• View in gallery
    Figure 1—

    Mean ± SEM body weight in mice (n = 12) before (time 0) and after treatment with BUP-HCl (1.5 mg/kg, SC). *Value differs significantly (P = 0.01) from the value at time 0. †Value differs significantly (P < 0.001) from the value at 4 hours. †Value differs significantly (P < 0.05) from the value at time 0.

  • View in gallery
    Figure 2—

    Mean ± SEM respiratory rate in mice (n = 12/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment [black squares]), BUP-HCl (1.5 mg/kg, SC [dark gray inverted triangles]), or BUP-SR (1.5 mg/kg, SC [light gray triangles]). *,†Within a time point, value differs significantly (*P = 0.01; †P < 0.001) from the value for the control group. †Within a time point, value differs significantly (P < 0.001) from the value for the BUP-HCl group. §Within a time point, value differs significantly (P < 0.05) from the value for the control group.

  • View in gallery
    Figure 3—

    Total activity in mice (n = 12 mice/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment), BUP-HCl, or BUP-SR. At each time point, the numbers of ambulatory movements, fine movements (eg, grooming), and rearing movements (eg, standing upright on hind limbs) were counted; the numbers of movements were then combined to yield a total activity score. Results reported are the mean ± SEM. *,†Within a time point, value differs significantly (*P < 0.001; †P = 0.01) from the value for the control group. †Within a time point, value differs significantly (P < 0.05) from the value for the BUP-HCl group. See Figure 2 for remainder of key.

  • View in gallery
    Figure 4—

    Gastrointestinal tract motility in mice (n = 12/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment), BUP-HCl, or BUP-SR. Results reported are the mean ± SEM number of fecal pellets for each mouse during a 60-minute period within a testing chamber. *Within a time point, value differs significantly (P < 0.001) from the value for the control group. †Within a time point, value differs significantly (P < 0.05) from the value for the BUP-HCl group. See Figure 2 for remainder of key.

  • View in gallery
    Figure 5—

    Mean ± SEM response latency during hot plate (A) and tail-flick (B) nociception tests in mice (n = 12/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment), BUP-HCl, or BUP-SR. The %MPE was calculated by use of the following equation: %MPE = ([TL – BL]/[CL – BL]) × 100, where TL is the response latency at a given time point, BL is the baseline response latency, and CL is the cutoff response latency. The BL represents the mean of 2 response latencies determined before treatment administration. The CL was 30 and 10 seconds for the hot plate and tail-flick nociception tests, respectively. *Within a time point, value differs significantly (P < 0.001) from the value for the control group. †Within a time point, value differs significantly (P < 0.001) from the value for the BUP-HCl group. †Within a time point, value differs significantly (P = 0.01) from the value for the control group. §Within a time point, value differs significantly (P < 0.05) from the value for the BUP-HCl group. See Figure 2 for remainder of key.

  • 1. Roughan JV, Flecknell PA. Buprenorphine: a reappraisal of its antinociceptive effects and therapeutic use in alleviating postoperative pain in animals. Lab Anim 2002; 36: 322343.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2. Fudala PJ, Bridge TP, Herbert S, et al. Office-based treatment of opiate addiction with a sublingual-tablet formulation of buprenorphine and naloxone. N Engl J Med 2003; 349: 949958.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3. Huang P, Kehner GB, Cowan A, et al. Comparison of pharmacological activities of buprenorphine and norbuprenorphine: norbuprenorphine is a potent opioid agonist. J Pharmacol Exp Ther 2001; 297: 688695.

    • Search Google Scholar
    • Export Citation
  • 4. Martin WR, Eades CG, Thompson JA, et al. The effects of morphine- and nalorphine-like drugs in the nondependent and morphine-dependent chronic spinal dog. J Pharmacol Exp Ther 1976; 197: 517532.

    • Search Google Scholar
    • Export Citation
  • 5. Bloms-Funke P, Gillen C, Schuettler AJ, et al. Agonistic effects of the opioid buprenorphine on the nociceptin/OFQ receptor. Peptides 2000; 21: 11411146.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6. Hawkinson JE, Acosta-Burruel M, Espitia SA. Opioid activity profiles indicate similarities between the nociceptin/orphanin FQ and opioid receptors. Eur J Pharmacol 2000; 389: 107114.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7. Negus SS, Bidlack JM, Mello NK, et al. Delta opioid antagonist effects of buprenorphine in rhesus monkeys. Behav Pharmacol 2002; 13: 557570.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8. Leander JD. Buprenorphine has potent kappa opioid receptor antagonist activity. Neuropharmacology 1987; 26: 14451447.

  • 9. Jasinski DR, Pevnick JS, Griffith JD. Human pharmacology and abuse potential of the analgesic buprenorphine: a potential agent for treating narcotic addiction. Arch Gen Psychiatry 1978; 35: 501516.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Cowan A, Doxey JC, Harry EJ. The animal pharmacology of buprenorphine, an oripavine analgesic agent. Br J Pharmacol 1977; 60: 547554.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Lizasoain I, Leza JC, Lorenzo P. Buprenorphine: bell-shaped dose-response curve for its antagonist effects. Gen Pharmacol 1991; 22: 297300.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12. Christoph T, Kogel B, Schiene K, et al. Broad analgesic profile of buprenorphine in rodent models of acute and chronic pain. Eur J Pharmacol 2005; 507: 8798.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Stokes EL, Flecknell PA, Richardson CA. Reported analgesic and anaesthetic administration to rodents undergoing experimental surgical procedures. Lab Anim 2009; 43: 149154.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Johnson RE, Fudala PJ, Payne R. Buprenorphine: considerations for pain management. J Pain Symptom Manage 2005; 29: 297326.

  • 15. Bell JR, Butler B, Lawrance A, et al. Comparing overdose mortality associated with methadone and buprenorphine treatment. Drug Alcohol Depend 2009; 104: 7377.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16. Evans HC, Easthope SE. Transdermal buprenorphine. Drugs 2003; 63: 19992012.

  • 17. Sittl R, Griessinger N, Likar R. Analgesic efficacy and tolerability of transdermal buprenorphine in patients with inadequately controlled chronic pain related to cancer and other disorders: a multicenter, randomized, double-blind, placebo-controlled trial. Clin Ther 2003; 25: 150168.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Kress HG. Clinical update on the pharmacology, efficacy and safety of transdermal buprenorphine. Eur J Pain 2009; 13: 219230.

  • 19. Foley PL, Liang H, Crichlow AR. Evaluation of a sustained-release formulation of buprenorphine for analgesia in rats. J Am Assoc Lab Anim Sci 2011; 50: 198204.

