Reassessment of buprenorphine in conditioned place preference: temporal and pharmacological considerations

Interaction between buprenorphine and norbuprenorphine in neonatal opioid withdrawal syndrome

Buprenorphine (BUP) is the preferred treatment for opioid use disorder during pregnancy but can cause neonatal opioid withdrawal syndrome (NOWS). Norbuprenorphine (NorBUP), an active metabolite of BUP, is implicated in BUP-associated NOWS. We hypothesized that BUP, a low-efficacy agonist of mu opioid receptors, will not antagonize NorBUP, a high-efficacy agonist of mu opioid receptors, in producing NOWS. To test this hypothesis, we treated pregnant Long-Evans rats with BUP (0, 0.01, 0.1 or 1mg/kg/day) ± NorBUP (1mg/kg/day) from gestation day 9 until pup delivery, and tested pups for opioid dependence using our established NOWS model. We used LC-MS-MS to quantify brain concentrations of BUP, NorBUP, and their glucuronide conjugates. BUP had little effect on NorBUP-induced NOWS, with the exception of 1mg/kg/day BUP significantly increasing NorBUP-induced NOWS by 58% in females. BUP and NorBUP brain concentrations predicted NOWS in multiple linear regression models. Interestingly, NorBUP contributed more to NOWS in females (βNorBUP = 51.34, p = 0.0001) than in males (βNorBUP = 19.21, P = 0.093), while BUP was similar for females (βBUP = 10.62, P = 0.0017) and males (βBUP = 11.38, P = 0.009). We are the first to report that NorBUP induces NOWS in the presence of BUP and it is more influential in females than males in the contribution of NorBUP to BUP-associated NOWS. These findings suggest that females are more susceptible to NorBUP-induced NOWS, and that treatment strategies that reduce prenatal NorBUP exposure may be more effective for females than males.

Endomorphin analog ZH853 shows low reward, tolerance, and affective-motivational signs of withdrawal, while inhibiting opioid withdrawal and seeking

Currently available μ-opioid receptor agonist pharmacotherapies for opioid use disorder possess adverse effects limiting their use and, despite treatment, rates of relapse remain high. We previously showed that endomorphin analog ZH853 had no effect in rodent models that predict abuse liability in humans. Here we extended these findings by examining dependence liability and reinforcing properties in female rats and male rats with previous opioid exposure. The potential use of ZH853 in managing opioid use disorder was evaluated by examining its effect on opioid-seeking behavior and withdrawal. We found that ZH853 did not induce locomotor activation in male and female mice and was not self-administered by female rats. Relative to morphine, ZH853 led to similar somatic signs of withdrawal, but low affective-motivational signs of withdrawal, and absent changes in ventral tegmental area K(+)-Cl(-) co-transporter expression associated with reward dysregulation. The low abuse liability of ZH853 was further supported in oxycodone self-administering male rats, where ZH853 substitution extinguished opioid-seeking behavior. ZH853 priming also did not reinstate morphine conditioned place preference. Lastly, ZH853 inhibited oxycodone-seeking behavior during relapse after forced abstinence and decreased the expression of morphine withdrawal. These findings suggest the potential use of ZH853 as a safer opioid medication for long-term treatment of pain and opioid use disorder.

An evaluation method for developing abuse-deterrent opioid formulations with agonist and antagonist combinations using conditioned place preference

Although opioids are useful narcotic analgesics in clinical settings, their misuse and addiction in the United States of America and other countries are rapidly increasing. Therefore, the development of abuse-deterrent formulations is an urgent issue. We herein investigated how to select the ratio of an opioid and the opioid receptor antagonist, naloxone in abuse-deterrent formulations for mice. The conditioned place preference (CPP) test was used to evaluate the rewarding effects of abused drugs. The opioids morphine (30 μmol/kg), oxycodone (3 μmol/kg), fentanyl (0.4 μmol/kg), and buprenorphine (0.5 μmol/kg) significantly induced place preference in mice. We also examined the optimal ratio of naloxone and opioids to inhibit the rewarding effects of the latter. Naloxone (3-5 μmol/kg) effectively inhibited place preference induced by the opioids tested. We calculated theoretical drug doses that exerted the same pharmacodynamic effects based on two parameters: μ-opioid receptor binding affinity and blood-brain barrier (BBB) permeability. Theoretical doses were very close to the drug doses at which mice showed place preference. Therefore, the CPP test is useful as a behavioral method for evaluating abuse-deterrent formulations of opioids mixed with an antagonist. The ratio of naloxone with opioids, at which mice did not show place preference, may be an effective index for developing abuse-deterrent formulations. Ratios may be calculated for other opioids based on μ-opioid receptor binding affinity and BBB permeability.

