Tooth pulp-evoked potentials in the monkey: cortical surface and intracortical distribution

Olfactory (Chemosensory) Event-Related Potentials

Chemosensory event-related potentials (CSERPs) have found their way into a number offields of research where they help to determine the function of both the trigeminal and the olfactory system. The investigation of chemosensory deficits in patients with Parkinson's or Alzheimer's disease is only one of the typical applications. It can be assumed that scientists involved in research on patients with multiple chemical sensitivities will also benefit from having access to objective data covering different aspects of the sense of smell.

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.

Multiple complex somatosensory systems in mature rat molars defined by immunohistochemistry

OBJECTIVE

Intradental sensory receptors trigger painful sensations and unperceived mechanosensitivity, but the receptor bases for those functions are only partly defined. We present new evidence here concerning complex endings of myelinated axons in rat molars.

DESIGN

We sectioned mature rat jaws in sagittal and transverse planes to analyze neural immunoreactivity (IR) for parvalbumin, peripherin, neurofilament protein, neurotrophin receptors, synaptophysin, calcitonin gene-related peptide (CGRP), or mas-related g-protein-receptor-d (Mrgprd).

RESULTS

We found two complex sensory systems in mature rat molar dentin that labeled with neurofilament protein-IR, plus either parvalbumin-IR or peripherin-IR. The parvalbumin-IR system made extensively branched, beaded endings focused into dentin throughout each pulp horn. The peripherin-IR system primarily made unbeaded, fork-shaped dentinal endings scattered throughout crown including cervical regions. Both of these systems differed from neuropeptide CGRP-IR. In molar pulp we found peripherin- and parvalbumin-IR layered endings, either near special horizontal plexus arrays or in small coiled endings near tangled plexus, each with specific foci for specific pulp horns. Parvalbumin-IR nerve fibers had Aβ axons (5-7μm diameter), while peripherin-IR axons were thinner Aδ size (2-5μm). Mechano-nociceptive Mrgprd-IR was only found in peripherin-IR axons.

CONCLUSIONS

Complex somatosensory receptors in rat molars include two types of dentinal endings that both differ from CGRP-IR endings, and at least two newly defined types of pulpal endings. The PV-IR neurons with their widely branched, synaptophysin-rich, intradentinal beaded endings are good candidates for endodontic non-nociceptive, low threshold, unperceived mechanoreceptors. The complex molar dentinal and pulpal sensory systems were not found in rat incisors.

Objective pain diagnostics: clinical neurophysiology

Neurophysiological techniques help in diagnosis, prognosis and treatment of chronic pain, and are particularly useful to determine its neuropathic origin. According to current standards, the diagnosis of definite neuropathic pain (NP) needs objective confirmation of a lesion or disease of somatosensory systems, which can be provided by neurophysiological testing. Lesions causing NP mostly concern the pain-temperature pathways, and therefore neurophysiological procedures allowing the specific testing of these pathways (i.e., A-delta and C-fibres, spino-thalamo-cortical tracts) are essential for objective diagnosis. Different techniques to stimulate selectively pain-temperature pathways are discussed. Of these, laser-evoked potentials (LEPs) appear as the easiest and most reliable neurophysiological method of assessing nociceptive function, and their coupling with autonomic responses (e.g., galvanic skin response) and psychophysics (quantitative sensory testing - QST) can still enhance their diagnostic yield. Neurophysiological techniques not exploring specifically nociception, such as standard nerve conduction velocities (NCV) and SEPs to non-noxious stimulation, should be associated to the exploration of nociceptive systems, not only because both may be simultaneously affected to different degrees, but also because some specific painful symptoms, such as paroxysmal discharges, may depend on specific alteration of highly myelinated A-beta fibres. The choice of techniques is determined after anamnesis and clinical exam, and tries to answer a number of questions: (a) is the pain-related to injury of somatosensory pathways?; (b) to what extent are different subsystems affected?; (c) are mechanisms and lesion site in accordance with imaging data?; (d) are results of use for diagnostic or therapeutic follow-up? Neuropathic pain (NP) affects more than 15 million people in Western countries, and its belated diagnosis leads to insufficient or delayed therapy. The use of neurofunctional approaches to obtain a "physiological photograph" of somatosensory function is therefore highly relevant, as it yields significant clues about the type and mechanisms of pain, thus prompting rapid and optimised therapy.

