Pain and the neuromatrix in the brain

The neuromatrix theory of pain proposes that pain is a multidimensional experience produced by characteristic "neurosignature" patterns of nerve impulses generated by a widely distributed neural network-the "body-self neuromatrix"-in the brain. These neurosignature patterns may be triggered by sensory inputs, but they may also be generated independently of them. Acute pains evoked by brief noxious inputs have been meticulously investigated by neuroscientists, and their sensory transmission mechanisms are generally well understood. In contrast, chronic pain syndromes, which are often characterized by severe pain associated with little or no discernible injury or pathology, remain a mystery. Furthermore, chronic psychological or physical stress is often associated with chronic pain, but the relationship is poorly understood. The neuromatrix theory of pain provides a new conceptual framework to examine these problems. It proposes that the output patterns of the body-self neuromatrix activate perceptual, homeostatic, and behavioral programs after injury, pathology, or chronic stress. Pain, then, is produced by the output of a widely distributed neural network in the brain rather than directly by sensory input evoked by injury, inflammation, or other pathology. The neuromatrix, which is genetically determined and modified by sensory experience, is the primary mechanism that generates the neural pattern that produces pain. Its output pattern is determined by multiple influences, of which the somatic sensory input is only a part, that converge on the neuromatrix.

Blocking NMDA receptors in the hippocampal dentate gyrus with AP5 produces analgesia in the formalin pain test

The hippocampus is an integral component of the "limbic" system and, as such, may contribute to the negative affect and avoidance motivation experienced during pain. A substantial body of evidence indicates that the hippocampus processes pain-related information, that some hippocampal neurons respond exclusively to painful stimulation, and that long-term anatomical changes occur in dentate gyrus neurons, following noxious physical stimulation. NMDA receptor antagonist drugs administered to the hippocampus interfere with long-term potentiation, learning, and memory; these same drugs, when applied to the spinal cord, prevent the long-term neurophysiological changes caused by noxious physical stimulation. This experiment tested whether blocking NMDA receptors in the hippocampal formation reduces nociceptive behaviors in an animal model of persistent human pain. The competitive NMDA receptor antagonist AP5 was injected into the dentate gyrus of alert, unrestrained rats either 5 min before or 15 min following the administration of a subcutaneous injection of formalin irritant. Pain behaviors in both acute and tonic phases of the formalin test were significantly reduced by AP5 treatments. These results support the hypothesis that the hippocampal formation is involved in pain-related neural processing and that NMDA receptor-sensitive mechanisms in the hippocampus are involved in pain perception and/or the expression of pain-related behaviors.

Central neuroplasticity and pathological pain

The traditional specificity theory of pain perception holds that pain involves a direct transmission system from somatic receptors to the brain. The amount of pain perceived, moreover, is assumed to be directly proportional to the extent of injury. Recent research, however, indicates far more complex mechanisms. Clinical and experimental evidence shows that noxious stimuli may sensitize central neural structures involved in pain perception. Salient clinical examples of these effects include amputees with pains in a phantom limb that are similar or identical to those felt in the limb before it was amputated, and patients after surgery who have benefited from preemptive analgesia which blocks the surgery-induced afferent barrage and/or its central consequences. Experimental evidence of these changes is illustrated by the development of sensitization, wind-up, or expansion of receptive fields of CNS neurons, as well as by the enhancement of flexion reflexes and the persistence of pain or hyperalgesia after inputs from injured tissues are blocked. It is clear from the material presented that the perception of pain does not simply involve a moment-to-moment analysis of afferent noxious input, but rather involves a dynamic process that is influenced by the effects of past experiences. Sensory stimuli act on neural systems that have been modified by past inputs, and the behavioral output is significantly influenced by the "memory" of these prior events. An increased understanding of the central changes induced by peripheral injury or noxious stimulation should lead to new and improved clinical treatment for the relief and prevention of pathological pain.

Age differences in nociception and pain behaviours in the rat

Much remains to be learned about the effects of ageing on pain. Studies of life-span changes in nociception and pain behaviours in the rat are equivocal making it difficult to draw firm conclusions. This paper reviews the available data and finds that age differences in nociception may be dependent on the pain test employed. Specifically, reflexive responses to nociceptive stimuli do not change with age while there may be no change or a linear decrease with age on more highly organized tests of nociception. Interestingly, age differences in pain behaviours on models of tissue injury and inflammation may not be linear. It is shown that important changes that begin at mid-life in neuroanatomy, neurochemistry and endogenous pain inhibition may be associated with alterations in pain sensitivity. Several testable hypotheses which might encourage future research in this domain are developed throughout this paper.

Nausea and vomiting during pregnancy: A prospective study of its frequency, intensity, and patterns of change

OBJECTIVE

Our purpose was to provide a detailed description of patterns of nausea and vomiting of pregnancy.

STUDY DESIGN

A prospective study was performed with 160 women who provided daily recordings of frequency, duration, and severity of nausea and vomiting.

RESULTS

Seventy-four percent of women reported nausea lasting a mean of 34.6 days. "Morning sickness" occurred in only 1.8% of women, whereas 80% reported nausea lasting all day. Only 50% of women were relieved by 14 weeks' gestation; 90% had relief by week 22. Data based on the McGill Nausea Questionnaire indicate that the nausea experienced by pregnant women is similar in character and intensity to the nausea experienced by patients undergoing cancer chemotherapy.

