Evidence for serotonin (5HT) binding sites on murine lymphocytes

The binding of 3H-labeled serotonin (or 5-hydroxytryptamine: 5HT) to mouse lymphocytes was investigated. It was shown to be highly specific, time-dependent, saturable and partly reversible. Saturation analysis demonstrated a Kd of 198 nM and B max of 3.53 nM. We studied receptor specificity by using different types of serotonin antagonists, and numerous other substances. Serotonin was found to be the most effective drug among those tested in inhibiting the binding of 3H-5HT, having an IC50 of 194 nM. The fact that 5HTP, a 5HT precursor, had no inhibitory capacity indicated the high specificity of these 5HT binding sites. Dopamine was somewhat able to competitively inhibit 5HT fixation (IC50 = 27,000 nM), whereas norepinephrine and histamine had no effect. Lastly, we investigated the cellular specificity of this binding, and observed that nonmacrophage peritoneal cells extensively bound serotonin under the same conditions as spleen cells. This is the first direct demonstration of 5-hydroxytryptamine receptors on mouse lymphocytes. The presence of these binding sites can contribute to the understanding of the suppressive effect of 5HT on mouse immunoreactivity.