EFFECTS OF DESCENDING INHIBITORY SYSTEMS ON THE C-FOS EXPRESSION IN THE RAT SPINAL-CORD DURING FORMALIN-INDUCED NOXIOUS-STIMULATION

This study provides morphological evidence for the activation of the d escending modulatory control by nociceptive afferent pathways. Fos-lik e immunoreactivity in the spinal dorsal horn is used as an indicator o f efficacy of transmission of noxious inputs at this level. Wistar rat s were anesthetized with nembutal and the spinal dorsolateral funiculu s was transected unilaterally at the level of T-11,T-12. Two days late r, an equal volume (0.2 mi) of formalin (5% in saline) was injected in to the plantar aspect of two hindpaws. After Ih of injection, rats wer e deeply anesthetized and killed for the immunocytochemical examinatio n of Fos-like protein product by using an immunocytochemical technique . The results show that the mean number of Fos protein-like immunoreac tive neurons is significantly lower in the superficial laminae and in the neck of the dorsal horn on the side ipsilateral to the intact dors olateral funiculus than that on the opposite side (i.e. 18.4 +/- 1.0 v s 30.0 +/- 1.3 and 33.9 +/- 0.2 vs 56.8 +/- 1.7, respectively). We con clude that the peripheral noxious inputs (which ascend via the ventral half of the spinal cord in this study) can activate the supraspinal d escending inhibitory systems, which in turn suppress the synthesis of Fos-like protein in the related dorsal horn neurons.