Increased breakpoints on a progressive ratio schedule reinforced by IV cocaine are associated with reduced locomotor activation and reduced dopamine efflux in nucleus accumbens shell in rats

Repeated dosing with cocaine produces strain-dependent effects on responding for conditioned reinforcement in Collaborative Cross mice

RATIONALE

Cocaine use disorder (CUD) is a highly heritable form of substance use disorder, with genetic variation accounting for a substantial proportion of the risk for transitioning from recreational use to a clinically impairing addiction. With repeated exposures to cocaine, psychomotor and incentive sensitization are observed in rodents. These phenomena are thought to model behavioral changes elicited by the drug that contribute to the progression into addiction, but little is known about how genetic variation may moderate these consequences.

OBJECTIVES

Here, we describe the use of two Collaborative Cross (CC) recombinant inbred mouse strains that either exhibit high (CC018/UncJ) or no (CC027/GeniUncJ) psychomotor sensitization in response to cocaine to measure phenotypes related to incentive sensitization after repeated cocaine exposures; given the relationship of incentive motivation to nucleus accumbens core (NAc) dopamine release and reuptake, we also assessed these neurochemical mechanisms.

METHODS

Adult male and female CC018/UncJ and CC027/GeniUncJ mice underwent Pavlovian conditioning to associate a visual cue with presentation of a palatable food reward, then received five, every-other-day injections of cocaine or vehicle. Following Pavlovian re-training, they underwent testing acquisition of a new operant response for the visual cue, now serving as a conditioned reinforcer. Subsequently, electrically evoked dopamine release was assessed using fast-scan cyclic voltammetry from acute brain slices containing the NAc.

RESULTS

While both strains acquired the Pavlovian association, only CC018/UncJ mice showed conditioned reinforcement and incentive sensitization in response to cocaine, while CC027/GeniUncJ mice did not. Voltammetry data revealed that CC018/UncJ, compared to CC027/GeniUnc, mice exhibited higher baseline dopamine release and uptake. Moreover, chronic cocaine exposure blunted tonic and phasic dopamine release in CC018/UncJ, but not CC027/GeniUncJ, mice.

CONCLUSIONS

Genetic background is a moderator of cocaine-induced neuroadaptations in mesolimbic dopamine signaling, which may contribute to both psychomotor and incentive sensitization and indicate a shared biological mechanism of variation.

The potent psychomotor, rewarding and reinforcing properties of 3-fluoromethamphetamine in rodents

3-fluoromethamphetamine (3-FMA), a derivative of methamphetamine (METH), produces behavioral impairment and deficits in dopaminergic transmission in the striatum of mice. The abuse potential of 3-FMA has not been fully characterized. The aim of this study was to evaluate the effects of 3-FMA on locomotor activity as well as its rewarding and reinforcing properties in the conditioned place preference (CPP) and self-administration procedures. Intravenous (i.v.) administration of 3-FMA (0.5 and 1.0 mg/kg) significantly increased locomotor activity in a dose-dependent manner in rats. In the CPP procedure, intraperitoneal administration of 3-FMA (10 and 30 mg/kg) produced a significant alteration in place preference in mice. In the self-administration paradigms, 3-FMA showed drug-taking behavior at the dose of 0.1 mg/kg/infusion (i.v.) during 2 hr sessions under fixed ratio schedules and high breakpoints at the dose of 0.3 and 1.0 mg/kg/infusion (i.v.) during 6 hr sessions under progressive ratio schedule of reinforcement in rats. A priming injection of 3-FMA (0.4 mg/kg, i.v.), METH (0.2 mg/kg, i.v.), or cocaine (2.0 mg/kg, i.v.) reinstated 3-FMA-seeking behavior after an extinction period in 3-FMA-trained rats during 2 hr session. Taken together, these findings demonstrate robust psychomotor, rewarding and reinforcing properties of 3-FMA, which may underlie its potential for compulsive use in humans.

Organic cation transporter 3 and the dopamine transporter differentially regulate catecholamine uptake in the basolateral amygdala and nucleus accumbens

Regional alterations in kinetics of catecholamine uptake are due in part to variations in clearance mechanisms. The rate of clearance is a critical determinant of the strength of catecholamine signaling. Catecholamine transmission in the nucleus accumbens core (NAcc) and basolateral amygdala (BLA) is of particular interest due to involvement of these regions in cognition and motivation. Previous work has shown that catecholamine clearance in the NAcc is largely mediated by the dopamine transporter (DAT), but clearance in the BLA is less DAT-dependent. A growing body of literature suggests that organic cation transporter 3 (OCT3) also contributes to catecholamine clearance in both regions. Consistent with different clearance mechanisms between regions, catecholamine clearance is more rapid in the NAcc than in the BLA, though mechanisms underlying this have not been resolved. We compared the expression of DAT and OCT3 and their contributions to catecholamine clearance in the NAcc and BLA. We found DAT protein levels were ~ 4-fold higher in the NAcc than in the BLA, while OCT3 protein expression was similar between the two regions. Immunofluorescent labeling of the two transporters in brain sections confirmed these findings. Ex vivo voltammetry demonstrated that the magnitude of catecholamine release was greater, and the clearance rate was faster in the NAcc than in the BLA. Additionally, catecholamine clearance in the BLA was more sensitive to the OCT3 inhibitor corticosterone, while clearance in the NAcc was more cocaine sensitive. These distinctions in catecholamine clearance may underlie differential effects of catecholamines on behavioral outputs mediated by these regions.

