Abstract
The M-current (IM) is a voltage-dependent, persistent K+ current so termed because it is strongly inhibited by the cholinergic agonist muscarine. The IM main function is to limit neuronal excitability by contrasting action potential firing. Although motoneurons are sensitive to acetylcholine, the role of IM in modulating their excitability is still controversial. The aim of the present report was to examine the presence of IM in hypoglossal motoneurons (HMs) and its role in the modulation of firing properties using an in vitro model of rat brainstem slice. For this purpose, we employed the whole-cell patch-clamp technique to record HM responses upon stimulation with either a standard IM deactivation voltage protocol or depolarizing current steps. Voltage commands from depolarized potential induced inward relaxations with the common characteristics of IM, comprising inhibition by either muscarine (10μM) or the selective IM inhibitor linopirdine (30μM). IM was pharmacologically distinguished from the hyperpolarization-activated inward-rectifying current and, within the -20 to -50mV range, deactivated with >100-ms time constant. Current-clamp experiments demonstrated that IM strongly regulated HM action potential firing, since both muscarine and linopirdine increased spike frequency whereas the M-channel opener retigabine (20μM) reduced it. Conversely, IM seemed uninvolved in the generation of the medium afterhyperpolarizing potential. Our results suggest that HMs possess IM, whose pharmacological modulation is an important tool to up- or down-regulate excitability, to be explored in experimental models of neurodegeneration.