Brain stimulation and morphine reward deficits in dopamine D2 receptor-deficient mice

Identification of the Risk Genes Associated With Vulnerability to Addiction: Major Findings From Transgenic Animals

Understanding risk factors for substance use disorders (SUD) can facilitate medication development for SUD treatment. While a rich literature exists discussing environmental factors that influence SUD, fewer articles have focused on genetic factors that convey vulnerability to drug use. Methods to identify SUD risk genes include Genome-Wide Association Studies (GWAS) and transgenic approaches. GWAS have identified hundreds of gene variants or single nucleotide polymorphisms (SNPs). However, few genes identified by GWAS have been verified by clinical or preclinical studies. In contrast, significant progress has been made in transgenic approaches to identify risk genes for SUD. In this article, we review recent progress in identifying candidate genes contributing to drug use and addiction using transgenic approaches. A central hypothesis is if a particular gene variant (e.g., resulting in reduction or deletion of a protein) is associated with increases in drug self-administration or relapse to drug seeking, this gene variant may be considered a risk factor for drug use and addiction. Accordingly, we identified several candidate genes such as those that encode dopamine D₂ and D₃ receptors, mGluR₂, M₄ muscarinic acetylcholine receptors, and α₅ nicotinic acetylcholine receptors, which appear to meet the risk-gene criteria when their expression is decreased. Here, we describe the role of these receptors in drug reward and addiction, and then summarize major findings from the gene-knockout mice or rats in animal models of addiction. Lastly, we briefly discuss future research directions in identifying addiction-related risk genes and in risk gene-based medication development for the treatment of addiction.

Acupuncture Modulates Intracranial Self-Stimulation of the Medial Forebrain Bundle in Rats

Acupuncture affects the central nervous system via the regulation of neurotransmitter transmission. We previously showed that Shemen (HT7) acupoint stimulation decreased cocaine-induced dopamine release in the nucleus accumbens. Here, we used the intracranial self-stimulation (ICSS) paradigm to evaluate whether HT stimulation regulates the brain reward function of rats. We found that HT stimulation triggered a rightward shift of the frequency–rate curve and elevated the ICSS thresholds. However, HT7 stimulation did not affect the threshold-lowering effects produced by cocaine. These results indicate that HT7 points only effectively regulates the ICSS thresholds of the medial forebrain bundle in drug-naïve rats.

Involvement of the dopaminergic system in the reward-related behavior of pregabalin

There has been an increase in cases of drug addiction and prescription drug abuse worldwide. Recently, pregabalin abuse has been a focus for many healthcare agencies, as highlighted by epidemiological studies. We previously evaluated the possibility of pregabalin abuse using the conditioned place preference (CPP) paradigm. We observed that a 60 mg/kg dose could induce CPP in mice and that pregabalin-rewarding properties were mediated through glutamate neurotransmission. Notably, the dopaminergic reward circuitry is also known to play a crucial role in medication-seeking behavior. Therefore, this study aimed to explore the possible involvement of dopaminergic receptor-1 in pregabalin-induced CPP. Mice were randomly allocated to receive saline or the dopamine-1 receptor antagonist SKF-83566 (0.03 mg/kg, intraperitoneal). After 30 min, the mice received either saline or pregabalin (60 mg/kg) during the conditioning phase. Among the control groups that received saline or SKF-83566, the time spent in the two conditioning chambers was not significantly altered. However, among the pregabalin-treated group, there was a marked increase in the time spent in the drug-paired chamber compared to the time spent in the vehicle-paired chamber. Notably, blocking dopamine-1 receptors with SKF-83566 completely prevented pregabalin-induced place preference, thus demonstrating the engagement of the dopaminergic system in pregabalin-induced reward-related behavior.

Oral supplementation with ginseng polysaccharide promotes food intake in mice

INTRODUCTION

Ginseng polysaccharide (GPS, same as Panax polysaccharide) is a kind of polysaccharide extracted from ginseng. It has been reported that GPS has the ability to activate innate immunity, regulates blood sugar balance, and improves antioxidant capacity, but the effect on feeding behavior and its mechanism remains unclear.

METHOD

To investigate the possible effect of GPS on feeding behavior of animals, mice were supplied with GPS in water, and food intake, hedonic feeding behavior, anxiety-like behavior, expression of appetite-regulation peptides in the central nervous system and glucose-related hormone levels in the serum of mice were measured.

