D(3) dopamine receptors are down-regulated in amphetamine sensitized rats and their putative antagonists modulate the locomotor sensitization to amphetamine

Inhibition of GSDMD-dependent pyroptosis decreased methamphetamine self-administration in rats

It is widely believed that the activation of the central dopamine (DA) system is crucial to the rewarding effects of methamphetamine (METH) and to the behavioral outcomes of METH use disorder. It was reported that METH exposure induced gasdermin D (GSDMD)-dependent pyroptosis in rats. The membrane pore formation caused by METH-induced pyroptosis may also contribute to the overflow of DA into the extracellular space and subsequently increase the DA levels in the brain. The present study firstly investigated whether the membrane pore information induced by GSDMD-dependent pyroptosis was associated with the increased DA levels in the ventral tegmental area (VAT) and nucleus accumbens (NAc) of rats self-administering METH and SY-SH5Y cells treated by METH. Subsequently, the effect of pore formation blockade or genetic inhibition of GSDMD on the reinforcing and motivational effect of METH was determined in rats, using the animal model of METH self-administration (SA). METH exposure significantly increased the activity of NLRP1/Cas-1/GSDMD pathway and the presence of pyroptosis, accompanied by the significantly increased DA levels in VTA and NAc. Moreover, intraperitoneal injections of disulfiram (DSF) or microinjection of rAAV-shGSDMD into VTA/NAc significantly reduced the reinforcing and motivational effect of METH, accompanied by the decreased level of DA in VTA and NAc. The results provided novel evidence that METH-induced pyroptosis could increase DA release in VTA and NAc via the NLRP1/Cas-1/GSDMD pathway. Additionally, membrane pores or GSDMD blockade could significantly reduce the reinforcing and motivational effect of METH. In conclusion, blocking GSDMD and membrane pore formation could be a promising potential target for the development of agents to treat METH use disorder.

Angiotensin II involvement in the development and persistence of amphetamine-induced sensitization: Striatal dopamine reuptake implications

Amphetamine (AMPH) exposure induces behavioural and neurochemical sensitization observed in rodents as hyperlocomotion and increased dopamine release in response to a subsequent dose. Brain Angiotensin II modulates dopaminergic neurotransmission through its AT1 receptors (AT1-R), positively regulating striatal dopamine synthesis and release. This work aims to evaluate the AT1-R role in the development and maintenance of AMPH-induced sensitization. Also, the AT1-R involvement in striatal dopamine reuptake was analysed. The sensitization protocol consisted of daily AMPH administration for 5 days and tested 21 days after withdrawal. An AT1-R antagonist, candesartan, was administered before or after AMPH exposure to evaluate the participation of AT1-R in the development and maintenance of sensitization, respectively. Sensitization was evaluated by locomotor activity and c-Fos immunostaining. Changes in dopamine reuptake kinetics were evaluated 1 day after AT1-R blockade withdrawal treatment, with or without the addition of AMPH in vitro. The social interaction test was performed as another behavioural output. Repeated AMPH exposure induced behavioural and neurochemical sensitization, which was prevented and reversed by candesartan. The AT1-R blockade increased the dopamine reuptake kinetics. Neither the AMPH administration nor the AT1-R blockade altered the performance of social interaction. Our results highlight the AT1-R's crucial role in AMPH sensitization. The enhancement of dopamine reuptake kinetics induced by the AT1-R blockade might attenuate the neuroadaptive changes that lead to AMPH sensitization and its self-perpetuation. Therefore, AT1-R is a prominent candidate as a target for pharmacological treatment of pathologies related to dopamine imbalance, including drug addiction and schizophrenia.

The Role of Dopamine D3 Receptors in Tobacco Use Disorder: A Synthesis of the Preclinical and Clinical Literature

Tobacco smoking is a significant cause of preventable morbidity and mortality globally. Current pharmacological approaches to treat tobacco use disorder (TUD) are only partly effective and novel approaches are needed. Dopamine has a well-established role in substance use disorders, including TUD, and there has been a long-standing interest in developing agents that target the dopaminergic system to treat substance use disorders. Dopamine has 5 receptor subtypes (DRD1 to DRD5). Given the localization and safety profile of the dopamine receptor D3 (DRD3), it is of therapeutic potential for TUD. In this chapter, the preclinical and clinical literature investigating the role of DRD3 in processes relevant to TUD will be reviewed, including in nicotine reinforcement, drug reinstatement, conditioned stimuli and cue-reactivity, executive function, and withdrawal. Similarities and differences in findings from the animal and human work will be synthesized and findings will be discussed in relation to the therapeutic potential of targeting DRD3 in TUD.

Exploring regulation and function of dopamine D3 receptors in alcohol use disorder. A PET [11C]-(+)-PHNO study

Preclinical studies support an important role of dopamine D3 receptors (DRD3s) in alcohol use disorder (AUD). In animals, voluntary alcohol consumption increases DRD3 expression, and pharmacological blockade of DRD3s attenuates alcohol self-administration and reinstatement of alcohol seeking. However, these findings have yet to be translated in humans. This study used positron emission tomography (PET) and [¹¹C]-(+)-PHNO to compare receptor levels in several dopamine D2 receptor (DRD2) and DRD3 regions of interest between AUD subjects in early abstinence (n = 17; 6.59 ± 4.14 days of abstinence) and healthy controls (n = 18). We recruited non-treatment seeking subjects meeting DSM-5 criteria for AUD. We examined the relationship between DRD2/3 levels and both alcohol craving and alcohol motivation/wanting, using a cue reactivity procedure and an intravenous alcohol self-administration (IVASA) paradigm, respectively. [¹¹C]-(+)-PHNO binding levels in AUD subjects were significantly lower than binding in HCs when looking at all DRD2/3 ROIs jointly (Wilk's Λ = .58, F(6,28) =3.33, p = 0.013, η²_p = 0.42), however there were no region-specific differences. Binding values demonstrate -12.3% and -16.1% lower [¹¹C]-(+)-PHNO binding in the SMST and SN respectively, though these differences did not withstand Bonferroni corrections. There was a positive association between [¹¹C]-(+)-PHNO binding in the SN (almost exclusively reflective of DRD3) and alpha (lower values reflect higher alcohol demand) in the APT after Bonferroni corrections (r = 0.66, p = 0.0080). This demonstrates that AUD subjects with lower DRD3 levels in the SN exhibit increased demand for alcohol. These results replicate previous findings demonstrating reduced DRD2/3 levels while also supporting a lack of DRD3 upregulation and potential downregulation in early abstinent AUD. Furthermore, the finding that binding in the SN is associated with alcohol demand warrants further examination.

