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- Reduction in the threshold and increase in magnitude of a response to a noxious substance. Usually due to inflammation.
- Allodynia: things that weren't painful before are now.
- Hyperalgesia: things that were painful before are even more painful now.
3 main instigators of sensitisation
- Arachidonic acid
Importance of TRPV1
- Polymodally activated (protons, heat, endovanilloids, eicosanoids etc)
- Undergoes sensitisation and adaptation
- Caterina et al, 2000: KO = decreased inflammation and thermal hyperalgesia (hence important for sensitisation)
Arachidonic acid sensitising pathways
- AA: potentiates ASIC-like currents
- PGE2: -> AKAP -> TRPV1
- -> EP4 -> Gs
- cAMP: potentiates HCN2
- cAMP -> Epac1 and PKA
- PKA: sensitises Nav1.8
- Epac1: sensitises Piezo2
- Epac1 -> PKCe
- PKCe: sensitises TRPV1
Bradykinin sensitising pathway
- BK -> B2 - >Gq -> PKC-epsilon -> sensitises TRPV1Bradykinin is activated by kallikrein activation in inflammation
NGF sensitising pathways
- NGF -> TrkA ... -> ... TRPV1
- two suggestions
- TrkA -> PKC -> reduced PIP2 -> reduced TRPV1 inhibition
- TrkA -> src kinase -> increased TRPV1 membrane expression
Direct effects of AA
- Potentiates transient and sustained parts of ASIC-like currents in DRG neurons
- Smith et al, 2007: adding AA increased peak and sustained part of AP induced by pH 6.3. Not due to AA effectors, as it was incubated with inhibitors of AA pathways.
Role of EP4 in sensitisation
- AA -> PGE2 -> EP4 -> Gs ...
- EP4 upregulated in inflammation
- Lin et al, 2006: Western blot - EP4 upreg in ipsi but not contra DRGs following peripheral inflammation; EP4 antag/knockdown -> attenuated therm/mech hypersensitivity, and attenuated PGE2 mediated sensitivity
Epac1 in sensitisation
- Activated by cAMP.
- Hucho et al, 2005: In IB+ DRG neurons, there is a pathway linking cAMP and PKCe, causing mechanical hyperalgesia. cAMP -> Epac -> PLCepsilon -> DAG -> PKCepsilon -> TRPV1 (Numakazi et al, 2002)
- Eijelkamp et al, 2013: Sensitises Piezo2, causing mechanical allodynia in neuropathic pain. Lots of evidence - DRG Epac1 upreg in neuropathic pain; Epac-1 selective cAMP analogue 8-pCPT sensitises mechanically evoked currents in sensory neurons, and induces long lasting allodynia in vivo that is abolished by Epac1 knockdown and attenuated by Piezo2 knockdown. Piezo2 causes large mechanically evoked currents, that are enhanced by Epac2 but unaffected by PKC/PKA. Maybe via PLCe increasing Ca.
- Eijelkamp et al suggest that:
- Nav1.8-ve: Epac -> mechanical allodynia
- Nac1.8+ve: Epac -> thermal hyperalgesia, or mechanical hyperalgesia in IB4+ neurons
- (no thermal hyperalgesia in Nav1.8 depleted mice, IB4+ are Nav1.8 + or -, mechanical hyperalgesia requires Nav1.8+)
HCN channels in sensitisation
- HCN2 and 4 sensitised directly by cAMP: activate at more depolarised potentials so firing frequency is increased. (remember HCN channels in heart being pacemakers - funny current)
- Momin et al, 2008: Blocked by HCN blockers, but not PKA inhibitors.
- Emery et al, 2011: HCN2 KO -> firing frequency not affected by increases in cAMP (via forskolin), but rescued by HCN transfection
- But Ih current not completely abolished - there must be another isoform (HCN4 presumably?)
Which of HCN2 and Nav1.8 is more important for sensitisation?
- HCN2: causing increased AP firing
- HCN2 KO -> carageenan-induced hyperalgesia fails to develop (Emery et al, 2011)
- Nav1.8 -> hyperalgesia onset delayed
What does PKA sensitise?
- Nav1.8: hyperpolarising shift in activation and an increase in peak current (England et al, 1996)
- TRPV1: (Bhave et al, 2000) - scaffolding complex needs to form between TRPV1, PKA/PKC, and AKAP79/150 (Zhang et al, 2008)
Bradykinin and sensitisation
- BK -> B2 -> PKC -> TRPV1
- Cesare and McNaughton, 1996: BK sensitises heat gated ion channels in DRG neurons via PKC - mimicked by PKC activator PMA, recovery from sensitisation prevented by phosphatase inhibitors
- Cesare et al, 1999: PKC-epsilon is the active isoform: 5 present in sensory neurons, but epsilon the only one trafficked to the membrane by BK. BK induced sensitisation suppressed by a specific inhibitor of PKC-e
- Numakazi et al, 2002: PKC phosphorylates TRPV1, potentiating currents and reducing temperature threshold (either from BK -> B2 or ATP -> P2Y). PMA caused direct phosphorylation of TRPV1.
NGF and development
NGF or TrkA (receptor) mutations during development -> cannot perceive noxious stimuli and inability to sweat (anhidrosis)
NGF and sensitisation
- NGF elevated in various inflammatory states
- NGF administration -> rapid and long lasting thermal and mechanical hyperalgesia (increased current amplitude and decreased activation threshold)
- Long lasting because of increased gene expression? Levels of TRPV1, P2X3, ASIC1, Nav all elevated after NGF administration.
- NGF -> sensitises TRPV1: mechanisms unclear.
Competing hypotheses for NGF sensitisation
- Because NGF mediated sensitisation is abolished in TRPV1 KO mice
- Julius group: NGF -> TrkA -> PLC -> reduced PIP2 -> reduced inhibition of TRPV1
- McNaughton group: NGF -> TrkA -> PI3K -> src kinase -> increased membrane translocation of TRPV1
Julius group - NGF sensitisation
- NGF -> TrkA -> PLC -> reduced PIP2 -> reduced inhibition of TRPV1
- Phosphoinosities inhibit TRPV1
- PIP2 mAB -> TRPV1 activation at 35 degrees (vs higher) (Chuang et al, 2001)
- Adding PIP2 to artificial lysosomes -> increase in TRPV1 activation threshold (Cao et al, 2013)
McNaughton group - NGF sensitisation
- NGF -> TrkA -> PI3K -> src kinase -> increased membrane translocation of TRPV1
- Zhuang et al, 2005: NGF -> PI3K -> src kinase
- src kinase phosphorylates Y200 on TRPV1, increasing membrane expression and causing larger currents in response to capsaicin
- Not seen in Y200 mutant or dom-neg src kinase
PIP2 depletion saga
- Lukacs et al, 2013: sensitisation vs adaptation of TRPV1 depends on PLC isoform and how it affects PIP and PIP2
- Saturating capsaicin -> opens TRPV1 -> Ca entry -> PLCdelta -> decreases PIP and PIP2 -> TRPV1 inhibition
- BK -> B2 -> PLCbeta -> decreases PIP2 but NOT PIP; activates PKCepsilon -> TRPV1 sensitisation
- Unclear what this means for NGF: TrkA activates PLCgamma