RESULTS

Ginseng polysaccharide significantly increased the average daily food intake in mice and promoted hedonic eating behavior. Meanwhile, the levels of serum glucose and glucagon were significantly reduced by GPS, and GPS promoted hypothalamic neuropeptide Y expression, inhibited proopiomelanocortin (POMC) expression, and reduced dopamine D1 receptor (DRD1) levels in the midbrain. We also found that the anxiety level of mice was significantly lower after GPS intake. In conclusion, oral supplementation with GPS promoted food intake in mice, most likely through the regulation of circulating glucose levels.

Determinants of opioid abuse potential: Insights using intracranial self-stimulation

Intracranial self-stimulation (ICSS) is one procedure that can be used for preclinical abuse potential assessment. In ICSS procedures, subjects with microelectrodes implanted into a brain-reward region are trained to press an operant response lever for pulses of electrical brain stimulation, and drugs are evaluated for their effectiveness to increase or "facilitate" ICSS responding (an abuse-related effect) or to depress ICSS responding (an abuse-limiting effect). ICSS has been used for decades to evaluate determinants of opioid abuse potential, and this article reviews pharmacological and biological determinants of opioid abuse potential as revealed by ICSS studies in rodents. One of the most important observations from ICSS studies is that abused mu opioid receptor (MOR) agonists like morphine often fail to produce abuse-related ICSS facilitation in opioid-naïve subjects, but several days of repeated opioid exposure is sufficient for opioid-induced facilitation to emerge. Future studies with ICSS could help (a) to clarify mechanisms that increase MOR agonist abuse potential during early opioid exposure or during chronic exposure leading to dependence, (b) to evaluate novel opioids either developed as candidate analgesics with reduced abuse potential or identified as designer opioids being synthesized and distributed for illicit use, and (c) to test candidate pharmacotherapies for treatment of opioid abuse in non-dependent and dependent subjects.

Evidence for differential opioid use disorder in schizophrenia in an addiction treatment population

Although people diagnosed with schizophrenia are known to have elevated risks of abuse and dependence for nicotine, alcohol, cocaine, and cannabis, it is less clear if schizophrenia is associated with higher rates of opioid use disorders compared to either the general population or individuals with other major psychiatric disorders. Here we examine a large publicly available database from substance abuse treatment centers to compare how frequently patients with schizophrenia report problems with heroin or other opioid drugs compared to other major drugs of abuse. For comparison, the pattern of substance abuse in schizophrenia is contrasted with individuals with major depression, bipolar disorder, and the entire sample of individuals seeking substance abuse treatment. We find that a significantly lower proportion of patients with schizophrenia are reported to have problems with heroin (5.1%) relative to the entire treatment population (18.2%). The schizophrenia sample also had a significantly lower proportion of individuals with a non-heroin opioid problem (7.2%) compared to the entire treatment population (14.8%), patients with depression (23%), and patients with bipolar disorder (17.3%). In contrast, the schizophrenia sample had significantly higher proportions of individuals with problems with alcohol, cocaine, and cannabis relative to the treatment population. Although these data do not allow conclusions on the relative rate of opioid addiction in schizophrenia compared to the general population, the results suggest a discrepancy in patterns of drug choice that may aid our understanding of schizophrenia and substance use comorbidity.

Preliminary evidence that negative symptom severity relates to multilocus genetic profile for dopamine signaling capacity and D2 receptor binding in healthy controls and in schizophrenia

Deficits in central, subcortical dopamine (DA) signaling may underlie negative symptom severity, particularly anhedonia, in healthy individuals and in schizophrenia. To investigate these relationships, we assessed negative symptoms with the Schedule for the Assessment of Negative Symptoms and the Brief Negative Symptom Scale (BNSS) and self-reported anhedonia with the Scales for Physical and Social Anhedonia (SPSA), Temporal Experience of Pleasure Scale, and Snaith-Hamilton Pleasure Scale in 36 healthy controls (HC), 27 siblings (SIB) of individuals with schizophrenia, and 66 individuals with schizophrenia or schizoaffective disorder (SCZ). A subset of participants (N = 124) were genotyped for DA-related polymorphisms in genes for DRD4, DRD2/ANKK1, DAT1, and COMT, which were used to construct biologically-informed multi-locus genetic profile (MGP) scores reflective of subcortical dopaminergic signaling. DA receptor type 2 (D2R) binding was assessed among a second subset of participants (N = 23) using PET scans with the D2R-selective, non-displaceable radioligand (N-[¹¹C]methyl)benperidol. Higher MGP scores, reflecting elevated subcortical dopaminergic signaling capacity, were associated with less negative symptom severity, as measured by the BNSS, across all participants. In addition, higher striatal D2R binding was associated with less physical and social anhedonia, as measured by the SPSA, across HC, SIB, and SCZ. The current preliminary findings support the hypothesis that subcortical DA function may contribute to negative symptom severity and self-reported anhedonia, independent of diagnostic status.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Neuroplasticity, axonal guidance and micro-RNA genes are associated with morphine self-administration behavior

