Genetic and pharmacological manipulation of mu opioid receptors in mice reveals a differential effect on behavioral sensitization to cocaine

Endogenous enkephalin is necessary for cocaine-induced alteration in glutamate transmission within the nucleus accumbens

Altered glutamate transmission within the nucleus accumbens (NAc) has been proposed as a central mechanism underlying behavioural sensitisation associated with repeated cocaine exposure. In addition to glutamate, enkephalin, an endogenous opioid peptide derived from proenkephalin, is necessary for the neuroadaptations associated with chronic cocaine. However, the influence of enkephalin on long-term changes in glutamate transmission within the NAc associated with cocaine-induced sensitisation has not been described. This study used knockout proenkephalin mice (KO) to study the influence of endogenous enkephalin on the adaptations in glutamate neurotransmission associated with repeated cocaine treatment. Wild-type (WT) and KO mice were treated with daily cocaine injections for 9 days to induce sensitisation. On days 15 and 21, the animals received a cocaine challenge and locomotor sensitisation was evaluated, and microdialysis was performed to determine accumbens glutamate content on day 21. No expression of behavioural sensitisation to cocaine was evidenced in the KO mice. Consistently, these showed no changes in glutamate transmission in the NAc associated with repeated cocaine. This study reveals the central role of enkephalin in regulating the glutamate mechanisms associated with cocaine sensitisation.

Enkephalin as a Pivotal Player in Neuroadaptations Related to Psychostimulant Addiction

Enkephalin expression is high in mesocorticolimbic areas associated with psychostimulant-induced behavioral and neurobiological effects, and may also modulate local neurotransmission in this circuit network. Psychostimulant drugs, like amphetamine and cocaine, significantly increase the content of enkephalin in these brain structures, but we do not yet understand the specific significance of this drug-induced adaptation. In this review, we summarize the neurochemical and molecular mechanism of psychostimulant-induced enkephalin activation in mesocorticolimbic brain areas, and the contribution of this opioid peptide in the pivotal neuroadaptations and long-term behavioral changes underlying psychostimulant addiction. There is evidence suggesting that adaptive changes in enkephalin content in the mesocorticolimbic circuit, induced by acute and chronic psychostimulant administration, may represent a key initial step in the long-term behavioral and neuronal plasticity induced by these drugs.

ProSAAS-derived peptides are regulated by cocaine and are required for sensitization to the locomotor effects of cocaine

To identify neuropeptides that are regulated by cocaine, we used a quantitative peptidomic technique to examine the relative levels of neuropeptides in several regions of mouse brain following daily intraperitoneal administration of 10 mg/kg cocaine or saline for 7 days. A total of 102 distinct peptides were identified in one or more of the following brain regions: nucleus accumbens, caudate putamen, frontal cortex, and ventral tegmental area. None of the peptides detected in the caudate putamen or frontal cortex were altered by cocaine administration. Three peptides in the nucleus accumbens and seven peptides in the ventral tegmental area were significantly decreased in cocaine-treated mice. Five of these ten peptides are derived from proSAAS, a secretory pathway protein and neuropeptide precursor. To investigate whether proSAAS peptides contribute to the physiological effects of psychostimulants, we examined acute responses to cocaine and amphetamine in the open field with wild-type (WT) and proSAAS knockout (KO) mice. Locomotion was stimulated more robustly in the WT compared to mutant mice for both psychostimulants. Behavioral sensitization to amphetamine was not maintained in proSAAS KO mice and these mutants failed to sensitize to cocaine. To determine whether the rewarding effects of cocaine were altered, mice were tested in conditioned place preference (CPP). Both WT and proSAAS KO mice showed dose-dependent CPP to cocaine that was not distinguished by genotype. Taken together, these results suggest that proSAAS-derived peptides contribute differentially to the behavioral sensitization to psychostimulants, while the rewarding effects of cocaine appear intact in mice lacking proSAAS.

Responses to drugs of abuse and non-drug rewards in leptin deficient ob/ob mice

RATIONALE

Although leptin receptors are found in hypothalamic nuclei classically associated with homeostatic feeding mechanisms, they are also present in brain regions known to regulate hedonic-based feeding, natural reward processing, and responses to drugs of abuse. The ob/ob mouse is deficient in leptin signaling, and previous work has found altered mesolimbic dopamine signaling and sensitivity to the locomotor activating effects of amphetamine in these mice.

