Role of hypothalamus nociceptin/orphanin FQ in pre-ovulatory luteinizing hormone surge of estrogen and progesterone-primed, ovariectomized rats

Abstract

Aim: To investigate the role of hypothalamus nociceptin/orphanin FQ (OFQ) and its endogenous receptor, the opioid receptor-like 1 receptor (ORL1 receptor) in the estrus cycle of female rats. Method: Radioimmunoassay was used to detect the effect of the intracerebroventricular (icv) administration of OFQ and/or the ORL1 receptor antagonist [Nphe1]Nociceptin(1-13)NH2, that is, NC13 on luteinizing hormone (LH) levels of estrogen- and progesterone (EBP)-primed, ovariectomized (OVX) rats (EBP-primed OVX rats). RT-PCR, Western blotting, and immunohistochemistry techniques were adopted to observe the changes of OFQ and the ORL1 receptor in the pre-optic area (POA) and the medial basal hypothalamus (MBH) of the estrus cycle of female rat. Results: Pre-ovulatory LH surges in EBP-primed, OVX rats were significantly reduced by icv administration of 20 and 200 nmol OFQ (P < 0.05), and the effect of 20 nmol OFQ could be abolished by pretreatment with 20 nmol NC13. The OFQ mRNA level in the POA on proestrus was lowered markedly compared to diestrus and estrus (P < 0.05), while the mRNA and protein levels of the ORL1 receptor showed no significant changes in the POA and MBH across the estrus cycle. Meanwhile, the number of OFQ-immunoreactive neurons in the medial POA, ventromedial hypothalamus, and the arcuate nucleus on pro-estrus was significantly decreased compared to diestrus and estrus (P < 0.05). Conclusion: The inhibitory effect of OFQ on the LH surge of EBP-primed, OVX rats and its downregulation in POA and MBH on pro-estrus suggests that it might play a negative modulatory role in the estrus cycle. © 2007 CPS and SIMM.