Characteristics of Temporal Summation of Second Pain Sensations Elicited by Brief Contact of Glabrous Skin by a Preheated Thermode

Charles J. Vierck Jr.1, Richard L. Cannon1, Gentry Fry1, William Maixner2, and Barry L. Whitsel3

1 Department of Neuroscience, University of Florida College of Medicine, Gainesville, Florida 32610-0244; 2 Dental Research Center, University of North Carolina School of Dentistry; and 3 Department of Physiology, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27599

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Vierck, Charles J., Jr., Richard L. Cannon, Gentry Fry, William Maixner, and Barry L. Whitsel. Characteristics of temporal summation of second pain sensations elicited by brief contact of glabrous skin by a preheated thermode. J. Neurophysiol. 78: 992-1002, 1997. Temporal summation of sensory intensity was investigated in normal subjects using novel methods of thermal stimulation. A Peltier thermode was heated and then applied in a series of brief (700 ms) contacts to different sites on the glabrous skin of either hand. Repetitive contacts on the thenar or hypothenar eminence, at interstimulus intervals (ISIs) of 3 s, progressively increased the perceived intensity of a thermal sensation that followed each contact at an onset latency >2 s. Temporal summation of these delayed (late) sensations was proportional to thermode temperature over a range of 45-53°C, progressing from a nonpainful level (warmth) to painful sensations that could be rated as very strong after 10 contacts. Short-latency pain sensations rarely were evoked by such stimuli and never attained levels substantially above pain threshold for the sequences and temperatures presented. Temporal summation produced by brief contacts was greater in rate and amount than increases in sensory intensity resulting from repetitive ramping to the same temperature by a thermode in constant contact with the skin. Variation of the interval between contacts revealed a dependence of sensory intensity on interstimulus interval that is similar to physiological demonstrations of windup, where increasing frequencies of spike train activity are evoked from spinal neurons by repetitive activation of unmyelinated nociceptors. However, substantial summation at repetition rates of >= 0.33 Hz was observed for temperatures that produced only late sensations of warmth when presented at frequencies <0.16 Hz. Measurements of subepidermal skin temperature from anesthetized monkeys revealed different time courses for storage and dissipation of heat by the skin than for temporal summation and decay of sensory intensity for the human subjects. For example, negligible heat loss occurred during a 6-s interval between two trials of 10 contacts at 0.33 Hz, but ratings of sensory magnitude decreased from very strong levels of pain to sensations of warmth during the same interval. Evidence that temporal summation of sensory intensity during series of brief contacts relies on central integration, rather than a sensitization of peripheral receptors, was obtained using two approaches. In the first, a moderate degree of temporal summation was observed during alternating stimulation of adjacent but nonoverlapping skin sites at 0.33 Hz. Second, temporal summation was significantly attenuated by prior administration of dextromethorphan, a N-methyl-D-aspartate receptor antagonist.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

Some forms of chronic pain are certain to include sensitization of central neurons as an important pathological component (e.g., Noordenbos 1959; Price et al. 1989; Woolf and Thompson 1991). Proposed mechanisms for long-term sensitization of central nocireceptive neurons, leading to allodynia and hyperalgesia, recently have focused on combined actions of peptides and excitatory amino acids (e.g., Kellstein et al. 1990; Urban and Randic 1984). Central to these models is the phenomenon of windup of central neuronal activity (temporal summation), which has been regarded as instrumental in the creation and/or maintenance of chronic pain (Coderre et al. 1993; Price 1991; Wilcox 1993). Windup is displayed by spinal multireceptive neurons when C nociceptors are activated repetitively at rates of >= 0.3 Hz (Mendell 1966; Price 1972; Price et al. 1971, 1977), producing a progressive enhancement of late responses. The sensitization is presumed to be due to central mechanisms, because the effect requires input from C nociceptors that reveal a declining response amplitude with stimulus repetition (Price et al. 1977), and because increased response magnitudes for central neurons or exaggerated behaviors in animal models of nociceptive sensitivity can be attenuated by N-methyl-D-aspartate (NMDA) receptor antagonists (Davies and Lodge 1987; Kristensen et al. 1992; Mao et al. 1992; Meller et al. 1993; Price 1972; Tal and Bennett 1993; Woolf and Thompson 1991).

Animal models of nociceptive sensitization have suggested that useful therapies for chronic pain might involve antagonism of NMDA receptors (Dickenson 1990), and confirmatory evidence is accumulating for humans. One approach has been to administer analgesics or local anesthesia during a period of substantial nociceptive input (e.g., during surgery), to assess whether postsurgical pain levels are reduced. This technique of preemptive analgesia has been reported to be effective in some cases but not others (Yaksh and Abram 1993). More direct evidence is provided by relief of neuropathic pain by NMDA receptor antagonism (Backonja et al. 1994; Eide et al. 1995; Felsby et al. 1995; Kristensen et al. 1992; Mathisen et al. 1995). Fundamental to these approaches is the assumption that tonic or chronic pain results from temporal summation via excessive activation of NMDA receptors in response either to abnormally high levels of short-term nociceptive input or continual input from nociceptors. One way of evaluating this possibility is to characterize temporal summation of late pain sensations elicited by repetitive stimulation of normal individuals and chronic pain patients (Arendt-Nielsen et al. 1995; Eide et al. 1995). That is, in addition to traditional assessments of chronic pain levels, it should be instructive to assess elicited pain and evaluate the time course of temporal summation and its decay. Such assessments would provide evidence for or against the presence of excessive levels of input from C nociceptors and/or abnormalities in NMDA receptor systems, particularly when combined with pharmacological intervention (Arendt-Nielsen et al. 1995a; Cooper et al. 1986; Price et al. 1994a). With this goal in mind, the present report characterizes pain sensations in normal subjects with a new technique of thermal stimulation, which preferentially produces second pain sensations and generates substantial temporal summation in response to series of brief skin contacts.

