EFFECT OF SUBTYPE-SELECTIVE OPIOID RECEPTOR BLOCKERS ON NITROUS-OXIDE ANTINOCICEPTION IN RATS

Nitrous oxide antinociception in rats was evaluated by the warm water tail withdrawal test following central pretreatment with blockers of various opioid receptor subtypes. The analgesic dose, 50% (AD50) value for nitrous oxide antinociception was significantly elevated by MR-2266 (which is relatively selective for κ-opioid receptors) and increased to a lesser degree by ICI-174,864 (which is selective for δ-opioid receptors). However, pretreatment with β-funaltrexamine (which is selective for μ-opioid receptors), even at extremely high doses, was ineffective in altering the AD50 for nitrous oxide antinociception. According to these findings, nitrous oxide antinociception, as evaluated in this paradigm, appears to be mediated by κ- and possibly δ- but not μ-opioid receptors. © 1990 The Italian Pharmacological Society.