Stress-induced analgesia in humans: endogenous opioids and naloxone-reversible depression of pain reflexes

Cigarette smoking, stress-induced analgesia and pain perception in men and women

This study examined gender differences in smoking-related analgesia and stress-induced analgesia (SIA), as a function of pain modality. Forty men (20 smokers, 20 nonsmokers) and 37 women (17 smokers) were tested twice for pain sensitivity to tourniquet ischemia, thermal heat, and cold pressor tests; once following mental stress and once following rest control, counterbalancing order. Cardiovascular and neuroendocrine responses to mental stress were also examined. While expected gender differences in pain sensitivity were observed, women smokers had greater threshold and tolerance times to ischemic pain than women nonsmokers (P<0.05) when pain testing followed rest. Male smokers had greater threshold and tolerance to cold pressor pain than male nonsmokers (P<0.05) after both rest and stress. Only women showed evidence for SIA, since women nonsmokers demonstrated greater ischemic pain threshold and tolerance following mental stress versus rest (P<0.05), and all women reported lower thermal heat pain unpleasantness after stress versus rest (P=0.05). Only nonsmokers showed expected inverse relationships between sympathetic and hypothalamic-pituitary-adrenal (HPA) axis reactivity measures and sensitivity to pain. Smokers showed evidence for blunted HPA-axis function at rest and stress. These results indicate that analgesia related to both being a smoker and stress is influenced by gender and pain modality. The reduced pain perception in smokers and absence of relationships between endogenous pain regulatory mechanisms and pain sensitivity may reflect a maladaptive response to chronic smoking.

Differential nociceptive deficits in patients with borderline personality disorder and self-injurious behavior: laser-evoked potentials, spatial discrimination of noxious stimuli, and pain ratings

Approximately 70-80% of women meeting criteria for borderline personality disorder (BPD) report attenuated pain perception or analgesia during non-suicidal, intentional self-mutilation. The aim of this study was to use laser-evoked potentials (LEPs) and psychophysical methods to differentiate the factors that may underlie this analgesic state. Ten unmedicated female patients with BPD (according to DSM-IV) and 14 healthy female control subjects were investigated using brief radiant heat pulses generated by a thulium laser and five-channel LEP recording. Heat pulses were applied as part of a spatial discrimination task (two levels of difficulty) and during a mental arithmetic task. BPD patients had significantly higher heat pain thresholds (23%) and lower pain ratings (67%) than control subjects. Nevertheless, LEP amplitudes were either normal (N1, P2, P3) or moderately enhanced in BPD patients (N2). LEP latencies and task performance did not differ between patients and control subjects. The P3 amplitudes, the vertex potential (N2-P2), and the N1, which is generated near the secondary somatosensory cortex, were significantly reduced during distraction by mental arithmetic in both groups. In addition, P3 amplitudes reflected task difficulty. This study confirms previous findings of attenuated pain perception in BPD. Normal nociceptive discrimination task performance, normal LEPs, and normal P3 potentials indicate that this attenuation is neither related to a general impairment of the sensory-discriminative component of pain, nor to hyperactive descending inhibition, nor to attention deficits. These findings suggest that hypoalgesia in BPD may primarily be due to altered intracortical processing similar to certain meditative states.

Mental stress inhibits pain perception and heart rate variability but not a nociceptive withdrawal reflex

AIM

Do distraction from- or attention to sural nerve stimulation affect pain, heart rate variability, and a spinal withdrawal reflex?

MATERIAL AND METHODS

In 26 male volunteers, electrical stimulation at the distal cutaneous receptive field of the sural nerve elicited pain and a nociceptive withdrawal reflex. Intensity of pain was rated on a numeric rating scale. Electromyographic reflex responses were measured from biceps femoris muscle. Cardiac autonomic function was estimated by heart rate variability measures and was expressed in the time domain as mean of RR-intervals for normal heart beats (mean-RR) and standard deviation of all normal RR-intervals (SD-NN) and, in the frequency domain, where pure vagal activity was assessed by high frequency power (0.15-0.4 Hz). Low frequency power (0.04-0.15 Hz) reflects both parasympathetic and sympathetic control. Effect parameters were recorded before and during random distraction and attention. Distraction from sural nerve stimulation was induced by a mental arithmetic test, paced auditory serial addition task (PASAT), while attention was induced by concentrating on painful foot stimulation.

RESULTS

Paced auditory serial addition task decreased mean-RR and SD-NN, frequency domain parameters, as well as pain (P<0.001). In contrast, PASAT did not change the spinal withdrawal reflex. Attention did not affect any effect parameter.

CONCLUSION

Distraction by PASAT altered autonomic activity and inhibited pain but failed to affect withdrawal reflex responses, while attention had no effect on either parameter. Psychological distraction and attention may have different effects on noxious evoked pain perception and autonomic activity. Pain relief during PASAT probably involves supraspinal mechanisms.

Cerebral perturbations provoked by prolonged exercise

This review addresses cerebral metabolic and neurohumoral alterations during prolonged exercise in humans with special focus on associations with fatigue. Global energy turnover in the brain is unaltered by the transition from rest to moderately intense exercise, apparently because exercise-induced activation of some brain regions including cortical motor areas is compensated for by reduced activity in other regions of the brain. However, strenuous exercise is associated with cerebral metabolic and neurohumoral alterations that may relate to central fatigue. Fatigue should be acknowledged as a complex phenomenon influenced by both peripheral and central factors. However, failure to drive the motorneurons adequately as a consequence of neurophysiological alterations seems to play a dominant role under some circumstances. During exercise with hyperthermia excessive accumulation of heat in the brain due to impeded heat removal by the cerebral circulation may elevate the brain temperature to >40 degrees C and impair the ability to sustain maximal motor activation. Also, when prolonged exercise results in hypoglycaemia, perceived exertion increases at the same time as the cerebral glucose uptake becomes low, and centrally mediated fatigue appears to arise as the cerebral energy turnover becomes restricted by the availability of substrates for the brain. Changes in serotonergic activity, inhibitory feed-back from the exercising muscles, elevated ammonia levels, and alterations in regional dopaminergic activity may also contribute to the impaired voluntary activation of the motorneurons after prolonged and strenuous exercise. Furthermore, central fatigue may involve depletion of cerebral glycogen stores, as signified by the observation that following exhaustive exercise the cerebral glucose uptake increases out of proportion to that of oxygen. In summary, prolonged exercise may induce homeostatic disturbances within the central nervous system (CNS) that subsequently attenuates motor activation. Therefore, strenuous exercise is a challenge not only to the cardiorespiratory and locomotive systems but also to the brain.

