Does nitrous oxide induce release of brain opioid peptides?

Increased concentrations of immunoreactive methionine-enkephalin but not leucine-enkephalin were measured in fractions of cerebrospinal fluid perfusate collected from ventricular-cisternally perfused, urethane-anesthetized rats exposed to a mixture of 75% nitrous oxide/25% oxygen for 60 min. These findings suggest that nitrous oxide might be capable of inducing the release of methionine-enkephalin in the central nervous system.