OBJECTIVES

We directly assessed responses to three drugs of abuse and non-drug rewards in the leptin-deficient ob/ob mouse.

METHODS

Ob/ob mice were tested in assays of sweet preference, novelty seeking, and drug reward/reinforcement.

RESULTS

In assays of novelty seeking, novel open field activity and operant sensation seeking were reduced in ob/ob mice, although novel object interaction and novel environment preference were comparable to wild types. We also found that ob/ob mice had specific phenotypes in regard to cocaine: conditioned place preference for 2.5 mg/kg was increased, while the locomotor response to 10 mg/kg was reduced, and cocaine self-administration was the same as wild types. Ob/ob mice also acquired self-administration of the potent opioid remifentanil, but breakpoints for the drug were significantly reduced. Finally, we found significant differences in ethanol drinking in ob/ob mice that correlated negatively with body weight and positively with operant sensation seeking.

CONCLUSIONS

In conclusion, ob/ob mice displayed task-specific deficits in novelty seeking and dissociable differences in reward/reinforcement associated with cocaine, remifentanil, and ethanol.

Enkephalin is essential for the molecular and behavioral expression of cocaine sensitization

Behavioral sensitization to cocaine is associated to neuroadaptations that contribute to addiction. Enkephalin is highly expressed in mesocorticolimbic areas associated with cocaine-induced sensitization; however, their influence on cocaine-dependent behavioral and neuronal plasticity has not been explained. In this study, we employed a knockout (KO) model to investigate the contribution of enkephalin in cocaine-induced behavioral sensitization. Wild-type (WT) and proenkephalin KO mice were treated with cocaine once daily for 9 days to induce sensitization. Additionally, to clarify the observations in KO mice, the same procedure was applied in C57BL/6 mice, except that naloxone was administered before each cocaine injection. All animals received a cocaine challenge on days 15 and 21 of the treatment to evaluate the expression of locomotor sensitization. On day 21, microdialysis measures of accumbal extracellular dopamine, Western blotting for GluR1 AMPA receptor (AMPAR), phosphorylated ERK2 (pERK2), CREB (pCREB), TrKB (pTrkB) were performed in brain areas relevant for sensitization from KO and WT and/or naloxone- and vehicle pre-treated animals. We found that KO mice do not develop sensitization to the stimulating properties of cocaine on locomotor activity and on dopamine release in the nucleus accumbens (NAc). Furthermore, pivotal neuroadaptations such as the increase in pTrkB receptor, pERK/CREB and AMPAR related to sensitized responses were absent in the NAc from KO mice. Consistently, full abrogation of cocaine-induced behavioral and neuronal plasticity after naloxone pre-treatment was observed. We show for first time that the proenkephalin system is essential in regulating long-lasting pivotal neuroadaptations in the NAc underlying behavioral sensitization to cocaine.

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.

15 years of genetic approaches in vivo for addiction research: Opioid receptor and peptide gene knockout in mouse models of drug abuse

The endogenous opioid system is expressed throughout the brain reinforcement circuitry, and plays a major role in reward processing, mood control and the development of addiction. This neuromodulator system is composed of three receptors, mu, delta and kappa, interacting with a family of opioid peptides derived from POMC (β-endorphin), preproenkephalin (pEnk) and preprodynorphin (pDyn) precursors. Knockout mice targeting each gene of the opioid system have been created almost two decades ago. Extending classical pharmacology, these mutant mice represent unique tools to tease apart the specific role of each opioid receptor and peptide in vivo, and a powerful approach to understand how the opioid system modulates behavioral effects of drugs of abuse. The present review summarizes these studies, with a focus on major drugs of abuse including morphine/heroin, cannabinoids, psychostimulants, nicotine or alcohol. Genetic data, altogether, set the mu receptor as the primary target for morphine and heroin. In addition, this receptor is essential to mediate rewarding properties of non-opioid drugs of abuse, with a demonstrated implication of β-endorphin for cocaine and nicotine. Delta receptor activity reduces levels of anxiety and depressive-like behaviors, and facilitates morphine-context association. pEnk is involved in these processes and delta/pEnk signaling likely regulates alcohol intake. The kappa receptor mainly interacts with pDyn peptides to limit drug reward, and mediate dysphoric effects of cannabinoids and nicotine. Kappa/dynorphin activity also increases sensitivity to cocaine reward under stressful conditions. The opioid system remains a prime candidate to develop successful therapies in addicted individuals, and understanding opioid-mediated processes at systems level, through emerging genetic and imaging technologies, represents the next challenging goal and a promising avenue in addiction research. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

