Acute dopamine D<Subscript>1</Subscript> and D<Subscript>2</Subscript> receptor stimulation does not modulate mismatch negativity (MMN) in healthy human subjects

Rationale Schizophrenia is commonly associated with an impairment in pre-attentive change detection, as represented by reduced mismatch negativity (MMN), an auditory event related potential. While the neurochemical basis of MMN has been linked to the integrity of the glutamatergic system involving N-methyl-d-aspartate (NMDA) receptors, the role of the dopaminergic system and in particular, the role of D₁ and D₂ receptors on MMN is yet to be determined. Objectives The aim of the present project was to investigate the acute effects of dopamine D₂ (bromocriptine) and D₁/D₂ (pergolide) receptor stimulation on the human MMN in healthy subjects. Methods Fifteen healthy male subjects participated in a double-blind, placebo-controlled, cross-over design in which each subject was tested under three acute treatment conditions separated by a 1-week wash out period; placebo, bromocriptine (2.5 mg) and pergolide (0.1 mg). The subjects were exposed to a duration-MMN paradigm with 50 ms standard tones (91%) and 100 ms deviant tones (9%). Results The results showed that neither D₂ receptor stimulation with bromocriptine, nor simultaneous D₁ and D₂ receptor stimulation with pergolide, modulated MMN. Conclusions These findings suggest that acute D₁ and D₂ receptor stimulation does not modulate MMN. While the role of dopamine cannot be completely ruled out, the findings support the view that the aberrant MMN reported in schizophrenia may be linked primarily to glutamate dysfunction involving NMDA receptors.     

Involvement of dopamine D<Subscript>1</Subscript> and D<Subscript>2</Subscript> receptors in the nucleus accumbens core and shell in inhibitory response control

Rationale Impaired inhibitory control over behavior is a key feature in various psychiatric disorders, and recent studies indicated an important role for dopamine D₁ and D₂ receptors and the nucleus accumbens (Acb) in this respect. Objective The present experiments were designed to study the role of dopamine D₁ and D₂ receptors in the Acb in inhibitory response control. Methods Rats were trained in a five-choice serial reaction time task and received bilateral infusions into the Acb core or shell of either SCH 23390 or eticlopride (representing selective dopamine D₁ and D₂ receptor antagonists, respectively). Subsequently, the effects of systemic amphetamine on inhibitory response control were examined. Results Eticlopride into either the Acb core or shell did not affect premature responding, a measure for inhibitory response control, but increased reaction time and errors of omission. In contrast, SCH 23390 into both regions reduced premature responding, slightly improved attentional performance in the core and increased errors of omission in the shell. Amphetamine robustly increased premature responding which was dose-dependently blocked by eticlopride in the Acb core and attenuated by eticlopride in the shell. In addition, amphetamine slightly decreased accuracy and reaction time, and these effects were inhibited by eticlopride in both regions. SCH 23390 infusion into the Acb core or shell did not alter amphetamine’s effects. Conclusion Our data provide evidence for the involvement of dopamine D₁ and D₂ receptors in the Acb core and shell in inhibitory response control and attentional performance.     

Effect of dopamine D<Subscript>3</Subscript> receptor blockade on renal function and glomerular size in diabetic rats

Dopamine D₂-like receptors, including D₂, D₃, and D₄ receptors, are involved in the regulation of glomerular hyperfiltration due to diabetes mellitus. These hemodynamic alterations represent a risk factor for the later development of diabetic nephropathy. The aim of the present study was to determine whether the D₃ receptor subtype modulates the diabetes-induced increase in glomerular filtration rate (GFR) in rats. Renal function was studied in Sprague–Dawley rats 14 days after induction of a moderate diabetes mellitus (DM) by streptozotocin and in non-diabetic controls (CON). Rats were orally treated either with the peripherally acting, selective dopamine D₃ receptor antagonist BSF 135170 (BSF, 10 mg/kg per day for 2 weeks) or with vehicle (VHC). Perfusion-fixed kidneys were used for estimation of glomerular volume. In conscious rats, which were treated with BSF, the DM-induced increase in fluid intake, urinary output, and renal sodium excretion was significantly less pronounced than in the vehicle group (DM-VHC). In the clearance experiments, GFR in CON was about 0.84±0.04 ml/min per 100 g body weight. The DM-VHC group presented a significant glomerular hyperfiltration (1.09±0.04 ml/min per 100 g body weight). Treatment with BSF significantly lowered GFR towards levels of CON. The estimated glomerular volume was 0.73±0.03×10⁶ m³ in the CON-VHC group and 0.86±0.04×10⁶ m³ in the DM-VHC animals. Interestingly, treatment with BSF decreased the glomerular volume in both groups. Irrespective of BSF treatment, kidney wet weight related to body weight was about 36% higher in DM animals compared with CON animals. We conclude that dopamine D₃ receptors represent a target for the modulation of diabetes-induced glomerular hyperfiltration. Therefore, the results encourage the testing of the possible beneficial effects of long-term D₃ receptor blockade on the development of diabetic nephropathy.     