Biased Opioid Antagonists as Modulators of Opioid Dependence: Opportunities to Improve Pain Therapy and Opioid Use Management

Opioid analgesics are effective pain therapeutics but they cause various adverse effects and addiction. For safer pain therapy, biased opioid agonists selectively target distinct μ opioid receptor (MOR) conformations, while the potential of biased opioid antagonists has been neglected. Agonists convert a dormant receptor form (MOR-μ) to a ligand-free active form (MOR-μ*), which mediates MOR signaling. Moreover, MOR-μ converts spontaneously to MOR-μ* (basal signaling). Persistent upregulation of MOR-μ* has been invoked as a hallmark of opioid dependence. Contrasting interactions with both MOR-μ and MOR-μ* can account for distinct pharmacological characteristics of inverse agonists (naltrexone), neutral antagonists (6β-naltrexol), and mixed opioid agonist-antagonists (buprenorphine). Upon binding to MOR-μ*, naltrexone but not 6β-naltrexol suppresses MOR-μ*signaling. Naltrexone blocks opioid analgesia non-competitively at MOR-μ*with high potency, whereas 6β-naltrexol must compete with agonists at MOR-μ, accounting for ~100-fold lower in vivo potency. Buprenorphine's bell-shaped dose-response curve may also result from opposing effects on MOR-μ and MOR-μ*. In contrast, we find that 6β-naltrexol potently prevents dependence, below doses affecting analgesia or causing withdrawal, possibly binding to MOR conformations relevant to opioid dependence. We propose that 6β-naltrexol is a biased opioid antagonist modulating opioid dependence at low doses, opening novel avenues for opioid pain therapy and use management.

Discriminative Stimulus and Low Abuse Liability Effects of Novel Endomorphin Analogs Suggest a Potential Treatment Indication for Opioid Use Disorder

Opioid dependence can be difficult to manage using existing pharmacotherapies. A long-acting opioid with low abuse liability that substitutes for a shorter-acting opioid may improve treatment of opioid use disorders (OUDs). We recently characterized an endomorphin (EM) analog (ZH853) that produced a longer duration of antinociception compared with morphine, but did not produce self-administration or several other adverse effects preclinically. Here, we further characterized ZH853 in tests of antinociception, abuse liability, and drug discrimination. A conditioned place preference (CPP) procedure, that included a locomotor activity assessment, was used to test abuse liability in rats. Subsequently, dopamine (DA) cell-somas located in the ventral tegmental area (VTA) from these rats were assessed by size using immunohistochemistry and Stereo Investigator software. A hot-plate antinociception test in male and female mice confirmed central penetration. Morphine-substitution effects of several EM analogs (ZH850, ZH831, and ZH853) were tested in a drug discrimination (DD) procedure in rats. Morphine produced dose-dependent CPP and locomotor sensitization and reduced the size of DA cell somas in VTA, whereas ZH853 did not produce any of these effects relative to control. The antinociceptive effects of ZH853 were μ-receptor selective since β-funaltrexamine antagonized these effects. Rats responded on a morphine-trained lever when injected with ZH831 and ZH853 during DD experiments. The favorable morphine-substitution effects of these EM analogs relative to their low abuse liability indicate promising novel compounds that may improve treatment of OUD. SIGNIFICANCE STATEMENT: In this experiment, we investigated the preclinical effects of novel endomorphin analogs for use as substitution therapies for opioid use disorder, a problem that has contributed to an opioid overdose epidemic. Several endomorphin analogs substituted for morphine without producing adverse effects, including reward behaviors associated with abuse liability. These compounds have the potential to become important additional tools to treat opioid use disorders.

Mu Opioid Receptor Agonist DAMGO Produces Place Conditioning, Abstinence-induced Withdrawal, and Naltrexone-Dependent Locomotor Activation in Planarians

