Altered thermal sensitivity in neurons injured by infraorbital nerve lesion.

Nerve lesions are common injuries. While peripheral sensitivity is lost, the partially regenerating nerve undergoes a complex transformation, occasionally leading to persistent pain syndromes. Changes of thermal perception following nerve injury have received little attention. This study investigates the sensitivity of trigeminal neurons after infraorbital nerve lesion in guinea-pigs. Cultured trigeminal neurons innervating the area of denervation were identified by retrograde transport of DiI deposited at the site of the lesion. The standardized protocol consisted of cold and heat stimulation starting from body temperature as well as application of menthol and capsaicin, while activation was quantified by Fura-2-based calcium microfluorimetry. Compared to neurons from control animals, DiI-positive neurons were similar in the percentage and extend of the responses to menthol and capsaicin. However, DiI-positive neurons were less responsive to cold stimulation and had a lower cold threshold when compared to DiI-negative or control neurons. At the same time, DiI-positive neurons were more responsive to heat stimulation and had a lower heat threshold compared to control neurons. In summary, the percentage of trigeminal neurons responsive to thermal or chemical stimulation did not change after axotomy. However, thermal sensitivity of these neurons was altered.