7-OH-DPAT injected into the accumbens reduces locomotion and sucrose ingestion: D3 autoreceptor-mediated effects?

CART in the regulation of appetite and energy homeostasis

The cocaine- and amphetamine-regulated transcript (CART) has been the subject of significant interest for over a decade. Work to decipher the detailed mechanism of CART function has been hampered by the lack of specific pharmacological tools like antagonists and the absence of a specific CART receptor(s). However, extensive research has been devoted to elucidate the role of the CART peptide and it is now evident that CART is a key neurotransmitter and hormone involved in the regulation of diverse biological processes, including food intake, maintenance of body weight, reward and addiction, stress response, psychostimulant effects and endocrine functions (Rogge et al., 2008; Subhedar et al., 2014). In this review, we focus on knowledge gained on CART's role in controlling appetite and energy homeostasis, and also address certain species differences between rodents and humans.

Differential Effect of the Dopamine D3 Agonist (±)-7-Hydroxy-2-(N,N-di-n-propylamino) Tetralin (7-OH-DPAT) on Motor Activity between Adult Wistar and Sprague-Dawley Rats after a Neonatal Ventral Hippocampus Lesion

The neonatal ventral hippocampal lesion (nVHL) has been widely used as an animal model for schizophrenia. Rats with an nVHL show several delayed behavioral alterations that mimic some symptoms of schizophrenia. Sprague-Dawley (SD) rats with an nVHL have a decrease in D₃ receptors in limbic areas, but the expression of D₃ receptors in Wistar (W) rats with an nVHL is unknown. The 7-Hydroxy-2-(N,N-di-n-propylamino) tetralin (7-OH-DPAT) has been reported as a D₃-preferring agonist. Thus, we investigated the effect of (±)-7-OH-DPAT (0.25 mg/kg) on the motor activity in male adult W and SD rats after an nVHL. The 7-OH-DPAT caused a decrease in locomotion of W rats with an nVHL, but it did not change the locomotion of SD rats with this lesion. Our results suggest that the differential effect of 7-OH-DPAT between W and SD rats with an nVHL could be caused by a different expression of the D₃ receptors. These results may have implications for modeling interactions of genetic and environmental factors involved in schizophrenia.

Yawning and locomotor behavior induced by dopamine receptor agonists in mice and rats

Dopaminergic (DA) agonist-induced yawning in rats seems to be mediated by DA D3 receptors, and low doses of several DA agonists decrease locomotor activity, an effect attributed to presynaptic D2 receptors. Effects of several DA agonists on yawning and locomotor activity were examined in rats and mice. Yawning was reliably produced in rats, and by the cholinergic agonist, physostigmine, in both the species. However, DA agonists were ineffective in producing yawning in Swiss-Webster or DA D2R and DA D3R knockout or wild-type mice. The drugs significantly decreased locomotor activity in rats at one or two low doses, with activity returning to control levels at higher doses. In mice, the drugs decreased locomotion across a 1000-10 000-fold range of doses, with activity at control levels (U-91356A) or above control levels [(+/-)-7-hydroxy-2-dipropylaminotetralin HBr, quinpirole] at the highest doses. Low doses of agonists decreased locomotion in all mice except the DA D2R knockout mice, but were not antagonized by DA D2R or D3R antagonists (L-741 626, BP 897, or PG01037). Yawning does not provide a selective in-vivo indicator of DA D3R agonist activity in mice. Decreases in mouse locomotor activity by the DA agonists seem to be mediated by D2 DA receptors.

Dopaminergic control of food choice: Contrasting effects of SKF 38393 and quinpirole on high-palatability food preference in the rat

The aim of the present study was to determine the behavioural effects of the selective dopamine D1 receptor agonist, SKF 38393, and of the selective dopamine D2/D3 receptor agonist, quinpirole, on the feeding performance of food-deprived rats in a model of food-preference behaviour. The animals were familiarised with a choice between a high-palatability, high-fat, high-sugar food (chocolate biscuits/cookies) and their regular maintenance diet. Following administration of either SKF 38393 (1.0-10.0 mg/kg, s.c.) or quinpirole (0.03-0.3 mg/kg, s.c.), the animals were observed throughout a 15-min test period, and their feeding behaviour was carefully monitored. Other behavioural categories were also observed. The resulting data were subject to a microstructural analysis to determine the loci of the behavioural effects. The results indicated that SKF 38393 and quinpirole had contrasting effects on the preference for the high-palatability chocolate food. SKF 38393 enhanced the preference, whereas quinpirole eliminated it. These data reinforce the view that forebrain dopamine mechanisms are closely involved in responses to high-palatability energy-dense food constituents, including chocolate. The data also indicate that pharmacological characterization is important, such that dopamine receptor subtypes appear to mediate contrasting effects on food preference for a high-fat, high-sugar food. Hence, brain dopamine appears to be involved in potentially complex ways in determining food preferences, and this may carry implications in the growing evidence for a link between brain dopamine and human obesity.

