Evidence for involvement of both D1 and D2 receptors in maintaining cocaine self-administration

Aversion-induced drug taking and escape behavior involve similar nucleus accumbens core dopamine signaling signatures

Dopamine release in the nucleus accumbens core (NAcC) has long been associated with the promotion of motivated behavior. However, inhibited dopamine signaling can increase behavior in certain settings, such as during drug self-administration. While aversive environmental stimuli can reduce dopamine, it is unclear whether such stimuli reliably engage this mechanism in different contexts. Here we compared the physiological and behavioral responses to the same aversive stimulus in different designs to determine if there is uniformity in the manner that aversive stimuli are encoded and promote behavior. NAcC dopamine was measured using fiber photometry in male and female rats during cocaine self-administration sessions in which an acutely aversive 90 dB white noise was intermittently presented. In a separate group of rats, aversion-induced changes in dopamine were measured in an escape design in which operant responses terminated aversive white noise. Aversive white noise significantly reduced NAcC dopamine and increased cocaine self-administration in both male and female rats. The same relationship was observed in the escape design, in which white noise reduced dopamine and promoted escape attempts. In both designs, the magnitude of the dopamine reduction predicted behavioral performance. While prior research demonstrated that pharmacologically reduced dopamine signaling can promote intake, this report demonstrates that this physiological mechanism is naturally engaged by aversive environmental stimuli and generalizable to non-drug contexts. These findings illustrate a common physiological signature in response to aversion that may promote both adaptive and maladaptive behavior.

Chemogenetic Inhibition of Dopamine D1-expressing Neurons in the Dorsal Striatum does not alter Methamphetamine Intake in either Male or Female Long Evans Rats

The biochemical and molecular substrates of methamphetamine (METH) use disorder remain to be elucidated. In rodents, increased METH intake is associated with increased expression of dopamine D1 receptors (D1R) in the dorsal striatum. The present study assessed potential effects of inhibiting striatal D1R activity on METH self-administration (SA) by rats. We microinjected Cre-activated adeno-associated viruses to deliver the inhibitory DREADD construct, hM4D (Gi) - mCherry, into neurons that expressed Cre-recombinase (D1-expressing neurons) in the dorsal striatum of male and female transgenic Long Evans rats (Drd1a-iCre#3). Two weeks later, we trained rats to self-administer METH. Once this behavior was acquired, intraperitoneal injections of clozapine-N-Oxide (CNO) or its vehicle (sterile water) were given to rats before each METH SA session to determine the effect of DREADD-mediated inhibition on METH intake. After the end of the experiments, histology was performed to confirm DREADD delivery into the dorsal striatum. There were no significant effects of the inhibitory DREADD on METH SA by male or female rats. Post-mortem histological assessment revealed DREADD expression in the dorsal striatum. Our results suggest that inhibition of D1R in the dorsal striatum does not suppress METH SA. It remains to be determined if activating D1R-expressing neurons might have differential behavioral effects. Future studies will also assess if impacting D1R activity in other brain regions might influence METH SA.

D1 receptor hypersensitivity in mice with low striatal D2 receptors facilitates select cocaine behaviors

Vulnerability for cocaine abuse in humans is associated with low dopamine D2 receptor (D2R) availability in the striatum. The mechanisms driving this vulnerability are poorly understood. In this study, we found that downregulating D2R expression selectively in striatal indirect-pathway neurons triggers a multitude of changes in D1 receptor (D1R)-expressing direct-pathway neurons, which comprise the other main subpopulation of striatal projection neurons. These changes include a leftward shift in the dose-response to a D1-like agonist that indicates a behavioral D1R hypersensitivity, a shift from PKA to ERK intracellular signaling cascades upon D1R activation, and a reduction in the density of bridging collaterals from D1R-expressing neurons to pallidal areas. We hypothesize that the D1R hypersensitivity underlies abuse vulnerability by facilitating the behavioral responses to repeated cocaine, such as locomotor sensitization and drug self-administration. We found evidence that littermate control mice develop D1R hypersensitivity after they are sensitized to cocaine. Indeed, D1-like agonist and cocaine cross-sensitize in control littermates and this effect was potentiated in mice lacking striatal D2Rs from indirect-pathway neurons. To our surprise, mice with low striatal D2Rs acquired cocaine self-administration similarly to littermate controls and showed no significant change in motivation to take cocaine but lower seeking. These findings indicate that downregulation of striatal D2Rs triggers D1R hypersensitivity to facilitate cocaine locomotor sensitization, which by itself was not associated with greater cocaine taking or seeking under the conditions tested.

Dopamine D1 and D3 receptor polypharmacology as a potential treatment approach for substance use disorder

In the search for efficacious pharmacotherapies to treat cocaine addiction much attention has been given to agents targeting dopamine D1 or D3 receptors because of the involvement of these receptors in drug-related behaviors. D1-like and D3 receptor partial agonists and antagonists have been shown to reduce drug reward, reinstatement of drug seeking and conditioned place preference in rodents and non-human primates. However, translation of these encouraging results to clinical settings has been limited due to a number of factors including toxicity, poor pharmacokinetic properties and extrapyramidal and sedative side effects. This review highlights the role of D1 and D3 receptors in drug reward and seeking, the discovery of D1-D3 heteromers and their potential as targets in the treatment of addiction.

Restructuring of basal ganglia circuitry and associated behaviors triggered by low striatal D2 receptor expression: implications for substance use disorders

Dopamine D2 receptors (D2Rs) consistently emerge as a critical substrate for the etiology of some major psychiatric disorders. Indeed, a central theory of substance use disorders (SUDs) postulates that a reduction in D2R levels in the striatum is a determining factor that confers vulnerability to abuse substances. A large number of clinical and preclinical studies strongly support this link between SUDs and D2Rs; however, identifying the mechanism by which low D2Rs facilitate SUDs has been hindered by the complexity of circuit connectivity, the heterogeneity of D2R expression and the multifaceted constellation of phenotypes observed in SUD patient. Animal models are well‐suited for understanding the mechanisms because they allow access to the circuitry and the genetic tools that enable a dissection of the D2R heterogeneity. This review discusses recent findings on the functional role of D2Rs and highlights the distinctive contributions of D2Rs expressed on specific neuronal subpopulations to the behavioral responses to stimulant drugs. A circuit‐wide restructuring of local and long‐range inhibitory connectivity within the basal ganglia is observed in response to manipulation of striatal D2R levels and is accompanied by multiple alterations in dopamine‐dependent behaviors. Collectively, these new findings provide compelling evidence for a critical role of striatal D2Rs in shaping basal ganglia connectivity; even among neurons that do not express D2Rs. These findings from animal models have deep clinical implications for SUD patients with low levels D2R availability where a similar restructuring of basal ganglia circuitry is expected to take place.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

Drug abuse and addiction cause widespread social and public health problems, and the neurobiology underlying drug actions and drug use and abuse is an area of intensive research. Drugs of abuse alter synaptic transmission, and these actions contribute to acute intoxication as well as the chronic effects of abused substances. Transmission at most mammalian synapses involves neurotransmitter activation of two receptor subtypes, ligand-gated ion channels that mediate fast synaptic responses and G protein-coupled receptors (GPCRs) that have slower neuromodulatory actions. The GPCRs represent a large proportion of neurotransmitter receptors involved in almost all facets of nervous system function. In addition, these receptors are targets for many pharmacotherapeutic agents. Drugs of abuse directly or indirectly affect neuromodulation mediated by GPCRs, with important consequences for intoxication, drug taking and responses to prolonged drug exposure, withdrawal and addiction. Among the GPCRs are several subtypes involved in presynaptic inhibition, most of which are coupled to the Gi/o class of G protein. There is increasing evidence that these presynaptic Gi/o-coupled GPCRs have important roles in the actions of drugs of abuse, as well as behaviors related to these drugs. This topic will be reviewed, with particular emphasis on receptors for three neurotransmitters, Dopamine (DA; D1- and D2-like receptors), Endocannabinoids (eCBs; CB1 receptors) and glutamate (group II metabotropic glutamate (mGlu) receptors). The focus is on recent evidence from laboratory animal models (and some evidence in humans) implicating these receptors in the acute and chronic effects of numerous abused drugs, as well as in the control of drug seeking and taking. The ability of drugs targeting these receptors to modify drug seeking behavior has raised the possibility of using compounds targeting these receptors for addiction pharmacotherapy. This topic is also discussed, with emphasis on development of mGlu2 positive allosteric modulators (PAMs).

