Fig. 1. Experimental gas inhalation. Surgery in all rats lasted for approximately 20 min, with a recovery period of 60 min. (A –D ) Each box (squares) represents one experimental group. With variation of the xenon concentration (30 and 70%), nitrogen concentration changed from 40% to 0%, while oxygen concentration was kept constant at 30%. In the steady state and control experiments, a various time delay until the end of the cerebral blood flow experiments (1 min) and cerebral glucose utilization experiments (45 min) is required because of the different tracer kinetics in these two methods for the assessment of a physiology at the same time. 18,37Therefore, to relate the cerebral blood flow measurement to the glucose utilization, the cerebral blood flow experiment was performed 15 min after the deoxyglucose pulse infusion in the respective cerebral glucose utilization experiment. In the short-term inhalation groups, the gas-sealed inhalation chamber was first flushed with 70% xenon for 45 s. The inhalation periods with 70% xenon for 2 or 5 min were then started. For local cerebral blood flow (LCBF) measurements, tracer infusion was started 60 s before decapitation. (A ) Conscious controls: rats breathed nitrogen–oxygen (Fio²= 0.3) for 45 or 60 min until either 2-[1-¹⁴C]deoxy-d-glucose (measurement of local cerebral glucose utilization [LCGU]) or 4-iodo-N -methyl-[¹⁴C]antipyrine (measurement of LCBF) was infused, respectively. Tracer infusion for the LCBF experiments began at minute 59, and decapitation was performed at minute 60. Tracer infusion for the LCGU experiments was performed as a pulse at minute 45, and decapitation was performed at minute 90. (B ) Steady state inhalation of 30% xenon: rats breathed 30% xenon in nitrogen–oxygen (Fio² = 0.3). After a xenon equilibration period within the rat of 30 min, LCGU experiments were started with a pulse infusion and lasted for another 45 min. LCBF experiments were performed after a xenon equilibration period within the rat of 44 min and lasted for another minute. (C ) Steady state inhalation of 70% xenon: rats breathed 70% xenon in oxygen (Fio²= 0.3). Except for the higher xenon concentrations used, the experiments were identical with the steady state inhalation experiments of 30% xenon. (D ) Short-term inhalation of 70% xenon: in one group, rats breathed 70% xenon in oxygen (Fio²= 0.3) for 1 min. Then 4-iodo-N -methyl-[¹⁴C]antipyrine infusion for the measurement of LCBF was started, which lasted for another minute. In the other group of short-term inhalation, rats breathed 70% xenon in oxygen (Fio²= 0.3) for 4 min, when 4-iodo-N -methyl-[¹⁴C]antipyrine infusion for the measurement of LCBF was started, which lasted for another minute.