Dopamine displays an identical apparent affinity towards functional dopamine D1 and D2 receptors in rat striatal slices: possible implications for the regulatory role of D2 receptors

The Dopamine D4 Receptor Regulates Gonadotropin-Releasing Hormone Neuron Excitability in Male Mice

Gonadotropin-releasing hormone (GnRH)-secreting neurons control fertility. The release of GnRH peptide regulates the synthesis and release of both luteinizing hormone (LH) and Follicle stimulation hormone (FSH) from the anterior pituitary. While it is known that dopamine regulates GnRH neurons, the specific dopamine receptor subtype(s) involved remain unclear. Previous studies in adult rodents have reported juxtaposition of fibers containing tyrosine hydroxylase (TH), a marker of catecholaminergic cells, onto GnRH neurons and that exogenous dopamine inhibits GnRH neurons postsynaptically through dopamine D1-like and/or D2-like receptors. Our microarray data from GnRH neurons revealed a high level of Drd4 transcripts [i.e., dopamine D4 receptor (D4R)]. Single-cell RT-PCR and immunocytochemistry confirmed GnRH cells express the Drd4 transcript and protein, respectively. Calcium imaging identified changes in GnRH neuronal activity during application of subtype-specific dopamine receptor agonists and antagonists when GABAergic and glutamatergic transmission was blocked. Dopamine, dopamine with D1/5R-specific or D2/3R-specific antagonists or D4R-specific agonists decreased the frequency of calcium oscillations. In contrast, D1/5R-specific agonists increased the frequency of calcium oscillations. The D4R-mediated inhibition was dependent on Gαi/o protein coupling, while the D1/5R-mediated excitation required Gαs protein coupling. Together, these results indicate that D4R plays an important role in the dopaminergic inhibition of GnRH neurons.

Detection of phasic dopamine by D1 and D2 striatal medium spiny neurons

KEY POINTS

Brief dopamine events are critical actors of reward-mediated learning in the striatum; the intracellular cAMP-protein kinase A (PKA) response of striatal medium spiny neurons to such events was studied dynamically using a combination of biosensor imaging in mouse brain slices and in silico simulations. Both D1 and D2 medium spiny neurons can sense brief dopamine transients in the sub-micromolar range. While dopamine transients profoundly change cAMP levels in both types of medium spiny neurons, the PKA-dependent phosphorylation level remains unaffected in D2 neurons. At the level of PKA-dependent phosphorylation, D2 unresponsiveness depends on protein phosphatase-1 (PP1) inhibition by DARPP-32. Simulations suggest that D2 medium spiny neurons could detect transient dips in dopamine level.

ABSTRACT

The phasic release of dopamine in the striatum determines various aspects of reward and action selection, but the dynamics of the dopamine effect on intracellular signalling remains poorly understood. We used genetically encoded FRET biosensors in striatal brain slices to quantify the effect of transient dopamine on cAMP or PKA-dependent phosphorylation levels, and computational modelling to further explore the dynamics of this signalling pathway. Medium-sized spiny neurons (MSNs), which express either D1 or D2 dopamine receptors, responded to dopamine by an increase or a decrease in cAMP, respectively. Transient dopamine showed similar sub-micromolar efficacies on cAMP in both D1 and D2 MSNs, thus challenging the commonly accepted notion that dopamine efficacy is much higher on D2 than on D1 receptors. However, in D2 MSNs, the large decrease in cAMP level triggered by transient dopamine did not translate to a decrease in PKA-dependent phosphorylation level, owing to the efficient inhibition of protein phosphatase 1 by DARPP-32. Simulations further suggested that D2 MSNs can also operate in a 'tone-sensing' mode, allowing them to detect transient dips in basal dopamine. Overall, our results show that D2 MSNs may sense much more complex patterns of dopamine than previously thought.

Phasic dopamine release drives rapid activation of striatal D2-receptors

Striatal dopamine transmission underlies numerous goal-directed behaviors. Medium spiny neurons (MSNs) are a major target of dopamine in the striatum. However, as dopamine does not directly evoke a synaptic event in MSNs, the time course of dopamine signaling in these cells remains unclear. To examine how dopamine release activates D2-receptors on MSNs, G protein activated inwardly rectifying potassium (GIRK2; Kir 3.2) channels were virally overexpressed in the striatum, and the resulting outward currents were used as a sensor of D2-receptor activation. Electrical and optogenetic stimulation of dopamine terminals evoked robust D2-receptor inhibitory postsynaptic currents (IPSCs) in GIRK2-expressing MSNs that occurred in under a second. Evoked D2-IPSCs could be driven by repetitive stimulation and were not occluded by background dopamine tone. Together, the results indicate that D2-receptors on MSNs exhibit functional low affinity and suggest that striatal D2-receptors can encode both tonic and phasic dopamine signals.

