Aging impairs galanin expression in luteinizing hormone-releasing hormone neurons: effect of ovariectomy and /or estradiol treatment

In the medial preoptic area and the diagonal band of Broca, a subset of LHRH neurons coexpresses galanin at 4- to 5-fold higher rate in female than male rats, suggesting that estradiol (E2) plays a key role in galanin gene expression within LHRH neurons. In the present studies we investigated the incidence of colocalization of these paptides in different age groups, i.e., 2-, 10-, 18-, and 24-month-old intact female Fisher rats; 24-month-old E2-treated rats; 24-month-old ovariectomized (OVX) rats; and 24-month-old OVX E2-treated rats with single or double labeling immunocytochemistry. For cell counting, we took advantage of the typical fusiform morphology of galanin-immunoreactive neurons that colocalize LHRH. The presence of both peptides in the same perikaryon was substantiated by double staining representative sections from each brain. Our observations indicate that the number of galanin/LHRH-coexpressing perikarya dramatically decreased with age. Although in 18-month-old rats a moderate decline was observed, in 24-month-old female rats no, or only a few, faintly stained, fusiform galanin-immunopositive perikarya were present. Although galanin was absent from LHRH neurons of aged rats, their LHRH content was not altered. E2 treatment of intact 24-month-old rats had no effect on the low incidence of colocalization. However, when OVX 24-month-old rats were E2 treated, the incidence of colocalization increased to the level seen in 2- or 10-month-old estrous animals. Our observations on the presence of colocalizing pericarya in intact and E2-treated aged animals provide further evidence for the key role of E2 in galanin gene expression within LHRH neurons and emphasize that some ovarian factor(s) may blunt this effect. (Endocrinology 134:324-330, 1994