One hour, but not six hours, of daily access to self-administered cocaine results in elevated levels of the dopamine transporter

Profiling prefrontal cortex protein expression in rats exhibiting an incubation of cocaine craving following short-access self-administration procedures

INTRODUCTION

Incubation of drug-craving refers to a time-dependent increase in drug cue-elicited craving that occurs during protracted withdrawal. Historically, rat models of incubated cocaine craving employed extended-access (typically 6 h/day) intravenous drug self-administration (IV-SA) procedures, although incubated cocaine craving is reported to occur following shorter-access IV-SA paradigms. The notoriously low-throughput of extended-access IV-SA prompted us to determine whether two different short-access IV-SA procedures akin to those in the literature result in qualitatively similar changes in glutamate receptor expression and the activation of downstream signaling molecules within prefrontal cortex (PFC) subregions as those reported previously by our group under 6h-access conditions.

METHODS

For this, adult, male Sprague-Dawley rats were trained to intravenously self-administer cocaine for 2 h/day for 10 consecutive days (2-h model) or for 6 h on day 1 and 2 h/day for the remaining 9 days of training (Mixed model). A sham control group was also included that did not self-administer cocaine.

RESULTS

On withdrawal day 3 or 30, rats were subjected to a 2-h test of cue-reinforced responding in the absence of cocaine and a time-dependent increase in drug-seeking was observed under both IV-SA procedures. Immunoblotting of brain tissue collected immediately following the cue test session indicated elevated phospho-Akt1, phospho-CaMKII and Homer2a/b expression within the prelimbic subregion of the PFC of cocaine-incubated rats. However, we failed to detect incubation-related changes in Group 1 metabotropic glutamate receptor or ionotropic glutamate receptor subunit expression in either subregion.

DISCUSSION

These results highlight further a role for Akt1-related signaling within the prelimbic cortex in driving incubated cocaine craving, and provide novel evidence supporting a potential role also for CaMKII-dependent signaling through glutamate receptors in this behavioral phenomenon.

The Cocaine and Oxycodone Biobanks, Two Repositories from Genetically Diverse and Behaviorally Characterized Rats for the Study of Addiction

The rat oxycodone and cocaine biobanks contain samples that vary by genotypes (by using genetically diverse genotyped HS rats), phenotypes (by measuring addiction-like behaviors in an advanced SA model), timepoints (samples are collected longitudinally before, during, and after SA, and terminally at three different timepoints in the addiction cycle: intoxication, withdrawal, and abstinence or without exposure to drugs through age-matched naive rats), samples collected (organs, cells, biofluids, feces), preservation (paraformaldehyde-fixed, snap-frozen, or cryopreserved) and application (proteomics, transcriptomics, microbiomics, metabolomics, epigenetics, anatomy, circuitry analysis, biomarker discovery, etc.Substance use disorders (SUDs) are pervasive in our society and have substantial personal and socioeconomical costs. A critical hurdle in identifying biomarkers and novel targets for medication development is the lack of resources for obtaining biological samples with a detailed behavioral characterization of SUD. Moreover, it is nearly impossible to find longitudinal samples. As part of two ongoing large-scale behavioral genetic studies in heterogeneous stock (HS) rats, we have created two preclinical biobanks using well-validated long access (LgA) models of intravenous cocaine and oxycodone self-administration (SA) and comprehensive characterization of addiction-related behaviors. The genetic diversity in HS rats mimics diversity in the human population and includes individuals that are vulnerable or resilient to compulsive-like responding for cocaine or oxycodone. Longitudinal samples are collected throughout the experiment, before exposure to the drug, during intoxication, acute withdrawal, and protracted abstinence, and include naive, age-matched controls. Samples include, but are not limited to, blood plasma, feces and urine, whole brains, brain slices and punches, kidney, liver, spleen, ovary, testis, and adrenal glands. Three preservation methods (fixed in formaldehyde, snap-frozen, or cryopreserved) are used to facilitate diverse downstream applications such as proteomics, metabolomics, transcriptomics, epigenomics, microbiomics, neuroanatomy, biomarker discovery, and other cellular and molecular approaches. To date, >20,000 samples have been collected from over 1000 unique animals and made available free of charge to non-profit institutions through https://www.cocainebiobank.org/ and https://www.oxycodonebiobank.org/.