Neuroadaptations in the ventral striatum (VS) and ventral midbrain (VMB) following chronic opioid administration are thought to contribute to the pathogenesis and persistence of opiate addiction. In order to identify candidate genes involved in these neuroadaptations, we utilized a behavior-genetics strategy designed to associate contingent intravenous drug self-administration with specific patterns of gene expression in inbred mice differentially predisposed to the rewarding effects of morphine. In a Yoked-control paradigm, C57BL/6J mice showed clear morphine-reinforced behavior, whereas DBA/2J mice did not. Moreover, the Yoked-control paradigm revealed the powerful consequences of self-administration versus passive administration at the level of gene expression. Morphine self-administration in the C57BL/6J mice uniquely up- or down-regulated 237 genes in the VS and 131 genes in the VMB. Interestingly, only a handful of the C57BL/6J self-administration genes (<3%) exhibited a similar expression pattern in the DBA/2J mice. Hence, specific sets of genes could be confidently assigned to regional effects of morphine in a contingent- and genotype-dependent manner. Bioinformatics analysis revealed that neuroplasticity, axonal guidance and micro-RNAs (miRNAs) were among the key themes associated with drug self-administration. Noteworthy were the primary miRNA genes H19 and micro-RNA containing gene (Mirg), processed, respectively, to mature miRNAs miR-675 and miR-154, because they are prime candidates to mediate network-like changes in responses to chronic drug administration. These miRNAs have postulated roles in dopaminergic neuron differentiation and mu-opioid receptor regulation. The strategic approach designed to focus on reinforcement-associated genes provides new insight into the role of neuroplasticity pathways and miRNAs in drug addiction.

Pathway-specific dopaminergic deficits in a mouse model of Angelman syndrome

Angelman syndrome (AS) is a neurodevelopmental disorder caused by maternal deletions or mutations of the ubiquitin ligase E3A (UBE3A) allele and characterized by minimal verbal communication, seizures, and disorders of voluntary movement. Previous studies have suggested that abnormal dopamine neurotransmission may underlie some of these deficits, but no effective treatment currently exists for the core features of AS. A clinical trial of levodopa (L-DOPA) in AS is ongoing, although the underlying rationale for this treatment strategy has not yet been thoroughly examined in preclinical models. We found that AS model mice lacking maternal Ube3a (Ube3a(m-/p+) mice) exhibit behavioral deficits that correlated with abnormal dopamine signaling. These deficits were not due to loss of dopaminergic neurons or impaired dopamine synthesis. Unexpectedly, Ube3a(m-/p+) mice exhibited increased dopamine release in the mesolimbic pathway while also exhibiting a decrease in dopamine release in the nigrostriatal pathway, as measured with fast-scan cyclic voltammetry. These findings demonstrate the complex effects of UBE3A loss on dopamine signaling in subcortical motor pathways that may inform ongoing clinical trials of L-DOPA therapy in patients with AS.

Gentiopicroside attenuates morphine rewarding effect through downregulation of GluN2B receptors in nucleus accumbens

AIMS

Gentiopicroside (Gent) is one of the secoiridoid compound isolated from Gentiana lutea. This compound exhibits analgesic activities and inhibits the expression of GluN2B-containing N-methyl-D-aspartate (NMDA) receptors in the anterior cingulate cortex in mice. Nucleus accumbens (NAc) is a forebrain structure known for its role in drug addiction. However, little is known about the role of Gent on morphine dependence and synaptic transmission changes in the NAc.

