COLOCALIZATION OF DOPAMINE AND SEROTONIN IN THE RAT PITUITARY-GLAND AND IN THE NUCLEI INNERVATING IT

The nerve terminals in the intermediate and posterior lobes of the rat pituitary gland are reported to show colocalization of serotonin and tyrosine hydroxylase. This study examined the extent of this colocaliz ation in the pituitary gland and in the nuclei considered to project t o the pituitary. In the intermediate lobe, two types of nerve fibers w ere encountered, one containing serotonin (5-HT-IR) and tyrosine hydro xylase (TH-IR) immunoreactivities and the other showing 5-HT-IR only. Instead, there was no colocalization in the posterior lobe. In the hyp othalamus, colchicine treatment with L-tryptophan and pargyline inject ions resulted in 5-HT-IR in some neurons in the dorsomedial, periventr icular and arcuate nuclei, some of which in the arcuate and periventri cular nuclei were also TH-IR. In the raphe nuclei no colocalization of 5-HT-IR and TH-IR was observed. Catecholamine neurotoxin, 6-hydroxydo pamine, abolished the 5-HT-IR and dramatically reduced the TH-IR in th e intermediate lobe nerve fibers. Both effects were prevented by cocai ne, a monoamine uptake inhibitor, but not by fluoxetine, a specific se rotonin uptake inhibitor. Serotonin neurotoxin p-chloroamphetamine (PC A) had no effect on intermediate lobe fibers, although it caused compl ete disappearance of 5-HT-IR from the posterior lobe nerve fibers. Thi s effect was prevented by fluoxetine. Our results indicate, that coloc alization of serotonin and TH observed in the intermediate lobe occurs both in the nerve terminals within the lobe and in some nuclei that i nnervate it. Furthermore, drug treatments suggest that serotonin in th e intermediate lobe is localized in catecholaminergic fibers, which do not posses a specific serotonin uptake mechanism.