Amphetamine- type reinforcement by dopaminergic agonists in the rat

Blocking the dopaminergic receptors within the hippocampal dentate gyrus reduced analgesic responses induced by restraint stress in the formalin test

Previous studies have shown that various receptors, including dopamine receptors, are expressed in the hippocampal dentate gyrus (DG). Besides, indicatively, dopamine receptors play an essential role in the modulation of pain perception. On the other hand, stressful experiences can produce analgesia, termed stress-induced analgesia (SIA). The current study examined the probable role of dopamine receptors within the DG in antinociception induced by restraint stress (RS). Ninety-seven male albino Wistar rats were unilaterally implanted with a cannula in the DG. Animals received intra-DG microinjections of SCH23390 or Sulpiride (0.25, 1, and 4 μg/rat) as D1-and D2-like dopamine receptor antagonists, respectively, five minutes before RS. Ten minutes after the end of the induction of RS for three hours, 50 μl 2.5% formalin was injected subcutaneously into the plantar surface of the hind paw to induce persistent inflammatory pain. Pain scores were evaluated at 5-minute intervals for 60 minutes. These findings showed that; exposure to RS for three hours produced SIA in both phases of the formalin test, while this RS-induced analgesia was attenuated in the early and late phases of the formalin test by intra-DG microinjection of SCH23390 and Sulpiride. The results of the present study suggested that both D1- and D2-like dopamine receptors in the DG have a considerable role in the induced analgesia by RS.

Drug addiction co-morbidity with alcohol: Neurobiological insights

Addiction is a chronic disorder that consists of a three-stage cycle of binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. These stages involve, respectively, neuroadaptations in brain circuits involved in incentive salience and habit formation, stress surfeit and reward deficit, and executive function. Much research on addiction focuses on the neurobiology underlying single drug use. However, alcohol use disorder (AUD) can be co-morbid with substance use disorder (SUD), called dual dependence. The limited epidemiological data on dual dependence indicates that there is a large population of individuals suffering from addiction who are dependent on more than one drug and/or alcohol, yet dual dependence remains understudied in addiction research. Here, we review neurobiological data on neurotransmitter and neuropeptide systems that are known to contribute to addiction pathology and how the involvement of these systems is consistent or divergent across drug classes. In particular, we highlight the dopamine, opioid, corticotropin-releasing factor, norepinephrine, hypocretin/orexin, glucocorticoid, neuroimmune signaling, endocannabinoid, glutamate, and GABA systems. We also discuss the limited research on these systems in dual dependence. Collectively, these studies demonstrate that the use of multiple drugs can produce neuroadaptations that are distinct from single drug use. Further investigation into the neurobiology of dual dependence is necessary to develop effective treatments for addiction to multiple drugs.

Lack of correlation between the activity of the mesolimbic dopaminergic system and the rewarding properties of pregabalin in mouse

RATIONALE

Pregabalin is a psychoactive drug indicated in the treatment of epilepsy, neuropathic pain, and generalized anxiety disorders. Pregabalin acts on different neurotransmission systems by inactivating the alpha2-delta subunit of voltage-gated calcium channels. In light of this pharmacological property, the hypothesis has been raised that pregabalin may regulate the mesolimbic dopamine pathway and thereby display a potential for misuse or abuse as recently observed in humans. Although some preclinical data support this possibility, the rewarding properties of gabapentinoid are still a matter for debate.

OBJECTIVE

The aim of this work was to evaluate the rewarding properties of pregabalin and to determine its putative mechanism of action in healthy mice.

RESULTS

Pregabalin alone (60 mg/kg; s.c.) produced a rewarding effect in the conditioned place preference (CPP) test albeit to a lower extent than cocaine (30 mg/kg; s.c.). Interestingly, when assessing locomotor activity in the CPP, the PGB60 group, similarly to the cocaine group, showed an increased locomotor activity. In vivo single unit extracellular recording showed that pregabalin had mixed effects on dopamine (DA) neuronal activity in the ventral tegmental area since it decreased the activity of 50% of neurons and increased 28.5% of them. In contrast, cocaine decreased 75% of VTA DA neuronal activity whereas none of the neurons were activated. Intracerebal microdialysis was then conducted in awake freely mice to determine to what extent such electrophysiological parameters influence the extracellular DA concentrations ([DA]ext) in the nucleus accumbens. Although pregabalin failed to modify this parameter, cocaine produced a robust increase (800%) in [DA]ext.

CONCLUSIONS

Collectively, these electrophysiological and neurochemical experiments suggest that the rewarding properties of pregabalin result from a different mode of action than that observed with cocaine. Further experiments are warranted to determine whether such undesirable effects can be potentiated under pathological conditions such as neuropathic pain, mood disorders, or addiction and to identify the key neurotransmitter system involved.

