Light and electron microscopic immunocytochemical analysis of the dopamine innervation of the rat visual cortex.

The dopaminergic innervation of the rat primary (area 17) and secondary (areas 18 and 18a) visual cortical areas was examined immunocytochemically using an antiserum directed against dopamine. This innervation was characterized by the differential density of the respective afferents within individual visual areas. Area 18, especially its rostral part, was observed to receive a considerable amount of dopaminergic axons, whereas areas 17 and 18a were sparsely innervated. The innervation of all layers of area 18 seemed to consist to a considerable extent of axonal branches of radial fibres ascending from layer VI to layer I. At the ultrastructural level, dopamine profiles were found to display similar characteristics in all visual areas. Dopamine labelled axon-terminals and axonal varicosities, examined in single and serial ultrathin sections, were seen to form primarily asymmetrical synaptic contacts with dendritic profiles. These observations suggest a 'specific' innervation of cytoarchitectonically distinct cortical regions by dopaminergic axons.