Understanding Somatosensation and Pain: The 2013 Dr. Paul Janssen Award Symposium

Resources

Journal Articles

Bushnell MC, Ceko M, Low LA. Cognitive and emotional control of pain and its disruption in chronic pain. Nat Rev Neurosci. 2013;14(7):502-11.

Chiu IM, Heesters BA, Ghasemlou N, et al. Bacteria activate sensory neurons that modulate pain and inflammation. Nature. 2013;501(7465):52-7.

Chung K, Deisseroth K. CLARITY for mapping the nervous system. Nat Methods. 2013;10(6):508-13.

Clapham DE, Runnels LW, Strübing C. The TRP ion channel family. Nat Rev Neurosci. 2001;2(6):387-96.

Deisseroth K. Optogenetics. Nat Methods. 2011;8(1):26-9.

Dunn KM, Saunders KW, Rutter CM, et al. Opioid prescriptions for chronic pain and overdose: a cohort study. Ann Intern Med. 2010;152(2):85-92.

Hensel H, Zotterman Y. The effect of menthol on the thermoreceptors. Acta Physiol Scand. 1951;24(1):27-34.

Julius D. TRP channels and pain. Annu Rev Cell Dev Biol. 2013;29:355-384.

Schjerning Olsen AM, Fosbøl EL, Lindhardsen J, et al. Duration of treatment with nonsteroidal anti-inflammatory drugs and impact on risk of death and recurrent myocardial infarction in patients with prior myocardial infarction: a nationwide cohort study. Circulation. 2011;123(20):2226-35.

Seminowicz DA, Laferriere AL, Millecamps M, et al. MRI structural brain changes associated with sensory and emotional function in a rat model of long-term neuropathic pain. Neuroimage. 2009;47(3):1007-14.

Seminowicz DA, Wideman TH, Naso L, et al. Effective treatment of chronic low back pain in humans reverses abnormal brain anatomy and function. J Neurosci. 2011;31(20):7540-50.

Tajerian M, Alvarado S, Millecamps M, et al. Peripheral nerve injury is associated with chronic, reversible changes in global DNA methylation in the mouse prefrontal cortex. PLoS One. 2013;8(1):e55259.

Takahashi K, Tanabe K, Ohnuki M, et al. Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell. 2007;131(5):861-72.

Wilcox CM, Cryer B, Triadafilopoulos G. Patterns of use and public perception of over-the-counter pain relievers: focus on nonsteroidal antiinflammatory drugs. J Rheumatol. 2005;32(11):2218-24.

Venkatachalam K. Montell C. TRP channels. Annu Rev Biochem. 2007;76:387-417.

Villemure C, Ceko M, Cotton VA, Bushnell MC. Insular cortex mediates increased pain tolerance in yoga practitioners. Cereb Cortex. 2013. [Epub ahead of print]

Yu J, Vodyanik MA, Smuga-Otto K, et al. Induced pluripotent stem cell lines derived from human somatic cells. Science. 2007;318(5858):1917-20.

Report

Institute of Medicine of the National Academies. Relieving Pain in America: A Blueprint for Transforming Prevention, Care, Education, and Research. Committee on Advancing Pain Research, Care, and Education. Board on Health Sciences Policy. Washington, DC: National Academies Press (U.S.); 2011.

Sponsors

This symposium was made possible with support from

Featured Speaker:
David Julius, University of California, San Francisco
 
Award Presenter:
William N. Hait, Janssen Research & Development

Highlights

• The TRP family of ion channels is one of the most diverse receptor families in mammals. The Julius lab has characterized three channels that are promising targets for pain therapies.
• TRPV1 and TRPM8, respectively the capsaicin and menthol receptors, serve as molecular thermometers in primary sensory neurons.
• TRP1A, the wasabi receptor, responds to a huge range of stimuli, such as mustard, onion, air pollutants, and some endogenously produced inflammatory molecules.
• A major goal in pain research is to understand how pain thresholds change after injury; some mechanisms may be mediated by TRP receptors.

Introduction

The Dr. Paul Janssen Award for Biomedical Research is presented annually to honor Janssen's legacy in drug development and biomedical research. This year, the award committee unanimously recognized David Julius for his discovery of the molecular mechanisms that control thermosensation (sensory perception of temperature) and for elucidating its role in the sensation of acute and inflammatory pain. His research has increased our understanding of both pain hypersensitivity and how neurons transmit pain signals to the brain.

