Behavioural and histological observations of sensory impairment caused by tight ligation of the trigeminal nerve in mice.

Dental treatments sometimes cause sensory impairment, especially in the region innervated by the third division of the trigeminal nerve. The most frequent symptoms are loss of sensation and abnormal sensation. Although most studies have addressed the neuropathic symptom "allodynia" using experimental animal models of the infraorbital nerve, there is little information regarding the sensory impairment that frequently occurs clinically. Therefore, different experimental models are required to clarify the mechanisms of the clinical effects, and previous experimental models have been limited to rats. Here, we report a sensory impairment model in mice whose mechanical touch threshold increased after tight ligation of the mental nerve. Habituation before surgery by mechanical touching of the face enabled us to observe the long-term chronological changes in sensation. The mechanical touch thresholds within the mental nerve region were measured for 70 postoperative (PO) days. Changes in the distribution of substance P (SP) were evaluated by immunohistochemistry to clarify the involvement of axonal flow in the sensory impairment and its recovery. The mechanical touch thresholds transiently increased by PO days 2-3, but decreased to the preoperative levels at around PO day 14. Apparent SP immunoreactivity was recognizable on the medial side to the ligation at PO days 2-3 and disappeared at PO day 7. These behavioural and immunohistochemical changes appeared to exhibit similar time courses, suggesting a possible relationship between them. Therefore, we suggest that our experimental mouse model could represent a new model for clarifying the mechanism of the sensory impairment caused by peripheral nerve injury.