Intermittent access cocaine self-administration produces psychomotor sensitization: effects of withdrawal, sex and cross-sensitization

RATIONALE

With repeated administration, the psychomotor activating effects of drugs such as cocaine or amphetamine can change in very different ways-showing sensitization or tolerance-depending on whether they are administered more or less intermittently. This behavioral plasticity is thought to reflect, at least in part, changes in dopamine (DA) neurotransmission, and therefore, may provide insights into the development of substance use disorders. Indeed, the most widely used preclinical model of cocaine addiction, which involves Long Access (LgA) self-administration procedures, is reported to produce tolerance to cocaine's psychomotor activating effects and effects on DA activity. In contrast, Intermittent Access (IntA) cocaine self-administration is more effective than LgA in producing addiction-like behavior, but sensitizes DA neurotransmission. There is, however, very little information concerning the effects of IntA experience on the psychomotor activating effects of cocaine.

OBJECTIVE

The objective of this study was to determine whether IntA experience produces psychomotor sensitization with similar characteristics to that produced by the intermittent, noncontingent administration of cocaine.

RESULTS

IntA to cocaine did indeed produce psychomotor sensitization that (1) was greater after a long (30 days) vs. short (1 day) period of withdrawal, (2) was greater in females than males, and (3) resulted in cross-sensitization to another psychomotor stimulant drug, amphetamine.

CONCLUSION

The tolerance sometimes associated with LgA cocaine self-administration has been cited in support of the idea that, in addiction, drug-seeking and drug-taking is motivated to overcome a DA deficiency and associated anhedonia. In contrast, the neurobehavioral sensitization associated with IntA cocaine self-administration favors an incentive-sensitization view.

Acute Nicotine Exposure Alters Ventral Tegmental Area Inhibitory Transmission and Promotes Diazepam Consumption

Nicotine use increases the risk for subsequent abuse of other addictive drugs, but the biological basis underlying this risk remains largely unknown. Interactions between nicotine and other drugs of abuse may arise from nicotine-induced neural adaptations in the mesolimbic dopamine (DA) system, a common pathway for the reinforcing effects of many addictive substances. Previous work identified nicotine-induced neuroadaptations that alter inhibitory transmission in the ventral tegmental area (VTA). Here, we test whether nicotine-induced dysregulation of GABAergic signaling within the VTA increases the vulnerability for benzodiazepine abuse that has been reported in smokers. We demonstrate in rats that nicotine exposure dysregulates diazepam-induced inhibition of VTA GABA neurons and increases diazepam consumption. In VTA GABA neurons, nicotine impaired KCC2-mediated chloride extrusion, depolarized the GABAA reversal potential, and shifted the pharmacological effect of diazepam on GABA neurons from inhibition toward excitation. In parallel, nicotine-related alterations in GABA signaling observed ex vivo were associated with enhanced diazepam-induced inhibition of lateral VTA DA neurons in vivo Targeting KCC2 with the agonist CLP290 normalized diazepam-induced effects on VTA GABA transmission and reduced diazepam consumption following nicotine administration to the control level. Together, our results provide insights into midbrain circuit alterations resulting from nicotine exposure that contribute to the abuse of other drugs, such as benzodiazepines.

Increased Alcohol Seeking in Mice Lacking Gpr88 Involves Dysfunctional Mesocorticolimbic Networks

BACKGOUND

Alcohol use disorder (AUD) is devastating and poorly treated, and innovative targets are actively sought for prevention and treatment. The orphan G protein-coupled receptor GPR88 is enriched in mesocorticolimbic pathways, and Gpr88 knockout mice show hyperactivity and risk-taking behavior, but a potential role for this receptor in drug abuse has not been examined.

METHODS

We tested Gpr88 knockout mice for alcohol-drinking and -seeking behaviors. To gain system-level understanding of their alcohol endophenotype, we also analyzed whole-brain functional connectivity in naïve mice using resting-state functional magnetic resonance imaging.

RESULTS

Gpr88 knockout mice showed increased voluntary alcohol drinking at both moderate and excessive levels, with intact alcohol sedation and metabolism. Mutant mice also showed increased operant responding and motivation for alcohol, while food and chocolate operant self-administration were unchanged. Alcohol place conditioning and alcohol-induced dopamine release in the nucleus accumbens were decreased, suggesting reduced alcohol reward in mutant mice that may partly explain enhanced alcohol drinking. Seed-based voxelwise functional connectivity analysis revealed significant remodeling of mesocorticolimbic centers, whose hallmark was predominant weakening of prefrontal cortex, ventral tegmental area, and amygdala connectional patterns. Also, effective connectivity from the ventral tegmental area to the nucleus accumbens and amygdala was reduced.

