Pain--mechanics and management

In the past two decades there has been remarkable progress in understanding the neural mechanisms of pain. However, chronic pain is poorly understood and, by definition, poorly managed. In addition to hyperactivity of the sympathetic nervous system and damage to normal inhibitory mechanisms, social and psychological factors play a major role in producing the disability of chronic pain. New approaches to manage chronic pain include nonopiate drugs, transcutaneous electral nerve stimulation and psychological and behavioral methods. A nervous system network has recently been described that suppresses pain. This analgesic action is mediated by endogenous opioid peptides (endorphins) and by biogenic amines. The analgesia network can be activated either by electral stimulation or by opiates such as morphine or methadone.

Leukotriene B4 produces hyperalgesia that is dependent on polymorphonuclear leukocytes

Leukotriene B4, at the same intracutaneous doses as bradykinin, reduced the nociceptive threshold in the rat paw. The mechanism of leukotriene B4-induced hyperalgesia was distinguished from that of the hyperalgesia elicited by prostaglandin E2 and bradykinin by its dependence on polymorphonuclear leukocytes and independence of the cyclooxygenation of arachidonic acid.

The role of stimulus intensity and stress in opioid-mediated analgesia

Rats exposed to a Pavlovian conditioning paradigm developed naloxone-reversible analgesia only when the intensity of a noxious unconditioned stimulus was suprathreshold and the level of stress was augmented. The time course of the onset of this conditioned analgesia was reproduced by systemic administration of morphine. These findings suggest that both a minimal level of stimulus intensity and stress are necessary for the activation of endogenous opioid-mediated analgesia.

Pain-induced vocalization in the rat and its modification by pharmacological agents

Vocalization was induced in rats by electrical stimulation of the tail (pain-induced vocalization), and its components were characterized in terms of latency, duration, frequency spectrum and energy. Noxious stimuli at threshold elicit a single vocalization component (V1). Increases in stimulus intensity produce additional discrete vocalization components (V2-Vn) with successively longer latencies, termed the vocalization afterdischarge (AD). The AD components are acoustically similar to each other but differ significantly from the V1 component. The duration, the specific acoustic measures and the sound energy of both V1 and AD components are positively correlated with intensity of the stimulus. The dependence of the V1 and AD components on the affective state of the rat was evaluated by comparing the acoustic characteristics of both components to those of stress-induced vocalizations, and by studying the effects of the anxiolytic drug diazepam and physical restraint on the threshold of V1 and AD. The AD components were markedly more dependent on the affective state of the rat then was the V1 component. A moderately low dose of morphine (3.0 mg/kg) also preferentially affected the AD component, suggesting that a significant portion of the action of morphine on pain-induced vocalization is mediated through its action on the affective state of the rat.

Glutathione redox cycle protects cultured endothelial cells against lysis by extracellularly generated hydrogen peroxide

We have examined the role of the glutathione redox cycle as an antioxidant defense mechanism in cultured bovine and human endothelial cells by disrupting the glutathione redox cycle at several points. Endothelial glutathione reductase was selectively inhibited with 1,3-bis(chloroethyl)-1-nitrosourea (BCNU). Cellular stores of reduced glutathione were depleted by reaction with diethylmaleate (DEM) or 1-chloro-2,4-dinitrobenzene (CDNB) or by inhibition of glutathione synthesis with buthionine sulfoximine (BSO). Whereas several strains of untreated bovine and human endothelial cells were resistant to lysis by enzymatically generated hydrogen peroxide, BCNU-treated cells were readily lysed in a time- and dose-dependent manner. Glucose-glucose oxidase-mediated lysis of BCNU-treated bovine endothelial cells was catalase-inhibitable and directly related to BCNU concentration and endogenous glutathione reductase activity. Pretreatment of bovine endothelial cells with BCNU did not potentiate lysis by distilled water, calcium ionophore, lipopolysaccharide, or hypochlorous acid. Depletion of cellular reduced glutathione by reaction with DEM or CDNB or by inhibition of glutathione synthesis by BSO also potentiated endothelial lysis by enzymatically generated hydrogen peroxide. Inhibition of endothelial glutathione reductase by BCNU or depletion of reduced glutathione by BSO increased endothelial susceptibility to lysis by hydrogen peroxide generated by phorbol myristate acetate-activated neutrophils. We conclude that the glutathione redox cycle plays an important role as an endogenous antioxidant defense mechanism in cultured endothelial cells.

