Pharmacological reviews
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The concept of multiple opioid receptors reconciles a large body of clinical and pharmacological data. Recent studies have shown that there are also multiple opioid binding sites. It would appear that there is considerable variability between species in both the specificity and selectivity of opioid receptors. ⋯ Already subspecies of mu, kappa, and sigma receptors are being postulated. Both pharmacological and neurochemical methods may reveal even more. Some of the newer kappa agonists differ in their pharmacology from the prototypic kappa agonist ethylketazocine.