How much do we know about pain? Which would you choose from 0-10? In the Neurobiology of Disease workshop, held in conjunction with the Society for Neuroscience meeting in October 2012 in New Orleans, Louisiana, US, the subject was persistent pain, and prominent researchers met to educate the new generation of pain researchers and summarize what has been achieved in the field. As a graduate student working in pain circuitry and sensory neuron development, I find it easy to lose track of why I study the basic mechanisms of pain development in the first place, and to overlook the complexities of pain as a disorder in patients. I felt fortunate to attend a workshop that covered a wide range of pain research, from the molecular and cellular levels to behavior and psychophysiology, and from basic research to translational applications.
For me, the highlight of the meeting was hearing a former soccer player and nursing school graduate Lori (not her real name) talk about suffering from complex regional pain syndrome (CRPS). CRPS is a type of dysfunctional pain. In Lori’s case, the problem started after she broke her foot during a soccer game. After the injury healed, there was still a lingering pain in the foot, which was persistently swollen and had an inflamed look, yet was actually 20 degrees colder than the rest of her body. The injured foot developed a severe allodynia to light brush but not to pressure. The immense pain led her whole body to respond, causing depression and insomnia. Unfortunately, most of the available painkillers had no effect on her pain. Relief from lumbar sympathetic block surgeries wore off after two weeks. After several lumbar surgeries, she used spinal cord stimulation and had an analgesic injection once in a while. Fortunately, four hours of intravenous ketamine treatment could relieve most of the pain. Now, after five years of CRPS, Lori is pain-free, enough that she can finally continue her studies in nursing school.
Like Lori, many patients whose pain cannot be easily categorized or diagnosed are suffering from lack of appropriate pain treatment. Since pain is notorious for being subjective and variable, some patients are even ignored, or their complaints are explained away as psychological hallucinations.
From bench to clinic, there is a long way to go. The conference provided a thorough summary of bench studies so far, from the perspective of molecular, cellular, and circuit levels, as well as from the systems view. There is special significance for researchers to sit down and look back at all the pain studies when chronic pain is no longer thought of as a simple symptom, but as the disease itself.
Learn from nature
David Julius from the University of California, San Francisco, US, is one of the researchers interested in learning about the somatosensory system specifically from nature. Over the years, his group has identified several extremely common natural products as the ligands for neuronal transient receptor potential (TRP) channels, including the TRPV1, TRPA1, and TRPM8 receptors.
To date, most of the natural ligands were found in plants. If plants can produce the natural ligands, why not animals, too? People experience immense pain when bitten by the Texas coral snake. By calcium imaging, Julius and coworkers found that a toxin, MitTx, in the Texas coral snake venom could activate acid-sensing ion channels (ASICs), which was confirmed in ASIC knockout mice. Since ASICs colocalize with TRPV1+ neurons, the activation of ASICs will lead to the stimulation of the pain pathway. This was proven by showing that the immense pain induced by MitTx was greatly decreased after ablation of spinal cord TRPV1+ afferents by capsaicin injection. Studies like this will continue to add specific natural ligands to the library of receptor activators to help people understand the specificity and the molecular mechanism of pain sensation.
Endogenous pain control
It is well known that pain can be modulated by emotion, mood, distraction, stress, and anxiety. This is exemplified by the story of the famous missionary and adventurer David Livingstone. When he was attacked by a lion and almost devoured, he was quite conscious of the attack while barely feeling any pain.
Livingstone credited that transcendent but out-of-place feeling to the mercy of God. Over 100 years later, people found that the origins of the "dreaminess feelings" experienced during danger and pain are actually in the human brain itself. There are several peptides secreted in the brain that lead to hallucinations and pain relief, including the endogenous opioid peptides leu-enkephalin, met-enkephalin, beta-endorphin, and dynorphin, of which the receptors are μ, Δ, and γ opioid receptors. These endogenous opioids also explain the "runner’s high," how expectations change the perception of pain, placebo effects, and the effects of acupuncture.
