Ontogeny of the dopamine D₂ receptor mRNA expressing cells in the human hippocampal formation and temporal neocortex

The study details the cellular expression of the dopamine D₂ receptor mRNA in the human temporal lobe during prenatal development. At 13 embryonic weeks (E13) D₂ mRNA was widely expressed in the temporal lobe. At this time point in the dentate gyrus D₂ mRNA positive cells first appeared at the outer border of the granular layer and their number increased with development. The CA1 exhibited the highest level of D₂ mRNA expression. By E19-25 the hippocampal formation underwent rapid morphological maturation. D₂ mRNA expression became more uniform and dense in the ammonic subfield. At all ages the subiculum appeared more mature morphologically but less intensely stained for D₂ mRNA than the ammonic fields. In the entorhinal cortex D₂ mRNA expression was most conspicuous in the future layer II at all ages. In the temporal neocortex D₂ mRNA-positive cells were detected in the subplate and cortical plate. Differentiation of the cortical plate was accompanied by concentration of D₂ mRNA-positive cells in layer V. The most conspicuous cells expressing D₂ mRNA were found in the marginal zone of all regions and resembled Cajal-Retzius cells in morphology and location. Density of putative Cajal-Retzius cells expressing D₂ mRNA decreased with development. They all but disappeared from the hippocampal areas by mid gestation, but in the temporal neocortex occasional cells were seen even at term. Early and widespread but region and cell type specific expression of D₂ receptor mRNA suggests an important role of this DA receptor subtype in prenatal development of the human temporal lobe.