Heterogeneous distribution of cytochrome oxidase activity in the rat substantia nigra: correlation with tyrosine hydroxylase and dynorphin immunoreactivities

Abstract

Cytochrome oxidase (COase) activity, an endogenous marker of neuronal activity, was examined in the substantia nigra of the adult rat at the light-microscopic level. In addition, the pattern of histochemical staining observed for COase activity was correlated with immunohistochemistry for tyrosine hydroxylase (a marker of dopaminergic neurons) and for dynorphin (a peptide present in afferents from the striatum). Differential oxidative metabolic activity was revealed in subregions of the substantia nigra by COase histochemistry. Neurons of the substantia nigra pars compacta (SNc), ventral tegmental area, and ventrally displaced dopaminergic neurons were characterized by little or no staining for COase. n contrast, the substantia nigra pars reticulata (SNr) possessed a heterogeneous distribution of COase activity that was characterized by denzer staining in the ventrolateral than the dorsomedial part of the nucleus throughout its rostrocaudal extent, with the exception of the most rostral levels. This pattern of COase activity was inversely correlated with the density of ventrally descending tyrosine hydroxylase-positive dendrites arising from the medial portion of the SNc, as well as with the density of dynorphin immunoreactivity. The results suggest that the SNc and SNr possess distinct levels of oxidative metabolic activity. Furthermore, within the SNr itself, different levels of COase activity characterize subpopulations of neurons which may be differentially regulated by both striatal and dopaminergic influences.