METHODS

Conditioned place preference (CPP) test and behavioral sensitization of locomotor activity were used to investigate drug-seeking related behaviors. Brain slices containing NAc were prepared, and whole-cell patch-clamp recordings were performed to record the excitatory postsynaptic currents (EPSCs). Expression of proteins was detected by Western blot analysis.

RESULTS

Systemic administration of Gent attenuated the CPP effect induced by morphine, but had no effect on morphine-induced behavioral sensitization. Gent significantly reversed overexpression of GluN2B-containing NMDA receptors and dopamine D2 receptors in NAc during the first week of morphine withdrawal. However, the compound did not affect the overexpression of GluN2A-containing NMDA receptors, GluA1, and dopamine D1 receptors. Lastly, Gent significantly reduced NMDA receptors-mediated EPSCs in the NAc.

CONCLUSION

Our study provides strong evidence that Gent inhibits morphine dependence through downregulation of GluN2B-containing NMDA receptors in the NAc.

Disruption of conditioned reward association by typical and atypical antipsychotics

Antipsychotic drugs are broadly classified into typical and atypical compounds; they vary in their pharmacological profile however a common component is their antagonist effects at the D2 dopamine receptors (DRD2). Unfortunately, diminished DRD2 activation is generally thought to be associated with the severity of neuroleptic-induced anhedonia. The purpose of this study was to determine the effect of the atypical antipsychotic olanzapine and typical antipsychotic haloperidol in a paradigm that reflects the learned transfer of incentive motivational properties to previously neutral stimuli, namely autoshaping. In order to provide a dosing comparison to a therapeutically relevant endpoint, both drugs were tested against amphetamine-induced disruption of prepulse inhibition as well. In the autoshaping task, rats were exposed to repeated pairings of stimuli that were differentially predictive of reward delivery. Conditioned approach to the reward-predictive cue (sign-tracking) and to the reward (goal-tracking) increased during repeated pairings in the vehicle treated rats. Haloperidol and olanzapine completely abolished this behavior at relatively low doses (100microg/kg). This same dose was the threshold dose for each drug to antagonize the sensorimotor gating deficits produced by amphetamine. At lower doses (3-30microg/kg) both drugs produced a dose-dependent decrease in conditioned approach to the reward-predictive cue. There was no difference between drugs at this dose range which indicates that olanzapine disrupts autoshaping at a significantly lower proposed DRD2 receptor occupancy. Interestingly, neither drug disrupted conditioned approach to the reward at the same dose range that disrupted conditioned approach to the reward-predictive cue. Thus, haloperidol and olanzapine, at doses well below what is considered therapeutically relevant, disrupts the attribution of incentive motivational value to previously neutral cues. Drug effects on this dimension of reward processing are an important consideration in the development of future pharmacological treatments for schizophrenia.

Mutant mouse models: genotype-phenotype relationships to negative symptoms in schizophrenia

Negative symptoms encompass diminution in emotional expression and motivation, some of which relate to human attributes that may not be accessible readily in animals. Additionally, their refractoriness to treatment precludes therapeutic validation of putative models. This review considers critically the application of mutant mouse models to the study of the pathobiology of negative symptoms. It focuses on 4 main approaches: genes related to the pathobiology of schizophrenia, genes associated with risk for schizophrenia, neurodevelopmental-synaptic genes, and variant approaches from other areas of neurobiology. Despite rapid advances over the past several years, it is clear that we continue to face substantive challenges in applying mutant models to better understand the pathobiology of negative symptoms: the majority of evidence relates to impairments in social behavior, with only limited data relating to anhedonia and negligible data concerning avolition and other features; even for the most widely examined feature, social behavior, studies have used diverse assessments thereof; modelling must proceed in cognizance of increasing evidence that genes and pathobiologies implicated in schizophrenia overlap with other psychotic disorders, particularly bipolar disorder. Despite the caveats and challenges, several mutant lines evidence a phenotype for at least one index of social behavior. Though this may suggest superficially some shared relationship to negative symptoms, it is not yet possible to specify either the scope or the pathobiology of that relationship for any given gene. The breadth and depth of ongoing studies in mutants hold the prospect of addressing these shortcomings.

