Characterization of serotonin transporter in blood lymphocytes of rats. Modulation by in vivo administration of mitogens

Serotonin transporter sites were characterized in blood lymphocytes of rats. Pharmacological characteristics of drug interactions were in concordance with recent studies in nervous and human immune cells. The potency order of inhibition of [3H]paroxetine binding was imipramine>citalopram>alaproclate>serotonin. Selective inhibitors of dopamine or noradrenaline transporters did not inhibit it. The specific binding of [3H]paroxetine was higher at intermediate than at low concentrations, and the plot of free vs. specific binding had a sigmoid shape. The affinity constant or Kd, 1.77 nM, was in close agreement with data obtained from kinetic studies (Kd=1.33 nM), which evidences that the equilibrium was reached. In addition, serotonin transporter was evaluated by lipopolysaccharide or concanavalin A administration in vivo (0.1 mg/kg, i.p., 18 h). After the treatment with lipopolysaccharide, no changes were observed in the numbers of sites or Bmax or in the affinity, Kd. The treatment with concanavalin A showed a significant reduction in Bmax and reduction in Kd. Additionally, serotonin and 5-hydroxyindoleacetic acid levels were determined in plasma and lymphocytes by high-performance liquid chromatography. Treatment with lipopolysaccharide produced a significant increased of serotonin levels in lymphocytes without changes in 5-hydroxyindoleacetic acid level; in plasma, it produced an increase in serotonin and 5-hydroxyindolacetic acid levels. In addition, serotonin synthesis was evaluated by adding 300 μM of tryptophan in the medium, which significantly increased serotonin levels in control lymphocytes. Moreover, the concentrations of 5-hydroxyindoleacetic acid was enhanced significantly, both in plasma and lymphocytes in the presence of tryptophan after treatment with lipopolysaccharide. The administration of concanavalin A significantly decreased plasma levels of serotonin, as well as the concentrations of serotonin and 5-hydroxyindoleacetic acid in lymphocytes. These results demonstrate the presence of serotonin transporter in lymphocytes of rat blood, the capacity for serotonin synthesis in lymphocytes, and the modulation of these parameters by systemic administration of mitogens. The findings of this work contribute to understanding the immunological role of serotonin and the communication of immune and nervous systems.