Somatotopic and laminar organization of fos‐like immunoreactivity in the medullary and upper cervical dorsal horn induced by noxious facial stimulation in the rat

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Abstract
The distribution of fos‐like‐immunoreactivity (fos‐LI) in the medullary and upper cervical dorsal horn was examined following noxious facial stimulation, in order to evaluate the use of fos as a marker for neuronal activation in trigeminal nociceptive pathways. Control animals that received urethane anesthesia and no facial stimulation showed substantial bilateral labeling in the trigeminal complex that was restricted to one rostrocaudal level, at the transition between the medullary dorsal horn (nucleus caudalis) and nucleus interpolaris. Noxious mechanical stimulation (pinch) of different facial sites produced labeling in the ipsilateral dorsal horn whose distribution varied predictably with the rostrocaudal and dorsoventral position of the facial stimulation site, such that rostral facial sites were represented rostrally in the dorsal horn and dorsal sites were represented ventrolaterally. The cornea was exceptional among the facial stimulation sites in that it had a specific representation at two distinct rostrocaudal levels, in C1 and the interpolaris‐caudalis transition region; the position of the rostral peak was somatotopically inappropriate, based on the representation of other facial sites. The proportion of labelling in laminae III–IV relative to laminae I–II was higher with noxious mechanical stimulation than with noxious thermal (55°C) or chemical (subcutaneous injection of capsaicin) stimulation. The proportion of labelling in laminae III–IV produced by electrical stimulation of the infraobital nerve was no greater than that produced by pinch. The results suggest that fos‐LI mapping can be a useful method for the investigation of somatotopy but is subject to serious limitations when used for the investigation of laminar organzation. 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 related to an involvement in autonomic function.