Behavioural effects in the rat of the putative dopamine D3 receptor agonist 7-OH-DPAT: comparison with quinpirole and apomorphine

Long-lasting behavioral effects of quinpirole exposure on zebrafish

The human brain matures into a complex structure, and to reach its complete development, connections must occur along exact paths. If at any stage, the processes are altered, interrupted, or inhibited, the consequences can be permanent. Dopaminergic signaling participates in the control of physiological functions and behavioral processes, and alterations in this signaling pathway are related to the pathogenesis of several neurological disorders. For this reason, the use of pharmacological agents able to interact with the dopaminergic signaling may elucidate the biological bases of such disorders. We investigated the long-lasting behavioral effects on adult zebrafish after quinpirole (a dopamine D2/D3 receptor agonist) exposure during early life stages of development (24 h exposure at 5 days post-fertilization, dpf) to better understand the mechanisms underlying neurological disorders related to the dopaminergic system. Quinpirole exposure at the early life stages of zebrafish led to late behavioral alterations. When evaluated at 120 dpf, zebrafish presented increased anxiety-like behaviors. At the open tank test, fish remained longer at the bottom of the tank, indicating anxiety-like behavior. Furthermore, quinpirole-treated fish exhibited increased absolute turn angle, likely an indication of elevated erratic movements and a sign of increased fear or anxiety. Quinpirole-treated fish also showed altered swimming patterns, characterized by stereotypic swimming. During the open tank test, exposed zebrafish swims from corner to corner in a repetitive manner at the bottom of the tank. Moreover, quinpirole exposure led to memory impairment compared to control fish. However, quinpirole administration had no effects on social and aggressive behavior. These findings demonstrate that dopaminergic signaling altered by quinpirole administration in the early life stages of development led to late alterations in behavioral parameters of adult zebrafish.

Disruptions in effort-based decision-making following acute optogenetic stimulation of ventral tegmental area dopamine cells

The dopamine system has been implicated in decision-making particularly when associated with effortful behavior. We examined acute optogenetic stimulation of dopamine cells in the ventral tegmental area (VTA) as mice engaged in an effort-based decision-making task. Tyrosine hydroxylase-Cre mice were injected with Cre-dependent ChR2 or eYFP control virus in the VTA. While eYFP control mice showed effortful discounting, stimulation of dopamine cells in ChR2 mice disrupted effort-based decision-making by reducing choice toward the lever associated with a preferred outcome and greater effort. Surprisingly, disruptions in effortful discounting were observed in subsequent test sessions conducted in the absence of optogenetic stimulation, however during these sessions ChR2 mice displayed enhanced high choice responding across trial blocks. These findings suggest increases in VTA dopamine cell activity can disrupt effort-based decision-making in distinct ways dependent on the timing of optogenetic stimulation.

Can Slot-Machine Reward Schedules Induce Gambling Addiction in Rats?

Similar to drugs of abuse, random-ratio reward schedules are highly motivating and, in humans, are thought to foster gambling addiction. Animal gambling models, however, have not yet demonstrated the compulsivity so characteristic of drug addiction. Three criteria have been used to evaluate addiction-like behavior in drug models: (1) response inhibition when reward is not available, (2) persistence under a progressive ratio schedule, in which the response-to-reward ratio is stretched, and (3) persistence in spite of punishment. We tested whether prolonged exposure (6 weeks) to a gambling-like reward schedule would induce addiction-like symptoms in rats. In two studies, separate groups were trained to respond to either random- or fixed-ratio schedules for food reward. We found that rats trained on random-ratio schedules showed higher response rates and dramatically shorter pauses after rewards. Tests of addiction-like behavior, however, were largely negative. Response rates were not different during cued no-reward periods nor when reward was coupled with punishment. We also found no group differences when food was devalued nor in reinstatement of reward-seeking after a 1-week delay. The sole exception to this pattern was that rats in the second experiment showed greater persistence on a progressive ratio test. After experiment two, subjects were also orally administered pramipexole, which caused increased perseveration during progressive ratio testing, especially in the random ratio group. While, it is possible that longer training or more appetitive rewards might have led to addiction-like behavior, our results, on the surface, suggest that random-ratio schedules are motivating but not addictive.

Alterations in effort-related decision-making induced by stimulation of dopamine D1, D2, D3, and corticotropin-releasing factor receptors in nucleus accumbens subregions

RATIONALE

Nucleus accumbens (NAc) dopamine (DA) plays an integral role in overcoming effort costs, as blockade of D₁ and D₂ receptors reduces the choice of larger, more-costly rewards. Similarly, the stress neuropeptide corticotropin-releasing factor (CRF) modulates DA transmission and mediates stress-induced alterations in effort-related choice.

OBJECTIVES

The current study explored how excessive stimulation of different DA receptors within the NAc core and shell alters effort-related decision-making and compared these effects to those induced by CRF stimulation.

METHODS

Male Long Evans rats were well-trained on an effort-discounting task wherein they choose between a low-effort/low-reward and a high-effort/high-reward lever where the effort requirement increased over blocks (2-20 presses). Dopamine D₁ (SKF 81297, 0.2-2 μg), D_2/3 (quinpirole, 1-10 μg), or D₃ (PD 128,907, 1.5-3 μg) receptor agonists, or CRF (0.5 μg), were infused into the NAc core or shell prior to testing.

RESULTS

Stimulation of D_2/3 receptors with quinpirole in the NAc core or shell markedly reduced the choice of high-effort option and increase choice latencies, without altering preference for larger vs smaller rewards. Stimulation of D₁ or D₃ receptors did not alter choice, although SKF 81297 infusions into the shell reduced response vigor. In comparison, core infusions of CRF flattened the discounting curve, reducing effortful choice when costs were low and increasing it when costs were high.

CONCLUSIONS

Excessive stimulation of NAc D₂ receptors has detrimental effects on effort-related decision-making. Furthermore, CRF stimulation induces dissociable effects on decision-making compared with those induced the effects of stimulation of different DA receptors.

Role of dopamine D2-like receptors and their modulation by adenosine receptor stimulation in the reinstatement of methamphetamine seeking

RATIONALE AND OBJECTIVE

Previous work has demonstrated that dopamine and adenosine receptors are involved in drug-seeking behaviors, yet the pharmacological interactions between these receptors in methamphetamine (MA) seeking are not well characterized. The present studies examined the role of the dopamine D₂-like receptors in MA seeking and identified the interactive effects of adenosine receptor stimulation.

METHODS

Adult male Sprague-Dawley rats were trained to lever press for MA in daily 2-h self-administration sessions on a fixed-ratio 1 schedule for 10 consecutive days. After 1 day of abstinence, lever pressing was extinguished in six daily extinction sessions. Treatments were administered systemically prior to a 2-h reinstatement test session.

RESULTS

An increase in MA seeking was observed following the administration of the dopamine D₂-like agonist, quinpirole, or the D₃ receptor agonist, 7-OH-DPAT. Stimulation of D₂ or D₄ receptors was ineffective at inducing MA seeking. Quinpirole-induced MA seeking was inhibited by D₃ receptor antagonism (SB-77011A or PG01037), an adenosine A₁ agonist, CPA, and an adenosine A_2A agonist, CGS 21680. MA seeking induced by a MA priming injection or D₃ receptor stimulation was inhibited by a pretreatment with the adenosine A₁ agonist, CPA, but not the adenosine A_2A agonist, CGS 21680.

