| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1 Division of Pharmacology, School of Pharmacy, University of Missouri-Kansas City, Kansas City, Missouri 64108; 2 W. M. Keck Foundation Center for Integrative Neuroscience and Departments of Anatomy and Physiology and 3 Laboratory Medicine, University of California San Francisco, San Francisco, California 94143
 |
ABSTRACT |
|---|
 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Many studies indicate that blood pressure control systems can attenuate pain (hypoalgesia) of short duration; however, we recently found exaggerated nociceptive responses (hyperalgesia) of persistent duration in the spontaneously hypertensive rat (SHR). Here, we used SHR, Dahl Salt-Sensitive (SS), and normotensive control rats to evaluate the contribution of sustained elevations in arterial pressure to nociceptive responses. Compared with Sprague-Dawley and/or Wistar-Kyoto controls, SHR were 1) hypoalgesic in the hot plate test and 2) hyperalgesic in longer latency tail and paw-withdrawal tests and in two models of inflammatory nociception. These differences were not observed between SS and salt-resistant controls fed a high-salt diet. Inflammatory hyperalgesia in SHR was correlated with neither paw edema nor the number of Fos-positive spinal cord neurons. Our results indicate that "pain" phenotype of the SHR is not restricted to hypoalgesia. This phenotype is related to genetic factors or to the autonomic systems that control blood pressure and not to sustained elevations in blood pressure, differences in spinal neuron activity, or inflammatory edema.
pain; inflammation; Dahl salt-sensitive rat; spontaneously hypertensive rat
| |
INTRODUCTION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
MANY STUDIES INDICATE THAT blood pressure control systems can attenuate the processing of brief nociceptive signals (9, 33, 34). For example, the spontaneously hypertensive rat (SHR), which is an important model of human primary (genetic) hypertension, exhibits longer nociceptive latencies than its normotensive Wistar-Kyoto (WKY) control in the hot plate test (15, 23, 38, 39, 56), tail-flick test (37, 47), and Randall-Siletto paw pressure tests (3). Similarly, hypertensive humans have higher pain thresholds and smaller pain responses in tests that involve acute noxious stimuli (1, 10, 55). Other studies comparing normotensive and hypertensive animals, however, indicate that the direction of this change in nociceptive processing is variable. In some studies, adult WKY rats and SHRs did not differ in the tail-flick test (26) or hot plate test (51, 53), and in studies of persistent nociception using the Formalin test, SHR exhibited normal (47) or exaggerated nociceptive responses (45). To test the hypothesis that qualitative differences in nociception between SHR and normotensive rats vary in direction with the pain test used, in the present studies, we compared nociceptive responses in SHRs and two normotensive control strains in multiple models of acute nociception (hot plate, tail-flick, and paw-flick tests) as well as in two models of persistent inflammatory pain (zymosan and Formalin tests). Indeed, although the number of people whom develop chronic pain and/or hypertension is large, few studies have addressed the contribution of hypertension to the development and maintenance of persistent pain.
With the use of two models of secondary (experimental) hypertension, Sitsen and de Jong (38, 39) found no association between blood pressure and hot plate latency and suggested that the high nociceptive thresholds in SHRs resulted from genetic factors rather than from sustained elevations in arterial pressure. To test this hypothesis in the present studies, we evaluated nociceptive responses in a different genetic model of primary hypertension, the Dahl salt-sensitive (SS) rat. This rat, but not its control, responds to daily administration of a high-salt diet with the rapid development of hypertension. Importantly, because different selection criteria [i.e., development of spontaneous hypertension in SHR (30) vs. hypertension induced by dietary salt in SS rats (4)] were used to derive the original strains, it is likely that the genes that underlie the development of hypertension in the SHRs and SS rats differ. If sustained elevations in blood pressure contribute to altered nociception in the SHR, then SS rats and SHRs should exhibit similar nociceptive responses compared with their normotensive controls.
| |
METHODS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Animals
Male WKY, SHR, and Sprague-Dawley (SD) rats were obtained from Charles River Laboratories (Hollister, CA). Inbred SS and salt-resistant (SR) rats were originally obtained by Dr. John Rapp [Medical College of Ohio (36)] and Harlan Sprague Dawley (Indianapolis), respectively, and then subsequently bred at the University of California San Franicsco (UCSF) animal facilities. Several days before surgery or testing, animals, weight matched at 270-330 g, were individually housed in standard clear plastic cages in a temperature-controlled room (20 ± 1°C) on a 12:12-h light-dark cycle (6 AM lights on) with food and water provided ad libitum. SS and SR rats were fed a high-salt diet (2%) for at least 7 days before assessment of nociceptive responses. The Institutional Animal Care and Use Committee of UCSF approved all of the following protocols.Surgical Procedures
Arterial catheterization. We constructed femoral arterial catheters by heat fusing a 4.5-cm length of polyethylene (PE)-10 tubing to a 14.5-cm length of PE-50 tubing. While the rats were under pentobarbital sodium anesthesia (50-60 mg/kg), we isolated the left femoral artery by blunt dissection with care taken not to injure the femoral vein or sciatic nerve. Through a small slit cut into the vessel, we advanced the catheter, prefilled with 100 IU /ml heparin, proximally to the renal bifurcation of the abdominal aorta. After securing the catheter to the vessel with 4-0 sutures, we tunneled the PE-50 end of the catheter under the skin, exteriorized it at the nape, and sutured it to the dorsal neck muscles (splenicus cervicus) (42). After the rats recovered from anesthesia, we returned animals to their cages and allowed them to recover for 3-5 days before testing.