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

Subjects ranging in age from 20 to 59 yr were trained to evaluate late sensations elicited by repetitive thermal stimulation of the glabrous skin of either hand. Two forms of psychophysical measurement were used: verbal ratings of peak sensory magnitude and movements of one hand that varied the voltage across a potentiometer and provided a record of the time course of increases and decreases in sensory magnitude.

Two forms of stimulation were employed. The first method mimicked natural conditions of nociceptive thermal stimulation (as when one touches a hot object). The target skin site was positioned 5 mm from the surface of a square (2.56 cm2) Peltier thermode that was preheated to temperatures ranging from 45 to 53°C. When activated, a spring-loaded solenoid moved the thermode 1 cm, ensuring unambiguous contact with the skin. The solenoid was activated for 700 ms, which usually produced only late thermal sensations for the stimulus intensities used. When rating sensory magnitude, the subjects attended to the peak of late sensations that occurred ~2 s after the probe left the skin on each presentation. In most experiments, the interstimulus interval (from onset to onset) was 3 s, but this parameter was varied in some experiments to ascertain its effects. The temperature of the probe was calibrated immediately before each testing session, using an Omega No. 450 ATH attachable surface temperature thermistor (probe No. ON-409-PP). Use of the same thermistor throughout the experiment ensured consistency from session to session and accurate relative values from our apparatus, but it is important to note that different thermistors give different absolute values, depending on the configuration of the probe and cable. Therefore, absolute temperatures measured at a thermode will differ between experimental setups.

The second method of stimulation used a 0.8 cm2 circular thermode that was maintained in constant contact with the skin, and triangular ramps of heat were applied repetitively. The ramp rate was 10°C/s; the duration of the triangular ramps was 1.5 s; and the interstimulus interval was 3 s for ramps from 40 to 50, 51, or 52°C and back to 40°C.

Because the late thermal sensations elicited by brief contact progressed slowly and were quite salient, the subjects were not distracted by other sensations elicited during contact of the probe with the skin. The verbal ratings for both methods were guided by a list of numerical values that were tied to verbal descriptors of sensory magnitude: 10 = warm (no pain); 20 = threshold pain; 30 = very weak pain; 40 = weak pain; 50 = neither strong nor weak pain; 60 = slightly strong pain; 70 = strong pain; 80 = very strong pain; 90 = nearly intolerable pain; 100 = intolerable pain. Values intermediate between two ratings on the list were used (e.g., 65), but no higher resolution was permitted.

To determine sensation latencies and plot the time-course of changes in sensory magnitude, the subjects manipulated either a finger-span device (Cooper et al. 1986) or a slide that produced output voltages in proportion to the extent of a motor movement. Voltage increased linearly with separation of the thumb and forefinger from a resting position of contact or with movement of the slide away from the subject. The voltages were amplified, digitized, and stored on computer for off-line analysis. The subjects were instructed to follow the time course of changes in sensation intensity (see RESULTS) but not attempt to simultaneously estimate peak amplitudes, which could interfere with determinations of peaklatencies.

The general strategy of the study was to test trained subjects repeatedly on each stimulus condition, in an attempt to obtain reliable, within-subjects comparisons for parametric variations of stimulation. Each hand was stimulated at one or more site in each daily session, and >= 3 min elapsed between the last stimulus delivered to one hand and the first stimulus to the other hand. The order of testing the hands within-sessions and stimulating different sites with different stimulus features within an experiment (e.g., when varying temperature or interstimulus interval) was varied randomly. Each stimulus condition was presented to each subject in at least six sessions. For each stimulus condition, the intention was to present and have subjects rate a certain number of stimuli; however, the subjects were instructed to remove their hand from the apparatus before receiving a stimulus that they anticipated would produce an intolerable level of pain. Stimulus levels and sequences were chosen that rarely produced ratings as high as 90 or caused a subject to withdraw the hand.

To evaluate changes in skin temperature that resulted from series of brief contacts of the preheated thermode with glabrous skin, stimulus sequences identical to those delivered in the psychophysical experiments were presented to three anesthetized monkeys as terminal procedures at the conclusion of another project. The animals were premedicated for tracheal intubation with 10 mg/kg ketamine and were maintained under surgical levels of anesthesia with isofluorane. Heart and respiratory rates and blood pressure were monitored continuously. A Yellow Springs thermistor (model 524 in a 25-g needle) was inserted beneath the epidermis at the site of thermal stimulation, which consisted of 700-ms contacts, as in the psychophysical experiments. The output of the thermistor was amplified and recorded continuously during series of skin contacts. The analog record was digitized and stored for off-line analysis. At the conclusion of the recording of skin temperatures, the animals received an overdose (80 mg/kg ip) of pentobarbital sodium (Nembutal) and were perfused through the heart with saline and then paraformaldehyde. The tract in the skin from insertion of the hypodermic needle containing the thermistor was filled with true blue dye, and the region of skin containing the needle tract was sectioned for measurement of the depth of the thermistor.

