Three homologous small conductance calcium-activated potassium (SK) channel subunits (SK1, SK2 & SK3) are expressed in distinct and overlapping patterns in mammalian central nervous system. SK channels likely mediate the medium afterhyperpolarization (mAHP), which plays an essential role in regulating neuron repetitive firing frequency. In spinal motoneurons (MNs) the mAHP duration is shorter on average in fast (F-type) MNs than that in slow (S-type) MNs. To better understand the molecular basis for mAHP, we determined the expression and sub-cellular distribution of SK channels in normal, axonally-injured, and developing spinal MNs in vivo using immunohistochemistry and quantitative confocal imaging techniques.

SK2 and SK3 channels are clustered on the surface membrane of MN soma and proximal dendrites. SK clusters are post-synaptically localized at synapses associated with cholinergic C-terminals. In complementary pattern, SK2-immunoreactive (-IR) and SK3-IR clusters are expressed in different subpopulations of rat and mouse spinal α-MNs; on average, SK3-IR MNs are smaller than SK2-IR MNs. Comparison of SK3 expression in rat soleus versus gastrocnemius MNs, together with intracellular electrophysiological data suggests that SK3-IR MNs are S-type whereas SK2-IR MNs are F-type. Moreover a subpopulation of motor axon terminals innervating slow muscle fibers expresses SK3 channels.

In postnatal developing mouse MNs, differential expression of SK2 and SK3 channels becomes apparent around the same time that muscle fiber differentiation occurs (around P9). The SK channel clustering develops in concert with the establishment and maturation of pre-synaptic cholinergic C-terminals, corresponding to the maturation of motor function.

Injury to the motor axon results in a decreased AHP duration in S-type MNs but an increased AHP duration or no change in F-type MNs. Here, we characterized the effects on SK3 channel clustering in rat spinal MNs following nerve crush. SK3 clusters appear unaltered until the 3rd day after axotomy. By the 8th day post-injury, the average sizes of SK3 clusters are much smaller than in the normal control MNs. In contrast, co-localized Kv2.1 clusters start to fragment and become reduced in size within hours following injury, suggesting differential regulation and dynamics of discrete channel populations at these synapses.