    • Search Google Scholar
    • Export Citation
  • 20. Foley PL, Henderson AL, Bissonette EA, et al. Evaluation of fentanyl transdermal patches in rabbits: blood concentrations and physiologic response. Comp Med 2001; 51: 239244.

    • Search Google Scholar
    • Export Citation
  • 21. Gades NM, Danneman PJ, Wixson SK, et al. The magnitude and duration of the analgesic effect of morphine, butorphanol, and buprenorphine in rats and mice. Contemp Top Lab Anim Sci 2000; 39: 813.

    • Search Google Scholar
    • Export Citation
  • 22. Flecknell PA. Chapter 5. In: Flecknell P, ed. Laboratory animal anaesthesia. 3rd ed. New York: Academic Press, 2009; 160.

  • 23. Carbone ET, Lindstrom KE, Diep S, et al. Duration of action of sustained-release buprenorphine in 2 strains of mice. J Am Assoc Lab Anim Sci 2012; 51: 815819.

    • Search Google Scholar
    • Export Citation
  • 24. Middleton JC, Tipton AJ. Synthetic biodegradable polymers as orthopedic devices. Biomaterials 2000; 21: 23352346.

  • 25. Perrin DEEJ. Polycaprolactone. In: Domb AJKJ, Wiseman DM, eds. Handbook of biodegradable polymers. Amsterdam: Harwood Academic Publishers, 1997; 6378.

    • Search Google Scholar
    • Export Citation
  • 26. Perrin DEEJ. Polyglycolide and polylactide. In: Domb AJKJ, Wiseman DM, eds. Handbook of biodegradable polymers. Amsterdam: Harwood Academic Publishers, 1997; 327.

    • Search Google Scholar
    • Export Citation
  • 27. Gunatillake PA, Adhikari R. Biodegradable synthetic polymers for tissue engineering. Eur Cell Mater 2003; 5: 116.

  • 28. Ide S, Minami M, Satoh M, et al. Buprenorphine antinociception is abolished, but naloxone-sensitive reward is retained, in mu-opioid receptor knockout mice. Neuropsychopharmacology 2004; 29: 16561663.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 29. Codd EE, Carson JR, Colburn RW, et al. JNJ-20788560[9-(8-azabicyclo[3.2.1]oct-3-ylidene)-9H-xanthene-3-carboxylic acid diethylamide], a selective delta opioid receptor agonist, is a potent and efficacious antihyperalgesic agent that does not produce respiratory depression, pharmacologic tolerance, or physical dependence. J Pharmacol Exp Ther 2009; 329: 241251.

    • Search Google Scholar
    • Export Citation
  • 30. Langford DJ, Bailey AL, Chanda ML, et al. Coding of facial expressions of pain in the laboratory mouse. Nat Methods 2010; 7: 447449.

  • 31. Matsumiya LC, Sorge RE, Sotocinal SG, et al. Using the Mouse Grimace Scale to reevaluate the efficacy of postoperative analgesics in laboratory mice. J Am Assoc Lab Anim Sci 2012; 51: 4249.

    • Search Google Scholar
    • Export Citation
  • 32. Jablonski P, Howden BO, Baxter K. Influence of buprenorphine analgesia on post-operative recovery in two strains of rats. Lab Anim 2001; 35: 213222.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 33. Brennan MP, Sinusas AJ, Horvath TL, et al. Correlation between body weight changes and postoperative pain in rats treated with meloxicam or buprenorphine. Lab Anim (N Y) 2009; 38: 8793.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 34. Wala EP, Holtman JR Jr. Buprenorphine-induced hyperalgesia in the rat. Eur J Pharmacol 2011; 651: 8995.

  • 35. Benyamin R, Trescot AM, Datta S, et al. Opioid complications and side effects. Pain Physician 2008; 11: S105S120.

  • 36. Dahan A, Yassen A, Bijl H, et al. Comparison of the respiratory effects of intravenous buprenorphine and fentanyl in humans and rats. Br J Anaesth 2005; 94: 825834.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 37. Walsh SL, Preston KL, Stitzer ML, et al. Clinical pharmacology of buprenorphine: ceiling effects at high doses. Clin Pharmacol Ther 1994; 55: 569580.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 38. Dahan A, Yassen A, Romberg R, et al. Buprenorphine induces ceiling in respiratory depression but not in analgesia. Br J Anaesth 2006; 96: 627632.

  • 39. Houmes RJ, Voets MA, Verkaaik A, et al. Efficacy and safety of tramadol versus morphine for moderate and severe postoperative pain with special regard to respiratory depression. Anesth Analg 1992; 74: 510514.

    • Search Google Scholar
    • Export Citation
  • 40. Wilder-Smith CH, Hill L, Wilkins J, et al. Effects of morphine and tramadol on somatic and visceral sensory function and gastrointestinal motility after abdominal surgery. Anesthesiology 1999; 91: 639647.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 41. Liles JH, Flecknell PA. The effects of buprenorphine, nalbuphine and butorphanol alone or following halothane anaesthesia on food and water consumption and locomotor movement in rats. Lab Anim 1992; 26: 180189.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 42. Roughan JV, Flecknell PA. Effects of surgery and analgesic administration on spontaneous behaviour in singly housed rats. Res Vet Sci 2000; 69: 283288.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 43. Carregaro AB, Luna SP, Mataqueiro MI, et al. Effects of buprenorphine on nociception and spontaneous locomotor activity in horses. Am J Vet Res 2007; 68: 246250.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 44. Marquez P, Baliram R, Kieffer BL, et al. The mu opioid receptor is involved in buprenorphine-induced locomotor stimulation and conditioned place preference. Neuropharmacology 2007; 52: 13361341.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 45. Wise RA, Bozarth MA. A psychomotor stimulant theory of addiction. Psychol Rev 1987; 94: 469492.

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Evaluation of an improved sustained-release buprenorphine formulation for use in mice

Jason R. HealyDepartment of Basic Pharmaceutical Sciences, School of Pharmacy, West Virginia University, Morgantown, WV 26506.

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Jennifer L. TonkinDepartment of Medicine, School of Medicine, West Virginia University, Morgantown, WV 26506.

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Stacey R. KamarecDepartment of Medicine, School of Medicine, West Virginia University, Morgantown, WV 26506.

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Mitchell A. SaludesDepartment of Medicine, School of Medicine, West Virginia University, Morgantown, WV 26506.

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Sherif Y. IbrahimDepartment of Medicine, School of Medicine, West Virginia University, Morgantown, WV 26506.

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Rae R. MatsumotoDepartment of Basic Pharmaceutical Sciences, School of Pharmacy, West Virginia University, Morgantown, WV 26506.