Dopamine D3 Receptor Antagonism Reverses the Escalation of Oxycodone Self-administration and Decreases Withdrawal-Induced Hyperalgesia and Irritability-Like Behavior in Oxycodone-Dependent Heterogeneous Stock Rats

Prescription opioids, such as oxycodone, are highly effective analgesics for clinical pain management, but approximately 25% of patients who are prescribed opioids misuse them, and 5%–10% develop an opioid use disorder (OUD). Effective therapies for the prevention and treatment of opioid abuse and addiction need to be developed. The present study evaluated the effects of the highly selective dopamine D₃ receptor antagonist VK4-116 ([R]-N-[4-(4-[3-chloro-5-ethyl-2-methoxyphenyl]piperazin-1-yl)-3-hydroxybutyl]-1H-indole-2-carboxamide) on oxycodone addictive-like behaviors. We used a model of extended access to oxycodone self-administration and tested the effects of VK4-116 on the escalation of oxycodone self-administration and withdrawal-induced hyperalgesia and irritability-like behavior in male and female rats. Pretreatment with VK4-116 (5–25 mg/kg, i.p.) dose-dependently decreased the escalation of oxycodone self-administration and reduced withdrawal-induced hyperalgesia and irritability-like behavior in opioid-dependent rats. These findings demonstrate a key role for D₃ receptors in both the motivation to take opioids and negative emotional states that are associated with opioid withdrawal and suggest that D₃ receptor antagonism may be a viable therapeutic approach for the treatment of OUD.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

HIV-1 proteins dysregulate motivational processes and dopamine circuitry

Motivational alterations, such as apathy, in HIV-1+ individuals are associated with decreased performance on tasks involving frontal-subcortical circuitry. We used the HIV-1 transgenic (Tg) rat to assess effect of long-term HIV-1 protein exposure on motivated behavior using sucrose (1–30%, w/v) and cocaine (0.01–1.0 mg/kg/infusion) maintained responding with fixed-ratio (FR) and progressive-ratio (PR) schedules of reinforcement. For sucrose-reinforced responding, HIV-1 Tg rats displayed no change in EC₅₀ relative to controls, suggesting no change in sucrose reinforcement but had a downward shifted concentration-response curves, suggesting a decrease in response vigor. Cocaine-maintained responding was attenuated in HIV-1 Tg rats (FR1 0.33 mg/kg/infusion and PR 1.0 mg/kg/infusion). Dose-response tests (PR) revealed that HIV-1 Tg animals responded significantly less than F344 control rats and failed to earn significantly more infusions of cocaine as the unit dose increased. When choosing between cocaine and sucrose, control rats initially chose sucrose but with time shifted to a cocaine preference. In contrast, HIV-1 disrupted choice behaviors. DAT function was altered in the striatum of HIV-1 Tg rats; however, prior cocaine self-administration produced a unique effect on dopamine homeostasis in the HIV-1 Tg striatum. These findings of altered goal directed behaviors may determine neurobiological mechanisms of apathy in HIV-1+ patients.

Anhedonia, Reduced Cocaine Reward, and Dopamine Dysfunction in a Rat Model of Posttraumatic Stress Disorder

BACKGROUND

Posttraumatic stress disorder (PTSD) co-occurs with substance use disorders at high rates, but the neurobiological basis of this relationship is largely unknown. PTSD and drug addiction each involve dysregulation of brain reward circuitry; therefore, the identification of pathology of the mesolimbic dopamine system may aid in understanding their functional relationship. Dopamine reward dysfunction also may be relevant to the mechanisms underlying the PTSD symptoms of anhedonia and emotional numbing.

METHODS

Single-prolonged stress (SPS) was used as a rat model of PTSD, and a series of behavioral and neuropharmacologic assays were applied to assess the impact of SPS on reward, cocaine intake, and components of the striatal dopamine system.

RESULTS

Exposure to SPS increased anhedonia-like behaviors and decreased the rewarding properties of cocaine compared with control handling. Altered cocaine intake during extended access self-administration sessions was observed in rats exposed to SPS, further suggesting a difference in the reinforcing properties of cocaine following severe stress. SPS reduced tissue content of dopamine and its metabolites in the striatum, as well as altered striatal dopamine transporter and D2, but not D1, receptor densities.

CONCLUSIONS

These results support a role for altered dopaminergic transmission in reduced reward function in PTSD. Pathology of the dopamine system and the degradation of reward processes may contribute to PTSD symptomology and have implications for co-occurring psychiatric disorders such as substance abuse or depression.

Addicted to palatable foods: comparing the neurobiology of Bulimia Nervosa to that of drug addiction

RATIONALE

Bulimia nervosa (BN) is highly comorbid with substance abuse and shares common phenotypic and genetic predispositions with drug addiction. Although treatments for the two disorders are similar, controversy remains about whether BN should be classified as addiction.

OBJECTIVES

Here, we review the animal and human literature with the goal of assessing whether BN and drug addiction share a common neurobiology.

RESULTS

Similar neurobiological features are present following administration of drugs and bingeing on palatable food, especially sugar. Specifically, both disorders involve increases in extracellular dopamine (DA), D1 binding, D3 messenger RNA (mRNA), and ΔFosB in the nucleus accumbens (NAc). Animal models of BN reveal increases in ventral tegmental area (VTA) DA and enzymes involved in DA synthesis that resemble changes observed after exposure to addictive drugs. Additionally, alterations in the expression of glutamate receptors and prefrontal cortex activity present in human BN or following sugar bingeing in animals are comparable to the effects of addictive drugs. The two disorders differ in regards to alterations in NAc D2 binding, VTA DAT mRNA expression, and the efficacy of drugs targeting glutamate to treat these disorders.