CONCLUSIONS

Gpr88 deletion disrupts executive, reward, and emotional networks in a configuration that reduces alcohol reward and promotes alcohol seeking and drinking. The functional connectivity signature is reminiscent of alterations observed in individuals at risk for AUD. The Gpr88 gene, therefore, may represent a vulnerability/resilience factor for AUD, and a potential drug target for AUD treatment.

Cocaine Potency at the Dopamine Transporter Tracks Discrete Motivational States During Cocaine Self-Administration

Although the dopamine transporter (DAT) is the primary site of action for cocaine, and the dopamine system is known to mediate the reinforcing effects of cocaine, the dopaminergic variations underlying individual differences in cocaine self-administration behaviors are not fully understood. Recent advances in the application of economic principles to operant tasks in rodents have allowed for the within-subject, within-session determination of both consummatory and appetitive responding for reinforcers. Here we combined a behavioral economics approach with cocaine self-administration and ex vivo voltammetric recording of dopamine signaling in the core of the nucleus accumbens of rats to determine the relationship between dopamine signaling and discrete aspects of cocaine taking and seeking. We found neither dopamine release or uptake tracked individual differences in cocaine consumption or the reinforcing efficacy of cocaine. Cocaine potency at the DAT was correlated with reinforcing efficacy, but was not related to cocaine consumption. Further, we introduce a novel analysis that determines perseverative responding within the same procedure, and find that cocaine potency at the DAT also tracks differences in perseverative responding. Together, we demonstrate that cocaine effects at the DAT determine the reinforcing efficacy of cocaine, and perseverative responding for sub-threshold doses of cocaine that do not maintain responding when presented in isolation. Surprisingly, we find that variations in cocaine potency do not account for differences in cocaine consumption, suggesting that satiation for cocaine is determined by other targets or mechanisms. Finally, we outline a novel approach for relating drug-target interactions and potency to discrete motivational states during a single self-administration session.

Cocaine Self-Administration Experience Induces Pathological Phasic Accumbens Dopamine Signals and Abnormal Incentive Behaviors in Drug-Abstinent Rats

Chronic exposure to drugs of abuse is linked to long-lasting alterations in the function of limbic system structures, including the nucleus accumbens (NAc). Although cocaine acts via dopaminergic mechanisms within the NAc, less is known about whether phasic dopamine (DA) signaling in the NAc is altered in animals with cocaine self-administration experience or if these animals learn and interact normally with stimuli in their environment. Here, separate groups of rats self-administered either intravenous cocaine or water to a receptacle (controls), followed by 30 d of enforced abstinence. Next, all rats learned an appetitive Pavlovian discrimination and voltammetric recordings of real-time DA release were taken in either the NAc core or shell of cocaine and control subjects. Cocaine experience differentially impaired DA signaling in the core and shell relative to controls. Although phasic DA signals in the shell were essentially abolished for all stimuli, in the core, DA did not distinguish between cues and was abnormally biased toward reward delivery. Further, cocaine rats were unable to learn higher-order associations and even altered simple conditioned approach behaviors, displaying enhanced preoccupation with cue-associated stimuli (sign-tracking; ST) but diminished time at the food cup awaiting reward delivery (goal-tracking). Critically, whereas control DA signaling correlated with ST behaviors, cocaine experience abolished this relationship. These findings show that cocaine has persistent, differential, and pathological effects on both DA signaling and DA-dependent behaviors and suggest that psychostimulant experience may remodel the very circuits that bias organisms toward repeated relapse.

SIGNIFICANCE STATEMENT

Relapsing to drug abuse despite periods of abstinence and sincere attempts to quit is one of the most pernicious facets of addiction. Unfortunately, little is known about how the dopamine (DA) system functions after periods of drug abstinence, particularly its role in behavior in nondrug situations. Here, rats learned about food-paired stimuli after prolonged abstinence from cocaine self-administration. Using voltammetry, we found that real-time DA signals in cocaine-experienced rats were strikingly altered relative to controls. Further, cocaine-experienced animals found reward-predictive stimuli abnormally salient and spent more time interacting with cues. Therefore, cocaine induces neuroplastic changes in the DA system that biases animals toward salient stimuli (including reward-associated cues), putting addicts at increasing risk to relapse as addiction increases in severity.

Escalation of cocaine consumption in short and long access self-administration procedures

BACKGROUND

Escalation of consumption is a hallmark of cocaine addiction. Many animal models reveal escalation by increasing the duration of drug access (e.g., 6-24 h/day) after longer histories of self-administration. We recently developed a method that reveals escalation early post-acquisition under shorter access conditions. However, whether or not rats will escalate cocaine consumption both early post-acquisition under short access (2 h/day) conditions, and later under long access (6 h/day) conditions, has not been demonstrated.