CXCR4 inhibition with AMD3100 attenuates amphetamine induced locomotor activity in adolescent Long Evans male rats

Adolescent psychostimulant abuse has been on the rise over the past decade. This trend has demonstrable ramifications on adolescent behavior and brain morphology, increasing risk for development of addiction during adolescence and in later adulthood. Neuroimmune substrates are implicated in the etiology of substance use disorders. To add to this body of work, the current study was developed to explore the role of a chemokine receptor, CXC Chemokine Receptor 4 (CXCR4), in the development of amphetamine (AMPH) sensitization. We targeted CXCR4 as it is implicated in developmental processes, dopaminergic transmission, neuroimmune responses, and the potentiation of psychostimulant abuse pathology. To evaluate the role of CXCR4 activity on the development of AMPH sensitization, a CXCR4 antagonist (Plerixafor; AMD3100) was administered to rats as a pretreatment variable. Specifically, adolescent Long Evans male rats (N = 37) were divided into four groups: (1) AMD3100 (IP, 4.0 mg/kg) + AMPH (IP, 4.0 mg/kg), (2) saline (SAL; 0.9% NaCl) + AMPH, (3) AMD3100 + SAL, and (4) SAL + SAL. Animals were first habituated to locomotor activity (LMA) chambers, then injected with a pretreatment drug (AMD3100 or SAL) followed by AMPH or SAL every other for four days. After a one-week withdrawal period, all animals were administered a low challenge dose of AMPH (IP, 1.0 mg/kg). AMPH-injected rats displayed significantly more locomotor activity compared to controls across all testing days. CXCR4 antagonism significantly attenuated AMPH-induced locomotor activity. On challenge day, AMD3100 pre-treated animals exhibited diminutive AMPH-induced locomotor activity compared to SAL pre-treated animals. Postmortem analyses of brain tissue revealed elevated CXCR4 protein levels in the striatum of all experimental groups. Our results implicate CXCR4 signaling in the development of AMPH sensitization and may represent an important therapeutic target for future research in psychostimulant abuse.

The psychoactive cathinone derivative pyrovalerone alters locomotor activity and decreases dopamine receptor expression in zebrafish (Danio rerio)

INTRODUCTION

Pyrovalerone (4-methyl-β-keto-prolintane) is a synthetic cathinone (beta-keto-amphetamine) derivative. Cathinones are a concern as drugs of abuse, as related street drugs such as methylenedioxypyrovalerone have garnered significant attention. The primary mechanism of action of cathinones is to inhibit reuptake transporters (dopamine and norepinephrine) in reward centers of the central nervous system.

METHODS

We measured bioenergetic, behavioral, and molecular responses to pyrovalerone (nM-µM) in zebrafish to evaluate its potential for neurotoxicity and neurological impairment.

RESULTS

Pyrovalerone did not induce any mortality in zebrafish larvae over a 3- and 24-hr period; however, seizures were prevalent at the highest dose tested (100 µM). Oxidative phosphorylation was not affected in the embryos, and there was no change in superoxide dismutase 1 expression. Following a 3-hr treatment to pyrovalerone (1-100 µM), larval zebrafish (6d) showed a dose-dependent decrease (70%-90%) in total distance moved in a visual motor response (VMR) test. We interrogated potential mechanisms related to the hypoactivity, focusing on the expression of dopamine-related transcripts as cathinones can modulate the dopamine system. Pyrovalerone decreased the expression levels of dopamine receptor D1 (~60%) in larval zebrafish but did not affect the expression of tyrosine hydroxylase, dopamine active transporter, or any other dopamine receptor subunit examined, suggesting that pyrovalerone may regulate the expression of dopamine receptors in a specific manner.

DISCUSSION

Further studies using zebrafish are expected to reveal new insight into molecular mechanisms and behavioral responses to cathinone derivates, and zebrafish may be a useful model for understanding the relationship between the dopamine system and bath salts.

Downregulation of thioredoxin-1 in the ventral tegmental area delays extinction of methamphetamine-induced conditioned place preference

BACKGROUND

Drug addiction is characterized by compulsive drug use and relapse. Thioredoxin-1 is emerging as an important modulator involved in the cellular protective response against a variety of toxic stressors. Previous study has reported that thioredoxin-1 overexpression prevents the acquisition of methamphetamine-conditioned place preference. Here, we aimed to investigate the effect of thioredoxin-1 on methamphetamine-conditioned place preference extinction and the possible mechanism.

METHODS

(a) An extinction procedure in mice was employed to investigate the effect of thioredoxin-1 on the extinction of methamphetamine-conditioned place preference. After the acquisition of methamphetamine-conditioned place preference, mice underwent the following procedures: the injection of thioredoxin-1 small interfering RNA in the ventral tegmental area followed by the post-conditioned place preference test, four days of extinction training followed by four days of recovery after surgery. (b) The levels of thioredoxin-1, dopamine D1 receptor, tyrosine hydroxylase, phosphorylated extracellular regulated kinase, and phosphorylated cyclic adenosine monophosphate response element binding protein were examined by using Western blot analysis.

RESULTS

Thioredoxin-1 downregulation in the ventral tegmental area delayed methamphetamine-conditioned place preference extinction. The expression of thioredoxin-1 was decreased in the ventral tegmental area of mice in control and negative groups after methamphetamine-conditioned place preference extinction, but not in the thioredoxin-1 siRNA group. The levels of dopamine D1 receptor, tyrosine hydroxylase, phosphorylated extracellular regulated kinase, and phosphorylated cyclic adenosine monophosphate response element binding protein were decreased in the ventral tegmental area, nucleus accumbens, and prefrontal cortex of mice in the control and negative groups after methamphetamine-conditioned place preference extinction, but were inversely increased in thioredoxin-1 siRNA group.

CONCLUSIONS

The results suggest that downregulation of thioredoxin-1 in the ventral tegmental area may delay methamphetamine-conditioned place preference extinction by regulating the mesocorticolimbic dopaminergic signaling pathway.

A selective D3 receptor antagonist YQA14 attenuates methamphetamine-induced behavioral sensitization and conditioned place preference in mice

AIM

We have reported that a selective dopamine D3 receptor antagonist YQA14 attenuates cocaine reward and relapse to drug-seeking in mice. In the present study, we investigated whether YQA14 could inhibit methamphetamine (METH)-induced locomotor sensitization and conditioned place preference (CPP) in mice.

METHODS

Locomotor activity was monitored in mice treated with METH (1 mg/kg, ip) daily on d 4-13, followed by a challenge with METH (0.5 mg/kg) on d 21. CPP was examined in mice that were administered METH (1 mg/kg) or saline alternately on each other day for 8 days (METH conditioning). YQA14 was injected intraperitoneally 20 min prior to METH or saline.

RESULTS

Both repetitive (daily on d 4-13) and a single injection (on the day of challenge) of YQA14 (6.25, 12.5 and 25 mg/kg) dose-dependently inhibited the acquisition and expression of METH-induced locomotor sensitization. However, repetitive injection of YQA14 (daily during the METH conditioning) did not alter the acquisition of METH-induced CPP, whereas a single injection of YQA14 (prior to CPP test) dose-dependently attenuated the expression of METH-induced CPP. In addition, the repetitive injection of YQA14 dose-dependently facilitated the extinction and decreased the reinstatement of METH-induced CPP.

CONCLUSION

Brain D3 receptors are critically involved in the reward and psychomotor-stimulating effects of METH. Thus, YQA14 deserves further study as a potential medication for METH addiction.

Dopamine D3 receptors as a therapeutic target for methamphetamine dependence

BACKGROUND

Methamphetamine (MA) use disorders are major public health problems nationally and worldwide and treatment remains an unmet need.

OBJECTIVES

(1) To review preclinical and clinical studies identifying the dopamine D3 receptor as a therapeutic target for substance use disorders (SUDs), including MA dependence, (2) to consider buspirone (Buspar®) as a potential medication based on its dopamine D3 receptor antagonist properties, and (3) to evaluate the safety and initial efficacy of buspirone in a pilot study of MA-dependent individuals.

