Dose-related effects of ibuprofen on pain-related potentials

The effect of induced optimism on early pain processing: indication by contact heat evoked potentials (CHEPs) and the sympathetic skin response (SSR)

Situationally induced optimism has been shown to influence several components of experimental pain. The aim of the present study was to enlarge these findings for the first time to the earliest components of the pain response by measuring contact heat evoked potentials (CHEPs) and the sympathetic skin response (SSR). Forty-seven healthy participants underwent two blocks of phasic thermal stimulation. CHEPs, the SSR and self-report pain ratings were recorded. Between the blocks of stimulation, the 'Best Possible Self' imagery and writing task was performed to induce situational optimism. The optimism manipulation was successful in increasing state optimism. It did, however, neither affect pain-evoked potentials nor the SSR nor self-report pain ratings. These results suggest that optimism does not alter early responses to pain. The higher-level cognitive processes involved in optimistic thinking might only act on later stages of pain processing. Therefore, more research is needed targeting different time frames of stimulus processing and response measures for early and late pain processing in parallel.

Electroencephalography and analgesics

To assess centrally mediated analgesic mechanisms in clinical trials with pain patients, objective standardized methods such as electroencephalography (EEG) has many advantages. The aim of this review is to provide the reader with an overview of present findings in analgesics assessed with spontaneous EEG and evoked brain potentials (EPs) in humans. Furthermore, EEG methodologies will be discussed with respect to translation from animals to humans and future perspectives in predicting analgesic efficacy. We searched PubMed with MeSH terms 'analgesics', 'electroencephalography' and 'evoked potentials' for relevant articles. Combined with a search in their reference lists 15 articles on spontaneous EEG and 55 papers on EPs were identified. Overall, opioids produced increased activity in the delta band in the spontaneous EEG, but increases in higher frequency bands were also seen. The EP amplitudes decreased in the majority of studies. Anticonvulsants used as analgesics showed inconsistent results. The N-methyl-D-aspartate receptor antagonist ketamine showed an increase in the theta band in spontaneous EEG and decreases in EP amplitudes. Tricyclic antidepressants increased the activity in the delta, theta and beta bands in the spontaneous EEG while EPs were inconsistently affected. Weak analgesics were mainly investigated with EPs and a decrease in amplitudes was generally observed. This review reveals that both spontaneous EEG and EPs are widely used as biomarkers for analgesic drug effects. Methodological differences are common and a more uniform approach will further enhance the value of such biomarkers for drug development and prediction of treatment response in individual patients.

Chemo-somatosensory evoked potentials: a sensitive tool to assess conditioned pain modulation?

Abstract Background: Chemo-somatosensory evoked potentials (CSSEPs) elicited by chemical stimulation (CO₂ gas) of the nasal mucosa have been shown to be sensitive enough to pick up even weak analgesic effects. With the present study we wanted to investigate whether CSSEPs are also a sensitive tool to capture endogenous pain inhibitory mechanisms elicited by conditioned pain modulation (CPM; where a first conditioning stimulus reduces the sensitivity for a second test stimulus) with a conditioning stimulus of rather low noxious load.

METHODS

Seventeen healthy participants were tested for CPM effects (conditioning stimulus: tonic heat pain with intensities around the pain threshold induced via a thermode; test stimulus: chemonasal stimulation (73% and 78% CO₂)) on CSSEPs and on self-report ratings.

RESULTS

We found significant CPM effects in the CSSEPS, with reduced amplitudes and prolonged latencies at several electroencephalogram (EEG) recording positions when using the lower CO₂ concentration (73% CO₂). In contrast to the visible inhibitory effects on the CSSEPs, subjective ratings of the test stimulus did not reflect CPM action.

DISCUSSION

The experimental pain model using CO₂ stimuli to elicit CSSEPs proved to be sensitive enough to capture weak CPM effects elicited by a conditioning stimulus of rather low noxious load. The usage of such mild noxious conditioning stimuli-in contrast to stimuli of higher noxious load (e.g., cold pressor test)-has the advantage that the activation of other types of pain inhibitory mechanisms in parallel (like attentional distraction, stress-induced analgesia) can be avoided.

Tonic stimulation of the pharyngeal mucosa causes pain and a reversible increase of inflammatory mediators

Objective and design

To develop a model of the inflammatory component of non-infectious sore throat using tonic stimulation and quantification of inflammatory mediators in pharyngeal lavage fluid.