Dopamine signaling in reward-related behaviors

Dopamine (DA) regulates emotional and motivational behavior through the mesolimbic dopaminergic pathway. Changes in DA mesolimbic neurotransmission have been found to modify behavioral responses to various environmental stimuli associated with reward behaviors. Psychostimulants, drugs of abuse, and natural reward such as food can cause substantial synaptic modifications to the mesolimbic DA system. Recent studies using optogenetics and DREADDs, together with neuron-specific or circuit-specific genetic manipulations have improved our understanding of DA signaling in the reward circuit, and provided a means to identify the neural substrates of complex behaviors such as drug addiction and eating disorders. This review focuses on the role of the DA system in drug addiction and food motivation, with an overview of the role of D1 and D2 receptors in the control of reward-associated behaviors.

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

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

Competitive dopamine receptor antagonists increase the equiactive cocaine concentration during self-administration

Competitive dopamine receptor antagonists increase the rate of cocaine self-administration. As the rate of self-administration at a particular unit dose is determined by the satiety threshold and the elimination half-life (t(½)) of cocaine, we investigated whether dopamine receptor antagonists altered these parameters in rats. The plasma cocaine concentration at the time of each self-administration was constant during a session demonstrating that this satiety threshold concentration represents an equiactive cocaine concentration. The plasma cocaine concentration at the time of self-administration was increased by SCH23390, consistent with pharmacological theory. In rats trained to reliably self-administer cocaine, SCH23390 had no effect on the plasma steady-state cocaine concentration produced by constant infusions of cocaine. Therefore, this antagonist had no effect on cocaine t(½) at a dose that accelerated cocaine self-administration. A constant cocaine infusion at a rate that maintained steady state concentrations above the satiety threshold stopped self-administration. SCH23390, or the D₂ dopamine receptor antagonist (-)eticlopride, reinstated self-administration in the presence of the constant cocaine infusion. This is consistent with SCH23390 and eticlopride raising the satiety threshold above the steady state level produced by the constant cocaine infusion. It is concluded that the antagonist-induced acceleration of cocaine self-administration is the result of a pharmacokinetic/pharmacodynamic interaction whereby the rate of cocaine elimination is faster at the higher concentrations, as dictated by first-order kinetics, so that cocaine levels decline more rapidly to the elevated satiety threshold. This results in the decreased interinjection intervals.

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.

Distinct contributions of dopamine receptors in the nucleus accumbens core or shell to established cocaine reinforcement under a second-order schedule

Nucleus accumbens dopamine is implicated in the primary and conditioned reinforcing properties of abused drugs. In the present study, specific impairments in responding for intravenous cocaine (0.3 mg/inf/0.1 ml/5 s) under a fixed-ratio 1 (FR-1) or second-order schedule (FI 15 min (FR10:S)) were investigated following infusion of the dopamine antagonist, alpha-flupenthixol, into either the nucleus accumbens core or shell. Infusion of alpha-flupenthixol into the core decreased cocaine intake under the FR-1 and second-order schedules. By comparison, blockade of nucleus accumbens shell dopamine receptors increased cocaine intake under the FR-1 schedule. Under the second-order schedule, cocaine intake and the number of responses was decreased. Effects on responding were more apparent after self-administered cocaine, when impairments in the latency to receive cocaine infusions were no longer evident. These results are discussed with reference to a role for nucleus accumbens shell dopamine in instrumental responding, and a role of nucleus accumbens core dopamine in incentive motivation, perhaps under the control of contextual stimuli.

A D2 antagonist enhances the rewarding and priming effects of a gambling episode in pathological gamblers

Previous research indicated shared neurochemical substrates for gambling and psychostimulant reward. This suggests that dopamine substrates may directly govern the reinforcement process in pathological gambling. To investigate this issue, the present study assessed the effects of the relatively selective dopamine D2 antagonist, haloperidol (3 mg, oral) on responses to actual gambling (15 min on a slot machine) in 20 non-comorbid pathological gamblers and 18 non-gambler controls in a placebo-controlled, double-blind, counterbalanced design. In gamblers, haloperidol significantly increased self-reported rewarding effects of gambling, post-game priming of desire to gamble, facilitation of reading speed to Gambling words, and gambling-induced elevation in blood pressure. In controls, haloperidol augmented gambling-induced elevation in blood pressure, but had no effect on other indices. The findings provide direct experimental evidence that the D2 substrate modulates gambling reinforcement in pathological gamblers.

Evidence for the involvement of ERbeta and RGS9-2 in 17-beta estradiol enhancement of amphetamine-induced place preference behavior

Estrogen enhances dopamine-mediated behaviors, which make women and female rats more sensitive to the effects of the psychostimulant drugs, cocaine and amphetamine. How cocaine and amphetamine elicit more robust behavioral responses in females remains unclear, but studies have shown that the Regulator of G-protein Signaling 9-2 (RGS9-2) protein is an important modulator of the behavioral responses to these drugs. Previously, we reported that 17-beta estradiol reduced RGS9-2 mRNA expression in the shell of the nucleus accumbens, but not the core. The present studies were designed to further evaluate the involvement of RGS9-2 in estradiol enhancement of amphetamine-induced place preference behavior and to examine which estrogen receptor subtype mediates the effect of estradiol. Female Sprague-Dawley rats were ovariectomized and treated for 14 days with an inert vehicle or 17-beta estradiol (by Silastic implant or injection [80 microg/kg]). 17-beta-Estradiol-treated female rats had enhanced amphetamine-induced conditioned place preference behavior compared to vehicle-treated, ovariectomized female rats. In situ hybridization histochemistry and Western blotting identified an inverse relationship between RGS9-2 protein expression in the nucleus accumbens shell and the hormonal enhancement of amphetamine-induced place preference behavior. A similar relationship was not found between place preference behavior and RGS9-2 expression in the accumbens core. Moreover, treatment of ovariectomized female rats with the selective estrogen receptor-beta agonist, diarylpropionitrile (1 mg/kg), for 2 weeks also facilitated amphetamine-induced place preference behavior and selectively reduced nucleus accumbens shell RGS9-2 protein expression. These data provide insight into a potential mechanism by which estrogen and/or sex modulate mesoaccumbal dopamine receptor signaling and possibly, addictive behaviors.

Levo-tetrahydropalmatine attenuates cocaine self-administration and cocaine-induced reinstatement in rats

RATIONALE

Levo-tetrahydropalmatine (l-THP) is an alkaloid constituent of plants from the botanical genera Corydalis and Stephania and is contained in many traditional Chinese herbal preparations. In addition to its low-affinity antagonism of D2 dopamine (DA) receptors, we report that l-THP functions as a higher-affinity antagonist at D1 DA receptors and interacts with D3 DA receptors, suggesting that it may be effective for the treatment of drug addiction. Accordingly, l-THP has been reported to reduce heroin craving and relapse in recovering addicts.

OBJECTIVE

This study investigated the effects of l-THP (3.75, 7.5, and 15.0 mg/kg, i.p.) on cocaine self-administration (SA) and cocaine-induced reinstatement.

MATERIALS AND METHODS

Rats were trained to self-administer cocaine and food by pressing separate response levers during sessions consisting of a multiple schedule of alternating 30-min FR4 cocaine and 15-min FR4 food reinforcement. During the cocaine components of each session, the available cocaine dose varied such that rats had access to low and high dose ranges in varying sequence on alternating days. After SA, cocaine-reinforced responding was extinguished, and effects of l-THP on cocaine-induced reinstatement (10 mg/kg, i.p.) were examined.

