Publications
APS Bulletin • Volume 15, number 2, Spring 2005
History of Pain
Marcia Meldrum, PhD, Department Editor
The Story of Pain Research at NIDCR, Part 2
This is the second in a two-part series of articles about the beginnings of the Neurobiology and Anesthesiology Branch (NAB). Part 1, “Dental Chickens and Puzzled Monkeys: The Story of Pain Research at NIDCR,” appeared in the November/ December 2004 issue of the APS Bulletin.
The researchers of the NAB made many important contributions to the investigation of pain—in clinical pharmacology, psychology, neurophysiology, neuroanatomy, and molecular biology.
Between the 1960s and the 1990s, a small research group in the National Institute of Dental Research (NIDR), now the National Institute of Dental and Craniofacial Research (NIDCR), evolved into one of the most productive programs in the pain field.
Testing Old and New Drugs for Pain Relief
An expanded program in the evaluation of drugs for dental anesthesia and analgesia was part of the plan for the newly reorganized pain branch, called the Anesthesiology Section. Edward Driscoll had cut back on his experimental work as he took on new administrative responsibilities at NIDR, but he continued to be an advocate for pain research. To staff the Anesthesiology Section, he and Ronald Dubner looked for people with expertise in psychology, pharmacology, and neurology as well as in dentistry. Richard Gracely, Raymond Dionne, and Mitchell Max were three key investigators who joined the NAB in the 1970s to work on the problem of refining clinical trial methods to better evaluate analgesics.
The subjectivity of pain report was a well-known difficulty in this research. When Richard Gracely was a psychology graduate student at Brown University, he began work on a verbal pain measurement tool, the Differential Descriptor Scale. This scale can differentiate the patients’ sensory perception of pain intensity from their affective experience of the pain’s “unpleasant” quality. He took this new tool to the NAB in 1975. For example, the scale can be used to demonstrate that the narcotic fentanyl decreases the perception of pain intensity, independent of its impact on affective quality, while the tranquilizing drug diazepam decreases the patient’s sense of distress, independent of perceived changes in the intensity (Gracely, McGrath, & Dubner, 1978). Gracely has since developed several innovative word scales for pain measurement: “You can’t just do magnitude measures of pain and really get a lot of meaning out of it unless you anchor the judgment some way Every word response is an anchor to a subjective level” (Gracely, 1998–1999).
Driscoll found in Raymond Dionne a dentist with the necessary pharmacology training to assess the various new analgesic agents being introduced. How large a dose was needed to prevent pain and what were the side effects? Could they replace general dental anesthesia? Dionne conducted trials of ibuprofen at the Medical College of Virginia in the mid-1970s, using the third molar extraction model developed by Stephen Cooper at Georgetown University. This model is extremely useful for acute pain studies; the procedure is common and often causes moderate to severe pain, with accompanying anxiety. The fact that every patient has four molars creates a built-in crossover control. Dionne gave patients an initial test dose before surgery, then a second dose when they requested pain relief postoperatively. Using this protocol, he found that “all the treatments looked alike!” He realized that he needed to consider the time of request for the second dose and the pain level at that point as the measure of effectiveness of the first dose. His review of the study data showed a huge difference among patients in second-dose request time. But his colleagues termed this new finding “just retrospective data dredging.” So Dionne restarted the study from the beginning and analyzed the data prospectively to establish the truth of his contention (Dionne, 1998).
Dionne continued his analgesic trial work after joining NIDR in 1978, evaluating the effects of premedication and postmedication on patient pain in third molar extractions. His studies established the effectiveness of preoperative medication with a nonsteroidal anti-inflammatory drug (NSAID) (e.g., flurbiprofen) combined with the use of a long-acting local anesthetic (e.g., etidocaine) as effective pain control for dental surgery (Dionne, Wirdzek, Fox, & Dubner, 1984). This approach to dental analgesia was endorsed at the National Institutes of Health (NIH) Consensus Development Conference in 1985.