Sex differences in thermal nociception and prostaglandin-induced thermal hypersensitivity in rhesus monkeys

The present study examined thermal nociception in 4 male and 4 female rhesus monkeys. In the first experiment, monkeys were tested 5 days/week for 4 consecutive weeks in a warm-water, tail-withdrawal assay of thermal nociception. Thermal nociception did not vary by sex or menstrual cycle phase. However, male monkeys tended to be slightly more sensitive to thermal stimuli than female monkeys in the follicular phase of the menstrual cycle. Thermal nociception did not correlate with estradiol or progesterone levels in female monkeys. In the second experiment, thermal hypersensitivity was induced by administering prostaglandin E(2) (0.0032 to 0.1 mg subcutaneously) into the tail. Prostaglandin E(2) produced slightly greater thermal hypersensitivity in male than in follicular phase female monkeys, but male and luteal phase female monkeys did not differ, and there was not a significant difference between follicular and luteal phase female monkeys. Exposure to the behavioral procedures produced similar increases in blood levels of the stress-related hormones adrenocorticotropic hormone and cortisol in male and female monkeys, which suggests that measures of thermal nociception or thermal hypersensitivity were not confounded by sex differences in stress responses. These results suggest that sex and gonadal hormone levels have only a minor influence on thermal nociception or thermal hypersensitivity in rhesus monkeys.

PERSPECTIVE

These modest effects of sex and gonadal hormone levels on measures of pain in non-human primates could be interpreted to support the hypothesis that sex differences in pain perception in humans are due more to sociocultural factors than to a biological imperative.

Endogenous Opioids and Chronic Pain Intensity

OBJECTIVE

To test whether endogenous opioid antinociceptive system dysfunction evidenced in response to acute pain stimuli is associated with increased clinical pain intensity in chronic pain sufferers, and to determine whether this association is moderated by disability level.

DESIGN

A double-blind, placebo-controlled, randomized crossover design. Subjects underwent laboratory acute finger pressure pain stimulation and ischemic pain stimulation under placebo and under opioid blockade with naloxone. The primary independent measures, reflecting degree of endogenous opioid antinociception, were opioid Blockade Effects derived to reflect the change elicited by naloxone in pain intensity ratings for the acute pain tasks. High and Low Disability groups were derived based on Pain Disability Index scores to allow examination of the influence of disability level on the relationship between Blockade Effects and chronic pain intensity.

SUBJECTS

Twenty-eight chronic low back pain sufferers.

OUTCOME MEASURE

Seven-day diary ratings of overall chronic pain intensity based on McGill Pain Questionnaire-Short Form total scores.

RESULTS

Greater daily chronic pain intensity was associated with greater placebo acute pain sensitivity in the laboratory (P < 0.05). Positive Blockade Effects (ie, presence of opioid analgesia) were associated as expected with lower placebo-condition acute pain sensitivity in the laboratory (P < 0.05). In main effects analyses, Blockade Effects were not associated significantly with daily chronic pain intensity. This absence of overall main effects was accounted for by significant opposing interactions between disability level and Blockade Effects (P < 0.05). Negative Blockade Effects (ie, absence of endogenous opioid analgesia to acute pain) in the High Disability group were associated with greater daily chronic pain intensity, consistent with the hypothesized effects of chronic pain-related opioid dysfunction. In contrast, Positive Blockade Effects (ie, effective opioid analgesia to acute pain) were associated with higher daily chronic pain intensity in the Low Disability group.

CONCLUSIONS

These results suggest that endogenous opioid antinociceptive system dysfunction may contribute to elevated acute and chronic pain sensitivity among more disabled chronic pain patients. Among less disabled patients, chronic pain may serve as a primer producing up-regulated opioid antinociceptive responses to acute pain

Noise stress and human pain thresholds: divergent effects in men and women

Considerable animal research suggests that exposure to noxious and nonnoxious fear-inducing stimuli can produce hypoalgesia. Although this effect is thought to generalize across species, only a few studies have examined the pain modulatory effects of nonnoxious fear-eliciting stimuli in humans. The present study examined whether exposure to a series of loud noise bursts would produce a fear-related hypoalgesia in male and female human subjects. Both subjective and physiologic measures (skin conductance level, heart rate) indicated that noise exposure resulted in fear, sympathetic arousal, and decreased pain reactivity in women (n = 20). In contrast, men (n = 20) did not experience fear or physiologic arousal, but reacted with surprise and increased pain reactivity. These findings provide additional evidence that hypoalgesia is mediated by fear and physiologic arousal. Although future studies should directly manipulate surprise, it appears that surprise without fear and physiologic arousal might enhance pain processing.

Sex-dependent components of the analgesia produced by athletic competition

Competing in various athletic events (track meet, basketball game, or fencing match) can produce analgesia to cold pressor stimuli in male and female college athletes compared with baseline assessments. This competition-induced analgesia has been attributed to the stress associated with competition, which has components related to both physical exercise and the cognitive aspects of competing. This study evaluated the analgesic effect of exercise-related stress, and that caused by the cognitively stressful components of competing independent of exercise. Cold pressor pain ratings were assessed after competition in a track meet and after treadmill exercise or sedentary video game competition in both athletes and nonathletes. As expected, competing in athletics resulted in a decrease in cold pressor ratings in both male and female athletes. Independent of athletic status, treadmill running induced analgesia in women, but not in males, whereas sedentary video game competition produced analgesia in men, but not in women. These findings suggest that different components of the competitive athletic experience might be responsible for the analgesic effects in a sex-dependent manner.

Differential effects of stress on escape and reflex responses to nociceptive thermal stimuli in the rat

Acute stress has been shown to increase latencies of nociceptive reflexes, and this effect is considered evidence for stress-induced analgesia. However, tests for nociception that rely on motivated operant escape assess cerebral processing of pain and could be modulated independent of reflex responses. We therefore compared the effects of an acute stressor (restraint) on escape responses and lick/guard reflexes to stimulation of the paws by a thermally regulated floor. Testing sessions included a pre-test exposure to 36 degrees C, followed by a test trial in which either escape from 44 or 36 degrees C or reflex responses to 44 degrees C were observed. Behavioral responses to stress were assessed during a three day period, with baseline testing on day 1, post-stress or control testing on day 2, and evaluation of long-term stress effects on day 3. On day 2, half the animals received 15 min of restraint stress, followed by 15-min pre-test and test trials. Licking and guarding responses to thermal stimulation during 44 degrees C test trials were significantly reduced by restraint stress, confirming previously reported stress effects on nociceptive reflexes. In contrast, learned escape responses to the same thermal stimulus were significantly enhanced after stress. The increase in operant sensitivity suggests that acute restraint, a form of psychological stress, produces hyperalgesia for a level of thermal stimulation that preferentially activates C nociceptors. These results are discussed in relation to studies involving physical or psychological forms of stress, different nociceptive stimuli, and assessment strategies used to evaluate thermal pain sensitivity.

Placebo and Nocebo responses, cortisol, and circulating beta-endorphin

OBJECTIVE

The experiment tested whether the placebo and nocebo responses could be mediated via modulation of stress.

METHODS

Ischemic pain was induced in healthy volunteers (N = 59). When pain reached "7" on a 10-point scale, two groups of subjects received information that a pain relieving (the Placebo group) or a pain increasing (the Nocebo group) substance was injected. All injections contained physiological saline. A third group received no information and no injection (the Natural History group). Pain ratings and blood samples for analysis of cortisol and beta-endorphin were obtained every 5 minutes after pain equal to seven until the experiment was terminated.