Janssen was an extraordinary pharmacologist and drug developer, William N. Hait of Janssen Research & Development told the audience. Janssen was one of the first chemist-biologists, scientists who modified drugs looking for ways to reduce side effects and improve therapeutic outcomes. He created drugs such as loperamide (Imodium) and fentanyl by modifying morphine and developed the first antipsychotic medication, based on his observations of chicken behavior. Four of the 80 drugs Janssen put on the market remain on the World Health Organization's list of essential medicines.

Like Janssen's, Julius's work is a testament to innovation and creativity. To answer a simple question—Why are hot peppers hot?—Julius used sophisticated molecular biology to identify a heat receptor activated by capsaicin, the active compound in chili peppers. Characterizing the receptor's pharmacological activity was the first step in understanding a series of vanilloid receptors that we now know as the TRP receptors for calcium flux. While investigating a second question—Why does menthol feel cool?—Julius identified another family of receptors that gave us a molecular understanding of how we perceive hot and cold. This body of work has identified many potential targets for pain therapeutics.

Our understanding of somatosensation and pain

In his acceptance speech, David Julius of the University of California, San Francisco, noted that pain is the most common reason people seek medical care; it affects 100 million people each year in the U.S. alone. Nociception, the ability to detect noxious stimuli that are intense enough to cause pain, is a submodality of touch that is closely linked with mechanoreception, thermoreception, and chemoreception. When these psychophysical modalities breach a threshold, they together create a crucial warning system, signaling tissue injury and initiating protective reflexes in the brain. But after experiencing acute pain, people often experience shifts in sensory thresholds marked by hypersensitivity to pressure and temperature. These shifts cause chronic pain. A major goal in pain research is to understand the maladaptive changes that lead to persistent or chronic pain syndromes in order to develop drugs that intervene in these processes.

Primary sensory neurons sense a wide array of noxious (painful) and innocuous stimuli of a chemical, thermal, or mechanical nature. (Image courtesy of David Julius)

Primary sensory neurons are the first messengers in the pain signaling pathway. They innervate regions of the body such as the lips, arms, and hands, and remarkably, sample a huge range of chemical and physical stimuli—from chemical irritants to pressure and temperature changes. These cells project to the spinal cord, where spinothalamic tract neurons carry the signal to the brain. Primary sensory neurons also release transmitters that send a flood of immune cells to the injured region and stimulate immune responses in the periphery. The panoply of inflammatory molecules released, including bradykinin and nerve growth factor (NGF), heightens pain sensitivity by increasing the responsiveness of primary sensory neurons to other stimuli (such as pressure and temperature).

Researchers have debated the specificity peripheral nerves have in detecting noxious stimuli. If there are subgroups of nociceptors that can detect distinct subsets of stimuli—such as intense heat but not innocuous warmth—identifying the molecular basis of the differences between these cells is important. Julius turned to the pharmacological study of natural products to address this question.

Plant-derived products have long been used to control pain. Indeed, opium from poppies and aspirin from willow bark form the basis of most analgesics available today. In his work, Julius focused on molecules that cause rather than suppress pain; specifically, capsaicin from chili peppers and menthol from mint. These molecules, along with compounds in other plants, such as isothiocyanates from wasabi and mustard and thiosulfinates from garlic and onion, presumably help the plant to ward off predators by activating their pain system.

Julius and his colleagues began by cloning the capsaicin receptor, which turned out to be a member of the transient receptor potential (TRP) receptor family first identified in flies 30 years ago. TRP ion channels are one of the most diverse receptor families in mammals, with some 30 subtypes. Most are still poorly understood but are thought to be modulated by a common signaling pathway. The researchers found that TRPV1, the capsaicin receptor channel, is closed at temperatures below 40°C but open at higher temperatures. This is the thermal threshold at which most people discriminate between warm and hot stimuli. Conversely, the menthol receptor channel, TRPM8, is closed at high temperatures but open below 25°C, the point of discrimination between cold and cool stimuli. Thus, Julius hypothesized that these two channels serve as molecular thermometers in primary sensory neurons. Eating hot pepper or mint chemically modulates the respective channel, making the channel easier to open and eliciting a psychophysical mimic of the hot or the cold experience.