Behavioral Assessment of Nociceptive Reflexes in Naïve and/or Zymosan-Treated Rats
Hot plate test. One day before testing, animals were acclimated to the unheated hot plate twice for 1 min. Rats were placed on the hot plate (52.5°C) until they licked their hind paw or exhibited escape (jumping) behavior. Two measurements, taken 1 h apart, were averaged.
Thermal threshold. Tail-flick latency was measured using a technique modified from Hargreaves et al. (13). Briefly, after the tail was blackened with ink (to reduce response latency), rats were acclimated to a clear Plexiglas box on a glass floor for at least 1 h. At specified time points, the stimulus was applied once to each of four designated bands of the tail, 1 in. (2.2 cm) apart. The average of the measurements was calculated. If the rat did not respond within 15.0 s, the heat was terminated to prevent tissue damage. Paw withdrawal latency was measured, as previously described (44), with a cut-off latency of 20 s. Short-latency tail flick, long-latency tail flick, and paw withdrawal were defined by latencies of ~4 ± 1.0, 7 ± 1.0, and 10 ± 1.0 s, respectively, achieved by adjusting the voltage intensity of the light bulb.
Mechanical threshold.
After 1 h of acclimation to Plexiglass cages with a wire mesh
bottom, mechanical threshold for paw withdrawal was assessed using the
up-down method modified by Chaplan et al. (2). Thresholds were determined by applying calibrated von Frey hairs (Stoelting, Wood
Dale, IL), numbers 3.61-5.18, to the center of the plantar hindpaw
with sufficient force to produce slight bending. First, an intermediate
von Frey hair was applied. If a brisk withdrawal response occurred
within 8 s, the next weaker hair was applied. In the absence of a
response, the next stronger hair was applied. This continued until the
stimulus threshold could be determined as follows: 50% g
threshold = (10[X+
])/10,000, where X is the
value (in log units) of the final von Frey hair used, 
is the
tabular value for the pattern of positive/negative responses, and 
is the mean difference (in log units) between von Frey hairs.
Zymosan-induced nociception. After the subcutaneous injection of zymosan (2 mg in 100 µl) into the midplantar paw, thermal and tactile paw-withdrawal thresholds were assessed as above. Each animal was used only once, i.e., zymosan injection was never repeated in the same animal.
Formalin Nociception
Each animal was transferred to a bedded 10 × 10 × 10-in. Plexiglas box in the laboratory, with food and water provided ad libitum, at least 16 h before testing. Such adaptation to the test environment decreases the variability associated with behavioral measurement in the Formalin test [Tjolsen et al. (48)]. After this acclimation period, we connected the arterial catheter to a pressure transducer (Kobe, Arvada) with PE-50 tubing.Cardiovascular recording. We began cardiovascular recording at least 20 min later; this time period allows blood pressure and heart rate (HR) in the awake animal to reach resting state (42). Mean arterial pressure (MAP) and HR measurements were recorded once per minute. Resting MAP and HR were calculated as the mean of five measurements collected just before the injection of Formalin. For the analysis of Formalin-induced increases in MAP and HR, the time intervals 1-5 were combined to yield phase 1, 11-15 were combined to yield the interphase, and 21-70 were combined to yield phase 2.
Behavioral assessment. The Formalin stimulus consisted of a 50-µl sc injection of Formalin [37% (wt/wt) formaldehyde, diluted to 1.25% in saline] into the midplantar region of the right hindpaw. Formalin-induced flinching and licking responses, both shown to be reliable measures of the central transmission of nociceptive signals in the Formalin test [Tjolsen et al. (48)], were evaluated. To quantify pain behavior during phase 1, the number of flinches or the number of seconds spent licking during the second, third, fourth, and fifth minute after injection were counted as previously described [Taylor et al. (44)]. From 8 to 90 min, flinches and time spent licking were counted for 2 min at 5-min intervals. These numbers were divided by two, yielding values per minute. With this method, behavior in two animals was simultaneously recorded by one observer at 1-2, 2-3, 3-4, 4-5, and then 8-10, 13-15, ... , 68-70 min after the Formalin injection. Each animal was used only once, i.e., Formalin injection was never repeated in the same animal.
