ROS in central pain

A role may appear for the use of WIN 55, 212-2 in treating central pain.


WIN 55, 212-2 is also known as [R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl) methyl] pyrolol [1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl) methanone mesylate] This mouthful of a drug is given subcutaneously. It is a capsaicin agonist (mimic) and what is really radical is that it stops free radicals. We have already written on reactive oxygen species and you can read about it using SEARCH at this site. Suffice it to say, it is bad, causes cancer and relates to inflammation and therefore pain.

Oxygen is the most abundant element on earth. We need oxygen gas, or O2 to live so we regard it highly. However, an oxygen radical is something else altogether. It forms something like a peroxide. This is such a powerful oxidizing agent that it combines with DNA or whatever to really mess things up. It can kill us. Free oxygen is still harmless in normal concentrations. it is one of the coincidences of life that if the atmosphere contained a little more the forests would ignite and if there were a little less, we would die.

When we speak of oxygen radicals, we are not talking about oxygen which has combined with another oxygen to make the gas. We are talking about a free radical, which combines with just about any chemical in the body and ruins it, just like oxygen combining with iron makes rust. In nature’s miracle, we have substances which pick up the free radicals before they can normally get to our DNA. We call these substances antioxidants. Vegetables, fruit and most of the stuff that is good for you is packed with antioxidants. If you trust Welch’s grape juice, it is supposed to be just the very thing for free radicals. Chemists prefer the term reactive oxygen species, just to confuse the rest of us. The abbreviation is ROS and if you want to read the literature, you might as well get used to ROS.

Now if you have been following along at all, you know that capsaicin comes from chili peppers and if injected under the skin causes a kind of allodynia. We have called it narrow allodynia because it lacks the mix of pains seen in central pain. Still, it is nice to see those grad students who have some idea what we are talking about when we mention “dysesthesia” or “allodynia”. They have to get fancy and divide alloynia from capsaicin into primary and secondary. The primary is in the spot of injection. The C fibers go up to the cord and hypersensitize A fibers coming in from the area in or near the injection site. (Please see Marshall Devor’s principle of “crossed afterdischarge” at this site) This recruitment spreads the area of allodynia BEYOND the area of injection. This is how you wind up with secondary allodynia around a skin wheal of capsaicin.

Lee et al in Pain 2007 Mar 20 show that capsaicin-induced mechanical hyperalgesia in dorsal horn neurons is due to reactive oxygen species. This is VERY important. ROS causes neuropathic pain. You will recall that phosphorylation means the attachment of high energy bonds of phosphate in order to activate body chemicals, which would otherwise remain inert. Once this “battery” is attached, the chemical is ready to interact.

Gao et al in Pain. 2007 Feb 19 have shown that ROS facilitate the phosphorylation of subunit 1 of the NMDA receptor (NR1)) (See subunit 1 using SEARCH) in the dorsal horn of the cord. The dorsal horn is just the penetration of nerve fibers into the back of the cord, coming in from either side.

It gets its name from the fact that on cross section slide preps, the spinal nerves connecting with central gray around the central canal of the cord reminded some anatomist of two horns. The exiting spinal nerves are called the anterior horns. The spinothalamic tract is medial to this. (It is helpful to think of the dorsal horn as the entry point for sensations where the spinal nerve comes in from the dorsal root ganglion outside the cord.) The neurons involved in recruitment hypersensitization in the dorsal horn are mostly what we call WDR or wide dynamic range neurons.* Just remember that ROS cause hypersensitization, at least in the WDR neurons.

Although ROS action seems to be true across the board for any kind of central sensitization (central pain neuron changes secondary to peripheral injury), it seems particularly to be the case in central pain. In peripheral nerve injury (PNI) there are downstream changes after NMDAR activation. Li et al have shwon in the J Neurosci. 2004 Sep 29;24(39):8494-9 upregulation in spinal dorsal horn calcium channel alpha2delta-1 subunit. Calcium is part of NMDA action, the CaV 2.2 channel especially in pain neurons. (See calcium channels elsewhere at this site using SEARCH)

Calcium and sodium channels control the two main excitatory fluxes of current across the neuronal membrane. Since you already know that NaV 1.3 is the fetal sodium channel which is produced in central pain you will be interested to know that NaV 1.3 greatly enhances hypersensitixation in the dorsal horn in central pain, as well as the ROS activates NMDARs. Central sensitization seems to lack this double whammy. No NaV 1.3 channels are produced in normal pain, which travels via the 1.8 voltage gated sodium channels (NaV 1.8). See Hains et al J Neurosci. 2003 Oct 1;23(26):8881-92.

The growth factors attemping to repair cord somehow turn on the genes which were active in the fetus to again make NaV1.3. Central sensitization seems to be ROS, whereas Central Pain seems to be reactive oxygen species PLUS Nav 1.3 ion channels. Free radicals plus NaV 1.3 just drives the pain neurons wild.

As you already know in CP hypersensitization, the number of glia quadruples, so they are really trying hard to regrow nerves, but failing this, all the extra growth factors cause tormentous amounts of neuroinflammation, making central pain the worst pain state known to man. Quadrupling may not sound like much, but in the homeostatic body, it is enough to drive the system out of control. A quadrupling of temperature would kill us. A quadrupling of pain response merely tortures us. It is all quite sad, and we hope the scientists can soon find a way either to stop the growth factors or block the neuroinflammation. Better yet, we hope they can regrow spinal cord.

