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Effects of lowering barometric pressure on guarding behavior, heart rate and blood pressure in a rat model of neuropathic pain
Neuroscience Letters 299 (2001) 17±20
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Effects of lowering barometric pressure on guarding behavior, heart rate and blood pressure in a rat model of neuropathic pain Jun Sato*, Keisuke Takanari, Sayaka Omura, Kazue Mizumura Department of Neural Regulation, Research Institute of Environmental Medicine, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464±8601, Japan Received 5 October 2000; received in revised form 1 December 2000; accepted 4 December 2000
Abstract We investigated whether lowering barometric pressure by 20 mmHg (LP) aggravates the guarding behavior suggestive of spontaneous pain following sciatic nerve chronic constriction injury (CCI) in rats. Systemic blood pressure (BP) and heart rate (HR) of unrestrained rats were recorded telemetrically during LP both before and after the CCI surgery. CCI rats showed guarding posture in normopressure conditions, and LP increased the cumulative time of this behavior. Baseline BP but not HR was increased following CCI. LP increased BP and HR of the rats only before the CCI surgery. Animals after CCI surgery showed variable (BP, HR) and transient (HR) responses to LP. These results indicate that (1) LP aggravated spontaneous pain and increased BP and HR in the CCI rats, and (2) CCI surgery in¯uenced BP and HR of rats. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Chronic constriction injury; Meteorological change; Spontaneous pain; Systemic blood pressure; Heart rate
Chronic constriction injury (CCI) of the sciatic nerve in rats produces many symptoms that approximate the clinical features of human neuropathic pain [1,4]. We recently found that a simulated low-barometric pressure environment (20 mmHg below the atmospheric pressure; LP) intensi®es the mechanical allodynia and hyperalgesia shown in the hind paw of CCI rats [12]. These ®ndings support reports that several types of pain due to pathological conditions in humans are aggravated by approaching low-pressure systems (for review, see Ref. [8]). In addition to these stimulus-evoked pain-related behavioral signs, CCI animals also demonstrated a guarding posture suggestive of spontaneous pain (ongoing pain without apparent external stimuli) [4]. One aim of the present study was to examine whether this guarding posture shown in CCI rats is also aggravated by LP exposure. Another aim was to see whether LP exposure resulted in changes in systemic blood pressure (BP) and heart rate (HR) in unrestrained CCI rats, a question prompted by earlier indirect evidence that sympathetic activity contributes to the LP effect on neuropathic pain in CCI rats [12]. All of the testing was performed in accordance with the * Corresponding author. Tel.: 181-52-789-3862; fax: 181-52789-3889. E-mail address: [email protected] (J. Sato).
guidelines of the International Association for the Study of Pain [14] and received approval from the Institutional Animal Care Committee of the Nagoya University. Twenty-®ve male Sprague±Dawley rats (200±250 g) were housed two to three per cage, under controlled temperature (22 ^ 18C) and on a 12 h light±dark cycle, and had free access to food and water. All surgical procedures were performed under surgically clean conditions and sodium pentobarbital anesthesia (60 mg/kg, i.p.). After receiving kanamycin sulfate (20 mg/kg, s.c.), the animals were monitored to ensure that food and water intake had returned to preoperative levels. Experimental neuropathy (CCI) was produced by loosely ligated the sciatic nerve, according to a method previously described [4]. Brie¯y, the right sciatic nerve was exposed at the mid-thigh level and the nerve was then ligated with four loose ligatures using chromic gut (4/0) spaced at about 1 mm. To measure the guarding behavior in a natural setting without intervention by the experimenter, CCI rats were placed individually in inverted transparent plastic cages (21 £ 12 £ 10 cm) with a mesh ¯oor. After 5 min of adaptation, the cumulative time that the rat held its foot off the ¯oor during a 5-min period was recorded. Foot lifts associated with locomotion or body repositioning were not counted. A preliminary observation indicated that foot lift-
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 0) 01 76 9- 9
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J. Sato et al. / Neuroscience Letters 299 (2001) 17±20
Fig. 1. LP exposure increased the spontaneous pain in the injured paw of CCI rats. The ®gure shows the change in cumulative duration of foot lifting during a 5-min measurement (mean ^ SEM). Pre: pre-LP exposure; mid: during LP exposure; post: post-LP exposure. LP exposure signi®cantly increased the cumulative duration of foot lifting of the injured paw but not of the uninjured paw. *P , 0:05, compared with the pre-exposure value (repeated-measures ANOVA with Fisher's PLSD test).
