Fig. 2. Opioids decrease pontine reticular formation (PRF) adenosine levels. ( A ) Data summarize PRF adenosine levels during dialysis with Ringer's solution and 10, 30, 100, and 300 μm morphine. * Significant ( P < 0.05) decrease from Ringer's control (0 μm). ( B ) The morphine (100 μm)–induced decrease in PRF adenosine was blocked by coadministration of naloxone (10 μm). These data illustrate that adenosine levels during coadministration of morphine and naloxone were not statistically different from adenosine levels during dialysis with Ringer's solution. ( C ) Dialysis with fentanyl (100 μm) caused a decrease in PRF adenosine levels. ( D ) A plate from a rat brain atlas 22 illustrates a microdialysis probe placed in the PRF for drug delivery and collecting endogenous adenosine. The digitized rat brain stem section at right shows a representative microdialysis site ( arrow ) in the PRF.