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Capsaicin evokes hypothermia independent of cannabinoid CB1 and CB2 receptors
Brain Research 1065 (2005) 147 – 151 www.elsevier.com/locate/brainres
Short Communication
Capsaicin evokes hypothermia independent of cannabinoid CB1 and CB2 receptors Zhe Ding, Alan Cowan, Scott M. Rawls* Department of Pharmaceutical Sciences and Pharmacology, Temple University School of Pharmacy and Medicine, 3307 North Broad Street, Philadelphia, PA 19140, USA Accepted 15 October 2005 Available online 23 November 2005
Abstract The present study investigated a potential role for cannabinoid CB1 and CB2 receptors in capsaicin-evoked hypothermia. Capsaicin (1 mg/ kg, s.c.) caused rapid and significant hypothermia in rats. Pretreatment with SR 141716A (1, 2.5 and 5 mg/kg, i.p.), a CB1 antagonist, or SR 144528 (1, 2.5 and 5 mg/kg, i.p.), a CB2 antagonist, did not affect capsaicin-induced hypothermia. In separate experiments, the hypothermia caused by WIN 55212-2 (5 mg/kg, i.m.), a cannabinoid agonist, was not significantly altered by capsazepine (10 and 30 mg/kg, i.p.) or SB 366791 (2 mg/kg, i.p.), a novel TRPV1 antagonist. These data suggest that capsaicin causes hypothermia by a CB1- and CB2-independent mechanism, and that WIN 55212-2 causes hypothermia by a TRPV1-independent mechanism. D 2005 Elsevier B.V. All rights reserved. Theme: Neurotransmitters Topic: Interactions between neurotransmitters Keywords: Capsaicin; Cannabinoid; WIN 55212-2; Hypothermia; SR 141716A; Vanilloid; SB 366791
Cannabinoid CB1 and CB2 receptors are thought to be involved in the pharmacological effects of capsaicin. For example, CB1 receptor activation by (R)-(+)-[2,3-dihydro-5methyl-3-[(4-merpholino)methyl]pyrrolo-[1,2,3-de]-1,4benzoxazin-6-yl](1-naphthyl)methanone (WIN 55212-2), a cannabinoid agonist, blocks capsaicin-evoked hyperalgesia in rats [12,13]. Cannabinoid agonists and antagonists also modulate food intake by acting on CB1 receptors located on capsaicin-sensitive sensory terminals [8]. More recent data indicate that CB2 receptor activation by WIN 55212-2 blocks capsaicin-induced guinea pig bronchial smooth muscle contraction [28]. Those data offer the possibility that cannabinoid receptors modulate other capsaicin-induced effects, such as hypothermia. Hypothermia is one of capsaicin’s most valuable effects in people residing in warm climate, but the hypothermic
* Corresponding author. Fax: +1 215 707 3678. E-mail address: [email protected] (S.M. Rawls). 0006-8993/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2005.10.026
mechanism of capsaicin is poorly understood [5,6,16,27]. It is thought that capsaicin causes hypothermia by activating transient receptor potential vanilloid 1 cation channels (TRPV1/VR1) [2,6,27]. This view is supported by the observation that capsazepine attenuates capsaicin-evoked hypothermia in rats [6]. On the other hand, the administration of capsazepine does not significantly alter the hypothermic effect of capsaicin in mice [5]. Recent data indicate that a significant proportion of capsaicin-evoked hypothermia in rats is blocked by SB 366791 [27]. SB 366791 is a new antagonist developed by GlaxoSmithKline that blocks TRPV1 receptors in humans and rats [9]. Prior work shows that SB 366791 prevents heat-, acid-, and capsaicin-induced activation of TRPV1 receptors [19]. In addition to TRPV1 receptors, a number of endogenous transmitters and peptides, such as monoamines, substance P, and glutamate, also participate in the hypothermic effects of capsaicin [10,16]. As part of a broader study into cannabinoid pharmacology, we investigated the effects of [N-(piperidin-1-yl)-5-(4-
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Fig. 1. Effect of a CB1 antagonist, SR 141716A (SR 141), on the hypothermia caused by capsaicin (CAP). (A) Temporal profile: a fixed dose of CAP (1 mg/kg, s.c.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to CAP injection, vehicle (VEH) or SR 141 (1, 2.5 or 5 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. One-way ANOVA on AUC0 – 210 did not reveal a significant drug effect between SR 141 and CAP [ F(3, 22) = 0.4714, P = 0.7056].
chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride] (SR 141716A), a CB1 antagonist, and [N-((1S)-endo-1,3,3-trimethyl bicyclo heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide)] (SR 144528), a CB2 antagonist, on capsaicin-evoked hypothermia in rats [23,24]. We also investigated the effect of capsazepine and SB 366791 on the hypothermia caused by WIN 55212-2, a cannabinoid agonist [1,3,15,20,21]. Male Sprague – Dawley rats (150 –175 g) were housed one per cage for 5 days before experimental use under controlled conditions and a 12-h light/dark cycle. Each rat was used once and treated in accordance with the National Institutes of Health and Declaration of Helsinki guidelines for the care and use of laboratory animals. Between 9 and 10 AM on the morning of the experiment, rats were placed in an environmental room that was maintained at a constant temperature of 21 T 0.3 -C and relative humidity of 52 T 2%. Following a 60-min acclimation interval, baseline temperature measurements were taken according to standard procedures in our laboratory [20,21]. A thermistor probe was lubricated and inserted 7 cm into the rectum. A digital thermometer was used to record body temperature. Rats were unrestrained during the temperature
readings, with only the tail being held between two fingers. Body temperature was recorded every 30 min during a 90-min baseline interval. Rats were injected with capsaicin (1 mg/kg, s.c.) by itself or with SR 141716A (1, 2.5 and 5 mg/kg, i.p.) or SR 144528 (1, 2.5 and 5 mg/kg, i.p.). Body temperature was recorded 30, 60, 90,150, and 210 min following capsaicin administration. In separate experiments, WIN 55212-2 (5 mg/kg, i.m.) was injected by itself or with SR 141716A (5 mg/kg, i.p.) or capsazepine (10 and 30 mg/kg, i.p.). In a final set of experiments, WIN 55212-2 (5 mg/kg, i.m.) was injected by itself or with SB 366791 (2 mg/kg, i.p.). Body temperature was recorded 30, 60, 90,150, and 210 min following WIN 55212-2 administration. Drugs were dissolved in a 10% cremophor – saline solution except for capsazepine and SB 366791, which were dissolved in a 50% DMSO – saline solution. For data analysis, three consecutive body temperature readings were averaged to establish a baseline temperature before drug administration. The change from baseline was determined and plotted as the mean change in body temperature T SEM versus time. Area under the body temperature time curve (AUC0 – 210) from 0 to 210 min postadministration was determined. Statistical analysis of differ-
Fig. 2. Effect of a cannabinoid CB2 antagonist, SR 144528 (SR 144), on the hypothermia caused by capsaicin (CAP). (A) Temporal profile: a fixed dose of CAP (1 mg/kg, s.c.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to CAP injection, vehicle (VEH) or SR 144 (1, 2.5 or 5 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. Oneway ANOVA on AUC0 – 210 did not reveal a significant drug effect between SR 144 and CAP [ F(3,20) = 2.350, P = 0.1031].