Differential fMRI activation to noxious heat and tactile stimuli in parasylvian areas of new world monkeys

Emerging evidence supports an important role of posterior parasylvian areas in both pain and touch processing. Whether there are separate or shared networks for these sensations remains controversial. The present study compared spatial patterns of brain activation in response to unilateral nociceptive heat (47.5°C) or innocuous tactile stimulation (8-Hz vibration) to digits through high-resolution functional magnetic resonance imaging (fMRI) in squirrel monkeys. In addition, the temporal profile of heat-stimulus-evoked fMRI Blood Oxygenation Level Dependent (BOLD) signal changes was characterized. By examining high-resolution fMRI and histological measures at both the individual and the group levels, we found that both nociceptive heat and tactile stimuli elicited activation in bilateral secondary somatosensory and ventral parietal areas (S2/PV) and in ipsilateral ventral somatosensory areas (VS) and retroinsula (Ri). Bilateral posterior insular cortex (pIns) and area 7b responded preferentially to nociceptive heat stimulation. Single voxels within each activation cluster showed robust BOLD signal changes during each block of nociceptive stimulation. Across animals (n=11), nociceptive response magnitudes of contralateral VS and pIns and ipsilateral Ri were significantly greater than corresponding areas in the opposite hemisphere. In sum, both distinct and shared areas in regions surrounding the posterior sylvian fissure were activated in response to nociceptive and tactile inputs in nonhuman primates.

Cortical representation area of human dental pulp

To elucidate the dental pulp-representing area in the human primary somatosensory cortex and the presence of A-beta fibers in dental pulp, we recorded somatosensory-evoked magnetic fields from the cortex in seven healthy persons using magnetoencephalography. Following non-painful electrical stimulation of the right maxillary first premolar dental pulp, short latency (27 ms) cortical responses on the magnetic waveforms were observed. However, no response was seen when stimulation was applied to pulpless teeth, such as devitalized teeth. The current source generating the early component of the magnetic fields was located anterior-inferiorly compared with the locations for the hand area in the primary somatosensory cortex. These results demonstrate the dental pulp representation area in the primary somatosensory cortex, and that it receives input from intradental A-beta neurons, providing a detailed organizational map of the orofacial area, by adding dental pulp to the classic "sensory homunculus".

Intranasal trigeminal function in subjects with and without an intact sense of smell

The intranasal trigeminal system is involved in the perception of odors. To investigate the cerebral processing of sensory information from the trigeminal nerve in detail we studied subjects with and without olfactory function using functional magnetic resonance imaging. A normosmic group (n=12) was compared with a group of anosmic subjects (n=11). For trigeminal stimulation gaseous CO(2) was used. Following right-sided stimulation with CO(2) controls exhibited a stronger right-sided cerebral activation than anosmic subjects. Stronger activation was found in controls compared to anosmic subjects for the right prefrontal cortex, the right somatosensory cortex (SI), and the left parietal insula. In contrast, relatively higher activation was found in anosmic subjects for the left supplementary motor area in the frontal lobe, the right superior and middle temporal lobe, the left parahippocampal gyrus in the limbic lobe, and the sub-lobar region of the left putamen and right insula which was mostly due to a decreased BOLD signal of controls in these areas. Additional conjunction analysis revealed that activated areas common to the two groups were the cerebellum and the right premotor frontal cortex. These data suggest that the processing of the trigeminally mediated information is different in the presence or absence of an intact sense of smell, pointing towards the intimate connection between the two chemosensory systems.

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.

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.

