Differences in hypothalamic serotonin between estrous phases and gender: an in vivo microdialysis study

The aim of the present study was to assess whether there are gender differences in (1) levels of extracellular serotonin (5-HT) in the forebrain, and (2) the effect on 5-HT of a reuptake inhibitor, paroxetine, or a releasing drug, fenfluramine. In vivo microdialysis was used to measure 5-HT in the hypothalamus of male and regularly cycling female rats. Hypothalamic 5-HT was significantly lower in estrous females (0.83 +/- 0.05 pg/sample, n = 33) than in male rats (1.04 +/- 0.06 pg, n = 38). Levels in diestrous females (0.98 +/- 0.09 pg, 12 = 38) were not significantly different from males. Paroxetine (1 mg/kg) increased hypothalamic 5-HT in males, and diestrous and estrous females to similar to 2 pg/sample. However, the increase in hypothalamic 5-HT produced by a maximally effective dose of paroxetine (10 mg/kg) was significantly greater in male rats and during diestrous than during estrous. D,L-Fenfluramine (10 mg/kg) evoked an increase in extracellular 5-HT to similar to 15 pg/sample in all groups. A higher dose of D,L-fenfluramine (20 mg/kg) produced a significantly greater increase in hypothalamic 5-HT in males than in females during estrous or diestrous. These results are consistent with other evidence that during estrous, when rats are responding to peak levels of estrogen and progesterone, 5-HT release is decreased. (C) 1998 Elsevier Science B.V.