Evaluation of Edema
While the rat was gently restrained, paw thickness was measured with a Mitutoyo pocket gauge microcaliper (Western Tool, Oakland, CA) as previously described(32). Three measurements were taken and averaged. In the zymosan studies, paw thickness was determined immediately after the measurement of mechanical threshold/thermal latency. This minimized the possible effect of restraint on behavioral responses. In the Formalin studies, paw thickness was measured 70 min after intraplantar injection.Immunocytochemistry
Two hours after Formalin injection, each rat was deeply anesthetized with pentobarbital sodium (100 mg/kg ip) and intracardially perfused with 100 ml of 0.1 M PBS (pH 7.4) followed by 500 ml of 10% Formalin in 0.1 M phosphate buffer. Next, the brain and lumbar spinal cord were removed, postfixed for an additional 4 h, and then cryoprotected overnight in 30% sucrose in 0.1 M PBS. Forty-micrometer frozen sections were cut in the transverse plane and collected in 0.05 M PBS. The sections were then washed with a solution of 0.05 M PBS, 1% normal goat serum, and 0.3% Triton X-100, incubated for 1 h at room temperature in 0.05 M PBS, 3% normal goat serum, and 0.3% Triton X-100. Sections were incubated for 40 h at room temperature in a rabbit anti-Fos antibody (kindly provided by Dr. Dennis Slamon, Univ. of California Los Angeles), as previously described (31). This antibody was diluted 1:21,000 and preabsorbed against acetone-dried liver powder for 1 h at 37°C and for 1 h at 4°C before use. The sections were again washed and incubated in biotinylated goat anti-rabbit IgG and avidin-biotin-peroxidase complex [method adapted from Hsu et al. (16)]. To visualize the immunoreaction product, we used a nickel-intensified diaminobenzidine protocol with a glucose-oxidase reaction [adapted from Llewellyn-Smith and Minson (21)]. Immunoreacted sections were mounted onto slides and then placed under a coverslip using Eukitt mounting medium (Calibrated Instruments, Hawthorne, NY). To quantify the number of Fos-like immunoreactive (Fos-LI) neurons, we selected four to six sections at the L4/5 segmental level under dark-field illumination, photographed them under bright-field illumination at low (4×) power, and then enlarged each negative 15×. The spinal cord gray matter was divided into four segments: 1) the superficial laminae, I, IIo, IIi, 2) the nucleus proprius, laminae III and IV, 3) the neck of the dorsal horn, laminae V/VI, and 4) the ventral horn. Blinded to treatment, one investigator manually counted the number of labeled neurons in each of the four to six photographs per animal, which were averaged for the statistical analysis.Paw Skin Temperature
In naïve rats, we assessed paw-surface temperature with a contact surface probe (model 427, Yellow Springs Instuments) and thermometer (model 4600, Yellow Springs Instruments). After the paw was extended, the probe was placed on the skin at the center of the plantar surface. We allowed 60 s for the skin temperature to reach steady state.Data Analysis and Statistics
One-way ANOVA was used to analyze thermal and tactile thresholds in the absence of plantar injection. Two-way ANOVA with strain and time as the between-subjects variables was used to analyze the responses to Formalin injection. Two-way ANOVA with strain and laminae as the between-subjects variables was used to analyze Fos expression after Formalin injection. If significant, these analyses were followed by appropriate one-way ANOVAs and/or post hoc tests.Materials
Stock solutions of Formalin [aqueous solution of 37% (wt/wt) formaldehyde, Fisher, Fair Lawn, NJ] were diluted in 0.9% isotonic saline (Baxter Healthcare, Deerfield, IL). Pentobarbital sodium was obtained from Abbott Laboratories (North Chicago, IL). The avidin-biotin-peroxidase complex was obtained from Vector Labs (Burlingame, CA). Zymosan was obtained from Sigma (St. Louis, MO).| |
RESULTS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Resting Blood Pressure, HR, and Paw-Surface Temperature
To demonstrate that blood pressure was indeed elevated in the SHR and SS models of genetic hypertension, we used indwelling arterial catheters to evaluate baseline MAP and HR in these strains and in their controls. As shown in Fig. 1, resting arterial blood pressure was significantly greater in SHRs compared with WKY or SD controls (P < 0.05) and significantly greater in SS rats compared with SR rats (P < 0.05). Resting HR was greater in SD and SS rats compared with the other strains.
|
Because sympathetic tone is higher in SHRs, it is possible that differences in local temperature could confound results in thermal withdrawal tests. Local temperature of the extremities can significantly affect reflex responses to thermal stimuli, as in the tail-flick test. However, paw skin temperature in SHRs (27.0 ± 0.8°C, n = 6) was not significantly different from that of WKY rats (27.7 ± 0.7°C, n = 6, P > 0.05).
Transient Tactile and Thermal Reflexes in SHR, WKY, and SD Rats
To test the hypothesis that differences between SHR and normotensive rats vary with the type of acute thermal pain test, we compared their responses in the hot plate test, in the short-latency thermal tail-flick test (n = 16-30), and in two long-latency thermal tests (tail flick and paw withdrawal). To compare responses to a tactile stimulus, we also evaluated paw-withdrawal responses to von Frey filaments. Figure 2A illustrates that hot plate latencies were longest in the SHRs. One-way ANOVA revealed a significant effect of strain [F(2,47) = 29.2, P < 0.0001]. Both short-latency tail-flick responses [Fig. 2B, F(2,72) = 2.6, P < 0.05] and longer latency responses [Fig. 2C, F(2,47) = 7.9, P < 0.005] elicited from the tail were shortest in SHRs. Similarly, thermal responses elicited from the paw were shortest in SHRs [Fig. 2D, F(2,47) = 10.5, P < 0.0005]. ANOVA of mechanical thresholds revealed a significant effect of strain [F(2,47) = 13.5, P < 0.0001]. Figure 2E illustrates a dissociation between blood pressure and von Frey thresholds that can be rank ordered as follows: SD > SHR > WKY.
|
Persistent Nociceptive Responses in the Setting of Inflammation in SHRs
Zymosan. To test the hypothesis that hypertension is associated with altered nociceptive responses in the setting of inflammation, we evaluated acute tactile and thermal nociceptive responses in awake, unrestrained rats after the injection of zymosan. Zymosan is the active inflammatory substance of brewer's yeast (Saccharomyces cerevisiae). Unlike carrageenan, which predominantly decreases thermal thresholds (Taylor and Basbaum, unpublished observations), and complete Freund's adjuvant, which predominantly reduces mechanical thresholds, zymosan has been shown to robustly decrease both thermal and mechanical thresholds (25, 35). Because baseline responses were different between the strains (as shown in Fig. 2), we describe the data not only in terms of actual latencies/threshold, but also as percent change from baseline.
