Figure 4. Capsaicin Sensitivity of Single Peripheral Nerve Fibers in the Saphenous Skin Nerve Preparation and for Sensory Neurons in Culture.

(A) An example of a polymodal C-fiber (C_MH) response to local application of a 1 mM capsaicin solution to its receptive field.

(B) The average firing rate of 11 capsaicin-sensitive C-fibers during the 2 min application.

(C) The proportion of C_M and C_MH fibers showing an excitatory response to capsaicin.

(D) The rate of firing of C_MH fibers during application of pH 5.0 saline solution to the receptive field; no increase in firing rate was found in any of the tested fibers during acid application (n = 20).

(E) Calcium imaging experiments were performed to measure the intracellular calcium changes in response to capsaicin applied to sensory neurons in culture. A typical experiment is shown, where a group of cells from naked mole-rat were monitored, the phase contrast image is shown on the left. An image of the Fura-2 fluorescence signal measured when excited at 340 nm is shown for the same cells under resting conditions and in the presence (right) and absence (middle) of 10 nM capsaicin.

(F) Two of the five cells shown displayed a large increase in intracellular calcium upon application of capsaicin (10 nM) as shown in the F340/F380 ratio.

(G) Quantification of the proportion of imaged sensory neurons showing an increase in intracellular calcium after application of 10 nM or 2 μM capsaicin to naked mole-rat or mouse sensory neurons (n = 69 and n = 16). No significant differences were observed in the proportion of capsaicin-sensitive neurons between mouse and naked mole-rat.