Individual differences in cocaine- and amphetamine-induced activation of male Sprague-Dawley rats: contribution of the dopamine transporter

In situ-gelling hydrophobized starch nanoparticle-based nanoparticle network hydrogels for the effective delivery of intranasal olanzapine to treat brain disorders

Intranasal (IN) delivery offers potential to deliver antipsychotic drugs with improved efficacy to the brain. However, the solubilization of such drugs and the frequency of required re-application both represent challenges to its practical implementation in treating various mental illnesses including schizophrenia. Herein, we report a sprayable nanoparticle network hydrogel (NNH) consisting of hydrophobically-modified starch nanoparticles (SNPs) and mucoadhesive chitosan oligosaccharide lactate (COL) that can gel in situ within the nasal cavity and release ultra-small penetrative SNPs over time. Hydrophobization of the SNPs enables enhanced uptake and prolonged release of poorly water soluble drugs such as olanzapine from the NNH depot through mucous and ultimately into the brain via the nose-to-brain (N2B) pathway. The hydrogel shows high in vitro cytocompatibility in mouse striatal neuron and human primary nasal cell lines and in vivo efficacy in an amphetamine-induced pre-clinical rat schizophrenia model, with IN-delivered NNH hydrogels maintaining successful attenuation of locomotor activity for up to 4 h while all other tested treatments (drug-only IN or conventional intraperitoneal delivery) failed to attenuate at any time point past 0.5 h. As such, in situ-gelling NNHs represent a safe excipient for the IN delivery of hydrophobic drugs directly to the brain using customized SNPs that exhibit high penetration and drug complexing properties to maximize effective drug delivery.

Dopamine-mediated striatal activity and function is enhanced in GlyRα2 knockout animals

The glycine receptor alpha 2 (GlyRα2) is a ligand-gated ion channel which upon activation induces a chloride conductance. Here, we investigated the role of GlyRα2 in dopamine-stimulated striatal cell activity and behavior. We show that depletion of GlyRα2 enhances dopamine-induced increases in the activity of putative dopamine D1 receptor-expressing striatal projection neurons, but does not alter midbrain dopamine neuron activity. We next show that the locomotor response to d-amphetamine is enhanced in GlyRα2 knockout animals, and that this increase correlates with c-fos expression in the dorsal striatum. 3-D modeling revealed an increase in the neuronal ensemble size in the striatum in response to D-amphetamine in GlyRα2 KO mice. Finally, we show enhanced appetitive conditioning in GlyRα2 KO animals that is likely due to increased motivation, but not changes in associative learning or hedonic response. Taken together, we show that GlyRα2 is an important regulator of dopamine-stimulated striatal activity and function.

α7 nicotinic acetylcholine receptor modulation of accumbal dopamine release covaries with novelty seeking

Heightened novelty-seeking phenotypes are associated with a range of behavioural traits including susceptibility to drug use. These relationships are recapitulated in preclinical models, where rats that exhibit increased exploratory activity in novel environments (high responders-HR) acquire self-administration of psychostimulants more rapidly compared to rats that display low novelty exploration (low responders-LR). Dopamine release dynamics in the nucleus accumbens (NAc) covaries with response to novelty, and differences in dopaminergic signalling are thought to be a major underlying driver of the link between novelty seeking and drug use vulnerability. Accumbal dopamine release is controlled by local microcircuits including modulation through glutamatergic and nicotinic acetylcholine receptor (nAChR) systems, but whether these mechanisms contribute to disparate dopamine signalling across novelty phenotypes is unclear. Here, we used ex vivo voltammetry in the NAc of rats to determine if α7 nAChRs contribute to differential dopamine dynamics associated with individual differences in novelty exploration. We found that blockade of α7 nAChRs attenuates tonic dopamine release evoked by low-frequency stimulations across phenotypes but that phasic release is decreased in LRs while HRs are unaffected. These stimulation frequency- and phenotype-dependent effects result in a decreased dynamic range of release exclusively in LRs. Furthermore, we found that differential α7 modulation of dopamine release in LRs is dependent on AMPA but not NMDA receptors. These results help to form an understanding of the local NAc microcircuitry and provide a potential mechanism for covariance of dopamine dynamics and sensitivity to the reinforcing effects of drugs of abuse.

Role of melanin-concentrating hormone in drug use disorders

Melanin-concentrating hormone (MCH) is a neuropeptide primarily transcribed in the lateral hypothalamus (LH), with vast projections to many areas throughout the central nervous system that play an important role in motivated behaviors and drug use. Anatomical, pharmacological and genetic studies implicate MCH in mediating the intake and reinforcement of commonly abused substances, acting by influencing several systems including the mesolimbic dopaminergic system, glutamatergic as well as GABAergic signaling and being modulated by inflammatory neuroimmune pathways. Further support for the role of MCH in controlling behavior related to drug use will be discussed as it relates to cerebral ventricular volume transmission and intracellular molecules including cocaine- and amphetamine-regulated transcript peptide, dopamine- and cAMP-regulated phosphoprotein 32 kDa. The primary goal of this review is to introduce and summarize current literature surrounding the role of MCH in mediating the intake and reinforcement of commonly abused drugs, such as alcohol, cocaine, amphetamine, nicotine and opiates.