Blockade of mesolimbic dopamine D<Subscript>3</Subscript> receptors inhibits stress-induced reinstatement of cocaine-seeking in rats

Rationale The dopamine (DA) D₃ receptor is preferentially expressed in the mesolimbic system. We have previously shown that selective D₃ receptor blockade by the novel D₃ antagonist SB-277011A inhibits cocaines reinforcing action and cocaine-induced reinstatement of cocaine-seeking behavior.Objective In the present study, we investigated whether SB-277011A similarly inhibits stress-induced reinstatement of cocaine-seeking behavior.Methods Rats were allowed to self-administer cocaine (0.5 mg/kg per infusion, 3 h per session) for 10–14 days, followed by a once-daily extinction session for 7–14 days during which saline was substituted for cocaine. Extinction criteria were fewer than ten lever-presses per 3-h session for at least 3 consecutive days. After cocaine-seeking behavior was extinguished, each animal was tested twice for footshock-stress-induced reinstatement, once with vehicle (25% hydroxypropyl--cyclodextrin) and once with one of three doses of SB-277011A in counterbalanced fashion.Results During the last 3 days of cocaine self-administration (SA), active lever-presses were approximately 100 per session under fixed-ratio 2 reinforcement (25 mg/kg cocaine per session). After extinction, intermittent footshock (10 min, 0.5 mA, 0.5 s on with a mean inter-shock interval of 40 s) robustly reinstated the cocaine-seeking behavior (8.4±3.6 active lever-presses in last extinction session to 35.3±5.2 in animals after footshock stress). Intraperitoneal (IP) injections of SB-277011A (3, 6, and 12 mg/kg) dose-dependently blocked stress-induced reinstatement of cocaine-seeking. Reinstatement was also blocked by microinjections of SB-277011A (1.5 g/0.5 l per side) bilaterally into the nucleus accumbens, but not into the dorsal striatum.Conclusions The mesolimic DA D₃ receptor plays an important role in mediating stress-induced reinstatement.     

Cocaine-induced locomotor activity and cocaine discrimination in dopamine D<Subscript>4</Subscript> receptor mutant mice

Rationale Previous studies have found a role for dopamine D₂-like receptors in many of the behavioral effects of cocaine, including its stimulation of locomotor activity and interoceptive discriminative-stimulus effects. However, given the lack of selectivity of most of the available pharmacological tools among D₂, D₃ and D₄ dopamine receptors, the roles of these specific receptors remain unclear. Objectives The roles of specific dopamine D₄ receptors in the behavioral effects of cocaine, including its locomotor stimulant and interoceptive discriminative-stimulus effects were investigated using dopamine D₄ receptor knockout (DA D₄R KO) and wild-type (WT) mice. Methods The mice were trained in daily sessions to discriminate IP injections of saline from cocaine (10 mg/kg). Responses on one of two response keys intermittently produced a food pellet; one response was reinforced in sessions following cocaine injection (10 mg/kg), and the other response was reinforced in sessions following saline injection. Each 20th response produced a food pellet (fixed-ratio, or FR20 schedule of reinforcement). The dose-effects of cocaine and its interaction with the D₂-like antagonist, raclopride, were assessed. Horizontal locomotor activity was also assessed in each genotype. Results As previously shown), cocaine was a more potent stimulant of locomotor activity in the DA D₄R KO mice compared to WT littermate mice. In addition, cocaine was more potent in producing discriminative-stimulus effects in DA D₄R KO mice (ED₅₀ value=0.50 mg/kg) compared to their WT littermates (ED₅₀ value=2.6 mg/kg). Raclopride shifted the cocaine dose-effect curve in both DA D₄R KO and WT mice, though the shift was greater for the DA D₄R KO mice. Conclusions The present results on the stimulation of activity and interoceptive/subjective effects of cocaine are consistent with the previously reported disregulation of dopamine synthesis in DA D₄R KO mice, and further suggest a role of the DA D₄R in vulnerability to stimulant abuse.     

