- Email: [email protected]
A novel model of primary and secondary hyperalgesia after mild thermal injury in the rat
Neuroscience Letters 254 (1998) 25–28
A novel model of primary and secondary hyperalgesia after mild thermal injury in the rat Natsuko Nozaki-Taguchi, Tony L. Yaksh* Department of Anesthesiology, 9500 Gilman Drive, University of California, San Diego, La Jolla, CA 92093-0818 USA Received 14 May 1998; received in revised form 28 July 1998; accepted 4 August 1998
Abstract Secondary hyperesthesia was investigated in a rat thermal injury model. After a mild focal thermal injury (52°C / 45 s) to the rat heel, the response latency for a thermal stimulus directed at the injured site was reduced (10 → 6 s; e.g. primary thermal hyperalgesia) but no change was seen at the distal site. Conversely, tactile threshold at the distal site was significantly reduced (15 → 5 g; e.g. secondary tactile allodynia) but much less so at the injured site. Magnitude of the secondary tactile allodynia paralleled the severity of the primary injury. Accordingly, this model has the same characteristics seen in human post-tissueinjury hyperesthetic states and provides a tool for the study of mechanisms underlying primary and secondary hyperesthesia. 1998 Elsevier Science Ireland Ltd. All rights reserved
Keywords: Primary hyperalgesia; Secondary hyperalgesia; Thermal hyperalgesia; Tactile allodynia
Injury to the skin increases the reported pain sensation otherwise evoked by a noxious thermal stimulus (thermal hyperalgesia) or an innocuous mechanical stimulus (tactile allodynia) [15]. These phenomenon are seen within the injured area (primary hyperalgesia) and are also known to extend into the uninjured skin surrounding the injury (secondary hyperalgesia/allodynia). A cutaneous burn in human models has been shown to produce hyperalgesia to heat and tactile allodynia at the site of injury and tactile allodynia in the undamaged tissue surrounding the injury [13]. Recently, we developed a rat model to study the mechanisms of primary hyperalgesia [9]. A mild (non-blistering) thermal injury to the hind paw produces significant thermal hyperalgesia in the injured area. To extend this model for the study of primary and secondary hyperalgesia, the distribution of primary/secondary hyperalgesia to a thermal and mechanical stimulus was defined. Male Holtzman–Sprague–Dawley rats (275-350 g; Harlan Industries, Indianapolis, IN, USA) were used for the experiment. All experiments were carried out according to
* Corresponding author. Tel.: +1 619 5433597; fax: +1 619 5436070; e-mail: [email protected]
a protocol approved by the Institutional Animal Care Committee of the University of California, San Diego. For the measurement of mechanical threshold, rats were placed in a plastic cage with a wire mesh bottom. Mechanical thresholds were measured using von Frey filaments with logarithmically incremental stiffness (0.41, 0.70, 1.20, 2.00, 3.63, 5.50, 8.50 and 15.10 g) (Stoelting, Wood Dale, IL, USA) to calculate the 50% probability thresholds for mechanical paw withdrawal, as previously described [4]. In brief, beginning with the 2.0 g probe, filaments were applied to the plantar surface of a hind paw for 6–8 s, in a stepwise ascending or descending order following negative or positive withdrawal responses, respectively, until six consecutive responses were noted. Withdrawal thresholds were then calculated according to the method of Dixon [7]. Testing sites for mechanical threshold are described in Fig. 1. In the first experiment, three points within and around the injured area (A, B, C: primary area), and two points outside the injured area (D, E: secondary area) were chosen for the assessment of primary and secondary hyperalgesia, respectively. Four points were randomly tested in each rat: one primary and one secondary point in the injured paw, and the corresponding two points in the non-injured paw, except for two rats which were tested in two primary