Thermal hyperalgesia.
As illustrated in Fig. 3
(top), zymosan decreased thermal latencies to a greater
extent in the SHR and SD strains than in the WKY strain. This was true
at 2.5 h after injection, whether latency values or percent
changes were analyzed. ANOVA of thermal latencies over time revealed a
significant effect of strain [F(2,119) = 11.1, P < 0.0001].
|
Tactile hyperalgesia. Figure 3 (middle) illustrates that zymosan decreased tactile thresholds to a greater extent in the SHR strain than in the WKY strain. This was true at 1.5 h after injection, whether gram threshold values or percent changes were analyzed. When analyzed as a percentage of baseline, both SHR and SD rats displayed a greater decrease in tactile threshold than did WKY rats. ANOVA of von Frey thresholds over time revealed a significant effect of strain [F(2,85) = 3.8, P < 0.05].
Edema. As noted above, hypertensive animals characteristically demonstrate heightened levels of sympathetic activity. Because sympathetic activity modulates inflammation, (17, 41), we considered the possibility that differences in inflammatory responses contributed to any differences in nociceptive responses. To test this hypothesis, we compared zymosan- and Formalin-induced edema in SHRs and their normotensive controls. As shown in Fig. 3 (bottom), SHRs exhibited less edema than either WKY or SD rats at the earlier time points. These difference resolved within 4 h of injection. ANOVA of paw thickness over time revealed a significant effect of strain [F(2,119) = 7.7, P < 0.0001].
Formalin. In this model, the intraplantar injection of dilute Formalin first produces a rapid-onset, short-lived phase of painlike behavior (phase 1). An intermediary quiescent period of 10-15 min is then followed by a longer, persistent phase (phase 2) (6, 48). We previously reported that SHRs exhibit exaggerated flinching behavior and cardiovascular nociceptive responses during both phases of the Formalin test. Because of reports that the WKY strain is not genetically consistent from vendor to vendor (19), we compared SHRs not only with WKY rats, but also with outbred, normotensive SD rats. Furthermore, to determine whether differences in spinal nociceptive processing contribute to these observations, we compared not only behavioral responses, but also paw thickness and the expression of spinal cord Fos-LI in SHRs and normotensive rats.
Nociceptive responses.
As illustrated in Fig.
4A, 1.5% Formalin produced
exaggerated flinching responses in the SHR compared with WKY or SD rats during phase 2. Phase 1 flinching responses did not differ. ANOVA of
flinching responses over time revealed a significant effect of strain
[F(2,410) = 6.0, P < 0.0001]. As illustrated in Fig. 4B, 1.5% Formalin produced
exaggerated MAP responses in the SHR compared with the WKY or SD rats
during phase 2. Phase 1 MAP responses were greater in the SD rats
compared with the WKY rats. ANOVA of blood pressure responses over time
revealed a significant effect of strain
[F(2,252) = 16.1, P < 0.0001].
|
Spinal cord Fos expression in WKY, SHR, and SD rats. Figure 4C illustrates that the number of Formalin-induced Fos-LI neurons in laminae I-II of the lumbar spinal cord was greatest in SD rats, whereas the number of neurons in laminae III-IV was fewest in the SD rats. Fos expression in other laminae of the dorsal horn did not differ.
Edema. As illustrated in Fig. 4D, Formalin-induced edema was smaller in the SHRs compared with the WKY rats. Edema in SD rats was not statistically smaller than in WKY rats. ANOVA of paw-thickness responses over time revealed a significant effect of strain [F(2,47) = 3.9, P < 0.05].
Acute and Persistent Nociceptive Responses in SS Hypertensive Rats
With the use of a different genetic model of primary hypertension, the SS rat, we next tested the hypothesis that sustained elevations in arterial pressure contribute to differences in nociception between normotensive and SHRs. Figure 1 demonstrates that 7 days of a high-salt diet led to elevated MAP in SS but not in control SR rats. Figure 5 illustrates that neither hot plate, tactile, nor tail-flick latencies differed between the SS and SR strains whether they were tested before or after salt administration (P > 0.05). As illustrated in Fig. 6 (top), SR rats showed only a modest difference in flinching responses during phase 1 compared with SS rats. Figure 6 (bottom) illustrates that SS rats exhibited smaller phase 2 MAP responses than SR rats. ANOVA of blood pressure responses across time revealed a significant effect of strain [F(1,202) = 14.3, P < 0.0001].