METHODS

All rats acquired cocaine self-administration (0.8 mg/kg, i.v.) under 2 h conditions, and then continued 2h self-administration for an additional 13 sessions. Then, rats were assigned either to 2 or 6h conditions, and self-administered cocaine (0.8 mg/kg, i.v.) for an additional 19 sessions. In addition, four cocaine-induced locomotor activity measurements were taken for each rat: before cocaine exposure, after non-contingent cocaine administration, and after escalation in the short and long access experimental phases.

RESULTS

Following acquisition, rats displayed a robust escalation of intake during 2 h sessions. Rats that self-administered cocaine in continued 2h sessions exhibited stable intake, whereas rats that self-administered cocaine in 6h sessions further escalated intake. Despite the second escalation in 6h rats, cocaine-induced locomotor activity did not differ between 2 and 6h rats.

CONCLUSIONS

Escalation of cocaine self-administration can occur in the same rats both early post-acquisition, and later under long access conditions. Importantly, this early post-acquisition period provides a new opportunity to determine the mechanisms first involved in the escalation phenomenon.

Adaptations of presynaptic dopamine terminals induced by psychostimulant self-administration

A great deal of research has focused on investigating neurobiological alterations induced by chronic psychostimulant use in an effort to describe, understand, and treat the pathology of psychostimulant addiction. It has been known for several decades that dopamine neurotransmission in the nucleus accumbens is integrally involved in the selection and execution of motivated and goal-directed behaviors, and that psychostimulants act on this system to exert many of their effects. As such, a large body of work has focused on defining the consequences of psychostimulant use on dopamine signaling in the striatum as it relates to addictive behaviors. Here, we review presynaptic dopamine terminal alterations observed following self-administration of cocaine and amphetamine, as well as possible mechanisms by which these alterations occur and their impact on the progression of addiction.

The development of a preference for cocaine over food identifies individual rats with addiction-like behaviors

RATIONALE

Cocaine dependence is characterized by compulsive drug taking that supercedes other recreational, occupational or social pursuits. We hypothesized that rats vulnerable to addiction could be identified within the larger population based on their preference for cocaine over palatable food rewards.

OBJECTIVES

To validate the choice self-administration paradigm as a preclinical model of addiction, we examined changes in motivation for cocaine and recidivism to drug seeking in cocaine-preferring and pellet-preferring rats. We also examined behavior in males and females to identify sex differences in this "addicted" phenotype.

METHODS

Preferences were identified during self-administration on a fixed-ratio schedule with cocaine-only, pellet-only and choice sessions. Motivation for each reward was probed early and late during self-administration using a progressive-ratio schedule. Reinstatement of cocaine- and pellet-seeking was examined following exposure to their cues and non-contingent delivery of each reward.

RESULTS

Cocaine preferring rats increased their drug intake at the expense of pellets, displayed increased motivation for cocaine, attenuated motivation for pellets and greater cocaine and cue-induced reinstatement of drug seeking. Females were more likely to develop cocaine preferences and recidivism of cocaine- and pellet-seeking was sexually dimorphic.

CONCLUSIONS

The choice self-administration paradigm is a valid preclinical model of addiction. The unbiased selection criteria also revealed sex-specific vulnerability factors that could be differentiated from generalized sex differences in behavior, which has implications for the neurobiology of addiction and effective treatments in each sex.

Temporal pattern of cocaine intake determines tolerance vs sensitization of cocaine effects at the dopamine transporter

The dopamine transporter (DAT) is responsible for terminating dopamine (DA) signaling and is the primary site of cocaine's reinforcing actions. Cocaine self-administration has been shown previously to result in changes in cocaine potency at the DAT. To determine whether the DAT changes associated with self-administration are due to differences in intake levels or temporal patterns of cocaine-induced DAT inhibition, we manipulated cocaine access to produce either continuous or intermittent elevations in cocaine brain levels. Long-access (LgA, 6 h) and short-access (ShA, 2 h) continuous self-administration produced similar temporal profiles of cocaine intake that were sustained throughout the session; however, LgA had greater intake. ShA and intermittent-access (IntA, 6 h) produced the same intake, but different temporal profiles, with 'spiking' brain levels in IntA compared with constant levels in ShA. IntA consisted of 5-min access periods alternating with 25-min timeouts, which resulted in bursts of high responding followed by periods of no responding. DA release and uptake, as well as the potency of cocaine for DAT inhibition, were assessed by voltammetry in the nucleus accumbens slices following control, IntA, ShA, and LgA self-administration. Continuous-access protocols (LgA and ShA) did not change DA parameters, but the 'spiking' protocol (IntA) increased both release and uptake of DA. In addition, high continuous intake (LgA) produced tolerance to cocaine, while 'spiking' (IntA) produced sensitization, relative to ShA and naive controls. Thus, intake and pattern can both influence cocaine potency, and tolerance seems to be produced by high intake, while sensitization is produced by intermittent temporal patterns of intake.

