The delta-opioid receptor antagonist naltrindole prevents sensitization to the conditioned rewarding effects of cocaine

Adult rat ultrasonic vocalizations and reward: Effects of propranolol and repeated cocaine administration

BACKGROUND

Mechanisms underlying psychostimulant euphoria remain poorly understood. In adult rats, positive emotional states are associated with alterations in 50-kHz ultrasonic vocalizations (USVs): specifically, "trill" calls are promoted over "flat" calls. Here, we investigated the effects of acute and repeated cocaine administration, and-based on previous findings with amphetamine-their possible dependence on beta-adrenergic receptors.

METHODS

Adult male Long-Evans rats received intraperitoneal drug or saline injections before daily USV recording. Fourteen 50-kHz call subtypes were analyzed. In Experiments 1 and 2, cocaine (1-10 mg/kg) and propranolol (10 mg/kg) were tested alone. In Experiment 3, propranolol/cocaine interactions were sought within a conditioned place preference (CPP) procedure. Experiment 4 investigated acute and chronic cocaine effects (Phase 1), and propranolol/cocaine interactions either in an open field (Phase 2) or within a CPP procedure (Phase 3).

RESULTS

In drug-naïve animals, cocaine increased the 50-kHz call rate, with sensitization developing rapidly. After more extended exposure, cocaine now also increased the relative prevalence of trill versus flat calls; effects on other subtypes were also revealed. The beta-blocker propranolol prevented neither cocaine CPP nor cocaine effects on USV emission or locomotion but exerted significant USV-related effects when given alone. CPP magnitude and USV-related measures were uncorrelated.

CONCLUSIONS

With long-term intraperitoneal administration, cocaine can alter the relative prevalence of several 50-kHz call subtypes; its ability to promote trill versus flat calls, in particular, is consistent with a positive affect interpretation. Cocaine's behavioral effects (i.e., USV-related, locomotor, CPP) appear independent of beta-adrenergic receptor activity.

Ayahuasca blocks ethanol preference in an animal model of dependence and shows no acute toxicity

ETHNOPHARMACOLOGICAL RELEVANCE

Ayahuasca, a psychoactive beverage prepared from Banisteriopsis caapi and Psychotria viridis, is originally used by Amazon-based indigenous and mestizo groups for medicinal and ritualistic purposes. Nowadays, ayahuasca is used in religious and shamanic contexts worldwide, and preliminary evidence from preclinical and observational studies suggests therapeutic effects of ayahuasca for the treatment of substance (including alcohol) use disorders.

AIM OF THE STUDY

To investigate the initial pharmacological profile of ayahuasca and its effects on ethanol rewarding effect using the conditioned place preference (CPP) paradigm in mice.

MATERIALS AND METHODS

Ayahuasca beverage was prepared using extracts of B. caapi and P. viridis, and the concentration of active compounds was assessed through high performance liquid chromatography (HPLC). The following behavioral tests were performed after ayahuasca administration: general pharmacological screening (13, 130, or 1300 mg/kg - intraperitoneally - i.p., and 65, 130, 1300, or 2600 mg/kg - via oral - v.o.); acute toxicity test with elevated doses (2600 mg/kg - i.p., and 5000 mg/kg - v.o.); motor activity, motor coordination, and hexobarbital-induced sleeping time potentiation (250, 500, or 750 mg/kg ayahuasca or vehicle - v.o.). For the CPP test, the animals received ayahuasca (500 mg/kg - v.o.) prior to ethanol (1.8 g/kg - i.p.) or vehicle (control group - i.p.) during conditioning sessions.

RESULTS

Ayahuasca treatment presented no significant effect on motor activity, motor coordination, hexobarbital-induced sleeping latency or total sleeping time, and did not evoke signs of severe acute toxicity at elevated oral doses. Ayahuasca pre-treatment successfully inhibited the ethanol-induced CPP and induced CPP when administered alone.

CONCLUSIONS

Our results indicate that ayahuasca presents a low-risk acute toxicological profile when administered orally, and presents potential pharmacological properties that could contribute to the treatment of alcohol use disorders.

Agonist-induced phosphorylation bar code and differential post-activation signaling of the delta opioid receptor revealed by phosphosite-specific antibodies

The δ-opioid receptor (DOP) is an attractive pharmacological target due to its potent analgesic, anxiolytic and anti-depressant activity in chronic pain models. However, some but not all selective DOP agonists also produce severe adverse effects such as seizures. Thus, the development of novel agonists requires a profound understanding of their effects on DOP phosphorylation, post-activation signaling and dephosphorylation. Here we show that agonist-induced DOP phosphorylation at threonine 361 (T361) and serine 363 (S363) proceeds with a temporal hierarchy, with S363 as primary site of phosphorylation. This phosphorylation is mediated by G protein-coupled receptor kinases 2 and 3 (GRK2/3) followed by DOP endocytosis and desensitization. DOP dephosphorylation occurs within minutes and is predominantly mediated by protein phosphatases (PP) 1α and 1β. A comparison of structurally diverse DOP agonists and clinically used opioids demonstrated high correlation between G protein-dependent signaling efficacies and receptor internalization. In vivo, DOP agonists induce receptor phosphorylation in a dose-dependent and agonist-selective manner that could be blocked by naltrexone in DOP-eGFP mice. Together, our studies provide novel tools and insights for ligand-activated DOP signaling in vitro and in vivo and suggest that DOP agonist efficacies may determine receptor post-activation signaling.

The new kisspeptin derivative - kissorphin (KSO) - attenuates acute hyperlocomotion and sensitization induced by ethanol and morphine in mice

Kissorphin (KSO) is a new peptide derived from kisspeptin-10. This peptide possesses neuropeptide FF (NPFF)-like biological activity in vitro; NPFF, in many cases, inhibits opioid and ethanol effects in rodents. Therefore, the current study explored the influence of KSO on acute ethanol- and morphine-induced hyperactivity, and on the development and expression of locomotor sensitization induced by these drugs. In the present study, sensitization to locomotor effects was induced by repeated exposure to ethanol (2.4 g/kg, intraperitoneally [i.p.], 1 × 4 days) or morphine (10 mg/kg, subcutaneously [s.c.], 1 × 7 days). We found that KSO (1-10 nmol/300 μL, intravenously [i.v.]) did not have an impact on locomotor activity of naïve mice. However, it reduced both acute ethanol- (10 nmol/300 μL) and morphine-induced hyperactivity (3 and 10 nmol/300 μL). Pretreatment of animals with KSO (10 nmol/300 μL), before every ethanol or morphine injection during development of sensitization or before the ethanol or morphine challenge, attenuated the development, as well as the expression of locomotor sensitization to both substances. Moreover, prior administration of the NPFF receptor antagonist RF9 (10 nmol/300 μL, i.v.) inhibited the ability of KSO (10 nmol/300 μL) to reduce the expression of ethanol and morphine sensitization. KSO given alone, at all used doses, did not influence the motor coordination measured via the rotarod test. The results from this study show that KSO effectively attenuated acute and repeated effects of ethanol and morphine. Thus, KSO possesses NPFF-like anti-opioid activity in these behavioral studies.