Unlike the behavioral effects planarians display when exposed to cocaine, amphetamines, cathinones, ethanol and sucrose, effects of opioid receptor agonists, especially mu opioid receptor agonists, are poorly defined in these flatworms. Here, we tested the hypothesis that planarians exposed to a selective mu opioid receptor agonist, DAMGO (0.1, 1, 10 µM), would display a triad of opioid-like effects (place conditioning, abstinence-induced withdrawal, and motility changes). DAMGO was selected versus morphine because of its greater mu opioid receptor selectivity. In place conditioning and abstinence experiments, the planarian light/dark test (PLDT) was utilized (i.e., planarians are placed into a petri dish containing water that is split into light and dark compartments and time spent in the compartments is determined). Planarians conditioned with DAMGO (1 µM) spent more time on the drug-paired side compared to water controls. In abstinence experiments, planarians exposed to DAMGO for 30 min were removed and then placed into water, where light avoidance (e.g. defensive responding) and depressant-like effects (i.e., decreased motility) were quantified. Compared to water controls, DAMGO-withdrawn planarians spent less time in the light (10 µM) and displayed decreased motility (1, 10 µM). Acute DAMGO exposure (1 µM) produced hypermotility that was antagonized by naltrexone (1, 10, 100 µM). In contrast, acute exposure to the kappa opioid receptor agonist U50,488H (0.1, 1, 10 µM) resulted in decreased motility. Our results show that a mu opioid agonist produces mammalian-like behavioral responses in planarians that may be related to addiction and suggest opioid-like behavioral effects are conserved in invertebrates.

Morphine-induced conditioned place preference and effects of morphine pre-exposure in adolescent and adult male C57BL/6J mice

RATIONALE

Given the increasing abuse of prescription opioids, particularly in adolescents, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).

OBJECTIVES

This study compared the conditioned rewarding effects of morphine, without (experiment 1) and with morphine pre-exposure (experiment 2), in adolescent and adult male mice.

METHODS

Experiment 1: On each of three consecutive days, one of the two conditioning sessions was preceded by an injection of a particular dose of morphine (0.1, 0.32, 1, 3.2, 10, 32, or 100 mg/kg, intraperitoneal) and the other by saline; place preference was tested on day 4. Experiment 2: Mice received once daily injections of saline or a particular dose of morphine (17.8 or 56 mg/kg) for 4 days, and 3 days later, place conditioning with morphine (0.32, 1, 3.2, or 10 mg/kg) began.

RESULTS

In both experiments, morphine induced conditioned place preference along similar inverted U-shaped dose-response curves in adolescent and adult mice, with maximal effects between 0.32 and 10 mg/kg. Morphine pre-exposure did not sensitize morphine-induced conditioned place preference; instead, tolerance occurred, but only in adults. Adolescents were more sensitive than adults to morphine-induced locomotor stimulation. Response to novelty predicted the locomotor stimulating effects of morphine in adolescents, but not its rewarding effects.

CONCLUSIONS

The rewarding effects of morphine were similar in adolescent and adult mice but showed differential tolerance after morphine pre-exposure. Adolescents were more sensitive than adults to the acute locomotor stimulating effects of morphine, consistent with dopamine systems involved in locomotor activity being overactive during adolescence.

Rewarding or aversive effects of buprenorphine/naloxone combination (Suboxone) depend on conditioning trial duration

Buprenorphine is used as a sublingual medication in the treatment of opioid dependence. However, its misuse by i.v. injection may limit its acceptability and dissemination. A buprenorphine/naloxone (ratio 4:1) combination has been developed to reduce diversion and abuse. So far, the relevance of this combination has not been investigated in the animal models traditionally used to study the reinforcing effects of drugs of abuse. The aim of this study was to compare the rewarding effects, assessed by conditioned place preference (CPP), of buprenorphine and buprenorphine/naloxone combination following i.v. administration in mice. Animals were treated with different doses of buprenorphine or buprenorphine/naloxone combination (ratio 4:1), and CPP conditioning trial duration was 5 or 30 min. At the longest trial duration, a bell-shaped dose-response curve was obtained with buprenorphine, which was shifted significantly to the right with naloxone combination. At the shortest trial duration, an aversive effect was observed with the buprenorphine/naloxone combination in animals, involving opioid receptor-like 1 (ORL1). These findings may explain the discrepancies reported in the literature as some authors have shown a reduced buprenorphine/naloxone misuse compared to buprenorphine in opioid abusers, while others have not.

A non-rewarding, non-aversive buprenorphine/naltrexone combination attenuates drug-primed reinstatement to cocaine and morphine in rats in a conditioned place preference paradigm

Concurrent use of cocaine and heroin is a major public health issue with no effective relapse prevention treatment currently available. To this purpose, a combination of buprenorphine and naltrexone, a mixed very-low efficacy mu-opioid receptor agonist/kappa-opioid receptor antagonist/nociceptin receptor agonist, was investigated. The tail-withdrawal and the conditioned place preference (CPP) assays in adult Sprague Dawley rats were used to show that naltrexone dose-dependently blocked the mu-opioid receptor agonism of buprenorphine. Furthermore, in the CPP assay, a combination of 0.3 mg/kg buprenorphine and 3.0 mg/kg naltrexone was aversive. A combination of 0.3 mg/kg buprenorphine and 1.0 mg/kg naltrexone was neither rewarding nor aversive, but still possessed mu-opioid receptor antagonist properties. In the CPP extinction and reinstatement method, a combination of 0.3 mg/kg buprenorphine and 1.0 mg/kg naltrexone completely blocked drug-primed reinstatement in cocaine-conditioned rats (conditioned with 3 mg/kg cocaine, drug prime was 3 mg/kg cocaine) and attenuated drug-primed reinstatement in morphine-conditioned rats (conditioned with 5 mg/kg morphine, drug prime was 1.25 mg/kg morphine). These data add to the growing evidence that a buprenorphine/naltrexone combination may be protective against relapse in a polydrug abuse situation.

