Immunocytochemical Characterisation of Catecholaminergic Neurons in the Rat Striatum Following Dopamine Depleting Lesions

Abstract

In rats that receive unilateral 6-hydroxydopamine lesions, neurons immunoreactive for catecholaminergic markers appear in the striatum. If these neurons contribute to the brain’s ability to convert L-DOPA to dopamine in a therapeutic context then they have obvious relevance for the treatment of Parkinson’s disease. We show that the tyrosine hydroxylase immunoreactive neurons found in the dopamine-depleted striatum differ in size, morphology and location from those that are immunopositive for aromatic L-amino acid decarboxylase or dopamine. The tyrosine hydroxylase immunoreactive neurons, which lie ventrally along a rostrocaudal continuum from the core of nucleus accumbens to the fundus of the striatum, have the morphological characteristics of projection cells. Meanwhile, aromatic L-amino acid decarboxylase or dopamine-immunoreactive neurons, found in subcallosal positions in the dorsomedial caudate-putamen of both the dopamine-depleted and intact hemispheres, have the features of local circuit neurons. These catecholaminergic enzymes may be induced as a pharmacological response to the almost complete loss of dopamine from the striatum, or their appearance may reflect a general compromise in neuronal function. We present preliminary evidence for neuronal loss from the most severely depleted regions of the striatal complex suggesting that certain catecholaminergic markers may be found in neurons about to die.