TRPA1 modulation of spontaneous and mechanically evoked firing of spinal neurons in uninjured, osteoarthritic, and inflamed rats

Abstract

Background: There is growing evidence supporting a role for TRPA1 receptors in the neurotransmission of peripheral mechanical stimulation. In order to enhance understanding of TRPA1 contributions to mechanotransmission, we examined the effects a selective TRPA1 receptor antagonist, A-967079, on spinal neuronal activity following peripheral mechanical stimulation in uninjured, CFA-inflamed, and osteoarthritc (OA) rats.

Results: Systemic injection of A-967079 (30 micromol/kg, i.v.) decreased the responses of wide dynamic range (WDR), and nociceptive specific (NS) neurons following noxious pinch stimulation of the ipsilateral hind paw in uninjured and CFA-inflamed rats. Similarly, A-967079 reduced the responses of WDR neurons to high-intensity mechanical stimulation (300 g von Frey hair) of the knee joint in both OA and OA-sham rats. WDR neuronal responses to low-intensity mechanical stimulation (10 g von Frey hair) were also reduced by A-967079 administration to CFA-inflamed rats, but no effect was observed in uninjured rats. Additionally, the spontaneous activity of WDR neurons was decreased after A-967079 injection in CFA-inflamed rats but was unaltered in uninjured, OA, and OA-sham animals.

Conclusions: Blockade of TRPA1 receptors disrupts transmission of high-intensity mechanical stimulation to the spinal cord in both uninjured and injured rats indicating that TRPA1 receptors have an important role in noxious mechanosensation in both normal and pathological conditions. TRPA1 receptors also contribute to the transmission of low-intensity mechanical stimulation, and to the modulation of spontaneous WDR firing, but only after an inflammatory injury.

Figure 1

Structure of A-967079.

Figure 2

In uninjured rats, (A) systemic administration of A-967079 (30 μmol/kg, i.v.) reduced WDR responses to noxious pinch (28 mm mini bulldog clamp for 10 sec), but not to 10-g von Frey stimulation (n = 5-7 per group) of the RF on the hind paw. (B) Spontaneous firing of WDR neurons was unaltered by A-967079 injection to uninjured rats (n = 7-12 per group). Representative ratemeters showing the responses of a single WDR neuron to; (C) 10-g von Frey hair stimulation (15 sec), and (D) a noxious pinch (28 mm mini bulldog clamp for 10 sec) both before and after injection of A-967079 (30 μmol/kg, i.v.) to uninjured rats. VF = 10-g von Frey hair stimulation, P = noxious pinch (mini bulldog clamp). Since vehicle did not alter WDR firing in animals receiving either a von Frey (n = 5) or pinch stimulus (n = 2), this data was combined for comparison. *p < 0.05, vs. baseline firing; ⁺p < 0.05 vs. vehicle-treated group (Fisher's LSD).

Figure 3

In CFA-inflamed rats, systemic administration of A-967079 (30 μmol/kg, i.v.) reduced (A) WDR responses to noxious pinch (28 mm mini bulldog clamp for 10 sec), and 10-g von Frey stimulation (n = 6 per group) of the RF on the hind paw. (B) Injection of A-967079 (30 μmol/kg, i.v.) also decreased the spontaneous firing of WDR neurons in CFA-inflamed rats (n = 6-12 per group). Since vehicle did not alter WDR firing in animals that received either a von Frey (n = 4) or pinch stimulus (n = 2), this data was combined for comparison. *p < 0.05, **p < 0.01 vs. baseline firing; ⁺⁺p < 0.01 vs. vehicle-treated group (Fisher's LSD).

Figure 4

Effects of A-967079 (30 μmol/kg, i.v.) on single WDR neurons recorded from CFA-inflamed rats. Both evoked and spontaneous firing was attenuated by injection of A-967079. VF = 10-g von Frey hair stimulation, char = characterization period, T = non-noxious tap, B = non-noxious brush, P = noxious pinch (mini bulldog clamp). The neuronal RF on the hind paw was stimulated.

Figure 5

(A) Systemic administration of A-967079 (30 μmol/kg, i.v.) reduced NS responses to a noxious pinch (28 mm mini bulldog clamp for 10 sec) in both uninjured and CFA-inflamed rats (n = 5-6 per group). (B) Representative ratemeters showing the responses of a single NS neuron to noxious pinch (10 sec) stimulation of the hind paw in uninjured (left), and CFA-inflamed (right) rats before and after injection of A-967079 (30 μmol/kg, i.v.). P = 10 sec pinch (mini bulldog clamp) stimulation. Since vehicle did not alter NS firing in CFA (n = 3) or uninjured animals (n = 2), this data was combined for comparison **p < 0.01 vs. baseline firing; ⁺p < 0.05, ⁺⁺p < 0.01 vs. vehicle-treated group (Fisher's LSD).

Figure 6

In OA and OA-sham rats, systemic administration of A-967079 (30 μmol/kg, i.v.) reduced (A) the responses of WDR neurons high-intensity von Frey hair stimulation (300 g for 10 s) of the knee joint, but did not alter the (B) spontaneous firing of WDR neurons (n = 7-8 per group). Since vehicle did not alter WDR firing in OA (n = 5) or OA-sham rats (n = 2), this data was combined for comparison. *p < 0.05, **p < 0.01 vs. baseline firing; ⁺p < 0.05, ⁺⁺p < 0.01 vs. vehicle-treated group (Fisher's LSD).

Figure 7

Effects of A-967079 (30 μmol/kg, i.v.) on single WDR neurons recorded from either (A) an OA-sham or (B) an OA rat. In both cases, A-967079 reduced the evoked responses of WDR neurons following high-intensity stimulation (300 g von Frey for 10 s), but did not change the spontaneous activity. R = gentle manual rubbing of the knee joint, VF = 300-g von Frey hair stimulation of the knee, char = characterization period.