The dopamine D<Subscript>1</Subscript> receptor agonist and D<Subscript>2</Subscript> receptor antagonist LEK-8829 attenuates reinstatement of cocaine-seeking in rats

Various dopaminergic drugs have been studied for their efficacy in the treatment of cocaine addiction. Pretreatment with either selective dopamine D₁ receptor agonists or selective dopamine D₂ receptor antagonists prevents reinstatement of cocaine-seeking in animal models of drug craving and relapse. We tested a novel ergoline derivative with combined D₁ agonistic and D₂ antagonistic effects, 9,10-didehydro-N-methyl-N-(2-propynyl)-6-methyl-8-aminomethylergoline bimaleate (LEK-8829), for its effects on cocaine-seeking in the intravenous cocaine self-administration model in rats. Pretreatment with systemic injections of LEK-8829 attenuated reinstatement of cocaine-seeking induced by cocaine priming injections and diminished cocaine intake in cocaine self-administration sessions. LEK-8829 itself did not induce reinstatement of cocaine-seeking and did not maintain intravenous self-administration. The results of our study indicate that LEK-8829 is a candidate medication for the treatment of cocaine craving in cocaine addiction.     

Non-uniform blockade of intrastriatal D<Subscript>2</Subscript>/D<Subscript>3</Subscript> receptors by risperidone and amisulpride

Rationale Atypical antipsychotic drugs have been shown to preferentially affect extrastriatal (mesolimbic) D₂/D₃ receptors over those within the striatum (nigrostriatal). The striatum does not contain exclusively nigrostriatal dopamine tracts, however. The caudate nucleus and ventral parts of the striatum primarily contain limbic and associative dopamine pathways more relevant to psychosis. Objectives We tested the hypothesis that two pharmacologically distinct atypical antipsychotic drugs, amisulpride and risperidone, would preferentially occupy of D₂/D₃ dopamine receptors in limbic and associative regions of the striatum. Methods Eight amisulpride-treated patients, six risperidone-treated patients and six age- and sex-matched healthy controls were recruited. Dynamic SPET studies were performed after bolus injection of [¹²³I]epidepride. Binding potential (BP) images were generated using a modified Logan method and aligned between subjects. Regions of interest (ROIs) were placed around head of caudate and putamen bilaterally on an average BP map derived from aligned control images. These ROIs were then applied user-independently to the BP maps for each subject to calculate BP for head of caudate and putamen. Mean occupancy of D₂/D₃ receptors in each ROI was determined by reference to the drug-free healthy volunteer group. Occupancy values for head of caudate and putamen were compared using paired Student’s t test. Results D₂/D₃ receptor occupancy was 42% in caudate and 31% in putamen for risperidone (t=5.9, df=11, p=0.0001) and 51% in caudate and 37% in putamen for amisulpride (t=11.1, df=15, p<0.0001). Conclusions Amisulpride and risperidone both show selective occupancy for limbic and associative D₂/D₃ receptors within the striatum.     

Current perspectives on the selective regulation of dopamine D<Subscript>2</Subscript> and D<Subscript>3</Subscript> receptors

Among the characterized dopamine receptor subtypes, D₂ receptor (D₂R) and D₃ receptor (D₃R) are the main targets of neuroleptics that are currently in use. In particular, D₃R is closely related to the etiology of schizophrenia and drug addiction. The spatial expression patterns of D₂R and D₃R are distinct in certain areas of the brain. D₂R are heavily expressed in the regions responsible for motor functions, whereas D₃R are more selectively expressed in the limbic regions, which are associated with cognitive and emotional functions. Therefore, disturbances in the motor and endocrine functions, which are the most serious problems caused by the current neuroleptics, are likely to result from the non-selective blockade of D₂R. Selective regulation of D₃R is needed to separate the desired therapeutic activities from unwanted side effects that result from promiscuous blockade of other receptors. D₂R and D₃R possess high sequence homology and employ similar signaling pathways, and it is difficult to selectively regulate them. In this review, we discuss the signaling mechanisms, intracellular trafficking, and desensitization properties of D₂R and D₃R. In addition, the proteins interacting with D₂R or D³R are discussed in relation to their roles in the regulation of receptor functions, followed by the current status of the development of selective D₃R ligands.     