RESULTS

l-THP produced a rightward and downward shift in the dose-response curve for cocaine SA and attenuated cocaine-induced reinstatement. l-THP also reduced food-reinforced responding and locomotor activity. However, reductions in cocaine SA were found at doses that failed to alter food-reinforced responding, and significant effects were not observed on food responding during reinstatement.

CONCLUSIONS

These findings suggest that l-THP is potentially useful for treating cocaine addiction.

Effects of intra-nucleus accumbens shell administration of dopamine agonists and antagonists on cocaine-taking and cocaine-seeking behaviors in the rat

RATIONALE

Dopamine signaling in the nucleus accumbens (NAc) plays an important role in regulating drug-taking and drug-seeking behaviors, but the role of D(1)- and D(2)-like receptors in this regulation remains unclear.

OBJECTIVES

Our objective was to study the role of NAc D(1)- and D(2)-like receptors in the reinstatement of cocaine-seeking behavior and the regulation of stabilized cocaine intake in rats.

METHODS

Using a within-session reinstatement procedure, whereby animals self-administer cocaine (90 min) and extinguish responding (150 min) in a single session, we assessed the ability of NAc microinfusions of the D(1) agonist SKF 81297 and the D(2) agonist 7-OH-DPAT to reinstate extinguished cocaine seeking. The effects of the D(1) antagonist SCH 23390 and the D(2) antagonist eticlopride pretreatment on agonist- and cocaine-primed reinstatement were also measured. Similar agonist and antagonist treatments were tested for their ability to modulate stabilized cocaine and sucrose self-administration.

RESULTS

Intra-NAc infusions of either SKF 81297 (0.3-3.0 microg) or 7-OH-DPAT (1.0-10.0 microg) dose-dependently reinstated cocaine seeking with greater efficacy in the medial core than in the shell subregion and at doses that also stimulated locomotor behavior. Intra-NAc shell infusions of SCH 23390 (1.0 microg) and eticlopride (3.0-10.0 microg) blocked cocaine-primed reinstatement (2.0 mg/kg, i.v.) and indiscriminately blocked reinstatement induced by either intra-NAc D(1) or D(2) agonists. Doses of agonists that triggered reinstatement failed to alter stabilized cocaine intake, whereas doses of antagonists that blocked reinstatement increased cocaine intake in the shell.

CONCLUSIONS

Both D(1) and D(2) receptors in the NAc play a prominent, and perhaps cooperative, role in regulating cocaine-taking and cocaine-seeking behaviors.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.

The mesolimbic dopamine system: the final common pathway for the reinforcing effect of drugs of abuse?

In this review we will critically assess the hypothesis that the reinforcing effect of virtually all drugs of abuse is primarily dependent on activation of the mesolimbic dopamine system. The focus is on five classes of abused drugs: psychostimulants, opiates, ethanol, cannabinoids and nicotine. For each of these drug classes, the pharmacological and physiological mechanisms underlying the direct or indirect influence on mesolimbic dopamine transmission will be reviewed. Next, we evaluate behavioral pharmacological experiments that specifically assess the influence of activation of the mesolimbic dopamine system on drug reinforcement, with particular emphasis on animal experiments using drug self-administration paradigms. There is overwhelming evidence that all five classes of abused drugs increase dopamine transmission in limbic regions of the brain through interactions with a variety of transporters, ionotropic receptors and metabotropic receptors. Behavioral pharmacological experiments indicate that increased dopamine transmission is clearly both necessary and sufficient to promote psychostimulant reinforcement. For the other four classes of abused substances, self-administration experiments suggest that although increasing mesolimbic dopamine transmission plays an important role in the reinforcing effects of opiates, ethanol, cannabinoids and nicotine, there are also dopamine-independent processes that contribute significantly to the reinforcing effects of these compounds.

Cocaine-induced alterations in dopamine receptor signaling: Implications for reinforcement and reinstatement

The transition from casual drug use to addiction, and the intense drug craving that accompanies it, has been postulated to result from neuroadaptations within the limbic system caused by repeated drug exposure. This review will examine the implications of cocaine-induced alterations in mesolimbic dopamine receptor signaling within the context of several widely used animal models of addiction. Extensive evidence indicates that dopaminergic mechanisms critically mediate behavioral sensitization to cocaine, cocaine-induced conditioned place preference, cocaine self-administration, and the drug prime-induced reinstatement of cocaine-seeking behavior. The propagation of the long-term neuronal changes associated with recurring cocaine use appears to occur at the level of postreceptor signal transduction. Repeated cocaine treatment causes an up-regulation of the 3',5'-cyclic adenosine monophosphate (cAMP)-signaling pathway within the nucleus accumbens, resulting in a dys-regulation of balanced D1/D2 dopamine-like receptor signaling. The intracellular events arising from enhanced D1-like postsynaptic signaling mediate both facilitatory and compensatory responses to the further reinforcing effects of cocaine.

Effects of dopamine indirect agonists and selective D1-like and D2-like agonists and antagonists on cocaine self-administration and food maintained responding in rats

A procedure is described for comprehensive evaluation of the effects of acute drug pretreatments on the reinforcing effects of cocaine using the rat self-administration assay in combination with a novel control assay of liquid-food maintained responding. In sessions comprised of five 20-min components, either complete dose-effect functions for cocaine self-administration or complete concentration-effect functions for liquid-food maintained responding were evaluated. The schedule of reinforcement (FR 5 TO 20-s), drug pretreatment doses and time intervals (0-30 min), and duration of sessions (108 min) were identical for cocaine- and food-reinforced test sessions. Whereas acute pretreatment with indirect dopamine agonists (D-amphetamine, GBR 12909) and D2-like agonists (7-OH-DPAT, quinelorane) produced dose-dependent leftward shifts in dose-effect functions for cocaine self-administration, D1-like agonists (SKF 82958, R-6-Br-APB) and dopamine antagonists (D1-like, SCH 39166; D2-like, eticlopride) shifted dose-effect functions for cocaine downward and rightward, respectively. Interestingly, with the indirect dopamine agonists but not the D2-like agonists, increased responding maintained by low cocaine doses was paralleled by increased responding maintained by low food concentrations. Moreover, three of the four direct agonists were moderately selective (< or =5-fold more potent) in decreasing cocaine self-administration relative to food maintained responding. When data were analyzed according to alterations in total cocaine intake, all of the agonists uniformly decreased total cocaine intake, whereas both antagonists increased total cocaine intake. Overall, this procedure was sensitive to leftward, downward and rightward shifts in cocaine dose-effect functions and should be useful for evaluating the nature of pharmacological interactions between novel compounds and self-administered cocaine, as well as the potential for altering cocaine self-administration selectively with candidate treatments for cocaine abuse and dependence.

Dopamine mediation of positive reinforcing effects of amphetamine in stimulant naïve healthy volunteers: results from a large cohort

A positive experience during a first encounter with a drug of abuse is predictive of subsequent use and might represent a vulnerability factor to develop addiction. This paper presents a meta-analysis of data acquired in 60 healthy volunteers who underwent a low-dose amphetamine challenge (0.3 mg/kg, i.v.) during imaging of dopamine D2 receptor availability with SPECT and the D2/D3 radiotracer [123I]IBZM. Amphetamine-stimulated DA release induced a small, significant and highly variable decrease in striatal D2 receptor availability (-8.3 +/- 6.7%). The magnitude of the decrease in D2 receptor availability was significantly associated with the positive reinforcing effects of the drug reported by the subject (r2 = 0.14, p = 0.003). Age was associated with decreased potency of dopamine to elicit positive reinforcing effects. This study indicates that both a large dopaminergic response and young age during a first encounter with a drug of abuse potential contribute to higher positive reinforcing effects.