Genetic deletion of the dopamine D3 receptor increases vulnerability to heroin in mice

Despite extensive research, the neurobiological risk factors that convey vulnerability to opioid abuse are still unknown. Recent studies suggest that the dopamine D3 receptor (D3R) is involved in opioid self-administration, but it remains unclear whether altered D3R availability is a risk factor for the development of opioid abuse and addiction. Here we used dopamine D3 receptor-knockout (D3-KO) mice to investigate the role of this receptor in the different phases of opioid addiction. D3-KO mice learned to self-administer heroin faster and took more heroin than wild-type mice during acquisition and maintenance of self-administration. D3R-KO mice also displayed higher motivation to work to obtain heroin reward during self-administration under progressive-ratio reinforcement, as well as elevated heroin-seeking during extinction and reinstatement testing. In addition, deletion of the D3R induced higher baseline levels of extracellular dopamine (DA) in the nucleus accumbens (NAc), higher basal levels of locomotion, and reduced NAc DA and locomotor responses to lower doses of heroin. These findings suggest that the D3R is critically involved in regulatory processes that normally limit opioid intake via DA-related mechanisms. Deletion of D3R augments opioid-taking and opioid-seeking behaviors. Therefore, low D3R availability in the brain may represent a risk factor for the development of opioid abuse and addiction.

Predicting the ergogenic response to methylphenidate

PURPOSE

Methylphenidate (MPH) and other stimulants have been shown to enhance physical performance. However, stimulant research has almost exclusively been conducted in young, active persons with a normal BMI, and may not generalize to other groups. The purpose of this study was to determine whether the ergogenic response to MPH could be predicted by individual level characteristics.

METHODS

We investigated whether weekly minutes of moderate-to-vigorous physical activity (MVPA), age, and BMI could predict the ergogenic response to MPH. In a double-blind, cross-over design 29 subjects (14M, 15F, 29.7 ± 9.68 years, BMI: 26.1 ± 6.82, MVPA: 568.8 ± 705.6 min) ingested MPH or placebo before performing a handgrip task. Percent change in mean force between placebo and MPH conditions was used to evaluate the extent of the ergogenic response.

RESULTS

Mean force was significantly higher in MPH conditions [6.39% increase, T(25) = 3.09, p = 0.005 118.8 ± 37.96 (± SD) vs. 111.8 ± 34.99 Ns] but variable (coefficient of variation:163%). Using linear regression, we observed that min MVPA (T(25) = -2.15, β = -0.400, p = 0.044) and age [T(25) = -3.29, β = -0.598, p = 0.003] but not BMI [T(25) = 1.67, β = 0.320 p = 0.109] significantly predicted percent change in mean force in MPH conditions.

CONCLUSIONS

We report that lower levels of physical activity and younger age predict an improved ergogenic response to MPH and that this may be explained by differences in dopaminergic function. This study illustrates that the ergogenic response to MPH is partly dependent on individual differences such as habitual levels of physical activity and age.

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

Paradoxical abatement of striatal dopaminergic transmission by cocaine and methylphenidate

We combined in vitro amperometric, optical analysis of fluorescent false neurotransmitters and microdialysis techniques to unveil that cocaine and methylphenidate induced a marked depression of the synaptic release of dopamine (DA) in mouse striatum. In contrast to the classical dopamine transporter (DAT)-dependent enhancement of the dopaminergic signal observed at concentrations of cocaine lower than 3 μM, the inhibitory effect of cocaine was found at concentrations higher than 3 μM. The paradoxical inhibitory effect of cocaine and methylphenidate was associated with a decrease in synapsin phosphorylation. Interestingly, a cocaine-induced depression of DA release was only present in cocaine-insensitive animals (DAT-CI). Similar effects of cocaine were produced by methylphenidate in both wild-type and DAT-CI mice. On the other hand, nomifensine only enhanced the dopaminergic signal either in wild-type or in DAT-CI mice. Overall, these results indicate that cocaine and methylphenidate can increase or decrease DA neurotransmission by blocking reuptake and reducing the exocytotic release, respectively. The biphasic reshaping of DA neurotransmission could contribute to different behavioral effects of psychostimulants, including the calming ones, in attention deficit hyperactivity disorder.

Rats classified as low or high cocaine locomotor responders: a unique model involving striatal dopamine transporters that predicts cocaine addiction-like behaviors

Individual differences are a hallmark of drug addiction. Here, we describe a rat model based on differential initial responsiveness to low dose cocaine. Despite similar brain cocaine levels, individual outbred Sprague-Dawley rats exhibit markedly different magnitudes of acute cocaine-induced locomotor activity and, thereby, can be classified as low or high cocaine responders (LCRs or HCRs). LCRs and HCRs differ in drug-induced, but not novelty-associated, hyperactivity. LCRs have higher basal numbers of striatal dopamine transporters (DATs) than HCRs and exhibit marginal cocaine inhibition of in vivo DAT activity and cocaine-induced increases in extracellular DA. Importantly, lower initial cocaine response predicts greater locomotor sensitization, conditioned place preference and greater motivation to self-administer cocaine following low dose acquisition. Further, outbred Long-Evans rats classified as LCRs, versus HCRs, are more sensitive to cocaine's discriminative stimulus effects. Overall, results to date with the LCR/HCR model underscore the contribution of striatal DATs to individual differences in initial cocaine responsiveness and the value of assessing the influence of initial drug response on subsequent expression of addiction-like behaviors.

