Widespread distribution of brain dopamine receptors evidenced with [125I]iodosulpride, a highly selective ligand

Continuous Dopaminergic Stimulation Improves Cortical Maladaptive Changes in Advanced Parkinson's Disease

BACKGROUND

With the progression of Parkinson's disease (PD), pulsatile treatment with oral levodopa causes maladaptive changes within basal ganglia-thalamo-cortical circuits, which are clinically expressed as motor fluctuations and dyskinesias. At the level of the motor cortex, these changes may be detected using transcranial magnetic stimulation (TMS), as abnormal corticospinal and intracortical excitability and absent response to plasticity protocols.

OBJECTIVE

We investigated the effect of continuous dopaminergic stimulation on cortical maladaptive changes related to oral levodopa treatment.

METHODS

Twenty patients with advanced PD were tested using TMS within 1 week before and again 6 months after the introduction of levodopa-carbidopa intestinal gel. We measured resting and active motor thresholds, input/output curve, short interval intracortical inhibition curve, cortical silent period, and response to intermittent theta burst stimulation. Patients were clinically assessed with Part III and Part IV of the Movement Disorders Society Unified Parkinson's Disease Rating Scale.

RESULTS

Six months after the introduction of levodopa-carbidopa intestinal gel, motor fluctuations scores (P = 0.001) and dyskinesias scores (P < 0.001) were reduced. Resting and active motor threshold (P = 0.012 and P = 0.015) and x-intercept of input/output curve (P = 0.005) were also decreased, while short-interval intracortical inhibition and response to intermittent theta bust stimulation were improved (P = 0.026 and P = 0.031, respectively). Changes in these parameters correlated with clinical improvement.

CONCLUSIONS

In patients with advanced PD, switching from intermittent to continuous levodopa delivery increased corticospinal excitability and improved deficient intracortical inhibition and abnormal motor cortex plasticity, along with amelioration of motor fluctuations and dyskinesias. Continuous dopaminergic stimulation ameliorates maladaptive changes inflicted by chronic pulsatile dopaminergic stimulation. © 2022 International Parkinson and Movement Disorder Society.

Measurement of Striatal Dopamine Release Induced by Neuropsychological Stimulation in Positron Emission Tomography With Dual Injections of [11C]Raclopride

OBJECTIVES

Positron emission tomography (PET) with [¹¹C]raclopride has been applied to measure changes in the concentration of endogenous dopamine induced by pharmacological challenge or neuropsychological stimulation by evaluating the binding potential (BP) between the baseline and activated state. Recently, to reliably estimate BP in the activated state, a new approach with dual-bolus injections in a single PET scan was developed. In this study, we investigated the feasibility of applying this dual-bolus injection approach to measure changes in endogenous dopamine levels induced by cognitive tasks in humans.

METHODS

First, the reproducibility of BP estimation using the dual-bolus injection approach was evaluated using PET scans without stimulation in nine healthy volunteers. A 90-min scan was performed with bolus injections of [¹¹C]raclopride administered at the beginning of the scan and 45 min after the first injection. BPs in the striatum for the first injection (BP₁) and second injection (BP₂) were estimated using an extended simplified reference tissue model, and the mean absolute difference (MAD) between the two BPs was calculated. The MAD was also compared with the conventional bolus-plus-continuous infusion approach. Next, PET studies with a cognitive reinforcement learning task were performed on 10 healthy volunteers using the dual-bolus injection approach. The BP₁ at baseline and BP₂ at the activated state were estimated, and the reduction in BP was evaluated.

RESULTS

In the PET scans without stimulation, the dual-bolus injection approach showed a smaller MAD (<2%) between BP₁ and BP₂ than the bolus-plus-continuous infusion approach, demonstrating good reproducibility of this approach. In the PET scans with the cognitive task performance, the reduction in BP was not observed in the striatum by either approach, showing that the changes in dopamine level induced by the cognitive tasks performed in this study were not sufficient to be detected by PET.

CONCLUSION

Our results indicate that the cognitive task-induced changes in dopamine-related systems may be complex and difficult to measure accurately using PET scans. However, the proposed dual-bolus injection approach provided reliable BP estimates with high reproducibility, suggesting that it has the potential to improve the accuracy of PET scans for measuring changes in dopamine concentrations.

A Historical Perspective on the Dopamine D3 Receptor

Before 1990, the multiplicity of dopamine receptors beyond D1 and D2 had remained a controversial concept, despite its substantial clinical implications, at a time when it was widely accepted that dopamine interacted with only two receptor subtypes, termed D1 and D2, differing one from the other by their pharmacological specificity and opposite effects on adenylyl cyclase. It was also generally admitted that the therapeutic efficacy of antipsychotics resulted from blockade of D2 receptors. Thanks to molecular biology techniques, the D3 receptor could be characterized as a distinct molecular entity having a restricted anatomical gene expression and different signaling, which could imply peculiar functions in controlling cognitive and emotional behaviors. Due to the structural similarities of D2 and D3 receptors, the search for D3-selective compounds proved to be difficult, but nevertheless led to the identification of fairly potent and in vitro and in vivo selective compounds. The latter permitted to confirm a role of D3 receptors in motor functions, addiction, cognition, and schizophrenia, which paved the way for the development of new drugs for the treatment of psychiatric disorders.

Dopamine dysregulation in psychotic relapse after antipsychotic discontinuation: an [18F]DOPA and [11C]raclopride PET study in first-episode psychosis

Although antipsychotic drugs are effective for relieving the psychotic symptoms of first-episode psychosis (FEP), psychotic relapse is common during the course of the illness. While some FEPs remain remitted even without medication, antipsychotic discontinuation is regarded as the most common risk factor for the relapse. Considering the actions of antipsychotic drugs on presynaptic and postsynaptic dopamine dysregulation, this study evaluated possible mechanisms underlying relapse after antipsychotic discontinuation. Twenty five FEPs who were clinically stable and 14 matched healthy controls were enrolled. Striatal dopamine activity was assessed as K_i^cer value using [¹⁸F]DOPA PET before and 6 weeks after antipsychotic discontinuation. The D2/3 receptor availability was measured as BP_ND using [¹¹C]raclopride PET after antipsychotic discontinuation. Healthy controls also underwent PET scans according to the corresponding schedule of the patients. Patients were monitored for psychotic relapse during 12 weeks after antipsychotic discontinuation. 40% of the patients showed psychotic relapse after antipsychotic discontinuation. The change in K_i^cer value over time significantly differed between relapsed, non-relapsed patients and healthy controls (Week*Group: F = 4.827, df = 2,253.193, p = 0.009). In relapsed patients, a significant correlation was found between baseline striatal K_i^cer values and time to relapse after antipsychotic discontinuation (R² = 0.518, p = 0.018). BP_ND were not significantly different between relapsed, non-relapsed patients and healthy controls (F = 1.402, df = 2,32.000, p = 0.261). These results suggest that dysfunctional dopamine autoregulation might precipitate psychotic relapse after antipsychotic discontinuation in FEP. This finding could be used for developing a strategy for the prevention of psychotic relapse related to antipsychotic discontinuation.

The role of norepinephrine in the pathophysiology of schizophrenia

Several lines of evidence have suggested for decades a role for norepinephrine (NE) in the pathophysiology and treatment of schizophrenia. Recent experimental findings reveal anatomical and physiological properties of the locus coeruleus-norepinephrine (LC-NE) system and its involvement in brain function and cognition. Here, we integrate these two lines of evidence. First, we review the functional and structural properties of the LC-NE system and its impact on functional brain networks, cognition, and stress, with special emphasis on recent experimental and theoretical advances. Subsequently, we present an update about the role of LC-associated functions for the pathophysiology of schizophrenia, focusing on the cognitive and motivational deficits. We propose that schizophrenia phenomenology, in particular cognitive symptoms, may be explained by an abnormal interaction between genetic susceptibility and stress-initiated LC-NE dysfunction. This in turn, leads to imbalance between LC activity modes, dysfunctional regulation of brain network integration and neural gain, and deficits in cognitive functions. Finally, we suggest how recent development of experimental approaches can be used to characterize LC function in schizophrenia.

Correction of head movement by frame-to-frame image realignment for receptor imaging in positron emission tomography studies with [11C]raclopride and [11C]FLB 457

OBJECTIVE

Positron emission tomography (PET) scans of imaging receptors require 60-90-min dynamic acquisition for quantitative analysis. Head movement is often observed during scanning, which hampers the reliable estimation of quantitative parameters. This study evaluated image-based motion correction by frame-to-frame realignment for PET studies with [¹¹C]raclopride and [¹¹C]FLB 457 acquired by an Eminence SET-3000GCT/X and investigated the effect of this correction on the quantitative outcomes.