What happens when the endogenous pain control is lost? This is the question that interests Frank Porreca, University of Arizona Health Sciences Center, Tucson, US. He showed how invisible sensations become measureable in the behavioral model of conditioned place preference (CPP). Mice put in cages with two distinct chambers will spend an equal amount of time in each chamber. Porreca showed that after nerve injury, a peripheral nerve block could relieve pain, which made the mice prefer the chamber associated with the pain block. However, this phenomenon was abolished when dopaminergic neuron activity was blocked by lidocaine injection into the ventral tegmental area (VTA) of the brain. Porreca concluded that the relief of pain was due to the release of dopamine in the mesolimbic system where the VTA is located. These studies suggest that abnormalities in the mesolimbic system could account for chronic pain in some patients. They also remind us that induction of endogenous pain controls could be less dangerous and addictive, while similarly efficient in relieving pain than exogenous painkiller treatments, and could be a future treatment direction.
The invention of functional magnetic resonance imaging (fMRI) has made pain psychophysiology studies possible. M. Catherine Bushnell, who until recently was at McGill University, Montreal, Canada, is now scientific director of the Division of Intramural Research of the National Center for Complementary and Alternative Medicine (NCCA), Bethesda, Maryland, US. Bushnell is a pioneer in the field of brain structural and functional changes during chronic pain. Using fMRI, her group has been looking into the brains of chronic pain patients who suffer from migraine, back pain, fibromyalgia, and other conditions. Their extensive studies have found that brain activities were changed in pain-relevant regions, such as the anterior cingulate cortex, somatosensory cortex, insula, and other areas, in chronic pain patients.
With advances in other non-invasive brain imaging methods, Bushnell’s group was able to use positron emission tomography (PET) to estimate endogenous dopamine release and opioid receptor binding potential. In normal conditions, human basal ganglia release dopamine in response to pain. It has been found that in fibromyalgia patients, there is a decrease of dopamine release, as well as a drop in opioid receptor binding potential upon muscle injury compared to controls.
Furthermore, alterations in structure (such as thickened gray matter and thinner frontal cortex) and function take place in chronic pain conditions. These studies strongly suggest that we cannot treat chronic pain only in regard to its symptoms; rather, it might require some effort aimed at restoration of the altered central nervous system.
Ion channels and burning pain
“Rare genetic disorders are the experiments of nature that can define molecular mechanisms in humans and identify therapeutic targets that are relevant to common disorders,” said Stephen Waxman from Yale University, New Haven, Connecticut, US, who taught us that abnormal pain can be caused by mutated ion channels expressed in the peripheral nervous system. Patients who suffer from primary erythromelalgia (which presents as severe burning pain triggered by mild warmth) usually bear mutations in the Nav1.7 ion channel that make this sodium channel easier to turn on and slower to turn off. For a pain disorder caused by a single ion channel, it is important to find specific drugs or blockers that could restore Nav1.7 normal function while abolishing the hypersensitivities. Meanwhile, we should keep our eyes open and continue searching for other genetic mutations that could lead to pain sensitization, or correlate with pain in other diseases such as arthritis.
Microglia and their contribution to pain sensitization
The involvement of microglia in injury-induced pain sensitization was first proposed when an increased number of microglia was observed in the spinal cord after nerve injury, accompanied by morphological and excitability changes of microglia. Michael Salter from the Hospital for Sick Children, Toronto, Canada, brought us a different story of pain sensitization from the perspective of microglia. It was well known that microglia proliferate in the somatosensory area of the spinal cord upon injury in the peripheral sensory nerves. However, it remained unclear whether microglia activation is the cause or a result of nerve injury-induced mechanical hypersensitivity. Work from Salter and his colleagues suggests the former to be true. They observed an increase of P2X4 (a purinergic receptor) expression in microglia corresponding to the development of mechanical allodynia upon nerve injury. Pharmacological blocking or RNAi silencing of P2X4 reversed pain behavior, confirming that P2X4 expression in microglia is important for nerve injury-induced hypersensitivity. They proposed a model where ATP release by injured tissue leads to increased P2X4 expression in the microglia, which in turn leads to BDNF release from microglia. BDNF then binds toTrkB-expressing neurons in lamina I of the dorsal spinal cord, downregulating the KCC2 chloride transporter and elevating neuronal chloride ion levels. This causes a switch of GABAA and glycine receptor function from inhibitory to excitatory, thus leading to neuropathic pain. These findings are significant for discovering new drug targets and developing new therapies for neuropathic pain.
