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Received September 30, 2003
Revised October 20, 2003
Accepted after revision November 5, 2003
1 Wright State University
* To whom correspondence should be addressed. E-mail: robert.fyffe{at}wright.edu.
Delayed rectifier K+ currents are involved in the control of a-motoneuron excitability, but the precise spatial distribution and organization of the membrane ion channels that contribute to these currents have not been defined. Voltage activated Kv2.1 channels have properties commensurate with a contribution to delayed rectifier currents and are expressed in neurons throughout the mammalian central nervous system. A specific antibody against Kv2.1 channel subunits was used to determine the surface distribution and clustering of Kv2.1 subunit containing channels in the cell membrane of a-motoneurons and other spinal cord neurons. In a-motoneurons, Kv2.1 immunoreactivity (-IR) was abundant in the surface membrane of the soma and large proximal dendrites, and was present also in smaller diameter distal dendrites. Plasma membrane-associated Kv2.1-IR in a-motoneurons was distributed in a mosaic of small irregularly shaped, and large disk-like, clusters. However, only small-medium clusters of Kv2.1-IR were observed in spinal interneurons and projection neurons, and some interneurons, including Renshaw cells, lacked demonstrable Kv2.1-IR. In a-motoneurons, dual immunostaining procedures revealed that the prominent disk-like domains of Kv2.1-IR are invariably apposed to presynaptic cholinergic C-terminals. Further, Kv2.1-IR colocalizes with immunoreactivity against postsynaptic muscarinic (m2) receptors at these locations. Ultrastructural examination confirmed the postsynaptic localization of Kv2.1-IR at C-terminal synapses, and revealed clusters of Kv2.1-IR at a majority of S-type, presumed excitatory, synapses. Kv2.1-IR in a-motoneurons is not directly associated with presumed inhibitory (F-type) synapses, nor is it present in presynaptic structures apposed to the motoneuron. Occasionally, small patches of extrasynaptic Kv2.1-IR labeling were observed in surface membrane apposed by glial processes. Voltage-gated potassium channels responsible for the delayed rectifier current, including Kv2.1, are usually assigned roles in the repolarization of the action potential. However, the strategic localization of Kv2.1 subunit containing channels at specific postsynaptic sites suggests that this family of voltage activated K+ channels may have additional roles and/or regulatory components.
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