Elevation of Extracellular Glutamate by Blockade of Astrocyte Glutamate Transporters Inhibits Cocaine Reinforcement in Rats via a NMDA-GluN2B Receptor Mechanism.

It is well established that glutamate plays an important role in drug-induced and cue-induced reinstatement of drug seeking. However, the role of glutamate in drug reward is unclear. In this study, we systemically evaluated the effects of multiple glutamate transporter (GLT) inhibitors on extracellular glutamate and dopamine (DA) in the nucleus accumbens (NAc), intravenous cocaine self-administration, intracranial brain-stimulation reward (BSR), and reinstatement of cocaine seeking in male and female rats. Among the five GLT inhibitors we tested, TFB-TBOA was the most potent. Microinjections of TFB-TBOA into the NAc, but not the ventral tegmental area (VTA), or dorsal striatum (DS), dose-dependently inhibited cocaine self-administration under fixed-ratio and progressive-ratio (PR) reinforcement schedules, shifted the cocaine dose-response curve downward, and inhibited intracranial BSR. Selective downregulation of astrocytic GLT-1 expression in the NAc by GLT-1 antisense oligonucleotides also inhibited cocaine self-administration. The reduction in cocaine self-administration following TFB-TBOA administration was NMDA GluN2B receptor dependent, and rats self-administering cocaine showed upregulation of GluN2B expression in NAc DA- and cAMP-regulated phosphoprotein 32 (DARPP-32)-positive medium-spiny neurons (MSNs). In contrast, TFB-TBOA, when locally administered into the NAc, VTA, or ventral pallidum (VP), dose-dependently reinstated cocaine-seeking behavior. Intra-NAc TFB-TBOA-evoked drug-seeking was long-lasting and NMDA/AMPA receptor dependent. These findings, for the first time, indicate that glutamate in the NAc negatively regulates cocaine's rewarding effects, while an excess of glutamate in multiple brain regions can trigger reinstatement of drug-seeking behavior.SIGNIFICANCE STATEMENT It is well known that glutamate plays an important role in relapse to drug seeking. However, the role of glutamate in drug reward is less clear. Here, we report that TFB-TBOA, a highly potent glutamate transporter (GLT) inhibitor, dose-dependently elevates extracellular glutamate and inhibits cocaine self-administration and brain-stimulation reward (BSR), when administered locally into the nucleus accumbens (NAc), but not other brain regions. Mechanistic assays indicate that cocaine self-administration upregulates NMDA-GluN2B receptor subtype expression in striatal dopaminoceptive neurons and activation of GluN2B by TFB-TBOA-enhanced glutamate inhibits cocaine self-administration. TFB-TBOA also reinstates cocaine-seeking behavior when administered into the NAc, ventral tegmental area (VTA), and ventral pallidum (VP). These findings demonstrate that glutamate differentially regulates cocaine reward versus relapse, reducing cocaine reward, while potentiating relapse to cocaine seeking.

The Impact of Heroin Self-Administration and Environmental Enrichment on Ventral Tegmental CRF1 Receptor Expression.

BACKGROUND: There is a strong link between chronic stress and vulnerability to drug abuse and addiction. Corticotropin releasing factor (CRF) is central to the stress response that contributes to continuation and relapse to heroin abuse. Chronic heroin exposure can exacerbate CRF production, leading to dysregulation of the midbrain CRF-dopamine-glutamate interaction. METHODS: Here we investigated the role of midbrain CRF1 receptors in heroin self-administration and assessed neuroplasticity in CRF1 receptor expression in key opioid addiction brain regions. RESULTS: Infusions of antalarmin (a CRF1 receptor antagonist) into the ventral tegmental area (VTA) dose dependently reduced heroin self-administration in rats but had no impact on food reinforcement or locomotor activity in rats. Using RNAscope in situ hybridization, we found that heroin, but not saline, self-administration upregulated CRF1 receptor mRNA in the VTA, particularly on dopamine neurons. AMPA GluR1 and dopamine reuptake transporter mRNA in VTA neurons were not affected by heroin. The western-blot assay showed that CRF1 receptors were upregulated in the VTA and nucleus accumbens. No significant changes in CRF1 protein expression were detected in the prefrontal cortex, insula, dorsal hippocampus, and substantia nigra. In addition, we found that 15 days of environmental enrichment implemented after heroin self-administration does not reverse upregulation of VTA CRF1 receptor mRNA but it downregulates dopamine transporter mRNA. CONCLUSIONS: Overall, these data suggest that heroin self-administration requires stimulation of VTA CRF1 receptors and upregulates their expression in brain regions involved in reinforcement. Such long-lasting neuroadaptations may contribute to continuation of drug use and relapse due to stress exposure and are not easily reversed by EE exposure.