ing was maximal about 1 week after CCI surgery; therefore, the effects of LP exposure were evaluated on post-operative day (POD) 7. An implantable radiotransmitter equipped with a BP transducer (Physiotel TA11PA-C4, Data Sciences International, St. Paul, MN) was implanted in the abdominal cavity. A catheter connected to the BP transducer was introduced into the abdominal aorta just below the renal arteries, and the transducer was ®xed to the abdominal muscles with
sutures. The incision was then closed with sutures. The instantaneous HR was calculated from intervals between a series of BP waves. BP and HR under the LP exposure were measured twice, on POD 0 (before CCI surgery, 7 days after radiotransmitter implantation) and on POD 7±10. Throughout measurement period, rats were kept individually in their own cages (45 £ 25 £ 20 cm) and given food and water ad libitum. All exposure experiments were started around 10:00 h and ended before 17:00 h. Before measuring the pre-exposure values, the rats were kept in the climate-controlled room (ambient temperature 228C, relative humidity 50%) for 60±90 min. The barometric pressure was decreased over 8 min to a level 20 mmHg below the atmospheric pressure, and kept at this level for 29 min. It was then returned to the baseline pressure, again over 8 min (total LP-exposure time was 45 min). A change of 20 mmHg was chosen because barometric pressure gradually falls by 10± 20 mmHg at the passage of low-pressure systems in natural weather patterns. Rats were randomly divided into two groups. In the ®rst group (n 17), the effect of LP exposure on spontaneous pain was tested by monitoring the guarding behavior at three times: shortly before lowering barometric pressure, at the lowest pressure, and shortly after returning to the baseline pressure. In the second group (n 8), the effect of LP exposure on HR and BP was investigated. Instantaneous BP and HR values were sampled (100 Hz) every 5 min for 3 s throughout the entire experimental
Fig. 2. Time courses of changes in systemic blood pressure (BP: A,B) and heart rate (HR: C,D) of pre- (A,C) and post-CCI rats (B,D) during LP exposure. Average values of instantaneous BP and HR sampled for 3 s every 5 min are plotted. Each 45-min period of pre-, mid(shaded area) and post- exposures contains nine sampling time-points (B1±9, L1±9 and A1±9, respectively). *P , 0:05, compared with B9 of each group, repeated-measures ANOVA with Fisher's PLSD test.
J. Sato et al. / Neuroscience Letters 299 (2001) 17±20
period and averaged using LabPRO software (Data Sciences International, St. Paul, MN). Forty-®ve min sampling periods (nine sampling points each) were established in the pre-, mid- and post-exposure periods for use in assessing the effect of LP exposure on these parameters. Results are expressed as the mean ^ SEM. Statistical comparisons were performed using repeated-measures analysis of variance (ANOVA) followed by a Fisher's protected least signi®cant difference (PLSD) test or a paired t-test, as appropriate. P , 0:05 was considered a signi®cant difference. By POD 7, all the CCI rats (n 17) often held the hind paw of the affected side off the ¯oor much of the time, which is suggestive of spontaneous pain induced by nerve injury. LP exposure signi®cantly increased the cumulative time the injured paw was lifted (pre vs. mid, P , 0:05) but not of the uninjured paw (Fig. 1). This effect disappeared after returning to the normal pressure (post). In a preliminary experiment, LP exposure did not alter the spontaneous postures of sham-operated control rats (data not shown). These results indicate that LP exposure aggravates spontaneous pain in CCI rats. The time courses of the changes in BP and HR during LP exposure are shown in Fig. 2. The average BP of the preCCI rats increased in the middle of the LP-exposure period (Fig. 2A, L4-L5), but in the post-CCI rats, the effect of LP exposure was not clear (Fig. 2B). The average HR of both pre- and post-CCI rats was clearly increased by the LP exposure (Fig. 2C,D). In the pre-CCI rats (Fig. 2C), HR was increased throughout the LP exposure period, while in the post-CCI rats it increased only in the early part of the exposure period (Fig. 2D, L1-L2). To evaluate the in¯uence of CCI surgery and LP exposure on the BP and HR of each rat, these values were averaged for the pre-, mid- and post-exposure periods, respectively (Fig. 3). As is seen in Fig. 3A (which should be compared with Fig. 3B), CCI surgery itself caused a clear increase in the basal