0304-3940/98/$19.00 1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304- 3940(98) 00648- X
26
N. Nozaki-Taguchi, T.L. Yaksh / Neuroscience Letters 254 (1998) 25–28
points. Animals with baseline thresholds below 10 g were excluded from the study (three rats). After measurement of baseline withdrawal threshold, thermal injury was induced by placing the plantar surface of one hind paw on a 52 ± 1°C hot plate for 45 s, unless otherwise stated, under 2% halothane anesthesia. A 10 g sand pouch was applied on the top of the paw, covering the heel portion, to provide a constant pressure. This exposure produced significant thermal hyperalgesia in the injured area without tissue damage such as blistering [9]. Withdrawal thresholds for four points were measured every 30 min for 3 h post injury. For the subsequent study of secondary hyperalgesia/allodynia, all measurements were performed on point D. Thermal nociceptive responses were assessed in a separate experiment. Following 45 s thermal injury, thermal withdrawal responses were tested at both the primary and the secondary area of the injured paw, as well as the corresponding area of the non-injured paw. A commercially available device, similar to that described by Hargreaves et al. [8], was used to assess thermal nociceptive responses. Characteristics of the device have been previously described [6]. Briefly, rats were placed individually in a Plexiglas
cubicle on a heated (30°C) glass surface, and the light from a focused projection bulb, located below the glass, was directed at the plantar surface of one hind paw. The thermal stimulus was presented randomly to the primary and secondary test areas shown in Fig. 3. Thermal withdrawal responses were measured every 30 min for 3 h post injury. The time interval between the application of the light and the hind paw withdrawal response was defined as the paw withdrawal latency (PWL: s). A bulb current was chosen that evoked an average response latency in normal untreated rats of 10 ± 1 s. Data were analyzed for statistical significance using twoway repeated measures analysis of variance (ANOVA: two within factors; time and paw) followed by Bonferroni/ Dunn’s post-hoc test for multiple comparisons. Baseline mean mechanical withdrawal thresholds (mean ± SEM) were 15.0 ± 0.0, 15.0 ± 0.0, 14.7 ± 0.4, 14.4 ± 0.4 and 14.2 ± 0.4 g for testing sites A, B, C, D and E, respectively. Following thermal injury, a mild but significant de-crease in the mechanical threshold was seen in the primary area of the injured paw at points A and B, ((A): F(6,36) = 2.73, P , 0.03, (B): F(6,24) = 5.06, P ,
Fig. 1. Time course of paw withdrawal threshold (g) to von Frey filament of injured paw (X) and non-injured paw (W) at each corresponding testing site. The middle drawing shows the right injured paw with the injured area shown as a shaded area and the testing sites as indicated (A, B, C: primary area; D, E: secondary area). Injury was given at time 0 which is shown in a dashed line in the graphs. All points represent the mean ± SEM of five to seven animals. (b: baseline threshold) Statistically significant differences from baseline threshold and from non-injured paw are represented as **P , 0.01, *P , 0.05 and ‡P , 0.01, †P , 0.05, respectively.
N. Nozaki-Taguchi, T.L. Yaksh / Neuroscience Letters 254 (1998) 25–28
Fig. 2. Time course of the tactile paw withdrawal threshold (g) of the injured paw (X) and the non-injured paw (W) when von Frey filaments were applied at test site D (see Fig. 1). Injury with a duration of 15 s (upper panel) and 30 s (lower panel) was given at time 0, as shown by the dashed line. All points represent the mean ± SEM of six animals. (b: baseline threshold) Statistically significant differences from baseline threshold and from non-injured paw are represented as **P , 0.01, *P , 0.05 and ‡P , 0.01, respectively.
