Comparison of two penicillamine-containing enkephalins: mu, not delta, activity produces analgesia

Two penicillamine-containing enkephalin analogs were compared for inhibition of mouse writhing after intracerebroventricular administration. The delta receptor-selective analog was less potent in inhibiting writhing than the non-selective analog. Inhibition of writhing produced by the delta-selective analog was antagonized by very low doses of naloxone, and produced additive analgesic effects with the delta receptor antagonist, ICI 174864. These results suggest that inhibition of writhing was produced by mu receptor agonist activity, and not delta receptor agonist activity.