CONCLUSIONS

These results demonstrate the sufficiency of dopamine D₃ receptors to reinstate MA seeking that is inhibited when combined with adenosine A₁ receptor stimulation.

Opposing effects of dopamine D1- and D2-like agonists on intracranial self-stimulation in male rats

Dopamine acts through dopamine Type I receptors (comprising D1 and D5 subtypes) and dopamine Type II receptors (comprising D2, D3, and D4 subtypes). Intracranial self-stimulation (ICSS) is 1 experimental procedure that can be used to evaluate abuse-related effects of drugs targeting dopamine receptors. This study evaluated effects of dopamine receptor ligands on ICSS in rats using experimental procedures that have been used previously to examine abused indirect dopamine agonists such as cocaine and amphetamine. Male Sprague-Dawley rats responded under a fixed-ratio 1 schedule for electrical stimulation of the medial forebrain bundle, and frequency of stimulation varied from 56-158 Hz in 0.05 log increments during each experimental session. Drug potency and time course were determined for the D1 ligands A77636, SKF82958, SKF38393, fenoldopam, and SCH39166 and the D2/3 ligands sumanirole, apomorphine, quinpirole, PD128907, pramipexole, aripiprazole, eticlopride, and PG01037. The high-efficacy D1 agonists A77636 and SKF82958 produced dose-dependent, time-dependent, and abuse-related facilitation of ICSS. Lower efficacy D1 ligands and all D2/3 ligands failed to facilitate ICSS at any dose or pretreatment time. A mixture of SKF82958 and quinpirole produced a mixture of effects produced by each drug alone. Quinpirole also failed to facilitate ICSS after regimens of repeated treatment with either quinpirole or cocaine. These studies provide more evidence for divergent effects of dopamine D1- and D2-family agonists on ICSS procedure in rats and suggest that ICSS may be a useful complement to other approaches for preclinical abuse potential assessment, in part because of the reproducibility of results. (PsycINFO Database Record

Lentiviral vector-mediated dopamine d3 receptor modulation in the rat brain impairs alcohol intake and ethanol-induced conditioned place preference

BACKGROUND

It has been reported that dopamine D3 receptor (D3R) knockout mice display similar ethanol (EtOH) consumption compared to wild types. In addition, studies with D3R pharmacological targeting were inconclusive.

METHODS

In the current study, we used both gain- and loss-of-function approaches to test the effects of central D3R manipulation on voluntary alcohol intake and EtOH-induced conditioned place preference (CPP) in rats. To this aim, we developed a lentiviral-mediated gene transfer approach to examine whether D3R knockdown (LV-siD3R) or overexpression (LV-D3R) in the nucleus accumbens (NAcc) is sufficient to modulate voluntary alcohol consumption and EtOH-CPP.

RESULTS

Using the standard 2-bottle choice drinking paradigm and an unbiased CPP procedure, our results indicated that, like the D3R selective antagonist SB-277011-A, LV-siD3R attenuated voluntary alcohol consumption. In contrast, LV-D3R increased EtOH intake with no effect on total fluid intake. Similarly, the D3R agonist 7-OH-DPAT also exacerbated EtOH intake. Interestingly, neither pharmacological nor genetic manipulation of D3R activity affected saccharin and quinine consumption and preference. More importantly, we report that LV-siD3R blocked, whereas LV-D3R exacerbated, EtOH-CPP.

CONCLUSIONS

These results support the notion that the D3R plays an important role in alcohol reward in rats and suggest that a key threshold range of D3R levels is associated with impaired alcohol consumption. Taken together, these findings demonstrate that the D3R is an essential component of the molecular pathways underlying the reinforcing properties of alcohol. Thus, medications targeting the D3Rs may be beneficial to tackle EtOH abuse and alcoholism in humans.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Role of cerebellar dopamine D(3) receptors in modulating exploratory locomotion and cataleptogenicity in rats

Dopamine D(3) receptors are highly expressed in the cerebellum; however, their pathophysiological functions are not fully understood. Here, we conducted microinjection studies to clarify the role of cerebellar D(3) receptors in modulating locomotion and cataleptogenicity in rats. Microinjection of the preferential D(3) agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) into lobe 9 of the cerebellum significantly reduced spontaneous locomotor activity with a U-shaped dose-response curve. The intracerebellar microinjection of 7-OH-DPAT did not elicit catalepsy by itself, but markedly potentiated catalepsy induction with a low dose (0.3mg/kg) of haloperidol. The catalepsy enhancement by 7-OH-DPAT occurred in a dose-dependent manner and was not associated with the locomotor inhibition. U-99194A (a selective D(3) antagonist) or AD-6048 (a preferential D(3) vs. D(2) antagonist) antagonized both the catalepsy enhancement and the locomotor inhibition with 7-OH-DPAT. In addition, U-99194A and AD-6048 per se significantly alleviated catalepsy induced by a high dose (0.5mg/kg) of haloperidol. Furthermore, microinjection of 7-OH-DPAT into the nucleus accumbens or the dorsolateral striatum neither affected spontaneous locomotor activity nor haloperidol (0.3mg/kg)-induced catalepsy. The present results illustrate for the first time the role of cerebellar D(3) receptors in modulating cataleptogenicity of antipsychotic agents, implying that blockade of cerebellar D(3) receptors contributes to the reduction of extrapyramidal side effects.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

Noradrenergic versus dopaminergic modulation of impulsivity, attention and monitoring behaviour in rats performing the stop-signal task: possible relevance to ADHD

RATIONALE

Deficient response inhibition is a prominent feature of many pathological conditions characterised by impulsive and compulsive behaviour. Clinically effective doses of catecholamine reuptake inhibitors are able to improve such inhibitory deficits as measured by the stop-signal task (SST) in humans and other animals. However, the precise therapeutic mode of action of these compounds in terms of their relative effects on dopamine (DA) and noradrenaline (NA) systems in prefrontal cortical and striatal regions mediating attention and cognitive control remains unclear.

OBJECTIVES

We sought to fractionate the effects of global catecholaminergic manipulations on SST performance by using receptor-specific compounds for NA or DA. The results are described in terms of the effects of modulating specific receptor subtypes on various behavioural measures such as response inhibition, perseveration, sustained attention, error monitoring and motivation.

RESULTS

Blockade of α2-adrenoceptors improved sustained attention and response inhibition, whereas α1 and β1/2 adrenergic receptor antagonists disrupted go performance and sustained attention, respectively. No relevant effects were obtained after targeting DA D1, D2 or D4 receptors, while both a D3 receptor agonist and antagonist improved post-error slowing and compulsive nose-poke behaviour, though generally impairing other task measures.

CONCLUSIONS

Our results suggest that the use of specific pharmacological agents targeting α2 and β noradrenergic receptors may improve existing treatments for attentional deficits and impulsivity, whereas DA D3 receptors may modulate error monitoring and perseverative behaviour.

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.