METHODS

First, an optimal method for estimating motion parameters was evaluated by computer simulation. Simulated emission sinograms were reconstructed to the PET images with or without attenuation correction using a µ-map of the transmission scan. Six motion parameters were estimated frame-by-frame by registering each frame of the PET images to several types of reference images and the reliability of registration was compared. Next, in [¹¹C]raclopride and [¹¹C]FLB 457 studies in normal volunteers, six motion parameters for each frame were estimated by the registration method determined from the simulation results. Head movement was corrected by realigning the PET images reconstructed with a motion-included µ-map in which a mismatch between the transmission and emission scans was corrected. After this correction, time-activity curves (TAC) for the striatum or cerebral cortex were obtained and the binding potentials of the receptors (BP_ND) were estimated using the simplified reference tissue model.

RESULTS

In the simulations, the motion parameters could be reliably estimated by registering each frame of the non-attenuation-corrected PET images to their early-phase frame. The motion parameters in the human studies were also obtained using the same method. After correction, a discontinuity of TACs in the striatum and cerebral cortex was remarkably improved and the BP_ND values in these regions increased. Compared to the motion-corrected PET images reconstructed using the measured µ-map, the images reconstructed using the motion-included µ-map did not result in a remarkable improvement of BP_ND in the striatum of [¹¹C]raclopride studies, while the BP_ND in the cerebral cortex changed in some [¹¹C]FLB 457 studies in which large head movement was observed.

CONCLUSIONS

In PET receptor imaging, head movement during dynamic scans can be corrected by frame-to-frame realignment. This method is easily applicable to clinical studies and provides reliable TACs and BP_ND.

Hippocampus Contributions to Food Intake Control: Mnemonic, Neuroanatomical, and Endocrine Mechanisms

Food intake is a complex behavior that can occur or cease to occur for a multitude of reasons. Decisions about where, when, what, and how much to eat are not merely reflexive responses to food-relevant stimuli or to changes in energy status. Rather, feeding behavior is modulated by various contextual factors and by previous experiences. The data reviewed here support the perspective that neurons in multiple hippocampal subregions constitute an important neural substrate linking the external context, the internal context, and mnemonic and cognitive information to control both appetitive and ingestive behavior. Feeding behavior is heavily influenced by hippocampal-dependent mnemonic functions, including episodic meal-related memories and conditional learned associations between food-related stimuli and postingestive consequences. These mnemonic processes are undoubtedly influenced by both external and internal factors relating to food availability, location, and physiological energy status. The afferent and efferent neuroanatomical connectivity of the subregions of the hippocampus is reviewed with regard to the integration of visuospatial and olfactory sensory information (the external context) with endocrine and gastrointestinal interoceptive stimuli (the internal context). Also discussed are recent findings demonstrating that peripherally derived endocrine signals act on receptors in hippocampal neurons to reduce (leptin, glucagon-like peptide-1) or increase (ghrelin) food intake and learned food reward-driven responding, thereby highlighting endocrine and neuropeptidergic signaling in hippocampal neurons as a novel substrate of importance in the higher-order regulation of feeding behavior.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

The Effects of Methylphenidate on Resting-State Functional Connectivity of the Basal Nucleus of Meynert, Locus Coeruleus, and Ventral Tegmental Area in Healthy Adults

Background: Methylphenidate (MPH) influences catecholaminergic signaling. Extant work examined the effects of MPH on the neural circuits of attention and cognitive control, but few studies have investigated the effect of MPH on the brain's resting-state functional connectivity (rsFC).

Methods: In this observational study, we compared rsFC of a group of 24 healthy adults who were administered an oral 45 mg dose of MPH with a group of 24 age and gender matched controls who did not receive MPH. We focused on three seed regions: basal nucleus of Meynert (BNM), locus coeruleus (LC), and ventral tegmental area/substantia nigra, pars compacta (VTA/SNc), each providing cholinergic, noradrenergic and dopaminergic inputs to the cerebral cortex. Images were pre-processed and analyzed as in our recent work (Li et al., 2014; Zhang et al., 2015). We used one-sample t-test to characterize group-specific rsFC of each seed region and two-sample t-test to compare rsFC between groups.

Results: MPH reversed negative connectivity between BNM and precentral gyri. MPH reduced positive connectivity between LC and cerebellum, and induced positive connectivity between LC and right hippocampus. MPH decreased positive VTA/SNc connectivity to the cerebellum and putamen, and reduced negative connectivity to left middle occipital gyrus.

Conclusion: MPH had distinct effects on the rsFC of BNM, LC, and VTA/SNc in healthy adults. These new findings may further our understanding of the role of catecholaminergic signaling in Attention Deficit Hyperactivity Disorder (ADHD) and Parkinson's disease and provide insights into the therapeutic mechanisms of MPH in the treatment of clinical conditions that implicate catecholaminergic dysfunction.

Topography and collateralization of dopaminergic projections to primary motor cortex in rats

Dopaminergic signaling within the primary motor cortex (M1) is necessary for successful motor skill learning. Dopaminergic neurons projecting to M1 are located in the ventral tegmental area (VTA, nucleus A10) of the midbrain. It is unknown which behavioral correlates are encoded by these neurons. The objective here is to investigate whether VTA-M1 fibers are collaterals of projections to prefrontal cortex (PFC) or nucleus accumbens (NAc) or if they form a distinct pathway. In rats, multiple-site retrograde fluorescent tracers were injected into M1, PFC and the core region of the NAc and VTA sections investigated for concomitant labeling of different tracers. Dopaminergic neurons projecting to M1, PFC and NAc were found in nucleus A10 and to a lesser degree in the medial nucleus A9. Neurons show high target specificity, minimal collateral branching to other than their target area and hardly cross the midline. Whereas PFC- and NAc-projecting neurons are indistinguishably intermingled within the ventral portion of dopaminergic nuclei in middle and caudal midbrain, M1-projecting neurons are only located within the dorsal part of the rostral midbrain. Within M1, the forelimb representation receives sevenfold more dopaminergic projections than the hindlimb representation. This strong rostro-caudal gradient as well as the topographical preference to dorsal structures suggest that projections to M1 emerged late in the development of the dopaminergic systems in and form a functionally distinct system.

Differential regulation of observational fear and neural oscillations by serotonin and dopamine in the mouse anterior cingulate cortex

RATIONALE

The aberrant regulation of serotonin (5-HT) and dopamine (DA) in the brain has been implicated in neuropsychiatric disorders associated with marked impairments in empathy, such as schizophrenia and autism. Many psychiatric drugs bind to both types of receptors, and the anterior cingulate cortex (ACC) is known to be centrally involved with empathy. However, the relationship between the 5-HT/DA system in the ACC and empathic behavior is not yet well known.

OBJECTIVES

We investigated the role of 5-HT/DA in empathy-like behavior and in the regulation of ACC neural activity.

METHODS

An observational fear learning task was conducted following microinjections of 5-HT, DA, 5-HT and DA, methysergide (5-HT receptor antagonist), SCH-23390 (DA D1 receptor antagonist), or haloperidol (DA D2 receptor antagonist) into the mouse ACC. The ACC neural activity influenced by 5-HT and DA was electrophysiologically characterized in vitro and in vivo.

RESULTS

The microinjection of haloperidol, but not methysergide or SCH-23390, decreased the fear response of observing mice. The administration of 5-HT and 5-HT and DA together, but not DA alone, reduced the freezing response of observing mice. 5-HT enhanced delta-band activity and reduced alpha- and gamma-band activities in the ACC, whereas DA reduced only alpha-band activity. Based on entropy, reduced complexity of ACC neural activity was observed with 5-HT treatment.

CONCLUSIONS

The current results demonstrated that DA D2 receptors in the ACC are required for observational fear learning, whereas increased 5-HT levels disrupt observational fear and alter the regularity of ACC neural oscillations.

Can antipsychotic treatment contribute to drug addiction in schizophrenia?

Individuals with schizophrenia are at very high risk for drug abuse and addiction. Patients with a coexisting drug problem fare worse than patients who do not use drugs, and are also more difficult to treat. Current hypotheses cannot adequately account for why patients with schizophrenia so often have a co-morbid drug problem. I present here a complementary hypothesis based on evidence showing that chronic exposure to antipsychotic medications can induce supersensitivity within the brain's dopamine systems, and that this in turn can enhance the rewarding and incentive motivational effects of drugs and reward cues. At the neurobiological level, these effects of antipsychotics are potentially linked to antipsychotic-induced increases in the striatal levels of dopamine D2 receptors and D2 receptors in a high-affinity state for dopamine, particularly at postsynaptic sites. Antipsychotic-induced dopamine supersensitivity and enhanced reward function are not inevitable consequences of prolonged antipsychotic treatment. At least two parameters appear to promote these effects; the use of antipsychotics of the typical class, and continuous rather than intermittent antipsychotic exposure, such that silencing of dopaminergic neurotransmission via D2/3 receptors is unremitting. Thus, by inducing forms of neural plasticity that facilitate the ability of drugs and reward cues to gain control over behaviour, some currently used treatment strategies with typical antipsychotics might contribute to compulsive drug seeking and drug taking behaviours in vulnerable schizophrenia patients.

