Anxiolytic effects of nitrous oxide in mice in the light-dark and holeboard exploratory tests

To investigate anxiolytic effects of nitrous oxide (N₂O), male mice were tested in two exploratory models — a two-chambered light-dark (L-D) unit and a holeboard. Tests were conducted inside a glovebag through which one of three mixtures of N₂O and oxygen (25, 50 and 75% N₂O) or room air (RA) was circulated at a flow rate of 101/min. The principal findings in the L-D unit were a concentration-related increase in number of interdepartmental transitions and a generalized increase in time spent on the light side. Nitrous oxide effectively elevated transitions in the L-D unit at a lower concentration (25% N₂O) than was required to increase locomotor activity in an open field (50% N₂O), suggesting that these two measures are at least partially independent; transitions might reflect a specific exploratory component of locomotor behavior. In the holeboard test, a concentration-related increase in number of head dips was observed. Pretreatment with naltrexone-HCl or saline vehicle revealed a contribution by an endogenous opioid-linked locomotor stimulant effect in some measures. A dose-related reversal by flumazenil of 50% N₂O-induced shifts in number of head dips and time spent head-dipping implicates a benzodiazepine receptor. Both paradigms, in particular the holeboard, should prove useful in future N₂O research.