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Severe Deficit in Brain Reward Function Associated with Fentanyl Withdrawal in Rats
Severe Deficit in Brain Reward Function Associated with Fentanyl Withdrawal in Rats Adrie W. Bruijnzeel, Ben Lewis, Lakshmi K. Bajpai, Timothy E. Morey, Donn M. Dennis, and Mark Gold Background: During the last decade, there has been a strong increase in the use of the mu-opioid receptor agonist fentanyl. The aim of these studies was to investigate the effects of fentanyl withdrawal on brain reward function and somatic withdrawal signs. Methods: Fentanyl and saline were chronically administered via minipumps. An intracranial self-stimulation procedure was used to provide a measure of brain reward function. Somatic signs were recorded from a checklist of opioid abstinence signs. Results: The opioid receptor antagonist naloxone induced a dose-dependent elevation in brain reward thresholds and somatic withdrawal signs in fentanyl-treated rats. Discontinuation of fentanyl administration resulted in a time-dependent elevation of brain reward thresholds and somatic withdrawal signs. Conclusions: These findings indicate that fentanyl withdrawal is associated with affective and somatic withdrawal signs. The severity of the deficit in brain reward function in this animal model suggests that affective fentanyl withdrawal symptoms may be a strong deterrent to abstinence. Key Words: Fentanyl, dependence, mu-opioid receptor, naloxone, reward, withdrawal
F
entanyl is a mu-opioid receptor agonist that is widely used as a preanesthetic medication, anesthetic adjunct during general anesthesia, and for the treatment of severe chronic pain (Erpels et al 1987; Gourlay 2001; Muijsers and Wagstaff 2001). During the last decade, there has been a strong increase in the medical use of fentanyl (Gilson et al 2004; Joranson et al 2000). Physical dependence rapidly develops after the administration of fentanyl (Thornton and Smith 1997). Discontinuation of fentanyl treatment often results in a somatic withdrawal syndrome, and a delayed discharge from the hospital compared with cases in which other analgesic narcotics are used (Dominguez et al 2003; Franck et al 1998, 2004). Preclinical experiments using the intracranial self-stimulation (ICSS) procedure have shown that abrupt cessation of administration of drugs of abuse such as alcohol and psychostimulants induce an elevation of brain reward thresholds (Bruijnzeel and Markou 2004; Cryan et al 2003; Epping-Jordan et al 1998; Schulteis et al 1995). In addition, the opioid antagonist naloxone elevates brain reward thresholds in morphine-dependent rats (Schulteis et al 1994). Elevations in brain reward thresholds are commonly interpreted as a decrease in the reward value of the stimulation and have been hypothesized to reflect a depressivelike state (Barr et al 2002; Cryan et al 2003). The administration of naloxone to morphine-dependent rats also suppresses instrumental responding for food and induces conditioned place aversion (Gellert and Sparber 1977; Schulteis et al 1994). These findings provide additional support for the notion that opioid withdrawal induces a negative affective state. Whether a somatic withdrawal syndrome is expressed upon the discontinuation of drug administration is dependent on the class of drug. A somatic withdrawal syndrome is commonly observed after the cessation of opioid administration. By con-
From the Departments of Psychiatry (AWB, BL), Anesthesiology (LKB, TEM, DMD, MG), and Pharmacology and Experimental Therapeutics (DMD, MG), College of Medicine, University of Florida, Gainesville, Florida. Address reprint requests to Adrie Bruijnzeel, Ph.D., Department of Psychiatry, McKnight Brain Institute, University of Florida, P.O. Box 100256, Gainesville, FL 32610-0244; E-mail: [email protected] Received February 27, 2005; revised March 31, 2005; revised July 6, 2006; accepted July 13, 2005.
0006-3223/06/$32.00 doi:10.1016/j.biopsych.2005.07.020
trast, discontinuation of the self-administration of amphetamine does not result in a somatic withdrawal syndrome but can result in a severe negative affective state (Barr et al 2002). Therefore, it has been hypothesized that the affective state associated with cessation of drug administration plays a more important role in the continuation of drug taking than the somatic withdrawal syndrome (Bruijnzeel and Gold, in press; Koob et al 1997). Clinical evidence indicates that discontinuation of chronic fentanyl administration results in a somatic withdrawal syndrome (Arnold et al 1990; Dominguez et al 2003; Franck et al 1998, 2004). Although these studies show that fentanyl induces physical dependence, little is known about the effects of discontinuation of fentanyl treatment on mood states. Therefore, the aim of this study was to investigate the effects of fentanyl withdrawal on brain reward function in rats using an ICSS procedure.
