Distribution of fos-like immunoreactivity in the medullary and upper cervical dorsal horn produced by stimulation of dural blood vessels in the rat.

Neurophysiological studies have generally failed to find evidence of a specific ascending pathway for visceral nociception. However, pain that arises from deep or visceral tissues typically differs from cutaneous pain, particularly in its diffuse, poorly localized quality. In this study, the c-fos mapping technique was used in order to investigate possible differences in the distribution of central neurons activated by afferent pathways from cutaneous and deep tissues that may be related to the differing quality of the sensations they evoke. The distribution of neurons in the upper cervical and medullary dorsal horn that displayed fos-like immunoreactivity (fos-LI) was examined following mechanical stimulation of dural blood vessels (transverse and superior sagittal sinuses), and was compared to that found following mechanical, thermal, and chemical stimulation of facial sites. Dural stimulation was carried out Brevital anesthesia in rats that had received a chronic surgical exposure of the transverse and superior sagittal sinuses 2 d earlier. Localized mechanical stimulation of the dural surface of the transverse sinus produced a predominantly ipsilateral increase in the number of fos-LI neurons in the medullary and upper cervical dorsal horn (primarily laminae I and V), and in the transition region between the trigeminal nucleus caudalis and interpolaris. Stimulation of the superior sagittal sinus produced increases in fos-LI labeling that were generally smaller than those produced by transverse sinus stimulation. The distribution of fos-LI labeling in the dorsal horn induced by dural stimulation differed from that induced by facial stimulation in two ways. (1) Dural stimulation produced a more diffuse distribution of fos-LI than facial stimulation in the dorsal horn. Whereas facial stimulation produced a dense, localized zone of fos-LI labeling in the dorsal horn, dural stimulation produced fos-LI labeling that extended from the midlevel of caudalis to C2/C3, and also extended across a large portion of the ventrolateral-to-dorsomedial axis of the dorsal horn. This distribution roughly corresponds to the representation of most of the dorsal half of the head and face. (2) Dural stimulation produced a more restricted laminar distribution of fos-LI labeling than facial stimulation, in that the dural-induced labeling in the superficial dorsal horn was primarily restricted to lamina I, whereas facial stimulation typically induced substantial labeling in both lamina I and the outer part of lamina II. These differences in the central organization of the afferent pathways from dural and facial sites may contribute to the differences in the quality of sensations evoked by these pathways.