The course of true medicine never runs smooth. We lament those who developed Central Pain as a result of attempting to walk again, however haltingly, from stem cell implants.
Not the least of troublesome matters for central pain patients is the necessity of learning myriad new terms. Although you have already been bombarded with them, you will need to bear with us for this to make sense.
ALLODYNIA. When a nonpainful stimulus is perceived as painful
EFFERENT, a nerve fiber running away from the brain
AFFERENT, a nerve fiber running toward the brain.
TRANSCRIPTION, the coding or writing by DNA for amino acid components, which will later be combined to make peptides or even proteins.
PAIN MODULATORY LIPIDS include anandamide, 2-arachidonoyl glycerol, N-arachidonoyl glycine, N-arachidonoyl gamma amino butyric acid, and N-arachidonoyl dopamine
Ron Tasker, the great genius of central pain during the 1950′s, who still sees patients in Toronto Canada did more than anyone in memory to set central pain in the crosshairs of neurosurgery. He discovered that pain is carried in the spinothalamic tract. He also was the first to firmly establish that normal pain (nociception) is different from nerve injury (neuropathic) pain. Prior to this doctors had merely regarded neuropathic pain as a more severe version of normal pain. Tasker’s progeny at Western Toronto Hospital are famous, not only for their great knowledge of nerve injury pain, but for neurosurgery in general.
Neighboring McGill in Quebec is also a powerhouse of pain research. This was a class act. Many of Tasker’s articles are still up to date for anyone wanting to familiarize themselves with central pain. This is the sign of a good clinician. His descriptions were careful enough that they stand good today. Central Pain is so confusing that many patients found they first had to read Tasker’s articles before they could find the words to describe what they were feeling. How great is that for a clinician!
In studying the cord as best they could under the light microscope, scientists of that day noted that when the cord was cut, the distant ends of afferent pain neurons attempted to reestablish connection with the brain. Today we would say that the outpouring of growth factors from the neuronal genes engenders powerful attempts to reconnect. Similarly, the efferent fibers attempt to reach again the peripherae. The efferents are often more successful by finding the outer skeleton of the old axonal tract and regrowing down it toward the peripherae. Also, sensitizing C fibers have no outer “skeleton” so regrowing down the skin is impossible for injury to those fibers. C fibers are the ones which sensitize other neurons once you get up in the DRG/DH.
The early work of Tasker sometimes gets overlooked because it is so easy for neurochemists to measure pain chemical in the dorsal root ganglion, which is just outside the cord and ine dorsal horn, which is the bundle of entering neurons traveling into the posterior cord, on either side. To conceive of central pain, one must think of three contributions. This can be compared to a three legged stool, to emphasize that EACH part must be kept in mind:
First) The dorsal root ganglion and dorsal horn. This area involves the presence of potent pain chemicals. These may be thought of as pain modulatory lipids and proinflammatory and inflammatory chemicals. On the skin, inflammation usually, but not always involves inflammation. However, infection need not be present in such things as chemical injury or sunburn. In neuroinflammation, most of the same pain chemicals are present, but there is no infection in neuropathic pain, so we do not necessarily see white cells, However, as one prior article at this site mentions, portions of T cell (a type of small lymphocyte which is one kind of white cell) receptors ARE increased in the dorsal root ganglion. In the brain, mast cells and other inflammatory cells are present in central pain. However, the inflammation is largely chemical.
Second) Going back to the DRG and the dorsal horn, chemists can easily measure a dramatic increase in inflammatory chemicals, which are numerous (eg. NMDA). They also can detect huge increases in kinases, which activate the pain chemicals (eg. MAPK and related p38ERK, FAAH etc.) and also increases in growth factors which turn ON genes to make inflammatory chemicals. There is even the production of unique Nav1.8 ion channels. The ease with which this investigation produces results tends to make neurochemists focus almost entirely on the DRG and Dorsal Horn. However, this is only ONE leg of the three. The second area of concern is sometimes now overlooked. This is the area of the sprouts. As many as fifty of these may grow out from what was one a single axon. It is not unlike what happens if you cut off the top of a tree, numerous sprouts may replace the single original top; hence, the name. These sprouts spontaneously produce pain signal. Tasker’s emphasis on deafferented sprouts has more or less been overshadowed by the proteomics work in the DRG, which produces predictable results.
In some poorly understood way, the nervous system behaves as if the sprouts send a signal to the DRG and dorsal horn, which causes them to “pump up the volume”. They are trying chemically to reconnect, but cannot do so. The sprouts are calling so loudly that they send spontaneous pain signal to the brain. The DRG and DH begin to listen so closely that they also send out powerful pain signal through pathways that are still intact which may be entirely uninjured. What is often forgotten is that both INJURED and UNINJURED neurons are firing.