Methods and Materials Male Wistar rats (Charles River, Raleigh, North Carolina) were anesthetized and prepared with electrodes in the medial forebrain bundle (anterior–posterior –.5 mm; medial–lateral ⫾ 1.7 mm; dorsal–ventral – 8.3 mm from dura). After 1 week of recovery, the rats were trained on a modified discrete-trial ICSS procedure (Bruijnzeel and Markou 2004; Kornetsky and Esposito 1979), and brain reward thresholds were assessed daily. Briefly, each trial began with the delivery of a noncontingent electrical stimulus, followed by a 7.5-sec response window within which the animal could respond (turn the response wheel) to receive a second contingent stimulus identical to the initial noncontingent stimulus. A test session consisted of four alternating series of descending and ascending current intensities. The current threshold was defined as the midpoint between stimulation intensities that supported responding and current intensities that failed to support responding. Thus, four threshold estimates were recorded, and the mean of these values was taken as the brain reward threshold for each subject on each test session. After stable baseline brain reward thresholds were achieved, rats were prepared with 14-day minipumps containing either saline or fentanyl citrate salt (1.2 mg/kg/day) dissolved in saline. In experiment 1, naloxone (.003–.03 mg/kg, subcutaneously) injections were administered according to a within-subjects Latin square design and started at least 6 days after the implantation of the minipumps. Naloxone and fentanyl were obtained from Sigma-Aldrich (St. Louis, Missouri). Naloxone was administered 1 min before the behavioral observations. Rats were observed for BIOL PSYCHIATRY 2006;59:477– 480 © 2005 Society of Biological Psychiatry
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Figure 1. Effects of naloxone-precipitated fentanyl withdrawal on (A) brain reward thresholds and (B) weighted somatic signs in rats either exposed to fentanyl (n ⫽ 8) or saline (n ⫽ 7). Brain reward thresholds are expressed as a percentage of the pretest day values. ##Elevations in brain reward thresholds or somatic signs compared with all other groups (p ⬍ .01). **Elevations in brain reward thresholds or somatic signs compared with those after administration of vehicle and .003 mg/kg of naloxone to rats chronically treated with fentanyl and compared with the corresponding saline-treated control groups (p ⬍ .01). ⫹⫹ Elevations in somatic signs compared with those after the administration of vehicle to rats chronically treated with fentanyl and compared with the corresponding saline-treated control group (p ⬍ .01). Data are expressed as means ⫾ SEM.
10 min in a Plexiglas observation chambers. Individual withdrawal signs were recorded and a total withdrawal score was calculated (Gellert and Holtzman 1978). The opioid-withdrawal scale consisted of graded signs and checked signs. Graded signs included escape attempts (n ⫽ 2– 4, score 1; n ⫽ 5–9, score 2; n ⱖ 10, score 3) and abdominal constrictions (a score of 2 per constriction). Checked signs included diarrhea (score 2), facial fasciculations or teeth chattering (score 2), ptosis (score 2), abnormal posture (score 3), erection or ejaculation (score 3), and irritability (score 3). Immediately after the observation period, brain reward thresholds and latencies were assessed. After 14 days, the minipumps were removed and body weights, somatic www.sobp.org/journal
signs, reward thresholds, and latencies were assessed. Blood samples were collected 7 and 14 days after the implantation of the fentanyl pumps and plasma fentanyl levels were assessed using liquid chromatography/tandem mass spectrometry.
Results Experiment 1A: Effects of Naloxone-Precipitated Fentanyl Withdrawal on Brain Reward Thresholds, Response Latencies, and Somatic Withdrawal Signs Administration of naloxone resulted in a dose-dependent elevation in brain reward thresholds in rats treated with fentanyl,
A.W. Bruijnzeel et al
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Figure 2. Effects of spontaneous fentanyl withdrawal on (A) brain reward thresholds (fentanyl, n ⫽ 7; saline, n ⫽ 7) and (B) weighted somatic signs (fentanyl, n ⫽ 8; saline, n ⫽ 7) in rats. Brain reward thresholds are expressed as a percentage of the pre-minipump explantation day values. **Elevations in brain reward thresholds or somatic signs compared with the corresponding saline-treated control group (p ⬍ .01). Data are expressed as means ⫾ SEM.