Dopamine agonists increase perseverative instrumental responses but do not restore habit formation in a rat model of Parkinsonism

Dopamine (DA) deafferentation of the dorsolateral striatum has been shown to prevent habit development, leaving instrumental behavior under action-outcome control that is persistently sensitive to modification of the motivational value of the reward. The present experiment further explored the basis of this dysfunction by examining the ability of intrastriatal DA agonist injections (D1 SKF 38393 or D2/D3 Quinpirole) during overtraining of a signaled instrumental task to restore habit formation in rats subjected to bilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway. Overtraining was followed by a test of goal sensitivity by satiety-specific devaluation of the reward. The results confirmed the impaired shift in performance from action to habit in control lesioned rats. However, lesioned rats repeatedly injected with quinpirole D2/D3 agonist showed an increase in non-rewarded instrumental responses (intertrials periods) during overtraining, suggesting the development of perseverative behavior. Following the procedure of devaluation, quinpirole D2/D3 agonist treatment, and to a lesser extent SKF 38393 D1 agonist, caused the persistence of sensitivity to reward devaluation, indicating clear goal-directed behavior despite extended training. This absence of restoration of habit formation by DA agonist treatment is discussed in the light of DA agonist effects in Parkinson patients.

Effects of the dopamine stabilizer, OSU-6162, on brain stimulation reward and on quinpirole-induced changes in reward and locomotion

Dysregulation of limbic dopamine (DA) neurotransmission results in abnormal positive or negative emotional states that characterize several mental disorders. Drugs that restore DA homeostasis are most likely to constitute effective treatments for such emotional disturbances. In this study, we investigated the effects of several doses of OSU-6162, a drug that belongs to a new class named "DA stabilizers", on brain stimulation reward. Because quinpirole produces, depending on the dose, a pre-synaptic depressant and a post-synaptic stimulatory effect on reward and locomotor activity, we also compared the ability of OSU-6162 and haloperidol to prevent these effects of the full DA agonist. Results show that OSU-6162 produced a dose-orderly reduction of reward with no change in the capacity of the animals to produce the operant response, and prevented, like haloperidol, both stimulatory and depressant effects of quinpirole on locomotor activity but only its reward stimulatory effect. The observed functional antagonism of OSU-6162 on these DA-dependent behaviors suggests that it may constitute an effective treatment for abnormal positive emotional state, and that it would be exempt of motor side-effects.

Maternal deprivation and handling modify the effect of the dopamine D3 receptor agonist, BP 897 on morphine-conditioned place preference in rats

RATIONALE

Maternal deprivation and handling can lead to a vulnerability to opiate dependence. However, the involvement of the dopamine D3 receptors has not been investigated.

OBJECTIVES

This study analysed the effects of a selective partial D3 receptor agonist, BP 897, on morphine-conditioned place preference (CPP) in deprived and handled rats.

MATERIALS AND METHODS

The effects of BP 897 were studied on the expression and the extinction of morphine CPP. Quantitative autoradiography of D2, D3 receptors and immunoautoradiography of dopamine transporter were performed in some saline- and morphine-treated rats 24 h after the place preference test.

RESULTS

Morphine (5 mg/kg) induced a more prolonged morphine CPP in deprived and handled rats than in control animals. BP 897 (0.5 or 2 mg/kg) enhanced the expression of morphine conditioning in control rats. Same doses did not change morphine conditioning in deprived rats. BP 897 (2 mg/kg) suppressed morphine CPP in handled rats. An increase in basal D2 receptor density in the mesencephalon of handled rats, which was suppressed after morphine CPP, was observed. A decrease in D2 receptor levels in morphine-treated deprived rats occurred in the nucleus accumbens.

CONCLUSIONS

This study shows that maternal deprivation and handling induced a prolonged morphine CPP, and different changes of D2/D3 receptor functioning revealed after morphine CPP. Early manipulations of infant-mother relationships may have different consequences on the balance of opioidergic and dopaminergic neurotransmission and may be of interest to reveal pharmacological properties of dopamine receptor partial agonists or antagonists potentially useful for therapeutic applications.

Effect of quinpirole on timing behaviour in the free-operant psychophysical procedure: evidence for the involvement of D2 dopamine receptors

RATIONALE

Operant timing behaviour is sensitive to dopaminergic manipulations. It has been proposed that this effect is mediated principally by D(2)-like dopamine receptors. However, we recently found that the effect of d-amphetamine on timing in the free-operant psychophysical procedure was mediated by D(1)-like dopamine receptors. It has not been established whether stimulation of D(2)-like receptors affects timing in this schedule.

OBJECTIVE

To examine the effects of a D(2)-like receptor agonist quinpirole on second-range timing and the ability of dopamine receptor antagonists to reverse quinpirole's effects.

MATERIALS AND METHODS

Rats responded on two levers (A and B) under a free-operant psychophysical schedule in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rates [percent responding on B (%B) vs time (t)] under each treatment; quantitative timing indices [T (50) (value of t when %B = 50) and Weber fraction] were compared among treatments.

RESULTS

Quinpirole (0.04, 0.08 mg kg(-1)) reduced T (50). This effect was attenuated by D(2)-like receptor antagonists haloperidol (0.05, 0.1 mg kg(-1)), eticlopride (0.04, 0.08 mg kg(-1)) and sulpiride (30, 60 mg kg(-1)), but not by the D(3) receptor-preferring antagonist nafadotride (0.5, 1 mg kg(-1)), the D(4) receptor antagonist L-745870 (1, 3 mg kg(-1)) or the D(1)-like receptor antagonist SKF-83566 (0.015 mg kg(-1)).

CONCLUSIONS

Results suggest that quinpirole reduced T (50) via an action at D(2) receptors. D(1)-like and D(2)-like receptors may mediate behaviourally similar but pharmacologically distinct effects on timing behaviour.

Yawning and hypothermia in rats: effects of dopamine D3 and D2 agonists and antagonists

RATIONALE

Identification of behaviors specifically mediated by the dopamine D2 and D3 receptors would allow for the determination of in vivo receptor selectivity and aide the development of novel therapeutics for dopamine-related diseases.

OBJECTIVES

These studies were aimed at evaluating the specific receptors involved in the mediation of D2/D3 agonist-induced yawning and hypothermia.

MATERIALS AND METHODS

The relative potencies of a series of D2-like agonists to produce yawning and hypothermia were determined. The ability of D3-selective and D2-selective antagonists to inhibit the induction of yawning and hypothermia were assessed and a series of D2/D3 antagonists were characterized with respect to their ability to alter yawning induced by a low and high dose of PD-128,907 and sumanirole-induced hypothermia.

RESULTS

D3-preferring agonists induced yawning at lower doses than those required to induce hypothermia and the D2-preferring agonist, sumanirole, induced hypothermia at lower doses than were necessary to induce yawning. The rank order of D3 selectivity was pramipexole > PD-128,907 = 7-OH-DPAT = quinpirole = quinelorane > apomorphine = U91,356A. Sumanirole had only D2 agonist effects. PG01,037, SB-277,011A, and U99,194 were all D3-selective antagonists, whereas haloperidol and L-741,626 were D2-selective antagonists and nafadotride's profile of action was more similar to the D2 antagonists than to the D3 antagonists.

CONCLUSIONS

D3 and D2 receptors have specific roles in the mediation of yawning and hypothermia, respectively, and the analysis of these effects allow inferences to be made regarding the selectivity of D2/D3 agonists and antagonists with respect to their actions at D2 and D3 receptors.

