Effect of baclofen on morphine-induced conditioned place preference, extinction, and stress-induced reinstatement in chronically stressed mice

Rationale and Objective

A stress-induced increase in excitability can result from a reduction in inhibitory neurotransmission. Modulation of gamma-aminobutyric acid (GABA)ergic transmission is an effective treatment for drug seeking and relapse. This study investigated whether baclofen, a GABA_B receptor agonist, had an impact on morphine-induced conditioned place preference (CPP), extinction, and stress-induced relapse in chronically stressed mice.

Methods

Chronic stress was induced by restraining mice for 2 h for seven consecutive days. We first investigated whether chronic stress influenced morphine-induced CPP, extinction, and stress-induced relapse in the stressed mice. Next, we investigated whether three different doses of baclofen influenced chronic stress as measured by the expression of morphine-induced CPP. We chose the most effective dose for subsequent extinction and reinstatement experiments. Reinstatement of morphine-induced CPP was induced by a 6-min forced swim stress. Locomotor activity was also measured for each test.

Results

Chronic stress facilitated the expression of morphine-induced CPP and prolonged extinction time. Forced swim stress primed the reinstatement of morphine-induced CPP in mice. Baclofen treatment affected the impact of chronic stress on different phases of morphine-induced CPP.

Conclusions

Our results showed that baclofen antagonized the effects of chronic stress on morphine-induced CPP. These findings suggest the potential clinical utility of GABA_B receptor-positive modulators as an anti-addiction agent in people suffering from chronic stress.     

Effects of the kappa opioid receptor antagonist, norbinaltorphimine, on stress and drug-induced reinstatement of nicotine-conditioned place preference in mice

Rationale

Several studies implicate stress as a risk factor for the development and maintenance of drug addictive behaviors and drug relapse. Kappa opioid receptor (KOR) antagonists have been shown to attenuate behavioral responses to stress and stress-induced reinstatement of cocaine and ethanol seeking and preference.

Objectives

In the current study, we determined whether the selective KOR antagonist, norbinaltorphimine (nor-BNI), would block stress-induced reinstatement of nicotine preference.

Methods

Adult Institute of Cancer Research mice were conditioned with 0.5 mg/kg nicotine, injected subcutaneously (s.c.) for 3 days and tested in the nicotine-conditioned place preference (CPP) model. After 3 days extinction, nor-BNI (10 mg/kg, s.c.) was administered 16 h prior to a priming dose of nicotine (0.1 mg/kg, s.c.), and mice were tested in the CPP model for nicotine-induced reinstatement of CPP. A separate group of mice was subjected to a 2-day modified forced swim test (FST) paradigm to induce stress after 3 days extinction from CPP. Mice were given vehicle or nor-BNI (10 mg/kg, s.c.) 16 h prior to each FST session.

Results

Nor-BNI pretreatment significantly attenuated stress-induced reinstatement of nicotine-CPP, but had no effect on nicotine-primed reinstatement.

Conclusions

Blockade of KORs by selective antagonists attenuates stress-induced reinstatement of nicotine-CPP. Overall, the kappa opioid system may serve as a therapeutic target for suppressing multiple signaling processes which contribute to maintenance of smoking, smoking relapse, and drug abuse in general.     

Assessment of the impact of pattern of cocaine dosing schedule during conditioning and reconditioning on magnitude of cocaine CPP, extinction, and reinstatement

Rationale and objective

We sought to examine the impact of differing cocaine administration schedules and dosing on the magnitude of cocaine conditioned place preference (CPP), extinction, and stress- and cocaine-induced reinstatement of CPP.

Methods

First, in C57Bl/6J mice, we investigated whether total cocaine administration or pattern of drug exposure could influence the magnitude of cocaine CPP by conditioning mice with a fixed-low dose (F_L; 7.5 mg/kg; total of 30 mg/kg), a fixed-high dose (F_H; 16 mg/kg; total of 64 mg/kg), or an ascending dosing schedule (Asc; 2, 4, 8, and 16 mg/kg; total of 30 mg/kg). Next, we investigated if cocaine or saline is more effective at extinguishing preference by reconditioning mice with either a descending dosing schedule (Desc; 8, 4, 2, and 1 mg/kg) or saline. Finally, we examined if prior conditioning and reconditioning history alters stress (~2–3-min forced swim test) or cocaine-induced (3.5 mg/kg) reinstatement.