Dynamic changes of touch- and laser heat-evoked field potentials of primary somatosensory cortex in awake and pentobarbital-anesthetized rats

In this investigation, changes of mechanical- (MEP) and laser-evoked potentials (LEP) in rat primary somatosensory cortex during the course of pentobarbital (PB) anesthesia were examined. Temporal analysis of changes in the magnitude and latency of MEP and LEP, EEG activity, gross motor behaviors, and the tail flick response following laser stimulation before, during, and after PB administration (50 mg/kg, i.p.) was performed and correlated in chronically implanted rats. During the wakeful condition, there were two major cortical components each following mechanical stimulation (MEP1 and MEP2, n=17) and laser stimulation (LEP1 and LEP2, n=10), respectively. After PB administration, the positive peak in MEP1 was enhanced, and all other components disappeared. These components returned with different time courses. Two hours after PB administration, when the rat had spontaneous movements and flexor reflexes, LEP2 showed reversed polarity. MEP2 returned gradually 3 h after PB administration when the rat regained its ability to execute coordinated movements. After 4 h, LEP1 began to reappear and LEP2 returned to its negative polarity. We found that PB facilitated Abeta fiber-related cortical evoked potential (MEP1), while differentially inhibited Adelta and C fiber-related components (MEP2, LEP1 and LEP2). Characterization of these anesthesia-induced changes in cortical output may be useful in studying the neural basis of tactile and pain sensations.

Patients with rheumatoid arthritis adapt differently to repetitive painful stimuli compared to healthy controls

The aim of the study was to investigate whether there are changes of the nociceptive system in patients with chronic inflammatory joint pain. A pain model was used which is based on the recording of cortical chemo-somatosensory event-related potentials (CSSERP) after nociceptive stimulation of the nasal mucosa with gaseous carbon dioxide (CO(2)). Twenty-five patients with rheumatoid arthritis (RA) were compared to healthy controls matched for age and gender. Responses to both different intensities of painful stimuli and constant intensities of series of 4 stimuli were analysed. When testing increasing CO(2) concentrations ratings and CSSERP amplitudes increased for both patients and controls. However, when repetitive stimulation was performed with an interval of 2s CSSERP amplitudes N1 were significantly greater in RA patients. It is hypothesized that chronic inflammatory joint pain changes nociceptive processing in terms of generalized changes of the nociceptive system which may amplify chronic pain.

Microelectrode studies of normal organization and plasticity of human somatosensory thalamus

Microelectrode studies of single units in the human thalamus during stereotactic surgery offer a unique opportunity to study the organization and plasticity of the sensory thalamus. In this review the authors present results using single-unit microelectrode recording in the mapping of human sensory thalamus in a variety of patients. First they outline the overall organization of the human sensory thalamus, including both somatosensory and pain pathways. They also show that the sensory maps for receptive and projection fields can be altered during pathologic states such as amputation and spinal transection. Additionally, the sensory maps show plasticity during states with abnormal patterns of motor activity, like dystonia. Lastly, they discuss the processing of painful and emotionally laden sensory experiences through the thalamus. The physiologic results of thalamic pain processing are discussed in relation to the sensory-limbic model of pain. The studies reviewed demonstrate the spectrum of stimulus processing and plasticity of both painful and nonpainful signals by the human thalamus.

Direct Evidence of Nociceptive Input to Human Anterior Cingulate Gyrus and Parasylvian Cortex

Many lines of evidence implicate the anterior cingulate cortex (ACC, Brodmann's area 24) and parasylvian cortex in pain perception. Clinical studies demonstrate alterations in pain and temperature sensation after lesions of these structures. Imaging studies reveal increased blood flow in ACC and parasylvian cortex, both ipsilateral and contralateral to painful stimuli. Additionally, painful stimuli evoke potentials that seem to arise from these cortical structures. Short-duration cutaneous stimulation with a CO(2) laser evokes pain-related potentials (LEPs) with a vertex maximum and an initial negative peak followed by a positive wave. The cutaneous laser stimulus evokes a pure pain sensation due to selective activation of cutaneous nociceptors. Electrical source modeling has suggested that the vertex maximum of the scalp LEP arises, in part, from generators in the cingulate gyrus and parasylvian cortex. Thus, imaging and electrophysiologic studies suggest that these cortical structures are activated by painful stimuli. However, these studies incorporate multiple assumptions and therefore do not establish the presence of nociceptive inputs to ACC and parasylvian cortex. We review our recent reports of intracranial potentials evoked by painful stimuli. These studies provide direct evidence of nociceptive inputs to the human ACC and parasylvian cortex.

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.

