Behavioural topography in the striatum: differential effects of quinpirole and D-amphetamine microinjections

The mesocortical dopaminergic system cannot explain hyperactivity in an animal model of attention deficit hyperactivity disorder (ADHD)- Spontaneously hypertensive rats (SHR)

BACKGROUND

Attention deficit hyperactivity disorder (ADHD) is one of the most prevalent neuropsychiatric disorders with morphological brain abnormalities. There is a growing body of evidence that abnormalities in the dopaminergic system may account for ADHD pathogenesis. However, it is not clear whether the dopaminergic system is hyper or hypoactive. To determine whether the DA neurons and/or axons deficiency might be the cause of the postulated dopaminergic hypofunction in spontaneously hypertensive rats (SHR, animal model of ADHD), this study examined the dopaminergic neurons and fibers in the brain tissues of SHRs and Wistar Kyoto rats (WKY, control animals). Here, we performed immunohistochemical tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) staining on brain sections collected on juveniles from SHR and WKY. Moreover, behavioral testing to examine the hyperactivity in the open field area was also elucidated.

RESULTS

The mesocortical dopaminergic system appears to be normal in juvenile SHR, as suggested by (i) no alteration in the area density of TH-immunoreactive (TH-ir) dopaminergic neurons in the ventral tegmental area (VTA), (ii) no alterations in the volume density of TH-ir fibers in layer I of the prelimbic (PrL) subregion of medial PFC (mPFC), (iii) no alteration in the percentage of TH-ir dopaminergic fibers in layer I of the PrL subregion of mPFC as revealed by TH and/or DBH immunoreactivity. Furthermore, the SHR showed increased locomotor activity than WKY in the open field test.

CONCLUSIONS

The demonstration of no alteration in mesocortical dopaminergic neurons and fiber in SHR raises some concern about the position of SHR as an animal model of the inattentive subtype of ADHD. However, these results strengthen this strain as an animal model of hyperactive/impulsive subtype ADHD for future studies that may elucidate the underlying mechanism mediating hyperactivity and test various treatment strategies.

Prenatal nicotine sex-dependently alters adolescent dopamine system development

Despite persistent public health initiatives, many women continue to smoke during pregnancy. Since maternal smoking has been linked to persisting sex-dependent neurobehavioral deficits in offspring, some consider nicotine to be a safer alternative to tobacco during pregnancy, and the use of electronic nicotine delivery systems is on the rise. We presently show, however, that sustained exposure to low doses of nicotine during fetal development, approximating plasma levels seen clinically with the nicotine patch, produces substantial changes in developing corticostriatal dopamine systems in adolescence. Briefly, pregnant dams were implanted on gestational day 4 with an osmotic minipump that delivered either saline (GS) or nicotine (3 mg/kg/day) (GN) for two weeks. At birth, pups were cross-fostered with treatment naïve dams and were handled daily. Biochemical analyses, signaling assays, and behavioral responses to cocaine were assessed on postnatal day 32, representative of adolescence in the rodent. GN treatment had both sex-dependent and sex-independent effects on prefrontal dopamine systems, altering Catechol-O-methyl transferase (COMT)-dependent dopamine turnover in males and norepinephrine transporter (NET) binding expression in both sexes. GN enhanced cocaine-induced locomotor activity in females, concomitant with GN-induced reductions in striatal dopamine transporter (DAT) binding. GN enhanced ventral striatal D2-like receptor expression and G-protein coupling, while altering the roles of D2 and D3 receptors in cocaine-induced behaviors. These data show that low-dose prenatal nicotine treatment sex-dependently alters corticostriatal dopamine system development, which may underlie clinical deficits seen in adolescents exposed to tobacco or nicotine in utero.

