Conditioning of opioid reinforcement: neuroanatomical and neurochemical substrates

Five Decades of Research on Opioid Peptides: Current Knowledge and Unanswered Questions

In the mid-1970s, an intense race to identify endogenous substances that activated the same receptors as opiates resulted in the identification of the first endogenous opioid peptides. Since then, >20 peptides with opioid receptor activity have been discovered, all of which are generated from three precursors, proenkephalin, prodynorphin, and proopiomelanocortin, by sequential proteolytic processing by prohormone convertases and carboxypeptidase E. Each of these peptides binds to all three of the opioid receptor types (μ, δ, or κ), albeit with differing affinities. Peptides derived from proenkephalin and prodynorphin are broadly distributed in the brain, and mRNA encoding all three precursors are highly expressed in some peripheral tissues. Various approaches have been used to explore the functions of the opioid peptides in specific behaviors and brain circuits. These methods include directly administering the peptides ex vivo (i.e., to excised tissue) or in vivo (in animals), using antagonists of opioid receptors to infer endogenous peptide activity, and genetic knockout of opioid peptide precursors. Collectively, these studies add to our current understanding of the function of endogenous opioids, especially when similar results are found using different approaches. We briefly review the history of identification of opioid peptides, highlight the major findings, address several myths that are widely accepted but not supported by recent data, and discuss unanswered questions and future directions for research. SIGNIFICANCE STATEMENT: Activation of the opioid receptors by opiates and synthetic drugs leads to central and peripheral biological effects, including analgesia and respiratory depression, but these may not be the primary functions of the endogenous opioid peptides. Instead, the opioid peptides play complex and overlapping roles in a variety of systems, including reward pathways, and an important direction for research is the delineation of the role of individual peptides.

Sigma receptor-induced heavy drinking in rats: Modulation by the opioid receptor system

Alcohol use disorder (AUD) is a major cause of morbidity and mortality worldwide, for which new efficacious treatments are necessary. The opioid receptor system is a mediator of the rewarding effects of alcohol; in particular, while activation of μ opioid receptors enhances ethanol intake in rodents, opioid-receptor antagonists, such as naloxone and naltrexone, reduce its pleasurable and reinforcing effects, thereby decreasing alcohol. Sigma receptors (Sig-Rs) have been proposed as modulators of the effects of alcohol and, therefore, as a potential new pharmacological target for AUD. Somewhat analogously to μ opioid ligands, SigR agonists increase, while SigR antagonists decrease alcohol intake in animal models of excessive alcohol drinking. However, a potential cross-talk between these two receptor systems in relation to alcohol consumption has so far not been investigated. Here, we addressed this question pharmacologically, by testing the effects of either activating or inhibiting opioid receptors on the heavy alcohol drinking induced by chronic stimulation of SigR in alcohol-preferring rats. We found that the opioid receptor agonist morphine, which per se increases ethanol intake, at a sub-threshold dose reduces the binge-like drinking induced by the repeated treatment with the SigR agonist 1,3-di-o-tolylguanidine (DTG); conversely, the opioid receptor antagonist naltrexone, which per se reduces ethanol intake, at a sub-threshold dose potentiates the DTG-induced binge-like drinking. Our data show a cross-talk between the opioid and SigR systems relevant to the modulation of alcohol drinking, which provides important insights into the neurobiology of AUD and may lead to the development of novel therapies, either standalone or in combination.

Genetic variants and early cigarette smoking and nicotine dependence phenotypes in adolescents

Background

While the heritability of cigarette smoking and nicotine dependence (ND) is well-documented, the contribution of specific genetic variants to specific phenotypes has not been closely examined. The objectives of this study were to test the associations between 321 tagging single-nucleotide polymorphisms (SNPs) that capture common genetic variation in 24 genes, and early smoking and ND phenotypes in novice adolescent smokers, and to assess if genetic predictors differ across these phenotypes.

Methods

In a prospective study of 1294 adolescents aged 12–13 years recruited from ten Montreal-area secondary schools, 544 participants who had smoked at least once during the 7–8 year follow-up provided DNA. 321 single-nucleotide polymorphisms (SNPs) in 24 candidate genes were tested for an association with number of cigarettes smoked in the past 3 months, and with five ND phenotypes (a modified version of the Fagerstrom Tolerance Questionnaire, the ICD-10 and three clusters of ND symptoms representing withdrawal symptoms, use of nicotine for self-medication, and a general ND/craving symptom indicator).

Results

The pattern of SNP-gene associations differed across phenotypes. Sixteen SNPs in seven genes (ANKK1, CHRNA7, DDC, DRD2, COMT, OPRM1, SLC6A3 (also known as DAT1)) were associated with at least one phenotype with a p-value <0.01 using linear mixed models. After permutation and FDR adjustment, none of the associations remained statistically significant, although the p-values for the association between rs557748 in OPRM1 and the ND/craving and self-medication phenotypes were both 0.076.

Conclusions

Because the genetic predictors differ, specific cigarette smoking and ND phenotypes should be distinguished in genetic studies in adolescents. Fifteen of the 16 top-ranked SNPs identified in this study were from loci involved in dopaminergic pathways (ANKK1/DRD2, DDC, COMT, OPRM1, and SLC6A3).

Impact

Dopaminergic pathways may be salient during early smoking and the development of ND.

The role of the opioid system in alcohol dependence

The role of the brain opioid system in alcohol dependence has been the subject of much research for over 25 years. This review explores the evidence: firstly describing the opioid receptors in terms of their individual subtypes, neuroanatomy, neurophysiology and ligands; secondly, summarising emerging data from specific neurochemical, behavioural and neuroimaging studies, explaining the characteristics of addiction with a focus on alcohol dependence and connecting the opioid system with alcohol dependence; and finally reviewing the known literature regarding opioid antagonists in clinical use for alcohol dependence. Further interrogation of how modulation of the opioid system, via use of MOP (mu), DOP (delta) and KOP (kappa) agents, restores the balance of a dysregulated system in alcohol dependence should increase our insight into this disease process and therefore guide better methods for understanding and treating alcohol dependence in the future.