Individual differences and social influences on the neurobehavioral pharmacology of abused drugs

The interaction of drugs with biologic targets is a critical area of research, particularly for the development of medications to treat substance use disorders. In addition to understanding these drug-target interactions, however, there is a need to understand more fully the psychosocial influences that moderate these interactions. The first section of this review introduces some examples from human behavioral pharmacology that illustrate the clinical importance of this research. The second section covers preclinical evidence to characterize some of the key individual differences that alter drug sensitivity and abuse vulnerability, related primarily to differences in response to novelty and impulsivity. Evidence is presented to indicate that critical neuropharmacological mechanisms associated with these individual differences involve integrated neurocircuits underlying stress, reward, and behavioral inhibitory processes. The third section covers social influences on drug abuse vulnerability, including effects experienced during infancy, adolescence, and young adulthood, such as maternal separation, housing conditions, and social interactions (defeat, play, and social rank). Some of the same neurocircuits involved in individual differences also are altered by social influences, although the precise neurochemical and cellular mechanisms involved remain to be elucidated fully. Finally, some speculation is offered about the implications of this research for the prevention and treatment of substance abuse.

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.

J. Neurochem. (2008) 105, 2122–2133.

Dopamine uptake inhibitors but not dopamine releasers induce greater increases in motor behavior and extracellular dopamine in adolescent rats than in adult male rats

Most life-long drug addiction begins during adolescence. Important structural and functional changes in brain occur during adolescence and developmental differences in forebrain dopamine systems could mediate a biologic vulnerability to drug addiction during adolescence. Studies investigating age differences in psychostimulant responses have yielded mixed results, possibly because of different mechanisms for increasing extracellular dopamine. Recent research from our laboratory suggests that adolescent dopamine systems may be most affected by selective dopamine uptake inhibitors. We investigated age-related behavioral responses to acute administration of several dopamine uptake inhibitors [methylphenidate, 1-{2-[bis-(4-fluorophenyl)methoxy]ethyl}-4-(3-phenylpropyl)piperazine (GBR12909), and nomifensine] and releasing agents [amphetamine and methylenedioxymethamphetamine (MDMA)] in adolescent and adult male rats. Methylphenidate and amphetamine effects on stimulated dopamine efflux were determined using fast-scan cyclic voltammetry in vivo. Dopamine uptake inhibitors but not dopamine releasing agents induced more locomotion and/or stereotypy in adolescent relative to adult rats. MDMA effects were greater in adults at early time points after dosing. Methylphenidate but not amphetamine induced much greater dopamine efflux in periadolescent relative to adult rats. Periadolescent male rats are particularly sensitive to psychostimulants that are DAT inhibitors but are not internalized and do not release dopamine. Immaturity of DAT and/or DAT associated signaling systems in adolescence specifically enhances behavioral and dopaminergic responses in adolescence.

Mass spectrometry screening reveals peptides modulated differentially in the medial prefrontal cortex of rats with disparate initial sensitivity to cocaine

To better understand why certain individuals are more vulnerable to cocaine abuse and addiction, we identify peptide markers associated with individual variation in sensitivity to the behavioral effects of cocaine. Previous studies in rats show that low, compared to high, cocaine responders are more sensitive to cocaine-induced behavioral plasticity (sensitization), exhibit enhanced conditioning to cocaine's rewarding effects, and are more motivated to self administer cocaine. In the current study, we combine matrix-assisted laser desorption/ionization mass spectrometry with multivariate statistical methods to analyze tissue extracts from rat dorsal striatum, nucleus accumbens, and medial prefrontal cortex (mPFC) to examine trends in peptide changes that coincide with behavioral phenotype. Peptide profiles of these three regions from individual animals were characterized via mass spectrometry. Resulting mass peaks that were statistically different between these groups were identified using principal component analysis. The mass peaks were then identified in pooled samples via multistage liquid chromatography mass spectrometry. A total of 74 peptides from 28 proteins were sequenced from defined brain regions. Statistically significant changes in peak intensities for seven peptides were found in the mPFC of rats given a single injection of 10 mg/kg cocaine, with low cocaine responders showing approximately 2-fold increase in peak intensities for the acetylated N terminus peptides of stathmin and Hint 1, as well as truncated ATP synthase. These results suggest that distinct peptide profiles in the mPFC are associated with individuals that exhibit reduced sensitivity to the behavioral effects of cocaine.