Alcoholism and the dopaminergic system: a review

Alcohol as well as other substances of abuse are reinforcing substances which manifest their effects through activation of the mesolimbic dopaminergic reward pathways of the brain. In animal genetic models of alcoholism, reduced dopamine levels and D2 dopamine receptor (DRD2) numbers have been found in the brains of alcohol-preferring animals. Dopamine receptor agonists reduce alcohol consumption, whereas antagonists, in general, show the opposite effect. Moreover, quantitative trait loci studies in animals suggest the DRD2 gene and the region proximate to this locus is a chromosomal "hot spot" for alcohol-related behaviors. Human studies provide additional support for connection between alcohol dependence and CNS dopaminergic function. In endocrinological studies, using dopamine receptor agonists, reduced dopaminergic activity has been found in more severe and more genetic types of alcoholics. Brain imaging studies are similarly revealing a diminished dopaminergic tone in alcoholics. Treatment of alcoholics with dopamine receptor agonists shows reduced alcohol consumption and improvements in other outcome measures. Molecular genetic studies in humans have identified an association of the Al allele of the DRD2 gene with alcoholism. Moreover, a diminished central dopaminergic function has been found in DRD2 A1 allele subjects using pharmacological, electrophysiological and neuropsychological studies. Further, treatment of alcoholics with a dopamine receptor agonist showed more salutary effects on alcoholics who carry than those who do not carry the DRD2 A1 allele. The A1 allele has also been associated with substance use disorders other than alcoholism, including and cocaine and nicotine dependence and polysubstance abuse. The emerging evidence suggests that the DRD2 is a reinforcement or reward gene. It could represent one of the most prominent single-gene determinants of susceptibility to severe substance abuse. However, the environment and other genes, when combined, still play the larger role.

Attenuation of cue-controlled cocaine-seeking by a selective D3 dopamine receptor antagonist SB-277011-A

Conditioned stimuli (CS) previously paired with drugs of abuse can elicit cravings in humans, relapse to drug use, and can also reinforce drug-seeking behavior in both humans and animals, events that are believed to be subserved in part by activation of the mesolimbic dopamine system. Converging anatomical, pharmacological, and behavioral evidence implicates dopamine D(3) receptors in the mechanisms underlying cue-controlled behaviors. The purpose of the present study was therefore to investigate the effects on cocaine-seeking behavior of a novel D(3) receptor antagonist, SB-277011-A, which is 100-fold more selective for D(3) over D(2) dopamine receptors. We have established previously that second-order schedules of reinforcement provide an animal model of cue-controlled drug-seeking both prior to and after cocaine has been self-administered. SB-277011-A dose-dependently decreased cocaine-seeking maintained by a cocaine-associated conditioned reinforcer in both the first, drug-free interval and also following self-administration of cocaine. At higher doses, SB-277011-A also increased the latency to receive the first CS presentation and cocaine infusion, thereby decreasing the number of cocaine infusions self-administered under the second-order schedule of reinforcement. SB-277011-A had no effect on cocaine intake under an FR-1 schedule of reinforcement, or on responding for sucrose under a second-order schedule of reinforcement, at any dose tested. These results therefore suggest that D(3) dopamine receptors may be critically involved in cue-controlled drug-seeking behavior independently of any interaction with the reinforcing effects of cocaine itself, and may therefore provide a therapeutic target in the treatment of relapse to cocaine use induced by CSs.

Novel (bisarylmethoxy)butylpiperidine analogues as neurotransmitter transporter inhibitors with activity at dopamine receptor sites

A series of (bisarylmethoxy)butylpiperidine derivatives was prepared and evaluated in vitro and in vivo to determine the structural requirements necessary for dual activity at the DAT and DA/5-HT receptor sites. These hybrid ligands, constructed by combining pharmacophores specific for the DAT and DA/5-HT receptors, could be useful drugs for treating cocaine addiction by assisting cocaine addicts in maintaining abstinence. The series was evaluated in vitro for DAT and DA/5-HT receptor activity and then selected compounds were tested in vivo for their effects on cocaine-induced hyperlocomotor activity (LMA). The majority of the new compounds demonstrated high to moderate affinity (4-191 nM) for the DAT with 4-hydroxy-4-phenylpiperidine analogues 14 and 15 possessing the greatest affinity. Compounds 15 and 22 exhibited the highest ratio of reuptake inhibition-to-binding (discrimination ratio, DR), 111 and 323, respectively. These derivatives had modest affinity and antagonistic activity for dopamine D(2)/D(3) receptors. Compounds 9 and 15 (DR=0.9 and 111, respectively) stimulated locomotor activity, whereas the other compounds suppressed this response. All compounds tested except for 17 and 21 attenuated cocaine-induced hyperlocomotion.

Substance abuse among patients with schizophrenia

Substance use in the general population is a significant public health problem. Problems associated with substance use are aggravated by concomitant psychiatric illness, particularly schizophrenia and schizoaffective disorder. Although there is a general agreement on the need to address this problem, a wide range of opinions exists on exactly what is the best modality. In this article, we provide a brief overview of the etiology and consequences of substance use in individuals with schizophrenia, followed by a more detailed review of pharmacological and psychotherapeutic trends in the treatment of this population. Research studies indicate that, while some evidence supports the self-medication hypothesis, individuals with schizophrenia or schizoaffective disorder frequently use substances for the same reasons and in the same manner as the general population. In the pharmacotherapy section, we briefly discuss the rationale for current medication strategies, their efficacy, and directions for future research. This is followed by an assessment of current psychotherapeutic interventions, their limitations, and potential modifications to improve treatment outcome. The research literature suggests that integrated treatment and well-tailored interventions that take into account psychosocial factors and motivation offer the most promise for the future. More controlled trials are necessary to validate this hypothesis.

Reinforcing effects of D2 dopamine receptor agonists and partial agonists in rhesus monkeys

Dopamine (DA) receptors play a role in the reinforcing effects of psychomotor stimulants and other drugs. Both D1 and D2 DA receptor agonists have been reported to function as positive reinforcers in maintaining self-administration in non-human subjects. The purpose of the present study was to evaluate, in monkeys, the reinforcing effects of DA D2 receptor agonists that vary in their efficacy as D2 agonists. Rhesus monkeys were prepared with venous catheters and lever pressing was maintained by i.v. cocaine (n=5, 0.03 mg/kg/inj) in daily baseline sessions (2 h/day, fixed ratio 25). Various doses of cocaine or D2 agonists were then made available for at least four to seven sessions, and until responding was stable. At least one dose of the higher-efficacy D2 agonists, R(-)-propylnorapomorphine (NPA) (n=4, 0.001-0.01 mg/kg/inj), R(-)-apomorphine (APO) (n=4, 0.003-0.1 mg/kg/inj) and R(+)-3-(3-hydroxyphenyl)-N-propylpiperidine HCl [R(+)-3-PPP] (n=4, 0.03-0.3 mg/kg/inj), functioned as a positive reinforcer in all the monkeys tested. In contrast, no dose of the lower-efficacy D2 agonists, R(+)-terguride (n=4, 0.001-0.3 mg/kg/inj) and S(-)-3-(3-hydroxyphenyl)-N-propylpiperidine HCl [S(-)-3-PPP] (n=4, 0.001-0.3 mg/kg/inj), maintained self-administration. In in vitro binding studies with monkey brain tissue NPA and terguride had high affinities for the D2 receptor, while APO had intermediate affinity, and the 3-PPPs had low affinity. Among the compounds that were reinforcers potency as a reinforcer was directly related to D2 affinity in three of the four monkeys, consistent with D2 receptor involvement in the reinforcing effect of these compounds. The data suggest that the efficacy at D2 receptors is directly related to the reinforcing effect.

Drug abuse and bipolar disorder: comorbidity or misdiagnosis?

Bipolar disorder is a common, severe and cyclic psychiatric illness. A strong association between alcohol dependence and bipolar disorder has been reported in numerous studies. The abuse of other drugs including cocaine, amphetamines, opiates, cannabis, and prescription medications in bipolar patients is also an important public health concern and has been less extensively investigated. This review examines the abuse of drugs other than alcohol or nicotine in people with bipolar disorder. The high rates of milder affective symptoms but not mania observed in patients in drug abuse treatment settings suggests the symptoms may in many cases be associated with the drug use. However, such patients presenting in psychiatric settings might be suffering from cyclothymic and related attenuated bipolar disorders (type II). Substance abuse may be associated with medication non-compliance, more mixed or dysphoric mania and possibly an earlier onset of affective symptoms and more hospitalizations. The pharmacotherapy of patients with bipolar disorder and drug abuse is examined, including evidence on the use of mood stabilizers, neuroleptics and the newer atypical antipsychotics in this population.

