D3 receptors and the actions of neuroleptics in the ventral striatopallidal system of schizophrenics

Location, Location, Location: The Expression of D3 Dopamine Receptors in the Nervous System

When the rat D3 dopamine receptor (D3R) was cloned and the distribution of its mRNA examined in 1990-1991, it attracted attention due to its peculiar distribution in the brain quite different from that of its closest relative, the D2 receptor. In the rat brain, the D3R mRNA is enriched in the limbic striatum as opposed to the D2 receptor, which is highly expressed in the motor striatal areas. Later studies in the primate and human brain confirmed relative enrichment of the D3R in the limbic striatum but also demonstrated higher abundance of the D3R in the primate as compared to the rodent brain. Additionally, in the rodent brain, the D3R in the dorsal striatum appears to be co-expressed with the D1 dopamine receptor-bearing striatal neurons giving rise to the direct output striatal pathway, although the picture is less clear with respect to the nucleus accumbens. In contrast, in the primate striatum, the D3R co-localizes with the D2 receptor throughout the basal ganglia as well as in extrastriatal brain areas. The relative abundance of the D3R in the limbic striatum, its output structures, secondary targets, and some of the other connected limbic territories may underpin its role in reward, drug dependence, and impulse control. Selective expression of D3R in the brain proliferative areas may point to its important role in the neural development as well as in neurodevelopmental abnormalities associated with schizophrenia and other developmental brain disorders.

Pharmacogenetics of Antipsychotic Treatment in Schizophrenia

Antipsychotics are the mainstay treatment for schizophrenia. There is large variability between individuals in their response to antipsychotics, both in efficacy and adverse effects of treatment. While the source of interindividual variability in antipsychotic response is not completely understood, genetics is a major contributing factor. The identification of pharmacogenetic markers that predict antipsychotic efficacy and adverse reactions is a growing area of research and holds the potential to replace the current trial-and-error approach to treatment selection in schizophrenia with a personalized medicine approach.In this chapter, we provide an overview of the current state of pharmacogenetics in schizophrenia treatment. The most promising pharmacogenetic findings are presented for both antipsychotic response and commonly studied adverse reactions. The application of pharmacogenetics to schizophrenia treatment is discussed, with an emphasis on the clinical utility of pharmacogenetic testing and directions for future research.

Ventral striatal islands of Calleja neurons control grooming in mice

The striatum comprises multiple subdivisions and neural circuits that differentially control motor output. The islands of Calleja (IC) contain clusters of densely packed granule cells situated in the ventral striatum, predominantly in the olfactory tubercle (OT). Characterized by expression of the D3 dopamine receptor, the IC are evolutionally conserved, but have undefined functions. Here, we show that optogenetic activation of OT D3 neurons robustly initiates self-grooming in mice while suppressing other ongoing behaviors. Conversely, optogenetic inhibition of these neurons halts ongoing grooming, and genetic ablation reduces spontaneous grooming. Furthermore, OT D3 neurons show increased activity before and during grooming and influence local striatal output via synaptic connections with neighboring OT neurons (primarily spiny projection neurons), whose firing rates display grooming-related modulation. Our study uncovers a new role of the ventral striatum's IC in regulating motor output and has important implications for the neural control of grooming.

Striatal topographical organization: Bridging the gap between molecules, connectivity and behavior