Results

We replicated and extended findings by Itzhak and Anderson (Addict. Biol. 17(4): 706–16, 2011) demonstrating that Asc conditioning produces a greater CPP than either the F_L or F_H conditioning schedules. The magnitude of extinction expressed was similar in the Desc reconditioned and saline groups. Moreover, only the saline, and not the Desc reconditioned mice, showed stress and cocaine-induced reinstatement of CPP.

Conclusions

Our results suggest that the schedule of cocaine administration during conditioning and reconditioning can have a significant influence on the magnitude of CPP and extinction of preference and the ability of cocaine or a stressor to reinstate CPP.     

Persistent reduction of cocaine seeking by pharmacological manipulation of adenosine A1 and A2A receptors during extinction training in rats

Rationale

Adenosine receptor stimulation and blockade have been shown to modulate a variety of cocaine-related behaviors.

Objectives

These studies identify the direct effects of adenosine receptor stimulation on cocaine seeking during extinction training and the persistent effects on subsequent reinstatement to cocaine seeking.

Methods

Rats self-administered cocaine on a fixed ratio one schedule in daily sessions over 3 weeks. Following a 1-week withdrawal, the direct effects of adenosine receptor modulation were tested by administering the adenosine A₁ receptor agonist, N⁶-cyclopentyladenosine (CPA, 0.03 and 0.1 mg/kg), the adenosine A_2A agonist, CGS 21680 (0.03 and 0.1 mg/kg), the presynaptic adenosine A_2A receptor antagonist, SCH 442416 (0.3, 1, and 3 mg/kg), or vehicle prior to each of six daily extinction sessions. The persistent effects of adenosine receptor modulation during extinction training were subsequently tested on reinstatement to cocaine seeking induced by cues, cocaine, and the dopamine D₂ receptor agonist, quinpirole.

Results

All doses of CPA and CGS 21680 impaired initial extinction responding; however, only CPA treatment during extinction produced persistent impairment in subsequent cocaine- and quinpirole-induced seeking. Dissociating CPA treatment from extinction did not alter extinction responding or subsequent reinstatement. Administration of SCH 442416 had no direct effects on extinction responding but produced dose-dependent persistent impairment of cocaine- and quinpirole-induced seeking.

Conclusions

These findings demonstrate that adenosine A₁ or A_2A receptor stimulation directly impair extinction responding. Interestingly, adenosine A₁ receptor stimulation or presynaptic adenosine A_2A receptor blockade during extinction produces lasting changes in relapse susceptibility.     

The facilitative effects of d-cycloserine on extinction of a cocaine-induced conditioned place preference can be long lasting and resistant to reinstatement

Rationale and objective

The N-methyl-d-aspartate receptor agonist, d-cycloserine (DCS), accelerates extinction of a cocaine-induced conditioned place preference (CPP) when given after daily extinction tests. Here, we studied the effects of DCS in rats given spaced-extinction sessions at 3- or 7-day intervals using two different extinction procedures.

Materials and methods

Rats were trained on a CPP (four cocaine, 10 mg/kg, i.p., and four saline pairings with one of two compartments). Immediately following the CPP test and all extinction tests (days 4, 7, 10, and 24, experiment 1), DCS (15 mg/kg, i.p.) or saline was administered. In experiment 2, extinction was conducted by exposing rats to the drug-paired cues for 2 or 20 min, three times, at 7-day intervals followed immediately by DCS or saline. After extinction, tests for retention and cocaine-induced reinstatement were given.

Results

In experiment 1, rats given DCS lost the cocaine CPP after one extinction trial, an effect that persisted for 2 weeks after the last DCS injection and that was resistant to cocaine-induced reinstatement. In experiment 2, extinction was facilitated by DCS compared to saline when rats received 2-min exposures to the conditioned stimulus. Longer 20-min exposures minus/plus repeated testing led to retention of extinction in both groups regardless of DCS treatment.

Conclusions

Extinction of appetitive conditioning is facilitated by DCS after 1–3 post-spaced trial injections, and retention is lasting and resistant to reinstatement. The facilitative effects appear early in extinction, but when extinction procedures are intensive, DCS appears to have no additional benefit.     

