Melzack & Katz, Pain. Part 14h: more about glia, glia-fication of (um..) nociceptive input

The paper, Pain

Most recent blogposts:

Part 14: Side trip out to the periphery! Part 14b: Prevention of pain neurotags is WAY easier than cure Part 14c: PW Nathan was an interesting pain researcher  Part 14d: Brain glia are from neuroectoderm and PNS glia are from neural crest Part 14e: The stars in our headsPart 14f: Gleeful about glia Part 14g: ERKs and MAPKs and pain

SEE ALL PREVIOUS BLOGPOSTS IN THIS SERIES LISTED AT THE END

The next bit of the gliopathy paper is about receptors, channels, and transporters involved in various situations to do with nociceptive provocation I think you could look at them as molecules of cell stress. They're defined by function: they aren't secreted, but they are expressed to perform duties such as activating the MAPK pathways we took a look at yesterday, preparing the conditions for the "synthesis, release, and uptake" of molecules that are secreted. In other words, metaphorically speaking, these aren't the urine - these are the kidneys that make the urine. 

So, here are some of them: 

RECEPTORS

1. P2X and P2Y receptors
ATP  activates P2X and P2Y receptors (which "gate microglial signaling for neuropathic pain," whatever that means..); anyway, peripheral nerve injury results in a bunch more P2X4, P2X7, P2Y6, and P2Y12 channels and receptors being expressed. 
Like having a smart phone, iPad, laptop, and desktop TV for every room of the home. Or like giving a seagull with diarrhoea a laxative. 

SOURCE

If you inhibit or delete the glial cell's capacity to make these, turns out neuropathic pain is decreased. Eight papers are cited for that little assertion. 

• If you are a mouse with no P2X4 making gene, it'll be great, because your microglia won't be able to bother your neurons with their screechy seagull dives and swoops and splats. Your inflammation won't last as long and neuropathic pain will be blunted, the paper says. 
• If you are a mouse with genes that make P2X4 and P2X7, and you take opioids, chronically, look out - your microglia will take that as the go-ahead to make way more. In other words, feeding the seagulls makes them, over time, even more numerous, raucous, pesky, and splatty. Yikes. 

2. Toll-like receptors or TLRs
These are like immune function recognition software. Here is the Wikipedia entry for TLR2. They're good guys, right? Well, it depends. 
TLR4 in spinal microglia is the one to look out for. Mice without the gene to make this, who are given neuropathic pain to deal with, do better than mice who have the gene. (But only if they are boy mice! How sexist is that?) Also, more of these kinds of receptors are activated by "chronic morphine".. more hyperalgesia, more seagull splat. But wait, that's not all - these receptors can incite more inflammation just by getting things that usually don't care about opioids, to ..care about opioids. Yikes again.  
Drugs that shut down TLR4 signaling can supposedly stop analgesic dependence, hyperalgesia, and opioid-withdrawal behaviour in rats. So that's nice. But somebody else showed that mice without TLR4's still had chronic morphine-induced hyperalgesia, so, who knows for sure..? 

3. Chemokine receptors
CX3CL1, aka fractalkine in humans, is a chemokine receptor exclusively expressed in microglia. Ding your nerve or inflame your joint, and your spinal microglia will glia-fully make a big whack of these. They act like an echo chamber, so we don't really need them in big numbers, do we? 
Another one, CCR2, a receptor for CCL2, is expressed by neurons as well. If this receptor is knocked out in mice, they don't get neuropathic pain. Cool! The paper adds, "Activation of CCR2 by CCL2 also rapidly modulates DRG neuronal sensitivity and spinal cord synaptic plasticity." I.e., sensitizes them. 

4. INF-y (interferon) receptors
Interferon-y bothers microglia in ways similar to LPS. Receptors for this become more numerous in microglia after nerve injury, inducing more reaction, more P2X4 upregulation, more of something called Lyn phosphorylation. In mice who have the capacity to make this receptor knocked out, you can ding their nerves all you want, but their microglia won't become excited, and the mice won't develop mechanical allodynia. The word used in the paper is "abrogate." So, that's good. 

Those were just microglia being stressed. 

Now, we turn our attention to astrocytes and satellite cells. These guys use gap junctions to signal, and a lot of the signalling is Ca2+. Remember, gap junctions allow "stuff" inside a cell to pass right into another cell. 

SOURCE
Gap junction

Gap junction structural proteins
Some of the structural proteins building these gap junctions are CX43 and CX30. Turns out CX43 becomes upregulated in astrocytes after nerve lesion, spinal cord injury. What does that mean? More gap junctions, more signalling. Why is that bad? Good question, something about increased releasing of ATP.. people say, if gap junction inhibitors are used, there is less inflammatory and neuropathic pain. In fact, injured mice in which both proteins cannot be made do not develop heat hyperalgesia or mechanical allodynia, period. Looks like satellite glial cells might do something a little bit different with these proteins though, because normal rats in which CX43 expression was blocked in SGCs, acted like they were in more pain. 

