A subtype of purinergic receptor on spinal microglial cells mediated visceral pain hypersensitivity in rats with chronic pancreatitis, and pharmacologic or genetic inhibition of this receptor improved hyperalgesia, according to a report in the November issue of Cellular and Molecular Gastroenterology and Hepatology.
“Our study may be the first to identify that P2X7 receptors in spinal microglia are upregulated in chronic pancreatitis, and that this upregulation is associated with the development of visceral hyperalgesia,” said Dr. Pei-Yi Liu at National Yang-Ming University in Taipei, Taiwan, and her associates. A common laboratory dye known as brilliant blue G, which is an antagonist of P2X7R, “not only attenuated but also prevented CP-related chronic visceral hyperalgesia,” the researchers reported.
Chronic pancreatitis causes intense, recurrent epigastric pain that is “difficult and frustrating” to control and can lead to malnutrition, narcotic analgesic addiction, and social and financial problems, said the researchers. Previously, they had linked visceral pain in murine CP to activation of spinal microglia, the main effector immune cells in the central nervous system. The molecular pathways remained unclear, but some research had implicated extracellular adenosine triphosphate (ATP) as well as purinergic receptors in the CNS. Because a purine receptor subtype known as P2X7 had been linked to neuropathic and inflammatory pain, the researchers wondered if it also facilitated visceral pain (Cell Mol Gastroenterol Hepatol. 2015 Jul 22. doi: 10.1016/j.jcmgh.2015.07.008). To explore that question, they created a CP model by injecting 2% trinitrobenzene sulfonic acid into the pancreatic ducts of male rats. They measured behavioral responses to mechanical and electrical stimulation and quantified spinal cord P2X7R levels with the help of standard laboratory assays. They also watched for changes in pain-related behaviors after blocking spinal cord P2X7R with brilliant blue G or knocking it down with short interfering RNA (siRNA).
Spinal P2X7R expression rose significantly after CP induction, as did levels of the OX-42 microglial marker in the dorsal horn of the spinal cord, said the investigators. Brilliant blue G and genetic knock down suppressed P2X7R expression, inhibited activation of spinal microglia, and “significantly attenuated” nociceptive behaviors, they added.
The researchers also pretreated some rats with brilliant blue G before inducing CP and saw that these rats exhibited significantly lower pain responses to mechanical and electrical stimuli compared with other CP rats. In fact, the nociceptive responses of the pretreated CP rats resembled those of non-CP control rats, the investigators said. Spinal tissue from pretreated rats also lacked signs of P2X7R upregulation, they noted.
Taken together, the data “indicate a critical role of P2X7R expressed in the spinal cord in the development of chronic visceral pain in CP,” concluded the researchers. Brilliant blue G inhibits voltage-gated sodium channels, which are known to contribute to chronic visceral pain, and “may represent an effective drug for the treatment of chronic pain in chronic pancreatitis patients,” they added.
The study was funded by Taipei Veterans General Hospital, National Science Council of Taiwan, and the Taiwan Ministry of Education Aim for Top University Grant. The investigators declared no competing interests.