7-OH-DPAT effects on latent inhibition: low dose facilitation but high dose blockade: Implications for dopamine receptor involvement in attentional processes

7-OH-DPAT is a dopamine D2/D3 agonist, which at low doses acts preferentially on D3 receptors but at high doses it acts on D2 and D3 receptors. The present study investigated the contribution of D3 and D2 receptors on latent inhibition (LI) by using two dose levels of 7-OH-DPAT: a low dose, 0.1 mg/kg (D3 receptor activation) and a high dose, 1.0 mg/kg, (D2/D3 receptor activation) in a conditioned emotional response (CER) paradigm. The LI Protocols included CS pre-exposure (10 or 40 CS alone trials), CER induction and a non-drug CER test phase. Additionally, the drug effects upon CER acquisition without LI were assessed using the same treatments and test environment pre-exposure protocols but without the tone CS. The effects of 7-OH-DPAT on crossing, rearing and grooming were also measured in an open field 1 day after the CER test phase. The results showed that the low dose 7-OH-DPAT treatment potentiated LI at 10 but not at 40 CS pre-exposures. The high dose 7-OH-DPAT treatment blocked LI at both the 10 and 40 stimulus pre-exposures; and it also induced hyperactivity. Thus, D3 stimulation induced by a low dose of 7-OH-DPAT can facilitate LI but these effects are contingent upon and are specific to the number of stimulus presentations. Altogether, these findings indicate that D3 stimulation can enhance attentional processes, but D2 stimulation can impair attentional processes.

Compulsive-like effects of repeated administration of quinpirole on drinking behavior in rats

We have previously reported that repeated administrations of quinpirole, a D2/D3 dopamine receptor agonist, facilitate instrumental behavior in rats given the choice between operant and free access to water (contrafreeloading: CFL). The goal of the present study was to investigate the effects of repeated daily administrations of quinpirole (0.5 mg/kg i.p.) on the appetitive versus the consummatory component of water-reinforced behavior, under two experimental conditions. Under one condition, the rats were given access to tap water according to an FR3 schedule of reinforcement. Under the second condition, the rats were given the choice between operant and free access to water. Five major findings were obtained. First, acutely quinpirole suppressed operant behavior and, therefore, water intake for at least 1h. Second, upon repeated administrations tolerance developed to the suppressant effect of quinpirole on instrumental behavior but only to a lesser extent to the antidipsic effect, dissociating the appetitive from the consummatory components of water-reinforced behavior. Third, in CFL conditions quinpirole induced a progressively larger preference for the operant access. Fourth, even when the rats were given the choice between free access to highly palatable saccharine (0.05 or 0.01%) solutions and operant access to tap water, quinpirole shifted the animals towards the operant access. Fifth, repeated quinpirole produced lasting consequences on drinking behavior, since after rehydration and under drug-free conditions quinpirole-pretreated rats ingested larger amounts of water than control rats. In conclusion, the repeated activation of D2/D3 receptors appears to induce the rats to perseverate in performing needless instrumental behavior.

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.

Facilitation of brain stimulation reward by MK-801 (dizocilpine) may be independent of D2-like dopamine receptor stimulation in rats

RATIONALE

Dopamine (DA) and glutamate (Glu) interactions in the mesocorticolimbic pathway may regulate motivation and reward and contribute to schizophrenia and drug abuse. We have recently demonstrated synergistic effects of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor blockade and D(2/3) DA receptor stimulation in brain stimulation reward (BSR).

OBJECTIVES

This study was conducted to explore interactions between DA and Glu systems in BSR using the NMDA receptor antagonist MK-801 and the DA receptor agonists 7-OH-DPAT and apomorphine.

METHODS

Systemic effects of these compounds were measured in male Sprague-Dawley rats using rate-frequency threshold analysis of ventral tegmental area (VTA) BSR (n=27). Effects of bilateral applications of MK-801 and 7-OH-DPAT into the nucleus accumbens (NAS) shell subregion were also investigated (n=10).

RESULTS

MK-801 (0.03 or 0.13 mg kg(-1) i.p. or 0.66 mug intra-NAS) reduced reward thresholds while 7-OH-DPAT (0.03 mg kg(-1) s.c. or 5.0 microg intra-NAS) or apomorphine (0.05 mg kg(-1), s.c.) increased this measure. MK-801 combined with apomorphine or with 7-OH-DPAT, systemically or in the NAS shell, induced additive effects.

CONCLUSIONS

Lack of interaction between DA agonists and MK-801 in this study contrasts with our previous work showing synergistic reward-decreased effects of AMPA/kainate receptor blockade and D(2/3) DA receptor stimulation in the NAS shell, and indicates possible independence of DA and N-methyl-D-aspartate (NMDA) receptor effects in VTA electrical self-stimulation.

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.

Silencing dopamine D3-receptors in the nucleus accumbens shellin vivoinduces changes in cocaine-induced hyperlocomotion

The dopamine D(3) receptor (D(3)R) is an important pharmacotherapeutic target for its potential role in psychiatric disorders and drug dependence. To further explore its function in rats, a regulatable lentivirus, Lenti-D3, holding the rat D(3)R cDNA, has been constructed as well as three nonregulatable lentiviruses, Lenti-D3-siRNA1, Lenti-D3-siRNA2 and Lenti-D3-siRNA3, expressing small hairpin RNAs, aimed at silencing D(3)R expression and specifically targeted against different regions of the D(3)R mRNA. In vitro, Lenti-D3 expressed D(3)R and could efficiently be blocked with Lenti-D3-Sils. These viruses were stereotaxically injected into the shell part of the nucleus accumbens (NAcc) and effects of passive cocaine delivery on locomotor activity were assessed. Manipulations of D(3)R levels induced changes in the locomotor stimulant effects of cocaine as compared to control treatment. Suppression of dopamine (DA) D(3)R in the NAcc by means of local knockdown (with Lenti-D3-Sils) increased locomotor stimulant effects, whereas its overexpression with Lenti-D3 drastically reduced them. The latter effects could be reversed when animals were fed doxycycline, which prevented lentiviral-mediated DA D(3)R overexpression in the NAcc. Gene expression assessed by quantitative RT-PCR confirmed very efficient gene knockdown in vivo in animals treated with Lenti-D3-Sils (> 93% silencing of D(3)R gene). Thus D(3)R expression significantly contributes to behavioural changes associated with chronic cocaine delivery.

Pharmacological inhibition of DA- and 5-HT activity blocks spontaneous and cocaine-activated behavior: reversal by chronic cocaine treatment

Recently it was shown that the combined pretreatment with low autoreceptor preferring dose levels of apomorphine (0.05 mg/kg) and 8-OHDPAT (0.05 mg/kg), which decrease dopaminergic and serotonergic activity, induces a profound behavioral inhibition and also blocks the stimulant effects of cocaine. In two experiments, we report that the acute blockade of spontaneous and cocaine locomotor stimulant effects by pretreatment with 8-OHDPAT (0.05 mg/kg) plus apomorphine (0.05 mg/kg) is dose-dependently (0.0 2.5, 5.0, and 10.0 mg/kg cocaine) reversed with repeated cocaine treatments. Using a paired vs. unpaired Pavlovian conditioning protocol, we found that this reversal by cocaine (10 mg/kg) of the inhibition by the combined 8-OHDPAT plus apomorphine pretreatment occurred for the paired but not the unpaired cocaine treatment. The findings suggest that this reversal of behavioral inhibition is mediated by the transformation of the drug cues generated by 8-OHDPAT and apomorphine into cocaine-conditioned stimuli which can activate behavior.