CONCLUSIONS

Although additional empirical studies are necessary, the synthesis of the two bodies of research presented here suggests that BN shares many neurobiological features with drug addiction. While few Food and Drug Administration-approved options currently exist for the treatment of drug addiction, pharmacotherapies developed in the future, which target the glutamate, DA, and opioid systems, may be beneficial for the treatment of both BN and drug addiction.

Continuous exposure to dizocilpine facilitates escalation of cocaine consumption in male Sprague-Dawley rats

BACKGROUND

Although the escalation of cocaine consumption is a hallmark of cocaine dependence, the neurobiological mechanisms that underlie this change in behavior are not well understood.

METHODS

This study used an extended access version of the drug self-administration procedure to explore how N-methyl-d-aspartate (NMDA) receptors are involved in escalation of cocaine consumption. Male Sprague-Dawley rats (n=59) were first trained to self-administer cocaine (0.33 mg/infusion, i.v.) under a fixed-ratio 1 (FR1) schedule of reinforcement. After training, rats were implanted with subcutaneous osmotic minipumps filled with vehicle or the non-competitive NMDAR antagonist, dizocilpine (0.2 or 0.4 mg/kg/d), and subsequently allowed to self-administer cocaine in 2h or 6h self-administration sessions.

RESULTS

In the 6h groups, vehicle-treated rats escalated cocaine self-administration across 15 self-administration sessions; rats treated with dizocilpine escalated cocaine self-administration at a greater rate and to a greater degree. Rats that self-administered cocaine during 2h sessions did not escalate consumption of cocaine under any treatment condition. Discontinuation of dizocilpine treatment in the 6h access condition led to a substantial decrease in cocaine consumption, down to pre-escalation levels, and then control rates of escalation thereafter. Despite large differences in intake under the FR1 schedule, post-escalation break point under a progressive ratio schedule of reinforcement did not differ between groups.

CONCLUSION

These data suggest that glutamate tone through NMDA receptors can play a dynamic role in regulating cocaine intake and escalation of consumption.

Extended access of cocaine self-administration results in tolerance to the dopamine-elevating and locomotor-stimulating effects of cocaine

Tolerance to the neurochemical and psychoactive effects of cocaine after repeated use is a hallmark of cocaine addiction in humans. However, comprehensive studies on tolerance to the behavioral, psychoactive, and neurochemical effects of cocaine following contingent administration in rodents are lacking. We outlined the consequences of extended access cocaine self-administration as it related to tolerance to the psychomotor activating, dopamine (DA) elevating, and DA transporter (DAT) inhibiting effects of cocaine. Cocaine self-administration (1.5 mg/kg/inj; 40 inj; 5 days), which resulted in escalation of first hour intake, caused reductions in evoked DA release and reduced maximal rates of uptake through the DAT as measured by slice voltammetry in the nucleus accumbens core. Furthermore, we report reductions in cocaine-induced uptake inhibition and a corresponding increase in the dose of cocaine required for 50% inhibition of DA uptake (Ki ) at the DAT. Cocaine tolerance at the DAT translated to reductions in cocaine-induced DA overflow as measured by microdialysis. In addition, cocaine-induced elevations in locomotor activity and stereotypy were reduced, while rearing behavior was enhanced in animals with a history of cocaine self-administration. Here, we demonstrate both neurochemical and behavioral cocaine tolerance in an extended-access rodent model of cocaine abuse, which allows for a better understanding of the neurochemical and psychomotor tolerance that develops to cocaine in human addicts. We demonstrate tolerance to the neurochemical and behavioral effects of cocaine following extended-access cocaine self-administration. With respect to neurochemistry, we show reduced cocaine-induced dopamine uptake inhibition, an increased dose of cocaine required for 50% inhibition of the dopamine transporter, and reduced cocaine-induced dopamine overflow. In addition, we show escalation of cocaine intake and reduced cocaine-induced locomotor activity following cocaine self-administration.

Short and long access to cocaine self-administration activates tyrosine phosphatase STEP and attenuates GluN expression but differentially regulates GluA expression in the prefrontal cortex

RATIONALE

Dephosphorylation of extracellular signal-regulated kinase (ERK) and cyclic AMP response element binding protein (CREB) in the dorsomedial prefrontal cortex (dmPFC) at the end of short access (ShA) cocaine self-administration is implicated in cocaine seeking. However, what receptors and phosphatases mediate this effect and whether ERK/CREB and related phospho-proteins in the dmPFC react similarly during early withdrawal from long access (LgA) cocaine self-administration are unknown.

OBJECTIVES

The effects of ShA vs. LgA cocaine self-administration on the phosphorylation of protein phosphatase 2A (PP2A) and striatal-enriched protein tyrosine phosphatase (STEP), as well as GluN and GluA receptor subtype expression in the dmPFC during early withdrawal, were compared.

METHODS

Rats self-administered cocaine or received saline during 2- or 6-h daily sessions for 10-11 days. Two hours after the final session, the dmPFC was dissected out and processed for immunoblotting.