Chemogenetic activation of dopamine neurons in the ventral tegmental area, but not substantia nigra, induces hyperactivity in rats

Hyperactivity is a core symptom in various psychiatric disorders, including attention-deficit/hyperactivity disorder, schizophrenia, bipolar disorders, and anorexia nervosa. Although hyperactivity has been linked to dopaminergic signalling, the causal relationship between midbrain dopamine neuronal activity and locomotor hyperactivity remains unknown. In this study, we test whether increased dopamine neuronal activity is sufficient to induce locomotor hyperactivity. To do so, we used designer receptors exclusively activated by designer drugs (DREADD) to chemogenetically enhance neuronal activity in two main midbrain dopamine neuron populations, i.e. the ventral tegmental area (VTA) and substantia nigra pars compacta (SN), in TH:Cre rats. We found that activation of VTA dopamine neurons induced a pronounced and long-lasting hyperactive phenotype, whilst SN dopamine neuron activation only modestly increased home cage locomotion. Furthermore, this hyperactive phenotype was replicated by selective activation of the neuronal pathway from VTA to the nucleus accumbens (NAC). These results show a clear functional difference between neuronal subpopulations in the VTA and SN with regards to inducing locomotor hyperactivity, and suggest that the dopaminergic pathway from VTA to NAC may be a promising target for the treatment of hyperactivity disorders.

Evidence of D2 receptor expression in the nucleus incertus of the rat

The nucleus incertus (NI), located in the caudal brainstem, mainly consists of GABAergic neurons with widespread projections across the brain. It is the chief source of relaxin-3 in the mammalian brain and densely expresses corticotropin-releasing factor type 1 (CRF1) receptors. Several other neurotransmitters, peptides and receptors are reportedly expressed in the NI. In the present investigation, we show the expression of dopamine type-2 (D2) receptors in the NI by reverse transcriptase-polymerase chain reaction (RT-PCR), western blotting (WB) and immunofluorescence (IF). RT-PCR did not show expression of D3 receptors. D2 receptor short isoform (D2S)-like, relaxin-3, CRF1/2 receptor and NeuN immunoreactivity were co-expressed in the cells of the NI. Behavioural effects of D2 receptor activation by intra-NI infusion of quinpirole (a D2/D3 agonist) were evaluated. Hypolocomotion was observed in home cage monitoring system (LABORAS) and novel environment-induced suppression of feeding behavioural paradigms. Thus the D2 receptors expressed in the NI are likely to play a role in locomotion. Based on its strong bidirectional connections to the median raphe and interpeduncular nuclei, the NI was predicted to play a role in modulating behavioural activity and the present results lend support to this hypothesis. This is the first evidence of expression of a catecholamine receptor, D2-like immunoreactivity, in the NI.

Increased drinking after intra-striatal injection of the dopamine D2/D3 receptor agonist quinpirole in the rat

RATIONALE

Dopamine D2 receptor hyperactivity has been implicated in the development of psychogenic polydipsia in schizophrenic patients. Repeated treatment with dopamine agonists, including the D2/D3 agonist quinpirole, has been shown to induce hyperdipsia in a number of animal models. Despite these observations, obtained with systemic administrations, little attempt has been made to investigate where in the brain dopamine agonists act to induce hyperdipsia.

OBJECTIVE

The present study investigates the effects of repeated intra-caudate infusions of quinpirole on the intake of water by rats tested under free-drinking conditions.

MATERIALS AND METHODS

Rats with bilateral cannulae placed into the anterior, central or posterior caudate received quinpirole microinfusions (1 μg/side) for five consecutive days in their home cage. Water intake was measured 15 and 60 min after the treatment.

RESULTS

When injected in the central caudate, quinpirole increased water intake, and this effect progressively increased over sessions, indicating the development of sensitization. When injected in the posterior caudate, the dipsogenic effect of quinpirole was less intense and did not undergo sensitization. The infusion of quinpirole in the anterior caudate did not affect drinking.

CONCLUSION

The present study shows that caudate D2/3 receptors play an important role in the development of quinpirole-induced hyperdipsia, an animal model of psychotic polydipsia.