Z. Ding et al. / Brain Research 1065 (2005) 147 – 151
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Fig. 3. Effect of a TRPV1 receptor antagonist (capsazepine, CPZ) or a cannabinoid CB1 antagonist (SR 141) on the hypothermia caused by WIN 55212-2 (WIN). (A) Temporal profile: a fixed dose of WIN (5 mg/kg, i.m.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to WIN, vehicle (VEH), CPZ (10 or 30 mg/kg, i.p.), or SR 141 (5 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. One-way ANOVA on AUC0 – 210 revealed a significant drug effect [ F(3,23) = 13.95, P < 0.0001]. *P < 0.05 versus WIN-treated group.
ences between groups was determined by a one-way analysis of variance (ANOVA) on the AUC0 – 210 means followed by a Dunnett’s or Tukey’s post hoc test. A probability level of P < 0.05 was considered to be statistically significant. Fig. 1 shows the effect of SR 141716A (1, 2.5 and 5 mg/ kg, i.p.) on the hypothermia evoked by a fixed dose of capsaicin (1 mg/kg, s.c.). Capsaicin produced a rapid hypothermia which peaked (2.4 T 0.3 -C) 30 min postinjection. SR 141716A did not affect capsaicin-induced hypothermia (Fig. 1B) ( P > 0.05). Fig. 2 shows the effect of SR 144528 (1, 2.5, and 5 mg/kg, i.p.) on the hypothermia evoked by capsaicin (1 mg/kg, s.c.). SR 144528 did not significantly affect the hypothermic response to 1 mg/kg of capsaicin (Fig. 2B) ( P > 0.05). The hypothermic response to a lower dose of capsaicin (0.25 mg/kg, s.c.) was not affected by SR 141716A (5 mg/kg, i.p.) or SR 144528 (5 mg/kg, i.p.) (data not shown). Fig. 3 shows the effect of SR 141716A (5 mg/kg, i.p.) or capsazepine (10 and 30 mg/kg, i.p.) on the hypothermic effect of WIN 55212-2 (5 mg/kg, i.m.). WIN
55212-2 caused a hypothermia that was rapid in onset and persistent in duration (Fig. 3A). The maximal hypothermic response (2.5 T 0.3 -C) to WIN 55212-2 occurred 60 min post-injection (Fig. 3A). The injection of capsazepine (30 mg/kg, i.p.) or SR 141716A (5 mg/kg, i.p.) by itself did not affect body temperature ( P > 0.05) (data not shown). For combined administration, SR 141716A (5 mg/kg, i.p.) blocked WIN 55212-2-induced hypothermia ( P < 0.01) (Fig. 3B). In contrast, capsazepine (10 and 30 mg/kg, i.p.) did not significantly alter the hypothermia caused by WIN 55212-2 (5 mg/kg, i.m.). To confirm our results with capsazepine, we investigated the effect of SB 36679 on WIN 55212-2-induced hypothermia (Fig. 4). SB 366791 (2 mg/kg, i.p.) did not affect body temperature compared to vehicle-treated rats ( P > 0.05) (Fig. 4B). For combined administration, SB 366791 (2 mg/kg, i.p.) did not significantly alter the hypothermic response to 5 mg/kg of WIN 55212-2 ( P > 0.05) (Fig. 4B). The present study demonstrates that cannabinoid CB1 and CB2 antagonists do not affect the hypothermia evoked
Fig. 4. Effect of a novel TRPV1 receptor antagonist, SB 366791 (SB 366) on the hypothermia caused by WIN 55212-2 (WIN). (A) Temporal profile: vehicle (VEH) or a fixed dose of WIN (5 mg/kg, i.m.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to WIN, VEH or SB 366 (2 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. Oneway ANOVA on AUC0 – 210 revealed a significant drug effect [ F(3,26) = 14.07, P < 0.0001]. +P < 0.05 versus vehicle plus vehicle (VEH + VEH) group.
