Opioid-dependent Rats Research Articles

M156 - Buprenorphine Decreases Fentanyl Vapor Self-Administration More Effectively in Opioid Dependent Rats

M156 - Buprenorphine Decreases Fentanyl Vapor Self-Administration More Effectively in Opioid Dependent Rats

A rat model of operant negative reinforcement in opioid-dependent males and females.

Avoidance of opioid withdrawal plays a key role in human opioid addiction. Here, we present a procedure for studying operant negative reinforcement in rats that was inspired by primate procedures where opioid-dependent subjects lever-press to prevent naloxone infusions. In Experiment 1, we trained rats (n = 30, 15 females) to lever-press to escape and then avoid mild footshocks (0.13-0.27mA) for 35days (30 trials/d). Next, we catheterized them and implanted minipumps containing methadone (10mg/kg/day) or saline. We then paired (4 times, single session) a light cue (20-s) with a naloxone infusion (20µg/kg, i.v) that precipitated opioid withdrawal. Next, we trained the rats to escape naloxone injections for 10days (30 trials/d). Each trial started with the onset of the opioid-withdrawal cue. After 20-s, the lever extended, and an infusion of naloxone (1 to 2.2µg/kg/infusion) began; a lever-press during an 11-s window terminated the withdrawal-paired cue and the infusion. In Experiment 2, we trained rats (n = 34, 17 females) on the same procedure but decreased the footshock escape/avoidance training to 20days. All rats learned to lever-press to escape or avoid mild footshocks. In both experiments, a subset, 56% (10/18) and 33% (8/24) of methadone-dependent rats learned to lever-press to escape naloxone infusions. We introduce an operant negative reinforcement procedure where a subset of opioid-dependent rats learned to lever-press to escape withdrawal-inducing naloxone infusions. The procedure can be used to study mechanisms of individual differences in opioid negative reinforcement-related behaviors in opioid-dependent rats.

Effect of TRV130 and methadone on fentanyl-vs.-food choice and somatic withdrawal signs in opioid-dependent and post-opioid-dependent rats.

The high efficacy mu-opioid receptor (MOR) agonist methadone is an effective opioid use disorder (OUD) medication used exclusively in opioid-dependent patients. However, methadone has undesirable effects that limit its clinical efficacy. Intermediate efficacy MOR agonists may treat OUD with fewer undesirable effects. We compared the effects of methadone with the intermediate efficacy MOR agonist TRV130 (oliceridine) on fentanyl-vs.-food choice and somatic withdrawal signs in opioid-dependent and post-opioid-dependent rats. Male rats (n = 20) were trained under a fentanyl-vs.-food choice procedure. Rats were then provided extended fentanyl (3.2 µg/kg/infusion) access (6 p.m.-6 a.m.) for 10 days to produce opioid dependence/withdrawal. Rats were treated with vehicle (n = 7), TRV130 (3.2 mg/kg; n = 8), or methadone (3.2 mg/kg; n = 5) three times per day after each extended-access session (8:30 a.m., 11 a.m., 1:30 p.m.). Withdrawal sign scoring (1:55 p.m.) and choice tests (2-4 p.m.) were conducted daily. Vehicle, TRV130, and methadone effectson fentanyl choice were redetermined in post-opioid-dependent rats. Vehicle-, TRV130-, and methadone-treated rats had similar fentanyl intakes during extended access. Vehicle-treated rats exhibited increased withdrawal signs and decreased bodyweights. Both methadone and TRV130 decreased these withdrawal signs. TRV130 was less effective than methadone to decrease fentanyl choice and increase food choice in opioid-dependent rats. Neither methadone nor TRV130 decreased fentanyl choice in post-opioid-dependent rats. Results suggest that higher MOR activation is required to reduce fentanyl choice than withdrawal signs in fentanyl-dependent rats. Additionally, given that TRV130 did not precipitate withdrawal in opioid-dependent rats, intermediate efficacy MOR agonists like TRV130 may facilitate the transition of patients with OUD from methadone to lower efficacy treatments like buprenorphine.

