Involvement of trigeminal subnucleus caudalis (medullary dorsal horn) in craniofacial nociceptive reflex activity

We have previously shown that an increase in electromyographic (EMG) activi ty of digastric (DIG) and masseter (MASS) muscles can be reflexly evoked by injection into the rat's temporomandibular joint (TMJ) region of the small -fibre excitant and inflammatory irritant mustard oil (MO). Since the trige minal (V) subnucleus caudalis (Vc, i.e. medullary dorsal horn) has traditio nally been viewed as an essential brainstem relay site of nociceptive infor mation from craniofacial tissues, an EMG study was carried out in 45 anaest hetized rats to determine if Vc is involved in the MO-evoked increases in j aw muscle EMG activity. The effects of histologically confirmed surgical or chemical lesions of Vc on this evoked EMG activity were tested in differen t groups of rats. MO injection into the left TMJ region of intact rats evok ed bilateral increases in EMG activity of DIG and MASS which could be signi ficantly reduced by surgical transection of the left caudal brainstem at th e obex level; MO injection into the right TMJ region in these same rats sti ll readily evoked increases in EMG activity. A sagittal section medial to V c or transection at the level of the second cervical spinal segment did not produce any significant reduction in the reflexly evoked EMG activity. Neu rones in Vc, as opposed to fibres of passage, appear to be important for th e MO-evoked EMG activity, since injection into Vc of the neurotoxic chemica l ibotenic acid significantly reduced the mustard oil-evoked EMG activity. The Vc also appears to play a role in the activation of contralateral V mot oneurons, as evidenced by the activation of the contralateral DTG and MASS muscles by the injection of MO into the left TMJ region of intact rats and by the reduction of this evoked EMG activity in the contralateral DIG and M ASS of rats with a surgical transection or ibotenic acid lesion of the left Vc. These findings suggest that Vc may be a critical element in the neural pathways underlying the reflex responses evoked bilaterally in DIG and MAS S muscles by noxious stimulation of the TMJ region. (C) 1999 International Association for the Study of Pain. Published by Elsevier Science B.V.