Distinct roles of D1 and D5 dopamine receptors in motor activity and striatal synaptic plasticity

Striatal D2 receptors and LTD: yes, but not where you thought they were

D1 and D2 dopamine receptors are expressed in disjoint subsets of striatal projection neurons, the direct and indirect pathways, respectively. This differential distribution of receptors forms the basis for explanations of many aspects of basal ganglia function and dysfunction, but it seems incompatible with some other important properties of striatal neurons. In this issue of Neuron, Wang et al. discover the mechanism of D2 sensitivity of long term depression at synapses on the striatal projection neuron. They show that D2 dependence of LTD does not depend on dopamine receptors of on the projection cell but is mediated by dopamine-induced changes in release of acetylcholine by interneurons that contact projection cells of both types.

Coincident signaling in mesolimbic structures underlying alcohol reinforcement

The medium spiny neurons (MSNs) of the nucleus accumbens function in a critical regard to examine and integrate information in the processing of rewarding behaviors. These neurons are aberrantly affected by drugs of abuse, including alcohol. However, ethanol is unlike any other common drug of abuse, due to its pleiotropic actions on intracellular and intercellular signaling processes. Intracellular biochemical pathways appear to critically contribute to long-term changes in the level of synaptic activation of these neurons, which have been implicated in ethanol dependence. Additionally, these neurons also display a fascinating pattern of up/down activity, which appears to be, at least in part, regulated by convergent activation of dopaminergic and glutamatergic (NMDA) inputs. Thus, dopaminergic and NMDA receptor-mediated synaptic transmission onto these neurons may constitute a critical site of ethanol action in mesolimbic structures. For instance, dopaminergic inputs alter the ability of ethanol to regulate NMDA receptor-mediated synaptic transmission onto accumbal MSNs. Prior activation of D1-signaling cascade through the cAMP-regulated phosphoprotein-32kD (DARPP-32) and protein phosphatase-1 (PP-1) pathway significantly attenuates ethanol inhibition of NMDA receptor function. Therefore, the interaction of D1-signaling and NMDA receptor signaling may alter NMDA receptor-dependent long-term synaptic plasticity, contributing to the development of ethanol-induced neuroadaptation of the reward pathway.

Dopamine D5 receptor localization on cholinergic neurons of the rat forebrain and diencephalon: a potential neuroanatomical substrate involved in mediating dopaminergic influences on acetylcholine release

The study of dopaminergic influences on acetylcholine release is especially useful for the understanding of a wide range of brain functions and neurological disorders, including schizophrenia, Parkinson's disease, Alzheimer's disease, and drug addiction. These disorders are characterized by a neurochemical imbalance of a variety of neurotransmitter systems, including the dopamine and acetylcholine systems. Dopamine modulates acetylcholine levels in the brain by binding to dopamine receptors located directly on cholinergic cells. The dopamine D5 receptor, a D1-class receptor subtype, potentiates acetylcholine release and has been investigated as a possible substrate underlying a variety of brain functions and clinical disorders. This receptor subtype, therefore, may prove to be a putative target for pharmacotherapeutic strategies and cognitive-behavioral treatments aimed at treating a variety of neurological disorders. The present study investigated whether cholinergic cells in the dopamine targeted areas of the cerebral cortex, striatum, basal forebrain, and diencephalon express the dopamine D5 receptor. These receptors were localized on cholinergic neurons with dual labeling immunoperoxidase or immunofluorescence procedures using antibodies directed against choline acetyltransferase (ChAT) and the dopamine D5 receptor. Results from this study support previous findings indicating that striatal cholinergic interneurons express the dopamine D5 receptor. In addition, cholinergic neurons in other critical brain areas also show dopamine D5 receptor expression. Dopamine D5 receptors were localized on the somata, dendrites, and axons of cholinergic cells in each of the brain areas examined. These findings support the functional importance of the dopamine D5 receptor in the modulation of acetylcholine release throughout the brain.