Impact of Morphine Dependence and Withdrawal on the Reinforcing Effectiveness of Fentanyl, Cocaine, and Methamphetamine in Rats.

Recent estimates suggest increased popularity of the concurrent use of opioids and stimulants, with over 50% of treatment-seeking opioid users reporting regular stimulant use. The goal of the current study was to determine how opioid dependence and withdrawal affect the reinforcing effects of fentanyl, cocaine, and methamphetamine. Male Sprague-Dawley rats were allowed to self-administer fentanyl under a progressive ratio (PR) schedule of reinforcement. Baseline evaluations of reinforcing effectiveness of fentanyl, cocaine, and methamphetamine were determined. Opioid dependence was then established by administering escalating doses of morphine (10–40 mg/kg) twice-daily for four days and subsequently maintained by once-daily injections of 40 mg/kg morphine. To evaluate the impact of opioid dependence and withdrawal on the self-administration of fentanyl, cocaine, and methamphetamine, sessions occurred either 12 or 20 h after the morphine, respectively. During opioid withdrawal, the fentanyl dose-response curve was shifted rightward with an increase in maximal effectiveness, whereas it was shifted rightward with a reduction in maximal effectiveness when evaluated in rats currently dependent on opioids, relative to baseline. The reinforcing effects of cocaine and methamphetamine were unchanged by either condition. The current studies provide direct evidence that the reinforcing effects of fentanyl are increased in opioid-withdrawn rats and reduced in opioid-dependent rats, relative to rats that are not physically dependent on opioids. These findings suggest that motivations to use opioids are dependent on the state of the individual whereas stimulants retain their reinforcing effects regardless of whether the individual is in an opioid-dependent or withdrawn state.

κ-Opioid receptor antagonism reverses heroin withdrawal-induced hyperalgesia in male and female rats

Although opioids are potent analgesics, a consequence of chronic opioid use is hyperalgesia during withdrawal, which may contribute to opioid misuse. Dynorphin, the endogenous ligand of κ-opioid receptors (KORs), is upregulated in opioid-dependent rats and in animal models of chronic pain. However, the role of KORs in opioid withdrawal-induced hyperalgesia remains to be determined. We hypothesized that KOR antagonism would reverse opioid withdrawal-induced hyperalgesia in opioid-dependent rats. Male and female Wistar rats received daily injections of heroin (2–6 mg/kg, SC) and were tested for mechanical sensitivity in the electronic von Frey test 4–6 h into withdrawal. Female rats required significantly more heroin than male rats to reach comparable levels of both heroin-induced analgesia and hyperalgesia (6 mg/kg vs. 2 mg/kg). Once hyperalgesia was established, we tested the effects of the KOR antagonists nor-binaltorphimine (norBNI; 30 mg/kg, SC) and 5′-guanidinonaltrindole (5′GNTI; 30 mg/kg, SC). When the animals continued to receive their daily heroin treatment (or saline treatment in the repeated saline group) five times per week throughout the experiment, both KOR antagonists reversed heroin withdrawal-induced hyperalgesia. The anti-hyperalgesia effect of norBNI was more prolonged in males than in females (14 days vs. 7 days), whereas 5′GNTI had more prolonged effects in females than in males (14 days vs. 4 days). The behavioral effects of 5′GNTI coincided with higher 5′GNTI levels in the brain than in plasma when measured at 24 h, whereas 5′GNTI did not reverse hyperalgesia at 30 min posttreatment when 5′GNTI levels were higher in plasma than in the brain. Finally, we tested the effects of 5′GNTI on naloxone-induced and spontaneous signs of opioid withdrawal and found no effect in either male or female rats. These findings indicate a functional role for KORs in heroin withdrawal-induced hyperalgesia that is observed in rats of both sexes.

Dopamine D3 Receptor Antagonism Reverses the Escalation of Oxycodone Self-administration and Decreases Withdrawal-Induced Hyperalgesia and Irritability-Like Behavior in Oxycodone-Dependent Heterogeneous Stock Rats.