Strain and cocaine-induced differential opioid gene expression may predispose Lewis but not Fischer rats to escalate cocaine self-administration

The aim of the present study was to investigate alterations in gene expression of opioid system components induced by extended access (18 h) cocaine self-administration and to determine the impact of genetic background in the vulnerability to escalate cocaine intake. Comparing two inbred rat strains, we previously reported that Lewis rats progressively escalated cocaine consumption compared to Fischer rats, in a new translational model of intravenous cocaine self-administration, which included 14 sessions of 18-h operant sessions in which rats were allowed to select the cocaine unit dose to self-administer. We compare here Fischer and Lewis rats in the gene expression of endogenous opioid peptides (Pomc, Penk, Pdyn) and cognate receptors (Oprm, Oprk and Oprd) in reward-related brain regions, after exposure to either cocaine self-administration or yoked-saline, in the aforementioned translational paradigm. We performed a correlation analysis between the mRNA level, found in the Dorsal Striatum (DS), Nucleus accumbens (NAcc) shell and core respectively, and individual cocaine intake. Our findings show that the gene expression of all the aforementioned opioid genes exhibit strain-dependent differences in the DS, in absence of cocaine exposure. Also, different strain-specific cocaine-induced mRNA expression of Oprm and Oprk was found in DS. Only few differences were found in the ventral parts of the striatum. Moreover, gene expression level of Pdyn, Penk, Oprk, and Oprm in the DS was significantly correlated with cocaine intake only in Fischer rats. Overall, these data shed light on potential genetic differences which may predispose of subjects to initiate and escalate cocaine consumption.

The Nociceptin Receptor as an Emerging Molecular Target for Cocaine Addiction

Cocaine addiction is a global public health and socioeconomic issue that requires pharmacological and cognitive therapies. Currently there are no FDA-approved medications to treat cocaine addiction. However, in preclinical studies, interventions ranging from herbal medicine to deep-brain stimulation have shown promise for the therapy of cocaine addiction. Recent developments in molecular biology, pharmacology, and medicinal chemistry have enabled scientists to identify novel molecular targets along the pathways involved in drug addiction. In 1994, a receptor that showed a great deal of homology to the traditional opioid receptors was characterized. However, endogenous and exogenous opioids failed to bind to this receptor, which led scientists to name it opioid receptor-like receptor, now referred to as the nociceptin receptor. The endogenous ligand of NOPr was identified a year later and named orphanin FQ/nociceptin. Nociceptin and NOPr are widely distributed throughout the CNS and are involved in many physiological responses, such as food intake, nociceptive processing, neurotransmitter release, etc. Furthermore, exogenous nociceptin has been shown to regulate the activity of mesolimbic dopaminergic neurons, glutamate, and opioid systems, and the stress circuit. Importantly, exogenous nociceptin has been shown to reduce the rewarding and addictive actions of a number of drugs of abuse, such as psychostimulants, alcohol, and opioids. This paper reviews the existing literature on the role of endogenous nociceptin in the rewarding and addictive actions of cocaine. The effect of exogenous nociceptin on these processes is also reviewed. Furthermore, the effects of novel small-molecule NOPr ligands on these actions of cocaine are discussed. Overall, a review of the literature suggests that NOPr could be an emerging target for cocaine addiction pharmacotherapy.

AMN082, a metabotropic glutamate receptor 7 allosteric agonist, attenuates locomotor sensitization and cross-sensitization induced by cocaine and morphine in mice

Previous studies have indicated that metabotropic glutamate receptors 7 (mGluR7s) are involved in drug addiction. However, the role of these receptors in drug-induced behavioral sensitization is unknown. The aim of the present study was to determine whether systemic injection of AMN082, a selective mGluR7 allosteric agonist, reduces the cocaine- and morphine-induced hyperactivity and the development and expression of locomotor sensitization, and also affects the reciprocal cross-sensitization to the stimulant effect of cocaine and morphine in mice. AMN082 (1.25-10.0 mg/kg, i.p.) did not have an impact on locomotion of naive mice and did not affect the acute cocaine- or morphine-induced hyperactivity, except the dose of 10 mg/kg that suppressed the locomotor effect of both drugs. Repeated exposure to cocaine or morphine (10 mg/kg, 5× every 3 days) gradually increased locomotion during induction of sensitization and after 4 (cocaine) or 7 day (morphine) withdrawal phase when challenged with cocaine (10 mg/kg, i.p.) or morphine (10 mg/kg, i.p.) on day 17 or 20, respectively. Pretreatment of animals with the lower doses of AMN082 (1.25-5.0 mg/kg, i.p.), 30 min before every cocaine or morphine injection during repeated drug administration or before cocaine or morphine challenge, dose-dependently attenuated the development, as well as the expression of cocaine or morphine locomotor sensitization. AMN082 also inhibited the reciprocal cross-sensitization between these drugs. Prior to administration of MMPIP (10 mg/kg, i.p.), a selective mGluR7 antagonist reversed the inhibitory effect of AMN082 on the development or expression of cocaine or morphine sensitization. These data indicate that AMN082 attenuated the development and expression of cocaine and morphine sensitization, and the reciprocal cross-sensitization via a mechanism that involves mGluR7s. Thus, AMN082 might have therapeutic implications not only in the treatment of cocaine or opioid addiction but also in the treatment of cocaine/opioid polydrug-abusers.

Higher sensitivity to the conditioned rewarding effects of cocaine and MDMA in High-Novelty-Seekers mice exposed to a cocaine binge during adolescence

RATIONALE

Exposure to drugs during adolescence can induce alterations in the central nervous system. The novelty-seeking personality trait influences differences observed among individuals exposed to drugs of abuse.

OBJECTIVES

Long-term effects of intensive pre-treatment with cocaine during adolescence or adulthood were evaluated in High- and Low-Novelty Seeker (HNS and LNS) mice. It was hypothesized that a cocaine binge during adolescence would increase sensitivity to the rewarding effects of cocaine and MDMA, especially in HNS animals, and modify the spontaneous behaviour of adult animals.

METHODS

Adolescent (PND 33) and adult (PND 60) mice were identified as HNS or LNS according to their performance in the hole-board test. Subsequently, they received pre-treatment with cocaine (three injections per day of an increasing dose for 10 days) or saline. Three weeks later, the mice performed the hole-board, elevated plus maze, spontaneous locomotor activity and cocaine- (1 mg/kg) or MDMA- (1.25 mg/kg) induced conditioning place preference (CPP) tests. In another set of mice, the effects of pre-treatment of cocaine during adulthood on MDMA- or cocaine-induced CPP were also evaluated 3 weeks later.

RESULTS

Only HNS mice treated with cocaine during adolescence acquired MDMA- or cocaine-induced CPP in adulthood. Moreover, pre-exposure to cocaine during adolescence caused subsequent behavioural alterations, including reduced exploratory behaviour and increased locomotor reactivity.

CONCLUSIONS

Cocaine binge administration during adolescence induces a higher sensitivity to the rewarding effects of MDMA and cocaine in HNS mice in adulthood. This may explain the greater vulnerability often seen among individuals exposed early in life to drugs of abuse.

Synergistic activity between the delta-opioid agonist SNC80 and amphetamine occurs via a glutamatergic NMDA-receptor dependent mechanism

Glutamate is known to cause the release of dopamine through a Ca(2+)-sensitive mechanism that involves activation of NMDA ionotropic glutamate receptors. In the current study, we tested the hypothesis that the delta opioid agonist SNC80 acts indirectly, via the glutamatergic system, to enhance both amphetamine-stimulated dopamine efflux from striatal preparations and amphetamine-stimulated locomotor activity. SNC80 increased extracellular glutamate content, which was accompanied by a concurrent decrease in GABA levels. Inhibition of NMDA signaling with the selective antagonist MK801 blocked the enhancement of both amphetamine-induced dopamine efflux and hyperlocomotion observed with SNC80 pretreatment. Addition of exogenous glutamate also potentiated amphetamine-stimulated dopamine efflux in a Mg(2+)- and MK801-sensitive manner. After removal of Mg(2+) to relieve the ion conductance inhibition of NMDA receptors, SNC80 both elicited dopamine release alone and produced a greater enhancement of amphetamine-evoked dopamine efflux. The action of SNC80 to enhance amphetamine-evoked dopamine efflux was mimicked by the GABA(B) antagonist 2-hydroxysaclofen. These cumulative findings suggest SNC80 modulates amphetamine-stimulated dopamine efflux through an intra-striatal mechanism involving inhibition of GABA transmission leading to the local release of glutamate followed by subsequent activation of NMDA receptors.