METHODS

Literature on the dopamine D3 receptor as a therapeutic target and on the potential of buspirone as a novel therapy for MA dependence was reviewed. The cardiovascular and subjective effects of intravenous MA challenge were assessed in five non-treatment seeking individuals. Participants met DSM-IV criteria for MA dependence and were treated subacutely (9 days) with buspirone (60 mg daily).

RESULTS

The literature identified the dopamine D3 receptor as a therapeutic target for MA dependence, a safe and approved medication, and a valuable opportunity to re-purpose buspirone for treating MA dependence and perhaps other SUDs. Pilot data (n = 5) indicated that buspirone is safe in MA-using individuals and comparison against historical placebo data from this laboratory suggested that at least some aspects of the subjective properties of MA may be diminished during buspirone treatment.

CONCLUSION

Future studies should include a small-scale, placebo-controlled Phase IIa trial of buspirone in MA dependence.

What is the role of the D3 receptor in addiction? A mini review of PET studies with [(11)C]-(+)-PHNO

The chronic use of drugs, including psychostimulants such as cocaine and amphetamine, has been associated with low D2/3 dopamine receptor availability, which in turn has been linked to poor clinical outcome. In contrast, recent studies focused on the D3 receptor (a member of the D2-like receptor family) suggest that chronic exposure to stimulant drugs can up-regulate this receptor subtype, which, in preclinical models, is linked to dopamine system sensitization - a process hypothesized to contribute to relapse in addiction. In this mini review we present recent human data suggesting that the D3 receptor may contribute to core features of addiction, and discuss the usefulness of the PET imaging probe [(11)C]-(+)-PHNO in investigating this question.

Dopamine D3 receptor-regulated NR2B subunits of N-methyl-d-aspartate receptors in the nucleus accumbens involves in morphine-induced locomotor activity

AIMS

Dopamine and glutamate receptors are densely expressed in the nucleus accumbens (NAc). Active interactions between these receptors contribute to the development of neuropsychiatric diseases, such as drug addiction and relapse. However, the molecular mechanisms underlying these interactions remain unclear.

METHODS

This study established a mouse model of intermittent morphine-induced mouse behavioral sensitization model. Western blot and electrophysiological recording methods were performed to directly identify the affective components of morphine behavioral sensitization.

RESULTS

Interval morphine administration could cause significant locomotor sensitization. Hyperlocomotion and behavioral locomotor sensitization were significantly suppressed when ifenprodil (5 mg/kg), a selective NR2B subunit-containing N-methyl-d-aspartate (NMDA) receptor antagonist, or nafadotride (25 μg/kg), a dopamine D3 receptor (D3R)-preferring antagonist, was coadministered with morphine. Western blot analysis showed that morphine behavioral sensitization induced a region-specific increase in phosphorylation of NR2B (pNR2B) and total levels of NR2B (NR2B) expression in the NAc. Systemically administered nafadotride attenuated behavioral locomotor sensitization induced by morphine and significantly reversed the overexpression of pNR2B and NR2B subunit-containing NMDA receptor in the NAc. NMDA receptor-mediated excitatory postsynaptic currents in the NAc were also significantly reduced by nafadotride.

CONCLUSIONS

These findings suggest that D3Rs are involved in morphine-induced behavioral locomotor sensitization in mice by regulating the NR2B subunits of NMDA receptors in the NAc.

Concurrent choice for social interaction and amphetamine using conditioned place preference in rats: effects of age and housing condition

BACKGROUND

Social interaction can serve as a natural reward that attenuates drug reward in rats; however, it is unknown if age or housing conditions alter the choice between social interaction and drug.

METHODS

Individually- and pair-housed adolescent and adult male rats were tested using conditioned place preference (CPP) in separate experiments in which: (1) social interaction was conditioned against no social interaction; (2) amphetamine (AMPH; 1mg/kg, s.c.) was conditioned against saline; or (3) social interaction was conditioned against AMPH.

RESULTS

Social interaction CPP was obtained only in individually-housed adolescents, whereas AMPH CPP was obtained in both individually-housed adolescents and adults; however, the effect of AMPH was not statistically significant in pair-housed adults. When allowed to choose concurrently between compartments paired with either social interaction or AMPH, individually-housed adolescents preferred the compartment paired with social interaction, whereas pair-housed adolescents preferred the compartment paired with AMPH. Regardless of housing condition, adults showed a similar preference for the compartments paired with either social interaction or AMPH.

CONCLUSIONS

Although some caution is needed in interpreting cross-experiment comparisons, the overall results suggest that individually-housed adolescents were most sensitive to the rewarding effect of social interaction, and this hypersensitivity to social reward effectively competed with AMPH reward.

Reduced expression of haloperidol conditioned catalepsy in rats by the dopamine D3 receptor antagonists nafadotride and NGB 2904

Haloperidol, a dopamine (DA) D2 receptor-preferring antagonist, produces catalepsy whereby animals maintain awkward posture for a period of time. Sub-threshold doses of haloperidol fail to produce catalepsy initially, however, when the drug is given repeatedly in the same test environment, gradual day-to-day increases in catalepsy are observed. More importantly, if sensitized rats are injected with saline instead of haloperidol they continue to be cataleptic in the test environment suggesting that environment-drug associations may play a role. DA D3 receptors have been implicated in a number of conditioned behaviors. We were interested if DA D3 receptors contribute to catalepsy sensitization and conditioning in rats. We tested this hypothesis using the DA D3 receptor-selective antagonist NGB 2904 (0.5, 1.8 mg/kg) and the DA D3 receptor-preferring antagonist nafadotride (0.1, 0.5 mg/kg). For 10 consecutive conditioning days rats were treated with one of the D3 receptor antagonists alone or in combination with haloperidol (0.25 mg/kg) and tested for catalepsy, quantified by the time a rat remained with its forepaws on a horizontal bar. On test day (day 11), rats were injected with saline or the D3 receptor antagonist and tested for conditioned catalepsy in the previously drug-paired environment. Rats treated with NGB 2904 or nafadotride alone did not develop catalepsy. Rats treated with haloperidol or haloperidol plus NGB 2904 or nafadotride developed catalepsy sensitization with repeated conditioning. When injected with saline they continued to exhibit catalepsy in the test environment--now conditioned. On the other hand, NGB 2904 (1.8 mg/kg) or nafadotride (0.5 mg/kg) given on the test day (after sensitization to haloperidol) significantly attenuated the expression of conditioned catalepsy. Our data suggest that the D3 receptor antagonist NGB 2904 (1.8 mg/kg) and nafadotride (0.5 mg/kg) significantly attenuate conditioned catalepsy in rats when given in test but not when given during sensitization. Results implicate DA D3 receptors in regulating the expression of conditioned catalepsy.