Involvement of the ghrelin system in the maintenance of oxycodone self-administration: converging evidence from endocrine, pharmacologic and transgenic approaches.

Ghrelin, an orexigenic hormone, has emerged as a critical biological substrate implicated in drug reward. However, the response of the ghrelin system to opioid-motivated behaviors and the role of ghrelin in oxycodone self-administration remain to be studied. Here, we investigated the reciprocal interactions between the endogenous ghrelin system and oxycodone self-administration behaviors in rats and the role of the ghrelin system in brain stimulation reward (BSR) driven by optogenetic stimulation of midbrain reward circuits in mice. Oxycodone self-administration significantly elevated plasma ghrelin, des-acyl ghrelin and growth hormone and showed no effect on plasma LEAP2, a newly identified endogenous ghrelin receptor (GHS-R1a) antagonist. Oxycodone self-administration produced significant decreases in plasma gastric inhibitory polypeptide and insulin. Acquisition of oxycodone self-administration significantly upregulated GHS-R1a mRNA levels in dopamine neurons in the ventral tegmental area (VTA), a brain region critical in drug reward. Pretreatment with JMV2959, a selective GHS-R1a antagonist, dose-dependently reduced oxycodone self-administration and decreased the breakpoint for oxycodone under a progressive ratio reinforcement in Long-Evans rats. The inhibitory effects of JMV2959 on oxycodone self-administration is selectively mediated by GHS-R1a as JMV2959 showed a similar effect in Wistar wildtype but not in GHS-R knockout rats. JMV2959 pretreatment significantly inhibited BSR driven by selective stimulation of VTA dopamine neurons, but not by stimulation of striatal GABA neurons projecting to the VTA in mice. These findings suggest that elevation of ghrelin signaling by oxycodone or oxycodone-associated stimuli is a causal process by which oxycodone motivates oxycodone drug-taking and targeting the ghrelin system may be a viable treatment approach for opioid use disorders.

Repeated cocaine administration upregulates CB2 receptor expression in striatal medium-spiny neurons that express dopamine D1 receptors in mice.

Cannabinoid CB2 receptors (CB2R) are importantly involved in drug reward and addiction. However, the cellular mechanisms underlying CB2R action remain unclear. We have previously reported that cocaine self-administration upregulates CB2R expression in midbrain dopamine (DA) neurons. In the present study, we investigated whether cocaine or heroin also alters CB2R expression in striatal medium-spiny neurons that express dopamine D1 or D2 receptors (D1-MSNs, D2-MSNs) and microglia. Due to the concern of CB2R antibody specificity, we developed three mouse CB2-specific probes to detect CB2R mRNA using quantitative RT-PCR and RNAscope in situ hybridization (ISH) assays. We found that a single injection of cocaine failed to alter, while repeated cocaine injections or self-administration dose-dependently upregulated CB2R gene expression in both brain (cortex and striatum) and periphery (spleen). In contrast, repeated administration of heroin produced a dose-dependent reduction in striatal CB2 mRNA expression. RNAscope ISH assays detected CB2R mRNA in striatal D1- and D2-MSNs, not in microglia. We then used transgenic CX3CR1eGFP/+ microglia reporter mice and D1- or D2-Cre-RiboTag mice to purify striatal microglia or ribosome-associated mRNAs from CX3CR1eGFP/+, D1-MSNs, or D2-MSNs, respectively. We found that CB2R upregulation occurred mainly in D1-MSNs, not in D2-MSNs or microglia, in the nucleus accumbens rather than the dorsal striatum. These findings indicate that repeated cocaine exposure may upregulate CB2R expression in both brain and spleen, with regional and cell type-specific profiles. In the striatum, CB2R upregulation occurs mainly in D1-MSNs in the nucleus accumbens. Given the important role of D1-MSNs in brain reward function, the present findings provide new insight into mechanisms by which brain CB2Rs modulate cocaine action.