Dopamine receptor inactivation in the caudate-putamen differentially affects the behavior of preweanling and adult rats

The irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) has been used to study the ontogeny of dopamine (DA) receptor functioning in young and adult rats. Most notably, systemic administration of EEDQ blocks the DA agonist-induced behaviors of adult rats, while leaving the behavior of preweanling rats unaffected. The purpose of the present study was to: (a) determine whether the age-dependent actions of EEDQ involve receptors located in the dorsal caudate-putamen (CPu) and (b) confirm that EEDQ's behavioral effects result from the inactivation of DA receptors rather than some other receptor type. In Experiment 1, EEDQ or DMSO was bilaterally infused into the CPu on PD 17 or PD 84. After 24h, rats were given bilateral microinjections of the full DA agonist R(-)-propylnorapomorphine (NPA) or vehicle into the dorsal CPu and behavior was assessed for 40 min. In Experiment 2, preweanling rats were treated as just described, except that DA receptors were protected from EEDQ-induced alkylation by administering systemic injections of D1 (SCH23390) and D2 (sulpiride) receptor antagonists. As predicted, microinjecting EEDQ into the dorsal CPu attenuated the NPA-induced locomotor activity and stereotypy of adult rats. In contrast, rats given bilateral EEDQ infusions on PD 17 exhibited a potentiated locomotor response when treated with NPA. Experiment 2 showed that DA receptor inactivation was responsible for NPA's actions. A likely explanation for these results is that EEDQ inactivates a sizable percentage of DA receptors on PD 17, but leaves the remaining receptors in a supersensitive state. This receptor supersensitivity, which probably involves alterations in G protein coupling, could account for NPA-induced locomotor potentiation. It is likely that adult rats to not show a similar EEDQ-induced change in receptor dynamics or DA receptor inactivation was more complete in older animals and effectively eliminated the expression of DA agonist-induced behaviors.

Importance of D1 and D2 receptors in the dorsal caudate-putamen for the locomotor activity and stereotyped behaviors of preweanling rats

Dopaminergic compounds often affect the unlearned behaviors of preweanling and adult rats differently, although the brain regions underlying these age-dependent behavioral effects have not been specified. A candidate brain region is the dorsal caudate-putamen (CPu); thus, a goal of the present study was to determine whether D1 and D2 receptors in the dorsal CPu are capable of modulating the unlearned behaviors of preweanling rats. In Experiments 1 and 2, selective and nonselective dopamine agonists were bilaterally microinjected into the dorsal CPu on postnatal day (PD) 18 and both locomotor activity and stereotypy were measured. In Experiment 3, the functional coupling of D1 and D2 receptors was assessed by microinjecting the D1 agonist SKF-82958 and the D₂/D₃ agonist quinpirole either alone or in combination. In Experiments 4 and 5, quinpirole and the D1 receptor antagonist SCH-23390, or SKF-82958 and the D2 receptor antagonist raclopride, were co-administered into the dorsal CPu to further assess whether a functional D1 or D2 receptor system is necessary for the expression of quinpirole- or SKF-82958-induced behaviors. Results showed that selective stimulation of D1 or D2 receptors in the dorsal CPu increased both the locomotor activity and stereotypy of preweanling rats. Receptor coupling was evident on PD 18 because co-administration of a subthreshold dose of SKF-82958 and quinpirole produced more locomotor activity than either agonist alone. Lastly, the dopamine antagonist experiments showed that both D1 and D2 receptor systems must be functional for SKF-82958- or quinpirole-induced locomotor activity to be fully manifested. When the present data are compared to results from non-ontogenetic studies, it appears that pharmacological manipulation of D1 and D2 receptors in the dorsal CPu affects the behavior of preweanling and adult rats in a generally similar manner, although some important age-dependent differences are apparent. For example, D1 and/or D2 agonists preferentially induce locomotor activity, and not intense stereotypy, in younger animals.

Psychiatric conditions and behavioural problems in adolescents with intellectual disabilities: correlates with autism

OBJECTIVE

To determine whether psychiatric and behavioural disorders occur more frequently in adolescents with autism and intellectual disabilities, compared with those without autism.

METHOD

A population-based case-control study was undertaken and 36 adolescents with autism were pairwise matched for age and IQ to 36 adolescents without autism. Caregivers were interviewed with structured psychiatric interview and questionnaire measures of psychiatric and behavioural problems.