RESULTS

Similar to previous findings after ShA cocaine, phospho-ERK and phospho-CREB in the dmPFC were decreased after LgA cocaine. Cocaine elevated phospho-PP2A (deactivation) and decreased phospho-STEP (activation) in both ShA and LgA cocaine rats. GluN1, GluN2B, and phospho-GluN2B Tyr1472 in the dmPFC were decreased after ShA and LgA cocaine. Further, a significant reduction of GluA2, GluA1, and phospho-GluA1 Ser845 was found only in LgA rats.

CONCLUSIONS

Activation of phospho-STEP may underlie ERK and CREB dephosphorylation in the dmPFC as well as internalization and degradation of GluN complexes during early withdrawal from both ShA and LgA cocaine self-administration, whereas differential alteration of AMPA receptor subunits after ShA and LgA cocaine self-administration depends on cocaine intake.

Dopamine and glutamate release in the dorsolateral caudate putamen following withdrawal from cocaine self-administration in rats

Evidence suggests that cocaine addiction may involve progressive neuroadaptive changes in the dorsolateral caudate putamen (dlCPu). While cocaine seeking following abstinence from chronic self-administration requires intact dlCPu function, in vivo neurotransmitter release in the dlCPu has not been investigated. The current study measured dlCPu dopamine (DA) and glutamate (GLU) release during drug seeking following limited or extended abstinence, as well as in response to a cocaine priming injection alone. Male, Sprague-Dawley rats self-administered cocaine (0.2mg/50μl infusion, i.v.) for 10days (2h/day). In vivo microdialysis occurred in the self-administration chamber after 1 and 14days of abstinence (Experiment 1). A separate set of animals that completed self-administration as well as drug naïve controls received a cocaine priming injection (20mg/kg) during concurrent microdialysis (Experiment 2). DA release increased during drug seeking in the self-administration context at both 1 and 14days post abstinence. In contrast, GLU release only increased after 1day of abstinence. Furthermore, animals with a cocaine self-administration history showed enhanced DA and GLU release following cocaine challenge as compared to drug naïve controls. These results indicate that chronic cocaine self-administration enhances dlCPu DA and GLU under both drug-paired context and drug-primed conditions.

A review of preclinical research demonstrating that drug and non-drug reinforcers differentially affect behavior

This review describes and summarizes current preclinical research revealing important differences between drug and non-drug reinforcers in terms of their effects on behavior. Despite research showing that drugs are not especially strong reinforcers in animals, a number of other behavioral differences potentially relevant to addiction have been reported in studies that have compared drug and non-drug reinforcers. Several of these effects appear only after long-term access to drugs. These include an escalation of drug intake, an increased persistence in responding for the drug, and a decreased sensitivity to the effects of punishers or other suppressors of drug seeking. Further differences between drug and non-drug reinforcers include the effects that reinforcer-paired stimuli have on behavior. Drug cues, as compared to food cues, have been shown to exert greater control over reinforcer-seeking behavior after periods of abstinence. Similarly, behavior previously reinforced by drugs, but not food, has been shown to be susceptible to stress-induced reinstatement after extinction. The behavioral differences between drug and non-drug reinforcers reviewed here may identify special features of drugs that lead to addiction.

Neural Changes Developed during the Extinction of Cocaine Self-Administration Behavior

The high rate of recidivism in cocaine addiction after prolonged periods of abstinence poses a significant problem for the effective treatment of this condition. Moreover, the neurobiological basis of this relapse phenomenon remains poorly understood. In this review, we will discuss the evidence currently available regarding the neurobiological changes during the extinction of cocaine self-administration. Specifically, we will focus on alterations in the dopaminergic, opioidergic, glutamatergic, cholinergic, serotoninergic and CRF systems described in self-administration experiments and extinction studies after chronic cocaine administration. We will also discuss the differences related to contingent versus non-contingent cocaine administration, which highlights the importance of environmental cues on drug effects and extinction. The findings discussed in this review may aid the development of more effective therapeutic approaches to treat cocaine relapse.

The science of making drug-addicted animals

Research involving animal models of drug addiction can be viewed as a sort of reverse psychiatry. Contrary to clinicians who seek to treat addicted people to become and remain abstinent, researchers seek to make drug-naïve animals addicted to a drug with known addictive properties in humans. The goals of this research are to better understand the neuroscience of drug addiction and, ultimately, to translate this knowledge into effective treatments for people with addiction. The present review will not cover the vast literature that has accumulated over the past 50 years on animal models of drug addiction. It is instead more modestly devoted to recent research spanning the past decade on drug self-administration-based models of addiction in the rat (the animal species most frequently used in the field), with a special focus on current efforts to model compulsive cocaine use as opposed to nonaddictive use. Surprisingly, it turns out that modeling compulsive cocaine use in rats is possible but more difficult than previously thought. In fact, it appears that resilience to cocaine addiction is the norm in rats. As in human cocaine users, only few individual rats would be vulnerable. This conclusion has several important implications for future research on the neuroscience of cocaine addiction and on preclinical medication development.

