Morphological changes of striatal dopaminergic presynaptic boutons following intrastriatal kainate injection

Bouton-sparing properties of kainic acid were re-investigated at the fine structural level. Two to 42 days after kainic acid (2.2-5.0 nmol) injection into the rat striatum, nigrostriatal dopaminergic boutons were examined at the injection site using tyrosine hydroxylase immunohistochemistry. Injection of the toxin produced marked dilatations of intensely tyrosine hydroxylase-positive boutons with tremendous amounts of vacuoles. Active astroglial processes reacted with those enlarged, immunoreactive boutons. Thus these boutons, if not all, are interpreted as being in the process of degeneration. Such enlarged boutons were observed more frequently at longer survival times than at shorter ones, suggesting an indirect effect of kainic acid resulting from severe loss of postsynaptic neuronal elements. Morphological changes described here may explain biochemical features that tyrosine hydroxylase level of the kainic acid affected striatum, following initial elevation, gradually decreases at a later stage.