Low and high cocaine locomotor responding male Sprague-Dawley rats differ in rapid cocaine-induced regulation of striatal dopamine transporter function

Adult outbred Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively). Importantly, LCRs and HCRs are distinguished by their differential responsiveness to acute cocaine-induced (but not baseline) locomotor activity, inhibition of the dopamine transporter (DAT) and resulting extracellular DA (HCR > LCR), as well as by repeated cocaine-induced locomotor sensitization and measures of cocaine's rewarding and reinforcing effects (LCR > HCR). Curiously, 30 min after acute cocaine HCRs exhibit greater DAT-mediated [(3)H]DA uptake into striatal synaptosomes than LCRs. To investigate this finding further, we measured locomotor activity, striatal [(3)H]DA uptake kinetics and DAT cell surface expression in LCRs and HCRs over an extended period (25-180 min) after a single relatively low-dose of cocaine (10 mg/kg, i.p.). HCRs exhibited the "predicted" locomotor response: a marked initial activation that returned to baseline by 120 min post-injection. While LCRs exhibited a >50% lower maximal locomotor response, this increase was sustained, lasting approximately 33% longer than in HCRs. At 25 min post-cocaine, maximal velocity (V(max)) of [(3)H]DA uptake was significantly higher by 25% in HCRs than LCRs, with no difference in affinity (K(m)). Despite the DAT V(max) difference, however, DAT surface expression did not differ between LCRs and HCRs. There was a similar trend (HCR > LCR) for DAT V(max) at 40 min, but not at 150 or 180 min. These findings suggest that, compared to LCRs, HCRs have an enhanced ability to rapidly up-regulate DAT function in response to acute cocaine, which may contribute to their more "normal" cocaine-induced locomotor activation.

Repeated amphetamine administration in rats revealed consistency across days and a complete dissociation between locomotor and hypothalamic-pituitary-adrenal axis effects of the drug

RATIONALE

Most drugs of abuse stimulate both locomotor activity and the hypothalamic-pituitary-adrenal (HPA) axis, but the relationship between the two responses within the same subjects and their reliabilities has been scarcely studied. Our objectives were to study: (1) the consistency and stability across time of locomotor and HPA activation induced by repeated d-amphetamine (AMPH); (2) the relationship between locomotor and hormonal responses to AMPH; and (3) the relationship between novelty-induced activity and both types of responses to the drug.

METHODS

Male adult rats were exposed to a novel environment to study the locomotor response. Later, they were injected with AMPH (2 mg/kg, sc) for 5 days. In Experiment 1, Plasma adrenocorticotropin (ACTH) and corticosterone levels in response to AMPH were studied on days 1, 3, and 5, and locomotor response on days 2 and 4. In Experiment 2, ACTH and corticosterone responses were studied on days 2 and 4, and locomotor response on days 1, 3, and 5.

RESULTS

Across days, both locomotor and HPA responses to the drug were consistent, but independent measures, unrelated to the reactivity to novelty. As measured by the area under the curve, the HPA response to AMPH desensitized with the repeated injection, whereas the initial locomotor response to the drug increased.

CONCLUSIONS

Dissociation exists between HPA and locomotor activation induced by AMPH, which seemed to be both reliable individual traits. Locomotor reactivity to novelty was related neither to HPA nor to locomotor responses to AMPH.

Role of the dopamine transporter in the action of psychostimulants, nicotine, and other drugs of abuse

A number of studies over the last two decades have demonstrated the critical importance of dopamine (DA) in the behavioral pharmacology and addictive properties of abused drugs. The DA transporter (DAT) is a major target for drugs of abuse in the category of psychostimulants, and for methylphenidate (MPH), a drug used for treating attention deficit hyperactivity disorder (ADHD), which can also be a psychostimulant drug of abuse. Other drugs of abuse such as nicotine, ethanol, heroin and morphine interact with the DAT in more indirect ways. Despite the different ways in which drugs of abuse can affect DAT function, one evolving theme in all cases is regulation of the DAT at the level of surface expression. DAT function is dynamically regulated by multiple intracellular and extracellular signaling pathways and several protein-protein interactions. In addition, DAT expression is regulated through the removal (internalization) and recycling of the protein from the cell surface. Furthermore, recent studies have demonstrated that individual differences in response to novel environments and psychostimulants can be predicted based on individual basal functional DAT expression. Although current knowledge of multiple factors regulating DAT activity has greatly expanded, many aspects of this regulation remain to be elucidated; these data will enable efforts to identify drugs that might be used therapeutically for drug dependence therapeutics.

Reduced sensitivity to the locomotor-stimulant effects of cocaine is associated with increased sensitivity to its discriminative stimulus properties

Outbred Long-Evans rats exhibit wide variation in their locomotor response to cocaine. Here, we investigated the relationship between these individual differences and interoceptive effects of cocaine in low cocaine responder (LCR) and high cocaine responder (HCR) phenotypes. Rats were trained to discriminate cocaine (10.0 mg/kg, intraperitoneally) from saline by repeated pairings of injections with one of two response levers. In subsequent tests for stimulus generalization to other cocaine doses (1.25-15.0 mg/kg), LCRs exhibited partial-to-full generalization at 1.85 and 2.5 mg/kg cocaine, respectively, whereas HCRs did not. When the selective 5-HT reuptake inhibitor fluoxetine (5.0 mg/kg) was coadministered with saline or different cocaine doses, we observed similar upward shifts in dose-response in both phenotypes. In contrast, coadministration of the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 0.3 mg/kg) led to partial substitution of DOI for cocaine and enhancement of the stimulus properties of 1.25 mg/kg cocaine in LCRs only. Finally, a retest of cocaine-induced locomotion after discrimination testing revealed marked behavioral sensitization in LCRs and modest changes in behavior in HCRs. Taken together, these results suggest that initial sensitivity to the locomotor-stimulant effects of cocaine is inversely related to its interoceptive properties and that differences in 5-HT systems may contribute to the phenotypic differences observed.