Differential effects of 7-OH-DPAT and apomorphine on hyperactivity induced by MK-801 (dizocilpine) in rats

Recent experiments from this laboratory demonstrated synergistic locomotor depressant effects of AMPA/kainate receptor blockade and D(2/3) dopamine (DA) receptor stimulation. This study explored functional interactions between DA and glutamate (Glu) systems using the NMDA receptor antagonist MK-801 and the DA receptor agonists 7-OH-DPAT and apomorphine. Using photocell locomotor activity boxes, systemic effects of MK-801 in combination with 7-OH-DPAT (0.03 mgkg(-1) SC, n=8) or a pre-synaptically effective dose of apomorphine (0.05 mgkg(-1) SC, n=6) were measured in male Sprague-Dawley rats. Effects of bilateral applications of MK-801 and 7-OH-DPAT into the nucleus accumbens (NAS) shell subregion were also investigated (n=7). When given alone, MK-801 (0.13 mgkg(-1) or 0.66 microg intra-NAS shell) increased horizontal locomotor activity, while 7-OH-DPAT (0.03 mgkg(-1)) or apomorphine (0.05 mgkg(-1)) decreased this measure. Co-administration of 7-OH-DPAT (systemically or into the NAS shell) completely blocked MK-801 induced hyperactivity. In contrast, MK-801 and apomorphine demonstrated additive effects. Stimulation of D(3) DA receptors may therefore block the hyperactivity induced by NMDA receptor antagonism, and the NAS shell is an important site for this interaction. The differential effects of the DA agonists on hyperactivity induced by NMDA receptor blockade support the proposal that 7-OH-DPAT may induce hypoactivity by stimulation of postsynaptic D(3) DA receptors.

Dopamine D3 modulation of locomotor activity and sleep in the nucleus accumbens and in lobules 9 and 10 of the cerebellum in the rat

Dopamine D3 receptors are present in the nucleus accumbens and in lobules 9 and 10 of the cerebellum. Their function is not fully understood. In the present study, the involvement of D3 receptors in locomotor activity and sleep in the two structures was investigated. The method used was the measure of locomotor activity in an open field and the measure of sleep and sleep/waking stages by electroencephalographic recordings in the rat. Dopamine receptor agonists (7-OH-DPAT, quinelorane and SKF38393) and antagonists (amisulpride, nafadotride and haloperidol) were microinjected into the two structures. Concerning locomotor activity, the results show that D3 receptor stimulation, as well as concomitant stimulation of D1 and D3 receptors, have opposite effects in the cerebellum and in the core of nucleus accumbens, with an inhibition of locomotion in the cerebellum and a stimulation of locomotion in the nucleus accumbens. D1 and D3 stimulation have mutual potentiating locomotor effects. The results also show that in the cerebellum, the effects are selective to lobules 9 and 10, and are not observed with microinjections into lobule 8. Concerning sleep, the effects of agonists and antagonists show a similar tendency in the cerebellum and nucleus accumbens, with a somnogenic effect of D2 blockage, an awakening effect of at least one of the D2/D3 agonists, and a dose-dependent awakening effect of the D2/D3 antagonists. In conclusion, this study suggests that dopamine D3 receptors in the cerebellum and nucleus accumbens are involved in several aspects of the regulation of locomotor activity and sleep.

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.

Cocaine- and amphetamine-regulated transcript in the nucleus accumbens participates in the regulation of feeding behavior in rats

The present studies aimed to determine whether cocaine- and amphetamine-regulated transcript (CART) peptide in the nucleus of accumbens shell (AcbSh) is implicated in the regulation of food intake. Bilateral intranuclear injections of CART peptide (55-102, 1 microg/microl/side) into the AcbSh decreased food intake with no change in locomotion activity and attenuated the orexigenic effect of the GABA(A) agonist muscimol (100 ng/microl/side) in male Sprague-Dawley rats. Decreased food intake after bilateral intranuclear injections of CART was more sustained in freely fed rats than in food-deprived rats, suggesting fuel availability is an important factor in modulating the function of CART in the regulation of feeding. Our anatomical findings indicate that in addition to the perifornical region and the arcuate nucleus, some neurons within the AcbSh also project within the AcbSh. Moreover, many of these efferent cells contain CART immunoreactivity, including those which reside within the AcbSh, suggesting that accumbal CART circuitry is involved in the central function of the nucleus accumbens. Furthermore, fasting suppressed CART mRNA levels in the AcbSh, paraventricular nucleus of the hypothalamus, arcuate nucleus, and the perifornical region, indicating that the Acb is sensitive to fuel availability to an extent similar to those regions in the hypothalamus. Our findings are the first to demonstrate that CART mRNA in the AcbSh is sensitive to metabolic challenges and that injection of CART peptide into the AcbSh has an inhibitory effect on food intake.

