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.