B.L. Flashcards
Descending control can be facilitatory as well as inhibitory. In what ways is descending control dynamic?
Changing balance between degree of inhibition + degree of facilitation on pain processing in spinal cord
Dynamic in..
Emotional states: (fear - inhibition tends to predominate)
Pathological states: central control systems change in fever / sickness response (heightened transmission of nociceptive information)
Development: neonate facilitation predominates (neonatal exquisitely sensitive to nociceptive input and can have major impact on future pain experiences)
Descending facilitation now known to contribute to phenomenon of central sensitisation
What did Rexe provide?
Rexe’s Laminar Organisation of the Spinal Cord (1970s)
On cytoarchitectonic grounds (shape/organisation of neurons in spinal cord grey matter): delineated a number of concentrically organised laminae
* Dorsal horn = laminae I-V * Ventral horn = laminae VI-X
Although described purely on anatomical grounds, neurons in different laminae also serve different functions
What did Rexe provide?
Rexe’s Laminar Organisation of the Spinal Cord (1970s)
On cytoarchitectonic grounds (shape/organisation of neurons in spinal cord grey matter): delineated a number of concentrically organised laminae
* Dorsal horn = laminae I-V * Ventral horn = laminae VI-X
Although described purely on anatomical grounds, neurons in different laminae also serve different functions
(lamination described by Rex may have functional relevance based on where different fibre types terminate in spinal dorsal horn)
What are thermoreceptors mainly innervated by?
Free nerve endings that are predominantly αδ and C
Nociceptors come in different ‘types’ - what are they based on
Type of stimulus required to activate them (e.g. high threshold mechanoreceptors, thermal nociceptors, polymodal)
Most polymodal respond to heat, high threshold mechanical + chemical activation
- Deep receptors: less well studied than cutaneous receptors (invasive procedures can affect what you are trying to study)
- Viscera: can cut/burn without feeling, but distention of gut will be felt - finding adequate stimulus to activate nociceptors is difficult
What are examples of morphological studies?
DEGENERATION: observed after sectioning dorsal roots or portions of them - central projections degenerate but with a different delay related to diameter of fibre (Aβ, Aδ or C)
(monitoring time course of degeneration means can deduce where different fibre types terminate in cord
AXONAL TRANSPORTS: cut peripheral nerves into dye/markers, transported into spinal cord
AUTORADIOGRAPHY: of spinal cord sections following injections of radioactive amino acids into dorsal root
- Lots of overlap between what these different fibres are connected to in the periphery
What is a technique for functional identification + anatomical organisation of primary afferent terminals in the dorsal horn?
Record from individual DRG cell in preparation with intact peripheral axon (so can test what it responds to: touch/pinch/heat/chemical etc)
*Recording from nerve must be intracellular
- Then inject with a dye after killing animal - allow transport of substance into the dorsal horn → histological sectioning to map where the terminations are of individual neurons that have been characterised physiologically
- Very technically challenging! But useful, functional information
What did Cervero 1996 review article show about skin mechanical input?
Info from diff experiments; input from skin/muscle/viscera and where the fibres ended in spinal cord grey matter
Skin mechanical input: αβ mainly in neck of dorsal horn (lamina III,IV&V), C low threshold fibre in lamina II (superficial dorsal horn)
What did Cervero 1996 review article show about skin mechanical input?
Info from diff experiments; input from skin/muscle/viscera and where the fibres ended in spinal cord grey matter
Skin mechanical input: αβ mainly in neck of dorsal horn (lamina III,IV&V), C low threshold fibre in lamina II (superficial dorsal horn)
What did Cervero 1996 review article show about skin nociceptors?
Skin nociceptors: αδ + C fibres mainly in lamina I + II
* some aδ in lamina V but very little C fibre input
What did Cervero 1996 review article show about skin nociceptors?
Lamination first described by Rex may have functional relevance based on where different fibre types terminate in spinal dorsal horn
What did Cervero 1996 review article show about skin nociceptors?
