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 axon branchlets embracing the initial segment of the Purkinje cell axon. Its electrical activity contributes to the control of the cerebellar cortical output through the Purkinje cell axon by generating an inhibitory field effect. In addition to the structural features of the Pinceau, its repertoire of voltage-gated ion channels is likely to be an important aspect of this function, Therefore, we investigated the fine structural 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 ultrastructural localization of potassium channel subunits was compared to the distribution of synapse-associated protein 90 (SAP90), a protein capable to induce in vitro clustering of K(V)1 proteins. With an improved preembedding technique including ultrasmall gold particles, silver enhancement 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 junctions, which connect the basket cell axonal branchlets. K(V)3.4 immunoreactivity is not concentrated in junctional regions but uniformly distributed over the Pinceau and the pericellular basket surrounding the Purkinje cell some. In contrast, voltage-activated sodium channels were 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 heterooligomeric delayed rectifier type K-V channels, being colocalized to septate-like junctions by interaction with SAP90.