The striatum represents the major hub of the basal ganglia, receiving projections from the entire cerebral cortex and it is assumed to play a key role in a wide array of complex behavioral tasks. Despite being extensively investigated during the last decades, the topographical organization of the striatum is not well understood yet. Ongoing efforts in neuroscience are focused on analyzing striatal anatomy at different spatial scales, to understand how structure relates to function and how derangements of this organization are involved in various neuropsychiatric diseases. While being subdivided at the macroscale level into dorsal and ventral divisions, at a mesoscale level the striatum represents an anatomical continuum sharing the same cellular makeup. At the same time, it is now increasingly ascertained that different striatal compartments show subtle histochemical differences, and their neurons exhibit peculiar patterns of gene expression, supporting functional diversity across the whole basal ganglia circuitry. Such diversity is further supported by afferent connections which are heterogenous both anatomically, as they originate from distributed cortical areas and subcortical structures, and biochemically, as they involve a variety of neurotransmitters. Specifically, the cortico-striatal projection system is topographically organized delineating a functional organization which is maintained throughout the basal ganglia, subserving motor, cognitive and affective behavioral functions. While such functional heterogeneity has been firstly conceptualized as a tripartite organization, with sharply defined limbic, associative and sensorimotor territories within the striatum, it has been proposed that such territories are more likely to fade into one another, delineating a gradient-like organization along medio-lateral and ventro-dorsal axes. However, the molecular and cellular underpinnings of such organization are less understood, and their relations to behavior remains an open question, especially in humans. In this review we aimed at summarizing the available knowledge on striatal organization, especially focusing on how it links structure to function and its alterations in neuropsychiatric diseases. We examined studies conducted on different species, covering a wide array of different methodologies: from tract-tracing and immunohistochemistry to neuroimaging and transcriptomic experiments, aimed at bridging the gap between macroscopic and molecular levels.

Going Back to Kahlbaum's Psychomotor (and GABAergic) Origins: Is Catatonia More Than Just a Motor and Dopaminergic Syndrome?

In 1874, Karl Kahlbaum described catatonia as an independent syndrome characterized by motor, affective, and behavioral anomalies. In the following years, various catatonia concepts were established with all sharing the prime focus on motor and behavioral symptoms while largely neglecting affective changes. In 21st century, catatonia is a well-characterized clinical syndrome. Yet, its neurobiological origin is still not clear because methodological shortcomings of hitherto studies had hampered this challenging effort. To fully capture the clinical picture of catatonia as emphasized by Karl Kahlbaum, 2 decades ago a new catatonia scale was developed (Northoff Catatonia Rating Scale [NCRS]). Since then, studies have used NCRS to allow for a more mechanistic insight of catatonia. Here, we undertook a systematic review searching for neuroimaging studies using motor/behavioral catatonia rating scales/criteria and NCRS published up to March 31, 2019. We included 19 neuroimaging studies. Studies using motor/behavioral catatonia rating scales/criteria depict cortical and subcortical motor regions mediated by dopamine as neuronal and biochemical substrates of catatonia. In contrast, studies relying on NCRS found rather aberrant higher-order frontoparietal networks which, biochemically, are insufficiently modulated by gamma-aminobutyric acid (GABA)-ergic and glutamatergic transmission. This is further supported by the high therapeutic efficacy of GABAergic agents in acute catatonia. In sum, this systematic review points out the difference between motor/behavioral and NCRS-based classification of catatonia on both neuronal and biochemical grounds. That highlights the importance of Kahlbaum's original truly psychomotor concept of catatonia for guiding both research and clinical diagnosis and therapy.

Pharmacogenetic tests for antipsychotic medications: clinical implications and considerations

Optimizing antipsychotic pharmacotherapy is often challenging due to significant variability in effectiveness and tolerability. Genetic factors influencing pharmacokinetics and pharmacodynamics may contribute to some of this variability. Research studies have characterized these pharmacogenetic relationships, and some genetic markers are now available as clinical tests. These advances in pharmacogenetics research and test availability have great potential to improve clinical outcomes and quality of life in psychiatric patients. For clinicians considering using pharmacogenetics, it is important to understand the clinical implications and also the limitations of markers included in currently available tests. This review focuses on pharmacokinetic and pharmacodynamic gene variants that are currently available in commercial genetic testing panels. Associations of these variants with clinical efficacy and adverse effects, as well as other clinical implications, in antipsychotic pharmacotherapy are discussed.

Pharmacogenetics of antipsychotic treatment in schizophrenia

Antipsychotics are the mainstay treatment for schizophrenia. There is large variability between individuals in their response to antipsychotics, both in efficacy and adverse effects of treatment. While the source of interindividual variability in antipsychotic response is not completely understood, genetics is a major contributing factor. The identification of pharmacogenetic markers that predict antipsychotic efficacy and adverse reactions is a growing area of research, and holds the potential to replace the current trial-and-error approach to treatment selection in schizophrenia with a personalized medicine approach.In this chapter, we provide an overview of the current state of pharmacogenetics in schizophrenia treatment. The most promising pharmacogenetic findings are presented for both antipsychotic response and commonly studied adverse reactions. The application of pharmacogenetics to schizophrenia treatment is discussed, with an emphasis on the clinical utility of pharmacogenetic testing and directions for future research.