|
|
| |
DISCUSSION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Hypoalgesia and Hyperalgesia in the SHR
An important conclusion from this study is that the nociception phenotype of the SHR is not primarily one of hypoalgesia. Rather, the SHR phenotype varies with the nociceptive test. Compared with normotensive WKY controls, SHRs were hypoalgesic in the hot plate test and hyperalgesic in tail- and paw-withdrawal tests and in the Formalin and zymosan models of inflammatory nociception. Because of evidence for genetic heterogeneity in the WKY rat control strain (19), we also compared SHRs to outbred SD rats and again found the SHR to be hypoalgesic in the hot plate test, not different in the short-latency tail-flick test, and hyperalgesic in longer latency tail- and paw-withdrawal tests and in the Formalin model of inflammatory nociception. We conclude that hypoalgesia in the SHR is restricted to a limited number of acute pain models such as the hot plate test.There are several possible explanations for our observation that the SHR phenotype varies with the nociceptive test. First, the genetic determinants of behavioral responses to noxious stimuli in SHR and normotensive rats may vary with the pain assay (7, 27, 28). In fact, the genetic mechanisms involved in the persistent inflammatory nociception of the Formalin and zymosan models are likely quite different from those involved in acute nociception (29). Second, because the orienting response that is associated with enhanced sensory processing (11, 40) is blunted in SHRs (43, 46), SHRs may be less able to appropriately adapt to the hot plate stimulus, leading to a dampened response to noxious stimulation. Third, the differences between SHR phenotype within the thermal modality may be related to the requirement for animal handling just before the hot plate test but not before the application of the thermal stimulus in the Hargreaves' test. Because the fight-or-flight stress and sympathetic responses are exaggerated in SHRs (24, 43, 50) and these responses have been associated with hypoalgesia, it is possible that the stress-induced analgesia (SIA) associated with the hot plate test is enhanced in SHRs, leading to a dampened response. In contrast to previous studies that used more traditional behavioral pain assays in restrained animals (3, 15, 23, 37-39, 47, 56), the current study evaluated paw- and tail-withdrawal latencies in the unrestrained rat. Because SIA might alter the behavioral phenotype of acute nociception in the SHR, we suggest that the differential response to stress is a major contributor to the different results obtained in the present and previous studies.
Nociception and Sustained Elevations in Arterial Pressure
By comparing SHR and SS rats with their respective controls, we found that the abnormal nociceptive responses in the SHR are related to mechanisms other than sustained elevations in blood pressure. Although the high-salt diet increased blood pressure in SS rats to a level comparable with that of SHRs, the differences in acute nociception observed in SHR, WKY, and SD rats were not found in the SS and SR rats. By contrast, with the use of older models of acute nociception, Friedman et al. (8) found that SS rats, compared with SR rats, exhibited longer tail-flick latencies during animal restraint and higher flinch-jump thresholds to electric shock. However, the differences in baseline responses observed before salt administration confound interpretation of those results.The present studies demonstrate that the exaggerated Formalin-evoked responses in SHRs were not observed in SS rats. In fact, the SS rats displayed smaller blood pressure responses compared with control, which could merely be due to a ceiling effect associated with the high resting blood pressure in this strain. A decoupling of blood pressure from nociception was also observed by Sitsen and de Jong (38, 39), who reported that experimentally induced hypertension did not produce increases in hot plate latency, by Tsai and Lin (49), who reported that SHRs exhibited longer hot plate latencies at 30 days of age, before the development of a robust hypertension, and by Ghione et al. (10), who found that pharmacological reduction of blood pressure in hypertensive humans did not alter their pain sensitivity.
Although zymosan hyperalgesia was greater in the SHR compared with the WKY rat, it was not different between the SHR and the SD rat. This makes it difficult to interpret the results with respect to differences in resting blood pressure. Similarly, blood pressure differences could not explain the results of Wiesenfeld-Hallin and colleagues, who found that SHR, WKY, and SD rats exhibit widely different painlike behaviors and primary afferent discharge after nerve injury (12, 22, 54). Rather, they suggested that a genetic predisposition unrelated to high blood pressure accounts for much of the variability in neuropathic pain behavior across different strains of mice (28), rats (5), or possibly humans.
We cannot rule out the possibility that these interstrain differences derive, in part, from an interaction of genotype and hypertension. For example, because the hypertension in the SHR existed for a much longer time (~8 wk) than the SS rat (~10 days), it is possible that long-term compensatory mechanisms in the SHR, but not the SS rat, include alterations in nociceptive pathways. The bulk of the evidence, however, suggests that blood pressure per se is not the primary factor mediating differences in nociception between SHR and normotensive rats.
Persistent Nociception in SHRs
In the zymosan test, we found SHRs to be hyperalgesic compared with their inbred normotensive WKY controls. Our results do not agree with Chipkin and Latranyi (3), who found that the intraplantar injection of yeast (the compound from which zymosan is purified) decreased paw pressure threshold in WKY and SD rats but increased paw pressure threshold in SHRs. Possible explanations for this discrepancy include 1) the use of paw pressure threshold in the earlier study versus the use of thermal and von Frey thresholds in our study; 2) the possible existence of a contaminant in yeast (other than zymosan) that produced antinociception in the SHR but not the WKY or SD rats; or 3) measurement of behavior in the earlier study at a single, 1-h time point, before peak changes in nociceptive threshold could be reached. Indeed, we found that SHRs exhibit hyperalgesia 1.5-2.5 h after intraplantar injection.To further evaluate inflammatory hyperalgesia, we studied persistent nociception in a second model, the Formalin test. We found exaggerated behavioral and cardiovascular responses to Formalin during phase 2 in the SHR. This did not correlate with an increase in the number of Formalin-induced Fos-positive neurons in the spinal cord dorsal horn or with an increase in Formalin-induced edema in the paw. On the contrary, paw edema was smaller in the SHR than in the WKY rat and, in the case of zymosan, was smaller in the SHR than in either normotensive strain. One possible explanation is that glucocorticoid-mediated inhibition of inflammation is exaggerated in the SHR. In fact, plasma levels of cortisol and corticosterone are greater in SHRs (14, 18) and in humans predisposed to hypertension (52), respectively.
