c-fos induction in the subnucleus caudalis following noxious mechanical stimulation of the oral mucous membrane.

c-fos expression in response to noxious mechanical stimulation of the peripheral receptive field was used to study the somatotopic representation of the oral mucous membrane and trigeminal cutaneous receptive fields in the trigeminal subnucleus caudalis and the cervical spinal dorsal horn. Fos-neurons (neurons with c-fos protein-like immunoreactivity) following stimulation of the cutaneous receptive field were found in laminae I and II of the first and second cervical segments of the spinal cord ipsilateral to the stimulation and exhibited a clear somatotopic segregation; i.e., the fos-neurons responding to receptive fields in the mandibular, maxillary, and ophthalmic divisions were arranged in a mediolateral sequence. On the other hand, fos-neurons responding to the oral mucous membrane were found in the similar laminae of the rostral pole of subnucleus caudalis bilaterally. They were all found in the dorsalmost part of the subnucleus without segregation; e.g., topographic distributions of fos-neurons resulting from the mandibular and maxillary gingival stimulations were indistinguishable. The central mechanism of trigeminal nociception is discussed with respect to the difference between intra- and extraoral receptive fields.