Induction and expression mechanisms of postsynaptic NMDA receptor-independent homosynaptic long-term depression

The induction of long-term depression (LTD) can be divided into two main forms, one dependent upon activation of postsynaptic NMDAR, and another independent of postsynaptic NMDAR. Non-postsynaptic NMDAR-LTD (non-NMDAR-LTD) occurs in many regions of the brain, and encompasses a wide variety of induction and expression mechanisms. In this article, the induction and expression mechanisms of such LTD in over 10 brain regions are described, with a number of common mechanisms compared across a large range of types of LTD. The article describes the involvement of different presynaptic or postsynaptic receptors in the induction of non-NMDAR-LTD, especially metabotropic glutamate receptors, cannabinoid receptors and dopamine receptors. An increase in presynaptic or postsynaptic intracellular Ca concentration is a key event in induction, commonly followed by activation of certain kinases, especially PKC, p38 MAPK and ERK. Expression mechanisms are either presynaptic via a reduction in release probability, or postsynaptic involving a decrease in AMPAR via phosphorylation of a glutamate receptor subunit, especially GluR2, followed by clathrin-mediated endocytosis. Retrograde signalling from postsynaptic to presynaptic occurs when induction is postsynaptic and expression is presynaptic.

ERK phosphorylation and FosB expression are associated with L-DOPA-induced dyskinesia in hemiparkinsonian mice

BACKGROUND

The dopamine precursor 3,4-dihydroxyphenyl-L-alanine (L-DOPA) is currently the most efficacious noninvasive therapy for Parkinson's disease. A major complication of this therapy, however, is the appearance of the abnormal involuntary movements known as dyskinesias. We have developed a model of L-DOPA-induced dyskinesias in mice that reproduces the main clinical features of dyskinesia in humans.

METHODS

Dyskinetic symptoms were triggered by repetitive administration of a constant dose of L-DOPA (25 mg/kg, twice a day, for 25 days) in unilaterally 6-hydroxydopamine (6-OHDA) lesioned mice. Mice were examined for behavior, expression of FosB, neuropeptides, and externally regulated kinase (ERK) phosphorylation.

RESULTS

Dyskinetic symptoms appear toward the end of the first week of treatment and are associated with L-DOPA-induced changes in DeltaFosB and prodynorphin expression. L-DOPA also induces activation of ERK1/2 in the dopamine-depleted striatum. Interestingly, elevated FosB/DeltaFosB expression occurs exclusively within completely lesioned regions of the striatum, displaying an inverse correlation with remaining dopaminergic terminals. Following acute L-DOPA treatment, FosB expression occurs in direct striatal output neurons, whereas chronic L-DOPA also induces FosB expression in nitric oxide synthase-positive striatal interneurons.

CONCLUSIONS

This model provides a system in which genetic manipulation of individual genes can be used to elucidate the molecular mechanisms responsible for the development and expression of dyskinesia.

Pre- and postsynaptic contributions to age-related alterations in corticostriatal synaptic plasticity

Aging creates deficits in motor performance related to changes in striatal processing of cortical information. This study describes age-related changes in corticostriatal snaptic plasticity and associated mechanisms, which may contribute to declines in motor behavior. Intracellular recordings revealed an age-related decrease in the expression of paired-pulse, posttetanic, and long-term potentiation (LTP). The age-related difference in LTP was associated with reduced sensitivity to block of N-methyl-D-aspartate (NMDA) receptors in the aged population. These age-related changes could not be explained by increased L-type Ca(2+)channel activity, since block of L-type Ca(2+) channels with nifedipine increased rather than decreased the age-related difference in long-term plasticity. Age-related increases in reactive oxygen species (ROS) modulation were also ruled out, since application of H(2)O(2) produced changes in synaptic function that were opposite to trends seen in aging, and addition of the antioxidant Trolox-C had a larger effect on long-term plasticity in young rats than in older rats. A robust age-related difference in long-term synaptic plasticity was found by studying synaptic plasticity following the blocking of D2 receptors with l-sulpiride, which may involve age-difference in NMDA receptor function. l-sulpiride consistently enabled a slow development of LTP at young (but not aged) corticostriatal synapses. However, No age differences were found in the sensitivity to the addition of the D2 receptor agonist quinpirole. These findings provide evidence for age-induced changes in the release properties of cortical terminals and in the functioning of postsynaptic striatal NMDA receptors, which may contribute to age-related deficits in striatum control of movement.

