SOMATOTOPIC AND LAMINAR ORGANIZATION OF FOS-LIKE IMMUNOREACTIVITY IN THE MEDULLARY AND UPPER CERVICAL DORSAL HORN INDUCED BY NOXIOUS FACIALSTIMULATION IN THE RAT

The distribution of fos-like-immunoreactivity (fos-LI) in the medullar y and upper cervical dorsal horn was examined following noxious facial stimulation, in order to evaluate the use of fos as a marker for neur onal activation in trigeminal nociceptive pathways. Control animals th at received urethane anesthesia and no facial stimulation showed subst antial bilateral labeling in the trigeminal complex that was restricte d to one rostrocaudal level, at the transition between the medullary d orsal horn (nucleus caudalis) and nucleus interpolaris. Noxious mechan ical stimulation (pinch) of different facial sites produced labeling i n the ipsilateral dorsal horn whose distribution varied predictably wi th the rostrocaudal and dorsoventral position of the facial stimulatio n site, such that rostral facial sites were represented rostrally in t he dorsal horn and dorsal sites were represented ventrolaterally. The cornea was exceptional among the facial stimulation sites in that it h ad a specific representation at two distinct rostrocaudal levels, in C 1 and the interpolaris-caudalis transition region; the position of the rostral peak was somatotopically inappropriate, based on the represen tation of other facial sites. The proportion of labelling in laminae I II-IV relative to laminae I-II was higher with noxious mechanical stim ulation than with noxious thermal (55-degrees-C) or chemical (subcutan eous injection of capsaicin) stimulation. The proportion of labelling in laminae III-IV produced by electrical stimulation of the infraorbit al nerve was no greater than that produced by pinch. The results sugge st that fos-LI mapping can be a useful method for the investigation of somatotopy but is subject to serious limitations when used for the in vestigation of laminar organization. The results also suggest that the interpolaris-caudalis transition region may have properties that are distinct from those of the rest of the trigeminal complex, possibly re lated to an involvement in autonomic function.