Abstract
Aging leads to the diminished pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH). Kisspeptin (Kp), the upstream regulator of the hypothalamic-pituitary-gonadal (HPG) axis, regulates GnRH synthesis and release through its cognate receptor, G-protein coupled receptor 54 (GPR54). In turn, GnRH regulates GPR54 expression. GnRH administration into the third ventricle has been shown to induce neurogenesis in different brain regions in old age. However, aging-associated changes in hypothalamic and extra-hypothalamic GPR54 expression were unclear. Therefore, the expression levels of GPR54 were evaluated in various brain regions of adult (age, 3-4 months) and old (age, 20-24 months) male Wistar rats in the present study. In the hypothalamus, mRNA and protein levels of Kp and GPR54 were identified to be significantly decreased in old age. Furthermore, GnRH1 expression in the hypothalamus was analyzed to observe the functional consequence of a reduced Kp-GPR54 system in the hypothalamus. It was found that hypothalamic GnRH1 levels were significantly decreased in old age. As GnRH regulates GPR54 levels, GPR54 was examined in extra-hypothalamic regions. GPR54 levels were found to be significantly decreased in the hippocampus and medulla and pons in old-age rats when compared to adult rats. Notably, GPR54 expression was observed in the frontal lobe, cortex, midbrain and cerebellum of adult and old-age rats; however, the difference between the two groups was not statistically significant. To the best of our knowledge, this is the first study that provides the quantitative distribution of GPR54 in different brain regions during aging. Thus, the reduced levels of Kp and its receptor, GPR54 in the hypothalamus could be cumulatively responsible for reduced levels of GnRH observed in old age.