Distinct roles for spinophilin and neurabin in dopamine-mediated plasticity

Protein phosphatase 1 plays a major role in the governance of excitatory synaptic activity, and is subject to control via the neuromodulatory actions of dopamine. Mechanisms involved in regulating protein phosphatase 1 activity include interactions with the structurally related cytoskeletal elements spinophilin and neurabin, synaptic scaffolding proteins that are highly enriched in dendritic spines. The requirement for these proteins in dopamine-related neuromodulation was tested using knockout mice. Dopamine D1-mediated regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor activity was deficient in both striatal and prefrontal cortical neurons from neurabin knockout mice; in spinophilin knockout mice this deficit was manifest only in striatal neurons. At corticostriatal synapses long-term potentiation was deficient in neurabin knockout mice, but not in spinophilin knockout mice, and was rescued by a D1 receptor agonist. In contrast, long-term depression was deficient in spinophilin knockout mice but not in neurabin knockout mice, and was rescued by D2 receptor activation. Spontaneous excitatory post-synaptic current frequency was increased in neurabin knockout mice, but not in spinophilin knockout mice, and this effect was normalized by D2 receptor agonist application. Both knockout strains displayed increased induction of GluR1 Ser(845) phosphorylation in response to D1 receptor stimulation in slices, and also displayed enhanced locomotor activation in response to cocaine administration. These effects could be dissociated from cocaine reward, which was enhanced only in spinophilin knockout mice, and was accompanied by increased immediate early gene induction. These data establish a requirement for synaptic scaffolding in dopamine-mediated responses, and further indicate that spinophilin and neurabin play distinct roles in dopaminergic signal transduction and psychostimulant response.

Association of the calcyon gene (DRD1IP) with attention deficit/hyperactivity disorder

Attention deficit/hyperactivity disorder (ADHD) is a childhood-onset disorder characterized by marked inattention, hyperactivity and impulsivity. The dopaminergic system has been hypothesized to be involved in the development of ADHD. Positive associations have been found for the dopamine receptors D1 and D5 genes, suggesting that other genes involved in D1/D5 signalling may also contribute to ADHD. In this study, we tested the calcyon gene (DRD1IP), which encodes a brain-specific D1-interacting protein involved in D1/D5 receptors calcium signalling, for association with ADHD. The inheritance of nine polymorphisms in the calcyon gene was examined in a sample of 215 nuclear families, with 260 affected children, using the transmission/disequilibrium test. The most common haplotype, designated C1, demonstrated significant evidence for excess transmission. Quantitative trait analyses of this haplotype showed significant relationships with both the inattentive (parent's rating, P=0.006; teacher's rating, P=0.003) and hyperactive/impulsive (parent's rating, P=0.004) dimensions of the disorder. Two of the nine marker alleles included in haplotype C1, rs4838721A located approximately 10 kb 5' of the gene and rs2275723C located 10 bp upstream of the exon 5 acceptor splice site, also showed significant evidence for association when analysed individually. As these two variants are not predicted to alter calcyon function, we screened the gene exons by sequencing. No variation in the coding region was identified, suggesting that a causal variant allele resides elsewhere in a regulatory sequence of the gene. These findings support the proposed involvement of the calcyon gene in ADHD and implicate haplotype C1 as containing a risk allele.

Importance of D(1) receptors for associative components of amphetamine-induced behavioral sensitization and conditioned activity: a study using D(1) receptor knockout mice

RATIONALE

Repeated exposure to psychostimulant drugs results in conditioned activity and behavioral sensitization. Nonassociative cellular changes are necessary for behavioral sensitization, while associative processes appear to modify the sensitized response.

OBJECTIVE

The purpose of the present study was to determine whether the absence of the D(1) receptor would disrupt associative processes modulating sensitization and conditioned activity.

METHODS

Wild-type and D(1) receptor knockout mice (i.e., D(1)-deficient mice) were injected with amphetamine (AMPH; 8 mg/kg, IP) before being placed in a previously novel test chamber (AMPH-Test group) or before being returned to the home cage (AMPH-Home group). Separate groups of mice were injected with saline (SAL) at the same time points. Distance traveled was measured 60 min each day, with the preexposure phase lasting 1 or 7 days. Sensitization was subsequently assessed after an injection of AMPH (1 mg/kg, IP), while conditioned activity was assessed after an injection of SAL.

RESULTS

After a 1-day preexposure phase, wild-type and D(1)-deficient mice exhibited similar patterns of sensitization and conditioned activity. After a 7-day preexposure phase, (1) D(1)-deficient mice exhibited more robust context-specific sensitization than wild-type mice, (2) only D(1)-deficient mice showed context-independent sensitization, and (3) only D(1)-deficient mice showed conditioned activity.

