Nociceptor Activation Flashcards
(15 cards)
Studying pain
Human studies
- biopsy analysis
- quantitative sensory tests
- genetic analysis
Rodent studies
- mouse pain model
- spontaneous and evoked receptors
- RNA sequencing
- transgenic mice
Cellular assays
- ex vivo and in vitro electrophysiology
- live cell imaging
- histology
Molecular assays
- X-ray crystallography
- recombinant expression
- mutagenesis
Studying pain - CIP
causes of congenital insensitivity to pain
- NTRK1 and PRDM12 = no neurones (anatomical)
- SCN9A (sensory abnormalities)
The somatosensory system and nociceptors
- mechanoreceptors sensitise to stimuli
- nociceptors keep on firing until the painful stimulus is removed
- nociceptors tuned to respond to potentially damaging stimuli
- nociceptors are often polymodal and can respond to thermal, chemical and mechanical stimuli
- some nociceptors only response to a subset of these stimuli e.g. heat
Nociceptor analysis
unbiased RNA sequencing
- transcriptome analysis of DRG neurones
- identified 17 sensory subgroups
- what do the groups functionally do?
retrograde tracing
- enables tracing of sensory neurones
- labels DRG
clustering analysis
- identifies 7 colonic sensory neurone subtypes based on transcriptome
- clusters validated with qPCR (mRNA), immunohistochemistry (protein) and calcium imaging (function)
- evidence for one subset mediating pain in IBS-C
Nociceptor activation - heat - capsaicin
- gives chillis a hot taste
- increased capsaicin concentrations = increased heat
- capsaicin = increased [Ca]i in DRGs, same as heat
- cloning and isolation studies found cDNA responsible for this increase in calcium in DRG neurones
- activated by heat at a similar threshold to heat-gated currents in DRG neurones
Nociceptor activation - heat - TRPV1
- KO mice = normal mechanosensitivity but no change in response to very high thermal stimuli
- KO doesn’t stop polymodal C fibres firing; suggests other heat receptors are also present
- activated at ~ 52 degrees Celsius
Nociceptor activation - heat - TRPM2+3
- TRPM3 activated at >40 degrees Celsius
- TRPM2 = thermosensitive
- TRPM2 KO = deficit in warm, not noxious sensation
Nociceptor activation - heat - TRPA1
hot and cold?
Nociceptor activation - heat - TRPV1/M3/A1
- TRPVA1 and TRPM3 double KO DRG neurones still respond to both heat and TRPA1 agonist (AITC)
- TRPA1 antagonist inhibits residual heat and AITC response
- triple KO DRG neurones do not respond to heat
- sensitivity recovers if reintroduce any of the three receptors
- function remains in single and double KO
TRPV1 antagonists
- none have passed clinical trials
- due to hyperthermia or burn risk
- genetic evidence in humans indicates a dominant role for TRPV1, so antagonists could be beneficial
Nociceptor activation - cold - TRPM8
- TRPs are candidate detectors
- TRPM8 threshold is ~25 degrees C and is activated by menthol
- TRPM8 KO mice provide evidence for role in cold sensing
Nociceptor activation - cold - TRPA1
- activated by mustard oil, wasabi and a range of electrophilic compounds
- species-dependent differences
- conflicting evidence in humans
TRPA1 appears to depend on redox state
- responds to warmth and cold in lipid bilayers
- when reduced, both responses are inhibited
- when oxidised, cold response is enhanced
- mechanism for temperature gating is unclear
Nociceptor activation - mechanical - piezo1+2
- mouse neuroblastoma cell line has robust mechanically-gated currents
- screen found piezo1
- siRNA against piezo1 = decreased currents
- piezo2 knockdown in DRG = reduced RA Is
- piezo2 required for touch but apparently not for detection of noxious stimuli
- piezo2 KO = mouse not born
- conditional KO = normal response to noxious stimuli but reduced response to innocuous stimuli
- piezo2 loss-of-function mutations in humans = loss of vibration detection, touch discrimination, joint propioception etc.
- mechanosensitivity remains in piezo2 KO do other mechanosensors must exist
Nociceptor activation - mechanical - Elkin1
- mechanosensitivity remains in piezo2 KO
- suggests other mechanosensor exist e.g. Elik1
- Elkin1 induces mechanosensitivity and functions independently of piezo2
- reduced touch sensation
- reduced Abeta and C fibre functionality
Nociceptor activation - chemical
- tissue acidosis can occur during inflammation, muscle ischaemia and tumour growth
- proton sensing: ASICs, TRP, K2Ps and GPCRs
- proton sensitivity can be modulated
- inhibiting ASICs blocks pain from mild acidosis
- inhibiting TRPV1 blocks pH5-induced pain
- inhibiting TRPV1 and ASICs provides additional pain relief
- pH4.3: inhibiting ASICs, TRPV1 and TRPA1 have no effect on acid pain
