High affinity dopamine binding to mouse thymocytes and Mytilus edulis (Bivalvia) hemocytes

The present report demonstrates that both mouse thymocytes and Mytilus edulis hemocytes contain a novel type of dopamine receptor. Scatchard analysis of these data revealed a single class of high-affinity binding sites with an affinity constant (Kd) of 6.6 nM, and a binding site density (Bmax) of 141 pmol/g protein in mice and a Kd of 7.6 nM and a Bmax of 66 pmol/g protein for the hemocytes. In older Mytilus the Kd was the same; however, there was a significant decrease in the Bmax (48 pmol/g protein; P greater than 0.05). Age changes were not evident for the mouse cells. The ability of a variety of other related drugs to displace specifically bound dopamine was investigated. The catecholamines (norepinephrine greater than epinephrine) and the dopamine agonist epinine were the most potent of the ligands tested and apomorphine and butaclamol were of moderate potency. This may serve to indicate that this dopamine binding site may be very different from the known D1 and D2 sites. This particular site may represent a third type of dopaminergic receptor. Furthermore, the hemocytes of Mytilus appear to be of two major types, namely, cells with a granular cytoplasm and those whose cytoplasm is agranular. Histofluorescence studies on the hemocytes reveal that a subpopulation of the granule-containing cells appears to contain serotonin.