Fig. 3. Activity-dependent [Cl⁻]_i gradient in the superficial dorsal horn.

a MQAE fluorescence in the SDH of a transverse spinal cord slice (left) and pseudocolor image (right) show heterogeneous Cl⁻ concentrations across the SDH. b Representative patch clamp recordings from SDH neurons illustrate the strong increase in the frequency of spontaneous inhibitory (left) and excitatory (right) post-synaptic currents upon capsaicin administration (2 µM). c Estimated [Cl⁻]_i from MQAE fluorescence lifetime of SDH neurons is not correlated to the distance from the dorsal white matter in the presence of TTX (1 µM), bicuculline (20 µM), strychnine (1 µM), CNQX (10 µM), AP5 (40 µM) to block synaptic activity (black; slope −0.007 mM µm^–1; n cells per point = 20–43, total = 179 cells; F = 0.4, P = 0.6), while the correlation is significant in presence of capsaicin to enhance synaptic activity (red; slope = –0.027 mM µm^–1; n cells per point = 24–49, total = 209 cells; F = 4.8, P = 0.03). Two-way-ANOVA, F_treatment = 17.2, P < 0.001. [Cl⁻]_i values in SDH neurons are binned every 20 µm. d In silico model of the differential impact of somatic synaptic input on [Cl⁻]_i (Δ[Cl⁻]_i) between LI and LII. A first scenario considers the frequency of excitatory events equal to the frequency of inhibitory events (solid line), which approximates baseline and in vivo conditions, whereas a second scenario considers a frequency of excitatory events ten times higher than the frequency of inhibitory events (dashed line), similar to what we observed experimentally in slices under capsaicin application. Simulations are based on extrusion capacities estimated in Fig. 2g. LI Lamina I, LII lamina II, IPSCs  inhibitory post-synaptic currents, EPSCs excitatory post-synaptic currents. Data are shown as mean ± S.E.M. *P < 0.05.