The TRPV1 channel is activated at temperatures above 40°C and the TRPM8 channel is activated at temperatures below 20°C. Together, the channels act as a molecular thermometer that allows people to identify innocuous and noxious temperature stimuli. (Image courtesy of David Julius)

The researchers confirmed their discovery by knocking out the genes encoding these receptor channels in transgenic mice. TRPV1-knockout mice can tolerate much higher temperatures when their tails are placed in hot water compared to control animals. Similarly, TRPM8 knockouts spend equal time on a very cold floor as on a normal-temperature floor when given the option—suggesting they cannot tell the difference—while control animals avoid the cold floor. By labeling either of these receptors in the ganglion, the researchers showed that the receptors are generally present in different subsets of cells, suggesting that mammals discriminate hot and cold in part because this information travels from the periphery to the central nervous system along different pipelines.

Antibodies for TRPV1 and TRPM8 stain different populations of primary sensory neurons, suggesting that neurons are specialized to detect particular types of stimuli. (Image courtesy of David Julius)

Julius's group and others are beginning to tease apart the complicated signaling machinery that modulates channel thresholds. For example, injecting inflammatory molecules such as bradykinin or NGF shifts the thermal threshold, sensitizing a normal mouse to a temperature increase. But such action has no effect on a mouse that lacks TRPV1 receptors. This suggests that TRP receptors are excellent targets for pain-modulating drugs, because these channels are polymodal signal integrators, merging signals from many different input streams. The best pain drugs would inhibit the sensitization of the channel in response to injury but would not disturb its normal temperature-sensing function, and new drugs in development are approaching this goal.

More recently, Julius's lab has explored the wasabi receptor channel, TRPA1. This channel is activated by an unusually broad range of stimuli: natural compounds in plants such as mustard and onion, air pollutants such as vehicle exhaust, molecules produced endogenously by inflammation in arthritic joints, and others. These stimuli are united not by the shape of the activating molecules but by their chemical reactivity. This diversity suggests that this channel could make an especially potent target for new pain medicines, Julius concluded.

Speakers:
Husseini Manji, Janssen Research & Development
M. Catherine Bushnell, National Institutes of Health
Clifford Woolf, Boston Children's Hospital; Harvard University

Highlights

• A wave of recent advances in neuroscience and stem cell biology will help pain researchers to develop novel potent analgesics.
• Chronic pain may alter the anatomy and function of pain modulation, and lifestyle factors may be important in reversing such changes.
• Nociceptors can be activated not only by mechanical and heat stimuli but also by pore-forming toxins secreted by bacteria.

New developments in pain treatment

The unmet need for new pain medications is staggeringly high, reported Husseini Manji of Janssen Research & Development. Available drugs are inadequate. Non-steroidal anti-inflammatory drugs have mediocre efficacy and cause thousands of deaths from gastrointestinal bleeding, while opioids can be addictive and can cause endocrinopathies. But scientific advances are laying the groundwork for new treatment approaches.

Recent work has confirmed that the central nervous system is highly plastic and that new neurons can form into adulthood. Researchers can now visualize in living animals the formation of new dendritic spines, the points at which cells form connections. New techniques for mapping neural circuitry could also allow researchers studying the human brain to detect changes in connectivity that occur subsequent to specific events or therapies. In addition, with the ability to make induced pluripotent cells from skin cells, researchers can create neurons or glial cells from people with specific pain-related conditions to test in new treatments. One challenge in developing pain drugs is that few therapeutic molecules can cross the blood–brain barrier to effectively target the central nervous system, but researchers are looking for ways to temporarily open this barrier. We are in the "golden age of neurosciences," Manji said. These and other advances can propel the field to develop much-needed pain medicines.

Understanding the neural basis of pain—does chronic pain change pain control?

Chronic pain can change the brain, altering its response to pain treatment. M. Catherine Bushnell of the National Institutes of Health explained that brain signaling resulting in the experience of chronic pain is extremely complex, and many of the pathways that modulate pain descend through divergent areas of the cortex, thalamus, brainstem, and cerebellum. For example, attentional state and emotional state modulate pain differently, both in terms of the brain regions involved and the resulting perceptual experience.