We conclude that mechanisms other than spinal nociceptive transmission and inflammatory edema contribute to exaggerated Formalin-induced behavioral and cardiovascular responses in the SHR. Because sympathetic activity is positively coupled to inflammatory hyperalgesia (17), it remains possible that exaggerated sympathetic activity in the SHR increases the development of peripheral sensitization mechanisms. Such a mechanism may accelerate the development of adjuvant-induced arthritis in SHRs (20). Future studies that evaluate the development of peripheral sensitization in the SHR are needed.
In summary, our results indicate that the pain phenotype of the SHR is not restricted to hypoalgesia. Rather, differences in behavioral responses to noxious stimuli between SHR and normotensive rats vary with the nociceptive test. SHR hyperalgesia appears to be related not to sustained elevations in blood pressure per se, spinal neuron activity, or inflammatory edema, but rather may be related to genetic factors or to autonomic systems that control blood pressure. If certain genes pleiotropically affect both hypertension and nociception, then quantitative trait locus mapping techniques of loci involved both in pain (28) and hypertension (36) could yield valuable information regarding differential nociceptive processing in hypertensive individuals.
Perspectives
Although the number of people who develop chronic pain and/or hypertension is large, few studies have addressed the contribution of hypertension to pain threshold. Fewer, if any, studies have evaluated nociception in hypertensives with chronic debilitating pain states. A limited number of animal and clinical studies have found that hypertensive subjects show increased pain threshold to acute noxious stimulation, but we suggest here that these results may be confounded by stress-induced analgesia. Indeed, when we minimized stress with models of acute nociception in unrestrained animals and more clinically relevant models of persistent nociception and inflammatory hyperalgesia, we observed exaggerated nociceptive responses in the SHR. Further animal studies as well as measurement of persistent pain in patients with hypertension will facilitate our understanding of how these factors contribute to persistent pain.| |
ACKNOWLEDGEMENTS |
|---|
This research was supported by National Institutes of Health Grants DA-08377 and NS-21445 to A. I. Basbaum and DA-10356 to B. K. Taylor.
| |
FOOTNOTES |
|---|
Address for reprint requests and other correspondence: B. Taylor, Div. of Pharmacology, School of Pharmacy, Univ. of Missouri, Kansas City, 2411 Holmes St., M3-C15, Kansas City, MO 64108-2792 (E-mail: taylorb{at}umkc.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 16 June 2000; accepted in final form 27 September 2000.
| |
REFERENCES |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
1.
Bruehl, S,
Carlson CR,
and
McCubbin JA.
The relationship between pain sensitivity and blood pressure in normotensives.
Pain
48:
463-467,
1992[ISI][Medline].
2.
Chaplan, SR,
Bach FW,
Pogrel JW,
Chung JM,
and
Yaksh TL.
Quantitative assessment of tactile allodynia in the rat paw.
J Neurosci Methods
53:
55-63,
1994[ISI][Medline].
3.
Chipkin, RE,
and
Latranyi MB.
Subplantar yeast injection induces a non-naloxone reversible antinociception in spontaneously hypertensive rats.
Brain Res
303:
1-6,
1984[ISI][Medline].
4.
Dahl, LK,
Heine M,
and
Tassinari L.
Role of genetic factors in susceptibility to experimental hypertension due to chronic excess salt ingestion.
Nature
194:
480-481,
1962[Medline].
5.
Devor, M,
and
Raber P.
Heritability of symptoms in an experimental model of neuropathic pain.
Pain
42:
51-67,
1990[ISI][Medline].
6.
Dubuisson, D,
and
Dennis SG.
The Formalin test: a quantitative study of the analgesic effects of morphine, meperidine and brainstem stimulation in rats and cats.
Pain
4:
161-174,
1977[ISI][Medline].
7.
Elmer, GI,
Pieper JO,
Negus SS,
and
Woods JH.
Genetic variance in nociception and its relationship to the potency of morphine-induced analgesia in thermal and chemical tests.
Pain
75:
129-140,
1998[ISI][Medline].
8.
Friedman, R,
Murphy D,
Persons W,
and
McCaughran JA, Jr.
Genetic predisposition to hypertension, elevated blood pressure and pain sensitivity: a functional analysis.
Behav Brain Res
12:
75-79,
1984[ISI][Medline].
9.
Ghione, S.
Hypertension-associated hypalgesia.
Hypertension
28:
494-504,
1996
10.
Ghione, S,
Rosa C,
Mezzasalma L,
and
Panattoni E.
Arterial hypertension is associated with hypalgesia in humans.
Hypertension
12:
491-497,
1988
11.
Graham, FK,
Putnam LE,
and
Leavitt LA.
Lead-stimulation effects of human cardiac orienting and blink reflexes.
J Exp Psychol
104:
175-182,
1975.
12.
Hao, J-X,
and
Wiesenfeld-Hallin Z.
Variability in the occurrence of ongoing discharges in primary afferents originating in the neuroma after peripheral nerve section in different strains of rats.
Neurosci Lett
169:
119-121,
1994[ISI][Medline].
13.
Hargreaves, K,
Dubner R,
Brown R,
Flores C,
and
Joris J.
A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia.
Pain
32:
77-88,
1988[ISI][Medline].
14.
Hashimoto, K,
Makino S,
Hirasawa R,
Takao T,
Sugawara M,
Murakami K,
Ono K,
and
Ota Z.
Abnormalities in the hypothalamo-pituitary-adrenal axis in spontaneously hypertensive rats during development of hypertension.
Endocrinology
125:
1161-1167,
1989[Abstract].
15.
Hoffmann, O,
Plesan A,
and
Wiesenfeld-Hallin Z.