Closing the gap between clinic and cage: sensori-motor and cognitive behavioural testing regimens in neurotoxin-induced animal models of Parkinson's disease

Animal models that make use of chemical toxins to adversely affect the nigrostriatal dopaminergic pathway of rodents and primates have contributed significantly towards the development of symptomatic therapies for Parkinson's disease (PD) patients. Although their use in developing neuro-therapeutic and -regenerative compounds remains to be ascertained, toxin-based mammalian and a range of non-mammalian models of PD are important tools in the identification and validation of candidate biomarkers for earlier diagnosis, as well as in the development of novel treatments that are currently working their way into the clinic. Toxin models of PD have and continue to be important models to use for understanding the consequences of nigrostriatal dopamine cell loss. Functional assessment of these models is also a critical component for eventual translational success. Sensitive behavioural testing regimens for assessing the extent of dysfunction exhibited in the toxin models, the degree of protection or improvement afforded by potential treatment modalities, and the correlation of these findings with what is observed clinically in PD patients, ultimately determines whether a potential treatment moves to clinical trials. Here, we review existing published work that describes the use of such behavioural outcome measures associated with toxin models of parkinsonism. In particular, we focus on tests assessing sensorimotor and cognitive function, both of which are significantly and progressively impaired in PD.

Dopamine D3 receptor knock-out mice display deficits in locomotor sensitization after chronic morphine administration

Locomotor sensitization is the progressive and enduring enhancement of locomotion induced by stimulants such as drugs, which alter rodent locomotion in a long-standing manner. The dopamine D3 receptor has been reported to play a role in morphine addiction. The aim of the present study was to investigate the role of dopamine D3 receptor in the morphine induced locomotor sensitization using dopamine D3 receptor knock-out mice. The dopamine D3 receptor knock-out mice did not display an enhanced behavioral response to acute morphine administration or develop an increased rate of locomotor sensitization to intermittent morphine administration. When 2mg/kg naloxone was co-administered with 10mg/kg morphine, morphine-induced locomotion sensitization in wild-type mice was significantly blocked while the locomotion in the D3 receptor knock-out mice was decreased. Then the wild-type mice were administered with dopamine D3 antagonist nafadotride. It was found that co-administration of morphine with nafadotride could effectively suppress the level of morphine induced behavioral sensitization. It was concluded that a loss of the dopamine D3 receptor gene may inhibit acute morphine induced hyperlocomotor activity and chronic morphine induced behavioral sensitization.

A functional role for the dopamine D3 receptor in the induction and expression of behavioural sensitization to ethanol in mice

RATIONALE

We previously reported that mice lacking dopamine D3 receptors (D3R) were resistant to behavioural sensitization to ethanol (EtOH). Since knockout mice have permanent receptor inactivation, we investigated how temporary pharmacological blockade and activation of D3Rs affected the induction or expression of EtOH sensitization.

MATERIALS AND METHOD

In induction studies, DBA/2 mice received 0, 10 or 75 mg/kg, subcutaneous (s.c.) of the D3R antagonist, U99194A ,before each of seven EtOH (2.2 g/kg, intraperitoneal) or saline injections. Locomotor activity (LMA) was assessed in activity chambers. In expression studies, mice received seven injections of EtOH or saline, followed 14 days later with U99194A or vehicle immediately before a test dose of EtOH (1.8 g/kg). In separate experiments, the effects of the D3R agonist PD128907 (0.01 mg/kg, s.c.) were similarly examined during and after EtOH sensitization.

RESULTS

Chronic co-administration of low-dose U99194A blocked the induction of EtOH sensitization, while acute U99194A had no effect in mice that were already sensitized. Chronic co-administration of PD128907 resulted in decreased LMA but this effect was also seen in saline-treated mice, suggesting a simple subtractive effect. In previously sensitized mice, acute PD128907 significantly attenuated the expression of EtOH sensitization, indicating an enhanced response to the drug.

CONCLUSIONS

Results suggest a modulatory role for the D3R in behavioural sensitization to EtOH, where D3R blockade is associated with induction and D3R stimulation is associated with expression of EtOH sensitization. A model is suggested to account for these complementary functions of the D3R at different stages of EtOH sensitization.

The D(3) dopamine receptor inhibits dopamine release in PC-12/hD3 cells by autoreceptor signaling via PP-2B, CK1, and Cdk-5

The function of the D(3) dopamine (DA) receptor remains ambiguous largely because of the lack of selective D(3) receptor ligands. To investigate the function and intracellular signaling of D(3) receptors, we established a PC-12/hD3 clone, which expresses the human D(3) DA receptor in a DA producing cell line. In this model, we find that the D(3) receptor functions as an autoreceptor controlling neurotransmitter secretion. Pre-treatment with 3,6a,11, 14-tetrahydro-9-methoxy-2 methyl-(12H)-isoquino[1,2-b] pyrrolo[3,2-f][1,3] benzoxanzine-1-carboxylic acid, a D(3) receptor preferring agonist, dose-dependently suppressed K+-evoked [3H]DA release in PC-12/hD3 cells but not in the control cell line. This effect was prevented by D(3) receptor preferring antagonists GR103691 and SB277011-A. Furthermore, activation of D(3) receptors significantly inhibits forskolin-induced cAMP accumulation and leads to transient increases in phosphorylation of cyclin-dependent kinase 5 (Cdk5), dopamine and cAMP-regulated phosphoprotein of M(r) 32 000 and Akt. Because we observed differences in Cdk5 phosphorylation as well as Akt phosphorylation after DA stimulation, we probed the ability of Cdk5 and phosphatidylinositol-3 kinase (PI3K) to influence DA release. Cdk5 inhibitors, roscovitine, or olomoucine, but not the PI3K inhibitor wortmannin, blocked the D(3) receptor inhibition of DA release. In a complimentary experiment, over-expression of Cdk5 potentiated D(3) receptor suppression of DA release. Pertussis toxin, 3-[(2,4,6-trimethoxyphenyl)methylidenyl]-indolin-2-one and cyclosporine A also attenuated D(3) receptor-mediated inhibition of DA release indicating that this phenomenon acts through Gi/oalpha and casein kinase 1, and phosphatase protein phosphatase 2B (calcineurin), respectively. In support of previous data that D(3) DA receptors reduce transmitter release from nerve terminals, the current results demonstrate that D(3) DA receptors function as autoreceptors to inhibit DA release and that a signaling pathway involving Cdk5 is essential to this regulation.

Differential susceptibility to ethanol and amphetamine sensitization in dopamine D3 receptor-deficient mice

RATIONALE

Dopamine D3 receptors (D3Rs) have been implicated in behavioral sensitization to various drugs of abuse, but their role in ethanol (EtOH) sensitization has not been directly examined. We used D3R knockout (D3 KO) mice to examine whether the D3R plays a permissive role in EtOH and amphetamine (AMPH) sensitization. We also investigated whether EtOH sensitization is accompanied by alterations in D3R mRNA expression or binding.

MATERIALS AND METHODS

After comparing EtOH sensitization in C57Bl/6 mice and DBA/2 mice, D3 KO, wild type (WT), and for comparison, D1 and D2 KOs received five biweekly injections of EtOH (2.2 g/kg, i.p.) or saline. Another group of D3 KOs and WT controls received six times AMPH (1.5 mg/kg, i.p.). D3R mRNA and binding were measured in EtOH-sensitized DBA/2 mice with in situ hybridization and [(125)I]-7-OH-PIPAT autoradiography, respectively.