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

An initial evaluation of temporal summation of late sensory magnitudes evoked by brief skin contact was carried out with five subjects. The thermode was preheated to a temperature of 45, 47, 49, 51, or 53°C and was applied briefly (700-ms duration, nominally) and repeatedly to the thenar or hypothenar eminence of the right or left hand (Fig. 1, left). Averaged ratings of late sensory magnitudes elicited by each of 20 successive contacts at an interstimulus interval (ISI) of 3 s revealed substantial temporal summation for the higher stimulus intensities. A notable feature of these data is that the first contact at all thermode temperatures elicited a sensation of warmth (i.e., was subthreshold for pain). Thus the differentiating features of the curves in Fig. 1, left, are the rate and amount of temporal summation of sensory magnitude from a common starting point. On the average, ratings of late pain sensations progressed from below pain threshold on the first contact to a level of very strong pain after 12 contacts at 53°C on the thenar eminence. For comparison, three of the five subjects rated the magnitude of early sensations (during contact and/or withdrawal of the thermode) in separate sessions of 53°C contact with the thenar eminence (Fig. 1, bottom right). There was detectable temporal summation of the early sensation, but the average estimate of the magnitude of first pain after 20 successive stimuli (25) only slightly exceeded pain threshold. In contrast, the average final rating of the magnitude of late pain in comparable sessions was 82. 


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FIG. 1. Left: brief contact of a preheated thermode with skin on thenar or hypothenar eminence of either hand of 5 subjects produced increases in late thermal sensory intensity across series of 20 presentations at an interstimulus interval (ISI) of 3 s. Numerical ratings of sensory intensity corresponded to categorical ratings that ranged from below pain threshold (20) to nearly intolerable (90; see METHODS). Rate of increasing sensory magnitude and maximal rating were proportional to thermode temperature. Top right: subepidermal skin temperature, sampled immediately before each of 20 stimuli presented to 3 anesthetized monkeys (baseline skin temperature), was related systematically to probe temperature but did not correspond to sensory ratings (e.g., 20th contact at 45°C elicited an average sensory rating below pain threshold, but 6th contact at 53°C, associated with same baseline skin temperature, elicited ratings of slightly strong pain on thenar eminence). Bottom right: for a series of 20 contacts at 53°C and an ISI of 3 s, early and late thermal sensations were rated separately by 3 subjects for stimulation of thenar and hypothenar eminence of each hand. Early sensations occasionally summated to levels slightly above pain threshold, in contrast to dramatic summation of late sensations to levels characterized as very strong pain.

To evaluate relationships between skin temperature and sensory ratings, subepidermal skin temperatures from glabrous skin of three monkeys were measured during sequences of thermode-skin contact at temperatures identical to those used psychophysically. The depth of the thermistor was determined by histological reconstruction in one case to be 1.17 mm below the skin surface. Comparable locations were confirmed grossly for the other two animals, but distortion of the tissue prevented accurate measurement of depths. In Fig. 1, top right, average baseline skin temperatures are shown (sampled immediately before each contact of the probe with the skin, before and after phasic responses to contact occurred). As expected, baseline skin temperature increased within each series, and the amount of change across series was related to the temperature of the thermode. However, these functions were not related systematically with the psychophysical ratings of sensory magnitude. For example, increases in baseline skin temperature were obtained across the 45°C series that did not produce temporal summation of sensory magnitude. Also, different verbal ratings were associated with a given change in skin temperature in series involving different probe temperatures. For example, an increase in baseline skin temperature of 1.5-1.7°C was associated with verbal ratings of 15-16 for associated stimuli in the 45°C series (stimuli 15-20) and with verbal ratings of 52-59 in the 53°C series (stimuli 6-7).

Having identified a thermode temperature (53°C) that produced substantial temporal summation at an ISI of 3 s (i.e., from below pain threshold to very strong pain within 10 presentations to the thenar eminence), the ISI was varied between series at this temperature. Figure 2 (left) reveals a substantial effect of ISI on temporal summation. For the thenar and hypothenar test sites, the rate of summation was related inversely to ISIs from 3 to 6 s, and little or no temporal summation occurred at an ISI of 6 s. The 700-ms contacts of the 53°C thermode with glabrous skin at an ISI of >= 6 s elicited a sensation of warmth but not pain. That is, this stimulus is not painful in the absence of temporal summation.


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FIG. 2. Left: verbal ratings of sensory magnitude are shown as functions of ISIs of 3-7 s for 20 brief contacts of a 53°C probe to thenar or hypothenar eminence of either hand of 3 subjects. Rate of increasing sensory magnitude and maximal rating were inversely proportional to ISI. Right: changes in subepidermal skin temperature across series of 20 stimuli presented to 3 anesthetized monkeys were not systematically related to ISI. For ISIs of 4-6 s, rates of baseline heat storage (measured before each stimulus) were comparable (top right), but ratings of late sensory magnitude were substantially different (left) for these stimulation frequencies. Similarly, peak phasic increases in skin temperature (responses) after each stimulus were not systematically rated to ISI (bottom right).