Conditioned increases in behavioral activity and accumbens dopamine levels produced by intravenous cocaine

In vivo microdialysis, behavioral activity assessments, and a conditioned place preference (CPP) test were used to investigate dopaminergic correlates of cocaine-conditioned behaviors. Over 12 days, rats were given either intravenous cocaine (4.2 mg/kg) or saline (6 cocaine and 6 saline infusions) daily in distinctively different environments. The following day, rats were tested in the cocaine- and saline-paired environments; 48 hr later, CPP was determined. The cocaine-associated environment elicited greater nucleus accumbens dopamine (NAcc DA) levels, hyperactivity, and place preference, though the emergence of DA increases was not in synchrony with peak behavioral activation. Although conditioned behavioral effects after repeated cocaine are well documented, direct evidence of increased NAcc DA in response to a cocaine-paired environment has not been previously reported. Discrepancies with previous work are attributed to a number of methodological differences.

Firing rate of nucleus accumbens neurons is dopamine-dependent and reflects the timing of cocaine-seeking behavior in rats on a progressive ratio schedule of reinforcement

The progressive ratio (PR) schedule of reinforcement is used to determine the reinforcing properties of rewards such as drugs of abuse. In this schedule, the animal is required to press a lever a progressively increasing number of times to receive a reward; the highest ratio obtained before the animal ceases responding is termed "breakpoint." We recorded neuronal spike activity from cells in the nucleus accumbens (NAc) of rats responding on a PR schedule for cocaine reinforcement. A common subtype of NAc cells demonstrated firing rates that varied according to the time between cocaine deliveries. The firing rate was inversely related to the NAc cocaine level predicted by a pharmacokinetic model. At higher response-to-reward ratios, inter-reward intervals were increased, resulting in a decrease in modeled cocaine level and a concomitant increase in firing rate over the session. The final increase in firing rate above a threshold value suggests a neural correlate of breakpoint. The effects of preadministration of dopamine D1 or D2 antagonists on the animals' behavior were similar in that both reduced breakpoint; however, each antagonist had markedly different effects on NAc cell firing. The D1 antagonist SCH23390 reduced firing rates, even at low cocaine levels, whereas the D2 antagonist eticlopride induced a rightward shift in the dose dependence of NAc cell firing relative to modeled cocaine level. Our results suggest that the firing of NAc cells reflects changes in cocaine levels and thereby contributes to the temporal spacing of self-administration and to the cessation of responding at breakpoint.

Dopamine modulates acute responses to cocaine, nicotine and ethanol in Drosophila

BACKGROUND

Drugs of abuse have a common property in mammals, which is their ability to facilitate the release of the neurotransmitter and neuromodulator dopamine in specific brain regions involved in reward and motivation. This increase in synaptic dopamine levels is believed to act as a positive reinforcer and to mediate some of the acute responses to drugs. The mechanisms by which dopamine regulates acute drug responses and addiction remain unknown.

RESULTS

We present evidence that dopamine plays a role in the responses of Drosophila to cocaine, nicotine or ethanol. We used a startle-induced negative geotaxis assay and a locomotor tracking system to measure the effect of psychostimulants on fly behavior. Using these assays, we show that acute responses to cocaine and nicotine are blunted by pharmacologically induced reductions in dopamine levels. Cocaine and nicotine showed a high degree of synergy in their effects, which is consistent with an action through convergent pathways. In addition, we found that dopamine is involved in the acute locomotor-activating effect, but not the sedating effect, of ethanol.

CONCLUSIONS

We show that in Drosophila, as in mammals, dopaminergic pathways play a role in modulating specific behavioral responses to cocaine, nicotine or ethanol. We therefore suggest that Drosophila can be used as a genetically tractable model system in which to study the mechanisms underlying behavioral responses to multiple drugs of abuse.

A comparison of the effects of risperidone, raclopride, and ritanserin on intravenous self-administration of d-amphetamine

These experiments were conducted to examine the effects of simultaneous blockade of dopamine D2 and 5-hydroxytryptamine2 (5-HT) receptor function on responding for intravenous infusions of d-amphetamine. Rats were trained to self-administer d-amphetamine intravenously according to a progressive ratio schedule of reinforcement, in which response requirements increased for successive infusions until responding extinguished. In the first experiment it was shown that increases in the unit infusion dose of d-amphetamine resulted in an increase in the number of amphetamine infusions earned. Thus, the strength of responding for d-amphetamine was linked to the dose of drug received. The mixed D2 and 5-HT2 receptor antagonist risperidone (0.1, 0.2, and 0.4 mg/kg) reduced responding for d-amphetamine (60 microg/kg infusion). The selective D2 antagonist raclopride (0.1, 0.2, and 0.4 mg/kg) also reduced responding for d-amphetamine. In contrast, the selective 5-HT2 antagonist ritanserin (0.63, 1.25, and 2.5 mg/kg) failed to alter amphetamine self-administration. Combined injections of raclopride (0.05 or 0.1 mg/kg) and ritanserin (2.5 mg/kg) were no more effective than injections of raclopride alone in reducing responding for d-amphetamine. Overall, these results suggest that 5-HT2 receptor blockade plays a negligible role in the rewarding effects of d-amphetamine measured by intravenous self-administration, and does not contribute to the suppressant effects of risperidone on this behavior.

Involvement of cAMP-dependent protein kinase in the nucleus accumbens in cocaine self-administration and relapse of cocaine-seeking behavior

cAMP-dependent protein kinase (PKA) in the nucleus accumbens (NAc) has been implicated in cocaine addiction because (1) cocaine reinforcement is mediated by dopamine receptors that modulate cAMP formation, and (2) repeated exposure to cocaine upregulates the cAMP system in NAc neurons. This study tested PKA involvement in cocaine self-administration and relapse of cocaine-seeking behavior by infusing cAMP analogs that activate or inhibit PKA into the NAc of rats. Bilateral intra-NAc infusions of the PKA inhibitor Rp-cAMPS reduced baseline cocaine self-administration, shifted the dose-response curve for cocaine self-administration to the left, and induced relapse of cocaine-seeking behavior after extinction from cocaine self-administration, consistent with an enhancement of cocaine effects in each paradigm. In contrast, pretreatment with intra-NAc infusions of a PKA activator, Sp-cAMPS or dibutyryl cAMP, increased baseline cocaine self-administration during the second hour of testing and shifted the dose-response curve to the right, consistent with an antagonist-like action. After extinction from cocaine self-administration, similar infusions of Sp-cAMPS induced generalized responding at both drug-paired and inactive levers. As an index of PKA activity in vivo, NAc infusions of Rp-cAMPS reduced basal levels of dopamine-regulated phosphoprotein-32 phosphorylation and blocked amphetamine-induced increases in cAMP response element-binding protein (CREB) phosphorylation. Conversely, NAc infusions of Sp-cAMPS increased phosphorylation of CREB. Together, these results suggest that sustained upregulation of the cAMP system in the NAc after repeated cocaine exposure could underlie tolerance to cocaine reinforcement, whereas acute inhibition of this system may contribute to drug craving and relapse in addicted subjects.

