Abstract

The effects of cations and GTP on morphine (µ) and enkephalin (δ) receptors were examined by using binding assays of [³H]naloxone to rat brain membrane preparations and [³H]diprenorphine or [³H]naloxone to neuroblastoma cell membranes. The potencies and Hill coefficients (n) of many opiate agonists and opioid peptides in competing with the binding of the labeled antagonist are reduced by Na⁺ (100 mM) and GTP (0.1 mM). These effects are qualitatively similar for both subtypes of opiate receptors. However, quantitatively, the effects of GTP are much more profound for morphine receptors than for enkephalin receptors and the effects of Na⁺ are dependent upon the type of labeled antagonist used rather than upon receptor type. Na⁺ does not alter the affinity of opiate antagonists. GTP reduces the affinity of naloxone to morphine-binding sites by a factor of 2.5. Mg²⁺ (5 mM) increases the potency of opiate agonists and enkephalins for both receptor sites. The combination of Na⁺, GTP, and Mg²⁺ further reduces the affinity of enkephalins and opiate agonists for enkephalin-binding sites and the affinity of Met- and Leu-enkephalin for morphine-binding sites. However, the combination of Na⁺, GTP, and Mg²⁺ partially restores the affinity of [D-Ala², Leu⁵]- and [D-Ala², D-Leu⁵]enkephalin and morphine for the morphine-binding sites. These differential effects of cations and nucleotide further emphasize the differences that exist between morphine and enkephalin receptors and indicate the complex interactions of cations and nucleotides with opiate-binding sites.