RESULTS

Compulsive behaviours and stereotypies were significantly more common in adolescents with autism.

CONCLUSIONS

Adolescents with autism are prone to compulsive behaviours and stereotypies as well as specific manifestations of anxiety, fears, and phobias.

The dopamine uptake inhibitor 3 alpha-[bis(4'-fluorophenyl)metoxy]-tropane reduces cocaine-induced early-gene expression, locomotor activity, and conditioned reward

Benztropine (BZT) analogs, a family of high-affinity dopamine transporter ligands, are molecules that exhibit pharmacological and behavioral characteristics predictive of significant therapeutic potential in cocaine addiction. Here, we examined in mice the effects of 3 alpha-[bis(4'-fluorophenyl)metoxy]-tropane (AHN-1055) on motor activity, conditioned place preference (CPP) and c-Fos expression in the striatum. AHN-1055 produced mild attenuation of spontaneous locomotor activity at a low dose (1 mg/kg) and weak stimulation at a higher dose (10 mg/kg). In parallel, the BZT analog significantly increased c-Fos expression in the dorsolateral caudoputamen at the high dose, whereas producing marginal decreases at low and moderate doses (1, 3 mg/kg) in both dorsal and ventral striatum. Interaction assays showed that cocaine's ability to stimulate locomotor activity was decreased by AHN-1055 treatment, but not by treatment with D-amphetamine. Such reduced ability did not result from an increase in stereotyped behavior. Another dopamine uptake inhibitor, nomifensine, decreased cocaine-induced locomotor activity but evoked by itself intense motor stereotypies. Remarkably, the BZT analog dose-dependently blocked cocaine-induced CPP without producing CPP when given alone, and blocked in conditioned mice cocaine-stimulated early-gene activation in the nucleus accumbens and dorsomedial striatum. These observations provide evidence that AHN-1055 does not behave as a classical psychomotor stimulant and that some of its properties, including attenuation of cocaine-induced striatal c-Fos expression, locomotor stimulation, and CPP, support its candidacy, and that of structurally related molecules, as possible pharmacotherapies in cocaine addiction.

Effects of dorsal striatal infusions of R(-)-propylnorapomorphine on kappa-opioid-mediated locomotor activity in the young rat: possible role of the indirect pathway

Stimulation of kappa-opioid receptors in the substantia nigra pars reticulata (SNPR) increases the locomotor activity of young rats: an effect blocked by systemic administration of a D2-like receptor agonist. Based on these initial findings, we proposed that: (a) D2-like receptors in the dorsal striatum are responsible for attenuating kappa-opioid-induced locomotor activity, and (b) the effects of D2-like receptor stimulation are mediated by the indirect pathway, which extends from the dorsal striatum to the SNPR via the globus pallidus (GP) and subthalamic nucleus (STN). To test the first hypothesis, young rats were given a systemic injection (i.p.) of saline or the kappa-opioid receptor agonist (+/-)-trans-U50,488 methanesulfonate salt (U50,488) on postnatal day (PD) 18. Later in the testing session, rats received bilateral infusions of vehicle or the D2-like receptor agonist R(-)-propylnorapomorphine (NPA) into the dorsal striatum, and the ability of NPA to block U50,488-induced locomotor activity was determined. To test the second hypothesis, rats were given sham or bilateral electrolytic lesions of the GP or STN on PD 16. Two days later, saline- and U50,488-induced locomotor activity was measured after systemic (i.p.) administration of vehicle or NPA. As predicted, dorsal striatal infusions of NPA attenuated the U50,488-induced locomotor activity of young rats. Contrary to our expectations, bilateral lesions of the GP or STN did not impair NPA's ability to block U50,488-induced locomotor activity. When considered together, these results suggest that: (a) stimulation of D2-like receptors in the dorsal striatum is sufficient to attenuate the kappa-opioid-mediated locomotor activity of young rats; and (b) the indirect pathway does not mediate the effects of D2-like receptor stimulation in this behavioral model.