Dynamic expression of tyrosine hydroxylase mRNA and protein in neurons of the striatum and amygdala of mice, and experimental evidence of their multiple embryonic origin

Emotional and motivational dysfunctions observed in Parkinson's disease, schizophrenia, and drug addiction are associated to an alteration of the mesocortical and mesolimbic dopaminergic pathways, which include axons projecting to the prefrontal cortex, the ventral striatum, and the amygdala. Subpopulations of catecholaminergic neurons have been described in the cortex and striatum of several mammals, but the presence of such cells in the adult amygdala is unclear in murine rodents, and in other rodents appears to show variations depending on the species. Moreover, the embryonic origin of telencephalic tyrosine hydroxylase (TH) cells is unknown, which is essential for trying to understand aspects of their evolution, distribution and function. Herein we investigated the expression of TH mRNA and protein in cells of the striatum and amygdala of developing and adult mice, and analyzed the embryonic origin of such cells using in vitro migration assays. Our results showed the presence of TH mRNA and protein expressing cells in the striatum (including nucleus accumbens), central and medial extended amygdala during development, which are persistent in adulthood although they are less numerous, generally show weak mRNA expression, and some appear to lack the protein. Fate mapping analysis showed that these cells include at least two subpopulations with different embryonic origin in either the commissural preoptic area of the subpallium or the supraopto-paraventricular domain of the alar hypothalamus. These data are important for future studies trying to understand the role of catecholamines in modulation of emotion, motivation, and reward.

Aberrant dopamine D2-like receptor function in a rodent model of schizophrenia

Based on the observation that antipsychotic medications display antagonist properties at dopamine D2-like receptors, aberrant dopamine signaling has been proposed to underlie psychosis in patients with schizophrenia. Thus, it is not surprising that considerable research has been devoted to understanding the mechanisms involved in the antipsychotic action of these compounds. It is important to note that the majority of these studies have been performed in "normal" experimental animals. Given that these animals do not possess the aberrant neuronal information processing typically associated with schizophrenia, the aim of the current study was to examine the dopamine D2 receptor system in a rodent model of schizophrenia. Here, we demonstrate that methylazoxymethanol acetate (MAM)-treated rats display an enhanced effect of quinpirole on dopamine neuron activity and an aberrant locomotor response to D2-like receptor activation, suggesting changes in postsynaptic D2-like receptor function. To better understand the mechanisms underlying the enhanced response to D2-like ligands in MAM-treated rats, we examined the expression of D2, D3, and dopamine transporter mRNA in the nucleus accumbens and ventral tegmental area by quantitative reverse transcription-polymerase chain reaction. MAM-treated rats displayed a significant increase in dopamine D3 receptor mRNA expression in the nucleus accumbens with no significant changes in the expression of the D2 receptor. Taken together, these data demonstrate robust alterations in dopamine D2-like receptor function in a rodent model of schizophrenia and provide evidence that preclinical studies examining the mechanisms of antipsychotic drug action should be performed in animal models that mirror aspects of the abnormal neuronal transmission thought to underlie symptoms of schizophrenia.

Effect of cyclosporin A on the uptake of D3-selective PET radiotracers in rat brain

INTRODUCTION

Four benzamide analogs having a high affinity and selectivity for D(3) versus D(2) receptors were radiolabeled with (11)C or (18)F for in vivo evaluation.

METHODS

Precursors were synthesized, and the four D(3) selective benzamide analogs were radiolabeled. The tissue distribution and brain uptake of the four compounds were evaluated in control rats and rats pretreated with cyclosporin A, a modulator of P-glycoprotein and an inhibitor of other ABC efflux transporters that contribute to the blood brain barrier. Micro-positron emission tomographic (PET) imaging was carried out for [(11)C]6 in a control and a cyclosporin A pretreated rat.