Upregulation of dopamine D3, not D2, receptors correlates with tardive dyskinesia in a primate model

Tardive dyskinesia (TD) is a delayed and potentially irreversible motor complication arising in patients chronically exposed to centrally active dopamine D2 receptor antagonists, including antipsychotic drugs and metoclopramide. The classical dopamine D2 receptor supersensitivity hypothesis in TD, which stemmed from rodent studies, lacks strong support in humans. To investigate the neurochemical basis of TD, we chronically exposed adult capuchin monkeys to haloperidol (median, 18.5 months; n = 11) or clozapine (median, 6 months; n = 6). Six unmedicated animals were used as controls. Five haloperidol-treated animals developed mild TD movements, and no TD was observed in the clozapine group. Using receptor autoradiography, we measured striatal dopamine D1, D2, and D3 receptor levels. We also examined the D3 receptor/preprotachykinin messenger RNA (mRNA) co-expression, and quantified preproenkephalin mRNA levels, in striatal sections. Unlike clozapine, haloperidol strongly induced dopamine D3 receptor binding sites in the anterior caudate-putamen, particularly in TD animals, and binding levels positively correlated with TD intensity. Interestingly, the D3 receptor upregulation was observed in striatonigral neurons. In contrast, D2 receptor binding was comparable to controls, and dopamine D1 receptor binding was reduced in the anterior putamen. Enkephalin mRNA widely increased in all animals, but to a greater extent in TD-free animals. These results suggest for the first time that upregulated striatal D3 receptors correlate with TD in nonhuman primates, adding new insights to the dopamine receptor supersensitivity hypothesis. The D3 receptor could provide a novel target for drug intervention in human TD.

L-3,4-Dihydroxy-6-[F-18]fluorophenylalanine positron emission tomography demonstrating dopaminergic system abnormality in the brains of obsessive-compulsive disorder patients

AIM

Obsessive-compulsive disorder (OCD) is a chronic disabling neuropsychiatric disorder. Current treatment modalities, such as pharmacological and behavioral methods, are sometimes unsatisfactory. The mesolimbic dopaminergic pathway is supposed to have a role in the pathogenesis of OCD. In this study, L-3,4-Dihydroxy-6-[F-18]fluorophenylalanine (F-18 FDOPA) positron emission tomography (PET) is exploited to investigate the possible abnormality of dopaminergic neuronal circuits in the brains of OCD patients in vivo.

METHODS

The study subjects were recruited after psychological assessment and gave written informed consent to participate. The F-18 FDOPA PET scans were performed on five OCD patients and six healthy volunteers at 120 min after 185 MBq of F-18 FDOPA intravenous injection. The PET results were analyzed with the Statistical Parametric Mapping tool.

RESULTS

Compared to the healthy subjects, the OCD brains showed increased dopaminergic metabolism in the left frontal premotor cortex (P < 0.001), along with trends toward an increase in the left posterior cingulate gyrus, the left cuneus, the left lingual gyrus, the right cuneus and precuneus, the right lingual gyrus, the right middle temporal gyrus, the left cerebellum, and the right cerebellum (P < 0.01).

CONCLUSION

Our observations suggest that the increased dopaminergic neuronal function in these brain areas may be implicated in the pathogenesis of OCD.

The role of dopamine signaling in epileptogenesis

Clinical and experimental studies implicate most neuromodulatory systems in epileptogenesis. The dopaminergic system has a seizure-modulating effect that crucially depends on the different subtypes of dopamine (DA) receptors involved and the brain regions in which they are activated. Specifically, DA plays a major role in the control of seizures arising in the limbic system. Studies performed in a wide variety of animal models contributed to illustrate the opposite actions of D1-like and D2-like receptor signaling in limbic epileptogenesis. Indeed, signaling from D1-like receptors is generally pro-epileptogenic, whereas D2-like receptor signaling exerts an anti-epileptogenic effect. However, this view might appear quite simplistic as the complex neuromodulatory action of DA in the control of epileptogenesis likely requires a physiological balance in the activation of circuits modulated by these two major DA receptor subtypes, which determines the response to seizure-promoting stimuli. Here we will review recent evidences on the identification of molecules activated by DA transduction pathways in the generation and spread of seizures in the limbic system. We will discuss the intracellular signaling pathways triggered by activation of different DA receptors in relation to their role in limbic epileptogenesis, which lead to the activation of neuronal death/survival cascades. A deep understanding of the signaling pathways involved in epileptogenesis is crucial for the identification of novel targets for the treatment of epilepsy.

Facilitation of working memory in humans by a d2 dopamine receptor agonist

Recent studies on the neurobiology of cognition have focused on the ability of the prefrontal cortex (PFC) to support processes of working memory, i.e, mnemonic processes by which information relevant for a correct response is temporarily maintained to be reevaluated or updated on a trial-by-trial basis. Of most recent interest is the role played by dopamine (DA) in spatial working memory processes of the principal sulcal region of the PFC. Although D1 DA receptors appear to modulate these mnemonic processes in monkeys, several lines of research suggest that D2 DA receptors could also be relevant to cognitive functions. Therefore, we assessed the effects of a specific D2 receptor agonist (bromocriptine) and placebo on visuospatial delayed response performance in human subjects. During delay periods of 0 or 8 sec, subjects were required to remember the spatial location of rapidly presented visual cues displayed in peripheral vision within a 360° circumference. The extent to which D2 receptor activation by bromocriptine facilitated working memory in the 8-sec delay condition relative to placebo performance was assessed. As a means of providing validation of bromocriptine's D2 receptor effect, maximum inhibition of prolactin (PRL) secretion, which is inhibited specifically by activation of D2 receptor sites, was determined. Additionally, tasks having no working memory component were administered to rule out nonspecific effects of bromocriptine on sensory, arousal, attentional, and motor factors. Results demonstrated a significant facilitatory effect of bromocriptine on spatial delayed response performance (i.e., 8-sec delay performance). Results could not be explained by nonspecific effects of bromocriptine. Thus, findings of this study suggest that spatial working memory is facilitated by D2 receptor activation. The role that DA may play in human cognitive processes is discussed within the larger theoretical framework of DA's general role in the facilitation of goal-directed behavior. In the case of cognition, DA may facilitate processes that serve to guide motivated behavior through complex environments.

Dopamine D(2) Antagonist-Induced Striatal Nur77 Expression Requires Activation of mGlu5 Receptors by Cortical Afferents

Dopamine D₂ receptor antagonists modulate gene transcription in the striatum. However, the molecular mechanism underlying this effect remains elusive. Here we used the expression of Nur77, a transcription factor of the orphan nuclear receptor family, as readout to explore the role of dopamine, glutamate, and adenosine receptors in the effect of a dopamine D₂ antagonist in the striatum. First, we investigated D₂ antagonist-induced Nur77 mRNA in D_2L receptor knockout mice. Surprisingly, deletion of the D_2L receptor isoform did not reduce eticlopride-induced upregulation of Nur77 mRNA levels in the striatum. Next, we tested if an ibotenic acid-induced cortical lesion could block the effect of eticlopride on Nur77 expression. Cortical lesions strongly reduced eticlopride-induced striatal upregulation of Nur77 mRNA. Then, we investigated if glutamatergic neurotransmission could modulate eticlopride-induced Nur77 expression. A combination of a metabotropic glutamate type 5 (mGlu5) and adenosine A_2A receptor antagonists abolished eticlopride-induced upregulation of Nur77 mRNA levels in the striatum. Direct modulation of Nur77 expression by striatal glutamate and adenosine receptors was confirmed using corticostriatal organotypic cultures. Taken together, these results indicate that blockade of postsynaptic D₂ receptors is not sufficient to trigger striatal transcriptional activity and that interaction with corticostriatal presynaptic D₂ receptors and subsequent activation of postsynaptic glutamate and adenosine receptors in the striatum is required. Thus, these results uncover an unappreciated role of presynaptic D₂ heteroreceptors and support a prominent role of glutamate in the effect of D₂ antagonists.

On the origin of cortical dopamine: is it a co-transmitter in noradrenergic neurons?