Altered electrodermal responsivity associated with clinical pain

We used certain physiologic maneuvers to perturb the autonomic nervous system (ANS) in an attempt to detect a link between the ANS and pain. In the unperturbed state, we found no difference in the electrodermal response among normal controls, preoperative patients (increased stress without pain) and postoperative patients (increased stress and pain). The electrodermal response elicited by autonomic maneuvers was significantly attenuated in postoperative patients but not in preoperative patients or in normal control subjects.

Thrombin-mediated release of factor VIII antigen from human umbilical vein endothelial cells in culture

We have examined the effects of purified human alpha-thrombin on factor VIII antigen (FVIII-Ag) release by human umbilical vein endothelial cells in culture. Alpha-thrombin induced a time and dose-dependent release of FVIII-Ag into supernatant medium. Alpha-thrombin-mediated FVIII-Ag release was not dependent on protein synthesis and was observed in both serum-free and serum-containing media. FVIII-Ag release, however, was prevented when the serine esterase activity of thrombin was inhibited. Pretreatment of human endothelial cells with alpha-thrombin, but not diisofluorophosphate-thrombin, prevented subsequent FVIII-Ag release by alpha-thrombin. Thrombin-mediated FVII-Ag release was not associated with significant 51Cr release from prelabeled endothelial monolayers. We conclude that alpha-thrombin induces release of preformed FVIII-Ag from human umbilical vein endothelial cells by a receptor-independent, nonlytic mechanism requiring serine esterase activity.

Post-operative pain: effect of extent of injury and attention

The relationship to pain level of extent of injury (as measured by number of teeth extracted) and attention paid to the injury (as measured by frequency of pain ratings) was studied in patients with dental postoperative pain. Patients had either 2 or 4 impacted wisdom teeth removed and rated their pain either 2 or 5 times during the experiment. A positive correlation was found between extent of injury and reported pain level as well as between frequency of pain rating and pain level. The correlation between frequency of pain rating and pain level was found only in patients with 4 teeth extracted. To our knowledge, this is the first study which quantitatively evaluates the relationship between amount of injury and level of pain. This study also suggests that the degree to which manipulations of psychological variables alter an individual's pain perception may depend on the extent of injury.

Analgesic responses to morphine and placebo in individuals with postoperative pain

The effects of placebo and varying doses of intravenous morphine were studied in 74 patients. All patients underwent extraction of impacted mandibular third molars. Two hours after onset of anesthesia all patients received a placebo (intravenous saline). One hour after the placebo administration each patient received either a second placebo or, 4, 6, 8 or 12 mg of morphine, double blind, via a hidden intravenous line. Pain level was evaluated 50 min after morphine administration using a visual analog scale. Pooled data from all patients produced a dose-response curve asymptotic by 8 mg. The mean pain relief following the second placebo was found to be between that obtained following hidden administration of 4 and 6 mg of morphine. When pain level reports for individuals were plotted two unexpected features appeared. First, no patient reported complete relief, even at the highest dose of morphine (12 mg). Second, pain level reports 50 min following each dose of morphine tended to be in two clusters. Within each cluster the average pain was independent of the dose of morphine administered. However, in groups receiving progressively higher doses of morphine, the percentage of patients within the low pain level cluster increased. These latter observations are most consistent with the concept that there is a step component for narcotic analgesia.