Involvement of insular muscarinic cholinergic receptors in morphine-induced conditioned place preference in rats

Rationale

Drug addiction represents a pathological usurpation of neural processes involved in learning and memory. Retrieval of drug-related memories can result in drug craving and relapse. Recently, the insula was identified as part of the neuronal circuit responsible for the processing of drug memory; however, its precise role remains unclear.

Objective

To investigate the involvement of insular muscarinic acetylcholine receptors (mAChRs) in the processing of drug memory.

Method

The morphine-induced conditioned place preference (CPP) was used to assess drug memory. All rats were first trained with morphine to establish the CPP. Sub-groups of these rats were used for contextual cue-induced CPP reinstatement. Other sub-groups of rats underwent extinction of the CPP, and 5 m/kg morphine was used for priming-induced CPP reinstatement. Microinjection of mAChR antagonists or agonists into the insula was performed prior to the CPP tests in order to evaluate their effect on CPP expression.

Results

Insular microinjections of the nonselective mAChR antagonist, scopolamine, and the M₁ antagonist, pirenzepine, significantly inhibited CPP expression in both contextual cue- and priming-induced CPP reinstatement; the M₁ agonist, MCN-A-343, and the M₄ antagonist, tropicamide, enhanced CPP expression. The M₄ agonist, LY2033298, inhibited CPP expression. The M₂ antagonist, methoctramine, and M₃ antagonist, 4-DAMP, had no effect on CPP expression.

Conclusion

Our results demonstrate that insular mAChRs play a role in the processing of drug memory. M₁ and M₄ mAChRs work paradoxically; M₁ activation and M₄ inhibition attenuate the expression of drug memory, while M₁ inhibition and M₄ activation augment the expression of drug memory.     

Beta-2 adrenergic receptors mediate stress-evoked reinstatement of cocaine-induced conditioned place preference and increases in CRF mRNA in the bed nucleus of the stria terminalis in mice

Rationale

Understanding the mechanisms responsible for stress-induced relapse is important for guiding treatment strategies aimed at minimizing the contribution of stress to addiction. Evidence suggests that these mechanisms involve interactions between noradrenergic systems and the neuropeptide corticotropin-releasing factor (CRF).

Objectives

The interaction between β-adrenergic receptors (ARs) and CRF as it relates to the reinstatement of cocaine-conditioned reward in response to a stressor was examined in mice. We hypothesized that β₂-ARs are required for stress-induced activation of CRF pathways responsible for reinstatement.

Methods

Stress-induced relapse was examined based on the re-establishment of cocaine-induced conditioned place preference (CPP; 4?×?15 mg/kg cocaine, i.p.) after extinction using forced swim (6 min at 22 °C) or an injection of the β₂-AR agonist, clenbuterol (4 mg/kg, i.p.). The CRF-R1 antagonist antalarmin (10 mg/kg, i.p.) or the β₂-AR antagonist ICI-118,551 (1 mg/kg, i.p.) were given 30 min prior to reinstating stimuli. Quantitative PCR was conducted in dissected bed nucleus of the stria terminalis (BNST) and amygdala, putative sources of CRF that contribute to reinstatement, to examine the effects of ICI-118,551 on swim-induced increases in CRF messenger RNA (mRNA) in mice with a cocaine history.

Results

Pretreatment with ICI-118,551 or antalarmin blocked swim-induced reinstatement of CPP. Reinstatement by clenbuterol was also blocked by antalarmin. ICI-118,551 pretreatment prevented swim-induced increases in CRF mRNA in the BNST. Effects in the amygdala were not observed.

Conclusions

These findings indicate that, during stress, norepinephrine, via β₂-ARs, either directly or indirectly activates CRF-releasing neurons in the BNST that interface with motivational neurocircuitry to induce reinstatement of cocaine-conditioned reward.     

Modification of the behavioral effects of morphine in rats by serotonin (5-HT)1A and 5-HT2A receptor agonists: antinociception, drug discrimination, and locomotor activity

Rationale

Indirect-acting serotonin (5-HT) receptor agonists can enhance the antinociceptive effects of morphine; however, the specific 5-HT receptor subtype(s) mediating this enhancement is not established.

Objective

This study examined interactions between morphine and both 5-HT_1A and 5-HT_2A receptor agonists in rats using measures of antinociception (radiant heat tail flick and warm water tail withdrawal), drug discrimination (3.2 mg/kg morphine versus saline), and locomotion.