Other ion channels

• K+ channel subunit Kir 4.1 in SGCs: silencing this leads to "pain hypersensitivity"
• Water channel aquaporin-4, or AQP4 in spinal cord astrocytes:  mice without this are hypoalgesic. 
• TRPM2 in microglia: mice in which this ion channel is knocked out don't have as much microglial activation, and reduced inflammatory and neuropathic pain behaviour. 

Transporters
GLT-1 and GLAST, in astroctyes: their job is to clear up glutamate from synapses. Their expression is inhibited by nerve injury. So, glutamate accumulates, which makes synapses very hyperactive. Can you spell "spontaneous pain"? Supplying spinal cords with GLT-1 genes seems to stop inflammatory and neuropathic responses, so, that's nice. 

Enzymes

• COX-1 and COX-2 in microglia in the dorsal horn: these help make prostaglandins, and are increased by cutting on nerves. So is..
• NADPH oxidase 2 (Nox2). If mice can't make Nox2, they have decreased oxidative stress, microglial frenzy, proinflammatory cytokine expression, and neuropathic pain. 
• G-protein-coupled receptor kinase (GRK2) in microglia: reduction of this (by inflammation) can lead to increase of p38 and interleukin-1beta - which can take acute pain over into chronic. Eyew. We don't want that. 

Transcriptional factors
c-Jun in astrocytes, STAT3 and NF-KB in microglia and astrocytes are upregulated with nerve injury and associated with enhanced, maintained, neuropathic pain.

Anti-inflammatory receptors
Glia do try to fight back... Spinal cord astrocytes make lipoxin receptor ALX and lipoxin A4, which are good guys. They can help cut down on/balance out morphine tolerance. Spinal cord microglia make cannabinoid receptor CB2; supplying CB2 agonists (helpers) suppresses microglial reaction and neuropathic pain. Mice unable to make CB2 receptors had enhanced neuropathic pain. The more of these receptors the better: mice who overexpressed these receptors didn't show any glial reactivity or neuropathic pain. 

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Tired yet? Yet more exciting molecules to wade through and learn about in the next blogpost. 
It's worth it. I'm sure grinding through this paper for awhile longer will ultimately be worth it, when we finally return to the Melzack and Katz paper. 

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Previous blogposts

Part 1 First two sentences Part 2 Pain is personal Also Pain is Personal addendum., Neurotags! Pain is Personal, Always.

Part 3a Pain is more than sensation: Backdrop Part 3b Pain is not receptor stimulation Part 3c: Pain depends on everything ever experienced by an individual

Part 4: Pain is a multidimensional experience across time

Part 5: Pain and purpose

Part 6a: Descartes and his era; Part 6b: History of pain - what’s in “Ref 4”?; Part 6c: History of pain, Ref 4, cont.. : There is no pain matrix, only a neuromatrix; Part 6d: History of Pain: Final takedown Part 6e: Pattern theories in the history of pain Part 6f: Evaluation of pain theories Part 6g: History of Pain, the cautionary tale. Part 6h: Gate Control Theory.

Part 7: Gate control theory has stood the test of time: Patrick David Wall;  Part 7b: Gate control: "The theory was a leap of faith but it was right!"

Part 8: Beyond the gate: Self as mayor Part 8b: 3-ring circus of self Part 8c: Getting objective about subjectivity

Part 9: Phantom pain - in the brain! Part 9b: Dawn of the Neuromatrix model Part 9c: Neuromatrix: MORE than just spinal projection areas in thalamus and cortex Part 9d: More about phantom body pain in paraplegics

Part 10: "We don't need a body to feel a body." Part 10b: Conclusion1: The brain generates its own experience of being in a body Part 10c:Conclusion 2: Your brain, not your body, tells you what you're feeling Part 10d: Conclusion 3: The brain's sense of "Self" can INclude missing parts, or EXclude actual parts, of the biological body Part 10e: The neural network that both comprises and moves "Self" is (only)modified by sensory experience
Part 11: We need a new conceptual brain model! Part 11b: Intro to a new conceptual nervous system Part 11c: Older brain models just don't cut it Part 11d: The NEW brain model!

Part 12: Action! 12b: Examining the motor system, first pass. 12c: Motor output and nervous systems - where they EACH came from Part 12d... deeper and deeper into basal ganglia Part 12e: Still awfully deep in basal ganglia Part 12f: Surfacing out of basal ganglia Part 12g: The Action-Neuromatrix 

Part 13: Pain and Neuroplasticity Part 13b: Managing neuroplasticity