RESULTS

All four compounds showed low brain uptake in control rats at 5 and 30 min post-injection; despite recently reported rat behavioral studies conducted on analogs 6 (WC-10) and 7 (WC-44). Following administration of cyclosporin A, increased brain uptake was observed with all four PET radiotracers at both 5 and 30 min post-intravenous injection. An increase in brain uptake following modulation/inhibition of the ABC transporters was also observed in the microPET study.

CONCLUSIONS

These data suggest that D3 selective conformationally-flexible benzamide analogs which contain a N-2-methoxyphenylpiperazine moiety are substrates for P-glycoprotein or other adenosine triphosphate (ATP)-binding cassette transporters expressed at the blood-brain barrier, and that PET radiotracers containing this pharmacophore may display low brain uptake in rodents due to the action of these efflux transporters.

Effect of dopamine D3 receptor gene polymorphisms and clozapine treatment response: exploratory analysis of nine polymorphisms and meta-analysis of the Ser9Gly variant

D2 blockade has been implicated in having a central role in antipsychotic response. However, treatment refractoriness, in spite of complete D2 blockade, as well as the efficacy of clozapine (CLZ) in a portion of this patient population, indicates the involvement of other factors as well. Several lines of evidence suggest a role for D3. Furthermore, an earlier meta-analysis by Jönsson et al. (2003) (n=233) suggested a role for genetic variation in the D3 gene. Relevant to this study, Jönsson et al. found the Ser allele of the D3 serine-to-glycine substitution at amino acid position 9 (Ser9Gly) polymorphism to be associated with worse CLZ response compared with the Gly allele. In this study, we attempt to validate these findings by performing a meta-analysis in a much larger sample (n=758). Eight other variants were also tested in our own sample to explore the possible effect of other regions of the gene. We report a negative but consistent trend across individual studies in our meta-analysis for the DRD3 Ser allele and poor CLZ response. A possible minor role for this single-nucleotide polymorphism cannot be disregarded, as our sample size may have been insufficient. Other DRD3 variants and haplotypes of possible interest were also identified for replication in future studies.

Integrated signaling in heterodimers and receptor mosaics of different types of GPCRs of the forebrain: relevance for schizophrenia

Receptor-receptor interactions within receptor heterodimers and receptor mosaics formed by different types of GPCRs represent an important integrative mechanism for signaling in brain networks at the level of the plasma membrane. The malfunction of special heterodimers and receptor mosaics in the ventral striatum containing D(2) receptors and 5-HT(2A) receptors in cortical networks may contribute to disturbances of key pathways involving ventral striato-pallidal GABA neurons and mediodorsal thalamic prefrontal glutamate neurons that may lead to the development of schizophrenia. The ventral striatum transmits emotional information to the cerebral cortex through a D(2) regulated accumbal-ventral pallidal-mediodorsal-prefrontal circuit which is of special interest to schizophrenia in view of the reduced number of glutamate mediodorsal-prefrontal projections associated with this disease. This circuit is especially vulnerable to D(2) receptor activity in the nucleus accumbens, since it produces a reduction in the prefrontal glutamate drive from the mediodorsal nucleus. The following D(2) receptor containing heterodimers/receptor mosaics are of special interest to schizophrenia: A(2A)-D(2), mGluR5-D(2), CB(1)-D(2), NTS(1)-D(2) and D(2)-D(3) and are discussed in this review. They may have a differential distribution pattern in the local circuits of the ventral striato-pallidal GABA pathway, predominantly located extrasynaptically. Specifically, trimeric receptor mosaics consisting of A(2A)-D(2)-mGluR5 and CB(1)-D(2)-A(2A) may also exist in these local circuits and are discussed. The integration of receptor signaling within assembled heterodimers/receptor mosaics is brought about by agonists and allosteric modulators. These cause the intramembrane receptor-receptor interactions, via allosteric mechanisms, to produce conformational changes that pass over the receptor interfaces. Exogenous and endogenous cooperativity is discussed as well as the role of the cortical mGluR2-5-HT(2A) heterodimer/receptor mosaic in schizophrenia (Gonzalez-Maeso et al. 2008). Receptor-receptor interactions within receptor heterodimer/receptor mosaics of different receptors in the ventral striatum and cerebral cortex give novel strategies for treatment of schizophrenia involving, e.g., monotherapy with either A(2A), mGluR5, CB(1) or NTS(1) agonists or combined therapies with some of these agonists combined with D(2)-like antagonists that specifically target the ventral striatum. In addition, a combined targeting of receptor mosaics in the ventral striatum and in the cerebral cortex should also be considered.

