Characterization of opioid receptors in nervous tissue

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Abstract
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 to $\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 concentrations of $\mu $- or $\delta $-agonists. An interesting phenomenon is the potentiation of activity when an enkephalin analogue is conjugated to tobacco mosaic virus by the group of R. Schwyzer.

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