Pre-treatment with high doses of cocaine decreases the reinforcing effects of cocaine in the conditioned place preference paradigm

The aim of the present study was to determine if pre-exposure to high doses of cocaine can subsequently alter the rewarding effects of this drug. Adult male mice received a pretreatment of physiological saline, or 12.5 or 25 mg/kg of cocaine (one injection a day for five days). After an interval of six days without injections, the rewarding effects of low doses of cocaine (0.5, 1 or 1.5 mg/kg) were evaluated in the conditioned place preference (CPP) paradigm. Doses of 1 and 1.5 mg/kg induced a clear CPP in animals pre-treated with saline but were ineffective in those pre-treated with 25 mg/kg of cocaine. Only the dose of 1.5 mg/kg induced CPP in mice pre-treated with 12.5 mg/kg of cocaine. Our results, which reveal a decrease in the conditioned rewarding effects of threshold doses of cocaine, demonstrate that exposure to high doses of this drug can alter the reward system.

Lithium chloride disrupts consolidation of morphine-induced conditioned place preference in male mice: the nitric oxide/cyclic GMP signaling pathway

Lithium effects on brain functions such as cognition, attention, learning and memory are well-established for ages; however, the way it affects these functions and its precise mechanism of action remains unknown. The purpose of this study was to determine the effects of lithium on the consolidation of morphine-associated conditioned place preference and the possible involvement of the NO/cGMP pathway. Using an unbiased conditioned place preference (CPP) model, the effects of lithium (1-100 mg/kg, i.p.), nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) (5-100 mg/kg, i.p.), nitric oxide precursor L-arginine (50-150 mg/kg, i.p.) and phosphodiesterase inhibitor sildenafil (5-40 mg/kg, i.p.) on the consolidation of morphine-induced CPP were assessed. In addition, the possible interaction between lithium, L-arginine and sildenafil or subeffective doses of lithium and L-NAME on the consolidation of morphine-induced contextual memory was evaluated. NMRI mice were used in all studies. Lithium (5-30 mg/kg, i.p.), immediately after conditioning trials, significantly reduced the time spent by mice in the reward-paired compartment. Although post-training administration of L-arginine, sildenafil or L-NAME had no significant effect on the consolidation of CPP, concomitant administration of L-arginine (50-150 mg/kg) and sildenafil (5-10 mg/kg) with lithium (30 mg/kg) prevented the impairing effect of lithium. Also, co-administration of sub-effective doses of lithium (1 mg/kg) and L-NAME (5 mg/kg) disrupted consolidation of CPP. However, delayed administration of effective doses of lithium, which shows specific effect on memory consolidation, did not affect morphine-induced CPP. Lithium seems to inhibit consolidation of morphine-induced CPP and this impairing effect might be via nitric oxide/cyclic GMP pathway.

Synthesis and in vivo brain distribution of carbon-11-labeled δ-opioid receptor agonists

Three new radiolabeled compounds, [(11)C]SNC80 ((+)-4-[(αR)-α-{(2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl}-3-[(11)C]methoxybenzyl-N,N-diethylbenzamide), N,N-diethyl-4-[3-methoxyphenyl-1-[(11)C]methylpiperidin-4-ylidenemethyl)benzamide and N,N-diethyl-4-[(1-[(11)C]methylpiperidin-4-ylidene)phenylmethyl]benzamide, were prepared as potential in vivo radiotracers for the δ-opioid receptor. Each compound was synthesized by alkylation of the appropriate desmethyl compounds using [(11)C]methyl triflate. In vivo biodistribution studies in mice showed very low initial brain uptake of all three compounds and no regional specific binding for [(11)C]SNC80. A monkey positron emission tomography study of [(11)C]SNC80 confirmed low brain permeability and uniform regional distribution of this class of opioid agonists in a higher species. Opioid receptor ligands of this structural class are thus unlikely to succeed as in vivo radiotracers, likely due to efficient exclusion from the brain by the P-glycoprotein efflux transporter.

Modulation of opiate-related signaling molecules in morphine-dependent conditioned behavior: conditioned place preference to morphine induces CREB phosphorylation

Opiate addiction is a chronic, relapsing behavioral disorder where learned associations that develop between the abused opiate and the environment in which it is consumed are brought about through Pavlovian (classical) conditioning processes. However, the signaling mechanisms/pathways regulating the mechanisms that underlie the responses to opiate-associated cues or the development of sensitization as a consequence of repeated context-independent administration of opiates are unknown. In this study we examined the phosphorylation levels of various classic signaling molecules in brain regions implicated in addictive behaviors after acute and repeated morphine administration. An unbiased place conditioning protocol was used to examine changes in phosphorylation that are associated with (1) the expression of the rewarding effects of morphine and (2) the sensitization that develops to this effect. We also examined the effects of a delta-receptor antagonist on morphine-induced conditioned behavior and on the phosphorylation of classic signaling molecules in view of data showing that blockade of delta-opioid receptor (deltaOR) prevents the development of sensitization to the rewarding effects of morphine. We find that CREB phosphorylation is specifically induced upon the expression of a sensitized response to morphine-induced conditioned behavior in brain areas related to memory consolidation, such as the hippocampus and cortex. A similar effect is also observed, albeit to a lesser extent, in the case of the GluR1 subunit of AMPA glutamate receptor. These increases in the phosphorylation levels of CREB and pGluR1 are significantly blocked by pretreatment with a deltaOR antagonist. These results indicate a critical role for phospho-CREB, AMPA, and deltaOR activities in mediating the expression of a sensitized response to morphine-dependent conditioned behavior.

14-Amino-4,5-epoxymorphinan derivatives and their pharmacological actions

14-Hydroxy-7,8-dihydromorphinone (oxymorphone) and its derivatives (oxycodone, naloxone, naltrexone) have become among the most important clinical agents to have been produced from opium. 14-Aminocodeinone and its 7,8-dihydro and morphinone derivatives are of more recent origin thanks to the work of Professor Gordon Kirby and his collaborators. The 14-amino parent compounds have proved of limited interest but their 14-acylamino- and 14-alkylamino derivatives have been extensively studied. The 4'-substituted cinnamoylamino-17-cyclopropylmethyl-7,8-dihydronormorphinones, C-CAM and M-CAM are the best available selective MOR irreversible antagonists and the related dihydrocodeinone MC-CAM, 4'-chloro-cinnamoylamino-17-cyclopropylmethyl-7,8-dihydronorcodeinone, is a long-acting MOR partial agonist with extended MOR-pseudoirreversible antagonist activity that could be a candidate for pharmacotherapy of opiate abuse/dependence.

Delta-opioid receptor antagonists prevent sensitization to the conditioned rewarding effects of morphine

BACKGROUND

Functional interactions between mu- and delta-opioid receptors (MOPr and DOPr, respectively) are implicated in morphine tolerance and dependence. The contribution of DOPr to the conditioned rewarding effects of morphine and the enhanced conditioned response that occurs after repeated morphine administration is unknown. This issue was addressed with the conditioned place preference procedure (CPP).

METHODS

Rats received home cage injections of saline or morphine (5.0 mg/kg/day x 5 days) before conditioning. For sensitization studies, DOPr antagonists (DOPr1/2: naltrindole, DOPr2: naltriben, DOPr1: 7-benzylidenenaltrexone) were administered before morphine injections. Conditioning sessions (2 morphine; 2 saline) commenced 3 days later. To assess the influence of acute DOPr blockade on the conditioning of morphine reward in naïve animals, 3 morphine and 3 saline conditioning sessions were employed. Antagonists were administered before morphine conditioning sessions.

RESULTS

Morphine was ineffective as a conditioning stimulus after two conditioning sessions in naïve rats. However, doses > or = 3.0 mg/kg produced significant CPP in morphine pre-exposed rats, confirming that sensitization develops to the conditioned rewarding effects of morphine. In animals that received morphine pre-exposure with naltrindole or naltriben but not 7-benzylidenenaltrexone, sensitization was prevented. No attenuation of morphine CPP was observed in animals that received DOPr antagonists acutely, before conditioning sessions.