[Operculo-insular responses to nociceptive skin stimulation in humans. A review of the literature]

CO2 laser stimulation selectively activates the endings of small myelinated A delta fibers, involved with non-myelinated C fibers in the processing of nociceptive information. Thus, potentials evoked by CO2 laser stimulation reflect the activation of cortical areas receiving inputs from the spinothalamic tract. In this article we review data on the early pain-related CO2 laser evoked potentials recorded on the scalp, or by intracortical electrodes, during presurgical assessment of patients with drug-resistant epilepsy. A combination of surface and depth recordings allows the description of early cortical pain responses in terms of latency, polarity and scalp topography. Such a technique also allows the localization of the anatomical generators of these early responses using dipolar source modeling of scalp-recorded evoked potentials, or intracortical recordings, in stereotactical conditions. The earliest response recorded on the scalp to CO2 laser stimulation was an N1-P1 dipolar potential field at a latency of 140-200 ms. The N1 and P1 maximal voltages are recorded in the temporal region contralateral to stimulation and mid-frontal region, respectively. Intracerebral electrodes record an activation of a dipolar cortical source in the same latency range located in the upper bank of the sylvian fissure, corresponding to the second somatosensory (SII) area ipsi- and contralateral to the stimulation and insular cortex. The SII-insular responses ipsilateral to stimulation are likely to be triggered via transcallosal fibers coming from the opposite SII area. The operculo-insular cortex contralateral to stimulation, activated through direct thalamocortical projections, is likely to represent the first step in the cortical processing of peripheral A delta fiber pain inputs.

Painful stimuli evoke potentials recorded from the parasylvian cortex in humans

Cutaneous stimulation of the face and hand with a CO2 laser in three awake patients evoked potentials (LEPs) recorded from the dominant left parasylvian cortex. These were recorded by means of a subdural grid of electrodes implanted for evaluation of epilepsy. Stimulation of the contralateral face resulted in waveforms consisting of a negative potential (N2, 162 +/- 5 ms; mean +/- SE) followed by a positive potential (P2, 340 +/- 18 ms). Both waves occurred at longer latency after hand than after facial stimulation. N2 and P2 potentials recorded from the grid correspond well in morphology to those recorded from the scalp in four additional patients tested with the same stimulation paradigm. The N2 waves recorded from the subdural grid occurred at significantly shorter latencies than did those recorded from the scalp (184 +/- 6 ms), but the P2 waves at the grid occurred at significantly longer latencies than did those recorded at the scalp (281 +/- 13 ms). The amplitudes of the potentials recorded from the grid were maximal over the parietal operculum both for contralateral stimulation of the face or hand and for ipsilateral stimulation of the face. Potentials also were recorded in this area after stimulation of the ipsilateral hand. The cortical distributions of these potentials suggest that their generators are located in the parietal operculum or in the insula, or in both, consistent with previous PET, magnetoencephalographic, and scalp LEP source analyses. These previous analyses provide indirect evidence of nociceptive input to parasylvian cortex because the interpretation of each analysis incorporates multiple assumptions. The present results are the first direct evidence of nociceptive input to the human parasylvian cortex.

Olfactory and intranasal trigeminal event-related potentials in anosmic patients

The aim of this study was to investigate the usefulness of chemosensory event-related potentials (CSERPs) in response to both olfactory and intranasal trigeminal stimulation in the diagnosis of anosmia. Forty-four patients participated. Gaseous CO2 was used for trigeminal stimulation, vanillin and H2S were used as olfactory stimulants. Event-related potentials to olfactory stimuli could not be obtained in any of the anosmic patients, indicating the complete loss of the sense of smell. However, all patients responded to stimulation of the trigeminal nerve with CO2. These data clearly demonstrate the clinical significance of CSERPs in the assessment of anosmia.

