Effect on extracellular 5-HT of reverse dialysis infusion of fentanyl in the DRN: (a) Mean baseline level of extracellular 5-HT was 0.6±0.1 pg sample−1 (n=11) in the rats infused with citalopram-free aCSF and 7.0±0.8 pg sample−1 (n=23) with citalopram in the aCSF. The open horizontal bar indicates the period of fentanyl infusion. Fentanyl produced a concentration-dependent decrease in extracellular 5-HT in the DRN: F(3,14)=3.56, P=0.0421. Similarly, in response to local fentanyl infusion, 5-HT was decreased in rats infused with the citalopram-free aCSF (F(1,9)=18.01, P=0.0022). Plots of 300 μM fentanyl and citalopram-free aCSF control data and asterisks indicating statistical significance were omitted for the sake of clarity. (b) AUC values expressed as the cumulative decrease below baseline from 0 to 2 h after reverse dialysis infusion of fentanyl. The effect was significant (F(4,19)=6.53, P=0.0018). *P<0.05, compared to vehicle.