Material or subjects

Forty-five healthy volunteers.

Treatment

Cold dry air.

Method

Tonic stimulation of the pharynx was achieved using a constant stream of cold dry air to the back of the throat. Following optimization of stimulation conditions (phase 1), pharyngeal pain, irritation, and swallowing discomfort were assessed using visual analog scales, and the concentration of inflammatory markers were measured in pharyngeal lavage fluid (phase 2).

Results

Optimum conditions for tonic pharyngeal stimulation were cold dry air at 12 °C, relative humidity 20 %, at a flow rate of 12 L/min for 15 min. Analysis of pharyngeal lavage fluid collected 5 min after stimulation showed significant increases in prostaglandin E₂ (P = 0.018), thromboxane B₂ (P < 0.001), and substance P (P < 0.001), but no increase in peptidoleukotriene. When the stimulus was removed, the level of inflammatory markers in pharyngeal lavage fluid returned to baseline by 30 min post-stimulation. These objective measures mirrored subjective pain ratings.

Conclusions

Tonic stimulation of the pharyngeal mucosa with cold dry air causes pain and an increase of inflammatory mediators which are reversible.

Electronic supplementary material

The online version of this article (doi:10.1007/s00011-013-0663-7) contains supplementary material, which is available to authorized users.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Assessing efficacy of non-opioid analgesics in experimental pain models in healthy volunteers: an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of analgesics and target the clinical indications for their use. This review, the second in a series of two, addresses how the efficacy of non-opioid analgesics have been assessed in human volunteers using experimental pain models.

METHODS

A literature search was completed for randomized controlled studies that included human experimental pain models, healthy volunteers and non-opioid analgesics.

RESULTS

Nonsteroidal anti-inflammatory drugs worked against various types of acute pain as well as in hyperalgesia. Analgesia from paracetamol was difficult to detect in experimental pain and the pain needed to be assessed with very sensitive methods like evoked brain potentials. The N-methyl-D-aspartate antagonists exemplified by ketamine generally needed strong, long-lasting or repeated pain in the skin for detectable analgesia, whereas pain in muscle and viscera generally was more easily attenuated. Gabapentin worked well in several models, particularly those inducing hyperalgesia, whereas lamotrigine was weak in modulation of experimental pain. Imipramine attenuated pain in most experimental models, whereas amitriptyline had weaker effects. Delta-9-tetrahydrocannabinol attenuated pain in only a few models.

CONCLUSIONS

Pain induction and assessment are very important for the sensitivity of the pain models. Generally, experimental pain models need to be designed with careful consideration of the pharmacological mechanisms and pharmacokinetics of analgesics. The drawback with the different study designs is also discussed. This knowledge can aid the decisions that need to be taken when designing experimental pain studies for compounds entering Phase I and II trials.

Assessment of Upper Respiratory Tract and Ocular Irritative Effects of Volatile Chemicals in Humans

Accurate assessment of upper respiratory tract and ocular irritation is critical for identifying and remedying problems related to overexposure to volatile chemicals, as well as for establishing parameters of irritation useful for regulatory purposes. This article (a) describes the basic anatomy and physiology of the human upper respiratory tract and ocular mucosae, (b) discusses how airborne chemicals induce irritative sensations, and (c) reviews practical means employed for assessing such phenomena, including psychophysical (e.g., threshold and suprathreshold perceptual measures), physiological (e.g., cardiovascular responses), electrophysiological (e.g., event-related potentials), and imaging (e.g., magnetic resonance imaging) techniques. Although traditionally animal models have been used as the first step in assessing such irritation, they are not addressed here since (a) there are numerous reviews available on this topic and (b) many rodents and rabbits are obligate nose breathers whose nasal passages differ considerably from those of humans, potentially limiting generalization of animal-based data to humans. A major goal of this compendium is to inform the reader of procedures for assessing irritation in humans and to provide information of value in the continued interpretation and development of empirical databases upon which future reasoned regulatory health decisions can be made.