Methods

Male Sprague–Dawley rats (n?=?7-8 per group) were used to examine the effects of morphine alone and in combination with DOM (5-HT_2A agonist) and 8-OH-DPAT (5-HT_1A agonist).

Results

DOM did not modify antinociceptive or discriminative stimulus effects while modestly attenuating locomotor-stimulating effects of morphine; the effect of DOM (0.32 mg/kg) on morphine-induced locomotion was prevented by the 5-HT_2A receptor-selective antagonist MDL 100907. In contrast, 8-OH-DPAT (0.032–0.32 mg/kg) fully attenuated the antinociceptive effects (both procedures), did not modify the discriminative stimulus effects, and enhanced (0.32 mg/kg) the locomotor-stimulating effects of morphine. These effects of 8-OH-DPAT were prevented by the 5-HT_1A receptor-selective antagonist WAY100635.

Conclusion

Agonists acting at 5-HT_1A or 5-HT_2A receptors do not modify all effects of mu opioid receptor agonists in a similar manner. Moreover, interactions between 5-HT and opioid receptor agonists vary significantly between rats and nonhuman primates, underscoring the value of comparing drug interactions across a broad range of conditions and in multiple species.     

Chronic psychosocial stress causes delayed extinction and exacerbates reinstatement of ethanol-induced conditioned place preference in mice

Rationale

We have shown previously, using an animal model of voluntary ethanol intake and ethanol-conditioned place preference (EtOH-CPP), that exposure to chronic psychosocial stress induces increased ethanol intake and EtOH-CPP acquisition in mice.

Objective

Here, we examined the impact of chronic subordinate colony (CSC) exposure on EtOH-CPP extinction, as well as ethanol-induced reinstatement of CPP.

Methods

Mice were conditioned with saline or 1.5 g/kg ethanol and were tested in the EtOH-CPP model. In the first experiment, the mice were subjected to 19 days of chronic stress, and EtOH-CPP extinction was assessed during seven daily trials without ethanol injection. In the second experiment and after the EtOH-CPP test, the mice were subjected to 7 days of extinction trials before the 19 days of chronic stress. Drug-induced EtOH-CPP reinstatement was induced by a priming injection of 0.5 g/kg ethanol.

Results

Compared to the single-housed colony mice, CSC mice exhibited increased anxiety-like behavior in the elevated plus maze (EPM) and the open field tests. Interestingly, the CSC mice showed delayed EtOH-CPP extinction. More importantly, CSC mice showed increased alcohol-induced reinstatement of the EtOH-CPP behavior.

Conclusion

Taken together, this study indicates that chronic psychosocial stress can have long-term effects on EtOH-CPP extinction as well as drug-induced reinstatement behavior and may provide a suitable model to study the latent effects of chronic psychosocial stress on extinction and relapse to drug abuse.     

Chronic curcumin treatment normalizes depression-like behaviors in mice with mononeuropathy: involvement of supraspinal serotonergic system and GABAA receptor

Rationale

Comorbid depression is commonly observed in individuals who suffer from neuropathic pain, which necessitates improved treatment. Curcumin, a phenolic compound derived from Curcuma longa, possesses both antinociceptive and antidepressant-like activities in animal studies, suggesting its possible usefulness in treating this comorbidity.

Objective

We investigated the effect of curcumin on depressive-like behaviors in mice with mononeuropathy, and explored the mechanism(s).

Methods

Chronic constriction injury (CCI) was produced by loosely ligating the sciatic nerves in mice. The nociceptive behaviors were examined using Hargreaves test, and the depressive-like behaviors were determined by forced swim test (FST) and tail suspension test (TST).

Results

After CCI injury, the neuropathic mice developed nociceptive and depressive-like behaviors, as shown by thermal hyperalgesia in Hargreaves test and protracted immobility time in FST and TST. Chronic treatment of neuropathic mice with curcumin (45 mg/kg, p.o., twice per day for 3 weeks) corrected their exacerbated nociceptive and depressive-like behaviors, which was abolished by chemical depletion of brain serotonin rather than noradrenaline. The paralleled antinociceptive and antidepressant-like actions of curcumin seem to be pharmacologically segregated, since intrathecal and intracerebroventricular injection of methysergide, a nonselective 5-HT receptor antagonist, separately counteracted the two actions of curcumin. Further, this antidepression was abrogated by repeated co-treatment with 5-HT_1A receptor antagonist WAY-100635 and greatly attenuated by acute co-treatment with GABA_A receptor antagonist bicuculline.