CONCLUSIONS

Traditional teachings about nausea and vomiting of pregnancy are contradicted by our findings. Standardized tools for measuring the distribution, duration, and intensity of nausea are applicable to the study of nausea and vomiting of pregnancy and could be used in clinical trials to assess palliative measures.

Long-term patterns of morphine dosage and pain intensity among cancer patients

Cancer pain remains a worldwide problem and some patients continue to be undermedicated because of concerns about tolerance and drug dependence. The aim of this study was to document the morphine intake of patients with chronic cancer pain in an inpatient palliative care unit and to describe the long-term pattern of morphine use and pain intensity in this patient population. With IRB approval and written informed consent, patients admitted over a 64-week period to the palliative care unit at the Royal Victoria Hospital, Montreal, were candidates for this study. Cancer patients receiving morphine for 30 days or longer who were able to complete the pain scale were included. Excluded were patients with a confused or clouded sensorium. Daily pain intensity was recorded by the PPI (0-5 scale) of the McGill pain questionnaire. The daily morphine consumption was recorded and the occurrence and intensity of breakthrough pain were also recorded. Of the 35 potential candidates for study, 17 patients with a mean age of 59 (14) years completed the study. Patients were followed up for a mean of 82 (52) days. The mean (S.D.) daily morphine dosage at study entry was 135 (127) mg, and the daily morphine dose at study completion was 244 (240) mg. There was no evidence that any patient rapidly developed tolerance to morphine. Pain was well controlled for most patients. For 10 of 17 patients, 93% reported PPI scores of either 0 or 1. Occasional breakthrough pain was experienced by 4 of these 10 patients. Four other patients reported 79% of their PPI scores as either 0 or 1, and 18% of the PPI scores as either 2 (discomforting) or 3 (disturbing), Thus 82% of patients had good to excellent pain control. Three of 17 patients spent more than four months in the unit and had less than good pain control. All of these patients had neuropathic cancer pain. These results support the conclusion that pain was well controlled for most cancer patients, and that increases in daily morphine dose, when it occurred, generally developed over a period of weeks to months, and a pattern of rapid escalation in morphine dose did not occur.

Age differences in the response to the formalin test in rats

We report the results of a study designed to assess age differences in the response to the formalin test, a model of tissue injury and inflammation, while controlling for differences in weight and motoric abilities in three groups of adult male Long-Evans rats: young (3 months old), middle-aged (18 months old), and old (24 months old). The first part of the study assessed initial differences in responsivity and found that the middle-aged group showed the greatest response, whereas the young and old groups did not differ from each other. In the second part of the study, the young and middle-aged animals were followed for a 4-month period. The formalin test was repeated at 2-month intervals. These results indicate that there may be an age-associated change in the sensitivity to tonic pain and that this sensitivity may peak at mid-life.

Pain--an overview

The neuromatrix theory of pain proposes that pain is a multidimensional experience produced by characteristic "neurosignature" patterns of nerve impulses generated by a widely distributed neural network--the "body-self neuromatrix"--in the brain. These neurosignature patterns may be triggered by sensory inputs, but they may also be generated independently of them. Pains that are evoked by noxious sensory inputs have been meticulously investigated by neuroscientists, and their sensory transmission mechanisms are generally well understood. In contrast, chronic pain syndromes, which are often characterized by severe pain associated with little or no discernible injury or pathology, remain a mystery. The neuromatrix theory of pain, however, provides a new conceptual framework that is consistent with recent clinical evidence. It proposes that the output patterns of the neuromatrix activate perceptual, homeostatic and behavioral programs after injury or pathology, or as a result of multiple other inputs that act on the neuromatrix. Pain, then, is produced by the output of a widely distributed neural network in the brain rather than directly by sensory input evoked by injury, inflammation or other pathology. The neuromatrix, which is genetically determined and modified by sensory experience, is the primary mechanism that generates the neural pattern that produces pain. Its output pattern is determined by multiple influences, of which the somatic sensory input is only a part, that converge on the neuromatrix.

Pain: an overview

Until the 1960s, pain was considered an inevitable sensory response to tissue damage. There was little room for the affective dimension of this ubiquitous experience, and none whatsoever for the effects of genetic differences, past experience, anxiety, or expectation. In recent years, great advances have been made in our understanding of the mechanisms that underlie pain and in the treatment of people who complain of pain. The roles of factors outside the patient's body have also been clarified. Pain is probably the most common symptomatic reason to seek medical consultation. All of us have headaches, burns, cuts, and other pains at some time during childhood and adult life. Individuals who undergo surgery are almost certain to have postoperative pain. Ageing is also associated with an increased likelihood of chronic pain. Health-care expenditures for chronic pain are enormous, rivalled only by the costs of wage replacement and welfare programmes for those who do not work because of pain. Despite improved knowledge of underlying mechanisms and better treatments, many people who have chronic pain receive inadequate care.

A case of referred pain evoked by remote light touch after partial nerve injury

An unusual case of referred pain is presented in which a 63-year-old man, who suffered a severe injury to his right hand and arm during young adulthood, describes the later development of dysesthesia and shooting pain in his arm subsequent to stimulation of the ipsilateral scalp, the temporal and infrazygomatic region of the face, and the back. Referred sensations of this type are usually reported following amputation of an arm. Clinical examination of the sensory and motor function of the arm and hand revealed partial damage to the radial, ulnar and median nerves as well as possible brachial plexus involvement. Interestingly, pain could be evoked by repeated light touches applied to the remote trigger areas suggesting the involvement of a 'wind-up'-like process.

