Transient Receptor Potential Vanilloid 1 Antagonists Prevent Anesthesia-induced Hypothermia and Decrease Postincisional Opioid Dose Requirements in Rodents

AVOIDANCE of intraoperative hypothermia and techniques to minimize postoperative pain compose two of the important tasks an anesthesiologist is responsible for. Intraoperative and perioperative hypothermia result from various factors, including the effects of general anesthesia itself (vasodilatation and inhibition of shivering-induced thermogenesis) and surgical conditions (wide operative fields, cold irrigation solutions, etc.).¹  Intraoperative and perioperative hypothermia can be associated with increased surgical site infections, increased bleeding, poor wound healing, and increased cardiovascular stress. Thus, intraoperative and perioperative hypothermia may contribute to increased morbidity and mortality associated with surgeries.¹  At the present time, the maintenance of perioperative normothermia relies on physical means of warming a patient (e.g., forced-air warming blanket, heated intravenous solutions). However, these methods are inadequate in many surgeries.^2–5  Moreover, according to some,^6,7  but not all,⁸  the turbulent air flow from forced air may be associated with postsurgical infections.

Poor postsurgical pain control results in increased suffering, diminished function, hospital-related complications including infections, cardiovascular issues, and bleeding, all leading to longer in-hospital stays.^9,10  Moreover, a strong link exists between acute postsurgical pain intensity and the risk of development of chronic pain.¹¹  Management of postoperative pain relies primarily on opioids and nonsteroidal antiinflammatory drugs. Excessive opioid use in the perioperative phase is associated with increased neurologic and respiratory morbidities.¹²  Nonsteroidal antiinflammatory drugs cause increased bleeding and negatively affect bone healing and kidney function.¹³  Multiple anesthetic techniques and drugs have been evaluated as candidates for preemptive analgesics with the goal of achieving an opioid-sparing effect in the postoperative period.¹⁴  A drug that can be safely used in the perianesthesia period and potentially demonstrate an opioid-sparing effect can be of exceptional use for an anesthesiologist.

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel expressed predominantly in pain-sensing neurons.¹⁵  This channel is activated by endogenous lipids, low pH, and temperature above 42°C, among other ligands.¹⁶  Tonic activation of TRPV1 by low pH in the abdomen affects thermoeffectors and suppresses deep body temperature.^17–19  When this activation is blocked by TRPV1 antagonists, these drugs produce hyperthermia along with the desired effect of analgesia.^16–18  Additional advancement of TRPV1 antagonists in large clinical trials was halted due to the side effect of hyperthermia,²⁰  although some studies demonstrated analgesia.²¹ 

In this study, we tested the hypothesis that TRPV1 antagonists given on anesthesia induction prevent the development of anesthesia-induced hypothermia and decrease the opioid dose requirement for the reduction of postincision hyperalgesia. We tested our hypothesis in a rodent model of anesthesia-induced hypothermia and postsurgical hyperalgesia.

For experiments performed at the University of Arizona, the guidelines of University of Arizona (Tucson, Arizona) Institutional Animal Use and Care Committee (IACUC) were followed and the University of Arizona IACUC approved all of the animal protocols. The experiments performed at the University of Pecs (Pecs, Hungary) were conducted under protocols approved by the IACUC of the University of Pecs and conformed to the guidelines from Directive 2010/63/EU of the European Parliament on the protection of animals used for scientific purposes.

Adult, male Sprague–Dawley rats (Envigo, Indianapolis, Indiana) were used and group housed in a temperature- and humidity-controlled room on a 12-h light/dark cycle. The adult rats had access to standard chow and water ad libitum, and the neonates were housed with the dam. Adult rats weighed approximately 250 to 300 g (University of Arizona experiments) or 519 ± 8 g (University of Pecs). Mice of both sexes had the Trpv1 gene either present (Trpv1^+/+) or missing (Trpv1^–/–), weighed approximately 19 g, and they were bred as described in details elsewhere.²²  All of the experiments were conducted in age- and body weight–matched animal groups.

The experimenter was blinded to treatment allocations while giving the injection of vehicle or AMG 517 and also while performing temperature measurements and behavioral testing. The experimenter did not know whether the animals received vehicle or AMG 517 during the testing, and blinding was revealed at the end of the experiments. No randomization methods were used to assign animals to condition.