αβ (large diameter) down into lamina IV (4 - where most proprioceptive/tactile input goes)
αδ fibres activated neurons in lamina I (and some in lamina V - deep dorsal horn)
C fibres predominantly lamina II
*Different neurons in dorsal horn have long processes (Neurons in lamina V can make contact with lamina I/II), therefore Termination in the lamina does not mean that neurons within those laminae are exclusively activated by that fibre type
How are dorsal horn neurons classified?
1970s: defined dorsal horn neurons based on their responses to cutaneous inputs (Class 1, 2 and 3)
1 = lam 3+4 2 = lam 5 3 = lam I
What are class 1 DH neurons?
Low threshold / mechanoreceptive
Only respond to LOW INTENSITY stimulation in periphery (mainly αβ, rapidly conducting myelinated):
Mainly in III (3) + IV (4)
= (but some in V/VI)
What are class 2 DH neurons?
WDR / polymodal / mutlireceptive / convergent / lam V
Respond to both HIGH intensity and LOW intensity stimuli e.g. heat, acid.
Predominantly lamina V. (input is αβ + aδ + MAYBE C). *important - does class 2 fibre have C fibre input?
What are class 3 DH neurons?
High threshold / nociceptive specific (NS)
Only respond to HIGH intensity (40 degree heat, pinch, chemicals, algesic chemicals), input almost exclusively αδ + C fibres (in different combinations).
Predominantly in lamina I (superficial, surface)
Know less about lamina I (surface) because its easier to record from lamina V. (deep) Surface of cord = pulsatile movement with circulatory system/respiration - recording from small neuron difficult. Deep in DH more stable and easier to make long lasting recordings (hours).
Workman & Lumb (1990). What did they show about class 2 DH neurons?
Lamina V recording - intact animals anaesthetised
After killing animal, inject dye through electrode
Histological sections - examine exactly where recording was from
L6 recording (input from hind paw) - stimulus-time histogram, number of APs per second
Cell responses to prod, brush, PINCH (main) and noxious heat. Firing rate for noxious heat same as brush, therefore - the way these neurons signal nociceptive input still great debate, as can’t just be frequency of firing if noxious heat has same rate as brush
Debate: which cells are important in pain pathways?
Class 3 in lamina I that only respond to nociceptive input, or class 2 in lamina V with low and high intensity input?
Lumb (1990). What shown about viscerosomatic neurons?
They are convergent, in the sense that they have input from the skin and the viscera
No private visceral pain pathways from gut to brain
- shares its pathways with somatic input (efficient)
- not very economical use of nervous system to have private pathway as not often aware of gut/heart etc.
Particular cell recorded from responds to splanchnic nerve (visceral) but also receptive field on flank (skin)
* basis of referred pain = brain learns that activity from this cell is from the skin * injury/pathology in gut, nociceptive afferents activated (visceral afferents) but brain interprets of skin
How can class 2 DH neurons be classified?
C +ve or C -ve
Functional significance of classification
Class 2 activated by low + high intensity stimuli, but response to high intensity stimulation could be mediated by αδ, C, or both.
Most are C-positive (90%)
Important subset are not activated by C fibres (C-negative) - no matter how high intensity the stimulus, can never evoke a C fibre response
How can you identify C positive vs C negative
Anaesthetised rat, recording from DH neuron in lamina V, electrical stimulus in receptive field on skin
Responds with APs at very short latency (because α fibres conduct more rapidly, get into spinal cord more quickly) - response to α fibres, probably low threshold
After delay (about 200ms) - get burst of APs that are conveyed by C fibres (because conducting much more slowly - less than 1m/second
Lab work: descending control of neurons with and without C fibre input - important differences in the way they are regulated by the brain
* both behavioural and clinical significance
What did Cervero 1996 review article show about fibre tract OUTPUT from DH neurons?
PSDC = post-synaptic dorsal column pathway
SCT = spinal-cervical tract
STT = spino-thalamic tract (note: many ascending pain pathways that are not SST)
SRT = spino-reticular tract
SMT = spino-mesencephalic tract
SCbT= spino-cerebellar tract
AM - autonomic motor system, SM - somatic motor system
What did Cervero 1996 review article show about DH OUTPUT to the STT?