RESULTS

C57Bl/6 mice expressed EtOH sensitization albeit to a lesser extent than DBA/2 mice. Compared to WT mice, D3 KOs were resistant to EtOH sensitization. The behavioral profile of D3 KOs was more similar to D1 KOs than D2 KOs, which also failed to develop EtOH sensitization. However, D3 KOs developed AMPH sensitization normally. EtOH sensitization was not accompanied by changes in either D3R mRNA or D3R binding in the islands of Calleja, nucleus accumbens, dorsal striatum, or cerebellum.

CONCLUSIONS

These results suggest a necessary role for the D3R in EtOH but not AMPH sensitization, possibly through postreceptor intracellular mechanisms. Results also suggest that different neurochemical mechanisms underlie sensitization to different drugs of abuse.

Differences in aggressive behavior and in the mesocorticolimbic DA system between A/J and BALB/cJ mice

The neurotransmitters DA and serotonin are known to be important modulators of aggression, but endogenous differences in these systems between aggressive and nonaggressive animals are poorly understood. To examine this issue, the mesocorticolimbic DA and serotonin systems of two mouse strains that differ in aggressive behavior, BALB/cJ and A/J, were analyzed using high performance liquid chromatography and quantitative receptor autoradiography. Significant differences in both serotonergic and dopaminergic systems were found between aggressive and nonaggressive mice. The nonaggressive A/J mice exhibited higher DA utilization in the nucleus accumbens and prefrontal cortex, higher D1 receptor expression in the rostral pole of the accumbens, and lower D2 receptor expression throughout the accumbens, as compared with aggressive BALB/cJ mice. Although correlative in nature, these data suggest that differences in mesocorticolimbic DA and serotonin systems may contribute to endogenous differences in aggression.

Role of dopamine D1-like receptors in methamphetamine locomotor responses of D2 receptor knockout mice

Behavioral sensitization to psychostimulants manifests as an increased locomotor response with repeated administration. Dopamine systems are accepted to play a fundamental role in sensitization, but the role of specific dopamine receptor subtypes has not been completely defined. This study used the combination of dopamine D2 receptor-deficient mice and a D1-like antagonist to examine dopamine D1 and D2 receptor involvement in acute and sensitized locomotor responses to methamphetamine. Absence of the dopamine D2 receptor resulted in attenuation of the acute stimulant effects of methamphetamine. Mutant and wild-type mice exhibited sensitization that lasted longer within the time period of the challenge test in the mutant animals. Pretreatment with the D1-like receptor antagonist SCH 23390 produced more potent reductions in the acute and sensitized locomotor responses to methamphetamine in D2 receptor-deficient mice than in wild-type mice; however, the expression of locomotor sensitization when challenged with methamphetamine alone was equivalently attenuated by previous treatment with SCH 23390. These data suggest that dopamine D2 receptors play a key role in the acute stimulant and sensitizing effects of methamphetamine and act in concert with D1-like receptors to influence the acquisition of methamphetamine-induced behavioral sensitization, traits that may influence continued methamphetamine use.

Chronic d-amphetamine depresses an imaging marker of arachidonic acid metabolism in rat brain

Acute d-amphetamine (d-Amph) administration to rats leads to the release of arachidonic acid (AA, 20:4n-6) as a second messenger following indirect agonism at dopamine D2-like receptors in the brain. We hypothesized that chronically administered d-Amph in rats also would alter brain AA metabolism and signalling. To test this, adult male rats were injected i.p. daily for 2 wk with saline or 2.5 mg/kg d-Amph. After a 1-d washout, the unanaesthetized rats were injected acutely with i.v. saline, 1 mg/kg quinpirole (a D2-like receptor agonist) or 5.0 mg/kg SKF-38393 (a D1-like receptor agonist), followed by i.v. [1-14C]AA. The AA incorporation coefficient k* (brain radioactivity/integrated plasma radioactivity), a marker of AA signalling and metabolism, was quantified using autoradiography in each of 62 brain regions. Compared with chronic saline, chronic d-Amph widely decreased baseline values of k* in brain regions having D2-like receptors. On the other hand, chronic amphetamine did not alter the k* responses to quinpirole seen in chronic saline-treated rats. SKF-38393 had minimal effects on k* in both chronic saline-treated and amphetamine-treated rats, consistent with D1-like receptors not being coupled to AA signalling. The ability of chronic d-Amph after 1-d washout to down-regulate baseline values of k* probably reflects neuroplastic changes in brain AA signalling, and may correspond to depressive behaviours noted following withdrawal from chronic amphetamine in humans and in rats.

The dopamine D3/D2 agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] protects against acute and cocaine-kindled seizures in mice: further evidence for the involvement of D3 receptors

Previous findings have demonstrated a protective role for dopamine D(3)/D(2) receptor agonists in the convulsant and lethal effects of acutely administered cocaine. Data are provided here to establish that the protection occurs through a D(3)-linked mechanism and that protection is extended to seizure kindling. The D(3) antagonist SB-277011-A [4-quinolinecarboxamide,N-[trans-4-[2-(6-cyano-3,4-dihydro-2(1H)-isoquinolinyl)ethyl]-cyclohexyl]-(9CI)] prevented the anticonvulsant effect of the D(3)/D(2) receptor agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] on cocaine-induced seizures. The protection afforded by the D(3)/D(2) agonist, (+)-PD-128,907, was eliminated in D(3) receptor-deficient mice. In D(2) receptor knockout mice, the anticonvulsant effects of (+)-PD-128,907 were preserved. (+)-PD-128,907 also prevented the acquisition and expression of cocaine-kindled seizures engendered by repeated daily dosing with 60 mg/kg cocaine. (+)-PD-128,907 also blocked the seizures induced in mice fully seizure kindled to cocaine. Although repeated dosing with cocaine increased the potency of cocaine to produce seizures and lethality (decreased ED(50) values), daily coadministration of (+)-PD-128,907 significantly prevented this potency shift. In mice treated daily with cocaine and (+)-PD-128,907, the density, but not the affinity, of D(3) receptors was increased. The specificity with which (+)-PD-128,907 acts upon this cocaine-driven process was demonstrated by the lack of a significant effect of (+)-PD-128,907 on seizure kindling to a GABA(A) receptor antagonist, pentylenetetrazol. Taken together and with literature findings, the data indicate that dopamine D(3) receptors function in the initiation of a dampening mechanism against the toxic effects of cocaine, a finding that might have relevance to psychiatric disorders of drug dependence, schizophrenia, and bipolar depression.

Biological treatments for amfetamine dependence : recent progress

Amfetamine abuse has grown into a worldwide epidemic. Methamfetamine, a derivative of amfetamine made from readily accessible chemicals, has plagued the US since the 1960s, with an alarming recent surge in the numbers of those meeting the criteria for amfetamine abuse and dependence. We review this problem using a computerised literature search (PubMed 1964-2007) to summarise knowledge from animal and human studies about treatments for amfetamine dependence, while exploring the potential of pharmacogenetics to help uncover new treatment targets. Several promising therapeutic targets have come from animal models of reward, drug-taking behaviour and withdrawal. Although preclinical and selected clinical results have been promising, clinical studies have yielded inconsistent results. To improve these outcomes, pharmacogenetic studies may be used to identify candidate alleles that predict therapeutic response. Exciting preclinical findings and a steady progression of clinical results offer hope for the development of a treatment for amfetamine dependence.