RESULTS

Pain increased in all groups, but there were significantly lower pain ratings in the Placebo group at 15 minutes after the injection, compared with the other two groups. Cortisol increased in all groups, but mostly so in the Nocebo group. Circulating beta-endorphin increased in all groups. Pain-ratings were not correlated with beta-endorphins or cortisol.

CONCLUSIONS

A placebo response, ie, a reduced pain level, was seen in the Placebo group at 15 minutes after the injection. The placebo response was not related to stress or to beta-endorphin. Expectation of a pain increase in the Nocebo group led to an increase in cortisol, but the expectation of pain increase and the resultant cortisol increase had no effect on pain.

Segmental and plurisegmental modulation of pressure pain thresholds during static muscle contractions in healthy individuals

The purpose of this study was to assess possible segmental (uni- and/or bilateral) and plurisegmental changes in pressure pain thresholds (PPTs) during static muscle contractions. Twenty-four healthy subjects (12 female, 12 male) performed a standardised isometric contraction with the dominant m. quadriceps femoris (MQF) and m. infraspinatus (MI), respectively. PPTs were assessed using pressure algometry at the contracting muscle, at the contralateral (resting) muscle and at a distant resting muscle (MI during contraction of MQF and vice versa). The PPT assessments were performed before, during and 30min. following each contraction. The contractions were held until exhaustion or for a maximum of 10 PPT assessments/muscle. During contraction of MQF PPTs increased compared to baseline at the middle ( p<0.001) and the end (p<0.001) of the contraction period at all assessed sites alike. During contraction of MI PPTs increased compared to baseline at the middle (p<0.001) and the end (p<0.007) of the contraction period at all sites. The increase was more pronounced at the contracting muscle compared to the contralateral (p<0.002; p<0.01) and the distant (p<0.002; p<0.002) site. No statistically significant difference was seen in PPTs between the latter two. Following the contractions PPTs returned to baseline. Submaximal isometric contraction of MQF and MI gave rise to a statistically significant increase in PPTs at the contracting muscle, the resting homologous contralateral muscle and at the distant resting muscle indicating that generalised pain inhibitory mechanisms were activated. Contraction of MI, but not of MQF, gave rise to an additional activation of unilateral segmental antinociceptive effects.

Different mechanisms of intrinsic pain inhibition in early and late inflammation

Neuroimmune interactions control pain through activation of opioid receptors on sensory nerves by immune-derived opioid peptides. Here we evaluate mechanisms of intrinsic pain inhibition at different stages of Freund's adjuvant-induced inflammation of the rat paw. We use immunohistochemistry and paw pressure testing. Our data show that in early (6 h) inflammation leukocyte-derived beta-endorphin, met-enkephalin and dynorphin A activate peripheral mu-, delta- and kappa-receptors to inhibit nociception. In addition, central opioid mechanisms seem to contribute significantly to this effect. At later stages (4 days), antinociception is exclusively produced by leukocyte-derived beta-endorphin acting at peripheral mu and delta receptors. Corticotropin-releasing hormone (CRH) is an endogenous trigger of these effects at both stages. These findings indicate that peripheral opioid mechanisms of pain inhibition gain functional relevance with the chronicity of inflammation.

Negative affect: effects on an evaluative measure of human pain

Prior work indicates that exposure to fear-inducing shock inhibits finger-withdrawal to radiant heat in humans (hypoalgesia), whereas anxiety induced by threat of shock enhances reactivity (hyperalgesia; Pain 84 (2000) 65-75). Although finger-withdrawal latencies are thought to reflect changes in pain sensitivity, additional measures of pain are needed to determine whether pain perception is altered. The present study examined the impact of negative affect on visual analog scale (VAS) ratings of fixed duration thermal stimuli. One hundred twenty-seven male and female human subjects were randomly assigned to one of three emotion-induction conditions: (1) negative affect induced by exposure to three brief shocks; (2) negative affect elicited by the threat of shock without presentation; and (3) neutral affect, with no intervention. VAS ratings were tested before and after emotion-induction. Results suggest that both negative affect manipulations reduced pain. Manipulation checks indicated that the emotion-induction treatments induced similar levels of fear but with different arousal levels. Potential mechanisms for affect induced changes in pain are discussed.

Pavlovian conditioning of opioid and nonopioid pain inhibitory mechanisms in humans

Learning processes such as respondent or Pavlovian conditioning are believed to play an important role in the development of chronic pain, however, their influence on the inhibition of pain has so far not been assessed in humans. The purpose of this study was the demonstration of Pavlovian conditioning of stress-induced analgesia in humans and the determination of its opioid mediation. In a differential classical conditioning paradigm two different auditory stimuli served as conditioned stimuli and mental arithmetic plus white noise as unconditioned stimulus. Subsequent to four conditioning trials naloxone or placebo was applied in a double-blind fashion on two test days. Both pain threshold and pain tolerance showed conditioned stress-induced analgesia. Pain tolerance was affected by naloxone whereas pain threshold was not. The data of this study show that stress analgesia can be conditioned in humans and that it is at least partially mediated by the endogenous opioid system. Learning processes also influence pain inhibitory processes in humans and this effect might play a role in the development of chronic pain.

Acupuncture mechanisms for clinically relevant long-term effects--reconsideration and a hypothesis

From the author's direct involvement in clinical research, the conclusion has been drawn that clinically relevant long-term pain relieving effects of acupuncture (>6 months) can be seen in a proportion of patients with nociceptive pain. The mechanisms behind such effects are considered in this paper. From the existing experimental data some important conclusions can be drawn: 1. Much of the animal research only represents short-term hypoalgesia probably induced by the mechanisms behind stress-induced analgesia (SIA) and the activation of diffuse noxious inhibitory control (DNIC). 2. Almost all experimental acupuncture research has been performed with electro-acupuncture (EA) even though therapeutic acupuncture is mostly gentle manual acupuncture (MA). 3. Most of the experimental human acupuncture pain threshold (PT) research shows only fast and very short-term hypoalgesia, and, importantly, PT elevation in humans does not predict the clinical outcome. 4. The effects of acupuncture may be divided into two main components--acupuncture analgesia and therapeutic acupuncture. A hypothesis on the mechanisms of therapeutic acupuncture will include: 1. Peripheral events that might improve tissue healing and give rise to local pain relief through axon reflexes, the release of neuropeptides with trophic effects, dichotomising nerve fibres and local endorphins. 2. Spinal mechanisms, for example, gate-control, long-term depression, propriospinal inhibition and the balance between long-term depression and long-term potentiation. 3. Supraspinal mechanisms through the descending pain inhibitory system, DNIC, the sympathetic nervous system and the HPA-axis. Is oxytocin also involved in the long-term effects? 4. Cortical, psychological, "placebo" mechanisms from counselling, reassurance and anxiety reduction.