A closer look at alcohol-induced changes in the ghrelin system: novel insights from preclinical and clinical data.

Ghrelin is a gastric-derived peptide hormone with demonstrated impact on alcohol intake and craving, but the reverse side of this bidirectional link, that is, the effects of alcohol on the ghrelin system, remains to be fully established. To further characterize this relationship, we examined (1) ghrelin levels via secondary analysis of human laboratory alcohol administration experiments with heavy-drinking participants; (2) expression of ghrelin, ghrelin receptor, and ghrelin-O-acyltransferase (GOAT) genes (GHRL, GHSR, and MBOAT4, respectively) in post-mortem brain tissue from individuals with alcohol use disorder (AUD) versus controls; (3) ghrelin levels in Ghsr knockout and wild-type rats following intraperitoneal (i.p.) alcohol administration; (4) effect of alcohol on ghrelin secretion from gastric mucosa cells ex vivo and GOAT enzymatic activity in vitro; and (5) ghrelin levels in rats following i.p. alcohol administration versus a calorically equivalent non-alcoholic sucrose solution. Acyl- and total-ghrelin levels decreased following acute alcohol administration in humans, but AUD was not associated with changes in central expression of ghrelin system genes in post-mortem tissue. In rats, alcohol decreased acyl-ghrelin, but not des-acyl-ghrelin, in both Ghsr knockout and wild-type rats. No dose-dependent effects of alcohol were observed on acyl-ghrelin secretion from gastric mucosa cells or on GOAT acylation activity. Lastly, alcohol and sucrose produced distinct effects on ghrelin in rats despite equivalent caloric value. Our findings suggest that alcohol acutely decreases peripheral ghrelin concentrations in vivo, but not in proportion to alcohol's caloric value or through direct interaction with ghrelin-secreting gastric mucosal cells, the ghrelin receptor, or the GOAT enzyme.

Cocaine reward is reduced by decreased expression of receptor-type protein tyrosine phosphatase D (PTPRD) and by a novel PTPRD antagonist.

Receptor-type protein tyrosine phosphatase D (PTPRD) is a neuronal cell-adhesion molecule/synaptic specifier that has been implicated in addiction vulnerability and stimulant reward by human genomewide association and mouse cocaine-conditioned place-preference data. However, there have been no reports of effects of reduced expression on cocaine self-administration. There have been no reports of PTPRD targeting by any small molecule. There are no data about behavioral effects of any PTPRD ligand. We now report (i) robust effects of heterozygous PTPRD KO on cocaine self-administration (These data substantially extend prior conditioned place-preference data and add to the rationale for PTPRD as a target for addiction therapeutics.); (ii) identification of 7-butoxy illudalic acid analog (7-BIA) as a small molecule that targets PTPRD and inhibits its phosphatase with some specificity; (iii) lack of toxicity when 7-BIA is administered to mice acutely or with repeated dosing; (iv) reduced cocaine-conditioned place preference when 7-BIA is administered before conditioning sessions; and (v) reductions in well-established cocaine self-administration when 7-BIA is administered before a session (in WT, not PTPRD heterozygous KOs). These results add to support for PTPRD as a target for medications to combat cocaine use disorders. 7-BIA provides a lead compound for addiction therapeutics.

CB2 receptor antibody signal specificity: correlations with the use of partial CB2-knockout mice and anti-rat CB2 receptor antibodies.

