Opiate receptor: cooperativity of binding observed in brain slices

Kinetic constants for binding of 3H-labeled morphine and naloxone were determined from Hill plot analyses and from experiments in which the concentration of tritiated drug was constant and that of nonlabeled drug varied. With brain slices, the binding of either drug exhibited strong positive cooperativity (Hill slope greater than 3); this was not observed in brain homogenates. Thus, in slices the relationship between opiate binding site and ligand may be more relevant physiologically and pharmacologically.