TRPV1 receptor desensitisation in humans following repeated capsaicin administration in a novel model for neurogenic inflammation.

D Raj, S Ghodasara, N.J Goulding. William Harvey Research Institute, Barts and the London SMD, London, UK

Acute inflammation is difficult to model invivo in humans. In this study we utilised the oral cavity, a host to significant numbers of neutrophils (PMN) as a novel window for collecting extravasated PMN after inducing neurogenic inflammation using a capsaicin stimulus. The three day repeated exposure to capsaicin orally facilitated an investigation into the desensitisation of TRPV1 ion channels in the mouth during this time period.

17 healthy, non-smoker volunteers (mean age 26, range 20-45) were recruited with informed consent with approval from Queen Mary Ethics committee. Individuals had no evidence of oral/inflammatory disease and were not taking immunomodulating medication. Participants provided saliva samples via a 30 second mouth rinse of 20ml (0.9%) saline which were afterwards spat into falcon tubes. Samples were taken 1 hour before, immediately before and 1 hour after a 20ml (10%) Tabasco sauce mouthwash on the first and third day of investigation. Per sample, salivary PMN were counted, assessed for viability and superoxide production was analysed with Flow Cytometry to confirm functional activation. Subjects endured repeated capsaicin exposure by doing capsaicin mouthwashes at 11am, 2pm, 6pm & 10pm on days 1 & 2. The PMN extravasation response on day 1 and 3 was compared.

We observed a substantial mean percentage increase in salivary PMN count (+92.1%, p=0.0007), viability (+75.3%, p=0.0005) and superoxide production (+35.6%, p=0.002) after capsaicin mouthwash on day 1, which is consistent with previous results using this model. Day 3 saliva analysis showed smaller, insignificant increase in PMN count (+36%), viability (+21.1%) and ROS production (+11%) after capsaicin mouthwash (all p>0.05) demonstrating an attenuated response to capsaicin after repeated exposure. Direct comparison of result values between day 1 and day 3 were not however different with statistical significance, indicating only a limited degree of desensitisation. Perceived pain and duration of discomfort after capsaicin mouthwashes decreased by day 3 in all subjects.

Capsaicin causes a prominent extravasation of PMN into saliva, these cells were more viable and activated indicating inflammation. A trend towards a reduced response to capsaicin can be achieved within three days of repeated capsaicin exposure which can be explained by TRPV1 ion channel desensitisation as seen in existing literature. This study adds to a line of experiments supporting the use of the oral cavity as a convenient, well-tolerated site for modelling acute inflammation invivo and furthermore to obtain recruited cells for further analysis.