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.

Withdrawal from extended-access cocaine self-administration results in dysregulated functional activity and altered locomotor activity in rats

Much work has focused on determining the consequences of cocaine self-administration on specific neurotransmitter systems, thus neglecting the global changes that occur. Previous imaging studies have focused on the effects of cocaine self-administration in the presence of high blood levels of cocaine, but have not determined the functional effects of cocaine self-administration after cocaine has cleared. Extended-access cocaine self-administration, where animals administer cocaine for 6 h each day, results in escalation in the rate of cocaine intake and is believed to model the transition from recreational use to addiction in humans. We aimed to determine the functional changes following acute (48 h) withdrawal from an extended-access, defined-intake self-administration paradigm (5 days, 40 injections/day, 6 h/day), a time point when behavioral changes are present. Using the 2-[(14) C]deoxyglucose method to measure rates of local cerebral glucose metabolism, an indicator of functional activity, we found reductions in circuits related to learning and memory, attention, sleep, and reward processing, which have important clinical implications for cocaine addiction. Additionally, lower levels of functional activity were found in the dorsal raphe and locus coeruleus, suggesting that cocaine self-administration may have broader effects on brain function than previously noted. These widespread neurochemical reductions were concomitant with substantial behavioral differences in these animals, highlighted by increased vertical activity and decreased stereotypy. These data demonstrate that behavioral and neurochemical impairments following cocaine self-administration are present in the absence of drug and persist after cocaine has been cleared.

Paradoxical tolerance to cocaine after initial supersensitivity in drug-use-prone animals

There is great interest in outlining biological factors and behavioral characteristics that either predispose or predict vulnerability to substance use disorders. Response to an inescapable novel environment has been shown to predict a "drug-use-prone" phenotype that is defined by rapid acquisition of cocaine self-administration. Here, we showed that response to novelty can also predict the neurochemical and behavioral effects of acute and repeated cocaine in rats. We used cocaine self-administration under a fixed-ratio 1 schedule followed by fast-scan cyclic voltammetry in brain slices to measure subsecond dopamine (DA) release and uptake parameters in drug-use-prone and -resistant phenotypes. Despite no significant differences in stimulated release and uptake, animals with high responses to a novel environment had DA transporters that were more sensitive to cocaine-induced uptake inhibition, which corresponded to greater locomotor activating effects of cocaine. These animals also acquired cocaine self-administration more rapidly and, after 5 days of extended access cocaine self-administration, high-responding animals showed robust tolerance to DA uptake inhibition by cocaine. The effects of cocaine remained unchanged in animals with low novelty responses. Similarly, the rate of acquisition was negatively correlated with DA uptake inhibition by cocaine after self-administration. Thus, we showed that tolerance to the cocaine-induced inhibition of DA uptake coexists with a behavioral phenotype that is defined by increased preoccupation with cocaine as measured by rapid acquisition and early high intake.

Increased vulnerability to cocaine in mice lacking dopamine D3 receptors

Neuroimaging studies using positron emission tomography suggest that reduced dopamine D(2) receptor availability in the neostriatum is associated with increased vulnerability to drug addiction in humans and experimental animals. The role of D(3) receptors (D(3)Rs) in the neurobiology of addiction remains unclear, however. Here we report that D(3)R KO (D(3)(-/-)) mice display enhanced cocaine self-administration and enhanced motivation for cocaine-taking and cocaine-seeking behavior. This increased vulnerability to cocaine is accompanied by decreased dopamine response to cocaine secondary to increased basal levels of extracellular dopamine in the nucleus accumbens, suggesting a compensatory response to decreased cocaine reward in D(3)(-/-) mice. In addition, D(3)(-/-) mice also display up-regulation of dopamine transporters in the striatum, suggesting a neuroadaptative attempt to normalize elevated basal extracellular dopamine. These findings suggest that D(3)R deletion increases vulnerability to cocaine, and that reduced D(3)R availability in the brain may constitute a risk factor for the development of cocaine addiction.

The behavioral pharmacology of effort-related choice behavior: dopamine, adenosine and beyond

For many years, it has been suggested that drugs that interfere with dopamine (DA) transmission alter the "rewarding" impact of primary reinforcers such as food. Research and theory related to the functions of mesolimbic DA are undergoing a substantial conceptual restructuring, with the traditional emphasis on hedonia and primary reward yielding to other concepts and lines of inquiry. The present review is focused upon the involvement of nucleus accumbens DA in effort-related choice behavior. Viewed from the framework of behavioral economics, the effects of accumbens DA depletions and antagonism on food-reinforced behavior are highly dependent upon the work requirements of the instrumental task, and DA-depleted rats show a heightened sensitivity to response costs, especially ratio requirements. Moreover, interference with accumbens DA transmission exerts a powerful influence over effort-related choice behavior. Rats with accumbens DA depletions or antagonism reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and show increased selection of low reinforcement/low cost options. Nucleus accumbens DA and adenosine interact in the regulation of effort-related functions, and other brain structures (anterior cingulate cortex, amygdala, ventral pallidum) also are involved. Studies of the brain systems regulating effort-based processes may have implications for understanding drug abuse, as well as symptoms such as psychomotor slowing, fatigue or anergia in depression and other neurological disorders.