Frequency of new-onset pathologic compulsive gambling or hypersexuality after drug treatment of idiopathic Parkinson disease

OBJECTIVE

To determine the frequency of new-onset compulsive gambling or hypersexuality among regional patients with Parkinson disease (PD), ascertaining the relationship of these behaviors to PD drug use.

PATIENTS AND METHODS

We retrospectively reviewed the medical records of patients from 7 rural southeastern Minnesota counties who had at least 1 neurology appointment for PD between July 1, 2004, and June 30, 2006. The main outcome measure was compulsive gambling or hypersexuality developing after parkinsonism onset, including the temporal relationship to PD drug use.

RESULTS

Of 267 patients with PD who met the study inclusion criteria, new-onset gambling or hypersexuality was documented in 7 (2.6%). All were among the 66 patients (10.6%) taking a dopamine agonist. Moreover, all 7 (18.4%) were among 38 patients taking therapeutic doses (defined as >/=2 mg of pramipexole or 6 mg of ropinirole daily). Behaviors were clearly pathologic and disabling in 5: 7.6% of all patients taking an agonist and 13.2% of those taking therapeutic doses. Of the 5 patients, 2 had extensive treatment for what was considered a primary psychiatric problem before the agonist connection was recognized.

CONCLUSION

Among the study patients with PD, new-onset compulsive gambling or hypersexuality was documented in 7 (18.4%) of 38 patients taking therapeutic doses of dopamine agonists but was not found among untreated patients, those taking subtherapeutic agonist doses, or those taking carbidopa/levodopa alone. Behaviors abated with discontinuation of agonist therapy or dose reduction. Because this is a retrospective study, cases may have been missed, and hence this study may reflect an underestimation of the true frequency. Physicians who care for patients taking these drugs should recognize the drug's potential to induce pathologic syndromes that sometimes masquerade as primary psychiatric disease.

The dopamine D2 receptors in high-affinity state and D3 receptors in schizophrenia: a clinical [11C]-(+)-PHNO PET study

The dopamine D(2) receptors exist in two states: a high-affinity state (D(2)(high)) that is linked to second messenger systems, is responsible for functional effects, and exhibits high affinity for agonists; and a low-affinity state that is functionally inert and exhibits lower affinity for agonists. The dopamine D(3) receptors have high-affinity for agonist (eg dopamine) and the existence of the two affinity states is controversial. Although preclinical studies in animal models of psychosis have shown a selective increase of D(2)(high) as the common pathway to psychosis, the D(3) has been suggested to be involved in the pathophysiology of psychosis. We report the first study of the D(2)(high) and D(3) in schizophrenia using the novel PET radiotracer, [(11)C]-(+)-PHNO. We recruited 13 patients with schizophrenia-spectrum disorder amidst an acute psychotic episode, drug free for at least 2 weeks, and 13 age-sex-matched healthy controls. The binding potential no-displaceable (BP(ND)) was examine in the main regions of interest (caudate, putamen, ventral striatum, globus pallidus, substantia nigra, and anterior thalamus) and in a voxel-wise analysis. The BP(ND) between patients and controls was not different in any of the regions. The voxel-wise analysis did not reveal any difference and no correlations were found between the BP(ND) and positive and negative syndrome scale subscales. Our results do not find support for the hypothesis linking psychosis to a selective increase in D(2)(high) and/or D(3) in schizophrenia. It is possible that receptors with high affinity are not accessible by [(11)C]-(+)-PHNO because they are occupied by endogenous dopamine, a possibility that can be ruled out in future experiments.