CONCLUSIONS

These data indicate a critical role of DOPr systems in mediating sensitization to the conditioned rewarding effects of morphine. The efficacy of naltrindole and naltriben in preventing the enhanced response to morphine suggest the specific involvement of DOPr2 in the sensitization process.

(+)-Morphine attenuates the (-)-morphine-produced conditioned place preference and the mu-opioid receptor-mediated dopamine increase in the posterior nucleus accumbens of the rat

An unbiased conditioned place preference paradigm and the microdialysis technique was used to evaluate the effect of (+)-morphine pretreatment on the conditioned place preference produced by (-)-morphine and the increased release of the dopamine produced by mu-opioid ligand endomorphin-1, respectively, in the posterior nucleus accumbens shell of the male CD rat. (-)-Morphine (2.5-10 microg) microinjected into the posterior nucleus accumbens shell dose-dependently produced the conditioned place preference. Pretreatment with (+)-morphine (0.1-10 pg) given into the posterior accumbens shell for 45 min dose-dependently attenuated the conditioned place preference produced by (-)-morphine (5 microg) given into the same posterior accumbens shell. However, higher doses of (+)-morphine (0.1 and 1 ng) were less effective in attenuating the (-)-morphine-produced conditioned place preference. Thus, like given systemically, (+)-morphine given into the posterior nucleus accumbens shell also induces a U-shaped dose-response curve for attenuating the (-)-morphine-produced conditioned place preference. Microinjection of mu-opioid agonist endomorphin-1 (1-10 microg) given into the ventral tegmental area dose-dependently increased the release of the extracellular dopamine in the posterior nucleus accumbens shell in the urethane-anesthetized rats. The increased dopamine caused by endomorphin-1 (10 microg) was completed blocked by the (+)-morphine (10 pg) pretreatment given into ventral tegmental area. It is concluded that (+)-morphine attenuates the (-)-morphine-produced conditioned place preference and the mu-opioid receptor-mediated increase of extracellular dopamine in the posterior nucleus accumbens shell of the rat.

The OPRD1 and OPRK1 loci in alcohol or drug dependence: OPRD1 variation modulates substance dependence risk

Eleven single-nucleotide polymorphisms (SNPs) spanning OPRD1 were examined in 1063 European Americans (EAs) (620 cases with substance dependence (SD), including 557 with alcohol dependence (AD), 225 with cocaine dependence (CD) and 111 with opioid dependence (OD), and 443 controls). Nominally significant associations (P<0.05) of five SNPs with SD were observed; only the association of the non-synonymous variant G80T with OD remained significant after correction for multiple testing using SNPSpD. Haplotype analyses with six tag SNPs indicated that a specific haplotype GCAACT, which harbors G80T G-allele and C921T C-allele, was significantly associated with AD (chi(2)=14.82, degrees of freedom (d.f.)=1, P<0.001), CD (chi(2)=9.19, d.f.=1, P=0.002) and OD (chi(2)=20.68, d.f.=1, P<0.001). Logistic regression analyses, with sex and age being considered, demonstrated that this haplotype had a risk effect on AD (P=0.03, beta=1.86, odds ratio (OR)=6.43) and especially on OD (P<0.001, beta=3.92, OR=50.57). Moreover, seven SNPs covering OPRK1 were examined in the majority of the above subjects (390 cases, including 327 AD, 177 CD and 97 OD subjects, and 358 controls). Although no significant differences in allele, genotype or haplotype frequency distributions were seen between cases and controls, a specific OPRK1 haplotype, GGCTTCT, was significantly associated with AD (chi(2)=8.12, d.f.=1, P=0.004). Logistic regression analyses also revealed its risk effect on AD (P=0.009, beta=1.06, OR=2.90). Population stratification artifact was not observed in the sample. Taken together, our findings supported a positive association between OPRD1 variants and SD, and a positive haplotypic association between OPRK1 and AD in EAs.

A neuropeptide-centric view of psychostimulant addiction

Drugs of abuse all share common properties classically observed in human beings and laboratory animals. They enhance neural firing and dopamine tone within the nucleus accumbens and produce progressively greater drug-induced motor responses defined as behavioural sensitization. They produce conditioned place preference, a behavioural model of incentive motivation, which highlights the role of environmental cues in drug addiction. They increase brain reward function as seen by a lowering of intracranial self-stimulation thresholds. And last but not least, they are self-administered, and sometimes even abused, and can trigger reinstatement of drug-seeking behaviour in animals extinguished from drug self-administration. It has long been considered that the reinforcing properties of virtually all drugs of abuse, more specifically psychostimulants, are primarily dependent on activation of the mesolimbic dopamine system. However, recent evidence raises the importance of dopamine-independent mechanisms in reward-related behaviours. The overwhelming body of evidence that indicates a critical role for the mesolimbic dopamine system in the reinforcing effect of psychostimulants should not mask the key contribution of other modulatory systems in the brain. This review summarizes the complex and subtle role of several neuropeptidergic systems in various aspects of addictive behaviours observed in laboratory animals exposed to psychostimulants. A special emphasis is given to the cannabinoid, opioid, nociceptin/orphanin FQ, corticotropin-releasing factor and hypocretin/orexin systems. The relevance of these systems viewed as potential therapeutic targets for drug addiction is discussed in the light of their narrow pharmacological profile and their effectiveness in preventing drug addiction at doses usually not accompanied by severe side effects.

Endogenous nociceptin (orphanin FQ) suppresses basal hedonic state and acute reward responses to methamphetamine and ethanol, but facilitates chronic responses

The opioid peptide nociceptin (orphanin FQ) suppresses drug reward, drug self-administration, and impedes some of the processes believed to underlie the transition to addiction. As virtually all previous studies have used administration of nociceptin receptor agonists to evaluate the role of nociceptin on addiction-like behavior, the current study used a pharmacological (nociceptin receptor antagonist) and genetic (nociceptin receptor knockout mice) approach to elucidate the role of endogenous nociceptin. The nociceptin receptor antagonist UFP-101 induced a modest place preference, and enhanced the conditioned place preference induced by methamphetamine. In agreement with this, nociceptin receptor knockout mice had slightly enhanced methamphetamine and ethanol conditioned place preferences compared to wild-type mice. This effect did not appear to depend on differences in learning ability, as nociceptin receptor knockout mice had slightly weaker-conditioned place aversions to lithium chloride, the kappa-opioid receptor agonist, U50488H, and the general opiate antagonist, naloxone. The development of behavioral sensitization to methamphetamine was lower in nociceptin receptor knockout mice, and attenuated by UFP-101 administration to wild-type mice. Additionally, ethanol consumption and preference in a two-bottle choice test was lower in nociceptin receptor knockout mice, though ethanol-stimulated locomotion was stronger. Whereas the rewarding effect of methamphetamine and ethanol following chronic treatment, as measured by place conditioning, strengthened in wild-type mice, this effect was absent in nociceptin receptor knockout mice. These results suggest that endogenous N/OFQ suppresses basal and drug-stimulated increases in hedonic state, and plays either a permissive or facilitatory role in the development of addiction.