Caffeine accelerates absorption and enhances the analgesic effect of acetaminophen

The aim of this study was to determine the analgesic effect of acetaminophen compared to a combination of both caffeine and acetaminophen or caffeine alone using tonic and phasic pain stimulation. Twenty-four subjects were treated orally with 1000 mg acetaminophen, 130 mg caffeine, and a combination of both in a 4-way crossover, double-blind, placebo-controlled study. Pharmacokinetics and analgesic effects were assessed by means of an experimental pain model based on pain-related cortical potentials after phasic stimulation of the nasal mucosa with CO(2) and based on pain ratings after tonic stimulation with dry air. Analgesic effects of acetaminophen and acetaminophen plus caffeine but not caffeine alone caused a significant reduction of pain-related cortical potentials beginning 30 minutes after medication. The combination demonstrated an enhanced effect throughout the observation time up to 3 hours. Caffeine accelerated acetaminophen absorption, indicated by enhanced early AUCs. Significant analgesic effects of the combination on tonic pain ratings were found throughout the observation time as compared to acetaminophen and placebo. In this study, caffeine enhanced and prolonged the analgesic activity of acetaminophen.

Gustatory stimulation influences the processing of intranasal stimuli

OBJECTIVES

Taste and smell interact. The aim of this study was to examine this interaction using gustatory and olfactory stimuli applied at the same time, which exhibited perceptual compatibility and incompatibility.

METHODS

Thirty-two, young, healthy normosmic subjects (16 men, 16 women) took part in two randomized sessions. Event-related potentials (ERP) were recorded in response to vanillin, or gaseous CO2. These two conditions were combined with three "taste conditions" including sweet taste, sour taste, and the intraoral presentation of an empty taste dispenser.

RESULTS

Vanillin responses were largest for the "sweet" condition, while they were smaller for the "sour condition". In contrast, responses to CO2 were largest under the "sour" condition, and smallest under the "sweet" condition. Moreover, during the "sweet" condition the latencies of P1 and N1 were shorter than in the "sour" condition, which was the other way around for CO2.

CONCLUSIONS

Results of the present investigation suggested that (1) the early processing of intranasal chemosensory stimuli is modulated through concomitant gustatory stimulation, and that (2) this modulation may depend, at least in part, on the contextual compatibility between intranasal and intraoral stimuli.

Intranasal trigeminal sensitivity in subjects with allergic rhinitis

Trigeminal nerve endings of the human nasal mucosa are activated by chemical, physical or thermal stimuli. Activation of these A(delta) and C fibers can be quantified through the recording of chemo-somatosensory event-related potentials (ERP). The aim of this study was to investigate whether allergy-related activation of trigeminal nerve endings leads to changes in their responsiveness to intranasal trigeminal stimulation. Gaseous carbon dioxide (CO(2)) stimuli were applied in three sessions (baseline, after NaCl solution and after allergen application) to the nasal mucosa of 13 subjects with allergic rhinitis. Chemo-somatosensory ERP were recorded, and subjects rated the intensity of rhinitis symptoms. Administration of allergen produced a significant shortening of chemo-somatosensory ERP peak latencies P1 and N1. Observed changes of latencies were in line with rhinitis symptoms subjects indicated during the session. In addition, there was a negative relation between the general symptom score and ERP peak latencies, obtained both at baseline and after allergen exposure. In conclusion, it is hypothesized that in patients suffering from allergic rhinitis, nasal itching and sneezing after allergen exposure are, at least in part, clinical correlates of the activation of trigeminal nerve endings due to local inflammatory mechanisms. The correlations between ERP latencies and the patients' symptoms indicate that ERP latencies may possess a predictive value of the subjects' responsiveness to allergens.

The influence of stimulus duration on the reliability of pain ratings after nociceptive stimulation of the nasal mucosa with CO2

Nociceptive stimuli consisting of short pulses of gaseous CO(2) applied to the nasal mucosa were investigated in 12 healthy volunteers (six males, six females) with respect to trial-to-trial variability. A statistical rather than a psychophysical approach was employed, focusing on test-retest reliability. According to a randomized, volunteer-blind crossover design, two repeated measurements for each of four stimulus durations (200, 400, 800 and 1600 ms) were performed on eight different days. During measurements, a total of 80 stimuli of 16 concentrations of CO(2) (40-70% v/v in 2% steps) was applied with an interstimulus interval at 30 s. The interday variability of the pain ratings was lowest at a stimulus duration of 400 ms and highest at 200 ms. The results show that stimulus duration may affect the statistical outcome of studies on pain, and should be considered when setting up an experimental pain model.

