2.4 Opioid Receptors and Opioid Peptides
The endogenous opioid peptides, beta-endorphin, met/leu-enkephalin and dynorphin preferentially bind to mu, delta and kappa opioid receptors, respectively (Nyberg & Hallberg, 2012; Reisine, 1995), and there is ample evidence that the concentrations of these peptides and receptors differ between males and females (see Huhn et al., 2018; Rasakham & Liu-Chen, 2011; Chartoff & Mavrikaki, 2015). Evaluating these sex differences is often difficult because the magnitude systematically waxes and wanes as a function of circulating ovarian hormones in females (e.g., Flores et al., 2003). Evaluation of these sex differences is also complicated by inconsistences across neuroanatomical regions and developmental stages, suggesting multiple regulatory roles of the endogenous opioid system that vary both within and across sexes.