The anesthesiology section was projected to be a major component of the new dental clinic, which opened at the clinical center at NIH in 1983. The board of scientific counselors recommended that its scope be extended beyond dental pain, as this was the only clinical pain research unit on the NIH campus. Mitchell Max, a neurologist, was recruited to head the clinic. He came from Memorial Sloan-Kettering Cancer Center, where he had trained with pain specialist Kathleen Foley and had studied the pharmacokinetics and pharmacodynamics of opioid drugs with Charles Inturrisi. Max developed a trial model to evaluate analgesics in chronic painful nerve disorders, such as diabetic neuropathy and postherpetic neuralgia. Collaborating with Gracely, Max employed a crossover trial design; an active placebo, to control for response based on subjective mood improvement; and verbal pain descriptors for the patients to use in keeping their “pain diaries.”
One possible new class of analgesics, suggested by the work of Peter Watson in Toronto, was the tricyclic antidepressants, which block the nervous system from reuptake (reabsorption) of the neurotransmitter norepinephrine (nor-adrenalin). The NIDR Pain Clinic’s trials of the tricyclics amitriptyline and desipramine demonstrated that these drugs are effective analgesics in diabetic neuropathy and postherpetic neuralgia and that their efficacy is independent of their impact on patient mood (Max et al., 1992).
Max described his studies as “hybrid” trials, in that he sought both to find useful analgesics and to correlate their action with a clearer understanding of pain mechanisms. “Clinical trials may be said to be aimed at one of two ideal goals to establish some biological principle in humans that you could then apply to lots of other things ...[or a] pragmatic study, just trying to understand what to do for that patient group. I think our aims fall in the middle” (Max, 1999).
A Laboratory Model for Chronic Pain
Max’s clinical trials with diabetic neuropathy and postherpetic neuralgia patients addressed the major enigma of the pain field: the mechanism involved in chronic neuropathic pain. The classic neuropathic pain disorders are the chronic regional pain syndromes (CRPS), first described by the Civil War surgeon Weir Mitchell as “the most terrible of all disorders” (Mitchell, 1872).
Researchers had tried to reproduce the symptoms of neuropathic pain in animals, but without success. One method was to inject the hind paws of rats with solutions causing brief inflammation, such as Freund’s adjuvant or capsaicin, and then test the rats for hyperalgesia and allodynia by placing them on warmed surfaces and timing the withdrawal latency. An improved method, developed by Kenneth Hargreaves of the NAB, was to place the rat in a glass box and direct a pinpoint radiant heat stimulus at the inflamed paw (Hargreaves, Dubner, Brown, Flores, & Jorvis, 1988). An attached photoelectric cell measured the withdrawal latency in fractions of seconds. The rats did show signs of hyperalgesia, but the symptoms were short lived and there was no evidence of spontaneous pain, as often appeared with CRPS.
In 1988, Gary Bennett, a physiological psychologist working in the NAB, and Yi-Kuan Xie, a visiting scientist from Beijing, tried tying loose ligatures around the sciatic nerve of the rat’s left hind leg in order to observe any temporary changes in the neuronal discharge. They left the rat overnight and came back the next day to find it guarding the affected paw in a very unusual way, refusing to let it touch the ground. This was the first observation of the chronic constriction injury model: a rat with symptoms similar to CRPS in humans. Bennett and Xie (1988) consulted Max and his colleagues at the clinical center, who recognized the characteristic guarding action of a CRPS patient. Excitedly, they tested various nonpainful and mildly painful stimuli on the rat’s paw and found hyperalgesia and allodynia responses similar to those observed in patients. And, although the rat continued to eat, sleep, and groom normally, the continued guarding behavior was evidence of some spontaneous pain.
Looking for human subjects to compare observations, Bennett found some CRPS patients at a clinic in Philadelphia and obtained their cooperation. As Bennett described these studies, “It got to the point where we were doing so many things so rapidly to these patients that we started videotaping them, because things were happening very quickly and some of these wonderful patients would come back two, three, four times for us; these people demonstrated courage, real courage” (Bennett, 1999). This was the beginning of a productive collaboration between Bennett in the lab and Max and Gracely in the clinic. Max observed: “Prior to this, the clinical work had been kind of isolated now every month, the basic scientists had a new animal observation [to be verified in patients]; it was a very exciting time” (Max, 1999).