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by capsaicin in rats [6,16,27]. These findings suggest that CB1 or CB2 receptor activation is not required for capsaicin to cause hypothermia in rats. In contrast, CB1 receptor blockade by SR 141716A abolished WIN 55212-2-evoked hypothermia, confirming that WIN 55212-2 decreases body temperature by activating CB1 receptors [7]. Capsazepine or SB 366791 by itself did not affect body temperature. These results reveal that an endogenous TRPV1 tone is not a major factor in the regulation of body temperature. WIN 55212-2evoked hypothermia was not significantly affected by capsazepine or SB 366791, suggesting that TRPV1 receptor activation is not a major factor in the hypothermic effects of cannabinoids. The lack of involvement of TRPV1 receptors in WIN 55212-2-evoked hypothermia is in direct contrast to serotonin, dopamine, glutamate, GABA, and nitric oxide systems, all of which participate in cannabinoid mediation of hypothermia [14,17,20,21,26]. It should be mentioned that cannabinoid and vanilloid systems do interact to affect the perception of pain [4,11 – 13,18,22,25]. Capsaicin causes hyperalgesia by activating specific subclasses of nociceptive C fibers, and the activation of peripheral cannabinoid receptors is thought to reduce the perception of pain caused by capsaicin administration [11,13,22,25]. In addition, capsazepine administration prevents the antihyperalgesic effects of cannabidiol, a nonpsychoactive constituent of marijuana [4]. Those data suggest that TRPV1 may be a molecular target for the antihyperalgesic actions of cannabidiol. Collectively, these studies leave little doubt that cannabinoid – vanilloid interactions are critical in mediating the perception of pain. In summary, we used the endpoint of hypothermia to investigate cannabinoid – vanilloid interactions in conscious rats. We observed that capsaicin causes hypothermia by a CB1- and CB2-independent mechanism, and that WIN 55212-2 produces hypothermia by a TRPV1independent mechanism. Hence, in spite of the fact that cannabinoid and vanilloid systems interact to regulate hyperalgesia, feeding, and smooth muscle contraction, the most harmonious conclusion from our results appears to be that CB1 and TRPV1 receptors mediate hypothermia by separate processes.
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Acknowledgments [16]
This work was supported by NIDA grant DA 13429. The authors thank Dr. Martin W. Adler for invaluable advice on body temperature experiments and Dr. Ronald J. Tallarida for advice on data analysis.
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Short Communication
Capsaicin evokes hypothermia independent of cannabinoid CB1 and CB2 receptors Zhe Ding, Alan Cowan, Scott M. Rawls* Department of Pharmaceutical Sciences and Pharmacology, Temple University School of Pharmacy and Medicine, 3307 North Broad Street, Philadelphia, PA 19140, USA Accepted 15 October 2005 Available online 23 November 2005
Abstract The present study investigated a potential role for cannabinoid CB1 and CB2 receptors in capsaicin-evoked hypothermia. Capsaicin (1 mg/ kg, s.c.) caused rapid and significant hypothermia in rats. Pretreatment with SR 141716A (1, 2.5 and 5 mg/kg, i.p.), a CB1 antagonist, or SR 144528 (1, 2.5 and 5 mg/kg, i.p.), a CB2 antagonist, did not affect capsaicin-induced hypothermia. In separate experiments, the hypothermia caused by WIN 55212-2 (5 mg/kg, i.m.), a cannabinoid agonist, was not significantly altered by capsazepine (10 and 30 mg/kg, i.p.) or SB 366791 (2 mg/kg, i.p.), a novel TRPV1 antagonist. These data suggest that capsaicin causes hypothermia by a CB1- and CB2-independent mechanism, and that WIN 55212-2 causes hypothermia by a TRPV1-independent mechanism. D 2005 Elsevier B.V. All rights reserved. Theme: Neurotransmitters Topic: Interactions between neurotransmitters Keywords: Capsaicin; Cannabinoid; WIN 55212-2; Hypothermia; SR 141716A; Vanilloid; SB 366791