0.002), but no change was seen at point (C): F(6,24) = 0.47, P . 0.5 (Fig. 1). A more profound decrease in mechanical threshold (allodynia) was observed in the secondary area ((D): F(6,36) = 4.33, P , 0.003, (E): F (6,30) = 3.98, P , 0.005) (Fig. 1). Significant decreases from baseline threshold were seen at 30, 60 and 90 min after the injury at point D, and at all observation time points after injury at point E. Non-injured paw showed no significant change from baseline after the injury at any point. However, tendency for an increase in the sensitivity was noted at the secondary area of the non-injured paw. To assess the effect of magnitude of primary injury in the secondary tactile allodynia, rats were exposed to shorter injury intensities (30 and 15 s). When animals were exposed to 30 s thermal injury, significant decreases in the threshold were likewise observed in the secondary area (D) of the injured paw at 30–90 min post injury (F(6.30) = 7.13, P , 0.0001, Fig. 2-bottom). However, when the exposure time was reduced to 15 s, no significant allodynia was observed in the secondary area (F(6,30) = 1.34, P . 0.2, Fig. 2-top).
27
When animals were exposed to a 45 s thermal injury, thermal nociceptive test showed decrease in PWL (thermal hyperalgesia) in the primary area of the injured paw (F (6,30) = 6.04, P , 0.0004, Fig. 3-left). A significant decrease from baseline PWL was seen at 30 min post injury. However, no change in the thermal nociceptive response was seen in the secondary area of the injured paw (F (6,30) = 1.09, P . 0.3, Fig. 3-right) nor in any part of the non-injured paw. The current study clearly demonstrates that a mild focal thermal injury of the rat paw produces significant thermal hyperalgesia and mild tactile allodynia in the injured area (primary hyperalgesia) and a profound tactile allodynia but no thermal hyperalgesia in the uninjured area of the injured paw (secondary tactile allodynia). These data demonstrate the classical features of the primary and secondary hyperalgesia seen after tissue injury in human models [5,13]. Although, technically, we could not show the precise spatial distribution of secondary hyperalgesia, the result from five points indicated that the tactile allodynia was more significant at sites away from the thermal injury. As the induction of thermal injury requires the whole foot to be placed on the hot plate, there was a possibility for tissue injury sufficient to produce peripheral nociceptor sensitization in the secondary area. However, this likelihood was discounted by the absence of thermal hyperalgesia in the secondary area. The magnitude of the secondary tactile allodynia increased with the severity of thermal injury. Thus, the initiation of post-injury secondary tactile allodynia is closely related with the peripheral nociceptive input at the time of injury. However, in our experiment, the duration of the decrease from baseline observed for the secondary tactile allodynia outlasted the duration of the primary hyperalgesia, as seen with thermal nociceptive testing. In addition, a 30 s/ 52°C exposure, which yielded no primary thermal hyperalgesia (Jun and Yaksh, unpublished data), was significant to evoke secondary tactile allodynia. These results suggest that the acute afferent input and not peripheral sensitization per se induced changes that persist beyond the time of input from the thermal injury. This may not agree with the results of human models reporting that the secondary hyperesthesia outside the injury does not outlast the primary hyperalgesia [12]. However, human psychophysical study have also shown that non-painful thermal stimulation can evoke secondary tactile allodynia [3]. The phenomenon of primary and secondary hyperesthesia has been studied extensively in human models [1,2,5,10]. Parallel studies with human and animal behavior and electrophysiological studies have been reported [2,14] in which tactile allodynia can be evoked at a receptive field distal to the injury. The mechanism of the secondary allodynia remains unclear. Kinnman and Levin [11] showed that capsaicin injection in the rat paw evokes a secondary tactile allodynia, emphasizing the role of small afferent inputs. In our own work, topical application of a mu-opiate agonist
28
N. Nozaki-Taguchi, T.L. Yaksh / Neuroscience Letters 254 (1998) 25–28
Fig. 3. Time course of paw withdrawal latency (PWL: s) of injured paw (X) and non-injured paw (W) after thermal injury at time 0 (dashed line). The middle drawing shows the right injured paw with the shaded area showing the injury and circle showing the testing area. Left and right panels show the results of thermal testing at the primary and secondary site, respectively. All points represent the mean ± SEM of six animals. (b: baseline PWL) Statistically significant difference from baseline PWL and from non-injured paw are represented as **P , 0.01 and ‡P , 0.01, respectively.
