The pinceau is a cerebellar structure formed by GABA-ergic descending basket cell axonal terminals converging on the initial axonal segment of Purkinje cell. Although it exerts a powerful inhibitory influence on the output of the cerebellar cortex, the exact ultrastructural organization and function of this structure are not understood. The unique morphology of the pinceau lies in several elements: lack of synaptic contacts between basket cell terminals and the initial axonal segment, the absence of postsynaptic densities, and heavy ensheathment of the initial segment with glial processes, making astrocytes the only intervening cellular element between the two types of axonal structures. Several plasticity related proteins in this area have been previously characterized by immunocytochemistry. Those include nitric oxide synthase, PSD95, GABA transporters, and several voltage gated potassium channel subunits. We are using a combination of confocal and electron microscopy, correlated photooxidation, EM and tomography to reveal the ultrastructural distribution of novel proteins and their relationships to morphological compartments in the pinceau.

We describe here Kv3.2, a recently discovered member of the Shaw-like subfamily of potassium channels, and the correlated distribution of other proteins described below. Pre-embedding immunolabeling, electron microscopy and tomography showed that Kv3.2 was selectively localized to basket cell axons.