Pain threshold monitoring during chronic constriction injury of the infraorbital nerve in rats

Background: The chronic constriction injury (CCI) of the infraorbital nerve (ION) has been used to establish an animal mode of trigeminal neuralgia (TN), but key parameters of the model have not been quantified until now. Objective: The aim of the study was to quantify a standard of pain threshold to evaluate a successful TN model in Sprague-Dawley (SD) rats. Methods: Forty-eight adult SD rats (200-220 g) underwent chronic constriction injury of the infraorbital nerve. The pain threshold was tested one day preoperatively (baseline) and day 1, 3, 7, 14, 28 postoperatively using the up-down method. At day 28, all the animals were killed by dislocation of the cervical spine and the trigeminal nerve specimens were removed for electron microscopy. Results: The baseline pain threshold was 14.40 +/- 0.87 g. Postoperatively, all the rats presented an initial reduced sensitivity to mechanical stimulation from day 1 (15.63 +/- 1.92 g) through 7 (17.39 +/- 1.43 g) after the surgery. At day 14, 32 (66.7%) began to show significant mechanical allodynia (0.71 +/- 0.43 g) which did not change significantly till day 28 (0.88 +/- 0.54 g). These animals were regarded as successful TN models with a 95% confidence interval of the pain threshold of 0.58-1.27 at Day 14. The electron microscopy demonstrated homogeneously demyelinated changes in those successful TN model animals rather than severe or mild epineurial lesions in those unsuccessful model animals. Conclusion: Our study showed that an animal TN model could be established with a two-week chronic constriction injury of the infraorbital nerve. The mechanical allodynia index <1.27 at Day 14 was suggested as a criterion for a successful model.