Figure 2, bottom right, shows peak increases in intradermal skin temperature that occurred in the interval from the beginning of each thermode-skin contact to the beginning of the next contact in each ISI series. These phasic increases in skin temperature were superimposed on a gradually increasing baseline and ranged in peak amplitude from 0.3 to 0.9°C. Phasic skin temperature responses increased slightly from the first to the last contact in the series, but there was no consistent relationship with the ISI. This result is quite different from the regular relationship between verbal ratings of sensory magnitude and ISI (Fig. 2, left).

Disparities between baseline skin temperatures and sensory magnitudes were especially evident in the ISI series. Although baseline skin temperatures increased from contacts 1-20 in series at each ISI (Fig. 2, top right), the relationship between heat storage by the skin and ratings of sensory magnitude was very weak across ISIs. For example, increases in baseline skin temperature for ISIs of 3 and 5 s were indistinguishable throughout the series, but the differences in temporal summation of pain magnitude for these ISIs were considerable. Similarly, the 20th contact at an ISI of 6 s was associated with a 1.13°C increase in baseline skin temperature (from the beginning of the series) and average verbal ratings of 35.4, and the 11th contact at an ISI of 3 s was associated with the same increase in skin temperature but average verbal ratings of 85.

Manipulation of ISIs showed that residual (summating) effects of individual stimuli on the growth of sensory magnitude were minimal or absent with repetition at intervals of 6 s (Fig. 2, left). However, the time course of decay in sensory magnitude might be quite different after temporal summation has occurred with stimulation at an ISI of 3 s. Accordingly, an experiment with three subjects involved presentation of a trial of 10 stimuli at 53°C and an ISI of 3 s, followed by an intertrial interval (ITI) of 6 s, and then a second trial of 10 stimuli at an ISI of 3 s. The verbal ratings of sensory magnitude showed that the effect of temporal summation had disappeared by 6 s after the first trial (Fig. 3, top right). Thus in normal subjects, interactive effects of individual thermal stimuli or of sets of stimuli that produce summation dissipate within 6 s.


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FIG. 3. Top left: baseline storage of heat (line graph) and phasic skin temperature responses (bar graph) for 3 anesthetized monkeys are plotted over 2 trials of 10 stimuli at 0.33 Hz (3 s ISI) that were separated by an intertrial interval (ITI) of 6 s. Baseline skin temperature did not decrease appreciably during 6-s ITI. Top right: in contrast to skin temperature recordings, sensory magnitude decreased markedly over ITIs of 4-6 s (- - -) between 2 trials of 10 contacts at an ISI of 3 s. Three subjects were stimulated on thenar and hypothenar eminence of either hand. Bottom: data from top right are replotted to reveal increases in rate of change in sensory magnitude for 2nd trial after ITIs of 4, 5, or 6 s. First stimulus of 2nd trial is aligned in each case with stimulus on 1st trial that produced a comparable rating of late sensory magnitude.

Recordings of baseline skin temperature revealed little or no decrement during a 6-s ITI after 10 stimuli at an ISI of 3 s (Fig. 3, top left). This finding is regarded as particularly telling evidence against the possibility that temporal summation of sensory intensity depends on storage of heat by the skin. There was no association between the baseline temperatures of the skin before the 10th and 11th stimuli (37.8°C for the stimuli that spanned the 6-s ITI), and the ratings of sensory intensities (71 and 11) associated with the same stimuli in identical presentations. Also, phasic skin temperature responses to contact did not change appreciably across the two trials of 10 stimuli (Fig. 3, top left).

Figure 3, bottom, reveals that the rate of temporal summation was greater for the second trial, after ITIs of 4, 5, or 6 s. For these panels, the beginning of the second trial is aligned with the stimulus in the first trial that produced a comparable rating of sensory intensity (e.g., with the 3rd stimulus in the trial preceding a 4-s delay). Statistical analysis of changes in the rate of temporal summation from comparable initial levels of sensory magnitude [analysis of variance (ANOVA) across 3 subjects and 4 sites of stimulation] revealed significant enhancement of temporal summation after ITIs of 4 s (F = 13.9, P = 0.001) and 5 s (F = 6.66; P = 0.02) but not for 6 s (F = 1.57; P = 0.22). Thus residual effects on temporal summation after 10 stimuli at 0.33 Hz became negligible after a delay of 6 s.

Documentation of the time course of changes in sensory magnitude relative to each stimulus in a series was accomplished by instructing five subjects to open the arms of a potentiometer in proportion to the rate of increase in magnitude of evoked thermal sensations, to reverse the direction when sensory magnitude decreased (beginning immediately after the peak sensory level), and to close the arms when the sensation disappeared. This finger-span technique provides an analog record of onset and peak latencies and the duration of thermal sensations. The subjects were instructed to attend to the time course of changes in sensory magnitude but not to scale peak sensory intensity by this technique. Experience with the finger span method (Yeomans et al. 1996) has shown that it is difficult for subjects to simultaneously represent the time course and the amplitude of the sensations. Therefore, the latencies of late sensation onset, peak amplitude, and offset shown in Fig. 4 were obtained from the time estimation recordings (finger span or slide), and maximal peak amplitudes were derived from verbal ratings in separate but identical sessions.