Measurement of pain

Pain is a personal, subjective experience influenced by cultural learning, the meaning of the situation, attention, and other psychologic variables. Approaches to the measurement of pain include verbal and numeric self-rating scales, behavioral observation scales, and physiologic responses. The complex nature of the experience of pain suggests that measurements from these domains may not always show high concordance. Because pain is subjective, patients' self-reports provide the most valid measure of the experience. The VAS and the MPQ are probably the most frequently used self-rating instruments for the measurement of pain in clinical and research settings. The MPQ is designed to assess the multidimensional nature of pain experience and has been demonstrated to be a reliable, valid, and consistent measurement tool. A short-form MPQ is available for use in specific research settings when the time to obtain information from patients is limited and when more information than simply the intensity of pain is desired. The DDS was developed using sophisticated psychophysical techniques and was designed to measure separately the sensory and unpleasantness dimensions of pain. It has been shown to be a valid and reliable measurement of pain with ratio-scaling properties and has recently been used in a clinical setting. Behavioral approaches to the measurement of pain also provide valuable data. Further development and refinement of pain measurement techniques will lead to increasingly accurate tools with greater predictive powers.

Psychostimulant drugs potentiate morphine analgesia in the formalin test

Recent research has shown that the psychostimulant drug dextroamphetamine can increase the analgesia produced by opioids. Despite the strong, positive results in human clinical subjects and in animals, this combination is rarely used in clinical practice. The purpose of this paper is to investigate whether the psychostimulant drug methylphenidate (MP) can potentiate morphine analgesia in the rat formalin test, and to compare its effectiveness to that of dextroamphetamine (AMP). The formalin test was used because its long-lasting pain of moderate intensity resembles human clinical pain. Two different drug administration times were used to observe whether the early phase of the formalin response would be differentially affected by the drugs. At Drug Administration Time 1, rats received morphine 30 min prior to the formalin injection (-30 min) and MP or AMP 20 min prior to the formalin injection (-20 min). At Drug Administration Time 2, rats received morphine 10 min prior to the formalin injection (-10 min) and MP or AMP immediately prior to the formalin injection (0 min). All drugs were given subcutaneously. The results indicate that low doses of MP or AMP potentiate the analgesic effects of morphine. The clinical value of these drug combinations merits further investigation in animals and in humans.

Potentiation of opioid analgesia by psychostimulant drugs: a review

Recent research has investigated drug combinations that enhance the analgesic effectiveness of their component substances. Many studies have examined the combination of opioids and psychostimulant drugs, such as amphetamine and methylphenidate. Despite the positive results reported in the literature, this combination is rarely used in clinical practice. The purpose of this paper is to review the literature on the opioid-amphetamine combination. Experiments with animal and human subjects provide convincing evidence that d-amphetamine or methylphenidate potentiate the analgesic effects of morphine. Psychostimulant drugs have been shown in animal studies to possess intrinsic analgesic properties and to have the ability to enhance the analgesic properties of opioids when both types of drugs are given in combination. Studies with human subjects have confirmed the enhancement of opioid analgesia by amphetamines and, in addition, have demonstrated that psychostimulant drugs produce a decrease in somnolence and an increase in general cognitive abilities. The greater cognitive alertness, moreover, allows the use of larger opioid doses, which can produce a substantial increase in analgesia. These results indicate another possible method to enhance the quality of life in patients with difficult pain problems. Although the enhanced cognitive effects are well established, the effects on pain need further study to determine the mechanisms of action and the drug combinations and administration patterns that would maximize their effects.

Phantom limbs in people with congenital limb deficiency or amputation in early childhood

It is widely believed that people who are congenitally limb-deficient or suffer a limb amputation at an early age do not experience phantom limbs. The present study reports on a sample of 125 people with missing limbs and documents phantom experiences in 41 individuals who were either born limb-deficient (n = 15) or underwent amputation before the age of 6 years (n = 26). These cases provide evidence that phantom limbs are experienced by at least 20% of congenitally limb-deficient subjects and by 50% of subjects who underwent amputations before the age of 6 years. The phantoms are detailed and can be described in terms of size, shape, position, movement and temporal properties. The perceptual qualities of the phantoms can also be described by sensory descriptors and are reported as painful by 20% of subjects with phantoms in the congenital limb deficient group and 42% of young amputees. It is argued that these phantom experiences provide evidence of a distributed neural representation of the body that is in part genetically determined.

Repeated cold water swim produces delayed nociceptive responses, but not analgesia, for tonic pain in the rat

Earlier studies have demonstrated that cold water swim (CWS) produces stress-induced analgesia in tests of brief, phasic pain and produces a delayed nociceptive response (DNR) for more prolonged tonic pain. The present study reports the effect of repeated CWS on tonic pain, as measured by the formalin test. One group of rats was exposed to a 3.5-min swim in 2 degrees C water immediately prior to the formalin injection, to a 1.5-min swim at 50 min, and to another 1.5-min swim at 100 min postformalin injection. Compared to the no-swim control group, subjects which received repeated CWS had dramatically altered formalin pain responses. Formalin responses began just over 3 h postformalin injection, peaked at 4 h, and were still present at 5 h. Inspection of individual responses revealed a substantial degree of variability in the onset of responses, although the magnitude and duration of the formalin pain response remained at the same levels as those of control subjects. The lack of a decrease in the magnitude and duration of the delayed formalin responses indicates that repeated CWS does not produce analgesia for tonic pain. The period of stress, therefore, produces pain suppression but not loss of the mechanisms that subsequently underlie the pain. Earlier controls have ruled out peripheral mechanisms (such as retention of the formalin in the paw tissue). Rather, a memory mechanism appears to have been indicated and it is not lost, but persists until it can be manifested. Further research is needed to study the mechanisms responsible for the DNR.

