Serotonin and immune response: effect of the amine on in vitro proliferation of rat lymphocytes

The effect of serotonin (5-hydroxytryptamine; 5HT) on the in vitro proliferation of mitogen-stimulated lymphocytes was studied in primary cultures of rat spleen cells.

5HT was added to the cultures 1 h prior to the mitogen, at final concentrations from 10(-13) up to 10(-2) M. T and B cell mitogens (concanavalin A, pokeweed mitogen and lipopolysaccharide) were used at suboptimal and optimal concentrations. The cell proliferation was measured 24-72 h after the addition of mitogen. The effect of each 5HT concentration was studied on a group of 6-12 animals and was expressed as a percentage of the control values obtained with mitogen alone.

No significant effect of 5HT at concentrations from 10(-13) to 10(-5) M was found. At concentrations of > or =10(-4) M, a regular dose-dependent inhibition of the lymphocyte proliferation appeared, the concentration producing the half-maximal effect being 6 x 10(-4) M. The observed suppression was not due to 5HT cytotoxicity toward spleen cells.

With the experimental system used, we failed to confirm an immunostimulatory effect of 5HT in the range of concentrations of its receptor sensitivities or lower, but found a clear-cut immunoinhibitory effect at higher concentrations.