Ontogenesis of Delta-opioid Receptors in Rat Brain Using [3H][D-Pen2,D-Pen5]enkephalin As a Binding Ligand

The ontogenesis of delta-opioid receptors has been studied in the postnatal period up to day 50 using the highly selective delta-site ligand [3H][D-Pen2,D-Pen5]enkephalin ([3H]DPDPE) in binding studies. Analyses of saturation curves revealed marked increases in binding capacities between the second and fourth postnatal week with little change in affinity. In contrast to findings with [3H][D-Ala2,D-Leu5]enkephalin binding could not be detected before postnatal day 10 which may be associated with the low affinity and specific activity of DPDPE. A low specific binding for [3H]DPDPE poses methodological problems in the use of this ligand for ontogenic studies.