The Involvement of Norepinephrine in Behaviors Related to Psychostimulant Addiction

Although it is generally accepted that the abuse-related effects of amphetamines and cocaine result from the activation of the brain dopaminergic (DA) system, the psychostimulants also alter other neurotransmitter systems. In particular, they increase extracellular levels of norepinephrine (NE) and serotonin by inhibiting respective plasma membrane transporters and/or inducing release. The present review will discuss the preclinical findings on the effects of the NE system modulation (lesions, pharmacological and genetic approaches) on behaviors (locomotor hyperactivity, behavioral sensitization, modification of intracranial self-stimulation, conditioned place preference, drug self-administration, extinction/reinstatement of drug seeking behavior) related to the psychostimulant addiction.

Predicting abuse potential of stimulants and other dopaminergic drugs: overview and recommendations

Examination of a drug's abuse potential at multiple levels of analysis (molecular/cellular action, whole-organism behavior, epidemiological data) is an essential component to regulating controlled substances under the Controlled Substances Act (CSA). We reviewed studies that examined several central nervous system (CNS) stimulants, focusing on those with primarily dopaminergic actions, in drug self-administration, drug discrimination, and physical dependence. For drug self-administration and drug discrimination, we distinguished between experiments conducted with rats and nonhuman primates (NHP) to highlight the common and unique attributes of each model in the assessment of abuse potential. Our review of drug self-administration studies suggests that this procedure is important in predicting abuse potential of dopaminergic compounds, but there were many false positives. We recommended that tests to determine how reinforcing a drug is relative to a known drug of abuse may be more predictive of abuse potential than tests that yield a binary, yes-or-no classification. Several false positives also occurred with drug discrimination. With this procedure, we recommended that future research follow a standard decision-tree approach that may require examining the drug being tested for abuse potential as the training stimulus. This approach would also allow several known drugs of abuse to be tested for substitution, and this may reduce false positives. Finally, we reviewed evidence of physical dependence with stimulants and discussed the feasibility of modeling these phenomena in nonhuman animals in a rational and practical fashion. This article is part of the Special Issue entitled 'CNS Stimulants'.

Adrenaline rush: the role of adrenergic receptors in stimulant-induced behaviors

Psychostimulants, such as cocaine and amphetamines, act primarily through the monoamine neurotransmitters dopamine (DA), norepinephrine, and serotonin. Although stimulant addiction research has largely focused on DA, medication development efforts targeting the dopaminergic system have thus far been unsuccessful, leading to alternative strategies aimed at abating stimulant abuse. Noradrenergic compounds have shown promise in altering the behavioral effects of stimulants in rodents, nonhuman primates, and humans. In this review, we discuss the contribution of each adrenergic receptor (AR) subtype (α1, α2, and β) to five stimulant-induced behaviors relevant to addiction: locomotor activity, conditioned place preference, anxiety, discrimination, and self-administration. AR manipulation has diverse effects on these behaviors; each subtype profoundly influences outcomes in some paradigms but is inconsequential in others. The functional neuroanatomy and intracellular signaling mechanisms underlying the impact of AR activation/blockade on these behaviors remain largely unknown, presenting a new frontier for research on psychostimulant-AR interactions.

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

A preclinical evaluation of the discriminative and reinforcing properties of lisdexamfetamine in comparison to D-amfetamine, methylphenidate and modafinil

Lisdexamfetamine dimesylate, which consists of L-lysine covalently bound to D-amfetamine, is the first prodrug for treating ADHD. Its metabolic conversion to yield D-amfetamine by rate-limited, enzymatic hydrolysis is unusual because it is performed by peptidases associated with red blood cells. Other stimulants shown to be effective in managing ADHD include D-amfetamine, methylphenidate and modafinil. All have the potential for misuse or recreational abuse. The discriminative and reinforcing effects of these compounds were determined in rats using a 2-choice, D-amfetamine (0.5 mg/kg, i.p.)-cued drug-discrimination test, and by substitution for intravenous cocaine in self-administration. Lisdexamfetamine (0.5-1.5 mg/kg [D-amfetamine base], p.o.) generalised to saline when tested 15 min post-dosing, but dose-dependently generalised to d-amfetamine at 60 min. At 120 min, its D-amfetamine-like effects were substantially diminished. At 15 min, methylphenidate (3.0-10 mg/kg, p.o.) and D-amfetamine (0.1-1.5 mg/kg, p.o.) dose-dependently generalised to the intraperitoneal D-amfetamine cue. Switching to the intraperitoneal route reduced the interval required for lisdexamfetamine to be recognised as D-amfetamine-like, but did not alter its potency. Switching to intraperitoneal injection increased the potency of methylphenidate and D-amfetamine by 3.4× and 2.2×, respectively. Modafinil (50-200 mg/kg, i.p.) generalised partially, but not fully, to d-amfetamine. Methylphenidate (0.1, 0.3, 1.0 mg/kg/injection, i.v.) maintained robust self-administration at the 2 highest doses. Neither lisdexamfetamine (0.05, 0.15 or 0.5 mg/kg/injection [D-amfetamine base], i.v.) nor modafinil (0.166, 0.498 or 1.66 mg/kg/injection, i.v.) served as reinforcers. The results reveal important differences between the profiles of these stimulants. Lisdexamfetamine did not serve as a positive reinforcer in cocaine-trained rats, and although it generalised fully to D-amfetamine, its discriminative effects were markedly influenced by its unusual pharmacokinetics.

