Differential maturation of ? and ? opioid receptors in the chick embryonic brain

Abstract

The developmental profiles of the binding of μ and δ opiate receptors agonists was investigated using the chick embryo brain. Binding of opioids was performed at embryonic days 5, 6, 15, 18, and 20 in the developing chick embryo brain. [³H]dihyromorphine was used as a μ ligand and with 5×10⁻⁷ M levorphanol for non-specific binding, and [³H](d-Ala²-d-Leu⁵)-enkephalin was used as a δ with 5×10⁻⁷ M (d-Ser-Gly-Phe-Leu-Thr)-enkephalin for non-specific binding. Crude membranes were prepared from whole brain at days, 5, 6 and cerebral hemispheres at days 15, 18, and 20 of embryonic age. Both μ and δ opiate receptors were present during early embryogenesis and as early as day 5. Analysis of binding sites revealed high and low affinity μ sites during early embryogenesis but only one δ site. By 18 days of embryonic age, only one μ site remained. This developmental change is interpreted as a transitory state of the receptor to the adult μ pattern. The presence of only one δ site is constant throughout embryonic age; it is high during early embryogenesis reaching a lower level by 18 days. The presence of a dual binding site pattern for the μ receptor in early embryogenesis is implicated to have a functional significance in the pluripotential role of the endogenous opioids in early development.