Fig. 5. Prefrontal cortex acetylcholine (ACh) release did not change as a function of time during 150 min of 1.5% isoflurane. Histograms emulate the time course of the morphine studies but show acetylcholine during 75 min of substantia innominata dialysis with Ringer’s ( first six bars ) followed by a second 75-min interval of substantia innominata Ringer’s dialysis ( second six bars ). Regression analyses revealed best fit by a linear model (Y = 0.2714 + (−0.001) × X). The regression analysis of variance was not significant, and the adjusted r 2showed that dialysis time accounted for 0% of the variance in acetylcholine release. PFC = prefrontal cortex.