7-OH-DPAT selectively reduces intake of both chow and high fat diets in different food intake regimens

Mesolimbic dopaminergic system activation correlates with ingestive behavior in numerous feeding regimens. DA release is enhanced by food intake following deprivation, amount of food consumed, and the palatability of the food consumed. The dopamine-3 receptor (D3-R) has a limited expression pattern that is restricted largely to the mesolimbic dopaminergic system. The D3-R has been hypothesized to inhibit DA-mediated reward, locomotion and motivation. To test the potential for an inhibitory role of the D3-R on food intake, we administered the D3-R agonist 7-OH-DPAT (5, 10 and 50 microg/kg ip) to rats that had ad libitum access to standard rodent chow (3.41 kcal/gm, 0.51 kcal/gm from fat) or a preferable, high fat (HF) (4.4 kcal/gm, 1.71 kcal/gm from fat). In the second set of experiments we administered 7-OH-DPAT (10, 50 and 100 microg/kg) to rats that had access to chow or HF diet for only 3 h per day (meal fed). In the third set of experiments we administered 7-OH-DPAT (10 and 50 microg/kg) to rats that had access to chow or HF diet after a 21-h food restriction. The 10 and 50 microg/kg doses significantly, but equally reduced intake of chow and HF diet in animals that were ad libitum fed. In animals that were meal-fed the dose response was effectively shifted to the right and the 10 microg/kg dose was ineffective at reducing intake. The 50 and 100 microg/kg doses significantly but equally reduced intake of both diets. In animals that were 21-h restricted and had access to chow both the 10 and 50 microg/kg doses were ineffective at reducing intake. However, in animals that had access to HF diet, 7-OH-DPAT dose-dependently reduced intake. These results support a potential role for the D3-R in ingestive behavior particularly in situations that involve a significant learned component.

Morphine-induced alterations of immune status are blocked by the dopamine D2-like receptor agonist 7-OH-DPAT

Morphine administration produces profound effects on the immune system, including reductions in natural killer cell activity, mitogen-induced lymphocyte proliferation, and cytokine production. Although it has been established that the activation of central nervous system (CNS) micro-opioid receptors by morphine induces immunomodulation, little is known about the neural mechanisms underlying such processes. Interestingly, it has been shown that the dopamine (DA) D2-like receptor agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) blocks the effect of morphine on a number of behaviors that are mediated by central dopamine pathways. The present study examined whether dopamine is involved in the immunomodulatory effects of morphine. In separate experiments, 7-OH-DPAT was administered either systemically (subcutaneous, s.c.) or centrally (intracerebroventricularly, i.c.v.) prior to morphine treatment in male Lewis rats. The results demonstrate that both systemic and central administration of 7-OH-DPAT attenuate the suppressive effect of morphine on several measures of immune status. Overall, these findings provide the first evidence that CNS dopaminergic mechanisms are directly involved in morphine-induced immunomodulation.

Behavioral effects of dopamine agonists and antagonists in MPTP-lesioned D3 receptor knockout mice

To test the modulatory role of D(3) receptors in normal and dopamine-depleted mice, D(3) receptor KO mice and wild-type (WT) littermates were administered saline, L-dopa/carbidopa (20/2 mg/kg ip), a preferential D(3)>D(2) agonist S32504, a D1+D(2)/D(3) agonist apomorphine, a selective D(3) antagonist S33084, or apomorphine with S33084 prior to and after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We monitored lines crossed in a 55-min session, average number of rears, and average number of grooming bouts. MPTP treatment produced equivalent 70% losses of dopamine fibers in the caudate putamen (CPu) and nucleus accumbens (NAC) of WT and D(3) KO mice as compared to their control (vehicle injected) counterparts. D(3) receptors were absent in KO mice, and the number of D(3) receptors was unaffected by MPTP-induced loss of DA terminals in WT mice. The results support a lack of involvement of the D(3) receptor for D1:D2 receptor-mediated behavioral activity (synergy). First, S32504 inhibited all behaviors and to a similar degree in D(3) KO and WT mice. Second, S33084 at the higher concentration increased number of lines crossed in response to high dose apomorphine in both D(3) KO and WT mice. Third, in nonlesioned mice, apomorphine-induced gnawing stereotypies were inhibited by S33084 in both D(3) KO and WT mice. Interestingly, the inhibition of apomorphine-induced gnawing was not apparent in MPTP-lesioned mice, and this stereotypy was elevated in D(3) KO-MPTP-lesioned mice. Thus, the suppressive effects of S32504 could be via D2 autoreceptor inhibition of DA release, and D2 receptor blockade by S33084 leads to release of that inhibition. This may be more apparent in MPTP-lesioned partially DA denervated mice.