The experiments were performed at ambient room temperature of 22°C. Before anesthetic induction, baseline core body temperature was recorded using a wireless temperature probe (microthermia 2, ThermoWorks, Alpine, Utah) inserted at least one inch beyond the anal sphincter in gently restrained rats. Rats were lightly restrained and exposed to 5% isoflurane (Drager vaporizer, Drager, Telford, Pennsylvania) in 2 L oxygen until visible movement stopped and were maintained at 2.5% isoflurane in 2 L oxygen. Alternatively, lightly restrained rats were injected with intraperitoneal ketamine (100 mg/kg; Western Medical, Acadia, California). The tail vein was accessed and vehicle (10% dimethyl sulfoxide, 10% Tween 80, and 80% saline), AMG 517, or ABT-102 was administered as a bolus. Core body temperature was recorded every 5 min until the end of the experiment. ABT-102 (Axon Medchem, Reston, Virginia) and AMG 517 (Selleckchem, Boston, Massachusetts) were dissolved in 10% dimethyl sulfoxide, 10% Tween 80, and 80% saline (all from Sigma-Aldrich, St. Louis, Missouri) before the tail vein injection. All of the experiments were conducted in age- and body weight–matched animal groups.

These experiments were performed at the University of Pecs.

Similar to previously published studies,²³  each mouse was implanted with an intraperitoneal catheter, which allowed for nonstressful administration of the drugs during the experiment. Mice were anesthetized with an intraperitoneal injection of a ketamine–xylazine mixture (81.7 and 9.3 mg/kg, respectively) and received gentamycin (6 mg/kg subcutaneously) as an antibiotic prophylaxis. Experiments were performed 3 to 7 days after the surgery. During surgery, the mouse was placed on a heating pad (model TMP-5a; Supertech Instruments UK Ltd., London, United Kingdom). After a small midline incision, a polyethylene (PE)-50 catheter filled with pyrogen-free saline was inserted into the peritoneal cavity. The internal end of the catheter was fixed to the left side of the abdominal wall with a suture; the free end of the catheter was tunneled under the skin to the nape, where it was exteriorized and heat sealed. The surgical abdominal wound was sutured in layers. The catheter was flushed with saline on the day after the surgery and every other day thereafter. On the day of the experiment, the mouse was placed in a cylindrical confiner and equipped with copper–constantan thermocouples (Omega Engineering, Stamford, Connecticut) to measure colonic temperature (a form of core body temperature) and tail skin temperature. The colonic thermocouple was inserted 3 cm deep beyond the anal sphincter and was fixed to the base of the tail with a loop of adhesive tape. The skin thermocouple was positioned on the lateral surface of tail at the border between the proximal and middle third of the tail and secured in placed with tape. The thermocouples were plugged into a datalogger device (Cole-Palmer, Vernon Hills, Illinois) connected to a computer. Mice in their confiners were then placed into a biochemistry incubator (model BJPX-Newark; Biobase, Jinan, China) set to an ambient temperature of 25°C, which is subneutral for mice in this setup. The intraperitoneal catheter was connected to a PE-50 extension filled with the drug of interest. The extension was passed through a port of the incubator and connected to a syringe.