STT: some cells of origin in lamina I + V, but also some in lamina VI (6)
*Often assumed that DH all sensory + ventral horn all motor (many cells of origin of the STT are in lamina VII(7), VIII(8) (ventral horn) and X(10) (around central canal)
What did Cervero 1996 review article show about DH tract OUTPUT to the SRT?
SRT: largely nociceptive, taking info to reticular formation in brainstem
cells of origin - lamina I, V, VII, VIII, X (STT without 6)
Often is collaterals of STT (tracts share common parts of pathways then send of branches and innervate different parts of neuroaxis)
What did Cervero 1996 review article show about DH tract OUTPUT to the PSDC?
Used to be considered purely low threshold (as the dorsal columns are)
* dorsal columns: input through dorsal root, axons don’t enter grey matter → straight to dorsal column nuclei * but there are collaterals at the lower spinal cord that do synapse within the cord - the neurons they synapse with form the post-synaptic dorsal column pathway (synapse between primary afferent and dorsal column nuclei) * now know this also subserves nociceptive function
- Cerebellum neglected in pain research although it lights up in MRI scans!!
- SCbT: just one way of getting to cerebellum, has cells of origin in lamina I+V, (also IV, VI, VII, VIII)
What did Cervero 1996 review article show about DH tract OUTPUT to the PSDC?
Used to be considered purely low threshold (as the dorsal columns are) - from 3/4/5
Dorsal columns: input through dorsal root, axons don’t enter grey matter → straight to dorsal column nuclei
But collaterals at the lower spinal cord do synapse within the cord - the neurons they synapse with form the post-synaptic dorsal column pathway (synapse between primary afferent and dorsal column nuclei)
* now know this also subserves nociceptive function
What did Cervero 1996 review article show about DH tract OUTPUT to the cerebellum?
SCbT: just one way of getting to cerebellum, has cells of origin in lamina 1+5, (also IV, VI, VII, VIII)
Cerebellum neglected in pain research although it lights up in MRI scans!!
Sensory discriminative pain?
Perception / detection
Intensity, location, duration
Somatosensory cortex
Specific thalamic nuclei: VPL, VPM, VMPo
Components of: STT, SCT, PSDC
Affective-cognitive pain?
Emotional response / Coping
Autonomic, motor, endocrine
Memory
Insular & cingulate cortex
Specific thalamic nucleus (VMPo)
Non-specific thalamic nuclei: medial thalamus
Hypothalamus, amygdala
Components of: STT, SCT, PSDC + SHT, SRT, SMT, SPBT
?
Some modulation takes place in spinal cord itself (gate-control theory)
Important modulation mechanism is descending control (nociceptive input up to brain triggers descending pathways that come back down and modulate transmission - self-regulating system)
*Self-regulating system can depress or enhance transmission (behavioural/clinical significance)
/?
Nociceptive info from skin & viscera conveyed to spinal cord in different classes of nociceptors
*broad = C vs A fibre (predominantly A delta)
What is a limitation of experiments that activate the PAG electrically?
Indiscriminate
Activating fibres of passage, therefore may be fibres originating from elsewhere and just passing through PAG
*could antidromically activate cells in PAG that are activating something else in brain
?.
Descending control emanates from many areas of the brain, just focusing on PAG for the purpose of these lectures!
Nociceptive input into PAG, but also to higher centres
RVM = rostro-ventral-medial medulla
- likely to be hub of descending control
- higher centres such as PAG project to RVM to exert effects at the level of the spinal cord
- Only a few neurons in PAG that project directly to the dorsal horn of the spinal cord
- evidence suggests that PAG exerts its effects through the RVM or through pontine areas
- RVM has 2 descending systems (one inhibitory, one facilitatory)
- may be two separate pathways
What hypothesis did Waters & Lumb (1997) test?