It would appear that injury to central neurons makes for a particularly fierce, but ineffective combact attempt with neurites forming at either cut end putting out extraordinary amounts of pain neurotransmitters, driven by growth factors from the surrounding glia. Only recently have we realized that in central neurons, the neurites appear at both ends, with pressure coming from below the injury as well as from above. The neurites do not reunite successfully in central neurons, although they may well do so in peripheral nerves. (See neurites elsewhere at this site–they are tremendously sensitized “fingers” or sprouts which grow out in many directions from cut neurons, attempting to effect nerve repair)

When Lee et al used ROS scavengers (TEMPOL and PBN) they blocked the SECONDARY but not the PRIMARY allodynia from capsaicin. This means the ROS are acting up in the cord, where the dorsal horn neurons are, to cause secondary allodynia. The culprit has been identified. Our thanks to Lee et al–we LOVE the University of Texas at Galveston. They have been turning out top pain experts for decades thanks to the likes of William Willis Jr. and Claire Hulsebosch. Theoretically, Welch’s grape juice and vitamin E (see Kim, et al in Pain. 2006 May 122(1-2):53-62) should be good for central pain. However, ROS forms like lightning from almost every metabolic chemical reaction in the body involving oxygen (which is just about every chemical step in the body) and is bound just as rapidly by antioxidants, and it is hard to know what it takes to make a dent in ROS mediated pain. Perhaps TEMPOL and PBN, or other ROS scavengers will become prototypes for drugs to help our neuropathic pain.

Kim already recommends Vitamin E for central sensitization. Central sensitization is, however, much much much LESS severe than fully elaborated central pain. Still, pain is annoying, and those with diabetic neuropathy now have a little hope. Vitamin E is fat stored, which means you can absorb too much of it, so consult your doctor before attemting any dietary or vitamin therapies.

As we have stated for some time, it looks like inflammation is a very big deal in all kinds of disease and where it happens it has the nasty habit of involving the central nervous system, what is called “central sensitization”. A myriad of conditions are now thought capable of being viewed as permanent or paroxysmal neurogenic inflammatory disease. Even asthma now falls into this category. See Lomia at Med Hypotheses. 2007 Mar 19. Thus, we find ourselves taking a closer look at ROS all the time.

We have already written about one cannabinoid receptor agonist (capsaicin works at the cannabinoid receptor) which may be useful for neuropathic pain, anandamide. (See using SEARCH). Now Kukushkin et al in J Exp Biol Med. 2006 Jul;142(1):39-42. have compared anandamide with WIN 55,212-2 (the drug spelled out at the start) and found that it is even better at helping central pain than anandamide.

WIN 55,212-2, a mixed CB1 and CB2 cannabinoid receptor agonist, has been shown to be effective against hyperalgesia and allodynia in peripheral neuropathic pain (See Ulugol et al Neurosci Lett. 2004 Nov 23;371(2-3):167-70.) In Kukushkin’s study, which thankfully was aimed at CENTRAL pain, it worked best in the spinal fluid, in a dose of 1 mg/kg/day. Now here is the real kicker. Kukushkin et al produced experimental Central Pain by injecting the sodium salt of PENICILLIN intraspinallsy. This is a new and simple way to create lab animals with central pain. We have already seen that erythrocin B and lasering, mechanical drop injury, and even mild cord displacement can cause central pain. Now we have yet another method of producing an animal model.

It is important to remember here that we are talking about the genuine article, central pain. The dorsal horn is part of the CENTRAL nervous system. Most of the pain articles we cite here are in models of peripheral nerve injury pain. Until recently we had no choice but to site information about peripheral nerve injury pain. The similarities with central pain are so great that we are beginning to hear speculation that central pain may actually combine peripheral and central elements. this would not be too surprising in view of the nature of EVOKED central pain, which requries some external stimulus to be applied, whereas the spontaneous variety of CP does not.

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C fibers are thought to be primarily chemical sensors (so it is not surprising that most in the survey describe central pain as a chemical burn or “like acid under my skin”.) However C fibers do respond to higher degrees of thermal and mechanical stimulus.

*A wide dynamic range neuron usually refers to an A fiber (larger in diameter and faster than C fibers) which is capable of responding to minimal as well as marked stimulus. The hypersensitization of A fibers occurs in the dorsal horn. The chemicals involved in dorsal horn hypersensitization include substance P, glutamate, aspartate, calcitonin gene related peptide (CGRP), and a gas, nitric oxide (see Jensen 1996 p. 82 in Campbell JN (ed) Pain 1996 – An Updated Review, IASP Press, Seattle, p.77-86.)

The CGRP step is especially interesting in light of the known beneficial effect of epibatidine, a nicotinic agonist which actually helps PNI pain but is toxic. Patent wise there is a current frenzy over agonists of nicotinic acetylcholine receptors (nAChR). Acetyl choiine is classically associated with muscle activity, but a different type of receptor is active in the dorsal horn. There are MANY subtypes of the nAChR. Subtype 42 is said by Bunnelle to be important in neuropathic pain while subtype 37 is important for autonomic activity. There are also muscle side effects to be considered.

Lately, the nAChRs have been divided according to subunits alphanumerically numbered with Greek letters and also numbers. One agonist, ABT 594 selects for the neuronal alpha 4 beta 2 nAChR (See Roberts et al J Pharmacol Exp Ther. 1998 May;285(2):777-86) Metabolic Pharmaceuticals is ready for Phase 2 human trials of ACV 1, hoping for dorsal horn blockade with a minimum of side effects. nAChRs seem to work via a descending inhibitory tract by activating an inhibitory site in the dorsal horn relating to CGRP release. CGRP is calcitonin gene related peptide (See more using SEARCH at this site).

The spinothalamic tract causes nerve injury pain when damaged; however, it is thought by many that BURNING pain is split off into the spinoreticular tract (which is very close to the ST tract fibers). According to Ranney’s review, the SR tract connects to the dorsolateral pontine tegmentum, the rostral ventral medulla, the dorsal medulla, the caudal medulla and the lateral hypothalamus.

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