As described before,²⁴  a rat designated to an experiment at the University of Pecs was implanted with an intraperitoneal and an intravenous (iv) catheter in the same surgery. The rat was anesthetized by ketamine–xylazine (55.6 and 5.5, respectively, intraperitoneally) and received an antibiotic (gentamycin, 6 mg/kg, subcutaneously) prophylactically. Experiments were performed 3 to 9 days after the surgery. After a small midline incision, a PE-50 catheter was inserted in the abdominal cavity. Similar as in mice, the internal end was fixed to the abdominal wall and the free end of the catheter was tunneled under the skin and exteriorized and sealed at the nape. The surgical wound was sutured in layers. For iv catheter implantation, a small longitudinal incision was made on the ventral surface of the neck, left of the trachea. The left jugular vein was exposed, freed from its surrounding connective tissue, and ligated. A silicone catheter (ID = 0.5 mm, OD = 0.9 mm) filled with heparinized (10 U/ml) saline was passed into the superior vena cava through the jugular vein and secured in place with ligatures. The free end of the catheter was knotted, tunneled under the skin to the nape, and exteriorized. The wound was sutured. Both the intraperitoneal and iv catheters were flushed on the day after the surgery and every other day. On the day of an experiment, the rats equipped with copper–constantan thermocouples (Omega Engineering) were placed in a confiner. The colonic thermocouple was inserted 10 cm beyond the anal sphincter. The skin thermocouple was positioned and fixed to the tail similarly as in mice. The thermocouples were plugged into a data logger (Cole-Parmer). Then, each rat in its confiner was transferred to a Plexiglas chamber of the four-chamber, open-circuit calorimeter integrated system (Oxymax Equal Flow, Columbus Instruments, Columbus, Ohio). The chamber was sealed, submerged into a temperature-controlled water bath, and continuously ventilated with room air (1,000 ml/min). The fractional concentration of oxygen was measured in the air entering and exiting the chamber, and the rate of oxygen consumption (Vo₂) was calculated according to the manufacturer’s instructions using the Oxymax Windows software (version 3.1, Columbus Instruments). The measurement of Vs Vo₂ is a standard method used to assess the thermogenesis of rodents.^{18,19,24–26}  The intraperitoneal and iv catheters were connected to PE-50 extensions filled with the drug of interest. Each extension was passed through a port of the chamber and connected to a syringe, which was placed in a syringe pump (model 975; Harvard Apparatus Inc., Holliston, Massachusetts) in case of the iv extension. All of the experiments were conducted at an ambient temperature of 23°C, which is subneutral for rats in this setup.

Baseline thermal and mechanical withdrawal latencies were obtained using Hargreaves apparatus²⁷  and von Frey filaments, as described previously,²⁸  at least 24 h before the surgery. The animals were anesthetized using isoflurane as described above and treated with either vehicle or AMG 517 via the tail vein injection immediately on induction and just before performing surgery for incisional hyperalgesia, as described by Brennan et al.²⁹  The animals were maintained under isoflurane anesthesia (2.5%) until the surgery was performed (within 20 min) and allowed to recover from anesthesia.

Assessment of thermal and mechanical hyperalgesia was performed at 24 h postsurgery and was followed by the evaluation of the dose–response to morphine (National Institute of Drug Abuse, injected subcutaneously in the back) of rats that received preincision TRPV1 antagonist or vehicle treatment. The effect of morphine on postsurgical hyperalgesia was assessed 30, 60, and 120 min postmorphine injection.

Sample sizes were determined based on our previous experience. Data on core body temperature, tail skin temperature, Vo₂, and thermal and mechanical withdrawal latencies were compared by two-way ANOVA, repeated measures followed by Bonferroni post hoc tests, as appropriate. In ANOVA, we selected treatment (TRPV1 antagonist or vehicle as a between-subjects factor) and time (as a repeated-measure factor) as independent variables, although the dependent variables were core body temperature, tail skin temperature, Vo₂, and thermal and mechanical withdrawal latencies. The criteria for statistical significance was P < 0.05, and the nature of hypothesis testing was two tailed. For statistical analysis, Sigmaplot 11.0 (Systat Software, San Jose, California) software was used. All of the data are reported as mean ± SD.

We first evaluated whether TRPV1 antagonists could prevent both volatile and intravenous anesthesia-induced hypothermia. We chose isoflurane and ketamine as our prototypical agents for volatile and intravenous anesthetics for various reasons. First, both of these agents have been well characterized in both humans and rodents with regard to their dose response and effect on thermoregulation. Second, respirometry chamber setup requires delivery of an anesthetic that is long lasting without an infusion or a vaporizer to correctly measure metabolic changes. A single intraperitoneal ketamine dose allowed us to perform these studies. We used two chemically different TRPV1 antagonists (AMG 517 and ABT-102). Importantly, both of these antagonists have already been shown in rodents and humans to induce hyperthermia in nonanesthetized subjects.^26,30  After obtaining baseline core body temperature, rats were anesthetized either using ketamine (fig. 1A) or isoflurane (5.0% induction, 2.5% maintenance in 2 L O₂; fig. 1B) and were injected with either vehicle or graded doses of intravenous AMG 517 (0.01 mg to 1 mg/kg) or with ABT-102 at 10 mg/kg. The doses of AMG 517 were chosen based on previous published studies.²⁶  As expected, both ketamine and isoflurane resulted in a significant decrease in the rats’ core body temperature, with an average decrease of approximately 1.5°C (fig. 1, A through C), as is observed in humans. AMG 517 dose dependently prevented either ketamine- or isoflurane-induced hypothermia (fig. 1, A and B and Supplemental Digital Content 1, https://links.lww.com/ALN/B526, for scatterplot). Importantly, AMG 517 did not cause hyperthermia while the animals were maintained under anesthesia (fig. 1, A and B). The effect was statistically significant (P < 0.001, two way ANOVA, Bonferroni post hoc test) at all of the doses and time points in the ketamine experiments and at doses 0.1 and 1.0 mg/kg in the isoflurane experiments. To rule out the possibility that this effect was restricted to the particular pharmacophore of AMG 517, we used a completely unrelated drug, ABT-102, at a dose shown previously to cause hyperthermia³¹  to prevent anesthesia-induced hypothermia. ABT-102 significantly (P < 0.001) prevented isoflurane-induced hypothermia at all of the time points tested (fig. 1C).

