Background & Aims
Arthritis, a common chronic inflammatory pain condition, can occur before adulthood(1) but its prevalence increases linearly with age(2). Given the expected rise in the number of older adults within the population(3), a concomitant increase in those with arthritis is anticipated, making it a significant public health concern. However, a systematic investigation of sex and age differences in development, maintenance and recovery from chronic inflammatory pain is still needed. That was the first goal of our experiments. The second goal is to determine if the melanocortin-4 receptor (MC4R) signaling pathway plays a role in these differences at various stages of the pain process as our prior work demonstrated pain relieving properties of MC4R antagonism(4,5). We are currently determining sex-, age-, and recovery-based differences in levels of MC4R-related proteins in pain-relevant brain regions from rats injected with Complete Freund’s Adjuvant (CFA), a model of human arthritis.
Methods
We injected CFA into the hind paw of male and female, adolescent and adult Wistar rats (10 per age/sex group). An additional 40 (10 per age/sex group) rats were saline injected. Rats were tested for mechanical (von Frey) and thermal (Hargreave’s) sensitivity prior to and, every week for 3, (experiment 1) or 11 (experiment 2) weeks after injections. Twenty-four hours after the last task rats were euthanized, brains collected, and pain-relevant brain regions punched for western blotting to determine levels of MC4R-related signaling proteins. We previously presented the data on experiment 1, demonstrating hypersensitivity in female, as compared to male, CFA-injected rats regardless of age. In experiment 2 we also determined recovery by summing the differences from baseline for mechanical and thermal sensitivity each week. To be considered recovered, the summed score needed to be 150% of baseline and maintained for two out of three weeks. We also measured paw edema in a subset of rats.
Results
Within the CFA-injected rats we found protracted mechanical and thermal sensitivity in females compared to males regardless of age. Adolescent females had the lowest recovery rates (60%) compared to other groups (70%- adolescent males and adult females; 90%- adult males) and adult males exhibited the fastest recovery from thermal sensitivity. However, thermal sensitivity was positively correlated with recovery scores in all groups except, adult males. In adult males, there was a positive correlation between mechanical and thermal sensitivity scores. Interestingly, rats that recovered exhibited increased edema compared to rats that did not recover. Within the adult CFA females, there was a significant positive correlation between edema and thermal sensitivity scores while male and female adolescent CFA rats showed a positive correlation between edema and weight. Lastly, in adult males there was a positive correlation between ipsi- and contralateral paw withdrawal scores.
Conclusions
We show that females are hypersensitive compared to males at all stages of the recovery process. The paw edema data suggests that in the chronic state, edema may buffer thermal nociception. We are currently analyzing paw tissue to determine differences in its makeup by age, sex, and recovery status. Thermal sensitivity was the rate limiting step to recovery in all but the adult males for which, mechanical and thermal sensitivity are equally important. This is interesting given that adult males may be experiencing a systemic pain response to a localized inflammatory agent. The correlation between edema and weight in the adolescent groups could be developmentally driven, as these rats transition into the adult category in the fourth week post-injection. We are completing subcellular fractionation and western blotting to determine differences in levels of MC4R related proteins in pain-centric brain regions based on sex, age, and recovery status in rats with chronic inflammatory pain.
References
(1) Nigrovic, PA, Colbert, RA, Holers, VM, Ozen, S, Ruperto, N, Thompson, SD, Wedderburn, lR, Yeung, RSM & Martini, A (2021). Biological classification of childhood arthritis: Roadmap to molecular nomenclature. Nat Rev Rheumatol, 17(5), 257-269. doi: 10.1038/s41584-021-00590-6.
(2) Woolf, AD (2015). Global burden of osteoarthritis and musculoskeletal diseases. BMC Musculoskelet Disord., 16(Supp1) S3. doi: 10.1186/1471-2474-16-S1-S3
(3) He, W, Goodkind, D & Kowal, P, U.S. Census Bureau (2016). An aging world: 2015. International
Population Reports P95/16-1, U.S. Government Publishing Office, Washington, DC.
(4) Avegno, EM, Lobell, TD, Itoga, CA, Baynes, BB, Whitaker, AM, Weera, MM, Edwards, S, Middleton, JW & Gilpin, NW (2018). Central amygdala circuits mediate hyperalgesia in alcohol-dependent rats. J
Neurosci, 38(36). doi: 10.1523/JNEUROSCI.0483-18.2018.
(5) Sharfman, NM, Kelley, LK, Secci, ME & Gilpin, NW (2022). Melanocortin-4 receptor signaling in the central amygdala mediates chronic inflammatory pain effects on nociception. Neuropharmacology,
210. doi: 10.1016/j.neuropharm.2022.109032
Presenting Author
Andrea F Jones
Poster Authors
Topics
- Models: Chronic Pain - Inflammatory