Ca 2+ concentration augmented the amplitudes of the afterpotentials. The afterdepolarization amplitude was selectively depressed by the stilbene derivatives, 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulphonate, disodium 3H 2O, and 4,4′-diisothiocyanatostilbene-2, 2′-disulphonic acid, disodium salt. Reduction in external Cl − and intracellular Cl − injection enhanced the afterdepolarization amplitude without affecting the afterhyperpolarization. The null potentials for the afterdepolarizations recorded with K acetate- and Cs acetate-electrodes were −68 and −63 mV, respectively. The slope of the null potential obtained with K acetate electrodes or Cs acetate electrodes was 49 and 53 mV, respectively, per log unit of the external Cl − concentration. Reduction in external K + markedly depressed the afterdepolarization and augmented the afterhyperpolarization in size, whereas rise in external K + markedly augmented the afterdepolarization and depressed the afterhyperpolarization. The null potential for the afterhyperpolarization recorded with K acetate electrodes was −94 mV. The slope of the null potential was 57 mV per log unit of the external K + concentration. Reduction in extracellular Na + concentration slightly depressed both the amplitudes of the afterdepolarization and afterhyperpolarization. A hybrid voltage-clamp analysis revealed a slow decaying inward current and a subsequent outward current that followed an action potential. Both the amplitudes of the inward current corresponding to afterdepolarization and the outward current corresponding to after-hyperpolarization were suppressed by addition of Co 2+. Reduction in extracellular Cl − concentration augmented the inward current amplitude without significantly affecting the outward current. These results indicate that the afterdepolarization is mainly due to an increase in a Ca 2+-activated Cl − conductance, while the afterhyperpolarization is mainly generated by an activation of Ca 2+-mediated K + conductance.