D<Subscript>2</Subscript> but not D<Subscript>1</Subscript> dopamine receptor stimulation augments brain signaling involving arachidonic acid in unanesthetized rats

Rationale and objectives Signal transduction involving the activation of phospholipase A₂ (PLA₂) to release arachidonic acid (AA) from membrane phospholipids, when coupled to dopamine D₁- and D₂-type receptors, can be imaged in rats having a chronic unilateral lesion of the substantia nigra. It is not known, however, if the signaling responses occur in the absence of a lesion. To determine this, we used our in vivo fatty acid method to measure signaling in response to D₁ and D₂ receptor agonists given acutely to unanesthetized rats. Methods [1-¹⁴C]AA was injected intravenously in unanesthetized rats, and incorporation coefficients k* for AA (brain radioactivity/integrated plasma radioactivity) were measured using quantitative autoradiography in 61 brain regions. The animals were administered i.v. the D₂ receptor agonist, quinpirole (1 mg kg⁻¹, i.v.), the D₁ receptor agonist SKF-38393 (5 mg kg⁻¹, i.v.), or vehicle/saline. Results Quinpirole increased k* significantly in multiple brain regions rich in D₂-type receptors, whereas SKF-38393 did not change k* significantly in any of the 61 regions examined. Conclusions In the intact rat brain, D₂ but not D₁ receptors are coupled to the activation of PLA₂ and the release of AA.     

Imaging apomorphine stimulation of brain arachidonic acid signaling <Emphasis Type="Italic">via</Emphasis> D<Subscript>2</Subscript>-like receptors in unanesthetized rats

Rationale and objective Because of the important role of dopamine in neurotransmission, it would be useful to be able to image brain dopamine receptor-mediated signal transduction in animals and humans. Administering the D₁–D₂ receptor agonist apomorphine may allow us to do this, as the D₂-like receptor is reported to be coupled to cytosolic phospholipase A₂ activation and arachidonic acid (AA) release from membrane phospholipid. Methods Unanesthetized adult rats were given intraperitoneally apomorphine (0.5 mg/kg) or saline, with or without pretreatment with 6 mg/kg intravenous raclopride, a D₂/D₃ receptor antagonist. [1–¹⁴C]AA was injected intravenously, then AA incorporation coefficients k*—brain radioactivity divided by integrated plasma radioactivity—markers of AA signaling, were measured using quantitative autoradiography in 62 brain regions. Results Apomorphine significantly elevated k* in 26 brain regions, including the frontal cortex, motor and somatosensory cortex, caudate-putamen, thalamic nuclei, and nucleus accumbens. Raclopride alone did not change baseline values of k*, but raclopride pretreatment prevented the apomorphine-induced increments in k*. Conclusions A mixed D₁–D₂ receptor agonist, apomorphine, increased the AA signal by activating only D₂-like receptors in brain circuits containing regions with high D₂-like receptor densities. Thus, apomorphine might be used with positron emission tomography to image brain D₂-like receptor-mediated AA signaling in humans in health and disease.     

Prolactinemia is uncoupled from central D<Subscript>2</Subscript>/D<Subscript>3</Subscript> dopamine receptor occupancy in amisulpride treated patients