Hypothesis: DL / L columns only INHIBIT HIGH threshold input: low threshold input remains intact in WDR cells- dampen down noxious info but allow animal responses to low threshold stimuli.
*Ventro-lateral PAG: dampening down all sensory transmission in DH (could not find evidence to support this - just came from a review)
What did Waters & Lumb (1997) show about DL/L PAG?
Anaesthetised rats: recorded in lamina V lumbar DH (Class 2: responds to non-noxious pinch and high intensity noxious heat)
Stimulated PAG with homocysteic acid (DLH)- see expected BP increase
Response of DH neuron: post-stimulus-time histogram - not much happens to pinch, but noxious heat has fewer APs (selective control of noxious over non-noxious transmission)
What did Waters & Lumb (1997) show about VL PAG?
Anaesthetised rats: recorded in lamina V lumbar DH (Class 2: responds to non-noxious pinch and high intensity noxious heat)
Stimulated PAG with homocysteic acid (DLH)- see expected BP decrease
- Post-stimulus-time histogram: response to non-noxious prod remains intact + response to noxious heat profoundly depressed
- THEREFORE DOES NOT SUPPORT THE PREVIOUS IDEA that DL/L is selective + VL non-selective: shows that both DL and VL produce selective depression of noxious inf.
What did Waters & Lumb (2007) show?
Effects of PAG activation on a C+ve + a C-ve neurone
Although BOTH neurons showed depression of responses to noxious stimulation, was not depression in all cases - some neurons were actually facilitated
C+ve: Inject DLH in PAG, reduce response to noxious pinch, then after excitatory amino acid, shows recovery
*C-ve - significant minority of neurons: apply noxious pinch (control): apply DLH and get INCREASE in response
Those with C fibre input were depressed
Those without C fibre input were enhanced
How did Waters and Lumb (2008) identify C positive vs C negative cells?
Isolated single neuron (tap receptive field + isolate receptive field).
Needle electrodes in skin - electrically activate cell (C conduct more slowly than A, so if synchronous discharge following electrical stimulus, can tell if C-ve)
Bunch of APs at very short latency = A fibres.
If turn up stimulus intensity (about 10x more than A fibres) then may see C fibres (longer latency) - watch oscilloscope + listen for APs
C +ve cells have nociceptive drive and their nociceptive input could be mediated by C fibre AND/OR A fibres
C-ve cells (minority), no matter how high stimulus, would never see C fibre evoked response - *these cells are FACILITATED by descending control. Therefore descending control seems to target C fibre responses?
What did Waters and Lumb (2008) show?
Isolated single neuron (tap receptive field + isolate receptive field).
Needle electrodes in skin - electrically activate cell (C conduct more slowly than A, so if synchronous discharge following electrical stimulus, can tell if C-ve)
Bunch of APs at very short latency = A fibres.
If turn up stimulus intensity (about 10x more than A fibres) then may see C fibres (longer latency) - watch oscilloscope + listen for APs
C +ve cells have nociceptive drive and their nociceptive input could be mediated by C fibre AND/OR A fibres
C-ve cells (minority), no matter how high stimulus, would never see C fibre evoked response - *these cells are FACILITATED by descending control. Therefore descending control seems to target C fibre responses?
Waters & Lumb (2008) - what did grouped data show?
Group data over 100 neurons?
Above 0 = facilitation, below 0 = inhbition
C +ve cells: nociceptor-evoked responses are depressed by PAG
C -ve cells: nociceptor-evoked responses are enhanced by PAG
Linear relationship when quantified (strength of C fibre input on X, % inhibition on Y)
Stronger C-fibre input (more APs in response to stimulation) = greater inhibition by PAG
Properties of C-fibres
C-fibre afferents
* pain poorly localised ‘aching’ in character * stimulation can produce intolerable pain * slowly conducting * second pain * depressed & loss of encoding when PAG activated
Properties of A fibres
A-fibre afferents
- well localised ‘pricking’ pain
- stimulation tolerable
- faster conduction
- first pain
What is the behaviour significance of PAG being selective for C-fibre input?