BP before LP exposure (pre) in all but one rat. Thus, the basal BP of the post-CCI rats was signi®cantly higher than that of the pre-CCI rats (P 0:03). Such an effect of CCI surgery in increasing the basal BP was also demonstrated in the pre-exposure period (B1±9) shown in Fig. 2B, as compared with Fig. 2A. In the measurements of HR, the basal value was higher after (post, Fig. 3D) than before (pre, Fig. 3C) CCI surgery in four rats, but lower in the others. Thus, on average, the CCI surgery did not significantly alter the HR value of the rats tested (see also Fig. 2C,D). A preliminary observation indicated that an interval of 1 week, which corresponds to the time between the preand post-CCI measurements in the present experiment, did not signi®cantly change the basal BP and HR in agematched control rats (data not shown). LP exposure increased the BP in all the pre-CCI rats (Fig. 3A). Average BP during the LP exposure (mid) was signi®cantly higher than the pre-exposure value (P , 0:05). This elevated BP tended to return toward the pre-exposure level,
19
but was still higher when the LP exposure ended (P , 0:05). LP exposure also increased the HR of all but one rat before CCI surgery (Fig. 3C). Average HR during the LP exposure (mid) was signi®cantly higher than the pre-exposure value (P , 0:05). This elevated HR fell nearly to the pre-exposure level after returning to normal pressure (post) in all but two rats. LP exposure also caused increases in BP and HR in ®ve and six animals, respectively, of the eight post-CCI rats (pre and mid, Fig. 3B,D). However, because the HR change was transient in these rats (Fig. 2D), the magnitude of the increase in HR during the LP-exposure period was smaller than in the pre-CCI rats. LP exposure caused decreases in
Fig. 3. Changes in systemic blood pressure (BP: A,B) and heart rate (HR: C,D) for each of eight rats due to CCI surgery and LP exposure. (A,C) Pre-CCI rats (before CCI surgery), (B,D) post-CCI rats (after CCI-surgery). Symbols show the average HR and BP of nine sampling time-points during the periods of pre-LP exposure (pre), during LP exposure (mid) and post-LP exposure (post). The data from the same rat are represented by the same symbol and connected by lines. Horizontal bars show the average for all rats in each period. Note the basal BP of post-CCI rats is signi®cantly higher than the pre-CCI value (pre, A,B) 1P 0:03, paired t-test). LP exposure signi®cantly increased both the HR and BP of the pre-CCI rats (A,C) *P , 0:05, compared with the pre-exposure value (repeated-measures ANOVA with Fisher's PLSD test). In the post-CCI rats (B,D), LP exposure decreased the HR and BP in some rats.
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J. Sato et al. / Neuroscience Letters 299 (2001) 17±20
both HR and BP in the remaining three and two rats, respectively, in this group. Moreover, these decreases continued even after returning to the normal pressure (post). On the whole, neither the HR nor BP of the post-CCI rats was signi®cantly changed by the LP environment (pre vs. mid; P 0:11 and 0.31, respectively). The present results indicated that the guarding behavior of CCI rats was exacerbated by the LP exposure (Fig. 1). Thus, with all other parameters constant, a drop in barometric pressure of 20 mmHg (within the range of natural environmental change) seems to aggravate not only stimulus-evoked pain [12] but also spontaneous pain in CCI rats. We recently [12] showed that lumbar sympathectomy blocked the aggravating effect of LP exposure on the mechanical allodynia and hyperalgesia in CCI rats, but did not alter any nociceptive responses in sham-operated control rats. This led us to speculate that LP exposure activates the sympathetic nervous system only in CCI rats. In the present study, therefore, we tested the effect of LP exposure on BP and HR (as indicators of autonomic response) in both pre- and post-CCI rats. LP exposure was shown to cause even larger increases in BP and HR in the pre-CCI than in post-CCI rats (Figs. 2 and 3). This implies that LP exposure provokes autonomic responses; in other words, it may activate the sympathetic nervous system in both groups of rats almost similarly, suggesting that the pain-aggravating process involves other mechanism than sympathetic efferent excitation that works only in the CCI rats. We can propose two hypotheses for such a mechanism. First, in the post-CCI rats, humoral catecholamines released from the adrenal medulla due to the environmental stress (LP exposure) might