Qualitative differences between C57BL/6J and DBA/2J mice in morphine potentiation of brain stimulation reward and intravenous self-administration

RATIONALE

The C57BL/6J (C57) and DBA/2J (DBA) mice are the most common genotypes used to identify chromosomal regions and neurochemical mechanisms of interest in opioid addiction. Unfortunately, outside of the oral two-bottle choice procedure, limited and sometimes controversial evidence is available for determining their relative sensitivity to the rewarding effects of morphine.

OBJECTIVES

The purpose of this study was to utilize classically accepted models of drug abuse liability to determine relative susceptibility to the rewarding effects of morphine.

METHODS

The ability of morphine or amphetamine to potentiate lateral hypothalamic brain stimulation and intravenous morphine self-administration (across three doses in a fixed ratio schedule and at the highest dose in progressive ratio schedules) was investigated in both genotypes.

RESULTS

In both measures, C57 and DBA mice differed dramatically in their response to morphine. Morphine potentiated rewarding stimulation in the C57 mice but antagonized it in the DBA mice. Consistent with these findings, intravenous morphine did not serve as a positive reinforcer in DBA mice under conditions that were effective in the C57 mice using a fixed ratio schedule and failed to sustain levels of responding sufficient to maintain a constant rate of drug intake under a progressive ratio schedule. In contrast, amphetamine potentiated the rewarding effects of brain stimulation similarly in the two genotypes.

CONCLUSIONS

These findings provide strong evidence that morphine is rewarding in the C57 genotype and not in the DBA genotype. Understanding their relative susceptibility is important given the prominence of these genotypes in candidate gene identification and gene mapping.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Targeting pain-depressed behaviors in preclinical assays of pain and analgesia: drug effects on acetic acid-depressed locomotor activity in ICR mice

AIMS

Pain depresses expression of many behaviors, and one goal of analgesic treatment is to restore pain-depressed behaviors. Assays that focus on pain-depressed behaviors may contribute to preclinical assessment of candidate analgesics.

MAIN METHODS

This study compared effects of the mu opioid receptor agonist morphine (an acknowledged analgesic), the dopamine receptor antagonist haloperidol (a non-analgesic sedative), the adenosine receptor antagonist caffeine (a non-analgesic stimulant) and the neurokinin-1 receptor antagonist CJ 11,974-01 (a candidate analgesic) on acetic acid-induced writhing (a traditional pain-stimulated behavior) and acetic acid-induced suppression of locomotor activity (a pain-depressed behavior) in male ICR mice. Drug effects on non-depressed (baseline) locomotor activity were also examined.

KEY FINDINGS

I.P. administration of acetic acid (0.18-1%) was equipotent in stimulating writhing and depressing locomotor activity. Morphine blocked both acid-induced stimulation of writhing and depression of locomotion, although it was 56-fold less potent in the assay of acid-depressed locomotion. Haloperidol and CJ 11,974-01 decreased acid-stimulated writhing, but failed to block acid-induced depression of locomotion. Caffeine had no effect on acid-stimulated writhing or acid-depressed locomotor activity, although it did increase non-depressed locomotion. Thus, morphine was the only drug to block both acid-stimulated writhing and acid-depressed locomotion.

SIGNIFICANCE

Complementary assays of pain-stimulated and pain-depressed behaviors may improve the predictive validity of preclinical studies that assess candidate analgesic drugs. The low potency of morphine to block acid-induced depression of locomotion suggests that locomotor activity may be a relatively insensitive measure for studies of pain-depressed behavior.

Effects of the dopamine stabilizer, OSU-6162, on brain stimulation reward and on quinpirole-induced changes in reward and locomotion

Dysregulation of limbic dopamine (DA) neurotransmission results in abnormal positive or negative emotional states that characterize several mental disorders. Drugs that restore DA homeostasis are most likely to constitute effective treatments for such emotional disturbances. In this study, we investigated the effects of several doses of OSU-6162, a drug that belongs to a new class named "DA stabilizers", on brain stimulation reward. Because quinpirole produces, depending on the dose, a pre-synaptic depressant and a post-synaptic stimulatory effect on reward and locomotor activity, we also compared the ability of OSU-6162 and haloperidol to prevent these effects of the full DA agonist. Results show that OSU-6162 produced a dose-orderly reduction of reward with no change in the capacity of the animals to produce the operant response, and prevented, like haloperidol, both stimulatory and depressant effects of quinpirole on locomotor activity but only its reward stimulatory effect. The observed functional antagonism of OSU-6162 on these DA-dependent behaviors suggests that it may constitute an effective treatment for abnormal positive emotional state, and that it would be exempt of motor side-effects.