Cannabinoid CB1 and CB2 receptors are thought to be involved in the pharmacological effects of capsaicin. For example, CB1 receptor activation by (R)-(+)-[2,3-dihydro-5methyl-3-[(4-merpholino)methyl]pyrrolo-[1,2,3-de]-1,4benzoxazin-6-yl](1-naphthyl)methanone (WIN 55212-2), a cannabinoid agonist, blocks capsaicin-evoked hyperalgesia in rats [12,13]. Cannabinoid agonists and antagonists also modulate food intake by acting on CB1 receptors located on capsaicin-sensitive sensory terminals [8]. More recent data indicate that CB2 receptor activation by WIN 55212-2 blocks capsaicin-induced guinea pig bronchial smooth muscle contraction [28]. Those data offer the possibility that cannabinoid receptors modulate other capsaicin-induced effects, such as hypothermia. Hypothermia is one of capsaicin’s most valuable effects in people residing in warm climate, but the hypothermic
* Corresponding author. Fax: +1 215 707 3678. E-mail address: [email protected] (S.M. Rawls). 0006-8993/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2005.10.026
mechanism of capsaicin is poorly understood [5,6,16,27]. It is thought that capsaicin causes hypothermia by activating transient receptor potential vanilloid 1 cation channels (TRPV1/VR1) [2,6,27]. This view is supported by the observation that capsazepine attenuates capsaicin-evoked hypothermia in rats [6]. On the other hand, the administration of capsazepine does not significantly alter the hypothermic effect of capsaicin in mice [5]. Recent data indicate that a significant proportion of capsaicin-evoked hypothermia in rats is blocked by SB 366791 [27]. SB 366791 is a new antagonist developed by GlaxoSmithKline that blocks TRPV1 receptors in humans and rats [9]. Prior work shows that SB 366791 prevents heat-, acid-, and capsaicin-induced activation of TRPV1 receptors [19]. In addition to TRPV1 receptors, a number of endogenous transmitters and peptides, such as monoamines, substance P, and glutamate, also participate in the hypothermic effects of capsaicin [10,16]. As part of a broader study into cannabinoid pharmacology, we investigated the effects of [N-(piperidin-1-yl)-5-(4-
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Fig. 1. Effect of a CB1 antagonist, SR 141716A (SR 141), on the hypothermia caused by capsaicin (CAP). (A) Temporal profile: a fixed dose of CAP (1 mg/kg, s.c.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to CAP injection, vehicle (VEH) or SR 141 (1, 2.5 or 5 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. One-way ANOVA on AUC0 – 210 did not reveal a significant drug effect between SR 141 and CAP [ F(3, 22) = 0.4714, P = 0.7056].
chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride] (SR 141716A), a CB1 antagonist, and [N-((1S)-endo-1,3,3-trimethyl bicyclo heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide)] (SR 144528), a CB2 antagonist, on capsaicin-evoked hypothermia in rats [23,24]. We also investigated the effect of capsazepine and SB 366791 on the hypothermia caused by WIN 55212-2, a cannabinoid agonist [1,3,15,20,21]. Male Sprague – Dawley rats (150 –175 g) were housed one per cage for 5 days before experimental use under controlled conditions and a 12-h light/dark cycle. Each rat was used once and treated in accordance with the National Institutes of Health and Declaration of Helsinki guidelines for the care and use of laboratory animals. Between 9 and 10 AM on the morning of the experiment, rats were placed in an environmental room that was maintained at a constant temperature of 21 T 0.3 -C and relative humidity of 52 T 2%. Following a 60-min acclimation interval, baseline temperature measurements were taken according to standard procedures in our laboratory [20,21]. A thermistor probe was lubricated and inserted 7 cm into the rectum. A digital thermometer was used to record body temperature. Rats were unrestrained during the temperature