Shedding light on treatment
After learning about some basic pain research and listening to the story of an actual patient, I thought it natural to ask the question, What will future pain treatments look like? Despite the long history of human beings dealing with pain, treatment is still quite rudimentary. Many questions remain about basic pain mechanisms and highly variable subtypes of pain, making the future a bit blurry.
It was good timing when Allan Basbaum, University of California, San Francisco, US, talked about a novel pain treatment. Since pain allodynia results partly from loss of central inhibition, Basbaum and colleagues ventured to reintroduce GABAergic inhibitory activity by transplanting GABAergic interneuron precursors from the medial ganglionic eminence (MGE) region of the mouse frontal cortex to the adult mouse spinal cord. Surprisingly, the transplanted immature GABAergic neurons underwent maturation and integration after transplantation, and finally became inhibitory neurons that could be activated by peripheral stimulation. Most amazingly, the transplantation reversed the persistent pain produced by peripheral nerve injury. Though the relief of pain was not correlated with the number of grafted inhibitory neurons, and inflammatory pain was unaffected, the MGE-derived neuron grafts had a long survival, were stable and safe, and are a big step towards stem cell treatment of chronic pain. In the future, the group will focus on human induced pluripotent stem cells (iPSCs) that could be differentiated into MGE neurons for clinical treatment.
The workshop lectures were closed by an exciting story from Clifford Woolf, Boston Children’s Hospital, Massachusetts, US, about highly specific blocking of pain using a charged lidocaine derivative QX-314. QX-314 only works intracellularly and cannot cross the cell membrane under normal conditions. But a combination of a channel opener such as the TRPV1 agonist capsaicin with QX-314 led to the latter entering the cell via TRPV1 channels, resulting in an anesthetic effect specifically on TRPV1-expressing sensory neurons, most of which are pain neurons. This drug regimen worked well on mouse models. However, even though the combination of QX-314 and capsaicin is very effective in blocking pain, it is not practical because application of capsaicin will lead to several minutes of excruciating pain. To overcome this limitation, the group combined a low dosage of lidocaine with QX-314. Since this can antagonize the TRPV1 and TRPA1 channels, this particular combination of blockers will lead to initial general anesthesia caused by lidocaine, followed by a more specific and lasting pain block caused by QX-314. The combination could be applied in the skin to achieve localized blocking, which could be useful for dental surgeries, joint replacement, local wounds, and intra-articular surgeries.
Yet there are many more conditions that cannot be treated by either of these two original approaches. Bearing that in mind, I went to the afternoon workshop entitled "Developing Novel Pain Treatments," where Min Zhuo, University of Toronto, Canada, and Jon Levine, University of California, San Francisco, US, asked the question, Which system would you focus on if you were given a drug company to solve chronic pain? Most of the attendees said “central nervous system,” as it is more general and avoids the problem of specificity in the origin of the pain.
I asked the hosts what they thought about the breakthroughs discussed by Woolf and Basbaum, and how those advances could change the future treatment of chronic pain patients. Levine mentioned that it is always great to see original ideas for pain treatment, yet there are still many questions concerning the translation of basic research to patients. He emphasized that there is still much work to do to improve and develop new pain models, since his clinical experience is that not many cases can be closely correlated and phenocopied by existing, widely accepted animal models. As a result, it remains to be seen if pain relief observed in animals would be similarly observed using the same methods in human beings. All in all, there is still a long way to go.
Shan Lou is a graduate student in the lab of Quifu Ma, Dana Farber Cancer Institute and Harvard Medical School, Boston, US, where she studies the development of mechanical sensory neurons in the dorsal root ganglia (DRG). Lou is the author of the paper, “Runx1 Controls Terminal Morphology and Mechanosensitivity of VGLUT3-expressing C-Mechanoreceptors,” which appeared recently in the Journal of Neuroscience (Lou et al., 2013).
Related Reading on PRF:
A New Pathway for Paradoxical Pain (17 Jan 2013)
Personalizing Pain Medicine (17 December 2012)
Transplanted GABAergic Cells Ease Pain (6 Jun 2012)
A New Chapter for Sodium Channels (30 July 2011)
A Pain Protection Plan in the CNS? (26 July 2011)
Nav1.7 Mutations Move Into the Mainstream (25 July 2011)