The endogenous opioid system in cocaine addiction: what lessons have opioid peptide and receptor knockout mice taught us?

Cocaine addiction has become a major concern in the UK as Britain tops the European 'league table' for cocaine abuse. Despite its devastating health and socio-economic consequences, no effective pharmacotherapy for treating cocaine addiction is available. Identifying neurochemical changes induced by repeated drug exposure is critical not only for understanding the transition from recreational drug use towards compulsive drug abuse but also for the development of novel targets for the treatment of the disease and especially for relapse prevention. This article focuses on the effects of chronic cocaine exposure and withdrawal on each of the endogenous opioid peptides and receptors in rodent models. In addition, we review the studies that utilized opioid peptide or receptor knockout mice in order to identify and/or clarify the role of different components of the opioid system in cocaine-addictive behaviours and in cocaine-induced alterations of brain neurochemistry. The review of these studies indicates a region-specific activation of the µ-opioid receptor system following chronic cocaine exposure, which may contribute towards the rewarding effect of the drug and possibly towards cocaine craving during withdrawal followed by relapse. Cocaine also causes a region-specific activation of the κ-opioid receptor/dynorphin system, which may antagonize the rewarding effect of the drug, and at the same time, contribute to the stress-inducing properties of the drug and the triggering of relapse. These conclusions have important implications for the development of effective pharmacotherapy for the treatment of cocaine addiction and the prevention of relapse.

Genetic association of GABA-A receptor alpha-2 and mu opioid receptor with cocaine cue-reactivity: evidence for inhibitory synaptic neurotransmission involvement in cocaine dependence

BACKGROUND

This pilot feasibility study examined the role of genetics in laboratory-induced cocaine craving.

METHODS

Thirty-four African American, cocaine-depend- ent male subjects underwent a baseline assessment, cue-exposure session, and genetic analysis. Subjects were classified as either cue-reactive or nonreactive.

RESULTS

Among single nucleotide polymorphism markers in 13 candidate genes examined for association with cocaine cue-reactivity, two were statistically significant: GABRA2 (coding for GABA-A receptor alpha-2 subunit; rs11503014, nominal p= .001) and OPRM1 (coding for mu opioid receptor; rs2236256, nominal p= .03).

CONCLUSIONS

These pilot results suggest that cocaine craving shows variability among cocaine-dependent subjects, and that GABRA2 and OPRM1 polymorphisms have differential influences on cocaine cue-reactivity, warranting studies in future research.

Utility of genetically modified mice for understanding the neurobiology of substance use disorders

Advances in our ability to modify the mouse genome have enhanced our understanding of the genetic and neurobiological mechanisms contributing to addiction-related behaviors underlying substance use and abuse. These experimentally induced manipulations permit greater spatial and temporal specificity for modification of gene expression within specific cellular populations and during select developmental time periods. In this review, we consider the current mouse genetic model systems that have been employed to understand aspects of addiction and highlight significant conceptual advances achieved related to substance use and abuse. The mouse models reviewed herein include conventional knock-out and knock-in, conditional knockout, transgenic, inducible transgenic, mice suitable for optogenetic control of discrete neuronal populations, and phenotype-selected mice. By establishing a reciprocal investigatory relationship between genetic findings in humans and genomic manipulations in mice, a far better understanding of the discrete neuromechanisms underlying addiction can be achieved, which is likely to provide a strong foundation for developing and validating novel therapeutics for the treatment of substance abuse disorders.