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FIG. 4. Top left: time course of late thermal sensations evoked by brief (700 ms) contact with thenar or hypothenar eminence by a thermode preheated to 51°C (5 subjects). Top right: triangular ramps (750 ms to peak temperatures of 50-52°C and 750 ms back to a baseline of 40°C) were applied to thenar eminence by a probe in constant contact with skin (6 subjects). Timing and duration of 2 successive stimuli are shown on abscissa (black-square). Latencies to onset, peak amplitude, and offset of sensations (symbols and connecting lines) were averaged across 30 stimuli. Peak amplitudes were derived from separate sessions in which sensory magnitude was rated verbally. Bottom: latencies for onset, peak, and offset of late thermal sensations elicited by 700 ms contact with the glabrous skin of one hand are shown for 3 subjects, using finger span method. Bottom left: average latencies over 8 successive contacts that generated measurable latencies in all subjects. Average latencies from stimulus onset across 8 contacts are shown bottom right. Standard deviations (within subjects) are for onset of late thermal sensations (from stimulus onset) and for times from sensory onset to peak and offset.

For 51°C stimulation of the thenar and hypothenar sites, 700-ms contact at an ISI of 3 s produced thermal sensations with onset latencies in excess of 2 s and durations <1 s (Fig. 4, top left). These temporal characteristics of evoked sensations did not change systematically throughout a series of contacts (Fig. 4, bottom) that elicited increasing peak amplitudes (Fig. 5).


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FIG. 5. Ratings of sensory magnitude across a series of 30 brief contacts (Tap: 5 subjects) or triangular ramps (6 subjects) of 51°C to thenar eminence of each hand are shown for an ISI of 3 s. A greater rate and amount of temporal summation for 700-ms contacts was observed for this temperature that produced comparable peak ratings for these 2 forms of stimulation.

The time course of thermal sensations produced by brief contact of the thermode with the skin was compared with sensations elicited in six subjects by the more traditional method of generating triangular ramps with a thermode in constant contact with the skin. For triangular ramps to temperatures of 50-52°C from a baseline of 40°C (total duration of 1.5 s), the onset of the thermal sensation was <1.5 s (overlapping with the descending ramp), and the duration of painful sensations was ~2.4 s (Fig. 4, top right). Thus for ramped thermal stimulation, the average onset latency was shorter and the average duration of the elicited sensation was longer than for brief contact. Also, the amount of temporal summation was less with ramped stimulation than for brief contact with a preheated thermode (Fig. 5). Using ramped stimulation, intensities that did not elicit a painful sensation on the first presentation (e.g., 50°C) produced little or no temporal summation, and intensities that generated temporal summation were painful at the inception of a series of stimuli (e.g., 51°C). Functions relating the rate and amount of temporal summation to probe temperature were considerably more compressed (less temporal summation) for ramped thermal stimulation.

To determine whether antagonism of NMDA receptor activation reduced the temporal summation produced by brief contacts with the skin, a preliminary experiment was conducted with two subjects who received an oral dose of 45 mg of dextromethorphan 2 h before a testing session consisting of eight trials of six contacts (700 ms, 52°C, 3-s ISI) to the thenar eminence of the right hand, with ITIs of 3.5 min (onset to onset of each trial). In control sessions, temporal summation of sensory intensity progressed within each trial to produce ratings of 40-60 after the sixth contact, in contrast to postdextromethorphan sessions that produced maximal ratings of 15-40 (Fig. 6, top). Based on this preliminary finding, a blinded experiment was conducted with two subjects who were tested similarly by eight trials of 10 contacts at 52 or 54°C in sessions that were preceded by no treatment, ingestion of 45 mg of dextromethorphan hydrobromide (Robitussin DM), or ingestion of the same volume of the vehicle (Guaifenesin syrup; Robitussin). Nine sessions (3 of each condition) were conducted on separate days, with random ordering of the three pretreatment conditions. The subjects were not aware of which ingested syrup contained dextromethorphan, but temporal summation was attenuated significantly only by this pretreatment (Fig. 6, bottom). Posthoc Sheffe tests for individual subjects revealed significant reduction of sensory magnitudes by dextromethorphan versus control sessions (P <0.001 for each subject) and by dextromethorphan versus vehicle sessions (P < 0.001 for each subject) but not for vehicle versus control sessions (P = 0.74 and 0.79). Thus a low dosage of dextromethorphan consistently and significantly attenuated, but did not eliminate, temporal summation of sensory intensity. An interesting feature of the effect of dextromethorphan is that it developed progressively during the early trials within testing sessions (Fig. 6).