have activated the peripheral nociceptive and/or mechanoreceptive ®bers in the injured sciatic nerve, resulting in spontaneous pain. It has been reported that cutaneous nociceptive ®bers became responsive to adrenaline after nerve injury [10]. A second possible explanation is that LP exposure might have directly increased sympathetic nerve activity in the sciatic nerve. If this is true, then sympathetic nerve volleys activate and sensitize nociceptive primary afferents through the mechanism of sympatho-nociceptor interactions that are developed after nerve injury [7,10,11]. Such a process could be based on the sympathetic denervation supersensitivity of nociceptive ®bers [5]. In support of this latter hypothesis, we recently found that LP exposure increased both renal and lumbar sympathetic nerve activities in normal rats (unpublished observation). In the present study, all the CCI rats exhibited a guarding posture, which was not observed in any of the sham-operated control rats. This ®nding indicates that CCI surgery caused neuropathy in all the rats in the present study. On the other hand, LP exposure did not consistently increase HR or BP in the post-CCI rats (Fig. 3B,D), which is interpreted as indicating that the CCI procedure caused abnormal autonomic responses to environmental stress in some rats. Other such alterations in CCI rats have been reported, including abnor-
mal basal skin temperature [3,13] and vasoconstrictor re¯ex pattern to the cold pressor test [9]. Similar autonomic dysfunctions have also been reported in some neuropathic patients [2,6]. At present, it is not possible to predict the individual autonomic dysfunction that will occur after nerve injury. Understanding the origin of such heterogeneous responses will clearly require further investigations. This work was partly supported by Grants-in-Aid for Scienti®c Research from the Japanese Ministry of Education, Science and Culture, and carried out as part of the `Ground Research Announcement for Space Utilization' promoted by NASDA and the Japan Space Forum. [1] Attal, N., Kayser, V., Jazat, F. and Guilbaud, G., Further evidence for `pain-related' behaviours in a model of unilateral peripheral mononeuropathy, Pain, 41 (1990) 235±251. [2] Baron, R. and Maier, C., Re¯ex sympathetic dystrophy: skin blood ¯ow, sympathetic vaso constrictor re¯exes and pain before and after surgical sympathectomy, Pain, 67 (1996) 317±326. [3] Bennett, G.J. and Ochoa, J.L., Thermographic observations on rats with experimental neuropathic pain, Pain, 45 (1991) 61±67. [4] Bennett, G.J. and Xie, Y., A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man, Pain, 33 (1988) 87±107. [5] Birder, L.A. and Perl, E.R., Expression of a2-adrenergic receptors in rat primary afferent neurones after peripheral nerve injury or in¯ammation, J. Physiol., 515 (1999) 533± 542. [6] Birklein, F., Riedl, B., NeundoÈfer, B. and Handwerker, H.O., Sympathetic vasoconstrictor re¯ex pattern in patients with complex regional pain syndrome, Pain, 75 (1998) 93±100. [7] Bossut, D.F., Shea, V.K. and Perl, E.R., Sympathectomy induces adrenergic excitability of cutaneous C-®ber nociceptors, J. Neurophysiol., 75 (1996) 514±517. [8] Jamison, R.N., In¯uence of weather on report of pain, IASP Newsl. July/August, (1996) 3±5. [9] Kurvers, H.A.J.M., Tangelder, G.J., De Mey, J.G.R., Slaaf, D.W., Beuk, R.J., van den Wildenberg, F.A.J.M., Kitslaar, P.J.E.H.M., Reneman, R.S. and Jacobs, M.J.H.M., Skin blood ¯ow abnormalities in a rat model of neuropathic pain: results of decreased sympathetic vasoconstrictor out¯ow? J. Auton. Nerv. Syst., 63 (1997) 19±29. [10] O'Halloran, K.D., Shea, V.K. and Perl, E.R., Action of epinephrine and other adrenergic agonists on C-®ber polymodal nociceptors after partial nerve injury, Soc. Neurosci. Abstr., 22 (1996) 1809. [11] Sato, J. and Perl, E.R., Adrenergic excitation of cutaneous pain receptors induced by peripheral nerve injury, Science, 29 (1991) 1608±1610. [12] Sato, T., Morimae, H., Seino, Y., Kobayashi, T., Suzuki, N. and Mizumura, K., Lowering barometric pressure aggravates mechanical allodynia and hyperalgesia in a rat model of neuropathic pain, Neurosci. Lett., 266 (1999) 21± 24. [13] Wakisaka, S., Kajander, K.C. and Bennett, G.J., Abnormal skin temperature and abnormal sympathetic vasomotor innervation in an experimental painful peripheral neuropathy, Pain, 46 (1991) 299±313. [14] Zimmermann, M., Ethical guidelines for investigators of experimental pain in conscious animals, Pain, 16 (1983) 109±110.