Prescription opioids, such as oxycodone, are highly effective analgesics for clinical pain management, but approximately 25% of patients who are prescribed opioids misuse them, and 5%–10% develop an opioid use disorder (OUD). Effective therapies for the prevention and treatment of opioid abuse and addiction need to be developed. The present study evaluated the effects of the highly selective dopamine D3 receptor antagonist VK4-116 ([R]-N-[4-(4-[3-chloro-5-ethyl-2-methoxyphenyl]piperazin-1-yl)-3-hydroxybutyl]-1H-indole-2-carboxamide) on oxycodone addictive-like behaviors. We used a model of extended access to oxycodone self-administration and tested the effects of VK4-116 on the escalation of oxycodone self-administration and withdrawal-induced hyperalgesia and irritability-like behavior in male and female rats. Pretreatment with VK4-116 (5–25 mg/kg, i.p.) dose-dependently decreased the escalation of oxycodone self-administration and reduced withdrawal-induced hyperalgesia and irritability-like behavior in opioid-dependent rats. These findings demonstrate a key role for D3 receptors in both the motivation to take opioids and negative emotional states that are associated with opioid withdrawal and suggest that D3 receptor antagonism may be a viable therapeutic approach for the treatment of OUD.

The effects of focused ultrasound pulsation of nucleus accumbens in opioid-dependent rats

Background: Deep Brain Stimulation (DBS) is the only modality proven to be effective on selective stimulation of the deep brain structures. It was previously reported that, by using DBS, stimulatio...

Suppression of acute morphine withdrawal syndrome by adenovirus-mediated β-endorphin in rats

Background: Endogenous β-endorphin (β-EP) in the central nervous system (CNS) is decreased upon opioid addiction. The current study examined whether exogenous β-EP, delivered using an adenoviral vector into the CNS could attenuate morphine withdrawal syndrome in rats. Methods: The model of opioid-dependent rats was set up by receiving subcutaneous injection of morphine using an escalating regimen for 6 days (5, 10, 20, 40, 50, 60 mg/kg, three times/day). The adenovirus mediated β-EP gene was constructed based on our previous work. The ilea of opioid-dependent rats were isolated and treated with the supernatant of Ad-NEP. The basic and naloxone-induced (4 μm/l) contractions of dependent ilea were recorded. The Ad-NEP was injected into the left lateral ventricle of the addition rats. The expression of the β-EP gene was verified by radioimmunoassay of the cerebrospinal fluid (CSF) and immunocytochemistry for β-EP. Withdrawal syndrome was evaluated after intraperitoneal injection of naloxone. Results: The contractions of dependent ilea were attenuated with supernatant containing β-EP expressed by Ad-NEP. Injection of the Ad-NEP resulted in significant increases in β-EP level in the CSF and β-EP-positive neurons. Rats receiving adenovirus carrying the β-EP gene had significantly less severe withdrawal symptoms upon naloxone challenge. Conclusions: Exogenous β-EP mediated by adenovirus could attenuate withdrawal syndrome in morphine-dependent rats.

Induction of c-Fos and zif268 in the nociceptive amygdala parallel abstinence hyperalgesia in rats briefly exposed to morphine

Opioid-induced analgesia can be followed by spontaneous pain in humans, and hyperalgesia in rodents. In this study, opioid-induced hyperalgesia was measured by the tail-flick test when acute abstinence was precipitated by administering naloxone to drug naive rats that had experienced morphine analgesia for only 30 min. In a further experiment, the drug treatment that previously caused opioid-induced hyperalgesia was found to increase neurons expressing nuclear c-Fos or zif268 proteins in extended amygdalar regions targeted by projections of the ascending spino-parabrachio-amygdaloid nociceptive pathway. Transcription factor induction, however, was not detected in multiple brain regions known to respond in parallel with the same extended amygdalar structures when (1) rats are exposed to interoceptive/physical stressors, or (2) naloxone is used to precipitate abstinence in opioid dependent rats. Surprisingly, in many regions c-Fos induction by morphine was reduced or blocked by naloxone, even though these subjects had also experienced the effects of morphine for 30 min prior to antagonist administration. It is suggested transcription factor induction during opioid hyperalgesia in non-dependent rats could support the induction or consolidation of neural plasticity in nociceptive amygdaloid circuitry previously suggested to function in bi-directional control of pain and expression of pain-related behaviors.