Cannabinoid CB1 receptors are highly expressed in the brain and functionally modulate presynaptic neurotransmitter release, while cannabinoid CB2 receptors (CB2Rs) were initially identified in the spleen and regarded as peripheral cannabinoid receptors. Recently, growing evidence indicates the presence of functional CB2Rs in the brain. However, this finding is disputed because of the specificity of CB2R antibody signals. We used two strains of currently available partial CB2-knockout (CB2-KO) mice as controls, four anti-rat or anti-mouse CB2R antibodies, and mRNA quantification to further address this issue. Western blot assays using the four antibodies detected a CB2R-like band at ~40 kD in both the brain and spleen. Notably, more bands were detected in the brain than in the spleen, and specific immune peptides blocked band detection. Immunohistochemical assays also detected CB2-like immunostaining in mouse midbrain dopamine neurons. CB2R deletion in CB2-KO mice may reduce or leave CB2R-like immunoreactivity unaltered depending on antibody epitope. Antibodies with epitopes at the receptor-deleted region detected a significant reduction in CB2R band density and immunostaining in N-terminal-deleted Deltagen and C-terminal-deleted Zimmer strain CB2-KO mice. Other antibodies with epitopes at the predicted receptor-undeleted regions detected similar band densities and immunostaining in wild-type and CB2-KO mice. Quantitative RT-PCR assays detected CB2 mRNA expression using probes that targeted upstream or downstream gene sequences but not the probe that targeted the gene-deleted sequence in Deltagen or Zimmer CB2-KO mice. These findings suggest that none of the tested four polyclonal antibodies are highly mouse CB2R-specific. Non-specific binding may be related to the expression of mutant or truncated CB2R-like proteins in partial CB2-KO mice and the use of anti-rat CB2 antibodies because the epitopes are different between rat and mouse CB2Rs.

Cannabinoid CB1 receptor neutral antagonist AM4113 inhibits heroin self-administration without depressive side effects in rats.

Cannabinoid CB1 receptors (CB1Rs) have been shown to be a promising target in medication development for the treatment of addiction. However, clinical trials with SR141716A (rimonabant, a selective CB1R antagonist/inverse agonist) for the treatment of obesity and smoking cessation failed due to unwanted side effects, such as depression, anxiety, and suicidal tendencies. Recent preclinical studies suggest that the neutral CB1R antagonist AM4113 may retain the therapeutic anti-addictive effects of SR141716A in nicotine self-administration models and possibly has fewer unwanted side effects. However, little is known about whether AM4113 is also effective for other drugs of abuse, such as opioids and psychostimulants, and whether it produces depressive side effects similar to SR141716A in experimental animals. In this study, we demonstrated that systemic administration of AM4113 (3 and 10 mg/kg) dose-dependently inhibited the self-administration of intravenous heroin but not cocaine or methamphetamine, whereas SR141716A (3 and 10 mg/kg) dose-dependently inhibited the self-administration of heroin and methamphetamine but not cocaine. In the electrical brain-stimulation reward (BSR) paradigm, SR141716A (3 and 10 mg/kg) dose-dependently increased the BSR stimulation threshold (i.e., decreased the stimulation reward), but AM4113 had no effect on BSR at the same doses, suggesting that SR141716A may produce aversive effects while AM4113 may not. Together, these findings show that neutral CB1R antagonists such as AM4113 deserve further research as a new class of CB1R-based medications for the treatment of opioid addiction without SR141716A-like aversive effects.

Selective dopamine D3 receptor antagonism significantly attenuates stress-induced immobility in a rat model of post-traumatic stress disorder.

Post-traumatic stress disorder (PTSD) is a debilitating psychiatric syndrome that occurs in individuals exposed to extremely threatening or traumatic events. In both animals and humans, dopamine (DA) function appears to be dysregulated in brain areas involved in the conditioned fear response(s) that underlie PTSD. In this study, we determined the effect of the selective DA D3 receptor antagonists YQA14A (6.25, 12.5 and 25 mg/kg i.p.) and SB-277011A (6 mg/kg i.p.) on tone-induced fear (assessed by measuring freeze time) in a modified version of the single-prolonged stress (SPS) model of PTSD in adult male Sprague-Dawley rats. Rats pretreated with vehicle and then subjected to restraint stress, forced swim and random foot shock (SPS) in the presence of a distinctive tone, displayed a significantly increased tone-induced contextual freeze time and fecal pellet mass following re-exposure to the tone. Rats pretreated with a single i.p. injection of 6.25 or 12.5 mg/kg of YQA14 or 6 mg/kg of SB-277011A showed significantly attenuated contextual freeze time in the presence of the tone when tested 14 days after exposure to SPS. Overall, our results indicate that selectively antagonizing DA D3 receptors significantly decreases freezing time caused by an environment previously associated with stress. If our findings can be extrapolated to humans with PTSD, they suggest that DA D3 receptors may play a role in the pathophysiology of PTSD, and may have therapeutic utility for the clinical management of PTSD.

Expression of functional cannabinoid CB2 receptor in VTA dopamine neurons in rats.