The role of D2-autoreceptors in regulating dopamine neuron activity and transmission

Dopamine D2-autoreceptors play a key role in regulating the activity of dopamine neurons and control the synthesis, release and uptake of dopamine. These Gi/o-coupled inhibitory receptors play a major part in shaping dopamine transmission. Found at both somatodendritic and axonal sites, autoreceptors regulate the firing patterns of dopamine neurons and control the timing and amount of dopamine released from their terminals in target regions. Alterations in the expression and activity of autoreceptors are thought to contribute to Parkinson's disease as well as schizophrenia, drug addiction and attention-deficit hyperactivity disorder (ADHD), which emphasizes the importance of D2-autoreceptors in regulating the dopamine system. This review will summarize the cellular actions of dopamine autoreceptors and discuss recent advances that have furthered our understanding of the mechanisms by which D2-receptors control dopamine transmission.

A receptor-based model for dopamine-induced fMRI signal

This report describes a multi-receptor physiological model of the fMRI temporal response and signal magnitude evoked by drugs that elevate synaptic dopamine in basal ganglia. The model is formulated as a summation of dopamine's effects at D1-like and D2-like receptor families, which produce functional excitation and inhibition, respectively, as measured by molecular indicators like adenylate cyclase or neuroimaging techniques like fMRI. Functional effects within the model are described in terms of relative changes in receptor occupancies scaled by receptor densities and neuro-vascular coupling constants. Using literature parameters, the model reconciles many discrepant observations and interpretations of pre-clinical data. Additionally, we present data showing that amphetamine stimulation produces fMRI inhibition at low doses and a biphasic response at higher doses in the basal ganglia of non-human primates (NHP), in agreement with model predictions based upon the respective levels of evoked dopamine. Because information about dopamine release is required to inform the fMRI model, we simultaneously acquired PET (11)C-raclopride data in several studies to evaluate the relationship between raclopride displacement and assumptions about dopamine release. At high levels of dopamine release, results suggest that refinements of the model will be required to consistently describe the PET and fMRI data. Overall, the remarkable success of the model in describing a wide range of preclinical fMRI data indicate that this approach will be useful for guiding the design and analysis of basic science and clinical investigations and for interpreting the functional consequences of dopaminergic stimulation in normal subjects and in populations with dopaminergic neuroadaptations.

Increased affinity of dopamine for D(2) -like versus D(1) -like receptors. Relevance for volume transmission in interpreting PET findings

Evidence indicates that dopamine (DA) mainly acts as a volume transmission (VT) transmitter through its release into the extracellular fluid where the D(1) -like and D(2) -like receptors are predominantly extrasynaptic. It was therefore of interest to compare the affinities of the two major families of DA receptors. [(3)H] raclopride /DA and [(3)H] SCH23390/DA competition assays compared the affinity of DA at D(2) -like and D(1) -like receptors in rat dorsal striatal membrane preparations as well as in membrane preparations from CHO cell lines stably transfected with human D(2L) and D(1) receptors. The IC(50) values of DA at D(2) -like receptors in dorsal striatal membranes and CHO cell membranes were markedly and significantly reduced compared with the IC(50) values of DA at D(1) -like receptors. These IC(50) values reflect differences in both the high and low affinity states. The K(iH) value for DA at [(3)H] raclopride-labeled D(2) -like receptors in dorsal striatum was 12 nM, and this can help explain PET findings that amphetamine-induced increases in DA release can produce an up to 50% decrease of [(11)C] raclopride binding in the dorsal striatum in vivo. These combined results give indications for the existence of striatal D(2) -like receptor-mediated DA VT at the local circuit level in vivo. The demonstration of a K(iH) value of 183 nM for DA at D(1) antagonist-labeled D(1) -like receptors instead gives a likely explanation for the failure of a reduction of D(1) -like receptor binding after amphetamine-induced DA release in PET studies using the D(1) -like antagonist radioligands [(11)C] SCH23390 and [(11)C] NNC. It seems difficult to evaluate the role of the extrasynaptic D(1) receptors in VT in vivo with the PET radioligands available for this receptor.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

Do preclinical findings of methamphetamine-induced motor abnormalities translate to an observable clinical phenotype?

