Impaired visceral pain-related functions of the midbrain periaqueductal gray in rats with colitis

The midbrain periaqueductal gray (PAG) is a key structure involved in the supraspinal modulation of pain. Previous studies have reported the association of gut inflammation-triggered chronic abdominal pain with structural and neuronal alterations within the PAG. However, whether PAG-executed visceral nociception processing and descending modulation are altered in gut pathology is not known. We used c-Fos immunohistochemistry and extracellular microelectrode recording in urethane-anesthetized male Wistar rats to evaluate the colitis-induced changes in visceral pain-related neuronal properties of the PAG and its descending outflow to visceral nociceptive neurons of the caudal ventrolateral medulla (CVLM). Analysis of c-Fos protein expression in inflamed animals has shown diminished activation of the lateral and ventrolateral PAG columns by noxious colorectal distension (CRD), although the nonstimulated c-Fos labeling in these PAG subdivisions was enhanced compared with that in controls. Microelectrode recording in the ventrolateral PAG revealed a colitis-elicited decrease in the proportion of CRD-excited neurons accompanied by an increase in the number of unresponsive cells and weakened reactions to the stimulation of CRD-inhibited PAG units. Colonic inflammation has also been found to cause a shift in the effects of ventrolateral PAG electrostimulation on CRD-excited CVLM neurons from being mostly inhibitory under normal conditions to excitatory in colitis. These findings identify impaired PAG functioning in ascending and descending visceral nociception control that may contribute to gut injury associated visceral hyperalgesia. The data obtained can benefit a better understanding of the supraspinal mechanisms involved in the pathogenesis of postinflammatory chronic abdominal pain.