[Recent contributions concerning the development and distribution of dopaminergic innervation in the cerebral cortex of the rat].

Tyrosine hydroxylase was used as an immunocytochemical marker of dopaminergic axons in the cerebral cortex either in fetuses or in postnatal life after lesion of the noradrenergic input. The lesion was controlled by the absence of dopamine-beta-hydroxylase immunoreactivity. Fluorescence histochemistry also allowed the specific visualization of the dopaminergic system following uptake of exogenous amines in tissue sections in presence of selective high affinity transport inhibitors. Two main dopaminergic (DA) subpopulations reach the medial cortex of the rat: 1) a deep one, first detected in the anterior frontal cortex on day 16 of embryonic life, was well developed at birth and extended caudally in layer V and/or layer VI toward the splenium of corpus callosum; 2) a superficial one was detected in layer I of the anterior cingulate cortex (area 24) on postnatal day 3 to 5 and invaded layer III from day 6 to 14. The adult distribution pattern and striking varicose aspect were not reached until day 21 to 30. In addition to these medial fields, a dopaminergic innervation of low density was detected laterally along a dorsal sagittal strip which encompassed several distinct cytoarchitectonic areas in the sensorimotor and visual cortex (medial and lateral agranular field, area 18b) as well as in discrete zones of the retrosplenial granular 29c, b, and agranular 29d areas. Several characteristics of this newly observed DA input were similar to that of the superficial field described in the anterior cingulate cortex; these similarities suggested that the subpopulation of DA neurons which provides projections to the anterior cingulate cortex could also contribute to the motor and visual cortex and thus play a role in sensorimotor integration. A DA terminal field was also demonstrated in the temporal part (ventral and caudal) of the hippocampal formation, the subiculum especially the prosubiculum and the adjacent CA1 hippocampal field being the main targets. This DA terminal field in the hippocampal formation matches with the area which projects toward the accumbens nucleus. Thus, the hippocampo-striatal projections which represent a link of functional importance between the limbic and central motor systems, could be modulated by the dopaminergic meso-cortico-limbic pathway. The predictive values of these data in the ascent of the phylogenetic scale are further considered.