Characterization of opioid receptors in nervous tissue

The concept that endogenous opioid peptides interact with at least two different receptor sites developed from several experimental approaches. First, when the peptides were assayed by their effects on two pharmacological and two binding models, the rank order of activity differed in these four systems. Secondly, naloxone had a smaller antagonist effect on delta-receptors in the mouse vas deferens than on its mu-receptors. Thirdly, the enkephalins and morphine each occupied less than half of the total number of sites available in brain homogenates. Fourthly, cold ligands of the delta-type protected the binding of tritiated delta-agonists better than that of mu-agonists, and vice versa. Finally, tritiated ethylketazocine binds the kappa-receptor sites in homogenates of guinea-pig brain. It is readily displaced by etorphine, which binds uniformly to mu-, delta- and kappa-receptors, but only by very high concentration of mu- or delta-agonists. An interesting phenomenon is the potentiation of activity when a enkephalin analogue is conjugated to tobacco mosaic virus by the group of R. Schwyzer.