This review summarizes the preclinical literature of the effects of methamphetamine (MA) on subcortical dopaminergic and GABAergic mechanisms underlying motor behavior with the goal of elucidating the clinical presentation of human MA-induced movement disorders. Acute and chronic MA exposure in laboratory animal can lead to a variety of motor dysfunctions including increased locomotor activity, stereotypies, diminished or enhanced response times, and parkinsonian-like features. With the exception of psychomotor impairment and hyperkinesia, MA-induced movement disorders are not well documented in humans. This review attempts to draw parallels between the animal and human changes in basal ganglia neurochemistry associated with MA exposure and offers explanations for why a parkinsonian phenotype is not apparent among individuals who use and abuse MA. Significant differences in the expression of neurotoxicity and presence of multiple environmental and pharmacologic confounds may account for the lack of a parkinsonian phenotype in humans despite evidence of altered dopamine function.

Pharmacokinetic-Pharmacodynamic Analysis of Antipsychotics-induced Extrapyramidal Symptoms based on Receptor Occupancy Theory Incorporating Endogenous Dopamine Release

We aimed to analyze the risks of extrapyramidal symptoms (EPS) induced by typical and atypical antipsychotic drugs using a common pharmacokinetic-pharmacodynamic (PK-PD) model based on the receptor occupancy. We collected the data for EPS induced by atypical antipsychotics, risperidone, olanzapine and quetiapine, and a typical antipsychotic, haloperidol from literature and analyzed the following five indices of EPS, the ratio of patients obliged to take anticholinergic medication, the occurrence rates of plural extrapyramidal symptoms (more than one of tremor, dystonia, hypokinesia, akathisia, extrapyramidal syndrome, etc.), parkinsonism, akathisia, and extrapyramidal syndrome. We tested two models, i.e., a model incorporating endogenous dopamine release owing to 5-HT2A receptor inhibition and a model not considering the endogenous dopamine release, and used them to examine the relationship between the D2 receptor occupancy of endogenous dopamine and the extent of drug-induced EPS. The model incorporating endogenous dopamine release better described the relationship between the mean D2 receptor occupancy of endogenous dopamine and the extent of EPS than the other model, as assessed by the final sum of squares of residuals (final SS) and Akaike's Information Criteria (AIC). Furthermore, the former model could appropriately predict the risks of EPS induced by two other atypical antipsychotics, clozapine and ziprasidone, which were not incorporated into the model development. The developed model incorporating endogenous dopamine release owing to 5-HT2A receptor inhibition may be useful for the prediction of antipsychotics-induced EPS.

Prediction and assessment of extrapyramidal side effects induced by risperidone based on dopamine D(2) receptor occupancy

In the present study, we attempted to predict the risk of extrapyramidal side effects of a selective monoaminergic antagonist, risperidone, by analyzing the correlation between the dopamine D(2) receptor occupancy and the degree of extrapyramidal side effects of the drug. The occupancies of D(2) and 5-HT(2) receptors at various doses of risperidone were calculated by means of a receptor occupancy theory. The extrapyramidal side effects after administration of risperidone were attempted to predict by theoretical analysis of the correlation between the calculated occupancies and the evidence of extrapyramidal symptoms using a ternary complex model. The pharmacokinetic/pharmacodynamic analysis utilized the data concerning the pharmacokinetics of risperidone and 9-hydroxyrisperidone (active metabolite), their binding affinities with D(2) and 5-HT(2) receptors, and the clinical evidence of extrapyramidal symptoms (Extrapyramidal Symptom Rating Scale: ESRS), gathered from the literature. The mean occupancy of 5-HT(2) receptors after the administration of regular doses of risperidone was suggested to be more than 90%, whereas the mean occupancy of D(2) receptors varied between 50-80%, depending on the dose. The correlation between the occupancy of D(2) receptors and the extrapyramidal symptoms could be successfully analyzed with a ternary complex model, showing the predictability of the model for the extrapyramidal side effects of risperidone. Since the estimated risk of the extrapyramidal side effects varied with the dose, the present method of predicting the extrapyramidal side effects of risperidone may provide a basis for developing a rational dosing regimen for the drug.

