Evidence for Multiple Opioid Receptors in the Human Posterior Pituitary

The distribution and quantification of opioid receptor types in post‐mortem human pituitary cryostat sections was determined by quantitative in vitro receptor autoradiography. Highly specific radioligands were used for each opioid receptor type, i.e. [125I]‐FK‐33‐824 for μ‐opioid sites, [125I][D.Ala2]‐Deltorphin‐I for δ‐opioid sites and 3H‐U69,593 for κ‐opioid sites. None of the five specimens tested exhibited any labelling in the anterior lobe of the pituitary for the three radioligands. As for the posterior pituitary, all 5 specimens contained both μ and κ‐opioid binding sites whereas none of them showed δ‐binding sites. The presence of both μ‐ and κ‐opioid binding sites in the human posterior pituitary contrasts with previous findings in the rat where only κ‐sites have been found. The present study could contribute to understanding of the functional action of opioids in the human pituitary.