Characteristics of sodium-induced increase in opiate antagonist binding sites in rat brain membranes

Sodium decreases agonist binding and increases antagonist binding to opiate receptor sites in brain membranes. This study characterizes in detail the 20-40% increase in [3H]naloxone binding caused by sodium. This increase in binding was specific to sodium (not mimicked by potassium or lithium) and was maximal at 10 mM NaCl. The sodium effect was reversible: washing membranes free of sodium restored binding to normal. Sodium increased Bmax, not KD, of [3H]naloxone binding. The sodium-induced increased binding was not inhibited by either N-ethylmaleimide (NEM) or phospholipase A2 at concentrations which inhibit 50-85% of normal [3H]naloxone binding. In NEM-treated membranes, the effect of sodium on increasing naloxone binding was actually increased. The regional distribution of these sodium-dependent sites were different from normal naloxone sites. These results suggest that physiological concentrations of sodium expose naloxone sites which differ in biochemical properties from naloxone sites assayed in the absence of sodium.