The effects of 7-OH-DPAT, quinpirole and raclopride on licking for sucrose solutions in the non-deprived rat

Pharmacological manipulations that alter dopamine (DA) at DA receptor subtypes produce reductions in feeding behaviour. What remains uncertain is the exact way in which these reductions in feeding are achieved as a consequence of differing drug actions at separate receptor subtypes. In this study our aim was to compare the anorectic effects of the preferential D3/D2 agonists 7-hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT) and quinpirole and the non-selective D2/D3 antagonist raclopride on the microstructure of licking responses in non-deprived rats. In a 20-min test, trained adult, male hooded rats had access to one of three solutions: 1%, 3% or 10% sucrose. 7-OH-DPAT (0.1-1.0 mg/kg, i.p.), quinpirole (0.03-0.3 mg/kg, s.c.), raclopride (0.03-0.3 mg/kg, i.p.) or vehicle were injected 20 min prior to the start of the licking test. A lickometer recorded the timing of each lick, from which the microstructural parameters of bout frequency and bout duration were also computed. All compounds reduced the mean bout duration, while 7-OH-DPAT and raclopride also brought about a compensatory increase in bout number. Analysis of the licking rates over the test session showed that 7-OH-DPAT, quinpirole and raclopride decreased the initial rate, without affecting the rate of decline of licking. Changes in licking microstructure (i.e. initial rate of licking and mean bout duration) after the administration of 7-OH-DPAT, quinpirole and raclopride, are consistent with an action of these dopaminergic compounds to reduce palatability.

Altered feeding responses in mice with targeted disruption of the dopamine-3 receptor gene

Dopamine signaling has been implicated in the control of food intake and body weight. In particular, dopamine is important in the control of meal size and number and is thought to mediate the response to metabolic deprivation states. In the present experiments, the authors assessed the role of the dopamine-3 receptor (D3R) in the feeding responses to 2-deoxy-D-glucose, mercaptoacetate, and peripheral insulin. All 3 compounds increased food intake in wild-type mice, but the hyperphagic responses were blunted in D3R-/- mice. In other experiments, D3R-/- mice were hyperresponsive to the administration of amylin and leptin relative to wild-type mice. These results support the hypothesis that D3Rs chronically inhibit the effects of adiposity hormones, thereby contributing to a net anabolic state.

Effects of the dopamine antagonist PD 152255 on juvenile rats' responses to dorsal stimulation, the transport response, and related behaviors

The authors gave 23- and 40-day-old rats doses of the dopamine D3 antagonist PD 152255 and tested them on transport response intensity, vertical cling catalepsy duration, and dorsal immobility duration. Administration of PD 152255 resulted in dose-dependent increases in transport response intensity in 40-day-old rats but was without effect in 23-day-old rats. Administration of PD 152255 caused increases in dorsal immobility durations in both 23- and 40-day-old subjects. The drug was without effect on vertical cling catalepsy. Results are discussed with respect to the role of D3 receptors in the transport response and the nature of D2-D3 receptor interactions.

The D3 dopamine receptor and substance dependence

Behavioral sensitization, the progressive and enduring enhancement of certain stimulant-induced behaviors following repetitive drug use, is mediated in part by dopaminergic pathways known to play a role in drug dependence. It has been theorized that sensitization underlies the development of drug craving and initiates addictive behaviors of drug dependence. We propose that down-regulation of D3 dopamine receptor function contributes to sensitization. Rodent locomotion is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation inhibiting and concurrent D1/D2 receptor activation stimulating locomotion. The D3 receptor has greater occupancy than D1 or D2 receptors following stimulant drug administration. Sensitization may therefore result in part from greater accommodation of the inhibitory D3 receptor "brake" on locomotion, leading to progressive locomotion increase following repeated stimulant exposure. Further study is needed to test this proposed model, and to clarify the role of individual dopamine receptor subtypes in sensitization and drug dependence.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

Biphasic effects of 7-OH-DPAT on the acquisition of responding for conditioned reward in rats

Previous studies have shown that dopamine (DA) receptor subtype-specific agonists differentially affect responding for conditioned reward D1-like agonists impair, whereas D2-like agonists enhance responding. The present study compared the effects of the D2-like agonists bromocriptine and 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT). Food-deprived rats (N=159) were preexposed to a chamber with two levers, one producing a tone (3 s) and the other turning the house lights off (3 s), for five 40-min sessions. In four subsequent 65-min conditioning sessions with the levers removed, the lights-off stimulus was paired with food (80 presentations per session). During two 40-min test sessions, the lights-off (CR) and tone (NCR) levers were replaced and responses at each lever were recorded. Confirming previous results, bromocriptine (0.50-5.0 mg/kg) dose-dependently enhanced responding on the lever producing conditioned reward. In contrast, 7-OH-DPAT had a biphasic effect on responding for conditioned reward. Low doses (0.10-0.25 mg/kg) reduced CR lever responding, whereas a higher dose of 1.0 mg/kg enhanced such responding. An intermediate dose of 0.50 mg/kg neither impaired nor enhanced CR lever responding. The biphasic profile of 7-OH-DPAT may arise through differential actions at D3 vs. D2 receptors or presynaptic vs. postsynaptic DA receptors at low and high doses, respectively.

