Inflammation is an ubiquitous response of the body to cellular damage and injury. It not only leads to increased production of inflammatory molecules such as prostaglandins, bradykinins, histamine, and nerve growth factor (NGF) that sensitize the neurons, but is also associated with acidosis due to a local physiological decrease in extracellular pH, to as low as 5.3. The addition of protons to the sensitized area results in increased membrane conductance and depolarization in sensory neurons involving the acid sensing ion channels (ASICs), the capsaicin sensitive transient receptor potential vanilloid type 1 receptor (TRPV1), and blockade of background potassium conductance. Collectively, this combination heightens the pain state. As expression and activation of the NGF receptor tropomyosin-related kinase A (TrkA) are critically linked to inflammation-associated nociceptor sensitization, this led us to hypothesize that decreased pH may contribute to this process by depolarizing the neuron and mobilizing more TrkA to the cell membrane. This hypothesis is premised by previous experiments in other neuronal populations demonstrating that increased neuronal activity mobilizes more of the neurotrophin receptor TrkB to the membrane. We explored this question utilizing an in vitro model of acidosis akin to that observed with inflammation. Primary sensory neurons were grown on coverslips at a physiological pH of 7.4. The pH of the media was decreased to an acidic pH of 6.5 in half of the cultures for 30 minutes, followed by fixation under permeablizing or non-permeablizing conditions. Immunocytochemical analysis revealed a significant increase in the mobilization of TrkA to the plasma membrane in response to acidosis (confirmed using a biotinylation assay) and an enhanced level of TrkA activation in response to brief NGF challenge. This rapid mobilization of TrkA was attenuated with the addition of proton-sensitive channel blockers capsazepine and amiloride, for the TRPV and ASIC channels respectively. Unexpectedly, the amount of activated TrkA was also increased at pH 6.5 in the absence of NGF challenge. Taken together, the data suggests that sensory neurons can be rapidly sensitized to NGF in response to a decrease in pH and as such likely plays a role in the sensitization and hyperalgesia associated with an inflammatory state.
| Identifer | oai:union.ndltd.org:USASK/oai:usask.ca:etd-11302008-204459 |
| Date | 10 December 2008 |
| Creators | Bray, Geoffrey E |
| Contributors | David Schreyer, Thomas Haas, Deborah Anderson, Valerie Verge |
| Publisher | University of Saskatchewan |
| Source Sets | University of Saskatchewan Library |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://library.usask.ca/theses/available/etd-11302008-204459/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Saskatchewan or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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