Effects of somatosensory cortical stimulation on expression of c-Fos in rat medullary dorsal horn in response to formalin-induced noxious stimulation

We examined the effects of epidural electrical stimulation of primary (SI) and secondary (SII) somatosensory cortex on expression of c-Fos protein in rat medullary dorsal horn neurons (Vc; trigeminal nucleus caudalis) in response to formalin-induced noxious stimulation. Epidural electrical stimulation (single pulse, 0.2 msec duration at 10 Hz) was applied to the left facial region SI or SII at three different stimulus intensities, 0.1, 0.5, and 1.0 mA for 60 min 0 or 2 hr after bilateral injection of formalin into the lower lip. SII stimulation at 1.0 mA immediately after injection of formalin, significantly decreased the number of Fos-positive cells in the right VcI/II by 32.4%. There was no significant change in the number of Fos-positive cells in the VcIII/IV. SII stimulation at 0.5 and 1.0 mA 2 hr after injection of formalin, significantly decreased the number of Fos-positive cells in the right VcI/II by 47.9% and 40.8%, but significantly increased the number of Fos-positive cells in the right VcIII/IV by 178.8% and 324.3%, respectively. In contrast, SI stimulation had no effect on expression of c-Fos in Vc. Possible direct corticotrigeminal projections were labeled anterogradely by injection of WGA-HRP into the SI and SII. In the Vc, labeled terminals were distributed mostly in the contralateral medial half of VcIII/IV and medullary reticular nucleus dorsalis but rarely in VcI/II. These results suggest that activation of SII-medullary fibers suppress nociceptive information from the oro-facial regions.