Figure 6.

Blockade of spinal 5-HT₃Rs and NMDARs abolished immediate-onset, descending facilitation and withdrawal LTP after intravenous fentanyl and morphine. Areas of C-fiber-evoked field potentials were normalized to predrug values (dotted line) and plotted against time (minutes). A, The 5-HT₃R antagonist granisetron and the NMDAR antagonist d-AP-5 were applied simultaneously to the spinal cord dorsum (top white bar; 1 mm and 100 μm, respectively). Intravenous infusion of fentanyl (black bar) still induced a depression, but withdrawal precipitated with spinal CTOP (bottom white bar; 10 μm) failed to elicit opioid-induced enhancement of synaptic transmission. B, Same experiment as in A but with intravenous morphine (black bar; dosing as in Fig. 1C).