Fig. 3. Time course of prefrontal cortical acetylcholine (ACh) release during dialysis delivery of morphine to the substantia innominata region of the basal forebrain. The figure plots prefrontal cortical acetylcholine release for each 12.5-min sample bin during the 75 min when morphine was delivered by dialysis. The four functions represent four morphine concentrations (1, 10, 100, and 1,000 μm) delivered to the substantia innominata dialysis probe. The  dashed horizontal line at 100% indicates acetylcholine release in prefrontal cortex during perfusion of the SI dialysis probe with Ringer’s (control). Each of the four drug concentrations was administered to three animals, and acetylcholine was measured in 72 dialysis samples. PFC = prefrontal cortex.