Oxytocin modulates dopamine-mediated reward in the rat subthalamic nucleus

The subthalamic nucleus (STh) is increasingly recognized as an important region involved in the motivation for drug reward. It is not yet known if dopamine, the neurotransmitter primarily responsible for reward signaling, is also involved in mediating reward-related activity in the STh. The neuropeptide oxytocin acts within the STh to reduce the rewarding effects of the psychostimulant methamphetamine, through a proposed interaction with dopamine. However, the mechanisms of this interaction are unclear. The current study aimed to determine whether (i) dopamine microinjected into the STh would result in a significant place preference following a single-trial conditioning session, (ii) co-administered dopamine receptor antagonist would block the formation of a conditioned place preference (CPP) for dopamine, (iii) co-administered oxytocin would prevent CPP for dopamine and (iv) whether the selective oxytocin antagonist desGly-NH(2),d(CH(2))(5)[D-Tyr(2),Thr(4)]OVT, when co-administered with oxytocin and dopamine, would reverse the effects of oxytocin and result in a CPP for dopamine. Results showed that male Sprague Dawley rats i) formed a preference for the context paired with dopamine (100 nmol/side) administration into the STh, which was prevented by co-administration of ii) the mixed dopamine receptor antagonist fluphenazine (10 nmol/side) or iii) oxytocin (0.6 pmol/side), [corrected] with the oxytocin effect on dopamine CPP reversed by the co-administration of the oxytocin receptor antagonist (3 nmol/side). These data suggest that dopamine neurotransmission in the STh produces rewarding effects that can be reduced by activation of local oxytocin receptors.

Bipolar disorder: involvement of signaling cascades and AMPA receptor trafficking at synapses

There is increasing evidence that severe mood disorders are associated with impairment of structural plasticity and cellular resilience. Cumulative data demonstrate that mood stabilizers regulate intracellular signaling cascades, including protein kinase C (PKC), PKA, mitogen-activated protein (MAP) kinase, glycogen synthase kinase 3-beta (GSK3-beta) and intracellular calcium, which are signaling pathways that regulate synaptic plasticity. In this context, it is noteworthy that a growing body of data indicates that the glutamatergic system, has a major role in neuronal plasticity and cellular resilience, might be involved in the pathophysiology and treatment of mood disorders. AMPA glutamate-receptor trafficking is important in synaptic plasticity and might play crucial roles in maintaining critical neuronal circuits associated with mood. Two clinically effective, structurally dissimilar, antimanic agents, lithium and valproate (VPA), down-regulate synaptic expression of AMPA receptor subunit GluR1 in hippocampus in chronically treated rats. This reduction in synaptic GluR1 by lithium and VPA is due to attenuated phosphorylation of GluR1 at a specific PKA site (residue 845 of GluR1), which is crucial for AMPA receptor insertion. By contrast,imipramine, which can provoke mania, increases synaptic expression of GluR1 in the hippocampus in vivo. Furthermore, there is ample evidence from preclinical and clinical research that the glutamatergic system is involved in the pathophysiology of mood disorders and that many of the somatic treatments used for mood disorders including antidepressants, mood stabilizers, atypical antipsychotic drugs and electroconvulsive therapy have both direct and indirect effects on the glutamatergic system. Given these findings, further research with medications that specifically affect the glutamatergic system is warranted. Recent studies in our lab have shown that riluzole, a FDA approved medicine that regulates the glutamatergic system, shows antidepressant efficacy in unipolar and bipolar depression. These studies indicate that regulation of glutamate-mediated synaptic plasticity might play a role in the treatment of mood disorders, and raise new avenues for novel therapies for this devastating illness.

