Chronic tooth pulp inflammation causes transient and persistent expressionof Fos in dynorphin-rich regions of rat brainstem

We have analyzed central Fos immunoreactivity (Fos-IR) brainstems of adult rats after three clinically relevant dental injuries: filled dentin (DF) ca vities that cause mild pulp injury and heal within 1-2 weeks; open pulp exp osures (PX) that cause gradual pulp loss and subsequent periodontal lesions ; and filled pulp exposures (PXF). By 1 week after Dr; cavities, no Fos-IR remained except for sites such as lateral-ventral periolivary nucleus (LVPO ) that had Fos-IR in all rats including controls. PX injury induced (1) a d elayed transient expression of Fos at 1-2 weeks at three loci (ipsilateral neurons :in dorsomedial nucleus oralis, paratrigeminal nucleus, and trigemi nal tract), (2) persistent ipsilateral Fos for at least 4 weeks after injur y in dynorphin (Dyn)-rich regions (rostral lateral solitary nucleus, periob ex dorsal nucleus caudalis), and (3) late Fos-IR at 2-4 weeks (bilateral su perficial cervical dorsal horn, contralateral dorsal nucleus caudalis, cont ralateral rostral lateral solitary nucleus). Rats with PXF injury were exam ined at 2 weeks, and they had greater numbers and more extensive rostro-cau dal distribution of Fos neurons than the PX group. One week after PX injury , Fos-IR neurons were found in regions with strong Dyn-IR central fibers. G o-expression of Dyn and Fos was found in some unusually large neurons of th e ipsilateral rostral lateral solitary nucleus, trigeminal tract, and dorsa l nucleus caudalis. Immunocytochemistry for the p75 low affinity neurotroph in receptor (p75NTR) or for calcitonin gene-related peptide (CGRP) showed n o consistent change in trigeminal central endings in any Fos-reactive brain stem areas, despite the extensive structural and cytochemical reorganizatio n of the peripheral endings of the dental neurons. The Fos responses of cen tral neurons to tooth injury have some unusual temporal and spatial pattern s in adult rats compared to other trigeminal injury models. (C) 2000 Elsevi er Science B.V. All rights reserved.