Effects of dopamine antagonists with different receptor blockade profiles on morphine-induced place preference in male mice

Haloperidol disrupts opioid-antinociceptive tolerance and physical dependence

Previous studies from our laboratory and others have implicated a critical role of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in opioid tolerance and dependence. Translational research targeting the CaMKII pathway is challenging, if not impossible, because of a lack of selective inhibitors. We discovered in a preliminary study that haloperidol, a butyrophenone antipsychotic drug, inhibited CaMKII, which led us to hypothesize that haloperidol can attenuate opioid tolerance and dependence by inhibiting CaMKII. The hypothesis was tested in two rodent models of opioid tolerance and dependence. Pretreatment with haloperidol (0.2-1.0 mg/kg i.p.) prevented the development of morphine tolerance and dependence in a dose-dependent manner. Short-term treatment with haloperidol (0.06-0.60 mg/kg i.p.) dose-dependently reversed the established morphine-antinociceptive tolerance and physical dependence. Correlating with behavioral effects, pretreatment or short-term treatment with haloperidol dose-dependently inhibited morphine-induced up-regulation of supraspinal and spinal CaMKIIα activity. Moreover, haloperidol given orally was also effective in attenuating morphine-induced CaMKIIα activity, antinociceptive tolerance, and physical dependence. Taken together, these data suggest that haloperidol attenuates opioid tolerance and dependence by suppressing CaMKII activity. Because haloperidol is a clinically used drug that can be taken orally, we propose that the drug may be of use in attenuating opioid tolerance and dependence.

Dissociable roles of mGlu5 and dopamine receptors in the rewarding and sensitizing properties of morphine and cocaine

RATIONALE

Drugs of abuse are initially used because of their rewarding properties. As a result of repeated drug exposure, sensitization to certain behavioral effects of drugs occurs, which may facilitate the development of addiction. Recent studies have implicated the metabotropic glutamate receptor 5 (mGlu5 receptor) in drug reward, but its role in sensitization is unclear. Stimulation of dopamine receptors plays an important role in drug reward, but not in the sensitizing properties of cocaine and morphine.

OBJECTIVE

This study aims to evaluate the role of mGlu5 and dopamine receptors in the development of cocaine- and morphine-induced conditioned place preference (CPP) and psychomotor sensitization.

MATERIALS AND METHODS

Rats were treated with the mGlu5 receptor antagonist MTEP (0, 1, 3, and 10 mg/kg, i.p.) or the dopamine receptor antagonist α-flupenthixol (0, 0.125, 0.25, and 0.5 mg/kg, i.p.) during place conditioning with either morphine (3 mg/kg, s.c.) or cocaine (15 mg/kg, i.p.). Furthermore, MTEP (1 mg/kg, i.p.) or α-flupenthixol (0.5 mg/kg, i.p.) was co-administered during cocaine (30 mg/kg, i.p.) or morphine (3.0 mg/kg, s.c.) pretreatment and psychomotor sensitization was tested 3 weeks post-treatment.

RESULTS

MTEP attenuated the development of morphine- but not cocaine-induced CPP. In contrast, MTEP suppressed the development of cocaine- but not morphine-induced psychomotor sensitization. α-Flupenthixol blocked the development of both cocaine- and morphine-induced CPP but did not affect the development of sensitization to either drug.

CONCLUSION

Dopamine receptor stimulation mediates cocaine and morphine reward but not sensitization. In contrast, the role of mGlu5 receptors in reward and sensitization is drug-specific.

Preclinical evidence of new opioid modulators for the treatment of addiction

IMPORTANCE OF THE FIELD

Addiction to opiates is one of the most severe forms of substance dependence, and despite a variety of pharmacological approaches to treat it, relapse is observed in a high percentage of subjects. New pharmacological compounds are necessary to improve the outcome of treatments and reduce adverse side effects. Moreover, drugs that act on the opioid system can also be of benefit in the treatment of alcohol or cocaine addiction. AREA COVERED BY THIS REVIEW: Recent preclinical studies of pharmacological agents for the treatment of opiate addiction (2008 to the present date).

WHAT THE READER WILL GAIN

The reader will be informed of the latest drugs shown in animal models to modify dependence on opiates and the reinforcing effects of these drugs. In addition, reports of the latest studies to test these compounds in models of other drug addictions are reviewed.

TAKE HOME MESSAGE

The classic clinical pharmacotherapy for opiate dependence, involving mu-opioid receptor agonists or antagonists, has not yielded a high success rate in humans. In pharmacotherapy for opioid dependence, new options are emerging and different pharmacological strategies are now being tested.

Enhanced tyrosine hydroxylase phosphorylation in the nucleus accumbens and nucleus tractus solitarius-A2 cell group after morphine-conditioned place preference

Although dopamine (DA) has been extensively implicated in the morphine-induced conditioned place preference (CPP; a measure of reward), noradrenaline (NA) and other systems may play a larger role than previously suspected. The mesolimbic DA system, comprised of projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAc), receives noradrenergic innervations from the nucleus tractus solitaries (NTS)-A2 cell group and is modulated by NA. The purpose of the present study was to evaluate the turnover of DA and NA in the NAc and the site-specific phosphorylation of TH in the NAc, VTA, and NTS on the CPP mice conditioned by morphine. A dose-effect curve for morphine-induced CPP (0.5-8 mg/kg, s.c.) was obtained using 6-day conditioning sessions followed by a CPP test. TH phosphorylation was determined by quantitative blot immunolabeling and immunohistochemistry using phosphorylation state-specific antibodies; NA and DA turnover was evaluated by high-performance liquid chromatography. Morphine-induced CPP phosphorylates TH at serine (Ser)40 but not Ser31 in NAc, which is associated with an enhanced of DA and NA turnover. We also found that morphine-induced CPP increased levels of TH phosphorylated at Ser31 and Ser40 in the NTS. The present study demonstrates that morphine-induced CPP might stimulate TH activity and accelerate DA and NA turnover in the NAc via a mechanism involving phosphorylation of TH.

Effect of the CB1 cannabinoid agonist WIN 55212-2 on the acquisition and reinstatement of MDMA-induced conditioned place preference in mice

Background

Numerous reports indicate that MDMA users consume other psychoactive drugs, among which cannabis is one of the most common. The aim of the present study was to evaluate, using the conditioned place preference, the effect of the cannabinoid agonist WIN 55,212-2 on the rewarding effects of MDMA in mice.

Methods

In the first experiment adolescent mice were initially conditioned with 1.25, 2.5 or 5 mg/kg of MDMA or 0.1 or 0.5 mg/kg of WIN and subsequently with both drugs. Reinstatement of the extinguished preference by priming doses was performed in the groups that showed CPP. In the second experiment, animals were conditioned with 2.5 or 5 mg/kg of MDMA and, after extinction, reinstatement of the preference was induced by 0.5 or 0.1 mg/kg of WIN.

Results

A low dose of WIN 55212-2 (0.1 mg/kg) increased the rewarding effects of low doses of MDMA (1.25 mg/kg), although a decrease in the preference induced by MDMA (5 and 2.5 mg/kg) was observed when the dose of WIN 55212-2 was raised (0.5 mg/kg). The CB1 antagonist SR 141716 also increased the rewarding effects of the lowest MDMA dose and did not block the effects of WIN. Animals treated with the highest WIN dose plus a non-neurotoxic dose of MDMA exhibited decreases of striatal DA and serotonin in the cortex. On the other hand, WIN 55212-2-induced CPP was reinstated by priming injections of MDMA, although WIN did not reinstate the MDMA-induced CPP.

