Differential Distribution of Kv2.1 Channels in Spinal Neurons

Kv2.1 channel subunits are widely expressed in the mammalian CNS. These channels underlie delayed rectifier currents and likely have important roles in the regulation of dendritic excitability and synaptic currents. Previously, using specific antibodies, Kv2.1 channels were localized to high density plasma membrane clusters restricted to the soma and proximal dendrites of cortical and hippocampal neurons. The Kv2.1 clusters in these cells are associated with a variety of specialized membrane regions such as subsurface cisternae as well as surface membrane apposed to astrocytic processes and to some presumed inhibitory synapses. Here, we analyzed the cellular and subcellular distribution of Kv2.1 channels in a variety of defined classes of spinal cord neurons, including motoneurons (MNs), interneurons, and dorsal spinocerebellar tract (DSCT) cells. Use of a specific anti Kv2.1 mAb (Upstate) revealed very large patches of immunostaining localized to the soma and proximal dendrites of MNs. Interneurons in laminae III-VIII and X expressed smaller surface punctae. Notably, DSCT cells and Renshaw cells express less intense Kv2.1 labeling, in much smaller patches, suggesting that the membrane organization of Kv2.1 is cell-type specific. The localization of Kv2.1 in MNs was further explored by double labeling with Abs against a variety of neurotransmitters and receptors. At the light microscope level, almost all of the large Kv2.1 clusters in MNs were closely apposed by large nerve terminals labeled by Abs against the vesicular acetylcholine transporter and appeared to be colocalized with postsynaptic m2 muscarinic receptors. Kv2.1 channels thus appear to be localized at cholinergic C-terminal synapses, which are characterized by their prominent subsurface cisternae. The relationship of Kv2.1 with C-terminals was confirmed at the ultrastructural level