but not in the control rats [Figure 1A; Group ⫻ Dose interaction: F (3,39) ⫽ 13.497, p ⬍ .0001]. The administration of naloxone did not alter the response latencies [F (3,39) ⫽1.189, ns]. Naloxone also induced a dose-dependent increase in somatic signs in the fentanyl-treated rats, but did not affect somatic signs in the control rats [Figure 1B; Group ⫻ Dose interaction: F (3,39) ⫽ 20.472, p ⬍ .0001]. The mean (⫾ SEM) free fentanyl level was 16.2 ⫾ 1.1 ng/mL plasma 7 days after the implantation of the minipumps, and 15.4 ng/mL plasma 14 days after the implantation of the minipumps. There was no difference between fentanyl levels 7 days and 14 days after the implantation of the minipumps [t (7) ⫽ .75, ns].
Experiment 1B: Effects of Spontaneous Fentanyl Withdrawal on Brain Reward Thresholds, Response Latencies, and Somatic Withdrawal Signs Explantation of the minipumps resulted in an elevation in brain reward thresholds in the rats treated with fentanyl, but did not affect the reward thresholds of the control rats [Figure 2A; Group ⫻ Dose interaction: F (8,96) ⫽ 10.048, p ⬍ .0001]. Explantation of the minipumps had no effect on the response latencies of the fentanyl- or saline-treated rats [F (8,96) ⫽ 1.464, ns]. After explantation of the minipumps, there was a significant increase in the somatic withdrawal score in the rats treated with fentanyl [Figure 2B; Group ⫻ Dose interaction: F (6,78) ⫽ 6.978, www.sobp.org/journal
480 BIOL PSYCHIATRY 2006;59:477– 480 p ⬍ .0001]. Explantation of the minipumps resulted in a lowering of the body weight of the fentanyl-treated rats, and did not affect the body weight of the control rats [F (12,156) ⫽ 20.783, p ⬍ .0001].
Discussion The results of this study indicate that abrupt cessation of fentanyl administration resulted in a time-dependent elevation in brain reward thresholds and somatic withdrawal signs. To our knowledge, this is the first study to report that discontinuation of fentanyl administration is associated with a severe deficit in brain reward function. Furthermore, it was shown that naloxone dose-dependently elevated brain reward thresholds and somatic withdrawal signs in rats chronically treated with fentanyl. In this study, the minipumps delivered 1.2 mg/kg of fentanyl per day for 14 days, and this resulted in stable plasma fentanyl levels of approximately 16 ng/mL. This is similar to fentanyl levels that have been reported in infants chronically treated with fentanyl. Leuschen and colleagues (1993) reported a mean plasma fentanyl level of 19.7 ng/mL in infants over a 90-hour time period. This finding is consistent with the results of another clinical study, which showed that plasma fentanyl levels were 13.9 ng/mL on day 6 of the opioid treatment (Arnold et al 1991). These findings indicate that plasma fentanyl levels in our study are similar to those in patients chronically treated with fentanyl. Chronic administration of opioid receptor agonists has been associated with a suppression of behavioral responses after the administration of opioid receptor antagonists (Dwoskin et al 1983; Higgins and Sellers 1994; Schulteis et al 2000; Stinus et al 1990). Schulteis and colleagues showed that the same doses of naloxone as used in our study induce a dose-dependent elevation in brain reward thresholds in rats chronically treated with morphine (Schulteis et al 1994). Our results extend the aforementioned study by demonstrating that antagonism of opioid receptors in rats chronically treated with fentanyl induces a deficit in brain reward function. Spontaneous opioid withdrawal in rodents closely models the affective- and somatic-symptomatology experienced by patients who discontinue the use of opioids. Our results show that discontinuation of fentanyl administration results in a deficit in brain reward function, which lasts for 2 days. We suggest that studies investigating the deficit in brain reward function associated with withdrawal from various opioid receptor agonists may contribute to the proper selection of opioid-based pain treatments, and thereby decrease the risk for the development of drug dependencies in addiction-prone subjects. Arnold JH, Truog RD, Orav EJ, Scavone JM, Hershenson MB (1990): Tolerance and dependence in neonates sedated with fentanyl during extracorporeal membrane oxygenation. Anesthesiology 73:1136 –1140. Arnold JH, Truog RD, Scavone JM, Fenton T (1991): Changes in the pharmacodynamic response to fentanyl in neonates during continuous infusion. J Pediatr 119:639 – 643. Barr AM, Markou A, Phillips AG (2002): A “crash” course on psychostimulant withdrawal as a model of depression. Trends Pharmacol Sci 23:475– 482.
