Regulation of Hypothalamic Neuropeptide Y Y1 Receptor Gene Expression during the Estrous Cycle: Role of Progesterone Receptors*

Neuropeptide Y (NPY) stimulates the release of GnRH in an estrogen (E2)-dependent manner, which is important in generating preovulatory GnRH surges. We tested the hypothesis that E2 up-regulates NPY's actions by stimulating NPY Y1 receptor (Y1r) gene expression through a mechanism mediated by E2's ability to induce progesterone (P) receptors (PRs). In initial experiments, a specific Y1r antagonist BIBP3226 was used to confirm the involvement of Y1r in the stimulatory effects of NPY on in vivo GnRH release. Hypothalamic Y1r messenger RNA (mRNA) levels were then measured using competitive RT-PCR and were found to be significantly increased at 1000, 1200, and 1400 h on proestrus compared with other times of the day or cycle stage. Ovariectomy eliminated these increases, and E2 treatment restored them. Additional P treatment produced even larger increases in Y1r mRNA levels. To assess the role of PRs in stimulating Y1r expression, proestrous rats were treated with PR antagonist or oil vehicle and killed at 1200 h. Treatment with PR antagonist completely blocked the proestrous rise in Y1r gene expression. In parallel experiments, the same in vivo PR antagonist treatments also blocked NPY stimulation of GnRH release in vitro. Together our findings reveal that 1) Y1r mRNA levels are increased during the late morning and afternoon of proestrus; 2) Y1r mRNA levels are similarly increased by E2, and to an even greater extent by additional P; and 3) PR antagonism blocks both increased Y1r mRNA and induction of GnRH responsiveness to NPY. These observations support the idea that E2 up-regulates GnRH neuronal responses to NPY through stimulation of Y1r gene expression, and that E2's actions are mediated by the induction and subsequent activation of PRs.