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Jeffrey H. WimsattDepartment of Medicine, School of Medicine, West Virginia University, Morgantown, WV 26506.

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Abstract

Objective—To evaluate analgesic effects of an improved sustained-release buprenorphine (BUP-SR) formulation administered to mice.

Animals—36 male Swiss-Webster mice.

Procedures—Mice were assigned to each of 3 treatment groups (n = 12 mice/group). Treatments were administered SC (vehicle [control treatment], 1.5 mg of buprenorphine hydrochloride [BUP-HCl]/kg, and 1.5 mg of BUP-SR/kg). Mice were evaluated (total activity, gastrointestinal tract motility, respiratory rate, cataleptic behavior, and tall-flick and hot plate nociception tests) to determine behavioral and physiologic responses at 4, 24, and 48 hours after treatment administration. Body weight and respiratory rate were measured before and at each time point after treatment administration.

Results—SC administration of BUP-SR resulted in significant antinociception effects for 48 hours for the hot plate and tall-flick nociception tests without substantial adverse effects. Gastrointestinal tract motility and total activity were higher at 4 hours for mice receiving BUP-SR than for mice receiving the vehicle, but values were the same between these groups at 24 and 48 hours. The BUP-SR group had a lower respiratory rate than did the control group at all times after treatment administration. Mice treated with BUP-SR had no significant changes in body weight during the study, whereas mice treated with BUP-HCl had a significant decrease in body weight at 24 and 48 hours.

Conclusions and Clinical Relevance—BUP-SR administration resulted in antinociception effects for 48 hours. Results of this study indicated that the improved BUP-SR formulation could be safely administered SC and conferred superior analgesia, compared with that for BUP-HCl, in mice.

Abstract

Objective—To evaluate analgesic effects of an improved sustained-release buprenorphine (BUP-SR) formulation administered to mice.

Animals—36 male Swiss-Webster mice.

Procedures—Mice were assigned to each of 3 treatment groups (n = 12 mice/group). Treatments were administered SC (vehicle [control treatment], 1.5 mg of buprenorphine hydrochloride [BUP-HCl]/kg, and 1.5 mg of BUP-SR/kg). Mice were evaluated (total activity, gastrointestinal tract motility, respiratory rate, cataleptic behavior, and tall-flick and hot plate nociception tests) to determine behavioral and physiologic responses at 4, 24, and 48 hours after treatment administration. Body weight and respiratory rate were measured before and at each time point after treatment administration.

Results—SC administration of BUP-SR resulted in significant antinociception effects for 48 hours for the hot plate and tall-flick nociception tests without substantial adverse effects. Gastrointestinal tract motility and total activity were higher at 4 hours for mice receiving BUP-SR than for mice receiving the vehicle, but values were the same between these groups at 24 and 48 hours. The BUP-SR group had a lower respiratory rate than did the control group at all times after treatment administration. Mice treated with BUP-SR had no significant changes in body weight during the study, whereas mice treated with BUP-HCl had a significant decrease in body weight at 24 and 48 hours.

Conclusions and Clinical Relevance—BUP-SR administration resulted in antinociception effects for 48 hours. Results of this study indicated that the improved BUP-SR formulation could be safely administered SC and conferred superior analgesia, compared with that for BUP-HCl, in mice.

Buprenorphine is a pharmacologically unique opioid narcotic used for the treatment of pain1 and opioid dependency.2 Buprenorphine is an oripavine derivative that has high affinity for the 3 classic opioid receptor subtypes (μ, δ, and κ) as well as the opioid receptor-like 1 subtype.3 It traditionally has been classified as a partial agonist of μ-opioid receptors,4 which is primarily responsible for its analgesic effects. Furthermore, buprenorphine is a partial agonist of the opioid receptor-like 1 subtype5,6 and an antagonist of both S-opioid7 and K-opioid8 receptors.

Buprenorphine is reported to be 25 to 50 times as potent as morphine,9 but there is a ceiling effect that can potentially reduce antinociceptive efficacy with progressively increasing doses. This has been commonly referred to as an inverted U-shaped or bell-shaped dose-response curve; this phenomenon has been observed with respect to antinociception, gastrointestinal tract motility, respiration, and physical dependence.10,11 However, experiments with an array of antinociception pain models suggest that buprenorphine, when administered at a correct dosage, can cause profound analgesic effects.12

Buprenorphine is a popular analgesic agent for use in veterinary medicine, especially in research animals.13 The pharmacokinetic profile for buprenorphine indicates slow association and dissociation kinetics for opioid receptors, thereby allowing long-lasting analgesic effects.14 Because of the paradoxical reversal effect for buprenorphine at high doses, the likelihood of overdose in humans and other animals is lower, compared with that for many other potent μ-opioid receptor agonists used for the treatment of pain.15 In addition, buprenorphine is highly lipophilic, which has facilitated the development of sustained-release transdermal patches in humans. These patches provide relief from pain for up to 96 hours and have been indicated for a variety of pain-related conditions, including severe chronic pain and pain associated with cancer.16–18 Although buprenorphine transdermal patches are currently used on humans, they are of little use on rodents, including mice, because of concerns about patch displacement or consumption of patches and opioid-induced toxicosis. Furthermore, sites for patch application require special preparation, and various locations may yield different absorption profiles because of differences in skin thickness that may alter patch kinetic characteristics.19,20

Although the reported duration of action for buprenorphine depends on the specific testing methods used, its long duration of action (3 to 5 hours or longer in mice)21 allows for longer intervals between doses, compared with the dosing interval for other commonly used opioid analgesics used for this purpose. Buprenorphine hydrochloride may be administered every 8 to 12 hours22; however, if doses or dosing intervals for BUP-HCl are not optimal, there is an increased likelihood of inadequate analgesia. In a recent study,23 administration of a BUP-SR formulation in mice resulted in antinociception for only 12 hours. A major drawback of sustained-release formulations in mice used in previous studies19,23 is that they cause skin ulcers at the site of injection. Sustained-release opioid formulations, if found to be safe, have the potential to provide more consistent analgesic relief and to result in labor savings and fewer instances of stressful animal handling.