Effects of post-training heroin and d-amphetamine on consolidation of win-stay learning and fear conditioning

It has been proposed that the reinforcing properties of drugs of abuse are due, in part, to their ability to enhance memory consolidation. To test this hypothesis, heroin (0.03-3 mg/kg, SC) and d-amphetamine (0.5-2 mg/kg, SC) were administered to male Sprague-Dawley rats immediately or 4 h after training on win-stay and fear conditioning tasks. On the win-stay, immediate post-training administration of lower doses of heroin and d-amphetamine enhanced acquisition, and probe tests further revealed that these drugs enhanced different aspects of learning. Higher doses had no effect or impaired performance, particularly when administered repeatedly. On fear conditioning, the memory-enhancing effects of immediate post-training administration of lower heroin and d-amphetamine doses were revealed only when a single tone-shock pairing procedure was employed. Therefore, under appropriate experimental conditions, mildly stimulatory doses of heroin and d-amphetamine enhanced the acquisition of tasks thought to involve different types of learning. These results support the hypothesis that one of the ways in which drugs of abuse such as opiates and psychomotor stimulants reinforce behavior is by enhancing memory consolidation processes.

Reward circuitry dysfunction in psychiatric and neurodevelopmental disorders and genetic syndromes: animal models and clinical findings

This review summarizes evidence of dysregulated reward circuitry function in a range of neurodevelopmental and psychiatric disorders and genetic syndromes. First, the contribution of identifying a core mechanistic process across disparate disorders to disease classification is discussed, followed by a review of the neurobiology of reward circuitry. We next consider preclinical animal models and clinical evidence of reward-pathway dysfunction in a range of disorders, including psychiatric disorders (i.e., substance-use disorders, affective disorders, eating disorders, and obsessive compulsive disorders), neurodevelopmental disorders (i.e., schizophrenia, attention-deficit/hyperactivity disorder, autism spectrum disorders, Tourette's syndrome, conduct disorder/oppositional defiant disorder), and genetic syndromes (i.e., Fragile X syndrome, Prader-Willi syndrome, Williams syndrome, Angelman syndrome, and Rett syndrome). We also provide brief overviews of effective psychopharmacologic agents that have an effect on the dopamine system in these disorders. This review concludes with methodological considerations for future research designed to more clearly probe reward-circuitry dysfunction, with the ultimate goal of improved intervention strategies.

Striatal signal transduction and drug addiction

Drug addiction is a severe neuropsychiatric disorder characterized by loss of control over motivated behavior. The need for effective treatments mandates a greater understanding of the causes and identification of new therapeutic targets for drug development. Drugs of abuse subjugate normal reward-related behavior to uncontrollable drug-seeking and -taking. Contributions of brain reward circuitry are being mapped with increasing precision. The role of synaptic plasticity in addiction and underlying molecular mechanisms contributing to the formation of the addicted state are being delineated. Thus we may now consider the role of striatal signal transduction in addiction from a more integrative neurobiological perspective. Drugs of abuse alter dopaminergic and glutamatergic neurotransmission in medium spiny neurons of the striatum. Dopamine receptors important for reward serve as principle targets of drugs abuse, which interact with glutamate receptor signaling critical for reward learning. Complex networks of intracellular signal transduction mechanisms underlying these receptors are strongly stimulated by addictive drugs. Through these mechanisms, repeated drug exposure alters functional and structural neuroplasticity, resulting in transition to the addicted biological state and behavioral outcomes that typify addiction. Ca²⁺ and cAMP represent key second messengers that initiate signaling cascades, which regulate synaptic strength and neuronal excitability. Protein phosphorylation and dephosphorylation are fundamental mechanisms underlying synaptic plasticity that are dysregulated by drugs of abuse. Increased understanding of the regulatory mechanisms by which protein kinases and phosphatases exert their effects during normal reward learning and the addiction process may lead to novel targets and pharmacotherapeutics with increased efficacy in promoting abstinence and decreased side effects, such as interference with natural reward, for drug addiction.

How addictive drugs disrupt presynaptic dopamine neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging.