Monoamine transporter and sodium channel mechanisms in the rapid pressor response to cocaine

Intravenous (I.V.) cocaine (0.03-3 mg/kg) produced dose-dependent, rapid, and brief increases in blood pressure (BP) in conscious rats pretreated with the dopamine receptor antagonist, SCH 23390. Monoamine uptake inhibitors structurally analogous to cocaine (cocaethylene, CFT, betaCIT, CPT, (+)-cocaine, norcocaine, and benztropine) also produced this rapid pressor response, whereas structurally unrelated uptake inhibitors with diverse monoamine transporter selectivities (BTCP, indatraline, GBR 12935, mazindol, nomifensine, and zimeldine) either did not produce a rapid pressor response or produced only a small pressor response. At nonconvulsant doses, the sodium channel blockers acetylprocainamide, dibucaine, dyclonine, prilocaine, proparacaine, quinidine, and tetracaine produced a small pressor response or no increase in BP. In rats implanted with telemetric devices, cocaine and its analog, CFT, produced a biphasic pharmacological response that consisted of an initial brief and abrupt behavioral arousal associated with a rapid, large increase in BP followed by prolonged, parallel increases in BP and locomotor activity. Pretreatment with SCH 23390 prevented the prolonged but not the initial rapid and brief pressor and activity responses to both cocaine and CFT administration. The present data suggest that the inhibition of dopamine, norepinephrine, or serotonin transporter functions, either alone or in combination, does not mediate the rapid pressor response to cocaine. The sodium channel-blocking action of cocaine per se does not appear to be involved in the rapid pressor response to cocaine. Finally, the present results confirm previous findings that dopaminergic mechanisms mediate the prolonged increases in BP and locomotor activity produced by cocaine.

Olanzapine attenuates the reinforcing effects of cocaine

The possibility that the atypical neuroleptic olanzapine can antagonize the ability of cocaine to produce both conditioned place preference and self-administration in rats was investigated. Pre-treatment with olanzapine (3.0, 4.5 mg/kg, but not 1.5 mg/kg) significantly attenuated conditioned place preference produced by cocaine (10 mg/kg). However, the higher dose of olanzapine administered alone resulted in conditioned place aversion. Pre-treatment with olanzapine also produced a dose-dependent decrease in cocaine self-administration (0.33 mg/infusion) under a fixed-ratio 2 schedule of reinforcement. Olanzapine produced a similar dose-responsive attenuation in operant responding for food (fixed-ratio 10) suggesting that olanzapine produces a nonspecific decrease in operant behavior. Pre-treatment with 4.5 mg/kg olanzapine significantly attenuated cocaine-induced hyperactivity, whereas lower olanzapine doses had little effect upon cocaine-induced hyperactivity. These results suggest that pre-treatment with olanzapine is capable of blocking the reinforcing effects of cocaine and illustrates the value of using multiple tests of reinforcement when evaluating the pharmacological effects of newer psychotherapeutic agents.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.

Behavioral sensitization to ethanol: genetics and the effects of stress

Some aspects of drug abuse syndromes may be influenced by sensitization to some drug effects. This enhancement of drug effect has been associated with prior drug exposure and with exposure to stressful stimuli. It has been postulated that sensitization to psychomotor stimulant drug effects influences sensitivity to drug reward. The drugs of abuse best characterized for sensitization phenomena include cocaine, amphetamine, and morphine. In general, ethanol's molecular mechanisms of action have been difficult to define relative to drugs with known receptor or transporter binding sites and, likewise, ethanol sensitization has been less thoroughly examined. Evidence supporting the existence of behavioral sensitization to ethanol, for genetic differences in the occurrence of ethanol sensitization, and for the influence of corticosterone on the development of ethanol sensitization is reviewed herein. There appear to be different genetic determinants of acute drug sensitivity and sensitization. Cross-sensitization between stress and ethanol suggest a potential role for hypothalamic-pituitary-adrenal (HPA) axis associated changes in ethanol sensitization, consistent with mechanisms likely contributing to sensitization to other abused drugs. Furthermore, glucocorticoid receptors appear to mediate both ethanol- and stress-induced sensitization to ethanol. A biological link between drug reward and drug sensitization involving HPA axis hormones may exist and, thus, study of the sensitization process may elucidate mechanisms relevant to drug abuse.

Opponent process model and psychostimulant addiction

There are many sources of reinforcement in the spectrum of cocaine dependence that contribute to the compulsive cocaine self-administration or loss of control of cocaine intake that constitutes the core of modern definitions of dependence. The development of withdrawal has long been considered an integral part of drug addiction but has lost its impact in the theorization of drug dependence because of new emphasis on the neurobiological substrates for the positive-reinforcing properties of drugs. The present treatise reviews the neurobiological substrates for the acute positive reinforcing effects of cocaine and what is beginning to be known about the neurobiological substrates of cocaine withdrawal. The concept of motivational or affective withdrawal is reintroduced, which reemphasizes opponent process theory as a model for the motivational effects of cocaine dependence. The same neural substrates hypothesized to be involved in the acute reinforcing properties of drugs (basal forebrain regions of nucleus accumbens and amygdala) are hypothesized to be altered during chronic drug treatment to produce the negative motivational states characterizing drug withdrawal. Within these brain regions, both the neurochemical system(s) on which the drug has its primary actions and other neurochemical systems may undergo adaptations to chronic presence of the drug. An understanding of the adaptations of the motivational systems of the brain accompanying cocaine dependence leads to important predictions not only about the etiology, treatment, and prevention of cocaine addiction but also about the vulnerability of these motivational systems in non-drug-induced psychopathology.

Dopamine D(1) antagonist SCH23390 attenuates self-administration of both cocaine and fentanyl in rats

To investigate the role of dopamine D(1) receptors in the reinforcing effects of cocaine and fentanyl, the effect of the D(1) antagonist SCH23390 on intravenous self-administration of these drugs was investigated in rats using a progressive ratio (PR) reinforcement schedule, during which the rats received the first three injections under an FR1 schedule. Then the number of lever presses required to deliver an injection (lever press ratio) increased by three after every three further injections. The last lever press ratio completed by each rat during each 6 h session was designated the breaking point. Breaking point values increased dose-dependently during both cocaine (0.125-1.00 mg/kg per injection) and fentanyl (0.25-2.00 μg/kg per injection) self-administration. Pretreatment with SCH23390 (0.01 mg/kg, s.c.) decreased breaking point values for both cocaine and fentanyl, reflecting a decrease in the reinforcing efficacy of the drugs. To determine whether the effect of SCH23390 was due to general suppression of the lever pressing response, the effect of SCH23390 (0.01 mg/kg, s.c.) on the performance of rats maintained by water-reinforcement was examined. SCH23390 suppressed performance only transiently, therefore general suppression appears to have little or no effect on the breaking point. These results suggest that dopamine D(1) receptors are involved in mediating the reinforcing effects of both the psychostimulant cocaine and the opiate fentanyl.

A neuroendocrine role in cocaine reinforcement

Cocaine stimulates the secretion of corticosterone and ACTH, probably through a CRF-related mechanism, indicating that the drug activates the HPA axis. Indeed, cocaine has been reported to produce anxiety and to precipitate episodes of panic attack during chronic use and withdrawal in humans and to induce anxiogenic behavior in animals. Cocaine also alters benzodiazepine receptor binding in discrete regions of the rat brain. Some of these changes in binding are obviously related to the convulsions and seizures which are often observed in an acute cocaine overdose. However, data from behavioral studies have suggested that some of these effects may be related directly to cocaine reinforcement since receptor changes also were observed when binding in the brains of rats that self-administered cocaine was compared with that from animals that had received identical yoked, but non-contingent infusions of the drug. In this regard, pretreatment with the benzodiazepine receptor agonists chlordiazepoxide and alprazolam decreased cocaine self-administration without decreasing food-reinforced responding, suggesting that these effects were specific for cocaine. Since this attenuation of self-administration was reversed by increasing the unit dose of cocaine, it is likely that these drugs were decreasing cocaine reinforcement. In contrast, exposure to stress increases vulnerability to self-administer psychostimulants. In these experiments, low-dose cocaine self-administration was related directly to stress-induced increases in plasma corticosterone, such that plasma corticosterone was always greater than 150 ng/ml for rats which subsequently self-administered cocaine at doses of 0.125 mg/kg/infusion or lower, suggesting a threshold for the hormone in cocaine reinforcement. In other experiments, bilateral adrenalectomy completely abolished the acquisition of intravenous cocaine self-administration in naive rats, while metyrapone decreased ongoing self-administration. In addition, ketoconazole pretreatment resulted in patterns of self-administration that were virtually indistinguishable from that observed during saline extinction, suggesting that plasma corticosterone is not only important, but may even be necessary for cocaine reinforcement. The mechanisms through which adrenocorticosteroids alter cocaine reinforcement remain to be determined, but there is increasing evidence that the mesocorticolimbic dopaminergic system is involved. In particular, the medial prefrontal cortex appears to be at least one brain region where dopamine and adrenocorticosteroids may interact to affect cocaine reinforcement.