CONCLUSIONS

Repeatedly treating D(1)-deficient mice with AMPH appears to cause a general increase in responsivity. The reason for this hyper-responsivity is uncertain, but it is possible that cues from the testing environment were unable to inhibit responding (i.e., associative processes were disrupted). Alternatively, compensatory mechanisms (e.g., increases in D(2)-like receptors) may affect processes underlying sensitization and conditioned activity.

Phenotypic studies on dopamine receptor subtype and associated signal transduction mutants: insights and challenges from 10 years at the psychopharmacology-molecular biology interface

BACKGROUND

Mutants with targeted gene deletion ('knockout') or insertion (transgenic) of D1, D2, D3, D4 and D5 dopamine (DA) receptor subtypes are complemented by an increasing variety of double knockout and transgenic-'knockout' models, together with knockout of critical components of DA receptor signalling cascades such as G alpha(olf)[G gamma7], adenylyl cyclase type 5, PKA [RIIbeta] and DARPP-32. However, it is increasingly recognised that these molecular techniques have a number of inherent limitations. Furthermore, there are poorly understood methodological factors that contribute to inconsistent phenotypic findings between laboratories.

OBJECTIVE

This review seeks to document the impact of DA receptor subtype and related transduction mutants on our understanding of the behavioural roles of these entities, primarily at the level of unconditioned psychomotor behaviour.

METHODS

It includes ethologically based and orofacial movement studies in our own laboratories, since these are the only studies to systematically compare each of the D1, D2, D3, D4 and D5 receptor and DARPP-32 signal transduction 'knockouts'.

DISCUSSION

There is a particular emphasis on identifying methodological factors that might influence phenotypic effects and account for inconsistencies. The findings are offered empirically to (1) specify the extent of phenotypic diversity among individual DA receptor subtypes and transduction components and (2) indicate relationships between D1, D2, D3, D4 and D5 receptor subtype proteins, associated G alpha(i)/G alpha(s)/G alpha(olf)[G gamma7]-adenylyl cyclase type 5-PKA [RIIbeta]-DARPP-32 signalling cascades and behaviour. The findings are also offered heuristically as a base for such phenotypic comparisons at additional levels of behaviour so that a yet more complete phenotypic profile might emerge.

Identification of human dopamine D1-like receptor agonist using a cell-based functional assay

AIM

To establish a cell-based assay to screen human dopamine D1 and D5 receptor agonists against compounds from a natural product compound library.

METHODS

Synthetic responsive elements 6 cAMP response elements (CRE) and a mini promoter containing a TATA box were inserted into the pGL3 basic vector to generate the reporter gene construct pCRE/TA/Luci. CHO cells were co-transfected with the reporter gene construct and human D1 or D5 receptor cDNA in mammalian expression vectors. Stable cell lines were established for agonist screening. A natural product compound library from over 300 herbs has been established. The extracts from these herbs were used for human D1 and D5 receptor agonist screenings.

RESULTS

A number of extracts were identified that activated both D1 and D5 receptors. One of the herb extracts, SBG492, demonstrated distinct pharmacological characteristics with human D1 and D5 receptors. The EC(50) values of SBG492 were 342.7 microg/mL for the D1 receptor and 31.7 microg/mL for the D5 receptor.

CONCLUSION

We have established a cell-based assay for high-throughput drug screening to identify D1-like receptor agonists from natural products. Several extracts that can active D1-like receptors were discovered. These compounds could be useful tools for studies on the functions of these receptors in the brain and could potentially be developed into therapeutic drugs for the treatment of central nervous system diseases.

Effects of galanin-like peptide (GALP) on locomotion, reproduction, and body weight in female and male mice