Rationale Atypical antipsychotic drugs are classically associated with lower propensity to extrapyramidal symptoms (EPS) and hyperprolactinemia than typical antipsychotic drugs. It has not been clarified why some atypical antipsychotic drugs, such as amisulpride, induce prolactin plasma concentration (PRL) elevation, but little EPS. Previous studies have found an association between striatal D₂/D₃ receptor occupancy and PRL in typical antipsychotic treated patients suggesting that PRL is a marker of central D₂/D₃ receptors blockade.Objective We have evaluated the relationship between PRL and central (striatum, temporal cortex and thalamus) D₂/D₃ receptor occupancy in amisulpride treated schizophrenic patients.Methods Single photon emission tomography (SPET) and [¹²³I]-epidepride were used to determine D₂/D₃ receptor occupancy in eight amisulpride treated patients. PRL was measured concurrently with the scans.Results The mean PRL was 1166 (range 499–1892 mIU/l) for a mean amisulpride dose of 406 mg/day (range 150–600 mg/day). Amisulpride plasma concentration and central D₂/D₃ receptor occupancy were positively correlated (r=0.83–0.89, df=4, P<0.05). No significant correlations were observed between PRL and amisulpride (daily dose or plasma concentration, P>0.05), or between PRL and central D₂/D₃ receptor occupancy (P>0.05).Conclusions Our findings show that amisulpride-induced hyperprolactinemia is uncoupled from central D₂/D₃ receptor occupancy. Amisulpride has poor blood–brain barrier penetration and reaches much higher concentration at the pituitary, which is outside the blood–brain barrier. Higher D₂/D₃ receptor occupancy at the pituitary gland than at central regions is a possible explanation for amisulpride PRL elevation with low EPS. Further studies evaluating pituitary D₂/D₃ receptor occupancy in vivo are necessary to confirm this hypothesis.This study was partially presented (poster) at the International Congress on Schizophrenia Research, Colorado Springs, USA, 2003 and received the Young Investigator Award.     

Occupancy of dopamine D<Subscript>1</Subscript>, D<Subscript>2</Subscript> and serotonin<Subscript>2A</Subscript> receptors in schizophrenic patients treated with flupentixol in comparison with risperidone and haloperidol

Rationale Flupentixol (FLX) has been used as a neuroleptic for nearly 4 decades. In vitro data show comparable affinity to dopamine D₂, D₁ and 5-HT_2A receptors and recently, FLX showed to be not inferior to risperidone in schizophrenic patients with predominant negative symptomatology, which was implicated with flupentixol’s interaction with 5-HT_2A and/or D₁ receptors. Objectives To assess in vivo receptor occupancy (RO) in patients clinically treated with FLX (n = 13, 5.7 ± 1.4 mg/day) in comparison with risperidone (RIS, n = 11, 3.6 ± 1.3 mg/day) and haloperidol (HAL, n = 11, 8.5 ± 5.5 mg/day). Materials and methods Each patient underwent two PET scans with 3-N-[¹¹C]methylspiperone (target: frontal 5-HT_2A), [¹¹C]SCH23390 (striatal D₁) or [¹¹C]raclopride (striatal D₂). RO was calculated as the percentage reduction of specific binding in comparison with healthy controls. Results D₂-RO under FLX was between 50% and 70%, indicating an ED₅₀ of about 0.7 ng/ml serum. 5-HT_2A and D₁-RO was 20 ± 10% and 20 ± 5% (mean, SEM). Under HAL, D₁-RO was 14 ± 6% and under RIS not significantly different from zero. Conclusions We were able to demonstrate a moderate 5-HT_2A and D₁ occupancy under clinically relevant doses of flupentixol, albeit lower than expected from in vitro data and clearly below saturation. Therefore, if flupentixol’s efficacy on negative symptoms is based on its interaction with 5-HT_2A and/or D₁ receptors, it should be highly dependent on serum concentration and thus on dosage and metabolism. However, these data suggest that mechanisms other than D₁ or 5-HT_2A antagonism may contribute to flupentixol’s efficacy on negative symptoms.     

Dopamine D<Subscript>3</Subscript> receptor antagonists improve the learning performance in memory-impaired rats

Rationale The dopamine D₃ receptor has been extensively studied in animal models of drug abuse and psychosis; however, less is known on its possible role in cognitive functions.Objectives This study investigated the effects of different D₃ antagonists and a partial agonist on spatial learning performance in a water labyrinth test.Methods Rats had to swim through a labyrinth system by making correct directional turns at three choice-points. The number of errors was recorded in three daily trials for 3 days.Results D₃ antagonists such as the highly selective SB-277011 (24 mg/kg p.o.) and RGH-1756 (1 mg/kg p.o.), the moderately selective U-99194A (12 mg/kg s.c.) and the selective partial D₃ agonist BP-897 (1 mg/kg i.p.) all significantly attenuated the learning deficit caused by FG-7142. Against scopolamine-induced amnesia, SB-277011 (24 mg/kg p.o.) was equally potent in showing protective efficacy; however, two times higher dose levels of U-99194A (24 mg/kg s.c.) and RGH-1756 (2 mg/kg p.o.) were required to attenuate the scopolamine-induced impairment. In contrast to the full antagonists, against scopolamine-induced amnesia, the partial agonist BP-897 (2 mg/kg i.p.) was inactive, even at the two times higher dose level.Conclusions These data suggest that dopamine D₃ receptor antagonists possess cognition-enhancing activity which may be of benefit in the treatment of cognitive dysfunction associated with several psychiatric disorders.     

