Distribution of dopamine D1 receptors in rat cortical areas, neostriatum, olfactory bulb and hippocampus in relation to endogenous dopamine contents.

The tritiated dopamine D1 antagonist SCH23390 was employed to determine the densities of D1 receptors in seven discrete and functionally identified cortical areas (cingulate, frontal, parietal, primary somatosensory, primary visual, retrosplenial and entorhinal-piriform) as well as in the neostriatum, hippocampus and olfactory bulbs. In addition, the tissue levels of the catecholamines NA, AD, DA, the indoleamine 5-HT and their main metabolites (MHPG, DOPAC, HVA, 3-MT, 5-HTP and 5-HIAA) were measured in the different regions by HPLC with electrochemical detection. The Scatchard analysis of saturation curves revealed the highest density of [3H]SCH23390 binding sites for the neostriatum, while the densities were 10-20 times lower for total cerebral cortex and hippocampus respectively. For the olfactory bulb and other cortical areas, D1 receptor densities were determined by equilibrium binding at a fixed radioligand concentration approaching saturation. The distribution of D1 receptors was heterogeneous with the greatest densities in entorhinal-piriform and cingulate cortices. The endogenous DA levels measured for all regions correlated with their metabolite (DOPAC, HVA and 3-MT) contents (r = 0.999; P less than 0.001). There was also a very good correlation (r = 0.981; P less than 0.001) between tissue DA and D1 receptor densities. This quantitative information reflects particular features of the organization of the DA systems and is discussed in relation to turnover and recently established aspects of the DA innervation.