Both lactide and caprolactone have been used extensively to prepare biodegradable polymers and copolymers for inclusion in sutures, medical devices, and drug delivery systems.24 Polylactide and polycaprolactone polymers and copolymers are hydrolyzed in the body to form lactic acid and hydroxycaproic acid, which are metabolized and eliminated through the Krebs cycle.25,26 These polymers have been evaluated for tissue reactions and toxic effects and were found to be safe and biocompatible in humans and other animals.27

Formulations containing N-methyl-2-pyrrolidone yield insufficient blood concentrations 72 hours after administration.19,23 Similarly, an improved BUP-SR created by use of a copolymer of lactide and caprolactone maintained plasma concentrations > 0.5 ng/mL at 72 hours, but the concentrations at that time point were inconsistent during analgesic testing conducted by one of the authors (JHW; unpublished data). The purpose of the study reported here was to evaluate that improved BUP-SR, which had been developed to provide sustained antinociceptive benefits for ≥ 48 hours. Optimally, the adverse effects profile of any novel or improved opioid analgesic must be compared with that of the drug it is intended to replace. This is especially important for sustained-release products because they have the potential for a long duration of effect. In particular, respiratory and gastrointestinal tract effects of the improved BUP-SR needed to be evaluated and compared with those for the established BUP-HCl treatment. Finally, opioids, including buprenorphine, can cause both excitatory and sedative effects, and these results may differ considerably depending on the animal species and treatment dose.22 Therefore, total animal activity and cataleptic behavior were tested to assess the behavioral effects attributable to the improved BUP-SR.

Materials and Methods

Animals—Male Swiss-Webster micea were used for the study. Mice were 8 weeks old and had a body weight of 33 to 41 g. Mice were housed separately in individually ventilated polysulfone cagesb with a light-to-dark cycle of 12 hours of light to 12 hours of darkness. Mice were provided with ad libitum access to foodc and water. Mice were allowed to acclimatize to their cages for 1 week prior to the start of the study. In addition, mice were allowed to acclimatize to the testing room for 2 days prior to the start of the study. All procedures were approved by the Institutional Animal Care and Use Committee at West Virginia University.

Experimental procedures—Mice were randomly assigned to treatment groups (n = 12 mice/group) by use of a random number table. Group size was determined on the basis of results of a power analysis.d Mice in a respective treatment group received a single dose of vehicle (control treatment), BUP-HCl, or BUP-SR. All treatments were administered SC in the dorsal aspect of the neck by means of 1-mL Luer-lock syringes and 25-gauge needles.

The control treatment consisted of the BUP-SR vehicle at a volume equal to that of the BUP-SR treatment. Mice in the BUP-HCl group received 1.5 mg of BUP-HCle/kg (volume, 0.17 to 0.21 mL), whereas mice in the BUP-SR group received 1.5 mg of BUP-SRf/kg (volume, 0.08 to 0.11 mL). The dose of buprenorphine was selected on the basis that it was within the range found to result in near maximal antinociceptive activity in mice for both the hot plate and tail-flick tests.28 The BUP-SR formulation consisted of a biodegradable liquid polymer dissolved in a biocompatible solvent. The solvent used was a combination of N-methyl-2-pyrrolidone and triacetin; this mixture was used to solubilize the buprenorphine base so that it could be drawn into a sustained-release biodegradable matrix. In a preliminary unpublished study conducted by one of the authors (JHW) that involved 8-week-old male Swiss-Webster mice (n = 5/group), SC administration of 100 μL of N-methyl-2-pyrrolidone caused toxic skin reactions, but SC administration of 100 μL of triacetin did not, leading to its incorporation into the BUP-SR formulation used in the present study.

Evaluation of effects—On the basis of prior preliminary pharmacokinetic profiling for samples obtained at 0, 3, 6, 12, 24, 48, 72, and 96 hours after buprenorphine administration (JHW; unpublished data), it was determined that blood concentrations would remain consistently high for at least 48 hours in all mice. Therefore, only time points that provided the best comparison between BUP-SR and BUP-HCl were selected for further evaluation. Thus, mice were evaluated before (time 0) and 4, 24, and 48 hours after administration of the vehicle, BUP-HCl, or BUP-SR. The 4-hour time point was chosen as the approximate point of peak effect, as determined on the basis of previous pharmacokinetic analyses of buprenorphine-treated mice.21

Body weight of each mouse was assessed before each evaluation period. After body weight was measured, all mice were allowed to acclimatize to the testing room for 60 minutes. Each mouse was then placed inside a testing chamber (a clear plastic box without bedding) and allowed to acclimatize for an additional 30 minutes. Respiratory rate for each mouse was determined after the 30-minute acclimatization period in this testing chamber. An investigator who was not aware of the treatment administered to each mouse counted each full inhalation and exhalation cycle for 10 seconds; each count was multiplied by 6. Total activity was then measured by use of an automated computer-counting activity monitoring system,g which consisted of two 16 × 16-photon beam arrays separated in height by 5 cm. Total activity was quantified for a 30-minute period. Ambulatory movements, fine movements (eg, grooming), and rearing movements (eg, standing upright on hind limbs) were each counted; the number of movements was then combined to yield a total activity score. Testing chambers were cleaned between mice. Gastrointestinal tract motility was determined as the number of fecal pellets produced by each mouse during the 60-minute period within the testing chamber.29

Hot plate nociception testing was then performed. Mice were placed in a plastic cylinderh atop a uniformly heated black anodized aluminum plate.i Time until the first sign of excessive shaking, lifting, or licking of the hind paws was determined and recorded as the response latency. A maximum of 30 seconds was used as a cutoff for response latency to avoid tissue damage. Two response latencies were recorded before drug administration (baseline) and used to ensure nociception reflexes were clinically normal; a mean baseline latency of 8 to 10 seconds was required before testing was allowed to proceed. Response latency was recorded at each of the respective time points after treatment. Data obtained were reported as the %MPE, which was indicative of antinociception activity and was calculated by use of the following equation:

article image

where TL is the response latency at a given time point, BL is the baseline response latency, and CL is the cutoff response latency.

Cataleptic behavior was assessed next. The forepaws of each mouse were draped over a horizontal metal rod located 3 cm above the bench surface, and the interval until the mouse disengaged both forepaws was recorded. The investigator who performed the measurements was not aware of the treatment administered to each mouse.

Finally, tail-flick antinociception testing was performed. Mice were positioned so that tails were in the beam of an overhead halogen light source.j Time until the first sign of a rapid tail flick was determined and recorded as the response latency. A maximum of 10 seconds was used as a cutoff for response latency to avoid tissue damage. Two values were recorded before treatment administration (baseline), and a mean baseline latency of 2 to 4 seconds was required before testing was allowed to proceed. Response latency was recorded at each of the respective time points after treatment. Data obtained were reported as the %MPE.

Data analysis—All data analyses including normality testing were performed with a statistical software package.11 Data were evaluated by means of repeated-measures ANOVAs to examine time- and treatment-dependent effects. Tukey and Bonferroni post hoc tests were used for pairwise comparisons. For all analyses, α was set at 0.05 and any value of P < 0.05 was considered significant.