We have recently reported the expression of functional cannabinoid CB2 receptors (CB2 Rs) in midbrain dopamine (DA) neurons in mice. However, little is known whether CB2 Rs are similarly expressed in rat brain because significant species differences in CB2 R structures and expression are found. In situ hybridization and immunohistochemical assays detected CB2 gene and receptors in DA neurons of the ventral tegmental area (VTA), which was up-regulated in cocaine self-administration rats. Electrophysiological studies demonstrated that activation of CB2 Rs by JWH133 inhibited VTA DA neuronal firing in single dissociated neurons. Systemic administration of JWH133 failed to alter, while local administration of JWH133 into the nucleus accumbens inhibited cocaine-enhanced extracellular DA and i.v. cocaine self-administration. This effect was blocked by AM630, a selective CB2 R antagonist. These data suggest that CB2 Rs are expressed in VTA DA neurons and functionally modulate DA neuronal activities and cocaine self-administration behavior in rats.

Cannabinoid CB2 receptors modulate midbrain dopamine neuronal activity and dopamine-related behavior in mice.

Cannabinoid CB2 receptors (CB2Rs) have been recently reported to modulate brain dopamine (DA)-related behaviors; however, the cellular mechanisms underlying these actions are unclear. Here we report that CB2Rs are expressed in ventral tegmental area (VTA) DA neurons and functionally modulate DA neuronal excitability and DA-related behavior. In situ hybridization and immunohistochemical assays detected CB2 mRNA and CB2R immunostaining in VTA DA neurons. Electrophysiological studies demonstrated that activation of CB2Rs by JWH133 or other CB2R agonists inhibited VTA DA neuronal firing in vivo and ex vivo, whereas microinjections of JWH133 into the VTA inhibited cocaine self-administration. Importantly, all of the above findings observed in WT or CB1(-/-) mice are blocked by CB2R antagonist and absent in CB2(-/-) mice. These data suggest that CB2R-mediated reduction of VTA DA neuronal activity may underlie JWH133's modulation of DA-regulated behaviors.

The selective dopamine D3 receptor antagonist SB-277011A attenuates drug- or food-deprivation reactivation of expression of conditioned place preference for cocaine in male Sprague-Dawley rats.

We determined the effect of the selective dopamine D3 receptor antagonist SB-277011A on reactivation of conditioned place preference (CPP) to cocaine elicited by priming injections of cocaine or exposure to food deprivation stress (21 h) in male Sprague-Dawley rats. Animals paired with the cocaine-associated chamber displayed a robust and consistent CPP response. This CPP was extinguished after repeated pairings of the conditioned stimuli (cocaine-paired chamber contextual cues) in the absence of the unconditioned stimulus (cocaine). Twenty-four hours later, the administration of 5 mg kg(-1) i.p. of cocaine (immediately before the test) or exposure to 21 h of food deprivation reactivated the expression of the cocaine-induced CPP. In contrast, administration of 1 ml kg(-1) i.p. of vehicle did not reactivate the CPP response. Administration of the selective dopamine D3 receptor antagonist SB-277011A (3-24 mg kg(-1) i.p.) 30 min before cocaine administration on the test day produced a significant attenuation of CPP reactivation. Reactivation of the CPP response produced by food deprivation was also significantly attenuated by SB-277011A (6 or 12 mg kg(-1) i.p.) given 30 min before the test session. SB-277011A (12 or 24 mg kg(-1) i.p.) did not itself produce reactivation of the CPP response. Overall, these results suggest that the reactivation of the incentive value of drug-associated cues by cocaine or food deprivation is attenuated by selective antagonism of D3 receptors.

The selective dopamine D3 receptor antagonist SB-277011-A significantly decreases binge-like consumption of ethanol in C57BL/J6 mice.

The authors show that the dopamine D3 antagonist SB-277011-A consistently and persistently decrease binge-like EtOH consumption at 30 mg/kg and it decreases binge-like consumption on days 3, 7, 9, and 11-13 at 15 mg/kg. These findings suggest that the brain's D3 receptor may also be involved in not only ethanol reward but binge drinking as well.

Increased vulnerability to cocaine in mice lacking dopamine D3 receptors.

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

A novel mGluR5 antagonist, MFZ 10-7, inhibits cocaine-taking and cocaine-seeking behavior in rats.