Decreased cocaine motor sensitization and self-administration in mice overexpressing cannabinoid CB₂ receptors

The potential involvement of the cannabinoid CB₂ receptors (CB₂r) in the adaptive responses induced by cocaine was studied in transgenic mice overexpressing the CB₂r (CB₂xP) and in wild-type (WT) littermates. For this purpose, the acute and sensitized locomotor responses to cocaine, conditioned place preference, and cocaine intravenous self-administration were evaluated. In addition, we assessed whether CB₂r were localized in neurons and/or astrocytes, and whether they colocalized with dopamine D1 and D2 receptors (D1Dr and D2Dr). Dopamine (DA) extracellular levels in the nucleus accumbens (NAcc), and gene expression of tyrosine hydroxylase (TH) and DA transporter (DAT) in the ventral tegmental area (VTA), and μ-opioid and cannabinoid CB₁ receptors in the NAcc were also studied in both genotypes. CB₂xP mice showed decreased motor response to acute administration of cocaine (10-20 mg/kg) and cocaine-induced motor sensitization compared with WT mice. CB₂xP mice presented cocaine-induced conditioned place aversion and self-administered less cocaine than WT mice. CB₂r were found in neurons and astrocytes and colocalized with D2Dr in the VTA and NAcc. No significant differences in extracellular DA levels in the NAcc were observed between genotypes after cocaine administration. Under baseline conditions, TH and DAT gene expression was higher and μ-opioid receptor gene expression was lower in CB₂xP than in WT mice. However, both genotypes showed similar changes in TH and μ-opioid receptor gene expression after cocaine challenge independently of the pretreatment received. Importantly, the cocaine challenge decreased DAT gene expression to a lesser extent in cocaine-pretreated CB₂xP than in cocaine-pretreated WT mice. These results revealed that CB₂r are involved in cocaine motor responses and cocaine self-administration, suggesting that this receptor could represent a promising target to develop novel treatments for cocaine addiction.

The rewarding action of acute cocaine is reduced in β-endorphin deficient but not in μ opioid receptor knockout mice

We have previously shown that β-endorphin plays a functional role in the rewarding effect of acute cocaine. Considering that β-endorphin has high affinity for the μ opioid receptor, we determined the role of this receptor in the rewarding action of acute cocaine. For comparison, we assessed the role of the μ opioid receptor in the rewarding effect of acute morphine. We also examined the effect of intracerebroventricular (i.c.v.) administration of β-funaltrexamine (β-FNA), an irreversible μ opioid receptor antagonist, on the rewarding action of acute cocaine as well as that of morphine. Using the conditioned place preference (CPP) paradigm as an animal model of reward, we first assessed the rewarding action of cocaine in mice lacking β-endorphin or the μ opioid receptor and their respective wild-type littermates/controls. Mice were tested for preconditioning place preference on day 1, conditioned once daily with saline/cocaine (30mg/kg, i.p.) or cocaine/saline on days 2 and 3, and then tested for postconditioning place preference on day 4. We next studied the rewarding action of acute morphine in μ knockout mice and their wild-type controls. The CPP was induced by single alternate-day saline/morphine (10mg/kg, s.c.) or morphine/saline conditioning. We finally determined the effect of β-FNA on CPP induced by cocaine or morphine in wild-type mice, in which mice were treated with saline or β-FNA (9ug/3μl; i.c.v.) a day prior to the preconditioning test day. Our results revealed that morphine induced a robust CPP in wild-type mice but not in mice lacking the μ opioid receptor or in wild-type mice treated with β-FNA. In contrast, cocaine induced CPP in μ knockout mice as well as in wild-type mice treated with β-FNA. On the other hand, cocaine failed to induce CPP in mice lacking β-endorphin. These results illustrate that β-endorphin is essential for the rewarding action of acute cocaine, but the μ opioid receptor may not mediate the regulatory action of endogenous β-endorphin.