readings, with only the tail being held between two fingers. Body temperature was recorded every 30 min during a 90-min baseline interval. Rats were injected with capsaicin (1 mg/kg, s.c.) by itself or with SR 141716A (1, 2.5 and 5 mg/kg, i.p.) or SR 144528 (1, 2.5 and 5 mg/kg, i.p.). Body temperature was recorded 30, 60, 90,150, and 210 min following capsaicin administration. In separate experiments, WIN 55212-2 (5 mg/kg, i.m.) was injected by itself or with SR 141716A (5 mg/kg, i.p.) or capsazepine (10 and 30 mg/kg, i.p.). In a final set of experiments, WIN 55212-2 (5 mg/kg, i.m.) was injected by itself or with SB 366791 (2 mg/kg, i.p.). Body temperature was recorded 30, 60, 90,150, and 210 min following WIN 55212-2 administration. Drugs were dissolved in a 10% cremophor – saline solution except for capsazepine and SB 366791, which were dissolved in a 50% DMSO – saline solution. For data analysis, three consecutive body temperature readings were averaged to establish a baseline temperature before drug administration. The change from baseline was determined and plotted as the mean change in body temperature T SEM versus time. Area under the body temperature time curve (AUC0 – 210) from 0 to 210 min postadministration was determined. Statistical analysis of differ-
Fig. 2. Effect of a cannabinoid CB2 antagonist, SR 144528 (SR 144), on the hypothermia caused by capsaicin (CAP). (A) Temporal profile: a fixed dose of CAP (1 mg/kg, s.c.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to CAP injection, vehicle (VEH) or SR 144 (1, 2.5 or 5 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. Oneway ANOVA on AUC0 – 210 did not reveal a significant drug effect between SR 144 and CAP [ F(3,20) = 2.350, P = 0.1031].
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Fig. 3. Effect of a TRPV1 receptor antagonist (capsazepine, CPZ) or a cannabinoid CB1 antagonist (SR 141) on the hypothermia caused by WIN 55212-2 (WIN). (A) Temporal profile: a fixed dose of WIN (5 mg/kg, i.m.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to WIN, vehicle (VEH), CPZ (10 or 30 mg/kg, i.p.), or SR 141 (5 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. One-way ANOVA on AUC0 – 210 revealed a significant drug effect [ F(3,23) = 13.95, P < 0.0001]. *P < 0.05 versus WIN-treated group.
ences between groups was determined by a one-way analysis of variance (ANOVA) on the AUC0 – 210 means followed by a Dunnett’s or Tukey’s post hoc test. A probability level of P < 0.05 was considered to be statistically significant. Fig. 1 shows the effect of SR 141716A (1, 2.5 and 5 mg/ kg, i.p.) on the hypothermia evoked by a fixed dose of capsaicin (1 mg/kg, s.c.). Capsaicin produced a rapid hypothermia which peaked (2.4 T 0.3 -C) 30 min postinjection. SR 141716A did not affect capsaicin-induced hypothermia (Fig. 1B) ( P > 0.05). Fig. 2 shows the effect of SR 144528 (1, 2.5, and 5 mg/kg, i.p.) on the hypothermia evoked by capsaicin (1 mg/kg, s.c.). SR 144528 did not significantly affect the hypothermic response to 1 mg/kg of capsaicin (Fig. 2B) ( P > 0.05). The hypothermic response to a lower dose of capsaicin (0.25 mg/kg, s.c.) was not affected by SR 141716A (5 mg/kg, i.p.) or SR 144528 (5 mg/kg, i.p.) (data not shown). Fig. 3 shows the effect of SR 141716A (5 mg/kg, i.p.) or capsazepine (10 and 30 mg/kg, i.p.) on the hypothermic effect of WIN 55212-2 (5 mg/kg, i.m.). WIN