Cocaine self-administration produces pharmacodynamic tolerance: differential effects on the potency of dopamine transporter blockers, releasers, and methylphenidate

The dopamine transporter (DAT) is the primary site of action for psychostimulant drugs such as cocaine, methylphenidate, and amphetamine. Our previous work demonstrated a reduced ability of cocaine to inhibit the DAT following high-dose cocaine self-administration (SA), corresponding to a reduced ability of cocaine to increase extracellular dopamine. However, this effect had only been demonstrated for cocaine. Thus, the current investigations sought to understand the extent to which cocaine SA (1.5 mg/kg/inf × 40 inf/day × 5 days) altered the ability of different dopamine uptake blockers and releasers to inhibit dopamine uptake, measured using fast-scan cyclic voltammetry in rat brain slices. We demonstrated that, similar to cocaine, the DAT blockers nomifensine and bupropion were less effective at inhibiting dopamine uptake following cocaine SA. The potencies of amphetamine-like dopamine releasers such as 3,4-methylenedioxymethamphetamine, methamphetamine, amphetamine, and phentermine, as well as a non-amphetamine releaser, 4-benzylpiperidine, were all unaffected. Finally, methylphenidate, which blocks dopamine uptake like cocaine while being structurally similar to amphetamine, shared characteristics of both, resembling an uptake blocker at low concentrations and a releaser at high concentrations. Combined, these experiments demonstrate that after high-dose cocaine SA, there is cross-tolerance of the DAT to other uptake blockers, but not releasers. The reduced ability of psychostimulants to inhibit dopamine uptake following cocaine SA appears to be contingent upon their functional interaction with the DAT as a pure blocker or releaser rather than their structural similarity to cocaine. Further, methylphenidate's interaction with the DAT is unique and concentration-dependent.

Cocaine dose and self-administration history, but not initial cocaine locomotor responsiveness, affects sensitization to the motivational effects of cocaine in rats

Cocaine addiction is a significant and complex disease. Part of this complexity is caused by the variability of the drug experience early in drug use (initial responsiveness, amount of use, etc.). In rats, individual differences in initial cocaine responsiveness and cocaine self-administration history both predict the development of cocaine sensitization, a putative mechanism contributing to the development of cocaine addiction. Here, we sought to determine the role of these factors and cocaine dose on the development of sensitization to cocaine's motivational effects during the earliest stages of self-administration. Rats were classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on acute cocaine-induced locomotor activity (10 mg/kg i.p.) before learning to self-administer cocaine (0.6 mg/kg/infusion i.v.) under a fixed ratio 1 (FR1) schedule of reinforcement. After acquisition, rats self-administered cocaine (0.6 or 1.2 mg/kg/infusion) under a progressive ratio (PR) schedule of reinforcement either immediately or after an additional five FR1 sessions (0.6 or 1.2 mg/kg/infusion). No LCR/HCR differences in sensitization were observed. However, regardless of LCR/HCR classification, exposure to the higher dose of cocaine produced sensitization to cocaine's motivational effects on the PR schedule (i.e., increased break points) and an escalation of consumption on the FR schedule. Thus, our results reveal a novel model for studying escalation and sensitization very early after acquisition and suggest that sensitization may be important in the earliest stages of the cocaine addiction process.

The rewarding and locomotor-sensitizing effects of repeated cocaine administration are distinct and separable in mice

Repeated psychostimulant exposure progressively increases their potency to stimulate motor activity in rodents. This behavioral or locomotor sensitization is considered a model for some aspects of drug addiction in humans, particularly drug craving during abstinence. However, the role of increased motor behavior in drug reward remains incompletely understood. Intracranial self-stimulation (ICSS) was measured concurrently with locomotor activity to determine if acute intermittent cocaine administration had distinguishable effects on motor behavior and perception of brain stimulation-reward (BSR) in the same mice. Sensitization is associated with changes in neuronal activity and glutamatergic neurotransmission in brain reward circuitry. Expression of AMPA receptor subunits (GluR1 and GluR2) and CRE binding protein (CREB) was measured in the ventral tegmental area (VTA), dorsolateral striatum (STR) and nucleus accumbens (NAc) before and after a sensitizing regimen of cocaine, with and without ICSS. Repeated cocaine administration sensitized mice to its locomotor-stimulating effects but not its ability to potentiate BSR. ICSS increased GluR1 in the VTA but not NAc or STR, demonstrating selective changes in protein expression with electrical stimulation of discrete brain structures. Repeated cocaine reduced GluR1, GluR2 and CREB expression in the NAc, and reductions of GluR1 and GluR2 but not CREB were further enhanced by ICSS. These data suggest that the effects of repeated cocaine exposure on reward and motor processes are dissociable in mice, and that reduction of excitatory neurotransmission in the NAc may predict altered motor function independently from changes in reward perception.