Mu opioid receptor mRNA in nucleus accumbens is elevated following dopamine receptor activation

We have previously demonstrated that continuous cocaine treatment for three days induces a marked but transient increase in mu opioid receptor (MOR) mRNA in nucleus accumbens (n. acc.); SCH 23390 and eticlopride, selective antagonists of D1- and D2-like dopamine (DA) receptors, respectively, blocked this cocaine-induced upregulation of MOR mRNA in n. acc. suggesting involvement of both subfamilies of DA receptors in the effect of cocaine (1,2). In the present study the ability of the selective DA D3 receptor antagonist, nafadotride (3,4), to prevent the cocaine-induced upregulation of MOR mRNA in n. acc. has been examined. Also, regulation of MOR mRNA following chronic administration of the DA agonists, SKF 38393, R(+)-6-Bromo-APB hydrobromide, or bromocriptine, has been studied. Male Sprague-Dawley rats were treated for 3 days with saline, cocaine, the DA receptor agonists or antagonist delivered by osmotic minipump. Expression of MOR mRNA in n. acc. was estimated by quantitative competitive polymerase chain reaction (PCR) assays following reverse transcription. Nafadotride (1.0 mg/kg/day) prevented the cocaine-induced upregulation of MOR mRNA in n. acc. When administered alone, nafadotride did not change the expression of MOR mRNA. The levels of MOR mRNA were elevated in n. acc. after 3 days treatment with each of the DA agonists, SKF 38393 (4.0 mg/kg/day), R(+)-6-Bromo-APB hydrobromide (4.0 mg/kg/day), or bromocriptine (5.0 mg/kg/day). Thus, DA agonists mimick the effect of cocaine on the expression of MOR mRNA in n. acc. These data confirm the involvement of dopaminergic mechanisms in the mediation of cocaine effects, indicate the comparability of actions of indirect and direct DA agonists, and point to the usefulness of cocaine as a tool to expose interaction between dopaminergic and opioid systems. The results suggest that activation of more than one type of DA receptor is required for the increased expression of MOR mRNA.

A genetic approach to study mechanisms of cocaine action

The brain dopamine system is thought to be the major target for the neuropharmacological actions of psychomotor stimulants such as cocaine. To investigate the mechanisms of cocaine action, we used a genetic approach, the gene-targeting technique, and generated D1 dopamine receptor mutant mice. Locomotor activity analysis in response to cocaine indicates that, in contrast to control mice which showed a dose-dependent increase in locomotion, D1 receptor mutant mice exhibited a dose-dependent decrease, suggesting that D1 receptors play an essential role in mediating such effects. Extracellular single unit recording of dopamine sensitive nucleus accumbens neurons in the D1 receptor mutant mice and control mice revealed a marked reduction in the inhibitory effects of cocaine and dopamine on the generation of action potentials, suggesting that D1 receptors play a fundamental role in cocaine- and dopamine-mediated neurophysiological effects within the nucleus accumbens. From these analyses, we conclude that the D1 dopamine receptor plays essential roles in mediating these effects of cocaine. In the future, the use of this powerful genetic approach will be essential for elucidating the molecular components of the signal transduction pathway leading to anatomical, cellular and behavioral changes upon cocaine administration and dopamine neurotransmission.

Neuronal spike activity in rat nucleus accumbens during cocaine self-administration under different fixed-ratio schedules

Chronic ensemble recording techniques were used to investigate neuronal activity in the nucleus accumbens in freely moving rats during different cocaine self-administration schedules. The issue of concern in this study was the role of nucleus accumbens in initiating and sustaining cocaine self-administration. Specifically, to determine the nature of the neuronal activity, either motor or motivational, which precedes the multiple bar presses required to self-administer cocaine and of the post-lever press neuronal response, we used conventional fixed ratio-5, fixed ratio-10, and modified fixed ratio-3 schedules. In the modified fixed ratio-3 schedule, the first lever press resulted in retraction of the lever for 2 s; the second lever press retracted the lever and turned on a cue light; the third lever press turned off the cue light and delivered cocaine (1.0 mg/kg) intravenously. In the fixed ratio-5 and -10 schedules, rats continuously pressed the lever 5 or 10 times, respectively, to obtain a single infusion of cocaine. Phasic alterations in neural spike activity were observed in 50% of nucleus accumbens neurons before (termed "anticipatory" responses) and after lever pressing for cocaine self-administration. Neurons with anticipatory responses typically exhibited such responses for all lever presses in the modified fixed ratio-3, fixed ratio-5, and fixed ratio-10 schedules, but instances were found when the activity correlate was absent. In addition, some neurons had a prominent alteration in firing rate lasting 1-5 min after cocaine self-administration, and some of these neurons also had anticipatory responses. When cocaine was eliminated during self-administration sessions, the post-lever press inhibitory responses were largely abolished or even reversed, whereas anticipatory responses were not markedly changed when rapid lever presses occurred before behavior ceased. Post-cocaine inhibitory responses compared between self-administered and passively administered cocaine were not significantly different between these two conditions. The results suggest that nucleus accumbens may be involved in initiating general reward-seeking behaviors and action which are not exclusively associated with cocaine self-administration. Moreover, the neuronal responses in the nucleus accumbens to cocaine self-administration may play an essential role in maintaining cocaine reinforcement.

Effects of dopamine agonists and antagonists on cocaine-induced operant responding for a cocaine-associated stimulus

The present study examined the effects of receptor subtype-selective dopamine agonists and antagonists on (i) cocaine-induced responding for a cocaine-associated stimulus and (ii) on responding for food and cocaine reinforcement. Rats implanted with intravenous catheters were trained to lever-press for food or cocaine reinforcers on an FR5-FR5 multiple schedule, which was preceded by a 5-min component during which only stimuli previously associated with the primary reinforcers were available response-contingently. (i) Non-contingent delivery of cocaine at the beginning of the stimulus component significantly increased responding for the cocaine-associated stimulus, compared to responding for the food-associated cue. Changes in the dose of cocaine administered non-contingently before the stimulus component resulted in an inverted U-shaped dose-effect curve in responding for the cocaine-associated cue. In subsequent experiments, pretreatment with the dopamine D2 receptor agonist bromocriptine (4.0-16.0 mg/kg IP) attenuated the cocaine-induced increase in responding for the cocaine-associated cue. In contrast, pretreatment with low doses of SDZ 208-911, a dopamine D2 partial agonist (0.025-0.1 mg/kg SC), further potentiated the cocaine-induced response. Pretreatment with low and medium doses of the dopamine D1 and D2 receptor subtype-selective antagonists SCH 23390 (D1; 5-10 micrograms/kg SC) and raclopride (D2; 100-200 micrograms/kg SC) blocked responding for cocaine-associated cues, with SCH 23390 acting more selectively than raclopride. At higher doses (SCH 23390: 20 micrograms/kg SC; raclopride: 400 micrograms/kg SC), both drugs produced non-selective effects by inhibiting responses for the food-associated cue. (ii) Varying the dose of cocaine self-administered during the multiple schedule resulted in an inverted U-shaped dose-effect curve during the cocaine components, while the number of food pellets earned remained unchanged. Pretreatment with bromocriptine selectively reduced the number of cocaine infusions obtained. The compensatory increases in responding for cocaine typically associated with SCH 23390, raclopride or SDZ 208-911 pretreatment were also observed under the present schedule conditions, although the effect did not reach statistical significance in the case of SCH 23390 and raclopride, possibly due to methodological constraints. The results indicate that the present rat model of cocaine-seeking behavior is sensitive to pharmacological manipulations and may yield important information regarding the neurobiological mechanisms underlying conditioned and unconditioned reinforcing aspects of cocaine addiction.