The role of dopamine D<Subscript>3</Subscript> compared with D<Subscript>2</Subscript> receptors in the control of locomotor activity: a combined behavioural and neurochemical analysis with novel,selective antagonists in rats

Background The role of dopamine D₃/D₂ receptors in the control of locomotion is poorly understood.Objectives To examine the influence of selective antagonists at D₃ or D₂ receptors on locomotion in rats, alone and in interaction with the preferential D₃ versus D₂ receptor agonist, PD128,907.Methods Affinities of ligands at rat D₂ and cloned, human hD₃, hD_2S, hD_2L and hD₄ sites were determined by standard procedures. Locomotion was monitored automatically in rats pre-habituated for 30 min to an open-field environment. Extracellular levels of dopamine (DA) were determined by dialysis in the nucleus accumbens and striatum. Drugs were given acutely via the systemic route.Results PD128,907, which preferentially recognised D₃ versus D₂ sites, biphasically reduced and enhanced locomotion at low (0.01–0.63 mg/kg) and high (2.5–10 mg/kg) doses, respectively. L741,626 and S23199, which behaved as preferential D₂ versus D₃ receptor antagonists, enhanced the reduction in locomotion evoked by the low dose of PD128,907, blocked the increase provoked by the high dose and suppressed spontaneous locomotion alone. Analogous findings were obtained with haloperidol and raclopride which showed equilibrated affinity at D₂ and D₃ receptors. UH232 and AJ76, which showed a mild preference for D₃ versus D₂ sites, did not modify the effect of a low dose of PD128,907, slightly enhanced the hyperlocomotion elicited by the high dose and exerted little influence on locomotion alone. S14297 and U99194, which acted as preferential D₃ versus D₂ receptor antagonists, abolished the reduction in locomotion elicited by a low dose of PD128,907, potentiated the induction of locomotion by a high dose, and failed to influence locomotion alone. The actions of S14297 were stereoselective inasmuch as they were mimicked by the racemic form, S11566, but not by the inactive enantiomer, S17777. In contrast to S14297, S11566 and U99194, however, S33084, SB269,652, GR218,231 and N-[-4-[-(1-naphtyl)piperazine-1-yl]butyl] anthracene-2-carboxamide (NGB-1), highly selective D₃ versus D₂ receptor antagonists, were inactive under all conditions. PD128,907 (0.01–10.0 mg/kg) suppressed dialysate levels of DA in the nucleus accumbens and striatum, actions blocked by L741,626 and haloperidol, yet unaffected by S14297 and S33084.Conclusions The facilitatory influence of a high dose of PD128,907 upon locomotion is mediated by postsynaptic D₂ receptors and, possibly, countered by their D₃ counterparts. Correspondingly, selective blockade of D₂ but not of D₃ receptors alone suppresses motor function. The reduction in locomotion provoked by a low dose of PD128,907 may be mediated by D₂ autoreceptors, but a role of postsynaptic D₃ receptors cannot be excluded. Finally, mechanisms underlying the contrasting influence of chemically diverse D₃ receptor antagonists upon locomotion remain to be elucidated.     