When PAG activated in response to threat/stress, can filter out info from C-fibre route and enhance info from A-fibre route
Unwanted distracting part of pain signal is filtered out, animal can continue with behaviour neccessary for survival, and also enhanced response to A-fibre stimulation e.g. move from stimulus
* also useful clinically…
C-versus A-fibre evoked spinal nociception in secondary hyperalgesia?
Series of human experiments showed that primary hyperalgesia includes activation of C-fibre nociceptors (some C fibres transmitted to spinal cord, some transmitted to brain & triggers descending pathway)
CENTRAL sensitisation that underlies secondary hyperalgesia is dependent on C nociceptor activation from the area of primary hyperalgesia
*C fibre input from area of primary hyperalgesia triggers events in spinal cord, enhanced by descending control systems, that turn up excitability in spinal cord
Treede & Margel (2000): a-nociceptors that are enhanced in central sensitisation, but facilitated by C-fibre nociceptive input from primary hyperalgesia, causes secondary hyperalgesia which is though a-fibre nociceptors
Role of differential descending control of C vs A-fibre evoked spinal nociception in secondary hyperalgesia?
Because C -ve cells are facilitated by descending control, could contribute towards the central sensitisation effect
- In reading: secondary hyperalgesia sensitive to punctate mechanical stimuli and not to heat - but this is not the case
- experiments - activated a fibre nociceptors with heat within secondary hyperalgesia region, showing facilitation
How can a / c fibres be activated preferentially with non-invasive technique?
Electrical stimulation of skin after using needles is indirect - could be missing cells / getting false negatives) - therefore wanted to find way of activating fibres independently
Therefore use different rates of skin heating
Experiments to show that fast ramps predominantely activate a fibres and fast ramp predominantely activates c fibres
- McMullan, Simpson & Lumb (2004)
- Leith, Wilson, Donaldson & Lumb (2007)
*Extra reading.
How do fast heat ramps work?
Fast rates of surface skin heating produces linear sub-surface rates of skin heating
Get A fibre activation (predominantly)
- Fast ramps actually activating CAPSAICIN-INSENSITIVE A-fibre heat nociceptors (TRPV1 cells that are resistant to capsaicin?)
- Can feel it becoming warmer until too hot
How do slow heat ramps work?
- If use slow rates of skin heating, activate C fibre nociceptors
- Slow ramps activating primary afferents expressing TRPV1 receptors (C unmyelinated - capsaicin sensitive)
TRPV1 expressing are pre-dominantely c fibres and vice versa?
*Odd sensation which suddenly becomes excruciating - no encoding of the temperature going up
What did McMullan and Lumb (2006) show about withdrawal to fast/slow ramps?
Selective effects of PAG on withdrawal responses to slow (2.5°C s⁻¹) vs fast (7.5°C s⁻¹) rates of skin heating
Heating apparatus with copper plate (wires into biceps femoris to measure withdrawal) - effect of activating PAG on response to C vs A fibre stimulation
Often use reflex approach first because less labour intensive than using DH (reflexes for proof of concept)
- Slow heating (c): after activating PAG (using DLH) - threshold goes up (anti-nociption)
- Fast heating (a): some changes but not significant (threshold went up a bit again)
What did McMullan and Lumb (2006) show about DH responses to fast/slow ramps?
Recording from class 2 C+ve neuron: activate using slow + fast skin heating: plot APs per second for every temp degree rise
Responses to fast ramps (a input) = LINEAR increase in FIRING rate = faithful, precise encoding, as temperature increases neuron fires more and more
Responses to slow ramps (C input) = some encoding of stimulus ‘fuzzy’ - unsure of stimulus then suddenly very painful
*Assumed PAG activation would ↓ C fibre response + keep A response intact- BUT;
Slow ramp (c)= control shows some encoding then drops off as temp increases - DLH injection causes loss of relationship with stimulus intensity + response *Reducing number of APs (magnitude) & disrupting encoding in the C-fibres
Fast ramp (a) = activate PAG: do decrease size/magnitude of response, but ENCODING is still present - still able to signal increase in temperature to noxious range
Therefore, when PAG is active (for Class 2 C+ve cells): disrupt info coming in through c fibres, whereas a fibres still able to encode (but reduce the magnitude of response) - A fibres still provide information about where and how much it hurts
What would need to be tested next in terms of McMullan & Lumb (2006) experiment?
