The caudal subnucleus caudalis (medullary dorsal horn) acts as an interneuronal relay site in craniofacial nociceptive reflex activity

Abstract

We have recently documented that bilateral increases in electromyographic (EMG) activity of digastric (DIG) and masseter (MASS) muscles can be evoked by injection into the rat's temporomandibular joint (TMJ) region of the small-fiber excitant and inflammatory irritant mustard oil and that this increased jaw muscle activity can be significantly reduced by extensive lesions of the trigeminal subnucleus caudalis (Vc). This study was carried out in 34 anaesthetized rats to test whether neurones in the caudal Vc are indeed of critical importance in these craniofacial nociceptive reflexes. The effects of micro-injection of the cellular neurotoxic chemical ibotenic acid in histologically confirmed sites of the caudal brainstem on the mustard oil-evoked EMG activity of ipsilateral and contralateral DIG and MASS were tested. Ibotenic acid micro-injection in the left caudal Vc significantly reduced the increased EMG activity of all four muscles evoked by mustard oil injection into the left TMJ region whereas mustard oil injection into the right TMJ region in these same rats still readily evoked an increase in EMG activity. In other groups of rats, ibotenic acid micro-injection into the rostral Vc, the C2 segment or the reticular formation at the obex level did not produce any significant reduction in the reflexly evoked EMG activity. These findings suggest that neurones in the caudal Vc may be critical elements in neural pathways underlying the reflex responses evoked in jaw muscles by noxious stimulation of the TMJ region.