Striatal Contributions to Reward and Decision Making: Making Sense of Regional Variations in a Reiterated Processing Matrix

The striatum is the major input nucleus of the basal ganglia. It is thought to play a key role in learning on the basis of positive reinforcement and in action selection. One view of the striatum conceives it as comprising a reiterated matrix of processing units that perform common operations in different striatal regions, namely synaptic plasticity according to a three-factor rule, and lateral inhibition. These operations are required for reinforcement learning and selection of previously reinforced actions. Analysis of the behavioral effects of circumscribed lesions of the striatum, however, suggests regional specialization of learning and decision-making operations. We consider how a basic processing unit may be modified by regional variations in neurochemical parameters, for example, by the gradient in density of dopamine terminals from dorsal to ventral striatum. These variations suggest subtle differences between dorsolateral and ventromedial striatal regions in the temporal properties of dopamine signaling, which are superimposed on regional differences in connectivity. We propose that these variations make sense in relation to the temporal structure of activity in striatal inputs from different regions, and the requirements of different learning operations. Dorsolateral striatal (DLS) regions may be subject to brief, precisely timed pulses of dopamine, whereas ventromedial striatal regions integrate dopamine signals over a longer time course. These differences may be important for understanding regional variations in the contribution to reinforcement of habits, versus incentive processes that are sensitive to the value of expected rewards.

Mapping the effects of the selective dopamine D2/D3 receptor agonist quinelorane using pharmacological magnetic resonance imaging

Dopamine agonists with a high affinity for D2 and D3 receptors have a biphasic effect on rodent locomotion, inducing hypolocomotion at low doses and hyperlocomotion at higher doses. Controversy surrounds the role of the D3 receptor in mediating the hypolocomotor response to low agonist doses. This study examines patterns of neuronal activation induced by varying doses of the D2/D3 receptor agonist quinelorane using blood oxygen level dependent (BOLD) pharmacological magnetic resonance imaging (phMRI), and compares them with corresponding behavioural responses. Quinelorane (3 microg/kg) induced hypolocomotion in rats naive to the testing environment, and in phMRI experiments increased neuronal activity within the anterior olfactory nuclei, nucleus accumbens and islets of Calleja, regions containing a high density of D3 receptors. A 30 microg/kg dose of quinelorane resulted in biphasic locomotor effects, with initial hypolocomotion followed by sustained hyperlocomotion. phMRI indicated that this higher dose increased cerebral activity within limbic and olfactory regions, as did the lower drug dose, but induced additional activation in the caudate-putamen and globus pallidus, areas dense in D2 receptors but containing few D3 receptors. The more restricted pattern of activation at low agonist doses and close temporal relationship between behavioural and BOLD signal responses to quinelorane suggest that those nuclei most dense in D3 receptors play a key role in mediating the hypolocomotor effects of quinelorane. However, the presence of D3 receptors in activated brain regions may be coincidental, and further studies are required to show definitively which class of receptors mediates agonist-induced hypolocomotion. In contrast, the activation of D2 receptors within the striatum appears necessary for quinelorane-induced hyperlocomotion.

The effect of electroconvulsive shock seizures on behaviour induced by dopaminergic agonists and on immobility in the Porsolt test

Male, Wistar rats were given a course of eight electroconvulsive shock seizures (ECS group) or matched handling (control group). They were then tested for locomotion and rearing (7 days post-ECS), for grooming and yawning (9 days post-ECS), and for immobility in the Porsolt test (7, 14 and 21 days post-ECS). Seven days post-seizure, the ECS group showed significantly more locomotion following intraperitoneal administration of apomorphine (0.2 mg/kg), but not following injections of amphetamine (1 mg/kg). Drug-induced rearing was not different in the ECS and control animals. Nine days post-seizure, the ECS group showed significantly more grooming induced by the D-1 dopamine receptor agonist, SKF 38393 (1 mg/kg), but no difference in the yawning induced by the D-2 dopamine receptor agonist, quinpirole (0.05 mg/kg). In the Porsolt test, immobility was decreased in the ECS animals at 7 and 14, but not at 21 days post-ECS. It is concluded that ECS increases activity in the dopaminergic systems of the rat brain for at least 1-2 weeks post-seizure. The beneficial effects of electroconvulsive therapy (ECT) may relate to these dopaminergic alterations.