prior, but not after, thermal injury prevents development of the off-site tactile allodynia, suggesting that opiate-sensitive terminals are responsible for the initiation of the allodynia, but is not necessary to sustain it (Nozaki-Taguchi and Yaksh, unpublished observations). This model of mild thermal injury in rats, thus provides us with a useful tool for the study of the mechanisms underlying the primary and secondary hyperalgesia/allodynia after injury. This work was supported by DA02110 (T.Y.) and in part by Chiba University (N. N.-T). We thank Linda Sorkin for her comments and suggestions in these observations, and Charlie Conway for his editorial help in preparing the manuscript. [1] Andersen, O.K., Felsby, S., Nicolaisen, L., Bjerring, P., Jensen, T.S. and Arendt-Nielsen, L., The effect of ketamine on stimulation of primary and secondary hyperalgesic areas induced by capsaicin–a double-blind, placebo-controlled, human experimental study, Pain, 66 (1996) 51–62. [2] Baumann, T.K., Simone, D.A., Shain, C.N. and LaMotte, R.H., Neurogenic hyperalgesia: the search for the primary cutaneous afferent fibers that contribute to capsaicin-induced pain and hyperalgesia, J. Neurophysiol., 66 (1991) 212–227. [3] Cervero, F., Gilbert, R., Hammond, R.G. and Tanner, J., Development of secondary hyperalgesia following non-painful thermal stimulation of the skin: a psychophysical study in man, Pain, 54 (1993) 181–189. [4] Chaplan, S.R., Bach, F.W., Pogrel, J.W., Chung, J.M. and Yaksh, T.L., Quantitative assessment of tactile allodynia in the rat paw, J Neurosci. Methods, 53 (1994) 55–63. [5] Dahl, J.B., Brennum, J., Arendt-Nielsen, L., Jensen, T.S. and Kehlet, H., The effect of pre- versus postinjury infiltration with
[6]
[7] [8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
lidocaine on thermal and mechanical hyperalgesia after heat injury to the skin, Pain, 53 (1993) 43–51. Dirig, D.M., Salami, A., Rathbun, M.L., Ozaki, G.T. and Yaksh, T.L., Characterization of variables defining hindpaw withdrawal latency evoked by radiant thermal stimuli, J. Neurosci. Methods, 76 (1997) 183–191. Dixon, W.J., Efficient analysis of experimental observations, Annu. Rev. Pharmacol. Toxicol., 20 (1980) 441–462. Hargreaves, K., Dubner, R., Brown, F., Flores, C. and Joris, J., A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia, Pain, 32 (1988) 77–88. Jun, J.H. and Yaksh, T.L., The effect of intrathecal gabapentin and 3-isobutyl gamma-aminobutyric acid on the hyperalgesia observed after thermal injury in the rat, Anesth. Analg., 86 (1998) 348–354. Kilo, S., Schmelz, M., Koltzenburg, M. and Handwerker, H.O., Different patterns of hyperalgesia induced by experimental inflammation in human skin, Brain, 117 (1994) 385–396. Kinnman, E. and Levine, J.D., Involvement of the sympathetic postganglionic neuron in capsaicin-induced secondary hyperalgesia in the rat, Neuroscience, 65 (1995) 283–291. Moiniche, S., Dahl, J.B. and Kehlet, H., Time course of primary and secondary hyperalgesia after heat injury to the skin, Br. J. Anaesth., 71 (1993) 201–205. Raja, S.N., Campbell, J.N. and Meyer, R.A., Evidence for different mechanisms of primary and secondary hyperalgesia following heat injury to the glabrous skin, Brain, 107 (1984) 1179– 1188. Simone, D.A., Sorkin, L.S., Oh, U., Chung, J.M., Owens, C., LaMotte, R.H. and Willis, W.D., Neurogenic hyperalgesia: central neural correlates in responses of spinothalamic tract neurons, J. Neurophysiol., 66 (1991) 228–246. Willis, W., Hyperalgesia and allodynia. In W. Willis (Ed.), Hyperalgesia and Allodynia: Summary and Overview, Raven Press, New York, 1992, pp. 1–11.