Essential conservation of D<Subscript>1</Subscript> mutant phenotype at the level of individual topographies of behaviour in mice lacking both D<Subscript>1</Subscript> and D<Subscript>3</Subscript> dopamine receptors

Rationale. In the absence of agonists and antagonists evidencing appropriate selectivities, individual and interactive properties of D₁ and D₃ dopamine receptors would be illuminated most powerfully by their co-deletion. Objectives. To define and contrast the behavioural phenotype of D₁/D₃ double knockout mice in comparison with wild types, and with individual D₁ and D₃ mutants. Methods. Behavioural phenotype was characterised using an ethologically based topographical technique. Results. On comparison with wild types, D₁/D₃ double mutants were characterised topographically as follows: increases in sniffing and locomotion, which evidenced delayed habituation; reductions in rearing free, rearing seated, grooming, chewing and stillness. Though the D₁/D₃ double mutant ethogram comprised elements of both single mutant D₁ and D₃ lines, this phenotype was largely reflective of the D₁ mutant component. Conclusions. Distinct patterns of initial exploratory behaviour and of temporal change over subsequent habituation were evident across the three genotypes, with particular conservation of the D₁ phenotype in D₁/D₃ double mutants. Under the present conditions, there was little systematic evidence for D₁:D₃ interactions in the regulation of these aspects of behaviour. Electronic Publication     

Clorgyline-induced switch from locomotion to mouthing in sensitization to the dopamine D<Subscript>2</Subscript>/D<Subscript>3 </Subscript>agonist quinpirole in rats: role of sigma and imidazoline I<Subscript>2</Subscript> receptors

Rationale. The monoamine oxidase inhibitor (MAOI) clorgyline, blocks locomotor sensitization to the D₂/D₃ dopamine agonist quinpirole and sensitizes self-directed mouthing behavior in rats by a mechanism independent of MAO inhibition. Clorgyline has a high affinity for imidazoline I₂ and sigma receptors, which could account for its effects on quinpirole sensitization. Objectives. To examine whether the effect of clorgyline on quinpirole sensitization is attributed to stimulation of either I₂ or sigma receptors. Methods. In one experiment, rats received injections of the I₂ receptor agonist 2-BFI (0.2 mg/kg, IP) or vehicle, 90 min prior to each injection of quinpirole (0.5 mg/kg, SC, × 8, twice weekly) or saline. A similar protocol was used to examine the effects of the MAOI Ro 41-1049 (10 mg/kg, SC) on quinpirole sensitization. Unlike clorgyline, Ro 41-1049 has no affinity for sigma or I₂ sites. An initial experiment demonstrated that intermittent injections of clorgyline (1 mg/kg, SC) are as effective as a continuous clorgyline administration (1 mg/kg per day via osmotic mini-pump) on quinpirole sensitization. Results. Like clorgyline, Ro 41-1049, but not 2-BFI, blocked the development of quinpirole-induced locomotor sensitization and induced instead sensitization of self-directed mouthing. Conclusions. Because Ro 41-1049 produced the same effects as clorgyline, and 2-BFI had no effects on quinpirole sensitization, it is unlikely that clorgyline exerts its effects via an action at sigma or I₂ receptors. Our results are consistent with the suggestion that clorgyline and Ro 41-1049 affect the behavioral response to quinpirole via the MAOI-displaceable quinpirole binding (MQB) site, and the hypothesis that the MQB site selects what motor output becomes sensitized to repeated injections of quinpirole. Electronic Publication     

The activity of pramipexole in the mouse forced swim test is mediated by D<Subscript>2</Subscript> rather than D<Subscript>3</Subscript> receptors

Rationale Recent studies have reported antidepressant-like activities of the dopamine D₂/D₃ agonist pramipexole in the chronic mild stress model and in the forced swim test, suggesting that D₃ receptor agonists may represent a new class of antidepressant drugs. However, the relative contribution of D₂ or D₃ receptors to the activity of pramipexole in these models is unclear.Objectives The aim of the current studies was to explore the role of dopamine D₂ and D₃ receptors in the activity of pramipexole in the mouse forced swim test.Methods The effect of pramipexole (0.1–3.2 mg/kg) in the mouse forced swim test was examined both in conjunction with D₂ and D₃ receptor antagonists (haloperidol (0.1–1 mg/kg) and LU-201640 (A-437203, 5.6–17.8 mg/kg), as well as in D₃ receptor knockout mice obtained on two different background strains (C57BL/6J and B6129SF2/J). Locomotor activity was also assessed following pramipexole administration.Results Pramipexole produced dose-dependent reductions in immobility in the forced swim test at doses that did not produce generalized increases in locomotor activity. LU-201640, the D₃ selective antagonist, failed to block the antidepressant-like effects of pramipexole. In contrast, the efficacy of pramipexole in the forced swim test was completely blocked by the D₂ antagonist, haloperidol. No baseline differences were observed between knockout and wild-type mice from either background strain in locomotor activity or in the forced swim test. Furthermore, in both background strains, pramipexole showed similar efficacy in the forced swim test for both wild-type and knockout mice.Conclusions Taken together, these studies suggest that the D₂ receptor rather than the D₃ receptor is important for the antidepressant-like activity observed for pramipexole in the mouse forced swim test.Portions of this work were presented at the 36th Winter Conference on Brain Research, Snowbird, UT, January 26–31, 2003.     

