Effects of ketamine on nociception and gastrointestinal motility in mice are unaffected by naloxone.

The present study investigates the effects of ketamine on nociception towards chemical and thermic stimuli and on gastrointestinal transit in mice. The reversibility of these effects by the opioid antagonist naloxone (10 mg/kg) was also assessed. Ketamine promoted dose-related analgesia in both the acetic acid-induced writhing and hot plate tests. Analgesia was not influenced by pretreatment with naloxone. Contrasting the constipation induced by opioids, ketamine enhanced gastrointestinal transit in a dose-dependent manner and this was not modified by naloxone. These results suggest that although ketamine can elicit analgesia, it does not activate opioid mechanisms in subanesthetic doses.