Using the self-administration of apomorphine and cocaine to measure the pharmacodynamic potencies and pharmacokinetics of competitive dopamine receptor antagonists

Competitive dopamine receptor antagonists accelerate psychomotor stimulant self-administration. According to pharmacological theory of competitive antagonism antagonists raise the equiactive agonist concentration. In the self-administration paradigm this is assumed to be the satiety threshold or C(min). The magnitude of the proportional increase in satiety threshold (agonist concentration ratio) as a function of antagonist dose should reflect the antagonist pharmacodynamic potency. The time course of this effect should reflect the rate of change of antagonist occupancy of receptors and, therefore, antagonist concentration, i.e. pharmacokinetics. Rats self-administered apomorphine or cocaine at a stable rate and were then injected i.v. with one of four competitive D₁-like or D₂-like dopamine receptor antagonists and the session continued. The agonist concentrations at the time of each self-administration (satiety thresholds) were calculated during the session. The antagonists accelerated self-administration of both agonists with a concomitant increase in the calculated satiety thresholds. The maximum agonist concentration ratio was proportional to the dose of antagonist. The time courses of the changes in agonist concentration ratio were independent of the agonist and of the dose of antagonist. Schild analysis of the maximum agonist concentration ratio as a function of the antagonist dose allowed apparent pA₂ (or K(dose)) to be measured. Antagonist K(dose) values should provide a quantitative basis for receptor identification in behavioral pharmacology. The assay system may also measure the pharmacokinetics of antagonist elimination from the brain. Agonist self-administration represents a sensitive in vivo pharmacological assay system that provides information useful for pharmacokinetic/pharmacodynamic modeling of antagonist effects.

Conditioned place preference and locomotor activity in response to methylphenidate, amphetamine and cocaine in mice lacking dopamine D4 receptors

Methylphenidate (MP) and amphetamine (AMPH) are the most frequently prescribed medications for the treatment of attention-deficit/hyperactivity disorder (ADHD). Both drugs are believed to derive their therapeutic benefit by virtue of their dopamine (DA)-enhancing effects, yet an explanation for the observation that some patients with ADHD respond well to one medication but not to the other remains elusive. The dopaminergic effects of MP and AMPH are also thought to underlie their reinforcing properties and ultimately their abuse. Polymorphisms in the human gene that codes for the DA D4 receptor (D4R) have been repeatedly associated with ADHD and may correlate with the therapeutic as well as the reinforcing effects of responses to these psychostimulant medications. Conditioned place preference (CPP) for MP, AMPH and cocaine were evaluated in wild-type (WT) mice and their genetically engineered littermates, congenic on the C57Bl/6J background, that completely lack D4Rs (knockout or KO). In addition, the locomotor activity in these mice during the conditioning phase of CPP was tested in the CPP chambers. D4 receptor KO and WT mice showed CPP and increased locomotor activity in response to each of the three psychostimulants tested. D4R differentially modulates the CPP responses to MP, AMPH and cocaine. While the D4R genotype affected CPP responses to MP (high dose only) and AMPH (low dose only) it had no effects on cocaine. Inasmuch as CPP is considered an indicator of sensitivity to reinforcing responses to drugs these data suggest a significant but limited role of D4Rs in modulating conditioning responses to MP and AMPH. In the locomotor test, D4 receptor KO mice displayed attenuated increases in AMPH-induced locomotor activity whereas responses to cocaine and MP did not differ. These results suggest distinct mechanisms for D4 receptor modulation of the reinforcing (perhaps via attenuating dopaminergic signalling) and locomotor properties of these stimulant drugs. Thus, individuals with D4 receptor polymorphisms might show enhanced reinforcing responses to MP and AMPH and attenuated locomotor response to AMPH.

Neuroleptics and operant behavior: The anhedonia hypothesis

Neuroleptic drugs disrupt the learning and performance of operant habits motivated by a variety of positive reinforcers, including food, water, brain stimulation, intravenous opiates, stimulants, and barbiturates. This disruption has been demonstrated in several kinds of experiments with doses that do not significantly limit normal response capacity. With continuous reinforcement neuroleptics gradually cause responding to cease, as in extinction or satiation. This pattern is not due to satiation, however, because it also occurs with nonsatiating reinforcement (such as saccharin or brain stimulation). Repeated tests with neuroleptics result in earlier and earlier response cessation reminiscent of the kind of decreased resistance to extinction caused by repeated tests without the expected reward. Indeed, withholding reward can have the same effect on responding under later neuroleptic treatment as prior experience with neuroleptics themselves; this suggests that there is a transfer of learning (really unlearning) from nonreward to neuroleptic conditions. These tests under continuous reinforcement schedules suggest that neuroleptics blunt the ability of reinforcers to sustain responding at doses which largely spare the ability of the animal to initiate responding. Animals trained under partial reinforcement, however, do not respond as well during neuroleptic testing as animals trained under continuous reinforcement. Thus, neuroleptics can also impair responding (though not response capacity) that is normally sustained by environmental stimuli (and associated expectancies) in the absence of the primary reinforcer. Neuroleptics also blunt the euphoric impact of amphetamine in humans. These data suggest that the most subtle and interesting effect of neuroleptics is a selective attenuation of motivational arousal which is (a) critical for goal-directed behavior, (b) normally induced by reinforcers and associated environmental stimuli, and (c) normally accompanied by the subjective experience of pleasure. Because these drugs are used to treat schizophrenia and because they cause parkinsonian-like side effects, this action has implications for a better understanding of human pathology as well as normal motivational processes.