Kappa opioid receptors on dopaminergic neurons are necessary for kappa-mediated place aversion

Kappa-opioid receptor (KOR) agonists have dysphoric properties in humans and are aversive in rodents. This has been attributed to the activation of KORs within the mesolimbic dopamine (DA) system. However, the role of DA in KOR-mediated aversion and stress remains divisive as recent studies have suggested that activation of KORs on serotonergic neurons may be sufficient to mediate aversive behaviors. To address this question, we used conditional knock-out (KO) mice with KORs deleted on DA neurons (DAT(Cre/wt)/KOR(loxp/loxp), or DATCre-KOR KO). In agreement with previous findings, control mice (DAT(Cre/wt)/KOR(wt/wt) or WT) showed conditioned place aversion (CPA) to the systemically administered KOR agonist U69,593. In contrast, DATCre-KOR KO mice did not exhibit CPA with this same agonist. In addition, in vivo microdialysis showed that systemic U69,593 decreased overflow of DA in the nucleus accumbens (NAc) in WT mice, but had no effect in DATCre-KOR KO mice. Intra- ventral tegmental area (VTA) delivery of KORs using an adeno-associated viral gene construct, resulted in phenotypic rescue of the KOR-mediated NAc DA response and aversive behavior in DATCre-KOR KO animals. These results provide evidence that KORs on VTA DA neurons are necessary to mediate KOR-mediated aversive behavior. Therefore, our data, along with recent findings, suggest that the neuronal mechanisms of KOR-mediated aversive behavior may include both dopaminergic and serotonergic components.

Regional mRNA expression of GABAergic receptor subunits in brains of C57BL/6J and 129P3/J mice: strain and heroin effects

C57BL/6J and 129 substrains of mice are known to differ in their basal levels of anxiety and behavioral response to drugs of abuse. We have previously shown strain differences in heroin-induced conditioned place preference (CPP) between C57BL/6J (C57) and 129P3/J (129) mice, and in the regional expression of several receptor and peptide mRNAs. In this study, we examined the contribution of the GABAergic system in the cortex, nucleus accumbens (NAc), caudate putamen (CPu) and the region containing the substantia nigra and ventral tegmental area (SN/VTA) to heroin reward by measuring mRNA levels of 7 of the most commonly expressed GABA-A receptor subunits, and both GABA-B receptor subunits, in these same mice following saline (control) or heroin administration in a CPP design. Using real-time PCR, we studied the effects of strain and heroin administration on GABA-A α1, α2, α3, β2, and γ2 subunits, which typically constitute synaptic GABA-A receptors, GABA-A α4 and δ subunits, which typically constitute extrasynaptic GABA-A receptors, and GABA-B R1 and R2 subunits. In saline-treated animals, we found an experiment-wise significant strain difference in GABA-Aα2 mRNA expression in the SN/VTA. Point-wise significant strain differences were also observed in GABA-Aα2, GABA-Aα3, and GABA-Aα4 mRNA expression in the NAc, as well as GABA-BR2 mRNA expression in the NAc and CPu, and GABA-BR1 mRNA expression in the cortex. For all differences, 129 mice had higher mRNA expression compared to C57 animals, with the exception of GABA-BR1 mRNA in the cortex where we observed lower levels in 129 mice. Therefore, it may be possible that known behavioral differences between these two strains are, in part, due to differences in their GABAergic systems. While we did not find heroin dose-related changes in mRNA expression levels in C57 mice, we did observe dose-related differences in 129 mice. These results may relate to our earlier behavioral finding that 129 mice are hyporesponsive to the rewarding effects of heroin.

The μ-opioid receptor and treatment response to naltrexone

AIMS

To evaluate the pharmacogenetic evidence relating to the use of opioid antagonists (in particular naltrexone) in treating patients with alcohol abuse problems.

METHODS

Narrative review of pre-clinical and clinical published research regarding genetic modulation of psychotropic effects produced by alcohol and the therapeutic effects of opioid antagonists.

RESULTS

Alcohol activates brain reward pathways, leading to positive reinforcement of alcohol seeking and consumption. Thus, the underlying biological mechanisms may be targets for treatment, particularly in the early stages of addiction development. Alcohol reward is in part mediated by endogenous opioids. A single-nucleotide polymorphism (SNP) within the OPRM1 gene, A118G, leading to an amino acid change (Asn40Asp) in the extracellular portion of the receptor, has been implicated in alcoholism as well as in drug addiction, pain sensitivity and stress response, and in animal and human studies relates to the alcohol-dependent phenotype as well as to the treatment response to the µ-opioid antagonist naltrexone.

CONCLUSION

The effect size reported in naltrexone clinical studies is often small, which may be due to heterogeneity among patients. Pharmacogenetic approaches may help guide us in the search for the appropriate treatment optimal for one patient's need.

Pain relief produces negative reinforcement through activation of mesolimbic reward-valuation circuitry

Relief of pain is rewarding. Using a model of experimental postsurgical pain we show that blockade of afferent input from the injury with local anesthetic elicits conditioned place preference, activates ventral tegmental dopaminergic cells, and increases dopamine release in the nucleus accumbens. Importantly, place preference is associated with increased activity in midbrain dopaminergic neurons and blocked by dopamine antagonists injected into the nucleus accumbens. The data directly support the hypothesis that relief of pain produces negative reinforcement through activation of the mesolimbic reward-valuation circuitry.

Neuropeptides as mediators of the early-life impact on the brain; implications for alcohol use disorders

The brain is constantly exposed to external and internal input and to function in an ever-changing environment we are dependent on processes that enable the brain to adapt to new stimuli. Exposure to postnatal environmental stimuli can interfere with vital adaption processes and cause long-term changes in physiological function and behavior. Early-life alterations in brain function may result in impaired ability to adapt to new situations, in altered sensitivity to challenges later in life and thereby mediate risk or protection for psychopathology such as alcohol use disorders (AUD). In clinical research the studies of mechanisms, mediators, and causal relation between early environmental factors and vulnerability to AUD are restricted and attempts are made to find valid animal models for studies of the early-life influence on the brain. This review focuses on rodent models and the effects of adverse and naturalistic conditions on peptide networks within the brain and pituitary gland. Importantly, the consequences of alcohol addiction are not discussed but rather neurobiological alterations that can cause risk consumption and vulnerability to addiction. The article reviews earlier results and includes new data and multivariate data analysis with emphasis on endogenous opioid peptides but also oxytocin and vasopressin. These peptides are vital for developmental processes and it is hypothesized that early-life changes in peptide networks may interfere with neuronal processes and thereby contribute the individual vulnerability for AUD. The summarized results indicate a link between early-life rearing conditions, opioids, and ethanol consumption and that the ethanol-induced effects and the treatment with opioid antagonists later in life are dependent on early-life experiences. Endogenous opioids are therefore of interest to further study in the early-life impact on individual differences in vulnerability to AUD and treatment outcome.

