Sumatriptan alleviates nitroglycerin-induced mechanical and thermal allodynia in mice

Abstract

The association between the clinical use of nitroglycerin (NTG) and headache has led to the examination of NTG as a model trigger for migraine and related headache disorders, both in humans and laboratory animals. In this study in mice, we hypothesized that NTG could trigger behavioural and physiological responses that resemble a common manifestation of migraine in humans. We report that animals exhibit a dose-dependent and prolonged NTG-induced thermal and mechanical allodynia, starting 30-60 min after intraperitoneal injection of NTG at 5-10 mg/kg. NTG administration also induced Fos expression, an anatomical marker of neuronal activity in neurons of the trigeminal nucleus caudalis and cervical spinal cord dorsal horn, suggesting that enhanced nociceptive processing within the spinal cord contributes to the increased nociceptive behaviour. Moreover, sumatriptan, a drug with relative specificity for migraine, alleviated the NTG-induced allodynia. We also tested whether NTG reduces the threshold for cortical spreading depression (CSD), an event considered to be the physiological substrate of the migraine aura. We found that the threshold of CSD was unaffected by NTG, suggesting that NTG stimulates migraine mechanisms that are independent of the regulation of cortical excitability.

Figure 1

Nitroglycerin (NTG) induces thermal hypersensitivity. Time course of thermal nociceptive thresholds, measured by hindpaw withdrawal latency to a radiant heat source (Hargreaves test), after various doses of NTG administration. Latency to respond is decreased in animals that received 5 or 10 mg/kg NTG compared with animals that received saline. n = 10–12 animals per group; *P < 1 × 10⁻²; **P < 5 × 10⁻². Neither 1.0 nor 0.5 mg/kg NTG significantly affected the response to radiant heat (P > 5 × 10⁻²).

Figure 2

Nitroglycerin (NTG) induces dose-dependent mechanical hypersensitivity. NTG triggers changes in hindpaw withdrawal thresholds to mechanical stimulation with von Frey monofilaments. Administration of 5 or 10 mg/kg NTG significantly reduces mechanical pain thresholds at 60, 120 and 240 min compared with animals treated with saline (n = 10–12 animals per group; *P = 3 × 10⁻²; **P = 9 × 10⁻³). Mechanical thresholds in animals treated with lower doses (0.5, 1.0 and 5.0 mg/kg NTG) are not statistically different from animals treated with saline.

Figure 3

Sumatriptan reverses nitroglycerin (NTG)-induced thermal hypersensitivity. Intrathecal injection of 0.06 μg sumatriptan or intraperitoneal (i.p.) injection of 600 μg/kg sumatriptan alleviates NTG-induced thermal hypersensitivity (10 mg/kg i.p. NTG) (anova analysis **P < 5 × 10⁻³; *P < 5 × 10⁻²). The difference between i.p. injection of 600 μg/kg sumatriptan and saline is not significant at other time points.

Figure 4

Sumatriptan reduces nitroglycerin (NTG)-induced mechanical hypersensitivity. Intrathecal (i.t.) 0.06 μg sumatriptan reduces NTG-induced mechanical hypersensitivity [10 mg/kg intraperitoneal (i.p.) NTG] (**P < 5 × 10⁻⁴; *P = 6 × 10⁻³ compared with animals that received an i.t. saline injection). Sumatriptan (600 μg/kg, i.p.) did not significantly change the mechanical pain threshold after NTG compared with i.p. saline injection.

Figure 5

Nitroglycerin (NTG)-induced Fos expression in the spinal cord and trigeminal nucleus. (A) Quantification of Fos-immunoreactive nuclei in the cervical spinal cord lamina I–II, III–V, VI–X, and trigeminal nucleus caudalis 2 h after systemic 10 mg/kg NTG or saline. n = 3 animals per group, *P < 5 × 10⁻³. Representative examples of Fos immunoreactivity in the trigeminal nucleus caudalis at 2 h after the administration of (B) saline or (C) NTG 10 mg/kg, shown here with a 20× objective.

Figure 6

Determination of the cortical spreading depression (CSD) threshold with nitroglycerin (NTG) treatment. Mean CSD threshold 30 min prior to, and 30 and 60 min subsequent to treatment with saline vehicle or NTG 10 mg/kg. No significant difference was noted in CSD threshold in vehicle (n = 7) vs. NTG-treated (n = 7) animals. (b) Panels: Leftmost panel shows appearance of intact skull and underlying cortex with 940-nm illumination. Stimulation pipette at bottom centre of image; field potential electrode at top right corner. Scale bar: 500 μm. 1–5: Ratiometric images showing progression of CSD. Line trace: Optical intrinsic signal CSD waveform at 940 nm, from region of interest at centre of each image. Time point of each image is labelled. Vertical scale bar: 2% reflectance change; horizontal scale bar: 1 min.