Effects of buprenorphine and Ro 15-4513 on delayed death and brain beta-endorphin levels in rats treated with cocaine or cocaine-ethanol

The present study was aimed at elucidating the relationship between brain beta-endorphin, which was estimated by the immunofluorescence method, and fatal drug toxicities due to cocaine and combined cocaine-ethanol administration, including the late fatal toxicities clinically noted. beta-endorphin is an endogenous opioid peptide, and its secretion has been suggested to be influenced by physiological stresses. Furthermore, since protection against these fatal toxicities has been previously reported to be provided by buprenorphine (a ligand for opioid receptors) and Ro 15-4513 (a ligand for benzodiazepine receptors), this study also focused on the relationship between the effects of these two ligands and the changes in brain beta-endorphin immunoreactivity. In the fatal toxicity study, a toxic dose (75 mg/kg, i.p.) of cocaine combined with and without ethanol (3 g/kg, i.p.) was administered to the rats, with and without buprenorphine (0.25, 0.5, 1 mg/kg, i.p.) or Ro 15-4513 (5, 10, 15 mg/kg, i.p.). All of the deaths that occurred in these animals were divided into two groups: early deaths with early toxic symptoms in which the drugs were detected in the tissue samples, and late deaths with late toxic symptoms in which no drugs were detected in the samples. Without the administration of buprenorphine or Ro 15-4513, the frequency of late deaths was higher in the cocaine group as compared to the cocaine-ethanol group. The total mortality rate was effectively attenuated by treatment with 0.25 mg/kg buprenorphine or 10 mg/kg Ro 15-4513. Following treatment with 1 mg/kg buprenorphine or 15 mg/kg Ro 15-4513, the frequency of late deaths was significantly enhanced in the cocaine group. The brain and liver cocaethylene concentrations were also attenuated in those groups in which the total mortality rates were attenuated. In the brain beta-endorphin immunoreactivity study, the number of beta-endorphin immunoreactive nerve cells at the arcuate nucleus was counted at 3 minutes or 24 hours after the drug treatment. At 3 minutes after the drug treatment, the number of weakly immunoreactive cells with photographic light absorption values greater than 50% was enhanced in the groups in which the frequency of late deaths had been increased. In the cocaine-ethanol groups treated with buprenorphine or Ro 15-4513, this enhancement of weakly immunoreactive cells was observed when the total mortality rate was increased, regardless of the type of death. At 24 hours after the drug treatment (50 mg/kg cocaine), an enhancement of the weakly immunoreactive cells only was observed in all of the groups in which the occurrence of toxicities had been enhanced, regardless of the type of toxicity. Therefore, it can be concluded that the enhancement of total brain beta-endorphin immunoreactivity was closely correlated with the increase in the frequency of total fatal toxicities, and that the enhancement of weakly immunoreactive cells was closely correlated with the increase in the frequency of delayed fatal toxicities.