Chronic pain in elderly people

Chronic pain in elderly people has only recently begun to receive serious empirical consideration. There is compelling evidence that a significant majority of the elderly experience pain which may interfere with normal functioning. Nonetheless, a significant proportion of these individuals do not receive adequate pain management. Three significant factors which may contribute to this are (1) lack of proper pain assessment; (2) potential risks of pharmacotherapy in the elderly; and (3) misconceptions regarding both the efficacy of nonpharmacological pain management strategies and the attitudes of the elderly towards such treatments. In this review the most commonly used assessment instruments and patterns of age differences in the experience of chronic pain are described and evidence for the efficacy of psychological pain management strategies for this group is reviewed.

Electrical stimulation of the cingulum bundle and surrounding cortical tissue reduces formalin-test pain in the rat

Surgical lesions of the cingulum bundle in humans produce marked decreases in severe pain associated with cancer, reflex sympathetic dystrophy and other forms of chronic pain. Similarly, a temporary block of the anterior cingulum bundle in the rat by microinjection of lidocaine produces significant decreases in formalin-pain and reduces autotomy following peripheral neurectomy. The present study explored the effect of electrical stimulation of the cingulum bundle/surrounding cortical tissue (CB/CT) on tonic pain in the rat. Experiment 1 examined changes in formalin-induced pain responses following a 2.5-min period (30 s/min for 5 min) of electrical stimulation of the CB/CT 15 min prior to the formalin injection. The stimulation produced a significant reduction of first-period and second-period pain responses. Experiment 2 examined changes in formalin-induced pain responses following a 2.5-min period (30 s/min for 5 min) of electrical stimulation of the CB/CT 20 min following the formalin injection. The stimulation produced a dramatic reduction in second-period pain responses which persisted for the duration of the 35-min post-stimulation test period. The fact that either electrical stimulation or surgical section of the CB/CT produces pain relief suggests that this region serves a complex role in pain processing. Since the cingulum bundle has major connections with all other structures of the limbic system, it is possible that electrical stimulation disrupts patterned activity in the system, which is known to play an especially important role in the affective-motivational dimension of pain.

The bee venom test: a new tonic-pain test

The present study describes a new test of tonic pain to be used as an animal model of persistent pain. First, pain responses and edema produced by subcutaneous injection of increasing doses of honey bee venom into the hind paw of the rat were quantified. Second, the effect of morphine and aspirin on the pain responses was investigated. Finally, the response to concurrent injections of bee venom and formalin was examined. Subcutaneous injection of bee venom produced local inflammation, tonic-pain responses lasting from 10 min to more than 1 h, and marked edema lasting from 3 h to more than 48 h. Increasing doses of bee venom produced higher mean pain scores and increased durations of responding. The time course of the edema did not follow the time course of the pain responses. Analgesia was produced by morphine and aspirin, indicating that the bee venom test can be used to test analgesic drugs. Concurrent administration of bee venom and formalin produced pain responses similar to formalin alone, with a less profound interphase depression and a longer duration. The data suggest that the bee venom test is a valid animal model of experimental tonic pain.

Restraint reduces formalin-test pain but the effect is not influenced by lesions of the hypothalamic paraventricular nucleus

Previous research indicates that the paraventricular nucleus of the hypothalamus (PVN) plays an important role in the development of stress-induced analgesia (SIA). Research implicating the PVN in SIA has generally employed the cold-water swim as the stressor and a phasic pain test, such as the tail-flick test, as the pain model. The present study, using the formalin test for tonic pain, investigated the effect of PVN lesions on (1) tonic pain responses and (2) SIA caused by 30 min of restraint. Male Long-Evans rats were randomly assigned to one of four groups. Two groups received electrolytic lesions of the PVN and two additional groups served as sham-operated controls. One group which received PVN lesions and one group which was sham-operated were exposed to 30 min of restraint immediately prior to a 0.05-ml injection of 2.5% formalin into the planter surface of one hindpaw. The remaining groups which either received PVN lesions or were sham-operated received the formalin injection without prior exposure to restraint. During the first phase of the formalin response, PVN lesions did not alter duration of paw elevation scores, but significantly increased duration of paw licking scores. A 30-min period of restraint had no effect on duration of paw elevation scores, but significantly decreased duration of paw licking scores. PVN lesions did not alter the significant decrease in paw licking scores as a result of restraint. During the second phase of the formalin response, PVN lesions did not alter either the duration of paw elevation scores or the duration of paw licking scores. A 30-min period of restraint significantly decreased duration of paw elevation scores, but had no effect on duration of paw licking scores. PVN lesions did not alter the significant decrease in paw elevation scores as a result of restraint. The results indicate that PVN lesions increase paw licking only during the first phase of the formalin response, with no other alterations in paw licking or duration of paw elevation. In addition, a 30-min period of restraint can produce short-term and long-term SIA for tonic pain. The short-term SIA is reflected as a decrease in paw licking, whereas the long-term SIA is reflected as a decrease in paw elevation. In addition, PVN lesions failed to alter SIA during both phases of the formalin test. The differential effect of restraint on pain responses during the two phases of the formalin test and the lack of effect of PVN lesions on SIA for tonic pain suggest that stress engages multiple endogenous pain inhibitory systems.

