Fig. 3. Morphine and fentanyl decreased adenosine levels in the substantia innominata (SI). (  A ) Microdialysis with morphine (100 μm) decreased SI adenosine levels. (  B ) Similarly, the Student  t test revealed a significant decrease in SI adenosine levels during dialysis with fentanyl (100 μm). (  C ) A coronal brain plate  (left ) illustrates placement of a microdialysis probe in the SI. A cresyl violet–stained brain section  (right ) confirms that the microdialysis site was localized to the SI. *  P < 0.05 .