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinson's disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

Intra-accumbens infusion of D(3) receptor agonists reduces spontaneous and dopamine-induced locomotion

The present study investigated whether PD 128907 and 7-OH-DPAT, described as preferential dopamine (DA) D(3) receptor agonists, produce hypolocomotion by acting at postsynaptic dopaminergic receptors within the nucleus accumbens. Bilateral infusion of PD 128907 (1.5 and 3 microg/0.5 microl) induced a dose-dependent hypolocomotion, whereas its enantiomer, PD 128908, was inactive. Local infusion of 7-OH-DPAT and the preferential DA autoreceptor agonist, B-HT 920, at the same dose range also decreased spontaneous locomotion. In addition, both drugs induced yawning with B-HT 920 producing the greatest effect. In the second experiment, the ability of these agonists to reduce the locomotor activity induced by intra-accumbens injection of DA (10 microg/0.5 microl) was studied. Pretreatment with either PD 128907 or 7-OH-DPAT (3 microg) reduced DA-induced hyperactivity. Local infusion of B-HT 920 (3 microg) failed to antagonise the locomotor effects of DA. Altogether these findings suggest that PD 128907 and 7-OH-DPAT induce hypolocomotion by acting in part at postsynaptic DA receptors. The possible role of D(2) and/or D(3) receptors in the mediation of these effects is discussed.

Association of dopamine D3-receptor gene variants with neuroleptic induced akathisia in schizophrenic patients: a generalization of Steen's study on DRD3 and tardive dyskinesia

Neuroleptic induced akathisia is a common and distressful extrapyramidal side effect of antipsychotic treatment. A significant proportion of the variability of its development has been left unexplained and has to be attributed to individual susceptibility. Since hereditary factors have been discussed in the etiology of acute akathisia (AA), part of the individual susceptibility might be of genetic origin. Moreover, AA is regarded as a forerunner of tardive dyskinesia, a drug-induced chronic movement disorder, which may be associated with homozygosity for the Ser9Gly variant of the DRD3 gene. Considering expression studies, which demonstrated functional variants of DRD3 polymorphisms, we investigated whether homozygosity for the Ser9Gly variant of the DRD3 gene is associated with AA. Homozygosity for the Ser9Gly variant of the DRD3 gene was connected to an 88% incidence of AA as compared with a considerably lower 46.9% incidence of AA in schizophrenic patients nonhomozygous for the 2-2 allele (exact P = 0.0223). Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:187-191, 2000.

The dopamine D3/D2 receptor agonist 7-OH-DPAT induces cognitive impairment in the marmoset

Previous work has shown that dopaminergic systems are involved in cognitive function in the common marmoset. The present study investigated the role of dopamine D3 receptors in cognitive performance in the marmoset. The effects of the putative dopamine D3 receptor agonist, 7-OH-DPAT, on performance of a same-day reversal visual object discrimination task were assessed using a miniature Wisconsin General Test Apparatus (WGTA). Within the same test session marmosets acquired a two-choice object discrimination initial task and a reversal task to criterion. 7-OH-DPAT (6-10 microg/kg) significantly impaired reversal task performance only, without affecting acquisition of the initial task. A higher dose of 25 microg/kg 7-OH-DPAT impaired initial task acquisition as well as reversal task acquisition, possibly as a consequence of a nonspecific influence on motor function. The dopamine D2 receptor antagonist (-)sulpiride (5-10 microg/kg) and the alpha2-receptor antagonist yohimbine (50 microg/kg) failed to attenuate the effects of 7-OH-DPAT (6 microg/kg) in this task. In contrast, the dopamine D2/D3 receptor antagonist raclopride (50 microg/kg) significantly attenuated the 7-OH-DPAT-induced impairment of reversal task performance. These results suggest that activation of dopamine D3 receptors produces a selective impairment of aspects of cognitive function in the marmoset.

Quantification of dopamine D3 receptor mRNA level associated with the development of amphetamine-induced behavioral sensitization in the rat brain

We hypothesized that changes in expression of dopamine (DA) D3 receptor gene in the rat brain would correlate with the behavioral sensitization induced by amphetamine (AMPH). In order to test this hypothesis, we measured D3 receptor mRNA levels in the striatum, nucleus accumbens and prefrontal cortex, in individual rats following AMPH treatment (2.5 mg/kg s.c., for 5 consecutive days) using a ribonuclease protection assay method. We observed similar levels of D3 receptor mRNA in saline and AMPH treated animals in each brain region examined. These results suggest behavioral sensitization to AMPH is not mediated through postsynaptic transcriptional regulation of D3 receptor.