The long-lasting effects of JDTic, a kappa opioid receptor antagonist, on the expression of ethanol-seeking behavior and the relapse drinking of female alcohol-preferring (P) rats

The current study assessed the effects of the selective kappa opioid antagonist JDTic on alcohol (EtOH)-seeking behavior, EtOH relapse, and maintenance responding for EtOH. Adult alcohol-preferring (P) rats were trained in 2-lever operant chambers to self-administer 15% EtOH (v/v) on a fixed-ratio 5 (FR-5) and water on a FR-1 schedule of reinforcement during 1-hr sessions. After 10 weeks, rats underwent extinction training for seven sessions. Rats were then maintained in their home cages for 3 weeks without EtOH access. All rats received an injection (s.c.) of 0, 1, 3, or 10 mg/kg JDTic (n=11-14/group) after the first week of the home cage period. Rats were then tested using the Pavlovian Spontaneous Recovery paradigm (PSR; an animal model of alcohol-seeking) for four sessions during which, responses on the EtOH and water levers were recorded but did not produce their respective reinforcer. Following PSR testing rats were returned to their home cages without access to EtOH for one week prior to the start of EtOH relapse testing. To examine EtOH relapse responding, rats were returned to the operant chambers and the EtOH (FR5) and water (FR1) levers were active. Finally, rats were then tested over 17 operant sessions to assess the effects of JDTic on maintenance responding for EtOH. Rats received 0, 1, 3, or 10 mg/kg JDTic (counterbalanced from the initial experiment) 30 minutes prior to the initial maintenance session. JDTic administered 14 and 25 days prior to testing dose-dependently reduced the expression of an EtOH PSR and relapse responding. In contrast, JDTic did not alter EtOH responding under maintenance conditions. Overall, the results of this study indicate that different mechanisms mediate EtOH self-administration under relapse and maintenance conditions and kappa opioid receptors are involved in mediating EtOH-seeking behavior and relapse responding but not on-going EtOH self-administration.

Role of gonadal hormones on mu-opioid-stimulated [³⁵S]GTPγS binding and morphine-mediated antinociception in male and female Sprague-Dawley rats

RATIONALE

Male rats are more sensitive to morphine-mediated antinociception than female rats. A role for gonadal hormones in this sex difference has not been clearly defined.

OBJECTIVES

To test the hypothesis that in vivo manipulation of gonadal hormones alters morphine-mediated G protein activation and leads to changes in morphine-mediated antinociception.

METHODS

Adult male and female rats were gonadectomized and treated with either estradiol or testosterone in the females or testosterone in the male for up to 10 days. The ability of morphine and the peptidic mu-opioid agonist [D-Ala(2), N-MePhe(4), Gly-ol]-enkephalin (DAMGO) to stimulate [(35)S]GTPγS binding was measured in brain slices. In separate groups of identically treated rats, the antinociceptive response to morphine was determined using the warm-water tail-withdrawal assay.

RESULTS

In the thalamus, morphine- and DAMGO-stimulated [(35)S]GTPγS binding was reduced by estradiol treatment of gonadectomized females compared to gonadectomized females treated with vehicle or testosterone. In the nucleus accumbens, the morphine-stimulated [(35)S]GTPγS binding was increased by estradiol treatment of gonadectomized females. In males, castration caused an increase in agonist-stimulated binding in the thalamus and a reduction in the amygdala compared with intact males. No significant changes were seen in mu-opioid agonist-stimulated [(35)S]GTPγS binding in other brain regions. There was no difference in antinociception following the systemic administration of morphine across the different hormonal manipulation conditions and the greater sensitivity of males was maintained irrespective of the treatment conditions.

CONCLUSIONS

The modulation of mu-opioid receptor activation of G proteins by manipulation of sex hormones is region-specific and not reflected in antinociceptive responsiveness to morphine.

Dopaminergic modulation of appetitive and aversive predictive learning

Temporal contiguity between two stimuli is insufficient for the establishment of a predictive relation between those stimuli. Rather, learning about predictive relations is influenced by a prediction error mechanism: the discrepancy between actual and expected outcomes. Although the neural substrates of contiguous stimuli presentation have been the focus of research for decades, relatively little empirical evidence exists with regard to the neural mechanisms of prediction error. Recent work has implicated the neurotransmitter dopamine in regulation of predictive learning. If dopamine modulates prediction error then it should do so despite the nature (appetitive or aversive) of the biological stimuli that serve to drive learning. The exact role of dopamine in appetitive and aversive predictive learning, however, remains the focus of continuous debate. This review focuses on the behavioural, neuropharmacological and electrophysiological evidence implicating dopamine in prediction error in appetitive and aversive predictive learning. In addition, recent work in the area of fear conditioning implicating other neurochemical substrates, namely opioids, in the process of prediction error is discussed. Finally, some predictions are made with regard to the neurochemical circuitry involved in modulating learning and behaviour based on prediction error.

Implication of activated astrocytes in the development of drug dependence: differences between methamphetamine and morphine

Astrocytes are a subpopulation of glial cells that directly affect neuronal function. This review focuses on the potential functional roles of astrocytes in the development of behavioral sensitization and rewarding effects induced by chronic treatment with drugs of abuse. In vitro treatment of cortical neuron/glia cocultures with either methamphetamine or morphine caused activation of astrocytes via protein kinase C (PKC). Purified cortical astrocytes were markedly activated by methamphetamine, whereas morphine had no such effect. Methamphetamine, but not morphine, caused a long-lasting astrocytic activation in cortical neuron/glia cocultures. Morphine-induced behavioral sensitization, assessed as hyperlocomotion, was reversed by 2 months of withdrawal from intermittent morphine administration, whereas behavioral sensitization to methamphetamine-induced hyperlocomotion was maintained even after 2 months of withdrawal. In vivo treatment with methamphetamine, which was associated with behavioral sensitization, caused PKC-dependent astrocytic activation in the mouse cingulate cortex and nucleus accumbens. Furthermore, the glial modulator propentofylline dramatically diminished the activation of astrocytes and the rewarding effect induced by methamphetamine and morphine. On the other hand, intra-nucleus accumbens and intra-cingulate cortex administration of astrocyte-conditioned medium aggravated the development of rewarding effects induced by methamphetamine and morphine. Furthermore, astrocyte-conditioned medium, but not methamphetamine itself, clearly induced differentiation of neural stem cells into astrocytes. These findings provide direct evidence that astrocytes may, at least in part, contribute to the development of the rewarding effects induced by drugs of abuse in the nucleus accumbens and cingulate cortex.