Lack of genotype effect on D1, D2 receptors and dopamine transporter binding in triple MOP-, DOP-, and KOP-opioid receptor knockout mice of three different genetic backgrounds

We investigated D1, D2 receptors and dopamine transporter (DAT) binding levels in mice lacking all three opioid receptors and wild-type (WT) mice on three different genetic backgrounds. Quantitative autoradiography was used to determine the level of radioligand binding to the D1 and D2 receptors and DAT labeled with [(3)H]SCH23390, [(3)H]raclopride, and [(3)H]mazindol, respectively in triple-opioid receptor knockout (KO) and WT maintained on C57BL/6 (B6) and 129/SvEvTac (129) as well as C57BL/6 x 129/SvPas (B6 x 129) strains. No significant genotype effect was observed in D1, D2 receptors and DAT binding in any regions analyzed in any of the strains studied, suggesting that a lack of all three opioid receptors does not influence D1, D2 receptors and DAT expression, irrespective of their genetic strain background. However, strain differences were observed in D1 binding between the three strains of mice studied. Lower levels of D1 binding were observed in the substantia nigra of B6 x 129 WT mice compared with the 129 WT mice and in the olfactory tubercle of B6 x 129 WT compared with B6 WT and 129 WT mice. Lower levels of D1 binding were observed in the caudate putamen of B6 x 129 KO mice compared with 129 KO mice. In contrast, no significant strain differences were observed in D2 and DAT binding between the three strains of mice in any regions analyzed. Overall, these results indicate a lack of modulation of the dopaminergic system by the deletion of all three opioid receptors regardless of different background strains.

Potentiating effect of morphine on oral Salmonella enterica serovar Typhimurium infection is μ-opioid receptor-dependent

Previous studies from our laboratory demonstrated that mice treated with morphine pellets are sensitized to Salmonella enterica, serovar Typhimurium infection. However, the opioid receptor antagonist, naltrexone, only partially blocked the effect of morphine, raising the possibility that the opioid might have some of its effects through a nonopioid receptor. To further clarify whether sensitization to infection is an opioid receptor-dependent phenomenon, μ-opioid receptor knockout (MORKO) mice were used in the present study. Wild-type (WT) and MORKO mice were treated with morphine and their sensitivity to oral Salmonella infection was assessed by mortality, bacterial burdens in gut associated lymphoid tissue and in blood and peritoneal fluid, and by levels of pro-inflammatory cytokines in plasma. MORKO animals treated with morphine were refractory to a sublethal dose of Salmonella, while similar treatment of WT animals resulted in 100% mortality. WT animals treated with morphine had high bacterial loads in all organs tested, while morphine-treated MORKO animals had no culturable Salmonella in any organs. Pro-inflammatory cytokine levels were elevated in morphine-treated WT but not MORKO mice infected with Salmonella. These results provide definitive evidence that the morphine-mediated enhancement of oral Salmonella infection is dependent on the μ-opioid receptor.

Transgenic mice in the study of drug addiction and the effects of psychostimulant drugs

The first transgenic models used to study addiction were based upon a priori assumptions about the importance of particular genes in addiction, including the main target molecules of morphine, amphetamine, and cocaine. This consequently emphasized the importance of monoamine transporters, opioid receptors, and monoamine receptors in addiction. Although the effects of opiates were largely eliminated by mu opioid receptor gene knockout, the case for psychostimulants was much more complex. Research using transgenic models supported the idea of a polygenic basis for psychostimulant effects and has associated particular genes with different behavioral consequences of psychostimulants. Phenotypic analysis of transgenic mice, especially gene knockout mice, has been instrumental in identifying the role of specific molecular targets of addictive drugs in their actions. In this article, we summarize studies that have provided insight into the polygenic determination of drug addiction phenotypes in ways that are not possible with other methods, emphasizing research into the effects of psychostimulant drugs in gene knockouts of the monoamine transporters and monoamine receptors.

