Glial Response after Inferior Alveolar Nerve Injury

Inferior alveolar nerve (IAN) is the sensory branch of trigeminal nerve that innervates dental pulp and periodontal tissue and conveys somatosensory information to sensory nuclei in the brainstem. Peripheral nerve fibers of the IAN are damaged during dental surgeries such as endodontic procedure and tooth extraction. In some cases, taste deficits have been reported even at 6 month after the surgery suggesting alterations in gustatory function of central nervous system (CNS).

   Peripheral injury to sensory nerves causes glial responses at the primary nerve terminals of the ascending sensory pathways in the CNS. In the responses, microglia alters neural activity in the primary sensory nuclei. Because some of afferent fibers of IAN form synapses onto gustatory neurons in the rostral nucleus of the solitary tract (rNST), the microglia response may change in activity of the gustatory neurons upon IAN damage, which may lead to taste deficits. In this study, we investigated distribution of microglia in rNST after inferior alveolar nerve (IAN) transection as a first step to understand effects of IAN damage on the gustatory function.

   IAN in male Sprague-Dawley rats (eight weeks old) was transected at the left mandibular angle under pentobarbital anesthesia. One week after the IAN transaction, a microglia-specific protein Iba1-immunoreactive (ir) cells were labeled with fluorescent immunohistochemistry in brainstem slices that include NST. Fluorescent images of the Iba1-ir cells were taken on a laser scanning confocal microscope. Glial responses were evaluated by counting pixels above threshold representing Iba1-immunoreactive products. Mean value was calculated for the pixel counts in defined six regions within NST. Comparison was made between IAN-transected and sham control groups using Mann-Whitney U test. Results with p < 0.05 was considered as significant.

   In NST, density of the pixels was significantly higher in the rostral-lateral region in the IAN-transected rats in comparison with the same region in sham rats. This result suggests taste deficits after dental surgery may be attributed to responses of rNST microglia to IAN damage.

   1. Boucher Y, Berteretche MV, Farhang F, Arvy MP, Azérad J, Faurion A. 2006. Taste deficits related to dental deafferentation: an electrogustometric study in humans. Eur J Oral Sci. 114(6):456-64.

   2. Shafer DM1, Frank ME, Gent JF, Fischer ME. 1999. Gustatory function after third molar extraction. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 87(4):419-28.