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FIG. 6. Top: 2 subjects received 8 trials of brief contacts of a 52°C thermode to thenar eminence at an ISI or 3 s and an ITI of 3.5 min. During control sessions, sensory magnitude increased reliably from below or near pain threshold to an intermediate level of pain on each trial. During comparable sessions that were conducted 2 h after each subject ingested 45 mg of dexamethasone, sensory ratings were reduced significantly. Bottom: 2 subjects received 8 trials of 10 contacts, as above, in 3 sessions preceded by no treatment (control) or ingestion of dexamethasone (Dex.) or vehicle for dexamethasone (Veh.). Bars represent cumulative ratings of stimuli 1-10 on successive trials (left to right). Dexamethasone reduced temporal summation, particularly for trials 2-8.

In several procedures designed to evaluate spatial radiation of temporal summation, brief contacts were presented to nonoverlapping but adjacent skin sites. In the first experiment of this type, 20 stimuli were presented to a distal or proximal site on the thenar eminence of three subjects, followed by 20 contacts to the other site without interruption. That is, the ISI was 3 s throughout the 40 stimuli that were presented in two series of 20 contacts to adjacent but nonoverlapping sites (700-ms contact, 50°C). As shown in the top panel of Fig. 7, spatial radiation from this procedure was not prominent. The sensory magnitude for the first stimulus in the second series appeared not to have been influenced by the preceding stimulation at an adjacent site, and the rates of temporal summation were not markedly different for the first and second series. Analysis of variance for individual subjects revealed no significant difference in sensory ratings for the first and second series of stimuli presented to two subjects (F = 0.9 and 3.2; P > 0.05), but a significant attenuation of sensory magnitude occurred over the second series for the third subject (F = 15.3; P <0.001).


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FIG. 7. Top: sensory ratings for 50°C stimulation of each hand of 3 subjects are shown for 2 successive series of 20 stimuli to adjacent but nonoverlapping sites on thenar eminence at an ISI of 3 s. First stimulus in series 2 was 21st stimulus in session. No significant transfer of temporal summation from 1 site to other was observed. Bottom: 3 different conditions of thenar stimulation are compared directly for 3 subjects stimulated by brief contacts of 55°C. Very rapid temporal summation was produced by stimulation of 1 site at an ISI of 3 s; a moderate rate and amount of temporal summation was produced by alternating stimulation of adjacent but nonoverlapping sites at 3-s ISI; and little or no temporal summation occurred for stimulation of 1 site at an ISI of 6 s. All error bars represent standard deviations.

As a second test of spatial radiation, stimulation at the two thenar sites was alternated over a series of 50 stimuli (700-ms contact, 3 s ISI) for two subjects. To maximize radiation, a high temperature was used (55°C). Interleaved control sessions involved stimulation of one site on the thenar eminence at an ISI of either 3 or 6 s to compare alternation with conditions of 55°C stimulation at a single site that produce considerable temporal summation (3 s ISI) or negligible summation (6-s ISI). Figure 6, bottom, shows that alternating stimulation of adjacent sites (3-s ISI) produced a moderate degree of temporal summation that clearly was less than that obtained with stimulation of one site at an ISI of 3 s, but greater than that obtained with same-site stimulation at an ISI of 6 s (i.e., without interposed stimulation at an adjacent site). ANOVA for individual subjects revealed significant spatial summation for the alternating condition (involving stimulation of each site at an ISI of 6 s but with interposed stimulation of an adjacent site) compared with same-site stimulation at an ISI of 6 s (F = 34.6 and 67.4; P < 0.01).

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

Methods are described for presenting thermal stimulation of the skin in a manner that selectively produces late sensations of warmth that are altered by repetitive stimulation to become painful late sensations. The skin was contacted briefly with a preheated thermode, in contrast to a more traditional method of eliciting temporal summation with triangular ramps produced by a thermode in constant contact with the skin (Price et al. 1977, 1994; Price 1972). Ramping thermal stimulation has been employed widely to elicit painful sensations (Dyck 1993) with the rationale that presumed inhibitory interactions between tactile and thermal stimulation (Melzack and Wall 1965) are minimized or avoided. However, the rate of temporal summation was shown to be greater for brief contacts than for ramps that reach the same temperature. The tactile component from brief contact in the present study ended ~1.5 s before onset of the late thermal sensation and therefore was unlikely to exert a significant modulatory effect on the magnitude or duration of the thermal sensation. Furthermore, brief contacts did not produce early (1st) pain sensations within a range of thermode temperatures that produced rapid and pronounced temporal summation of late sensations. In contrast, ramped stimulation readily produces a first pain sensation (Price et al. 1977), and it is more difficult to distinguish between early and late sensations with this method. Thus concerning potential sources of inhibition of the late sensations that temporally summate, ramped stimulation is disadvantageous because activation of myelinated nociceptors (generating 1st pain sensations) appears to produce more inhibition of late pain sensations than does stimulation of low-threshold myelinated afferents (Chung et al. 1984).

Brief contact of a preheated thermode with the skin closely mimics forms of focal thermal stimulation that elicit pain in natural circumstances, whereas increases in skin temperature without a tactile component generally are associated with changes in environmental temperature that elicit thermoregulatory responses and are not local or nociceptive. In contrast to the widely promoted view that the presence of tactile stimulation is to be avoided, there are experimental circumstances in which it is advantageous for investigating differences in neural representations of tactile and thermal sensations. For example, the method involving brief contact with the skin has been used to reveal that cerebral cortical responses to repetitive stimulation by 28 or 52°C are localized separately in somatotopically appropriate regions of primary somatosensory cortex (in areas 3b and 1 for contact by the lower temperature and areas 3a and 2 for the higher temperature) (Tommerdahl et al. 1996a).