Absence of conditioned place preference or reinstatement with bivalent ligands containing mu-opioid receptor agonist and delta-opioid receptor antagonist pharmacophores

Treatment of pain with opioids is limited by their potential abuse liability. In an effort to develop analgesics without this side effect, a series of bivalent ligands containing a mu-opioid receptor agonist pharmacophore connected to a delta-opioid receptor antagonist pharmacophore through variable-length spacers (16-21 atoms) was synthesized. Members of this series [mu-opioid receptor (M)-delta-opioid receptor (D)-agonist (A)-antagonists (N): MDANs] are antinociceptive in the tail flick assay, but antinociceptive tolerance and physical dependence do not develop to ligands having spacers with 19-21 atoms. The current studies compared the rewarding properties of three bivalent ligands (MDAN-16, -19 and -21) and a mu-opioid receptor agonist (MA-19) to those of morphine in the conditioned place preference assay in mice after i.v. administration. Place preference developed to morphine and to MA-19, but not to the MDANs. The responses to MDAN-16 were highly variable, although place preference of borderline significance appeared to develop. Reinstatement was also evaluated after extinguishing morphine conditioned place preference; morphine and MA-19, but not the MDANs, reinstated morphine conditioned place preference. Taken together, these results suggest that the bivalents are less rewarding compared to morphine in opioid-naïve mice and do not induce reinstatement in previously morphine-preferring mice. The lack of a conditioned place preference response for MDAN-19 and -21, compared to the equivocal results with MDAN-16, suggests a minimum distance requirement between mu-opioid receptor and delta-opioid receptor recognition sites. This requirement may reflect the binding of MDAN-19 and -21 to mu-opioid receptor-delta-opioid receptor heterodimeric receptors that block reward but not antinociception.

dextro-Morphine attenuates the morphine-produced conditioned place preference via the sigma(1) receptor activation in the rat

An unbiased conditioned place preference paradigm was used to evaluate the effect of dextro-morphine on the morphine-produced reward in male CD rats. Morphine sulfate (1-10 mg/kg) given intraperitoneally dose-dependently produced the conditioned place preference. Pretreatment with dextro-morphine at a dose from 0.1 to 3 microg/kg given subcutaneously dose-dependently attenuated the morphine-produced conditioned place preference. However, dextro-morphine at a higher dose 100 microg/kg did not affect the morphine-produced conditioned place preference. Thus, dextro-morphine pretreatment induces a U-shaped dose-response curve for attenuating the morphine-produced conditioned place preference. The attenuation of the morphine-produced conditioned place preference was reversed by the pretreatment with the sigma(1) receptor antagonist BD1047 (N-[2-(3,4-Dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide. dextro-Morphine or BD1047 given alone did not affect the baseline place conditioning. It is concluded that dextro-morphine attenuated the morphine-produced conditioned place preference via the sigma(1) receptor activation.

The behavioral actions of lithium in rodent models: leads to develop novel therapeutics

For nearly as long as lithium has been in clinical use for the treatment of bipolar disorder, depression, and other conditions, investigators have attempted to characterize its effects on behaviors in rodents. Lithium consistently decreases exploratory activity, rearing, aggression, and amphetamine-induced hyperlocomotion; and it increases the sensitivity to pilocarpine-induced seizures, decreases immobility time in the forced swim test, and attenuates reserpine-induced hypolocomotion. Lithium also predictably induces conditioned taste aversion and alterations in circadian rhythms. The modulation of stereotypy, sensitization, and reward behavior are less consistent actions of the drug. These behavioral models may be relevant to human symptoms and to clinical endophenotypes. It is likely that the actions of lithium in a subset of these animal models are related to the therapeutic efficacy, as well the side effects, of the drug. We conclude with a brief discussion of various molecular mechanisms by which these lithium-sensitive behaviors may be mediated, and comment on the ways in which rat and mouse models can be used more effectively in the future to address persistent questions about the therapeutically relevant molecular actions of lithium.

Co-localization of GABA with other neuroactive substances in the basal ganglia

The dorsal striatum (caudate putamen) contains two types of GABAergic medium spiny neurons (MSNs) that are distinguished by the expression of either the opioid peptide, enkephalin, or the opioid peptide, dynorphin, as well as the tachykinin substance P. Pharmacological studies suggest that these peptides modulate local neurotransmission in the striatum in response to direct and indirect dopamine agonists. In contrast, GABA appears to have minimal impact within the striatum under these conditions. The actions of the peptide cocktail are dependent on the cellular distribution of their receptors in the striatal network. The net result of their actions is a homeostatic response that regulates striatal output and balances dopamine and glutamate receptor stimulation.

Mu-opioid receptors are not involved in acute cocaine-induced locomotor activity nor in development of cocaine-induced behavioral sensitization in mice

Although mu-opioid receptors have been extensively investigated for their role in drug reinforcement, little is known about the contribution of these receptors to the acute and sensitized locomotor response to cocaine. In this study mu-opioid receptor involvement in acute cocaine-induced locomotor activity and in the development of cocaine-induced behavioral sensitization was evaluated using mu-opioid receptor knockout mice and chronic naltrexone (NTX) pretreatment as models. In addition, co-administration of the specific mu-opioid receptor antagonist CTOP with repeated saline or cocaine injections was used to establish the involvement of mu-opioid receptors in sensitization to the locomotor stimulant effects of cocaine. The acute locomotor response to cocaine (3, 10, 20, or 30 mg/kg i.p.) of mu-opioid receptor knockout or chronic NTX pretreated mice was not different from the cocaine response of their respective controls. With respect to cocaine-induced behavioral sensitization, induced by daily injections of 20 mg/kg cocaine for 11 subsequent days, mu-opioid receptor knockout mice developed behavioral sensitization to the locomotor stimulant effects of cocaine (challenge 10 mg/kg i.p.) comparable to wild-type littermates and the mu-opioid receptor antagonist CTOP did not affect cocaine-induced sensitization either. However, mice that were pretreated with NTX exhibited augmented cocaine-induced behavioral sensitization relative to placebo pretreated controls, which may be ascribed to increased delta-opioid receptor levels as has been described for chronic NTX pretreated mice. The present findings suggest that mu-opioid receptors are not required for the acute locomotor response to cocaine nor are they essential for the development of cocaine-induced behavioral sensitization.

Differential conditioned place preference responses to endomorphin-1 and endomorphin-2 microinjected into the posterior nucleus accumbens shell and ventral tegmental area in the rat

An unbiased conditioned place preference (CPP) paradigm was used to evaluate the reward effects of endogenous mu-opioid receptor ligands endomorphin-1 (EM-1) and endomorphin-2 (EM-2) from the mesolimbic posterior nucleus accumbens (Acb) shell and the ventral tegmental area (VTA) in CD rats. EM-1 (1.6-8.1 nmol) microinjected into posterior Acb shell produced CPP, whereas EM-2 (8.7-17.5 nmol) given into the same Acb shell produced conditioned place aversion (CPA). EM-1 (1.6-16.3 nmol) microinjected into the VTA produced CPP, whereas EM-2 (8.7 and 17.5 nmol) given into the same VTA site did not produce any effect, but at a high dose (35 nmol) produced CPP. EM-1 (3.3 nmol) or EM-2 (17.5 nmol) microinjected into the nigrostriatal substantia nigra was not significantly different from vehicle-injected groups. D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) at 94.13 pmol or 3-methoxynaltrexone at 0.64 pmol microinjected into the posterior Acb shell blocked EM-1-induced CPP and EM-2-induced CPA. At a higher dose, CTOP (941.3 pmol) and 3-methoxynaltrexone (6.4 pmol) produced CPA and CPP, respectively. Coadministration with antiserum against dynorphin A(1-17) (Dyn) (10 microg) microinjected into the posterior Acb shell blocked EM-2-induced CPA. However, it did not affect EM-1-induced CPP. It is concluded that EM-1 and EM-2 produce site-dependent CPP and CPA, respectively, by stimulation of different subtypes of mu-opioid-receptors; stimulation of one subtype of mu-opioid-receptor at the posterior Acb shell and VTA by EM-1 induces CPP, whereas stimulation of another subtype of mu-opioid receptor at the posterior Acb shell, but not the VTA, by EM-2 induces the release of Dyn to produce CPA.

