3.4.2.2. Delta Receptors
Androgens decrease delta opioid receptor expression in neuronal cells. Nandrolone treatment decreases delta receptor binding and delta receptor mRNA expression in hybrid cell lines naturally expressing delta receptors. Interestingly, these effects are independent of androgen receptors as evidenced by the observation that (1) androgenic decreases in receptor density are observed in a cell line that does not express androgen receptors and (2) androgenic decreases in receptor density are observed in the presence of an androgen receptor antagonist (Pasquariello et al., 2000). Androgen-induced decreases in delta opioid receptor density is widespread across brain regions. In rats, for instance, castration increases delta opioid receptor density in the olfactory bulb, thalamus, midbrain, and hypothalamus, and this effect is reversed completely by administration of testosterone (Takayama et al., 1990). Similar to mu receptors, androgen-induced decreases in delta receptors may be explained, in part, by receptor down-regulation induced by androgenic increases in endogenous enkephalin concentrations.