Physostigmine effects on serotonin uptake in human blood platelets

Platelet serotonin (5HT) uptake was measured in 18 subjects administered physostigmine salicylate in a double-blind, placebo crossover design. In comparison to placebo, the drug caused a significant transient depression in mood, as measured by self- and observer-rated depression scores. In addition, physostigmine significantly increased platelet counts while independently decreasing the maximum velocity (Vmax) of platelet serotonin uptake. Physostigmine administration did not significantly affect the affinity constant (Km) for platelet serotonin uptake. The data are interpreted as being consistent with the postulate that platelet serotonin uptake may be decreased in depressed patients via cholinergic mechanisms.