Different sensitivity of pain-related chemosensory potentials evoked by stimulation with CO2, tooth pulp event-related potentials, and acoustic event-related potentials to the tranquilizer diazepam

Comparison of bromazepam and ibuprofen influence on tooth pulp-evoked potentials in humans

Introduction/Objective Somatosensory evoked potentials are a neurophysiological tool for testing the effects of drugs in humans and animals. The aim of this study was to estimate the way that bromazepam and ibuprofen had on tooth pulp-evoked potentials (TPEPs) after non-painful stimuli, as well as to detect possible differences in this activity. Methods Sixty young healthy subjects were included in the study. They were arranged into three groups: ibuprofen, bromazepam and placebo. To record TPEPs response, dental pulp was electrically stimulated through intact enamel with non-painful stimuli. For stimulation and registration, we used Xltek Protektor 32 system, software EPWorks, version 5.0. The experiment consisted of two testing sessions. Five recordings were performed in each session. The first test session was before, and the second was 45 minutes after administration of a single dose of the ibuprofen (400 mg), bromazepam (1.5 mg) or placebo. Results The results of the present study exhibit that both ibuprofen and bromazepam significantly increased all the latencies; ibuprofen decreased amplitudes of all the waves except the first one (p < 0.05), and bromazepam decreased amplitudes of all the waves except the first one (p < 0.05); placebo did not modified TPEPs waves (p > 0.05). Additionally, there were no significant differences in influence on TPEPs between bromazepam and ibuprofen (p > 0.05). Conclusion Our study showed that both bromazepam and ibuprofen had the same influence on TPEPs after non-painful stimuli. That indicates that anxiolytic dose of bromazepam affects neurotransmission in the same manner as non-opioid analgesics ibuprofen.

Central encoding of the strength of intranasal chemosensory trigeminal stimuli in a human experimental pain setting

An important measure in pain research is the intensity of nociceptive stimuli and their cortical representation. However, there is evidence of different cerebral representations of nociceptive stimuli, including the fact that cortical areas recruited during processing of intranasal nociceptive chemical stimuli included those outside the traditional trigeminal areas. Therefore, the aim of this study was to investigate the major cerebral representations of stimulus intensity associated with intranasal chemical trigeminal stimulation. Trigeminal stimulation was achieved with carbon dioxide presented to the nasal mucosa. Using a single-blinded, randomized crossover design, 24 subjects received nociceptive stimuli with two different stimulation paradigms, depending on the just noticeable differences in the stimulus strengths applied. Stimulus-related brain activations were recorded using functional magnetic resonance imaging with event-related design. Brain activations increased significantly with increasing stimulus intensity, with the largest cluster at the right Rolandic operculum and a global maximum in a smaller cluster at the left lower frontal orbital lobe. Region of interest analyses additionally supported an activation pattern correlated with the stimulus intensity at the piriform cortex as an area of special interest with the trigeminal input. The results support the piriform cortex, in addition to the secondary somatosensory cortex, as a major area of interest for stimulus strength-related brain activation in pain models using trigeminal stimuli. This makes both areas a primary objective to be observed in human experimental pain settings where trigeminal input is used to study effects of analgesics.

The intranasal trigeminal system

Many odors activate the intranasal chemosensory trigeminal system where they produce cooling and other somatic sensations such as tingling, burning, or stinging. Specific trigeminal receptors are involved in the mediation of these sensations. Importantly, the trigeminal system also mediates sensitivity to airflow. The intranasal trigeminal and the olfactory system are closely connected. With regard to central nervous processing, it is most interesting that trigeminal stimuli can activate the piriform cortex, which is typically viewed as the primary olfactory cortex. This suggests that interactions between the two systems may form at a relatively early stage of processing. For example, there is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity, probably on account of the lack of interaction in the central nervous system. Decreased trigeminal sensitivity may also be responsible for changes in airflow perception, leading to the impression of congested nasal airways.