Next, we studied whether AMG 517 prevention of anesthesia-induced hypothermia was specific to the blockade of TRPV1. After the induction of anesthesia with ketamine, Trpv1^+/+ and Trpv1^–/– mice were infused with AMG 517 (1 mg/kg) or its vehicle in a nonstressful manner, via a preimplanted catheter. To avoid technical problems associated with the catheterization of small murine veins, the catheter was implanted in the peritoneal cavity. Because mice are more sensitive to anesthetics than rats, ketamine was injected at 50 mg/kg intraperitoneally in these experiments. In Trpv1^+/+ mice, ketamine caused hypothermia with a nadir of 35.2° ± 0.4°C (P < 0.001) at 20 min after injection (fig. 2A). The core body temperature of the mice remained significantly (P = 0.005) lower than the baseline, even at 240 min after ketamine injection in the vehicle-treated group. When the Trpv1^+/+ mice were treated with AMG 517 instead of the vehicle, the ketamine-induced hypothermia was prevented at 40 min postinjection, and from that point the core body temperature of the mice did not decrease below the baseline throughout the experiment. The core body temperature of the AMG 517-treated Trpv1^+/+ mice was significantly (P < 0.05) higher than that of controls from 40 to 240 min after injection of ketamine. In Trpv1^–/– mice, ketamine caused marked hypothermia between 10 and 240 min postinjection with similar dynamics as observed in the vehicle-treated Trpv1^+/+ mice. In contrast with the Trpv1^+/+ mice, the administration of AMG 517 did not prevent the ketamine-induced hypothermia in Trpv1^–/– mice (fig. 2B). There was no significant difference between the core body temperature of AMG 517-treated and vehicle-treated Trpv1^–/– mice throughout the experiment.

We investigated which autonomic thermoeffectors contribute to the effect of AMG 517 on anesthesia-induced hypothermia. First, we administered AMG 517 or its vehicle after intraperitoneal injection of saline to confirm that AMG 517 causes hyperthermia in rats (fig. 3A). The administration of AMG 517 caused a pronounced increase of core body temperature with a peak of 38.9° ± 0.2°C at 80 min postinjection (P < 0.001). The core body temperature of the AMG 517–treated rats was significantly higher than that of controls between 30 and 240 min. The rise of core body temperature was accompanied by elevated Vo₂ in the AMG 517–treated animals, which was increased compared with controls from 30 to 190 min postinjection. Although an elevated Vo₂ can theoretically result from both shivering and nonshivering (brown fat) thermogenesis, in rats nonshivering thermogenesis is of greater importance.³²  These experiments were performed at a low ambient temperature of 23°C, which was needed for the development of anesthesia-induced hypothermia. In this subneutral environment, the tail skin temperature of the rats remained continuously low because of cold-induced vasoconstriction in both treatment groups. The effects of AMG 517 on core body temperature and autonomic thermoeffectors (fig. 3A) are in agreement with our previous results of the mechanisms of TRPV1 antagonist-induced hyperthermia.^18,19 

In ketamine-treated rats, the iv administration of AMG 517 prevented the anesthesia-induced hypothermia (fig. 3B) in accordance with our findings described earlier (fig. 1A). The core body temperature of the AMG 517–treated rats was significantly higher than that of the controls from 70 to 240 min after drug administration. Regarding the thermoeffector pattern, we found that the ketamine-induced decrease of Vo₂ was markedly (P < 0.001) blunted after AMG 517 administration as compared with vehicle-treated rats. Interestingly, we recoded a transient yet significant increase in the tail skin temperature of AMG 517–treated rats compared with controls. The dynamics of the core body temperature response to ketamine were somewhat different in Trpv1^–/– mice compared with their Trpv1^+/+ littermates. This was presumably caused by the characteristic thermoeffector pattern (i.e. enhanced vasoconstriction, hypometabolism, and hyperactivity), which is used by Trpv1^–/– mice to regulate their core body temperature²³  and can be differently influenced by ketamine.

