Serotonergic Elements of the Mammalian Pituitary*

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Abstract
Serotonin [5-hydroxytryptamine (5-HT)] is a constituent of the mammalian pituitary gland. Light microscopic and EM immunocytochemistry were used to locate immunoreactivity in the pituitary glands of mice and bats. In addition the distribution of endogenous 5-HT immunoreactivity was compared with that of exogenous [3H]5-HT taken up by elements of the pituitaries and detected by radioiautography. 5-HT immunoreactivity was found in neurites in the posterior and intermediate lobes of the glands. These neurites also took up [3H]5-HT. The neural elements reactive with antisera to 5-HT or which labeled with [3H]5-HT differed in their distribution from neurites that labeled with [3H]dopamine or [3H]norepinephrine; lesioning of catecholaminergic neurites with 6-hydroxydopamine had no effect on the serotonergic structures. 5-HT immunoreactivity was also found in endocrine cells of the anterior lobes of the pituitaries of both species. No 5-HTwas detected in neural elements of the anterior lobe, and in fact, no staining with nerve-specific antibodies to neurofilament proteins was found in the anterior pituitary. the ultrastructure of the granules of the 5-HT-immunoreactive endocrine cells suggested that they might be a subset of gonadotrophs, the cell type previously shown to take up exogenous [3H]5-HT. 5-HT immunoreactivity was found within the granules of these cells. In bats, 5-HT immunoreactivity was present in the anterior pituitaries of active animals when 5-HT levels were high, but was greatly diminished in glands of hibernating animals when the 5-HT content was low. Apparently some cells that may be gonadotrophs contain endogenous 5-HT and also have an uptake mechanism for the amine. The function of 5-HT in the anterior pituitary remains to be determined, but 5-HT has previously been shown to inhibit the secretagogue action of LHRH on gonadotrophs.