Possible novel pharmacodynamic action of cocaine: cardiovascular and behavioral evidence

Intravenous cocaine (0.03-3 mg/kg) produced two distinct and temporally separable effects in rats. One is an initial, large, and brief increase in blood pressure (BP) and heart rate (HR) of a rapid onset (abrupt hemodynamic stimulation). A rapid, brief, and intense behavioral arousal accompanied this abrupt hemodynamic stimulation. The other effect of cocaine is a prolonged locomotor activation of a relatively slower onset. Prolonged increases in BP and HR accompanied this locomotor effect. The threshold doses of cocaine to produce abrupt hemodynamic stimulation and locomotion are 0.03 and 0.3 mg/kg, respectively. Dopamine receptor antagonists, SCH 23390 or eticlopride, at a 0.03 mg/kg dose antagonized the locomotion and the parallel prolonged increases in BP and HR, but not the initial brief behavioral arousal and abrupt hemodynamic stimulation responses to cocaine. Peripheral dopamine receptor antagonist, domperidone, altered neither behavioral nor cardiovascular effects of cocaine. Chlorisondamine (1 mg/kg), an autonomic ganglionic blocker, did not alter either the initial brief behavioral arousal or the locomotor responses to cocaine, but it prevented the cardiovascular changes that accompanied both these behavioral responses. Norepinephrine, a direct adrenergic vasoconstrictor, although produced rapid and large increase in BP, did not cause abrupt behavioral arousal or locomotor activation. Unlike cocaine, monoamine reuptake inhibitors that are selective for norepinephrine (nisoxetine, 0.1-1 mg/kg) or serotonin (fluoxetine, 0.3-3 mg/kg) produced neither brief behavioral arousal and abrupt hemodynamic stimulation nor locomotor activation. Dopamine-selective reuptake inhibitor, GBR 12,909, also did not elicit the initial brief behavioral arousal and abrupt hemodynamic stimulation. But, GBR 12,909, like cocaine, produced a prolonged locomotor effect and parallel increases in BP and HR. These effects of GBR 12,909 were prevented by SCH 23,390 and eticlopride, but not by domperidone. Similar to cocaine, cardiovascular, but not the locomotor effects of GBR 12,909 were prevented by chlorisondamine. Lidocaine (0.3-3 mg/kg), a sodium channel blocker and local anesthetic, produced neither behavioral nor physiological changes. Both cocaine (3 mg/kg) and GBR 12,909 (1 mg/kg) increased plasma norepinephrine and epinephrine concentrations. These increases were antagonized by both eticlopride and SCH 23,390. These results indicate that behavioral and cardiovascular effects of cocaine are intricately related with respect to the molecular mechanisms involved. Two pharmacodynamic actions of cocaine appear to mediate these effects. One is a dopamine-dependent while the other is a monoamine- and sodium channel-independent novel action. The former mediates cocaine's locomotor effect and the accompanying prolonged increases in BP and HR, while the latter mediates the initial brief behavioral arousal and the accompanying abrupt hemodynamic stimulation.

Neuropharmacological evidence for the role of dopamine in ventral pallidum self-stimulation

The present study examines the role of dopaminergic neurotransmission in modulating the reinforcing effect of ventral pallidum (VP) intracranial self-stimulation (ICSS). Fifty four adult rats were implanted with a monopolar moveable stimulating electrode in the VP. Rate-frequency functions were determined by logarithmically decreasing the number of pulses in a stimulation train from a value that sustained maximal responding to one that did not sustain responding. After the ICSS thresholds stabilized, the animals received treatments with several doses of cocaine and of various selective drugs acting at the level of DA receptor subtypes. Their effects on threshold and asymptotic rate were analyzed. Cocaine produced a significant decrease in ICSS threshold but had no significant effect on the asymptotic rate. A significant decrease in ICSS threshold was also seen with the D3 agonist 7-OH-DPAT. This was associated with a decrease rather than an increase in performance. D1 and D2 DA receptor blockers (haloperidol, SCH-23390, raclopride and sulpiride) produced a dose dependent increase in ICSS threshold and a decrease in the maximal rate. The results suggest that DA plays a modulatory role in VP intracranial self-stimulation, and that D1, D2 and D3 receptors are involved in the mediation of this effect, although to different extents.

Self-administration of the D1 agonist SKF 82958 is mediated by D1, not D2, receptors

We have previously reported that two D1 dopamine agonists, SKF 82958 and SKF 77434, are readily self-administered by rats. However, due to the limited selectivities of these agents, it was not possible to attribute their reinforcing effects exclusively to their D1 actions. To assess the relative involvement of D1 and D2 receptors in SKF 82958 reinforcement, rats were pretreated 30 min before self-administration sessions with either the D1-selective antagonist (+)SCH 23390 or the D2-selective antagonist raclopride. The D1 antagonist (+)SCH 23390 (5-20 micrograms/kg, SC) produced significant, dose-related (compensatory) increases in SKF 82958; in contrast, the D2 antagonist raclopride (25-400 micrograms/kg, SC) did not significantly increase SKF 82958 self-administration, although raclopride did increase cocaine self-administration. Compensatory increases in self-administration rates are thought to reflect antagonist-induced reductions in drug reinforcement. Hence, we conclude that SKF 82958 self-administration depends on activation of a D1-regulated reinforcement substrate.

Effects of monoamine reuptake inhibitors on cocaine self-administration in rats

The objective of this study was to investigate the effects of acute administration of monoamine reuptake inhibitors on cocaine self-administration in rats. Pretreatment with GBR 12909 (1-5.6 mg/kg, IV), a dopamine-selective reuptake inhibitor, produced a dose-dependent and large reduction in the self-administration of cocaine (1 mg/kg/infusion). The 3- and 5.6-mg/kg doses of GBR 12909 produced downward shifts in the dose-response curves for cocaine (0.3-3 mg/kg/infusion) self-administration. Unlike GBR 12909, the norepinephrine-selective reuptake inhibitors, desipramine and nisoxetine, at a 10-mg/kg dose produced small, but significant, reductions in the self-administration of cocaine (1 mg/kg/infusion). The 10-mg/kg dose of fluoxetine, a serotonin-selective reuptake inhibitor, produced a small, but not significant, reduction in the self-administration of cocaine. The 10-mg/kg dose of desipramine, nisoxetine, or fluoxetine produced brief respiratory distress and motor abnormalities immediately following IV injections, thereby suggesting that this dose is close to the toxic range for all three drugs. Desipramine, nisoxetine, or fluoxetine at nontoxic doses of 1 and 3 mg/kg had no significant effects on cocaine self-administration. These data indicate that the acute enhancement of endogenous dopaminergic activity by pretreatment with dopamine reuptake inhibitor reduces the total intake of cocaine, thus supporting the hypothesis that the dopamine is critically involved in the reinforcing properties of cocaine. The data also suggest that the acute enhancements in the endogenous norepinephrine or serotonin systems by nontoxic doses of norepinephrine- or serotonin-selective reuptake inhibitors do not appear to alter the reinforcing properties of cocaine.

Effect of (-)-DS 121 and (+)-UH 232 on cocaine self-administration in rats

The novel dopamine autoreceptor antagonists (-)-DS 121 and (+)-UH 232 were tested for their ability to alter cocaine self-administration behavior in rats reinforced on a progressive ratio (PR) schedule. (-)-DS 121 (15 mg/kg) and (+)-UH 232 (30 mg/kg) produced significant decreases in breaking point. (-)-DS 121 produced variable results on rate of cocaine intake on an FR1 schedule, indicating that rate may on occasion be insensitive to changes in cocaine reinforcement. In animals previously trained to self-administer cocaine, (-)-DS 121 failed to maintain responding when substituted for cocaine. This profile suggests that (-)-DS 121 is a promising new candidate for the treatment of cocaine abuse.