Current and voltage clamp studies of the spike medium afterhyperpolarization of hypoglossal motoneurons in a rat brain stem slice preparation

Whole-cell patch clamp recordings were performed on hypoglossal motoneurons (HMs) in a brain stem slice preparation from the neonatal rat. The medium afterhyperpolarization (mAHP) was the only afterpotential always present af ter single or multiple spikes, making it suitable for studying its role in firing behavior. At resting membrane potential (-68.8 +/- 0.7 mV), mAHP (23 +/- 2 ms rise-time and 150 +/- 10 ms decay) had 9.5 +/- 0.7 mV amplitude, was suppressed in Ca2+-free medium or by 100 nM apamin, and reversed at -94 mV membrane potential. These observations suggest that mAHP was due to act ivation of Ca2+-dependent, SK-type K+ channels. Carbachol (10-100 mu M) rev ersibly and dose dependently blocked the mAHP and depolarized HMs (both eff ects prevented by 10 mu M atropine). Similar mAHP block was produced by mus carine (50 mu M) In control solution a constant current pulse (1 s) induced HM repetitive firing with small spike frequency adaptation. When the mAHP was blocked by apamin, the same current pulse evoked much higher frequency firing with strong spike frequency adaptation. Carbachol also elicited fast er firing and adapting behavior. Voltage clamp experiments demonstrated a s lowly deactivating, apamin-sensitive K+ current (l(AHP)) which could accoun t for the mAHP. l(AHP) reversed at -94 mV membrane potential, was activated by depolarization as short as 1 ms, decayed with a time constant of 154 +/ - 9 ms at -50 mV, and was also blocked by 50 mu M carbachol. These data sug gest that mAHP had an important role in controlling firing behavior as clea rly demonstrated after its pharmacological block and was potently modulated by muscarinic receptor activity.