McMullan & Lumb (2006) was conducted on C+ve cells: showed A + C fibre inputs into C-positive were both depressed in their magnitude
Not yet tested with heat ramps, what happens to C-negative cells when activate PAG
*Hypothesis: if repeat on C-ve cells, wouldn’t get the same depression that is seen in C+ve cells (because PAG thought to facilitate C-ve cells, which would add to effect of central sensitisation, where A fibres are facilitated in area of secondary hyperalgesia!)
Significance of lamina I?
Lots of the nociceptive input coming in at primary afferents (αδ) terminates predominantly in lamina I
Contains majority of nociceptive-specific neurons *less known about the descending control of these superficial cells (difficult to record lamina I)
Different approach: instead of electrophysiology, activate αδ/C fibres using skin heating - but expression of cFos protein to identify where cells were activated & the effects of descending control
What is cFos?
Immediate early gene, encodes Fos protein contained in neuron nuclei
Turned on by Ca entry into activated cell
Fos protein positive neurons can be visualised by immunohistochemistry
Noxious stimulus is powerful stimulus for activation of cFos gene
What did Koutsikou et al (2007) do?
Anaesthetised rats, stimulated hind paw (fast/slow heat ramp)
Repeat in separate populations of cells (separate sets of animals):
* one set = activate A fibres (control) * one set = activate C fibres (control) * one set = A fibres + PAG activation (DLH) * one set = C fibres + PAG stimulation (DLH)
Electrode into PAG: Inject DLH & record BP
Counted Fos neurons in control + after PAG stimulation (if inhibitory, see fewer neurons)
What did Koutsikou et al (2007) show about slow skin heating?
2 animals: kill + prepare spinal cord
Superficial dorsal horn (I+II); black dots = Fos +ve nuclei following peripheral stimulation with slow heat ramps (C)
Repeat in different group of animals - precede stimulus to skin with injection of amino acid to PAG - reduces amount of CFos positive nuclei
Even in the superficial DH; PAG having inhibitory effect (fewer cFos positive neurons)
Effect of PAG stimulation on cFos induction in the spinal response to fast & slow rates of skin heating
What did Koutsikou et al (2007) show about slow and fast skin heating?
Pooled data from their experiments (lam III-VI were electrophysiological recordings)
Slow (C fibres): PAG = DEPRESSION of C-fibre evoked activity in DEEP dorsal horn (lamina V) & SUPERFICIAL dorsal horn (I-II) *consistent with electrophysiological experiments
(remember, electrophysiological experiments showed no significant effect on A-evoked activity in deep horn?
Fast ramps (a): PAG has no effect in deep dorsal horn (III-VI) but DEPRESSION of A fibre evoked activity in SUPERFICIAL
Superficial and deep dorsal horn probably have different roles in nociceptive processing
One idea: superficial cells important in triggering descending control systems? Further investigation needed
What hypothesis was generated from the effects of NSAIDs?
- NSAIDs = COX inhibitors therefore inhibit PGE₂ production
Microinjected PGE₂ into PAG —> affect nociceptive processing?
- Therefore NSAIDs have some CNS action
- But unsure if this was C or A fibre activity mediated
What did Leith et al (2007) about COX inhibitors?
Non-selective COX inhibitor (for proof of concept)
- Slow (C): non-selectively inhibit COX isoforms by VL-PAG injection, ↑ withdrawal threshold (ANTI-nociceptive effect)
- Fast (A): still an effect, but shorter lasting, and less profound
COX-1 inhibitor: same results as before (sustained anti-nociceptive effect on C fibre stimulation, shorter effect on A fibre stim)
COX 2: no effect on either stimulus
- Conclusion: ANTI-nociceptive effect of COX-1 inhibitors, but not COX-2 inhibitors, in VL PAG (not DL/L PAG)
- By inhibiting COX, getting change in threshold; therefore, prostaglandins are tonically active in the PAG (they have ongoing activity, and therefore have ability to set the level of nociceptive spinal processing - COX inhibitors change this level of activity)
What did Leith et al (2007) about COX inhibitors?