55212-2 caused a hypothermia that was rapid in onset and persistent in duration (Fig. 3A). The maximal hypothermic response (2.5 T 0.3 -C) to WIN 55212-2 occurred 60 min post-injection (Fig. 3A). The injection of capsazepine (30 mg/kg, i.p.) or SR 141716A (5 mg/kg, i.p.) by itself did not affect body temperature ( P > 0.05) (data not shown). For combined administration, SR 141716A (5 mg/kg, i.p.) blocked WIN 55212-2-induced hypothermia ( P < 0.01) (Fig. 3B). In contrast, capsazepine (10 and 30 mg/kg, i.p.) did not significantly alter the hypothermia caused by WIN 55212-2 (5 mg/kg, i.m.). To confirm our results with capsazepine, we investigated the effect of SB 36679 on WIN 55212-2-induced hypothermia (Fig. 4). SB 366791 (2 mg/kg, i.p.) did not affect body temperature compared to vehicle-treated rats ( P > 0.05) (Fig. 4B). For combined administration, SB 366791 (2 mg/kg, i.p.) did not significantly alter the hypothermic response to 5 mg/kg of WIN 55212-2 ( P > 0.05) (Fig. 4B). The present study demonstrates that cannabinoid CB1 and CB2 antagonists do not affect the hypothermia evoked
Fig. 4. Effect of a novel TRPV1 receptor antagonist, SB 366791 (SB 366) on the hypothermia caused by WIN 55212-2 (WIN). (A) Temporal profile: vehicle (VEH) or a fixed dose of WIN (5 mg/kg, i.m.) was injected at 0 min, as indicated by the arrow. Thirty minutes prior to WIN, VEH or SB 366 (2 mg/kg, i.p.) was injected. DT b is the change in body temperature from baseline (time 0). (B) AUC0 – 210: the effect is AUC0 – 210 and is determined from at least 6 rats. Oneway ANOVA on AUC0 – 210 revealed a significant drug effect [ F(3,26) = 14.07, P < 0.0001]. +P < 0.05 versus vehicle plus vehicle (VEH + VEH) group.
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by capsaicin in rats [6,16,27]. These findings suggest that CB1 or CB2 receptor activation is not required for capsaicin to cause hypothermia in rats. In contrast, CB1 receptor blockade by SR 141716A abolished WIN 55212-2-evoked hypothermia, confirming that WIN 55212-2 decreases body temperature by activating CB1 receptors [7]. Capsazepine or SB 366791 by itself did not affect body temperature. These results reveal that an endogenous TRPV1 tone is not a major factor in the regulation of body temperature. WIN 55212-2evoked hypothermia was not significantly affected by capsazepine or SB 366791, suggesting that TRPV1 receptor activation is not a major factor in the hypothermic effects of cannabinoids. The lack of involvement of TRPV1 receptors in WIN 55212-2-evoked hypothermia is in direct contrast to serotonin, dopamine, glutamate, GABA, and nitric oxide systems, all of which participate in cannabinoid mediation of hypothermia [14,17,20,21,26]. It should be mentioned that cannabinoid and vanilloid systems do interact to affect the perception of pain [4,11 – 13,18,22,25]. Capsaicin causes hyperalgesia by activating specific subclasses of nociceptive C fibers, and the activation of peripheral cannabinoid receptors is thought to reduce the perception of pain caused by capsaicin administration [11,13,22,25]. In addition, capsazepine administration prevents the antihyperalgesic effects of cannabidiol, a nonpsychoactive constituent of marijuana [4]. Those data suggest that TRPV1 may be a molecular target for the antihyperalgesic actions of cannabidiol. Collectively, these studies leave little doubt that cannabinoid – vanilloid interactions are critical in mediating the perception of pain. In summary, we used the endpoint of hypothermia to investigate cannabinoid – vanilloid interactions in conscious rats. We observed that capsaicin causes hypothermia by a CB1- and CB2-independent mechanism, and that WIN 55212-2 produces hypothermia by a TRPV1independent mechanism. Hence, in spite of the fact that cannabinoid and vanilloid systems interact to regulate hyperalgesia, feeding, and smooth muscle contraction, the most harmonious conclusion from our results appears to be that CB1 and TRPV1 receptors mediate hypothermia by separate processes.
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Acknowledgments [16]
This work was supported by NIDA grant DA 13429. The authors thank Dr. Martin W. Adler for invaluable advice on body temperature experiments and Dr. Ronald J. Tallarida for advice on data analysis.
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