Glial cells are the cells that nurture the nervous system, but they may go further. Restraining them may help relieve pain that currently available drugs cannot alleviate
Helen's left foot slipped from the clutch pedal, and her ankle buckled against the floor of the car. At that moment the feeling was like a slight sprain, she recalls, but the pain never subsided. In fact, it intensified. The slightest touch, even the gentle friction of the bed sheets, ignited flames of electricity up her leg. "Because of the pain I couldn't speak, but inside I was screaming," the young Englishwoman wrote in an online diary about the mysterious situation that would torment her for the next three years.
The chronic pain suffered by people like Helen is different from the warning slap of sudden acute pain. Acute pain is the body's most intense alarm feeling, and its purpose is to prevent us from causing ourselves further damage. This pain is also called pathological pain because an external factor, such as tissue damage, produces signals that reach the brain where they are decoded as pain. But imagine that the searing pain of a real injury never stops, even after the wound heals, or that everyday sensations turn into agony: "I couldn't shower... the water was like daggers," Helen recalls. "The vibrations of the car, someone walking on a wooden floor, people talking, a gentle breeze... would awaken the uncontrollable pain. Normal painkillers... even morphine, had no effect. I felt like my mind was playing tricks on me."
Unfortunately, Helen was right. Her chronic pain was due to a malfunction of the pain circuits in her body, which caused them to trigger a false alarm, known as neuropathic pain (or peripheral pain), because it results from a malfunction of the nerve cells themselves. When the false alarm reaches the brain, the suffering it evokes is as real as any life-threatening pain, but it does not go away, and doctors often stand helpless in the face of it.
Recent studies finally reveal why normal pain relievers fail in the face of neuropathic pain: the drugs focus on the nerve cells only, while the source of the pain may be a functional problem of non-neuronal cells called glial cells that reside in the brain and spinal cord. New insights into the way in which these cells, whose role is to nurture the activity of nerve cells, get out of balance and damage the nerve activity bring forth new ideas for the treatment of chronic pain. The studies also provide a surprising perspective on an unfortunate possible consequence of taking painkillers: narcotic addiction.
Circles of pain and their breaking
To understand what causes the pain to persist after the wound has healed, you must understand what causes the pain in the first place. Although the sensation of injury is ultimately absorbed by the brain, the nerve cells that produce it do not reside in it but in the spinal cord, where they gather sensory information from the entire body. The cell bodies of the nerve cells in the dorsal root ganglion, DRG, represent the first step in a three-step circuit of pain sensing. They are packed like bunches of grapes in the seams between the vertebrae of the spine, like a blazer with two rows of buttons running from the tailbone to the skull. Each nerve cell in the DRG, similar to a person spreading both arms out to the sides, sends out two thin extensions, called fibers or axons. One axon is sent to survey a tiny, remote area of the body. The second axon goes out to the spinal cord, where it makes contact with a nerve cell that will transmit the signal to a chain of nerve cells in the spinal cord - the second step in the pain cycle. These cells in the spinal cord transmit the sensation of pain from the DRG cells, up the spinal cord to the last stage: the brain stem and finally the cerebral cortex. Pain signals originating from the left side of the body cross the spinal cord to the other side and reach the right brain, signals from the right side are sent to the left brain.
Impairment of the flow of information in each of the three stages of the pain cycle can relieve acute pain. Local anesthetics, such as novocaine used by dentists to painlessly extract teeth, anesthetize the ends of the axons in the injection area and prevent the cells from firing electrical signals. Epidural anesthesia, often used to prevent pain during childbirth, blocks pain signals at the second stage of the circuit, where the axon bundles of DRG cells enter the spinal cord and meet the spinal neurons. This blockage leaves the mother fully conscious and she can experience the birth and assist in the birth of her baby without pain. A morphine injection works at the same stage and blocks the transmission of pain signals in the spinal nerves, but leaves sensations that are not painful. In contrast, general anesthesia during surgery impairs the processing of information in the cerebral cortex, and causes the patient to be completely unaware of any sensory input coming from neural pathways outside the brain.
Our body's natural pain relievers work on the same three links of the pain cycle. An adrenaline-charged soldier on the battlefield may suffer a serious injury without being aware of it, because his cerebral cortex ignores the pain signals while dealing with the external situation that endangers the soldier's life and strongly affects his emotions. During natural childbirth, the woman's body releases small amounts of proteins called endorphins that dull the transmission of pain signals reaching the spinal cord.
Hormones, emotional states and many other factors can significantly alter our perception of pain by regulating signaling in the pain pathways. Also, many processes and biological substances that change the flow rate of molecules in ion channels in the nerve cells, contribute to the regulation of the sensitivity of the nerve cells themselves. When we are injured, these factors can loosen the control of nerve firing, making it easier for the nerve cell that transmits pain signals to work.