Influence of sweet tasting solutions on opioid withdrawal

The effect of the ingestion of palatable fluids on the suppression of opioid withdrawals in the opioid-dependent rats was studied. Physical dependence was induced by administration of morphine over a period of 6 days. Withdrawals were precipitated with naloxone (1 mg/kg, s.c.) 4 h after the last morphine injection on the 6th day. Test drugs (10–30% sucrose solution) were given orally for 2 h prior to naloxone-induced withdrawal in 14 h water deprived rats. Somatic signs of withdrawals were scored by using the global Gellert–Holtzman rating scale. Animals pretreated with low doses of sucrose solution (10–15%) did not produce any effect on the global withdrawal scale whereas a significant decrease on the global withdrawal scores was observed at higher doses of sucrose solution (20–30%) as compared to the controls. All the individual behavioral signs of withdrawals were significantly suppressed in a dose-related manner at higher doses of sucrose solutions whereas minimal suppression was observed for facial fasciculation/teeth chattering. These findings provide support that ingestion of high concentrations of sucrose solutions for shorter duration may activate the endogenous opioid system and appears to have an important role in modifying morphine withdrawals.

Behavioral and molecular changes elicited by acute administration of SR141716 to Δ 9-tetrahydrocannabinol-tolerant rats: an experimental model of cannabinoid abstinence

Whether chronic cannabinoid consumption produces a dependent state comparable to that occurring with other drugs (e.g. the appearance of withdrawal signs when consumption is interrupted), and whether chronic cannabinoid consumption increases the risk of consuming other drugs of greater addictive power, are probably the two questions relating to cannabinoid addiction that provoke the most controversy. The present study was designed to further explore these two questions in laboratory animals. Firstly, we examined the effects of an acute challenge with SR141716 (an antagonist for the cannabinoid CB 1 receptor) in Δ 9-tetrahydrocannabinol (Δ 9-THC)-tolerant rats. This antagonist has been reported to precipitate a cannabinoid withdrawal syndrome. Thus, the administration of SR141716 to Δ 9-THC-tolerant rats reduced inactivity in the open-field test and enhanced responses as tremor, turning and retropulsion—these responses that were only slightly enhanced in control rats. The administration of SR141716 increased the plasma prolactin and the corticosterone concentration in controls, but these increases were much lesser in Δ 9-THC-tolerant rats. In addition, CRF-mRNA levels in the paraventricular hypothalamic nucleus, while reduced in SR141716-treated controls, were significantly increased in Δ 9-THC-tolerant rats. The analysis of endocannabinoids also revealed that the administration of SR141716, which was mostly inactive in control rats, was able to reverse the changes in anandamide or 2-arachidonoylglycerol concentrations found in Δ 9-THC-tolerant rats, in the striatum, limbic forebrain, diencephalon, cerebellum and brainstem, but not in the midbrain and hippocampus. As a second objective, we evaluated whether Δ 9-THC-tolerant rats were more vulnerable to morphine in a self-administration paradigm. The Δ 9-THC-tolerant and control rats self-administered morphine to a similar extent, in concordance with the similar values of dopaminergic activity in limbic and motor regions. In summary, our data indicate that Δ 9-THC-tolerant rats were not more vulnerable to the reinforcing properties of morphine. However, they responded to the blockade of CB 1 receptors by exhibiting slightly but possibly relevant differences in behavioral, endocrine and molecular parameters compared to the response in non-tolerant rats. This is indicative of the existence of a withdrawal syndrome in cannabinoid-tolerant rats that is mild compared with abstinence in opioid-dependent rats.