Third) Then, of course, there is the thalamus, which generates pain signal itself and is overburdened with almost the same pain chemical as are infesting the DRG. Thus, we have three areas, the thalamus, the sprouts, and the DRG/DH which are packed with hyped up pain genes, NMDA and related chemicals and the kinases which drive them, as well as inappropriate ion channels. The steps are discrete but the chemicals which drive each of the three aspects are nearly and possibly entirely identical. The mirroring or “echo” happens at all three places.
Tasker emphasized the sprouting area. He also focused on mechanism and sometimes grouped central pain and peripheral nerve injury (where sprouting also occurs) together under the term, “deafferentation pain”, which means a severing of afferent neuron(s). The cord need not actually be severed, and based on the surveys, the most severe CP seems to occur in those who retain some cord function. This is probably because evoked central pain is so much more severe than spontaneous central pain, and evoked CP requires some ability of the cord or cord bypass pathways to continue firing (cord bypass is probably accomplished by afferents which travel with blood vessels). Today’s neurochemists tend use “central pain” and “PNI” separately, AND to look mostly at the DRG/DH. A few focus on the thalamus, and speak of “thalamic pain”, particularly if the CP occurs after stroke. At this site, we often speak of “nerve injury pain” in the same way Tasker spoke of “deafferentation pain”.
The neuroradiologists studying pain have begun to look at the brain, in particular the cortex, where the post-central gyrus or SI tells WHERE the pain is, the parietal cortex or SII tells what kind of pain it is, and the insular cortex (operculum) tells the painfulness of the pain.
There are also contributions in the prefrontal cortex, which connects to emotional centers, including the cingulum which input to the affective results of pain. The problem with placebo research, which seems to suppress the prefrontal area is that this tells us nothing about what is going on in other areas. The person may still have the pain but FEEL more reassured about it. Stimulation of the primary MOTOR cortex, which is just on the other side of the central sulcus or dip from SI suppresses central pain, but we do not know why. It also risks the creation of phantom limbs which have CP in them. These feel every bit as real as actual limbs.
In the brain itself, those fibers that do survive will be reassigned to neighboring brain. This is known as allocation. For example, if the thumb is cut off, its quasi-dedicated neurons in the brain will become devoted to the index finger by the plastic changes of allocation.
One of the greats at Hopkins, formerly of Tasker’s group, has theorized that the brain does not like to the shut off from the external environment, and when the afferent nerves of light touch are destroyed, the brain recruits the more durable pain fibers in order to keep in touch with the outside. This is termed “misallocation” and is still a viable theory as part of what is going in central pain. The pain fibers are kicked into gear to replace the sensory loss which is always present to some degree in central pain. (see article on misallocation elsewhere at this site)
With time, the distant axon of the pain nerve dies, for lack of nourishment by the cell body which is up in the dorsal root ganglion, near the cord. The proximal end continues to “sprout” in a futile attempt to reestablish contact with its feeder length of axon. Sprouts are very very hyperactive and generate spontaneous meaningless pain. The more sprouting, the worse the central pain. Sprouting occurs as an attempt to heal, but the failure to connect is known as deafferentation. Many scientists today still refer to central pain as deafferentation pain. Tasker used the term and we are discovering how correct he was.
In lab animals, when the earliest successful attempts at reestablishing motor function after severing of the cord were reported, no mention was made of any pain behavior. This was probably missed. Stem cells are multipotential cells which are very primitive. They have not differentiated into the various cell types. Since local factors can cause differentiation, it was theorized that injection of stem cells might cause them to develop into neurons. This hope was realized in rats and other lab animals. Not surprisingly, it was not too long before it was tried in humans. The desire to walk again is strong.
Imagine the disappointment when some of those with injected stem cells developed central pain. The growth factors that were attempting to lead the stem cells into differentiation caused sprouting and of course this led to central pain. The disappointment was overwhelming. It appeared that the cure might be worse than the disease. Pain might be worse than paralysis. That was the end of the story. Or was it?
It Sweden the great Karolinska Institute has long been associated with pain research. Jorgen Boivie, who corresponds with painonline is at Karolinska. Zsusanna Weisenfeld-Hallin developed one of the early animal models for central pain. She first injected the rats with erythrocin B to make the tissue more sensitive to light and then lasered the cord. This produced a good yield of rats with central pain. This was a very important step in convincing clinicians that the condition was real. The little rats chewed off their legs (autotomy) in an attempt to be rid of the distal pain so characteristic of CP. They also could not stand light touch or modest temperature increase, precisely as the condition appears in humans, who do not chew off legs, but tend to lose personality and certainly acquire agony.