A study of the quantal (all-or-none) change in reflex latency produced by opiate analgesics

The properties of opiate-induced changes of tail-flick latency were studied in the rat. (1) Morphine and pentazocine produced a stepwise increase in latency which rose from near baseline to cut-off (usually greater than 20 sec) in less than 30 sec. Abrupt return to pre-treatment latencies was observed either spontaneously or when the rat was back-titrated with the narcotic antagonist naloxone. (2) The proportion of rats showing this stepwise change increased with increasing dose; however, the step itself was independent of dose. The same step was produced by a slow, constant infusion of morphine but was not produced by ice-water stress or barbiturate administration. (3) Increasing heat intensity to the tail shortened the baseline latency and raised the mean dose of morphine required to produce a step latency increase. (4) A step increase in latency was also observed when paw withdrawal instead of tail-flick was measured. We hypothesize that the analgesic behavior described partly defines the operating characteristics of an intrinsic endorphin-mediated analgesia system which mediates narcotic suppression of withdrawal reflexes.

Role of pain in placebo analgesia

The hypothesis that perceived pain intensity can influence placebo analgesia was tested. One hundred and seven subjects rated their pain from from 0 to 10 on a visual analog scale after a standard wisdom tooth extraction. The expected course of such postoperative pain in the absence of therapy or placebo is a steady increase; this was confirmed by blind administration of the placebo. When placebos were given intravenously in view of the patients, some (placebo nonresponders) reported that their pain increased, whereas others (placebo responders) reported that their pain either decreased or remained the same over the next 60 min. A placebo response was more likely to occur if the pain rating 5 min prior to placebo administration (initial pain) was greater than 2.6. Furthermore, placebo responders with initial pain above this 2.6 level reported significantly greater mean analgesia than those with lower initial pain. Indeed, responders with initial pain less than 2.6 reported no change in pain during the 60 min after administration of a placebo. When their initial pain level was greater than 2.6, they reported a steady decline in pain over this period. However, above the 2.6 level there was no obvious relationship between the magnitude of the placebo analgesia and the initial pain.

The mechanism of placebo analgesia

The effect of naloxone on dental postoperative pain was studied to examine the hypothesis that endorphins mediate placebo analgesia. All patients had extraction of impacted mandibular third molars with diazepam, N2O, and local block with mepivacaine. 3 h and 4 h after surgery naloxone or a placebo was given under randomised, double-blind conditions. Pain was evaluated on a visual analogue scale. Patients given naloxone reported significantly greater pain than those given placebo. Patients given placebo as their first drug was either placebo responders, whose pain was reduced or unchanged, or nonresponders whose pain increased. Naloxone given as a second drug produced no additional increase in pain levels in nonresponders but did increase pain levels of placebo responders. Nonresponders had a final mean pain rating identical to that of responders who received naloxone as their second drug. Thus the enhancement of reported pain produced by naloxone can be entirely accounted for by its effect on placebo responders. These data are consistent with the hypothesis that endorphin release mediates placebo analgesia for dental postoperative pain.

Observations on the analgesic effects of needle puncture (acupuncture)

The present study was undertaken in order to investigate the analgesic effect of needle puncture in a small self-selected group of patients with chronic or acute pain, and to examine the factors which determine success or failure of this treatment modality. We have found that in chronic painful conditions, needle puncture may be very effective in producing at least transient analgesia. It also can produce permanent relief of acute (self-limited) pains. Needle puncture was not helpful in the management of pain resulting from nerve damage. High score on psychometric indicators of anxiety and depression is a significant predictor os successful needle puncture analgesia in patients with chronic pain. Comparison of our results to studies of counterirritation indicate that the analgesia produced by needle puncture involves a mechanism similar to that of counterirritation-induced analgesia.

Stereotaxic map of the muscle fibers in the indirect flight muscles of Drosophila melanogaster

It is possible to monitor the electrical activity of the motor neurons of Drosophila by recording the electrical activity of the muscle fibers. We have found that it is possible to specify the location of the subcuticular terminations and to describe the orientation within the thorax for the individual muscle fibers, because of the large size of the fibers and because the surface anatomy of Drosophila is known in detail. A map has been made to indicate the location of the muscle fibers with respect to superficial landmarks. The importance of the stereotaxic map for physiological studies is discussed.