Rapid substrate-induced down-regulation in function and surface localization of dopamine transporters: rat dorsal striatum versus nucleus accumbens

The dopamine transporter (DAT) substrates dopamine, d-amphetamine (AMPH), and methamphetamine are known to rapidly and transiently reduce DAT activity and/or surface expression in dorsal striatum and heterologous expression systems. We sought to determine if similar substrate-induced regulation of DATs occurs in rat nucleus accumbens. In dorsal striatum synaptosomes, brief (15-min) in vitro substrate pre-exposure markedly decreased maximal [(3)H]dopamine uptake velocity whereas identical substrate pre-exposure in nucleus accumbens synaptosomes produced a smaller, non-significant reduction. However, 45 min after systemic AMPH administration, maximal ex vivo [(3)H]dopamine uptake velocity was significantly reduced in both brain regions. Protein kinase C inhibition blocked AMPH's down-regulation of DAT activity. DAT synaptosomal surface expression was not modified following either the brief in vitro or in vivo AMPH pre-exposure but was reduced after a longer (1-h) in vitro pre-exposure in both brain regions. Together, our findings suggest that relatively brief substrate exposure results in greater down-regulation of DAT activity in dorsal striatum than in nucleus accumbens. Moreover, exposure to AMPH appears to regulate striatal DATs in a biphasic manner, with an initial protein kinase C-dependent decrease in DAT-mediated uptake velocity and then, with longer exposure, a reduction in DAT surface expression.

Individual differences in cocaine-induced locomotor activity in male Sprague-Dawley rats and their acquisition of and motivation to self-administer cocaine

RATIONALE

Factors that increase an individual's susceptibility to cocaine dependence remain largely unknown. We have previously shown that adult outbred male Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor activity following the administration of a single dose of cocaine (10 mg/kg, i.p.). Furthermore, LCR/HCR classification predicts dopamine transporter function/inhibition, cocaine-induced locomotor sensitization, and cocaine-conditioned place preference.

OBJECTIVES

The present study assessed LCR/HCR classification and the development of locomotor sensitization on the latency to acquire cocaine self-administration and motivation to self-administer cocaine.

RESULTS

LCRs and HCRs did not differ in their latency to acquire low-dose cocaine self-administration (0.25 mg/kg/infusion over 12 s, fixed ratio 1 schedule of reinforcement). In a follow-up experiment, repeated experimenter-administered injections of cocaine (10 mg/kg, i.p.) resulted in locomotor sensitization for LCRs, but not HCRs; nonetheless, all rats exhibited decreased latency to acquire cocaine self-administration compared to the first experiment. Repeated cocaine preexposure and LCR/HCR classification predicted break point when rats responded for cocaine under a progressive ratio schedule of reinforcement (0.25, 0.5, and 1.0 mg/kg/infusion; multiple exposure>single exposure, LCR>HCR), but there was no interaction between these variables.

CONCLUSIONS

Although LCR/HCR classification did not predict the rate of acquisition of cocaine self-administration under these conditions, LCR rats demonstrated greater responding for cocaine after acquisition (PR). Thus, these findings demonstrate the relevance of using the LCR/HCR model when studying susceptibility to cocaine dependence.

Disparate cocaine-induced locomotion as a predictor of choice behavior in rats trained in a delay-discounting task

Heightened impulsivity and differential sensitivity to a drug's behavioral effects are traits that, individually, have been associated with chronic drug use and dependence. Here, we used an animal model to test whether individual differences in cocaine-induced activity are predictive of impulsive choice behavior. Adult, male Sprague-Dawley rats were given cocaine (10mg/kg, i.p.) and classified into low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response in an open-field arena. Rats were then trained in a delay-discounting task that offers a choice between immediately delivered, but smaller reinforcements, or larger reinforcements that are delivered after a delay. We also examined the effects of amphetamine (AMPH; 0.3-1.0mg/kg) and the 5-HT1A agonist 8-OH-DPAT (0.3-1.0mg/kg) on delay-discounting. Lastly, all rats were retested in the open-field to determine if phenotypes were stable. We observed baseline differences in choice behavior between the groups, with HCRs behaving more impulsively (i.e., choosing the small reinforcement) compared to LCRs. AMPH decreased choice of the large reinforcement in LCRs, but did not alter choice in HCRs. Impulsive choice was increased in both phenotypes following 8-OH-DPAT, with LCRs exhibiting changes across a wider range of delays. When cocaine-induced open-field behavior was retested, responses in LCRs were similar whereas HCRs showed evidence of tolerance. Our results suggest that differential sensitivity to cocaine-induced locomotion is predictive of impulsivity and the potential neurobiological differences in LCRs and HCRs may provide insight into mechanisms contributing to vulnerability for chronic drug use and/or dependence.

