Inhibition of growth in young mice treated with pentazocine: reversal by naltrexone

One week old mice were injected subcutaneously once daily with d,1-methadone (5 mg/kg), pentazocine, naltrexone, naloxone, nalorphine or nalbuphine, each at 10 mg/kg. The remaining half of each litter was used as control. Only methadone and pentazocine groups showed reduced weight gain after 3 weeks of treatment (P < 0.01). Injection of pentazocine in dosages of 5-20 mg/kg inhibited weight gain and protein synthesis in a dose-related manner. The incorporation of labeled leucine was followed in brain, liver and muscles. Methadone and pentazocine groups showed a significant decrease in protein synthesis in all tissues studied. The nalbuphine, nalorphine, naloxone, and naltrexone-treated groups incorporated leucine normally, correlating with normal weight gain. These data suggest that pentazocine, unlike the other mixed agonist-antagonists and antagonists, adversely affects the growth of very young animals when administered chronically. A specific opioid effect is suggested by the fact that naltrexone given concomitantly with the pentazocine prevents development of the biochemical lesion.