Conditioned Opioid Withdrawal Decreases Nociceptin/Orphanin FQ Levels in the Frontal Cortex and Olfactory Tubercle

Clinical evidence suggests that individuals experiencing drug withdrawal can become conditioned to environmental situations, whereby previously neutral stimuli can produce symptoms of withdrawal. It is believed that this “conditioned withdrawal” can have motivational significance, but the neurobiological basis for conditioned withdrawal is unknown. The goal of this study was to determine adaptations in endogenous opioid systems that may be responsible for expression of conditioned withdrawal. Opioid-dependent rats trained to lever press for food were exposed to tone and scent cues in the presence of naloxone or saline. Naloxone but not saline predictably suppressed responding for food. One month later and in a post-dependent state, all rats again were exposed to the cues but not naloxone. The conditioned cues alone suppressed responding for food in the rats previously paired with naloxone, but no suppression was seen in rats previously paired with saline. Radioimmunoassay (RIA) analysis for nociceptin/orphanin FQ (nociceptin), met-enkephalin-Arg-Phe (MEAP), and dynorphin A (dyn A) was performed from dissections of various brain regions of the rats undergoing conditioned withdrawal. Significant reductions in nociceptin peptide levels were seen in the frontal cortex and olfactory tubercle of these rats. Unconditioned opioid withdrawal and unconditioned footshock stress produced different patterns of opioid peptide regulation in separate groups of rats. These results shed light on adaptations of endogenous opioid systems to conditioned cues, stress, and withdrawal, all factors that play a role in motivating drug intake.

Further evidence for a role of NMDA receptors in the locus coeruleus in the expression of withdrawal syndrome from opioids

To examine a role of N-methyl- d-aspartate (NMDA) receptors in the locus coeruleus (LC) in the expression of the withdrawal signs from opioids, rats were continuously infused with morphine (a μ-opioid agonist, 26 nmol/μl per h) or butorphanol (a μ/δ/κ-mixed opioid agonist, 26 nmol/μl per h) intracerebroventricularly (i.c.v.) through osmotic minipumps for 3 days. An LC injection of NMDA (0.1 and 1 nmol/5 μl) induced withdrawal signs in opioid-dependent animals. However, it did not precipitate any abnormal behaviors in saline-treated control rats. The expression of the withdrawal signs precipitated by NMDA (1 nmol/5 μl), glutamate (10 nmol/5 μl), or naloxone (an opioid antagonist, 24 nmol/5 μl) was completely blocked by pretreatment with a NMDA antagonist, MK-801 (5-methyl-10,11-dihydro-5H-dibenzo[ a,d]cycloheptan-5,10-imine), 0.1 mg/kg, i.p. In animals that had been infused with opioids in the same manner, naloxone (48 nmol/5 μl, i.c.v.) precipitated withdrawal signs and increased extracellular glutamate levels in the LC of opioid-dependent rats measured by in vivo microdialysis method. Pretreatment with MK-801, however, did not affect the increases of glutamate levels in the LC. These results further demonstrate that the expression of opioid withdrawal induced by an expeditious release of glutamate in the LC region of opioid-dependent animals might be mainly mediated by the postsynaptic NMDA receptors.

A protein kinase inhibitor, H-7, blocks naloxone-precipitated changes in dopamine and its metabolites in the brains of opioid-dependent rats

The influence of an inhibitor of cAMP-dependent protein kinase and protein kinase C, H-7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine], on naloxone (an opioid receptor antagonist)-precipitated withdrawal signs and changes in levels of dopamine (DA) and its metabolites in morphine- or butorphanol-dependent rats was investigated. Animals were infused continuously with morphine (a μ-opioid receptor agonist) or butorphanol (a μ/δ/κ mixed opioid receptor agonist) for 3 days. Naloxone precipitated withdrawal syndrome and decreased the levels of DA in the cortex, striatum, and midbrain; 3,4-dihydroxyphenylacetic acid (DOPAC) in the cortex, striatum, limbic areas, and midbrain; and homovanilic acid (HVA) in the striatum, limbic areas, and midbrain regions. In animals rendered dependent on butorphanol, the results obtained were similar to those of morphine-dependent rats except for the changes in DOPAC levels. Concomitant infusion of H-7 and opioid blocked both the expression of withdrawal signs and the decreases in DA, DOPAC, and HVA levels in a dose-dependent manner. These results suggest that the enhancement of cAMP-dependent protein kinase and/or protein kinase C activity accompanying the increase of DA neuron activity during continuous infusion of opioids leads to an abrupt reduction in levels of DA and its metabolites precipitated by naloxone, which is intimately involved in the expression of physical dependence on opioids.