Conclusion

Curcumin can normalize the depressive-like behaviors of neuropathic mice, which may be independent of the concurrent analgesic action and possibly mediated via the supraspinal serotonergic system and downstream GABA_A receptor.     

Blockade of 5-HT2A receptors in the medial prefrontal cortex attenuates reinstatement of cue-elicited cocaine-seeking behavior in rats

Rationale

The action of serotonin (5-HT) at the 5-HT_2A receptor subtype is thought to be involved in cocaine-seeking behavior that is motivated by exposure to drug-associated cues and drug priming. 5-HT_2A receptors are densely clustered in the ventromedial prefrontal cortex (vmPFC), an area that plays a role in mediating cocaine-seeking behavior.

Objectives

This study examined the hypothesis that M100907, a 5-HT_2A receptor antagonist, infused directly in the vmPFC attenuates cue- and cocaine-primed reinstatement of cocaine-seeking behavior.

Methods

Rats trained to self-administer cocaine (0.75?mg/kg, i.v.) paired with light and tone cues underwent extinction training during which operant responses produced no consequences. Once behavior extinguished, rats were tested for reinstatement of responding elicited by either response-contingent presentations of the cocaine-paired light/tone cues or by cocaine-priming injections (10?mg/kg, i.p.) within 1?min after pretreatment with microinfusions of M100907 (0.1, 0.3, 1.0, or 1.5???g/0.2???l/side) into the vmPFC.

Results

Intra-vmPFC M100907 decreased cue-elicited reinstatement at the two highest doses (1.0 and 1.5???g) but produced only a slight decrease in cocaine-primed reinstatement that was not dose dependent. The decrease in cue reinstatement was not likely due to impaired ability to respond since intra-vmPFC M100907 infusions had minimal effect on cocaine self-administration and no effect on cue-elicited sucrose-seeking behavior, or spontaneous or cocaine-induced locomotion. M100907 infusions into the adjacent anterior cingulate cortex had no effect on cue reinstatement.

Conclusions

The results suggest that the blockade of 5-HT_2A receptors in the vmPFC selectively attenuates the incentive motivational effects of cocaine-paired cues.     

Influence of abstinence and intervals between extinction trials on the expression of cocaine-conditioned place preference in adolescent rats

Rationale

Disruption of acquired drug–cue associations can effectively decrease relapse. The benefits of extinction training as opposed to abstinence have been reported. Timing of extinction trials is an important variable. Finding an effective extinction regimen can optimize addiction therapies.

Objective

To determine the effects of different drug-free periods on cocaine-conditioned place preference (CPP) in rats that either did or did not receive non-reinforced exposure to drug-associated stimuli.

Materials and methods

Male adolescent rats were trained for cocaine-CPP (5, 10, or 15 mg/kg, i.p.) in a biased manner for 8 days and then tested following different intervals.

Results

Rats treated with 15 mg/kg cocaine displayed high and equal CPP on the first test, performed 1, 4, 7, or 14 days following conditioning. Expression of CPP during the test performed 1 day after conditioning was equal in the groups conditioned with 5, 10, or 15 mg/kg cocaine. When the interval before the first test was extended to 14 days, the group treated with 5 mg/kg did not show CPP. Rats treated with the three doses and tested repeatedly at 1, 7, and 14 days did not display CPP on the third test. CPP after treatment with 10 or 15 mg/kg cocaine was already extinguished in the second test but only for an interval of 1–14 days.

Conclusions

Maintenance of CPP was evident at least 2 weeks after forced abstinence. Extinguished CPP can be obtained after a single extinction trial, performed close to original training and followed by prolonged abstinence. However, with low doses of cocaine, abstinence alone may be sufficient to disrupt drug–cue associations.     