Macronutrient-specific dietary selection in rodents and its neural bases

The only evidence for nutrient selection comes from baseline or treatment effects on nutrient intakes that are qualitatively similar when sensorily contrasting forms of each macronutrient are investigated and/or dietary compositions and strains of rat or mouse are different within or between laboratories. By that criterion the only potential case of a treatment reliably altering macronutrient selection identified in the present review of the literature is d-norfenfluramine, fluoxetine and paraventricular serotonin (5-HT) reducing the intake of dextrin-containing diets at early dark. The only clear example of reverse effects of an agonist and an antagonist on dietary intake was found with serotonergic agents. Claims for catecholaminergic or opioid involvement in protein intake and peptidergic involvement in carbohydrate intake were not substantiated. There remain the issues of which learnt macronutrient-specific postgastric actions and sensory cues from the affected diet rely on the neural pathway(s) on which the drug is acting to alter dietary selection. Until experiments address these questions, the neural bases of nutrient-specific appetites will remain unknown. Drug effects must be consistent across differently textured and flavoured versions of each macronutrient tested.

Dopamine D3 receptor mutant and wild-type mice exhibit identical responses to putative D3 receptor-selective agonists and antagonists

Previous studies using a variety of drugs with different affinities for the dopamine (DA) D3 receptor suggested that this receptor is involved in regulating motor activity and hypothermia. However, the in vivo selectivity of many of these compounds has been repeatedly questioned. To examine the precise roles of the DA D3 receptor in motor activity and hypothermic responses, we used mutant mice lacking the DA D3 receptor to evaluate the in vivo effects of several putative D3 receptor-selective agonists and antagonists. Using automated photocell activity chambers, we observed that the decreases in locomotor activity produced by putative D3 receptor-selective agonists as well as increases in locomotor activity produced by putative D3 receptor antagonists are identical in D3 receptor mutant and wild-type mice. In addition, the hypothermia produced by the putative D3 receptor-selective agonist PD 128907 is identical in both groups of mice. Based on these findings, we propose that D3 receptors are unlikely to be involved in these effects and we caution that the putative D3 ligands that have been used to reach conclusions regarding the functional roles of D3 receptors lack the necessary in vivo selectivity to support such conclusions.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

Effect of antidepressant drugs administered repeatedly on the dopamine D3 receptors in the rat brain

Previous studies have indicated that antidepressant drugs displaying different pharmacological profiles, administered repeatedly, increase the locomotor hyperactivity induced by various dopaminomimetics, among others by quinpirole. As this drug, according to a recent study, shows high affinity not only for dopamine D2 but also for dopamine D3 receptors, the question arises if dopamine D3 receptors are involved in the increase in quinpirole-elicited locomotor hyperactivity induced by repeated treatment with antidepressant drugs. In the present study we administered imipramine, amitriptyline, citalopram and mianserin (in a dose of 10 mg/kg p.o., twice a day, 14 days) to male Wistar rats and then (+/-)-7-OH-DPAT (7-hydroxy-dipropylaminotetralin), a dopamine D3 receptor agonist, was given (3 mg/kg s.c.). Hyperlocomotion induced by (+/-)-7-OH-DPAT was significantly increased by repeated administration of antidepressant drugs. The receptor autoradiography technique with [3H]7-OH-DPAT as a radioligand was applied to measure the effects of antidepressant drugs treatment on the dopamine D3 receptors in the islands of Calleja and in the shell of the nucleus accumbens septi, which are brain regions with highly selective expression of dopamine D3 receptors. The biochemical studies indicated that in both examined brain regions there was an increase in the binding of [3H]7-OH-DPAT following the repeated administration of antidepressant drugs. In some cases this increase was also observed after the acute administration of antidepressants. The results obtained in the present study indicate that antidepressant drugs administered repeatedly enhance the responsiveness of dopamine D3 receptors, probably via an increase in the density of these receptors. This mechanism is probably similar to that observed already in the case of dopamine D2 receptors. Therefore it is hypothesized that dopamine D3 receptors are also involved in the increased responsiveness to dopamine D3 receptor agonists observed after antidepressants.

Specific changes in food intake elicited by blockade or activation of glutamate receptors in the nucleus accumbens shell

Blockade of non-N-methyl-D-aspartic acid (NMDA) ionotropic glutamate receptors in the nucleus accumbens shell (AcbSh) with 6,7-dinitroquinoxaline-2,3-dione (DNQX) elicits intense feeding in satiated rats. In order to determine whether or not this feeding is part of a general behavioral activation, we observed the effect of intra-AcbSh DNQX injections on intake of solid food, liquid food, and water, and on gnawing behavior. In addition, we investigated the possibility that activation of a subset of these receptors with (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) could suppress feeding. DNQX significantly increased intake of solid and liquid food, but did not significantly affect water intake or gnawing behavior. Furthermore, injections of AMPA into the AcbSh suppressed deprivation-induced feeding and intake of a palatable 5% sucrose solution without affecting water intake in water-deprived rats. Taken together, these data suggest that DNQX is acting on a system specifically involved with the regulation of food intake.