Clinical pharmacology of analgesics assessed with human experimental pain models: bridging basic and clinical research

The medical impact of pain is such that much effort is being applied to develop novel analgesic drugs directed towards new targets and to investigate the analgesic efficacy of known drugs. Ongoing research requires cost-saving tools to translate basic science knowledge into clinically effective analgesic compounds. In this review we have re-examined the prediction of clinical analgesia by human experimental pain models as a basis for model selection in phase I studies. The overall prediction of analgesic efficacy or failure of a drug correlated well between experimental and clinical settings. However, correct model selection requires more detailed information about which model predicts a particular clinical pain condition. We hypothesized that if an analgesic drug was effective in an experimental pain model and also a specific clinical pain condition, then that model might be predictive for that particular condition and should be selected for development as an analgesic for that condition. The validity of the prediction increases with an increase in the numbers of analgesic drug classes for which this agreement was shown. From available evidence, only five clinical pain conditions were correctly predicted by seven different pain models for at least three different drugs. Most of these models combine a sensitization method. The analysis also identified several models with low impact with respect to their clinical translation. Thus, the presently identified agreements and non-agreements between analgesic effects on experimental and on clinical pain may serve as a solid basis to identify complex sets of human pain models that bridge basic science with clinical pain research.

Subtype-selective GABAA receptor mimetics--novel antihyperalgesic agents?

Agonists at the benzodiazepine-binding site of ionotropic gamma-aminobutyric acid (GABA(A)) receptors are in clinical use as hypnotics, anxiolytics, and anticonvulsants since the early 1960. Analgesic effects of classical benzodiazepines have occasionally been reported in certain subgroups of patients suffering from chronic pain or after spinal delivery through intrathecal catheters. However, these drugs are generally not considered as analgesics but should in fact be avoided in patients with chronic pain. Recent evidence from genetically modified mice now indicates that agents targeting only a subset of benzodiazepine (GABA(A)) receptors should provide pronounced antihyperalgesic activity against inflammatory and neuropathic pain. Several such compounds have been developed recently, which exhibit significant antihyperalgesia in mice and rats and appear to be devoid of the typical side-effects of classical benzodiazepines.

Diazepam-induced disruption of classically-conditioned fear-potentiation of late-latency auditory evoked potentials is prevented by flumazenil given before, but not after, CS/US pairing

Classical fear conditioning involves pairing a neutral conditional stimulus (CS) with an aversive unconditional stimulus (US). Subsequent presentation of the CS alone induces fear responses. Acquisition of conditioned fear is thought to involve learning of the CS/US association, followed by memory consolidation. Recently we reported that the N1/P2 auditory evoked potential was enhanced by fear conditioning in humans. Diazepam 10 mg, given before CS/US pairing, prevented subsequent expression of fear potentiation when the response was elicited, 1 week later, in the presence of the CS. In this experiment, we examined whether this effect of diazepam was caused by disruption of the formation of CS/US associations or by disruption of consolidation. The benzodiazepine antagonist flumazenil was used to block the effect of diazepam either during the association period or during subsequent consolidation. Forty-two male volunteers (18-35 years) participated in two sessions separated by 7 days. In Session One, they ingested diazepam 10 mg or placebo: 60 minutes later they received flumazenil 1 mg or saline intravenously (i.v.). Then they received 20 presentations of a light (CS), 50% of which terminated with electric shock (US). This was followed by a second infusion of flumazenil or saline. Subjects received placebo/saline/saline (Group 1), diazepam/saline/saline (Group 2), diazepam/flumazenil/saline (Group 3) and diazepam/saline/flumazenil (Group 4). In Session Two, the CS was presented without the US; 50% of CS presentations terminated with a sound pulse; an equal number of sound pulses were presented without the CS. Auditory evoked potentials were recorded at Cz. In Session Two, CS presentation enhanced the auditory N1/P2 potential in placebo-treated subjects (Group 1). This enhancement was prevented by diazepam (Group 2). Flumazenil reversed diazepam's effect on fear potentiation if it was administered before conditioning (Group 3), but not if it was administered afterwards (Group 4). The results confirm that diazepam prevents the acquisition of fear conditioning in humans, and suggest that it disrupts the formation of CS/US associations, rather than the consolidation of fear memory.

