Expression of postsynaptic Ca2+‐activated K+ (SK) channels at C‐bouton synapses in mammalian lumbar α‐motoneurons
Authors: Deardorff Adam S. , Romer Shannon H. , Deng Zhihui , Bullinger Katie L. , Nardelli Paul , Cope Timothy C. , Fyffe Robert E. W. 
Published: November 2012

Key points
Spinal cord α‐motoneurons display strong membrane immunoreactivity (IR) against small‐conductance calcium‐activated potassium channel (SK) isoform SK2, and a specific subpopulation of motoneurons also express SK3‐IR.
Rat α‐motoneurons expressing SK3‐IR are significantly smaller, have significantly longer after‐hyperpolarization half‐decay time, significantly larger after‐hyperpolarization amplitude and significantly slower axon conduction velocity than α‐motoneurons that lack SK3‐IR.
Motoneuron pools innervating slow‐twitch muscles have a higher percentage of SK3‐IR α‐motoneurons than those innervating fast‐twitch muscles.
Expression of SK3 may contribute to variability in after‐hyperpolarization duration and amplitude across different types of rat α‐motoneurons and may be a molecular factor differentiating between slow‐ and fast‐type motoneurons.
In the soma and proximal dendrites of α‐motoneurons, large clusters of SK2 and SK3 channel subunits appose cholinergic C‐boutons and colocalize with muscarinic type 2 receptors and Kv2.1 channels, which suggests a novel cellular mechanism for state‐dependent regulation of neuronal excitability.
Abstract  Small‐conductance calcium‐activated potassium (SK) channels mediate medium after‐hyperpolarization (AHP) conductances in neurons throughout the central nervous system. However, the expression profile and subcellular localization of different SK channel isoforms in lumbar spinal α‐motoneurons (α‐MNs) is unknown. Using immunohistochemical labelling of rat, mouse and cat spinal cord, we reveal a differential and overlapping expression of SK2 and SK3 isoforms across specific types of α‐MNs. In rodents, SK2 is expressed in all α‐MNs, whereas SK3 is expressed preferentially in small‐diameter α‐MNs; in cats, SK3 is expressed in all α‐MNs. Function‐specific expression of SK3 was explored using post hoc immunostaining of electrophysiologically characterized rat α‐MNs in vivo. These studies revealed strong relationships between SK3 expression and medium AHP properties. Motoneurons with SK3‐immunoreactivity exhibit significantly longer AHP half‐decay times (24.67 vs. 11.02 ms) and greater AHP amplitudes (3.27 vs. 1.56 mV) than MNs lacking SK3‐immunoreactivity. We conclude that the differential expression of SK isoforms in rat and mouse spinal cord may contribute to the range of medium AHP durations across specific MN functional types and may be a molecular factor distinguishing between slow‐ and fast‐type α‐MNs in rodents. Furthermore, our results show that SK2‐ and SK3‐immunoreactivity is enriched in distinct postsynaptic domains that contain Kv2.1 channel clusters associated with cholinergic C‐boutons on the soma and proximal dendrites of α‐MNs. We suggest that this remarkably specific subcellular membrane localization of SK channels is likely to represent the basis for a cholinergic mechanism for effective regulation of channel function and cell excitability.