Fig. 8: Functional classes of SPB neurons show distinct alterations in tuning after central sensitization.

a Experimental design. b Left: cardinal modality (left) and mechanical sub-modality (right) tuning of SPB neurons (SPBNs) before (grey) and after capsaicin injection (red). **P < 0.01 (Two-tailed Wilcoxon matched pairs signed rank test comparing angle, P = 0.0005). c Pie-chart of the 3 functionally defined classes within SPB neurons. Wide-dynamic range (WDR, Red) SPBNs are defined as those that responded to both low and high intensity stimuli but encoded the stimuli intensity in their response amplitude. High-threshold (HT, Blue) SPBNs responded primarily to the 2.0 g VF filament (HT stimulus). Low-Threshold (LT, Black) SPBNs only responded to brush and/or the 0.16 g VF filament (LT stimulus). d Mechanical sub-modality tuning of LT (left), WDR (middle), and HT (right) SPBN sub-groups before (grey) and after capsaicin injection (red). Dotted circle is 1 unit vector in diameter. *P < 0.05 (Two-tailed Wilcoxon matched pairs signed rank test WDR: P = 0.0391, HT = 0.0117). e Radar chart showing the response to CICADA ligands within LT (left), WDR (middle), and HT (right) SPBN sub-groups. The spoke length is the population normalized response to CICADA ligands. There was a significant main effect of CICADA agonist response (F (3.885, 65.48) = 3.377), P = 0.015, and a significant interaction between class and CICADA agonist response (F (14, 118) = 3.143), P = 0.0003 (Mixed-effects restricted maximum likelihood model). Post hoc testing was performed comparing CICADA agonist response across functional groups, *Q < 0.05. (HT vs WDR, Q = 0.0105, LT vs WDR, Q = 0.0105). f Proposed model and putative roles for identified excitatory populations in the development of capsaicin induced allodynia.