Divergent effect of the selective D3 receptor agonist pd-128,907 on locomotor activity in Roman high- and low-avoidance rats: relationship to NGFI-A gene expression in the Calleja islands

RATIONALE

The inbred Roman high- (RHA-I) and low-avoidance (RLA-I) rats, differing in dopaminergic activity and novelty/substance-seeking profiles, may be a suitable model to study the implication of the dopaminergic system in vulnerability to drug abuse. Differences in D3 receptor binding recently described between the two strains (Guitart-Masip M, Johansson B, Fernández-Teruel A, Cañete T, Tobeña A, Terenius L, Giménez-Llort L, Neuroscience 142:1231-1243, 2006b) may be important in shaping the aforementioned differences in novelty seeking.

OBJECTIVE

The aim of the present work was to study the effect of D3 receptor activation on novelty-induced locomotor activity in these two strains of rats.

MATERIALS AND METHODS

We administered saline and PD-128,907 (0.01 and 0.1 mg/kg), a putative D3 receptor agonist, to the Roman rats and studied the locomotor activity when animals were placed in a novel environment. Thereafter, by means of in situ hybridization, nerve growth factor inducible clone A (NGFI-A) mRNA was measured in the striatum and the Calleja islands of these animals.

RESULTS

We found that RLA-I rats showed stronger locomotor inhibition than RHA-I rats after PD-128,907 administration. Moreover, RLA-I rats showed stronger reduction of NGFI-A mRNA in the Calleja islands than RHA-I rats.

CONCLUSIONS

These results, together with previous findings, suggest that differences in D3 receptor expression in the Calleja islands may contribute to the divergent behavioral effect of PD-128,907 administration in the two strains of Roman rats.

The role of the cannabinoid type 1 receptor and down-stream cAMP/DARPP-32 signal in the nucleus accumbens of methamphetamine-sensitized rats

Blockade of the cannabinoid type 1 (CB(1)) receptor could suppress methamphetamine self-administration; however, the cellular mechanism remains unclear. In this study, we intended to investigate the significance of brain CB(1) receptors on the development of behavioral sensitization to methamphetamine. Male Sprague-Dawley rats treated with chronic methamphetamine (4 mg/kg, i.p.) for either 7 or 14 days developed behavioral sensitization to methamphetamine (1 mg/kg) at withdrawal day 7. A progressive decrease in numbers of CB(1) receptor (both Bmax and mRNA) but increase in binding affinity (Kd) was noticed during withdrawal days 3 to 7. Microinjection of CB(1) antagonist [5-(4-bromophenyl)-1-(2,4-dichlorophenyl)-4-ethyl-N-(1-piperidinyl)-1H-pyrazole-3-carboxamide] into the nucleus accumbens (NAc) at withdrawal day 7, significantly suppressed the behavioral sensitization to methamphetamine. In NAc brain slices preparation, acute incubation with CB(1) agonist (1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexan-1-ol (CP 55940) dose-dependently enhanced cAMP accumulation in sensitized rats; no change was noticed in control groups. Consequently, treatment of CP 55940 induced a dose-dependent (10 nmol/L-10 micromol/L) phosphorylation on down-stream dopamine and cAMP-regulated phosphoprotein of Mr 32 000 (DARPP-32)/Thr34 in sensitized rats, while only 10 micromol/L CP 55940 was able to enhance the phosphoDARPP-32/T34 in control groups. Alternatively, both basal activity of calcineurin (PP-2B) and CP 55940-induced changes in the amount of PP-2B in the NAc were both decreased in sensitized rats, but not in controls. Overall, we demonstrated that brain CB(1) receptor and its down-stream cAMP/DARPP-32/T34/PP-2B signaling are profoundly altered in methamphetamine-sensitized animals.

Dual alteration of limbic dopamine D1receptor-mediated signalling and the Akt/GSK3 pathway in dopamine D3receptor mutants during the development of methamphetamine sensitization

The central dopamine system plays significant roles in motor activity and drug-induced behavioural sensitization. Our goal was to determine the significance of dopamine D(3) receptors in the development of behavioural sensitization to methamphetamine, assessed with D(3) receptor mutant mice. The absence of D(3) receptors significantly increased the behavioural responses to acute methamphetamine and evoked a faster rate of behavioural sensitization to chronic methamphetamine. In addition, both D(3) receptor protein and mRNA levels in the limbic forebrain decreased in sensitized wild-type mice. Further analyses indicated that D(1)-dependent behavioural sensitization and the number of limbic D(1) receptors increased in sensitized D(3) mutants as compared with sensitized wild-type mice. Consistent with this finding, we observed higher levels of D(1) receptor-evoked cAMP accumulation and basal phosphoDARPP-32/Thr34 in the limbic forebrain of D(3) mutants than wild-type mice and the difference was more pronounced after chronic methamphetamine treatment. We also observed an increase in phospho-extracellular signal-regulated kinase 2 but a decrease in phosphoAkt/Ser473 and phosphoglycogen synthase kinase 3 (GSK3)-alpha/beta in the limbic forebrain of D(3) mutants compared with wild-type mice after methamphetamine treatment. The convergent results implicate D(3) receptors as a negative regulator of the development of methamphetamine sensitization. A compensatory up-regulation of D(1) receptor-mediated signals, in addition to an altered Akt/GSK3 pathway, could contribute to the accelerated development of behavioural sensitization.

Dopamine D3 receptor antisense oligodeoxynucleotide potentiates imipramine-induced dopaminergic behavioural supersensitivity

Chronic antidepressant treatments result in the potentiation of dopaminergic transmission in the mesolimbic dopamine system revealed as an increased motor response to dopamine D2-like agonists. On the basis of the involvement of this system in the control of motivation and reward-related behaviour, which are impaired in depression, it has been suggested that such supersensitivity might play an important role in the mechanism of action of these drugs. Several studies have provided evidence suggesting a role of dopamine D3 receptors in mediating antidepressant-induced increased motor response to dopamine agonists. To test this hypothesis, we studied the effect of the intracerebroventricular infusion of a dopamine D3 receptor antisense oligodeoxynucleotide (10 microg/3 microl, 2-3 daily injections) on the expression of imipramine-induced supersensitivity (20 mg/kg daily intraperitoneal injections for 21 days) to the motor effect of the dopamine D2-like receptor agonist quinpirole (a single 0.3 mg/kg subcutaneous injection 24-48 h after imipramine withdrawal). The results show that a treatment previously shown to reduce the synthesis of dopamine D3 receptors, rather than resulting in an inhibitory effect, potentiated the ability of imipramine to induce dopaminergic motor supersensitivity. The present results suggest that increased dopamine D3 receptor expression following antidepressant treatments is not involved in the mechanism of dopaminergic supersensitivity, and are consistent with evidence supporting an inhibitory role for dopamine D3 receptors in motor activity, both in normal and in sensitized subjects.