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A.W. Bruijnzeel et al Bruijnzeel AW, Gold MS (in press): The role of corticotropin-releasing factorlike peptides in cannabis, nicotine and alcohol dependence. Brain Res Brain Res Rev [Available online February 2005]. Bruijnzeel AW, Markou A (2004): Adaptations in cholinergic transmission in the ventral tegmental area associated with the affective signs of nicotine withdrawal in rats. Neuropharmacology 47:572–579. Cryan JF, Hoyer D, Markou A (2003): Withdrawal from chronic amphetamine induces depressive-like behavioral effects in rodents. Biol Psychiatry 54: 49 –58. Dominguez KD, Lomako DM, Katz RW, Kelly HW (2003): Opioid withdrawal in critically ill neonates. Ann Pharmacother 37:473– 477. Dwoskin LP, Neal BS, Sparber SB (1983): Yohimbine exacerbates and clonidine attenuates acute morphine withdrawal in rats. Eur J Pharmacol 90:269 –273. Epping-Jordan MP, Watkins SS, Koob GF, Markou A (1998): Dramatic decreases in brain reward function during nicotine withdrawal. Nature 393:76 –79. Erpels FA, Vermeyen KM, Janssen LA, Van Houwe P, Moens P, Hanegreefs GH (1987): Isoflurane-fentanyl anesthesia for coronary bypass surgery in patients with good left ventricular function. Acta Anaesthesiol Belg 38: 231–239. Franck LS, Naughton I, Winter I (2004): Opioid and benzodiazepine withdrawal symptoms in paediatric intensive care patients. Intensive Crit Care Nurs 20:344 –351. Franck LS, Vilardi J, Durand D, Powers R (1998): Opioid withdrawal in neonates after continuous infusions of morphine or fentanyl during extracorporeal membrane oxygenation. Am J Crit Care 7:364 –369. Gellert VF, Holtzman SG (1978): Development and maintenance of morphine tolerance and dependence in the rat by scheduled access to morphine drinking solutions. J Pharmacol Exp Ther 205:536 –546. Gellert VF, Sparber SB (1977): A comparison of the effects of naloxone upon body weight loss and suppression of fixed-ratio operant behavior in morphine-dependent rats. J Pharmacol Exp Ther 201:44 –54. Gilson AM, Ryan KM, Joranson DE, Dahl JL (2004): A reassessment of trends in the medical use and abuse of opioid analgesics and implications for diversion control: 1997–2002. J Pain Symptom Manage 28:176 –188. Gourlay GK (2001): Treatment of cancer pain with transdermal fentanyl. Lancet Oncol 2:165–172. Higgins GA, Sellers EM (1994): Antagonist-precipitated opioid withdrawal in rats: Evidence for dissociations between physical and motivational signs. Pharmacol Biochem Behav 48:1– 8. Joranson DE, Ryan KM, Gilson AM, Dahl JL (2000): Trends in medical use and abuse of opioid analgesics. JAMA 283:1710 –1714. Koob GF, Caine SB, Parsons L, Markou A, Weiss F (1997): Opponent process model and psychostimulant addiction. Pharmacol Biochem Behav 57: 513–521. Kornetsky C, Esposito RU (1979): Euphorigenic drugs: Effects on the reward pathways of the brain. Fed Proc 38:2473–2476. Leuschen MP, Willett LD, Hoie EB, Bolam DL, Bussey ME, Goodrich PD et al (1993): Plasma fentanyl levels in infants undergoing extracorporeal membrane oxygenation. J Thorac Cardiovasc Surg 105:885– 891. Muijsers RB, Wagstaff AJ (2001): Transdermal fentanyl: An updated review of its pharmacological properties and therapeutic efficacy in chronic cancer pain control. Drugs 61:2289 –2307. Schulteis G, Ahmed SH, Morse AC, Koob GF, Everitt BJ (2000): Conditioning and opiate withdrawal. Nature 405:1013–1014. Schulteis G, Markou A, Cole M, Koob GF (1995): Decreased brain reward produced by ethanol withdrawal. Proc Natl Acad Sci U S A 92:5880 –5884. Schulteis G, Markou A, Gold LH, Stinus L, Koob GF (1994): Relative sensitivity to naloxone of multiple indices of opiate withdrawal: A quantitative dose-response analysis. J Pharmacol Exp Ther 271:1391–1398. Stinus L, Le Moal M, Koob GF (1990): Nucleus accumbens and amygdala are possible substrates for the aversive stimulus effects of opiate withdrawal. Neuroscience 37:767–773. Thornton SR, Smith FL (1997): Characterization of neonatal rat fentanyl tolerance and dependence. J Pharmacol Exp Ther 281:514 –521.
