The establishment and maintenance of chronic pain involves not only neuronal pathways, but also components of the immune system. Peripheral mast cells and their mediators are known to be involved in the development of hyperalgesia. In the CNS, mast cells are mainly studied in the thalamus and sex differences exist in the response of thalamic mast cells to nociceptive signals. Mast cells are also localised in the spinal dura mater, but less is known about their role in chronic pain states, particularly in peripheral inflammation. In this study, an animal model of inflammatory pain induced by intraplantar carrageenan injection was used to study sex differences in dural mast cell properties and their relation to nociceptive behaviour.
Lumbar and thoracic dura mater samples were retrieved from vehicle- and carrageenan-treated male and female rats and stained for mast cells using toluidine blue. Cells were manually counted and classified as either intact or degranulated. Heat hyperalgesia after carrageenan injections was assessed using the Hargreaves test, mechanical allodynia was quantified with Von Frey filaments and cold allodynia was evaluated with the acetone test.
Percentage of degranulated mast cells was significantly increased at 3, 24 and 72 hrs post carrageenan in the thoracic dura mater of female animals. Density of both total and degranulated mast cells was significantly enhanced at 24 and 72 hrs after carrageenan injections in the lumbar dura mater of female rats. No significant effects of carrageenan on mast cell properties were detected in male animals.
The intraplantar injection of carrageenan caused robust mechanical, heat and cold nociceptive behaviour in both male and female rats. Intrathecal application of the mast cell blocker cromolyn prevented carrageenan-induced mast cell activation in female rats but failed to influence nociceptive behaviour. No significant differences in mechanical, heat and cold nociception between cromolyn- and shamtreated animals were found in both sexes. The results indicate that spinal dural mast cells are not necessary for the development of hyperalgesia and allodynia after carrageenan-induced peripheral inflammation.