However, this unrestrained state may last too long and leave the DRG cells in a state of over-excitation, causing them to fire pain signals without external stimulation. This condition is the primary cause of neuropathic pain. The heightened nerve sensitivity can also cause abnormal sensations of tingling, burning, tickling, and tingling (paresthesia) or, as in Helen's Dagger Shower, amplify subtle sensations of touch or temperature to painful levels (allodynia).
Efforts to understand how nerve cells in the pain circuit become hypersensitive after injury have naturally focused on the question of what goes wrong in the nerve cells. These studies yielded some clues, but not the complete picture. My research and that of many colleagues have shown, for example, that the very act of firing pain signals changes the activity of genes in the nerve cells of the pain circuit. Some of the genes whose expression the pain signals control include the code for creating ion channels in the nerve cell wall. Other genes are responsible for coding substances that increase cell sensitivity. The intense activation of DRG cells following the tissue injury can therefore cause in these cells the same sensitizing changes that later cause neuropathic pain. But our research and that of other laboratories also show that nerve cells are not the only ones that respond to painful injury and release substances that increase nerve sensitivity.
The number of glial cells is much greater than the number of nerve cells in the spinal cord and brain. They do not fire nerve signals like neurons, but they have interesting and important properties that affect nerve activity. Glial cells maintain the chemical environment surrounding the nerve cells. They provide the energy that sustains the neurons, but also absorb the neurotransmitters that the neurons release when they signal to neighboring neurons. Sometimes, glial cells can even provide neurotransmitters to enhance or modulate nerve signal transmission. When nerve cells are damaged, glial cells release growth factors that improve the survival and recovery of the nerve cells, and they also release substances that alert the immune system cells to fight the infection and start the recovery process. However, recent studies show that these activities of glial cells, which are supposed to nurture the nerve cells and improve their function, can also prolong the duration of the excess nerve excitement.
The suspicion falls on glial cells
Already more than a century ago, scientists knew that glial cells respond to injuries. In Germany, in 1894, Franz Nissel noticed that after a nerve injury there was a marked change in glial cells at the meeting points between the nerve fibers and between the spinal cord or the brain. Microglial cells proliferate, and cells of a larger type, called astrocytes because of their stellate shape, fatten up and fill with thick bundles of fibers that reinforce their intracellular skeleton.
It was suggested that these responses of glial cells help repair the nerve cells after the injury, but their mechanism of action was not clear. Moreover, if the injury, such as an ankle sprain, is far from the spinal cord, the astrocytes do not respond, obviously, to the direct injury, but to changes in the relay point between the DRG and the nerve cells of the spinal cord. These observations indicated that astrocytes and microglial cells monitor the physiological properties of pain-sensing neurons.
Over the past twenty years, it has been discovered that glial cells contain many mechanisms for detecting electrical nerve activity. The various mechanisms include channels that sense potassium and other ions that release the nerves that fire electrical signals, as well as receptors on the surface of the cell that sense the neurotransmitters used by nerve cells for synaptic communication. The main neurotransmitters that neurons release are glutamate, ATP, and nitric oxide, and they are recognized by glial cells, but there are many more. This array of sensors allows glial cells to review the electrical activity in neural circuits throughout the body and brain and respond to changing physiological conditions.
Once scientists understood the extent of glial cell responses to neural activity, research attention was directed toward the suspicious behavior of these supporting cells at relay points of pain sensations. If glial cells do monitor nerve conduction, do they also influence it? Exactly 100 years after Nissel observed glial cells responding to nerve injury, a simple experiment tested for the first time the hypothesis that glial cells might contribute to the development of chronic pain. In 1994, Steven T. Meler and his colleagues at the University of Iowa injected mice with a toxin that selectively kills astrocytes, and then tested whether the animals' sensitivity to painful stimulation decreased. The answer was negative and indicated that astrocytes do not play a prominent role in the transmission of acute pain signals.
The scientists then treated the rats with a substance that stimulates nerve fibers and causes the animals to gradually develop chronic pain, similar to Helen's experience long after the car accident irritated the nerves in her ankle. Animals injected with the astrocyte toxin developed much less chronic pain and the conclusion was that astrocytes are somehow responsible for the development of chronic pain after nerve injury.
Other studies have discovered how glial cells release many types of molecules that can increase the sensitivity of nerve cells in the DRG and spinal cord that transmit pain signals to the brain, such as growth factors and some of those neurotransmitters produced by the nerve cells themselves. Scientists now understand that glial cells interpret rapid nerve firing and the changes it provokes as distress signals from the nerve cells. In response, glial cells release molecules that increase sensitivity to help the nerve cells in their signaling work and start their recovery process.