Regulation of cellular plasticity and resilience by mood stabilizers: the role of AMPA receptor trafficking

There is increasing evidence from a variety of sources that severe mood disorders are associated with regional reductions in brain volume, as well as reductions in the number, size, and density of glia and neurons in discrete brain areas. Although the precise pathophysiology underlying these morphometric changes remains to be fully elucidated, the data suggest that severe mood disorders are associated with impairments of structural plasticity and cellular resilience. In this context, it is noteworthy that a growing body of data suggests that the glutamaiergic system (which is known to play a major role in neuronal plasticity and cellular resilience) may be involved in the pathophysiology and treatment of mood disorders. Glutamate α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) GluR1 receptor trafficking plays a critical role in regulating various forms of neural plasticity. It is thus noteworthy that recent studies have shown that structurally dissimilar mood stabilizers lithium and valproate regulate GluR1 receptor subunit trafficking and localization at synapses. These studies suggest that regulation of glutamatergically mediated synaptic plasticity may play a role in the treatment of mood disorders, and raises the possibility that agents more directly affecting synaptic GluR1 represent novel therapies for these devastating illnesses.

Oxytocin directly administered into the nucleus accumbens core or subthalamic nucleus attenuates methamphetamine-induced conditioned place preference

Accumulating evidence indicates that the neuropeptide oxytocin (OXY) may modulate reward-related behavioural responses to methamphetamine (METH) administration. Limited research has examined the effect of OXY on METH-induced conditioned place preference (CPP) and little is known about the neural mechanisms involved. A Fos immunohistochemistry study recently demonstrated that peripheral OXY administration reduced METH-induced Fos expression within the nucleus accumbens (NAc) core and subthalamic nucleus (STh) in rats. The current study aimed to (i) investigate the effect of systemically administered OXY on METH-induced CPP, (ii) determine the effectiveness of a single-trial CPP procedure with METH, in order to (iii) evaluate whether pretreatment with OXY injected directly into the NAc core or the STh attenuates METH-induced CPP. Results showed that male Sprague Dawley rats learned to associate unique compartmental cues with METH (1 mg/kg, i.p.) such that they spent more time in the METH-paired compartment and less time in the saline-paired compartment. Pretreatment with systemic OXY (0.6 mg, i.p.), or OXY (0.6 ng, i.c.) microinjected into the NAc core or the STh prior to METH administration attenuated the formation of a CPP to METH. This provides further evidence that OXY acts within either the NAc core or the STh to reduce the rewarding effects of METH administration.

Comparison of the antinociceptive and antirewarding profiles of novel bifunctional nociceptin receptor/mu-opioid receptor ligands: implications for therapeutic applications

The nociceptin receptor (NOPr), a member of the opioid receptor family, is a target for the treatment of pain and drug abuse. Nociceptin/orphanin FQ (N/OFQ), the endogenous peptide for NOPr, not only modulates opioid antinociception, but also blocks the rewarding effects of several abused drugs, such as morphine, cocaine, and amphetamine. We hypothesized that NOPr agonists, with bifunctional activity at the mu-opioid receptor (MOPr), may function as nonaddicting analgesics or as drug abuse medications. Bifunctional small-molecule NOPr agonists possessing different selectivities and efficacies at MOPr were evaluated in an acute thermal antinociception assay, and for their ability to induce conditioned place preference (CPP) and their effect on morphine-induced CPP. 1-(1-Cyclooctylpiperidin-4-yl)-indolin-2-one) (SR14150), a high-affinity NOPr partial agonist, with low MOPr affinity and efficacy, produced analgesia that was naloxone-reversible. SR14150 did not induce CPP alone, nor did it attenuate morphine-induced CPP. 3-Ethyl-1-(1-(4-isopropylcyclohexyl)piperidin-4-yl)-indolin-2-one (SR16507), which has high affinity for both NOPr and MOPr, full agonist activity at NOPr, and partial agonist activity at MOPr, was also a potent analgesic and produced CPP alone, but also modestly attenuated morphine CPP. 1-(1-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)piperidinl-4-yl)-indolin-2-one (SR16835), a NOPr full agonist and low-affinity MOPr partial agonist, was not antinociceptive, did not produce CPP alone, but attenuated morphine CPP. Our results suggest that NOPr full-agonist activity is required to modulate opioid-induced reward, whereas a bifunctional NOPr/MOPr partial agonist profile may be suitable as a nonaddicting analgesic. The opioid-modulating effects of the NOPr ligands may be used effectively to produce better medications for treatment of drug abuse and pain.