Pre-clinical studies suggest that negative allosteric modulators (NAMs) of the metabotropic glutamate receptor subtype 5 (mGluR5), including 2-methyl-6-(phenylethynyl)pyridine (MPEP), 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and fenobam are highly effective in attenuating drug-taking and drug-seeking behaviors. However, both MPEP and MTEP have no translational potential for use in humans because of their off-target effects and short half-lives. Here, we report that 3-fluoro-5-[(6-methylpyridin-2-yl)ethynyl]benzonitrile (MFZ 10-7), a novel mGluR5 NAM, is more potent and selective than MPEP, MTEP and fenobam in both in vitro binding and functional assays. Similar to MTEP, intraperitoneal administration of MFZ 10-7 inhibited intravenous cocaine self-administration, cocaine-induced reinstatement of drug-seeking behavior and cocaine-associated cue-induced cocaine-seeking behavior in rats. Although MFZ 10-7 and MTEP lowered the rate of oral sucrose self-administration, they did not alter total sucrose intake. Further, MFZ 10-7 appeared to be more potent than MTEP in inducing downward shifts in the cocaine dose-response curve, but less effective than MTEP in attenuating sucrose-induced reinstatement of sucrose-seeking behavior. MFZ 10-7 and MTEP had no effect on basal locomotor behavior. These findings not only provide additional evidence supporting an important role for mGluR5 in cocaine reward and addiction, but also introduce a new tool for both in vitro and in vivo investigations with which to further characterize this role.

The selective D3 receptor antagonist SB-277011A attenuates morphine-triggered reactivation of expression of cocaine-induced conditioned place preference.

We examined the effect of acute administration of the selective D3 receptor antagonist SB-277011A on morphine-triggered reactivation of cocaine-induced conditioned place preference (CPP) in adult male Sprague-Dawley rats. Repeated pairing of animals with 15 mg/kg i.p. of cocaine HCl or vehicle to cue-specific CPP chambers produced a significant CPP response compared to animals paired only with vehicle in both chambers. Expression of the CPP response to cocaine was then extinguished by repeatedly giving the animals vehicle injections in the cocaine-paired chambers. The magnitude of the CPP response after extinction was not significantly different from that of animals paired only with vehicle. Expression of the extinguished CPP response was reactivated by acute administration of 5 mg/kg i.p. of morphine but not by vehicle. Acute administration of 6 or 12 mg/kg i.p. (but not 3 mg/kg) of SB-277011A significantly attenuated morphine-triggered reactivation of the cocaine-induced CPP. SB-277011A itself (12 mg/kg i.p.) did not reactivate the extinguished CPP response. Overall, SB-277011A decreases the incentive motivational actions of morphine. The present findings suggest that central D3 dopamine receptors are involved in relapse to cocaine-seeking behavior, that a final common neural mechanism exists to mediate the incentive motivational effects of psychostimulants and opiates, and that selective dopamine D3 receptor antagonists constitute promising compounds for treating addiction.

Blockade of dopamine D3 receptors in the nucleus accumbens and central amygdala inhibits incubation of cocaine craving in rats.

Cue-induced drug seeking progressively increases over time of withdrawal from drug self-administration in rats, a phenomenon called 'incubation of craving'. The underlying mechanisms have been linked to increased expression of brain-derived neurotrophic factor and GluR2-lacking AMPA receptors in the mesolimbic dopamine (DA) system and also to increased extracellular signal-regulated kinase activation in the central amygdala (CeA). However, it remains unclear whether any DA mechanism is also involved in incubation of craving. Recent research demonstrates that cue-induced cocaine seeking appears to parallel increased DA D3 , but not D1 or D2 , receptor expression in the nucleus accumbens (NAc) of rats over time of withdrawal, suggesting possible involvement of D3 receptors (D3 Rs) in incubation of cocaine craving. Here, we report that systemic or local administration of SB-277011A, a highly selective D3 R antagonist, into the NAc (core and shell) or the CeA, but not the dorsal striatum or basolateral amygdala, significantly inhibits expression of incubation of cocaine craving in rats after 2-30 days of withdrawal from previous cocaine self-administration but had no effect on sucrose-seeking behavior in rats after 10-30 days of withdrawal. These data suggest that DA D3 Rs in both the NAc and the CeA play an important role in incubation of cocaine craving in rats and support the potential utility of D3 R antagonists in the treatment of cocaine addiction.

