TABLE 7

Discrimination of opioid agonist and antagonist binding

The effects of many of the treatments that diminish agonist binding are more pronounced in the presence of sodium chloride. Although EGTA preferentially chelates Ca²⁺ over Mg²⁺ or Mn²⁺, EDTA binds them all. Both EDTA and EGTA will chelate the heavy metals. Note that binding is extremely sensitive to trypsin, which at only 1 µg/ml lowers agonist binding by approximately 90% and antagonist binding by approximately a third. From the literature (Pert et al., 1973; Creese et al., 1975; Pasternak and Snyder, 1975c; Pasternak et al., 1975b,c; Wilson et al., 1975).

Treatment	Effect on Receptor Binding
Treatment	Agonist	Antagonist
Ions
Na¹⁺	↓	—^a
Mg²⁺ and Mn²⁺	↑	−
Cu²⁺ and Ni²⁺ and Fe²⁺	↓↓	↓
Anions	—	—
Chelators
EDTA	↓	—^a
EGTA	—
Enzymes
Trypsin	↓↓	↓
Chymotrypsin	↓	↓
Phospholipase A	↓	↓
Reagents
N-Ethylmaleimide	↓	—
Iodoacetate	↓	—
Carbodiimide	↓	—
Temperature (0° compared with 25°C)	↓	↑

↵^a Binding may increase due to the dissociation of endogenous agonists in brain tissue.