Hyperdopaminergic tone in HIV-1 protein treated rats and cocaine sensitization

In the United States, one-third of infected individuals contracted Human Immunodeficiency Virus-1 (HIV-1) via injecting drugs with contaminated needles or through risky behaviors associated with drug use. Research demonstrates concomitant administration of psychostimulants and HIV-1-proteins damage neurons to a greater extent than viral proteins or the drug alone. To model the onset of HIV-1-infection in relation to a history of drug use, the current research compared behavior and extracellular dopamine and metabolite levels following Tat(1-86) infusions in animals with and without a history of cocaine (Coc) experience (10 mg/kg; i.p.; 1 injection/day × 9 days). Animals receiving a behaviorally sensitizing regimen of Coc demonstrated a decrease in extracellular dopamine concentration in the nucleus accumbens, consistent with evidence describing up-regulation of dopamine transporter uptake. Contrary to this effect, Tat(1-86) microinfusion into the nucleus accumbens following the sensitizing regimen of Coc caused a significant increase in extracellular dopamine levels (nM) within 48 h with no difference in percent of baseline response to Coc. After 72 h, Tat + Coc treated animals demonstrated a blunted effect on potassium-stimulated extracellular dopamine release (percent of baseline) with a corresponding decrease in expression of behavioral sensitization to Coc challenge. A persistent decrease in extracellular dopamine metabolite levels was found across all time-points in Tat-treated animals, regardless of experience with Coc. The current study provides evidence for divergent neurochemical and behavioral outcomes following Tat-treatment; contingent upon experience with Coc.

Sex differences and ovarian hormones in animal models of drug dependence

Increasing evidence indicates the presence of sex differences in many aspects of drug abuse. Most studies reveal that females exceed males during the initiation, escalation, extinction, and reinstatement (relapse) of drug-seeking behavior, but males are more sensitive than females to the aversive effects of drugs such as drug withdrawal. Findings from human and animal research indicate that circulating levels of ovarian steroid hormones account for these sex differences. Estrogen (E) facilitates drug-seeking behavior, while progesterone (P) and its metabolite, allopregnanalone (ALLO), counteract the effects of E and reduce drug seeking. Estrogen and P influence other behaviors that are affiliated with drug abuse such as drug-induced locomotor sensitization and conditioned place preference. The enhanced vulnerability to drug seeking in females vs. males is also additive with the other risk factors for drug abuse (e.g., adolescence, sweet preference, novelty reactivity, and impulsivity). Finally, treatment studies using behavioral or pharmacological interventions, including P and ALLO, also indicate that females show greater treatment effectiveness during several phases of the addiction process. The neurobiological basis of sex differences in drug abuse appears to be genetic and involves the influence of ovarian hormones and their metabolites, the hypothalamic pituitary adrenal (HPA) axis, dopamine (DA), and gamma-hydroxy-butyric acid (GABA). Overall, sex and hormonal status along with other biological risk factors account for a continuum of addiction-prone and -resistant animal models that are valuable for studying drug abuse prevention and treatment strategies.

Drug-induced plasticity contributing to heightened relapse susceptibility: neurochemical changes and augmented reinstatement in high-intake rats

A key in understanding the neurobiology of addiction and developing effective pharmacotherapies is revealing drug-induced plasticity that results in heightened relapse susceptibility. Previous studies have demonstrated that increased extracellular glutamate, but not dopamine, in the nucleus accumbens core (NAcc) is necessary for cocaine-induced reinstatement. In this report, we examined whether drug-induced adaptations that are necessary to generate cocaine-induced reinstatement also determine relapse vulnerability. To do this, rats were assigned to self-administer cocaine under conditions resulting in low (2 h/d; 0.5 mg/kg/infusion, i.v.) or high (6 h/d; 1.0 mg/kg/infusion, i.v.) levels of drug intake since these manipulations produce groups of rats exhibiting differences in the magnitude of cocaine-induced reinstatement. Approximately 19 d after the last session, cocaine-induced drug seeking and extracellular levels of glutamate and dopamine in the NAcc were measured. Contrary to our hypothesis, high-intake rats exhibited a more robust cocaine-induced increase in extracellular levels of dopamine but not glutamate. Further, increased reinstatement in high-intake rats was no longer observed when the D(1) receptor antagonist SCH-23390 was infused into the NAcc. The sensitized dopamine response to cocaine in high-intake rats may involve blunted cystine-glutamate exchange by system x(c(-)). Reduced (14)C-cystine uptake through system x(c(-)) was evident in NAcc tissue slices obtained from high-intake rats, and the augmented dopamine response in these rats was no longer observed when subjects received the cysteine prodrug N-acetyl cysteine. These data reveal a role for drug-induced NAcc dopamine in heightened relapse vulnerability observed in rats with a history of high levels of drug intake.