Pain is modulated by a complex network of signaling pathways, many of which descend from the cortex. The perceptual experience of pain via each of these pathways might differ. For example, asking someone how strong a sensation is, as opposed to how much it bothers them, may result in two different answers. (Image courtesy of M. Catherine Bushnell)

Multiple studies have found that people with chronic pain have alterations in these descending pathways, such as changes in white matter tracts, signs of inflammation, decreased cortical thickness, and increased markers of gliosis and neuronal death. Bushnell's group took a close look at cortical grey matter, which has been shown to shrink in chronic pain-related brain areas across conditions including fibromyalgia, irritable bowel syndrome, and temperomandibular joint disorder. In a longitudinal rat study her group found that nerve-injured rats' prefrontal cortices were smaller than controls'. The change was not detected until 20 weeks after injury, suggesting the shrinking occurs gradually in chronic conditions.

Studies by Bushnell and others also suggest that changes in grey matter are age-dependent. While older patients with chronic pain show decreased grey matter thickness compared to controls, younger patients show the opposite pattern (increased grey matter thickness compared to controls). Bushnell speculated that hypertrophy in younger patients might reflect the brain's attempt to attenuate symptoms by running pain-modulation circuits in overdrive. In older brains, such compensation might cease to be effective and instead cause damage due to use-dependent excitotoxicity. A better understanding of how the brains of older and younger people respond to chronic pain will inform better treatment.

In addition to identifying anatomical and physiological changes related to chronic pain, recent work on which Bushnell collaborated found that the brains of mice with chronic nerve injury show significantly less DNA methylation—a chemical tag that regulates gene expression—in two brain areas closely linked to pain modulation.

There are hints that these changes may be reversible. For example, in patients who had successful procedures to remove chronic pain (surgery for chronic back pain and hip replacements for osteoarthritis), prefrontal cortical thickness increased in the six months after the surgery. Other research suggests that yoga practitioners have more cortical thickness than non-practitioners in brain regions linked to chronic pain modulation; these individuals also experience less age-related cortical grey matter loss. Similarly, rodents living in an enriched environment also seem to be protected against hyperalgesia and do not show changes in global patterns of DNA methylation after nerve injury. Bushnell concluded that this body of work suggests chronic pain may alter the anatomy and function of pain modulation and lifestyle factors may be important in reversing such changes.

Activating nociceptors with bacterial toxins

Clifford Woolf of Boston Children's Hospital and Harvard University described his group's recent findings on bacterial activation of nociceptors. Data in the literature suggest that both bacterial and viral pathogens can act on pain receptors, but very little is known about how they do so. Isaac Chiu, a researcher in Woolf's lab, led studies on the bacterium methicillin-resistant Staphylococcus aureus to answer this question.

Pain is one of the first signs of infection; injecting live bacteria into the hindpaws of mice causes the animals to exhibit signs of intense pain. The group used S. aureus labeled with Green Fluorescent Protein (GFP) to follow its distribution in the paw: the bacteria began to accumulate between 1 and 3 hours after injection, but after 6 hours immune cells such as neutrophils and macrophages had entered the region. These immune cells peaked at 24 hours, clearing the bacteria.

The researchers hypothesized that incoming immune cells would cause pain by activating nociceptors, but pain occurred before the flood of immune cells began. To their surprise, the pain response intensified when a neutralizing antibody was applied to destroy the immune cells. Further experiments revealed that pain is driven by bacterial numbers, not the presence of immune cells. To investigate whether the pathogens can directly activate nociceptors, the group exposed in vitro primary sensory neurons to S. aureus. A subset of neurons expressing the wasabi receptor, TRPA1, became activated, while neurons expressing other nociceptors did not. The same effect was observed when the cells were exposed to Streptococcus pneumoniae, another type of bacteria known to cause pain.

Formyl peptides and alpha-hemolysin, two molecules produced in the bacterial cell, can directly and selectively activate nociceptors. (Image courtesy of Clifford Woolf)

How exactly do bacteria activate pain receptors? A rare type of receptor called the formyl peptide receptor, which is present in the olfactory system, is known to respond to bacteria. Woolf's group examined whether formyl peptides present in bacteria might activate nociceptors. The molecules turned out to selectively activate a subset of TRPA1 receptors. S. aureus secretes a broad range of toxins, and the researchers found that this secreted solution also activated a subset of nociceptors. One molecule in the toxic solution is a pore-forming toxin called alpha-hemolysin, which inserts into red blood cells and causes them to hemolyze. When added to a dish of sensory neurons, alpha-hemolysin inserts itself only into capsaicin-positive (TRPV1) nociceptors. These bacterially-produced molecules—alpha-hemolysin and formyl peptides—constitute a novel class of activators of the pain system, Woolf concluded.