The relationship between duration of initial alcohol exposure and persistence of molecular tolerance is markedly nonlinear

The neuronal calcium- and voltage-activated BK potassium channel is modulated by ethanol, and plays a role in behavioral tolerance in vertebrates and invertebrates. We examine the influence of temporal parameters of alcohol exposure on the characteristics of BK molecular tolerance in the ventral striatum, an important component of brain reward circuitry. BK channels in striatal neurons of C57BL/6J mice exhibited molecular tolerance whose duration was a function of exposure time. After 6 h exposure to 20 mm (0.09 mg%) ethanol, alcohol sensitivity was suppressed beyond 24 h after withdrawal, while after a 1 or 3 h exposure, sensitivity had significantly recovered after 4 h. This temporally controlled transition to persistent molecular tolerance parallels changes in BK channel isoform profile. After withdrawal from 6 h, but not 3 h alcohol exposure, mRNA levels of the alcohol-insensitive STREX (stress axis-regulated exon) splice variant were increased. Moreover, the biophysical properties of BK channels during withdrawal from 6 h exposure were altered, and match the properties of STREX channels exogenously expressed in HEK 293 cells. Our results suggest a temporally triggered shift in BK isoform identity. Once activated, the transition does not require the continued presence of alcohol. We next determined whether the results obtained using cultured striatal neurons could be observed in acutely dissociated striatal neurons, after alcohol administration in the living mouse. The results were in remarkable agreement with the striatal culture data, showing persistent molecular tolerance after injections producing 6 h of intoxication, but not after injections producing only 3 h of intoxication.

Cocaine-insensitive dopamine transporters with intact substrate transport produced by self-administration

BACKGROUND

Psychomotor stimulant drugs such as cocaine and amphetamine activate brain dopamine (DA) neurotransmission and support self-administration in humans and laboratory animals. Cocaine amplifies DA signaling by blocking the DA transporter (DAT), and this has been described as the most important mechanism underlying cocaine's reinforcing effects. Amphetamine has the added mechanism of reverse transport of intracellular DA through the DAT.

METHODS

We used cocaine and amphetamine self-administration under a fixed-ratio 1 schedule followed by microdialysis in freely moving rats to measure extracellular DA levels and fast scan cyclic voltammetry in brain slices to measure subsecond DA release and uptake parameters.

RESULTS

Following a high dose (1.5 mg/kg intravenous) cocaine self-administration paradigm (40 injections/day × 5 days), the DAT was markedly less sensitive to cocaine, as measured by microdialysis and voltammetry in the nucleus accumbens core. In contrast, the DAT substrate amphetamine retained the same efficacy at the DAT in cocaine self-administering animals, and amphetamine did not mimic cocaine's effect on the DAT when self-administered. A single session of cocaine self-administration caused a significant decrease in the ability of cocaine to inhibit the DAT, a finding that may provide a neurochemical basis for rapid tolerance. The effects of cocaine returned to normal within a few weeks following cessation of self-administration.

CONCLUSIONS

Here, we, for the first time, demonstrate an in vivo, pharmacologically induced alteration in the sensitivity of the DAT to cocaine that is specific to cocaine, spares DAT and substrate/releaser interactions, and is independent of maximal rate of DA uptake (V(max)).

Periadolescent exposure to cannabinoids alters the striatal and hippocampal dopaminergic system in the adult rat brain

In a previous work, we have shown that chronic administration of the cannabinoid agonist CP 55,940 (CP) during periadolescence increases cocaine self-administration in adult female rats, while it produces no such effect in males (Higuera-Matas et al., 2008). To extend these findings, we have analysed here the brains of the rats used as subjects in this previous work to evaluate the impact of the interaction between CP exposure and cocaine self-administration on dopaminergic parameters. We evaluated the levels of the dopamine transporter (DAT), and the D1- (D1R) and D2-type (D2R) dopaminergic receptors, as well as tyrosine hydroxylase (TH) mRNA in dopaminergic areas of the adult, cocaine self-administered, rat brain that had been chronically exposed to CP or vehicle (VH) during periadolescence. Control groups with CP/VH exposure and no self-administration experience were also included. In adult females, CP administration induced an up-regulation of DAT in the caudate-putamen that was maintained after cocaine self-administration. In males, CP induced an increase in the D1Rs content in the nucleus accumbens shell, which was not evident after cocaine self-administration. CP also reduced the expression of D2Rs in CA1 irrespective of sex. Finally, an increase in D1Rs was observed in the substantia nigra following cocaine self-administration. These findings suggest that a dopaminergic component modulated by cannabinoids may underlie the enhanced cocaine self-administration previously observed in adult female rats.