TRPV1 antagonists cause hyperthermia in unanesthetized rodents and humans (for a review, see Romanovsky et al.¹⁷ ). Therefore, it was important to evaluate whether TRPV1 antagonists administered while the animals were anesthetized cause hyperthermia in the postanesthesia period. We addressed this question in both ketamine and isoflurane-induced anesthesia conditions. First, we administered AMG 517 (or its vehicle) to ketamine-treated rats and recorded their core body temperature for an extended, that is, 480-min, time period (fig. 4A). Long-term experiments can be exhaustive for the rats, thus, to reduce the potential distress of the experimental animals, we randomly selected four animals per group only for these experiments. Animals would typically recover from ketamine–anesthesia approximately 60 min postinjection. We found that the infusion of AMG 517 prevented the ketamine-induced hypothermia, as expected from our previous results in the current study (figs. 1A and 3B). Importantly, however, the core body temperature of the AMG 517–treated rats did not significantly differ from their baseline core body temperature even at 480 min postinjection (fig. 4A and Supplemental Digital Content 2, https://links.lww.com/ALN/B526). Similarly, after maintaining animals under isoflurane anesthesia for 20 min, the gas was turned off and the animals recovered from anesthesia in the next 15 min with their core body temperature approaching back to baseline at 50 min postcessation of anesthesia (fig. 4B). Importantly, while AMG 517 prevented isoflurane-induced hypothermia, it did not result in hyperthermia after anesthesia was terminated (20 to 70 min, as well as 24 h after AMG 517 injection; fig. 4B).

TRPV1 antagonists were primarily developed as analgesics. However, due to the unwanted side effect of hyperthermia, it is unlikely that they would be used in the postoperative period when the patient is not under anesthesia. Hence, we tested the hypothesis that TRPV1 antagonists given at anesthesia induction, before the incision is performed, decrease the opioid dose requirement for the reduction of postincision hyperalgesia. Both vehicle and AMG 517–pretreated animals developed thermal and mechanical hyperalgesia 24 h after incisional surgery (fig. 5). The extent of hyperalgesia development was not different in both groups. When we analyzed the antihyperalgesic response of morphine by comparing paw withdrawal latencies at various time points postmorphine injection, the morphine antihyperalgesia was significantly greater at the 0.3 and 0.6 mg/kg doses for thermal hypersensitivity (two-way repeated-measures ANOVA, Bonferroni post hoc test) in the AMG 517–treated animals in comparison with vehicle-treated animals (fig. 5A). Similarly, morphine reduction of incision-induced mechanical hyperalgesia was apparent at a lower dose of 0.3 mg/kg in AMG 517–treated animals but not in vehicle-treated animals (fig. 5B).

We tested the hypothesis that TRPV1 antagonism prevents anesthesia-induced hypothermia. In addition, we determined that an additional potential benefit of the use of preincision TRPV1 antagonists may be to decrease opioid dosing required to reduce postincisional hypersensitivity. We found that two structurally distinct TRPV1 antagonists were able to prevent anesthesia-induced hypothermia after injected or volatile-induced general anesthesia. Importantly, TRPV1 antagonists did not cause hyperthermia in the setting of general anesthesia. Our data demonstrate that the prevention of anesthesia-induced hypothermia is specific to TRPV1 blockade, because it was absent in TRPV1 knockout mice. TRPV1 antagonists likely prevent anesthesia-induced hypothermia primarily by acting on brown fat thermogenesis. Finally, preincision TRPV1 antagonist treatment resulted in a decrease in the postincision morphine dose for antihyperalgesic effects in incisional hypersensitivity. These preclinical data suggest that TRPV1 antagonists may be useful in the perioperative phase, both to maintain normothermia and to reduce the requirements for opioid treatment of postoperative hyperalgesia.