A review of mechanisms of response to pain therapy: why voodoo works

A person's response to pain treatment may be due to factors other than the direct effect of analgesics. Amelioration of pain is often associated with modalities that appear to make no scientific sense. This review outlines the mechanisms of pain amelioration other than that by medication. These mechanisms help to explain why pain relief may follow the administration of allopathic modalities. Pain is modulated, enhanced, or diminished by both cerebral and peripheral mechanisms. Cerebral factors include the placebo response, psychological phenomena, and conscious cognitive activation. In addition to evoking endogenous opioids, these central mechanisms activate antinociceptive pathways beginning in the limbic forebrain and relayed in the periaqueductal gray matter to primary afferent nociceptive sites in the spinal cord dorsal horn (and medullary nucleus caudalis). Obvious peripheral factors that may diminish pain perception are those that decrease afferent stimuli. Paradoxically, stimulation of afferent neurons may also ameliorate pain by activating spinal or supraspinal inhibitory mechanisms. Finally, improvement in pain or other symptoms is often falsely attributed to a therapy when remission occurs because the underlying illness has run its normal course.

The nociceptive flexion reflex in humans -- review article

The nociceptive flexion reflex (NFR) is a physiological, polysynaptic reflex allowing for painful stimuli to activate an appropriate withdrawal response. NFR is easily measurable in clinical setting, and is a reliable and objective tool for measurement of an individual's pain experience. An exhaustive review of the literature, covering multiple search engines, indicates that the NFR method is valuable in studying the impact of diverse pharmacological and non-pharmacological interventions on the flexion reflex, in conditions of acute pain and in healthy volunteers. More recently, the NFR method has gained particular attention as a research tool in studies of central sensitization and persistent or chronic pain.

Nociceptive Sensitivity and Control: Hypo- and Hyperalgesia Under Two Different Modes of Coping

Abstract. Effects of perceived control on nociceptive sensitivity were investigated in an experimental arrangement with N = 40 healthy volunteers in which the duration of painful pressure stimuli was made contingent on success in a tracking task. Perceived control over the pain duration was manipulated through varying the frequency of success in the tracking task. The amount of painful stimulation applied in the high and low control conditions was balanced by a yoked-control design. Pain sensitivity was measured before and after the tracking task by means of a thermal sensory analyzer. Pain sensitivity was found to decrease in the low control condition (hypoalgesia), and to increase slightly in the high control condition (hyperalgesia). These effects are explained with reference to a dual process model of coping.

Pain and emotion: effects of affective picture modulation

OBJECTIVE AND METHODS

Two experiments examined the impact of viewing unpleasant, pleasant, and neutral photographic slides on cold-pain perception in healthy men and women. In each experiment, participants viewed one of three slide shows (experiment 1 = fear, disgust, or neutral; experiment 2 = erotic, nurturant, or neutral) immediately before a cold-pressor task. Skin conductance and heart rate were recorded during the slide shows, whereas visual analog scale ratings of pain intensity and unpleasantness thresholds and pain tolerance were recorded during the cold-pressor task.

RESULTS

Viewing fear and disgust slides decreased pain intensity and unpleasantness thresholds, but only the fear slides decreased pain tolerance. In contrast, viewing erotic, but not nurturant, slides increased pain intensity and unpleasantness threshold ratings on the visual analog scale in men, whereas neither nurturant nor erotic slides altered pain tolerance.

CONCLUSIONS

These results are consistent with a motivational priming model that predicts that unpleasant affective states should enhance pain and that pleasant affective states should attenuate it.

Expectations of analgesia do not affect spinal nociceptive R-III reflex activity: an experimental study into the mechanism of placebo-induced analgesia

The purpose of this study was to investigate whether placebo analgesia is mediated by the release of beta-endorphin. In addition to subjective pain reports, we included an objective physiological parameter of nociception reflected by the opioid sensitive nociceptive R-III reflex. Placebo consisted of strong suggestions of pain relief and an intravenous injection of saline. Forty minutes after placebo, either the opioid antagonist naloxone or saline was administered intravenously without subjects noticing (hidden). Sixty healthy males, aged 18-30 years, voluntarily participated in this study. Subjects were randomized into one of four groups: group 1 received placebo and hidden naloxone, group 2 received hidden naloxone only, group 3 received placebo and hidden saline and group 4 received hidden saline only. Pain was induced by electrical stimulation of the sural nerve and evaluated with a visual analogue scale (VAS). In addition, changes in the magnitude of the nociceptive R-III reflex activity were assessed. We determined to what extent R-III reflex activity and subjective pain reports were decreased by placebo and we investigated whether these placebo-induced changes in reflex activity and subjective pain reports were naloxone reversible. Furthermore, we measured the degree of association between pain relief as measured on VAS and changes in R-III reflex activity. Finally, the role of beta-endorphin was assessed by measuring plasma endorphin levels before and after the administration of placebo. This study could not demonstrate a placebo effect as measured on VAS and R-III responses. The administration of placebo did not appear to have an effect on the release of beta-endorphins. Consistently, the antagonizing effects of naloxone were negligible. A subgroup analysis of those who did show a placebo response as indicated on the VAS did not support the supposition that beta-endorphin is released due to placebo suggestion. It is suggested that intensified stimuli and a more effective procedure to induce placebo analgesia (e.g. conditioning) may produce a proper placebo effect.

Lack of pressure pain modulation by heterotopic noxious conditioning stimulation in patients with painful osteoarthritis before, but not following, surgical pain relief

To investigate the influence of chronic nociceptive pain on endogenous pain modulation, the effect of heterotopic noxious conditioning stimulation (HNCS) on perception of various somatosensory modalities was assessed in 15 patients with painful osteoarthritis of the hip. Thirteen patients were re-assessed when pain-free 6-14 months following surgery. Sex- and age matched healthy subjects assessed at similar time intervals served as controls. The effects of HNCS were tested using the upper extremity submaximal effort tourniquet test. Subjects rated tourniquet-induced pain intensity on a visual analogue scale (VAS). Quantitative sensory testing (QST) was performed contralaterally to the maximally painful area in 13 patients and contralaterally to the second most painful area in two patients (i.e. lateral thigh n = 12, frontal thigh n = 1, lateral calf n = 2). Sensibility was assessed before, during and 45 min following the tourniquet test. Perception thresholds to light touch were assessed using von Frey filaments and pressure pain thresholds by pressure algometry. Perception thresholds to non-painful and painful warmth and cold were determined using a Thermotest. In both sessions, patients rated the tourniquet-induced pain higher than controls at the start (P < 0.003 and P < 0.006, respectively), but not at the end of the tourniquet test. Decreased sensitivity to light touch (P < 0.001) and innocuous cold (P < 0.002) was seen during the tourniquet in patients and controls alike, on both occasions, while perception thresholds to innocuous warmth and heat pain remained unaffected. In the first session, pressure pain thresholds increased during the tourniquet test in controls (P < 0.002), but not in patients. In the second session, pressure pain thresholds increased during the tourniquet test in controls (P < 0.001) and in patients (P < 0.02). In conclusion, no pressure pain modulation was induced by HNCS in patients before surgery, as opposed to controls, suggesting a dysfunction in systems subserving 'diffuse noxious inhibitory controls' (DNIC). Normal pressure pain modulation induced by HNCS was seen when patients were re-assessed in a pain-free state following surgery, indicating that the dysfunction of DNIC had been maintained by chronic nociceptive pain.

