The TrkA receptor mediates experimental thermal hyperalgesia produced by nerve growth factor: Modulation by the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75NTR) and its activation of the sphingomyelin signaling cascade are essential for mechanical hypersensitivity resulting from locally injected nerve growth factor (NGF). Here the roles of the same effectors, and of the tropomyosin receptor kinase A (TrkA) receptor, are evaluated for thermal hyperalgesia from NGF. Sensitivity of rat hind paw plantar skin to thermal stimulation after local sub-cutaneous injection of NGF (500ng) was measured by the latency for paw withdrawal (PWL) from a radiant heat source. PWL was reduced from baseline values at 0.5-22h by ∼40% from that in naïve or vehicle-injected rats, and recovered to pre-injection levels by 48h. Local pre-injection with a p75NTR blocking antibody did not affect the acute thermal hyperalgesia (0.5-3.5h) but hastened its recovery so that it had reversed to baseline by 22h. In addition, GW4869 (2mM), an inhibitor of the neutral sphingomyelinase (nSMase) that is an enzyme in the p75NTR pathway, also failed to prevent thermal hyperalgesia. However, C2-ceramide, an analog of the ceramide produced by sphingomyelinase, did cause thermal hyperalgesia. Injection of an anti-TrkA antibody known to promote dimerization and activation of that receptor, independent of NGF, also caused thermal hyperalgesia, and prevented the further reduction of PWL from subsequently injected NGF. A non-specific inhibitor of tropomyosin receptor kinases, K252a, prevented thermal hyperalgesia from NGF, but not that from the anti-TrkA antibody. These findings suggest that the TrkA receptor has a predominant role in thermal hypersensitivity induced by NGF, while p75NTR and its pathway intermediates serve a modulatory role

The p75NTR SIGNALING CASCADE MEDIATES MECHANICAL HYPERALGESIA INDUCED BY NERVE GROWTH FACTOR INJECTED INTO THE RAT HIND PAW

Nerve Growth Factor (NGF) augments excitability of isolated rat sensory neurons through activation of the p75 neurotrophin receptor (p75NTR) and its downstream sphingomyelin signaling cascade, wherein neutral sphingomyelinase(s) (nSMase), ceramide, and the atypical PKC (aPKC), PKMζ, are key mediators. Here we examined these same receptor-pathways in vivo for their role in mechanical hyperalgesia from exogenous NGF. Mechanical sensitivity was tested by the number of paw withdrawals in response to 10 stimuli (PWF = n/10) by a 4g von Frey hair (VFH, testing “allodynia”) and by 10g and 15g VFHs (testing “hyperalgesia”). NGF (500 ng/10 µl) injected into the male rat’s plantar hind paw induced long lasting ipsilateral mechanical hypersensitivity. Mechano-hypersensitivity, relative to baseline responses and to those of the contralateral paw, developed by 0.5–1.5h and remained elevated at least for 21–24h, Acute intraplantar pre-treatment with nSMase inhibitors, GSH or GW4869, prevented the acute hyperalgesia from NGF (at 1.5h) but not that at 24h. A single injection of N-acetyl sphingosine (C2-ceramide), simulating the ceramide produced by nSMase activity, induced ipsilateral allodynia that persisted for 24h, and transient hyperalgesia that resolved by 2h. Intraplantar injection of hydrolysis-resistant mPro-NGF, selective for the p75NTR over the TrkA receptor, gave very similar results to NGF and was susceptible to the same inhibitors. Hyperalgesia from both NGF and mPro-NGF was prevented by paw pre-injection with blocking antibodies to rat p75NTR receptor. Finally, intraplantar (1 day before NGF) injection of mPSI, the myristolated pseudosubstrate inhibitor of PKCζ/PKMζ, decreased the hyperalgesia resulting from NGF or C2-ceramide, although scrambled mPSI was ineffective. The findings indicate that mechano-hypersensitivity from peripheral NGF involves the sphingomyelin signaling cascade activated via p75NTR, and that a peripheral aPKC is essential for this sensitization

Endothelin receptors and pain.

International audienceThe endogenous endothelin (ET) peptides participate in a remarkable variety of pain-relatedprocesses. Pain that is elevated by inflammation, by skin incision, by cancer, during a Sickle Cell Disease crisis and by treatments that mimic neuropathic and inflammatory pain and are all reduced by local administration of antagonists of endothelin receptors. Many effects of endogenously released endothelin are simulated by acute, local subcutaneous administration of endothelin, which at very high concentrations causes pain and at lower concentrations sensitizes the nocifensive reactions to mechanical, thermal and chemical stimuli. PERSPECTIVE: In this paper we review the biochemistry, second messenger pathways and hetero-receptor coupling that are activated by ET receptors, the cellular physiological responses to ET receptor activation, and the contribution to pain of such mechanisms occurring in the periphery and the CNS. Our goal is to frame the subject of endothelin and pain for a broad readership, and to present the generally accepted as well as the disputed concepts, including important unanswered questions