Conclusions

These results confirm that the cannabinoid system plays a role in the rewarding effects of MDMA and highlights the risks that sporadic drug use can pose in terms of relapse to dependence. Finally, the potential neuroprotective action of cannabinoids is not supported by our data; on the contrary, they are evidence of the potential neurotoxic effect of said drugs when administered with MDMA.

Effect of adolescent exposure to WIN 55212-2 on the acquisition and reinstatement of MDMA-induced conditioned place preference

The present study employs a conditioned place preference procedure (CPP) to examine the effects of exposure to the cannabinoid agonist WIN 55212-2 (WIN) (0.1 and 0.5mg/kg) during adolescence on the reinforcing properties of +/-3,4-methylenedioxymetamphetamine hydrochloride (MDMA) (1.25 and 2.5mg/kg) in mice. On postnatal day (PD) 27, animals received a daily injection of the assigned treatment on 5 consecutive days, and three days later the place conditioning procedure was initiated (PD 35). The results suggest that pre-exposure to cannabinoids strengthens the properties of MDMA and favors reinstatement of the craving for the drug, which endorses the gateway hypothesis.

Social experiences affect reinstatement of cocaine-induced place preference in mice

RATIONALE

Drug addiction is a multifactorial disorder resulting from an interaction between genetic and environmental factors, and negative and positive environmental conditions may increase or reduce, respectively, vulnerability to drug addiction.

OBJECTIVES

The influence of different social experiences on the acquisition, extinction, and reinstatement of a cocaine-induced conditioned place preference (CPP) was evaluated.

METHODS

In experiment 1, adolescent and adult male OF1 mice housed under four different conditions (grouped, isolated, crowded, and cohabitating with a female) were conditioned with 50, 12.5, or 3.125 mg/kg of cocaine. All mice underwent extinction sessions until the CPP was extinguished. The effects of cocaine priming injections on the reinstatement of CPP were then evaluated. In experiment 2, the effect of different social experiences on the maintenance and reinstatement of cocaine-CPP in adult mice was studied.

RESULTS

Although housing conditions did not affect the acquisition of cocaine-CPP, it did modify reinstatement after extinction. Adolescent mice living in crowded conditions or cohabitating with a female did not present reinstatement after cocaine priming. Similarly, neither isolated adult mice nor adults cohabitating with a female presented reinstatement. In grouped adult mice, isolation after acquisition of the CPP and social defeat before reinstatement increased the vulnerability to reinstatement induced by cocaine priming. Conversely, both exposure to females and a brief social interaction undermined cocaine-induced reinstatement.

CONCLUSIONS

Social experiences modify vulnerability to reinstatement, acting as prevention or risk factors in the development of drug addiction.

Acquisition and reinstatement of MDMA-induced conditioned place preference in mice pre-treated with MDMA or cocaine during adolescence

Those who take ecstasy are more likely to consume other drugs than non-users with cocaine abuse being reported by 75.5% of high school student MDMA (+/- 3,4-methylenedioxymetamphetamine hydrochloride) users. The aim of this work was to evaluate the effects of exposure during adolescence to MDMA, cocaine or to both drugs on the MDMA-induced conditioned place preference (CPP) in adult mice. Animals received two daily administrations of saline, 10 mg/kg of MDMA, 25 mg/kg of cocaine or 10 mg/kg of MDMA plus 25 mg/kg of cocaine over 3 days (from PD28 to 30). Three weeks after pre-treatment, the MDMA-induced CPP procedure was initiated (PD52). Acquisition of CPP was induced with a sub-threshold dose of MDMA (1.25 mg/kg) only in animals treated during adolescence with MDMA alone. Preference was established in all the groups after conditioning with 10 mg/kg of MDMA, while the time required to achieve extinction was longer in those pre-treated with cocaine or MDMA alone (46 and 28 sessions, respectively). Moreover, preference was reinstated with progressively lower priming doses of MDMA in mice pre-treated with MDMA or cocaine alone. These results demonstrate that early exposure to MDMA or cocaine induces long-lasting changes that last until adulthood and modify the response of animals to MDMA.

Memantine blocks sensitization to the rewarding effects of morphine

Knowledge regarding the specific brain changes and neural plasticity processes produced by repeated drug exposure may be used to advance the understanding of the neurobiology of addiction in order to design appropriate medications. In the present study, the influence of N-methyl-d-aspartate (NMDA) glutamatergic receptors on sensitization to the motor and rewarding effects of morphine was evaluated. The effects of pre-exposure to saline or 20 mg/kg morphine plus the NMDA receptor antagonist memantine (10 or 20 mg/kg) on motor activity and place conditioning induced by a low dose of morphine (2 mg/kg) were assessed. The dose of 2 mg/kg of morphine was ineffective in mice pre-exposed to saline but induced a clear conditioned place preference (CPP) in those pre-exposed to morphine. Conversely, animals pre-exposed to morphine plus memantine did not acquire CPP. Only those pre-exposed to morphine presented an increased motor response to morphine 2 mg/kg. Our results demonstrate that NMDA glutamatergic receptors are implicated in the development of sensitization to the conditioned rewarding effects of morphine.

The preferential dopamine D3 receptor antagonist S33138 inhibits cocaine reward and cocaine-triggered relapse to drug-seeking behavior in rats

We have previously reported that selective dopamine (DA) D3 receptor antagonists are effective in a number of animal models of drug addiction, but not in intravenous drug self-administration, suggesting a limited ability to modify drug reward. In the present study, we evaluated the actions ofS33138, a novel partially selective D3 receptor antagonist, in animal models relevant to drug addiction. S33138, at doses of 0.156 or 0.625 mg/kg (i.p.), attenuated cocaine-enhanced brain-stimulation reward (BSR), and the highest dose tested (2.5 mg/kg) produced a significant aversive-like rightward shift in BSR rate-frequency reward functions. Further, S33138 produced biphasic effects on cocaine self-administration, i.e., a moderate dose (2.5 mg/kg, p.o.) increased, while a higher dose (5 mg/kg, p.o.) inhibited, cocaine self-administration. The increase in cocaine self-administration likely reflects a compensatory response to a partial reduction in drug reward after S33138. In addition, S33138 (0.156-2.5 mg/kg, p.o.) also dose-dependently inhibited cocaine-induced reinstatement of drug-seeking behavior. The reduction in cocaine-enhanced BSR and cocaine-triggered reinstatement produced by lower effective doses (e.g., 0.156 or 0.625 mg/kg) of 533138 is unlikely due to impaired locomotion, as lower effective doses of S33138 decreased neither Ymax levels in the BSR paradigm, rotarod performance, nor locomotion. However, the higher doses (2.5 or 5 mg/kg) of S33138 also significantly inhibited sucrose self-administration and rotarod performance, suggesting non-D3 receptor-mediated effects on non-drug reward and locomotion. These data suggest that lower doses of S33138 interacting essentially with D3 receptors have pharmacotherapeutic potential in treatment of cocaine addiction, while higher doses occupying D2 receptors may influence locomotion and non-drug reward.