A novel model of primary and secondary hyperalgesia after mild thermal injury in the rat Natsuko Nozaki-Taguchi, Tony L. Yaksh* Department of Anesthesiology, 9500 Gilman Drive, University of California, San Diego, La Jolla, CA 92093-0818 USA Received 14 May 1998; received in revised form 28 July 1998; accepted 4 August 1998
Abstract Secondary hyperesthesia was investigated in a rat thermal injury model. After a mild focal thermal injury (52°C / 45 s) to the rat heel, the response latency for a thermal stimulus directed at the injured site was reduced (10 → 6 s; e.g. primary thermal hyperalgesia) but no change was seen at the distal site. Conversely, tactile threshold at the distal site was significantly reduced (15 → 5 g; e.g. secondary tactile allodynia) but much less so at the injured site. Magnitude of the secondary tactile allodynia paralleled the severity of the primary injury. Accordingly, this model has the same characteristics seen in human post-tissueinjury hyperesthetic states and provides a tool for the study of mechanisms underlying primary and secondary hyperesthesia. 1998 Elsevier Science Ireland Ltd. All rights reserved
Keywords: Primary hyperalgesia; Secondary hyperalgesia; Thermal hyperalgesia; Tactile allodynia
Injury to the skin increases the reported pain sensation otherwise evoked by a noxious thermal stimulus (thermal hyperalgesia) or an innocuous mechanical stimulus (tactile allodynia) [15]. These phenomenon are seen within the injured area (primary hyperalgesia) and are also known to extend into the uninjured skin surrounding the injury (secondary hyperalgesia/allodynia). A cutaneous burn in human models has been shown to produce hyperalgesia to heat and tactile allodynia at the site of injury and tactile allodynia in the undamaged tissue surrounding the injury [13]. Recently, we developed a rat model to study the mechanisms of primary hyperalgesia [9]. A mild (non-blistering) thermal injury to the hind paw produces significant thermal hyperalgesia in the injured area. To extend this model for the study of primary and secondary hyperalgesia, the distribution of primary/secondary hyperalgesia to a thermal and mechanical stimulus was defined. Male Holtzman–Sprague–Dawley rats (275-350 g; Harlan Industries, Indianapolis, IN, USA) were used for the experiment. All experiments were carried out according to
* Corresponding author. Tel.: +1 619 5433597; fax: +1 619 5436070; e-mail: [email protected]
a protocol approved by the Institutional Animal Care Committee of the University of California, San Diego. For the measurement of mechanical threshold, rats were placed in a plastic cage with a wire mesh bottom. Mechanical thresholds were measured using von Frey filaments with logarithmically incremental stiffness (0.41, 0.70, 1.20, 2.00, 3.63, 5.50, 8.50 and 15.10 g) (Stoelting, Wood Dale, IL, USA) to calculate the 50% probability thresholds for mechanical paw withdrawal, as previously described [4]. In brief, beginning with the 2.0 g probe, filaments were applied to the plantar surface of a hind paw for 6–8 s, in a stepwise ascending or descending order following negative or positive withdrawal responses, respectively, until six consecutive responses were noted. Withdrawal thresholds were then calculated according to the method of Dixon [7]. Testing sites for mechanical threshold are described in Fig. 1. In the first experiment, three points within and around the injured area (A, B, C: primary area), and two points outside the injured area (D, E: secondary area) were chosen for the assessment of primary and secondary hyperalgesia, respectively. Four points were randomly tested in each rat: one primary and one secondary point in the injured paw, and the corresponding two points in the non-injured paw, except for two rats which were tested in two primary
0304-3940/98/$19.00 1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304- 3940(98) 00648- X
26
N. Nozaki-Taguchi, T.L. Yaksh / Neuroscience Letters 254 (1998) 25–28