Behavioral sensitization, alternative splicing, and d3 dopamine receptor-mediated inhibitory function

Behavioral sensitization, the progressive and enduring augmentation of certain behaviors following repetitive drug use, alters rodent locomotion in a long-standing manner. The same dopamine pathways playing an important role in drug dependence and psychosis also play a critical role in sensitization. Individual dopamine receptor subtypes have markedly different functional responses to stimulation, with D3 dopamine receptor stimulation inhibiting rodent locomotion. The D3 receptor has highest affinity of the dopamine receptor subtypes for dopamine, and is occupied to a greater degree following stimulant drug administration. D3 receptor activity may be regulated through the expression of an alternatively spliced, truncated receptor isoform (termed 'D3nf') altering receptor localization and function via dimerization with the full-length subunit. The expected physiological response to repetitive drug administration is tolerance. Tolerance of D3 receptor inhibition of locomotion would contribute to sensitization to stimulant drugs. We hypothesize that repetitive D3 receptor stimulation contributes to the development of behavioral sensitization through decreased responsivity of D3-receptor-mediated locomotor inhibition. Increased D3nf expression may direct altered receptor localization and subsequent release of D3-receptor-mediated inhibition, contributing to the expression of sensitization. These hypotheses follow directly from the affinities of the receptor subtypes for dopamine; dopamine concentrations following stimulant administration; the effects of individual dopamine receptor subtype stimulation on locomotion; and the expected homeostatic response of the system to perturbation by drug. Clarifying these mechanisms underlying sensitization may suggest new interventions for neuropsychiatric conditions in which dopamine plays an important role, including psychosis, drug dependence, and Parkinson's disease. This information may also elucidate a previously unrecognized mechanism regulating receptor trafficking and desensitization.

Divergent anatomical pattern of D1 and D3 binding and dopamine- and cyclic AMP-regulated phosphoprotein of 32 kDa mRNA expression in the Roman rat strains: Implications for drug addiction

Autoradiography analysis of D1, D2 and D3 dopamine receptors and in situ hybridization analysis of mRNA for dopamine and cAMP regulated phosphoprotein of 32 kDa (DARPP-32) were performed in brains of naïve Roman high avoidance (RHA) and Roman low avoidance (RLA) inbred rats. These strains, genetically selected for high (RHA) or extremely low (RLA) active avoidance acquisition in the two-way shuttle box, differ in indices of dopaminergic activity along with sensation/novelty and substance-seeking behavioral profiles. The present study shows no differences in D2 receptor binding between the two strains. In contrast, the D1 and D3 receptor binding in the nucleus accumbens was higher in RHA-I rats, whereas RLA-I rats show higher D3 binding in the Calleja islands. Together with previous evidence showing behavioral and presynaptic differences related to the dopamine system, the present results suggest a higher dopaminergic tone at the nucleus accumbens shell in RHA-I rats. Besides, the comparison of the expression pattern of DARPP-32 mRNA with that of dopamine receptor binding revealed a mismatch in some amygdala nuclei. In some cortical structures (prelimbic and cingulate cortices, the dentate gyrus) as well as in the central amygdala, RHA-I rats showed higher DARPP-32 mRNA expression than RLA-I rats. Hence, RHA-I and RLA-I rats may be a useful tool to identify dopamine-related mechanisms that predispose to drug and alcohol dependence.

Methamphetamine reward in mice as assessed by conditioned place preference test with Supermex sensors: effect of subchronic clorgyline pretreatment

Recent studies in our laboratory have shown that methamphetamine (METH)-induced hyperlocomotion and behavioral sensitization in mice were inhibited by clorgyline, an irreversible monoamine oxidase inhibitor. In this study, the effect of clorgyline pretreatment on METH-induced rewarding effect was assessed by a conditioned place preference (CPP) test, using an apparatus developed with Supermex sensors (infrared pyroelectric sensors). Although intact male ICR mice showed significant CPP for METH (0.5 mg/kg, i.p.), pretreatment with subchronic clorgyline (0.1 and 10 mg/kg, s.c.) did not affect the magnitude of CPP. At a dose of 1 mg/kg, pretreatment of the mice with clorgyline showed a similar CPP index in both saline/saline and METH/saline pairing groups. During the conditioning session, the mice did not express behavioral sensitization to METH. Pretreatment with clorgyline (0.1, 1, and 10 mg/kg) decreased striatal apparent monoamine turnover in a dose-dependent manner. These results indicated that clorgyline pretreatment (0.1 and 10 mg/kg) did not influence the METH-induced rewarding effect in mice, although pretreatment of the mice with clorgyline at a dose of 1 mg/kg appeared to influence the CPP for METH.

Risperidone pretreatment prevents elevated locomotor activity following neonatal hippocampal lesions

Long-standing behavioral abnormalities emerge after puberty in rats following neonatal hippocampal lesion, providing a developmental model of abnormal rat behavior that may have predictive validity in identifying compounds effective in treating symptoms of schizophrenia. We sought to test the predictive validity of the neonatal hippocampal lesion model in identifying preventive treatment for first-episode psychosis. We determined the effect of risperidone, recently studied for prevention of first-episode psychosis, on the development of elevated locomotor activity following neonatal hippocampal lesions. Rat pups received hippocampal or sham lesions on postnatal day 7, followed by treatment with risperidone or vehicle from postnatal days 35 to 56. Locomotor activity in response to novelty, amphetamine, and nocturnal locomotion were determined on postnatal day 57. Low-dose risperidone (45 microg/kg) pretreatment prevented elevated locomotor activity in some, but not all, of the behavioral tasks following neonatal hippocampal lesions. In contrast, higher risperidone pretreatment was less effective in preventing elevated locomotor activity following neonatal hippocampal lesions. Because low risperidone dosages were also found to be effective in preventing first-episode psychosis in human studies, these data support the predictive validity of the hippocampal lesion model in identifying medications for prevention of first-episode psychosis. Additionally, these data support the use of low-dose risperidone in psychosis prevention, and suggest the possibility that higher risperidone doses could be less effective in this application.

Acute and repeated intravenous cocaine-induced locomotor activity is altered as a function of sex and gonadectomy

The present experiment examined the effects of sex and gonadectomy on cocaine-induced locomotor activity via intravenous (IV) cocaine. Male, female, castrated (CAST), and ovariectomized (OVX) rats received daily IV cocaine injections (3.0 mg/kg/injection) for 13 consecutive days. Locomotor activity was measured in automated activity chambers for 60 min following the baseline-saline administration and after the 1st and 13th cocaine injections. Observational time sampling was also performed, and the observational data were grouped into locomotor and orofacial composite incidence scores. Females exhibited more cocaine-induced locomotor activity, rearing, and locomotor incidence compared to males. The orofacial data revealed a sex difference in the expression of behavioral sensitization: females exhibited more orofacial behaviors than males after repeated, but not acute, cocaine injection. Females exhibited more cocaine-induced locomotor activity, rearing, and locomotor incidence compared to OVX rats, but exhibited less orofacial incidence following acute cocaine administration. There were no differences between male and CAST rats. CAST rats showed more locomotor incidence than OVX after repeated, but not acute, cocaine injection. CAST rats exhibited behavioral sensitization, whereas OVX rats' locomotor incidence did not change with repeated cocaine injection. CAST rats showed less orofacial incidence than OVX after acute, but not repeated, cocaine injection. These findings demonstrate sex differences in response to IV cocaine and replicate earlier findings which show that OVX attenuates increased locomotor activity in females. Furthermore, these findings suggest that IV cocaine administration produces behavioral differences between male and female rats in the absence of circulating gonadal hormones.

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.

