A study of central opioid receptor involvement in nitrous oxide analgesia in mice.

This study was undertaken to assess the sensitivity of nitrous oxide (N2O) analgesia to antagonism by intrathecally (IT) and intracerebroventricularly (ICV) administered antagonists selective for kappa- and mu-opioid receptors. Male ICR mice were pretreated IT or ICV with the kappa antagonist nor-binaltorphimine (nor-BNI), 1 or 50 nmol, respectively, or distilled water (control), then exposed to N2O (50% or 75% in oxygen). Compared with IT control mice, IT nor-BNI-pretreated mice responded with significantly less analgesia. Compared with ICV control mice, ICV nor-BNI-pretreated mice also showed markedly reduced analgesic response. Other mice were pretreated IT or ICV with either the selective and irreversible mu antagonist beta-funaltrexamine (beta-FNA, 5.0 micrograms) or distilled water (control). When exposed to N2O 24 h later, beta-FNA-pretreated and control mice exhibited comparable analgesic responses. These preliminary results suggest that N2O analagesia in mice may involve spinal and supraspinal kappa-opioid receptors but not mu-opioid receptors.