Dissociation of increased 5-hydroxyindoleacetic acid levels and physical dependence: the effects of naloxone.

1. A slow release emulsion of naloxone (naloxone SR) was administered subcutaneously to rats in an attempt to induce physical dependence of the morphine type. 2. Naltrexone (2.5 mg/kg), injected i.p., failed to elicit an abstinence syndrome in rats treated with 75, 100 or 150 mg/kg naloxone SR for 24, 48, or 72 h. 3. Naloxone SR had no effect on the whole brain levels of noradrenaline, dopamine, homovanillic acid or serotonin. 4. Naloxone SR caused an apparent dose-related increase in the brain levels of 5-hydroxyindoleacetic acid. 5. These results show that while naloxone does not induce physical dependence of the morphine type, it may, like morphine, increase the brain serotonin turnover rate. 6. It is proposed that the increase in brain serotonin turnover rate may not be causally related to physical dependence on morphine-like drugs but may be a property of drugs containing the basic opiate molecular structure.