Scalp topography and dipolar source modelling of potentials evoked by CO2 laser stimulation of the hand

CO2 laser evoked potentials to hand stimulation recorded using a scalp 19-channel montage in 11 normal subjects consistently showed early N1/P1 dipolar field distribution peaking at a mean latency of 159 ms. The N1 negativity was distributed in the temporoparietal region contralateral to stimulation and the P1 positivity in the frontal region. The N1/P1 response was followed by 3 distinct components: (1) N2a reaching its maximal amplitude at the vertex and ipsilaterally to the stimulated hand, (2) N2b mostly distributed in the frontal region, and (3) P2 with a mid-central topography. Brain electrical source analysis showed that this sequence was explained, with a residual variance below 5%, by a model including two dipoles in the upper bank of the Sylvian fissure of each hemisphere, a frontal dipole close to the midline, and two anterior medial temporal dipoles, thus suggesting a sequential activation of the two second somatosensory areas, anterior cingulate gyrus and the amygdalar nuclei or the hippocampal formations, respectively. This model fitted well with the scalp field topography of grand average responses to stimulation of left and right hand obtained across all subjects as well as when applied to individual data. Our findings suggest that the second somatosensory area contralateral to the stimulation is the first involved in the building of pain-related responses, followed by ipsilateral second somatosensory area and limbic areas receiving noxious inputs from the periphery.

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.

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.

Analgesic effects of dihydrocodeine and tramadol when administered either in the morning or evening

The aim of the study was to investigate the analgesic effects of two opioids [dihydrocodeine (DHC) and tramadol] when administered either in the morning or evening. The experimental technique used is based on chemosomatosensory event-related potentials (CSSERPs) in response to painful chemical stimuli that are applied to the nasal mucosa. Eighteen healthy volunteers participated in the experiments. The study followed a controlled, randomized, double-blind, sixfold, cross-over design. Thus, each of the three medications (90 mg DHC, 50 mg tramadol, or placebo) was perorally administered to all subjects on different days at 08:00 or 20:00 h. Measurements were performed before and 60, 120, 240, and 360 min after administration of the medication. In addition to the assessment of CSSERP, subjects rated the intensity of the stimuli. Moreover, unspecific drug effects were monitored by means of acoustical event-related potentials and the subjects' performance in a video game. The results indicated that the painful intensity of the chemical stimuli strongly increased during evening sessions. In addition, both DHC and tramadol exerted stronger analgesic effects when administered in the evening. Thus, an inflexible scheme of prescription might produce either an increase of pain in the morning due to insufficient analgesia or the unnecessary overdosing of analgesics in the evening.

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

1. The aim of this study was to investigate the sensitivity of pain-related potentials used in experimental pain models to the non-specific effects of the tranquilizer diazepam. Pain-related potentials were recorded after painful stimulation of the nasal mucosa with CO2 and after painful stimulation of the tooth pulp. Acoustically evoked potentials were measured in order to compare their sensitivity to the tranquilizer diazepam with the sensitivity of the pain-related potentials. 2. Twenty volunteers participated in this randomised, double-blind, three-fold crossover study. Measurements were obtained before and 20 min after the administration of the drug. Event-related potentials were recorded after painful stimulation of the nasal mucosa with CO2 (two stimulus intensities: 60% v/v and 70% v/v CO2), after painful stimulation of the tooth pulp (two stimulus intensities: 2.2 x and 3.3 x detection threshold), and after non-painful acoustical stimulation of the right ear. The subjects rated the perceived intensity of the painful stimuli by means of a visual analogue scale. In addition the spontaneous EEG was analysed in the frequency domain and the vigilance of the subjects was assessed in a tracking task. 3. Diazepam reduced significantly the amplitudes of the event-related potentials after painful stimulation of the tooth pulp and after acoustical stimulation. In contrast only a small, statistically non-significant reduction could be found after painful stimulation with CO2. The pain ratings of the painful stimuli were not affected by diazepam. Diazepam reduced the performance of the tracking task. A decrease of arousal could be found in the alpha 2-range, whereas in the beta 2 and the theta-range the power density increased under diazepam. 4. We demonstrated that event-related potentials after painful stimulation of the nasal mucosa with CO2 are less affected by the nonspecific effects of the tranquilizer diazepam than event-related potentials after painful stimulation of the tooth pulp. The effects of diazepam on the tracking task, the spontaneous EEG and the event-related potentials clearly confirm its sedative properties. Diazepam had no analgesic effect measurable by pain intensity estimates.