Galanin-like peptide (GALP) has been implicated in the neuroendocrine regulation of both feeding and reproduction. In male rodents and primates, intracerebroventricular (icv) infusions of GALP stimulate luteinizing hormone (LH) release, induce Fos expression in brain areas implicated in feeding and reproduction, and affect food intake and body weight in rodents. In gonad-intact and castrated male rats, icv administration of GALP also stimulates male sexual behavior. While the effects of GALP on male physiology and behavior are well documented, no studies have addressed such a role of GALP in females. We tested the effects of icv GALP infusions on LH release, locomotor activity, motor control, and body weight regulation in adult ovariectomized female mice hormonally primed with estradiol benzoate and progesterone. In addition, sexually-experienced male and female mice were treated with GALP and tested for sexual behavior. In females, GALP reduced open-field locomotor activity, the ability to maintain grip on an accelerating rotarod, and 24-h body weight in a dose-dependent manner. GALP also increased LH secretion in female mice, an effect that was blocked by pre-treatment with Antide, a gonadotropin-releasing hormone (GnRH) type-1 receptor antagonist. GALP infusions slightly decreased the occurrence of lordosis behavior in female mice and significantly increased the latencies with which females displayed receptivity. Unlike previous reports in male rats, GALP inhibited male sexual behavior in mice. Our data indicate that in female mice, GALP stimulates LH release via GnRH, and decreases body weight, motor control, and locomotor activity via GnRH-independent pathways. Furthermore, our sexual behavior and locomotor findings suggest species-specific differences in the mechanism and/or location of GALP action in the brains of rats and mice.

Pathological synaptic plasticity in the striatum: implications for Parkinson's disease

Repetitive stimulation of the corticostriatal pathway can cause either a long-lasting increase, or an enduring decrease in synaptic strength, respectively referred to as long-term potentiation (LTP), and long-term depression (LTD), both requiring a complex sequence of biochemical events. Once established, LTP can be reversed to control levels by a low-frequency stimulation (LFS) protocol, an active phenomenon defined "synaptic depotentiation", required to erase redundant information. In the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease (PD), striatal synaptic plasticity has been shown to be impaired, though chronic treatment with l-dopa was able to restore it. Interestingly, a consistent number of l-dopa-treated animals developed involuntary movements, resembling human dyskinesias. Strikingly, electrophysiological recordings from the dyskinetic group of rats demonstrated a selective impairment of synaptic depotentiation. This survey will provide an overview of plastic changes occurring at striatal synapses. The potential relevance of these findings in the control of motor function and in the pathogenesis both of Parkinson's disease and l-dopa-induced motor complications will be discussed.

Ethological resolution of behavioural topography and D1-like versus D2-like agonist responses in congenic D5 dopamine receptor mutants: identification of D5:D2-like interactions

The phenotypic ethogram of congenic dopamine D(5) receptor "knockout" mice was evaluated. Each individual topography of behaviour within the natural repertoire was assessed over the extended course of initial exploration of and subsequent habituation to the environment, and following challenge with a series of D(1)-like agonists. Over initial exploration, D(5)-null mice evidenced a modest reduction in locomotion and a modest increase in sifting. Subsequent habituation revealed additional phenotypic effects, primarily overall reduction in grooming and delayed habituation of rearing. Among D(1)-like agonists, A 68930 stimulates both adenylyl cyclase and a putative D(1)-like receptor coupled to stimulation of phospholipase C-mediated phosphoinositide hydrolysis; conversely, SK&F 83959 stimulates phosphoinositide hydrolysis but not adenylyl cyclase while SK&F 83822 stimulates adenylyl cyclase but not phosphoinositide hydrolysis. Though programmed grooming syntax and episodic seizure activity induced by A 68930 and SK&F 83822 were unaltered, grooming induced by SK&F 83959 was reduced in D(5) mutants. Stereotyped, ponderous locomotion induced by the D(2)-like agonist RU 24213 was enhanced in D(5) mutants. Phenotypic and pharmacological characterisation of congenic D(5)-null mice at an ethological level identifies novel functional roles for the D(5) receptor in mediating discrete topographies of behaviour relating to exploration, sequential motor coordination, and how these processes change over the course of interaction with and habituation to the environment. Additionally, they indicate the involvement of phosphoinositide hydrolysis and D(5):D(2)-like interactions in regulating these processes.

Striatal synaptic plasticity: Implications for motor learning and Parkinson's disease

Changing the strength of synaptic connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. Plastic changes appear to follow a regional specialization and underlie the specific type of memory mediated by the brain area in which plasticity occurs. Thus, long-term changes occurring at excitatory corticostriatal synapses should be critically involved in motor learning. Indeed, repetitive stimulation of the corticostriatal pathway can cause either a long-lasting increase or an enduring decrease in synaptic strength, respectively referred to as long-term potentiation (LTP), and long-term depression, both requiring a complex sequence of biochemical events. Once established, LTP can be reversed to control levels by a low-frequency stimulation protocol, an active phenomenon defined "synaptic depotentiation," required to erase redundant information. In the 6-hydroxydopamine rat model of Parkinson's disease (PD), striatal synaptic plasticity has been shown to be impaired, although chronic treatment with levodopa was able to restore it. Of interest, a consistent number of L-dopa-treated animals developed involuntary movements, resembling human dyskinesias. Strikingly, electrophysiological recordings from the dyskinetic group of rats demonstrated a selective impairment of synaptic depotentiation. This survey will provide an overview of plastic changes occurring at striatal synapses. The potential relevance of these findings in the control of motor function and in the pathogenesis both of PD and L-dopa-induced motor complications will be discussed.