Plasma norepinephrine levels, vagal tone index, and flexor reflex threshold in premature neonates receiving intravenous morphine during the postoperative period: a pilot study

OBJECTIVE

The goal of this study was to evaluate the effects of a single dose of intravenous morphine on postoperative pain in extremely premature neonates after thoracotomy.

DESIGN

Descriptive correlational study.

PATIENTS

Twenty-four critically ill mechanically ventilated premature neonates with a mean gestational age of 26.1 +/- 2.1 (SD) weeks and a postnatal age of 13.8 +/- 8.1 (SD) days.

OUTCOME MEASURES

Plasma norepinephrine (NE) levels, vagal tone index (VTI), and flexor reflex threshold were measured preoperatively, immediately before, and 20 and 60 minutes after the administration of the first postoperative dose of morphine (0.1 mg/kg).

RESULTS

One-way repeated-measures ANOVA revealed no significant change in plasma NE levels from baseline levels (df[2,32] = 2.40, p = 0.11). Pre- and postmorphine VTI values were significantly lower than preoperative values (df[3,60] = 6.04, p = 0.0012), but no significant differences were found between pre- and postmorphine VTI values. Neonates (n = 10) who had a flexor reflex response during the postoperative period demonstrated no significant differences in the force required to elicit a flexor reflex across the four measurements (df[3,27] = 0.76, p = 0.53); however, the flexor reflex responses were significantly less vigorous during the postoperative period than at baseline.

CONCLUSIONS

Findings from this pilot study suggest that the dose of morphine commonly used to treat postoperative pain in premature neonates does not affect NE, VTI, and flexor reflex threshold values within 1 hour of administration.

The pain inhibiting pain effect: an electrophysiological study in humans

This study examines the counterirritation phenomenon of experimental pain in human subjects. Phasic pain induced by intracutaneous electrical stimuli was simultaneously applied with tonic pain induced by ischemic muscle work. Pain ratings, spontaneous EEG and evoked potentials were measured. We found a significant reduction of phasic pain ratings during and 10 min after tonic pain. The late somatosensory evoked potentials as neurophysiological correlates of phasic pain sensation were attenuated until 20 min after tonic pain offset. The extent of phasic pain relief due to concomitant tonic pain was small but significant, comparable to the effect of a regular systemic dose of a narco-analgesic drug in this experimental pain model. On the other hand, no modulations in the late components of the auditory evoked potential and the power spectrum of the spontaneous EEG were observed. These variables reflect the attention and vigilance of the subject and are well-known to be affected by opioids. The only exception was an increase of beta power, which might reflect hyperarousal during tonic pain. These results support the suggestion, that the analgesic effect of heterotopic noxious stimulation in humans is based on the activation of a specific inhibitory pain control system. Systemic release of endogenous opioids is unlikely to be involved, because the typical effects of opioids on the EEG were not observed.

Fear and anxiety: divergent effects on human pain thresholds

Animal studies suggest that fear inhibits pain whereas anxiety enhances it; however it is unclear whether these effects generalize to humans. The present study examined the effects of experimentally induced fear and anxiety on radiant heat pain thresholds. Sixty male and female human subjects were randomly assigned to 1 of 3 emotion induction conditions: (1) fear, induced by exposure to three brief shocks; (2) anxiety, elicited by the threat of shock; (3) neutral, with no intervention. Pain thresholds were tested before and after emotion induction. Results suggest that findings from animal studies extend to humans: fear resulted in decreased pain reactivity, while anxiety led to increased reactivity. Pain rating data indicated that participants used consistent subjective criteria to indicate pain thresholds. Both subjective and physiological indicators (skin conductance level, heart rate) confirmed that the treatment conditions produced the targeted emotional states. These results support the view that emotional states modulate human pain reactivity.

Theoretical review: altered pain regulatory systems in chronic pain

This review synthesizes the existing literature regarding the relationship between resting blood pressure and pain sensitivity, and the literature indicating possible endogenous opioid dysfunction in chronic pain. Adaptive interactions between the cardiovascular and pain regulatory systems occur in healthy individuals, with greater blood pressure associated with decreased acute pain sensitivity. Endogenous opioids appear necessary for full expression of this relationship. There is ample evidence indicating diminished endogenous opioid CSF/plasma levels in chronic pain patients, yet little is known about the functional effects of these opioid changes. A theoretical model is proposed based upon the literature reviewed suggesting progressive dysfunction in endogenous opioid systems with increasing chronic pain duration. This dysfunction is hypothesized to result in dysregulation of normally adaptive relationships between the cardiovascular and pain regulatory systems, resulting in increased chronic pain intensity and increased acute pain sensitivity among chronic pain patients. Preliminary data are consistent with the hypothesis of progressive opioid changes resulting in dysfunctional alterations in the adaptive blood pressure-pain relationship. Clinical implications of this theory are discussed.

Clinical and theoretical issues in self-injurious behavior

This article presents an overview of pathological self-injurious behavior (SIB). Historical and cultural aspects, epidemiology, classification and clinical aspects and pathogenesis are described. The importance of comprehensive assessment of symptomatology and functions of SIB for treatment planning are discussed.

Effects of an opioid antagonist on pain intensity and withdrawal reflexes during induction of hypnotic analgesia in high- and low-hypnotizable volunteers

The aim of this investigation was to study the effect of suggestions of hypnotic analgesia on spinal pain transmission and processing. Pain intensity and amplitude of nociceptive withdrawal reflexes to electrical stimuli were measured in 10 high- and 10 low-hypnotizable subjects during two sessions taking place at least 24 h apart under five conditions of: (1) pre-hypnosis; (2) neutral hypnotic relaxation; (3) suggestions of hypnotic analgesia; (4) suggestions of hypnotic analgesia after injections of either naloxone (1 ml, 1 mg/ml) or saline (1 ml) under double-blinded conditions; and (5) post-hypnosis. The conditions of naloxone or saline were allocated at random to either Day 1 or Day 2 in a double-blinded fashion. Results showed significant reductions of pain intensity during hypnotic analgesia, and a significant reduction in nociceptive reflexes during hypnotic analgesia on Day 1 in the highly hypnotizable group. No differences were found for low-hypnotizable subjects. The results support previous findings that pain intensity as well as the nociceptive reflex can be modulated by suggestions of hypnotic analgesia. While no effect of naloxone on pain intensity was found during hypnotic analgesia, naloxone significantly reversed the suppressive effect of suggestions of hypnotic analgesia on reflexes in high-hypnotizable subjects. Subsequent analysis showed that the effect of naloxone was associated with the intensity of the stimulus needed to elicit a reflex, and was unrelated to hypnotic susceptibility when controlling for stimulus intensity. These results suggest that the effect of naloxone was related to the greater stimulus intensities needed to elicit a reflex in the high-hypnotizable group, rather than to hypnosis or hypnotic susceptibility in itself. It is unclear why greater stimulus intensities were needed in high-hypnotizable subjects and further studies are needed. Copyright 1998 European Federation of Chapters of the International Association for the Study of Pain.