Contrasting effects of μ opioid receptor and δ opioid receptor deletion upon the behavioral and neurochemical effects of cocaine

Conventional brain microdialysis was used to assess basal and cocaine-induced dopamine (DA) levels in the nucleus accumbens of wildtype (WT) C57BL/6J mice and mice with constitutive deletion of ether mu- or delta-opioid receptors (MOR or DOR knockout [KO], respectively). Locomotor activity was assessed in these same animals. Basal locomotor activity of DOR KO was elevated relative to MOR KO, but did not differ from that of WT mice. DOR mice, but not WT or MOR KO, exhibited a significant increase in activity in response to an injection of saline. The acute administration of cocaine produced a dose-related increase in locomotor activity in the three genotypes. The locomotor activating effects of a low dose (10 mg/kg) of cocaine were enhanced in DOR KO mice whereas the locomotor activating effects of both a low and higher (20 mg/kg) dose of cocaine were reduced in MOR KO animals. Microdialysis studies revealed no difference between genotypes in basal DA levels. Acute administration of cocaine, but not saline, increased DA levels in WT and KO animals. Paradoxically, however, the magnitude of this effect was smaller in DOR KO as compared with that in either WT or MOR KO. These data indicate that constitutive deletion of either MOR or DOR results in contrasting effects upon responsiveness to cocaine, which is consistent with the distinct phenotypes previously described for these mutants.

Asymmetric serial interactions between ethanol and cocaine in taste aversion learning

Although the interaction between ethanol and cocaine is well documented, it has generally been limited to situations in which the two drugs are given concurrently. Little exists on the interaction between ethanol and cocaine when one drug is given prior to the other. In Experiment 1, female Long-Evans rats were given five exposures to ethanol (2 g/kg ip) or vehicle prior to taste aversion conditioning with cocaine (32 mg/kg sc) for a total of five conditioning trials. In Experiment 2, rats were given five exposures to cocaine (32 mg/kg sc) or vehicle prior to taste aversion conditioning with ethanol (2 g/kg ip) for a total of five conditioning trials. Ethanol-preexposed, cocaine-conditioned animals (Experiment 1) displayed attenuated aversions to the cocaine-associated solution, drinking significantly greater amounts of saccharin than vehicle-preexposed, conditioned subjects. Conversely, cocaine-preexposed, ethanol-conditioned animals (Experiment 2) displayed robust aversions to the ethanol-associated solution, drinking levels comparable to those consumed by vehicle-preexposed, conditioned subjects and drinking significantly less than controls. Although the basis for these asymmetric effects is not known, they may have implications for abuse vulnerability in that drug history may impact subsequent drug toxicity that, in turn, may alter drug acceptability.

Chronic intermittent amphetamine pretreatment enhances future appetitive behavior for drug- and natural-reward: interaction with environmental variables

Appetitive behavior for drug and sexual reward is enhanced in animals with a history of amphetamine-experience. The present experiment investigated whether prior exposure to a sensitizing regimen of amphetamine treatment would 'globally' enhance future appetitive behaviors of adult male Sprague-Dawley rats, and whether the drug preexposure-environment or intermittency of administration would affect this development. Reward appetite was compared in drug-experienced versus drug-naive rats using amphetamine place-preference conditioning (CPP) and a natural-incentive sensitization task, which measured appetitive approach for food and sexual reward. Experiment I found that 10 daily exposures to 1 mg/kg amphetamine did not alter future psychostimulant CPP, regardless of abstinence schedule. Although daily exposure to a higher amphetamine dose also did not alter appetitive behavior when measured after 2-weeks drug abstinence in Experiment II, alternate-day amphetamine experience (5.0 mg/kg, twice-a-day) in an initially unfamiliar environment persistently enhanced future amphetamine CPP and appetitive behavior for natural reward. Identical treatment administered in the homecage did not. Furthermore, sensitized reward-seeking behaviors were not globally evident. Animals that showed sensitized amphetamine CPP did not show sensitized food-seeking behavior and vice versa. Thus, the environment surrounding chronic psychostimulant drug experience can greatly affect subsequent reward appetite, but the sensitized expression may be individually determined.

Differentially altered mGluR1 and mGluR5 mRNA expression in rat caudate nucleus and nucleus accumbens in the development and expression of behavioral sensitization to repeated amphetamine administration

Altered glutamatergic transmission in the striatum may be implicated in behavioral sensitization to repeated amphetamine (AMPH) administration. Quantitative in situ hybridization histochemistry was performed to define the effects of acute and chronic AMPH exposures on mRNA expression of Group I metabotropic glutamate receptors (mGluRs) in the striatum. Behavioral ratings indicated that the motor activity of rats was significantly higher after the final of five daily AMPH injections (4 mg/kg, i.p.) than that after the first of five daily AMPH, indicative of the development of behavioral sensitization. Moreover, the motor activity of rats treated with five daily AMPH was significantly greater than that of rats treated with five daily saline in response to a 2 mg/kg challenge dose of AMPH 7, 14, 28, and 60 days after the discontinuation of drug treatments, indicative of the persistent expression of behavioral sensitization. Three hours after acute administration of AMPH to naive rats, mGluR1 and mGluR5 mRNA expression in the dorsal (caudatoputamen) and ventral (nucleus accumbens) striatum showed no change as compared to acute saline injection. In rats that developed behavioral sensitization to repeated AMPH, mGluR1 levels in the dorsal and ventral striatum were increased by 53% and 43%, respectively, 3 h after the final AMPH treatment. However, this change did not persist during withdrawal since it was not observed 7, 14, and 28 days after the discontinuation of AMPH treatment. Conversely, mGluR5 levels were markedly reduced 3 h after the final of five daily AMPH treatments in the entire striatum of sensitized rats (34% and 77% of controls in the dorsal and ventral striatum, respectively). The reduction persisted at 7, 14, and 28 days of withdrawal. These results reveal a close linkage between striatal Group I mGluR gene expression and behavioral sensitization to AMPH. This may indicate functional implications of the two subtypes of Group I mGluRs in the regulation of behavioral sensitization to the dopamine stimulant.

Ethanol-induced conditioned place aversion in mice

Previous studies have shown that ethanol produces conditioned place preference (CPP) in mice when injections are given immediately before exposure to the conditioned stimulus (CS). Paradoxically, however, injection of ethanol immediately after the CS produces conditioned place aversion (CPA). Four experiments were conducted to characterize the parametric boundaries of CPA produced by post-CS ethanol exposure. Experiment 1 showed that CPA is positively related to ethanol dose, with significant CPA at 2 and 4 g / kg, but not at 1 g / kg. Experiment 2 revealed an inverse relationship between CPA and trial duration, i.e. significant CPA occurred when the trial duration was 5, 15 or 30 min, but not when it was 60 or 90 min. Experiment 3 indicated that ethanol pre-exposure (eight daily injections) significantly reduced subsequent development of CPA. Finally, experiment 4 showed that repeated exposure to the CS alone (six 30 min exposures to each CS) after CS-ethanol pairings produced complete extinction of CPA. The same extinction procedure also completely eliminated CPP induced by pre-CS injections of ethanol. Overall, these studies demonstrate that CPA induced by post-CS ethanol injection is influenced by many of the same variables that affect CPP produced by pre-CS ethanol injection in mice. However, these findings do not resolve the issue of whether the 'before-versus-after' effect in ethanol place conditioning is better explained by assuming ethanol produces only rewarding effects or by assuming that ethanol produces both rewarding and aversive effects.