Demonstration of competition between endogenous dopamine and [11C]raclopride binding in in vitro brain slices using a dynamic autoradiography technique

To elucidate the mechanism of in vivo binding competition between radioligand and endogenously released transmitter, we examined the influence of depolarization-induced dopamine (DA) release on [11C]raclopride-specific binding to D2 receptors in slices of living brain tissues using dynamic positron autoradiography. Rat brain slices were incubated in a chamber with [11C]raclopride in oxygenated medium at 34 degrees C for 150 min. Two-dimensional images of radioactivity in the slices were recorded on a storage phosphor screen and dynamic changes were measured. When the brain slices were exposed to the depolarization agents (25 mM K+, 50 mM K+, and 20 microM veratridine), the percentage inhibition of striatal [11C]raclopride-specific binding was 22 +/- 4%, 44 +/- 8% and 54 +/- 7% of the control, respectively. The percentage inhibition of [11C]raclopride-specific binding during each depolarization treatment agreed proportionally with the amount of DA released into the medium. However, preexposure of brain slices to the same depolarization treatment (50 mM K+) did not affect the [11C]raclopride-specific binding, suggesting that the reduction in receptor density and/or affinity was not involved in the decrease of [11C]raclopride-specific binding. [11C]Raclopride-specific binding decreased dose-dependently in the presence of exogenously added DA (range 0.005-3mM). The synaptic DA concentration during each depolarization treatment estimated using three different methods. These results suggest that the decrease of [11C]raclopride-specific binding to striatal slices following evoked DA release is due to competition between endogenous DA and raclopride. It is unlikely that changes in D(2) receptor density or in affinity of the receptors for raclopride are involved. These results provide supportive evidence for in vivo binding competition between radioligand and endogenous neurotransmitter.

A single exposure to morphine induces long-lasting behavioural and neurochemical sensitization in rats

Repeated exposure to drugs of abuse causes persistent behavioural sensitization and associated adaptations in striatal neurotransmission, which is thought to play an important role in certain aspects of drug addiction. Remarkably, even a single exposure to psychostimulant drugs such as amphetamine or cocaine can be sufficient to elicit long-lasting sensitization. The present study was designed to evaluate whether long-lasting behavioural and neurochemical sensitization can also be evoked by a single exposure to morphine, an opiate drug of abuse. Rats were pretreated once with morphine (2, 10 or 30 mg/kg). Three weeks later, the locomotor effects of morphine and amphetamine, as well as the electrically evoked release of [3H]dopamine and [14C]acetylcholine from slices of nucleus accumbens and caudate-putamen, was assessed. In morphine-pretreated rats, the psychomotor effects of morphine and amphetamine were sensitized. In addition, the electrically evoked release of [3H]dopamine and [14C]acetylcholine was augmented in slices of nucleus accumbens and caudate-putamen from morphine-pretreated animals. Although the sensitization of the locomotor effect of morphine was less profound than previously observed after repeated intermittent morphine treatment, the enduring behavioural and neurochemical consequences of a single and repeated intermittent morphine treatment appear to be highly comparable. We therefore conclude that a single exposure to morphine induces long-lasting behavioural sensitization and associated neuroadaptations.

A single administration of interleukin-1 or amphetamine induces long-lasting increases in evoked noradrenaline release in the hypothalamus and sensitization of ACTH and corticosterone responses in rats

Single administration of the cytokine interleukin-1beta (IL-1) or the psychostimulant amphetamine causes long-term sensitization of the hypothalamus pituitary adrenal (HPA) axis, i.e. enhanced adrenocorticotropine hormone (ACTH) and corticosterone responses weeks later. HPA responses to these stimuli involve activation of hypothalamic corticotropin-releasing hormone (CRH) neurons by noradrenergic projections to the paraventricular nucleus (PVN). In search of the underlying mechanisms, we studied the temporal pattern of HPA sensitization in relation to (1) the reactivity of noradrenergic projections to the PVN and (2) altered secretagogue production in hypothalamic CRH neurons. Single exposure to IL-1 or amphetamine induced cross-sensitization of ACTH and corticosterone responses 11 and 22 days later, but not after 42 days. Amphetamine-induced HPA sensitization was not accompanied by increased costorage of arginine vasopressin (AVP) in CRH terminals, as found previously after IL-1 pretreatment. The reactivity of noradrenergic terminals was assessed by measuring the electrically evoked release of [3H]-noradrenaline from superfused PVN slices. Single administration of amphetamine and IL-1 induced a long-lasting (up to 22 days) increase (up to 165%) of evoked noradrenaline release. This indicates that single exposure to psychostimulants or to cytokines can induce a long-lasting increase in stimulus-secretion coupling in brainstem noradrenergic neurons that innervate the PVN. This common, long-lasting functional change may underlie, at least in part, IL-1- and amphetamine-induced HPA cross-sensitization. In addition, increased AVP signalling by hypothalamic CRH neurons appears to play a role in IL-1-induced, but not in amphetamine-induced, HPA sensitization.