The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) supports intravenous self-administration and induces conditioned place preference in the rat

We recently reported that the mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) reduces intravenous self-administration of ketamine and, to a lesser extent, heroin in rats. We also found that MPEP potentiates conditioned place preference induced by these drugs, suggesting that the reduction of self-administration results from an MPEP-induced potentiation of the rewarding effect of the self-administered drug. The aim of the present study was to examine whether MPEP has intrinsic positive reinforcing and rewarding effects. In experiment 1, rats were trained to self-administer either ketamine [0.5 mg/kg/infusion, 2 h sessions, fixed-ratio (FR) 3] or heroin (0.05 mg/kg/infusion, 1 h sessions, FR 10), followed by a number of substitution sessions with MPEP (1 mg/kg/infusion) or saline. In experiment 2, drug-naïve rats were allowed to acquire intravenous self-administration of MPEP (1 mg/kg/infusion, 2 h sessions, FR 3) or saline. In experiment 3, rats were subjected to a single-trial unbiased conditioned place preference protocol with MPEP (0.3-10 mg/kg i.v., 20 min conditioning). It was found that (1) substitution with MPEP in rats which had learned to self-administer ketamine or heroin resulted in stable self-administration behavior, whereas substitution with saline resulted in a typical extinction profile, (2) drug-naïve rats learned to self-administer MPEP, but not saline, and self-administration remained stable for at least 7 sessions, and (3) MPEP induced dose-dependent place preference with a minimal effective dose of 3 mg/kg. These data clearly demonstrate that MPEP has (weak) positive reinforcing and rewarding effects when administered i.v.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

The effects of buprenorphine on fentanyl withdrawal in rats

RATIONALE

Fentanyl is a potent mu-opioid receptor agonist that is widely used for the treatment of severe chronic pain. Discontinuation of fentanyl administration has been shown to induce a negative emotional state.

OBJECTIVES

The aim of the present studies was to investigate the effects of the partial mu-opioid receptor agonist buprenorphine on the negative emotional state associated with precipitated and spontaneous fentanyl withdrawal in rats.

MATERIALS AND METHODS

Fentanyl and saline were chronically administered via osmotic minipumps. A discrete-trial 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

Naloxone induced a deficit in brain reward function in rats chronically treated with fentanyl. Buprenorphine dose-dependently prevented the naloxone-induced deficit in brain reward function. Discontinuation of fentanyl administration was also associated with a deficit in brain reward function. After explantation of the minipumps, the administration of buprenorphine induced a potentiation of brain reward function in the fentanyl-withdrawing rats, but did not affect brain reward function of saline-treated control rats. Buprenorphine prevented the somatic withdrawal signs associated with spontaneous fentanyl withdrawal and attenuated the somatic signs associated with precipitated fentanyl withdrawal.

CONCLUSIONS

Buprenorphine prevents affective and somatic fentanyl withdrawal signs. Moreover, buprenorphine is rewarding in rats previously exposed to fentanyl, but not in opioid-naïve rats. This pattern of results suggests that buprenorphine may be an effective treatment for the anhedonic-state associated with fentanyl withdrawal, but further study of buprenorphine's abuse potential is warranted.

Rewarding effects of 3,4-methylenedioxymethamphetamine ("Ecstasy") in dominant and subordinate OF-1 mice in the place preference conditioning paradigm

We tested the ability of 3,4-methylenedioxymethamphetamine (MDMA) to induce conditioned place preference (CPP) in dominant and subordinate OF-1 mice subjected to cohabitation and repeated sessions of agonistic confrontation, as well as in non-confronted mice. We selected doses of MDMA (2, 6, 10 mg/kg) previously reported to induce CPP in mice and we measured expression of c-Fos evoked by the treatments in non-confronted mice. MDMA induced c-Fos protein in several corticolimbic regions involved in drug-induced reward. Mice were exposed to brief sessions of agonistic confrontation on 5 consecutive days. Determinations of circulating hormones and drug conditioning tests were carried out on completion of the encounters. The results of hormone assays indicated that dominant mice had higher serum concentrations of testosterone, but lower levels of corticosterone, than submissive mice. Post-conditioning tests after drug conditioning (4 injections of MDMA or saline on alternate days) showed that MDMA significantly produced CPP at doses of 2 and 6 mg/kg, but not at 10 mg/kg, an inverted U-shaped pattern of conditioning that was invariable in non-confronted, dominant and subordinate mice. These results demonstrate that the endocrine and behavioural correlates linked to social status and social stress in mice are not paralleled by significant changes in the rewarding efficacy of MDMA in the CPP paradigm under the specific conditions tested.

