TABLE 6.

Expression of opioid receptor mRNAs in gonadotropin-releasing hormone (GnRH) and KNDy neurons in female mammals.

Neurons	Receptors	Expression rates	Species	Gonadal states	Methods	References
GnRH	μ-	0%	Rat	Proestrus	ISH	Mitchell et al., 1997
	μ-	0%	Rat	Intact/OVX/OVX + E2/OVX + E2 + P4	ISH	Sannella and Petersen, 1997
	μ-	33.3%	Guinea pig	OVX	ISH + IHC1	Zheng et al., 2005
	μ-	0%	Rat	OVX + E2	ISH	Tsuchida et al., 2021
	δ-	0%	Rat	Intact/OVX/OVX + E2/OVX + E2 + P4	ISH	Sannella and Petersen, 1997
	κ-	0%	Rat	Proestrus	ISH	Mitchell et al., 1997
	κ-	0%	Rat	Intact/OVX/OVX + E2/OVX + E2 + P4	ISH	Sannella and Petersen, 1997
	κ-	95.4%	Sheep	luteal phase	IHC	Weems et al., 2016
	κ-	95.4%	Rat	OVX + E2 + P4	IHC	Weems et al., 2016
KNDy	μ-	0.4%	Rat	OVX + low E2	ISH	Tsuchida et al., 2021
	κ-	20%	Mouse	OVX/OVX + E2	ISH	Navarro et al., 2009
	κ-	33%	Mouse	OVX	qRT-PCR2	Ikegami et al., 2017
	κ-	97.8%	Sheep	luteal phase	IHC	Weems et al., 2016
	κ-	62%	Rat	OVX + low E2	ISH	Tsuchida et al., 2020

ISH, in situ hybridization; IHC, immunohistochemistry.

¹GnRH neurons were detected by IHC.

²Oprk1 (coding KOR) expression in pooled KNDy cells was analyzed by qRT–PCR.