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 examined in order to differentiate the Fos activation related to nociceptive processing in contrast to that due to somatoautonomic processing. The inflammation 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 identified in bilateral spinal trigeminal nucleus (VSP), nucleus tractus solitarius (NTS), ventrolateral medulla (VLM) and inferior medial olivary nucleus (IOM). At the level of the trigeminal subnucleus interpolaris/caudalis (Vi/Vc) transition zone, there was a selective induction of Fos-like immunoreactivity (LI) in the VSP and NTS, when compared to control rats (anesthesia with or without S-cut). A major portion of the Fos-LI in the VSP at the level of the caudal Vc was apparently activated by S-cut. Bilateral adrenalectomy or a unilateral vagotomy resulted in a selective reduction of inflammation-induced Fos-LI in the VSP at the Vi/Vc transition zone (P<0.05) and NTS (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 orofacial 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 responses, orofacial deep tissue injury also is coupled to somatovisceral and somatoautonomic processing that contribute to central neural activation.