Alterations in the level of OFQ/N-IR in rat brain regions by cocaine

We have previously shown that administration of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the opioid receptor-like (ORL-1) receptor, into the lateral ventricles or VTA blocked cocaine sensitization. In the present study, we determined the effect of acute and chronic cocaine treatment on the level of endogenous OFQ/N in rat brain regions. Male Sprague Dawley rats were tested for motor activity in response to saline or cocaine (20 mg/kg) injection once daily for three consecutive days. To determine the effect of single or repeated cocaine administration on the level of OFQ/N, rats were sacrificed 1 h following saline or cocaine injection either on day 1 or 3, respectively. Additional groups of rats were treated similarly with saline or cocaine on days 1-3 and sacrificed or tested for locomotor sensitization on day 8. Consistent with previous studies, repeated cocaine administration induced locomotor sensitization to a challenge dose of cocaine (7.5 mg/kg) given on day 8. Measurements of tissue content of OFQ/N-IR using radioimmunoassay indicated that the rat hypothalamus and striatum, respectively, contained the highest and lowest levels of the peptide among the brain regions tested. Acute cocaine decreased the level of OFQ-IR in the rat midbrain and to a lesser extent in the striatum. On the other hand, the level of OFQ/N was higher in rats treated with cocaine on days 1-3 and sacrificed on day 8. These findings suggest that endogenous OFQ/N may be involved in the actions of cocaine and possibly in cocaine-induced motor stimulation and locomotor sensitization.

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.

The role of beta-endorphin in the acute motor stimulatory and rewarding actions of cocaine in mice

RATIONALE

Opioid receptor antagonists have been shown to attenuate the rewarding and addictive effects of cocaine. Furthermore, cocaine has been shown to cause the release of beta-endorphin, an endogenous opioid peptide.

OBJECTIVE

We assessed whether this neuropeptide would play a functional role in cocaine-induced motor stimulation and conditioned place preference (CPP).

MATERIALS AND METHODS

Mice lacking beta-endorphin and their wild-type littermates were habituated to motor activity chambers for 1 h, then injected with cocaine (0, 15, 30, or 60 mg/kg, intraperitoneally) or morphine (0, 5, or 10 mg/kg, subcutaneously), and motor activity was recorded for 1 h. In the CPP paradigm, mice were tested for baseline place preference on day 1. On days 2 and 3, mice received an alternate-day saline/cocaine (15, 30, or 60 mg/kg) or saline/morphine (10 mg/kg) conditioning session and then tested for postconditioning place preference on day 4.

RESULTS

Cocaine-induced motor stimulation and CPP were both reduced in mice lacking beta-endorphin. On the other hand, motor stimulation and CPP induced by morphine were not altered in mutant mice.

CONCLUSION

The present results demonstrate that the endogenous opioid peptide beta-endorphin plays a modulatory role in the motor stimulatory and rewarding actions of acute cocaine.

A role for mu opioid receptors in cocaine-induced activity, sensitization, and reward in the rat

RATIONALE

Considerable evidence suggests that the endogenous opioid system plays a role in mediating the behavioral effects of psychostimulants. Opioidergic drugs have been shown to have profound effects on cocaine-induced behavioral sensitization and conditioned reward. However, the role specifically of the mu opioid receptor in this regard is unclear as most previous pharmacological studies have used nonselective opioid receptor ligands.

OBJECTIVES

The objective of this series of experiments was to elucidate the role of mu opioid receptors in the behavioral effects of cocaine in the rat.

MATERIALS AND METHODS

Adult male rats were used to assess the effects of the selective mu opioid receptor antagonist D: -Phe-Cys-Tyr-D: -Trp-Arg-Thr-Pen-Thr (CTAP) on acute hyperactivity, locomotor sensitization, and conditioned place preference induced by cocaine. Intracerebroventricular administration of CTAP, 4 microg, was paired with peripheral injections of cocaine, 10-15 mg/kg.

RESULTS

Mu receptor blockade significantly attenuated cocaine-induced hyperactivity, as well as the development of behavioral sensitization. Pretreatment with CTAP also prevented the development of conditioned place preference to cocaine. Administration of CTAP alone had neither effect on locomotor activity nor did it demonstrate aversive or rewarding properties.

CONCLUSIONS

These results suggest that activation of mu opioid receptors by endogenous opioids is an important contributor to cocaine-induced hyperactivity and the development of behavioral sensitization and conditioned reward.