F
entanyl is a mu-opioid receptor agonist that is widely used as a preanesthetic medication, anesthetic adjunct during general anesthesia, and for the treatment of severe chronic pain (Erpels et al 1987; Gourlay 2001; Muijsers and Wagstaff 2001). During the last decade, there has been a strong increase in the medical use of fentanyl (Gilson et al 2004; Joranson et al 2000). Physical dependence rapidly develops after the administration of fentanyl (Thornton and Smith 1997). Discontinuation of fentanyl treatment often results in a somatic withdrawal syndrome, and a delayed discharge from the hospital compared with cases in which other analgesic narcotics are used (Dominguez et al 2003; Franck et al 1998, 2004). Preclinical experiments using the intracranial self-stimulation (ICSS) procedure have shown that abrupt cessation of administration of drugs of abuse such as alcohol and psychostimulants induce an elevation of brain reward thresholds (Bruijnzeel and Markou 2004; Cryan et al 2003; Epping-Jordan et al 1998; Schulteis et al 1995). In addition, the opioid antagonist naloxone elevates brain reward thresholds in morphine-dependent rats (Schulteis et al 1994). Elevations in brain reward thresholds are commonly interpreted as a decrease in the reward value of the stimulation and have been hypothesized to reflect a depressivelike state (Barr et al 2002; Cryan et al 2003). The administration of naloxone to morphine-dependent rats also suppresses instrumental responding for food and induces conditioned place aversion (Gellert and Sparber 1977; Schulteis et al 1994). These findings provide additional support for the notion that opioid withdrawal induces a negative affective state. Whether a somatic withdrawal syndrome is expressed upon the discontinuation of drug administration is dependent on the class of drug. A somatic withdrawal syndrome is commonly observed after the cessation of opioid administration. By con-
From the Departments of Psychiatry (AWB, BL), Anesthesiology (LKB, TEM, DMD, MG), and Pharmacology and Experimental Therapeutics (DMD, MG), College of Medicine, University of Florida, Gainesville, Florida. Address reprint requests to Adrie Bruijnzeel, Ph.D., Department of Psychiatry, McKnight Brain Institute, University of Florida, P.O. Box 100256, Gainesville, FL 32610-0244; E-mail: [email protected] Received February 27, 2005; revised March 31, 2005; revised July 6, 2006; accepted July 13, 2005.
0006-3223/06/$32.00 doi:10.1016/j.biopsych.2005.07.020
trast, discontinuation of the self-administration of amphetamine does not result in a somatic withdrawal syndrome but can result in a severe negative affective state (Barr et al 2002). Therefore, it has been hypothesized that the affective state associated with cessation of drug administration plays a more important role in the continuation of drug taking than the somatic withdrawal syndrome (Bruijnzeel and Gold, in press; Koob et al 1997). Clinical evidence indicates that discontinuation of chronic fentanyl administration results in a somatic withdrawal syndrome (Arnold et al 1990; Dominguez et al 2003; Franck et al 1998, 2004). Although these studies show that fentanyl induces physical dependence, little is known about the effects of discontinuation of fentanyl treatment on mood states. Therefore, the aim of this study was to investigate the effects of fentanyl withdrawal on brain reward function in rats using an ICSS procedure.