The mu opioid receptor is involved in buprenorphine-induced locomotor stimulation and conditioned place preference

The analgesic effect of buprenorphine is mediated via the mu opioid receptor (MOP). In the present study, using mice lacking the MOP and their wild-type littermates, we determined the role of the MOP in buprenorphine-induced locomotor stimulation and conditioned place preference (CPP). Buprenorphine (3 mg/kg) increased motor activity in wild-type but not in MOP knockout mice, showing the motor stimulatory action of buprenorphine is mediated via the MOP. When the mice were given the same treatment once daily for 5 consecutive days and challenged with buprenorphine on day 11, the motor stimulatory action of buprenorphine was enhanced in wild-type but not in MOP knockout mice, showing sensitization developed to the motor stimulatory action of buprenorphine and this phenomenon was mediated via the MOP. Likewise, buprenorphine induced CPP in wild-type mice after four alternate-day saline/buprenorphine (3 mg/kg) injections paired with olfactory and visual cues. However, buprenorphine failed to induce CPP in MOP knockout mice. In contrast, amphetamine (1 mg/kg) induced a comparable CPP in wild-type and MOP knockout mice. Together, the present results suggest that the ability of buprenorphine to increase motor activity and induce locomotor sensitization and CPP is mediated via the MOP.

SR 16435 [1-(1-(bicyclo[3.3.1]nonan-9-yl)piperidin-4-yl)indolin-2-one], a novel mixed nociceptin/orphanin FQ/mu-opioid receptor partial agonist: analgesic and rewarding properties in mice

We identified a novel nociceptin/orphanin FQ (NOP)/mu-opioid receptor agonist, SR 16435 [1-(1-(bicyclo[3.3.1]nonan-9-yl)piperidin-4-yl)indolin-2-one], with high binding affinity and partial agonist activity at both receptors. It was hypothesized that SR 16435 would produce antinociception and yet, unlike morphine, would have diminished rewarding properties and tolerance development. Antinociception was assessed in mice using the tail-flick assay, whereas behavioral and rewarding effects were assessed using the place conditioning (PC) paradigm. PC was established by pairing drug injections with a distinct compartment. Behavioral effects were measured after acute and repeated drug administration, and the test for PC was carried out 24 h after four drug- and vehicle-pairing sessions. SR 16435 produced an increase in tail-flick latency, but SR 16435-induced antinociception was lower than that observed with morphine. Given that naloxone blocked SR 16435-induced antinociception, it is highly likely that this effect was mediated by mu-opioid receptors. Compared with morphine, chronic SR 16435 treatment resulted in reduced development of tolerance to its antinociceptive effects. SR 16435-induced conditioned place preference (CPP) was evident, an effect that was probably mediated via mu-opioid receptors, as it was reversed by coadministration of naloxone. NOP agonist activity was also present, given that SR 16435 decreased global activity, and this effect was partially reversed with the selective NOP antagonist, SR 16430 [1-(cyclooctylmethyl)-4-(3-(trifluoromethyl)phenyl)piperidin-4-ol]. Naloxone, however, also reversed the SR 16435-induced decrease in activity, indicating that both opioid and NOP receptors mediate this behavior. In summary, the mixed NOP/mu-opioid partial agonist SR 16435 exhibited both NOP and mu-opioid receptor-mediated behaviors.

Development of peptidic dopamine transporter inhibitors via aromatic modification-mediated conformational restriction

The dopamine transporter plays an important role in the molecular mechanism of cocaine dependence. It is suggested that inhibitors of the dopamine transporter would have strong therapeutic potential. Here we report that aromatic modification can constrain a linear peptide into the beta-turn conformation, which is preferred by the dopamine transporter. On the basis of this finding, a novel selective and competitive peptidic inhibitor of the dopamine transporter was developed. The peptide binds to the dopamine- and cocaine-binding site of the dopamine transporter and has behavioral effects different from those of cocaine in mice.

Effects of venlafaxine and desipramine on heroin-induced conditioned place preference in the rat

Venlafaxine, an antidepressant with serotonin and noradrenaline reuptake inhibiting properties, has been reported to reduce acquisition (but not maintenance) of heroin intravenous self-administration (IVSA) in rats. The present study investigated whether this phase-dependent effect is due to an antidepressant-induced attenuation of the rewarding effect of heroin, as assessed in the conditioned place preference (CPP) paradigm. In order to study the effects of venlafaxine and the tricyclic antidepressant desipramine on acquisition and expression of heroin CPP, both compounds were administered prior to the conditioning sessions (together with heroin), or prior to the expression test after conditioning, respectively. As clinical evidence indicates that antidepressants require repeated administration for full efficacy, additional experiments were performed in which both antidepressants were administered for 2 weeks prior to conditioning, or for 1 week prior to the expression test, respectively. When tested alone, heroin [0.05-3.16 mg/kg intraperitoneally (i.p.)] produced a dose-dependent CPP, whereas the antidepressants (1-21.5 mg/kg i.p.) produced neither a CPP nor a conditioned place aversion (CPA). For both antidepressants (10 mg/kg i.p.), neither acute nor repeated pretreatment affected acquisition or expression of heroin (0.5 mg/kg) CPP. Thus, the present study does not support the hypothesis that the previously observed attenuation of acquisition of heroin IVSA by venlafaxine is due to an antidepressant-induced attenuation of the rewarding effect of heroin. It is conceivable, however, that the rewarding effect of the 0.5 mg/kg dose of heroin was too pronounced to be susceptible to modulation by antidepressants. Alternatively, the modulation of acquisition of heroin IVSA in the previous study may be related to mechanisms that cannot be modelled with the CPP paradigm.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Rats maintained chronically on buprenorphine show reduced heroin and cocaine seeking in tests of extinction and drug-induced reinstatement

