Adult and developing human cerebella exhibit different profiles of opioid binding sites

The binding of [³H][D-Ala², MePhe⁴, Gly-ol⁵]enkephaline (DAGO), [³H][D-Pen^2,5]enkephalin (DPDPE), [³H]ethylketocyclazocine (EKC), and [³H][Met⁵]enkephalin (MET) was used to examine η-,δ-,κ-, and η-receptors, respectively, in the developing (birth to postnatal day 19) and adult human cerebellum. Specific and saturable binding of all ligands was recorded in developing brains, and of [³H]DAGO, [³H]DPDPE, and [³H]EKC in adult cerebellum; all data fit a single homogeneous binding site for each ligand. However, the ontogenic profile of opioid receptor subtypes differed. δ- and κ-receptor capacities were 7.8- and 3.6-fold, respectively, greater in infant cerebellum than in adults. The η-receptor decreased over 7-fold in both binding affinity and capacity after day 2; by adulthood, the binding affinity was the same as in newborns but only one-half the binding capacity was recorded. The concentration of ζ-receptors was 20-fold greater in subjects 2-19 days of age than in newborns. These data demonstrate the presence, and distinct developmental profiles, opioid receptors in human cerebellum. Although the function of η-,ζ-, and κ-receptors in human cerebellum are unclear, the growth-related ζ-receptor is present at a time of cell replication and differentiation but is not detected in mature cerebellum.