Chronic hyperammonemia alters motor and neurochemical responses to activation of group I metabotropic glutamate receptors in the nucleus accumbens in rats in vivo

Hyperammonemia leads to altered cerebral function and neurological alterations in patients with hepatic encephalopathy. We studied the effects of hyperammonemia in rats on the modulation by group I metabotropic glutamate receptors (mGluR) of motor and neurochemical functions in vivo. Locomotion induced by injection of the mGluR agonist DHPG into nucleus accumbens was increased in hyperammonemic rats. In control rats DHPG increased extracellular dopamine (ca. 400%) but not glutamate. In contrast, in hyperammonemic rats DHPG increased extracellular glutamate (ca. 600%), while DHPG-induced dopamine increase was reduced. Blocking mGluR1 receptor with CPCCOEt prevented all DHPG effects, indicating that this receptor mediates its locomotor and neurochemical effects. Hyperammonemic rats showed increased (32%) mGluR1alpha, but not mGluR5 content in nucleus accumbens. These results show that modulation of locomotor and neurochemical functions by mGluRs in nucleus accumbens is strongly altered in hyperammonemia. These alterations may contribute to the neurological alterations in hyperammonemia and liver failure.

Striatal dopaminergic stimulation produces c-Fos expression in the PPT and an increase in wakefulness

Striatal activation can modify activity in cortical areas related to specific striatal functions possibly through a process of disinhibition within the basal ganglia. Anatomical studies have shown substantial GABAergic innervation from these nuclei to the pedunculopontine tegmental nucleus (PPT). Thus, dopaminergic stimulation of the striatum could produce PPT disinhibition and result in non-specific cortical activation. To test this hypothesis, d-amphetamine was infused both into the striatum of freely moving rats for motor and electrocorticographic recordings, and into the striatum of animals under deep anesthesia for c-Fos immunohistochemistry. The results show that intrastriatal amphetamine increases wakefulness independent of motor activity, and it increases c-Fos expression in the PPT and adjacent areas. They also suggest that the striatum participates in non-specific cortical activation probably as a result of its relationship with the PPT.

Stimulation of dopamine D2 receptors in the nucleus accumbens inhibits inflammatory pain

Previous studies suggest that dopamine in the nucleus accumbens links noxious or mesolimbic stimulation with the feedback inhibition of nociception. To test the hypothesis that pharmacological agonism at dopamine receptors in the nucleus accumbens elicits antinociception, we bilaterally microinjected dopamine D1- and D2-receptor subtype selective drugs, and then evaluated behavioral responses to noxious intraplantar formalin. While the D1-selective agonist SKF 38393 was without effect at a dose of 0.5 nmol/side, the D2-selective agonist quinpirole dose-dependently (0.05-5.0 nmol/side, bilateral) inhibited the persistent phase of formalin-induced nociception. This was blocked by pre-administration of a selective D2-dopaminergic antagonist raclopride (0.3 nmol/side, bilateral). Quinpirole did not produce overt behavioral effects and did not change rotarod latency. Our results indicate that quinpirole acts at dopamine D2 receptors in the nucleus accumbens to inhibit persistent nociception at doses that circumvent confounding non-specific motor deficits, namely, sedation and motor coordination.