A single exposure to amphetamine is sufficient to induce long-term behavioral, neuroendocrine, and neurochemical sensitization in rats

Repeated treatment with psychostimulant drugs causes long-lasting behavioral sensitization and associated neuroadaptations. Although sensitization induced by a single psychostimulant exposure has also been reported, information on the behavioral and neurochemical consequences of a single psychostimulant exposure is sparse. Therefore, to evaluate whether behavioral sensitization evoked by single and repeated psychostimulant pretreatment regimens represent the same neurobiological phenomenon, the time-dependent expression of behavioral, neurochemical, and neuroendocrine sensitization after a single exposure to amphetamine was investigated in rats. A single exposure to amphetamine (5 mg/kg, i.p.) caused context-independent sensitization of the locomotor effects of amphetamine, which intensified over time. Thus, sensitization to amphetamine was marginal at 3 d after treatment and more evident after 1 week, whereas 3 weeks after treatment, profound sensitization, as well as cross-sensitization, to cocaine was observed. Amphetamine pretreatment caused an increase in the electrically evoked release of [(3)H]dopamine from nucleus accumbens, caudate putamen, and medial prefrontal cortex slices and of [(14)C]acetylcholine from accumbens and caudate slices. The hyperreactivity of dopaminergic nerve terminals appeared to parallel the development of locomotor sensitization, i.e., whereas hyperreactivity of accumbens dopaminergic terminals increased between 3 d and 3 weeks after treatment, the hyperreactivity of medial prefrontal dopaminergic terminals decreased. Pre-exposure to amphetamine also sensitized the hypothalamus-pituitary-adrenal axis response to amphetamine at 1 and 3 weeks, but not at 3 d after treatment. Because these data closely resemble those reported previously for repeated amphetamine pretreatment, it is concluded that a single exposure to amphetamine is sufficient to induce long-term behavioral, neurochemical, and neuroendocrine sensitization in rats.

Towards an integration of biochemical and biophysical models of neuronal information processing: a case study in the nigro-striatal system

The experimental and theoretical study of intracellular biochemical signaling mechanisms lags considerably behind our understanding of electrical processes of neuronal membranes. Both signaling processes, however, are extensively intertwined and can be analyzed and modeled using formally similar mathematical tools. With the nigro-striatal system as an example, we review various formal approaches to describe metabotropic signaling in dopamine- and calcium-dependent pathways and their interactions with electrical membrane processes. These demonstrate the feasibility of synthetic modeling and afford insights into a variety of specific signaling mechanisms. Extending and linking hitherto isolated models has the potential to transcend descriptive levels and to provide a fuller understanding of the molecular basis of macroscopic information processing in the central nervous system.

Opposing role of dopamine D1 and D2 receptors in modulation of rat nucleus accumbens noradrenaline release