An advantage of ramped thermal stimulation is that skin temperature can be controlled during the ISI by returning the thermode temperature to a level that does not activate nociceptors. Accordingly, to evaluate influences of skin temperature on sensory ratings, recordings of subepidermal skin temperature were obtained during presentation of the paradigms of brief contact that were used for the psychophysical experiments. Manipulation of the temperature of the thermode and the interval between skin contacts showed that the skin stores heat in proportion to the temperature of the thermode and in inverse proportion to the interval between contacts. However, these functions did not match the relationships between thermode temperature or ISI and the growth of sensory intensity. Most telling was the finding that heat loss by the skin after a trial of 10 contacts at 0.33 Hz was negligible over an ITI (6 s) in which sensory magnitude decreased from maximal to minimal values. Thus storage of heat by the skin may contribute to, but does not determine the rate of, temporal summation or its decline from a series of brief contacts.

During and after brief thermal stimulation of the skin surface, a gradient of heat develops in depth and time, and the most superficial and/or lowest threshold receptors are activated first. Adaptation rates for the receptors activated and the decay rate for local skin temperature determine the duration of receptor activation. For the present study, glabrous skin sites were stimulated because a type of myelinated nociceptor (type II AMH or mechano- and heat-sensitive A fiber nociceptors) with relatively low thresholds (46°C) is absent or scarce (Treede et al. 1995) in contrast to hairy skin. Thus the thermal gradient from brief contacts to glabrous skin could potentially activate type I AMHs, with median heat thresholds of 53°C, or C fiber nociceptors, with median thresholds of 41°C (LaMotte and Campbell 1977; Tillman et al. 1995), or low threshold C fiber receptors subserving warmth (Swerup 1995; LaMotte and Campbell 1977). For activation of these heat-sensitive receptors in glabrous skin, response latencies in the CNS are on the order of 2 s because of a slow peripheral conduction velocity (C afferents) or a long response latency (type I AMHs). Because the endings of C fibers can penetrate the epidermis and are likely to be located relatively superficially (Cauna 1980; Novotny and Gommert-Novotny 1988) and because of low thresholds for activation (Tillman et al. 1995), the late sensations elicited by brief contact seem certain to be dominated by input from C thermoreceptors (Yeomans et al. 1996). These neurophysiological findings, coupled with demonstrations in the present study of substantial temporal summation of late sensations elicited by brief repetitive contact with glabrous skin by preheated thermodes, are consistent with assertions that windup of spinal neuronal activity depends on activation of C afferents (Mendell 1966; Price et al. 1977). This linkage is likely for the intensities of thermal stimulation that elicited temporal summation in the present study (49-53°C at the skin surface). However, because of the possibility that type I AMHs are activated also by subepidermal temperatures in excess of 53°C, second pain from high levels of thermal stimulation cannot be regarded as strictly related to input from C nociceptors (Sinclair and Stokes 1964).

Not only are the phenomena of windup and temporal summation of pain intensity regarded as dependent on activation of C afferents, but also it has seemed likely that stimulation of nociceptors is required. A commonly expressed hypothesis is that stimulation of C nociceptors is required to produce high rates of discharge among nocireceptive central neurons, which opens NMDA receptor channels, rendering these neurons more responsive to input from nociceptive and nonnociceptive afferents (e.g., Liu and Sandkuhler 1995). This enhanced sensitivity of nocireceptive central neurons (thought to underlie allodynia and hyperalgesia) is assumed not to be triggered by input from nonnociceptive afferents. However, in the present study, temporal summation of thermal sensations to very strong levels of pain could be produced by repetition of a stimulus that produced only sensations of warmth when presented at frequencies of <= 0.14 Hz. This finding opens the possibility that temporal summation to painful levels does not depend on activation of nociceptors. Alternatively, some nociceptors may be activated by temperatures that elicit only sensations of warmth, or nociceptor activity may be recruited especially by brief contacts of the preheated thermode at frequencies >0.14 Hz.

The supposition that temporal summation of thermal sensations for stimulation frequencies >0.14 Hz results from central integration and not from peripheral sensitization of nociceptors is difficult to establish with absolute certainty. Convincing evidence has been provided by a demonstration that peripheral nociceptor discharge is suppressed progressively by repetitive stimulation that produces windup of central cells (Price et al. 1977). This result was obtained for stimulation of hairy skin by ramped activation of a contact thermode, and further evidence relevant to the present study is needed for glabrous skin and brief contact by thermodes regulated to different temperatures. Another method of testing central versus peripheral effects involves pharmacological antagonism of NMDA receptors (Davies and Lodge 1987; Dickenson and Sullivan 198); this has been assumed to represent only a central effect. Using ramped thermal stimulation (Price et al. 1994) or brief contacts by a preheated thermode (present study), doses of 30-45 mg of dextromethorphan, an NMDA receptor antagonist (Church et al. 1985), clearly attenuated temporal summation. However, information is needed on whether the effects of this agent on temporal summation might be attributable to actions at peripheral NMDA receptors that have been identified recently (Carlton et al. 1995; Liu et al. 1994).