Results

Animals—Mice tolerated the injections well. No skin reactions were detected at the site of injection for any mice during the study period.

Body weight—Repeated-measures ANOVA revealed a significant (P < 0.001) difference in body weight among time points. However, Bonferroni post hoc analysis revealed that body weight for the BUP-SR group did not differ significantly from body weight for the control group at 4, 24, and 48 hours. Bonferroni post hoc analysis also revealed that body weight for the BUP-HCl group did not differ significantly from body weight of the control group at 4, 24, and 48 hours and that body weight did not differ significantly between the BUP-SR and BUP-HCl groups at 4, 24, and 48 hours.

A 1-way repeated-measures ANOVA revealed a significant (P < 0.001) difference in body weight for the BUP-HCl group among time points. Tukey post hoc analysis revealed a significant (P = 0.01) increase in body weight (2.4% increase) for the BUP-HCl group at 4 hours, compared with body weight at 0 hours (Figure 1). Additionally, there was a significant (P < 0.001) decrease in body weight for the BUP-HCl group between 4 and 24 hours (4.9% decrease) as well as between 4 and 48 hours (4.1% decrease). Similarly, there was a significant (P = 0.01) decrease in body weight from 0 to 24 hours (2.7% decrease) and 0 to 48 hours (1.8% decrease).

Figure 1—
Figure 1—

Mean ± SEM body weight in mice (n = 12) before (time 0) and after treatment with BUP-HCl (1.5 mg/kg, SC). *Value differs significantly (P = 0.01) from the value at time 0. †Value differs significantly (P < 0.001) from the value at 4 hours. †Value differs significantly (P < 0.05) from the value at time 0.

Citation: American Journal of Veterinary Research 75, 7; 10.2460/ajvr.75.7.619

Respiratory rate—Repeated-measures ANOVA revealed a significant (P < 0.001) difference in respiratory rate among treatment groups and among time points. Bonferroni post hoc analysis revealed that administration of BUP-SR resulted in a significant decrease in respiratory rate, compared with that for the control group, at 4 (P < 0.001), 24 (P < 0.001), and 48 (P < 0.05) hours (Figure 2). Bonferroni post hoc analysis also revealed that the BUP-HCl group had a significant (P = 0.01) decrease in respiratory rate, compared with the respiratory rate for the control group, at 4 hours; however, there was not a significant difference in respiratory rates between these groups at 24 and 48 hours. Bonferroni post hoc analysis revealed no significant difference in respiratory rate between the BUP-SR and BUP-HCl groups at 4 hours, but the respiratory rate for the BUP-SR group was significantly (P < 0.001) lower than the respiratory rate for the BUP-HCl group at 24 and 48 hours.

Figure 2—
Figure 2—

Mean ± SEM respiratory rate in mice (n = 12/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment [black squares]), BUP-HCl (1.5 mg/kg, SC [dark gray inverted triangles]), or BUP-SR (1.5 mg/kg, SC [light gray triangles]). *,†Within a time point, value differs significantly (*P = 0.01; †P < 0.001) from the value for the control group. †Within a time point, value differs significantly (P < 0.001) from the value for the BUP-HCl group. §Within a time point, value differs significantly (P < 0.05) from the value for the control group.

Citation: American Journal of Veterinary Research 75, 7; 10.2460/ajvr.75.7.619

Figure 3—
Figure 3—

Total activity in mice (n = 12 mice/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment), BUP-HCl, or BUP-SR. At each time point, the numbers of ambulatory movements, fine movements (eg, grooming), and rearing movements (eg, standing upright on hind limbs) were counted; the numbers of movements were then combined to yield a total activity score. Results reported are the mean ± SEM. *,†Within a time point, value differs significantly (*P < 0.001; †P = 0.01) from the value for the control group. †Within a time point, value differs significantly (P < 0.05) from the value for the BUP-HCl group. See Figure 2 for remainder of key.

Citation: American Journal of Veterinary Research 75, 7; 10.2460/ajvr.75.7.619

Figure 4—
Figure 4—

Gastrointestinal tract motility in mice (n = 12/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment), BUP-HCl, or BUP-SR. Results reported are the mean ± SEM number of fecal pellets for each mouse during a 60-minute period within a testing chamber. *Within a time point, value differs significantly (P < 0.001) from the value for the control group. †Within a time point, value differs significantly (P < 0.05) from the value for the BUP-HCl group. See Figure 2 for remainder of key.

Citation: American Journal of Veterinary Research 75, 7; 10.2460/ajvr.75.7.619

Total activity—Repeated-measures ANOVA revealed a significant difference in total activity among treatment groups (P < 0.05) and among time points (P < 0.001). Bonferroni post hoc analysis revealed that the BUP-SR group had a significantly (P = 0.01) higher total activity, compared with total activity for the control group, at 4 hours (Figure 3); however, total activity did not differ significantly between these groups at 24 and 48 hours. Bonferroni post hoc analysis also revealed that the BUP-HCl group had a significantly (P < 0.001) higher total activity, compared with total activity for the control group, at 4 hours, but total activity did not differ significantly between these groups at 24 and 48 hours. Total activity did not differ significantly between the BUP-SR and BUP-HCl groups at 4 hours but did differ significantly between these groups at 24 and 48 hours.

Gastrointestinal tract motility—Repeated-measures ANOVA revealed a significant difference in gastrointestinal tract motility among treatment groups (P = 0.01) and among time points (P < 0.001). Bonferroni post hoc analysis revealed that the BUP-SR group had significantly (P < 0.001) less gastrointestinal tract motility, compared with gastrointestinal tract motility for the control group, at 4 hours (Figure 4); however, gastrointestinal tract motility did not differ significantly between these groups at 24 and 48 hours. Bonferroni post hoc analysis also revealed that the BUP-HCl group had significantly (P < 0.001) less gastrointestinal tract motility, compared with gastrointestinal tract motility for the control group, at 4 hours; however, gastrointestinal tract motility did not differ significantly between these groups at 24 and 48 hours. There was no significant difference in gastrointestinal tract motility between the BUP-SR and BUP-HCl groups at 4 and 24 hours; however, the BUP-SR group had significantly less gastrointestinal tract motility, compared with that of the BUP-HCl group, at 48 hours.