Dopamine signaling in the nucleus accumbens of animals self-administering drugs of abuse

Abuse of psychoactive substances can lead to drug addiction. In animals, addiction is best modeled by drug self-administration paradigms. It has been proposed that the crucial common denominator for the development of drug addiction is the ability of drugs of abuse to increase extracellular concentrations of dopamine in the nucleus accumbens (NAcc). Studies using in vivo microdialysis and chronoamperometry in the behaving animal have demonstrated that drugs of abuse increase tonic dopamine concentrations in the NAcc. However, it is known that dopamine neurons respond to reward-related stimuli on a subsecond timescale. Thus, it is necessary to collect neurochemical information with this level of temporal resolution, as achieved with in vivo fast-scan cyclic voltammetry (FSCV), to fully understand the role of phasic dopamine release in normal behavior and drug addiction. We review studies that investigated the effects of drugs of abuse on NAcc dopamine levels in freely moving animals using in vivo microdialysis, chronoamperometry, and FSCV. After a brief introduction of dopamine signal transduction and anatomy and a section on current theories on the role of dopamine in natural goal-directed behavior, a discussion of techniques for the in vivo assessment of extracellular dopamine in behaving animals is presented. Then, we review studies using these techniques to investigate changes in phasic and tonic dopamine signaling in the NAcc during (1) response-dependent and -independent administration of abused drugs, (2) the presentation of drug-conditioned stimuli and operant behavior in self-administration paradigms, (3) drug withdrawal, and (4) cue-induced reinstatement of drug seeking. These results are then integrated with current ideas on the role of dopamine in addiction with an emphasis on a model illustrating phasic and tonic NAcc dopamine signaling during different stages of drug addiction. This model predicts that phasic dopamine release in response to drug-related stimuli will be enhanced over stimuli associated with natural reinforcers, which may result in aberrant goal-directed behaviors contributing to drug addiction.

Dopamine receptors in the learning, memory and drug reward circuitry

As primary targets of a variety of abused drugs G-protein-coupled dopamine receptors in the brain play an important role in mediating the various drug-induced alterations in neural and psychological processes thought to underlie the transition from voluntary drug use to habitual and progressively compulsive drug-taking. This review considers the functional involvement of the five major dopamine receptor subtypes in drug reinforcement and reward and discusses the development of addiction as a series of learning transitions from initial goal-directed behaviour to pathological stimulus-response habits in which drug-seeking behaviours are automatically elicited and maintained by cues and stimuli associated with drug rewards.

Norepinephrine and stimulant addiction

No pharmacotherapies are approved for stimulant use disorders, which are an important public health problem. Stimulants increase synaptic levels of the monoamines dopamine (DA), serotonin and norepinephrine (NE). Stimulant reward is attributable mostly to increased DA in the reward circuitry, although DA stimulation alone cannot explain the rewarding effects of stimulants. The noradrenergic system, which uses NE as the main chemical messenger, serves multiple brain functions including arousal, attention, mood, learning, memory and stress response. In pre-clinical models of addiction, NE is critically involved in mediating stimulant effects including sensitization, drug discrimination and reinstatement of drug seeking. In clinical studies, adrenergic blockers have shown promise as treatments for cocaine abuse and dependence, especially in patients experiencing severe withdrawal symptoms. Disulfiram, which blocks NE synthesis, increased the number of cocaine-negative urines in five randomized clinical trials. Lofexidine, an alpha(2)-adrenergic agonist, reduces the craving induced by stress and drug cues in drug users. In addition, the NE transporter (NET) inhibitor atomoxetine attenuates some of d-amphetamine's subjective and physiological effects in humans. These findings warrant further studies evaluating noradrenergic medications as treatments for stimulant addiction.

Alpha 1-noradrenergic system role in increased motivation for cocaine intake in rats with prolonged access

In rodents, extended access to cocaine produces an escalation in cocaine self-administration that has face and construct validity for human compulsive drug intake. Here we report that rats with six-hour access (long access, LgA) to cocaine self-administration produced a higher breakpoint for cocaine using a progressive-ratio schedule than rats with one-hour access (short access, ShA), and prazosin (alpha 1 receptor antagonist) reduced the higher breakpoint for cocaine in LgA rats. Additionally, the number of neurons with alpha 1-adrenergic receptor-like immunoreactivity in the bed nucleus of stria terminalis (BNST) was found to be much lower in LgA rats than in ShA and drug-naive rats. In contrast, UK14304 (alpha 2 receptor agonist) and betaxolol (beta 1 receptor antagonist) had no effect on cocaine self-administration in either group. The data suggest that activation of the alpha 1-noradrenergic system, perhaps in the BNST, is associated with increased motivation for cocaine in rats with extended access.