Chiral Resolution of the Enantiomers of the Slow-Onset Dopamine Reuptake Inhibitor CTDP-32476 and Their Activities.

An improved synthesis was developed for CDTP-32476, a potent slow-onset dopamine reuptake blocker that may have utility as a treatment for cocaine abuse. The enantiomers of the compound were separated by fractional crystallization with N-acetylleucine enantiomers. An X-ray crystal structure was obtained of the RR enantiomer paired with N-acetyl-d-leucine. Chiral chromatography showed that the resolved enantiomers were pure with little contamination by the other enantiomer. The enantiomers were tested for their ability to block the reuptake of monoamines at their respective transporters and to stimulate locomotor activity in mice. Both enantiomers potently blocked the reuptake of dopamine and stimulated locomotor activity in mice. The RR enantiomer that corresponds to the active RR enantiomer of methylphenidate was slightly more potent at the dopamine reuptake site. The RR enantiomer also was found to be about twice as selective for the dopamine transporter relative to the norepinephrine transporter, which may have clinical implications. A method for designing slow-onset stimulants is proposed since there is increasing evidence that such activity is an important factor in stimulants that may have limited abuse potential.

The acute administration of the selective dopamine D(3) receptor antagonist SB-277011A reverses conditioned place aversion produced by naloxone precipitated withdrawal from acute morphine administration in rats.

We examined the effect of SB-277011A, a selective D(3) receptor antagonist, on the conditioned place aversion (CPA) response associated with naloxone-induced withdrawal from acute morphine administration in male Sprague-Dawley rats. Morphine (5.6 mg/kg i.p.) was given, followed 4 hrs later by naloxone (0.3 mg/kg i.p.) and prior to placing the animals in one specific chamber of the test apparatus. All animals were subjected to 2 of these trials. A significant CPA occurred in animals that received an i.p. injection of vehicle 30 minutes prior to the measurement of chamber preference. The pretreatment of animals (30 minutes prior to testing) with 3 mg/kg i.p. of SB-277011A did not significantly alter the CPA compared to animals treated with vehicle (1 ml/kg i.p. of deionized distilled water). In contrast, the acute pretreatment of animals with 6, 12 or 24 mg/kg i.p. of SB-277011A significantly decreased the CPA compared to vehicle-treated animals. In fact, the 12 and 24 mg/kg doses of SB-277011A significantly increased the time spent in the chamber where animals were paired with morphine and naloxone. These results suggest that the selective antagonism of D(3) receptors attenuates the CPA produced by a model of naloxone-induced withdrawal from acute morphine dependence.

YQA14: a novel dopamine D3 receptor antagonist that inhibits cocaine self-administration in rats and mice, but not in D3 receptor-knockout mice.

The dopamine (DA) D3 receptor is posited to be importantly involved in drug reward and addiction, and D3 receptor antagonists have shown extraordinary promise as potential anti-addiction pharmacotherapeutic agents in animal models of drug addiction. SB-277011A is the best characterized D3 receptor antagonist in such models. However, the potential use of SB-277011A in humans is precluded by pharmacokinetic and toxicity problems. We here report a novel D3 receptor antagonist YQA14 that shows similar pharmacological properties as SB-277011A. In vitro receptor binding assays suggest that YQA14 has two binding sites on human cloned D3 receptors with K(i-High) (0.68 × 10(-4) nM) and K(i-Low) (2.11 nM), and displays > 150-fold selectivity for D3 over D2 receptors and > 1000-fold selectivity for D3 over other DA receptors. Systemic administration of YQA14 (6.25-25 mg/kg) or SB-277011A (12.5-25 mg/kg) significantly and dose-dependently reduced intravenous cocaine self-administration under both low fixed-ratio and progressive-ratio reinforcement conditions in rats, while failing to alter oral sucrose self-administration and locomotor activity, suggesting a selective inhibition of drug reward. However, when the drug dose was increased to 50 mg/kg, YQA14 and SB-277011A significantly inhibited basal and cocaine-enhanced locomotion in rats. Finally, both D3 antagonists dose-dependently inhibited intravenous cocaine self-administration in wild-type mice, but not in D3 receptor-knockout mice, suggesting that their action is mediated by D3 receptor blockade. These findings suggest that YQA14 has a similar anti-addiction profile as SB-277011A, and deserves further study and development.