Dopamine (DA) and noradrenaline (NA) in the prefrontal cortex (PFC) modulate superior cognitive functions, and are involved in the aetiology of depressive and psychotic symptoms. Moreover, microdialysis studies in rats have shown how pharmacological treatments that induce modifications of extracellular NA in the medial PFC (mPFC), also produce parallel changes in extracellular DA.To explain the coupling of NA and DA changes, this article reviews the evidence supporting the hypothesis that extracellular DA in the cerebral cortex originates not only from dopaminergic terminals but also from noradrenergic ones, where it acts both as precursor for NA and as a co-transmitter.Accordingly, extracellular DA concentration in the occipital, parietal and cerebellar cortex was found to be much higher than expected in view of the scarce dopaminergic innervation in these areas.Systemic administration or intra-cortical perfusion of alpha(2)-adrenoceptor agonists and antagonists, consistent with their action on noradrenergic neuronal activity, produced concomitant changes not only in extracellular NA but also in DA in the mPFC, occipital and parietal cortex.Chemical modulation of the locus coeruleus by locally applied carbachol, kainate, NMDA or clonidine modified both NA and DA in the mPFC.Electrical stimulation of the locus coeruleus led to an increased efflux of both NA and DA in mPFC, parietal and occipital cortex, while in the striatum, NA efflux alone was enhanced.Atypical antipsychotics, such as clozapine and olanzapine, or antidepressants, including mirtazapine and mianserine, have been found to increase both NA and DA throughout the cerebral cortex, likely through blockade of alpha(2)-adrenoceptors. On the other hand, drugs selectively acting on dopaminergic transmission produced modest changes in extracellular DA in mPFC, and had no effect on the occipital or parietal cortex.Acute administration of morphine did not increase DA levels in the PFC (where NA is diminished), in contrast with augmented dopaminergic neuronal activity; moreover, during morphine withdrawal both DA and NA levels increased, in spite of a diminished dopaminergic activity, both increases being antagonised by clonidine but not quinpirole administration.Extensive 6-hydroxy dopamine lesion of the ventral tegmental area (VTA) decreases below 95% of control both intra- and extracellular DA and DOPAC in the nucleus accumbens, but only partially or not significantly in the mPFC and parietal cortex.The above evidence points to a common origin for NA and DA in the cerebral cortex and suggests the possible utility of noradrenergic system modulation as a target for drugs with potential clinical efficacy on cognitive functions.

Astrocyte expression of D2-like dopamine receptors in the prefrontal cortex

The dopaminergic system is of crucial importance for understanding human behavior and the pathogenesis of many psychiatric and neurological conditions. The majority of studies addressing the localization of dopamine receptors (DR) examined the expression of DR in neurons, while its expression, precise anatomical localization and possible function in glial cells have been largely neglected. Here we examined the expression of D2-like family of DR in neuronal and glial cells in the normal human brain using immunocytochemistry and immunofluorescence. Tissue samples from the right orbitomedial (Brodmann’s areas 11/12), dorsolateral (areas 9/46) and dorsal medial (area 9) prefrontal cortex were taken during autopsy from six subjects with no history of neurological or psychiatric disorders, formalin-fixed, and embedded in paraffin. The sections were stained using novel anti-DRD2, anti-DRD3, and anti-DRD4 monoclonal antibodies. Adjacent sections were labeled with an anti-GFAP (astroglial marker) and an anti-CD68 antibody (macrophage/microglial marker). The pyramidal and non-pyramidal cells of all three regions analyzed had strong expression of DRD2 and DRD4, whereas DRD3 were very weakly expressed. DRD2 were more strongly expressed in layer III compared to layer V pyramidal neurons. In contrast, DRD4 receptors had a stronger expression in layer V neurons. The most conspicuous finding was the strong expression of DRD2, but not DRD3 or DRD4, receptors in the white matter fibrous astrocytes and in layer I protoplasmic astrocytes. Weak DRD2-immunoreactivity was also observed in protoplasmic astrocytes in layers III and V. These results suggest that DR-expressing astrocytes directly participate in dopaminergic transmission of the human prefrontal cortex.

Antagonist-D2S-dopamine receptor interactions in intact recombinant Chinese hamster ovary cells [corrected]

D(2)-type dopamine receptors are major recognition sites for antipsychotic drugs. There are two splice variants: D(2S) and D(2L) with an additional 29 amino acid sequence in the third intracellular loop. Only little comparative information is hitherto available about their pharmacological properties and none of these studies dealt with intact cell systems. This prompted us to investigate the binding properties of [(3)H]-raclopride, a hydrophilic benzamide, and [(3)H]-spiperone, a highly hydrophobic butyrophenone, to intact CHO cells expressing recombinant human D(2L)-receptors. Presently, we have repeated and extended this experimental approach to the human D(2S)-receptors in the same cell system. Except for a slower dissociation of [(3)H]-spiperone from D(2S), the binding properties of these and other antagonists were not significantly different for both isoforms (P > 0.05). The very slow dissociation of the atypical antipsychotic clozapine was surprising in light of its low affinity. Two experiments pointed out the existence of non-competitive interactions between raclopride and spiperone for D(2S) as well as D(2L) (A. Packeu, J. P. De Backer & G. Vauquelin, in preparation). Alongside the different physicochemical properties of these ligands, this finding fits with a model wherein the hydrophilic raclopride approaches the D(2L)-receptor from the aqueous phase, while the hydrophobic spiperone approaches the receptor by lateral diffusion between the membrane lipids. These different modes of approach could imply the existence of topologically distinct ligand binding sites at D(2)-receptors.

Non-competitive interaction between raclopride and spiperone on human D-receptors in intact Chinese hamster ovary cells

We recently investigated the binding properties of the antagonists [(3)H]-raclopride and [(3)H]-spiperone to intact Chinese hamster ovary cells expressing recombinant human D(2long)-dopamine receptors (CHO-D(2L) cells). Compared with saturation binding with [(3)H]-raclopride, raclopride reduced [(3)H]-spiperone binding with to low potency in competition binding experiments. The present findings illustrate the ability of spiperone to inhibit [(3)H]-raclopride binding non-competitively. While raclopride only decreases the apparent K(D) of [(3)H]-raclopride in saturation binding experiments, spiperone only decreases the number of sites to which [(3)H]-raclopride binds with high affinity. Also, while the IC(50) of raclopride depends on the concentration of [(3)H]-raclopride in competition experiments, this is not the case for spiperone. Kinetic studies reveal that the binding of raclopride at its high affinity sites does not affect the association of subsequently added [(3)H]-spiperone nor the rebinding of freshly dissociated [(3)H]-spiperone to the same or surrounding receptors. Yet, spiperone does not affect the dissociation rate of [(3)H]-raclopride and raclopride does not affect the (genuine) dissociation rate of [(3)H]-spiperone. The easiest way to interpret the present findings in molecular terms is to assume that D(2L)-receptors or their dimeric complexes possess two distinct binding sites: one with high affinity/accessibility for [(3)H]-raclopride and the other one with high affinity/accessibility for [(3)H]-spiperone. The ability of bound spiperone to inhibit high affinity raclopride binding while the reverse is not the case suggests for the occurrence of non-reciprocal allosteric interactions. These new findings could point at the occurrence of allosteric interactions between different classes of D(2)-receptor antagonists.

Dopaminergic signals in primary motor cortex

Brainstem monoamine areas such as the ventral tegmental area (VTA) send dopaminergic projections to the cerebral cortex that are widely distributed across different cortical regions. Whereas the projection to prefrontal areas (PFC) has been studied in detail, little is known about dopaminergic projections to primary motor cortex (M1). These projections have been anatomically characterized in rat and primate M1. Primates have even denser dopaminergic projections to M1 than rats. The physiological role, the effects of dopaminergic input on the activity of M1 circuits, and the behavioral function of this projection are unknown. This review explores the existing anatomical, electrophysiological and behavioral evidence on dopaminergic projections to M1 and speculates about its functional role. The projection may explain basic features of motor learning and memory phenomena. It is of clinical interest because of its potential for augmenting motor recovery after a brain lesion as well as for understanding the symptomatology of patients with Parkinson's disease. Therefore, targeted investigations are necessary.

Quantitative PET analysis of the dopamine D2 receptor agonist radioligand 11C-(R)-2-CH3O-N-n-propylnorapomorphine in the human brain

It has been demonstrated in vitro that the dopamine D(2) receptor has 2 interconvertible affinity states for endogenous dopamine, referred to as the high- and the low-affinity states. (11)C-(R)-2-CH(3)O-N-n-propylnorapomorphine ((11)C-MNPA) is a new agonist radioligand for in vivo imaging of the high-affinity state of dopamine D(2) receptors using PET. In the present study, the kinetics of (11)C-MNPA were examined for the first time, to our knowledge, in the human brain and analyzed using quantitative approaches with or without an arterial input function.

METHODS

A 90-min dynamic PET scan was obtained for 10 healthy men after an intravenous injection of (11)C-MNPA. The binding potential (BP(ND)) was calculated using the indirect kinetic method, a kinetic compartment analysis with a metabolite-corrected arterial input function. BP(ND) was also calculated by the simplified reference tissue model (SRTM) and transient equilibrium methods, both with the cerebellum as the reference brain region. The results of the quantitative methods were compared in a cross-validation approach.

RESULTS

The highest regional radioactivity was observed in the putamen. BP(ND) values obtained by kinetic analysis were 0.82 +/- 0.09, 0.59 +/- 0.11, and 0.28 +/- 0.06, respectively, in the putamen, caudate, and thalamus. BP(ND) values obtained by the SRTM and transient equilibrium methods were in good agreement with those obtained by the indirect kinetic method (r = 0.98 and r = 0.93, respectively). For all quantification methods, the BP(ND) values based on data acquired from 0 to 60 min were in good agreement with those based on data acquired from 0 to 90 min (r = 0.90-0.99).