Preprodynorphin mediates locomotion and D2 dopamine and mu-opioid receptor changes induced by chronic 'binge' cocaine administration

Evidence suggests that the kappa-opioid receptor (KOP-r) system plays an important role in cocaine addiction. Indeed, cocaine induces endogenous KOP activity, which is a mechanism that opposes alterations in behaviour and brain function resulting from repeated cocaine use. In this study, we have examined the influence of deletion of preprodynorphin (ppDYN) on cocaine-induced behavioural effects and on hypothalamic-pituitary-adrenal axis activity. Furthermore, we have measured mu-opioid receptor (MOP-r) agonist-stimulated [(35)S]GTPgammaS, dopamine D(1), D(2) receptor and dopamine transporter (DAT) binding. Male wild-type (WT) and ppDYN knockout (KO) mice were injected with saline or cocaine (45 mg/kg/day) in a 'binge' administration paradigm for 14 days. Chronic cocaine produced an enhancement of locomotor sensitisation in KO. No genotype effect was found on stereotypy behaviour. Cocaine-enhanced MOP-r activation in WT but not in KO. There was an overall decrease in D(2) receptor binding in cocaine-treated KO but not in WT mice. No changes were observed in D(1) and DAT binding. Cocaine increased plasma corticosterone levels in WT but not in KO. The data confirms that the endogenous KOP system inhibits dopamine neurotransmission and that ppDYN may mediate the enhancement of MOP-r activity and the activation of the hypothalamic-pituitary-adrenal axis after chronic cocaine treatment.

Cocaine exposure during adolescence affects anxiety in adult mice

Psychostimulant drugs such as cocaine have profound and long-lasting neurobiological effects, which may affect anxiety or social behaviors. These actions could be greater when cocaine is administered during a developmental period such as adolescence. The present work attempts to further clarify the long-lasting effects of cocaine administration on mice, examining three major variables: age; pattern of drug administration; and housing conditions. Adolescent (postnatal day 26) or early adult mice (postnatal day 46) were exposed to a daily or binge cocaine administration and 15 days later their behavior was evaluated, the mice being housed either in isolation or in groups during this stage. After a period free of drug, the behaviors evaluated were: spontaneous and cocaine-induced motor activity; anxiety, using the elevated plus maze; the social profile, assessed in a social interaction test. Daily cocaine administration increased avoidance and flee in isolated adolescent-treated mice and decreased social contacts in those which were grouped. On the other hand, the binge pattern modified the anxiety of the grouped adolescent-treated mice evidenced by the increase in time spent on the open arms of the plus maze. An increase in spontaneous and cocaine-induced motor activity was shown in animals after a daily pre-treatment. The results are discussed in terms of presenting cocaine-induced behavioral changes within a specific temporal window and depending on the three variables studied.

An antisense oligodeoxynucleotide to the mu opioid receptor attenuates cocaine-induced behavioral sensitization and reward in mice

Numerous studies support a role for the endogenous opioid system in cocaine-influenced behavior. Few of these studies, however, selectively delineate a role for the mu opioid receptor (MOR) in this regard. This investigation examined if the MOR modulates cocaine-induced behavior in mice using a 17-base antisense oligodeoxynucleotide (AS ODN) directed against the MOR coding sequence 16-32. Specifically, cocaine-induced behavioral sensitization and conditioned reward were investigated. For the sensitization study, C57BL/6J mice received eight intermittent i.c.v. infusions of saline, mismatch oligodeoxynucleotide (ODN) (20 microg/4 microl) or AS ODN (20 microg/4 microl) over 20 days. Mice also received concomitant once daily i.p. injections of saline (4 ml/kg) or cocaine (15 mg/kg) for 10 days. There was a 7-day withdrawal period, after which all mice were challenged with cocaine (15 mg/kg) to test for behavioral sensitization. For the conditioned place preference (CPP) study, mice received five i.c.v. infusions of mismatch ODN or MOR AS ODN (days 1-5). An unbiased counterbalanced conditioning procedure was used where mice were conditioned with saline (4 ml/kg, i.p.) and cocaine (15 mg/kg, i.p.) on alternate days for four sessions (days 3-6). Mice were tested on day 7 for CPP. Immediately following testing, [3H]DAMGO (D-Ala2, N-Me-Phe4, Gly-ol5-enkephalin) receptor binding to brain homogenates was conducted. MOR AS attenuated cocaine-induced behavioral sensitization and conditioned reward. MOR AS ODN also reduced [3H]DAMGO binding. Collectively, these findings implicate the MOR as playing an important neuromodulatory role in the behavioral effects of cocaine in mice.