Inhibitory effects of diltiazem, an L-type Ca 2+ channel blocker, on naloxone-increased glutamate levels in the locus coeruleus of opioid-dependent rats

To investigate the effects of diltiazem, an L-type Cat 2+ channel blocker, on naloxone-precipitated withdrawal and elevations of glutamate levels in the locus coeruleus (LC) of opioid-dependent rats, animals were continuously infused with 26 nmol/μl/h of morphine or butorphanol intracerebroventricularly (i.c.v.) via osmotic minipumps for 3 days. l.c.v. injection of naloxone (an opioid-receptor antagonist, 48 nmol/5 μl) precipitated withdrawal signs and increased extracellular fluid levels of glutamate in the LC of morphineor butorphanol-dependent rats measured by in vivo microdialysis. Meanwhile, concomitant infusion of opioids with diltiazem (10 or 100 nmol/μl/h) inhibited the withdrawal signs and prevented the elevations of glutamate levels in the LC. These results suggest that an expeditious release of glutamate in the LC region regulated by L-type Ca 2+ channels mediated system plays a role in the expression of withdrawal signs from opioids.

Pharmacological studies on drug dependence in rodents: Dependence on opioids and CNS depressants.

Physical and psychic dependence on opioids and CNS depressants in rodents were examined using the drug-admixed food (DAF) method. A comparison of several methods for developing physical dependence on opioids was made. The DAF method has the advantage of rapidly inducing a high degree of physical dependence without causing morbidity or mortality. When morphine-dependent rats were pretreated with several opioids, naloxone-precipitated weight loss was suppressed in a dose-dependent manner. A procedure for the development of severe physical dependence on CNS depressants was also established. Drug concentrations were rapidly increased until animals showed moderate to severe CNS depression, and then this condition was maintained for at least 10 days. With this procedure, animals became severely dependent on CNS depressants. Another technique, intermittent infusion, was developed that has been used to quantify short-acting CNS depressant dependence potential. The sedative effects of pentobarbital were used as an index in the determination of the injection intervals. These results suggest that the DAF method and the new approaches are useful tools for assessing the physical dependence potential of new drugs. Moreover, oral self-administration and weight pulling procedures were utilized along with the DAF method. Procedures for the oral self-administration of opioids and CNS depressants were established. Opioid-dependent rats pulled the weight to obtain the DAF even though they had free access to normal food. This weight-pulling procedure may be useful for assessing the degree of reinforcing effects for drugs in rats.

Pharmacological Studies on Drug Dependence in Rodents: Dependence on Opioids and CNS Depressants

Physical and psychic dependence on opioids and CNS depressants in rodents were examined using the drug-admixed food (DAF) method. A comparison of several methods for developing physical dependence on opioids was made. The DAF method has the advantage of rapidly inducing a high degree of physical dependence without causing morbidity or mortality. When morphine-dependent rats were pretreated with several opioids, naloxone-precipitated weight loss was suppressed in a dose-dependent manner. A procedure for the development of severe physical dependence on CNS depressants was also established. Drug concentrations were rapidly increased until animals showed moderate to severe CNS depression, and then this condition was maintained for at least 10 days. With this procedure, animals became severely dependent on CNS depressants. Another technique, intermittent infusion, was developed that has been used to quantify short-acting CNS depressant dependence potential. The sedative effects of pentobarbital were used as an index in the determination of the injection intervals. These results suggest that the DAF method and the new approaches are useful tools for assessing the physical dependence potential of new drugs. Moreover, oral self-administration and weight pulling procedures were utilized along with the DAF method. Procedures for the oral self-administration of opioids and CNS depressants were established. Opioid-dependent rats pulled the weight to obtain the DAF even though they had free access to normal food. This weight-pulling procedure may be useful for assessing the degree of reinforcing effects for drugs in rats.