Genetic differences in morphine sensitivity, tolerance and withdrawal in rats.
Brain Res
806:
232-237,
1998[ISI][Medline].
16.
Hsu, S,
Raine L,
and
Fanger H.
A comparative study of the antiperoxidase method and an avidin-biotin complex method for studying polypeptide hormones with radioimmunoassay antibodies.
Am J Clin Pathol
75:
734-738,
1981[ISI][Medline].
17.
Janig, W,
Levine JD,
and
Michaelis M.
Interactions of sympathetic and primary afferent neurons following nerve injury and tissue trauma.
Prog Brain Res
113:
161-184,
1996[ISI][Medline].
18.
Kenyon, CJ,
Panarelli M,
Holloway CD,
Dunlop D,
Morton JJ,
Connell JM,
and
Fraser R.
The role of glucocorticoid activity in the inheritance of hypertension: studies in the rat.
J Steroid Biochem Mol Biol
45:
7-11,
1993[ISI][Medline].
19.
Kurtz, TW,
Montano M,
Chan L,
and
Kabra P.
Molecular evidence of genetic heterogeneity in Wistar-Kyoto rats: implications for research with the spontaneously hypertensive rat.
Hypertension
13:
188-192,
1989
20.
Levine, JD,
Dardick SJ,
Roizen MF,
Helms C,
and
Basbaum AI.
Contribution of sensory afferents and sympathetic efferents to joint injury in experimental arthritis.
J Neurosci
6:
3423-3429,
1986[Abstract].
21.
Llewellyn-Smith, IJ,
and
Minson JB.
Complete penetration of antibodies into vibratome sections after glutaraldehyde fixation and ethanol treatment: light and electron microscopy for neuropeptides.
J Histochem Cytochem
40:
1741-1749,
1992[Abstract].
22.
Luo, L,
and
Wiesenfeld-Hallin Z.
Genetic factors may influence the development of spinal reflex hyperexcitability following sciatic nerve section in the rat.
Neurosci Lett
169:
122-125,
1994[ISI][Medline].
23.
Maixner, W,
Touw KB,
Brody MJ,
Gebhart GF,
and
Long JP.
Factors influencing the altered pain perception in the spontaneously hypertensive rat.
Brain Res
237:
137-145,
1982[ISI][Medline].
24.
McCarty, R.
Stress, behavior and experimental hypertension.
Neurosci Biobehav Rev
7:
493-502,
1983[ISI][Medline].
25.
Meller, ST,
and
Gebhart GF.
Intraplantar zymosan as a reliable, quantifiable model of thermal and mechanical hyperalgesia in the rat.
Eur J Pain
1:
43-52,
1997[Medline].
26.
Meller, ST,
Lewis SJ,
Brody MJ,
and
Gebhart GF.
Age, strain and anesthetic dependent differences in the nociceptive responses produced by i.v. 5-HT in the rat.
Brain Res
587:
88-94,
1992[ISI][Medline].
27.
Mogil, JS,
Kest B,
Sadowski B,
and
Belknap JK.
Differential genetic mediation of sensitivity to morphine in genetic models of opiate antinociception: influence of nociceptive assay.
J Pharmacol Exp Ther
276:
532-544,
1996
28.
Mogil, JS,
Wilson SG,
Bon K,
Lee SE,
Chung K,
Raber P,
Pieper JO,
Hain HS,
Belknap JK,
Hubert L,
Elmer GI,
Chung JM,
and
Devor M.
Heritability of nociception. I. Responses of 11 inbred mouse strains on 12 measures of nociception.
Pain
80:
67-82,
1999[ISI][Medline].
29.
Mogil, JS,
Wilson SG,
Bon K,
Lee SE,
Chung K,
Raber P,
Pieper JO,
Hain HS,
Belknap JK,
Hubert L,
Elmer GI,
Chung JM,
and
Devor M.
Heritability of nociception. II. "Types" of nociception revealed by genetic correlation analysis.
Pain
80:
83-93,
1999[ISI][Medline].
30.
Okamoto, K,
and
Aoki K.
Development of a strain of spontaneously hypertensive rats.
Jap Circul J
27:
282-293,
1963.
31.
Peterson, MA,
Basbaum AI,
Abbadie C,
Rohde DS,
McKay WR,
and
Taylor BK.
The differential contribution of capsaicin-sensitive afferents to behavioral and cardiovascular measures of brief and persistent nociception and to Fos expression in the Formalin test.
Brain Res
755:
9-16,
1997[ISI][Medline].
32.
Petricevic, M,
Wanek K,
and
Denko CW.
A new mechanical method for measuring rat paw edema.
Pharmacology
16:
153-158,
1978[ISI][Medline].
33.
Randich, A,
and
Gebhart GF.
Vagal afferent modulation of nociception.
Brain Res Rev
17:
77-99,
1992[Medline].
34.
Randich, A,
and
Maixner W.
Interactions between cardiovascular and pain regulatory systems.
Neurosci Biobehav Rev
8:
343-367,
1984[ISI][Medline].
35.
Randich, A,
Meller ST,
and
Gebhart GF.
Responses of primary afferents and spinal dorsal horn neurons to thermal and mechanical stimuli before and during zymosan-induced inflammation of the rat hindpaw.
Brain Res
772:
135-148,
1997[ISI][Medline].
36.
Rapp, JP.
Genetic analysis of inherited hypertension in the rat.
Physiol Rev
80:
135-72,
2000
37.
Saavedra, JM.