Prenatal ethanol preferentially enhances reactivity of the dopamine D1 but not D2 or D3 receptors in offspring

Reports of prenatal ethanol (ETOH) effects on the dopamine system are inconsistent. In an attempt to clarify this issue, dams were given 35% ethanol-derived calories as the sole nutrient source in a liquid diet from the 10th through the 20th day of gestation (ETOH). Controls were pair-fed (PF) an isocaloric liquid diet or given ad libitum access to laboratory chow (LC). Prenatal exposure to both liquid diets reduced body weight of offspring relative to LC controls, more so for ETOH than for PF exposure. Prenatal ETOH also decreased litter size and viability, relative to both LC and PF control groups. On postnatal days 21-23, male and female offspring were given an injection of saline vehicle or one of eight specific dopamine receptor agonists or antagonists. Immediately after injection subjects were placed in individual observation cages, and over the following 30 min, eight behaviors (square entries, grooming, rearing, circling, sniffing, yawning, head and oral movements) were observed and quantified. No prenatal treatment effects on drug-induced behaviors were observed for dopamine D2 (Apomorphine, DPAT or Quinpirole) or D3 (PD 152255, Nafadotride, Apo or Quin effects on yawning) receptor agonists or antagonists, or for the vehicle control. In contrast, prenatal treatment effects were seen with drugs affecting the dopamine D1 receptor. Both D1 agonists (SKF 38393) and antagonists (SCH 23390 and high doses of spiperone) altered behaviors, especially oral and sniffing behaviors, in a manner which suggested enhanced dopamine D1 drug sensitivity in both ETOH and PF offspring relative to LC controls. These results suggest that at this age, both sexes experience a prenatal undernutrition-linked increase in the behavioral response to dopamine D1 agonists and antagonists, which can be intensified by gestational exposure to alcohol.

Simultaneous AMPA/kainate receptor blockade and dopamine D2/3 receptor stimulation in the nucleus accumbens decreases brain stimulation reward in rats

Interactions between dopamine (DA) and glutamate (GLU) in the mesocorticolimbic pathway of the brain may influence motivation and reward. Previous work from this laboratory has demonstrated that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor blockade may potentiate decreases in exploratory motor activity induced by the DA D(2/3) receptor agonist 7-OH-DPAT in the nucleus accumbens septi (NAS). This study investigated the interaction of AMPA/kainate receptor antagonists CNQX or NBQX with 7-OH-DPAT on ventral tegmental area (VTA) brain stimulation reward (BSR). Effects of these compounds, alone and combined, were measured in male Sprague-Dawley rats stereotaxically implanted with a unilateral VTA electrode and bilateral guide cannulae in the NAS core or shell subregions. Rate-frequency analysis was used to assess BSR frequency thresholds and maximum response rates of rats trained to lever-press for reinforcing electrical stimulation. When given alone, CNQX (0.5 microg), NBQX (0.5 microg), or 7-OH-DPAT (5.0 microg) did not affect BSR frequency thresholds. Co-administration of CNQX or NBQX with 7-OH-DPAT synergistically increased BSR frequency thresholds, indicative of decreased reward. These data indicate that simultaneous AMPA/kainate receptor blockade and DA D(2/3) receptor stimulation in the NAS may act synergistically to inhibit motivated behaviours such as electrical brain self-stimulation.

Effects of dopamine D3 receptor antagonists on spontaneous and agonist-reduced motor activity in NMRI mice and Wistar rats: comparative study with nafadotride, U 99194A and SB 277011

Studies investigating the role of the dopamine D3 receptor in the regulation of motor activity of rodents have used several ligands; however, there have been few comparative studies using agonist-antagonist interactions. In the present study, we compared the effects of dopamine D3 antagonists with different levels of selectivity over D2 receptors (nafadotride, U 99194A and SB 277011) on motor activity as well as on agonist-induced hypoactivity, in mice and rats. Horizontal and vertical movements were measured in photocell activity cages. 7-Hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT) and PD 128907 were used as dopaminergic agonists. Both dose-dependently inhibited motor activity in mice and vertical activity in rats, while decreasing horizontal activity of rats at doses of 0.01 and 0.1 mg/kg s.c., with no effect (7-OH-DPAT) or stimulation (PD 128907) at the 1 mg/kg dose. In mice habituated to the activity cage, nafadotride (0.1-3 mg/kg i.p.) caused a dose-dependent decrease in motor activity but did not affect the hypomotility evoked by either 7-OH-DPAT (0.1 mg/kg) or PD 128907 (0.1 mg/kg). In habituated rats it had no significant effect on motor activity and was not able to antagonize the hypoactivity caused by PD 128907 (0.1 mg/kg s.c.). U 99194A (5, 10 and 20 mg/kg s.c.) dose-dependently and significantly increased motor activity in mice and inhibited the effects of both agonists. In rats, nafadotride produced considerable motor stimulation and significantly inhibited the PD 128907-induced decrease in horizontal, but not in vertical, activity. SB 277011 (15-45 mg/kg p.o.) significantly increased motor activity in mice and partially blocked the action of 7-OH-DPAT on vertical, but not on horizontal, activity while against PD 128907, its significant inhibitory effect was restricted to a single dose (20 mg/kg). In habituated rats, SB 277011 (13.5, 20 and 30 mg/kg p.o.) exerted no significant effects on motor activity and did not antagonize the hypoactivity caused by PD 128907. Considerable species differences and movement-type differences (horizontal versus vertical) were observed between the effects of the tested dopamine D2/D3 ligands on motor activity in rodents. The antagonists also differed markedly in the robustness of their action. The poorly D3 selective antagonist, nafadotride, had little effect on motor behaviour. The moderately selective U 99194A exerted marked stimulatory effects on motility, and potently inhibited the actions of agonists. SB 277011, a highly selective dopamine D3 receptor antagonist, showed limited ability to influence the motor activity of rodents.

The 5-HT1A receptor and behavioral stimulation in the rat: effects of 8-OHDPAT on spontaneous and cocaine-induced behavior

RATIONALE

The contribution of the 5-HT(1A) somato-dendritic autoreceptor populations to spontaneous and cocaine-induced locomotion is unclear.

OBJECTIVES

To use a low dose range of +/-8-hydroxy-2-(di- n-propylamino)tetralin (8-OHDPAT) to preferentially stimulate 5-HT(1A) autoreceptors and a medium 8-OHDPAT dose range to stimulate both 5-HT(1A) autoreceptors and postsynaptic receptors as pretreatments prior to either saline or cocaine.

METHODS

In experiment 1, either a medium dose of 8-OHDPAT (0.4 mg/kg) or a low dose (0.05 mg/kg) was given as pretreatments 20 min before five separate 20-min open-field tests. In experiment 2, the pretreatments were changed to a low dose range of 8-OHDPAT (0.01-0.05 mg/kg), with or without WAY 100635 (0.01-0.05 mg/kg). In experiment 3, the 8-OHDPAT pretreatments (0.01, 0.025 or 0.05 mg/kg) were administered 20 min prior to saline or cocaine (10 mg/kg) tests. In experiment 4, a medium dose range (0.2-0.3 mg/kg) was given 20 min prior to saline or cocaine (10 mg/kg) tests.

RESULTS

Experiment 1 showed that 8-OHDPAT (0.4 mg/kg) tended to increase locomotor activity but that pretreatment with 0.05 mg/kg severely suppressed locomotor activity. In experiment 2, 8-OHDPAT in the low dose range inhibited locomotor activity and this effect was reversed by co-administration of WAY 100635. Experiment 3 showed that the low-dose 8-OHDPAT pretreatment reduced locomotor activity in saline but not cocaine tests. In experiment 4, 8-OHDPAT in the medium dose range enhanced locomotor activity in cocaine tests.