Nucleus accumbens core acetylcholine is preferentially activated during acquisition of drug- vs food-reinforced behavior

Acquisition of drug-reinforced behavior is accompanied by a systematic increase of release of the neurotransmitter acetylcholine (ACh) rather than dopamine, the expected prime reward neurotransmitter candidate, in the nucleus accumbens core (AcbC), with activation of both muscarinic and nicotinic ACh receptors in the AcbC by ACh volume transmission being necessary for the drug conditioning. The present findings suggest that the AcbC ACh system is preferentially activated by drug reinforcers, because (1) acquisition of food-reinforced behavior was not paralleled by activation of ACh release in the AcbC whereas acquisition of morphine-reinforced behavior, like that of cocaine or remifentanil (tested previously), was, and because (2) local intra-AcbC administration of muscarinic or nicotinic ACh receptor antagonists (atropine or mecamylamine, respectively) did not block the acquisition of food-reinforced behavior whereas acquisition of drug-reinforced behavior had been blocked. Interestingly, the speed with which a drug of abuse distributed into the AcbC and was eliminated from the AcbC determined the size of the AcbC ACh signal, with the temporally more sharply delineated drug stimulus producing a more pronounced AcbC ACh signal. The present findings suggest that muscarinic and nicotinic ACh receptors in the AcbC are preferentially involved during reward conditioning for drugs of abuse vs sweetened condensed milk as a food reinforcer.

Beta-endorphin and drug-induced reward and reinforcement

Although drugs of abuse have different acute mechanisms of action, their brain pathways of reward exhibit common functional effects upon both acute and chronic administration. Long known for its analgesic effect, the opioid beta-endorphin is now shown to induce euphoria, and to have rewarding and reinforcing properties. In this review, we will summarize the present neurobiological and behavioral evidences that support involvement of beta-endorphin in drug-induced reward and reinforcement. Currently, evidence supports a prominent role for beta-endorphin in the reward pathways of cocaine and alcohol. The existing information indicating the importance of beta-endorphin neurotransmission in mediating the reward pathways of nicotine and THC, is thus far circumstantial. The studies described herein employed diverse techniques, such as biochemical measurements of beta-endorphin in various brain sites and plasma, and behavioral measurements, conducted following elimination (via administration of anti-beta-endorphin antibodies or using mutant mice) or augmentation (by intracerebral administration) of beta-endorphin. We suggest that the reward pathways for different addictive drugs converge to a common pathway in which beta-endorphin is a modulating element. Beta-endorphin is involved also with distress. However, reviewing the data collected so far implies a discrete role, beyond that of a stress response, for beta-endorphin in mediating the substance of abuse reward pathway. This may occur via interacting with the mesolimbic dopaminergic system and also by its interesting effects on learning and memory. The functional meaning of beta-endorphin in the process of drug-seeking behavior is discussed.

Genetic variation in dopamine pathways differentially associated with smoking progression in adolescence

OBJECTIVE

To clarify the nature of the association between dopamine genes and smoking by examining whether genetic variability in components of the dopamine pathway could explain refined phenotypes in adolescent smoking progression.

METHOD

Data are from an ongoing prospective study of the long-term outcome of early risk factors studied since birth. At age 15 years, 220 participants (108 males, 112 females) completed a self-report questionnaire measuring smoking behavior and were genotyped for five dopamine gene variants.

RESULTS

Smoking initiation was related to allelic variation in the dopamine D4 receptor gene (DRD4), whereas smoking continuation and dependence showed association with the dopamine D2 receptor gene (DRD2). Adolescents with the seven-repeat allele of the common DRD4 exon 3 polymorphism had rates of ever smoking that were significantly higher than in those with other genotypes. Once smoking started, carriers of the T allele of a single nucleotide polymorphism of DRD2 (rs4648317) reported higher rates of current smoking and scored higher on nicotine dependence than their allelic counterparts. Among current smokers, intention to quit was significantly lower in adolescents homozygous for the 10-repeat allele of the common dopamine transporter 3' untranslated region polymorphism.

CONCLUSIONS

Our results provide preliminary evidence of genetic influences on different stages of smoking and suggest the importance of specific dopamine genes in smoking progression in adolescence.

Dopamine-2 receptors in the arcuate nucleus modulate cocaine-seeking behavior

Beta-endorphin is an endogenous opioid peptide, implicated in the behavioral effects of drugs of abuse. It is synthesized in the arcuate nucleus and secreted into the nucleus accumbens. In the present study, we examined the interaction between arcuate nucleus dopaminergic cells and accumbal beta-endorphin, during cocaine exposure. Using microdialysis, we found that blockade of arcuate dopamine-2 receptors with a selective antagonist significantly attenuated cocaine-induced increases of beta-endorphin levels in the nucleus accumbens. Moreover, rats chronically exposed to cocaine using the self-administration paradigm displayed extinction-like behavior following blockade of dopamine-2 receptors. These findings indicate that dopaminergic neurons in the arcuate nucleus may induce the secretion of beta-endorphin in the nucleus accumbens, and that they are implicated in the cocaine reward pathway.

Direct evidence of astrocytic modulation in the development of rewarding effects induced by drugs of abuse

Long-term exposure to pyschostimulants and opioids induced neuronal plasticity. Accumulating evidence suggests that astrocytes actively participate in synaptic plasticity. We show here that a glial modulator propentofylline (PPF) dramatically diminished the activation of astrocytes induced by drugs of abuse, such as methamphetamine (METH) and morphine (MRP). In vivo treatment with PPF also suppressed both METH- and MRP-induced rewarding effects. On the other hand, intra-nucleus accumbens (N.Acc.) administration of astrocyte-conditioned medium (ACM) aggravated the development of rewarding effects induced by METH and MRP via the Janus kinase/signal transducers and activators of transcription (Jak/STAT) pathway, which modulates astrogliosis and/or astrogliogenesis. Furthermore, ACM, but not METH itself, clearly induced the differentiation of multipotent neuronal stem cells into glial fibrillary acidic protein-positive astrocytes, and this effect was reversed by cotreatment with the Jak/STAT inhibitor AG490. Intra-cingulate cortex (CG) administration of ACM also enhanced the rewarding effect induced by METH and MRP. In contrast to ACM, intra-N.Acc. administration of microglia-conditioned medium failed to affect the rewarding effects of METH and MRP in mice. These findings suggest that astrocyte-, but not microglia-, related soluble factors could amplify the development of rewarding effect of METH and MRP in the N.Acc. and CG. The present study provides direct evidence that astrocytes may, at least in part, contribute to the synaptic plasticity induced by drugs of abuse during the development of rewarding effects induced by psychostimulants and opioids.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Elevated platelet vesicular monoamine transporter 2 in former heroin addicts maintained on methadone

The brain vesicular monoamine transporter (VMAT2) is essential for neuronal monoamine storage and regulation of monoaminergic neurotransmission. We demonstrated previously a high degree of similarity between the pharmacodynamic characteristics of platelet and brain VMAT2. Opioids induce increase of dopamine release in limbic structures. In the present study we assessed the VMAT2 pharmacodynamic characteristics using high affinity [(3)H]dihydrotetrabenazine (TBZOH) binding to platelets of former male heroin addicts maintained on methadone (n = 12) compared to age-matched healthy controls (n = 13). A significant increase (19%, p < 0.05) in platelet VMAT2 density (Bmax) was observed in the methadone treated patients compared to controls. There was no significant difference in the affinity of [(3)H]TBZOH to its platelet binding site. The increased VMAT2 density may reflect a compensatory attempt to prevent vesicular depletion due to chronic methadone exposure.