Amphetamine- and cocaine-induced conditioned place preference and concomitant psychomotor sensitization in mice with genetically inactivated melanin-concentrating hormone MCH(1) receptor

The melanin-concentrating hormone MCH(1) receptor has been proposed to exert an inhibitory control on monoaminergic (especially dopaminergic) activity within the mesolimbic system, which underpins drug seeking and reward. That hypothesis predicts that an inactivation of these receptors should enhance the sensitivity to drug rewarding effects. To test that prediction, we examined the propensity of mice lacking the melanin-concentrating receptor (MCH(1) KO) and their intact counterparts (WT) to form cocaine- and amphetamine-induced conditioned place preference. The conditioned rewarding effects induced by 0.375, 0.75, 1.5 and 3 mg/kg amphetamine were assessed in two sub-experiments and those induced by 1, 2, 4 and 8 mg/kg cocaine in two other sub-experiments. All mice were tested under saline for place preference 24 h following four every-other-day conditioning trials and an initial pre-conditioning session under saline. Most of the cocaine and amphetamine doses induced place preference, but without any genotype difference being revealed. Also, none of the cocaine doses induced psychomotor sensitization during conditioning, whereas amphetamine generated clear-cut dose-dependent sensitization in both genotypes. Albeit MCH(1) KO mice exhibited higher levels of psychomotor activation, the rates of sensitization were comparable across genotypes at 1.5 and 3 mg/kg amphetamine. Moreover, 0.375 and especially 0.75 mg/kg amphetamine produced a slight but yet significant sensitization in MCH(1) KO but not in their WT counterparts. Despite such an effect, the results cannot be considered as unambiguously supportive of the tested prediction.

Individual differences in initial low-dose cocaine-induced locomotor activity and locomotor sensitization in adult outbred female Sprague-Dawley rats

Sex and individual differences are important considerations when studying cocaine responsiveness. We have previously shown that male Sprague-Dawley (S-D) rats can be classified as low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor activity following a single dose of cocaine (10 mg/kg, i.p.). Further, this distinction was found to predict dopamine transporter function, cocaine-induced locomotor sensitization, cocaine conditioned place preference and motivation to self-administer cocaine. Here we investigated whether or not individual differences in cocaine-induced locomotor activity and locomotor sensitization exist in female S-D rats. Female rats exhibited a broad range of locomotor activation following either a 5 or 10 mg/kg cocaine injection, allowing for classification as LCRs or HCRs. When administered over 7 days, both doses induced locomotor sensitization in female LCRs/HCRs. However, the magnitude of effects produced by 5 mg/kg cocaine in female LCRs/HCRs was more comparable to that produced by 10 mg/kg in male LCRs/HCRs, both of which, interestingly, developed sensitization in this study. These findings suggest that female S-D rats, like male S-D rats, can be classified as LCRs/HCRs and highlight the importance of accounting for dose when studying sex and individual differences to the effects of cocaine.

Simultaneous expression of cocaine-induced behavioral sensitization and conditioned place preference in individual rats

Conditioned place preference and locomotor sensitization are rodent behavioral models commonly used to investigate the actions of drugs of abuse. However, few studies have examined both paradigms in the same group of animals. We were interested in developing a combined protocol which successfully induced both conditioned place preference and sensitization simultaneously in cocaine-treated Sprague-Dawley rats in order to test the hypothesis that the magnitude of these two phenomena would be positively correlated. We used an open-field with a removable place preference insert to assess these measures independently. Cocaine-conditioned animals demonstrated a significant shift in preference for the drug-paired compartment and a sensitized locomotor response which was not observed in saline-conditioned animals challenged with cocaine. There was no significant relationship between locomotor sensitization and conditioned place preference in individual animals. We further examined these results with respect to each rat's initial response to cocaine, response to a novel environment and central zone entries in an open-field. Locomotor sensitization demonstrated an inverse correlation with the initial cocaine response. In contrast, conditioned place preference demonstrated an inverse correlation with the centre response. These results demonstrate that the combination of the acute cocaine response and the centre response in a novel open-field environment can be used to indicate the propensity of a given rat to exhibit either behavioral sensitization or conditioned place preference; however, it seems that sensitization and place preference are not necessarily co-expressed to a similar extent in the same individual animal.

Individual differences in response to novelty predict prefrontal cortex dopamine transporter function and cell surface expression

Previous research has shown that individual differences in response to novelty predict self-administration and the locomotor response to psychostimulant drugs of abuse. The aim of the present study was to determine if individual differences in response to novelty based on inescapable or free-choice novelty tests predict dopamine transporter (DAT) function and trafficking as well as nicotine-induced modulation of DAT. Results show that the maximal velocity (Vmax) of [3H]dopamine uptake into prefrontal cortex (PFC) synaptosomes correlated negatively with the locomotor response to inescapable novelty. In contrast, Vmax correlated positively with novelty place preference in the free-choice novelty test. The divergent correlations between DAT and the two behavioral phenotypes suggest a differential contribution of DAT in these phenotypes, which are known not to be isomorphic. Furthermore, rats categorized as high responders to inescapable novelty had lower Vmax values, which were accompanied by less DAT expression at the cell surface in PFC compared with low responders, suggesting that inherent individual differences in DAT cellular localization may underlie the differential response to novelty. Compared with the saline control, nicotine increased Vmax and cell surface DAT immunoreactivity in PFC from high responders but not from low responders. Similarly, nicotine increased Vmax and cell surface DAT in PFC in rats classified as low in novelty place preference but not in rats classified as high in novelty place preference. Thus, despite the different behavioral phenotypes, the pharmacological effect of nicotine to increase DAT function and cell surface expression was apparent, such that rats with inherently lower DAT function show a greater sensitivity to the neurochemical effect of nicotine.