Encoding of the subjective intensity of sharp dental pain

This paper is a review and a discussion of our own pain research over the last decade. It is of a methodological and theoretical character and deals with preparation technique, choice of electrodes, control experiments involving pulpotomy and reliability tests of psychophysical methods for pain measurements, and the neuronal population encoding of sharp dental pain. The electrophysiological recording technique selectively picks up electrical activity induced in pulpal A-delta nerve fibers. The sensation of pain was quantified by means of an intermodal matching technique, finger span (PAS), in combination with sensory verbal descriptors covering a range from very, very weak to maximal pain. When a cold stimulus, ethyl chloride, was applied on the tooth surface a close agreement was demonstrated between intradental A-delta nerve activity (INA) and the sensation magnitude of pain (PAS) with respect to curve amplitude and time course. The high covariation of the neural and perceptual response measures indicated a good internal validity and confirmed also the basic soundness and the applicability of the procedures employed. For the purpose of further analyzing the functional relation of INA to PAS we studied specifically the effect of cold stimuli of different intensity on the integrated nerve response. Only sharp, shooting pain was accepted as a sensorial, perceptual correlate of the intradental A-delta nerve activity. Since an increase in amplitude was generally accompanied by an increase in duration of the responses, the fundamental question was raised how to best describe and characterize the neural and perceptual responses so that they most adequately reflect the information processing of the intensive aspect of sharp dental pain.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the antinociceptive effects of imipramine, tramadol and anpirtoline

The pain relieving properties of imipramine (100 mg orally), tramadol (150 mg orally), and anpirtoline (60 mg orally) were compared in 16 healthy subjects in a cross-over, double-blind, randomized, and placebo-controlled study. Anpirtoline exhibits analgesia which is possibly mediated via serotoninergic pathways, whereas tramadol exerts its effects at opioid receptors. The pain-relieving effect of the tricyclic antidepressant imipramine may involve both serotoninergic and opioid mechanisms. Chemo-somatosensory event-related potentials (CSSERP) were recorded after painful stimulation of the nasal mucosa with carbon dioxide. Subjects rated the perceived intensity of the stimuli by means of a visual analogue scale. In addition, acoustically evoked responses were recorded, the spontaneous EEG was analyzed in the frequency domain, the subjects' vigilance was assessed in a tracking task, and side effects of the drugs were monitored. Anpirtoline and tramadol produced a decrease of both CSSERP amplitudes and subjective estimates of pain, the effects of the former compound being greater. In contrast, after administration of imipramine no change of CSSERP amplitudes could be detected, whereas the subjective estimate of pain intensity decreased significantly. This was accompanied by a significant decrease of arousal indicating that pain relief produced by acute administration of imipramine was primarily related to its sedation action. The analgesic properties of anpirtoline were demonstrated in man. Tramadol was characterized as a week opioid analgesic. In contrast, imipramine appeared to produce its pain-relieving effects predominantly by non-specific actions. It is hypothesized that different analgesics may change ERP sources in a drug-specific manner.

Reversible pain and tactile deficits associated with a cerebral tumor compressing the posterior insula and parietal operculum

Extensive psychophysical tests were conducted on a patient with a well circumscribed tumor located just inferior and posterior to the retroinsular cortex of the right hemisphere. Statistically significant laterality differences were observed, with the left hand exhibiting: (1) a higher mechanical pain threshold, (2) a higher heat pain threshold, (3) a greater cold pain tolerance, and (4) a poorer ability to discriminate roughness. The patient was re-examined 2.5 months after operative removal of the tumor and was found to have regained normal sensitivity in his left hand. Pre- and postoperative MRIs showed resolution of the tumor's mass effect on the retroinsular and neighboring parietal operculum, which likely included the second somatosensory cortex. This dramatic change in sensory capacity signifies an essential role for the posterior insula and parietal operculum in normal pain and tactile perception.

