Masseteric inflammation-induced Fos protein expression in the trigeminal interpolaris/caudalis transition zone: contribution of somatosensory-vagal-adrenal integration

The effects of vagotomy and adrenalectomy on the expression of Fos protein in brainstem neurons following the inflammation of masseter muscle were exa mined in order to differentiate the Fos activation related to nociceptive p rocessing in contrast to that due to somatoautonomic processing. The inflam mation was induced by a unilateral injection of complete Freund's adjuvant (CFA) into the masseter muscle under methohexital anesthesia after a small skin-cut (S-cut). After the CFA injection, Fos positive neurons were identi fied in bilateral spinal trigeminal nucleus (VSP), nucleus tractus solitari us (NTS), ventrolateral medulla (VLM) and inferior medial olivary nucleus ( IOM). At the level of the trigeminal subnucleus interpolaris/caudalis (Vi/V c) transition zone, there was a selective induction of Fos-like immunoreact ivity (LI) in the VSP and NTS, when compared to control rats (anesthesia wi th or without S-cut). A. major portion of the Fos-LI in the VSP at the leve l of the caudal Vc was apparently activated by S-cut. Bilateral adrenalecto my or a unilateral vagotomy resulted in a selective reduction of inflammati on-induced Fos-LI in the VSP at the Vi/Vc transition zone (P < 0.05) and NT S (P < 0.05), but had less effect on Fos-LI in the caudal Vc. These results suggest that the inflammation of the masseter muscle, an injury of orofaci al deep tissue, results in a widespread change in neuronal activity in the VSP and NTS that depends in part on the integrity of the adrenal cortex and vagus. Thus, in addition to somatotopically organized nociceptive response s, orofacial deep tissue injury also is coupled to somatovisceral and somat oautonomic processing that contribute to central neural activation. (C) 199 9 Elsevier Science B.V. All rights reserved.