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Effects of lowering barometric pressure on guarding behavior, heart rate and blood pressure in a rat model of neuropathic pain Jun Sato*, Keisuke Takanari, Sayaka Omura, Kazue Mizumura Department of Neural Regulation, Research Institute of Environmental Medicine, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464±8601, Japan Received 5 October 2000; received in revised form 1 December 2000; accepted 4 December 2000
Abstract We investigated whether lowering barometric pressure by 20 mmHg (LP) aggravates the guarding behavior suggestive of spontaneous pain following sciatic nerve chronic constriction injury (CCI) in rats. Systemic blood pressure (BP) and heart rate (HR) of unrestrained rats were recorded telemetrically during LP both before and after the CCI surgery. CCI rats showed guarding posture in normopressure conditions, and LP increased the cumulative time of this behavior. Baseline BP but not HR was increased following CCI. LP increased BP and HR of the rats only before the CCI surgery. Animals after CCI surgery showed variable (BP, HR) and transient (HR) responses to LP. These results indicate that (1) LP aggravated spontaneous pain and increased BP and HR in the CCI rats, and (2) CCI surgery in¯uenced BP and HR of rats. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Chronic constriction injury; Meteorological change; Spontaneous pain; Systemic blood pressure; Heart rate
Chronic constriction injury (CCI) of the sciatic nerve in rats produces many symptoms that approximate the clinical features of human neuropathic pain [1,4]. We recently found that a simulated low-barometric pressure environment (20 mmHg below the atmospheric pressure; LP) intensi®es the mechanical allodynia and hyperalgesia shown in the hind paw of CCI rats [12]. These ®ndings support reports that several types of pain due to pathological conditions in humans are aggravated by approaching low-pressure systems (for review, see Ref. [8]). In addition to these stimulus-evoked pain-related behavioral signs, CCI animals also demonstrated a guarding posture suggestive of spontaneous pain (ongoing pain without apparent external stimuli) [4]. One aim of the present study was to examine whether this guarding posture shown in CCI rats is also aggravated by LP exposure. Another aim was to see whether LP exposure resulted in changes in systemic blood pressure (BP) and heart rate (HR) in unrestrained CCI rats, a question prompted by earlier indirect evidence that sympathetic activity contributes to the LP effect on neuropathic pain in CCI rats [12]. All of the testing was performed in accordance with the * Corresponding author. Tel.: 181-52-789-3862; fax: 181-52789-3889. E-mail address: [email protected] (J. Sato).
guidelines of the International Association for the Study of Pain [14] and received approval from the Institutional Animal Care Committee of the Nagoya University. Twenty-®ve male Sprague±Dawley rats (200±250 g) were housed two to three per cage, under controlled temperature (22 ^ 18C) and on a 12 h light±dark cycle, and had free access to food and water. All surgical procedures were performed under surgically clean conditions and sodium pentobarbital anesthesia (60 mg/kg, i.p.). After receiving kanamycin sulfate (20 mg/kg, s.c.), the animals were monitored to ensure that food and water intake had returned to preoperative levels. Experimental neuropathy (CCI) was produced by loosely ligated the sciatic nerve, according to a method previously described [4]. Brie¯y, the right sciatic nerve was exposed at the mid-thigh level and the nerve was then ligated with four loose ligatures using chromic gut (4/0) spaced at about 1 mm. To measure the guarding behavior in a natural setting without intervention by the experimenter, CCI rats were placed individually in inverted transparent plastic cages (21 £ 12 £ 10 cm) with a mesh ¯oor. After 5 min of adaptation, the cumulative time that the rat held its foot off the ¯oor during a 5-min period was recorded. Foot lifts associated with locomotion or body repositioning were not counted. A preliminary observation indicated that foot lift-
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 0) 01 76 9- 9
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J. Sato et al. / Neuroscience Letters 299 (2001) 17±20
Fig. 1. LP exposure increased the spontaneous pain in the injured paw of CCI rats. The ®gure shows the change in cumulative duration of foot lifting during a 5-min measurement (mean ^ SEM). Pre: pre-LP exposure; mid: during LP exposure; post: post-LP exposure. LP exposure signi®cantly increased the cumulative duration of foot lifting of the injured paw but not of the uninjured paw. *P , 0:05, compared with the pre-exposure value (repeated-measures ANOVA with Fisher's PLSD test).