Peripheral and central mechanisms of cutaneous hyperalgesia

Hyperalgesia after cutaneous injury can be divided into two phenomena: Primary hyperalgesia occurs at the site of injury and is characterized by hyperalgesia to mechanical and heat stimuli. Secondary hyperalgesia occurs outside the injury site and is characterized by mechanical hyperalgesia only. Hyperalgesia in inflammatory processes corresponds to primary hyperalgesia. Hyperalgesia in referred pain and neuropathic pain resembles secondary hyperalgesia (Table 3). Evidence for the latter would be strengthened if hyperalgesia to cooling stimuli, which is observed in neuropathic pain, was also demonstrated in referred pain and in secondary hyperalgesia. Some of the more likely neural mechanisms to explain primary and secondary hyperalgesia are illustrated in Fig. 8. Primary hyperalgesia to heat stimuli has a counterpart in the sensitization of peripheral nociceptors to heat stimuli (Fig. 8A), leading to similar changes in central neurons. In addition, the enlargement of the mechanical receptive field of primary afferent nociceptors to include the site of injury may account for the primary hyperalgesia to mechanical stimuli (Fig. 8B). In the literature, there are some contradictions with respect to the stimulus modalities to which hyperalgesia and sensitization occur. In spite of the well-documented sensitization of primary afferent nociceptors to heat stimuli, there are few studies on its molecular mechanisms. On the other hand, there is pharmacological evidence for a peripheral mechanism of primary mechanical hyperalgesia, but little direct evidence that nociceptors can be sensitized to mechanical stimuli by injury. This contradiction should spawn further investigations into the mechanical response properties of nociceptors and into the molecular mechanisms of heat sensitization. Secondary hyperalgesia to mechanical stimuli is likely due to the sensitization of central pain signalling neurons (CPSNs). This sensitization could involve only input from nociceptors (Fig. 8C), since mechanical pain thresholds after a cutaneous injury are of the same order as those of nociceptors. Central sensitization could also be the result of enhanced connectivity between low-threshold mechanoreceptors and CPSNs (Fig. 8D). This form of sensitization may account for the pain to light touch associated with neuropathic pain. Receptive field plasticity is a prevalent property of dorsal horn neurons and probably plays a vital role with regard to hyperalgesia. The molecular mechanisms of synaptic plasticity are currently subject to intense experimental investigation and may provide new insights on the mechanisms of pain and hyperalgesia.

Differences in human evoked potentials related to olfactory or trigeminal chemosensory activation

The aim of the present study was to determine, whether there are differences in the topographical distribution of chemosensory evoked potentials (CSEPs) due to stimulation with different odorous substances. The odorants used in the study which mainly excited the olfactory nerve were vanillin and acetaldehyde; those which additionally excited the trigeminal nerve were sulphur dioxide and ammonia. Twelve subjects participated in the study. The subjects separately estimated the intensity of the odorous and of the painful/pricking sensation caused by the stimuli, and described the odorous qualities in their own words. CSEPs were recorded from 7 positions. After stimulation with "olfactory" substances maximum CSEP amplitudes were recorded at parieto-central sites, and after stimulation with "trigeminal" substances maximum amplitudes were obtained at the vertex. Following stimulation with ammonia and sulphur dioxide amplitudes were largest contralateral to the stimulated nostril. In contrast, little difference in CSEP amplitudes was observed between hemispheres after stimulation with vanillin or acetaldehyde. Thus, the topographical distribution of CSEP amplitudes may provide information with regard to the sensory system (olfactory or trigeminal) activated by the presentation of an odorous stimulus.

Dose-related analgesic effects of flupirtine

1. Flupirtine is a novel and, in all probability, centrally acting, analgesic. The present investigation was conducted in order to investigate dose-related effects of perorally administered flupirtine in man, with special regard to specifically analgesic actions, employing a model based on pain-related chemosomatosensory evoked potentials and subjective intensity estimates of painful stimuli. 2. Plasma concentrations of flupirtine measured 2 h after dosing linearly increased as a function of the administered dose. 3. It was possible to reproduce our own previously obtained results, which established the analgesic action of 200 mg flupirtine administered perorally. 4. Intensity estimates linearly decreased as a function of the administered dose, whereas chemosomatosensory evoked potential amplitudes non-linearly changed in relation to the administered dose. 5. In the spontaneous EEG, a dose-dependent increment in the power-spectra was observed, and this mainly in the alpha- and beta-range.