CONCLUSION

The regional distribution of (11)C-MNPA binding was in good agreement with previous PET studies of dopamine D(2) receptors in the human brain using antagonist radioligands. The results support routine use of the SRTM and transient equilibrium methods, that is, methods that do not require an arterial input function and need a scan time of only about 60 min. (11)C-MNPA should thus be useful for clinical research on the pathophysiology of neuropsychiatric disorders and estimation of dopamine D(2) receptor occupancy by dopaminergic drugs.

11C-NMSP/ 18F-FDG microPET to monitor neural stem cell transplantation in a rat model of traumatic brain injury

PURPOSE

To investigate whether (11)C-N-methylspiperone ((11)C-NMSP) microPET could be used for imaging neural stem cells (NSCs) transplantation in a rat model of traumatic brain injury.

METHODS

NSCs were induced to express dopamine receptor type 2 (DRD(2)), then confirmed by RT-PCR, Western blotting and immunocytochemistry. Eighteen rats were subjected to focal traumatic brain injury in the right parietal lobe and then assigned randomly to the transplantation group and the control group. NSCs labeled with 5-bromo-2-deoxyuridine (BrdU) were transplanted into the cerebral lesion of the transplantation group. MicroPET scan using (11)C-NMSP and (18)F-FDG were performed to detect the DRD(2) expression of transplanted NSCs and the regional glucose metabolism in the cerebral lesion, respectively. Behavioral neurological function of rats were also tested.

RESULTS

Histological analysis identified viable NSCs. Western blotting and immunofluorescence showed high level of NSCs-induced DRD(2) expression. Immunostaining demonstrated high levels of survived BrdU+ and DRD(2)+ donor cells in the cerebral lesion 2 weeks after transplantation. The lesion-to-normal contralateral ratio (L/N ratio) of (11)C-NMSP in the cerebral lesion decreased significantly from 97% to 68% after injury and increased dramatically to 137% 1 day after the transplantation and then decreased gradually. Glucose metabolism showed a decrease of 35% in the cerebral lesion 1 day after injury and recovered to 87% 2 weeks after transplantation. The behavioral neurological function of the transplantation group was significantly improved compared with the control group.

CONCLUSIONS

This study verified that (11)C-NMSP microPET can be used to assess the NSCs-induced DRD(2) expression in rat model.

Antagonist-radioligand binding to D2L-receptors in intact cells

D(2)-dopamine receptors mediate most of the physiological actions of dopamine and are important recognition sites for antipsychotic drugs. Earlier binding studies were predominantly done with broken cell preparations with the tritiated D(2)-receptor antagonists [(3)H]-raclopride, a hydrophilic benzamide, and [(3)H]-spiperone, a highly hydrophobic butyrophenone. Here we compared [(3)H]-raclopride and [(3)H]-spiperone binding properties in intact Chinese Hamster Ovary cells stably expressing recombinant human D(2L)-receptors. Specific binding of both radioligands occurred to a comparable number of sites. In contrast to the rapid dissociation of [(3)H]-raclopride in both medium only and in the presence of an excess of unlabelled ligand [(3)H]-spiperone dissociation was only observed in the latter condition, and it was still slower than in broken cell preparations. However, this could not explain the pronounced difference in the potency of some unlabelled ligands to compete with both radioligands. To integrate these new findings, a model is proposed in which raclopride approaches the receptor from the aqueous phase, while spiperone approaches the receptor by lateral diffusion within the membrane.

Microtubule-associated STOP protein deletion triggers restricted changes in dopaminergic neurotransmission

The microtubule-associated stable tubule only polypeptide (STOP) protein plays a key-role in neuron architecture and synaptic plasticity. Recent studies suggest that schizophrenia is associated with alterations in the synaptic connectivity. Mice invalidated for the STOP gene display phenotype reminiscent of some schizophrenic-like symptoms, such as behavioral disturbances, dopamine (DA) hyper-reactivity, and possible hypoglutamatergia, partly improved by antipsychotic treatment. In the present work, we examined potential alterations in some DAergic key proteins and behaviors in STOP knockout mice. Whereas the densities of the DA transporter, the vesicular monoamine transporter and the D(1) receptor were not modified, the densities of the D(2) and D(3) receptors were decreased in some DAergic regions in mutant versus wild-type mice. Endogenous DA levels were selectively decreased in DAergic terminals areas, although the in vivo DA synthesis was diminished both in cell bodies and terminal areas. The DA uptake was decreased in accumbic synaptosomes, but not significantly altered in striatal synaptosomes. Finally, STOP knockout mice were hypersensitive to acute and subchronic locomotor effects of cocaine, although the drug equally inhibited DA uptake in mutant and wild-type mice. Altogether, these data showed that deletion of the ubiquitous STOP protein elicited restricted alterations in DAergic neurotransmission, preferentially in the meso-limbic pathway.

Nicotine improves cognitive deficits of dopamine transporter knockout mice without long-term tolerance

Various studies suggest a dysfunction of nicotinic neurotransmission in schizophrenia and establish that patients suffering from schizophrenia and attention deficit hyperactivity disorder (ADHD) have a high tobacco consumption, potentially for the purpose of self-medication. Owing to its neuroprotective and procognitive effects, transdermal nicotine was proposed to be an effective treatment of some neurodegenerative and psychiatric diseases. Mice deficient in the dopamine transporter (DAT KO) exhibit a phenotype reminiscent of schizophrenia and ADHD, including hyperdopaminergia, hyperactivity, paradoxical calming by methylphenidate and cognitive deficits, some of which being improved by antipsychotic agents. We recently demonstrated that nicotinic receptor content and function were profoundly modified in DAT KO mice. In this study, we assessed the effects of a chronic nicotine treatment in the drinking water on the nicotine-induced locomotion, anxiety status and learning performance. Chronically nicotine-treated DAT KO mice were always hypersensitive to the hypolocomotor effect of nicotine without tolerance and did not exhibit the anxiogenic effect of nicotine treatment observed in WT mice. Very interestingly, both acute and chronic nicotine treatments greatly improved their deficits in the cued and spatial learning, without eliciting tolerance. We speculate that the procognitive effects of nicotine in DAT KO mice are related to the upregulation of alpha7 nicotinic receptors in the hippocampus, amygdala, and prelimbic cortex, all areas involved in cognition. Data from our studies on DAT KO mice shed light on the nicotine self-medication in psychiatric patients and suggest that nicotinic agonists could favorably lead to additional therapy of psychiatric diseases.

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

RATIONALE

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Endosomal location of dopamine receptors in neuronal cell cytoplasm

Five subtypes of dopamine receptor exist in two subfamilies: two D(1)-like (D(1) and D(5)) and three D(2)-like (D(2), D(3) and D(4)). We produced novel monoclonal antibodies against all three D(2)-like receptors and used them to localize receptors in Ntera-2 (NT-2) cells, the human neuronal precursor cell line. Most of the immunostaining for all three receptors colocalized with mannose-6-phosphate receptor, a marker for late endosomes formed by internalization of the plasma membrane. This result was obtained with antibodies against three different epitopes on the D(3) receptor, to rule out the possibility of cross-reaction with another protein, and controls without primary antibody or in the presence of competitor antigen were completely negative. In rat cerebral cortex and hippocampus, some of the dopamine receptor staining was found in similar structures in neuronal cell cytoplasm. Only some of the neurons were positive for dopamine receptors and the pattern was consistent with previously-reported patterns of innervation by dopamine-producing neurons. Endosomal dopamine receptors may provide a useful method for identifying cell bodies of dopamine-responsive neurons to complement methods that detect only active receptors in the neuronal cell membrane.

Functional alterations of nicotinic neurotransmission in dopamine transporter knock-out mice

Mice lacking the dopamine (DA) transporter (DAT) gene exhibit a phenotype reminiscent of schizophrenia and attention deficit hyperactivity disorder (ADHD), including hyperDAergia, hyperactivity and deficits in cognitive performance, which are alleviated by antipsychotic agents. Numerous studies suggest a dysfunction of nicotinic neurotransmission in schizophrenia and show increased tobacco intake in schizophrenic and ADHD patients, possibly as a self-medication. Thus, we examined the potential alteration of nicotinic neurotransmission in DAT knock-out (KO) mice. We showed that constitutively hyperDAergic DAT KO mice exhibited modifications in nicotinic receptor density in an area- and subtype-dependent manner. In some DAergic areas, the small decrease in the beta2* nicotinic subunit (nAChR) density contrasted with the higher decrease and increase in the alpha6* and alpha7 nAChR densities, respectively. Mutant mice were hypersensitive to the stimulant locomotor effects of nicotine at low doses, probably due to enhanced nicotine-induced extracellular DA level. They also showed hypersensitivity to the hypolocomotion induced by nicotine. In contrast, no hypersensitivity was observed for other nicotine-induced behavioral effects, such as anxiety or motor activity in the elevated plus maze. Co-administration of nicotinic agonists at sub-active doses elicited opposite locomotor effects in wild-type and DAT KO mice, as reported previously for methylphenidate. Interestingly, such a co-administration of nicotinic agonists induced synergistic hypolocomotion in DAT KO mice. These findings show that a targeted increase of DA tone can be responsible for significant adaptations of the cholinergic/nicotinic neurotransmission. This study may provide potential leads for the use of nicotine or combined nicotinic agonists for the therapy of psychiatric disorders.