ing was maximal about 1 week after CCI surgery; therefore, the effects of LP exposure were evaluated on post-operative day (POD) 7. An implantable radiotransmitter equipped with a BP transducer (Physiotel TA11PA-C4, Data Sciences International, St. Paul, MN) was implanted in the abdominal cavity. A catheter connected to the BP transducer was introduced into the abdominal aorta just below the renal arteries, and the transducer was ®xed to the abdominal muscles with
sutures. The incision was then closed with sutures. The instantaneous HR was calculated from intervals between a series of BP waves. BP and HR under the LP exposure were measured twice, on POD 0 (before CCI surgery, 7 days after radiotransmitter implantation) and on POD 7±10. Throughout measurement period, rats were kept individually in their own cages (45 £ 25 £ 20 cm) and given food and water ad libitum. All exposure experiments were started around 10:00 h and ended before 17:00 h. Before measuring the pre-exposure values, the rats were kept in the climate-controlled room (ambient temperature 228C, relative humidity 50%) for 60±90 min. The barometric pressure was decreased over 8 min to a level 20 mmHg below the atmospheric pressure, and kept at this level for 29 min. It was then returned to the baseline pressure, again over 8 min (total LP-exposure time was 45 min). A change of 20 mmHg was chosen because barometric pressure gradually falls by 10± 20 mmHg at the passage of low-pressure systems in natural weather patterns. Rats were randomly divided into two groups. In the ®rst group (n 17), the effect of LP exposure on spontaneous pain was tested by monitoring the guarding behavior at three times: shortly before lowering barometric pressure, at the lowest pressure, and shortly after returning to the baseline pressure. In the second group (n 8), the effect of LP exposure on HR and BP was investigated. Instantaneous BP and HR values were sampled (100 Hz) every 5 min for 3 s throughout the entire experimental
Fig. 2. Time courses of changes in systemic blood pressure (BP: A,B) and heart rate (HR: C,D) of pre- (A,C) and post-CCI rats (B,D) during LP exposure. Average values of instantaneous BP and HR sampled for 3 s every 5 min are plotted. Each 45-min period of pre-, mid(shaded area) and post- exposures contains nine sampling time-points (B1±9, L1±9 and A1±9, respectively). *P , 0:05, compared with B9 of each group, repeated-measures ANOVA with Fisher's PLSD test.
J. Sato et al. / Neuroscience Letters 299 (2001) 17±20
period and averaged using LabPRO software (Data Sciences International, St. Paul, MN). Forty-®ve min sampling periods (nine sampling points each) were established in the pre-, mid- and post-exposure periods for use in assessing the effect of LP exposure on these parameters. Results are expressed as the mean ^ SEM. Statistical comparisons were performed using repeated-measures analysis of variance (ANOVA) followed by a Fisher's protected least signi®cant difference (PLSD) test or a paired t-test, as appropriate. P , 0:05 was considered a signi®cant difference. By POD 7, all the CCI rats (n 17) often held the hind paw of the affected side off the ¯oor much of the time, which is suggestive of spontaneous pain induced by nerve injury. LP exposure signi®cantly increased the cumulative time the injured paw was lifted (pre vs. mid, P , 0:05) but not of the uninjured paw (Fig. 1). This effect disappeared after returning to the normal pressure (post). In a preliminary experiment, LP exposure did not alter the spontaneous postures of sham-operated control rats (data not shown). These results indicate that LP exposure aggravates spontaneous pain in CCI rats. The time courses of the changes in BP and HR during LP exposure are shown in Fig. 2. The average BP of the preCCI rats increased in the middle of the LP-exposure period (Fig. 2A, L4-L5), but in the post-CCI rats, the effect of LP exposure was not clear (Fig. 2B). The average HR of both pre- and post-CCI rats was clearly increased by the LP exposure (Fig. 2C,D). In the pre-CCI rats (Fig. 2C), HR was increased throughout the LP exposure period, while in the post-CCI rats it increased only in the early part of the exposure period (Fig. 2D, L1-L2). To evaluate the in¯uence of CCI surgery and LP exposure on the BP and HR of each rat, these values were averaged for the pre-, mid- and post-exposure periods, respectively (Fig. 3). As is seen in Fig. 3A (which should be compared with Fig. 3B), CCI surgery itself caused a clear increase in the basal