After the molecular identification and characterization of the TRPV1 ion channel, antagonists were developed as potential nonopioid analgesics. However, this class of molecules was associated with the side effect of hyperthermia in both animals and in humans halting additional development for the treatment of pain.¹⁷  Although hyperthermia is a major side effect, we hypothesized that, in the controlled setting of anesthesia in the operating room, this side effect could be exploited for therapeutic effect to pharmacologically counter anesthetic-induced hypothermia. AMG 517 and ABT-102 are structurally distinct TRPV1 antagonists that have been demonstrated to produce hyperthermia in rodents and humans.^16,31  Both compounds prevented anesthesia-induced hypothermia. The use of two structurally unrelated TRPV1 antagonists suggests a specific interaction of the molecules with the TRPV1 channel for prevention of anesthesia-induced hypothermia. This conclusion was further supported by the absence of the prevention of anesthesia-induced hypothermia in TRPV1 knockout mice.

Interestingly, despite its long half-life of approximately 20 h,³³  AMG 517 did not induce hyperthermia in the postanesthesia period at the doses used. Multiple possibilities could explain this paradoxical observation. First, the duration of AMG 517 thermoregulatory action may be different than its plasma half-life, as is seen with other drugs.^34,35  Second, general anesthetics inhibit autonomic cold-defense effectors (brown fat thermogenesis and cutaneous vasoconstriction),^36,37  an effect that may be counterbalanced by their possible disinhibition by TRPV1 antagonists. Finally, general anesthetics such as isoflurane have been demonstrated to activate TRPV1, which can desensitize TRPV1,³⁸  resulting in its inability to be further antagonized by AMG 517. If the lack of hyperthermic effect in the postanesthesia phase is also confirmed in humans, it may help mitigate one of the concerns regarding the potential use of TRPV1 antagonists in the perioperative phase.

We observed that anesthesia-induced hypothermia was not altered in TRPV1 knockout mice, suggesting that TRPV1 is not the primary mediator for anesthesia-induced hypothermia. Nevertheless, the effect of AMG 517 depended on the expression of TRPV1. Because AMG 517 also prevented the anesthesia-induced hypothermia in wild-type mice in addition to rats, our findings demonstrate that antagonism of TRPV1 for the prevention of anesthesia-induced hypothermia is not species specific. Previous studies have demonstrated that AMG 517 causes hyperthermia in both rodents and humans,^18,19,26,39  suggesting that its thermoregulatory effects are preserved in humans. Additional studies are needed to determine whether AMG 517 can prevent anesthesia-induced hypothermia in humans.

In the present studies, the TRPV1 antagonist was administered immediately after anesthesia induction to test whether TRPV1 antagonism can reverse mild hypothermia that occurs as soon as anesthesia is induced and prevent the development of additional hypothermia with ongoing anesthesia. A question that we did not directly address in this article is whether the treatment with the TRPV1 antagonist can reverse an already established hypothermic state. The experimental protocol used in these studies necessitated the induction of general anesthesia that was followed by the injection of the TRPV1 antagonist. As a result, almost all of the animals had mild hypothermia (approximately 0.25° to 0.50°C) before the TRPV1 antagonist was administered. This also means that activation thresholds of the different thermoeffectors could already be shifted as a consequence of general anesthesia.⁴⁰  Under these conditions, TRPV1 antagonists were able maintain normothermia while the animals were under anesthesia.

In the experiments evaluating the mechanism behind TRPV1 antagonist prevention of anesthesia-induced hypothermia, we evaluated both vasomotor tone and oxygen consumption. In unanesthetized animals, brown fat thermogenesis and skin vasoconstriction were the mechanisms by which the TRPV1 antagonist induced hyperthermia.¹⁸  In anesthetized animals as well, TRPV1 antagonist prevention of anesthesia-induced hypothermia was associated with increasing thermogenesis. We observed a transient increase in tail skin temperature, suggesting a change in vasomotor tone. However, this effect was neither pronounced nor long lasting and may have been brought about by a downward shift in the threshold deep body temperature for vasodilation by ketamine, which in itself was not enough to overcome the vasoconstrictor effect of the low ambient temperature used. However, AMG 517 disinhibits thermogenesis,^19,26  so the core body temperature of ketamine-anesthetized rats moved up compared with ketamine-anesthetized controls without AMG 517 and crossed the threshold deep body temperature for vasodilation.