points. Animals with baseline thresholds below 10 g were excluded from the study (three rats). After measurement of baseline withdrawal threshold, thermal injury was induced by placing the plantar surface of one hind paw on a 52 ± 1°C hot plate for 45 s, unless otherwise stated, under 2% halothane anesthesia. A 10 g sand pouch was applied on the top of the paw, covering the heel portion, to provide a constant pressure. This exposure produced significant thermal hyperalgesia in the injured area without tissue damage such as blistering [9]. Withdrawal thresholds for four points were measured every 30 min for 3 h post injury. For the subsequent study of secondary hyperalgesia/allodynia, all measurements were performed on point D. Thermal nociceptive responses were assessed in a separate experiment. Following 45 s thermal injury, thermal withdrawal responses were tested at both the primary and the secondary area of the injured paw, as well as the corresponding area of the non-injured paw. A commercially available device, similar to that described by Hargreaves et al. [8], was used to assess thermal nociceptive responses. Characteristics of the device have been previously described [6]. Briefly, rats were placed individually in a Plexiglas
cubicle on a heated (30°C) glass surface, and the light from a focused projection bulb, located below the glass, was directed at the plantar surface of one hind paw. The thermal stimulus was presented randomly to the primary and secondary test areas shown in Fig. 3. Thermal withdrawal responses were measured every 30 min for 3 h post injury. The time interval between the application of the light and the hind paw withdrawal response was defined as the paw withdrawal latency (PWL: s). A bulb current was chosen that evoked an average response latency in normal untreated rats of 10 ± 1 s. Data were analyzed for statistical significance using twoway repeated measures analysis of variance (ANOVA: two within factors; time and paw) followed by Bonferroni/ Dunn’s post-hoc test for multiple comparisons. Baseline mean mechanical withdrawal thresholds (mean ± SEM) were 15.0 ± 0.0, 15.0 ± 0.0, 14.7 ± 0.4, 14.4 ± 0.4 and 14.2 ± 0.4 g for testing sites A, B, C, D and E, respectively. Following thermal injury, a mild but significant de-crease in the mechanical threshold was seen in the primary area of the injured paw at points A and B, ((A): F(6,36) = 2.73, P , 0.03, (B): F(6,24) = 5.06, P ,
Fig. 1. Time course of paw withdrawal threshold (g) to von Frey filament of injured paw (X) and non-injured paw (W) at each corresponding testing site. The middle drawing shows the right injured paw with the injured area shown as a shaded area and the testing sites as indicated (A, B, C: primary area; D, E: secondary area). Injury was given at time 0 which is shown in a dashed line in the graphs. All points represent the mean ± SEM of five to seven animals. (b: baseline threshold) Statistically significant differences from baseline threshold and from non-injured paw are represented as **P , 0.01, *P , 0.05 and ‡P , 0.01, †P , 0.05, respectively.
N. Nozaki-Taguchi, T.L. Yaksh / Neuroscience Letters 254 (1998) 25–28
Fig. 2. Time course of the tactile paw withdrawal threshold (g) of the injured paw (X) and the non-injured paw (W) when von Frey filaments were applied at test site D (see Fig. 1). Injury with a duration of 15 s (upper panel) and 30 s (lower panel) was given at time 0, as shown by the dashed line. All points represent the mean ± SEM of six animals. (b: baseline threshold) Statistically significant differences from baseline threshold and from non-injured paw are represented as **P , 0.01, *P , 0.05 and ‡P , 0.01, respectively.
0.002), but no change was seen at point (C): F(6,24) = 0.47, P . 0.5 (Fig. 1). A more profound decrease in mechanical threshold (allodynia) was observed in the secondary area ((D): F(6,36) = 4.33, P , 0.003, (E): F (6,30) = 3.98, P , 0.005) (Fig. 1). Significant decreases from baseline threshold were seen at 30, 60 and 90 min after the injury at point D, and at all observation time points after injury at point E. Non-injured paw showed no significant change from baseline after the injury at any point. However, tendency for an increase in the sensitivity was noted at the secondary area of the non-injured paw. To assess the effect of magnitude of primary injury in the secondary tactile allodynia, rats were exposed to shorter injury intensities (30 and 15 s). When animals were exposed to 30 s thermal injury, significant decreases in the threshold were likewise observed in the secondary area (D) of the injured paw at 30–90 min post injury (F(6.30) = 7.13, P , 0.0001, Fig. 2-bottom). However, when the exposure time was reduced to 15 s, no significant allodynia was observed in the secondary area (F(6,30) = 1.34, P . 0.2, Fig. 2-top).