The dopamine D(3) receptor Ser9Gly polymorphism modulates prepulse inhibition of the acoustic startle reflex

BACKGROUND

The dopamine D(3) receptor (DRD(3)) is suspected to modulate prepulse inhibition (PPI) in animals and humans, but definite conclusions cannot be drawn due to lack of selective DRD(3) ligands. The Ser9Gly polymorphism is a common variant of the DRD(3) gene and determines the gain of function of the D(3) receptor. This is the first study to examine the influence of the DRD(3) Ser9Gly polymorphism on human PPI.

METHODS

Prepulse inhibition was measured in 101 healthy male subjects presented with 75-dB and 85-dB prepulses at 30-, 60-, and 120-msec prepulse-pulse intervals. Subjects were grouped according to their DRD(3) status into a Gly/Gly, a Ser/Gly, and a Ser/Ser group.

RESULTS

Analyses of variance showed that at all prepulse and interval conditions, Gly/Gly individuals had the lowest PPI and the greatest onset latency facilitation and Ser/Ser individuals had the highest PPI and the lowest onset latency facilitation, while Ser/Gly individuals were intermediate.

CONCLUSIONS

These results suggest that PPI is modulated by the D(3) receptor and its levels depend on the Ser9Gly polymorphism.

Basal ganglia shape abnormalities in the unaffected siblings of schizophrenia patients

BACKGROUND

Abnormalities of basal ganglia structure in schizophrenia have been attributed to the effects of antipsychotic drugs. Our aim was to test the hypothesis that abnormalities of basal ganglia structure are intrinsic features of schizophrenia by assessing basal ganglia volume and shape in the unaffected siblings of schizophrenia subjects.

METHOD

The study involved 25 pairs of schizophrenia subjects and their unaffected siblings and 40 pairs of healthy control subjects and their siblings. Large-deformation, high-dimensional brain mapping was used to obtain surface representations of the caudate, putamen, and globus pallidus. Surfaces were derived from transformations of anatomic templates, and shapes were analyzed using reduced-dimensional measures of surface variability (i.e., principal components and canonical analysis). Canonical functions were derived using schizophrenia and control groups and were then used to compare shapes in the sibling groups. To visualize shape differences, maps of the estimated surface displacement between groups were created.

RESULTS

In the caudate, putamen, and globus pallidus, the degree of shape abnormality observed in the siblings of the schizophrenia subjects was intermediate between the schizophrenia and control subjects. In the schizophrenia subjects, significant correlations were observed between measures of caudate, putamen, and globus pallidus structure and the selected measures of lifetime psychopathology.

CONCLUSIONS

Attenuated abnormalities of basal ganglia structure are present in the unaffected siblings of schizophrenia subjects. This finding implies that basal ganglia structural abnormalities observed in subjects with schizophrenia are at least in part an intrinsic feature of the illness.

The ventral striatum as an interface between the limbic and motor systems

Over the next 2 years,CNS Spectrumswill be publishing a series of articles on neuroanatomy. The purpose of these articles is to broaden knowledge and interest in neuroanatomy, with a special reference to some key brain structures that are important for neuropsychiatry. Interest in nuclear structures and hodology, in connectivity and circuitry between brain regions, and in neurochemical associations has increased in the last 3 decades due to new neuroanatomical staining methods, brain imaging, and new treatments, such as deep brain stimulation.These columns will enliven an understanding of the clinical neuroscience interface but also provide a solid framework of contemporary neuroanatomy for psychiatrists and neurologists.

The first in the series reviews the ventral striatum. Henk J. Groenewegen, MD, PhD, in a column dedicated to the late Lennart Heimer, MD, reveals the importance of this structure and its connectivity for a contemporary understanding of brain-behavior relationships. In earlier conceptions, the basal ganglia were solely related to motor function, uninvolved with emotion or cognition. This conception arose from a misunderstanding of basic neuroanatomy, which has been unravelled by careful neuroanatomical studies in the last 30 years with new tissue staining and tracing techniques.The basal ganglia are the main target structures of the limbic system, hence the motion in emotion.

Disruption of prepulse inhibition of the startle reflex by the preferential D(3) agonist ropinirole in healthy males

RATIONALE

Emerging evidence from agonist-antagonist studies suggests a role for the dopamine D(3) receptor subtype in the regulation of PPI in animals, but such evidence is lacking for human subjects.