Interaction between the dopamine D4 receptor and the serotonin transporter promoter polymorphisms in alcohol and tobacco use among 15-year-olds

Early onset of alcohol and tobacco use during adolescence increases the risk for establishing a substance use disorder in adulthood. Both alcohol and nicotine stimulate the dopamine (DA) and the serotonin (5-HT) systems. The DA system has been implicated in the mediation of the rewarding effects of self-administered drugs of abuse. A possible role of an interaction between these neurotransmitter systems in substance use behavior has been suggested but is as yet unknown. The present study was designed to examine the influence of the DA D4 receptor (DRD4) and the serotonin transporter (5-HTT) genotype and their interaction on adolescent alcohol and tobacco experimentation. Participants were from a longitudinal study of a birth cohort consisting initially of 384 children from a high-risk community sample. At the age of 15 years, adolescents completed a self-report questionnaire measuring tobacco and alcohol consumption. DNA was taken from 305 participants (146 boys, 159 girls) and genotyped for the DRD4 exon III and the 5-HTTLPR polymorphisms. The DRD4 7-repeat allele was associated with greater smoking and drinking involvement in boys. In girls, a significant DRD4 x 5-HTT interaction was detected. Girls without the DRD4 7-repeat allele and who were homozygous for the long allele of 5-HTTLPR displayed the highest smoking and drinking activity. The genetic and potential molecular background underlying adolescent vulnerability to substance abuse is discussed.

Activation of muscarinic and nicotinic acetylcholine receptors in the nucleus accumbens core is necessary for the acquisition of drug reinforcement

Neurotransmitter release in the nucleus accumbens core (NACore) during the acquisition of remifentanil or cocaine reinforcement was determined in an operant runway procedure by simultaneous tandem mass spectrometric analysis of dopamine, acetylcholine, and remifentanil or cocaine itself. Run times for remifentanil or cocaine continually decreased over the five consecutive runs of the experiment. Intra-NACore dopamine, acetylcholine, and drug peaked with each intravenous remifentanil or cocaine self-administration and decreased to pre-run baseline with half-lives of approximately 10 min. As expected, remifentanil or cocaine peaks did not vary between the five runs. Surprisingly, however, drug-contingent dopamine peaks also did not change over the five runs, whereas acetylcholine peaks did. Thus, the acquisition of drug reinforcement was paralleled by a continuous increase in acetylcholine overflow in the NACore, whereas the overflow of dopamine, the expected prime neurotransmitter candidate for conditioning in drug reinforcement, did not increase. Local intra-accumbens administration by reverse microdialysis of either atropine or mecamylamine completely and reversibly blocked the acquisition of remifentanil reinforcement. Our findings suggest that activation of muscarinic and nicotinic acetylcholine receptors in the NACore by acetylcholine volume transmission is necessary during the acquisition phase of drug reinforcement conditioning.

Altered extracellular striatal in vivo biotransformation of the opioid neuropeptide dynorphin A(1-17) in the unilateral 6-OHDA rat model of Parkinson's disease

The in vivo biotransformation of dynorphin A(1-17) (Dyn A) was studied in the striatum of hemiparkinsonian rats by using microdialysis in combination with nanoflow reversed-phase liquid chromatography/electrospray time-of-flight mass spectrometry. The microdialysis probes were implanted into both hemispheres of unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats. Dyn A (10 pmol microl(-1)) was infused through the probes at 0.4 microl min(-1) for 2 h. Samples were collected every 30 min and analyzed by mass spectrometry. The results showed for the first time that there was a difference in the Dyn A biotransformation when comparing the two corresponding sides of the brain. Dyn A metabolites 1-8, 1-16, 5-17, 10-17, 7-10 and 8-10 were detected in the dopamine-depleted striatum but not in the untreated striatum. Dyn A biotransformed fragments found in both hemispheres were N-terminal fragments 1-4, 1-5, 1-6, 1-11, 1-12 and 1-13, C-terminal fragments 2-17, 3-17, 4-17, 7-17 and 8-17 and internal fragments 2-5, 2-10, 2-11, 2-12, and 8-15. The relative levels of these fragments were lower in the dopamine-depleted striatum. The results imply that the extracellular in vivo processing of the dynorphin system is being disturbed in the 6-OHDA-lesion animal model of Parkinson's disease.

Brain substrates for increased drug seeking during protracted withdrawal

Studies are reviewed indicating that both increased anxiety and altered hedonic processing accompany protracted withdrawal from opiates. Increased anxiety may be most apparent in response to stress, whereas decreased motivation for natural rewards but increased interest in drugs reveals substantial alterations in hedonic values. Our recent work indicates that increased norepinephrine (NE) release in the bed nucleus of the stria terminalis (BNST) may underlie anxiety associated with protracted withdrawal. Altered plasticity in afferents to the ventral tegmental area (VTA; accumbens, amygdala and lateral hypothalamus), or in the VTA itself, may be involved in the altered hedonic processing that occurs during protracted withdrawal. We hypothesize that conditioned release of NE in the BNST in response to stressors (including drug-associated stimuli) may elevate anxiety which then augments the reward value of drugs by a negative reinforcement mechanism. We also propose that plasticity in VTA neurons and their afferents during chronic drug exposure and protracted withdrawal decreases the valence of natural rewards whereas sensitization occurs to the motivational effects of drugs that increases their motivational valence. The combination of anxiety, decreased valence of natural rewards, and sensitized incentive for drugs make a potent formula for relapse and drug seeking during protracted withdrawal.

The neurobiology of addiction

Drug addiction includes complex neurobiological and behavioural processes. Acute reinforcing effects of drugs of abuse are responsible for the initiation of drug addiction, whereas the negative consequences of drug abstinence have a crucial motivational significance for relapse and maintenance of the addictive process. The mesocorticolimbic system represents a common neuronal substrate for the reinforcing properties of drugs of abuse. Both dopamine and opioid transmission play a crucial role in this reward pathway. Common neuronal changes have also been reported during the abstinence to different drugs of abuse that could underlie the negative motivational effects of withdrawal. These changes include decreased dopaminergic activity in the mesolimbic system and a recruitment of the brain stress pathways. All drugs of abuse interact with these brain circuits by acting on different molecular and neurochemical mechanisms. The existence of bidirectional interactions between different drugs of abuse, such as opioids and cannabinoids, provides further findings to support this common neurobiological substrate for drug addictive processes.