GABA in the nucleus accumbens shell participates in the central regulation of feeding behavior

We have demonstrated previously that injections of 6, 7-dinitroquinoxaline-2,3-dione into the nucleus accumbens shell (AcbSh) elicits pronounced feeding in satiated rats. This glutamate antagonist blocks AMPA and kainate receptors and most likely increases food intake by disrupting a tonic excitatory input to the AcbSh, thus decreasing the firing rate of a population of local neurons. Because the application of GABA agonists also decreases neuronal activity, we hypothesized that administration of GABA agonists into the AcbSh would stimulate feeding in satiated rats. We found that acute inhibition of cells in the AcbSh via administration of the GABAA receptor agonist muscimol or the GABAB receptor agonist baclofen elicited intense, dose-related feeding without altering water intake. Muscimol-induced feeding was blocked by coadministration of the selective GABAA receptor blocker bicuculline, but not by the GABAB receptor blocker saclofen. Conversely, baclofen-induced feeding was blocked by coadministration of saclofen, but was not affected by bicuculline. Furthermore, we found that increasing local levels of GABA by administration of a selective GABA-transaminase inhibitor, gamma-vinyl-GABA, elicited robust feeding in satiated rats, suggesting a physiological role for endogenous AcbSh GABA in the control of feeding. A mapping study showed that although some feeding can be elicited by muscimol injections near the lateral ventricles, the ventromedial AcbSh is the most sensitive site for eliciting feeding. These findings demonstrate that manipulation of GABA-sensitive cells in the AcbSh can have a pronounced, but specific, effect on feeding behavior in rats. They also constitute the initial description of a novel and potentially important component of the central mechanisms controlling food intake.

Evidence to suggest that agonist modulation of hyperlocomotion is via post-synaptic dopamine D2 or D3 receptors

It has been suggested that a sub-population of dopamine D3 receptors is located pre-synaptically and these serve as autoreceptors in dopamine projection areas such as the nucleus accumbens/ventral striatum. To study further the physiological role and synaptic location of the dopamine D3 receptor, we have investigated the in vivo effect of the D3/D2 receptor agonist quinelorane on amphetamine-induced hyperactivity and extracellular dopamine release from the nucleus accumbens of the conscious rat. Amphetamine increased dopamine release to 202 +/- 34% of pre-injection control values, but quinelorane at 2.5 micrograms/kg, a dose which effectively blocked amphetamine-induced hyperlocomotion, had no significant effect on amphetamine-induced dopamine release. These data suggest that hyperlocomotion is mediated via post-synaptic rather than pre-synaptic dopamine receptors. Since quinelorane has significant affinity for the dopamine D3 receptor, these effects may be via post-synaptic D3 receptors; however, D2 receptor effects cannot be disregarded. In summary, these data indicate that the quinelorane effect on amphetamine-stimulated hyperlocomotion is not mediated via D3 or D2 autoreceptors, but rather a population of receptors located post-synaptically, which appear to mediate the inhibition of rat locomotor activity.

Evidence for the occurrence of depressant EEG effects after stimulation of dopamine D3 receptors: a computerized study in rabbits

The putative dopamine D3 receptor agonist, (+/-) 7-OH-di-n-propylaminotetralin (+/- 7-OH-DPAT), induced depressant effects on rabbit EEG at the dose of 1 mg/kg i.v. Bromocriptine, a preferential dopamine D2 receptor agonist, induced EEG activation at the dose of 0.5 mg/kg i.v. Although the lack of very selective ligands makes it difficult to discriminate between D2- and D3- dependent effects, these findings suggest that -unlike D2 receptors-dopamine D3 receptors may mediate depressant effects on the electrocorticogram.

Aminotetralin drugs and D3 receptor functions. What may partially selective D3 receptor ligands tell us about dopamine D3 receptor functions?

The dopamine D3 receptor gene was identified by Sokoloff and colleagues in 1990. This finding rapidly gained the interest of the scientific community because this unexpected dopamine receptor subtype may play an important role in the antipsychotic activity of neuroleptic drugs. It recognizes most neuroleptics with a high affinity, and its brain distribution is restricted mainly to the ventral part of the striatal complex. However, the characterization and the subsequent identification of functions of the D3 receptor were hampered initially by at least four important factors that are still partially unresolved: (1) the absence of selective drugs that can discriminate between the D2 and D3 receptor subtype functions in vivo, (2) the lack of apparent coupling with GTP-dependent proteins, (3) the absence of effects on second messenger systems, and (4) the low level of expression of this receptor in brain tissue; these factors have contributed to tempering the interest of scientists. However, this situation has begun to change with the identification of [3H]7-hydroxy-N,N-(di-n-propyl)-2-aminotetralin ([3H]7-OH-DPAT), the first selective ligand for the dopamine D3 receptor. Although its binding selectivity for the D3 versus the D2 receptor is somewhat artificial, the potentially important impact of identification of a function for the D3 receptor encouraged scientists to use this aminotetralin compound for in vivo studies with, however, limited success. This commentary is focused on the impact and controversies generated by the use of 7-OH-DPAT and its congeners, on new conceptual views that may arise from this research, and on what partially selective D3 receptor ligands may tell us about dopamine D3 receptor functions.

