Effects of the serotonin-uptake inhibitor, fluoxetine, on immunoreactive serotonin innervation in the rat pituitary gland.

Serotonin (5-HT)-immunoreactive innervation can be demonstrated in both the pars intermedia and pars nervosa of the rat pituitary gland. Fluoxetine hydrochloride, a 5-HT uptake inhibitor, was administered in single or multiple doses to adult male Sprague-Dawley rats to examine the possibility that 5-HT present in pituitary fibers may be due to uptake of the indoleamine. Paraffin-embedded sections of pituitaries from fluoxetine-treated rats revealed diminution or loss of 5-HT fiber staining in the intermediate, but not in the neural lobe, in all animals examined. Multiple doses of fluoxetine produced the most consistent effects on immunostaining. These observations along with results of our previous studies suggest that the intermediate lobe innervation immunoreactive for 5-HT may be due to uptake of indoleamine from extrinsic sources, while the pars nervosa terminals containing 5-HT most likely originate from 5-HT neuronal perikarya in the lower brainstem or hypothalamus.