Chronic pain afflicts more than 100 million Americans and significantly affects the quality of life of those suffering. Opioid receptor agonists are effective analgesics but have severe dose-limiting side effects and display drug tolerance with prolonged use. Understanding key pathways that modulate pain is critical to developing analgesics that are more effective.
In my dissertation, I investigated the importance of three key purinergic receptor signaling pathways in nociception: P2Y1, P2Y12 and P2Y13. Purinergic receptors respond to nucleotides released from damaged tissue and/or immune cells and have long been implicated in modifying nociceptive responses, however, the P2Y G-protein coupled purinergic receptor family is just starting to be investigated.
P2Y1, P2Y12 and P2Y13 receptors are activated by the nucleotide adenosine diphosphate (ADP), however, the receptors have opposing effects because the P2Y1 receptor is Gq/11-coupled, whereas the P2Y12 and P2Y13 receptors are Gi/o-coupled. Traditionally, Gq-coupled receptors are pro-nociceptive while Gi/o-coupled receptors are anti-nociceptive. In order to understand the implications of antagonistic signaling between the P2Y1 and P2Y12/P2Y13 receptors during inflammatory pain, the signaling cascades for each individual receptor must first be elucidated.
For the first part of my dissertation, I tested the hypothesis that P2Y1 Gq-coupled receptors signal through conventional protein kinase C isoforms (cPKC) and that these isoforms mediate the behavioral effects of P2Y1 receptor activation. I discovered that the cPKC isoform PKCα is expressed predominately in the IB4+ population, similar to the P2Y1 receptor, and that antagonism of PKCα attenuates P2Y1 receptor-mediated mechanical hypersensitivity. These results demonstrate the importance of cPKCs in the P2Y1 signaling cascade and detail how P2Y1 receptors function. Next, I tested the hypothesis that P2Y12/P2Y13 Gi/o-coupled signaling is important for maintaining basal nociceptive tone and modulating pro-nociceptive signaling. I discovered that all three ADP-activated receptors, P2Y1, P2Y12 and P2Y13, are active at baseline and likely maintain basal nociceptive tone. Additionally, P2Y12 and P2Y13 receptors are capable of attenuating signaling through pro-inflammatory Gs-coupled pathways, validating the importance of P2Y12 and P2Y13 receptors as potential analgesics.
These results provide insight into the signaling cascades of P2Y1, P2Y12 and P2Y13 receptors and advance our understanding of nociceptive signaling
