We have demonstrated that luteinizing hormone-releasing hormone (LHRH) and galanin are colocalized in a subset of hypothalamic LHRH neurons and the degree of colocalization is estrogen-dependent. In the female rat hypothalamus, the number of LHRH cells coexpressing galanin is four to five-fold higher than in the male. Progesterone seems to blunt the effect of estrogen on galanin gene expression, since in pregnant, lactating or old animals the degree of colocalization is similar to that seen in the male animals. Our observations also indicate that galanin can be considered as a marker for sexual differentiation, since in male postnatally-castrated animals, whose hypothalami were masculinized by sexual steroids, estradiol induces galanin gene expression if administered in adulthood. Our functional data indicate that galanin regulates reproductive functions at both the hypothalamic and pituitary levels. We have also shown that high levels of progesterone and/or prolactin induce enkephalin gene expression in dopaminergic neurons of the arcuate nucleus. Enkephalin is not produced by this subset of neurons in intact female or male rats; however, in pregnant, lactating or old animals enkephalin appears in dopaminergic neurons of the arcuate nucleus. Enkephalin, co-produced with dopamine, is probably able to antagonize partially the inhibitory effect of dopamine on prolactin secretion.