Enhanced Cdk5 activity and p35 translocation in the ventral striatum of acute and chronic methamphetamine-treated rats

The cyclin-dependent kinase Cdk5 and DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of Mr 32 kDa)-dependent signaling have been implicated in the regulation of dopaminergic neurotransmission after chronic cocaine treatment. In this study, we examined if Cdk5 signaling participates in the behavioral and biochemical effect of acute and chronic methamphetamine (METH) treatment. We found that Cdk5 activity and the membrane fraction of p35 protein, a Cdk5 activator, in the ventral striatum increased transiently after an injection of 4 mg/kg METH, while intra-accumbens treatment with a Cdk5 inhibitor, roscovitine, prevented the acute METH-induced locomotor activation. The phosphorylation of DARPP-32 at both Thr75 and Thr34 was differentially regulated after acute METH treatment, but the levels of total Cdk5, p35, and DARPP-32 remained the same. To determine if Cdk5 signaling was associated with behavior sensitization to METH, rats that received repetitive injections of METH (4 mg/kg) for 14 consecutive days were analyzed at withdrawal day 7. The results indicate that Cdk5 activity and p35 translocation in the ventral striatum were upregulated in METH-sensitized rats; treatment with roscovitine in the nucleus accumbens effectively suppressed the 1 mg/kg METH-induced behavioral sensitization. Concomitantly, a decrease in the amount of PP-2A and DARPP-32 phosphorylation at Thr34, but an increase in phosphorylation of DARPP-32/Thr75, was observed in the ventral striatum of sensitized rats. The overall results demonstrate that Cdk5/p35 and downstream signaling in the ventral striatum play a critical role in the effects of acute METH treatment as well as the development of behavioral METH sensitization.

Diurnal rhythms in quinpirole-induced locomotor behaviors and striatal D2/D3 receptor levels in mice

Dopaminergic drugs, including the D2/D3 agonist quinpirole, produce lasting changes in the brain that lead to altered behavioral responses. The action of these drugs is dosing time-dependent; in fruit flies, behavioral response to quinpirole shows a marked circadian variability. Here we demonstrate diurnal rhythm-dependent variations both in quinpirole-induced locomotor behaviors and in striatal D2 and D3 protein levels in mice. We found opposing diurnal rhythms in striatal D2 and D3 protein levels, resulting in a high D2/D3 ratio during the day and a low D2/D3 ratio at night. Protracted quinpirole treatment differentially altered striatal D2/D3 rhythms depending on the time of injection (i.e., day or night). When quinpirole-induced locomotor activity was analyzed for 90 min, we found hypomotility after the first day or nighttime drug injection. By the seventh injection, daytime quinpirole treatment produced clear hyperactivity while nighttime quinpirole treatment continued to induce a significant initial hypoactivity followed by a hyperactivity period. Our data indicate that quinpirole-induced long-term alterations in the brain include dosing time-dependent changes in dopamine receptor rhythms. Therefore, we propose that diurnal mechanisms, which participate in drug-induced long-term changes in the dopamine system, are important for the development of dopaminergic behaviors.

Sex differences and repeated intravenous nicotine: behavioral sensitization and dopamine receptors

The present study examined the sex-dependent expression of behavioral sensitization as well as changes of dopamine (DA) transporters and D1, D2, and D3 receptors following repeated intravenous nicotine administration. Male and female Sprague-Dawley rats were implanted with indwelling jugular catheters, equipped with subcutaneous intravenous injection ports. Rats were habituated to activity chambers for 3 days and were subsequently administered 15-s bolus injections of intravenous nicotine (50 microg/kg/ml) 1/day for 21 days. Animals were placed in activity chambers for 60 min immediately after the 1st and 21st nicotine injection. Observational time sampling was also performed. Brains were subsequently removed and frozen for autoradiographic DA transporter/DA receptor analysis on the afternoon females were in proestrus. With one exception, no robust sex differences were observed for locomotor activity or any rearing measures either during baseline or after initial nicotine injection. Females exhibited markedly more behavioral sensitization of locomotor activity, rearing, duration of rearing, and incidence of observed rearing. There were no sex differences in the number of D1 or D2 receptors. Females exhibited an increased number of DA transporters and decreased D3 receptors in the NAcc, relative to males. Multiple regression analyses suggest that D3 receptors and DA transporters in various striatal and NAcc subregions differentially predicted nicotine-induced behaviors for males and females. Collectively, these findings demonstrate that repeated intravenous nicotine produces sex differences in the expression of behavioral sensitization, and suggest that nicotine-induced changes of DA transporters and D3 receptors are partly responsible for increased behavioral sensitization in female rats.

Dopamine D3 receptor stimulation promotes the proliferation of cells derived from the post-natal subventricular zone

In the adult mammalian brain, neural stem cells persist in the subventricular zone (SVZ) where dopamine D3 receptors are expressed. Here, we demonstrate that addition of 1 microm apomorphine increases cell numbers in post-natal SVZ cell cultures. This effect was prevented by a co-treatment with haloperidol, sulpiride or U-99194A, a D3-preferring antagonist, and mimicked by the dopamine D3 receptor selective agonist 7-hydroxy-dipropylaminotetralin (7-OH-DPAT). EC50 values were 4.04 +/- 1.54 nm for apomorphine and 0.63 +/- 0.13 nm for 7-OH-DPAT, which fits the pharmacological profile of the D3 receptor. D3 receptors were detected in SVZ cells by RT-PCR and immunocytochemistry. D3 receptors were expressed in numerous beta-III tubulin immunopositive cells. The fraction of apoptotic nuclei remained unchanged following apomorphine treatment, thus ruling out any possible effect on cell survival. In contrast, proliferation was increased as both the proportion of nuclei incorporating bromo-deoxyuridine and the expression of the cell division marker cyclin D1 were enhanced. These findings provide support for a regulatory role of dopamine over cellular dynamics in post-natal SVZ.

Behavioral sensitization to intermittent morphine in mice is accompanied by reduced adrenocorticotropine but not corticosterone responses

The present study was aimed to test the hypothesis that behavioral sensitization to intermittent administration of morphine is accompanied by sensitization of adrenocorticotropine (ACTH) and corticosterone responses and with signs of hyperactivity of the hypothalamic-pituitary-adrenocortical function in mice. Male mice were injected subcutaneously with 40 mg/kg morphine or saline every 72 h for 16 days (in total, six injections were performed) and the effects were compared to those after single drug injection. Hormones were investigated 60 min after the last (6th) morphine or saline injection, i.e. 3 days after the 5th injection of intermittent treatments. Locomotor sensitization was confirmed in a separate series. ACTH levels in response to the last morphine injection of the repeated dosage regimen were found to be lower compared to those in acutely morphine-treated mice. Morphine administration was followed by increases in plasma corticosterone, but no significant differences between the acutely and repeatedly treated groups were observed. The body weight of morphine-treated mice showed a characteristic pattern with decreases measured the day after morphine administration. No statistically significant differences in adrenal and thymus weights were found. In conclusion, behavioral sensitization to morphine in mice is accompanied by a blunted rather than an enhanced ACTH response to drug injection. Unchanged levels of plasma corticosterone demonstrate an absence of tolerance and possible involvement of ACTH unrelated mechanisms needs further verification. Intermittent administration of morphine for 2 weeks failed to induce marked signs of glucocorticoid overexposure.