Through careful comparison studies, Bennett and his colleagues established that the rat’s behavioral responses to pain were very similar to those of the CRPS patients. They recorded the nerve activity patterns in the area of the rat’s injury, which were characterized by persistent, spontaneous firing. Even in the absence of any direct stimulus, the rat’s sensory system behaved exactly as it would in response to pain. They hypothesized that when certain injuries or inflammatory conditions triggered persistent firing over a long period of time, the ongoing nociceptive input created a toxic excitation of the brain and spinal cord cells, such that the entire system was chronically primed to signal pain in response to any stimulus (Gracely, Lynch, & Bennett, 1992). The chronic constriction injury model helped to open new avenues of research into the mechanisms of all forms of neuropathic pain and the search for effective treatments.
Mapping the Spinal Cord
The underlying theme in all the NAB projects was the dynamic activity-dependent plasticity of the nervous system. “Pain” emerged from these studies as a complex set of responses to environmental situations. Dubner’ s monkey behavioral studies (see Part 1 in the November-December 2004 APS Bulletin) and the work of Dionne, Gracely, Max, and Bennett on pain and analgesia in humans were paralleled by continuing neuroanatomical work by Steve Gobel and, after 1976, by Maryann Ruda (with Jan Hylden and Haruhide Hayashi).
Gobel’s studies with the electron microscope traced the axons and dendrites of the trigeminal system and the spinal cord, identifying those neurons, which projected to the brain and mapped the network of local interneurons within the spinal cord. In the late 1970s, Ruda used several newly developed methods to label and trace synaptic pathways (e.g., tagging cellular compounds with fluorescent antibodies or horseradish peroxidase or using nucleic acid probes that bind to cellular DNA). With these techniques, she was able to follow the activity of the recently identified chemical messengers of the nervous system—the neurotransmitters and neuropeptides. By the mid-1980s, a number of these mediators had been found to be active in the nervous system’s response to pain.
The Dynorphin Connection
In 1982, Ruda demonstrated that the endogenous opioid enkephalin made direct synaptic contact with spinal cord neurons projecting to the thalamus. Another neuropeptide of particular interest was dynorphin, a substance that normally “lies low” in the system but becomes highly active in response to noxious stimuli. Although conspicuously absent from the nerves in the skin, this peptide is widely distributed in the brain and spinal cord. Ruda’s studies showed that dynorphin synapsed directly onto the thalamic projection neurons and was also active within the complex network of interneurons.
While working on the molecular biology of neuropeptides in Erminio Costa’s lab at National Institute of Mental Health (NIMH) in the early 1980s, Michael Iadarola measured dynorphin in the spinal cord by labeling and tracing its antibodies. He had also developed a method to clone the precursor protein predynorphin, but he needed a biological system that would allow him to look at real-time changes in levels of the peptide. In the literature, he discovered an inflammation model used in the hind limbs of rats and realized that he could use the opposite, unaffected, limb as a control. Using this method, he found that the painful synaptic input triggered a genetic regulatory sequence, called the enhancosome, that activated the transcription of the precursor protein and thus of dynorphin itself.
In 1986, when Iadarola was invited to give a seminar at the NAB, the group realized that his genetic research would be an excellent complement to their work. Two years later, Iadarola and Ruda collaborated on an important paper in Proceedings of the National Academy of Sciences, where they documented the increased biosynthesis of dynorphin in the spinal cord of the rat in response to brief inflammation of the hind leg. Dynorphin is first the messenger RNA of predynorphin and then of dynorphin itself. Iadarola commented, “The dynorphin connection weaves everything together in our work” (Iadarola, 1999; Ruda, Iadarola, Cohen, & Young, 1988).
This impressive evidence of dynorphin activity in response to inflammation led the researchers to hypothesize that this peptide might be one of the body’ s endogenous analgesics. In normal animals, however, dynorphin proved to have an algesic effect; the animal became more sensitive to heat and touch stimuli. An extended series of studies by Robert Caudle, a research fellow at NIDR, showed that dynorphin plays at least two roles in the nervous system’s response to pain: inhibiting pain signals in acute injury, but having an excitatory, intensifying effect on neural transmission in persistent, chronic pain.
Further Studies
Working with colleagues, including Ke Ren, Marino DeLeon, and Richard Nahin, Ruda concentrated in her later work on problems of gender differences in pain and the effects of pain on neonatal development. She showed that there are crucial differences in pain responses between male and female animals and in different phases of the females’ reproductive cycles, commenting, “It makes no sense to assume that male and female processing of pain are the same” (Ruda, 1999). Her team also examined the responses of rats to painful stimuli as newborns and as adults, demonstrating that the experience of pain during neonatal development alters neural circuitry and has long-term sensory and behavioral consequences (Ruda, Ling, Hohmann, Peng, & Tachibana, 2000).
