This study attempted to distinguish between μ (morphine) and δ [(D-Ala2-D-Leu5)-enkephalin; DADLE] receptors, with regard to both in vivo effects (analgesia and gastrointestinal motility) and the location of binding activity in the brain. Analgesia and motility are distinguishable both by dose (intracere-broventricular) and by ligand selectivity with μ ligands more potent for the former and δ for the latter. Tolerance and cross-tolerance are exhibited for both effects, with the relationships between μ and δ ligand potencies preserved. In vitro receptor binding revealed an affinity decrease for δ in medulla and an increase in medulla and diencephalon for μ receptors after tolerance development to morphine. The results indicate that the μ receptors in medulla and diencephalon mediate analgesia, while medullary δ receptors control motility.
ASJC Scopus subject areas
- Cellular and Molecular Neuroscience