Electrolytic lesions and pharmacological inhibition of the dorsal raphe nucleus prevent stressor potentiation of morphine conditioned place preference in rats

Rationale Exposure to a single session of uncontrollable inescapable shock (IS), but not to identical controllable escapable shock, produces a potentiation of morphine's rewarding properties that is unusual in that the stressor can be given a number of days before the drug administration in an environment quite different from the drug context. Many other behavioral outcomes of stressors that depend on the uncontrollability of the stressor are mediated by alterations in serotonergic (5-HT) neurons within the dorsal raphe nucleus (DRN).Objectives The present experiments examined the role of the DRN and 5-HT in mediating the effect of IS on the rewarding properties of morphine as assessed by conditioned place preference (CPP).Methods In experiment 1, subjects received small electrolytic lesions of the DRN and were tested for morphine (3.0 mg/kg, SC) CPP after IS or control treatment. In experiment 2, subjects received an intra-DRN microinjection of the 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 1.0 g/0.5 l) either before IS or before morphine (3.0 mg/kg, SC) injections during CPP testing.Results IS potentiated morphine CPP in controls, but both DRN lesion and intra-DRN 8-OH-DPAT, either before IS or before morphine administration, completely blocked this effect.Conclusions These data implicate alterations in DRN 5-HT neurons in the potentiation of morphine reward produced by uncontrollable stress.     

Cannabidiol enhances consolidation of explicit fear extinction in humans

Rationale

Whilst Cannabidiol (CBD), a non-psychotomimetic cannabinoid, has been shown to enhance extinction learning in rats, its effects on fear memory in humans have not previously been studied.

Objectives

We employed a Pavlovian fear-conditioning paradigm in order to assess the effects of CBD on extinction and consolidation.

Method

Forty-eight participants were conditioned to a coloured box (CS) with electric shocks (UCS) in one context and were extinguished in a second context. Participants received 32 mg of CBD either following before or after extinction in a double-blind, placebo-controlled design. At recall, 48 h later, participants were exposed to CSs and conditioning contexts before (recall) and after (reinstatement) exposure to the UCS. Skin conductance and shock expectancy measures of conditioned responding were recorded throughout.

Results

Successful conditioning, extinction and recall were found in all three treatment groups. CBD given post-extinction enhanced consolidation of extinction learning as assessed by shock expectancy. CBD administered at either time produced trend level reduction in reinstatement of autonomic contextual responding. No acute effects of CBD were found on extinction.

Conclusions

These findings provide the first evidence that CBD can enhance consolidation of extinction learning in humans and suggest that CBD may have potential as an adjunct to extinction-based therapies for anxiety disorders.     

No evidence for enhanced extinction memory consolidation through noradrenergic reuptake inhibition—delayed memory test and reinstatement in human fMRI

Rationale

One promising approach in the current ambition to maximise treatment benefit for anxiety disorders is the pharmacological enhancement of cognitive–behavioural treatment efficacy, which can be experimentally modelled by pharmacological enhancement of extinction learning/consolidation. Noradrenaline (NA) is involved in memory consolidation, and NAergic innervations are found in brain areas implicated in fear conditioning and extinction.

Objectives

Thus, to enhance extinction memory consolidation through boosted NAergic signalling, we administered 4 mg reboxetine (RBX) immediately after extinction learning (day 2, 24 h after conditioning on day 1) in a randomised, placebo (PLC)-controlled design. At a delayed memory test (day 8), we probed cued and contextual fear and extinction memories before and after a reinstatement manipulation.

Results

After reinstatement, we find significantly enhanced amygdala and posterior hippocampus activation in the RBX group, areas implicated in fear memory expression, while the PLC group exhibited enhanced activation in areas associated with extinction memory expression (vmPFC, anterior hippocampus). No group differences were found in skin conductance responses.

Conclusions

Thus, our data do not support our hypothesis that enhancement of NA signalling may facilitate extinction memory consolidation and provide preliminary evidence that this might rather enhance fear memories on a neural but not physiological (skin conductance responses) level.     

Topiramate increases the rewarding properties of cocaine in young-adult mice limiting its clinical usefulness

Rationale

Topiramate is an anticonvulsant drug which has been evaluated as a therapeutic option for the treatment of cocaine addiction during the last decade.

Objectives

The purpose of this study was to evaluate the effects of topiramate on the reinforcing actions of cocaine. To this aim, the topiramate-mediated regulation of acquisition and extinction phases of the cocaine conditioned place preference (CPP) was assessed in young-adult mice using three experimental designs.

