Steroidal regulation of hypothalamic neuropeptide Y release and gene expression.

Neuropeptide Y (NPY) readily stimulates the release of hypothalamic LHRH and pituitary LH release in intact and gonadal steroid-primed gonadectomized rats. We have now tested the hypothesis that the release and synthesis of hypothalamic NPY may be regulated by gonadal steroids. To measure the effects of gonadal hormones on NPY release, a permanent push-pull cannula was implanted in the anterior pituitary (AP) of sham castrated (controls) or castrated (CAST) male rats, and 1 week later, the AP was perfused with artificial cerebrospinal fluid over a 3-4 h period. NPY concentrations in the perfusates collected at 10-min intervals were measured by RIAs. The NPY release pattern in the AP was episodic in both intact and CAST rats, and the frequency of NPY episodes was similar in two groups. However, the amount of NPY detected in the AP of CAST rats was significantly less than that of intact rats because the mean rate of release and the amplitude of NPY episodes in the perfusates of CAST rats were significantly reduced. This observation of attenuated hypothalamic NPY output in vivo and previous evidence of decreased hypothalamic NPY contents after CAST implied that the synthesis of hypothalamic NPY may be regulated by testicular secretions. Therefore, the effects of testosterone (T)-replacement on preproNPY messenger RNA (mRNA) in the medial basal hypothalamus (MBH) was evaluated. Rats were CAST and received either empty or T-filled Silastic capsules sc. Two weeks later, the level of perproNPY mRNA in the MBH was determined by solution hybridization/ribonuclease protection assay using a complementary RNA probe complementary to the rat NPY precursor mRNA. We observed that the levels of preproNPY mRNA were 2-fold higher in the MBH of T-replaced CAST as compared to control CAST rats. These findings are consistent with the hypothesis that gonadal steroids enhance the neurosecretory activity of hypothalamic NPYergic neurons, and for the first time reveal a coupling between the level of gene expression and the secretion of a neuropeptide involved in the regulation of hypothalamic LHRH and pituitary LH release.