Enhancing effects of morphine on methamphetamine-induced reinforcing behavior and its association with dopamine release and metabolism in mice

Polydrug abuse has become a significant problem worldwide, and the combined use of methamphetamine (MA) and morphine (M) is now highly prevalent among addicts. In the present study, we investigated the neurobehavioral effects of repeated treatment regimens of these drugs (i.p. administration of 0.75 mg/kg/day MA, 5 mg/kg/day M, and their combination for five consecutive days followed by once weekly for five consecutive weeks) in mice. In addition, we used an in vivo microdialysis technique to study the changes in extracellular concentrations of dopamine (DA) and its metabolites in the mouse striatum after challenge administration of these drugs. The results showed that systemic M increased MA-induced conditioned place preference (CPP), as revealed by higher CPP values which were also maintained for a longer duration compared with those induced by an identical dose of MA or M alone. Subsequent to challenge with combined MA and M, mice exhibited an increase in stereotyped behavior, which appeared to be associated with an elevation of extracellular concentration of DA in the striatum. Our findings suggest that M not only produces synergistic effects on MA-induced CPP, but also interacts with MA to induce stereotyped behavioral sensitization which is mediated by an increase in DA outflow in the striatum. These findings provide insight into the behavioral and neurochemical basis responsible for the combined abuse liability of MA and M.

Effects of extended cocaine conditioning in the reinstatement of place preference

Rats allowed extended access to cocaine self-administration develop a number of symptoms of addiction, such as greater susceptibility to drug-induced relapse. Using the conditioned place preference (CPP), the number of conditioning training sessions was increased in order to augment exposure to contextual cues associated with the effects of a drug. Mice were conditioned with a steady dose of 6 or 25 mg/kg of cocaine for 4, 8, 12, 16, 20 or 40 days. Weekly sessions of extinction followed the establishment of preference, after which a priming dose of cocaine was administered to reinstate the extinguished preference. The magnitude of the place preference effect was equal in all groups, independently of the number of conditioning sessions. The persistence of the place preference was not related with the number of sessions. Higher responsiveness to reinstatement of the extinguished preference occurred only with an intermediate number of conditioning sessions. In this way, the relation between the number of training sessions and vulnerability to relapse appeared to follow an inverted U-shaped function. Our results suggest that increasing the number of conditioning sessions from 12 to up to 16, without increasing the amount of drug administered, can be of great use in the study of vulnerability to relapse.

Effects of reversible inactivation of the ventral tegmental area on the acquisition and expression of morphine-induced conditioned place preference in the rat

The mesolimbic dopaminergic system that projects from the ventral tegmental area (VTA) to the nucleus accumbens is critical for initiation of opioid reinforcement and reward-related effects of drugs of abuse. In the present study, the effects of reversible inactivation of VTA on acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in rats. One hundred and eighty-one adult male albino Wistar rats were used in these experiments. Reversible inactivation of VTA was done through the unilateral and bilateral intra-VTA microinjection of 2% lidocaine during the acquisition and expression of morphine (5 mg/kg; s.c.)-induced CPP. Animal displacement, conditioning score and locomotor activity were recorded by Ethovision software. The results showed that bilateral but not unilateral intra-VTA administration of lidocaine significantly decreases the acquisition (P<0.01) and expression (P<0.05) of morphine-induced CPP compared to their respective saline-microinjected groups. Moreover, intra-VTA administration of lidocaine had no effect on locomotor activity in these experiments. Our results further support the idea that VTA may play an important role in the acquisition of morphine-induced CPP. In addition, there is no functional lateralization in the VTA for acquisition and/or expression of morphine-induced CPP in the rat.

Effect of adolescent exposure to MDMA and cocaine on acquisition and reinstatement of morphine-induce CPP

It is well known that an elevated percentage of ecstasy users also consume cocaine. Recently, it has been reported that a high frequency of heroin smokers first consumed heroin under the effects of ecstasy with the hope of reducing the stimulant effects of the latter drug. The aim of the present study was to evaluate the effect of exposure to MDMA and cocaine during adolescence on morphine-induced conditioned place preference (CPP) and reinstatement in adulthood. In the first experiment, adolescent mice were exposed to six injections of MDMA and three weeks later their response to the reinforcing properties of 40 mg/kg of morphine was evaluated using the CPP paradigm. All the treatment groups developed the same magnitude of morphine-induced preference and, after CPP was extinguished, it was restored in all the groups with a priming dose of 10 mg/kg of morphine. Only mice that had been treated with 10 or 20 mg/kg of MDMA had their morphine-induced preference reinstated after receiving only 5 mg/kg of morphine. In the second experiment, adolescent mice were similarly treated with six administrations of cocaine (25 mg/kg) or cocaine plus MDMA (5, 10 or 20 mg/kg), and their response to morphine-induce CPP was evaluated three weeks later. Similarly to the first experiment, all the groups developed a preference for the morphine-paired compartment, but this preference was not reinstated with a priming dose of 10 mg/kg of morphine following extinction, as was the case among the control animals. These results lead us to hypothesize that periadolescent MDMA exposure alters responsiveness to the rewarding properties of morphine, highlighting MDMA as a gateway drug whose use may increase the likelihood of dependence on other drugs.

Effects of scopolamine on morphine-induced conditioned place preference in mice

It is well known that the cholinergic system plays a crucial role in learning and memory. Psychopharmacological studies in humans and animals have shown that a systemic cholinergic blockade may induce deficits in learning and memory. Accumulated studies have indicated that learning and memory play an important role in drug addition. In the present study, in order to get a further understanding about the functions of the cholinergic system in drug-related learning and memory, we examined the effects of scopolamine (0.5, 1.0 and 2.0 mg/kg) on morphine-induced conditioned place preference (CPP). Two kinds of morphine exposure durations (4 days and 12 days) were used. The main finding was that all doses of scopolamine enhanced the extinction of morphine-induced CPP in mice treated with morphine for 12 days. However, in mice treated with morphine for 4 days, all doses of scopolamine did not inhibit morphine-induced CPP. The highest dose (2.0 mg/kg) of scopolamine even significantly delayed the extinction of morphine-induced CPP. Our results suggest that the effects of a systemic cholinergic blockade on morphine-induced CPP depend on the morphine exposure time.

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.

Rewarding effects and reinstatement of MDMA-induced CPP in adolescent mice

Although the rewarding effects of 3,4-methylenedioxy-metamphetamine (MDMA) have been demonstrated in self-administration and conditioned place preference (CPP) procedures, its addictive potential (ie, the vulnerability to relapse, measured by its ability to induce reinstatement of an extinguished response), remains poorly understood. In this study, the effects of MDMA (5, 10, and 20 mg/kg) on the acquisition, extinction and reinstatement of CPP were evaluated in mice, using two different protocols during acquisition of CPP. In the first experiment, animals were trained using a two-session/day schedule (MDMA and saline for 4 consecutive days), whereas in the second experiment, they were trained using an alternating day schedule (MDMA and saline each 48 h). After extinction, the ability of drug priming to reinstate CPP was evaluated. In Experiment 1, MDMA did not significantly increase the time spent in the drug-paired compartment during the post-conditioning (Post-C) test, although the preference was evident a week afterwards, lasting between 2 and 21 weeks. No reinstatement was observed after MDMA priming. In Experiment 2, all doses produced CPP in Post-C, which lasted between 1 and 4 weeks. MDMA induces reinstatement at doses up to 4 times lower than those used in conditioning. The analyses of brain monoamines revealed that the daily schedule of treatment induces a non-dose-dependent decrease in dopamine and serotonin (5-HT) in the striatum, whereas the alternating schedule produces a dose-dependent decrease of 5-HT in the cortex. These results demonstrate that MDMA produces long-lasting rewarding effects and reinstatement after extinction, suggesting the susceptibility of this drug to induce addiction.