[123I]Epidepride binding to cerebellar dopamine D2/D3 receptors is displaceable: Implications for the use of cerebellum as a reference region

The low density of cerebellar dopamine D(2)/D(3) receptors provides the basis for using the cerebellum as a representation of free- and non-specifically bound radioligand in positron emission tomography (PET) and single photon emission computed tomography (SPECT) studies. With the development of ultra high-affinity dopamine D(2)/D(3) ligands like [(123)I]epidepride, [(18)F]fallypride, and [(11)C]FLB-457, quantification of extrastriatal low density receptor populations including the cerebellum is possible with important implications for calculation of binding parameters. [(123)I]epidepride-SPECT was performed in 23 patients with schizophrenia before and after 3 months of antipsychotic treatment with either risperidone (n=14) or zuclopenthixol (n=9). In the unblocked situation and partially blocked situation, the average distribution volumes were 5.2+/-1.3 mL/mL and 4.0+/-0.8 mL/mL, respectively. The paired distribution volumes were reduced by 22+/-15% (mean+/-SD) after antipsychotic treatment (p<0.0001, paired Student's t-test). From the paired distribution volumes in cerebellum and extrastriatal regions, the average distribution volume representing free and non-specifically bound [(123)I]epidepride was calculated to be 3.3+/-0.8 mL/mL. Both the % [(123)I]epidepride fraction of plasma radioactivity (p>0.76) and the plasma [(123)I]epidepride concentration (p>0.45) were unchanged after antipsychotic treatment (paired Student's t-test). These results strongly suggest the presence of "non-negligible" specific [(123)I]epidepride binding to dopamine D(2)/D(3) receptors in the cerebellum. Using the cerebellum as a representation of free and non-specifically bound radioligand and neglecting the specifically bound component may lead to results that erroneously imply that antipsychotic drugs bind to extrastriatal dopamine D(2)/D(3) receptors with a higher affinity than to striatal dopamine D(2)/D(3) receptors.

Striatal neuroadaptation and rescue of locomotor deficit by l-dopa in aphakia mice, a model of Parkinson's disease

Preferential neurodegeneration of dopaminergic neurons in the ventral substantia nigra of the midbrain is a hallmark of Parkinson's disease. The homeobox transcription factor Pitx3 is similarly and selectively expressed in the same neurons. Pitx3 deficiency in a natural mouse mutant, the aphakia mouse, was correlated with the loss of these neurons and with a deficit in locomotor activity. We now report that the locomotor deficit of aphakia mice is established by 40 days of age and that it can be rescued by injection of l-dopa. We further show that downstream striatal correlates of the midbrain neuronal losses in aphakia mice, as assessed by dopamine transporter binding and expression of dopamine receptors, enkephalin, dynorphin and neurotensin, are highly similar to neuroadaptive responses observed following rapid neurodegeneration induced by neurotoxin administration in adult animals or following the progressive neurodegenerative processes as seen in Parkinson patients. Taken collectively, these data support the idea that the aphakia mice represent a selective model of dopaminergic deficiency that closely resembles the midbrain and striatal neuropathology associated with Parkinson's disease, and this suggests that these mice are a good model to assess therapies for Parkinson's disease as well as to understand the susceptibility of these neurons to neurodegeneration.

Effects of physiological and pharmacological stimuli on dopamine release in the rat globus pallidus

A major aspect of understanding functions of the globus pallidus (GP) within the basal ganglia is the significance of its dopamine innervation. Here, we used in vivo-microdialysis in rats to characterize pallidal dopamine release in response to a number of physiological and pharmacological stimuli known to activate dopamine neurons. Results reveal that an aversive stimulus, i.e. handling for 20 min, significantly increased dialysate dopamine in the globus pallidus to about 130% of baseline levels. Likewise, a novel and appetitive stimulus, i.e. presentation of unfamiliar, palatable food, significantly elevated pallidal dopamine to about 150% of baseline levels both in rats which did and did not consume the food reward. These findings provide evidence that increases of dopamine (DA) efflux may largely reflect stimulus saliency implicating an involvement of pallidal dopamine signalling in control of behaviour governed by salient stimuli. Results further showed that reverse microdialysis of D-amphetamine and cocaine in augmenting concentrations of 0.1-100 microM elevated dialysate dopamine in a concentration-dependent manner suggesting a role of pallidal dopamine in mediating behavioural effects of psychostimulant drugs.

Inactivation of the rat dorsal striatum impairs performance in spatial tasks and alters hippocampal theta in the freely moving rat

We analysed the interaction between the dorsal striatum (motor coordination and planning) and the hippocampus (sensory information processing and integration) during performance of goal-directed tasks. The performance of rats that had been injected with different doses of the D(2)-antagonist Sulpiride into the dorsal striatum was tested in an egocentric 4-arm maze task that tests striatal functions. Furthermore, hippocampal EEGs were recorded before, during and after inactivation of the dorsal striatum via injections of Sulpiride of rats that were performing a continuous alternation task. Injection of 5 microl of 100 mM Sulpiride increased the number of errors committed in the egocentric 4-arm maze (p < 0.01), indicating that the dorsal striatum is involved in motor control and motor memory recall in such a task. In the recording study, the same dose of Sulpiride injected into the dorsal striatum had powerful effects on the hippocampal EEG. The main activity in the theta range (5-10 Hz) was shifted from higher frequencies in the 8-10 Hz range to lower frequencies in the 5-7 Hz range (p < 0.005). The impairment in the behavioural egocentric task after Sulpiride injection, and the effects of Sulpiride on hippocampal theta shows that there is a functional interaction between the dorsal striatum and the hippocampus. While the dorsal striatum coordinates the execution of complex motor programs, the hippocampus integrates spatial and other sensory information required for the planning and execution of goal-directed movements.

Co-release of noradrenaline and dopamine in the cerebral cortex elicited by single train and repeated train stimulation of the locus coeruleus

Background

Previous studies by our group suggest that extracellular dopamine (DA) and noradrenaline (NA) may be co-released from noradrenergic nerve terminals in the cerebral cortex. We recently demonstrated that the concomitant release of DA and NA could be elicited in the cerebral cortex by electrical stimulation of the locus coeruleus (LC). This study analyses the effect of both single train and repeated electrical stimulation of LC on NA and DA release in the medial prefrontal cortex (mPFC), occipital cortex (Occ), and caudate nucleus. To rule out possible stressful effects of electrical stimulation, experiments were performed on chloral hydrate anaesthetised rats.

Results

Twenty min electrical stimulation of the LC, with burst type pattern of pulses, increased NA and DA both in the mPFC and in the Occ. NA in both cortices and DA in the mPFC returned to baseline within 20 min after the end of the stimulation period, while DA in the Occ reached a maximum increase during 20 min post-stimulation and remained higher than baseline values at 220 min post-stimulation. Local perfusion with tetrodotoxin (TTX, 10 μM) markedly reduced baseline NA and DA in the mPFC and Occ and totally suppressed the effect of electrical stimulation in both areas.

A sequence of five 20 min stimulations at 20 min intervals were delivered to the LC. Each stimulus increased NA to the same extent and duration as the first stimulus, whereas DA remained elevated at the time next stimulus was delivered, so that baseline DA progressively increased in the mPFC and Occ to reach about 130 and 200% the initial level, respectively.

In the presence of the NA transport (NAT) blocker desipramine (DMI, 100 μM), multiple LC stimulation still increased extracellular NA and DA levels.

Electrical stimulation of the LC increased NA levels in the homolateral caudate nucleus, but failed to modify DA level.

Conclusion

The results confirm and extend that LC stimulation induces a concomitant release of DA and NA in the mPFC and Occ.

The different time-course of LC-induced elevation of DA and NA suggests that their co-release may be differentially controlled.

Changes of dopamine-sensitive cyclic AMP-generating system in the rat hippocampus as a function of age

The dopamine (DA) D(1) and D(2) receptors coupled to the 3',5'-cyclic adenosine monophosphate (cAMP)-generating system were studied in membrane particles of the dorsal hippocampus in 3 (considered to be young), 12 (considered to be adult) and 24 (considered to be old) month male Sprague-Dawley rats. Activation of D(1) receptors with DA, apomorphine or SKF 82526 enhanced accumulation of cAMP in the hippocampus of rats of the three age groups examined. This stimulatory effect was significantly reduced in adult rats. No further changes were noticeable in old animals. D(2) receptors negatively coupled to cAMP generation were demonstrated by incubating hippocampus membrane particles with SCH 23390 plus DA or with D(2) receptor agonists quinpirole or bromocriptine. The D(2) inhibitory effect on cAMP generation was unchanged in the three age groups. No difference was detectable between young, adult and old rats in the activation of cAMP production by forskolin.