Figure 5—
Figure 5—

Mean ± SEM response latency during hot plate (A) and tail-flick (B) nociception tests in mice (n = 12/group) 4, 24, and 48 hours after treatment with a vehicle (control treatment), BUP-HCl, or BUP-SR. The %MPE was calculated by use of the following equation: %MPE = ([TL – BL]/[CL – BL]) × 100, where TL is the response latency at a given time point, BL is the baseline response latency, and CL is the cutoff response latency. The BL represents the mean of 2 response latencies determined before treatment administration. The CL was 30 and 10 seconds for the hot plate and tail-flick nociception tests, respectively. *Within a time point, value differs significantly (P < 0.001) from the value for the control group. †Within a time point, value differs significantly (P < 0.001) from the value for the BUP-HCl group. †Within a time point, value differs significantly (P = 0.01) from the value for the control group. §Within a time point, value differs significantly (P < 0.05) from the value for the BUP-HCl group. See Figure 2 for remainder of key.

Citation: American Journal of Veterinary Research 75, 7; 10.2460/ajvr.75.7.619

Hot plate nociception testing—Peak antinociception effects were evident at 4 hours for both the BUP-SR and BUP-HCl groups. Repeated-measures ANOVA revealed a significant (P 0.001) difference in latency for hot plate nociception testing among treatments groups and among time points. Bonferroni post hoc analysis revealed that the BUP-SR group had significantly greater antinociception, compared with antinociception for the control group, at 4 (P < 0.001), 24 (P < 0.001), and 48 (P = 0.01) hours (Figure 5). Bonferroni post hoc analysis also revealed that the BUP-HCl group had significantly (P < 0.001) greater antinociception, compared with antinociception for the control group, at 4 hours, but antinociception did not differ significantly between these groups at 24 and 48 hours. There was no significant difference in antinociception between the BUP-SR and BUP-HCl groups at 4 hours, but the BUP-SR group had significantly (P < 0.001) greater antinociception, compared with antinociception for the BUP-HCl group, at 24 and 48 hours.

Cataleptic behavior—Repeated-measures ANOVA revealed a significant (P = 0.01) difference in cataleptic behavior among time points. Bonferroni post hoc analysis revealed that cataleptic behavior for the BUP-SR group did not differ significantly, compared with cataleptic behavior for the control group, at 4, 24, and 48 hours. Bonferroni post hoc analysis revealed that cataleptic behavior for the BUP-HCl group also did not differ significantly, compared with cataleptic behavior for the control group, at 4, 24, and 48 hours. Cataleptic behavior did not differ significantly between the BUP-SR and BUP-HCl groups at 4, 24, or 48 hours.

Tail-flick nociception testing—Peak antinociception effects were evident at 4 hours for both the BUP-SR and BUP-HCl groups (Figure 5). Repeated-measures ANOVA revealed a significant (P < 0.001) difference in latency for tail-flick nociception testing among treatment groups and among time points. Bonferroni post hoc analysis revealed that the BUP-SR group had significantly greater antinociception, compared with antinociception for the control group, at 4 (P < 0.001), 24 (P < 0.001), and 48 (P = 0.01) hours (Figure 5). Bonferroni post hoc analysis also revealed that the BUP-HCl group had greater antinociception, compared with antinociception for the control group, at 4 hours (P < 0.001), but antinociception did not differ significantly between these groups at 24 and 48 hours. Antinociception did not differ significantly between the BUP-SR and BUP-HCl groups at 4 hours; however, the BUP-SR group had significantly greater antinociception, compared with antinociception for the BUP-HCl group, at 24 (P < 0.001) and 48 (P < 0.05) hours.

Discussion

The study reported here was conducted to determine the antinociceptive effects of an improved BUP-SR formulation with reduced opioid-related adverse effects. A BUP-SR that can be administered SC is advantageous for a number of reasons. Clinical pain assessment in rodents is challenging, even with refined assessment tools such as the mouse grimace scale.30,31 Thus, continuous analgesia is more desirable than a fixed intermittent dosing regimen, which leaves open the possibility of wide swings in drug concentrations, periods of inadequate pain relief, and inadvertent instances of noncompliance with treatment administration. Furthermore, repeated administration of BUP-HCl has the potential to cause adverse effects, such as respiratory depression, gastrointestinal tract stasis, and reduced food consumption and body weight loss, at the time of peak concentrations without the benefit of continuous analgesic relief.32 Loss of body weight has been reported in buprenorphine-treated animals.33 Results for the present study are consistent with those of previous studies because a loss in body weight was detected at 24 and 48 hours after administration in mice treated with BUP-HCl. Surprisingly, mice treated with BUP-SR did not have significant weight loss up to 48 hours after administration. Although this result should be confirmed, it is possible it reflected smaller variations in buprenorphine blood concentrations and, conceivably, fewer adverse effects when BUP-SR was used.

The behavioral effects after administration of commercially available BUP-SR formulations have been reported.19,23 In those studies,19,23 skin lesions were detected as early as the first day after administration. To our knowledge, particularly for Swiss-Webster mice, the BUP-SR formulation used in the study reported here is the first that diminishes the risk for skin lesions at the site of injection.

Antinociception effects can range from 12 to 72 hours after administration of BUP-SR formulations; disparities in these reported findings may be explained by differences in experimental methods, blood concentration, or degrees of binding to species’ opioid receptors or the development of hyperalgesia related to higher peak concentrations.34 In addition, differences in antinociceptive effects and development of hyperalgesia may reflect differences among rodent species or mouse strains. The potential for hyperphagia in rodents (especially rats) may not be diminished after administration of BUP-SR, compared with the potential for hyperphagia in rodents administered BUP-HCl.19 To optimize benefits for the use of BUP-SR formulations in mice, whereby they would undergo long periods without observation during routine use, it is important to screen for potential adverse effects, including effects on respiration, gastrointestinal tract motility, body weight, total activity, and cataleptic behavior. There are multiple clinically relevant adverse effects associated with opioid narcotic use, with respiratory depression being one of the most dangerous.35 In the present study, there was a significant decrease in respiratory rate for mice administered BUP-SR, compared with that in mice administered the control treatment, for the duration of the study (48 hours). However, tidal volume was not measured. Clinical studies36,37 indicate that there is a ceiling effect for buprenorphine with regard to respiratory depression, which thereby minimizes the risk when compared with the risk for respiratory depression with conventional μ-opioid receptor agonists. Interestingly, it has been suggested in 1 study38 that analgesic efficacy may not be constrained by this ceiling effect.