Inbred Lewis and Fischer 344 rat strains differ not only in novelty- and amphetamine-induced behaviors, but also in dopamine transporter activity in vivo

Inbred Lewis (LEW) and Fischer 344 (F344) rats are differentially sensitive to drugs of abuse, making them useful for studying addiction-related neural mechanisms. Here, we investigated whether strain differences in dopamine transporters (DATs) in dorsal striatum (dSTR) and/or nucleus accumbens (NAc) may help to explain their behavioral differences. The behavior of male LEW and F344 rats was assessed in an open-field arena during habituation to novelty and after an i.v. infusion of saline and/or 0.5 mg/kg d-amphetamine (AMPH). In vitro measures of DAT binding, protein and cell-surface expression, as well as in vitro and in vivo measures of function, were used to compare DATs in dSTR and NAc of these two strains. We found that LEW rats exhibited higher novelty- and AMPH-induced locomotion, but F344 rats exhibited greater AMPH-induced rearing and stereotypy. An initial habituation session with i.v. saline minimized the strain differences in AMPH-induced behaviors except that the more frequent AMPH-induced rearing in F344 rats persisted. Strain differences in DAT total protein and basal activity were also observed, with LEW rats having less protein and slower in vivo clearance of locally applied DA in dSTR and NAc. AMPH inhibited in vivo DA clearance in dSTR and NAc of both strains, but to a greater extent in F344 rats. Taken together, the lower basal DAT function in LEW rats is consistent with their greater novelty-induced locomotor activation, whereas the greater inhibition of DA clearance by AMPH in F344 rats is consistent with their marked AMPH-induced rearing behavior.

GPR37 associates with the dopamine transporter to modulate dopamine uptake and behavioral responses to dopaminergic drugs

The orphan G protein-coupled receptor 37 (GPR37) is a substrate of parkin; its insoluble aggregates accumulate in brain samples of Parkinson's disease patients. We report here that GPR37 interacts with the dopamine transporter (DAT) and modulates DAT activity. GPR37 and DAT were found colocalized in mouse striatal presynaptic membranes and in transfected cells and their interaction was confirmed by coimmunoprecipitation assays. Gpr37-null mutant mice showed enhanced DAT-mediated dopamine uptake in striatal membrane samples, with a significant increase in the number of plasma membrane DAT molecules. The null mutant mice also exhibited a decrease in cocaine-induced locomotor activity and in catalepsy induced by dopamine receptor antagonists. These results reveal the specific role of GPR37, a putative peptidergic G protein-coupled receptor, in modulating the functional expression of DAT and the behavioral responses to dopaminergic drugs.

C57BL/6J mice show greater amphetamine-induced locomotor activation and dopamine efflux in the striatum than 129S2/SvHsd mice

Inbred strains of mice have served as valuable models for studying genetic susceptibility to drug addiction, an alternative to genetically modified mouse models. This is the first study comparing amphetamine (AMPH) effects on locomotor stimulation and dopamine efflux between two inbred strains of mice C57BL/6J and 129S2/SvHsd, frequently used as background strains for production of genetically engineered mice. There were no significant differences in basal locomotor activity and basal dopamine levels between the two strains. However, C57BL/6J mice showed greater AMPH-stimulated locomotor activity and AMPH-induced striatal dopamine efflux than 129S2/SvHsd mice. The differential AMPH effects could not be explained by differences in presynaptic dopamine components such as surface and total dopamine transporter (DAT) expression levels, striatal dopamine contents, and DAT activity. C57BL/6J and 129S2/SvHsd mice are excellent models for future identification of genetic, molecular, and behavioral components related to individual vulnerability to AMPH addiction. This study emphasizes the importance of mouse strain selections in the production of genetically modified mice for investigating phenotypes and mechanisms of psychostimulants.

Individual differences in novelty- and cocaine-induced locomotor activity as predictors of food-reinforced operant behavior in two outbred rat strains

A goal of the current study was to determine if individual differences in cocaine-induced locomotion, which has been shown in outbred Sprague-Dawley (SD) rats to be correlated with differential function of dopamine transporters, were also evident in Long-Evans (LE) rats. Another objective was to determine if differences in locomotion following exposure to novelty or cocaine predicted food-reinforced behavior. Between-strain comparisons of open-field activity revealed similar effects of 10 mg/kg cocaine, although increases in rearing were prominent in LE rats. Both strains exhibited robust individual differences in cocaine-induced locomotion, with nearly identical ambulatory behavior observed in low and high cocaine responders (LCRs and HCRs, respectively) from the two strains. In a cued-discrimination operant task, LE rats learned the contingency in fewer sessions, whereas SD rats obtained more food pellets at fixed ratio 10 and maintained higher progressive ratio (PR) breakpoints. HCRs from both strains also tended to maintain higher PR breakpoints; low and high responders to novelty (LR and HR, respectively) had no consistent differences in food-reinforced behavior. Overall, these studies suggest that wide individual differences in cocaine-induced behavior are common to SD and LE strains and certain differences in food-reinforced behavior are associated with HCRs compared to LCRs.

