Aberrant TRPV1 expression in heat hyperalgesia associated with trigeminal neuropathic pain

Abstract

Trigeminal neuropathic pain is a facial pain syndrome associated with trigeminal nerve injury. However, the mechanism of trigeminal neuropathic pain is poorly understood. This study aimed to determine the role of transient receptor potential vanilloid 1 (TRPV1) in heat hyperalgesia in a trigeminal neuropathic pain model. We evaluated nociceptive responses to mechanical and heat stimuli using a partial infraorbital nerve ligation (pIONL) model. Withdrawal responses to mechanical and heat stimuli to vibrissal pads (VP) were assessed using von Frey filaments and a thermal stimulator equipped with a heat probe, respectively. Changes in withdrawal responses were measured after subcutaneous injection of the TRP channel antagonist capsazepine. In addition, the expression of TRPV1 in the trigeminal ganglia was examined. Mechanical allodynia and heat hyperalgesia were observed in VP by pIONL. Capsazepine suppressed heat hyperalgesia but not mechanical allodynia. The number of TRPV1-positive neurons in the trigeminal ganglia was significantly increased in the large-diameter-cell group. These results suggest that TRPV1 plays an important role in the heat hyperalgesia observed in the pIONL model.

Keywords: TRPV1; heat hyperalgesia.; trigeminal neuropathic pain.

Figure 1

Time course of the changes in mechanical (A) and heat (B) sensitivities in vibrissal pad after pIONL. Vibrissal pads ipsilateral to nerve injury and sham operation were stimulated with noxious stimuli. Open circles, Sham group (n = 5); closed circles, pIONL group (n = 5). P values were calculated by two-factor repeated-measure ANOVA corrected with Holm method (10 hypotheses). * P < 0.05 and ** P < 0.01 (Sham vs. pIONL).

Figure 2

Effects of TRPV1 antagonist capsazepine (CPZ) on behavioral experiments. At 25-28 days after pIONL, none (pre), 50 μL of DMSO, 10 pmol/50 μL of CPZ and 100 pmol/50 μL of CPZ were intraperitoneally injected. After 30 min, mechanical (A) and heat (B) sensitivities in vibrissal pad were examined. P values were calculated by two-factor repeated-measure ANOVA corrected with Holm method (6 hypotheses). * P < 0.05, ** P < 0.01 (pre vs. DMSO, DMSO vs. 10 pmol/50 μL of CPZ and DMSO vs. 100 pmol/50 μL of CPZ in each operation).

Figure 3

Fluorescent photomicrographs of TRPV1-immunoreactive cells in TG on sham-operated side (A), nerve ligation side (B) and in the absence of a primary antibody. (C). Arrows indicate TRPV1-positive neuron. Bar, 100 μm.

Figure 4

Proportion of TRPV1-positive neurons in TG neurons in the pIONL group (n = 4) and the Sham group (n = 4). (A) The means of ratios of TRPV1-positive cells to all cells in TG neurons. P = 0.952 (Welch test). (B) The mean of the major axis of TRPV1-positive cells. Total TRPV1-positive cell numbers are indicated in parentheses. P < 0.001 (Welch test). (C) Ratios of TRPV1-positive cells classified by cell size (C). Cells were classified into 3 groups (< 20 μm, 20 to 30 μm, > 30 μm) by the major axis of cells. The ratios of TRPV1-positive cells in each group to total TRPV1-positive cell number are shown. P < 0.001 (Fisher's exact test).