Function of galanin in the anterior pituitary of estrogen-treated Fischer 344 rats: autocrine and paracrine regulation of prolactin secretion.

Estrogen is a robust stimulator of galanin synthesis and secretion in the anterior pituitary. Galanin is colocalized in lactotrophs in the estrogen-treated anterior pituitary, and its roles in lactotroph function are still being elucidated. In the present studies, we quantified the phenotypes of estrogen-treated Fischer 344 rat anterior pituitary cells expressing the galanin gene by dual in situ hybridization. The total population of galanin-positive pituitary cells increased from undetectable levels to 16% of all cells after 2 weeks of estrogen treatment. More than 90% of the galanin-positive cells coexpressed PRL messenger RNA, and one-third of the lactotrophs expressed galanin messenger RNA. We hypothesized that galanin in the anterior pituitary may contribute to the heterogeneous secretion of PRL, and that one of the functions of galanin is to regulate PRL secretion in an autocrine/paracrine manner. To test this hypothesis, we performed the reverse hemolytic plaque assay combined with in situ hybridization to measure PRL secretion and galanin gene expression within the same individual cells. PRL secretion from galanin-positive lactotrophs was significantly greater than that from galanin-negative lactotrophs. Moreover, treatment with galanin antiserum significantly attenuated PRL secretion from galanin-positive cells, and treatment with galanin significantly enhanced PRL secretion from galanin-negative lactotrophs. In conclusion, these data provide direct evidence that galanin derived from the estrogen-treated anterior pituitary stimulates PRL secretion in both autocrine and paracrine manners.