Another essential group of molecules produced by glial cells in response to injury or neural distress is the cytokines. The origin of the name comes from the word "cytokinetic", which means cell movement. Cytokines act as a powerful chemical beacon that immune system cells follow to reach the site of injury. Think of the needle in the haystack problem that your immune system has to solve when it comes to find a tiny thorn in the end of your finger. Powerful cytokines released from the cells damaged by the thorn accelerate the cells of the immune system to reach from the blood and lymph fluid to the tip of the finger to fight the infection and start the healing process. They also induce changes in the local tissue and blood vessels that facilitate the work of the immune system cells and promote recovery, but they also cause redness and swelling. The total effects of the cytokines are known as inflammation.
The example of the thorn shows how effective the cytokines are in directing the cells of the immune system to the wound, but what is more impressive is how much the little thorn can hurt. The pain is much greater than the tiny damage caused to the tissue. Soon, even the area around the thorn swells and becomes tender, even though the cells around the injured area are not damaged. The peripheral pain is caused by additional activity of pro-inflammatory cytokines: they significantly increase the sensitivity of the pain fibers. Hypersensitive pain receptors near the injured area are the body's way of getting us to lay down on the area so it can heal.
As a rule, nerve cells are not the source of cytokines in the nervous system, but glial cells. And just as cytokines can hypersensitize the nerve endings surrounding the fingertip thorn, the cytokines released by glial cells in the spinal cord in response to intense pain signals can spread to nearby nerve cells and hypersensitize them as well. That is, a cycle of hypersensitive nerve cells may develop that fire at an increased rate and cause glial cells to switch to an active state where they release more sensitizing factors and cytokines in an attempt to alleviate the distress of the nerve cells, but instead they prolong it. When such a situation happens, the pain can originate from within the spinal cord, from nerve fibers that were not directly damaged.
The initial responses of glial cells to injury are beneficial to the healing process, but if they are too strong or prolonged, the result is chronic, intractable pain. Several research groups have documented feedback loops that may stimulate glial cells to prolong the release of sensitizing factors and inflammatory signals that cause neuropathic pain, and many are exploring ways to inhibit these processes. This work even led to finding ways to increase the effectiveness of narcotic substances used to treat acute pain.
Stop the source of the pain
In the past, all treatments for chronic pain were aimed at reducing the level of nerve activity, but the pain cannot be alleviated if glial cells continue to stimulate the nerve cells. Insights into the way in which glial cells may be drawn into the vicious circle that increases nerve cell sensitivity are leading to the development of new approaches to treat recalcitrant glial cells in hopes of stopping the underlying source of neuropathic pain. Experimental efforts to treat neuropathic pain by altering the activity of glial cells therefore focus on silencing glial cells themselves, blocking inflammatory molecules and signals, and providing anti-inflammatory signals.
In animal experiments, for example, Joyce A. DeLeo and her colleagues at the Dartmouth School of Medicine showed that a chemical called propantophylline suppresses astrocyte activity and therefore suppresses chronic pain. The antibiotic minocycline prevents both nerve cells and glial cells from producing pro-inflammatory cytokines and nitric oxide, and also reduces the migration of microglial cells towards the injury sites, and therefore it can be assumed that the drug will be able to prevent the overactivation of glial cells.
A similar approach focuses on Toll-like receptors (TLR), proteins found on the membrane of glial cells that recognize various indicators of cell distress and encourage glial cells to secrete cytokines. Linda R. Watkins from the University of Colorado at Boulder and her colleagues showed in animals that the use of an experimental compound that blocks a certain type of TLR on glial cells in the spinal cord, TLR-4, succeeds in treating neuropathic pain caused by damage to the sciatic nerve. Interestingly, naloxone, a drug used to blunt the effects of opiates during addiction treatment, also blocks the response of glial cells to TLR-4 activation. Indeed, Watkins showed in rats that naloxone could completely eliminate neuropathic pain.
Another common drug, actually an ancient pain reliever, can succeed where many others fail: marijuana, the medical use of which is legal in several states. Substances found in the marijuana plant mimic natural compounds found in the brain called cannabinoids. These compounds activate certain receptors on nerve cells and regulate the transmission of nerve signals.
But there are two types of cannabinoid receptors in the brain and the rest of the nervous system, CB1 and CB2, and they have different functions. Activation of the CB2 receptor relieves pain, while activation of CB1 receptors causes the psychoactive effect of marijuana. The CB2 receptor is not found on nerve cells that transmit pain but on glial cells. When cannabinoids bind to CB2 receptors on microglial cells they reduce their level of inflammatory signals. Recent studies have revealed that when chronic pain develops, the number of CB2 receptors on microglial cells increases, a sign that the cells are valiantly trying to trap more cannabinoids nearby to provide pain relief. Now, pharmaceutical companies are trying hard to develop pain relievers that will act on CB2 receptors without turning people into mastoids.