Tooth pulp-evoked potentials in the trigeminal brainstem nuclear complex

The surface and depth distributions of mandibular canine, tooth pulp-evoked potentials (TPEPs) in the trigeminal brainstem nuclear complex were studied in anesthetized cats. Three pairs of positive-negative waves or components were elicited from each trigeminal brainstem nucleus (main sensory, MSN; oralis, NO; interpolaris, NI; caudalis or medullary dorsal horn, NC). The location and dipole orientation of the current generator source for each pair of components in each nucleus were determined by using the topographic amplitude distribution of TPEPs in both their normal-reference and inverted polarities and the isoelectric contour line. The current sources for all components were the following: MSN--dorsomedial subnucleus; NO--dorsolateral portion; NI--dorsomedial portion; NC--medial part of superficial and intermediate laminae. These loci are consistent with the central terminal zones of mandibular tooth pulp afferents reported in previous neuroanatomical studies. Measurements of mean peak latencies suggest that tooth pulp A beta afferents contribute to the putatively presynaptic (P1-N1) and monosynaptic (P2-N2) components found in all trigeminal brainstem nuclei and that A delta afferents contribute to the later and possibly polysynaptic components (P3-N3) in the same nuclei. The pertinence of these findings to the theory that both non-nociceptive and nociceptive intradental inputs project to rostral and caudal nuclei are discussed.

Nociceptive responses of trigeminal neurons in SII-7b cortex of awake monkeys

A cluster of trigeminal nociceptive neurons was located in the lateral sulcus on the upper bank of the frontoparietal operculum in a region bordering between cortical areas SII and 7b. These neurons were isolated in cortical cell layers IV and V-VI. All nociceptive neurons responded exclusively to noxious mechanical stimulation of cutaneous receptive fields on the face/head or intraoral tissue. Sustained noxious mechanical stimulation elicited slowly adapting responses that accurately encoded the duration of the stimulation. Prolonged discharges following removal of noxious stimulation were not observed. These nociceptive specific neurons poorly encoded graded noxious stimuli. Trigeminal somatosensory neurons within and surrounding the SII-7b cluster were not topographically organized according to divisions of the trigeminal nerve, laterality of receptive fields, or division of face/head and intraoral receptive fields. The thalamocortical, corticocortical and indirect corticolimbic connectivities of SII and area 7b and the possible role of SII-7b nociceptive neurons in learning, memory and avoidance behaviors are discussed.

Cortical nociceptive responses and behavioral correlates in the monkey

Experiments were performed to characterize cerebral cortical activity and pain behavior elicited by electrical stimulation of the tooth pulp in unanesthetized monkeys. Four monkeys were trained on two different operant paradigms: two on a simple escape task and two on an appetitive tolerance-escape task. All monkeys were implanted with bipolar stimulating electrodes in the right maxillary canine tooth and subdural recording electrodes over the left primary (SI) and/or secondary (SII) somatosensory cortices. Subdural tooth pulp-evoked potentials (TPEPs) recorded over the SII consisted of components P1 (27.5 ms), N1 (40.3 ms), P2 (84.0 ms), N2 (163.5 ms), P3 (295.3 ms), and N3 (468.0 ms). The long latency component (P3-N3) was found exclusively over the SII and was elicited by high intensity stimulation. The appearance of component P3-N3 required the recruitment of A delta nerve fibers into the maxillary nerve compound action potential and was correlated with high frequencies of escape. Administration of morphine sulfate (4 mg/kg, i.m.) caused a contemporaneous reduction in escape frequency and in the amplitude of P3-N3 recorded over the SII. The relationships between TPEP amplitude, escape behavior and A delta nerve fiber activity strongly suggest that the SII is involved with nociception and pain behavior.

Cortical responses to painful CO2 stimulation of nasal mucosa; a magnetoencephalographic study in man

Magnetic and electrical responses to CO2 stimulation of the nasal mucosa were recorded in 4 subjects. The magnetic responses peaked at 350-400 msec, coinciding with the negative vertex potential. The field pattern suggested neural activation in the upper bank of the sylvian fissure near the lateral end of the central sulcus, i.e., at or near SII. The results also suggest that this area contributes to the generation of the electrical vertex potential.