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

Aim:To investigate the role of hypothalamus nociceptin/orphanin FQ(OFQ)andits endogenous receptor,the opioid receptor-likel receptor(ORL1 receptor)in theestrus cycle of female rats.Method:Radioimmunoassay was used to detect theeffect of the intracerebroventricular(icy)administration of OFQ and/or the ORL1receptor antagonist[Nphe~1]Nociceptin(1-13)NH2,that is,NC13 on luteinizinghormone(LH)levels of estrogen- and progesterone(EBP)-primed,ovariectomized(OVX)rats(EBP-primed OVX rats).RT-PCR,Western blotting,and immunohis-tochemistry techniques were adopted to observe the changes of OFQ and theORL1 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 200nmol OFQ(P<0.05),and the effect of 20 nmol OFQ could be abolished by pretreat-merit with 20 nmol NC13.The OFQ mRNA level in the POA on pro-estrus waslowered markedly compared to diestrus and estrus(P<0.05),while the mRNA andprotein levels of the ORL1 receptor showed no significant changes in the POAand MBH across the estrus cycle.Meanwhile,the number of OFQ-immunoreac-tive neurons in the medial POA,ventromedial hypothalamus,and the arcuatenucleus on pro-estrus was significantly decreased compared to diestrus and es-tins(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 sug-gests that it might play a negative modulatory role in the estrus cycle.