Naloxone reversible decrease in pain sensitivity in young and adult spontaneously hypertensive rats.
Brain Res
209:
245-249,
1981[ISI][Medline].
38.
Sitsen, JM,
and
de Jong W.
Hypoalgesia in genetically hypertensive rats (SHR) is absent in rats with experimental hypertension.
Hypertension
5:
185-190,
1983
39.
Sitsen, JM,
and
de Jong W.
Observations on pain perception and hypertension in spontaneously hypertensive rats.
Clin Exp Hypertens
6:
1345-1356,
1984.
40.
Sokolov, EN.
Perception and the Conditioned Reflex. Oxford: Plenum, 1963.
41.
Taiwo, YO,
and
Levine JD.
Kappa- and delta-opioids block sympathetically dependent hyperalgesia.
J Neurosci
11:
928-932,
1991[Abstract].
42.
Taylor, B,
Peterson MA,
and
Basbaum A.
Persistent cardiovascular and behavioral nociceptive responses to subcutaneous Formalin require peripheral nerve input.
J Neurosci
15:
7575-7584,
1995[Abstract].
43.
Taylor, BK,
Holloway DH,
and
Printz MP.
A unique cholinergic deficit in the spontaneously hypertensive rat: physostigmine reveals a bradycardia response associated with sensory stimulation.
J Pharmacol Exp Ther
268:
1081-1090,
1994
44.
Taylor, BK,
Peterson MA,
and
Basbaum AI.
Early nociceptive events contribute to the temporal profile, but not the magnitude, of the tonic response to subcutaneous Formalin.
J Pharmacol Exp Ther
280:
876-883,
1997
45.
Taylor, BK,
Peterson MA,
and
Basbaum AI.
Exaggerated cardiovascular and behavioral nociceptive responses to subcutaneous Formalin injection in the spontaneously hypertensive rat.
Neurosci Lett
201:
9-12,
1995b[ISI][Medline].
46.
Taylor, BK,
and
Printz MP.
Habituation of startle-associated cardiovascular responses: the orienting reflex may be impaired in hypertensive rats.
Physiol Behav
60:
919-925,
1996[Medline].
47.
Tchakarov, L,
Abbott FV,
Ramirez-Gonzalez M,
and
Kunos G.
Naloxone reverses the antinociceptive action of clonidine in spontaneously hypertensive rats.
Brain Res
328:
33-40,
1985[ISI][Medline].
48.
Tjolsen, A,
Berge O-G,
Hunskaar S,
Rosland JH,
and
Hole K.
The Formalin test: an evaluation of the method.
Pain
51:
5-17,
1992[ISI][Medline].
49.
Tsai, CF,
and
Lin MT.
Pain sensitivity, thermal capability, and brain monoamine turnover in hypertensive rats.
Am J Physiol Regulatory Integrative Comp Physiol
253:
R910-R916,
1987
50.
Tucker, DC,
and
Johnson AK.
Behavioral correlates of spontaneous hypertension.
Neurosci Biobehav Rev
5:
463-471,
1981[ISI][Medline].
51.
Wang, Y,
Cheng CY,
Wang JY,
and
Lin JC.
Enhanced antinociception of clonidine in spontaneously hypertensive rats involves a presynaptic noradrenergic mechanism.
Pharmacol Biochem Behav
59:
109-114,
1998[ISI][Medline].
52.
Watt, GC,
Harrap SB,
Foy CJ,
Holton DW,
Edwards HV,
Davidson HR,
Connor JM,
Lever AF,
and
Fraser R.
Abnormalities of glucocorticoid metabolism and the renin-angiotensin system: a four-corners approach to the identification of genetic determinants of blood pressure.
J Hypertens
10:
473-482,
1992[ISI][Medline].
53.
Wendel, OT,
and
Bennett B.
The occurence of analgesia in an animal model of hypertension.
Life Sci
29:
515-521,
1981[ISI][Medline].
54.
Wiesenfeld-Hallin, Z,
Hao J,
Xu X,
Aldskogius H,
and
Seiger A.
Genetic factors influence the development of mechanical hypersensitivity, motor deficits and morphological damage after transient spinal cord ischemia in the rat.
Pain
55:
235-241,
1993[ISI][Medline].
55.
Zamir, N,
and
Shuber E.
Altered pain perception in hypertensive humans.
Brain Res
201:
471-474,
1980[ISI][Medline].
56.
Zamir, N,
Simantov R,
and
Segal M.
Pain sensitivity and opioid activity in genetically and experimentally hypertensive rats.
Brain Res
184:
299-310,
1980[ISI][Medline].
This article has been cited by other articles:
|
|
 |
|
  M. B. Calzavara, W. A. Medrano, R. Levin, S. R. Kameda, M. L. Andersen, S. Tufik, R. H. Silva, R. Frussa-Filho, and V. C. Abilio Neuroleptic Drugs Revert the Contextual Fear Conditioning Deficit Presented by Spontaneously Hypertensive Rats: A Potential Animal Model of Emotional Context Processing in Schizophrenia? Schizophr Bull, February 16, 2008; (2008) sbn006v1. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. B. Mahinda, B. M. Lovell, and B. K. Taylor Morphine-Induced Analgesia, Hypotension, and Bradycardia Are Enhanced in Hypertensive Rats Anesth. Analg., June 1, 2004; 98(6): 1698 - 1704. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
P < 0.05 SHR vs. SD,
§P < 0.05 SS vs. SR, 
P < 0.05 SD
vs. WKY. bpm, Beats/min.
