3.4.2. Opioid Receptors
Early studies using the nonselective opioid ligand, [3H]naltrexone, reported that castration increases opioid receptors in whole brain relative to intact male rats, and this effect is reversed by treatment with testosterone propionate (TP; Han &Fishman, 1979; Han & Fishman, 1985). Later studies using ligands with greater specificity reported androgenic effects on opioid receptors that differ across subtype and brain region. In some but not all cases, there are sufficient data to compare androgenic effects on both peptide and receptor concentrations to draw functional inferences.