Orphanin FQ and behavioral sensitization to cocaine

The recently identified endogenous ligand for the ORL-1 (opioid receptor-like) receptor, orphanin FQ, has been shown to induce hypolocomotion and to decrease extracellular dopamine levels in the nucleus accumbens (N.Acc) after intraventricular (ICV) administration. This study investigated the effect of intraventral tegmental area (VTA) administration of orphanin FQ on the hyperlocomotor effects of peripheral cocaine administration and on the development of behavioral sensitization to cocaine. The administration of cocaine (40 mg/kg IP) once daily for 3 days to male Sprague-Dawley rats resulted in an enhanced locomotor response to a subsequent challenge of cocaine (10 mg/kg IP) 5 days later. The bilateral administration of orphanin FQ (10 microg/side or 30 microg/side) into the VTA 5-10 min prior to the administration of cocaine (40 mg/kg IP) produced a transient (15-30 min) decrease in the hyperlocomotor response to cocaine on day 1 but not on days 2 and 3 of the sensitization paradigm. Such orphanin FQ pretreatment on days 1-3 had no effect on the development of a sensitized response to cocaine (10 mg/kg IP) 5-7 days after the last orphanin FQ injection. However, repeated intra-VTA administration of orphanin FQ (30 microg/side) alone for 3 days resulted in a sensitized response to a single dose of cocaine (10 mg/kg IP) given 5-7 days later. These results indicate that orphanin FQ decreases the activity of mesolimbic dopamine neurons via an action in the VTA, an effect that is both transient and demonstrates rapid tolerance, and consequently, is insufficient to prevent the development of cocaine sensitization. The ability of the peptide to induce cocaine sensitization when administered alone despite its acute inhibitory effects is unique and requires further study to elucidate the mechanisms responsible.

Differential knockdown of delta-opioid receptor subtypes in the rat brain by antisense oligodeoxynucleotides targeting mRNA

Two antisense oligodeoxynucleotides (A-ODN), targeting delta-opioid receptor mRNA (DOR) and two mismatch ODN sequences (mODN) were continuously infused for 24 days into the lateral brain ventricles of Wistar rats. The density of delta-opioid receptors in rat brain homogenates was measured by saturation binding experiments using four selective ligands, two agonists ([D-Ala2, Glu4]-deltorphin and DPDPE) and two antagonists (Dmt-Tic-OH and naltrindole), and by immunoblotting SDS solubilized receptor protein. In brain membranes of mODN or saline-infused rats, the rank order of delta-opioid receptor density, calculated by Bmax values of the four delta-opioid receptor ligands, was: [D-Ala2, Glu4]deltorphin approximately Dmt-Tic-OH approximately naltrindole (86-118 fmo/mg protein) > DPDPE (73.6+/-6.3 fmol/mg protein). At the end of the 24 day infusion of A-ODN targeting DOR nucleotide sequence 280299 (A-ODN280-299), the Bmax of DPDPE (62.4+/-3.2 fmol/mg protein) was significantly higher than that of Dmt-Tic-OH (31.5+/-3.9 fmol/mg protein). Moreover, both the Kd value for DPDPE saturation binding and the Ki value for Dmt-Tic-OH displacement by DPDPE were halved. In contrast, an A-ODN treatment targeting exon 3 (A-ODN741-760) decreased the specific binding of [D-Ala2, Glu4]deltorphin and Dmt-Tic-OH significantly less (67%-81%) than the binding of DPDPE (53%), without changes in DPDPE Ki and KD values. No A-ODN treatment modified the specific binding of the micro-opioid agonist DAMGO and of the k-selective opioid receptor ligand U69593. On the Western blot of solubilized striatum proteins, A-ODN(280-299) and A-ODN(741-760) downregulated the levels of the DOR protein, whereas the corresponding mODN were inactive. The 24-day infusion of A-ODN(280-299) inhibited the rat locomotor response to [D-Ala2, Glu4]deltorphin but not to DPDPE. Intracerebroventricular (i.c.v.) infusion of A-ODN(741-760) reduced the locomotor responses to both delta-opioid receptor agonists, whereas mODN infusion never affected agonist potencies. In conclusion, these results demonstrate that 24-day continuous i.c.v. infusion of A-ODN targeting the nucleotide sequence 280-299 of DOR can differentially knockdown delta1 and delta2 binding sites in the rat brain.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

Continued trends in the conditioned place preference literature from 1992 to 1996, inclusive, with a cross-indexed bibliography

In light of the overwhelming response to the previous publication in Neuroscience and Biobehavioral Reviews (1993, 17, 21-41) regarding trends in place conditioning (either preference or aversion), the present work constitutes a five-year follow-up to review the empirical research in this behavioral paradigm from 1992 to 1996, inclusively. The behavioral technique has grown as indicated by the number of publications over the last five years which equals those authored over the 35 years covered by our last survey. The previous work used descriptive statistics to explore topical issues, whereas the present work discusses trends since that time and hopes to provide an exhaustive bibliography of the CPP literature, including articles, published abstracts, book chapters and reviews, as well as providing a cross-index of identified key words/drugs tested.

Opioid reward mechanisms: a key role in drug abuse?

There is increasing evidence to implicate the mesolimbic dopamine system in the rewarding effects of drugs of abuse such as opioids, psychostimulants, and alcohol, and in addition endogenous opioids may play a key role in the underlying adaptive mechanisms. Opioid agonists with affinity for µ and delta opioid receptors are rewarding, whereas opioid agonists with affinity for kappa receptors are aversive. These opposing motivational effects are paralleled by an increase and decrease, respectively, of dopamine release in the nucleus accumbens. Opposite effects are induced in response to selective antagonists for these different receptor types, pointing to tonically active endogenous opioid reward mechanisms. Withdrawal from chronic morphine results in sensitization for opioid reward; an effect that is counteracted by kappa opioid agonists. The rewarding effects of psychostimulants such as cocaine and amphetamine, mediated by the mesolimbic dopamine pathway, are modulated by opioid mechanisms in both directions: sensitization by morphine pretreatment, inhibition by kappa receptor agonists. A modulatory role of endogenous opioids is also suggested from biochemical data, showing increased dynorphin and kappa receptor expression after chronic cocaine treatment. Alcohol reward involves the mesolimbic reward system also, and opioids modulate this behaviour. Naltrexone as well as selective µ and delta opioid receptor antagonists decrease alcohol consumption in operant conditioning models. Biochemical approaches point to a functional deficit of endogenous opioids in genetic models exhibiting high prevalence for alcohol intake. The therapeutic implications of these data are discussed.Key words: reward mechanisms, endogenouos opioid systems, psychostimulants, alcohol.

Naltrindole, a selective delta-opioid receptor antagonist, potentiates the lethal effects of cocaine by a central mechanism of action

The potentiation of the toxic and lethal effects of cocaine by the selective delta-opioid receptor antagonist naltrindole was explored in unrestrained, unanesthetized rats that received a continuous intravenous infusion of cocaine until death. The lethal dose of cocaine was lowered dose dependently in animals administered naltrindole intracisternally (3.0-30 microg), but not intravenously (30-300 microg). There was also a decrease in the lethal dose of cocaine following an injection of the nonselective opioid antagonist naltrexone, but not naloxone. However, the seizure-producing dose of cocaine was decreased dose dependently in rats that received naltrindole, regardless of the route of administration, naloxone, or naltrexone. In contrast, the effect of cocaine on heart rate was altered only by centrally administered naltrindole or intravenous naltrexone, with a dose of 30 microg naltrindole and 10 mg/kg naltrexone abolishing the bradycardic effect of cocaine. Despite this, neither naltrindole nor naltrexone changed the hypertensive effect of cocaine. Higher doses of naltrindole (100 microg i.c.) produced significant increases in heart rate and mean arterial pressure and were not tested in combination with cocaine. Because the lethal dose of cocaine was reduced only when naltrindole was administered intracisternally, the potentiation of the lethal effects of cocaine by naltrindole is through a central mechanism of action that may involve changes in cardiovascular function.

A proposed test battery and constellations of specific behavioral paradigms to investigate the behavioral phenotypes of transgenic and knockout mice

Behavioral phenotyping of transgenic and knockout mice requires rigorous, formal analyses. Well-characterized paradigms can be chosen from the established behavioral neuroscience literature. This review describes (1) a series of neurological and neuropsychological tests which are effectively used as a first screen for behavioral abnormalities in mutant mice, and (2) a series of specific behavioral paradigms, clustered by category. Included are multiple paradigms for each category, including learning and memory, feeding, analgesia, aggression, anxiety, depression, schizophrenia, and drug abuse models. Examples are given from the experiences of the authors, in applying these experimental designs to transgenic and knockout mice. Extensive references for each behavioral paradigm are provided, to allow new investigators to access the relevant literature on behavioral methodology.