Methods

Topiramate (50 mg/kg, p.o.) was given as follows: (1) during cocaine (1 and 25 mg/kg, i.p.) conditioning sessions (4 days) and cocaine (25 mg/kg) post-conditioning session; (2) 2 weeks before and during cocaine conditioning (25 mg/kg); and (3) during extinction of CPP induced by cocaine (25 mg/kg). In the first experimental design, changes in tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene expressions were measured in the ventral tegmental area (VTA).

Results

Topiramate significantly increased cocaine-induced CPP and delayed or failed to produce extinction after the first cocaine reinstatement extinction in the first and second experiments. Furthermore, treatment with topiramate after place conditioning blocked the extinction of cocaine-induced CPP. TH and DAT gene expression in the VTA was significantly lower both with topiramate alone and in combination with cocaine compared with animals receiving only cocaine.

Conclusions

These findings suggest that topiramate increases the rewarding properties of cocaine, at least in part, by regulating dopaminergic signaling in the mesolimbic circuit. Consequently, the results of this study do not support the use of topiramate for the treatment of problems related to cocaine dependence.

Highlights

? Topiramate increases the rewarding properties of cocaine in CPP? Topiramate alters dopaminergic signaling in the mesolimbic circuit? Topiramate delays the extinction of cocaine-induced CPP? TH and DAT gene expression in the VTA decreases with topiramate and/or with cocaine? Results show that it should limit the use of topiramate in cocaine-dependent subjects

     

Effects of cocaine combined with a social cue on conditioned place preference and nucleus accumbens monoamines after isolation rearing in rats

Rationale

Social interaction during drug exposure can potentiate cocaine reward. Isolation rearing (ISO) during adolescence increases social interaction and may amplify this potentiation.

Objectives

The objectives of this study are to determine whether ISO alters conditioned place preference (CPP) for cocaine when combined with a social cue and to determine whether ISO alters the effects of cocaine when combined with social cue on nucleus accumbens shell (NAcS) dopamine (DA) and serotonin (5-HT).

Methods

Male and female rats were either ISO or group (GRP) reared for 4 weeks during adolescence. CPP was performed using a low dose of cocaine (2 mg/kg or saline) with or without exposure to a novel same-sex conspecific during conditioning. In vivo microdialysis was performed using the same parameters.

Results

ISO rats engaged in more social and aggressive behaviors during conditioning relative to GRP. Cocaine reduced social and aggressive behaviors in all rats. CPP was not influenced by rearing condition. Cocaine produced significant CPP, and a social cue produced CPP only in males. In contrast, the interaction of cocaine and a social cue on NAcS DA and 5-HT differed depending upon rearing condition. In isolates, cocaine-induced DA was attenuated, while cocaine plus a social cue produced potentiated DA and 5-HT.

Conclusions

Exposure to a low dose of cocaine in the presence of a social cue produced additive effects on CPP while producing synergistic effects on DA and 5-HT in the NAcS of ISO rats. The aversive effects of this compound stimulus may negate the rewarding effects in isolates.     

CB1 antagonism: interference with affective properties of acute naloxone-precipitated morphine withdrawal in rats

Rationale

Modulation of the endocannabinoid system has been found to interfere with opiate withdrawal. The potential of activation and blockade of the endocannabinoid system to prevent the aversive-affective state of naloxone-precipitated morphine withdrawal (MWD) was investigated in a one-trial conditioned place aversion (CPA) paradigm.

Objective

CPA provides a sensitive measure of the motivational effects of acute MWD. The potential of the fatty acid amide hydrolase (FAAH) inhibitors, URB597 and PF-3845, the CB₁ antagonist/inverse agonist, AM251, and the neutral CB₁ antagonists, AM4113 and AM6527 (oral), to interfere with establishment of a MWD-induced CPA was investigated. As well, the potential of AM251 and AM4113 to interfere with reinstatement of a previously established MWD-induced CPA was investigated.

Materials and methods

Using a one-trial place conditioning paradigm, rats were administered naloxone (1 mg/kg, subcutaneous (sc)) 24 h after receiving a high dose of morphine (20 mg/kg, sc) and were placed on the conditioning floor. To determine the effect of each pretreatment drug on the establishment of the MWD-induced CPA, URB597 (0.3 mg/kg, intraperitoneally (ip)), PF-3845 (10 mg/kg, ip), AM251 (1 or 2.5 mg/kg, ip), AM4113 (1 or 2.5 mg/kg, ip), and AM6527 (5 mg/kg, oral) were administered prior to conditioning.