Concurrent activation of dopamine D1 and D2 receptors is required to evoke neural and behavioral phenotypes of cocaine sensitization

Repeated exposure to psychomotor stimulants produces a striking behavioral syndrome involving repetitive, stereotypic behaviors that occur if an additional exposure to the stimulant is experienced. The same stimulant exposure produces specific alterations in gene expression patterns in the striatum. To identify the dopamine receptor subtypes required for the parallel expression of these acquired neural and behavioral responses, we treated rats with different D1-class and D2-class dopamine receptor agonists and compared the responses of drug-naive rats with those of rats given previous intermittent treatment with cocaine. In rats exposed to repeated cocaine treatment, the effects of a subsequent challenge treatment with either a D1-class agonist (SKF 81297) or a D2-class agonist (quinpirole) were not significantly different from those observed in drug-naive animals: the drugs administered singly did not induce robust stereotyped motor behaviors nor produce significantly striosome-predominant expression of early genes in the striatum. In contrast, challenge treatment with the D1-class and D2-class agonists in combination led to marked and correlated increases in stereotypy and striosome-predominant gene expression in the striatum. Thus, immediately after repeated psychomotor stimulant exposure, only the concurrent activation of D1 and D2 receptor subclasses evoked expression of the neural and behavioral phenotypes acquired through repeated cocaine exposure. These findings suggest that D1-D2 dopamine receptor synergisms underlie the coordinate expression of both network-level changes in basal ganglia activation patterns and the repetitive and stereotypic motor response patterns characteristic of psychomotor stimulant sensitization.

Group III metabotropic glutamate receptors and D1-like and D2-like dopamine receptors interact in the rat nucleus accumbens to influence locomotor activity

Evidence for functional interactions between metabotropic glutamate (mGlu) receptors and dopamine (DA) neurotransmission is now clearly established. In the present study, we investigated interactions between group III mGlu receptors and D1- and D2-like receptors in the nucleus accumbens (NAcc). Administration, into the NAcc, of the selective group III mGlu receptor agonist, AP4, resulted in an increase in locomotor activity, which was blocked by pretreatment with the group III mGlu receptor antagonist, MPPG. In addition, pretreatment with AP4 further blocked the increase in motor activity induced by the D1-like receptor agonist, SKF 38393, but potentiated the locomotor responses induced by either the D2-like receptor agonist, quinpirole, or coinfusion of SKF 38393 and quinpirole. MPPG reversed the effects of AP4 on the motor responses induced by D1-like and/or D2-like receptor activation. These results confirm that glutamate transmission may control DA-dependent locomotor function through mGlu receptors and further indicate that group III mGlu receptors oppose the behavioural response produced by D1-like receptor activation and favour those produced by D2-like receptor activation.

Effects of individual and concurrent stimulation of striatal D1 and D2 dopamine receptors on electrophysiological and behavioral output from rat basal ganglia

Bilateral infusions of d-amphetamine into the rat ventral-lateral striatum (VLS) were previously shown to cause a robust behavioral activation that was correlated temporally with a net increase in firing of substantia nigra pars reticulata (SNpr) neurons, a response opposite predictions of the basal ganglia model. The current studies assessed the individual and cooperative contributions of striatal D1 and D2 dopamine receptors to these responses. Bilateral infusions into VLS of the D1/D2 agonist apomorphine (10 microg/microl/side) caused intense oral movements and sniffing, and an overall increase in SNpr cell firing to 133% of basal rates, similar to effects of d-amphetamine. However, when striatal D2 receptors were stimulated selectively by infusions of quinpirole (30 microg/microl/side) + the D1 antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390; 10 microg/microl/side), no behavioral response and only modest and variable changes in SNpr cell firing were observed. Selective stimulation of striatal D1 receptors by (+/-) 6-chloro-APB hydrobromide (SKF 82958; 10 microg/microl/side) + the D2 antagonist cis-N-(1-benzyl-2-methyl-pyrrolidin-3-yl)-5-chloro-2-methoxy-4-methyl-aminobenzamide (YM 09151-2; 2 microg/microl/side) caused a weak but sustained increase in oral movements and modestly increased SNpr cell firing, but neither response was of the magnitude observed with apomorphine. When the two agonists were infused concurrently, however, robust oral movements and sniffing again occurred over the same time period that a majority of SNpr cells exhibited marked, sometimes extreme and fluctuating, changes in firing (net increase, 117% of basal rates). These data confirm that concurrent striatal D1/D2 receptor stimulation elicits a strong motor activation that is correlated temporally with a net excitation rather than inhibition of SNpr firing, and reveal that D1 and D2 receptors interact synergistically within the striatum to stimulate both forms of output.