Craving for alcohol and drugs in animals and humans: biology and behavior

Research studies indicate that sites and pathways for appetitive drive states, that are located in the limbic system, appear to be responsible for normal and pathological craving for alcohol and other addicting drugs. Pathological craving for alcohol and drugs in humans has been substantiated by animal studies, which have identified neurosubstrates and neurotransmitters associated with behavioral models of addiction. Repetitive administration of alcohol and drugs appears to affect hedonic homeostasis of the appetitive drives leading to the hedonic alleostasis where negative reinforcement exceeds positive returns despite continued drug use. Neuroimaging studies have concentrated on areas in the brain related to reward or reinforcement of alcohol/drug use, but the technique can be employed to find support for a neurosubstrate to distinguish normal craving or "liking" from pathological craving or "wanting" a drug. Identifying the neurobasis of "wanting" a drug long after not "liking it" is central to understanding pathological craving and loss of control over drug use in addiction in humans. Neuroimaging is currently the only method to directly visualize sites for craving in the brain in humans. Neuroimaging techniques will provide methods, which are not possible in animals, for studying addictive disease in humans.

Naltrexone and alcohol drinking in mice lacking beta-endorphin by site-directed mutagenesis

Alcohol-induced activation of the opioid system may contribute to the reinforcing properties of alcohol. This study investigated whether elimination of beta-endorphin (BE) synthesis via site-directed mutagenesis in embryonic stem cells would alter alcohol intake in mice. Both BE-deficient and wildtype (WT) mice generated from the targeted stem cells were backcrossed for nine generations onto a C57BL/6 background, and were maintained with ad libitum food and water. Mice had access to alcohol (10% v/v) under the following conditions: 24 h, scheduled access for 2 h/day, following acute (1 or 2 days) or chronic (5 weeks) alcohol deprivation, and scheduled access following six doses of naltrexone (0.125-16.0 mg/kg BW, ip) or saline treatment. Alcohol intake was similar in BE-deficient and WT mice given chronic access to alcohol, but greater in BE-deficient compared with WT mice during the first 10 days of scheduled access to alcohol, but not after more extensive experience with scheduled access. BE-deficient, but not WT mice, increased alcohol intake following 2 days, but not 1 day or 5 weeks, of deprivation. Naltrexone reduced alcohol drinking both in BE-deficient and WT mice, suggesting that drinking is mediated, in part, by activation of opioid receptors in both genotypes.

The effect on opioid peptides in the rat brain, after chronic treatment with the anabolic androgenic steroid, nandrolone decanoate

In recent years, an increase in abuse of anabolic androgenic steroids (AAS) has been seen among individuals not directly connected to sports. Clinical evidence suggests that abuse of these steroids may result in profound changes in personality, expressed by depressive symptoms, irritability and increased aggression. It is still unknown whether these alterations are related to changes in any particular transmitter system or whether they are persistent or reversible. In this study we focused on AAS effect on the endogenous dynorphin and enkephalin system in the brain. Male rats were given intramuscular injections of the AAS nandrolone decanoate (15 mg/kg), once daily for 2 weeks. The levels of the opioid peptide immunoreactivities (ir) were assessed by radioimmunoassay in two groups immediately after the treatment and in two other groups after additional 3 weeks without any drug treatment (recovery period). The result indicates that chronic AAS treatment increased the activity in the dynorphin B- and Met-enkephalin-Arg(6)Phe(7)-ir in the hypothalamus, striatum and periaqueductal gray (PAG) compared to controls. In addition, the steroid induced an imbalance between the dynorphin and the enkephalin opioid system in the nucleus accumbens, hypothalamus and PAG. This imbalance remained after the recovery period. Since increased peptide activity was found in brain regions regulating emotions, dependence, defensive reactions and aggression, it was suggested that the actual endogenous opioid systems are involved in previously reported AAS-induced changes in these behaviours.

D3 dopamine and kappa opioid receptor alterations in human brain of cocaine-overdose victims

Cocaine is thought to be addictive because chronic use leads to molecular adaptations within the mesolimbic dopamine (DA) circuitry, which affects motivated behavior and emotion. Although the reinforcing effects of cocaine are mediated primarily by blockade of DA uptake, reciprocal signaling between DA and endogenous opioids has important implications for understanding cocaine dependence. We have used in vitro autoradiography and ligand binding to map D3 DA and kappa opioid receptors in the human brains of cocaine-overdose victims. The number of D3 binding sites was increased one-to threefold over the nucleus accumbens and ventromedial sectors of the caudate and putamen from cocaine-overdose victims, as compared to age-matched and drug-free control subjects. D3 receptor/cyclophilin mRNA ratios in the nucleus accumbens were increased sixfold in cocaine-overdose victims over control values, suggesting that cocaine exposure also affects the expression of D3 receptor mRNA. The number of kappa opioid receptors in the nucleus accumbens and other corticolimbic areas from cocaine fatalities was increased twofold as compared to control values. Cocaine-overdose victims exhibiting preterminal excited delirium had a selective upregulation of kappa receptors measured also in the amygdala. Understanding the complex regulatory profiles of DA and opioid synaptic markers that occur with chronic misuse of cocaine may suggest multitarget strategies for treating cocaine dependence.

Activation of reward circuitry in human opiate addicts

The neurobiological mechanisms of opiate addictive behaviour in humans are unknown. A proposed model of addiction implicates ascending brainstem neuromodulatory systems, particularly dopamine. Using functional neuroimaging, we assessed the neural response to heroin and heroin-related cues in established opiate addicts. We show that the effect of both heroin and heroin-related visual cues are maximally expressed in the sites of origin of ascending midbrain neuromodulatory systems. These context-specific midbrain activations predict responses to salient visual cues in cortical and subcortical regions implicated in reward-related behaviour. These findings implicate common neurobiological processes underlying drug and drug-cue-related effects.

Drugs of abuse and the brain

New insights into our understanding of drug abuse and addiction have revealed that the desire to use drugs and the process of addiction depend on effects on brain function. Drugs of abuse have been hypothesized to produce their rewarding effects by neuropharmacological actions on a common brain reward circuit called the extended amygdala. The extended amygdala involves the mesolimbic dopamine system and specific subregions of the basal forebrain, such as the shell of the nucleus accumbens, the bed nucleus of the stria terminalis, and the central nucleus of the amygdala. The psychomotor stimulants cocaine and amphetamine activate the mesolimbic dopamine system; opiates activate opioid peptide receptors within and independent of the mesolimbic dopamine system. Sedative hypnotics alter multiple neurotransmitter systems in this circuitry, including: 1) gamma aminobutyric acid; 2) dopamine; 3) serotonin; 4) glutamate; and 5) opioid peptides. Nicotine and tetrahydrocannabinol both activate mesolimbic dopamine function and possibly opioid peptide systems in this circuitry. Repeated and prolonged drug abuse leads to compulsive use, and the mechanism for this transition involves, at the behavioral level, a progressive dysregulation of brain reward circuitry and a recruitment of brain stress systems such as corticotropin-releasing factor. The molecular mechanisms of signal transduction in these systems are a likely target for residual changes in that they convey allostatic changes in reward set point, which lead to vulnerability to relapse.