Results

AM251 (2.5, but not 1 mg/k), AM4113, and AM6527, but not URB597 or PF-3845, interfered with the establishment of the MWD-induced CPA. AM251 and AM4113 did not prevent reinstatement of the CPA.

Conclusions

Neutral antagonism of the CB₁ receptor reduces the aversive affective properties of morphine withdrawal.     

Role of gamma-aminobutyric acid type A (GABAA) receptor subtypes in acute benzodiazepine physical dependence-like effects: evidence from squirrel monkeys responding under a schedule of food presentation

Rationale

Assays of schedule-controlled responding can be used to characterize the pharmacology of benzodiazepines and other GABA_A receptor modulators, and are sensitive to changes in drug effects that are related to physical dependence.

Objective

The present study used this approach to investigate the role of GABA_A receptor subtypes in mediating dependence-like effects following benzodiazepine administration.

Methods

Squirrel monkeys (n?=?6) were trained on a fixed-ratio schedule of food reinforcement. Initially, the response rate-decreasing effects of chlordiazepoxide (0.1–10 mg/kg; nonselective GABA_A receptor agonist), zolpidem (0.032–1.0 mg/kg; α1 subunit-containing GABA_A subtype-preferring agonist), and HZ-166 (0.1–10 mg/kg; functionally selective α2 and α3 subunit-containing GABA_A receptor agonist) were assessed. Next, acute dependence-like effects following single injections of chlordiazepoxide, zolpidem, and HZ-166 were assessed with flumazenil (0.1–3.2 mg/kg; nonselective GABA_A receptor antagonist). Finally, acute dependence-like effects following zolpidem administration were assessed with βCCt and 3-PBC (0.1–3.2 mg/kg and 0.32–10 mg/kg, respectively; α1 subunit-containing GABA_A receptor antagonists).

Results

Chlordiazepoxide, zolpidem, and HZ-166 produced dose- and time-dependent decreases in response rates, whereas flumazenil, βCCT, and 3-PBC were ineffective. After the drug effects waned, flumazenil produced dose-dependent decreases in response rates following administration of 10 mg/kg chlordiazepoxide and 1.0 mg/kg zolpidem, but not following any dose of HZ-166. Further, both βCCT and 3-PBC produced dose-dependent decreases in response rates when administered after 1.0 mg/kg zolpidem.

Conclusions

These data raise the possibility that α1 subunit-containing GABA_A receptors play a major role in physical dependence-related behaviors following a single injection of a benzodiazepine.     

Discriminative stimulus effects of pregnanolone in rhesus monkeys

Rationale

Neuroactive steroids and benzodiazepines can positively modulate GABA by acting at distinct binding sites on synaptic GABA_A receptors. Although these receptors are thought to mediate the behavioral effects of both benzodiazepines and neuroactive steroids, other receptors (e.g., extrasynaptic GABA_A, N-methyl-d-aspartate (NMDA), σ₁, or 5-HT₃ receptors) might contribute to the effects of neuroactive steroids, accounting for differences among positive modulators.

Objective

The current study established the neuroactive steroid pregnanolone as a discriminative stimulus to determine whether actions in addition to positive modulation of synaptic GABA_A receptors might contribute to its discriminative stimulus effects.

Methods

Four rhesus monkeys discriminated 5.6 mg/kg pregnanolone while responding under a fixed-ratio 10 schedule of stimulus-shock termination.

Results

Positive modulators acting at benzodiazepine, barbiturate, or neuroactive steroid sites produced ≥80 % pregnanolone-lever responding, whereas drugs acting primarily at receptors other than synaptic GABA_A receptors, such as extrasynaptic GABA_A, NMDA, σ₁, and 5-HT₃ receptors, produced vehicle-lever responding. Flumazenil antagonized the benzodiazepines midazolam and flunitrazepam, with Schild analyses yielding slopes that did not deviate from unity and pA₂ values of 7.39 and 7.32, respectively. Flumazenil did not alter the discriminative stimulus effects of pregnanolone.

Conclusion

While these results do not exclude the possibility that pregnanolone acts at receptors other than synaptic GABA_A receptors, they indicate a primary and possibly exclusive role of synaptic GABA_A receptors in its discriminative stimulus effects. Reported differences in the effects of benzodiazepines and neuroactive steroids are not due to differences in their actions at synaptic GABA_A receptors.