Food cravings, endogenous opioid peptides, and food intake: a review

Extensive research indicates a strong relationship between endogenous opioid peptides (EOPs) and food intake. In the present paper, we propose that food cravings act as an intervening variable in this opioid-ingestion link. Specifically, we argue that altered EOP activity may elicit food cravings which in turn may influence food consumption. Correlational support for this opioidergic theory of food cravings is provided by examining various clinical conditions (e.g. pregnancy, menstruation, bulimia, stress, depression) which are associated with altered EOP levels, intensified food cravings, and increased food intake.

Objective evidence of decreased pain perception in normotensives at risk for hypertension

Results from laboratory and naturalistic studies have demonstrated decreased subjective pain ratings in hypertensives and individuals at risk for hypertension. Based on previous evidence that the nociceptive withdrawal reflex may provide an objective index of pain threshold in humans, the present study examined the intensity of sural nerve stimulation required to elicit nociceptive withdrawal in offspring of hypertensives and normotensives. Participants included 60 men and 56 women who were normotensive, 18-23 years of age, and predominately Caucasian. To assess the nociceptive withdrawal reflex, ascending and descending intensities of electrical stimulation were applied over the sural nerve while electromyographic activity was recorded from the ipsilateral biceps femoris muscle. Analyses of the intensity of electrical stimulation required to reach the thresholds for nociceptive withdrawal and subjective pain revealed a pattern of hypoalgesia in individuals at risk for hypertension. First, significantly higher intensities were required to elicit nociceptive withdrawal in offspring of hypertensives versus normotensives. Second, offspring of hypertensives endured significantly more intense stimulation before reporting pain. Third, both parental history of hypertension and resting systolic blood pressure were significant independent predictors of stimulation intensity at nociceptive withdrawal reflex and subjective pain thresholds. These results confirm and extend previous observations of an association between risk for hypertension and hypoalgesia, and suggest that hypoalgesia should be examined as a potential predictor of progressive blood pressure increases in individuals at risk for hypertension.

Cerebrospinal fluid and plasma beta-endorphin in combat veterans with post-traumatic stress disorder

Opioid-mediated analgesia develops in experimental animals following traumatic stress and increased opioid-mediated analgesia has been observed in combat veterans with post-traumatic stress disorder (PTSD). These observations have led to the hypothesis that increased central nervous system (CNS) opioidergic activity exists in patients with PTSD. However, direct CNS data on opioid peptide concentrations and dynamics in patients with PTSD are lacking. We withdrew cerebrospinal fluid (CSF) via a flexible, indwelling subarachnoid catheter over a 6-h period and determined hourly CSF concentrations of immunoreactive beta-endorphin (ir beta END) in 10 well-characterized combat veterans with PTSD and nine matched normal volunteers. Blood was simultaneously withdrawn to obtain plasma for ir beta END. PTSD symptom clusters, as measured by the CAPS, were correlated with neuroendocrine data. Mean CSF ir beta END was significantly greater in patients with PTSD compared with normals and there was a negative correlation between the ir beta END and PTSD intrusive and avoidant symptoms of PTSD. No intergroup difference between plasma ir beta END was found, nor was there a significant correlation between CSF and plasma ir beta END. Immunoreactive beta-lipotropin (ir beta LPH) and pro-opiomelanocortin (irPOMC), both precursors of beta END, were much more plentiful in human CSF than was beta-endorphin itself, as has been previously reported. It remains to be determined whether the increased CNS opioid concentrations predate traumatic stress, thereby conferring a vulnerability to dissociative states and PTSD itself, or result from the trauma. The negative correlation between CSF ir beta END and avoidant and intrusive symptoms suggests that CNS hypersecretion of opioids might constitute an adaptive response to traumatic experience. Poor correlation between CSF and plasma ir beta END limits use of plasma measures to assess CNS opioid activity.

Effects of stress and shock anticipation on prepulse inhibition of the startle reflex

The effects of shock anticipation and attention to external stimuli on prepulse inhibition (PPI) were compared. In the threat-of-shock experiment, acoustic startle stimuli were presented with and without prepulses when aversive shocks were or were not anticipated. In the control experiment, startle and prepulse stimuli were delivered during periods with attended or ignored external stimuli. In the threat-of-shock experiment, startle was potentiated (fear-potentiated startle) and PPI was increased by shock anticipation. A gradual reduction in the overall PPI throughout the experiment was also found. In the control experiment, only PPI was increased in the attend condition. The PPI level remained constant throughout the experiment. The increase in PPI in the threat and attend conditions may have resulted from an increase in the general level of alertness that facilitated the processing of the prepulse. The gradual decrease in PPI in the threat experiment was hypothesized to result from a progressive deficit in sensory functioning due to the stressful nature of repeated shock anticipation.

Determinants of pain expression in hospitalized burn patients

This study examines the relationship between extent of injury, degree and type of psychological distress and self-report of pain in burn survivors. One hundred eighty burn patients were interviewed within 2 weeks of their burn trauma. Using a visual analogue scale to assess subjective pain and pain relief, and self-report measures of post-traumatic stress symptoms and general psychological distress, we assessed the relationship between PTSD symptoms, general distress and pain. Subjective pain was unrelated to sex, ethnicity, or total body surface area burned. The most important correlate of subjective pain was general psychological distress. Intrusive PTSD symptoms had no independent power to predict the variance in pain scores. However, among women, more severe avoidant symptoms were associated with greater subjective pain.

Modulatory influence on somatosensory perception from vibration and heterotopic noxious conditioning stimulation (HNCS) in fibromyalgia patients and healthy subjects

In order to assess the function of endogenous mechanisms modulating somatosensory input in fibromyalgia (FM), the effect of vibratory stimulation (VS) and heterotopic noxious conditioning stimulation (HNCS) on perception of various somatosensory modalities was assessed. Ten female FM patients and 10 healthy, age-matched, females participated. VS (100 Hz) was applied to the left forearm for 45 min and quantitative sensory testing (QST) was performed within the vibrated area and in the right thigh before, during and 45 min following vibration. Pressure pain thresholds (PPTs) were assessed by pressure algometry. Perception thresholds to non-painful cold (CT) and warmth (WT), heat pain thresholds (HPTs), cold pain thresholds (CPTs) and stimulus-response curves of pain intensity as a function of graded nociceptive heat stimulation were assessed using a Peltier element based thermal stimulator. The effects of HNCS were tested using the upper extremity submaximal effort tourniquet test. Subjects rated tourniquet induced pain intensity on a visual analogue scale (VAS). QST was performed in the right thigh before, during and 60 min following the tourniquet. FM patients did not differ from controls in the response to VS. There was a local increase of PPTs during vibration (P < 0.001) and of WTs following vibration (P < 0.001). HPTs increased in the forearm and in the thigh (P < 0.009) during vibration. CTs and sensitivity to suprathreshold heat pain were not influenced by VS. The intensity of pain induced by the tourniquet did not differ between groups. PPTs increased during the tourniquet in controls (P < 0.001) but not in FM patients (difference between groups P < 0.001). Decreased sensitivity to non-painful cold (P < 0.001) and non-painful warmth (P < 0.001) was seen during and following (P < 0.001; P < 0.05, respectively) the tourniquet in both groups alike. HPTs and perception of suprathreshold heat pain remained unaffected in both groups. In conclusion, FM patients did not differ from healthy controls in their response to vibration, but no modulation of pressure pain was induced by HNCS, as opposed to controls, suggesting a dysfunction in systems subserving 'diffuse noxious inhibitory controls' (DNIC).