Animal models of substance abuse and addiction: implications for science, animal welfare, and society

Substance abuse and addiction are well recognized public health concerns, with 2 NIH institutes (the National Institute on Drug Abuse and the National Institute on Alcohol Abuse and Alcoholism) specifically targeting this societal problem. As such, this is an important area of research for which animal experiments play a critical role. This overview presents the importance of substance abuse and addiction in society; reviews the development and refinement of animal models that address crucial areas of biology, pathophysiology, clinical treatments, and drug screening for abuse liability; and discusses some of the unique veterinary, husbandry, and IACUC challenges associated with these models.

Extended daily access to cocaine results in distinct alterations in Homer 1b/c and NMDA receptor subunit expression within the medial prefrontal cortex

Human cocaine addicts show altered function within the basal ganglia and the medial prefrontal cortex (mPFC) and altered glutamate function within these areas is postulated to be critical for cocaine addiction. The present project utilized a highly valid animal model of cocaine addiction, to test whether excessive use of cocaine alters glutamate function within these brain areas. Rats were trained to lever-press for i.v. saline vehicle or cocaine (0.25 mg/infusion) over seven 1-h daily sessions, after which, saline controls and half of cocaine self-administering animals (brief access condition) received 10 more 1-h daily sessions, whereas the remaining cocaine animals received 10 additional 6-h daily sessions (extended access condition). One, 14, or 60 days after the last self-administration session, animals were sacrificed. Tissue samples from the ventral tegmental area (VTA), nucleus accumbens (N.Acc) core and shell, and mPFC were analyzed by immunoblotting for expression of Homer1b/c, Homer2a/b, mGluR1, mGluR5, NR2a, and NR2b subunits of the NMDA receptor. Brief and extended access to cocaine failed to alter protein levels within the VTA, and produced transient and similar changes in N.Acc protein expression, which were more pronounced in the core subregion. In contrast, extended access to cocaine resulted in distinct and long lasting alterations of protein expression within the mPFC that included: increased levels of Homer1b/c at 1 day, NR2b at 14 days, and NR2a at 60 days, of withdrawal. These data support the notion that altered NMDA function within the mPFC may contribute, in part, to the transition to excessive uncontrolled consumption of cocaine.

Unrestricted access to methamphetamine or cocaine in the past is associated with increased current use

Laboratory animals allowed to self-administer stimulants for extended periods of time escalate drug intake compared to animals that self-administer under temporally limited conditions. To our knowledge, this phenomenon has not been systematically investigated in humans. We interviewed 106 (77 male, 29 female) methamphetamine (Meth) and 96 (81 male, 15 female) cocaine (Coc) users to determine if they had experienced discrete period(s) of unrestricted access to unlimited quantities of Meth or Coc in the past. Fifty-eight Meth users and 53 Coc users reported having a discrete period of unrestricted access in the past, but not in the present. Meth-using participants with a prior history of unrestricted access reported significantly more current Meth use, compared to Meth users with no prior history of unrestricted access. Specifically, these participants reported more days used in the past 30 d, more days of use per week, greater use per day and greater total use per week (p<0.05 for each). Coc-using participants with a prior history of unrestricted access also reported significantly more current Coc use, compared to Coc users with no prior history of unrestricted access. This was true across all measures of current use for these participants, including more days used in the past 30 d, more days of use per week, greater use per day, and higher total use per week (p<0.02 for each). Taken together, these results suggest that a history of unrestricted access to stimulants is associated with long-lasting increases in stimulant use.

Dopamine uptake changes associated with cocaine self-administration

The present study was designed to reveal the relationship between cocaine-induced dopamine uptake changes and patterns of cocaine self-administration observed under a fixed-ratio schedule. Cocaine was intravenously infused into anesthetized rats, according to inter-infusion intervals obtained from self-administering animals, and dopamine uptake changes (apparent K(m)) were assessed in the nucleus accumbens using voltammetry. The data demonstrate that cocaine-induced dopamine transporter (DAT) inhibition accounts for the accumbal dopamine fluctuations, which are associated with the cyclic regularity of cocaine intake observed during self-administration. Specifically, the inter-infusion intervals that are maintained during cocaine self-administration correlate with the maintenance of a rapidly changing level of dopamine uptake inhibition, which appears to be tightly regulated. Furthermore, this maintained level of dopamine uptake inhibition was found to shift upward using intervals from animals that had shown an escalation in the rate of cocaine self-administration. Although no significant change in the apparent K(m) was revealed in animals that exhibited an escalation in the rate of cocaine intake, an increased dopamine uptake rate was found suggesting an upregulation of DAT number in response to a history of high cocaine intake. This is the first demonstration of the tight correlation that exists between the level of dopamine uptake inhibition and rates of cocaine self-administration. Moreover, a new mathematical model was created that quantitatively describes the changes in cocaine-induced dopamine uptake and correctly predicts the level of dopamine uptake inhibition. This model permits a computational interpretation of cocaine-induced dopamine uptake changes during cocaine self-administration.

Essential value of cocaine and food in rats: tests of the exponential model of demand

RATIONALE

To provide a prospective test of the predictive adequacy of the exponential model of demand (Hursh and Silberberg, Psych Rev 115(1):186-198, 2008).