Effects of sleep on pain-related somatosensory evoked magnetic fields in humans

We investigated the effects of sleep on pain-related somatosensory evoked magnetic fields (SEFs) following painful electrical stimulation to identify the mechanisms generating them in both fast A-beta fibers relating to touch and slow A-delta fibers relating to pain. While the subjects were awake, non-painful and painful electrical stimulations were applied, and while asleep, painful stimulation was applied to the left index finger. During awake, five components (1M-5M) were identified following both non-painful and painful stimulation, but the 4M and 5M at around 70-100 ms and 140-180 ms, respectively, were significantly enhanced following painful stimulation. During sleep, 1M and 2M generated in the primary somatosensory cortex (SI) did not show a significant change, 3M in SI showed a slight but significant amplitude reduction, and 4M and 5M generated in both SI and the secondary somatosensory cortex (SII) were significantly decreased in amplitude or disappeared. The 4M and 5M are complicated components generated in SI and SII ascending through both A-beta fibers and A-delta fibers. They are specifically enhanced by painful stimulation due to an increase of signals ascending through A-delta fibers, and are markedly decreased during sleep, because they much involve cognitive function.

Pain in the trigeminal system: irritation of the nasal mucosa using short- and long-lasting stimuli

The paper describes methods which allow intranasal irritation using short- and long-lasting painful stimuli in humans. Short-lasting pain is induced by gaseous CO(2), while long-lasting pain is induced by a stream of dry air. Both models have been explored regarding their major determinants, e.g. stimulus duration, stimulus intensity, or repeated stimulation. Short-lasting, non-inflammatory pain stimuli seem to provide specific indicators of A(delta)-fiber function, while responses to long-lasting, inflammatory pain appear to be indicative of C-fiber function. Responses to both types of painful stimuli are modulated by analgesic drugs. As these well-investigated models allow the detailed and precise analysis of modulatory effects on intranasal nociception, they appear to be suited for the investigation of subtle changes of intranasal irritation, e.g. induced by environmental agents.

Assessment of intranasal trigeminal function

Intranasal trigeminal function is more and more understood as an integral part of human chemosensory perception. Sensations like burning, stinging, warmth, coolness, or itching are produced by almost all odorants so that they can be perceived by anosmics. Electrophysiological responses to trigeminal stimuli allow the specific assessment of trigeminally mediated information at different levels of processing including the periphery or the cortex. Information regarding the localization of these processes can be derived from magnetoencephalographic recordings or functional imaging data. When using these techniques in combination with psychophysical measures, it seems to be possible to specifically describe how and where the processing of irritation takes place, how it may interact with olfactory mediated sensations, and how it is modulated, e.g. by environmental influences or analgesic drugs.

Dose dependent time course of the analgesic effect of a sustained-release preparation of tramadol on experimental phasic and tonic pain

1. The aim of this study was to investigate the analgesic effect and its duration of a new sustained-release preparation of tramadol in an experimental pain model based on pain-related chemosomatosensory evoked potentials (CSSEPs) and subjective intensity estimates of painful phasic and tonic stimuli. 2. Twenty volunteers participated in a randomised, double-blind, three-fold cross-over study. Measurements were obtained before and 0.5, 1, 4, 6, and 12 h after administration of the drug (100 mg, 200 mg and placebo orally). CSSEPs were recorded after stimulation of one nostril with phasic, painful CO2 pulses. The other nostril was stimulated with a constant stream of dry air, which produced a tonic painful sensation. Subjects rated the perceived intensity of phasic and tonic stimuli via visual analogue scales. In order to test for nonspecific effects, acoustic evoked potentials (AEPs) were recorded, the spontaneous EEG was analysed in the frequency domain, the subject's vigilance was assessed in a tracking task, and the side effects of the drug were monitored. 3. The sustained-release preparation of tramadol produced a significant dose-related decrease in CSSEP amplitudes when compared with placebo. The reduction in amplitudes outlasted the observation period of 12 h, demonstrating the prolonged duration of the analgesic effect. 4. A dose-related significant decrease could be observed for the estimates of tonic pain. Similar to the decrease of amplitudes of the CSSEP, the reduction of the ratings of tonic pain outlasted the observation period of 12 h. The observed slight decrease in the estimates of phasic pain under medication did not reach a statistically significant level when compared with placebo. No significant effect could be demonstrated for the perception of the phasic and the tonic pain as determined by the McGill-Questionnaire. 5. A significant dose-related increase in the estimates of the side effects 'drowsiness', 'vertigo' and 'sickness' but not for 'tiredness' could be demonstrated.