Blocking inflammatory cytokines with currently used anti-inflammatory drugs, such as anakinra (marketed as Kineret) and etanercept (marketed as Enbrel), also relieves neuropathic pain in experimental animals. Aside from dampening inflammatory signals, several research groups have shown that adding anti-inflammatory cytokines, such as interleukin-10 (IL-10) and IL-2, can alleviate neuropathic pain in animals. Two existing drugs, pentoxifylline and AV411, both inhibit inflammation by stimulating cells to produce IL-10. Furthermore, various research groups have been able to alleviate neuropathic pain for a period of up to 4 weeks by introducing the genes encoding IL-10 and IL-2 into the muscle or spinal cord of animals.
Few of these drugs have already reached clinical trials for the treatment of pain in humans, including AV411, which in Japan is already being used as an anti-inflammatory treatment after a stroke. Tests in Australia showed that pain patients reduced their morphine dose on their own while receiving the drug, a sign that AV411 contributed to their pain relief. But AV411 may act through mechanisms beyond relieving pain generated by inflammation, thus highlighting a surprising twist in the glial cell and pain plot.
restore balance
Morphine is one of the most powerful pain relievers known to us, but doctors are so afraid of its diabolical properties that many of them will not give full morphine treatment even to terminal cancer patients. Morphine, like heroin, opium and modern narcotics, such as OxyContin, numbs pain by weakening the communication between nerve cells in the spinal cord, thus reducing the transmission of pain signals.
Unfortunately, the power of morphine and other narcotics to block pain rapidly diminishes after repeated use, a property known as tolerance. Stronger and more frequent doses are required to achieve the same effect. Patients with chronic pain can become addicted, and their pain is doubled due to the limiting dependence on the drug. When it comes to such large quantities of narcotics, some doctors fear they will be suspected of not writing legal prescriptions but drug dealers, and they are often forced to limit their patients to doses that are no longer effective in relieving their suffering. Some of the patients are therefore forced to commit crimes and obtain illegal prescriptions to alleviate the unbearable pain, and some commit suicide to end their suffering. A new finding linking pain relief to glial cells and drug addiction shows that glial cells are responsible for creating tolerance to heroin and morphine.
Suspicions regarding the involvement of glial cells in tolerance to narcotic substances arose for the first time following an observation that showed that just like a heroin addict who stops consuming the drug at once, so too patients dependent on narcotic pain relievers who stop taking the drug suddenly suffer from a painful withdrawal syndrome. The patients (and heroin addicts) become so sensitive to the extreme that even normal sounds and light are extremely painful for them. The similarity of these symptoms to the increased sensitivity in neuropathic pain suggests a common cause.
In 2001, Ping Song and Zhi-Chi Zhao from the Shanghai Institute of Physiology tested whether glial cells are involved in the development of morphine tolerance. When the researchers gave rats repeated doses of morphine, they saw that the number of active astrocytes in their spinal cords increased. The changes caused to glial cells by repeated injections of morphine were the same as those observed in the spinal cord after injury or when neuropathic pain develops. When the scientists eliminated the astrocytes with the same poison used by Meller to inhibit the development of neuropathic pain in rats, the tolerance to morphine decreased significantly. That is, glial cells contribute in some way to the development of tolerance.
Many research groups have since tried to block various signals between nerve cells and glial cells (for example, by silencing certain cytokine receptors on glial cells) and to test whether tolerance to morphine is affected by this. These studies show that blocking inflammatory signals from and to glial cells does not change the sensation of acute pain at all, but if the blocking substances are injected with morphine, a lower dose of morphine is needed to achieve the same relief and the duration of the morphine's activity is doubled. These findings show that glial cells counteract the pain-relieving effect of morphine.
The fact that glial cells undercut the efficacy of morphine is consistent with their fundamental role in maintaining the balanced activity of neural circuits. Narcotic substances dull the sensitivity of pain circuits, and glial cells respond by releasing active substances that increase the sensitivity of nerve cells to return the nerve circuits to a normal level of activity. Over time, the influence of glial cells increases the sensitivity of the pain cells, and when the pain-numbing effect of heroin or narcotic drugs suddenly disappears due to rapid withdrawal from the drug, the nerve cells fire intensely, causing hypersensitivity and painful withdrawal symptoms. In experimental animals, it is possible to significantly reduce the painful withdrawal symptoms with the help of drugs that block responses of glial cells.
Modulating the activity of glial cells may therefore be the key not only to alleviating chronic pain but also to reducing the likelihood that people treated with narcotic painkillers will become addicts. Drugs targeting glial cells would have long since given a tremendous boost to long-running attempts to treat two major causes of human misery and tragedy. But in the past the connection between nerve cells and pain and between addiction escaped the eyes of the scientists who ignored glial cells, the essential partner of the nerve cells.
