Enkephalinomimetic drugs
Enkephalinomimetic drugs can be classified according to their direct or indirect mechanism of action. The majority of them, in particular morphine, stimulate enkephalinergic receptors directly. Acetorphan acts indirectly by increasing the concentration of endogenous enkephalins in the digestive tract, but its effect is only peripheral because it does not cross the blood-brain barrier.
Directly acting enkephalinomimetics should be classified according to their predominant effect on types and subtypes of receptors, but literature data are too complex or too imprecise or even contradictory so that one cannot do it correctly.
The predominance of central or peripheral effects of enkephalinomimetic agents results from the facility with which they penetrate into the brain or not.
Agonists, with high affinity for opioid receptors but with low activating capacity, act in the presence of more powerful agonists as antagonists: it is the case of pentazocine and especially of nalorphine with respect to morphine.
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