The role of dopamine receptors in the modulation of nucleus accumbens noradrenaline release was investigated in superfused rat brain slices. At concentrations of </=1 microM, dopamine enhanced, whereas at higher concentrations dopamine inhibited electrically evoked [3H]noradrenaline release. The D1 receptor agonist SKF-38393 increased, whereas the D2 agonist quinpirole inhibited evoked [3H]noradrenaline release. These effects were attenuated by the D1 antagonist SCH-23390 and the D2 antagonist (-)-sulpiride, respectively, indicating that accumbens noradrenaline release is regulated by stimulatory D1 and inhibitory D2 receptors. Whereas (-)-sulpiride enhanced, SCH-23390 did not reduce evoked accumbens [3H]noradrenaline release, indicating a tonic activation of D2 receptors only. Given the similar apparent affinity of dopamine for D1 and D2 receptors in striatal slices, the lack of tonic D1 receptor activation suggests that D1, unlike D2, receptors are extrasynaptically localized. No dopaminergic modulation of noradrenaline release was observed in rat medial prefrontal cortex or amygdala slices. To examine the regulation of accumbens noradrenaline release under conditions of increased dopaminergic activity, measurements were made using slices of amphetamine-pretreated rats. In these slices, the electrically evoked release of [3H]dopamine and [3H]noradrenaline was enhanced. The increasing effect of (-)-sulpiride on noradrenaline release was augmented, and SCH-23390 almost completely reversed this enhancement of [3H]noradrenaline release. These data suggest that whereas although under a moderate dopaminergic tone, accumbens noradrenaline release is mainly regulated by inhibitory D2 receptors, under circumstances of increased dopaminergic activity, recruitment of extrasynaptic stimulatory D1 receptors contributes to enhancement of noradrenaline release.

Influence of dopamine on GABA release in striatum: evidence for D1-D2 interactions and non-synaptic influences

Striatal slices from the rat were preincubated with [3H]GABA and superfused in the presence of nipecotic acid and aminooxyacetic acid, inhibitors of high-affinity GABA transport and GABA aminotransferase, respectively. GABA efflux was estimated by monitoring tritium efflux, 98% of which was in the form of [3H]GABA. The following three major observations were made: (1) The overflow of GABA evoked by electrical field stimulation (8 Hz) was increased two-fold by SKF-38393 (10 microM), an agonist at the D1 family of dopamine receptors. This increase was completely blocked by the D1 receptor antagonist SCH-23390 (10 microM). However, SCH-23390 had no effect on GABA overflow when given alone. Thus, dopamine agonists appear to exert an excitatory influence on GABA release; however, this effect was not elicited by endogenous dopamine under the conditions of this experiment. (2) Electrically evoked GABA overflow was reduced 50% by quinpirole (10 microM), an agonist at the D2 family of dopamine receptors, and this effect was blocked by the D2 antagonist sulpiride (10 microM). Moreover, exposure to sulpiride alone caused a 60% increase in GABA overflow, and this effect was abolished by 3-iodotyrosine (2 mM), a dopamine synthesis inhibitor. Thus, D2 agonists appear to exert an inhibitory influence on dopamine release, an effect that can be exerted by endogenous stores of dopamine. (3) The stimulatory effect of SKF-38393 was attenuated by quinpirole, whereas the sulpiride-induced increase in GABA efflux was attenuated by SCH-23390. Sulpiride also increased [3H]GABA efflux during KCl-induced depolarization, an effect that was antagonized by SCH-23390 as in the case of electrical stimulation. However, although tetrodotoxin did not alter the stimulatory effect of sulpiride, it did block the ability of SCH-23390 to antagonize the sulpiride-induced increase in GABA overflow. These latter results suggest that there is an interaction between D1 and D2 receptors whereby the effects of dopamine mediated via D1 sites are inhibited by an action on D2 sites. In conclusion, our results suggest that (i) dopamine agonists can exert an excitatory influence on depolarization-induced GABA release within neostriatum via D1 receptors and an inhibitory influence via D2 receptors; (ii) under the conditions of these experiments, endogenous dopamine fails to act on D1 sites but does exert an inhibitory influence via D2 sites; and (iii) there is an interaction between D1 and D2 receptors such that the actions of dopamine mediated via D1 sites are inhibited as a result of the concomitant actions exerted via D2 sites.

In vivo striatal binding of the D1 antagonist SCH 23390 is not modified by changes in dopaminergic transmission

The in vivo striatal binding of [3H]SCH 23390, an antagonist of the D1 dopamine receptors, was investigated in mice submitted to pretreatment to either decrease (gammabutyrolactone 750 mg/kg, i.p.) or, increase (3,4-dihydroxyphenylalanine (L-DOPA) 200 mg/kg i.p. plus dexamphetamine 4 mg/kg, s.c.) dopaminergic transmission. Such conditions failed to modify [3H]SCH 23390 binding. However, we observed that dopamine (at concentrations > or = 1 microM), reduced the in vitro binding of [3H]SCH 23390 in membrane fractions. These results suggest that modifications in dopamine neurotransmission do not alter the in vivo quantification of D1 receptors with [3H]SCH 23390, for example, in studies that use positron emission tomography.