Properties of complex hallucinations associated with deficits in vision

Complex visual hallucinations are known to occur in individuals with impaired vision yet whose emotional and intellectual functions are within the normal range. These hallucinations, which were first reported by Charles Bonnet in 1760, have been described in many case studies, but have not been analyzed empirically to determine their major properties. In the present study, sixty complex hallucinators labeled as Charles Bonnet hallucinators were administered a questionnaire to determine the properties of their hallucinations. Combined use of multiple-correspondence analysis and hierarchical cluster analysis reveals a set of features that characterize the 'typical' Charles Bonnet hallucinatory experience: the hallucinators' experience occurs while they are alert and with the eyelids open; a sharply focused image appears suddenly, without any apparent trigger or voluntary control; the hallucination is present for seconds, does not move during this time, then suddenly vanishes. These features are discussed in terms of a 'dimension' of hallucinatory/perceptual experience, which ranges from discrete perceptual experiences to multiple, changing experiences. Possible mechanisms that underlie the Charles Bonnet hallucinations are discussed.

Delayed nociceptive response following cold-water swim in the formalin test: possible mechanisms of action

Exposure of animals to aversive events produces stress-induced analgesia. A common method of producing stress in animals is the cold-water swim (CWS). The present series of experiments examines the effect of CWS on tonic pain, as measured by the formalin test, and explores possible mechanisms of action. Experiment 1 demonstrates that a 3.5-min swim in 2 degrees C water produces a delayed nociceptive response (DNR), characterized by a prolonged period of no formalin responding which then begins and continues during the time when control animals, which have not received the CWS, are finished responding. The delayed response begins at 50-60 min postformalin injection, peaks at 80 min, and is still present at 120 min. Experiment 2 indicates that paw temperature effects are not responsible for the DNR, although core body temperature effects are a possible mechanism. However, systematic delays in the formalin injection following the CWS (Experiment 3) drastically altered the DNR even though core body temperature remained unchanged, suggesting that a decrease of core body temperature is insufficient to account for the DNR. Experiment 4 demonstrates that the NMDA antagonist MK-801 administered prior to the CWS dramatically reduces the DNR. The present experiment is the first study that reports a delay as long as 60 min in pain responding. It is concluded that the delayed response to formalin injection is the result of complex interactions involving peripheral mechanisms and central neuronal plasticity in which activity initiated by a noxious input persists after the cessation of the input as a consequence of a stressful event such as the cold-water swim.

Positive intrasurgical suggestion fails to affect postsurgical pain

The present study explored the effect of positive intrasurgical suggestion during the anesthetic state on postsurgical pain. One-half of the patients who were undergoing elective cholecystectomy or hysterectomy received strong positive intrasurgical suggestion directed specifically towards reducing pain. The control patients received information about pain without suggestion content. There was no effect on postsurgical pain measured by the McGill Pain Questionnaire and a visual analogue scale. The lack of effect on postsurgical pain indicates that intrasurgical suggestion does not provide a therapeutic method to achieve pain control.

Hypophysectomy produces analgesia and paraventricular lesions have no effect on formalin-induced pain

Chemical destruction of the pituitary gland has been shown to alleviate severe cancer pain in a substantial proportion of patients. The underlying mechanisms, however, remain controversial. The present study investigated the effects of hypophysectomy in the formalin test, which provides an animal model of tonic pain, and attempted to determine a possible neural mechanism to explain the effectiveness of the procedure. Hypophysectomized rats displayed significantly less pain behavior in the formalin test than control rats that underwent a sham hypophysectomy, implicating the pituitary gland in the modulation of tonic pain. Since the paraventricular nucleus of the hypothalamus (PVN) provides a major source of input to the pituitary gland, the effects of electrolytic lesions of the PVN on tonic pain were examined. The results failed to show a significant effect of PVN lesions on pain responses in the formalin test. The results suggest that the pituitary gland modulates tonic pain in the formalin test and that the test provides an animal model to study possible mechanisms which underlie the relief of severe cancer pain by hypophysectomy. However, since PVN lesions did not affect the response of rats in the formalin test, it is possible that the PVN is only one of multiple interacting neural and endocrine structures that influence the functions of the pituitary.

Analgesia induced by morphine microinjection into the lateral hypothalamus of the rat

Earlier studies have demonstrated that electrical stimulation of the lateral hypothalamus (LH) produces analgesia in tests of brief, phasic pain and more prolonged tonic pain. The present experiment examined the effect of morphine microinjection into the LH on formalin-induced pain. The results show that a 10-micrograms morphine microinjection significantly decreases pain responses. Moreover, the decrease persists for the duration of the 25-min test period, which began 25 min after the formalin injection. These findings suggest that electrical stimulation of the LH engages an endogenous opiate system which is capable of producing prolonged analgesia.

Phantom limb experiences in congenital limb-deficient adults

This case report describes phantom limbs in four adults with congenital limb deficiency. Case 1, with congenital absence of the left arm below the elbow, received a minor injury of the stump at age 16 and subsequently developed a full-length phantom arm, hand and fingers. Cases 2 and 3, at the ages of 26 and 30, underwent minor surgery of the lower limbs and subsequently developed vivid, full-length phantoms of their legs. Case 4, aged 31, was born with a shortened and deformed right leg, and underwent amputation of the foot and deformed part of the leg at age 3. Beginning at age 6, she developed a full-length phantom leg and foot that touched the floor, which persisted for more than 20 years. The descriptions by these four adults lend credibility to earlier reports of phantom limbs in congenital limb-deficient children. They also indicate that these phantoms may persist into adulthood.

