Serotonin in human tears.
It has been postulated that serotonergic receptors are present in the corneal epithelium and that their activation by serotonin released from subepithelial corneal nerves raises the level of cyclic AMP, which in turn stimulates active CI-secretion by the corneal epithelium. We looked for serotonin in tears because these bathe the corneal epithelium. Twenty-two normal subjects, 14 women and 8 men between 25 and 60 years of age (average 36.5 +/- 9 years), participated in this study. Twenty microliters of tears were collected in capillary tubes after trigeminal stimulation (pepper in the nose) and were immediately analysed using high-performance liquid chromatography with electrochemical detection. Serotonin was identified and measured in every subject. Its mean concentration was 2.74 +/- 1.99 ng/ml. No difference was found between women and men. Emotionally evoked tears were obtained from two subjects. The amount of serotonin in these samples was much higher than in the samples obtained by trigeminal stimulation. These results demonstrate for the first time that serotonin is present in human tears. The possible role of this serotonin in chloride transport in the corneal epithelium and the probable difference between "emotional" and stimulated tears warrants further investigation.