DIFFERENTIAL EXPRESSION OF FOS PROTEIN AFTER TRANSECTION OF THE RAT INFRAORBITAL NERVE IN THE TRIGEMINAL NUCLEUS CAUDALIS

To determine the effects of nerve injury on Fos expression, temporal a nd spatial distributions of Fos-positive neurons in the trigeminal nuc leus caudalis were examined after tissue injury for isolation of the i nfraorbital nerve as controls and transection of this nerve as well as noxious chemical stimulation by formalin injection in adult rats. Fos immunoreactivity was markedly elevated in laminae I and II of the onl y ipsilateral nucleus caudalis 2 h after these surgical procedures and noxious chemical stimulation. The distributions of Fos-positive neuro ns were restricted rostro-caudally following formalin injection and ti ssue injury compared to transection of the infraorbital nerve. One day after tissue injury and nerve transection, however, Fos-positive neur ons were distributed bilaterally in laminae III and IV extending rostr o-caudally and medio-laterally in this nucleus, and this persisted ove r the 2-week study period. The number of Fos-positive neurons in the s ide ipsilateral to nerve transection was markedly less than that in th e contralateral side whereas positive neurons in the tissue injured ra ts were distributed symmetrically along the rostro-caudal axis. There was no difference in the contralateral sides between nerve transection and tissue injury groups. The rostro-caudal level showing reduction i n Fos expression corresponded roughly to the sites of central terminat ion of the injured nerve in this nucleus, suggesting a role for the pr imary afferents in the reduction of Fos expression in laminae III and IV neurons of the ipsilateral nucleus caudalis. (C) 1997 Elsevier Scie nce B.V.