Continuous isoflurane flow protocol. “On-target” trials of muscimol microinjection into the mesopontine tegmentum are those which extended anesthesia time beyond that attributable to residual circulating isoflurane (ISO). These define the location of the MPTA. Symbols indicate results of individual trials in the mouse indicated. A Time to emergence from anesthesia after stopping the flow of isoflurane, marked by the onset of rhythmic limb pacing movements, averaged 5.0 min (solid horizontal line) ± 3.5 min (± SD, dashed horizontal lines) in 12 DBA/2 strain mice (all males). B Like A but in trials in which muscimol was microinjected into the mesopontine tegmentum at the time isoflurane flow was stopped. Subtracting the average time attributable to residual isoflurane from A (solid horizontal line marked with a red arrow) yields the bonus time attributable to the muscimol. Those trials in which bonus time exceeded the mean baseline isoflurane anesthesia time by 3 × SD were considered statistically significant. Only 2 trials, in mouse #6 and #7, met this criterion. C Same as A., but results from 10 B6 mice, all females except for #13, 14, 22. D Same as B. B6 mice. 6 trials in 5 mice reached statistical significance

Continuous isoflurane flow protocol. “On-target” trials of muscimol microinjection into the mesopontine tegmentum are those which extended anesthesia time beyond that attributable to residual circulating isoflurane (ISO). These define the location of the MPTA. Symbols indicate results of individual trials in the mouse indicated. A Time to emergence from anesthesia after stopping the flow of isoflurane, marked by the onset of rhythmic limb pacing movements, averaged 5.0 min (solid horizontal line) ± 3.5 min (± SD, dashed horizontal lines) in 12 DBA/2 strain mice (all males). B Like A but in trials in which muscimol was microinjected into the mesopontine tegmentum at the time isoflurane flow was stopped. Subtracting the average time attributable to residual isoflurane from A (solid horizontal line marked with a red arrow) yields the bonus time attributable to the muscimol. Those trials in which bonus time exceeded the mean baseline isoflurane anesthesia time by 3 × SD were considered statistically significant. Only 2 trials, in mouse #6 and #7, met this criterion. C Same as A., but results from 10 B6 mice, all females except for #13, 14, 22. D Same as B. B6 mice. 6 trials in 5 mice reached statistical significance

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The mesopontine tegmental anesthesia area (MPTA) was identified in rats as a singular brainstem locus at which microinjection of minute quantities of GABAergic agents rapidly and reversibly induces loss-of-consciousness and a state of general anesthesia, while lesioning renders animals insensitive to anesthetics at normal systemic doses. Obtaining...