CONCLUSIONS

It is suggested that the facilitatory effect of 8-OHDPAT on cocaine-induced locomotor stimulation is mediated by inhibition of 5-HT(1A) somato-dendritic autoreceptors.

Effects of quinpirole on operant conditioning: perseveration of behavioral components

Quinpirole (QNP) is reported to elicit repetitive spontaneous behaviors as well as reduce extinction of operant responses. To determine whether these effects represent perseveration of learned behaviors, behavioral components were examined during the acquisition and extinction of operant responses. Rats, receiving either 0, 0.08, or 0.60 mg/kg QNP were trained to nose poke to receive water. The lower dose interfered with acquisition, but once learned, behavioral characteristics were normal. The higher dose produced excessive time in the drinking well when water was delivered. When water was withheld, the control and 0.08 mg/kg dose groups altered their behavior by initially increasing nose poke duration, followed by a progressive extinction of the operant response. The higher dose group, however, did not modify the characteristics of their behaviors, but continued to perform the behavioral sequence in the absence of reward. These effects are not ascribable to generalized locomotor activation in that response rates during reinforced responses, as well as at the beginning of the extinction phase, did not differ significantly across treatment groups. These results indicate that perseveration effects of QNP are not accountable by general behavioral arousal, nor are specific to extinction. Instead, these effects appear to reflect reduced adaptability of learned behavioral patterns to changes in reinforcement contingencies.

Neuroprotective and neurotrophic effect of apomorphine in the striatal 6-OHDA-lesion rat model of Parkinson's disease

We investigated the possible neuroprotective effect of the dopamine (DA) receptor agonist R-apomorphine (R-APO) within the striatal 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease. In one group of rats, R-APO administration (10 mg/kg/day, s.c.) started 15 min before 6-OHDA-injection. In a second group, R-APO administration started 24 h after lesion induction. Both groups received R-APO chronically for 11 days. Testing was carried out 2 weeks post-lesioning. R-APO treatment, whether started before or after the lesion induction, significantly reduced both the amphetamine-induced ipsiversive rotation and the size of the lesion at the level of the substantia nigra. Moreover, the dopamine cell shape and size resembled that observed in intact animals. R-APO treatment had no effect on the number of cells in the substantia nigra of intact rats, but significantly increased the number of cells in the ventral tegmental area (VTA), suggesting selective neurotrophic properties of R-APO in this region. R-APO treatment significantly attenuated the 6-OHDA-induced striatal DA depletion and DOPAC/DA ratios were normalized. Finally, an acute injection of 10 mg/kg R-APO was unable to scavenge 6-OHDA or MPP(+)-induced hydroxyl radicals as determined with the in vivo salicylate trapping technique. These data provide further evidence of the neurorescuing properties of R-APO. At least at the dose used in this study, this effect possibly occurs via mechanisms other than scavenging of hydroxyl radicals. In intact rats, we also show neurotrophic effects of the R-APO treatment. These seem to be limited to the VTA.

Differential effects of dopaminergic agents on locomotor sensitisation and on the reinstatement of cocaine-seeking and food-seeking behaviour

RATIONALE

The dopaminergic pathways are involved in natural and drug reward related processes.

OBJECTIVES

To compare the respective involvement of the dopaminergic receptors D1, D2 and D3 in natural-seeking versus drug-seeking behaviour and evaluate any concomitant expression of locomotor sensitisation.

METHODS

In separate experiments, male Wistar rats were trained to self-administer cocaine (0.25 mg/infusion) or to press a lever to obtain food pellets. Following a prolonged period of extinction, reinstatement of lever responding was measured following non-contingent food delivery, cocaine (15 mg/kg), D1-like (SKF 82958, 0.25 mg/kg), D2-like (quinelorane, 0.25 mg/kg) and D3-like (7-OHDPAT, 0.25 mg/kg) agonists. To demonstrate parallel expression of behavioural sensitisation, locomotor activity was recorded during the reinstatement sessions.

RESULTS

Cocaine and quinelorane administrations reinstated cocaine-seeking behaviour and induced the expression of locomotor sensitisation, whereas SKF 82958 and 7-OHDPAT had either no effect or non-specific effects. In the food-seeking experiment, we found that quinelorane and 7-OHDPAT did not reinstate lever pressing. Cocaine increased responding on both active and inactive levers, whereas SKF 82958 had a more specific effect with higher responding on the previously food-associated lever.

CONCLUSIONS

Our results indicate that expression of locomotor sensitisation and reinstatement of cocaine-seeking but not food-seeking behaviours are in part supported by common dopaminergic substrates, among which the D2 receptors play a crucial role.

Dopaminergic and serotonergic autoreceptor stimulation effects are equivalent and additive in the suppression of spontaneous and cocaine induced locomotor activity

We used the D(2) receptor agonist, apomorphine (APO) and the 5-HT(1A) receptor agonist, 8-OHDPAT (8OH) in a low dose range to stimulate autoreceptors and in this way assess the separate and combined effects of reduced DA and 5-HT activity upon spontaneous and cocaine induced locomotor behavior. Two separate experiments were conducted. In the first experiment, separate groups of rats (N=10) were tested with either saline, 8OH, APO or 8OH plus APO (0.01, 0.025, 0.05 mg/kg). At 0.05 mg/kg, 8OH and APO induced similar dose related decreases (up to approximately 50%) in locomotor activity. The combined 8OH plus APO treatment induced dose-related decreases in locomotion (approximately 90%). At the 0.05 mg/kg dose level, the drug treatments given separately blocked cocaine induced increases in activity and the 8OH and APO inhibitory effects were again additive. In the second experiment, separate groups (N=10) received saline, 0.05 mg/kg APO, 0.05 mg/kg 8OH or 0.05 mg/kg APO plus 0.05 mg/kg 8OH. As in the first experiment, the 8OH and APO given separately reduced locomotor activity by approximately 50% and when given together, locomotor activity was virtually eliminated (reduced 80-90%). When the combined APO/8OH group also received the 5-HT(1A) antagonist, WAY 100635 (0.05 mg/kg), the effect on activity was equivalent to 0.05 mg/kg APO alone. Ex vivo neurochemical measurement of dopamine (DA) and serotonin (5-HT) metabolism confirmed that the APO decreased DA turnover, 8OH decreased 5-HT turnover and the combined treatment reduced both the DA and 5-HT turnover. Thus, for both spontaneous and cocaine induced locomotor behavior, the low dose 8OH and APO treatments suppressed locomotor activity and these effects were additive. These findings indicate that DA and 5-HT systems contribute separately to motoric activation. These results suggest that it is important to consider both DA and 5-HT contributions to disorders of motoric impoverishment such as Parkinson's disease as well as to hyperkinetic states such as those induced by stimulant drugs.

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.