In vivo evidence for preferential role of dopamine D3 receptor in the presynaptic regulation of dopamine release but not synthesis

Brain microdialysis was used to investigate the effects of the putative dopamine D3 receptor agonist (+/-)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) on dopamine release, metabolism and synthesis in the dorsal striatum and nucleus accumbens of awake rats. The drug administered i.p. dose dependently decreased the release, metabolism and synthesis of dopamine in both brain areas. The potency of 7-OH-DPAT to decrease dopamine release was found to be higher in the nucleus accumbens than in the dorsal striatum (ED50 for nucleus accumbens 0.0096 mg/kg, i.p.; for dorsal striatum 0.068 mg/kg, i.p.). Dopamine metabolism, assessed by measuring 3,4-dihydroxyphenylacetic acid extracellular levels, and dopamine synthesis, determined as 3,4-dihydroxyphenylalanine output following perfusion with the L-aromatic acid decarboxylase inhibitor 3-hydroxybenzylhydrazine (10(-5) M), were decreased at higher dose ranges of 7-OH-DPAT (ED50 for decrease of 3,4-dihydroxyphenylalanine output in nucleus accumbens 0.124 mg/kg, i.p.; in dorsal striatum 0.101 mg/kg, i.p.). The hypomotility of rats induced by 7-OH-DPAT in doses of 0.002-0.25 mg/kg, i.p., was shown to correlate with the decreased dopamine release in the nucleus accumbens. Pretreatment of animals with 7-OH-DPAT at the putative dopamine D3 receptor 'selective' dose of 0.05 mg/kg, i.p., was found to prevent the increase of dopamine release but not the increase in metabolism in the dorsal striatum of freely moving rats induced by (+)-AJ76, cis (+)-(1S,2R)-5-methoxy-1-methyl-1-2-(n-propylamino)tetralin HCI (7 mg/kg, i.p.) and haloperidol (0.1 mg/kg, i.p.). Local application of 7-OH-DPAT by addition into the perfusing medium also resulted in a preferential decrease of dopamine release in the nucleus accumbens as compared with the dorsal striatum (EC50 for nucleus accumbens 1.9 nM; for dorsal striatum 11.3 nM). The present results give further support to the hypothesis that the dopamine D3 autoreceptor is preferentially involved in the presynaptic regulation of dopamine release, while the D2 autoreceptor controls dopamine synthesis.

Dopamine autoreceptor reserve in vitro: possible role of dopamine D3 receptors

Receptor alkylation in vivo with N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ) decreased the potency, but not the efficacy, of the dopamine D3 receptor-preferring agonist 7-hydroxy-N, N-di-n-propyl-2-aminotetralin (7-OH-DPAT) at synthesis modulating dopamine autoreceptors in striatal and olfactory tubercle slices stimulated by 30 mM K+. In contrast, 7-OH-DPAT was ineffective in slices exposed to forskolin (10 microM). The results support the presence of autoreceptor reserve in vitro, and the partial involvement of dopamine D3 receptors in the autoregulation of dopamine synthesis.

Motor actions of 7-OH-DPAT in normal and reserpine-treated mice suggest involvement of both dopamine D2 and D3 receptors

In non-habituated mice, 7-hydroxy-N,N-di-n-propylaminotetralin (7-OH-DPAT, 0.04-10 mg/kg s.c.) potently and rapidly suppressed species-typical behaviours and induced frozen postures, with only occasional evidence of weak behavioural stimulation occurring at 5-10 mg/kg. This inhibitory effect was reversed by the dopamine D1 receptor agonist 2,3,4,5-tetrahydro-7,8-di-hydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF 38393, 10 mg/kg i.p.). 7-OH-DPAT (3-10 mg/kg) did not reinstate locomotion in 4 h habituated mice, either when administered alone or in conjunction with a threshold dose of SKF 38393 (3 mg/kg). By contrast, 7-OH-DPAT (0.2-10 mg/kg) dose-dependently reversed the akinesia of 24 h reserpine-treated mice. This response was blocked by the dopamine D2 receptor antagonist raclopride (10 mg/kg i.p.), but not by the dopamine D1 receptor antagonist (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepine-7-ol hemimaleate (SCH 23390, 0.05 mg/kg i.p.), and was potentiated synergistically by coinjection of SKF 38393 (3 mg/kg). These and earlier data suggest the motor inhibitory effects of 7-OH-DPAT (low doses) in normal animals are mediated by dopamine autoreceptors (D2 and/or D3), whilst its motor stimulant actions in normal (high doses) and in dopamine-depleted, supersensitive animals, are mediated by dopamine D2 receptors.