Effects of fenfluramine and ritanserin on prolactin response to insulin-induced hypoglycemia in obese patients: evidence for failure of the serotoninergic system.

In order to study the hypothesized impairment of the serotoninergic system in human obesity, an insulin tolerance test (ITT) was carried out on 12 obese normoprolactinemic women and on 6 normal-weight women before (A) and after (B) the administration of a serotoninergic drug, fenfluramine (60 mg twice a day per os for 7 days). After a washout period, a new ITT (C) followed the administration of fenfluramine at the same dose, associated with a specific S2 blocker receptor agent, ritanserin (30 mg/day for the first 2 days and 20 mg/day for a further 5 days). In obese subjects, the prolactin (PRL) response to ITT A was reduced as compared to the controls: in 6 patients ('nonresponders') the PRL levels did not change, while in the other 6 ('responders') they increased (p less than 0.003) but less than in the controls (p less than 0.02). In normal-weight subjects, the administration of fenfluramine alone or with ritanserin did not modify the PRL response to ITT. In the responders, the serotoninergic drug normalized the PRL response to ITT while significantly improving it in the nonresponders; these effects were not antagonized by ritanserin. In conclusion, our data suggest that the serotoninergic system of obese patients is impaired and that the different secretory pattern observed in the two groups before and after fenfluramine may reflect differing degrees of this impairment.