Similar decrease in spontaneous morphine abstinence by methadone and RB 101, an inhibitor of enkephalin catabolism

1 The dual inhibitor of enkephalin degrading enzymes, RB 101, is able to block endogenous enkephalin metabolism completely, leading to potent antinociceptive responses potentiated by blockade of CCK B receptors. In this study we have investigated the effects induced by RB 101 given alone, or with the CCK B antagonist, PD‐134,308, on a model of spontaneous morphine withdrawal and substitutive maintenance in rats. 2 Animals were chronically treated with morphine for 7 days followed, 36 h after the interruption of drug administration, by a maintenance treatment for 5 days with methadone (2 mg kg −1 , i.p.), clonidine (0.025 mg kg −1 , i.p.), RB 101 (40 mg kg −1 , i.p.), PD‐134,308 (3 mg kg −1 , i.p.) or a combination of RB 101 plus PD‐134,308. Several behavioural observations were made during this period in order to evaluate the acute effects as well as the consequence of chronic maintenance induced on spontaneous withdrawal by the different treatments. 3 Methadone was the most effective compound in decreasing the spontaneous withdrawal syndrome after acute administration. Both, methadone and RB 101 had similar effectiveness in reducing opiate abstinence during the period of substitutive treatment. PD‐134,308 did not show any effect when administered alone and did not modify the effect of RB 101. 4 Naloxone (1 mg kg −1 , s.c.) failed to precipitate any sign of withdrawal when injected at the end of the chronic maintenance treatment suggesting that, under the present conditions, methadone and RB 101 did not induce significant physical opiate‐dependence. 5 The mildness of the side effects induced by chronic RB 101, suggests that systemically active inhibitors of enkephalin catabolism could represent a promising treatment in the maintenance of opiate addicts.