When an embryo forms, it is first a little ball, then more flattened, and then it folds on itself. The place where it folds forms into a primitive spinal cord and brain. This part of the embryo is known as the neural crest. Positioned in sequence along the length of the neural crest are concentrations of a certain type of cell. Each little bunch is known as a placode. In the head region, each placode develops into one of the cranial nerves. The opthalmic placode throws off cells which have already specialized and have gone through mitosis. All the other placodes throw off cells in the midst of dividing–they have not specialized as yet.
You already know that the presence of tyrosine kinase A (TrkA) is a marker or signal that a given nerve cell is a PAIN neuron. Somewhere along the line, some neural stem cells are directed to begin gene expression which will turn them into TrkA positive neurons, or pain neurons. Certain molecules induce our DNA to begin to transcribe its code, to turn on, selectively. These are known as transcription factors. They are closely linked to the production of nerve cell growth factors. One such group is known as the Runx1 group of transcription factors.
The brilliant scientists at Karolinska, including Dr. Weisenfeld-Hallin, discovered that Runx1/AML1 transcription factor “selectively regulates development and survival of TrkA nociceptive sensory neurons.” See Marmigere et al, Nat Neurosci. 2006 Feb;9(2):180-7. They considered the gene cascade underlying TrkA production and found that a molecule known as neurogenin-2 directed neurons to become TrkA positive.
Ota and Ita noted that the expression of Brn3 identified neural crest cells as eventual sensory neurons (see Dev Dyn. 2006 Mar;235(3):646-55.) They also discovered that bone morphogenetic protein was a positive regulator of neurogenin-2. Fibroblast growth factor was found to suppress neurogenin-2 through what is known as Notch signaling.
Now, the brilliant scientists at Karolinska have done it again. See Hofsteter et al, Nat Neurosci. 2005 Mar;8(3):346-53. See also comment in Nat Neurosci. 2005 Mar;8(3):259-60.
Since allodynia obviously limits the usefulness of intraspinal neural stem cell grafts, these amazing researchers, now using a weight drop model of Central Pain (as developed by UTMB researcher Claire Hulsebosh, another correspondent with painonline) have now shown that sprouting can be prevented and differentiation directed away from the formation of TrkA positive cells by direction. This direction of neural stem cells is accomplished by application of neurogenin-2 early in the life of the neural stem cell.
The Karolinska scientists reported:
“Transduction of neural stem cells with neurogenin-2 before transplantation suppressed astrocytic differentiation of engrafted cells and prevented graft-induced sprouting and allodynia. Transduction with neurogenin-2 also improved the positive effects of engrafted stem cells, including increased amounts of myelin in the injured area, recovery of hindlimb locomotor function and hindlimb sensory responses, as determined by functional magnetic resonance imaging. These findings show that stem cell transplantation into injured spinal cord can cause severe side effects and call for caution in the consideration of clinical trials.”
This is a clear caution, but it indicates we can start the show again. Transduce the neural stem cells with neurogenin-2 (to prevent their becoming TrkA positive) and then implant them.
We should emphasize that this work is about avoiding the creation of central pain, not curing pre-existing central pain. Ordinary humans have already undergone embryonic life and possess many TrkA positive sensory neurons, or pain neurons. There is presently no way to kill them once they have already differentiated, although one recent article at this site suggests that scientists are looking at ways to do just that.
These studies on neurogenin-2 required an amazing cooperation and coordination between different departments and disciplines. Stockholm takes second seat to no one in this matter. We wish scientists in the U.S. were free to pursue such avenues, but since they are not, we are grateful to see the incredible work coming out of other nations.
Stem cells in most countries come from infertility clinics. The reproductive specialist induces superovulation by recombinant follicle stimulating hormone (FSH) and then during a surgical procedure harvests as many eggs as is practical. These eggs are then fertilized by insertion of a sperm cell and if it “takes”, they are frozen. Perhaps 15-30 eggs may be harvested and fertilized. Sometimes only a fraction of those are actually used for implantation into the mother. The rest are discarded except where they may be donated to stem cell centers. A few places extract stem cells from aborted fetuses, but this is unnecessary.
We respect the motives of those who oppose stem cell research, but believe they are misguided and should not fear that tiny groups of cells which will be discarded anyway by infertility clinics (since the mother has already gotten pregnant with another fertilized ovum) constitute human life.
It is true that they are potential human life but so are any egg and sperm and nature herself casts these off with regularity. A blastocyst has maybe 78 cells and the fertilized frozen embryos typically do not go beyond 256 cells. We certainly have to draw the line at some point. However, if the embryo is to be discarded by the clinic, it seems that respect for life favors the use of the cells to promote human life, rather than to discard them.
If you have thought of joining one of the teams going overseas for stem cell injection, perhaps you should contact Karolinska in Stockholm to see how they plan to proceed to prevent the development of allodynia in humans. If you already have central pain, you certainly do NOT need more of it. If you do NOT have central pain, yo most certainly do not want it. Severe CP is the most severe pain state known to man. Walking is not worth it.