Nerve growth factor (
NGF) augments the excitability of isolated rat sensory neurons through activation of the
p75 neurotrophin receptor (p75(NTR)) and its downstream
sphingomyelin signaling cascade, wherein neutral
sphingomyelinase(s) (nSMase),
ceramide, and the
atypical protein-kinase C (
aPKC), protein-
kinase M zeta (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 4-g von Frey hair (VFH, testing "
allodynia") and by 10 and 15g VFHs (testing "
hyperalgesia").
NGF (500ng/10μL) 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,
glutathione (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 p75(NTR) over the
tyrosine kinase (
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 p75(NTR) receptor. Finally, intraplantar (1day 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 p75(NTR), and that a peripheral aPKC is essential for this sensitization.