Buprenorphine is being introduced as a maintenance therapy in opioid addiction, but it is not clear how buprenorphine will affect co-use of cocaine in opioid users. We examined the effects of chronic buprenorphine (BUP0: 0.0 mg/kg/day; BUP1.5: 1.5 mg/kg/day; BUP3: 3.0 mg/kg/day) on the locomotor activity effects of acute heroin (0.25 mg/kg, subcutaneously (s.c.)) and cocaine (20 mg/kg, intraperitoneally (i.p.)). Buprenorphine had no effect on the stimulatory effect of heroin, but potentiated the locomotor response to cocaine. To investigate further the interactions between buprenorphine (BUP1.5 and BUP3), heroin (0.125, 0.25 and 0.375 mg/kg, s.c.), and cocaine (10, 20 and 30 mg/kg, i.p.), we used in vivo microdialysis and high-performance liquid chromatography to analyze extracellular levels of dopamine (DA) in the nucleus accumbens (NAc). Buprenorphine attenuated the heroin-induced rise in NAc DA, but greatly potentiated the cocaine-induced rise. Finally, we examined the potential of the highest dose of buprenorphine (BUP3) to reduce heroin and cocaine seeking in the presence of drug-associated cues under extinction conditions and in tests for reinstatement induced by heroin (0.25 mg/kg, s.c.), cocaine (20 mg/kg, i.p.), and 15-min footshock stress (0.8 mA, 0.5 s/shock, 40 s mean OFF time) in rats trained to self-administer both drugs. Buprenorphine reduced heroin and cocaine seeking during extinction and following acute heroin and cocaine priming injections, but had no effect on stress-induced reinstatement. These results indicate that the suppression of responding following priming injections of drugs did not result from reduced motor activity, but possibly from a reduction in the salience of drug-associated cues induced by chronic buprenorphine treatment.

Place Conditioning and Locomotor Effects of N-Substituted, 4′,4′′-Difluorobenztropine Analogs in Rats

Previous studies demonstrated that analogs of benztropine [3alpha-(diphenyl-methoxy)tropane (BZT)] bind to the dopamine (DA) transporter with high affinity, inhibit DA uptake, but do not maintain rates of responding in self-administration procedures comparable with those maintained by cocaine. Some BZT analogs have an onset of action that is slower than that for cocaine that may contribute to this decreased effectiveness. In addition, some BZT analogs have affinity for muscarinic-M1 receptors that may interfere with reinforcing effects. The present study assessed effects of BZT analogs in place-conditioning procedures designed to accommodate variations in onset of effect. BZT analogs with variations in relative affinities for the DA transporter over M1 receptors from equal [AHN 1-055 (3alpha-[bis(4'-fluorophenyl)methoxy]-tropane)] to 16-fold [JHW 007 (N-(n-butyl)-3alpha-[bis(4'-fluorophenyl)methoxy]-tropane)] were compared with cocaine and the muscarinic antagonist, atropine. Cocaine (10-20 mg/kg) but not atropine (1.0-5.6 mg/kg) produced dose-related place conditioning. The N-methyl-substituted BZT analog, AHN 1-055, was without significant effects at doses that ranged from 0.3 to 3.0 mg/kg and when administered up to 90 min before conditioning trials. In contrast, effects of AHN 2-005 (N-allyl-3alpha-[bis(4'-fluorophenyl)methoxy]-tropane; 0.1-10.0 mg/kg) were significant, and those of JHW 007 approached significance when administered 45 min but not immediately or 90 min before trials. Atropine blocked the effect of AHN 2-005 and approached significant antagonism of cocaine. The present study further supports and extends previous results showing minimal preclinical indications of abuse liability of BZT analogs and suggests that these differences from cocaine are not entirely accounted for by a slower onset of action or muscarinic M1 receptor affinity.