Induction of neurogenesis in the adult rat subventricular zone and neostriatum following dopamine D3 receptor stimulation

Discrete regions of the adult CNS, including the subventricular zone (SVZ), do retain the capacity for neurogenesis. These progenitor cells may represent a potential new source of cells for replacement therapies in neuroregenerative diseases. An understanding of the microenvironmental signals regulating neurogenesis in the adult brain would facilitate the development of such therapeutic approaches. A particularly strong expression of dopamine D(3) receptor mRNA occurs in the proliferative SVZ during prenatal and early postnatal ontogeny. Although its expression diminishes following development, a restricted D(3) receptor expression persists in this region through adulthood, coincident with continued proliferation in this region. Here, we demonstrate a two-fold induction of cell proliferation (BrdU incorporation) in the SVZ and rostral migratory stream of the adult Sprague-Dawley rat brain following intrasubventricular administration of the dopamine D(3) receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) for 2 weeks. The number of BrdU-positive cells was elevated ten-fold from very low baseline levels in the neighbouring neostriatum, another region known to express D(3) receptors. These striatal BrdU-positive cells appeared within 3 days following intracerebral infusion of 7-OH-DPAT and were distributed homogeneously throughout the striatum following systemic administration. This suggests that these cells originate from resident progenitor cells rather than the SVZ. Dopamine D(3) receptor activation may serve as a proneuronal differentiation signal as 60-70% of the new cells had neuronal markers following 7-OH-DPAT infusion. These results suggest that the dopamine D(3) receptor may be a good drug target for cell replacement strategies, particularly because of the fact that its expression is almost exclusively limited to the nervous system.

Blockade by the cannabinoid CB1 receptor antagonist, rimonabant (SR141716), of the potentiation by quinelorane of food-primed reinstatement of food-seeking behavior

It has been shown previously that the selective cannabinoid CB1 receptor antagonist, rimonabant (SR141716), reduced the intake of palatable food as well as the self-administration of several drugs of abuse, suggesting that endocannabinoid systems play a role in brain reward function. The present study investigated whether a cannabinoid step was involved in food-seeking behavior induced by explicit stimuli, using an operant reinstatement procedure in rats. Experimental sessions consisted of a 15-min food rewarded period, followed by a 45-min extinction period. Rimonabant did not affect the response reinstatement induced by noncontingent delivery of food pellets, but prevented (0.03-0.3 mg/kg) the potentiation by quinelorane, a dopamine D3 receptor-preferring agonist, of food-seeking behavior. A possible link between cannabinoid processes and D3- and/or D2-mediated dopaminergic transmission was further investigated by studying Fos protein expression in cortico-limbic structures in D3 (D3-/-) and D2 (D2-/-) knockout mice. Rimonabant (10 mg/kg) increased Fos immunoreactivity in the prefrontal cortex (pFCortex) and in the shell but not the core of the nucleus accumbens (NAcc). Fos induction by this dose of rimonabant was not seen in mice lacking CB1 receptors, providing clear evidence for the involvement of CB1 receptors. In the NAcc shell, the effect of rimonabant was suppressed in D3-/-, but remained unchanged in D2-/- mice. In contrast, Fos expression by rimonabant in the pFCortex was impervious to D2 or D3 receptor deletion. In conclusion, these data indicate first that rimonabant prevented the enhancement by quinelorane of the appetitive value of food pellets unexpectedly delivered during extinction and second that rimonabant effects might involve D3 receptor-mediated processes. Overall, these results are consistent with the notion that endocannabinoid functions control brain reward processes and in particular the capacity of explicit stimuli to precipitate food-seeking behavior.

Effects of quinpirole on behavioral extinction

Behavioral effects of quinpirole (QNP), a dopamine D(2) receptor agonist, suggest it impacts neural mechanisms mediating goal-directed behaviors, as well as behavioral extinction following removal of a primary reinforcer. The present study investigated the effect of QNP on behavioral extinction following the omission of contingent reinforcement, and whether this effect is related to acquisition or processes specific to extinction. Rats were trained on a continuous reinforcement schedule to nose-poke for water reward. Using a free-operant procedure, rats completed approximately 70 responses for each of four consecutive days. On the fifth day reward was withheld. Rats were assigned to one of five groups in which they received 0.3 mg/kg QNP ip either during the first day (acquisition phase), the second 2 days (maintenance phase), the last day (extinction phase), or during all days. A fifth group received vehicle injections. Rats receiving QNP during the acquisition and maintenance phase did not differ significantly from the control group during the extinction phase, although they demonstrated reduced response rates on days they received QNP. However, rats treated during the extinction phase or during all phases demonstrated a significant reduction in the rate of extinction. This effect cannot be attributed to an increase in general behavioral arousal because response rates for reinforced responses did not differ significantly among groups following acquisition of the behavior. The reduced extinction effect does not appear to be related to abnormalities in the initial behavior-reward association, but instead may result from enhanced engagement of learned behavioral patterns, or from interference of signals associated with removal of predicted reinforcement.

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.

Behavioral response profiles following drug challenge with dopamine receptor subtype agonists and antagonists in developing rat

As part of an investigation into the effects of gestational ethanol (ETOH) exposure on the developing dopamine (DA) system, pregnant Sprague-Dawley rats were exposed to one of three conditions: ETOH, pair-fed (PF) to the ETOH group, or ad libitum lab chow controls (LC). In this paper we report behavioral drug challenge effects for offspring of the two control groups (PF and LC). Male and female pups between postnatal days (PNDs) 21 and 23 in age were exposed to one of three intraperitoneal/subcutaneous doses of one of eight drugs chosen to assess the functional status of the DA D(1), D(2), and D(3) receptor subtype, or a saline control. Agonists were SKF 38393, apomorphine (APO), quinpirole (QUIN), and 7-hydroxy-N,N-di-n-propyl-2-amino-tetralin [7-OH-DPAT (DPAT)]; antagonists were spiperone (SPIP), SCH 23390, and two recently developed D(3) antagonists nafadotride (NAF) and PD 152255. Immediately following drug injection, pups were placed in observation cages, where eight behaviors (square entries, grooming, circling, rearing, sniffing, head and oral movements, and yawning) were scored at 3-min intervals for 30 min. Classic behavioral profiles were generally obtained for the high-dose mixed agonists APO, DPAT, and QUIN, which potently increased square entries, rearing, and sniffing, while reducing grooming and head movements. However, low-dose APO had no effect on behavior. The D(1) agonist, SKF 38393, had a strikingly different behavioral profile; it had no effect on square entries at any dose, while increasing grooming and sniffing at the medium dose. The D(1) antagonist, SCH 23390, profoundly decreased all behaviors except oral and head movements, especially at high doses. In contrast, the effects of the D(2) antagonist, SPIP, were limited to increasing sniffing at the medium dose. The two putative D(3) antagonists, NAF and PD 152255, presented strikingly different profiles. NAF induced a pattern of behavioral suppression that resembled the profile of high-dose SCH, while high-dose PD 152255 stimulated behavior. The failure of low-dose APO to have any effect on behavior suggests that the D(2) autoreceptor is not functional in preweanling rats. This hypothesis is further supported by the lack of behavioral suppression seen with low-dose QUIN and DPAT. Failure of NAF to produce behavioral activation at low doses and the stimulatory effects seen with PD 152255 suggests that either the D(3) autoreceptor, the postsynaptic D(3) receptor, or both are not fully functional at this age as well.

