It is well known that hypothalamic endogenous opioid peptides (EOP) exert an inhibitory influence on LH release, and that a restraint on this inhibitory tone triggers preovulatory LHRH and LH hypersecretion. Recent evidence suggests that hypothalamic neuropeptide Y (NPY) is an important component of the neural circuitry that participates in induction of the LH surge. We have reported previously that blockade of EOP influence by the opioid receptor antagonist, naloxone (NAL), stimulated NPY accumulation in the median eminence (ME) in association with increased LH release in estradiol-17 beta-primed ovariectomized rats. To evaluate whether a restraint on the EOP system will result in an increase in NPY synthesis, the effects of NAL infusion on preproNPY messenger RNA (mRNA) levels in the medial basal hypothalamus were studied by solution hybridization/RNase protection assay and by in situ hybridization. NAL (2 mg/0.6 ml.h) or saline (SAL, 0.6 ml/h) was infused for 3 h (1100-1400 h) via an intrajugular cannula in estradiol-17 beta-primed ovariectomized rats. In accord with previous studies, NAL infusion significantly increased plasma LH levels at 1400 h. concomitant with this activation of LH release, NPY gene expression was also augmented. As compared with initial control levels at 1100 h, preproNPY mRNA levels in the medial basal hypothalamus increased at 1400 h in SAL (106%)- and NAL (202%)-infused rats, and at this time, preproNPY mRNA levels were significantly higher in NAL-infused rats than in SAL-infused rats. In situ hybridization studies showed that NAL infusion significantly increased the preproNPY mRNA signal at 1400 h mainly in the rostral and middle regions of the arcuate nucleus (ARC) as compared with that seen at 1100 and 1400 h in SAL-infused rats. To examine further the relationship between the NAL-induced increase in LH release and increase in NPY gene expression, the effects of NPY mRNA antisense oligonucleotides (oligos) on NPY levels in the ME-ARC and on plasma LH levels were studied in NAL-infused rats. In NAL-infused rats, intracerebroventricular administration of a NPY antisense oligo (25 micrograms/rat) at 1000, 1200, and 1300 h decreased NPY levels in the ME-ARC and blocked the increase in plasma LH levels at 1500 h, whereas control missense oligos had no effect. Collectively, these results show that a decrease in opioid influence rapidly augments NPY gene expression in a subpopulation of neurons in the ARC, and support the hypothesis that a disinhibition from opioid influence acutely promotes NPY synthesis and release, which is necessary for phasic LH discharge in rats.
