Differential c-fos-like protein expression in mechanically versus chemically induced visceral nociception

The expression of c-fos-like protein has been suggested to be a marker for neuronal activity in nociceptive processing. The immunohistochemical detection of this protein was used to determine if different visceral noxious stimuli induce distinct patterns in the rat spinal cord. We have developed a mechanical visceral pain model which is based on the acute distention of the duodenum yielding a quantifiable behavioral endpoint, writhing-like activity. One hour following either intraperitoneal injection of acetic acid or the distention of the duodenum via a chronically implanted balloon catheter, the animals were processed for the immunocytochemical detection of c-fos-like protein in the spinal cord. Characteristic patterns of c-fos-like immunoreactivity were observed following each type of stimulus that differed in spinal laminar and segmental distribution, number of neurons expressing fos-like immunoreactivity and staining intensity. The chemical noxious stimulus induced c-fos bilaterally in laminae I and X predominantly in the thoraco-lumbar region of the spinal cord. In contrast, the mechanical noxious stimulus induced a greater number and more intense neuronal c-fos-like protein expression in laminae I-VI, IX and X. These data provide further evidence that there is a differential nociceptive modulation in mechanical noxious visceral stimulation.