Molecular, chemical, and anatomical characterization of globus pallidus dopamine D2 receptor mRNA-containing neurons

Essential for normal movement, the globus pallidus (GP) is a prominent nucleus whose neurons project to all other basal ganglia nuclei. The GP is composed of at least two distinct neuron populations. GP neurons of the rodent contain either the calcium-binding protein parvalbumin (PV) or preproenkephalin (PPE) mRNA, differentially innervate several basal ganglia structures, and have distinct immediate early gene responses to dopamine agonists or antagonists. Recent research has revealed that dopamine directly influences GP neurons, with D2 receptors contributing to both pre- and postsynaptic effects of dopaminergic agents. The existence of D2 mRNA-expressing (D2+) GP neurons has been established, but little is known concerning their numbers, regional distribution, or relationship to pallidal subpopulations identified on the basis of PV immunocytochemistry, PPE mRNA, or axonal targets. Detection of pallidal D2 mRNA with a 35S-cRNA probe revealed that D2+ neurons are found throughout the GP, comprising approximately one-half of pallidal neurons, but they are most dense within a dorsoventral band in lateral GP. While a substantial proportion (42-51%) of all chemically and anatomically labeled pallidal neuron subpopulations expressed D2 transcript, the D2+ neurons exhibited both population-based and regional heterogeneities. Overall, the pallidostriatal cells had a greater density of D2 mRNA than did pallidosubthalamic cells. Also, compared to other pallidal regions, the ventromedial GP contained fewer D2+ cells, and the PPE mRNA-expressing cells in this region had lower densities of D2 mRNA per neuron. These results reveal heterogeneous chemical and anatomical properties of the extensive population of D2+ GP neurons, a potential cellular substrate for dopamine's effects in pallidum.

How laminar frontal cortex and basal ganglia circuits interact to control planned and reactive saccades

How does the brain learn to balance between reactive and planned behaviors? The basal ganglia (BG) and frontal cortex together allow animals to learn planned behaviors that acquire rewards when prepotent reactive behaviors are insufficient. This paper proposes a new model, called TELOS, to explain how laminar circuitry of the frontal cortex, exemplified by the frontal eye fields, interacts with the BG, thalamus, superior colliculus, and inferotemporal and parietal cortices to learn and perform reactive and planned eye movements. The model is formulated as fourteen computational hypotheses. These specify how strategy priming and action planning (in cortical layers III, Va and VI) are dissociated from movement execution (in layer Vb), how the BG help to choose among and gate competing plans, and how a visual stimulus may serve either as a movement target or as a discriminative cue to move elsewhere. The direct, indirect and hyperdirect pathways through the BG are shown to enable complex gating functions, including deferred execution of selected plans, and switching among alternative sensory-motor mappings. Notably, the model can learn and gate the use of a What-to-Where transformation that enables spatially invariant object representations to selectively excite spatially coded movement plans. Model simulations show how dopaminergic reward and non-reward signals guide monkeys to learn and perform saccadic eye movements in the fixation, single saccade, overlap, gap, and delay (memory-guided) saccade tasks. Model cell activation dynamics quantitatively simulate seventeen established types of dynamics exhibited by corresponding real cells during performance of these tasks.

Measurement of dopamine D2-like receptors in postmortem CNS and pituitary: differential regional changes in schizophrenia

In situ radioligand binding with autoradiography and anti-human dopamine D(2) receptor antibodies with Western blots have been used to measure the density of dopamine D(2)-like receptors in the caudate-putamen and pituitary from schizophrenic subjects who did or did not have residual antipsychotic drugs in their tissue at death. There was a significant decrease in the Ki for haloperidol displaceable [(125)I]iodosulpride binding in the pituitary (p < 0.01) and caudate-putamen (p < 0.05) from subjects with schizophrenia with residual drugs in their tissue. There was a significant decrease in the density of [(125)I]iodosulpride in the pituitary (p < 0.001) and a strong trend to a decrease in binding in the caudate-putamen (p = 0.055) from subjects with schizophrenia. By contrast, [(3)H]spiperone binding was decreased in the caudate-putamen (p < 0.05) with a trend to decreased binding in the pituitary (p = 0.07) from subjects with schizophrenia. There was no difference in the density of dopamine D(2) receptors in the caudate-putamen from subjects with schizophrenia (p = 0.31). All the findings on receptor densities were independent of drug status. [(125)I]iodosulpride binds to the dopamine D(2&3) receptors. We have shown that there is no change in the dopamine D(2) receptor in the caudate-putamen from subjects with schizophrenia and therefore, these data would be consistent with there being a decrease in the dopamine D(3) in the caudate-putamen from subjects with schizophrenia. Since dopamine D(3) receptors are absent or present at low concentrations in the pituitary, our data would suggest the dopamine D(2) receptor is decreased in that tissue from schizophrenic subjects.

Lesion of substantia nigra pars compacta by the GluR5 agonist ATPA

Dopamine (DA) released by substantia nigra pars compacta (SNc) neurons is a key regulator of motor activity. A deficiency in the striatum DA content due to SNc degeneration is a characteristic of Parkinson's disease. The involvement of excitotoxic mechanisms in this pathology has been suggested. The kainate receptor subunit GluR5 has been identified in a few basal ganglia but it is strongly expressed in SNc. Here we examine whether (RS)-2-amino-3-(3-hydroxy-5-tbutylisoxazol-4-yl) propanoic acid (ATPA), a selective agonist of GluR5, induces damage in dopaminergic (DAergic) neurons. ATPA (13 nmol) was administered to rat SNc. Immediately after recovery from surgery, the rats displayed ipsilateral turning. This behavior disappeared in subsequent days. The administration of the D1/D2 agonist, apomorphine (1 mg/kg, s.c.) 1 and 2 weeks after ATPA-infusion also induced ipsilateral turning. Histological studies-performed 21 days after ATPA-infusion-showed a lesion of the lateral and central part of the SNc, where a significant loss (36%) of DAergic cells was detected by tyrosine hydroxylase immunohistochemistry. The lesion was restricted to the SNc, since no damage or glial reaction was observed in the substantia nigra pars reticulata as assessed by Nissl staining, tomato lectin staining for microglial cells and GFAP immunohistochemistry for astrocytes.

IN CONCLUSION

(1). ATPA-infusion induces neuronal damage in the SNc in the rat and (2). the behavioral effects of unilateral infusion of ATPA are consistent with DAergic alterations in basal ganglia.

The role of the globus pallidus D2 subfamily of dopamine receptors in pallidal immediate early gene expression

The globus pallidus plays an important role in basal ganglia circuitry, representing the first relay nucleus of the 'indirect pathway' of striatal efferents. In contrast to the well-characterized actions of dopamine on striatal neurons, the functional role of the dopamine innervation of globus pallidus is less well understood. Previous research showed that systemic administration of either a dopamine D2 receptor antagonist or combined dopamine D1 and D2 receptor agonists induces Fos, the protein product of the immediate early gene c-fos, in neurons of globus pallidus [Ruskin and Marshall (1997) Neuroscience 81, 79-92]. To determine whether the ability of the D2 receptor antagonist, sulpiride, to induce Fos in rat pallidal neurons is mediated by D2-like receptors in striatum or globus pallidus, intrastriatal or intrapallidal sulpiride infusions were conducted. The diffusion of intrastriatal sulpiride was estimated by measuring this antagonist's competition for N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced D2 receptor inactivation. The phenotype of the striatal neurons expressing Fos after intrastriatal infusion was assessed by combining Fos immunocytochemistry with D2 receptor mRNA in situ hybridization. Intrastriatal infusions of (-)-sulpiride (10-200 ng) dose-dependently increased the number of striatal cells expressing Fos; and the Fos-immunoreactive striatal cells were D2 receptor mRNA-expressing, the same population in which systemic D2 receptor antagonists induce Fos. Intrastriatal infusions of high (5 microg), but not low (10-200 ng), (-)-sulpiride doses also induced Fos in globus pallidus cells but the sulpiride appeared to spread to the globus pallidus. Direct intrapallidal infusions of (-)-sulpiride (50-100 ng) dose-dependently induced Fos in globus pallidus with minimal influence on striatum or other basal ganglia structures. Using sensitive in situ hybridization conditions, prominent labeling of D2 receptor mRNA was evident in globus pallidus. D2 receptor mRNA was densest in a lateral 200 microm wide band that follows the curvature of the pallidal/striatal boundary. Cellular analysis revealed silver clusters associated with D2 receptor mRNA labeling over globus pallidus neurons that were immunoreactive for neuron-specific nuclear protein. These results strongly suggest that the dopaminergic innervation of globus pallidus, acting through D2-like receptors internal to this structure, can control gene expression in pallidal neurons.