BP before LP exposure (pre) in all but one rat. Thus, the basal BP of the post-CCI rats was signi®cantly higher than that of the pre-CCI rats (P 0:03). Such an effect of CCI surgery in increasing the basal BP was also demonstrated in the pre-exposure period (B1±9) shown in Fig. 2B, as compared with Fig. 2A. In the measurements of HR, the basal value was higher after (post, Fig. 3D) than before (pre, Fig. 3C) CCI surgery in four rats, but lower in the others. Thus, on average, the CCI surgery did not significantly alter the HR value of the rats tested (see also Fig. 2C,D). A preliminary observation indicated that an interval of 1 week, which corresponds to the time between the preand post-CCI measurements in the present experiment, did not signi®cantly change the basal BP and HR in agematched control rats (data not shown). LP exposure increased the BP in all the pre-CCI rats (Fig. 3A). Average BP during the LP exposure (mid) was signi®cantly higher than the pre-exposure value (P , 0:05). This elevated BP tended to return toward the pre-exposure level,
19
but was still higher when the LP exposure ended (P , 0:05). LP exposure also increased the HR of all but one rat before CCI surgery (Fig. 3C). Average HR during the LP exposure (mid) was signi®cantly higher than the pre-exposure value (P , 0:05). This elevated HR fell nearly to the pre-exposure level after returning to normal pressure (post) in all but two rats. LP exposure also caused increases in BP and HR in ®ve and six animals, respectively, of the eight post-CCI rats (pre and mid, Fig. 3B,D). However, because the HR change was transient in these rats (Fig. 2D), the magnitude of the increase in HR during the LP-exposure period was smaller than in the pre-CCI rats. LP exposure caused decreases in
Fig. 3. Changes in systemic blood pressure (BP: A,B) and heart rate (HR: C,D) for each of eight rats due to CCI surgery and LP exposure. (A,C) Pre-CCI rats (before CCI surgery), (B,D) post-CCI rats (after CCI-surgery). Symbols show the average HR and BP of nine sampling time-points during the periods of pre-LP exposure (pre), during LP exposure (mid) and post-LP exposure (post). The data from the same rat are represented by the same symbol and connected by lines. Horizontal bars show the average for all rats in each period. Note the basal BP of post-CCI rats is signi®cantly higher than the pre-CCI value (pre, A,B) 1P 0:03, paired t-test). LP exposure signi®cantly increased both the HR and BP of the pre-CCI rats (A,C) *P , 0:05, compared with the pre-exposure value (repeated-measures ANOVA with Fisher's PLSD test). In the post-CCI rats (B,D), LP exposure decreased the HR and BP in some rats.
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J. Sato et al. / Neuroscience Letters 299 (2001) 17±20
both HR and BP in the remaining three and two rats, respectively, in this group. Moreover, these decreases continued even after returning to the normal pressure (post). On the whole, neither the HR nor BP of the post-CCI rats was signi®cantly changed by the LP environment (pre vs. mid; P 0:11 and 0.31, respectively). The present results indicated that the guarding behavior of CCI rats was exacerbated by the LP exposure (Fig. 1). Thus, with all other parameters constant, a drop in barometric pressure of 20 mmHg (within the range of natural environmental change) seems to aggravate not only stimulus-evoked pain [12] but also spontaneous pain in CCI rats. We recently [12] showed that lumbar sympathectomy blocked the aggravating effect of LP exposure on the mechanical allodynia and hyperalgesia in CCI rats, but did not alter any nociceptive responses in sham-operated control rats. This led us to speculate that LP exposure activates the sympathetic nervous system only in CCI rats. In the present study, therefore, we tested the effect of LP exposure on BP and HR (as indicators of autonomic response) in both pre- and post-CCI rats. LP exposure was shown to cause even larger increases in BP and HR in the pre-CCI than in post-CCI rats (Figs. 2 and 3). This implies that LP exposure provokes autonomic responses; in other words, it may activate the sympathetic nervous system in both groups of rats almost similarly, suggesting that the pain-aggravating process involves other mechanism than sympathetic efferent excitation that works only in the CCI rats. We can propose two hypotheses for such a mechanism. First, in the post-CCI rats, humoral catecholamines released from the adrenal medulla due to the environmental stress (LP exposure) might