Morphine-induced place preference: Involvement of cholinergic receptors of the ventral tegmental area

In the present study, the effects of intra-ventral tegmental area injections of cholinergic agents on morphine-induced conditioned place preference were investigated by using an unbiased 3-day schedule of place conditioning design in rats. The conditioning treatments with subcutaneous injections of morphine (0.5-7.5 mg/kg) induced a significant dose-dependent conditioned place preference for the drug-associated place. Intra-ventral tegmental area injection of an anticholinesterase, physostigmine (2.5 and 5 microg/rat) or nicotinic acetylcholine receptor agonist, nicotine (0.5 and 1 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant conditioned place preference. Furthermore, intra-ventral tegmental area administration of muscarinic acetylcholine receptor antagonist, atropine (1-4 microg/rat) or nicotinic acetylcholine receptor antagonist, mecamylamine (5 and 7.5 microg/rat) dose-dependently inhibited the morphine (5 mg/kg)-induced place preference. Atropine or mecamylamine reversed the effect of physostigmine or nicotine on morphine response respectively. The injection of physostigmine, but not atropine, nicotine or mecamylamine, into the ventral tegmental area alone produced a significant place aversion. Moreover, intra-ventral tegmental area administration of the higher doses of physostigmine or atropine, but not nicotine or mecamylamine decreased the locomotor activity. We conclude that muscarinic and nicotinic acetylcholine receptors in the ventral tegmental area may critically mediate the rewarding effects of morphine.

The effect of l-stepholidine, a novel extract of Chinese herb, on the acquisition, expression, maintenance, and re-acquisition of morphine conditioned place preference in rats

The effect of L-stepholidine (SPD), a novel alkaloid extract of the Chinese herb Stephania with partial dopamine D1 receptor agonistic and D2 receptor antagonistic dual actions, on morphine conditioned place preference (CPP) was studied. Daily injection of morphine (10 mg/kg, i.p.) for 6 days induced CPP in rats, and daily treatment with SPD at 10 or 20 mg/kg before morphine injection dose-dependently attenuated morphine-induced CPP. On the day following acquisition of morphine CPP, a single administration of SPD at 10 or 20 mg/kg failed to block the expression of CPP. However, daily administration of SPD at 20 mg/kg for 7 days attenuated the maintenance of CPP. Morphine-induced CPP extinguished after a 21-day saline training and then a single injection of morphine (3 mg/kg, i.p.) induced re-acquisition of morphine CPP; however, pretreatment with SPD at 10 or 20 mg/kg 30 min before morphine injection dose-dependently blocked morphine (3 mg/kg, i.p.)-induced re-acquisition of morphine CPP. Furthermore, our data indicate that SPD had no effect on food-induced CPP or state-dependent learning, suggesting that the observed effect of SPD does not result from an inhibition of general learning ability. These results demonstrate that SPD can inhibit acquisition, maintenance, and re-acquisition of morphine conditioned place preference and suggest its potential for treatment of opioid addiction.

Morphine-induced place preference: Involvement of the central amygdala NMDA receptors

In the present study, the effects of bilateral injections of N-methyl-d-aspartate (NMDA) receptor agonist and/or antagonist into the central amygdala (CeA) on the acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Animals that received 3 daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg) indicated a significant preference for compartment paired with morphine in a dose dependent manner. Intra-CeA administration of the NMDA (0.01, 0.1 or 1 microg/rat) with an ineffective dose of morphine (1 mg/kg, s.c.) elicited a significant CPP. Administration of the non-competitive NMDA receptor antagonist, MK-801 (0.1, 0.3 or 0.5 microg/rat), into the central amygdala dose-dependently inhibited the morphine (6 mg/kg, s.c.)-induced place preference. Furthermore, intra-CeA administration of MK-801 (0.25, 0.5 or 1 microg/rat) reduced the response induced by NMDA (1 microg/rat, intra-CeA) plus morphine (1 mg/kg, s.c.). Neither NMDA nor MK-801 alone produce a significant place preference or place aversion. Moreover, intra-CeA injection of NMDA but not MK-801 before testing significantly increased the expression of morphine (6 mg/kg, s.c.)-induced place preference. NMDA or MK-801 injections into the CeA had no effects on locomotor activity on the testing sessions. These results suggest that the NMDA receptor mechanisms in the central amygdala may be involved in the acquisition and expression of morphine-induced place preference.

Effects of early methylphenidate exposure on morphine- and sucrose-reinforced behaviors in adult rats: relationship to dopamine D2 receptors

Methylphenidate is commonly used to treat Attention Deficit Hyperactivity Disorder (ADHD) in school-aged children, and there is an increasing trend to prescribe methylphenidate to younger preschool-aged children. While the efficacy of methylphenidate is not in question, there is evidence that early methylphenidate treatment may have long-term effects on later drug responsiveness. The goal of this study was to determine whether early exposure to methylphenidate would alter morphine-induced conditioned place preference (CPP) and sucrose-reinforced lever-pressing in young adult rats. We also assessed whether early methylphenidate exposure would impact dopamine D(2) binding sites. Sprague-Dawley rats were treated with methylphenidate (0, 2, or 5 mg/kg) once a day from PD 11-PD 20. On PD 60, morphine-induced CPP or sucrose-reinforced lever-pressing was assessed. A 10-day CPP procedure was used, which included 1 preconditioning day, 8 conditioning days, and 1 test day. After CPP testing, D(2) receptor binding was determined in striatal and accumbal tissue samples. In the sucrose experiment, rats were trained to lever-press on a progressive ratio schedule for one sucrose pellet. Results showed that early exposure to methylphenidate (5 mg/kg) increased the magnitude of morphine-induced CPP. Exposure to methylphenidate did not alter the number of D(2) binding sites, however, there were positive correlations between the number of D(2) binding sites and the strength of the CPP. In the sucrose-reinforced lever-press experiment, rats exposed to methylphenidate (2 and 5 mg/kg) had higher break points than saline controls. These results suggest that early exposure to methylphenidate alters reward system functioning, thereby making these systems more sensitive to appetitive stimuli.

The drug-induced helplessness test: an animal assay for assessing behavioral despair in response to neuroleptic treatment

RATIONALE

Neuroleptic dysphoria encompasses a range of unpleasant subjective responses and, as a result, is difficult to study in preclinical animal models.

OBJECTIVE

Based on the learned helplessness model of depression, increases in escape failures (EFs) in the drug-induced helplessness test (DH) are proposed to reflect drug-induced depressive-like state, a contributing factor to neuroleptic dysphoria in humans.