Stimulation of different skin sites can be employed to evaluate spatial radiation, which can be exaggerated in chronic pain conditions. For example, alternating stimulation at separate but adjacent sites at an ISI of 3 s stimulates each peripheral site once every 6 s, and this rate of stimulation at a single site produces little or no temporal summation (see Fig. 2). However, central neurons with receptive fields common to the two sites of alternating stimulation receive input at 0.33 Hz, which does produce summation for stimulation of a single site. The amount of summation from alternation should be less than that obtained from stimulation of a single site at 0.33 Hz unless the peripheral receptive fields overlap extensively. Also the central summation could be enhanced by an NMDA-receptor-sensitive expansion of central receptive fields (Ren et al. 1985). Peripheral sensitization of each site could be enhanced by mutual influences on the sites from a lateral spread of inflammation (LaMotte et al. 1992), but secondary hyperalgesia adjacent to a cutaneous injury is not revealed by thermal stimulation (Simone et al. 1989). In the present experiment, evidence in favor of central summation was obtained by the finding that alternating stimulation of adjacent sites on the thenar eminence at 0.33 Hz produced temporal summation that was intermediate in rate and amount between that obtained with stimulation of either site alone at ISIs of 3 or 6 s (see Fig. 6). This finding is suggestive of central radiation but contrasts with another study using alternating ramped stimulation of adjacent sites on hairy skin that produced temporal summation in excess of that observed with stimulation at a single site (Price et al. 1977).

In contrast to the central summation that was revealed by alternating stimulation of two sites, another paradigm that involved switching sites revealed no central summation. When 20 stimuli were applied to one location at 0.33 Hz, followed by a 6 s interval and then another 20 stimuli to an adjacent site, no interaction was observed. This is suggestive of mechanisms for spatial interaction that have been described at the cerebral cortical level (Whitsel et al. 1991). For numerous forms of repetitive somatosensory stimulation, regions of primary somatosensory cortex (SI) that are highly responsive to a given stimulus exert powerful inhibitory influences on nearby ensembles of neurons that are less effectively driven (Tommerdahl et al. 1996b). Thus for the paradigm involving extended series of stimuli to one site and then another, repetitive excitation of a subset of SI cortical neurons that are maximally excited at the first site would suppress the activity of neurons with partially overlapping receptive fields that are less effectively excited at the first site than at the second site. In contrast, for alternating stimulation of the same sites, the two subsets of neurons with partially overlapping receptive fields would be activated to some extent at a rate of 0.33 Hz, which could produce some temporal summation.

Given the different phenomena of temporal summation that can be demonstrated for repetitive contact of a preheated thermode with the skin, how could this technique be used optimally to test the possibility that some pathological pain states reflect abnormalities of temporal summation and responsible transmitter systems? The relationship of temporal summation to temperature of the thermode provides a basis for comparing chronic pain patients with a normal population of subjects. For chronic pain conditions involving tonic input from C nociceptors (and/or deficiencies in endogenous opioid antinociception), temporal summation might well occur for stimulation of affected skin regions by abnormally low temperatures. However, individual differences in sensitivity of normal subjects and variations in sensitivity to stimulation at different skin sites makes it difficult to show that temporal summation occurs at lower temperatures for a select group of subjects. A suggested alternative is to identify for each individual the lowest temperature that produces ratings that plateau at a criterion level of summated pain for a standard frequency (e.g., 0.33 Hz) at a single site and then compare temporal summation for lower frequencies of stimulation across groups of subjects. That is, the status of neurotransmitter/receptor systems responsible for temporal summation should be revealed by documenting the rate of temporal summation and decay. Once an effective temperature has been identified for a subject, parametric variation of the interval between contacts---both within and between series and sites---will reveal time constants for the processes of temporal summation and decay and spatial radiation. In the present study of normal subjects, these relationships were quite orderly, stable, and consistent across individuals, and the time constants for temporal summation and decay of thermal sensations were considerably shorter than effects that have been demonstrated after more extreme methods of stimulation that may simulate pathological conditions. If exaggerations of central excitability that last for minutes or hours after tetanic peripheral stimulation at intensities that excite C afferents are representative of central pathological states, as postulated (Dickenson 1990; Liu and Sandkuhler 1995; Woolf and Thompson 1991), then our expectation is that time constants for temporal summation and decay of late thermal sensations will be extended considerably during expression of some forms of chronic pain.

    ACKNOWLEDGEMENTS

  The technical support of A. Azam is acknowledged gratefully.

  This work was supported by the National Institutes of Health Grants NS-07261, NS-34979, and DE-07509 and Brain and Spinal Cord Injury Rehabilitation Trust Fund, Florida.

    FOOTNOTES

  Address for reprint requests: C. J. Vierck, Dept. of Neuroscience, University of Florida College of Medicine, Gainesville, FL 32610-0244.

  Received 13 January 1997; accepted in final form 22 April 1997.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

0022-3077/97 $5.00 Copyright ©1997 The American Physiological Society