Getting specialized: presynaptic and postsynaptic dopamine D2 receptors

Dopamine (DA) signaling controls many physiological functions ranging from locomotion to hormone secretion, and plays a critical role in addiction. DA elevation, for instance in response to drugs of abuse, simultaneously activates neurons expressing different DA receptors; how responses from diverse neurons/receptors are orchestrated in the generation of behavioral and cellular outcomes, is still not completely defined. Signaling from D2 receptors (D2Rs) is a good example to illustrate this complexity. D2Rs have presynaptic and postsynaptic localization and functions, which are shared by two isoforms in vivo. Recent results from knockout mice are clarifying the role of site and D2 isoform-specific effects thereby increasing our understanding of how DA modulates neuronal physiology.

Getting specialized: presynaptic and postsynaptic dopamine D2 receptors

Dopamine (DA) signaling controls many physiological functions ranging from locomotion to hormone secretion, and plays a critical role in addiction. DA elevation, for instance in response to drugs of abuse, simultaneously activates neurons expressing different DA receptors; how responses from diverse neurons/receptors are orchestrated in the generation of behavioral and cellular outcomes, is still not completely defined. Signaling from D2 receptors (D2Rs) is a good example to illustrate this complexity. D2Rs have presynaptic and postsynaptic localization and functions, which are shared by two isoforms in vivo. Recent results from knockout mice are clarifying the role of site and D2 isoform-specific effects thereby increasing our understanding of how DA modulates neuronal physiology.

Biological substrates of reward and aversion: a nucleus accumbens activity hypothesis

The nucleus accumbens (NAc) is a critical element of the mesocorticolimbic system, a brain circuit implicated in reward and motivation. This basal forebrain structure receives dopamine (DA) input from the ventral tegmental area (VTA) and glutamate (GLU) input from regions including the prefrontal cortex (PFC), amygdala (AMG), and hippocampus (HIP). As such, it integrates inputs from limbic and cortical regions, linking motivation with action. The NAc has a well-established role in mediating the rewarding effects of drugs of abuse and natural rewards such as food and sexual behavior. However, accumulating pharmacological, molecular, and electrophysiological evidence has raised the possibility that it also plays an important (and sometimes underappreciated) role in mediating aversive states. Here we review evidence that rewarding and aversive states are encoded in the activity of NAc medium spiny GABAergic neurons, which account for the vast majority of the neurons in this region. While admittedly simple, this working hypothesis is testable using combinations of available and emerging technologies, including electrophysiology, genetic engineering, and functional brain imaging. A deeper understanding of the basic neurobiology of mood states will facilitate the development of well-tolerated medications that treat and prevent addiction and other conditions (e.g., mood disorders) associated with dysregulation of brain motivation systems.

Decreased brain reward function during nicotine withdrawal in C57BL6 mice: evidence from intracranial self-stimulation (ICSS) studies

Deficits in brain reward function during nicotine withdrawal may serve as an important substrate for negative reinforcement that contributes to the persistence of the tobacco habit in human smokers. The ability to assess withdrawal-associated reward deficits in genetically modified mice may facilitate understanding of the neurobiological mechanisms of nicotine dependence. Here, we assessed the effects of nicotine withdrawal on brain reward function in mice, as measured by intracranial self-stimulation (ICSS) thresholds. Male C57BL6 mice were trained in a discrete-trial current-threshold ICSS procedure until stable reward thresholds were obtained. Mice then received experimenter-administered saline or nicotine (2 mg/kg/injection salt; x4 daily) injections for 7 consecutive days, and ICSS thresholds assessed for 3 days after cessation of injections. Thresholds were unaltered in nicotine- and saline-treated mice after cessation of injections, indicating that this treatment regimen was not sufficient to induce withdrawal-associated reward deficits. Next, mice were implanted subcutaneously with osmotic minipumps delivering a constant daily amount of saline or nicotine (24 mg/kg/day; free-base), with pumps surgically removed 13 days later. The nicotinic receptor antagonist mecamylamine (2 mg/kg) elevated ICSS thresholds in nicotine- but not saline-treated mice when administered 8-10 days after pump implantation. Similarly, reward thresholds were elevated in nicotine-treated mice 12-72 h after minipump removal. These data demonstrate that antagonist-precipitated or spontaneous withdrawal from nicotine delivered via osmotic minipumps induced reward deficits in mice. Further, these findings highlight the potential utility of the ICSS procedure for assessing this important affective component of nicotine withdrawal in genetically modified mice.