Methods and Materials Male Wistar rats (Charles River, Raleigh, North Carolina) were anesthetized and prepared with electrodes in the medial forebrain bundle (anterior–posterior –.5 mm; medial–lateral ⫾ 1.7 mm; dorsal–ventral – 8.3 mm from dura). After 1 week of recovery, the rats were trained on a modified discrete-trial ICSS procedure (Bruijnzeel and Markou 2004; Kornetsky and Esposito 1979), and brain reward thresholds were assessed daily. Briefly, each trial began with the delivery of a noncontingent electrical stimulus, followed by a 7.5-sec response window within which the animal could respond (turn the response wheel) to receive a second contingent stimulus identical to the initial noncontingent stimulus. A test session consisted of four alternating series of descending and ascending current intensities. The current threshold was defined as the midpoint between stimulation intensities that supported responding and current intensities that failed to support responding. Thus, four threshold estimates were recorded, and the mean of these values was taken as the brain reward threshold for each subject on each test session. After stable baseline brain reward thresholds were achieved, rats were prepared with 14-day minipumps containing either saline or fentanyl citrate salt (1.2 mg/kg/day) dissolved in saline. In experiment 1, naloxone (.003–.03 mg/kg, subcutaneously) injections were administered according to a within-subjects Latin square design and started at least 6 days after the implantation of the minipumps. Naloxone and fentanyl were obtained from Sigma-Aldrich (St. Louis, Missouri). Naloxone was administered 1 min before the behavioral observations. Rats were observed for BIOL PSYCHIATRY 2006;59:477– 480 © 2005 Society of Biological Psychiatry
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Figure 1. Effects of naloxone-precipitated fentanyl withdrawal on (A) brain reward thresholds and (B) weighted somatic signs in rats either exposed to fentanyl (n ⫽ 8) or saline (n ⫽ 7). Brain reward thresholds are expressed as a percentage of the pretest day values. ##Elevations in brain reward thresholds or somatic signs compared with all other groups (p ⬍ .01). **Elevations in brain reward thresholds or somatic signs compared with those after administration of vehicle and .003 mg/kg of naloxone to rats chronically treated with fentanyl and compared with the corresponding saline-treated control groups (p ⬍ .01). ⫹⫹ Elevations in somatic signs compared with those after the administration of vehicle to rats chronically treated with fentanyl and compared with the corresponding saline-treated control group (p ⬍ .01). Data are expressed as means ⫾ SEM.
10 min in a Plexiglas observation chambers. Individual withdrawal signs were recorded and a total withdrawal score was calculated (Gellert and Holtzman 1978). The opioid-withdrawal scale consisted of graded signs and checked signs. Graded signs included escape attempts (n ⫽ 2– 4, score 1; n ⫽ 5–9, score 2; n ⱖ 10, score 3) and abdominal constrictions (a score of 2 per constriction). Checked signs included diarrhea (score 2), facial fasciculations or teeth chattering (score 2), ptosis (score 2), abnormal posture (score 3), erection or ejaculation (score 3), and irritability (score 3). Immediately after the observation period, brain reward thresholds and latencies were assessed. After 14 days, the minipumps were removed and body weights, somatic www.sobp.org/journal
signs, reward thresholds, and latencies were assessed. Blood samples were collected 7 and 14 days after the implantation of the fentanyl pumps and plasma fentanyl levels were assessed using liquid chromatography/tandem mass spectrometry.
Results Experiment 1A: Effects of Naloxone-Precipitated Fentanyl Withdrawal on Brain Reward Thresholds, Response Latencies, and Somatic Withdrawal Signs Administration of naloxone resulted in a dose-dependent elevation in brain reward thresholds in rats treated with fentanyl,
A.W. Bruijnzeel et al
BIOL PSYCHIATRY 2006;59:477– 480 479
Figure 2. Effects of spontaneous fentanyl withdrawal on (A) brain reward thresholds (fentanyl, n ⫽ 7; saline, n ⫽ 7) and (B) weighted somatic signs (fentanyl, n ⫽ 8; saline, n ⫽ 7) in rats. Brain reward thresholds are expressed as a percentage of the pre-minipump explantation day values. **Elevations in brain reward thresholds or somatic signs compared with the corresponding saline-treated control group (p ⬍ .01). Data are expressed as means ⫾ SEM.
but not in the control rats [Figure 1A; Group ⫻ Dose interaction: F (3,39) ⫽ 13.497, p ⬍ .0001]. The administration of naloxone did not alter the response latencies [F (3,39) ⫽1.189, ns]. Naloxone also induced a dose-dependent increase in somatic signs in the fentanyl-treated rats, but did not affect somatic signs in the control rats [Figure 1B; Group ⫻ Dose interaction: F (3,39) ⫽ 20.472, p ⬍ .0001]. The mean (⫾ SEM) free fentanyl level was 16.2 ⫾ 1.1 ng/mL plasma 7 days after the implantation of the minipumps, and 15.4 ng/mL plasma 14 days after the implantation of the minipumps. There was no difference between fentanyl levels 7 days and 14 days after the implantation of the minipumps [t (7) ⫽ .75, ns].