Pain: past, present and future

Descartes' concept that pain is produced by a direct, straight-through transmission system from injured tissues in the body to a pain centre in the brain has dominated pain research and therapy until recently. The gate control theory of pain, published in 1965, proposes that a mechanism in the dorsal horns of the spinal cord acts like a gate which inhibits or facilitates transmission from the body to the brain on the basis of the diameters of the active peripheral fibers as well as the dynamic action of brain processes. As a result, psychological variables such as past experience, attention and other cognitive activities have been integrated into current research and therapy on pain processes. The gate control theory, however, is not able to explain several chronic pain problems, such as phantom limb pain, which require a greater understanding of brain mechanisms. A new theory of brain function, together with recent research that has derived from it, are described. They throw light on complex pain problems and have important implications for basic assumptions in psychology.

Visual hallucinations and mental state. A study of 14 Charles Bonnet syndrome hallucinators

Complex visual hallucinations are usually a sign of acute psychopathology or gross cognitive impairment, but may also occur in people with visual deficits--the Charles Bonnet Syndrome. The mental state of 14 Charles Bonnet hallucinators was assessed using four psychological tests: the Beck Depression Inventory, the State-Trait Anxiety Inventory, the Mini-Mult, and the Mini-Mental State Examination. Results are consistent with earlier reports suggesting that these hallucinations are not due to psychopathology or compromised cognitive functioning. It is proposed that these complex visual hallucinations represent ongoing neural activity in the visual system following eye damage.

The habenula and pain: repeated electrical stimulation produces prolonged analgesia but lesions have no effect on formalin pain or morphine analgesia

Recent studies have found that electrical stimulation of the habenula or microinjection of morphine into it reduces pain in several pain tests. The present study explored additional properties of the habenula. Expt. 1 examined the influence of the duration of stimulation on the duration of poststimulation analgesia in the formalin test. Expt. 2 was carried out to determine whether destruction of the habenula would affect either baseline pain levels or analgesia produced by morphine administered systemically in the formalin test. The results showed that the duration of analgesia is related to the duration of electrical stimulation. However, habenular lesions did not affect baseline pain levels or morphine analgesia. These studies support earlier evidence that manipulation of the habenula can produce analgesia, but suggest that it is not tonically active in modulating pain or necessary for the analgesic effects of systemically administered morphine.

The role of NMDA receptor-operated calcium channels in persistent nociception after formalin-induced tissue injury

The contribution of intracellular calcium to central sensitization and persistent nociception in response to tissue injury in rats was examined following the subcutaneous injection of formalin into the hindpaw. Formalin injury-induced nociceptive behaviors were enhanced by intrathecal pretreatment with the calcium ionophore A23187 or the calcium channel agonist Bay-K8644. Conversely, formalin nociceptive responses were reduced by intrathecal pretreatment with the calcium chelator Quin 2 or the calcium channel antagonists verapamil and nifedipine. Each of these agents affected the tonic, but not the acute, phase of the formalin response. The enhancement in formalin nociceptive behavior in rats treated with L-aspartate or L-glutamate was reversed by combined pretreatment with the noncompetitive NMDA antagonist MK-801, but not by nifedipine or the non-NMDA excitatory amino acid antagonist 6-cyano-7-dinitroquinoxaline-2,3-dione. In rats not treated with excitatory amino acids, the analgesic effect of MK-801 was also significantly greater than that produced by nifedipine. Furthermore, combining nifedipine with MK-801 did not produce a significantly greater analgesic effect than MK-801 alone. The results suggest that central sensitization and persistent nociception following formalin-induced tissue injury are dependent on the influx of calcium through predominantly NMDA receptor-operated (and to a lesser extent voltage-gated) calcium channels.

The contribution of excitatory amino acids to central sensitization and persistent nociception after formalin-induced tissue injury

The contribution of excitatory amino acids (EAAs) to the development of central sensitization and persistent nociception in response to tissue injury in rats was examined following the subcutaneous injection of formalin into the hindpaw. Formalin-induced nociceptive behaviors were enhanced by intrathecal pretreatment with the EAAs L-glutamate and L-aspartate. An enhancement of the formalin nociceptive response was also produced by intrathecal pretreatment with the receptor-selective EAA agonists NMDA and trans-(+/- )-1-amino-1,3-cyclopentane dicarboxylic acid (ACPD), but not (R,S)-alpha-amino-3-hydroxy-5-methylisozazole-4-propionic acid hydrobromide (AMPA). The effect of NMDA was enhanced by a combined administration with AMPA or APCD. Formalin nociceptive responses were dose-dependently reduced by intrathecal pretreatment with the NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (APV) and (+)-MK-801 hydrogen maleate, but not the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione or the selective metabotropic EAA receptor antagonist 2-amino-3-phosphonopropionic acid. The results suggest that EAAs acting at the NMDA receptor contribute to central sensitization and persistent nociception following subcutaneous formalin injection.