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.

Enhanced sensitivity of the nucleus accumbens proenkephalin system to alcohol in rats selectively bred for alcohol preference

Evidence suggests that alcohol-induced activation of the endogenous opioid system is part of a neurobiological mechanism that may be functionally involved in alcohol reinforcement and high alcohol drinking behavior. We postulate that a genetic predisposition toward alcohol drinking is accompanied by increased responsiveness of the opioid system to alcohol. To test this hypothesis, the present study compared the effect of an acute alcohol challenge on enkephalin gene expression in discrete brain regions which are high in preproenkephalin (PPENK) mRNA content and/or are important in mediating alcohol reward in rats selectively bred for alcohol preference (P) or nonpreference (NP). PPENK mRNA content was measured by in situ hybridization performed with a 36 base oligonucleotide probe for PPENK mRNA and was quantified using a computerized image-analysis system. Blood alcohol concentration (BAC) and rate of alcohol elimination following alcohol infusion were similar in P and NP rats. P and NP rats did not differ in basal content of PPENK mRNA in any of the brain areas examined prior to onset of infusion. An intragastric (I.G.) infusion of alcohol (2.5 g/kg b.wt) produced a significant increase in PPENK mRNA in the nucleus accumbens (both shell and core) of P but not NP rats at 1 h after the onset of infusion which coincided with the time at which peak BAC was attained. In contrast, at 8 h after the onset of the alcohol infusion, when BAC was falling toward baseline, PPENK mRNA was decreased in the nucleus accumbens of both P and NP rats and in the anterior striatum and amygdala of NP rats. The results suggest that enhanced responsiveness of the enkephalinergic system to alcohol is associated with, and may be functionally involved in, mediating high alcohol drinking behavior.

Effect of mu opioid receptor blockade on alcohol intake in rats bred for high alcohol drinking

Beta-funaltrexamine (beta-FNA), a selective mu opioid receptor antagonist, when administered in doses of 10.0, 15.0, and 20.0 mg/kg b.wt., decreased alcohol but not water intake in a dose-dependent manner in rats selectively bred for high alcohol intake (HAD line). Beta-FNA also suppressed the intake of a saccharin solution containing alcohol without altering the intake of a similar solution without alcohol. The results suggest that beta-FNA may prove useful as a pharmacotherapeutic agent for the treatment of alcohol dependence. In a second study, pituitary beta-endorphin gene expression (proopiomelanocortin or POMC messenger ribonucleic acid-mRNA) was compared in another pair of rat lines selectively bred for high or low alcohol intake (alcohol-preferring or P and alcohol-nonpreferring or NP lines). A repeated alcohol challenge (1.0 g/kg b.wt./day, IP for 4 days) produced a greater increase in POMC mRNA in the anterior and neurointermediate lobes of the pituitary of P rats compared with NP rats. The results suggest that a genetic predisposition toward high alcohol drinking may be associated with increased responsiveness of the opioid system to alcohol.

Kappa2 opioid receptors in limbic areas of the human brain are upregulated by cocaine in fatal overdose victims

Cocaine is thought to be addictive because chronic use leads to molecular adaptations within the mesolimbic dopamine (DA) circuitry that affect motivated behavior and emotion. Although the reinforcing effects of cocaine are mediated primarily by blocking DA reuptake into the presynaptic nerve terminal, reciprocal signaling between DA and endogenous opioids has important implications for cocaine dependence. The present study used the opioid antagonist 6 beta-[125iodo]-3,14-dihydroxy-17-cyclopropylmethyl-4,5 alpha-epoxymorphinan ([125I]IOXY) after pretreatment with the site-directed acylating agents 2-(p-ethoxybenzyl)-1-diethylaminoethyl-5-isothiocyanatobenzimid iazole -HCl (mu-selective) and N-phenyl-N-[1-(2-(4-isothiocyanato)-phenethyl)-4-piperidinyl]-p ropana mide-HCl (delta-selective) to examine the effect of cocaine exposure on the distribution and density of kappa2 receptors in autopsy studies of human cocaine fatalities. The selective labeling of the kappa2 receptor subtype was demonstrated by competition binding studies, which gave a pharmacological signature (IOXY >/= (+)-bremazocine >> U50,488 >/= U69,593) distinct from either the kappa1 or kappa3 receptor subtypes. Visualization of [125I]IOXY labeling revealed that kappa2 receptors localize to mesocortical and subcortical limbic areas, including the cingulate, entorhinal, insular, and orbitofrontal cortices and the nucleus accumbens and amygdala. The number of kappa2 receptors in the nucleus accumbens and other limbic brain regions from cocaine fatalities was increased twofold as compared with age-matched and drug-free control subjects. Cocaine overdose victims, who experienced paranoia and marked agitation before death, also had elevated densities of kappa2 receptors in the amygdala. These findings demonstrate for the first time that kappa2 receptor numbers are upregulated by cocaine exposure. The molecular adaptation of kappa2 receptor numbers may play a role in the motivational incentive associated with episodes of binge cocaine use and in the dysphoria that follows abrupt cocaine withdrawal.

Drug abuse: hedonic homeostatic dysregulation

Understanding the neurobiological mechanisms of addiction requires an integration of basic neuroscience with social psychology, experimental psychology, and psychiatry. Addiction is presented as a cycle of spiralling dysregulation of brain reward systems that progressively increases, resulting in compulsive drug use and a loss of control over drug-taking. Sensitization and counteradaptation are hypothesized to contribute to this hedonic homeostatic dysregulation, and the neurobiological mechanisms involved, such as the mesolimbic dopamine system, opioid peptidergic systems, and brain and hormonal stress systems, are beginning to be characterized. This framework provides a realistic approach to identifying the neurobiological factors that produce vulnerability to addiction and to relapse in individuals with a history of addiction.