Determinants of cocaine self-administration by laboratory animals

The reinforcing effect of a drug is that effect that increases the probability that the drug will be self-administered again. Like other drug effects, a reinforcing effect is the result of an interaction between organism, drug and environment. Laboratory research using animal subjects has helped elucidate the contribution of each of these factors to the self-administration of cocaine. A substantial amount of research indicates that increased dopamine neurotransmission in the brain, particularly in mesolimbic and mesocortical regions, plays a major role in cocaine self-administration. Both indirect and direct dopamine agonists can function as positive reinforcers in animals, whereas noradrenergic and serotonergic (5-HT, 5-hydroxytryptamine) agonists have not been found to do so. In addition, evidence suggests that dopamine but not noradrenaline (norepinephrine) or serotonin antagonists can attenuate the reinforcing effect of cocaine. Environmental factors have also been shown to be critical determinants of the reinforcing effect of cocaine. The schedule of reinforcement essentially determines the rate and pattern of drug-maintained behaviour. In addition, punishing self-administration, increasing the value of alternative reinforcers that are available, and increasing the cost of cocaine have all been shown to decrease the reinforcing effect of cocaine. With regard to organismic factors, recent research has suggested that there are significant genetic determinants of cocaine consumption. Taken together these research findings in animals imply that certain individuals may be more sensitive to the reinforcing effect of cocaine but that cocaine abuse can be decreased by pharmacological or behavioural means or by a combination of the two.

Stereotyped and complex motor routines expressed during cocaine self-administration: results from a 24-h binge of unlimited cocaine access in rats

RATIONALE

Cocaine binges represent the most severe form of cocaine taking due to high levels of cocaine consumed and a potential loss of self-control over cocaine taking. Experimentally, regulation of intravenous cocaine self-administration is observed during binges as revealed by nearly constant sequential inter-infusion intervals, suggesting that pharmacological and unconditioned behavioral effects of cumulative cocaine leave intact its reinforcing effect.

OBJECTIVE

To elucidate factors that contribute to the patterning of cocaine self-administration behavior during binges, the current study quantifies the expression of specific motor routines that emerge from the beginning of a cocaine infusion until the next cocaine reinforcement is received, in six separate rats over 24 h of continuous access.

MATERIALS AND METHODS

During each 24-h cocaine binge, behavior was continuously monitored using a force plate actometer that provides reliable quantitation of rodent behaviors, including rotational behavior, locomotor activity, and focused stereotypy.

RESULTS

Corroborating earlier results, each rat accumulated between 4 and 11 mg/kg/h during periods of active cocaine self-administration. All rats displayed a similar narrow range of unconditioned behavioral responses to cocaine, yet each rat varied with regard to the intensity and specific predominant pattern of behavioral activation. Focused stereotyped behavior, in particular, was apparent in all rats and continued for as long as cocaine self-administration behavior remained active.

CONCLUSIONS

The current results support the hypothesis that individual differences in cocaine's pharmacodynamics may contribute to specific behaviors expressed during cocaine self-administration, but leave unresolved whether or not intense unconditioned behavior (e.g., focused stereotypy) conflicts with, or contributes to, ongoing self-administration behavior.

Relationships between locomotor activation and alterations in brain temperature during selective blockade and stimulation of dopamine transmission

It is well known that the dopamine (DA) system plays an essential role in the organization and regulation of brain activational processes. Various environmental stimuli that induce locomotor activation also increase DA transmission, while DA antagonists decrease spontaneous locomotion. Our previous work supports close relationships between locomotor activation and brain and body temperature increases induced by salient environmental challenges or occurring during motivated behavior. While this correlation was also true for psychomotor stimulant drugs such as methamphetamine and MDMA, more complex relationships or even inverted correlations were found for other drugs that are known to increase DA transmission (i.e. heroin and cocaine). In the present study we examined brain, muscle and skin temperatures together with conventional locomotion during selective interruption of DA transmission induced by a mixture of D1 and D2 antagonists (SCH-23390 and eticlopride at 0.2 mg/kg, s.c.) and its selective activation by apomorphine (APO; 0.05 and 0.25 mg/kg, i.v.) in rats. While full DA receptor blockade decreased spontaneous locomotion, it significantly increased brain, muscle and skin temperatures, suggesting metabolic brain activation under conditions of vasodilatation (or weakening of normal vascular tone). In contrast, APO strongly decreased skin temperature but tended to decrease brain and muscle temperatures despite strong hyperlocomotion and stereotypy. The brain temperature response to APO was strongly dependent on basal brain temperature, with hypothermia at high basal temperatures and weak hyperthermia at low temperatures. While supporting the role of DA in locomotor activation, these data suggest more complex relationships between drug-induced alterations in DA transmission, behavioral activation and metabolic brain activation.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

Self-administration of mixtures of fenfluramine and amphetamine by rhesus monkeys