Multiple phantom limbs in a child

This case report describes multiple phantom feet in a child after amputation of a leg. The subject is a 16-year-old girl who was born with a right leg 10 cm shorter than the left and who at the age of 6 was amputated below the right knee so that she could wear a prosthesis that would give her normal mobility. The girl reports that she subsequently experienced 2 phantom feet and 3 sets of phantom toes which have persisted to the present time. Each phantom has a distinct size, length and position in relation to the others and each is also the site of vivid sensations such as heat, tickle, and fatigue as well as voluntary and involuntary movement. She also describes sensations that resemble sensations experienced before the amputation: one of her phantom feet feels flat and locked into a forward position which corresponds with the actual shape and position of her congenitally deformed amputated foot. The implications of multiple phantoms are discussed with reference to recent concepts of phantom limbs.

Analgesic and aversive effects of naloxone in BALB/c mice

Opioid antagonists have been shown to produce dose-dependent analgesia in the formalin test in BALB/c mice. In light of this paradoxical finding, the motivational-affective property of naloxone was examined in BALB/c mice. Naloxone produced a conditioned place aversion at doses which were also found to produce analgesia in the formalin test (1 and 10 mg/kg). In addition, the analgesia produced by 1 mg/kg naloxone was completely abolished in mice pretreated with nor-binaltorphimine, a highly selective kappa-opioid antagonist. Norbinaltorphimine on its own, however, had no effect. These results suggest that the analgesic actions of naloxone may be due to an interaction with kappa receptors.

Phantom limb pain

A phantom limb is universally experienced after a limb has been amputated or its sensory roots have been destroyed. A complete break of the spinal cord also often leads to a phantom body below the level of the break. Furthermore, phantom breasts, genitals and other body areas occur in a substantial number of people after surgical removal or denervation of the body part. The most astonishing feature of a phantom limb (or other body area) is its incredible "reality" to the person. An examination of phantom limb phenomena has led to a new theory. It is proposed that we are born with a widespread neural network--the "neuromatrix"--for the body-self, which is subsequently modified by experience. The neuromatrix imparts a pattern--the "neurosignature"--on all inputs from the body, so that experiences of one's own body have a quality of self and are imbued with affective tone and cognitive meaning. The theory is presented with supporting evidence as well as implications for research.

Amitriptyline produces analgesia in the formalin pain test

A dose of 20 mg/kg of amitriptyline reduced pain in the second phase of the formalin test, which is an animal model of long-lasting pain in humans. Since the analgesic effect was produced by a single dose, which is insufficient to produce an antidepressant effect, these results indicate that amitriptyline has analgesic properties that are independent of its antidepressant properties.

The Charles Bonnet syndrome: 'phantom visual images'

The Charles Bonnet syndrome is a condition in which individuals experience complex visual hallucinations without demonstrable psychopathology or disturbance of normal consciousness. An analysis of the sixty-four cases described in the literature reveals that the syndrome can occur at any age though it is more common in elderly people. Reduction in vision, due to peripheral eye pathology as well as pathology within the brain, is associated with the syndrome. Individual hallucinatory episodes can last from a few seconds to most of the day. Episodes can occur for periods of time ranging from days to years, with the hallucinations changing both in frequency and in complexity during this time. The hallucinations may be triggered or stopped by a number of factors which may exert their effect through a general arousal mechanism. People, animals, buildings, and scenery are reported most often. These images may appear static, moving in the visual field, or animated. Emotional reaction to the hallucinations may be positive or negative. Several theories have been proposed to account for the hallucinations. This paper highlights the sensory deprivation framework, with particular emphasis on the activity in the visual system after sensory loss that produces patterns of nerve impulses that, in turn, give rise to visual experience.

Labor pain: effect of maternal position on front and back pain

The purpose of this study was to determine whether women in labor report less pain when they are in a vertical (sitting or standing) position than in a horizontal (side-lying or supine) position. Pain scores were obtained from 60 women in early labor (dilation 2-5 cm) who alternated between the two positions. The results show that about 35% of women feel less front pain and 50% feel less back pain when they are in a vertical position than in a horizontal position. The decrease in continuous back pain (83%) was particularly impressive, but the front and back pains associated with contractions were significantly diminished as well. These results, taken together with those of earlier studies, indicate that many women in early labor have less pain and are generally more comfortable in a vertical than in a horizontal position. Since early labor comprises a substantial proportion of the entire process of labor and delivery, any simple procedure which alleviates pain without danger to mother or child, such as shifting from a horizontal to a vertical position, should be promoted and employed.

Injury prior to neurectomy alters the pattern of autotomy in rats. Behavioral evidence of central neural plasticity

A common property of phantom limb pain is that a preamputation lesion continues to be felt in the same location of the phantom limb after amputation. A model of the phantom limb in the rat is provided by sectioning the sciatic and saphenous nerves. This procedure leads to self-mutilation of the denervated hindpaw, a behavior known as autotomy. There is strong evidence that autotomy is a response to painful or dysesthetic sensations referred to the anesthetic limb. The present study examined the hypothesis that the site of autotomy behavior can be altered by an injury given prior to denervation. Experiment 1 evaluated the effects of a thermal injury applied (under sodium pentobarbital anesthesia) to the medial or lateral hindpaw and digits before or after sciatic and saphenous nerve transections in order to determine whether autotomy is directed specifically to a previously injured site. The results revealed that autotomy onset occurred in the injured region of the paw in a significantly greater proportion of rats, compared to an uninjured control group, if the thermal injury had been induced before denervation: 100% of the rats with medial paw injury induced prior to neurectomy initiated autotomy in the medial digits, and 55.6% of rats with lateral paw injury initiated autotomy in the lateral digits. Rats injured after autotomy showed no such preference (medial, 33.3% and lateral, 37.5%) relative to the uninjured controls (medial, 33.3% and lateral, 17%). These results suggest that central cells, sensitized by the thermal injury, contribute to enhanced autotomy in the absence of further inputs from the injured paw.(ABSTRACT TRUNCATED AT 250 WORDS)