Sensitization to the behavioral effects of cocaine: modulation by dynorphin and kappa-opioid receptor agonists

Several lines of evidence suggest an involvement of the mesolimbic dopamine (DA) system in the mediation of psychostimulant-induced sensitization. It is also apparent that endogenous opioid peptide systems can modulate the activity of this same DA system. Psychostimulant-induced alterations in opioid peptide gene expression have also been reported. In this review, evidence will be presented that demonstrates that the administration of kappa-opioid agonists can prevent the initiation of behavioral sensitization to cocaine and that such treatment is also effective in preventing alterations in mesolimbic DA neurotransmission that occur as a consequence of repeated cocaine administration. The putative role of opioid-DA interactions in the modulation of psychostimulant-induced sensitization will also be discussed.

The neurobiology of social play behavior in rats

Social play behavior is one of the earliest forms of non-mother-directed social behavior appearing in ontogeny in mammalian species. During the last century, there has been a lot of debate on the significance of social play behavior, but behavioral studies have indicated that social play behavior is a separate and relevant category of behavior. The present review provides a comprehensive survey of studies on the neurobiology of social play behavior. Evidence is presented that opioid and dopamine systems play a role in the reward aspect of social play behavior. The role of cholinergic, noradrenergic and opioid systems in attentional processes underlying the generation of social play behavior and the involvement of androgens in the sexual differentiation of social play behavior in rats is summarized. It is concluded that there is not only behavioral, but also neurobiological evidence to suggest that social play behavior represents a separate category of behavior, instead of a precursor for adult social, sexual or aggressive behavior.

Dopaminergic and opiate agonists and antagonists differentially decrease multiple schedule responding maintained by sucrose/ethanol and sucrose

Similar neurobiological mechanisms are hypothesized to influence ethanol- and food-related reinforcement processes. This study examined the ability of compounds with dopaminergic or opiate activity to selectively alter responding maintained by a sucrose/ethanol solution in comparison to a sucrose solution. Long-Evans rats were trained to press a lever using 5% sucrose/10% ethanol and 5% sucrose as the reinforcers on a multiple Fixed Ratio 4 Fixed Ratio 4 schedule of reinforcement. When stable responding was established, the effects of intraperitoneally administered amphetamine (0.0-3.0 mg/kg), haloperidol (0.0-1.0 mg/kg), morphine (0.0-10.0 mg/kg), and naloxone (0.0-10.0 mg/kg) were examined on total session reinforcer presentation and presentation of each reinforcer within individual multiple schedule components. Prior to drug treatment, the total number of reinforcer presentations of the sucrose/ethanol solution was significantly greater than sucrose reinforcer presentations, suggesting the sucrose/ethanol solution was a more efficacious reinforcer. All agents administered decreased responding maintained by sucrose/ethanol and sucrose. The dose-effect curves for sucrose/ethanol were shifted to the left compared to sucrose, suggesting that although the compounds did not selectively impact sucrose/ethanol-maintained responding, sucrose/ethanol-maintained responding was more sensitive to the effects of these compounds.

Differential metabolism of dynorphins in substantia nigra, striatum, and hippocampus

To map the proteolytic enzymes metabolizing dynorphins in brain structures, size-exclusion chromatography linked to electrospray ionization mass spectrometry was used. Enzymes extracted from rat hippocampus, striatum, and substantia nigra were tested for their capability of converting dynorphin-related peptides. Dynorphin A was the most resistant to proteolytic conversion, whereas Big dynorphin and dynorphin B-29 were slowly converted to dynorphin A and dynorphins A and B, respectively. Dynorphin B and alpha-neoendorphin were the least resistant. Dynorphin B was rapidly converted to Leu-enkephalin in the striatum and hippocampus but to Leu-enkephalin-Arg6 in the substantia nigra. alpha-Neoendorphin was converted to Leu-enkephalin in all tissues investigated.

Elevations of nucleus accumbens dopamine and DOPAC levels during intravenous heroin self-administration

Extracellular dopamine and DOPAC (3,4-dihydroxyphenylacetic acid) levels in nucleus accumbens were sampled by microdialysis and quantified with high-performance liquid chromatography during intravenous heroin self-administration sessions in rats. Dopamine levels in 10 and 20 min samples were elevated following the first injection of each session, reaching a plateau of elevation within the first two or three injections and falling back toward baseline only when drug access was terminated. Elevations were in the range of 150-300% when unit dosages of 0.05-0.2 mg/kg were given. Increasing the work requirement from FR-1 to FR-10 did not appear to alter the degree of elevation of dopamine levels, and dopamine levels fell during extinction while lever-pressing rates increased 20-fold. While animals compensated for unit dose changes between 0.05 and 0.2 mg/kg/injection, adjusting their response rate such that the same hourly drug intake and the same asymptotic dopamine levels were maintained across these conditions, at 0.4 mg/kg/injection hourly drug intake and asymptotic dopamine levels were elevated beyond the levels sustained by the lower doses. These findings confirm that self-administered doses of intravenous heroin are sufficient to activate the mesolimbic dopamine system and suggest that significant heroin "craving" can emerge when dopamine levels are still moderately elevated, long before the development of dopamine depletion associated with opiate withdrawal.

The delta 2-opioid receptor antagonist naltriben selectively attenuates alcohol intake in rats bred for alcohol preference

The relative importance of different opioid receptor types in mediating alcohol drinking behavior compared with the intake of other ingesta can be determined by characterizing the effects of selective opioid antagonists on the intake of various ingesta. Nonselective opioid receptor antagonists suppress the intake of many ingesta including alcohol, food, water, and sweets. Two distinct subtypes of delta-opioid receptors, delta 1 and delta 2, have recently been identified in rodent brain. We have previously reported that naltrindole (NTI), which blocks both delta 1 and delta 2 receptors, suppresses both alcohol and saccharin intake in rats selectively bred for high alcohol preference (P line). We now report that naltriben (NTB), an opioid antagonist that is selective for delta 2-opioid receptors, suppresses alcohol intake in rats of the P line and the effect appears to be both specific for alcohol and independent of alcohol palatability. NTB may reduce alcohol intake by attenuating the reinforcing pharmacological properties of alcohol.

Sertindole antagonizes morphine-, cocaine-, and methamphetamine-induced place preference in the rat

The motivational effect of sertindole, a limbic selective antipsychotic drug, was investigated in rats using a non-biased conditioned place preference method which could reliably detect the reinforcing effects of morphine, cocaine and methamphetamine. Sertindole (0.01-1.0 mg/kg, s.c.), like haloperidol and fluphenazine, produced neither place preference nor place aversion. However, sertindole, at a dose of 1.0 mg/kg, s.c., completely abolished the place preferences induced by morphine (8.0 mg/kg, i.p.), cocaine (4.0 mg/kg, i.p.) and methamphetamine (2.0 mg/kg i.p.). These results strongly suggest that sertindole itself does not induce a rewarding effect and may have therapeutic value in the treatment of drug abuse.