Phenotypic analysis of dopamine receptor knockout mice; recent insights into the functional specificity of dopamine receptor subtypes

The functional specificity of dopamine receptor subtypes remains incompletely understood, in part due to the absence of highly selective agonists and antagonists. Phenotypic analysis of dopamine receptor knockout mice has been instrumental in identifying the role of dopamine receptor subtypes in mediating dopamine's effects on motor function, cognition, reward, and emotional behaviors. In this article, we provide an update of recent studies in dopamine receptor knockout mice and discuss the limitations and future promise of this approach.

Altered plasticity of the human motor cortex in Parkinson's disease

Interventional paired associative stimulation (IPAS) to the contralateral peripheral nerve and cerebral cortex can enhance the primary motor cortex (M1) excitability with two synchronously arriving inputs. This study investigated whether dopamine contributed to the associative long-term potentiation-like effect in the M1 in Parkinson's disease (PD) patients. Eighteen right-handed PD patients and 11 right-handed age-matched healthy volunteers were studied. All patients were studied after 12 hours off medication with levodopa replacement (PD-off). Ten patients were also evaluated after medication (PD-on). The IPAS comprised a single electric stimulus to the right median nerve at the wrist and subsequent transcranial magnetic stimulation of the left M1 with an interstimulus interval of 25 milliseconds (240 paired stimuli every 5 seconds for 20 minutes). The motor-evoked potential amplitude in the right abductor pollicis brevis muscle was increased by IPAS in healthy volunteers, but not in PD patients. IPAS did not affect the motor-evoked potential amplitude in the left abductor pollicis brevis. The ratio of the motor-evoked potential amplitude before and after IPAS in PD-off patients increased after dopamine replacement. Thus, dopamine might modulate cortical plasticity in the human M1, which could be related to higher order motor control, including motor learning.

Subthalamic nucleus lesion reverses motor abnormalities and striatal glutamatergic overactivity in experimental parkinsonism

Subthalamic nucleus (STN) is a target of choice for the neurosurgical treatment of Parkinson's disease (PD). The therapeutic effect of STN lesion in PD is classically ascribed to the rescue of physiological activity in the output structures of the basal ganglia, and little is known about the possible involvement of the striatum. In the present study, therefore, we electrophysiologically recorded in vitro single striatal neurons of DA-depleted rats unilaterally lesioned by 6-hydroxydopamine, treated or not with therapeutic doses of levodopa (l-DOPA), or with a consecutive ipsilateral STN lesion. We show that the beneficial motor effects produced in parkinsonian rats by STN lesion or l-DOPA therapy were paralleled by the normalization of overactive frequency and amplitude of striatal glutamate-mediated spontaneous excitatory postsynaptic currents (sEPSCs). Since neither l-DOPA treatment nor STN lesion affected sEPSCs kinetic properties, the reversal of these abnormalities in striatal excitatory synaptic transmission can be attributable to the normalization of glutamate release.

Rationale for and use of NMDA receptor antagonists in Parkinson's disease

N-Methyl-d-aspartate (NMDA) glutamate receptors are a class of excitatory amino acid receptors, which have several important functions in the motor circuits of the basal ganglia, and are viewed as important targets for the development of new drugs to prevent or treat Parkinson's disease (PD). NMDA receptors are ligand-gated ion channels composed of multiple subunits, each of which has distinct cellular and regional patterns of expression. They have complex regulatory properties, with both agonist and co-agonist binding sites and regulation by phosphorylation and protein-protein interactions. They are found in all of the structures of the basal ganglia, although the subunit composition in the various structures is different. NMDA receptors present in the striatum are crucial for dopamine-glutamate interactions. The abundance, structure, and function of striatal receptors are altered by the dopamine depletion and further modified by the pharmacological treatments used in PD. In animal models, NMDA receptor antagonists are effective antiparkinsonian agents and can reduce the complications of chronic dopaminergic therapy (wearing off and dyskinesias). Use of these agents in humans has been limited because of the adverse effects associated with nonselective blockade of NMDA receptor function, but the development of more potent and selective pharmaceuticals holds the promise of an important new therapeutic approach for PD.