MATERIALS AND METHODS

Effects of the typical antipsychotic haloperidol and the atypical antipsychotics risperidone, olanzapine, aripiprazole, quetiapine, and clozapine were investigated in the DH test. We further characterized this test by examining compounds affecting motor function, cognition, anxiety, and those with antidepressant activity.

RESULTS

The antipsychotics haloperidol, risperidone, aripiprazole, and olanzapine, all increased EFs, while quetiapine had no effect, and clozapine reduced EFs. Amphetamine, diazepam, and ciproxifan, had no effect on EFs. Scopolamine significantly reduced EFs and MK-801 showed a trend toward reducing EFs at doses not significantly sti mulating locomotor activity. Subchronic, but not acute, imipramine and subchronic fluoxetine significantly reduced EFs at doses significantly suppressing locomotor activity. Dissociation appears to exist between performance in the DH test and compound effects on catalepsy or locomotor activity.

CONCLUSIONS

After discussing potential alternative interpretations of the drug-induced changes of EFs, we propose the DH test as a useful test for assessing a drug-induced, depressive-like state that may contribute to neuroleptic dysphoria.

Prenatal cocaine alters later responses to morphine in adult male mice

Mice prenatally exposed to cocaine (25 mg/kg), physiological saline or non-treated during the last 6 days of pregnancy were evaluated as adults for the rewarding properties of 2 mg/kg of morphine, using the conditioned place preference (CPP) procedure. Likewise, isolated animals underwent a social interaction test with conspecifics after receiving the same morphine dose. Unlike control or animals pre-treated with saline, subjects prenatally treated with cocaine did not develop CPP with this dose of morphine. Only cocaine-exposed animals showed increased threat, avoidance and fleeing during the social encounter. No differences in motor effects of morphine were observed. Analysis of monoamines revealed effects of housing conditions, isolated animals having fewer DOPAC but higher levels of HVA than those grouped, but in both groups there was a decrease in DOPAC in cocaine- and saline-treated mice. Prenatal cocaine exposure decreases the response to the rewarding properties of drugs in mature offspring. They also implicate cocaine consumption during pregnancy could affect the response of offspring to take other drugs of abuse.

Morphine-conditioned single-trial place preference: role of nucleus accumbens shell dopamine receptors in acquisition, but not expression

RATIONALE

A large body of evidence indicates an involvement of the mesolimbic dopamine (DA) pathway innervating the ventral striatum in the motivational effects of drug abuse.

OBJECTIVE

The goal of the study is to clarify the role of DA D1 and D2 receptors of the rat nucleus accumbens (NAc) shell and core in the motivational effects of morphine as studied by conditioned place preference (CPP).

METHODS

The effect of the intracerebral infusion of DA antagonists specific for DA D1 (SCH 39166) and D2 receptors (L-sulpiride) was studied in a single-trial place conditioning paradigm with fixed assignment of the drug to the unpreferred compartment.

RESULTS

Morphine induced significant CPP at all the doses tested (0.5, 1.0, and 2.0 mg/kg, subcutaneously). A dose of 1.0 mg/kg was selected for further studies. Intra-NAc shell infusion of SCH 39166 and L-sulpiride at doses of 25 and 50 ng/1 microl per side impaired the acquisition of CPP by morphine. No effect was observed at 12.5 ng/1 microl per side. Intra-NAc core infusion of SCH 39166 (12.5, 25, and 50 ng/1 microl per side) did not affect the acquisition of morphine-induced CPP, while L-sulpiride (12.5, 25, and 50 ng/1 microl per side) impaired CPP acquisition only at the dose of 50 ng/1 microl per side. No effect on morphine-induced CPP was observed when the DA antagonists were infused into the NAc shell or core 10 min before the test session.

CONCLUSION

These results indicate that DA D1 and D2 receptors in the NAc shell are involved in the acquisition of morphine-induced CPP.

Social stress is as effective as physical stress in reinstating morphine-induced place preference in mice

RATIONALE

Relapse to drug-seeking in abstinent heroin addicts and reinstatement in experimental animals are observed when exposed to drug-associated stimuli or cues, the drug itself, and stressful events. It has been shown that footshock-induced stress increases the rewarding effects of opiates, delays extinction, and induces the reinstatement of drug-seeking. However, the effects of social stress on the reinstatement of opiate-seeking after extinction has not been studied.

OBJECTIVES

The role of physical (restraint and tail pinch) and social (social defeat) stressors on the reinstatement of morphine-induced conditioned place preference (CPP) was evaluated.

METHODS

Adult male OF1 mice were conditioned with 10, 20, or 40 mg/kg of morphine or saline. Only morphine-conditioned animals acquired CPP. All mice underwent extinction sessions until the CPP was extinguished. Then, the effects of physical or social stress on the reinstatement of CPP were evaluated. Morphine- and saline-conditioned animals were exposed to the respective stressor or control stress condition immediately or 15 min before reinstatement tests. In experiment 1, animals underwent restraint for 15 min. In experiment 2, animals were exposed to tail pinch or placed in a cage without any manipulation for 15 min. In experiment 3, animals performed an agonistic encounter with an isolated or anosmic mouse or were placed in a cage without any social contact or manipulation.

RESULTS

Restraint, tail pinch, and social defeat in an agonistic encounter with an isolated mouse produce the reinstatement of CPP in morphine-conditioned animals.

CONCLUSIONS

These data demonstrate that social stress is as effective as physical stress in reinstating morphine-seeking.

Morphine withdrawal syndrome: Involvement of the dopaminergic system in prepubertal male and female mice

Morphine (MOR) withdrawal signs are more marked in males than in females. Considering that the influence of the dopaminergic system on these differences is unclear, we analyzed dopamine (DA) and dihydroxyphenylacetic-acid (DOPAC) brain levels during naloxone (NAL)-precipitated withdrawal as well as the involvement of D(1) and D(2) receptors in the expression of MOR withdrawal in either sex. Prepubertal Swiss-Webster mice received MOR (2 mg/kg, i.p.) twice daily for 9 days. On the tenth day, dependent animals received NAL (6 mg/kg, i.p.) after MOR and were sacrificed 30 min later. DA and DOPAC concentrations were determined in different brain areas using HPLC with electrochemical detection. Other pool of mice received either a D(1) (SCH 23390; 0.2 mg/kg, i.p.) or D(2) (raclopride; 0.3 mg/kg, i.p.) receptor antagonist before NAL and withdrawal signs were evaluated. DA and DOPAC levels only decreased in striatum and cortex of withdrawn males. Conversely, both DA receptor antagonists decreased the expression of MOR withdrawal signs in either sex. The neurochemical sex differences described here could partially explain the behavioral sex differences observed during MOR withdrawal. Additionally, SCH-23390 and raclopride effects suggest an important role of both DA receptors in the expression of MOR withdrawal in males and females.

Morphine reward in dopamine-deficient mice

Dopamine has been widely implicated as a mediator of many of the behavioural responses to drugs of abuse. To test the hypothesis that dopamine is an essential mediator of various opiate-induced responses, we administered morphine to mice unable to synthesize dopamine. We found that dopamine-deficient mice are unable to mount a normal locomotor response to morphine, but a small dopamine-independent increase in locomotion remains. Dopamine-deficient mice have a rightward shift in the dose-response curve to morphine on the tail-flick test (a pain sensitivity assay), suggesting either a decreased sensitivity to the analgesic effects of morphine and/or basal hyperalgesia. In contrast, dopamine-deficient mice display a robust conditioned place preference for morphine when given either caffeine or l-dihydroxyphenylalanine (a dopamine precursor that restores dopamine throughout the brain) during the testing phases. Together, these data demonstrate that dopamine is a crucial component of morphine-induced locomotion, dopamine may contribute to morphine analgesia, but that dopamine is not required for morphine-induced reward as measured by conditioned place preference.