Similar to traditional μ-opioid receptor agonists, buprenorphine can cause a decrease in gastrointestinal tract motility,10 which leads to concerns about constipation. The BUP-SR evaluated in the present study caused an initial decrease in gastrointestinal tract motility, as was evident at 4 hours after administration. However, a rebound effect was detected because gastrointestinal tract motility responses at 24 and 48 hours mirrored responses for the control treatment. In future studies, pure μ-opioid receptor agonists could be evaluated and their effects compared with those for BUP-SR to evaluate respiration, gastrointestinal tract motility, and efficacy for a sustained-release formulation in situations where buprenorphine's analgesic properties may be insufficient.39,40

An increase in total activity has been reported in rodents treated with buprenorphine.10,41–44 Those results are consistent with results for both the BUP-SR and BUP-HCl groups at 4 hours after administration in the present study. Activation of μ-opioid receptors is crucial for buprenorphine-induced hyperactivity.44 It has been suggested45 that increases in opioid-induced activity may correlate with the tendency toward human drug reuse. In the present study, hyperactivity was observed in mice receiving BUP-SR only at 4 hours, after which activity returned to a level comparable to that of the control group, whereas substantial antinociceptive effects were sustained for at least 48 hours after administration.

Future studies with the BUP-SR formulation used in the present study should be conducted to determine variations in antinociception that may exist between animals with acute and chronic pain as well as variations in efficacy for various types of pain. Buprenorphine can be an effective analgesic agent with respect to multiple types of pain that reflect a complex assembly of specific pain modalities.12 Side-by-side comparisons that use methods to induce long-lasting pain, in addition to the methods used in the present study to induce short-term pain, will further advance the development of optimized use of BUP-SR and other opioid formulations.

A BUP-SR could be beneficial for postsurgical treatment of mice because it could provide more continuous analgesia, more stable opioid binding of receptors, and more stable blood concentrations as well as reducing handling stress. The novel BUP-SR in the present study sustained significant antinociceptive activity in mice for up to 48 hours as assessed by the use of thermal nociception testing.

Results for the present study were obtained from mice that had not undergone surgical intervention. Thus, there is the potential that these results may differ from those of mice undergoing surgery and receiving the same BUP-SR. Additional studies are needed to optimize the dosing regimen and efficacy. It is anticipated that improved alleviation of pain and distress will directly improve animal well-being and reduce experimental variation and may potentially reduce the number of mice needed in future studies. Overall, the data reported here support the potential use of this improved BUP-SR for painful procedures in mice to better alleviate pain and distress for an extended period of at least 24 hours without the need for administration of additional doses.

ABBREVIATIONS

BUP-HCl

Buprenorphine hydrochloride

BUP-SR

Sustained-release buprenorphine

%MPE

Percentage of the maximum possible effect

a.

Taconic, Germantown, NY.

b.

Sealsafe Next IVC Blue Line, Tecniplast USA Inc, Philadelphia, Pa.

c.

2018 Teklad global 18% protein rodent diet, Harlan Laboratories Inc, Indianapolis, Ind.

d.

GraphPad Instat 3.0, GraphPad Software Inc, San Diego, Calif.

e.

Buprenex, 0.3 mg/mL, Reckitt Benckiser Pharmaceuticals Inc, Richmond, Va.

f.

0.6 mg/mL, provided by WildPharm, Windsor, Colo.

g.

San Diego Instruments, San Diego, Calif.

h.

10.2 cm ID × 15.2 cm length, IITC Life Science Inc, Woodland Hills, Calif.

i.

27.5 × 26.3 × 1.5 cm, 53°C, IITC Life Science Inc, Woodland Hills, Calif.

j.

IITC Life Science Inc, Woodland Hills, Calif.

k.

GraphPad Prism 4.0, GraphPad Software Inc, San Diego, Calif.

References

  • 1. Roughan JV, Flecknell PA. Buprenorphine: a reappraisal of its antinociceptive effects and therapeutic use in alleviating postoperative pain in animals. Lab Anim 2002; 36: 322343.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2. Fudala PJ, Bridge TP, Herbert S, et al. Office-based treatment of opiate addiction with a sublingual-tablet formulation of buprenorphine and naloxone. N Engl J Med 2003; 349: 949958.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3. Huang P, Kehner GB, Cowan A, et al. Comparison of pharmacological activities of buprenorphine and norbuprenorphine: norbuprenorphine is a potent opioid agonist. J Pharmacol Exp Ther 2001; 297: 688695.

    • Search Google Scholar
    • Export Citation
  • 4. Martin WR, Eades CG, Thompson JA, et al. The effects of morphine- and nalorphine-like drugs in the nondependent and morphine-dependent chronic spinal dog. J Pharmacol Exp Ther 1976; 197: 517532.

    • Search Google Scholar
    • Export Citation
  • 5. Bloms-Funke P, Gillen C, Schuettler AJ, et al. Agonistic effects of the opioid buprenorphine on the nociceptin/OFQ receptor. Peptides 2000; 21: 11411146.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6. Hawkinson JE, Acosta-Burruel M, Espitia SA. Opioid activity profiles indicate similarities between the nociceptin/orphanin FQ and opioid receptors. Eur J Pharmacol 2000; 389: 107114.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7. Negus SS, Bidlack JM, Mello NK, et al. Delta opioid antagonist effects of buprenorphine in rhesus monkeys. Behav Pharmacol 2002; 13: 557570.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8. Leander JD. Buprenorphine has potent kappa opioid receptor antagonist activity. Neuropharmacology 1987; 26: 14451447.

  • 9. Jasinski DR, Pevnick JS, Griffith JD. Human pharmacology and abuse potential of the analgesic buprenorphine: a potential agent for treating narcotic addiction. Arch Gen Psychiatry 1978; 35: 501516.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Cowan A, Doxey JC, Harry EJ. The animal pharmacology of buprenorphine, an oripavine analgesic agent. Br J Pharmacol 1977; 60: 547554.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Lizasoain I, Leza JC, Lorenzo P. Buprenorphine: bell-shaped dose-response curve for its antagonist effects. Gen Pharmacol 1991; 22: 297300.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12. Christoph T, Kogel B, Schiene K, et al. Broad analgesic profile of buprenorphine in rodent models of acute and chronic pain. Eur J Pharmacol 2005; 507: 8798.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Stokes EL, Flecknell PA, Richardson CA. Reported analgesic and anaesthetic administration to rodents undergoing experimental surgical procedures. Lab Anim 2009; 43: 149154.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Johnson RE, Fudala PJ, Payne R. Buprenorphine: considerations for pain management. J Pain Symptom Manage 2005; 29: 297326.

  • 15. Bell JR, Butler B, Lawrance A, et al. Comparing overdose mortality associated with methadone and buprenorphine treatment. Drug Alcohol Depend 2009; 104: 7377.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16. Evans HC, Easthope SE. Transdermal buprenorphine. Drugs 2003; 63: 19992012.

