Inhibition of 5-hydroxytryptamine uptake in human platelets by antidepressant agents in vivo

The accumulation of ¹⁴C-5-hydroxytryptamine (¹⁴C-5-HT) in platelet-rich plasma (PRP) and the concentration of 5-hydroxytryptamine (5-HT) in whole blood of patients treated with the antidepressant agents zimelidine (2×100 mg. daily), desipramine (2×75 mg daily), and clomipramine (2×75 mg daily) were examined before and during the treatment. Clomipramine and zimelidine markedly reduced the accumulation of ¹⁴C-5-HT and the concentration of 5-HT in the blood. Desipramine had a weaker, but significant effect. Added to the PRP in vitro clomipramine was ten-times more potent than norzimelidine, the active metabolite of zimelidine, and 60- and 300-times more active than desipramine and zimelidine, respectively in inhibiting the accumulation of ¹⁴C-5-HT. Analysis of plasma concentrations of zimelidine and norzimelidine showed that the decreased blood 5-HT and the inhibition of ¹⁴C-5-HT accumulation in platelets was mainly produced by norzimelidine. The inhibition of the ¹⁴C-5-HT accumulation and the decrease in blood 5-HT by desipramine were significantly correlated to the log plasma concentration of desipramine. It is concluded that the decrease in blood 5-HT caused by these agents is due to the inhibition of 5-HT uptake in platelets. The half-life of the decrease in blood 5-HT after clompramine and zimelidine was about 5 days. The return to normal 5-HT level after withdrawal of the drugs was 14 days or longer. These observations might indicate that only the newly formed platelets can accumulate 5-HT.