Mechanisms underlying the hyperalgesic responses triggered by joint activation of TLR4.
BACKGROUND
Toll-like receptors (TLRs) including TLR4 and their signal pathways contribute to the pathogenesis of arthritis. Herein, we evaluated the mechanisms underlying the hyperalgesic response caused by TLR4 activation in the tibio-tarsal joint in mice.


METHODS
Joint inflammatory hyperalgesia was induced by intra-articular (ia) injection of LPS (lipopolysaccharide- TLR4 agonist) in C57BL/6, TLR4, TLR2, MyD88, TRIF, TNFR1/2 and IL-1R1 knockout (-/-) mice. Joint hyperalgesia was evaluated using an electronic von Frey. Neutrophil recruitment was assessed by MPO activity. Joint levels of cytokines were measured by ELISA.


RESULTS
Firstly, it was shown that LPS injected into the joints causes a dose- and time-dependent reduction in the mechanical nociceptive threshold. The TLR4 activation in the joint triggers mechanical hyperalgesia and neutrophil migration, which was abolished in TLR4-/-and MyD88-/-, but not in TLR2-/-and TRIF-/-mice. Besides, joint administration of LPS increased the release of TNF-α, IL-1β, and KC/CXCL1, which were reduced in TLR4-/-and MyD88-/-, but not in TRIF-/-mice. In agreement, the LPS-induced joint nociceptive effect was decreased in TNFR1/2-/-and IL-1R1-/-mice or in mice pre-treated with a CXCR1/2 selective antagonist (DF2156A).


CONCLUSIONS
These results suggest that TLR4 activation in the joint produces articular hyperalgesia via MyD88 signaling pathway. Moreover, this pathway is involved in the cascade of events of articular hyperalgesia through mechanisms dependent on cytokines and chemokines production. Thus, TLR4/MyD88 signaling pathway inhibitors might be useful for the treatment of inflammatory joint pain.
Department of Pharmacology, Ribeirão Preto Medical School, University of São Paulo, São Paulo, Avenida Bandeirantes, 3900, 14049-900 Ribeirão Preto, São Paulo, Brazil; Fundação Oswaldo Cruz-FIOCRUZ Mato Grosso do Sul, Rua Gabriel Abrão 92, 79081-746 Campo Grande, Mato Grosso do Sul, Brazil. Electronic address: anaguerrero@fiocruz.br.
Ana T G
AT
Guerrero
Department of Pharmacology, Ribeirão Preto Medical School, University of São Paulo, São Paulo, Avenida Bandeirantes, 3900, 14049-900 Ribeirão Preto, São Paulo, Brazil. Electronic address: larifarm@gmail.com.
Larissa G
LG
Pinto
Department of Pharmacology, Ribeirão Preto Medical School, University of São Paulo, São Paulo, Avenida Bandeirantes, 3900, 14049-900 Ribeirão Preto, São Paulo, Brazil. Electronic address: fdqcunha@fmrp.usp.br.
Fernando Q
FQ
Cunha
Department of Pharmacology, Ribeirão Preto Medical School, University of São Paulo, São Paulo, Avenida Bandeirantes, 3900, 14049-900 Ribeirão Preto, São Paulo, Brazil. Electronic address: shferrei@fmrp.usp.br.
Sérgio H
SH
Ferreira
Department of Pharmacology, Ribeirão Preto Medical School, University of São Paulo, São Paulo, Avenida Bandeirantes, 3900, 14049-900 Ribeirão Preto, São Paulo, Brazil. Electronic address: jcafilho@usp.br.
Jose C
JC
Alves-Filho
Departamento de Patologia, Centro de Ciências Biológicas, Universidade Estadual de Londrina, Paraná, Rod. Celso Garcia Cid Km380 PR445, 86057-970 Londrina, Paraná, Brazil. Electronic address: waldiceujr@yahoo.com.br.
Waldiceu A
WA
Verri
Department of Pharmacology, Ribeirão Preto Medical School, University of São Paulo, São Paulo, Avenida Bandeirantes, 3900, 14049-900 Ribeirão Preto, São Paulo, Brazil. Electronic address: thicunha@usp.br.
Thiago M
TM
Cunha
D000818:Animals; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D006930:Hyperalgesia / Q000139:chemically induced / Q000378:metabolism*; D018836:Inflammation Mediators / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000633:toxicity; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D013640:Tarsal Joints / Q000187:drug effects / Q000378:metabolism*; D051197:Toll-Like Receptor 4 / Q000378:metabolism*
D018836:Inflammation Mediators; D008070:Lipopolysaccharides; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4
Arthritis; Cytokines; Hyperalgesia; Joint pain; TLR4