Effect of microiontophoretic application of dopamine on subthalamic nucleus neuronal activity in normal rats and in rats with unilateral lesion of the nigrostriatal pathway

The subthalamic nucleus (STN) receives dopamine inputs from the substantia nigra but their implication in the pathophysiology of parkinsonism is still debated. Extracellular microrecordings were used to study the effect of microiontophoretic injection of dopamine and the D1 receptor agonist SKF 38393 on the activity of STN neurons in normal and 6-hydroxydopamine-lesioned rats under urethane anaesthesia. Dopamine and SKF induced an increase in the firing rate of the majority of STN neurons in both normal and 6-OHDA rats. In rats with 6-OHDA lesions, the percentage of firing rate increase did not differ from that of controls. When GABA, glutamate and dopamine were all applied to the same individual STN neurons, GABA induced an inhibitory effect and glutamate and dopamine caused an excitatory effect in both groups. This excitatory response was suppressed by the application of GABA. Systemic administration of apomorphine provoked a decrease in the firing rate of STN neurons in rats with 6-OHDA lesions. These results show that dopamine exerts an excitatory influence on STN neurons, suggesting that the inhibitory effect induced by the systemic injection of apomorphine is due to the GABAergic inputs from the globus pallidus as predicted by the current model of basal ganglia organization. In addition, we show that dopamine, GABA and glutamate can act on the same STN neuron and that GABA can reverse the excitatory effect of dopamine and glutamate, suggesting the predominant influence of GABAergic inputs to the subthalamic nucleus.

Intrasubthalamic injection of 6-hydroxydopamine induces changes in the firing rate and pattern of subthalamic nucleus neurons in the rat

The subthalamic nucleus (STN) receives dopaminergic projections from the substantia nigra pars compacta (SNc). To investigate the role of direct and indirect dopaminergic influences on STN neurons, the spontaneous activity was studied in four groups of animals: normal rats, rats with intrasubthalamic or intranigral injection of 6-hydroxydopamine (6-OHDA), and sham STN injection rats by using extracellular recordings 4 weeks postsurgery. After intrasubthalamic injection of 6-OHDA, the mean firing rate significantly decreased (7.29 +/- 0.39 spikes/sec, P < 0.01 vs. 11.13 +/- 0.59 spikes/sec in normal or 11.26 +/- 0.57 spikes/sec in sham group), and the percentage of STN neurons discharging regularly decreased significantly (81%, P < 0.05 vs. 90% in normal group or P < 0.01 vs. 92% in sham group) and that of bursty cells increased (19%, P < 0.05 vs. 10%; in normal group or P < 0.01 vs. 8% in sham group). In the group of rats with SNc lesion, the firing rate of subthalamic neurons did not show a significant difference (11.61 +/- 0.81 spikes/sec) compared with normal group. However, the firing pattern was dramatically changed: 74% of cells exhibited bursty pattern and only 26% of cells discharged regularly or slightly irregularly. Immunohistochemical results showed that intrasubthalamic injection of 6-OHDA induced a marked degeneration of dopaminergic cells in the lateral part of the ipsilateral SNc, whereas 6-OHDA injection into the SNc induced a total in situ lesion of dopamine cells. These results suggest that the SNc exerts an excitatory influence on STN neurons and that the loss of this dopaminergic projection could, at least partially, account for the changes in the firing pattern of STN neurons in the 6-OHDA rat model of parkinsonism.

Error analysis for quantification of [(11)C]FLB 457 binding to extrastriatal D(2) dopamine receptors in the human brain

To estimate receptor binding of ligand by positron emission tomography (PET) without an arterial input function, several quantitative approaches based on the use of a reference region have been proposed. We compared three approaches for quantifying extrastriatal D(2) dopamine receptors using [(11)C]FLB 457. The PET measurements were performed on seven healthy men. Binding potential (BP) of [(11)C]FLB 457 was calculated by the reference tissue model method, transient equilibrium method, and late time method. The reference tissue model describes the time-activity curve in a brain region in terms of that in the reference region, assuming that the levels of nondisplaceable radioligand binding in both regions are the same. The transient equilibrium theoretically occurs when the derivative for specific binding is zero. With the late time method, BP is calculated by integrating a late part of the time-activity curve. BP values obtained by all methods were in good agreement with those obtained by the kinetic approach, and the highest coefficient of correlation was observed in the reference tissue model method. In the simulation study, the error of BP calculated by the reference tissue model method was smallest. Moreover, the effect of the difference in the influx rate constant K(1) between the brain and the reference regions on BP was nearly avoided as theoretically predicted. We concluded that the reference tissue model method is most suitable for calculating BP of extrastriatal D(2) dopamine receptors with [(11)C]FLB 457.

Neuroprotective role of dopamine against hippocampal cell death

Glutamate excitotoxicity plays a key role in the induction of neuronal cell death occurring in many neuropathologies, including epilepsy. Systemic administration of the glutamatergic agonist kainic acid (KA) is a well characterized model to study epilepsy-induced brain damage. KA-evoked seizures in mice result in hippocampal cell death, with the exception of some strains that are resistant to KA excitotoxicity. Little is known about the factors that prevent epilepsy-related neurodegeneration. Here we show that dopamine has such a function through the activation of the D2 receptor (D2R). D2R gene inactivation confers susceptibility to KA excitotoxicity in two mouse strains known to be resistant to KA-induced neurodegeneration. D2R-/- mice develop seizures when administered KA doses that are not epileptogenic for wild-type (WT) littermates. The spatiotemporal pattern of c-fos and c-jun mRNA induction well correlates with the occurrence of seizures in D2R-/- mice. Moreover, KA-induced seizures result in extensive hippocampal cell death in D2R-/- but not WT mice. In KA-treated D2R-/- mice, hippocampal neurons die by apoptosis, as indicated by the presence of fragmented DNA and the induction of the proapoptotic protein BAX. These results reveal a central role of D2Rs in the inhibitory control of glutamate neurotransmission and excitotoxicity.

Dopaminergic innervation of the subthalamic nucleus in the normal state, in MPTP-treated monkeys, and in Parkinson's disease patients

The existence of a dopaminergic innervation of the subthalamic nucleus (STN) has been demonstrated in rats but has remained controversial in primates. The aim of the present study was first to demonstrate the existence of a dopaminergic innervation of the STN in monkeys using tracing methods and then to quantify the loss of dopaminergic fibers in the parkinsonian state in monkeys and humans. Following injection of Fluoro-Gold into the STN of a vervet monkey (Cercopithecus aethiops), retrogradely labeled neurons were found to be scattered in all dopaminergic areas of the mesencephalon. Injection of biotin dextran amine into dopaminergic areas A8 and A9 of two monkeys resulted in anterogradely labeled axons located throughout the whole extent of the STN. Labeled axons that also expressed tyrosine hydroxylase (TH) were reconstructed from serial sections. Some terminal axonal arborizations had profuse branching and occupied much of the STN, and others were restricted to small portions of the nucleus. In TH-immunoreactive sections, numerous sparse, fine, and varicose TH-positive fibers were observed in the STN of normal monkeys and humans. Quantification of these TH-positive fibers revealed a 51% loss of TH-positive fibers in MPTP-intoxicated monkeys and a 65% loss in Parkinson's disease patients compared with their respective controls. These findings demonstrate the existence of a dopaminergic innervation of the STN in primates. The loss of dopaminergic innervation in MPTP-intoxicated monkeys and in Parkinson's disease patients may directly affect the activity of STN neurons and could participate in the hyperactivity of the structure.

Functional changes of the basal ganglia circuitry in Parkinson's disease

The basal ganglia circuitry processes the signals that flow from the cortex, allowing the correct execution of voluntary movements. In Parkinson's disease, the degeneration of dopaminergic neurons of the substantia nigra pars compacta triggers a cascade of functional changes affecting the whole basal ganglia network. The most relevant alterations affect the output nuclei of the circuit, the medial globus pallidus and substantia nigra pars reticulata, which become hyperactive. Such hyperactivity is sustained by the enhanced glutamatergic inputs that the output nuclei receive from the subthalamic nucleus. The mechanisms leading to the subthalamic disinhibition are still poorly understood. According to the current model of basal ganglia organization, the phenomenon is due to a decrease in the inhibitory control exerted over the subthalamic nucleus by the lateral globus pallidus. Recent data, however, suggest that additional if not alternative mechanisms may underlie subthalamic hyperactivity. In particular, given the reciprocal innervation of the substantia nigra pars compacta and the subthalamic nucleus, the dopaminergic deficit might influence the subthalamic activity, directly. In addition, the increased excitatory drive to the dopaminergic nigral neurons originating from the hyperactive subthalamic nucleus might sustain the progression of the degenerative process. The identification of the role of the subthalamic nucleus and, more in general, of the glutamatergic mechanisms in the pathophysiology of Parkinson's disease might lead to a new approach in the pharmacological treatment of the disease. Current therapeutic strategies rely on the use of L-DOPA and/or dopamine agonists to correct the dopaminergic deficit. Drugs capable of antagonizing the effects of glutamate might represent, in the next future, a valuable tool for the development of new symptomatic and neuroprotective strategies for therapy of Parkinson's disease.