Behavioral sensitization to cocaine in rats: evidence for temporal differences in dopamine D<Subscript>3</Subscript> and D<Subscript>2</Subscript> receptor sensitivity

Rationale

Cocaine-induced changes in D₂ receptors have been implicated in the expression of sensitized behavioral responses and addiction-like behaviors; however, the influence of D₃ receptors is less clear.

Objectives

To characterize the effects of repeated cocaine administration on the sensitivity of rats to D₂- and D₃-mediated behaviors, as well as the binding properties of ventral striatal D₂-like and D₃ receptors.

Methods

Pramipexole was used to assess the sensitivity of rats to D₃/D₂ agonist-induced yawning, hypothermia, and locomotor activity, 24 h, 72 h, 10, 21, and 42 days after repeated cocaine or saline administration. The locomotor effects of cocaine (42 day) and the binding properties of ventral striatal D₂-like and D₃ receptors (24 h and 42 days) were also evaluated.

Results

Cocaine-treated rats displayed an enhanced locomotor response to cocaine, as well as a progressive and persistent leftward/upward shift of the ascending limb (72 h–42 day) and leftward shift of the descending limb (42 days) of the pramipexole-induced yawning dose–response curve. Cocaine treatment also decreased B _max and K _d for D₂-like receptors and increased D₃ receptor binding at 42 days. Cocaine treatment did not change pramipexole-induced hypothermia or locomotor activity or yawning induced by cholinergic or serotonergic agonists.

Conclusions

These studies suggest that temporal differences exist in the development of cocaine-induced sensitization of D₃ and D₂ receptors, with enhancements of D₃-mediated behavioral effects observed within 72 h and enhancements of D₂-mediated behavioral effects apparent 42 days after cocaine. These findings highlight the need to consider changes in D₃ receptor function when thinking about the behavioral plasticity that occurs during abstinence from cocaine use.

     

Rewarding effects elicited by cocaine microinjections into the ventral tegmental area of C57BL/6 mice: involvement of dopamine D<Subscript>1</Subscript> and serotonin<Subscript>1B</Subscript> receptors

Rationale The role of ventral tegmental area (VTA) in mediating the rewarding effects of cocaine has not been extensively studied.Objectives We used the intracranial self-administration (ICSA) procedure to assess the involvement of the VTA in the rewarding effects of cocaine, and the effect of dopamine (DA) D₁- and serotonin (5-HT)_1B-receptor antagonists on ICSA of cocaine.Methods Adult male C57BL/6 mice were stereotaxically implanted, unilaterally, with a guide cannula either 1.5 or 2.3 mm above the VTA. After 1 week, mice were trained to discriminate between the two arms of a Y-maze over seven daily sessions, one arm being reinforced by intracranial cocaine microinjections. Starting from session 8, the D₁ and 5-HT_1B-receptor antagonists were injected IP pre-test each day over five consecutive sessions.Results Mice injected into the VTA rapidly exhibited a preference for the cocaine-reinforced arm, whatever the dose of cocaine available (30 pmol or 150 pmol per injection), reaching optimum ICSA performance within 5 days. In contrast, mice injected 0.8 mm above the VTA did not discriminate between the arms of the maze and performed at random, except for one subject. Once the ICSA response was acquired, systemic pre-injections of either the D₁ (SCH23390; 25 g/kg IP) or 5-HT_1B (GR127935; 0.5 mg/kg IP) antagonist disrupted this behavior. Replacement of each antagonist by vehicle led to the reinstatement of intra-VTA cocaine self-administration.Conclusions The results of the present study suggest that VTA neurons play a critical role in mediating the rewarding effects of acute cocaine and that both D₁ and 5-HT_1B receptors modulate these effects.