OBJECTIVES

In Experiment 1, to measure the 'essential value' (the propensity to defend consumption with changes in price) of cocaine and food in a demand analysis (functional relation between price and consumption) by means of the exponential model; in Experiment 2, to test whether the model's systematic underestimation of cocaine consumption in Experiment 1 was due to weight loss; and in Experiment 3, to evaluate the effects of cocaine on the essential value of food.

MATERIALS AND METHODS

In Experiment 1, demand curves for food and cocaine were determined by measuring consumption of these goods in a multiple schedule over a range of fixed ratios; in Experiment 2, a demand curve for only cocaine was determined; and in Experiment 3, demand for food was determined in the absence of cocaine.

RESULTS

In Experiment 1, the exponential equation accommodated high portions of variance for both curves, but systematically underestimated cocaine demand; in Experiment 2, this predictive underestimation of the equation was eliminated; and in Experiment 3, the essential value of food was greater than in Experiment 1.

CONCLUSIONS

The exponential model of demand accommodated the data variance for all cocaine and food demand curves. Compared to food, cocaine is a good of lower essential value.

Heightened drug-seeking motivation following extended daily access to self-administered cocaine

Rats allowed extended daily access (6 h) to cocaine, consume high doses of the drug and escalate their cocaine intake over days, resembling the pattern of cocaine use seen in human addicts. The current study was designed to test whether such animals would also demonstrate the heightened motivation to seek cocaine seen in human addicts. Rats were trained to lever press for i.v. cocaine (0.25 mg/infusion) over a 5-day period of 1 h sessions. Subjects were then assigned to either a brief-access (1 h/day) or an extended-access condition for an additional 10 days. Control rats lever pressed for i.v. saline. Following the final self-administration session animals were tested for their motivation to receive cocaine in an operant runway apparatus. Extended-access animals exhibited significantly higher motivation for cocaine in the runway (where they received 1.0 mg/kg cocaine i.v. upon goal-box entry) as was evident by faster run times and less ambivalence about entering the goal box (i.e. retreat behavior) than either brief-access or control subjects. Brief and extended-access animals, tested in the Elevated Plus Maze, exhibited comparable and significant increases in anxiety following a single 1.0 mg/kg i.v. injection of cocaine, as compared to saline control animals that were challenged with i.v. saline infusion. Together, these data suggest that extended access to cocaine results in an especially high motivation for the drug that is not accounted for by reductions in the anxiogenic properties of cocaine.

Essential value of cocaine and food in rats: tests of the exponential model of demand

RATIONALE

To provide a prospective test of the predictive adequacy of the exponential model of demand (Hursh and Silberberg, Psych Rev 115(1):186-198, 2008).

OBJECTIVES

In Experiment 1, to measure the 'essential value' (the propensity to defend consumption with changes in price) of cocaine and food in a demand analysis (functional relation between price and consumption) by means of the exponential model; in Experiment 2, to test whether the model's systematic underestimation of cocaine consumption in Experiment 1 was due to weight loss; and in Experiment 3, to evaluate the effects of cocaine on the essential value of food.

MATERIALS AND METHODS

In Experiment 1, demand curves for food and cocaine were determined by measuring consumption of these goods in a multiple schedule over a range of fixed ratios; in Experiment 2, a demand curve for only cocaine was determined; and in Experiment 3, demand for food was determined in the absence of cocaine.

RESULTS

In Experiment 1, the exponential equation accommodated high portions of variance for both curves, but systematically underestimated cocaine demand; in Experiment 2, this predictive underestimation of the equation was eliminated; and in Experiment 3, the essential value of food was greater than in Experiment 1.

CONCLUSIONS

The exponential model of demand accommodated the data variance for all cocaine and food demand curves. Compared to food, cocaine is a good of lower essential value.

Surgical adrenalectomy with diurnal corticosterone replacement slows escalation and prevents the augmentation of cocaine-induced reinstatement in rats self-administering cocaine under long-access conditions

The loss of control over cocaine use and persistently heightened susceptibility to drug relapse that define human cocaine addiction are consequences of drug-induced neuroplasticity and can be studied in rats self-administering cocaine under conditions of daily long access (LgA) as escalating patterns of drug intake and heightened susceptibility to reinstatement. This study investigated the potential contribution of elevated glucocorticoids at the time of LgA cocaine self-administration (SA) to these behavioral indices of addiction-related neuroplasticity. Rats provided 14 days of 6-h access (LgA) to cocaine showed a progressive escalation of SA and were more susceptible to cocaine-induced reinstatement (10 mg/kg, i.p.) compared to rats self-administering under short-access (ShA; 2 h) conditions. A surgical adrenalectomy and corticosterone replacement (ADX/C) regimen that eliminated SA-induced increases in corticosterone (CORT) while maintaining the diurnal pattern of secretion failed to alter SA or reinstatement in ShA rats but slowed escalation and attenuated later reinstatement in LgA rats when applied before but not after chronic LgA SA testing. Although the contribution of other adrenal hormones cannot be ruled out, these data suggest that elevated glucocorticoids at the time of cocaine exposure may be required for the effects of LgA SA on cocaine intake and later reinstatement. The inability of daily CORT administration before daily ShA SA, at a dose that reproduced the response during LgA SA, to mimic the effects of LgA SA suggests that elevated glucocorticoids during SA may play a permissive role in cocaine-induced neuroplasticity that contributes to addiction.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.