Blockade by naloxone of cocaine-induced hyperactivity, reverse tolerance and conditioned place preference in mice

Cocaine-induced hyperactivity was inhibited by a single administration of naloxone (2 and 5 mg/kg, i.p.), an opioid receptor antagonist, and naloxone administered prior to and during the chronic injection of cocaine attenuated the development of both cocaine-induced reverse tolerance and conditioned place preference (CPP). Dopamine (DA) receptor supersensitivity which developed in cocaine-induced reverse tolerant or CPP mice, was also inhibited by naloxone. Furthermore, naloxone reduced an apomorphine-induced striatal dopaminergic action, climbing behavior. Therefore, the present studies suggest that cocaine-induced dopaminergic behaviors, such as hyperactivity, reverse tolerance and CPP, may be commonly produced via activation of an opioid receptor. The development of DA receptor supersensitivity may be a possible common mechanism of cocaine-induced reverse tolerance and CPP, since cocaine-induced changes in sensitivity to apomorphine, as well as apomorphine-induced climbing behavior in mice, were both inhibited by naloxone.

Probes for narcotic receptor mediated phenomena. 23. Synthesis, opioid receptor binding, and bioassay of the highly selective delta agonist (+)-4-[(alpha R)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]- N,N-diethylbenzamide (SNC 80) and related novel nonpeptide delta opioid receptor ligands

The highly selective delta (delta) opioid receptor agonist SNC 80 [(+)-4- [(alpha R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N ,N- diethylbenzamide, (+)-21] and novel optically pure derivatives were synthesized from the enantiomers of 1-allyl-trans-2,5-dimethylpiperazine (2). The piperazine (+/-)-2 was synthesized, and its enantiomers were obtained on a multigram scale in > 99% optical purity by optical resolution of the racemate with the camphoric acids. The absolute configuration of (+)-2 was determined to be 2S,5R by X-ray analysis of the salt with (+)-camphoric acid. Since the chirality of the starting material was known, and the relative configuration of compounds (-)-21, (-)-22, and (+)-23 were obtained by single-crystal X-ray analysis, the assignment of the absolute stereochemistry of the entire series could be made. Radioreceptor binding studies in rat brain preparations showed that methyl ethers (+)-21 (SNC 80) and (-)-25 exhibited strong selectivity for rat delta receptors with low nanomolar affinity to delta receptors and only micromolar affinity for rat mu (mu) opioid receptors. Compounds (-)-21, (-)-22, and (-)-23 showed micromolar affinities for delta opioid receptors. The unsubstituted derivative (+)-22 and the fluorinated derivative (-)-27 showed > 2659- and > 2105-fold delta/mu binding selectivity, respectively. The latter derivatives are the most selective ligands described in the new series. Studies with some of the compounds described in the isolated mouse vas deferens and guinea pig ileum bioassays revealed that all were agonists with different degrees of selectivity for the delta opioid receptor. These data show that (+)-21 and (+)-22 are potent delta receptor agonists and suggest that these compounds will be valuable tools for further study of the delta opioid receptor at the molecular level, including its function and role in analgesia and drug abuse.

Naloxone inhibits the reinforcing and motivational aspects of cocaine addiction in mice

The opioid receptor antagonist naloxone is known to influence a wide range of behavioral effects of cocaine, including its addictive property. In the present study the effects of different doses of naloxone and naloxone-methyl-iodide, a methylated analogon of naloxone that does not penetrate the blood-brain barrier, on the action of cocaine in the intravenous self-administration and conditioned place preference (CCP) paradigm were assessed. Systemic naloxone, but not naloxone-methyl-iodide, dose-dependently suppressed cocaine intake during self-administration and decreased the preference for the cocaine-associated compartment in the CCP paradigm. A significant blockade of cocaine's effects was only present at a relatively high dose of NLX (1.0 mg/kg, s.c.). In addition, NLX produced a rightward shift in the inverted U-shaped dose-response curve for cocaine reward during self-administration, indicating a decrease in sensitivity for the reinforcing effects of cocaine. These data demonstrate that blockade of opioid receptors in the brain block both the reinforcing and conditioned motivational effects of cocaine. An interaction between endogenous opioid systems and local dopaminergic systems is suggested in mediating the effects of NLX on cocaine.

Role of delta-opioid receptors in mediating the aversive stimulus effects of morphine withdrawal in the rat

An unbiased place preference conditioning procedure was used to examine the role of delta-opioid receptors in mediating the aversive effects of opioid withdrawal. Rats were implanted s.c. with two pellets each containing placebo or 75 mg morphine. Single-trial conditioning sessions with saline and the opioid receptor antagonists naloxone (0.001-1.0 mg/kg, s.c.), naltrindole (0.01-3.0 mg/kg, s.c.) or naltriben (0.01-3.0 mg/kg, s.c.) commenced 4 days later. During these conditioning sessions, physical signs of withdrawal were also quantified. Tests of conditioning were conducted on day 5. Naloxone in doses of 0.01-1.0 mg/kg produced significant conditioned place aversions in morphine-implanted animals. A dose of 0.01 mg/kg produced few physical withdrawal signs whereas higher doses resulted in marked wet dog shakes, body weight loss ptosis and diarrhea. No such effects were observed in control (placebo-implanted) animals. Administration of the selective delta-opioid receptor antagonists naltrindole and naltriben produced dose-related place aversions in morphine-implanted animals. The magnitude of these effects did not differ from that observed with naloxone. The minimum effective doses of naltrindole and naltriben were 0.1 mg/kg. Doses of 0.1-1.0 mg/kg produced few, if any, somatic signs of withdrawal whereas higher doses of these antagonists only produced diarrhea and wet-dog shakes. Other withdrawal signs were absent. In contrast to the opioid receptor antagonists tested, the dopamine D1 receptor antagonist SCH23390 failed to produced conditioned place aversions or physical signs of withdrawal in morphine-pelleted animals. These data demonstrate that the selective blockade of either delta- or mu-opioid receptors is sufficient to induce conditioned aversive effects in morphine-dependent animals. They also indicate that physical symptoms associated with precipitated morphine withdrawal differ depending upon the opioid receptor antagonist employed.

Differential role of delta-opioid receptors in the development and expression of behavioral sensitization to cocaine

The present study was designed to determine whether the selective delta-opioid receptor antagonist naltrindole hydrochloride can prevent the expression and development of sensitization to the locomotor-activating effects of cocaine. Rats were sensitized to the motor stimulant effects of cocaine (20 mg/kg i.p. x 3 days). 48 h after withdrawal of pretreatment, rats were pretreated with naltrindole (0.1-1.0 mg/kg s.c.) or its vehicle and 15 min later challenged with either saline or the sensitizing dose of cocaine. In a second set of experiments, naltrindole (0.1-1.0 mg/kg s.c.) or its vehicle were given in combination with either saline or cocaine (20 mg/kg i.p.) for 3 days. Activity in response to saline and to cocaine (20 mg/kg i.p.) was assessed on days 4 and 5, respectively. Additional experiments determined whether naltrindole prevents the development of sensitization to the locomotor-activating effects of nicotine: naltrindole (0.3, 1.0 mg/kg s.c.) or its vehicle were given in combination with nicotine (0.6 mg/kg s.c.) for 3 days. Naltrindole blocked the development but not expression of sensitization to the locomotor-activating effects of cocaine. In contrast, naltrindole failed to modify nicotine-induced sensitization in nicotine-treated animals. These data suggest that delta-opioid receptors are involved in the development but not expression of behavioral sensitization to cocaine.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered this year include stress: tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.