3H-Codeine Binding in the Guinea Pig Lower Brain Stem

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Abstract
Saturable binding of (–)-3H-codeine was found in the guinea pig medulla (KD = 5.6 × 10–7M, Bmax = 1.4 pmol/mg protein), whereas little stereospecific binding was detected (KD = 4.4 × 10–5M). The saturable binding of (–)-3H-codeine was slightly enhanced by Na+ and by Mg++ but not by Li++ and Ca++. The enhancement appears to be due to an increase in the number of receptor sites. (–)-3H-Codeine binding was displaced by (–)- and (+)-codeine, morphine, (–)- and (+)-methadone but not by barbiturates. Naloxone, at a high concentration (1 × 10–5M), inhibits the binding by only 40%. This agrees with our previously published data which shows that the optical isomers of codeine had significant antitussive effects in the cat, these effects not being antagonized by naloxone. A class of opiate antitussive receptors, which are less naloxone-sensitive and less stereoselective than the mu receptors, is implicated.