27
When animals were exposed to a 45 s thermal injury, thermal nociceptive test showed decrease in PWL (thermal hyperalgesia) in the primary area of the injured paw (F (6,30) = 6.04, P , 0.0004, Fig. 3-left). A significant decrease from baseline PWL was seen at 30 min post injury. However, no change in the thermal nociceptive response was seen in the secondary area of the injured paw (F (6,30) = 1.09, P . 0.3, Fig. 3-right) nor in any part of the non-injured paw. The current study clearly demonstrates that a mild focal thermal injury of the rat paw produces significant thermal hyperalgesia and mild tactile allodynia in the injured area (primary hyperalgesia) and a profound tactile allodynia but no thermal hyperalgesia in the uninjured area of the injured paw (secondary tactile allodynia). These data demonstrate the classical features of the primary and secondary hyperalgesia seen after tissue injury in human models [5,13]. Although, technically, we could not show the precise spatial distribution of secondary hyperalgesia, the result from five points indicated that the tactile allodynia was more significant at sites away from the thermal injury. As the induction of thermal injury requires the whole foot to be placed on the hot plate, there was a possibility for tissue injury sufficient to produce peripheral nociceptor sensitization in the secondary area. However, this likelihood was discounted by the absence of thermal hyperalgesia in the secondary area. The magnitude of the secondary tactile allodynia increased with the severity of thermal injury. Thus, the initiation of post-injury secondary tactile allodynia is closely related with the peripheral nociceptive input at the time of injury. However, in our experiment, the duration of the decrease from baseline observed for the secondary tactile allodynia outlasted the duration of the primary hyperalgesia, as seen with thermal nociceptive testing. In addition, a 30 s/ 52°C exposure, which yielded no primary thermal hyperalgesia (Jun and Yaksh, unpublished data), was significant to evoke secondary tactile allodynia. These results suggest that the acute afferent input and not peripheral sensitization per se induced changes that persist beyond the time of input from the thermal injury. This may not agree with the results of human models reporting that the secondary hyperesthesia outside the injury does not outlast the primary hyperalgesia [12]. However, human psychophysical study have also shown that non-painful thermal stimulation can evoke secondary tactile allodynia [3]. The phenomenon of primary and secondary hyperesthesia has been studied extensively in human models [1,2,5,10]. Parallel studies with human and animal behavior and electrophysiological studies have been reported [2,14] in which tactile allodynia can be evoked at a receptive field distal to the injury. The mechanism of the secondary allodynia remains unclear. Kinnman and Levin [11] showed that capsaicin injection in the rat paw evokes a secondary tactile allodynia, emphasizing the role of small afferent inputs. In our own work, topical application of a mu-opiate agonist
28
N. Nozaki-Taguchi, T.L. Yaksh / Neuroscience Letters 254 (1998) 25–28
Fig. 3. Time course of paw withdrawal latency (PWL: s) of injured paw (X) and non-injured paw (W) after thermal injury at time 0 (dashed line). The middle drawing shows the right injured paw with the shaded area showing the injury and circle showing the testing area. Left and right panels show the results of thermal testing at the primary and secondary site, respectively. All points represent the mean ± SEM of six animals. (b: baseline PWL) Statistically significant difference from baseline PWL and from non-injured paw are represented as **P , 0.01 and ‡P , 0.01, respectively.