Suppression of an inhibitory jaw reflex by the anticipation of pain in man

Electromyographic recordings (EMGs) were made from the active masseter muscle, of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in 15 human subjects. In control sequences, the reflex had a mean latency and duration (+/- S.E.M.) of 45.4 +/- 1.3 msec and 47.9 +/- 2.8 msec, respectively. Significant decreases in the reflex as well as increases in heart rate and anxiety levels assessed by a visual analogue scale, occurred when the subjects were stressed by the anticipation of receiving painful electrical stimuli above the ankle (P < 0.00005; Student's t-tests). During such sequences, the magnitude of the reflex measured by integration of the EMG, was reduced by 47.7 +/- 5.6%. This effect involved a reduction in both the duration and depth of the inhibitory wave. It occurred regardless of whether the painful stimuli were applied during or after the recording of the reflex and of whether the baseline activity in the muscle was inadvertently raised or lowered during the stressful sequences. It is concluded that stress induced by the anticipation of pain, can markedly reduce an inhibitory jaw reflex in man by exerting an influence on the reflex pathway prior to the motoneurones.

Modulation of pressure pain thresholds during and following isometric contraction in patients with fibromyalgia and in healthy controls

This study aimed at evaluating the influence of submaximal isometric contraction on pressure pain thresholds (PPTs) in 14 fibromyalgia (FM) patients and 14 healthy volunteers, before and after skin hypoesthesia. PPTs were determined with pressure algometry over m. quadriceps femoris before, during and following an isometric contraction. Maximum voluntary contraction (MVC) was assessed using a computerized dynamometer. A contraction of 22% MVC on average was held until exhaustion (max. 5 min) and PPTs were assessed every 30 sec. A local anesthetic cream and a control cream were applied following a double-blind design and PPTs were reassessed. In healthy volunteers PPTs increased during contraction (P < 0.001), then decreased after the end of contraction (P < 0.001) but remained above precontraction values during the 5 min of post-contraction assessments (P < 0.001). In FM patients PPTs decreased in the middle of the contraction period (P < 0.05) and remained below precontraction levels during the rest of the contraction period (P < 0.05) and during the 5 min of post-contraction assessment (immediately post-contraction NS; 2.5 min post-contraction P < 0.01; 5 min post-contraction P < 0.05). The normalized PPTs were significantly lower in patients than in controls during contraction (start P < 0.01; middle P < 0.001; end P < 0.001) and at all times during post-contraction assessments (P < 0.001). Anesthetic cream raised PPTs at rest in controls (P < 0.01) but not in FM patients, and did not influence contraction or post-contraction PPTs in either group. Therefore, the increased pressure pain sensibility in FM patients is more pronounced deep to the skin. The observed decrease of PPTs during isometric contraction in FM patients could be due to sensitization of mechanonociceptors caused by muscle ischemia and/or dysfunction in pain modulation during muscle contraction.

The formalin test: scoring properties of the first and second phases of the pain response in rats

The formalin test is increasingly used as a model of injury-produced pain but there is no generally accepted method of pain rating. To examine the properties of various pain rating methods we established dose-response relations for formalin injected in the plantar surface of one hind paw, and the analgesic effects of morphine and amphetamine using the most frequently reported behavioural measures of pain (favouring, lifting, licking and flinching/shaking of the injured paw) and combinations of these. Licking, elevation and favouring of the injected paw showed a biphasic response at all formalin doses. Flinching varied in form across the time course of formalin, and the biphasic nature of the behaviour was not as apparent. In untreated rats all these behaviours were infrequent. Flinching and favouring were increased after injection of local anaesthetic into the paw but remained negligible relative to the effect of formalin. Grooming other than that directed to the injected paw was elevated in a dose-dependent manner by formalin. Intercorrelations between the behaviours were different for the initial response and the second phase. Correlational analysis indicated that no single behavioural measure was a strong predictor of formalin, morphine and amphetamine dose. A simple sum of time spent licking plus elevating the paw, or the weighted pain score of Dubuisson and Dennis (1977), were superior to any single measure (r ranging from 0.75 to 0.86). Addition of flinching and favouring to the combined pain score using multiple regression did not increase variance explained. Depending on the measure used, a sedative dose of pentobarbital produced apparent analgesia, hyperalgesia or no effect. The interphase depression of pain, as well as the analgesic effects of morphine and amphetamine, were all associated with increased motor activation. Power analysis indicated that using a moderate dose of formalin and a combined pain score gave the greatest power to detect differences in pain. It was also found that pain scores increase with ambient temperature and that rat strains may differ in formalin pain sensitivity.

Hypnotic analgesia reduces R-III nociceptive reflex: further evidence concerning the multifactorial nature of hypnotic analgesia

Mechanisms of hypnotic analgesia were investigated by examining changes in the R-III, a nociceptive spinal reflex, during hypnotic reduction of pain sensation and unpleasantness. The R-III was measured in 15 healthy volunteers who gave VAS-sensory and VAS-affective ratings of an electrical stimulus during conditions of resting wakefulness, suggestions for hypnotic analgesia, and attempted suppression of the reflex during non-hypnotic conditions. The H-reflex was also measured to monitor and control for general changes in alpha-motoneuron excitability. Hypnotic sensory analgesia was related to reduction in the R-III after controlling for changes in the H-reflex (R2 = 0.51, P < 0.003), suggesting that hypnotic sensory analgesia is at least in part mediated by descending antinociceptive mechanisms that exert control at spinal levels in response to hypnotic suggestion. The relationship between hypnotic affective analgesia and reduction in R-III approached significance (R2 = 0.26; P = 0.053). Reduction in R-III was 67% as great and accounted for 51% of the variance in reduction of pain sensation. In turn, reduction in pain sensation was 75% as great and accounted for 77% of the variance in reduction of unpleasantness. The results suggest that 3 general mechanisms may be involved in hypnotic analgesia. The first, implicated by reductions in R-III, is related to spinal cord antinociceptive mechanisms. The second, implicated by reductions in pain sensation over and beyond reductions in R-III, may be related to brain mechanisms that serve to prevent awareness of pain once nociception has reached higher centers, as suggested by Hilgard.(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise-induced analgesia and the role of reactivity in pain sensitivity