Non-selective COX inhibitor (for proof of concept)
- Slow (C): non-selectively inhibit COX isoforms by VL-PAG injection, ↑ withdrawal threshold (ANTI-nociceptive effect)
- Fast (A): still an effect, but shorter lasting, and less profound
COX-1 inhibitor: same results as before (sustained anti-nociceptive effect on C fibre stimulation, shorter effect on A fibre stim)
COX 2: no effect on either stimulus
- Conclusion: ANTI-nociceptive effect of COX-1 inhibitors, but not COX-2 inhibitors, in VL PAG (not DL/L PAG)
- Inhibiting COX = change in threshold; therefore, PGs are TONICALLY ACTIVE in PAG (have ongoing activity, and can set level of nociceptive spinal processing - COX inhibitors change this level of activity)
How did Leith et al (2007) show that PGE2/COX inhibition preferentially modulates C-fibre evoked responses?
Area under curve = area between baseline and effect (quantitative measure of any effect)
PRO-nociceptive effect of PGs on slow ramp (area under curve decreases)
Give non-selective COX inhibitor: ANTI-nociceptive effect on the slow ramps more than the fast ramps (mimicked by COX-1 inhibition - SC-560)
No effect from COX2 manipulation
- Demonstrates that these descending control systems are also manipulated in the clinical setting with commonly used drugs
Summary: PAG prostaglandins
- PGE2 in the VL-PAG but not the DL/L-PAG has a pro-nociceptive effect on C- but not A-nociceptor-evoked spinal processing
- COX-1 but not COX-2 products have pro-nociceptive effect on acute spinal nociception
- … and effects on C-greater than on A-nociceptor evoked activity
- COX-1 products are tonically active and have the capacity to set the gain of acute spinal nociception
How did Leith et al (2007) show that PGE2/COX inhibition preferentially modulates C-fibre evoked responses?
AUC = area between baseline and effect (quantitative measure of any effect)
PRO-nociceptive effect of PGs on slow ramp (area under curve decreases)
Give non-selective COX inhibitor (ketoprofen): ANTI-nociceptive effect on the slow ramps more than the fast ramps (mimicked by COX-1 inhibition - SC-560)
No effect from COX2 inhibition
Demonstrates that these descending control systems are also manipulated in the clinical setting with commonly used drugs
Summary: prostaglandins on PAG
PGE2 is PRONOCICEPTIVE on C-nociceptor evoked spinal processing, when acting in VL-PAG, (but not DL/L-PAG)
COX-1 but not COX-2 PROCDUCTS have pro-nociceptive effect on acute spinal nociception
- … and effects on C-greater than A-nociceptor evoked activity
- COX-1 products are tonically active and have the capacity to set the gain of acute spinal nociception
Where do neurons in PAG project?
Very few neurons in PAG project directly to DH (influences DH, but indirectly)
Receiving info from higher cortex, amygdala etc
Evidence that it acts through pontine noradrenergic systems
- Most importantly: through rostral ventromedial medulla (RVM)
- Two pathways: one inhibitory, one facilitatory from cells in RVM
PAG can manipulate activity in RVM and affect balance between inhibitory and facilitatory effects
Amygdala / hypothalamus etc.- areas of limbic system, instrumental in autonomic and emotional responses to pain: communicate with PAG which communicates with RVM
In addition to spinal mechanisms, what is required for hyperalgesia and allodynia? (cardinal signs of chronic pain)
Descending control from RVM
*remember: RVM itself is influenced by PAG
Inflammatory and neuropathic models:
* formalin test * mustard oil hyperalgesia * spinal nerve ligation
- all have demonstrated the integrity of the systems from the RVM
What are the cells in the RVM?
