Ionic mechanisms underlying the depolarizing and hyperpolarizing afterpotentials of single spike in guinea-pig...

Intracellular 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 Co2+ or Cd2+, reduction in extracellular Ca2+, and intracellular EGTA injection. On the other hand, elevation of extracellular Ca2+ 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 3H2O, 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 afterhyperpolarization were suppressed by addition of Co2+. 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.