The present study was designed to evaluate whether pain perception and pain tolerance are altered by submaximal aerobic exercise. Sixty male volunteers were randomly assigned to one of two control or experimental groups in the first of two sessions. In session 1, baseline measures of pain tolerance and pain perception were obtained for half of the subjects in each of the experimental and control groups, respectively. In addition, all subjects completed the Reactivity Scale, followed by estimation of their maximum aerobic power (VO2 max) using the Canadian Home Fitness Test. In session 2, the subjects in the two experimental groups exercised for 12 min by climbing a double step to pre-recorded musical cadences, working on average at 63% VO2 max, whereas the subjects in the two control groups spent approximately 12 min completing two short unrelated questionnaires. Measures of pain tolerance and pain perception were obtained from all subjects after exercising or completing questionnaires. Pain tolerance was assessed by the amount of time (up to 10 min) that subjects could voluntarily endure a 2300 g pressure to the index finger of their dominant hand. Pain perception was defined by participants' intensity ratings on an 11-point scale, made at 30 and 60 s. The results showed that reactivity and exercise were significant predictors of pain tolerance, together accounting for approximately 22% of the variance. The finding that submaximal workloads produce analgesia supports the potential usefulness of exercise in therapeutic intervention.

Bilateral lesions of the amygdala attenuate analgesia induced by diverse environmental challenges

This study was designed to evaluate the role of the amygdala, particularly its central nucleus, in the induction of analgesia elicited by environmental challenges. Rats with large, radiofrequency lesions centered in the central nucleus were found to display significantly attenuated analgesic responses to three different challenges: cat exposure, acute footshock, and re-exposure to an environment associated with footshock. These findings show that the amygdala plays an important role in the elicitation of analgesia by each of the environmental challenges tested. Since the amygdala has been shown to play a critical role in fear, these findings suggest that the analgesia elicited by these challenges involves a substantial fear component. Moreover, the finding that amygdala lesions significantly reduced the analgesia elicited by a non-noxious unconditional stimulus (cat exposure) strongly suggests that these lesions disrupt the expression of analgesia rather than producing a learning impairment. And finally, the findings of this study support the suggestion that fear-elicited analgesia is triggered by activation of a projection from amygdala to periaqueductal gray which forms one component of an integrated 'defensive behavioral system.'

Cessation of long-term naltrexone therapy and self-injury: a case study

The cessation of long-term Naltrexone administration was investigated through a double-blind, placebo-controlled, withdrawal design in a 28-year-old, profoundly mentally retarded woman with a history of severe self-injurious behavior (SIB). The subject had previously exhibited a dramatic decrease in SIB subsequent to Naltrexone administration. At the end of 1 year of continuous drug treatment, the subject displayed a near-zero rate of SIB episodes. This near-zero rate continued through placebo and no-drug phases of the study and at 6-month (no-drug) follow-up. This trend suggests not only that Naltrexone may be effective in reducing SIB, but that long-term administration may produce durable results after treatment cessation. Findings are discussed in relation to the endogenous opioid system theories of SIB.

Beta-endorphin response to exercise and mental stress in patients with ischemic heart disease

We compared symptomatic, hemodynamic and opioid responses of heart disease patients to exercise testing and a stressful public speaking task. Plasma beta-endorphins were measured at rest and immediately post stress. Nineteen of 50 patients had angina during exercise; 31 had asymptomatic ischemia. No patient had angina during the speech, but two had ECG changes and 39% had radionuclide changes indicating ischemia. Patients with asymptomatic ischemia on exercise had a significantly greater beta-endorphin response than those with angina. Public speaking elicited a significantly larger beta-endorphin increase relative to change in double product (an index of stress) than did exercise.

CONCLUSIONS

(1) Patients with silent vs painful ischemia experience a greater beta-endorphin response to exercise. (2) beta-endorphin response to a speech stressor is greater than to exercise when controlled for an index of stress. (3) Increased beta-endorphin response to a speech stressor may partially explain the predominance of silent ischemia during psychological stress.

Stress and endogenous opioids: behavioral and circulatory interactions

Endogenous opioid peptides are the basis of a diverse system of complex neuroregulatory and endocrine mechanisms. While relatively quiescent in the resting state, these peptides are released during intense stimulation and modify, in a number of ways, circulatory homeostatic mechanisms. The endogenous opioids, primarily via endorphins and enkephalins, are capable of influencing circulatory responses to stress at the behavioral, the endocrinological, and the neural level. Recent research in humans and animals has described several roles for opioids in regulation of the circulatory stress response, and has also provided clues about the significance of opioid dysregulation in the pathophysiology of stress. Increased understanding of the basic mechanisms of stress and endogenous opioids will clarify the potential roles of opioids in important pharmacologic and behaviorally based therapeutics.

Properties and functions of human placental opioid system

Human placental villus tissue contains opioid receptors and peptides. Kappa opioid receptors (the only type present in this tissue) were purified with retention of their binding properties. The purified kappa receptor is a glycoprotein with an apparent molecular weight of 63,000. Two opioid receptor mediated functions were identified in trophoblast tissue, namely regulation of acetylcholine and hormonal (human chorionic gonadotrophin and human placental lactogen) release. Placental content of kappa receptors increases with gestational age. Term placental content of kappa receptors correlates with route of delivery (higher in those abdominally obtained). Opioid use and/or abuse during pregnancy affects placental receptor content at delivery, as well as its mediated functions. Opioid peptides identified in placental extracts were beta-endorphin, methionine enkephalin, leucine enkephalin and dynorphins 1-8 and 1-13. Dynorphin 1-8 seem to be the predominant opioid peptide present in placental villus tissue.

Quantitative evaluation of hypnotically suggested hyperaesthesia and analgesia by painful laser stimulation

The ability to reduce both clinically and experimentally induced pain by hypnotic suggestion of analgesia is well known. However, the nature of hypnotic analgesia still remains uncertain. Attempts to demonstrate and identify specific psychophysiological mechanisms have, so far, been unsatisfactory. Methodological problems in inducing pain and monitoring physiological responses may be the reason for this lack of success. In the present study, we have attempted to eliminate some of these methodological problems. The sensory and pain thresholds to laser stimulation were determined, and the laser-evoked brain potentials were measured for 8 highly hypnotically susceptible subjects in 3 conditions: (1) waking state, (2) suggestion of hyperaesthesia, (3) suggestion of analgesia. The thresholds were reduced during induced hyperaesthesia and increased during analgesia. During hyperaesthesia sensations could be evoked by laser intensities which were below intensities that could be perceived in the awake state. The amplitude of the evoked brain potentials increased during hyperaesthesia and decreased during analgesia. The latency of the potential remained constant. The perception of pain during hypnosis can change very fast, indicating that slow endogenous mechanisms may play only a minor role in suggested hyperaesthesia/analgesia.