Purpose: Serotonin is biochemically present in the iris and ciliary body of animals and humans. Controversial findings are reported about the concentrations of serotonin in aqueous humor with respect to plasma in humans. The aim of this study was to evaluate the levels of serotonin both in aqueous humor and plasma in human subjects. Methods: In 50 patients with glaucoma or cataract, plasma and aqueous humor serotonin levels were measured by HPLC with electrochemical detection. Serotonin plasma levels were also measured in 25 healthy subjects as controls. Results: In all patients with cataract or glaucoma, the aqueous humor serotonin concentration is significantly lower than that in plasma [1.14±0.29 (SEM) vs. 5.33±1.03 ng/ml, p < 0.01]. Furthermore, in the same patients and in 25 healthy controls, serotonin plasma levels were similar. Conclusion: Our study shows that serotonin is present in human aqueous humor and its concentration is 4 times lower than in plasma.

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