Cross-reinstatement by cocaine and amphetamine of morphine-induced place preference in mice

The cross-reinstatement by psychostimulants of a conditioned place preference (CPP) induced by morphine was evaluated in mice. In Experiment 1, we examined the effects of a single dose of cocaine and amphetamine on a previously extinguished morphine CPP. After acquisition of CPP induced by morphine (40 mg/kg), animals underwent daily extinction sessions of 15 min duration until the CPP was extinguished. Subsequently, animals received a non-contingent injection of cocaine (25 mg/kg) or amphetamine (4 mg/kg), which produced the reinstatement of the extinguished morphine-induced CPP. In Experiment 2, we evaluated the reinstating effects of several priming doses of cocaine (Experiment 2A) or amphetamine (Experiment 2B). As in the first experiment, after conditioning with morphine (40 mg/kg), mice underwent daily 15 min extinction sessions. When the preference was no longer evident, we tested the effects of cocaine (0, 6.25, 12.5, 25 and 50 mg/kg) and amphetamine (0, 0.5, 1, 2 and 4 mg/kg) on the reinstatement of CPP. Doses from 12.5 mg/kg of cocaine upward and doses from 1 mg/kg of amphetamine upward effectively reinstated CPP. Our results demonstrate cross-reinstatement with psychostimulants and opiates, suggesting that in abstinent individuals, drug exposure can produce craving for the previously abused drug and relapse.

Role of the cholinergic system in the rat basolateral amygdala on morphine-induced conditioned place preference

The effects of intra-basolateral amygdala (intra-BLA) injections of physostigmine, atropine, nicotine and/or mecamylamine on morphine-induced conditioned place preference (CPP) in rats was investigated by using an unbiased 3-day schedule of place conditioning design. Animals that received 3 daily injections of morphine (0.5-10 mg/kg) subcutaneously (s.c.) or saline (1.0 ml/kg, s.c.) showed a significant preference for compartment paired with morphine. The maximum response was observed with 7.5 mg/kg of the opioid. Administration of the anticholinesterase drug, physostigmine (1, 3 and 5 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant CPP. Injections of antimuscarinic receptor agent, atropine (1, 4 and 7 microg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. The injections of nicotine (0.75, 1 and 2 microg/rat) potentiated the morphine (0.5 mg/kg)-induced place preference, while the nicotinic receptor antagonist, mecamylamine (1, 3 and 6 microg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. Furthermore, administration of atropine (7 microg/rat) but not mecamylamine (6 microg/rat) reduced the response induced by different doses of physostigmine plus morphine. Moreover, mecamylamine (6 microg/rat) but not atropine (7 microg/rat) reduced the response induced by different doses of nicotine plus morphine. It is concluded that the muscarinic and nicotinic receptor mechanisms in the BLA may be involved in the acquisition of morphine-induced place preference.

Long-lasting rewarding effects of morphine induced by drug primings

To evaluate the persistence of the rewarding effects of morphine, the acquisition, maintenance, extinction and reinstatement of a conditioned place preference (CPP) was assessed in OF1 mice. In Experiment 1, the persistence of morphine-induced CPP was evaluated weekly. Mice showed CPP after four sessions of conditioning with 5, 10, 20 and 40 mg/kg of morphine, which lasted 0, 1, 2 and 4 weeks, respectively. In Experiment 2, after four sessions of conditioning with 40 mg/kg of morphine, the effects of four schedules of extinction differing in the time interval (2, 4, 6 or 8 weeks) between sessions were evaluated. CPP was no longer evident after 8 weeks for the groups examined each 2 or 8 weeks and after 12 weeks for the groups examined each 4 and 6 weeks. After extinction, the reinstating effects of a priming dose of 20 mg/kg of morphine were demonstrated. This procedure of extinction/reinstatement was repeated with a decreasing priming dose of morphine (10, 5, 2.5 and 1.25 mg/kg) until a noneffective dose was found. These results show that morphine-induced CPP is very persistent over time, suggesting that drug exposure induces long-lasting changes in the brain, which supports the idea that drug addiction must be considered as a chronic, lifelong disorder.

NMDA glutamate but not dopamine antagonists blocks drug-induced reinstatement of morphine place preference

The effects of dopaminergic and glutamatergic antagonists on the drug-induced reinstatement of a previously extinguished morphine conditioned place preference (CPP) in mice were evaluated. Following extinction of a place preference induced by morphine (40 mg/kg), a non-contingent injection of the dopaminergic antagonists SCH 23390 (0.125, 0.5 mg/kg), raclopride (0.3, 1.2 mg/kg), haloperidol (0.1, 0.2 mg/kg) and the dopamine (DA) release inhibitor CGS 10746B (1, 10 mg/kg) or glutamatergic NMDA antagonists memantine (10, 20, 40 mg/kg) and MK-801 (0.1, 0.2, 0.3 mg/kg) alone or with 10 mg/kg morphine was given. Neither the dopaminergic nor the glutamatergic antagonists alone reinstated the place preference. Dopamine antagonists failed to block the morphine-induced reinstatement of place preference while memantine and MK-801 blocked it with intermediate and high doses. These results suggest that drug-induced reinstatement of place preference may be largely independent of dopamine and more closely related to glutamatergic neurotransmission.

Repeated administration of dopaminergic agents in the dorsal hippocampus and morphine-induced place preference

The aim of the present experiments was to investigate whether repeated intra-hippocampal CA1 (intra-CA1) administration of dopaminergic agents can affect morphine-induced conditioned place preference (CPP). Effects of repeated intra-CA1 injections of dopamine (DA) receptor agonists and antagonists on morphine-induced CPP in rats were investigated using an unbiased 3-day schedule of place conditioning. Animals receiving once-daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner: the maximum response was observed with 3 mg/kg morphine. Three days' intra-CA1 injections of apomorphine (0.25-1 microg/rat) followed by 5 days free of the drug, significantly decreased morphine CPP (1 and 3 mg/kg, s.c.). Moreover, pre-treatment with the highest dose of apomorphine (1 microg/rat) altered the effect of morphine to an aversive response. The morphine (1 and 3 mg/kg) CPP was also significantly decreased in animals that previously received three intra-CA1 injections of SKF 38393 (2-9 microg/rat), quinpirole (1-3 microg/rat) or sulpiride (1-3 microg/rat), and significantly increased in animals that had previously received three intra-CA1 injections of SCH 23390 (0.02 microg/rat). The 3-day pre-treatment with apomorphine, SKF 38393 or quinpirole reduced locomotor activity in the test session, while SCH 23390 and sulpiride did not have any influence on locomotor activity. It is concluded that repeated injections of DA receptor agents in the dorsal hippocampus, followed by 5 days free of the drugs, can affect morphine reward.