Moderator:
David S. Bredt, Janssen Research & Development
 
Panelists:
Allan I. Basbaum, University of California, San Francisco
M. Catherine Bushnell, National Institutes of Health
David Julius, University of California, San Francisco
Husseini Manji, Janssen Research & Development
Clifford Woolf, Boston Children's Hospital; Harvard University

Panel discussion

The symposium concluded with a panel discussion moderated by David S. Bredt of Janssen Research & Development. Bredt began by asking Julius which TRP channels he considers most amenable to drug development. Julius replied that TRPV1 is promising because it is well known to be involved in inflammatory pain. The first wave of drugs blocking TRPV1 caused a drop in basal body temperature, but a new generation lacks this side effect. TRPA1 is also a good target, as it has been implicated in airway disorders and inflammatory conditions inside the lung.

Next, Bredt asked Allan I. Basbaum of the University of California, San Francisco, about the prospects of translating his group's work—on nerve transplantation as a pain treatment—from animal models to humans. Basbaum conceded that it is difficult to determine clinical condition in animal models, but he is optimistic, particularly because transplanted neurons integrated well into host circuitry. In general, pain drugs pose particular challenges: there is often a strong placebo effect, it can be difficult to measure pain levels in animal models, and it is necessary to develop drugs that can cross the blood–brain barrier to affect the central nervous system. Basbaum believes that administering pain drugs to the spinal cord might avoid some of these problems, but pharmaceutical companies have not shown interest in this approach because it does not involve medication taken in pill form. Basbaum also noted that the bar for drug development is exceptionally high for new pain drugs compared to medications to treat other diseases, such as cancer. A cancer drug that improves survival by just five months would be a blockbuster, but a drug showing such an effect in pain would probably not even be developed, despite providing meaningful benefits for patients. New pain drugs need to be better than existing ones and need to have better side-effect profiles.

Bredt asked Bushnell whether a decrease in grey matter could cause chronic pain, rather than the reverse effect that her group reported. Bushnell responded that their work in rats showed that brain changes occur several months after injury. The effect of the injury may not be direct, but it does appear to start a cascade of events that leads to brain alterations. Researchers studying animal models of pain generally examine the animals immediately after the injury, but it is also important to reexamine them later, when the pain is chronic. Bushnell also noted that the rats developed anxiety and cognitive deficits at about the same time as prefrontal cortex changes emerged; people with chronic pain also complain of cognitive deficits. Studies suggest that chronic pain can lead to many effects in the brain.

Finally, Bredt asked Woolf to identify recent developments in the field that are most likely to lead to new pain medicines. Woolf noted that, ironically, while the science of pain is beginning to flourish, many pharmaceutical companies have abandoned this therapeutic area—perhaps because single therapies active across all pain classes have not emerged. Woolf advocated a much more sophisticated approach that takes into account the fact that some therapies will be uniquely suitable for one sort of pain or one cohort of patients but not for others. Acute pain differs from chronic pain, and nerve injury pain differs from inflammatory pain. Researchers should embrace the complexity in the molecular basis of pain and recognize that pain occurs at multiple levels of the nervous system.

An audience member asked whether cancer pain might have an evolutionary role. Julius responded that it could be part of the general logic of pain as a warning system, with cancer pain warning of the presence of a tumor. But Basbaum noted that he sees no evolutionary value to cancer pain: since the tumor is expected to kill the host, it is not necessarily signaling anything. Chronic pain has no value, which is why he thinks of it as a disease state. Woolf added that more than 100 years ago people with chronic pain would not have survived, so such pain is unlikely to be acted upon by evolution.

How are noxious stimuli encoded and detected?

How do detectors transduce signals?

How are sensitivity thresholds set, and how are they reset following injury?

How can nociceptors be targeted for analgesia?

How can advances in neuroscience and other fields be harnessed for the development of novel pain therapies?

What are the differences in the ways that the brains of younger versus older people respond to chronic pain?

What kinds of lifestyle factors might play a role in reversing or attenuating chronic pain?

Which still-unknown types of stimuli can activate nociceptors?