have activated the peripheral nociceptive and/or mechanoreceptive ®bers in the injured sciatic nerve, resulting in spontaneous pain. It has been reported that cutaneous nociceptive ®bers became responsive to adrenaline after nerve injury [10]. A second possible explanation is that LP exposure might have directly increased sympathetic nerve activity in the sciatic nerve. If this is true, then sympathetic nerve volleys activate and sensitize nociceptive primary afferents through the mechanism of sympatho-nociceptor interactions that are developed after nerve injury [7,10,11]. Such a process could be based on the sympathetic denervation supersensitivity of nociceptive ®bers [5]. In support of this latter hypothesis, we recently found that LP exposure increased both renal and lumbar sympathetic nerve activities in normal rats (unpublished observation). In the present study, all the CCI rats exhibited a guarding posture, which was not observed in any of the sham-operated control rats. This ®nding indicates that CCI surgery caused neuropathy in all the rats in the present study. On the other hand, LP exposure did not consistently increase HR or BP in the post-CCI rats (Fig. 3B,D), which is interpreted as indicating that the CCI procedure caused abnormal autonomic responses to environmental stress in some rats. Other such alterations in CCI rats have been reported, including abnor-
mal basal skin temperature [3,13] and vasoconstrictor re¯ex pattern to the cold pressor test [9]. Similar autonomic dysfunctions have also been reported in some neuropathic patients [2,6]. At present, it is not possible to predict the individual autonomic dysfunction that will occur after nerve injury. Understanding the origin of such heterogeneous responses will clearly require further investigations. This work was partly supported by Grants-in-Aid for Scienti®c Research from the Japanese Ministry of Education, Science and Culture, and carried out as part of the `Ground Research Announcement for Space Utilization' promoted by NASDA and the Japan Space Forum. [1] Attal, N., Kayser, V., Jazat, F. and Guilbaud, G., Further evidence for `pain-related' behaviours in a model of unilateral peripheral mononeuropathy, Pain, 41 (1990) 235±251. [2] Baron, R. and Maier, C., Re¯ex sympathetic dystrophy: skin blood ¯ow, sympathetic vaso constrictor re¯exes and pain before and after surgical sympathectomy, Pain, 67 (1996) 317±326. [3] Bennett, G.J. and Ochoa, J.L., Thermographic observations on rats with experimental neuropathic pain, Pain, 45 (1991) 61±67. [4] Bennett, G.J. and Xie, Y., A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man, Pain, 33 (1988) 87±107. [5] Birder, L.A. and Perl, E.R., Expression of a2-adrenergic receptors in rat primary afferent neurones after peripheral nerve injury or in¯ammation, J. Physiol., 515 (1999) 533± 542. [6] Birklein, F., Riedl, B., NeundoÈfer, B. and Handwerker, H.O., Sympathetic vasoconstrictor re¯ex pattern in patients with complex regional pain syndrome, Pain, 75 (1998) 93±100. [7] Bossut, D.F., Shea, V.K. and Perl, E.R., Sympathectomy induces adrenergic excitability of cutaneous C-®ber nociceptors, J. Neurophysiol., 75 (1996) 514±517. [8] Jamison, R.N., In¯uence of weather on report of pain, IASP Newsl. July/August, (1996) 3±5. [9] Kurvers, H.A.J.M., Tangelder, G.J., De Mey, J.G.R., Slaaf, D.W., Beuk, R.J., van den Wildenberg, F.A.J.M., Kitslaar, P.J.E.H.M., Reneman, R.S. and Jacobs, M.J.H.M., Skin blood ¯ow abnormalities in a rat model of neuropathic pain: results of decreased sympathetic vasoconstrictor out¯ow? J. Auton. Nerv. Syst., 63 (1997) 19±29. [10] O'Halloran, K.D., Shea, V.K. and Perl, E.R., Action of epinephrine and other adrenergic agonists on C-®ber polymodal nociceptors after partial nerve injury, Soc. Neurosci. Abstr., 22 (1996) 1809. [11] Sato, J. and Perl, E.R., Adrenergic excitation of cutaneous pain receptors induced by peripheral nerve injury, Science, 29 (1991) 1608±1610. [12] Sato, T., Morimae, H., Seino, Y., Kobayashi, T., Suzuki, N. and Mizumura, K., Lowering barometric pressure aggravates mechanical allodynia and hyperalgesia in a rat model of neuropathic pain, Neurosci. Lett., 266 (1999) 21± 24. [13] Wakisaka, S., Kajander, K.C. and Bennett, G.J., Abnormal skin temperature and abnormal sympathetic vasomotor innervation in an experimental painful peripheral neuropathy, Pain, 46 (1991) 299±313. [14] Zimmermann, M., Ethical guidelines for investigators of experimental pain in conscious animals, Pain, 16 (1983) 109±110.