ABSTRACT
Title
Toll like receptors in pathological pain: new potential pharmacological targets?
Authors
F. Comelli and B. Costa
Dept. of Biotechnology and Bioscience, University of Milano-Bicocca, Milano
Dept. of Biotechnology and Bioscience, University of Milano-Bicocca, Milano
Abstract
It has become evident in recent years that the expression and functionality of Toll-like receptors (TLRs), transmembrane pattern-recognition receptors that initiate signals in response to diverse pathogen-associated molecular patterns (PAMPs), extends well beyond the boundaries of immune cells. In fact, they can be expressed by all types of central nervous system residing cells, including microglia. Increasing evidence, indicating that the production of inflammatory and algogen factors by activated spinal microglia plays a crucial role in the initiation and/or maintenance of pathological pain,highlighted TLRs, especially TLR4 and TLR2, as key contributors in triggering microglial activation. In fact, recent studies have shown the involvement of TLR signalling in the regulation of pain-associated molecules, which contribute to central sensitization, and thus, to behavioural hypersensitivity.
In this context, we demonstrated that TLR2 and TLR4 knockout mice submitted to a chronic constriction of sciatic nerve (CCI, neuropathy model) or an intraplantar injection of complete Freund’s adjuvant (CFA, chronic inflammation model), developed less pain hypersensitivity and the treatment of CCI and CFA mice with a TLR4 antagonist (IAXO-101) resulted into a significant attenuation of behavioural hypersensitivity. Accordingly, TLR2 and TLR4 mRNA expression increased in the spinal cord (L4-L6 tract) of CCI and CFA wild type mice so confirming a key role of such receptors during neuropathic and chronic inflammatory pain conditions. All together these results were consistent with the localization and the role of TLR2 and TLR4 on activated microglia. Probably, TLR2 and TLR4 lack or blockade determined an attenuated pain response by avoiding inflammatory and pronociceptive mediator release by activated microglia. By contrast, our studies revealed that neither the acute inflammatory pain nor the tonic pain requested TLR2 or TLR4 involvement. In fact, λ-carrageenan-evoked hypersensitivity in TLR2 and TLR4 knockout mice was super-imposable to that observed in wild-type mice, indicating that these receptors were not required for the pain signalling elicited by an acute inflammatory stimulus. TLR2 and TLR4 did not even contribute to persistent pain development, as suggested by examining nociceptive responses in the formalin test, an animal model that mimics a persistent pain produced by peripheral tissue injury and inflammation. The absence of TLR2 and TLR4 overexpression in the spinal cord of unhealthy mice and the lack of antinociceptive efficacy following the pharmacological block by the TLR4 antagonist confirmed these results.
In conclusion, our results highlighted a key role of microglial TLR2 and TLR4 in mediating exclusively chronic pain, so suggesting the possibility of a therapeutic benefit of blocking TLR2 and TLR4 in the treatment of such a pain. It is important to note, as expected, that TLR2 and TLR4 deficiency or block can only attenuated, but not completely abolished pain responses, indicative of numerous pathways mediating the complex signalling of chronic pain. So, their pharmacological block could represent an adjuvant therapy to use in combination with common analgesic drugs, active in acute pain, but often unsatisfactory for chronic pain relief.
In this context, we demonstrated that TLR2 and TLR4 knockout mice submitted to a chronic constriction of sciatic nerve (CCI, neuropathy model) or an intraplantar injection of complete Freund’s adjuvant (CFA, chronic inflammation model), developed less pain hypersensitivity and the treatment of CCI and CFA mice with a TLR4 antagonist (IAXO-101) resulted into a significant attenuation of behavioural hypersensitivity. Accordingly, TLR2 and TLR4 mRNA expression increased in the spinal cord (L4-L6 tract) of CCI and CFA wild type mice so confirming a key role of such receptors during neuropathic and chronic inflammatory pain conditions. All together these results were consistent with the localization and the role of TLR2 and TLR4 on activated microglia. Probably, TLR2 and TLR4 lack or blockade determined an attenuated pain response by avoiding inflammatory and pronociceptive mediator release by activated microglia. By contrast, our studies revealed that neither the acute inflammatory pain nor the tonic pain requested TLR2 or TLR4 involvement. In fact, λ-carrageenan-evoked hypersensitivity in TLR2 and TLR4 knockout mice was super-imposable to that observed in wild-type mice, indicating that these receptors were not required for the pain signalling elicited by an acute inflammatory stimulus. TLR2 and TLR4 did not even contribute to persistent pain development, as suggested by examining nociceptive responses in the formalin test, an animal model that mimics a persistent pain produced by peripheral tissue injury and inflammation. The absence of TLR2 and TLR4 overexpression in the spinal cord of unhealthy mice and the lack of antinociceptive efficacy following the pharmacological block by the TLR4 antagonist confirmed these results.
In conclusion, our results highlighted a key role of microglial TLR2 and TLR4 in mediating exclusively chronic pain, so suggesting the possibility of a therapeutic benefit of blocking TLR2 and TLR4 in the treatment of such a pain. It is important to note, as expected, that TLR2 and TLR4 deficiency or block can only attenuated, but not completely abolished pain responses, indicative of numerous pathways mediating the complex signalling of chronic pain. So, their pharmacological block could represent an adjuvant therapy to use in combination with common analgesic drugs, active in acute pain, but often unsatisfactory for chronic pain relief.