Sensitivity of late-latency auditory and somatosensory evoked potentials to threat of electric shock and the sedative drugs diazepam and diphenhydramine in human volunteers

Late-latency auditory and somatosensory evoked potentials are sensitive to some centrally acting drugs and to certain psychological interventions. In this experiment we compared the effects of acute doses of a benzodiazepine, diazepam and an H(1) histamine receptor-blocking sedative, diphenhydramine, on auditory and somatosensory evoked potentials within the latency range 100-500 ms in a fear conditioning paradigm. Twelve healthy males (18-30 years) participated in three sessions at weekly intervals in which they received diazepam 10mg, diphenhydramine 75 mg and placebo in a balanced, double-blind, crossover protocol. One hundred and twenty min after diphenhydramine or 60 min after diazepam, they underwent an 8 min recording period in which auditory evoked potentials elicited by 40 ms, 95 dB[A], 1 kHz tones, and somatosensory evoked potentials elicited by a mildly painful electric shock (1.8 mA, 50 ms) were recorded at Cz (vertex). Each session consisted of four blocks of trials in which either the sound pulse or the shock was presented. Alternate blocks were designated SAFE or THREAT ('context' conditions); in THREAT blocks subjects were warned that shocks would be delivered via electrodes placed on the wrist (electrodes were removed during SAFE blocks). In one SAFE and one THREAT block, the sound stimuli and shocks (shocks were delivered only in the THREAT block) were preceded by a 2 s conditioned stimulus (CS: a red light) ('cue' condition). Diazepam, but not diphenhydramine, reduced the amplitude of the P2 auditory evoked potential. The THREAT context was associated with increased N1 and reduced N2 potential amplitudes. The CS had no effect on the amplitudes, but markedly reduced the latencies of the N1, P2 and N2 potentials under the THREAT condition. Diazepam reduced the amplitudes of the somatosensory potential evoked by the shock; the CS shortened the latencies of the later components of the response. Diazepam and diphenhydramine were approximately equi-sedative in the doses used in this experiment, as judged by visual analogue self-rating scales. The results indicate that the suppression of late-latency auditory and somatosensory evoked potentials by diazepam is not simply a reflection of sedation. Late-latency evoked potentials can be modified by an aversive CS, but the components that are sensitive to the CS are different from those that are sensitive to diazepam.

Diazepam suppresses the acquisition but not the expression of 'fear-potentiation' of the acoustic startle response in man

Sudden auditory stimuli elicit a short-latency muscular response (acoustic startle response) which is enhanced during presentation of a Pavlovian conditioned stimulus (CS) that has previously been paired with an aversive unconditioned stimulus (US) ('fear-potentiation'). In rodents, acute treatment with benzodiazepines blocks both the acquisition of fear-potentiation and the expression of fear-potentiation induced by prior exposure to CS/US pairing. We examined the effect of diazepam on the acquisition and expression of fear-potentiation of the acoustic startle response in man. Forty-six male volunteers (18-30 years) participated in two sessions separated by 7 days. In session 1, they were exposed to 20 2-s presentations of a light (CS), 50% of which terminated with an electric shock to the wrist (1.8 mA, 50 ms: US). Somatosensory potentials evoked by the US were recorded from the scalp at Cz, and skin conductance responses from electrodes taped to the second and fourth fingers. In session 2, the CS was presented 20 times without the US; a random 50% of CS presentations terminated with a sound pulse (40-ms 115-dB 1-kHz); an equal number of sound pulses was presented without the CS. Electromyographic responses of the orbicularis oculi muscle to the acoustic stimuli were recorded from electrodes placed on the lower eyelid, late-latency auditory evoked potentials were recorded at Cz, and skin conductance responses from electrodes taped to the second and fourth fingers. In each session, alertness was measured using visual analogue self-rating scales and critical flicker fusion frequency. Subjects received placebo or diazepam 10mg in the two sessions in a double-blind protocol: group 1 (n 12) placebo/placebo; group 2 (n 11) placebo/diazepam; group 3 (n 12) diazepam/placebo; group 4 (n 11) diazepam/diazepam. Diazepam reduced alertness as measured by visual-analogue self-rating scales and critical flicker fusion frequency. In session 1, diazepam reduced the amplitude of the somatosensory potentials and skin conductance responses evoked by the CS. In session 2, the acoustic startle response, the N1/P2 auditory evoked response and the skin conductance response evoked by the sound stimuli were enhanced in the presence of the CS. This fear-potentiation was attenuated in subjects who received diazepam in session 1, but was not affected by the treatment given in session 2. The results indicate that diazepam blocks the acquisition of fear-potentiation of startle responses in man, as in animals, but does not prevent the expression of a previously learned response.

