Nociceptin system does not affect MAC of volatile anaesthetics

Background: Nociceptin is the endogenous agonist of the opioid receptor‐like (ORL) 1 receptor (NOP), and both nociceptin and NOP are widely expressed in the brain and spinal cord, which are target organs of general anaesthetics. As nociceptin has been reported to be involved in modulating pain mechanisms and stress responses, it is possible that the activity of the nociceptin system affects the anaesthetic potency of general anaesthetics. To address this possibility, we investigated the minimum alveolar concentrations (MACs) of various volatile anaesthetics in nociceptin receptor knockout mice (NOP–/–) and wild‐type mice (NOP+/+). Methods: We used male NOP–/– mice and NOP+/+ mice. MACs for halothane, isoflurane and sevoflurane were determined by the tail‐clamp method. Results: MACs for halothane, isoflurane and sevoflurane in NOP–/– mice were 1.60 (SD 0.06), 1.68 (0.08) and 3.36 (0.07)%, respectively. In NOP+/+ mice, MACs for halothane, isoflurane and sevoflurane were 1.59 (SD 0.07), 1.72 (0.07) and 3.38 (0.09)%, respectively. Conclusion: MACs in NOP–/– mice did not significantly differ from those in NOP+/+ mice for halothane, isoflurane and sevoflurane. This result suggests that the nociceptin system does not affect the anaesthetic potency of volatile anaesthetics.