Previous research with psychostimulants has suggested a negative relationship between both potency and efficacy as a reinforcer and serotonergic potency, particularly relative to dopaminergic potency. The present experiment was designed to examine the relationship between the serotonergic activity and efficacy as a reinforcer by allowing rhesus monkeys (n=5) to self-administer amphetamine mixed with a serotonin releaser, fenfluramine. Additionally, the role of 5-HT2 receptors in the interaction between amphetamine and fenfluramine was investigated using ketanserin, a selective 5-HT2 receptor antagonist. Amphetamine and fenfluramine were combined in ratios of, respectively, 1:1 to 1:10 on a mg/kg basis and made available for self-administration under a progressive-ratio schedule of reinforcement. Amphetamine (0.0056-0.1 mg/kg/injection) functioned as a positive reinforcer with sigmoidal or biphasic dose-response functions. The addition of fenfluramine to amphetamine decreased the maximum responding, at least at the highest dose ratio (1:10, amphetamine/fenfluramine), in all monkeys. When measured after the pretreatment of ketanserin (1.0-3.0 mg/kg, i.m.), the self-administration of the mixture of amphetamine and fenfluramine at a ratio of 1:10 decreased in three monkeys and was unaffected in the fourth. These results support the notion of a negative influence of increased serotonergic neurotransmission on reinforcing efficacy of drugs that act via monoamine systems. However, the involvement of 5-HT2 receptors in the interaction between the serotonergic system and the reinforcing efficacy still remains equivocal.

Selective dopamine D3 receptor antagonism by SB-277011A attenuates cocaine reinforcement as assessed by progressive-ratio and variable-cost-variable-payoff fixed-ratio cocaine self-administration in rats

In rats, acute administration of SB-277011A, a highly selective dopamine (DA) D(3) receptor antagonist, blocks cocaine-enhanced brain stimulation reward, cocaine-seeking behaviour and reinstatement of cocaine-seeking behaviour. Here, we investigated whether SB-277011A attenuates cocaine reinforcement as assessed by cocaine self-administration under variable-cost-variable-payoff fixed-ratio (FR) and progressive-ratio (PR) reinforcement schedules. Acute i.p. administration of SB-277011A (3-24 mg/kg) did not significantly alter cocaine (0.75 mg/kg/infusion) self-administration reinforced under FR1 (one lever press for one cocaine infusion) conditions. However, acute administration of SB-277011A (24 mg/kg, i.p.) progressively attenuated cocaine self-administration when: (a) the unit dose of self-administered cocaine was lowered from 0.75 to 0.125-0.5 mg/kg, and (b) the work demand for cocaine reinforcement was increased from FR1 to FR10. Under PR (increasing number of lever presses for each successive cocaine infusion) cocaine reinforcement, acute administration of SB-277011A (6-24 mg/kg i.p.) lowered the PR break point for cocaine self-administration in a dose-dependent manner. The reduction in the cocaine (0.25-1.0 mg/kg) dose-response break-point curve produced by 24 mg/kg SB-277011A is consistent with a reduction in cocaine's reinforcing efficacy. When substituted for cocaine, SB-277011A alone did not sustain self-administration behaviour. In contrast with the mixed DA D(2)/D(3) receptor antagonist haloperidol (1 mg/kg), SB-277011A (3, 12 or 24 mg/kg) failed to impede locomotor activity, failed to impair rearing behaviour, failed to produce catalepsy and failed to impair rotarod performance. These results show that SB-277011A significantly inhibits acute cocaine-induced reinforcement except at high cocaine doses and low work requirement for cocaine. If these results extrapolate to humans, SB-277011A or similar selective DA D(3) receptor antagonists may be useful in the treatment of cocaine addiction.

Methamphetamine discrimination and in vivo microdialysis in squirrel monkeys

RATIONALE

Characterization of changes in dopamine activity associated with the discriminative-stimulus effects of methamphetamine (MA) and related stimulants will aid our understanding of the role of dopamine in mediating the subjective effects of this drug class.

OBJECTIVES

Squirrel monkeys were studied to explore the relationship between discriminative-stimulus effects of psychomotor stimulant drugs and their ability to increase extracellular dopamine levels in the caudate nucleus.

METHODS

The ability of MA, cocaine and methylphenidate (0.01-0.32 mg/kg) to produce MA-like discriminative-stimulus effects was assessed in monkeys trained to discriminate i.m. injections of 0.32 mg/kg MA from saline. In addition, the effects of a range of MA doses (0.01-0.32 mg/kg) and selected doses of cocaine, methylphenidate and the GBR 12909 analog AM2517 on extracellular dopamine in the caudate nucleus were determined by microdialysis.

RESULTS

MA, cocaine and methylphenidate produced dose-related increases in responding on the MA-associated lever and fully substituted at higher doses. In microdialysis studies, doses of MA, cocaine, methylphenidate, and AM2517 that produced 100% MA-lever responding produced comparable increases in caudate dopamine (to approximately 250% of control values). However, comparable increases in extracellular dopamine also were observed following a lower dose of MA (0.1 mg/kg) that produced, on average, 42% MA-lever responding. Moreover, increases in dopamine levels following administration of 0.03 and 0.1 mg/kg MA persisted after responding on the MA-associated lever had subsided.

CONCLUSIONS

Taken together, these results support a prominent role for dopamine in MA-like discriminative-stimulus effects, but are consistent with the additional involvement of other neurochemical actions.

