Fos-like proteins in the lumbosacral spinal cord following noxious and non-noxious colorectal distention in the rat.

The rat lumbosacral spinal cord was immunocytochemically stained for Fos-like immunoreactivity following repetitive colorectal distention (CRD) to 20, 40 or 80 mm Hg. Following all 3 distention pressures, Fos-like immunoreactive (Fos-ir) nuclei were observed primarily in laminae I-II, V-VII and X, although some labeled nuclei were observed in laminae III-IV. Eighty mm Hg CRD resulted in significantly more Fos-ir nuclei than 20 or 40 mm Hg CRD. Morphological examination of the colon revealed clear signs of inflammation following 80 but not 20 mm Hg CRD. Acute 20 mm Hg CRD is a non-noxious stimulus, suggesting that both noxious and non-noxious visceral stimuli can induce Fos-like immunoreactivity in the spinal cord. It is suggested that activation of neuropeptide-containing small-diameter primary afferents is necessary, although not necessarily sufficient, to induce c-fos in the rat spinal cord.