ULTRASTRUCTURAL-LOCALIZATION OF SHAKER-RELATED POTASSIUM CHANNEL SUBUNITS AND SYNAPSE-ASSOCIATED PROTEIN 90 TO SEPTATE-LIKE JUNCTIONS IN RAT CEREBELLAR PINCEAUX

The Pinceau is a paintbrush-like network of cerebellar basket cell axo n branchlets embracing the initial segment of the Purkinje cell axon. Its electrical activity contributes to the control of the cerebellar c ortical output through the Purkinje cell axon by generating an inhibit ory field effect. In addition to the structural features of the Pincea u, its repertoire of voltage-gated ion channels is likely to be an imp ortant aspect of this function, Therefore, we investigated the fine st ructural distribution of voltage-activated potassium (K(V)1.1, K(V)1.2 , K(V)3.4) and sodium channel proteins in the Pinceau. The ultrastruct ural localization of potassium channel subunits was compared to the di stribution of synapse-associated protein 90 (SAP90), a protein capable to induce in vitro clustering of K(V)1 proteins. With an improved pre embedding technique including ultrasmall gold particles, silver enhanc ement and gold toning, we could show that antibodies recognizing K(V)1 .1, K(V)1.2 and SAP90 are predominantly localized to septate-like junc tions, which connect the basket cell axonal branchlets. K(V)3.4 immuno reactivity is not concentrated in junctional regions but uniformly dis tributed over the Pinceau and the pericellular basket surrounding the Purkinje cell some. In contrast, voltage-activated sodium channels wer e not detected in the Pinceau, but localized to the Purkinje cell axon initial segment. The results suggest that K(V)1.1 and K(V)1.2 form he terooligomeric delayed rectifier type K-V channels, being colocalized to septate-like junctions by interaction with SAP90.