TRPV1 antagonists were originally being developed as analgesics. Given the likely importance of TRPV1 in the development and maintenance of postsurgical pain, we studied the possible effect of preincision TRPV1 blockade on the development of postincision hyperalgesia and the dose of morphine required to reduce it. Preincision TRPV1 blockade did not prevent the development of postincision thermal and mechanical hyperalgesia (fig. 5). This finding is in agreement with previous studies demonstrating limited efficacy of TRPV1 antagonists alone in incisional pain.⁴¹  However, this is in contrast with other studies that have demonstrated the key role of TRPV1 in the development of incisional thermal hyperalgesia using TRPV1 knockout animals.⁴²  It is possible that either higher or repeated preincision dosing of TRPV1 antagonists is required to replicate results obtained in knockout animals. Alternatively, preincision TRPV1 antagonists may reduce ongoing pain, which is not tested in these assays. Importantly, preincision TRPV1 blockade resulted in the appearance of opioid antihyperalgesia at lower doses. Multiple possible mechanisms might underlie this observation. First, preincision TRPV1 blockade may decrease central sensitization by blocking primary afferent barrage or central TRPV1 activation.^43,44  Second, TRPV1 antagonism may block acute opioid tolerance, because activation of TRPV1 has been implicated as one of the mechanisms underlying acute opioid tolerance and opioid-induced hyperalgesia.^45,46 

Future studies may help to determine whether advancement of TRPV1 antagonists to clinical practice is possible. After the discovery of hyperthermia as a major side effect of TRPV1 antagonists, multiple approaches were taken to separate the analgesic and hyperthermic effects of TRPV1 antagonists. One study compared different pharmacophores necessary for the hyperthermic versus analgesic effects.¹⁹  Using a similar approach, future studies may focus on developing pharmacophores designed to enhance the antihypothermia effect while retaining the analgesia. The clinical consequences of anesthesia-induced hypothermia are on bleeding, postoperative myocardial infarction, and surgical site infection. Although conceivable, it would be important to study whether maintenance of normothermia by TRPV1 antagonists prevents these consequences. Moreover, TRPV1 antagonists also affect the mortality rate in murine models of systemic inflammation, both in age-specific and inflammation-specific (septic vs. aseptic) ways.^39,47  These findings suggest that the use of TRPV1 antagonists during surgery may have additional advantages.

In summary, this study demonstrates the possibility of a novel use of TRPV1 antagonists to prevent anesthesia-induced hypothermia while simultaneously decreasing postoperative opioid requirements. Because both drugs described here (AMG 517 and ABT-102) have already been advanced to human trials, this hypothesis can be evaluated in humans undergoing anesthesia.

The authors thank Jessica Hansen, B.S., Department of Pharmacology, University of Arizona, Tucson, Arizona, for her help conducting the experiments.

Supported by a Foundation for Anesthesia Education and Research grant (Schaumburg, Illinois; to Dr. Patwardhan), a University of Arizona Career Development grant (Tucson, Arizona; to Dr. Patwardhan), and in part by the Research Fund of the Medical School, University of Pecs (Pecs, Hungary; grant No. KA-2016-15) and the New National Excellence Program of the Hungarian Ministry of Human Capacities (Budapest, Hungary; UNKP-16-4-III).

Drs. Patwardhan and Porreca declare a financial interest in Catalina Pharmaceuticals (Tucson, Arizona), which licenses the intellectual property involved in this research. Drs. Patwardhan, Porreca, and Romanovsky are founders of Catalina Pharma Inc. (Tucson, Arizona) and hold a provisional patent for the use of transient receptor potential vanilloid 1 antagonists in the prevention of anesthesia-induced hypothermia. Dr. Romanovsky has consulted for Abbott Laboratories (Chicago, Illinois), AbbVie (Chicago, Illinois), Amgen Inc. (Thousand Oaks, California), Japan Tobacco Inc. (Geneva, Switzerland), Teva Pharmaceutical Industries Ltd. (North Wales, Pennsylvania), and other pharmaceutical companies, and his research has been supported by Abbott Laboratories, AbbVie, and Amgen Inc. The other authors declare no competing interests.