Sensitization to the rewarding effects of morphine depends on dopamine

The influence of dopamine (DA) on sensitization to the rewarding effects of morphine was evaluated. The effects of pre-treatment with saline or morphine plus naloxone, CGS 10746B, haloperidol, SCH 23390 and raclopride, on the place conditioning induced by 2 mg/kg morphine were evaluated. This dose was ineffective in saline pre-treated animals but induced a clear conditioned place preference in mice pre-treated with morphine, CGS 10746B or haloperidol. Conversely, animals pre-treated with morphine plus naloxone, CGS 10746B, SCH 23390, raclopride and the high dose of haloperidol did not acquire place preference. Our results demonstrated that DA release and subsequent DA D1 and D2 receptor activation is essential for the development of sensitization to the rewarding effects of morphine.

GHB ameliorates naloxone-induced conditioned place aversion and physical aspects of morphine withdrawal in mice

RATIONALE

Gamma-hydroxybutyric acid (GHB) is a naturally occurring substance in the brain, the administration of which has proved useful in the treatment of the opiate withdrawal symptoms in humans.

OBJECTIVES

The aim of the present work was to validate this beneficial effect on the physical and motivational aspects of morphine withdrawal in mice.

METHODS

In a first experiment, animals rendered morphine-dependent were conditioned to develop a place aversion (CPA) to the compartment paired with naloxone administration in a two-chamber apparatus. The conditioning phase consisted of three pairings of either naloxone (0.250 mg/kg) or vehicle in one compartment, both with similar time allotments during the preconditioning test. During the testing phase, mice were again allowed to explore the entire apparatus. GHB (6, 12.5, 25, and 50 mg/kg) was administered during either the acquisition or expression phase of this conditioning. In a second experiment, the capacity of GHB to ameliorate the intensity of physical signs of morphine withdrawal was evaluated.

RESULTS

GHB blocked CPA in both phases: administered during acquisition (from 12.5 mg/kg and higher) as well as in the expression phase (from 6 mg/kg, except for 25 mg/kg). It also decreased the intensity of physical signs of morphine withdrawal to near control levels measured by the modified Gellert-Holtzman scale (25 mg/kg and higher). Decreases in jumping, body shakes, and paw tremor were also observed.

CONCLUSIONS

Our results support the idea that GHB ameliorates both aspects of morphine withdrawal, physical as well as motivational signs.

Cannabinoid agonist-induced sensitisation to morphine place preference in mice

The influence of the endocannabinoid system on the sensitisation to the rewarding effects of morphine in the place conditioning paradigm was evaluated. In mice pretreated with morphine this drug induces place preference with lower doses. Pretreatment with non-rewarding doses of the cannabinoid agonist WIN 55,212-2 (0.5 and 1 mg/kg) induces sensitisation to the rewarding effects of morphine. However, the pretreatment with the cannabinoid antagonist SR 144716A plus morphine or WIN 55,212-2 blocks it. Our results suggest the existence of cross-talk between cannabinoids and opiates on the sensitisation to morphine and the implication of the endocannabinoid system in the process of sensitisation to opiates.

Effects of NMDA receptor antagonists (MK-801 and memantine) on the acquisition of morphine-induced conditioned place preference in mice

Several studies have shown that the systemic administration of a variety of N-methyl-D-aspartate (NMDA) receptor antagonists can block the development or expression of conditioned place preference (CPP) induced by rewarding drugs such as morphine. In the present study, we examined the effects of different doses of two non-competitive NMDA receptor antagonists, MK-801 (0.1, 0.2 and 0.3 mg/kg) and memantine (2.5, 5, 10, 20 and 40 mg/kg), in CPP induced by 40 mg/kg of morphine in male mice. The CPP was carried out with an unbiased procedure in terms of initial spontaneous preference. Animals received the different doses of drugs in the conditioning sessions. MK-801 and memantine, at all doses used, produced neither place preference nor place aversion, but the higher doses of memantine (20 and 40 mg/kg) were able to completely block morphine-induced CPP. The present data show that the NMDA receptor antagonists MK-801 and memantine have no reinforcing properties but memantine is capable of preventing the acquisition of morphine-induced CPP. These results suggest that the development of morphine-induced CPP may be closely related to NMDA receptors and that the glutamatergic system can modulate opiate reward.

The Type IV phosphodiesterase inhibitor rolipram interferes with drug-induced conditioned place preference but not immediate early gene induction in mice

Behavioural effects of psychostimulant and opiate drugs are mediated in part by cAMP pathways operating in the nucleus accumbens. Degradation of cAMP occurs through the action of phosphodiesterases, such as the Type IV phosphodiesterases (PDE4s) that are found throughout the brain. To examine the potential role of PDE4 in reward-mediated behaviour, we measured the effects of rolipram, a PDE4 selective inhibitor, on cocaine (18 mg/kg i.p.) and morphine (5 mg/kg s.c.) conditioned place preference in Swiss Webster mice. Rolipram (0, 0.2 or 1.0 mg/kg i.p.) given 30 min prior to drug administration dose-dependently reduced conditioning due to both cocaine and morphine. However, rolipram did not affect place preference induced by food, nor did it prevent the expression of a previously established place preference conditioned by cocaine or morphine. In a second experiment, rolipram administered 30 min prior to a single cocaine injection (50 mg/kg i.p.), did not alter cocaine-induced c-Fos expression in the caudate putamen or nucleus accumbens core. However, rolipram, but not cocaine, induced c-Fos in the nucleus accumbens shell. These results indicate that elevation of cAMP in neurons that express PDE4s may attenuate the rewarding properties of cocaine and morphine, but does not alter the cocaine signalling cascade that induces c-Fos expression. Thus, PDE4-mediated regulation of cAMP levels could underlie the establishment of reward valence to abused drugs.

Agonist-like or antagonist-like treatment for cocaine dependence with methadone for heroin dependence: two double-blind randomized clinical trials

Concurrent abuse of cocaine and heroin is a common problem. Methadone is effective for opioid dependence. The question arises as to whether combining agonist-like or antagonist-like medication for cocaine with methadone for opioid dependence might be efficacious. Two parallel studies were conducted. One examined sustained release d-amphetamine and the other risperidone for cocaine dependence, each in combination with methadone. In total, 240 subjects (120/study) were recruited, who were both cocaine and heroin dependent and not currently receiving medication. All provided consent. Both studies were carried out for 26 weeks, randomized, double-blind and placebo controlled. Study I compared sustained release d-amphetamine (escalating 15-30 or 30-60 mg) and placebo. Study II examined risperidone (2 or 4 mg) and placebo. All subjects underwent methadone induction and were stabilized at 1.1 mg/kg. Subjects attended clinic twice/week, provided urine samples, obtained medication take-home doses for intervening days, and completed self-report measures. Each had one behavioral therapy session/week. In Study I, reduction in cocaine use was significant for the 30/60 mg dose compared to the 15/30 mg and placebo. Opioid use was reduced in all groups with a trend toward greater reduction in the 30/60 mg d-amphetamine group. In Study II, methadone reduced illicit opioid use but cocaine use did not change in the risperidone or placebo groups. There were no adverse medication interactions in either study. The results provide support for the agonist-like (d-amphetamine) model in cocaine dependence treatment but not for antagonist-like (risperidone) treatment. They coincide with our previous reports of amphetamine or risperidone administered singly in cocaine-dependent individuals.