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.

Peripheral and central nervous changes in patients with rheumatoid arthritis in response to repetitive painful stimulation

It has been observed that patients with rheumatoid arthritis (RA) respond differently to repetitive painful stimulation. The present study investigated whether this is related to the peripheral or central nervous nociceptive system. EEG-derived potentials and the negative mucosal potential (NMP) from the respiratory epithelium were recorded in response to painful intranasal stimulation with gaseous CO(2). Differences between groups (12 RA patients, 12 controls) were found when stimuli were presented at short intervals. While the NMP did not differ between groups, patients had larger cortical responses to the first stimuli of a series of painful stimuli. This may indicate that in RA central nervous changes of nociceptive processing are present.

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.

Auditory and electroencephalographic effects of midazolam and alpha-hydroxy-midazolam in healthy subjects

AIMS

Whereas cortical EEG effects of benzodiazepines are well characterized, information about benzodiazepine effects in other areas of the central nervous system is sparse. This study investigated the action of midazolam and its active metabolite alpha-hydroxy-midazolam on different parts of the auditory pathway in six healthy volunteers in a randomized, double-blind, three-way cross-over study.

METHODS

Acoustically evoked short (SLP) and middle (MLP) latency potentials, transitory evoked otoacoustic emissions (TEOAE), and EEG power spectra were analysed after short i. v. injections of placebo, or 0.15 mg kg-1 midazolam, or alpha-hydroxy-midazolam, respectively.

RESULTS

All subjects fell asleep during the 4 min infusion of active drug. SLP showed a significant transient increase of Jewett wave V 10 min after injection for midazolam and alpha-hydroxy-midazolam while the latency of wave I was unchanged. Both benzodiazepines induced a marked and long-lasting MLP amplitude decrease for 240 min with slow recovery over the following 360 min. No changes of TEOAE were observed. In agreement with earlier reports, increases in EEG beta activity and decreases in alpha activity were observed after administration of either drug.

CONCLUSIONS

Systemically administered benzodiazepines modulate the auditory pathway above the level of the cochlea. While SLP changes were closely associated with sedation and high plasma benzodiazepine concentrations, MLP effects persisted for hours after sedation even at low benzodiazepine plasma levels. Evoked potentials may therefore be more sensitive than EEG as a tool to monitor benzodiazepine effects.

Midazolam does not influence intravenous fentanyl-induced analgesia in healthy volunteers

The effects of saline and intravenous midazolam (0.5, 1, and 2 mg per 70 kg) in combination with intravenous fentanyl (0.1 mg/70 kg) were examined on pain induced by a cold pressor test. Healthy volunteers (six females, six males) were enrolled in a prospective, double-blind, randomized, crossover trial in which mood and psychomotor performance were also examined. Five minutes and 135 min postinjection subjects immersed their forearm in ice cold water for 3 min while assessments of pain were recorded. During the first immersion, subjects reported significantly lower pain intensity and bothersomeness ratings after having been injected with fentanyl, relative to the saline condition, but the addition of midazolam neither increased nor decreased pain reports. During the second immersion (approximately 2.5 h postinjection) pain ratings did not differ between the drug and saline conditions. Mood-altering and psychomotor-impairing effects of the drug combination were dose related. We conclude that midazolam at the doses and route of administration tested neither potentiates nor decreases the analgesia produced by fentanyl in a cold-pressor pain assay.

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.

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.