Background & Aims
Chronic stress is a risk factor for chronic pain development in humans. Studies in rodents using social defeat stress (SDS) demonstrated to trigger social avoidance behavior and chronic allodynia in animals subjected to chronic stress, and that was associated with neuraxial pain signaling changes. Toll-like receptor 4 (TLR4) has been involved in neuropathic and inflammatory pain phenotypes by provoking inflammatory pathway activation and neuronal sensitization. The role of TLR4 expressed in the dorsal root ganglia (DRG) in the chronic stress model still needs to be better understood. Thus, our project investigates whether SDS-induced in juvenile mice promotes acute pain or predisposes to chronic pain in adulthood and whether TLR4 receptors can play a key role in promoting neuronal hypersensitization during and after stress.
Methods
C57BL/6 male mice (4-5 weeks) were subjected to 6 sessions of SDS, which consisted of 10 minutes of physical contact with the aggressor (CD1 retired male breeder). After each session, mice were kept in the same box separated by a plexiglass divider with small holes allowing constant sensorial contact between aggressor CD1 and the stressed C57. Twice gaps of 2 days without session were applied in this protocol in between stress sessions. Mechanical threshold of the paws was measured by von Frey filament, and the Hargraves method was used for thermal withdrawal evaluation. Body weight and water intake were monitored throughout the experiment. At the end of 10 days of protocol, C57 was single-housed for 11 days and had DRG, spinal cord, and brain collected and processed for molecular analysis of TLR4 expression.
Results
The behavioral data suggests that juvenile mice developed mechanical allodynia after 2 or 4 SDS sessions compared to control mice. The same was observed after 6 SDS sessions, day 11 of the protocol.
In addition, stressed mice showed a significantly lower thermal withdrawal than the control group.
Flow cytometry data demonstrated that TLR4 in the dorsal root ganglia or spinal cord was not over-expressed on day 11.
Conclusions
Our study showed that chronic stress induces chronic pain phenotype in the juvenile C57 mice, and TLR-4 is not the main agent in the early phase of pain behavior. However, we are currently conducting a study in the late phase of the protocol to verify TLR4 expression changes in the brain of stressed C57.
References
1. Pagliusi, M., Jr, Bonet, I. J. M., Lemes, J. B. P., Oliveira, A. L. L., Carvalho, N. S., Tambeli, C. H., Parada, C. A., & Sartori, C. R. (2020). Social defeat stress-induced hyperalgesia is mediated by nav 1.8+ nociceptive fibers. Neuroscience letters, 729, 135006. https://doi.org/10.1016/j.neulet.2020.135006
2. Golden, S. A., Covington, H. E., 3rd, Berton, O., & Russo, S. J. (2011). A standardized protocol for repeated social defeat stress in mice. Nature protocols, 6(8), 1183–1191. https://doi.org/10.1038/nprot.2011.361
3. Pagliusi, M. O. F., Jr, & Sartori, C. R. (2019). Social Defeat Stress (SDS) in Mice: Using Swiss Mice as Resident. Bio-protocol, 9(6), e3197. https://doi.org/10.21769/BioProtoc.3197