Intracranial self-stimulation (ICSS) in rodents to study the neurobiology of motivation

It has become increasingly important to assess mood states in laboratory animals. Tests that reflect reward, reduced ability to experience reward (anhedonia) and aversion (dysphoria) are in high demand because many psychiatric conditions that are currently intractable in humans (e.g., major depression, bipolar disorder, addiction) are characterized by dysregulated motivation. Intracranial self-stimulation (ICSS) can be utilized in rodents (rats, mice) to understand how pharmacological or molecular manipulations affect the function of brain reward systems. Although many different methodologies are possible, we will describe in this protocol the use of medial forebrain bundle (MFB) stimulation together with the 'curve-shift' variant of analysis. This combination is particularly powerful because it produces a highly reliable behavioral output that enables clear distinctions between the treatment effects on motivation and the treatment effects on the capability to perform the task.

Behavioral genetic contributions to the study of addiction-related amphetamine effects

Amphetamines, including methamphetamine, pose a significant cost to society due to significant numbers of amphetamine-abusing individuals who suffer major health-related consequences. In addition, methamphetamine use is associated with heightened rates of violent and property-related crimes. The current paper reviews the existing literature addressing genetic differences in mice that impact behavioral responses thought to be relevant to the abuse of amphetamine and amphetamine-like drugs. Summarized are studies that used inbred strains, selected lines, single-gene knockouts and transgenics, and quantitative trait locus (QTL) mapping populations. Acute sensitivity, neuroadaptive responses, rewarding and conditioned effects are among those reviewed. Some gene mapping work has been accomplished, and although no amphetamine-related complex trait genes have been definitively identified, translational work leading from results in the mouse to studies performed in humans is beginning to emerge. The majority of genetic investigations have utilized single-gene knockout mice and have concentrated on dopamine- and glutamate-related genes. Genes that code for cell support and signaling molecules are also well-represented. There is a large behavioral genetic literature on responsiveness to amphetamines, but a considerably smaller literature focused on genes that influence the development and acceleration of amphetamine use, withdrawal, relapse, and behavioral toxicity. Also missing are genetic investigations into the effects of amphetamines on social behaviors. This information might help to identify at-risk individuals and in the future to develop treatments that take advantage of individualized genetic information.

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

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

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Differences in behavioural effects of amphetamine and dopamine-related gene expression in wild-type and homozygous CCK2 receptor deficient mice

Neuropeptide cholecystokinin (CCK) interacts with dopamine in the regulation of motor activity and motivations. Therefore, in CCK(2) receptor deficient mice the behavioural effects of repeated amphetamine administration and changes in dopamine-related gene expression were studied. Four-day amphetamine (1 mg/kg) treatment induced a significantly stronger motor sensitization in homozygous mice compared to their wild-type littermates. However, in the conditioned place preference test the action of amphetamine was more pronounced in wild-type animals. As opposed to wild-type mice, amphetamine (1-3 mg/kg) did not cause a significant conditioned place preference in homozygous mice. The expression of Tyhy gene was elevated in the mesolimbic structures and Drd2 gene was down-regulated in the mesencephalon of saline-treated homozygous mice in comparison with respective wild-type group. Four-day treatment with amphetamine induced a significant increase in the expression of Tyhy in the mesencephalon, striatum and mesolimbic structures of wild-type mice, whereas in homozygous mice a similar change was evident only in the mesencephalon. Also, the expression of Drd1 gene in the striatum and Drd2 gene in the mesolimbic structures of wild-type mice were up-regulated under the influence of amphetamine. In conclusion, the present study established differences in the behavioural effects of amphetamine in wild-type and homozygous mice. The increased tone of dopaminergic projections from the mesencephalon to mesolimbic structures is probably related to increased amphetamine-induced motor sensitization in homozygous mice. The lack of development of up-regulation of Drd1 and Drd2 genes after repeated treatment with amphetamine probably explains the reduced place conditioning in CCK(2) receptor deficient mice.