Fig. 4

TrkB⁺ neurons are necessary and sufficient to convey mechanical allodynia after nerve injury. a Schematic of CFA injection and behavior tests following ablation of TrkB⁺ neurons. Mechanical hypersensitivity in control Avil^iDTR (black bar), TrkB^CreERT2::Avil^iDTR (white bar), and sham-injected (gray bar) mice 48 h after CFA injections as measured by b von Frey filaments (t-test, p = 0.886), c dynamic brush stimuli (t-test; p = 0.537), and d cotton swab stimuli (t-test; p = 0.242). All mice received two diphtheria toxin injections 7 days and 10 days before CFA treatment. e Paw withdrawal frequencies in control (Rosa26^ChR2 mice without Cre, black bar), and CFA-injected (white bar) or sham (grey bar)-injected paw of TrkB^CreERT2::Rosa26^ChR2 mice upon stimulation with 473 nm blue light. No significant differences under baseline conditions and 48 h after CFA injection (Mann–Whitney test; p = 0.886). f Schematic of SNI and behavioral tests following ablation of TrkB⁺ neurons. g von Frey mechanical thresholds indicating that ablation of TrkB⁺ neurons abolished the development of mechanical allodynia after SNI in TrkB^CreERT2::Avil^iDTR mice (white circles) as compared to Avil^iDTR controls (black circles) (n = 7 for both sets, two-way RM ANOVA; p = 0.001 followed by a Bonferroni post hoc test). Sham-operated mice (grey circles) did not develop mechanical hypersensitivity. h, i Reduced dynamic allodynia in ablated TrkB^CreERT2::Avil^iDTR mice (white bar) as compared to littermate controls (black bar; t-test p = 0.016) stimulated with a h brush or i cotton swab. Sham-operated mice did not develop dynamic allodynia. j Nociceptive behavior evoked by optogenetic stimulation of the paws of Rosa26^ChR2 mice without Cre (black bars) and TrkB^CreERT2::Rosa26^ChR2 (white bars) mice after SNI, or ipsilateral paws (grey bars) of sham-operated mice (two-way RM ANOVA; p = 0.001). k Cross sections of lumbar spinal cord from TrkB^CreERT2::Rosa26^ChR2 mice labeled for c-fos (red) and IB4 (green) after 1 min exposure to 15 Hz blue light. Representation of section taken from an uninjured mouse and a section from an injured mouse at 7 days post SNI. l Quantification of the number of c-fos positive cells in laminae I, II, and III/V of the lumbar spinal cord within a 40 μm section. Data are shown for SNI, non-injured and sham-operated TrkB^CreERT2::Rosa26^ChR2 mice with or without light, and control SNI Rosa26^ChR2 mice without Cre. Baseline indicates pre-ablation and pre-treatment. Error bars indicate SEM. Scale bars in k, 40 μm