Demonstration of nondeclarative sequence learning in mice: development of an animal analog of the human serial reaction time task

In this paper, we demonstrate nondeclarative sequence learning in mice using an animal analog of the human serial reaction time task (SRT) that uses a within-group comparison of behavior in response to a repeating sequence versus a random sequence. Ten female B6CBA mice performed eleven 96-trial sessions containing 24 repetitions of a 4-trial sequence. During the 12th session, the repeating sequence was replaced with the random sequence halfway through the session. Reaction time (RT) to respond to an illuminated nose-poke was recorded, and performance was compared at the halfway point in each session to test for any change in behavior. For learning effect, RTs decreased over the no-switch repeating-sequence sessions. For interference effect, behavior did not change appreciably at the halfway point during the last repeating-sequence session. However, RTs deteriorated significantly after the switch from repeating to random sequences halfway through session 12. The mice demonstrated a robust interference effect when switched from repeating to random sequences. This pattern of behavior in humans performing the SRT is interpreted as evidence of nondeclarative sequence learning. The similarity between the human and mouse SRTs will enable more direct comparisons of mouse-human nondeclarative memory behavior and will provide a useful behavioral end-point in mouse-models of basal ganglia dysfunction.

The Dopamine D1 Antagonist Reduces Ethanol Reward for C57BL/6 Mice

BACKGROUND

Dopamine D1 antagonist effects on behaviors related to obtaining and consuming ethanol remain unclear. The highly selective D1 antagonist ecopipam (SCH 39166), which has no effect on the serotonin system, was used to evaluate the role of D1 receptors in ethanol reward and its potential for treating alcohol abuse by determining its effect on several measures of ethanol reward in C57BL/6 (B6) mice.

METHODS

Ecopipam (0.025-0.2 mg/kg) effects on instrumental and contingent consummatory responses and on noncontingent consummatory responses for ethanol and water reward were determined in food-restricted male mice trained to lever-respond for 12% ethanol delivered on a fixed ratio-4 reinforcement schedule. The mice were tested for 15-min sessions under preprandial (high-hunger and low-thirst) and postprandial (low-hunger and high-thirst) test conditions.

RESULTS

Ecopipam dose-dependently reduced instrumental and consummatory responses for ethanol and ethanol intake when tested under hunger- or thirst-motivated conditions with free access to water. Under thirst motivation with no access to an alternate fluid source, lever responses for ethanol and water were similar; however, ecopipam reduced responding for ethanol more than responding for water reward. When given concurrent free access to the same fluid delivered for lever pressing, animals made more contacts for ethanol than for water; ecopipam reduced free ethanol but not water contacts.

CONCLUSIONS

Ecopipam attenuated ethanol reward at doses that did not affect water reward, indicating an effect independent of reductions in motor system function or general motivation and arousal. Ecopipam also reduced ethanol reward to the same degree under hunger, thirst, or sated conditions, again indicating that it affected ethanol reward at doses that did not grossly affect general motivational states. These data suggest that ecopipam may reduce ethanol reward with few side effects and that it warrants further investigation as a pharmacological tool for treating alcohol abuse.

Risperidone reduction of amphetamine-induced self-injurious behavior in mice

The behavioral and neurochemical effects of high doses of amphetamine administered to BALB/c mice were examined in the presence and absence of co-administered haloperidol (a D2 antagonist), SCH 23390 (a D1 antagonist) and risperidone (a mixed 5-HT2/D2 antagonist). It was observed that mice displayed a dose-dependent increase in stereotypic behavior, oral dyskinesia, and self-injurious behavior (SIB) in response to amphetamine treatment. Furthermore, agents that blocked the SIB reversed the amphetamine-induced release of serotonin. This effect was unrelated to hyperthermia or non-specific sedation (as assessed by measurement of motor activity). These data are interpreted in the context of the underlying basis of murine SIB involving both dopaminergic and serotonergic activation and demonstrate the efficacy of risperidone in treating these behaviors.