Kinetic characteristics of newly synthesized 3H-5-HT in the brain of control and reserpinized mice. Evidence for the heterogeneous distribution of 5-HT in serotoninergic neurons.

Following an intravenous administration of 3H-tryptophan to mice, 3H-serotonin rapidly accumulated in the brain, with a maximum around the 20th min after the injection. In mice whole monoamine oxidase activity was blocked by pargyline (75 mg/kg i.p.), reserpine pretreatment (5 mg/kg, 24 h before death) did not alter the initial accumulation of 3H-5-HT indicating that the rate of 5 HT synthesis was similar in control and reserpinized animals. When monoamine oxidase was not blocked, the accumulation of 3H-5-HT was significantly lower in reserpine-treated mice than in controls. However, the specific activity of 3H-5-HT accumulated in brain was 2-3 times higher in reserpinized (45 h) than in control mice. Since neither the specific activity of the precursor 3H-tryptophan, the reuptake process for 3H-5-HT nor the absolute synthesis rate of 5-HT was altered in the brain of reserpinized mice, it is proposed that the differences observed in the specific activity of 3H-5-HT reflected the presence of 3H-5-HT in different compartments in control and reserpinized mice. Serotonin would be distributed in a "storage" and a "functional" compartments in serotoninergic neurons under normal conditions whereas only the functional compartment would exist 18-48 h after the administration of reserpine.