The colocalization of TrkB and 5-HT in RVM neurons projecting to spinal dorsal horn and the elimination of intra-RVM BDNF-induced hyperalgesia by the descending 5-HT depletion. Aa, Distribution of retrograde tracer FG expression in neurons of the RVM area at 7 d after microinjection in the lumbar spinal dorsal horn. Ab, A high-magnification view of the inset outlined in Aa shows the spinal projecting RVM neurons and their dendrites expressing FG. Ac–Af, Combination of double immunostaining with retrograde FG tracking further demonstrates the expression of TrkB (Ac) in the most FG-labeled RVM neurons (Ad) and the colocalization of 5-HT immunoreactivity (IR, Ae) with TrkB-IR in the FG-labeled neurons in the RVM (Af). Scale bars: 200 μm (Aa), 50 μm (Ab–Af). B, Microinjection of BDNF (100 fmol) induces a prolonged thermal hyperalgesia in control gene-treated but not Tph-2 shRNA-treated rats. C, The same dose of BDNF results in mechanical allodynia in control but not Tph-2 shRNA-treated animals at 4–6 h after gene transfer in the RVM. *p < 0.05; **p < 0.01, BDNF versus ACSF in control shRNA-treated rats; #p < 0.05, BDNF versus ACSF in Tph-2 shRNA-treated animals.