Analgesic effects of propyphenazone in comparison to its combination with caffeine

OBJECTIVE

The aim of the study was to investigate whether the analgesic effect of propyphenazone (PROP) was increased when it was administered in combination with caffeine (CAFF).

METHODS

For assessment of analgesia a model was chosen based on chemo-somatosensory event-related potentials (CSSERP) elicited by stimulation of the nasal mucosa. Twenty healthy volunteers participated in the experiments. The study followed a placebo-controlled, randomised, double-blind, 5-fold cross-over design. Each of the 5 medications (400 mg PROP, 600 mg PROP, 400 mg PROP + 100 mg CAFF, 600 mg PROP + 150 mg CAFF, placebo) was orally administered. Experiments were separated by at least 5 days. In addition to assessment of CSSERP, subjects estimated the intensity of the stimulus. Drug effects unrelated to nociception were monitored, and in addition, the plasma levels of PROP were also analysed.

RESULTS

While 400 mg PROP did not significantly reduce the amplitude of CSSERP in comparison to placebo, all other medications produced a significant decrease in amplitudes. For both dosages of PROP, there was a significant amplification of the antinociceptive effect of PROP by CAFF, as indicated by the decrease in CSSERP amplitude. A significant effect of the factor "drug" was also found in the spontaneous EEG, indicating an arousal reaction after CAFF. No significant differences between plasma levels of PROP were found when applied either alone or in combination with CAFF.

CONCLUSION

The significant increase in the antinociceptive effect of PROP when administered together with caffeine appears to be related either to amplification of PROP's antinociceptive actions by CAFF or an atinociceptive effect of CAFF itself.

Effects of azapropazone on pain-related brain activity in human subjects

1. The dose-related effects of azapropazone on (i) event-related and spontaneous EEG-activity and (ii) the subjects' pain ratings were investigated using an experimental human pain model based on both chemo-somatosensory event-related potentials (CSSERP) and subjects' pain ratings. 2. Healthy subjects (n = 20) participated in a placebo-controlled, randomized, double-blind, four-way cross-over study. Single doses of azapropazone (300 mg, 600 mg and 1200 mg) and placebo were administered intravenously. Each experiment consisted of five sessions (before and 1, 2, 4 and 8 h after administration of the medication). Each session lasted for approximately 40 min. In the first 20 min, pain was induced by short CO2-stimuli presented to the right nostril (phasic pain; interstimulus interval 30 s) and EEG was recorded from five positions. CSSERPs were obtained in response to painful CO2-stimuli. In the following 20 min period, tonic pain was induced by a constant stream of dry air introduced in the left nostril. Subjects rated the intensity of both phasic and tonic pain by means of a visual analogue scale. Additionally, a frequency analysis of the spontaneous EEG was performed. 3. Azapropazone reduced the pain-related CSSERP-amplitudes at frontal and parietal recording positions. This topographical pattern was observed in previous studies with opioids, while NSAIDs such as flurbiprofen and ketoprofen exerted effects at frontal and central positions. In contrast to other NSAIDs, administration of azapropazone resulted in a reduction of the frequency bands alpha 1, delta and theta of the spontaneous EEG. At the subjective level, analgesic effects of azapropazone were observed in the ratings of tonic pain. 4. Analgesic properties of azapropazone were demonstrated in man. The topographical pattern of the changes in the CSSERPs and the effects on EEG background activity suggest a central component of the analgesic action of azapropazone.