Psychomotor-activating effects mediated by dopamine D(2) and D(3) receptors in the nucleus accumbens

The contribution made by specific dopamine receptor subtypes to the induction of motor behaviors has not been firmly established. Here, we first characterized the behavioral effects induced by a D(2)-class receptor agonist, bromocriptine, following injections into the nucleus accumbens (Acb). Bromocriptine showed an atypical D(2)-class receptor agonist profile, having no observable effect on a range of motor behaviors. However, when coadministered with the D(1)-class receptor agonist SKF 38393, bromocriptine showed a typical D(2)-class receptor agonist profile, enhancing locomotor activity and suppressing spontaneous yawning. We then administered the dopamine receptor antagonists L-741626 and nafadotride, which possess relative selectivity for D(2) and D(3) receptors, respectively, prior to injections of dopamine agonists into the Acb. Nafadotride significantly reduced the locomotor-enhancing effects elicited by the coadministration of SKF 38393 and the D(2)-class receptor agonist (+)-PD 128907 into the Acb, and also attenuated the effects induced by the combination of SKF 38393 and bromocriptine, although not significantly so. L-741626 mildly attenuated the locomotor effects elicited by both drug combinations. Taken together, these results suggest that both D(2) and D(3) receptors in the Acb contribute to the expression of heightened psychomotor activation.

Dynamic dopamine receptor interactions in the core and shell of nucleus accumbens differentially coordinate the expression of unconditioned motor behaviors

Many neurochemical and behavioral functions mediated by dopamine require the dynamic interaction between dopamine receptors. We examined the behavioral effects evoked by microinjections of drugs with relative selectivity for specific dopamine receptors into the nucleus accumbens (Acb). The results showed that, at behaviorally inactive doses, the dopamine D1-class receptor agonist SKF 38393 switched the behavioral profile induced by injections of the dopamine D2-class receptor agonist quinpirole into the Acb, from sedation, yawning, and motor inhibition to hyperactive-like behavior. Further, the effects of injections of the dopamine D2-class receptor agonist (+)-PD 128907 into the shell of Acb, including suppression of rearing, locomotion, and grooming, and induction of oral dyskinesia, yawning, and sedation, could not be distinguished from those elicited by (+)-PD 128907 following infusions into the core of Acb. However, the behavioral effects elicited by coadministration of SKF 38393 and (+)-PD 128907 into the core or the shell of Acb showed a striking anatomical specificity. The infusion of SKF 38393 plus (+)-PD 128907 into the core, but not into the shell, of Acb modified the pattern of responses induced by (+)-PD 128907, inducing behavioral hyperactivity. These results suggest critical differences in the functional interaction between dopamine receptors in the core and the shell of the Acb and reveal a mechanism of behavioral switching in the core of Acb by virtue of which dopamine D1-class receptors regulate the transition from states of behavioral suppression to states of heightened psychomotor arousal.

The role of nigral and thalamic output pathways in the expression of oral stereotypies induced by amphetamine injections into the striatum

Microinjections of amphetamine into the ventrolateral striatum (VLS) elicit a striking behavioral syndrome characterized by compulsive oral and forelimb motor stereotypies. The neural pathways that mediate these behavioral responses downstream from the striatum have not yet been identified. In a series of experiments, we investigated the involvement of the substantia nigra pars reticulata (SNr) and the ventromedial nucleus of the thalamus (VMT) in the mediation of this behavioral syndrome. We demonstrated that lidocaine-induced reversible inactivation of the SNr reduced amphetamine-induced stereotyped biting and gnawing behaviors, suggesting that the nigral output pathway plays a significant role in the expression of these behavioral responses. In turn, injections of lidocaine into the VMT only transiently reduced amphetamine-stimulated biting and increased stereotyped gnawing and paw nibbling, suggesting that the expression of oral stereotypies induced by amphetamine injections into the VLS is not dependent on thalamocortical feedback.