In the late 1990s, Iadarola and collaborators (e.g., Hsiu-Ying T. Yang, Jun Gu, and Donna Messersmith) worked on describing the multistep genetic expression of the precursor proteins for dynorphin and enkephalin under painful and stressful stimuli. His lab helped to develop the new methods required for these experiments, including transient transfection (infection of a cell with nucleic acid to induce replication). He and his coworkers also employed subtraction cloning to identify new pain-relevant genes in the dorsal horn and developed an innovative gene-transfer model that triggers meningeal cells to secrete beta-endorphin into the cerebrospinal fluid.
Epilogue
In 1996, Ron Dubner left NIDR to establish a new program in Oral and Craniofacial Biology at the University of Maryland. The following year, the NAB was reorganized as the Pain and Neurosensory Mechanisms Branch (PNMB) headed by Ray Dionne. Despite often lukewarm support from the parent institute, Dionne, Iadarola, and Max have continued the tradition at NIDCR—the PNMB remains the major site of pioneering pain research at NIH. n
References
Bennett, G. J. (1999). Oral history interview (MS Coll 112). Los Angeles: UCLA, John C. Liebeskind History of Pain Collection.
Bennett, G. J., & Xie, Y. K. (1988). A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain, 33(1), 87–107.
Dionne, R. A. (1998). Oral history interview (MS Coll 112). Los Angeles: UCLA, John C. Liebeskind History of Pain Collection.
Dionne, R. A., Wirdzek, P. R., Fox, P. C, & Dubner, R. (1984). Suppression of postoperative pain by the combination of a nonsteroidal anti-inflammatory drug, flurbiprofen, and a long-acting local anesthetic, etidocaine. Journal of the American Dental Association, 108, 598–601.
Gracely, R. H. (1998–1999). Oral history interview (MS Coll 112). Los Angeles: UCLA, John C. Liebeskind History of Pain Collection.
Gracely, R. H., Lynch, S. A., & Bennett, G. J. (1992). Painful neuropathy: Altered central processing maintained dynamically by peripheral input. Pain, 51(2), 175–194.
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Iadarola, M. J. (1999). Oral history interview (MS Coll 112). Los Angeles: UCLA, John C. Liebeskind History of Pain Collection.
Max, M. B. (1999). Oral history interview (MS Coll 112). Los Angeles: UCLA, John C. Liebeskind History of Pain Collection.
Max, M. B., Lynch, S. A., Muir, J., Shoaf, S. E., Smoller, B., & Dubner, R. (1992). Effects of desipramine, amitriptyline, and fluoxetine on pain in diabetic neuropathy. New England Journal of Medicine, 326, 1250–1256.
Mitchell, S. W. (1872). Injuries of nerves and their consequences. Philadelphia: Lippincott.
Ruda, M. A. (1999). Oral history interview (MS Coll 112). Los Angeles: UCLA, John C. Liebeskind History of Pain Collection.
Ruda, M. A., Iadarola, M. J., Cohen, L. V., & Young, W. S., III. (1988). In situ hybridization histochemistry and immunocytochemistry reveal an increase in spinal dynorphin biosynthesis in a rat model of peripheral inflammation and hyperalgesia. Proceedings of the National Academy of Sciences, 85, 622–626.
Ruda, M. A., Ling, Q. D., Hohmann, A. G, Peng, Y. B., & Tachibana, T. (2000). Altered nociceptive neuronal circuits after neonatal peripheral inflammation. Science 289, 628–631.
Marcia Meldrum, PhD, is Co-Director of the John C. Liebeskind History of Pain Project at NIDCR. This article is based on her research while she was the DeWitt Stetten Fellow in the History of the Biomedical Sciences at NIH, 1998–1999. An earlier, illustrated version of Parts 1 and 2 of this article is on the NIH Web site at: http://history.nih.gov /exhibits /pain/ index.html.
Please direct your comments or suggestions about this article or department to Marcia Meldrum, PhD, Department Editor, at mlynnmel@earthlink.net.