prior, but not after, thermal injury prevents development of the off-site tactile allodynia, suggesting that opiate-sensitive terminals are responsible for the initiation of the allodynia, but is not necessary to sustain it (Nozaki-Taguchi and Yaksh, unpublished observations). This model of mild thermal injury in rats, thus provides us with a useful tool for the study of the mechanisms underlying the primary and secondary hyperalgesia/allodynia after injury. This work was supported by DA02110 (T.Y.) and in part by Chiba University (N. N.-T). We thank Linda Sorkin for her comments and suggestions in these observations, and Charlie Conway for his editorial help in preparing the manuscript. [1] Andersen, O.K., Felsby, S., Nicolaisen, L., Bjerring, P., Jensen, T.S. and Arendt-Nielsen, L., The effect of ketamine on stimulation of primary and secondary hyperalgesic areas induced by capsaicin–a double-blind, placebo-controlled, human experimental study, Pain, 66 (1996) 51–62. [2] Baumann, T.K., Simone, D.A., Shain, C.N. and LaMotte, R.H., Neurogenic hyperalgesia: the search for the primary cutaneous afferent fibers that contribute to capsaicin-induced pain and hyperalgesia, J. Neurophysiol., 66 (1991) 212–227. [3] Cervero, F., Gilbert, R., Hammond, R.G. and Tanner, J., Development of secondary hyperalgesia following non-painful thermal stimulation of the skin: a psychophysical study in man, Pain, 54 (1993) 181–189. [4] Chaplan, S.R., Bach, F.W., Pogrel, J.W., Chung, J.M. and Yaksh, T.L., Quantitative assessment of tactile allodynia in the rat paw, J Neurosci. Methods, 53 (1994) 55–63. [5] Dahl, J.B., Brennum, J., Arendt-Nielsen, L., Jensen, T.S. and Kehlet, H., The effect of pre- versus postinjury infiltration with
[6]
[7] [8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
lidocaine on thermal and mechanical hyperalgesia after heat injury to the skin, Pain, 53 (1993) 43–51. Dirig, D.M., Salami, A., Rathbun, M.L., Ozaki, G.T. and Yaksh, T.L., Characterization of variables defining hindpaw withdrawal latency evoked by radiant thermal stimuli, J. Neurosci. Methods, 76 (1997) 183–191. Dixon, W.J., Efficient analysis of experimental observations, Annu. Rev. Pharmacol. Toxicol., 20 (1980) 441–462. Hargreaves, K., Dubner, R., Brown, F., Flores, C. and Joris, J., A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia, Pain, 32 (1988) 77–88. Jun, J.H. and Yaksh, T.L., The effect of intrathecal gabapentin and 3-isobutyl gamma-aminobutyric acid on the hyperalgesia observed after thermal injury in the rat, Anesth. Analg., 86 (1998) 348–354. Kilo, S., Schmelz, M., Koltzenburg, M. and Handwerker, H.O., Different patterns of hyperalgesia induced by experimental inflammation in human skin, Brain, 117 (1994) 385–396. Kinnman, E. and Levine, J.D., Involvement of the sympathetic postganglionic neuron in capsaicin-induced secondary hyperalgesia in the rat, Neuroscience, 65 (1995) 283–291. Moiniche, S., Dahl, J.B. and Kehlet, H., Time course of primary and secondary hyperalgesia after heat injury to the skin, Br. J. Anaesth., 71 (1993) 201–205. Raja, S.N., Campbell, J.N. and Meyer, R.A., Evidence for different mechanisms of primary and secondary hyperalgesia following heat injury to the glabrous skin, Brain, 107 (1984) 1179– 1188. Simone, D.A., Sorkin, L.S., Oh, U., Chung, J.M., Owens, C., LaMotte, R.H. and Willis, W.D., Neurogenic hyperalgesia: central neural correlates in responses of spinothalamic tract neurons, J. Neurophysiol., 66 (1991) 228–246. Willis, W., Hyperalgesia and allodynia. In W. Willis (Ed.), Hyperalgesia and Allodynia: Summary and Overview, Raven Press, New York, 1992, pp. 1–11.