Tonic versus phasic pain: dose-related effects of ketoprofen

Only recently has a new experimental technique been developed which combines tonic and phasic painful stimulation. By means of this technique the non-steroidal anti-inflammatory drug (NSAID) ibuprofen has been shown to produce a dose-related decrease in heterotopically applied phasic and tonic pain. The present study aimed to investigate the dose-related effects of the NSAID ketoprofen (50, 100, and 150 mg i.v.) when tonic and phasic stimuli were applied homotopically. Eighteen healthy volunteers participated in the double-blind, randomized, placebo-controlled study. After an initial training session subjects took part in four experiments, each of which was divided into three sessions (before, 30, and 120 min after drug administration). During each session 45 painful phasic CO2 stimuli of three concentrations were presented to the left nostril in randomized order (duration 200 ms; interval 40 s; 45%, 52%, and 59% v/v CO2). The left nostril was additionally stimulated with a constant stream of dry air, which produced a tonic painful sensation described as dull and burning. Subjects rated the intensity of the painful stimuli by means of visual analogue scales. Chemosomatosensory event-related potentials (CSSERPs) were recorded in response to phasic painful CO2 stimuli. Ketoprofen reduced the subjects' estimates of tonic pain in a dose-related manner. In contrast, given the special conditions of homotopic application of tonic and phasic painful stimuli, estimates of phasic pain increased significantly, corresponding to a significant increase in CSSERP amplitudes. An explanation of this inverse effect of the drug on responses to tonic and phasic pain may be a lateralized interaction between both C-fiber and A delta-fiber systems at a spinal or peripheral level.

Effects of flurbiprofen enantiomers on pain-related chemo-somatosensory evoked potentials in human subjects

1. The aim of the study was to investigate the analgesic effects of flurbiprofen enantiomers using an experimental pain model based on both chemo-somatosensory event-related potentials (CSSERP) and subjective pain ratings. 2. Healthy female volunteers (n = 16, age 23-36 years) participated in a placebo-controlled, randomised, double-blind, four-way crossover study. Single doses of (S)-flurbiprofen (50 mg), (R)-flurbiprofen (50 and 100 mg) and placebo were administered orally. Measurements were taken before and 2 h after administration of the medications. During each measurement, 32 painful stimuli of gaseous carbon dioxide (200 ms duration, interval approximately 30 s) of two concentrations (60 and 65% CO2 v/v) were applied to the right nostril. EEG was recorded from five positions and CSSERP were obtained in response to the painful CO2- stimuli. Additionally, subjects rated the perceived intensity of the painful stimuli by means of a visual analogue scale (VAS). 3. The CSSERP-amplitude P2, a measure of analgesic effect, decreased after administration of both (R)- and (S)-flurbiprofen, while it increased after placebo. This was statistically significant at recording positions C4 (P < 0.01) and Fz (P < 0.05). The analgesia-related decreases in evoked potential produced by (R)-flurbiprofen were dose-dependent. Comparing similar doses of (R)- and (S)-flurbiprofen, the decrease in CSSERP-amplitudes produced by the (S)-enantiomer was somewhat more pronounced, indicating a higher analgesic potency. 4. The present data indicate that both enantiomers of flurbiprofen produce analgesic effects. Since (R)-flurbiprofen caused only little toxicity in rats as compared with the (S)-enantiomer or the racemic compound, a reduction of the quantitatively most important side effects in the gastrointestinal tract might be achieved by employing (R)-flurbiprofen in pain therapy.

Effects of peripherally and centrally acting analgesics on somato-sensory evoked potentials

1. The effects of aspirin 1000 mg, paracetamol 1000 mg, codeine 60 mg on somatosensory evoked potentials (SEPs) were measured in a four-way cross-over study. 2. SEPs were elicited by electrical stimulation of the skin overlying the digital nerve at intensities close to pain threshold. 3. Amplitudes and latencies of both early and late SEPs were recorded, as well as first sensory threshold and subjective pain threshold. 4. None of the study medications affected the amplitude or latency of the late SEP components (100-250 ms post-stimulus). The amplitude of early components (15-30 ms post-stimulus) was also unaffected, but aspirin shortened the latency 30 min after ingestion. 5. Sensory detection and pain threshold to electrical skin stimulation were also unaffected by any of the study medications despite subjective central effects with codeine.

[Non-steroidal anti-inflammatory drugs (NSAIDs): Antirheumatic, anti-pyretic or analgesic drugs?]

This manuscript reviews some aspects of the unorthodox development of analgesics. Most of them were not discovered by systematic research for analgesics; rather they were developed primarily as antipyretic or antirheumatic drugs that later became analgesics due to clinical observations. One reason for this development may be that the definition and quantification of analgesic effects under clinical circumstances is rather difficult and a clear dose-response relationship almost impossible to obtain. These drawbacks limit the discovery of new analgesics. On the other hand, modern insights into the site of action and the mode of action of antipyretic analgesics are likely to further the discovery of new and better analgesics.