Sensitization processes in drug addiction

In 1993, Robinson and Berridge published their first review that laid out the incentive sensitization theory of addiction (Robinson and Berridge 1993 Brain Res Rev 18:247). Its basic point is that repeated exposure to drugs of abuse causes hypersensitivity to drugs and drug-associated stimuli of the neural circuits mediating incentive salience, an important way in which motivational stimuli influence behavior. In laymen's terms, it states that this drug-induced hypersensitivity of motivational circuitry would mediate an increase in drug "wanting," thus being responsible for the dramatically exaggerated motivation for drugs displayed by addicts. This theory has been exceptionally influential, as evidenced by the fact that the original review paper about this theory (Robinson and Berridge 1993 Brain Res Rev 18:247) has been cited 2,277 times so far, and subsequent updates of this view (Robinson and Berridge 2000 Addiction 95(Suppl 2):S91; Robinson and Berridge 2001 Addiction 96:103; Robinson and Berridge 2003 Ann Rev Psychol 54:25) have been cited 274, 297, and 365 times, respectively, adding up to more than 3,200 citations within 15 years. The present chapter aims to delineate the merits and limitations of the incentive sensitization view of addiction, and whether incentive sensitization occurs in humans. We conclude that since incentive sensitization most prominently occurs after the first few drug exposures, it may represent an important initial step in the addiction process. During the expression of full-blown addiction, characterized by loss of control over drug intake and use of large quantities of drugs, the expression of incentive sensitization may be transiently suppressed. However, detoxification and the gradual disappearance of tolerance and withdrawal symptoms may unmask sensitization, which could then play an important role in the high risk of relapse.

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.

Parsing the Addiction Phenomenon: Self-Administration Procedures Modeling Enhanced Motivation for Drug and Escalation of Drug Intake

Investigators who study drug addiction are fortunate to have access to excellent animal models. Such models will be invaluable in the assessment of factors involved in the progression of drug addiction. The relevance of these findings, however, will depend on the general understanding of how each model is related to drug addiction. The present review focuses on several procedures that were designed to model the addiction process and questions whether these models are tapping into the same underlying process or whether each is addressing a unique feature. Furthermore, various factors (e.g., rate of drug onset, dose magnitude, early drug history, periods of abstinence) influencing the progression of these addiction-like changes in behavior are discussed.

Demand for cocaine and food over time

When reinterpreted, data from Ahmed and Koob [Ahmed, S.H., Koob, G.F., Transition from moderate to excessive drug intake: Change in hedonic set point. Science 1998; 282:298-301.] show that the reinforcing strength of cocaine, an inessential good, increases with experience. However, no such effect obtains with a homeostatically regulated good such as food. The present study evaluated whether this difference could serve to distinguish abused drugs from biologically necessary goods. In Experiment 1, five rats from Christensen, Silberberg, Hursh, Huntsberry and Riley [Christensen, C.J., Silberberg, A., Hursh, S.R., Huntsberry, M.E., Riley, A.L., Essential value of cocaine and food in rats: tests of the exponential model of demand. Psychopharmacology 2008;198(2):221-229.] earned cocaine under a Fixed-Ratio 3 schedule for 7 sessions. Thereafter, in a demand procedure identical to that in Christensen et al., demand was re-assessed by measuring consumption at Fixed Ratios between 3 and 560. In Experiment 2, five different rats from Christensen et al. had their food demand curves re-determined using an identical procedure as the first. When fit with the exponential model, the second determination of cocaine demand in Experiment 1 showed greater essential value than the first, indicating that strength increased with cocaine exposure. In Experiment 2, the re-determined food demand curves showed no change from their initial determination. These results show that the strength of cocaine, but not food, increases with increased experience. Measures of time-based changes in essential value may serve as a basis for distinguishing addictive from non-addictive reinforcers.

Behavioral and electrophysiological indices of negative affect predict cocaine self-administration

The motivation to seek cocaine comes in part from a dysregulation of reward processing manifested in dysphoria, or affective withdrawal. Learning is a critical aspect of drug abuse; however, it remains unclear whether drug-associated cues can elicit the emotional withdrawal symptoms that promote cocaine use. Here we report that a cocaine-associated taste cue elicited a conditioned aversive state that was behaviorally and neurophysiologically quantifiable and predicted subsequent cocaine self-administration behavior. Specifically, brief intraoral infusions of a cocaine-predictive flavored saccharin solution elicited aversive orofacial responses that predicted early-session cocaine taking in rats. The expression of aversive taste reactivity also was associated with a shift in the predominant pattern of electrophysiological activity of nucleus accumbens (NAc) neurons from inhibitory to excitatory. The dynamic nature of this conditioned switch in affect and the neural code reveals a mechanism by which cues may exert control over drug self-administration.