7-Nitroindazole blocks conditioned place preference but not hyperactivity induced by morphine

The effects of 7-nitroindazole (7-NI), a neural nitric oxide synthase (nNOS) inhibitor, on spontaneous locomotor activity, morphine-induced hyperactivity, acquisition of place conditioning and morphine-induced conditioned place preference (CPP) were evaluated in male mice. In experiment 1, animals treated with 7-NI (25, 50 and 100mg/kg), morphine (40 mg/kg) or morphine (40 mg/kg) plus 7-NI (25, 50 or 100mg/kg) were placed in an actimeter for 3h. In experiment 2, animals treated with the same drugs and doses were conditioned following an unbiased procedure. 7-NI did not affect the spontaneous locomotor activity or hyperactivity induced by morphine. However, the moderate and high doses of 7-NI produced conditioned place aversion (CPA) and the lowest dose blocked morphine-induced CPP. Our results suggest that nitric oxide is involved in the rewarding properties of morphine but not in its motor effects.

Involvement of dopamine receptors of the dorsal hippocampus on the acquisition and expression of morphine-induced place preference in rats

In the present study, the effects of bilateral intrahippocampal CA1 injections of dopamine receptor agonists and antagonists on the acquisition and expression of morphine-induced place preference were examined in male Wistar rats. Subcutaneous administration of different doses of morphine sulphate (0.5-10 mg/kg) produced a conditioned place preference (CPP) dose-dependently. Using a 3-day schedule of conditioning, it was found that dopamine D1 receptor agonist, SKF 38393 (0.01-1 microg/rat), dopamine D1 receptor antagonist, SCH 23390 (0.25-1 microg/rat), dopamine D(2/3) receptor agonist, quinpirole (0.3-3 microg/rat) or dopamine D2 receptor antagonist, sulpiride (0.04-5 microg/rat) did not produce significant place preference. The administration of SKF 38393 (1 microg/rat) significantly potentiated the acquisition of morphine (0.5 and 2.5 mg/kg)-induced place preference. This potentiation was reversed by SCH 23390 (1 microg/rat) pretreatment. Quinpirole injection (0.3 microg/rat) induced CPP in combination with the lower doses of morphine but decreased the response of the higher doses of morphine. These responses of quinpirole were reversed by sulpiride (5 microg/rat) pretreatment. SCH 23390 or sulpiride reduced the acquisition of morphine (7.5 mg/kg)-induced place preference. The administration of sulpiride, but not other drugs, during acquisition showed an increase in the locomotor activity on the testing days. SKF 38393, SCH 23390 or sulpiride, but not quinpirole when used before testing, reduced the expression of morphine-induced place preference. Sulpiride, but not other drugs, increased locomotion when used before testing. It is concluded that dorsal hippocampal dopamine receptors may play an active role in morphine reward.

Effects of a dopamine D3 receptor ligand, BP 897, on acquisition and expression of food-, morphine-, and cocaine-induced conditioned place preference, and food-seeking behavior in rats

The present study addressed the role of dopaminergic D(3) receptors (D(3)R) in motivational processes in rats. The effects of the selective D(3)R partial agonist, BP 897 (0.25-1 mg/kg, i.p.), on the establishment and the expression of conditioned place preference (CPP) supported by food, morphine (4 mg/kg, s.c.), or cocaine (2 mg/kg, s.c.) were investigated using an unbiased, one-compartment, place-conditioning procedure. When administered alone, BP 897 (0.05-2 mg/kg, i.p.) did not support CPP; on the contrary, conditioned place avoidance (CPA) was observed at 1 mg/kg, suggesting that this dose of BP 897 could be perceived as aversive. When given before each cocaine injection during the conditioning phase, BP 897 (1 mg/kg) prevented the establishment of CPP, and a single administration of BP 897 (0.5 and 1 mg/kg) before the test session impaired the expression of cocaine CPP. In contrast, neither the establishment nor the expression of food- and morphine-CPP were significantly altered by BP 897 (up to 1 mg/kg), whereas the full but less selective D(3)/D(2)R agonists, 7-OH-DPAT (0.5-2 mug/kg, s.c.) and quinelorane (1 mug/kg, s.c.), prevented the acquisition of food CPP. In a within-session extinction schedule of lever pressing for food, BP 897 (0.06-2 mg/kg) was ineffective in potentiating response reinstatement induced by the noncontingent delivery of two food pellets, in contrast with quinelorane and 7-OH-DPAT where previous studies showed to be efficient in this respect (Duarte et al, 2003). These results indicate that BP 897 has no positive appetitive value on its own, and that a moderate degree of stimulation of D(3)R is not sufficient to modulate food-primed food-seeking behavior or alter incentive motivation for food, morphine, and/or their associated cues. However, D(3)R are likely involved in the perception of the rewarding value of cocaine and cocaine-paired cues. This suggests that the appetitive effects of cocaine are subserved by mechanisms different, at least in part, from those of morphine and food, and that D(3)R play a role only in the former.

Memantine presents different effects from MK-801 in motivational and physical signs of morphine withdrawal

Adaptive changes in neural systems due to chronic opiate exposure are related to the neural plasticity phenomenon, NMDA receptors being implicated in these processes, e.g. tolerance, dependence or withdrawal. In this work, we investigated the effect of two non-competitive NMDA antagonists, memantine and MK-801, in motivational (Conditioned Place Aversion paradigm, CPA) and physical aspects of morphine withdrawal. After the induction of morphine dependence, animals in which the CPA was studied, received memantine (5 and 10 mg/kg) or MK-801 (0.3-0.006 mg/kg) either during the acquisition (conditioning) or expression (test) phase of this procedure. Both drugs were capable of inhibiting conditioned aversion when administered in any phase. In a second experiment, the effects of these drugs were evaluated in the intensity of the physical signs of withdrawal, only memantine administration being efficient. In addition to these studies, the intensity of morphine dependence was investigated under the blockade of NMDA receptors, i.e. morphine was co-administered with memantine or MK-801. These animals did not develop CPA and present less intensity in the physical signs of morphine withdrawal. Our results support the idea that NMDA receptors are involved in the behavioural changes and therefore in the neural adaptations produced by repeated morphine administration.

Suppression of morphine-induced conditioned place preference by l-12-chloroscoulerine, a novel dopamine receptor ligand

The effect of l-12-chloroscoulerine (l-CSL), a novel ligand with dual dopamine D1 receptor agonistic and D2 receptor antagonistic actions, on the development of morphine-induced conditioned place preference (CPP) was investigated in mice. Morphine (10 mg/kg)-induced place preference was dose dependently suppressed by coadministration of l-CSL (5, 10 and 20 mg/kg), which induced neither place preference nor place aversion when administered alone at a dose of 20 mg/kg. The D1 receptor antagonist SCH23390 (0.1 mg/kg) suppressed, whereas the D2 receptor agonist (+/-)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin (PPHT) (0.5 mg/kg) had no influence on the development of morphine-induced place preference. However, SCH23390 (0.1 mg/kg) did not affect, whereas PPHT (0.5 mg/kg) reversed the suppressive effect of l-CSL on the development of morphine-induced place preference. These results indicate that l-CSL suppresses the development of place preference of morphine by blocking D2 receptors.