DISTRIBUTION OF FOS-LIKE IMMUNOREACTIVITY IN THE CAUDAL BRAIN-STEM OFTHE RAT FOLLOWING NOXIOUS CHEMICAL-STIMULATION OF THE TEMPOROMANDIBULAR-JOINT

Central expression of the protooncogene c-fos was used to examine area s receiving noxious sensory input from the rat temporomandibular joint (TMJ). Fos-like immunoreactivity (Fos-LI) in the caudal brainstem was visualized 2 hours after unilateral injection of the small-fiber-spec ific excitant/inflammatory irritant mustard oil into the TMJ region. C ontrol animals received injection of either mustard oil into the subcu taneous fascia overlying the masseter muscle or mineral oil vehicle in to the TMJ region. In all groups, Fos-LI was consistently observed ips ilaterally in the spinal trigeminal nucleus and cervical dorsal horn a nd, bilaterally, in the nucleus of the solitary tract and the ventrola teral medulla. The expression of Fos-LI ipsilaterally in the paratrige minal nucleus was variable. Within the trigeminal sensory complex, Fos -LI was restricted to subnucleus caudalis and the caudal portions of s ubnucleus interpolaris near the level of the obex. Approximately 12% o f Fos-LI cells in subnucleus caudalis and in the cervical dorsal horn were found in laminae III-VI. Compared to TMJ mustard oil injection, m ineral oil injection produced less Fos-LI at all rostrocaudal levels, whereas subcutaneous mustard oil injection produced less Fos-LI in cau dal subnucleus caudalis but similar amounts in the cervical dorsal hor n. Neither of these injections yielded significant ipsilateral respons es in subnucleus caudalis, indicating that Fos-LI in this region follo wing TMJ mustard oil injection could be ascribed solely to small-fiber stimulation in the deep TMJ region. The wide rostrocaudal distributio n of Fos-LI within the caudal brainstem reflects the distribution of T MJ-responsive nociceptive neurons that may underlie the spread and ref erral of pain from the TMJ region. (C) 1995 Wiley-Liss. Inc.