ULTRASTRUCTURAL CHARACTERIZATION OF THE ACETYLCHOLINE INNERVATION IN ADULT-RAT NEOSTRIATUM

The ultrastructural features of acetylcholine axon terminals (varicosi ties) in adult rat neostriatum were characterized by electron microsco py after immunostaining with a sensitive monoclonal antibody against r at choline acetyltransferase. Several hundred single sections from the se varicosities were analysed for shape, size and content, presence of a synaptic membrane specialization, and composition of the microenvir onment. An equivalent number of unlabeled varicosities selected at ran dom from the same micrographs were similarly examined. The immunostain ed varicosity profiles were relatively small and seldom showed a junct ional membrane specialization. Stereological extrapolation to the whol e volume of these varicosities indicated that less than 10% were synap tic. Far fewer dendritic spines were juxtaposed to these predominantly asynaptic profiles than to their unlabeled counterparts. This differe nce seemed imputable to the low synaptic incidence of the acetylcholin e varicosities and was consistent with the view that these are randoml y distributed in relation to surrounding elements. The bulk of the dat a was suggestive of volume transmission. This raised the possibility t hat, in such a densely innervated area, a basal level of acetylcholine is permanently maintained around all cellular elements, contributing to the modulatory properties of this transmitter. This basal level of acetylcholine could also serve as a regulatory signal controlling the expression of different receptor subtypes in neurons, glia and blood v essels.