Endogenous opiates and behavior: 2004

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

Age-specific behavioral responses to psychostimulants in mice

This study investigated the influence of age on the behavioral responses elicited by psychostimulants in male CD-1 mice. Behavioral activity including locomotion and stereotypy was measured following acute or repeated administration of cocaine, methylphenidate, amphetamine or saline to postweanling (24 days old), periadolescent (33 days old) and adult (60 days old) mice. Postweanling mice exhibited less total and ambulatory activity than periadolescent mice following a single acute injection of cocaine (20 or 30 and 30 mg/kg, respectively). Further, postweanling mice showed less total activity than both periadolescent and adult mice at a dose of 10 mg/kg methylphenidate. Less stereotypy was also seen in postweanling mice when compared to adolescent mice after 30 mg/kg amphetamine. Seven daily injections of cocaine resulted in a heightened behavioral response on day 7 as compared to day 1, indicative of behavioral sensitization in adult and periadolescent, but not postweanling mice. Repeated methylphenidate resulted in increased total activity in adult, but not periadolescent or postweanling mice. None of the animals were sensitized to the behavioral activating effects of amphetamine. The magnitude of behavioral response and the development of sensitization were dependent upon the age of the animal and the agent tested.

Increased gabaergic input to ventral tegmental area dopaminergic neurons associated with decreased cocaine reinforcement in mu-opioid receptor knockout mice

There is general agreement that dopaminergic neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens and prefrontal cortex play a key role in drug reinforcement. The activity of these neurons is strongly modulated by the inhibitory and excitatory input they receive. Activation of mu-opioid receptors, located on GABAergic neurons in the VTA, causes hyperpolarization of these GABAergic neurons, thereby causing a disinhibition of VTA dopaminergic neurons. This effect of mu-opioid receptors upon GABA neurotransmission is a likely mechanism for mu-opioid receptor modulation of drug reinforcement. We studied mu-opioid receptor signaling in relation to cocaine reinforcement in wild-type and mu-opioid receptor knockout mice using a cocaine self-administration paradigm and in vitro electrophysiology. Cocaine self-administration was reduced in mu-opioid receptor knockout mice, suggesting a critical role of mu-opioid receptors in cocaine reinforcement. The frequency of spontaneous inhibitory post-synaptic currents onto dopaminergic neurons in the ventral tegmental area was increased in mu-opioid receptor knockout mice compared with wild-type controls, while the frequency of spontaneous excitatory post-synaptic currents was unaltered. The reduced cocaine self-administration and increased GABAergic input to VTA dopaminergic neurons in mu-opioid receptor knockout mice supports the notion that suppression of GABAergic input onto dopaminergic neurons in the VTA contributes to mu-opioid receptor modulation of cocaine reinforcement.

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.

Somatostatin coupling to adenylyl cyclase activity in the mouse retina

The peptide somatostatin-14 (SRIF) acts in the mammalian retina through its distinct receptors (sst(1-5)). Scarce information is available on SRIF function in the retina, including the elucidation of transduction pathways mediating SRIF action. We have investigated SRIF and SRIF receptor modulation of adenylyl cyclase (AC) activity in both wild-type (WT) retinas and sst1 or sst2 knock-out (KO) retinas, which are known to over-express sst2 or sst1 receptors respectively. In WT retinas, application of SRIF compounds does not affect forskolin-stimulated AC activity. In contrast, activation of sst1 or sst2 receptors inhibits AC in the presence of sst2 or sst1 receptor antagonists respectively. Results from sst1 KO retinas demonstrate that either SRIF or the sst2 receptor preferring agonist octreotide, pertussis toxin-dependently inhibit AC activity. In contrast, in sst2 KO retinas, neither SRIF nor CH-275, an sst1 receptor agonist, are found to influence AC activity. As revealed by immunoblotting experiments, in sst1 KO retinas, levels of G(o)alpha proteins are 60% higher than in WT retinas and this increase in G(o)alpha protein levels is concomitant with an increase in sst2A receptor expression. We conclude that interactions between sst1 and sst2 receptors may prevent SRIF effects on AC activity. In addition, we suggest that the density of sst2 receptors and/or G(o)alpha proteins may represent the rate-limiting factor for the sst2 receptor-mediated inhibition of AC.