Nafadotride administration increases D1 and D1/D2 dopamine receptor mediated behaviors

The administration of nafadotride, given at doses known to block the D3 dopamine receptors (0.75, 1.5, 3 mg/kg i.p.) increased locomotor activity both in naive and habituated rats and counteracted the hypothermia but not the hypomotility induced by a low dose of the putative D3 dopamine agonist (+/-)-7-hydroxy-2-(di-N-propylamino)-tetralin (7-OH-DPAT; 0.04 mg/kg). Nafadotride did not antagonize either the motor effects induced by different doses of the D2 agonist quinpirole (0.05 and 0.3 mg/kg) or the hypermotility induced by 7-OH-DPAT given at a dose (0.32 mg/kg) stimulating D2 dopamine receptors. The same nafadotride doses potentiated the grooming behavior induced by the D1 dopamine agonist SKF 38393 (10 mg/kg i.p.) as well as the stereotyped response to the D1/D2 agonist apomorphine (0.5 mg/kg s.c.). Stereotyped behavior was also observed in rats concomitantly treated with nafadotride and the D2 agonist quinpirole. As the activation of D1 dopamine receptors plays an important role in the occurrence of stereotypies, the results suggest that the blockade of D3 receptors by nafadotride could have favored D1/D2 dopamine receptor-mediated behaviors by potentiating D1 receptor function.

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.

Effects of 7-OH-DPAT on cocaine-seeking behavior and on re-establishment of cocaine self-administration

Effects of the D2-like dopamine agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT), on cocaine-seeking behavior and re-establishment of cocaine self-administration were examined. Rats were trained to lever press for cocaine infusions (0.25 mg/kg iv). Some were then tested for cocaine-seeking behavior (i.e., lever presses in the absence of cocaine re-inforcement) immediately following acute 7-OH-DPAT (0.001, 0.01, 0.1, or 1.0 mg/kg sc) or saline administration. Others were tested immediately or 2-23 h following repeated daily 7-OH-DPAT (1.0 mg/kg sc) or saline administration for extinction of cocaine-seeking behavior, cocaine re-instatement of cocaine-seeking behavior, and re-establishment of cocaine self-administration following extinction. 7-OH-DPAT-induced changes in locomotion were also assessed. Cocaine-experienced animals exhibited cross-tolerance to the transient hypoactivity produced by acute 7-OH-DPAT administration. Acute administration of low doses (0.01-0.1 mg/kg) of 7-OH-DPAT attenuated cocaine-seeking behavior, whereas the highest dose (1.0 mg/kg) initially attenuated, then increased, cocaine-seeking behavior. In animals tested immediately following one of the repeated administrations, 7-OH-DPAT did not alter cocaine self-administration, but sensitized locomotion. Repeated 7-OH-DPAT administration also increased cocaine-seeking behavior when administered 0 h, but not 2 or 4 h, before cocaine priming (15 mg/kg ip) and testing. In animals tested 17-23 h following one of the repeated administrations, cocaine-seeking behavior and re-establishment of cocaine self-administration were attenuated, but maintenance of self-administration following re-establishment, cocaine re-instatement of extinguished cocaine-seeking behavior, and spontaneous locomotion were unaltered. The findings suggest that following repeated administration, 7-OH-DPAT produces a transient increase (<2 h) in incentive motivation for cocaine that is followed by a protracted decrease in incentive motivation for cocaine.

Preclinical evaluation of GBR12909 decanoate as a long-acting medication for methamphetamine dependence

Methamphetamine (METH) abuse is a growing health problem, and no treatments for METH dependence have been identified. The powerful addictive properties of METH are mediated by release of dopamine (DA) from nerve terminals in mesolimbic reward pathways. METH stimulates DA release by acting as a substrate for DA transporter (DAT) proteins, thereby triggering efflux of DA from cells into the synapse. We have shown that blocking DAT activity with high-affinity DA uptake inhibitors, like GBR12909, can substantially reduce METH-evoked DA release in vitro, suggesting GBR12909 may have potential as a pharmacotherapy for METH dependence. The purpose of the present study was to examine the neurobiological effects of a long-acting oil-soluble preparation of GBR12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-hydroxy-3-phenylpropyl) piperazinyl decanoate, or GBR-decanoate). Male rats received GBR-decanoate (480 mg/kg, i.m.) or its oil vehicle, and were tested using a variety of methods one and two weeks later. Ex vivo autoradiography showed that GBR-decanoate decreases DAT binding in DA-rich brain regions. In vivo microdialysis in the nucleus accumbens revealed that GBR-decanoate elevates baseline levels of extracellular DA and antagonizes the ability of METH to evoke DA release. The dopaminergic effects of GBR-decanoate were sustained, lasting for at least two weeks. Rats pretreated with GBR-decanoate displayed enhanced locomotor responses to novelty at one week, but not two weeks, postinjection. Administration of the D(2)/D(3) receptor agonist quinpirole (10 and 100 microg/kg, s.c.) decreased locomotor activity and suppressed plasma prolactin levels; quinpirole-induced responses were not altered by GBR-decanoate. Thus, GBR-decanoate is able to elevate basal synaptic DA levels and block METH-evoked DA release in a persistent manner, without significant perturbation of DA receptor function. The findings suggest that GBR-decanoate, or similar long-acting agents, should be evaluated further as potential treatment adjuncts in the management of METH addiction in humans.

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.

Neuroprotective effects of D3 dopamine receptor agonists

The effects of D(3) receptor activation are unresolved at this time, but may have practical implications in the treatment of Parkinson's disease (PD). As a result of assessing the neuroprotective effects of the direct-acting D(3) preferring dopamine (DA) agonist pramipexole (PPX), we have observed that drugs which psossess D(3) affinity increase the production of a DA neurotrophic factor in tissue culture. This molecule is increased by treatment with PPX, is constitutively produced by DA neurons in culture, and possesses a molecular weight of approximately 35kDa. It is hypothesized that this molecule may be the so-called DA autotrophic factor referred to by many authors over the past two decades. Interestingly, the protein is oxidant-labile and, therefore, D(3) agonists which increase its production and also possess antioxidant capacity would provide unique neuroprotective benefits to patients with PD. However, many questions remain. Although the data supporting this notion are strong, it is clear that other unknown characteristics of DA agonists, including increased production of anti-apoptotic proteins, are also involved. This manuscript will review this concept in the context of tissue culture strategies of neuroprotection. Although no conclusion can be made at this time, it is clear that direct comparisons of the neuroprotective effects of direct-acting DA agonists in mesencephalic culture can provide considerable insight into the mechanistic actions of anti-dopaminergic drugs.

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.

Effects of low dosages of apomorphine on maternal responsiveness in lactating rats

Lactating rats (day 7 +/- 1 postpartum) were observed during a 1-h reunion with their pups 4 h after separation from them and 10 min after subcutaneous injection of saline (SAL; 0.1 ml) or low dosages of the dopamine agonist, apomorphine (APO; 0.1 or 0.25 mg/kg). Although APO did not affect latency to sniff pups or retrieve the first pup, there were dosage-dependent delays in onset of licking and nursing pups, and decreases in retrieval and grouping of pups, nursing duration, and litter weight gain. The alterations in maternal responsiveness among APO-treated dams were related to increased carrying and mouthing of pups and markedly increased sniffing of pups, bedding, and cage. Duration of time spent licking pups, exploring, and self-grooming did not differ between groups. Thus, certain APO-induced stereotypic behaviors interfered with the normal sequence of maternal behavior by exaggerating some components and delaying others. These results are relevant to disturbances in maternal behavior caused by hyperreactivity or by other drugs that increase dopaminergic activity, such as cocaine.