Low and high locomotor responsiveness to cocaine predicts intravenous cocaine conditioned place preference in male Sprague-Dawley rats

Outbred, male Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on cocaine-induced locomotor activity in an open-field arena. This difference reflects cocaine's ability to inhibit the striatal dopamine transporter and predicts development of sensitization. To investigate the relationship between initial cocaine locomotor responsiveness and cocaine reward, here we first classified rats as either LCRs or HCRs in a conditioned place preference (CPP) apparatus. Subsequently, we conducted cocaine conditioning trials, twice-daily over 4 days with vehicle and cocaine (10 mg/kg, i.p. or 1 mg/kg, i.v.). When cocaine was administered by the i.p. route, similar to previous findings in the open-field, LCRs and HCRs were readily classified and locomotor sensitization developed in LCRs, but not HCRs. However, cocaine CPP was not observed. In contrast, when cocaine was administered by the i.v. route, the LCR/HCR classification not only predicted sensitization, but also CPP, with only LCR rats exhibiting sensitization and cocaine conditioning. Our findings show that the initial locomotor response to cocaine can predict CPP in male Sprague-Dawley rats under conditions when place conditioning develops, and that LCRs may be more prone to develop conditioning in the context of cocaine reward.

Regulation of genes involved in dopamine transporter modulation by acute cocaine in rat striatum

It is well established that acute administration of psychostimulants alters dopamine transport. However, the exact mechanism of this modulation is still unknown. In this study we examined the mRNA levels of several proteins involved in the various proposed processes following cocaine administration. The expression levels of several immediate early genes were also studied. This was performed in rat striatum using real-time quantitative PCR. As expected, a marked increase of the immediate early genes Fos, Egr1 and Egr3 was observed. Egr2 was also found up-regulated. Among the different genes studied only Synaptotagmin4 in the SNARE family and Synphilin1 in the synaptic vesicles binding family were modulated by acute cocaine treatment. Interestingly, acute amphetamine treatment did not increase either Synaptotagmin4 and Synphilin1 mRNA levels, although increases in early genes expression were noted.

Environmental enrichment decreases cell surface expression of the dopamine transporter in rat medial prefrontal cortex

Rats raised in an enriched environmental condition (EC) exhibit a decreased (35%) maximal velocity (V(max)) of [3H]dopamine (DA) uptake in medial prefrontal cortex (mPFC) compared with rats raised in an impoverished condition (IC); however, no differences between EC and IC groups in V(max) for [3H]DA uptake were found in nucleus accumbens and striatum. Using biotinylation and immunoblotting techniques, the present study examined whether the brain region-specific decrease in DA transporter (DAT) function is the result of a reduction in DAT cell surface expression. In mPFC, nucleus accumbens and striatum, total DAT immunoreactivity was not different between EC and IC groups. Whereas no differences in cell surface expression of DAT were found in nucleus accumbens and striatum, DAT immunoreactivity in the biotinylated cell surface fraction of mPFC was decreased (39%) in EC compared with IC rats, consistent with the magnitude of the previously observed decrease in V(max) for [3H]DA uptake in mPFC in EC rats. These results suggest that the decrease in DAT cell surface expression in the mPFC may be responsible for decreased DAT function in the mPFC of EC compared with IC rats, and that there is plasticity in the regulatory mechanisms mediating DAT trafficking and function.

Rapid regulation of the dopamine transporter: role in stimulant addiction?

Dopamine (DA) and the DA transporter (DAT) play important roles in psychomotor stimulant behavioral activation and reward. By understanding how DAT activity is regulated, we will better appreciate its contribution to normal neurotransmission and to brain diseases like drug addiction. DAT is regulated long-term by chronic drug administration. It is also regulated in a rapid, dynamic fashion by many factors--including brief exposure to DAT substrates, e.g. DA and amphetamine, and inhibitors, e.g. cocaine. We found that individual differences in the initial and sensitized locomotor responsiveness of rats to cocaine reflect differences in in vivo DAT function. Our ex vivo studies have further suggested that differences in basal and/or rapid cocaine-induced expression of functional DATs in striatum contribute to the differences in initial responsiveness. Studies in model systems have demonstrated that short-term DAT regulation occurs by altered transporter trafficking, and thereby cell surface expression. For example, a rapid, complex regulation of DAT by DA is suggested. Amphetamine causes DAT internalization into early endosomal compartments whereas cocaine appears to up-regulate surface expression of DAT. Future studies are needed to confirm these observations in neurons, as well as to elucidate the mechanisms of rapid DAT endocytic trafficking at neuronal synapses.