Effect of aripiprazole, a partial dopamine D2 receptor agonist, on increased rate of methamphetamine self-administration in rats with prolonged session duration

Aripiprazole is a dopamine (DA) D(2) receptor partial agonist, approved by the Food and Drug Administration (FDA) for the treatment of schizophrenia. DA receptor partial agonists have been previously assessed as potential therapeutic agents for cocaine dependence. The present experiment examined the effect of aripiprazole on methamphetamine self-administration in a rodent model of an increasing drug self-administration with prolonged session duration. Wistar rats were allowed to self-administer methamphetamine (0.05 mg/kg/injection, intravenously) in either 1-h (short access: ShA rats) or 6-h sessions (long access: LgA rats). After 15 sessions, the dose-response function of methamphetamine was determined under either a progressive- or a fixed-ratio schedule. Next, the effect of aripiprazole (0.3-10 mg/kg, subcutaneuously (s.c.)) on the dose-response function was examined. LgA rats exhibited an increasing rate of methamphetamine self-administration. Responding for methamphetamine by LgA rats was higher than that of ShA rats under both schedules. Pretreatment with aripiprazole shifted the dose-response function of methamphetamine to the right in both LgA and ShA rats. However, the effect of aripiprazole was greater in LgA than ShA rats. In in vitro receptor binding assay, no change in the level of D(2) DA receptors in the nucleus accumbens and the striatum was found in any group. The present data suggest increased sensitivity of the dopaminergic system to aripiprazole in LgA rats compared with ShA rats. However, mechanisms other than downregulation of D(2) DA receptors in the nucleus accumbens and the striatum may be responsible for the increased sensitivity of the dopaminergic function in LgA rats.

Effects of dose and session duration on cocaine self-administration in rats

Previous studies showed that an extended 6-h session duration produced an increasing rate of cocaine self-administration in rats. The present study further investigated the effect of dose and session duration on cocaine self-administration. Eight groups of rats (4 doses x 2 session durations) self-administered one of four cocaine doses (0.25, 0.5, 1, and 2 mg/kg/injection) in either 1- or 6-h sessions under a fixed-ratio schedule. In another experiment, two other groups of rats self-administered 0.5 mg/kg/injection of cocaine in either 3- or 12-h sessions. Cocaine self-administration increased at all doses in 6-h sessions but not in 1-h sessions. Cocaine intake (milligram/kilogram) reached an asymptote earlier at a higher dose, but the rate of responding increased faster when the dose was lower. In ShA rats, the cocaine dose-response function was higher in rats at the two higher unit doses than at the lower doses. Cocaine self-administration increased in 6- and 12-h sessions, but not in 1- and 3-h sessions. The increase in self-administration was faster and greater in 12-h sessions than 6-h sessions. The data suggest that cocaine self-administration increases at various doses with prolonged access and that an increase in the rate of responding is positively and inversely associated with session duration and unit dose, respectively. Results also imply that cocaine intake reaches a ceiling faster at high doses even under short session duration. Therefore, high doses or prolonged access to cocaine are more likely to result in a pattern of cocaine intake that reflects compulsive use.

Changes in levels of D1, D2, or NMDA receptors during withdrawal from brief or extended daily access to IV cocaine

We previously reported that brief (1 h), but not extended (6 h), daily access to cocaine results in a sensitized locomotor response to cocaine and in elevated c-Fos immunoreactivity and DAT binding in the nucleus accumbens (N.Acc) core. In order to better our understanding of the neural adaptations mediating the transition from controlled drug use to addiction, the current experiments were set to further explore the neural adaptations resulting from these two access conditions. Rats received either brief daily access to saline or cocaine, or brief daily access followed by extended daily access to cocaine. Subjects were then sacrificed either 20 min, or 14 or 60 days, after the last self-administration session. Samples of the ventral tegmental area (VTA), N.Acc core and shell, dorsal striatum, and medial prefrontal cortex (mPFC) were taken for analysis of D1 ([3H]SCH-23390), D2 ([3H]Spiperone), and NMDA ([3H]MK-801) receptor binding (using the method of receptor autoradiography). At 20 min into withdrawal, D2 receptors were elevated and NMDA receptors were reduced in the mPFC of the brief access animals while D1 receptors were elevated in the N.Acc shell of the extended access animals, compared to saline controls. D2 receptors were reduced in the N.Acc shell of the brief access animals compared to saline controls after 14 days, and compared to extended access animals after 60 days of withdrawal. In summary, extended access to cocaine resulted in only transient changes in D1 receptors binding. These results suggest that the development of compulsive drug use is largely unrelated to changes in total binding of D2 or NMDA receptors.