A low-voltage activated, transient calcium current is responsible for the time-dependent depolarizing inward rectification of rat neocortical neurons in vitro

Intracellular recordings were obtained from rat neocortical neurons
in vitro. The current-voltage-relationship of the neuronal membrane
was investigated using current- and single-electrode-voltage-clamp
techniques. Within the potential range up to 25 mV positive to the
resting membrane potential (RMP: –75 to –80 mV) the steady state
slope resistance increased with depolarization (i.e. steady state
inward rectification in depolarizing direction). Replacement of
extracellular NaCl with an equimolar amount of choline chloride
resulted in the conversion of the steady state inward rectification
to an outward rectification, suggesting the presence of a
voltage-dependent, persistent sodium current which generated the
steady state inward rectification of these neurons. Intracellularly
injected outward current pulses with just subthreshold intensities
elicited a transient depolarizing potential which invariably
triggered the first action potential upon an increase in current
strength. Single-electrode-voltage-clamp measurements reveled that
this depolarizing potential was produced by a transient calcium
current activated at membrane potentials 15–20 mV positive to the
RMP and that this current was responsible for the time-dependent
increase in the magnitude of the inward rectification in
depolarizing direction in rat neocortical neurons. It may be that,
together with the persistent sodium current, this calcium current
regulates the excitability of these neurons via the adjustment of
the action potential threshold.