Amphetamine primes motivation to gamble and gambling-related semantic networks in problem gamblers

Previous research suggests that gambling can induce effects that closely resemble a psychostimulant drug effect. Modest doses of addictive drugs can prime motivation for drugs with similar properties. Together, these findings imply that a dose of a psychostimulant drug could prime motivation to gamble in problem gamblers. This study assessed priming effects of oral D-amphetamine (AMPH) (30 mg) in a within-subject, counter-balanced, placebo-controlled design in problem gamblers (n=10), comorbid gamblerdrinkers (n=6), problem drinkers (n=8), and healthy controls (n=12). Modified visual analog scales assessed addictive motivation and subjective effects. A modified rapid reading task assessed pharmacological activation of words from motivationally relevant and irrelevant semantic domains (Gambling, Alcohol, Positive Affect, Negative Affect, Neutral). AMPH increased self-reported motivation for gambling in problem gamblers. Severity of problem gambling predicted positive subjective effects of AMPH and motivation to gamble under the drug. There was little evidence that AMPH directly primed motivation for alcohol in problem drinkers. On the reading task, AMPH produced undifferentiated improvement in reading speed to all word classes in Nongamblers. By contrast, in the two problem gambler groups, AMPH improved reading speed to Gambling words while profoundly slowing reading speed to motivationally irrelevant Neutral words. The latter finding was interpreted as directly congruent with models, which contend that priming of addictive motivation involves a linked suppression of motivationally irrelevant stimuli. This study provides experimental evidence that psychostimulant-like neurochemical activation is an important component of gambling addiction.

Treatment with the atypical antipsychotic, olanzapine, prevents the expression of amphetamine-induced place conditioning in the rat

Place conditioning (PC) experiments were conducted as a means to further elaborate the treatment potential of the atypical antipsychotic, olanzapine (OLZ), for stimulant abuse. The resulting preference/aversion provides an indirect measure of the incentive salience (i.e., euphoria/dysphoria) produced by a drug. Male Sprague-Dawley rats (n=48) were conditioned in two unique environments (i.e., vertical vs. horizontal stripped walls, large vs. small grid flooring) using injections (1.0 mg/kg ip) of either amphetamine (AMPH) or saline (SAL). On average, animals displayed a significant preference for the AMPH-paired location after 2.5 weeks of conditioning (five pairings each of AMPH and SAL). Once the preference was established, animals were pretreated (60 min) with a single dose of OLZ (0.0, 0.56, 1.0 or 1.5 mg/kg sc) given on the test (AMPH-free) day. For the following week's test, animals were injected with SAL (1.0 mg/kg ip) in an attempt to recapture the side preference exhibited before OLZ treatment. OLZ treatment prevented the expression of the AMPH-conditioned preference and reduced locomotor activity. Inhibition of preference resulted from the highest dose of OLZ (1.5 mg/kg), while the inhibition of locomotor activity occurred across all three doses. Additionally, while the effects on preference were no longer apparent by the SAL test the following week (reversible), the activity was still depressed during the SAL tests in animals that had experienced the highest dose of OLZ (1.5 mg/kg). Control experiments, in which OLZ was used as the conditioning drug, suggest that OLZ itself possesses no aversive effects in the PC paradigm, and may even produce a preference for the drug-paired chamber. Because the AMPH preference is dependent on dopamine (DA) release in the nucleus accumbens (NAcc), these experiments suggest that OLZ pretreatment interferes with the rewarding, as well as the subjective effects of AMPH.

Comparison of the reinforcing efficacy of two dopamine D2-like receptor agonists in rhesus monkeys using a progressive-ratio schedule of reinforcement

High-efficacy D2-like dopamine (DA) receptor agonists can function as positive reinforcers when made available to animals for intravenous self-administration under a fixed-ratio (FR) schedule of reinforcement. In a previous study, however, low-efficacy D2-like agonists failed to maintain self-administration under an FR schedule, suggesting that agonist efficacy is directly related to efficacy as a positive reinforcer. To examine this hypothesis further, the present study compared two D2-like DA receptor agonists that maintained FR responding, but differ in their D2-like receptor efficacy and selectivity, using a procedure designed to rank-order drugs according to their efficacy as reinforcers. Rhesus monkeys (n=5) were prepared with chronic, indwelling intravenous catheters and allowed to self-administer cocaine (0.1 mg/kg/injection) or saline on different days under a progressive-ratio (PR) schedule. When responding was stable, doses of the full D2-like agonist R(-)-propylnorapomorphine (NPA) or the partial D2-like agonist R(-)-apomorphine (APO) were made available for self-administration in the test sessions. Both compounds maintained self-administration with sigmoidal or biphasic dose-response functions. Surprisingly, the lower efficacy agonist APO was the more efficacious positive reinforcer. This result fails to support the hypothesis that D2-like receptor efficacy is directly related to efficacy as a reinforcer. It is possible that other pharmacological effects, e.g., D1 receptor activity, influenced self-administration.