ON & OFF Cells
- Project to DH laminae I, II & V
- Respond to manipulations of PAG
- Affected by morphine / analgesics acting at the PAG
Broad influence - whole body receptive fields, widespread SC and TG projections (single site within RVM can affect inputs from face/foot)
Fire reciprocally & each cell type fires together (if on cells on, off cells are off, vice versa)
Difference between ON and OFF cells?
OFF cells: net inhibitory effect: tail flick (TF) latency longer when active (if specifically activate off cells, tail flick will be at higher temperature)
ON cells: net facilitatory effect: tail flick (TF) latency shorter when cells active
(if can specifically activate on cells, tail flick will be at lower temperature)
What did Fields et al (2006) show?
Lightly anaesthetised rat - tail blackened (absorbs heat) & heat ramp over tail - latency to tail flick measured (indicates the response to noxious stimulus)
Oscilloscope trace: APs recorded from cell in RVM. Off cell typically firing then just before tail flick, stops firing. On cell starts to fire at tail flick.
Also third type of cell: neutral cell
Hypothesis: OFF cells are inhibitory to spinal nociceptive processing - therefore when switch off - the tail flicks. ON cells: FACILITATE spinal nociceptive processing - only when they become active that tail flick occurs
What did Fields et al (2006) show about morphine?
Morphine INHIBITS ON cells - could be a mechanism behind its anti-nociceptive effects? It also ACTIVATES the OFF cells
Why has there been lots of interest in serotonin? What discovery has this led to?
Off cells: most serotonin in CNS is in RAPHE nuclei (part of RVM), when it became evident that RVM critical in descending control - focus on the effect of 5HT on these systems
Clinical trials - do not prove to be efficacious in clinical setting (relief of pain)
Combine recordings with intracellular labelling of cells (identify off cell, recover it from histological material and stain for 5HT - it is NEUTRAL cells that are serotonergic, not on/off cells)
As move from acute to chronic pain (still don’t know what triggers the change) - hypothesis that OFF cells (serotonergic cells?) may take on DIFFERENT characteristics when moving from acute → chronic pain
*facilitatory pathway may predominate in chronic pain? when manipulate animal models of chronic pain - RVM important
Porecca et al (2002) - what did they show?
Review: brought together evidence for descending facilitation from RVM in inflammatory hyperalgesia
Also brought together evidence from NP pain models & inflammatory models
1ry hyperalgesia (site of injury) + 2ry hyperalgesia (develops due to central sensitisation) → high input into DH, transmitted to brain (some in STT) → collaterals into RVM → descending pathways with FACILITATORY effect on dorsal horn
*one of the mechanisms to increase excitability of dorsal horn
Nociceptive response is facilitated by EXCITATORY projections from the RVM → spinal cord
* if block this, block the pain response
Based on Porecca (2002) review - how have labs manipulated RVM circuit to study effects on nociception?
- cut descending pathways (e.g. cut VLF) - can reduce hyperalgesia / allodynia
- manipulate activity in RVM (e.g. lidocaine injection, neurotensin antagonist [important NT in system - antagonist reduces chronic pain], block NO / CCK & excitatory amino acids): requirement for RVM in inflammatory hyperalgesia
What did Bee & Dickenson (2007) show about the RVM in NP pain?
Anaesthetised rats, spinal nerve ligation, recording lamina V WDR cells (AP rate in response to diff skin temps)
Control NP model: stimulus intensity still ENCODED by Class II WDR neurons (one of their features).
Block RVM with lignocaine: reduced deep dorsal horn response in 64% of NORMAL rats and in 81% of SNL rats
Normal rat: RVM block reduced response to NOXIOUS input
SNL rat: RVM block also reduced responses to NON-noxious stimuli
Increase in the proportion of DH neurons showing a post-lignocaine reduction activity in SNL suggests that the strength of these facilitatory influences INCREASES after neuropathy.
The effect of lignocaine in RVM may be due to blockade of facilitatory On cells.
