Nociceptors 1

Nociceptors are receptors that respond only to actual or imminent tissue damage Several types: High threshold mechanoreceptors: mostly Aδ Thermal nociceptors: mostly Aδ Polymodal nociceptors: mostly C: nociceptors that respond to more than one modality (mechanical, heat, chemical etc) 2

Nerve fibres are of different diameters Large diameter, myelinated: fast conduction Small diameter, unmyelinated: slow conduction Conduction velocity also relates to function... 3

Conduction velocity and function Aα, Aβ Low threshold mechanoreceptors Large myelinated fibres - fast (>30 m/s) Aδ Fast nociceptors/cold receptors - bare nerve endings Small myelinated fibres - slower (5-30 m/s) C: Slow nociceptors/warm and cold receptors - bare nerve endings Small unmyelinated fibres - very slow conduction (0.5-2 m/s) NOCICEPTORS 4

Nociceptor activation 5

Responses of nociceptors Polymodal nociceptor: response to mechanical stimulation Polymodal nociceptor: response to heat 6

Multiple stimuli activate (or sensitise) nociceptors...how are they detected? Inflammatory mediators (prostaglandins, histamine, serotonin, substance P) Substances released from damaged cells: ATP, K +, bradykinin Stimuli causing direct tissue damage: heat, low pH 7

Pain pathways: - fast and slow pain - gate control theory - referred pain 8

Fast and slow pain 9

Reducing the pain: the gate control theory Light touch or rubbing inhibits pain: why? Inhibitory connection in the spinal cord Here’s how it works 10

Reducing the pain: the gate control theory Aβ fibre inactive Inhibitory interneurone inactive C fibre strongly activates projection (second order) neurone Aβ fibre active Inhibitory interneurone active Inhibitory interneurone reduces C fibre activation of projection neurone This is the basis of TENS (transcutaneous electrical nerve stimulation): widely used in pain control TENS: stimulate here 11

Referred pain (1) 12

Referred pain (2) 13

Nociceptor sensitisation causes hyperalgesia Burn injury applied to A and D causes hyperalgesia at A, B and C: in the injured area and far beyond 14

Nociceptor transduction: TRPV1 15

How can we study nociceptor transduction? Not easy: terminals are small and buried in connective tissue We need an alternative approach 16

Partial solution: use the soma as a model of the terminal DRG soma synthesises ion channels…. …which are transported along the axon In vivo…. 17

In culture…. DRG soma synthesises ion channels…. No axon, so they appear in the soma So we can record the ion channels by patch clamping the soma Partial solution: use the soma as a model of the terminal Useful points: The soma is accessible so: Can apply Ca 2+ imaging Can use all varieties of patch clamping Problems:  Mixture of channels from terminal, soma and axon  Change of phenotype in culture 18

Dorsal root ganglion (DRG) neurones in culture Soma Processes 19

Cultured DRG neurone response to heat see Cesare & McNaughton PNAS 1996; Current Opinion in Neurobiology

Cultured DRG neurone response to heat: heat-activated ion channels Nagy & Rang

What kind of ion channel is activated by heat? Heat-gated channel TRPV1 Voltage gated channel They’re distant relatives: definite evolutionary relationship 22

Responses of TRPV1 Heat-activated current of TRPV1 see Caterina et al 1997; Tominaga et al

Responses of TRPV1 Chilli-activated current of TRPV1! 24

Responses of TRPV1 TRPV1 also responds to acid pH - just like polymodal nociceptors 25

Responses of TRPV1 Interaction between acid and heat response of TRPV1: acid sensitises TRPV1 to heat 26

Innocuous thermal sensing 27

warm cold Sensory spots on the back of the wrist Blix 1882, taken from Norrsell et al Brain Res Bull 48: (1999) Spots/cm 2 Cold: 1.0 – 9.0 Warm: 0.4 –

Cold receptor (epidermis) Warm receptor (dermis) Skin thermoreceptors 29

Warm receptor activity Warm receptor activitySpike frequency Darian-Smith et al J Neurophysiol 42: (1979) 30

Cold receptor activity Cold receptor activitySpike frequency Darian-Smith et al J Neurophysiol 36: (1973) 31

1 s 30 °C 25 °C 20 °C 15 °C 10 °C 5 °C Recordings from human cold fibres Cold receptors: Steady state firing vs. temperature Campero et al J Physiol 535: (2001) Spike frequency vs. temperature 32

Warm and cold thermoreceptors Warm receptors Cold receptors Patapoutian et al Nature Rev Neurosci 4: (2003) 33

Innocuous cold transduction: TRPM8 34

Heat-activated Cold-activated Thermally activated TRP channels 35

Thermosensitive TRP channels TRPM8 TRPV3/4 TRPV1 (VR1) TRPV2 (VRL1) 36

Cold-induced depolarisation is potentiated by menthol From Reid & Flonta, Nature 413:480 (2001) 37

Cold activates an inward current which is sensitised by menthol From Reid & Flonta, Nature 413:480 (2001) 38

Cold-activated current: adaptation and recovery From Reid & Flonta, Nature 413:480 (2001) 39

Reading for this lecture: Purves et al chapter 9 (give particular emphasis to the part up to page 198, but please read the rest of the chapter too); chapter 10 (all) Nicholls et al chapter 17 pages see also chapter 18 pages Kandel et al chapters 21-24