Properties of persistent sodium conductance and calcium conductance of layer V neurons from cat sensorimotor cortex in vitro

Abstract

Properties of the persistent Na conductance and the Ca conductance of layer V neurons from cat sensorimotor cortex were examined in an in vitro slice preparation by use of a single microelectrode, somatic voltage clamp, current clamp, intra- and extracellular application of blocking agents and extracellular ion substitution. The persistent Na current (INaP) attained its steady level within 2-4 ms of a step change in voltage at every potential where it could be examined directly [to about 40 mV positive to resting potential (RP)]. Because of its fast onset INaP can be activated during a single excitatory postsynaptic potential(EPSP) and can influence the subsequent voltage time course and cell excitability. Application of a depolarizing holding potential .gtoreq.20 mV positive to RP could inactivate spikes, thus allowing examination of INaP at voltages positive to spike threshold. At every potential where INaP was visible, it was mixed with a slow outward current. After depressing K currents with blocking agents, INaP could be observed during depolarizations to about 40 mV positive to RP where it is normally hidden by the larger outward currents. Indirect evidence suggests that INaP is present and large during prolonged depolarizations > 50 mV positive to RP. INaP and spike Na channels are apparently separate. After blockade of INaP and Na spikes, Ca2+ spikes could be evoked only if K currents were first depressed. The Ca2+-dependent nature of the regenerative potentials was indicated by their disappearance when Co2+ or Mn2+ was substituted for Ca2+ in the perfusate and by the appearance of greatly enhanced potentials of similar form when Ba2+ was substituted for Ca2+. Ba2+ substitution greatly enhanced evoked and spontaneous synaptic potentials. Apparently, little or no Ca2+ conductance is activated in the voltage range 25 mV positive to RP where INaP is the dominant ionic current. Ca2+ currents could be controlled during voltage clamp only in a limited voltage range corresponding to the amplitude of a small, slow, regenerative Ca2+ response seen during current clamp. Large, uncontrolled Ca2+ currents first appeared at a larger depolarization corresponding to the threshold of a faster, larger Ca2+-spike component seen during current clamp. Ca2+ conductance is evidently distributed from soma to dendrites with the largest Ca2+ currents being generated in the dendrites.