Behavioral and electrophysiological aspects of nitrous oxide dependence

We examined the effect of 70% nitrous oxide (N2O) on locomotion and visual-evoked potentials (VEP) in rats. The animals exposed to N2O showed an initial decrease of locomotion, followed by development of tolerance and unaltered motor activity during N2O withdrawal. Similarly, an initial decrease of VEP amplitudes was followed by tolerance to N2O. In addition, some amplitudes (N2-P3, P3-N3, and N3-P4) exceeded the control values, indicating an increase of neuronal excitability of the visual system during a long lasting exposure to N2O. The increase of VEP amplitudes was further potentiated by cessation of this gas. The VEP latencies after initial increase, returned to normal and remained unaltered during N2O withdrawal suggesting that the speed of neurotransmission is not essentially changed during chronic exposure to N2O. However, a significant increase of neuronal excitability during chronic N2O exposure, which further increased by cessation of N2O, could be of clinical importance. Therefore, monitoring of VEP, particularly the amplitude values, may significantly improve a detection of altered neuronal excitability during anaesthesia and drug withdrawal.