  • 17. Sittl R, Griessinger N, Likar R. Analgesic efficacy and tolerability of transdermal buprenorphine in patients with inadequately controlled chronic pain related to cancer and other disorders: a multicenter, randomized, double-blind, placebo-controlled trial. Clin Ther 2003; 25: 150168.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Kress HG. Clinical update on the pharmacology, efficacy and safety of transdermal buprenorphine. Eur J Pain 2009; 13: 219230.

  • 19. Foley PL, Liang H, Crichlow AR. Evaluation of a sustained-release formulation of buprenorphine for analgesia in rats. J Am Assoc Lab Anim Sci 2011; 50: 198204.

    • Search Google Scholar
    • Export Citation
  • 20. Foley PL, Henderson AL, Bissonette EA, et al. Evaluation of fentanyl transdermal patches in rabbits: blood concentrations and physiologic response. Comp Med 2001; 51: 239244.

    • Search Google Scholar
    • Export Citation
  • 21. Gades NM, Danneman PJ, Wixson SK, et al. The magnitude and duration of the analgesic effect of morphine, butorphanol, and buprenorphine in rats and mice. Contemp Top Lab Anim Sci 2000; 39: 813.

    • Search Google Scholar
    • Export Citation
  • 22. Flecknell PA. Chapter 5. In: Flecknell P, ed. Laboratory animal anaesthesia. 3rd ed. New York: Academic Press, 2009; 160.

  • 23. Carbone ET, Lindstrom KE, Diep S, et al. Duration of action of sustained-release buprenorphine in 2 strains of mice. J Am Assoc Lab Anim Sci 2012; 51: 815819.

    • Search Google Scholar
    • Export Citation
  • 24. Middleton JC, Tipton AJ. Synthetic biodegradable polymers as orthopedic devices. Biomaterials 2000; 21: 23352346.

  • 25. Perrin DEEJ. Polycaprolactone. In: Domb AJKJ, Wiseman DM, eds. Handbook of biodegradable polymers. Amsterdam: Harwood Academic Publishers, 1997; 6378.

    • Search Google Scholar
    • Export Citation
  • 26. Perrin DEEJ. Polyglycolide and polylactide. In: Domb AJKJ, Wiseman DM, eds. Handbook of biodegradable polymers. Amsterdam: Harwood Academic Publishers, 1997; 327.

    • Search Google Scholar
    • Export Citation
  • 27. Gunatillake PA, Adhikari R. Biodegradable synthetic polymers for tissue engineering. Eur Cell Mater 2003; 5: 116.

  • 28. Ide S, Minami M, Satoh M, et al. Buprenorphine antinociception is abolished, but naloxone-sensitive reward is retained, in mu-opioid receptor knockout mice. Neuropsychopharmacology 2004; 29: 16561663.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 29. Codd EE, Carson JR, Colburn RW, et al. JNJ-20788560[9-(8-azabicyclo[3.2.1]oct-3-ylidene)-9H-xanthene-3-carboxylic acid diethylamide], a selective delta opioid receptor agonist, is a potent and efficacious antihyperalgesic agent that does not produce respiratory depression, pharmacologic tolerance, or physical dependence. J Pharmacol Exp Ther 2009; 329: 241251.

    • Search Google Scholar
    • Export Citation
  • 30. Langford DJ, Bailey AL, Chanda ML, et al. Coding of facial expressions of pain in the laboratory mouse. Nat Methods 2010; 7: 447449.

  • 31. Matsumiya LC, Sorge RE, Sotocinal SG, et al. Using the Mouse Grimace Scale to reevaluate the efficacy of postoperative analgesics in laboratory mice. J Am Assoc Lab Anim Sci 2012; 51: 4249.

    • Search Google Scholar
    • Export Citation
  • 32. Jablonski P, Howden BO, Baxter K. Influence of buprenorphine analgesia on post-operative recovery in two strains of rats. Lab Anim 2001; 35: 213222.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 33. Brennan MP, Sinusas AJ, Horvath TL, et al. Correlation between body weight changes and postoperative pain in rats treated with meloxicam or buprenorphine. Lab Anim (N Y) 2009; 38: 8793.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 34. Wala EP, Holtman JR Jr. Buprenorphine-induced hyperalgesia in the rat. Eur J Pharmacol 2011; 651: 8995.

  • 35. Benyamin R, Trescot AM, Datta S, et al. Opioid complications and side effects. Pain Physician 2008; 11: S105S120.

  • 36. Dahan A, Yassen A, Bijl H, et al. Comparison of the respiratory effects of intravenous buprenorphine and fentanyl in humans and rats. Br J Anaesth 2005; 94: 825834.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 37. Walsh SL, Preston KL, Stitzer ML, et al. Clinical pharmacology of buprenorphine: ceiling effects at high doses. Clin Pharmacol Ther 1994; 55: 569580.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 38. Dahan A, Yassen A, Romberg R, et al. Buprenorphine induces ceiling in respiratory depression but not in analgesia. Br J Anaesth 2006; 96: 627632.

  • 39. Houmes RJ, Voets MA, Verkaaik A, et al. Efficacy and safety of tramadol versus morphine for moderate and severe postoperative pain with special regard to respiratory depression. Anesth Analg 1992; 74: 510514.

    • Search Google Scholar
    • Export Citation
  • 40. Wilder-Smith CH, Hill L, Wilkins J, et al. Effects of morphine and tramadol on somatic and visceral sensory function and gastrointestinal motility after abdominal surgery. Anesthesiology 1999; 91: 639647.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 41. Liles JH, Flecknell PA. The effects of buprenorphine, nalbuphine and butorphanol alone or following halothane anaesthesia on food and water consumption and locomotor movement in rats. Lab Anim 1992; 26: 180189.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 42. Roughan JV, Flecknell PA. Effects of surgery and analgesic administration on spontaneous behaviour in singly housed rats. Res Vet Sci 2000; 69: 283288.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 43. Carregaro AB, Luna SP, Mataqueiro MI, et al. Effects of buprenorphine on nociception and spontaneous locomotor activity in horses. Am J Vet Res 2007; 68: 246250.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 44. Marquez P, Baliram R, Kieffer BL, et al. The mu opioid receptor is involved in buprenorphine-induced locomotor stimulation and conditioned place preference. Neuropharmacology 2007; 52: 13361341.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 45. Wise RA, Bozarth MA. A psychomotor stimulant theory of addiction. Psychol Rev 1987; 94: 469492.

Contributor Notes

Ms. Tonkin's present address is Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.

Dr. Healy was supported by a diversity supplement from the National Institutes of Health (DA013583).

The authors thank Dr. Kristie Brock, Dr. Ying Huang, Amber Forrisi, and Brandi Underwood for technical assistance.

Address correspondence to Dr. Wimsatt (jwimsatt@hsc.wvu.edu).