Experiment 1B: Effects of Spontaneous Fentanyl Withdrawal on Brain Reward Thresholds, Response Latencies, and Somatic Withdrawal Signs Explantation of the minipumps resulted in an elevation in brain reward thresholds in the rats treated with fentanyl, but did not affect the reward thresholds of the control rats [Figure 2A; Group ⫻ Dose interaction: F (8,96) ⫽ 10.048, p ⬍ .0001]. Explantation of the minipumps had no effect on the response latencies of the fentanyl- or saline-treated rats [F (8,96) ⫽ 1.464, ns]. After explantation of the minipumps, there was a significant increase in the somatic withdrawal score in the rats treated with fentanyl [Figure 2B; Group ⫻ Dose interaction: F (6,78) ⫽ 6.978, www.sobp.org/journal
480 BIOL PSYCHIATRY 2006;59:477– 480 p ⬍ .0001]. Explantation of the minipumps resulted in a lowering of the body weight of the fentanyl-treated rats, and did not affect the body weight of the control rats [F (12,156) ⫽ 20.783, p ⬍ .0001].
Discussion The results of this study indicate that abrupt cessation of fentanyl administration resulted in a time-dependent elevation in brain reward thresholds and somatic withdrawal signs. To our knowledge, this is the first study to report that discontinuation of fentanyl administration is associated with a severe deficit in brain reward function. Furthermore, it was shown that naloxone dose-dependently elevated brain reward thresholds and somatic withdrawal signs in rats chronically treated with fentanyl. In this study, the minipumps delivered 1.2 mg/kg of fentanyl per day for 14 days, and this resulted in stable plasma fentanyl levels of approximately 16 ng/mL. This is similar to fentanyl levels that have been reported in infants chronically treated with fentanyl. Leuschen and colleagues (1993) reported a mean plasma fentanyl level of 19.7 ng/mL in infants over a 90-hour time period. This finding is consistent with the results of another clinical study, which showed that plasma fentanyl levels were 13.9 ng/mL on day 6 of the opioid treatment (Arnold et al 1991). These findings indicate that plasma fentanyl levels in our study are similar to those in patients chronically treated with fentanyl. Chronic administration of opioid receptor agonists has been associated with a suppression of behavioral responses after the administration of opioid receptor antagonists (Dwoskin et al 1983; Higgins and Sellers 1994; Schulteis et al 2000; Stinus et al 1990). Schulteis and colleagues showed that the same doses of naloxone as used in our study induce a dose-dependent elevation in brain reward thresholds in rats chronically treated with morphine (Schulteis et al 1994). Our results extend the aforementioned study by demonstrating that antagonism of opioid receptors in rats chronically treated with fentanyl induces a deficit in brain reward function. Spontaneous opioid withdrawal in rodents closely models the affective- and somatic-symptomatology experienced by patients who discontinue the use of opioids. Our results show that discontinuation of fentanyl administration results in a deficit in brain reward function, which lasts for 2 days. We suggest that studies investigating the deficit in brain reward function associated with withdrawal from various opioid receptor agonists may contribute to the proper selection of opioid-based pain treatments, and thereby decrease the risk for the development of drug dependencies in addiction-prone subjects. Arnold JH, Truog RD, Orav EJ, Scavone JM, Hershenson MB (1990): Tolerance and dependence in neonates sedated with fentanyl during extracorporeal membrane oxygenation. Anesthesiology 73:1136 –1140. Arnold JH, Truog RD, Scavone JM, Fenton T (1991): Changes in the pharmacodynamic response to fentanyl in neonates during continuous infusion. J Pediatr 119:639 – 643. Barr AM, Markou A, Phillips AG (2002): A “crash” course on psychostimulant withdrawal as a model of depression. Trends Pharmacol Sci 23:475– 482.
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