Methods: Sprague-Dawley rats (about 200g body weight) were injected with a retrograde tracer, fluorogold into the submedia (SM), parabrachial n. (PB), lateral hypothalamus (LH) or medial ventroposterior thalamic n. (VPM), 7d before injection of an inflammatory agent, complete Freund's adjuvant (CFA), into the left masseter muscle. Rats were perfused at 2h after CFA injection and brain stem tissues were processed for fos-fluorogold double immunocytochemistry.
Results: At the level of the trigeminal subnuclei interpolaris/caudalis transition (Vi/Vc) zone, the largest amount of neurons in ventral portion of the Vi/Vc zone showed the fos-positive immunoreactivity. Most of inflammation induced neuronal activation at the Vi/Vc zone projected to the SM. Almost fos-positive neurons did not project to the VPM.
Conclusion: The activation of SM neurons may modulate the trigeminal pain process after orofacial deep tissue injury.