Involvement of apamin-sensitive SK channels in spike frequency adaptation of neurons in nucleus tractus solitarii of the rat

We delineated the role of Ca2+-activated K+ channels in the phenomenon of s pike frequency adaptation (SFA) exhibited by neurons in the caudal region o f nucleus tractus solitarius (cNTS) using intracellular recording coupled w ith the current-clamp technique in rat brain slices, Intracellular injectio n of a constant depolarizing current evoked a train of action potentials wh ose discharge frequency declined rapidly to a lower steady-state level of i rregular discharges. This manifested phenomenon of SFA was found to be rela ted to extracellular Ca2+. Low Ca2+ (0.25 mM) or Cd2+ (0.5 mM) in the perfu sing medium resulted in a significant increase in the adaptation time const ant (tau(adap)) and an appreciable reduction in the percentage adaptation o f spike frequency (F-adap) in addition, the evoked discharges were converte d from an irregular to a regular pattern, accompanied by a profound increas e in mean firing rate. intriguingly, similar alterations in tau(adap), F-ad ap, discharge pattern and discharge rate were elicited by apamin (1 mu M), a selective blocker for small-conductance Ca2+-activated K+ (SK) channels. On the other hand, charybdotoxin (0.1 mu M), a selective blocker for large- conductance Ca2+-activated K+ channels, was ineffective. Our results sugges t that SK channels of cNTS neurons may subserve the generation of both SFA and irregular discharge patterns displayed by action potentials evoked with a prolonged depolarizing current.