Pain and paresthesia in patients with healed burns: an exploratory study

The present study was designed to examine the prevalence and characteristics of painful and paresthetic sensations in a group of patients with healed burns. Adult patients who had been hospitalized for burn injuries during a 7-yr period were contacted and given a structured interview that included a series of questions about their present condition. Patients' medical charts were reviewed to obtain relevant demographic and medical information. The results show that abnormal sensations in healed burns are frequently reported as long as several years after the injury. Of 104 patients interviewed 1 yr or more after a burn injury, 82% reported paresthetic sensations such as tingling, stiffness, cold sensations, and numbness; and 35% complained of pain in the scarred tissue. The prevalence of these sensations was not related to age, sex, or etiology of the burns, but was associated with burn size and skin grafting. The theoretical and clinical implications of these results are discussed with particular emphasis on the need to pursue research on the long-term adverse effects of burn injuries.

Central neural mediators of secondary hyperalgesia following heat injury in rats: neuropeptides and excitatory amino acids

The contribution of C-fiber neuropeptides and excitatory amino acids (EAAs) as central mediators of secondary hyperalgesia was assessed by examining the effects of intrathecal (i.t.) administration of receptor-selective agonists and antagonists on foot-withdrawal latencies (from 48 degrees C water), both before and after heat injury of the contralateral hindpaw. The hyperalgesia which develops in the hindpaw contralateral to a heat injury, could be reproduced in uninjured rats following i.t. injection of substance P, neurokinin A and N-methyl-D-aspartate (NMDA) but not following calcitonin gene related peptide (CGRP), neurokinin B, kainate or (R,S)-alpha-amino-3-hydroxy-5-methylisozazole-4-propionic acid hydrobromide (AMPA). Contralateral hyperalgesia was reversed by the substance P antagonist Arg1,D-Pro2,D-Phe2-D-His9-substance P, and the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (APV), but not by the non-NMDA EAA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). When the limb of the injured hindpaw was pretreated with the C-fiber neurotoxin capsaicin, hyperalgesia in the contralateral hindpaw was unaffected. Furthermore, prior to injury, the capsaicin pretreatment itself produced hyperalgesia in the contralateral hindpaw. The results give support for a contribution of both C-fiber neuropeptides and EAAs to central nervous system plasticity and secondary hyperalgesia following heat injury.

Sensory and pharmacological modulation of pain

Traditional neurosurgical methods to control pain have been replaced by a variety of modulation techniques. The major types of modulation are pharmacological, physical, and psychological. Important advances are occurring in the development of all three modulation approaches.

Auricular transcutaneous electrical nerve stimulation (TENS) reduces phantom limb pain

The present paper evaluates the efficacy of low frequency, high intensity auricular transcutaneous electrical nerve stimulation (TENS) for the relief of phantom limb pain. Auricular TENS was compared with a no-stimulation placebo condition using a controlled crossover design in a group of amputees with (1) phantom limb pain (Group PLP), (2) nonpainful phantom limb sensations (Group PLS), and (3) no phantom limb at all (Group No PL). Small, but significant, reductions in the intensity of nonpainful phantom limb sensations were found for Group PLS during the TENS but not the placebo condition. In addition, 10 min after receiving auricular TENS, Group PLP demonstrated a modest, yet statistically significant decrease in pain as measured by the McGill Pain Questionnaire. Ratings of mood, sleepiness, and anxiety remained virtually unchanged across test occasions and sessions, indicating that the decrease in pain was not mediated by emotional factors. Further placebo-controlled trials of auricular TENS in patients with phantom limb pain are recommended in order to evaluate the importance of electrical stimulation parameters such as pulse width and rate, and to establish the duration of pain relief.

The role of the cingulum bundle in self-mutilation following peripheral neurectomy in the rat

Peripheral neurectomy in rats is followed by self-mutilation of the denervated zone (autotomy), which is assumed to represent an index of pain or dysesthesia associated with deafferentation. This study examines the role of the cingulum bundle in mediating autotomy behavior. Rats were given bilateral injections of 1 microliter of 0.5% bupivicaine (a local anesthetic) or saline into the cingulum bundle immediately prior to, and on Days 1, 7, 14, and 21 after, sectioning the sciatic and saphenous nerves. Bupivicaine injections into the cingulum produced a significant delay in the onset of self-mutilation and reduced the overall degree of autotomy. Furthermore, the delayed onset of autotomy exceeded the duration of the anesthetic block produced by bupivicaine.

Central nervous system plasticity in the tonic pain response to subcutaneous formalin injection

Evidence is presented which suggests that central neural changes occur during the brief early phase after subcutaneous formalin injection that are essential for the expression of pain during the long-lasting (tonic) later phase. First, tonic pain responses to subcutaneous formalin injections are abolished only if the injected hindpaw is locally anesthetized at the time of injection as well as the time of testing (30-60 min later). Second, tonic formalin pain is substantially reduced by brief spinal anesthesia given 5 min before, but not 5 min after the formalin injection.