OBJECTIVES

This study examines the effect of the preferential D(3) agonist ropinirole on PPI in humans.

METHODS

PPI was tested in 12 healthy men in three sessions associated with ropinirole 0.25 mg, ropinirole 0.5 mg, or placebo according to a balanced, crossover, double-blind design. Two prepulses (75- and 85-dB white noise bursts) and two lead intervals (50 and 80 ms) were employed.

RESULTS

Ropinirole 0.5 mg significantly reduced prepulse inhibition (PPI) with both prepulses at the 80-ms lead intervals. There was no effect of treatment on startle amplitude and habituation.

CONCLUSIONS

These results suggest a role for the dopamine D(3) receptor in the mediation of human PPI, although a contribution from ropinirole's agonistic activity at the D(2) receptor cannot be entirely excluded. Firm conclusions on the role of the D(3) receptor in the modulation of human PPI can only be drawn with the use of genetic approaches or more selective ligands for this receptor.

Selective dopamine D3 receptor antagonists enhance cortical acetylcholine levels measured with high-performance liquid chromatography/tandem mass spectrometry without anti-cholinesterases

The present study compared the effects of two selective dopamine (DA) D(3) receptor antagonists, SB-277011A (3, 10 and 30 mg/kg i.p.) and SB-414796A (3, 10 and 30 mg/kg i.p.) on extracellular levels of acetylcholine (ACh) in the rat medial prefrontal cortex (mPFC) by using a LC/MS-MS analytical method that permitted the detection of ACh without the necessity of adding acetylcholinesterase inhibitors to the perfusate. Furthermore, the present LC/MS-MS method permitted the simultaneous measurement of the respective concentrations of SB-277011A and SB-414796A in the same extracellular samples from the mPFC. The systemic administration of both selective DA D(3) receptor antagonists produced a significant increase in extracellular levels of Ach compared to vehicle-treated animals, which was associated with increases in extracellular concentrations of SB-277011A and SB-414796. Overall, the present findings further strengthen the likelihood of a modulation of cortical cholinergic function through a DA D(3)-mediated mechanism and suggest that selective DA D(3) receptor antagonism may be beneficial in the treatment of psychiatric diseases, such as schizophrenia, which are characterized by cognitive dysfunction.

Computational elucidation of the structural basis of ligand binding to the dopamine 3 receptor through docking and homology modeling

The dopamine subtype 3 receptor (D3) is a promising therapeutic target for the treatment of cocaine addiction, schizophrenia, Parkinson's disease, and other disorders, but little is known about the binding of ligands to D3 at the atomic level. In the present study, binding of 29 known ligands to the D3 receptor was modeled computationally using four D3 receptor models which were obtained from homology modeling. The predicted binding models were validated with experimental data from site-directed mutagenesis, structure-activity relationship studies, and affinity labeling studies. Docking scores calculated for these 29 ligands correlate reasonably well with the experimentally determined binding affinities. A pharmacophore model is proposed that describes the binding of ligands at a single D3 receptor binding site and offers insights into the binding of structurally diverse D3 ligands to this receptor.

Cerebral D2 and 5-HT2 receptor occupancy in Schizophrenic patients treated with olanzapine or clozapine

We report the results of a double-blind, randomized prospective trial on D2 and 5-HT2 receptor occupancy and the clinical effects of olanzapine versus clozapine in a sample of neuroleptic-refractory schizophrenic patients. Receptor occupancy was evaluated in different cortical areas and in basal ganglia using [18F] fluoro-ethyl-spiperone ([18F] FESP) and positron emission tomography (PET). A total of 15 neuroleptic-free patients completed the study undergoing a baseline and a post-treatment PET scan (olanzapine, nine patients, one female; clozapine, six patients, three female) 8 weeks after starting treatment. PET data were analysed both by regions of interest and on a voxel-by-voxel basis using Statistical Parametric Mapping (SPM96). Olanzapine and clozapine induced a similar and significant inhibition of [18F] FESP binding index in the cortex. In the basal ganglia, receptor occupancy was significantly higher with olanzapine than with clozapine (p=0.0018). By contrast, no differences in receptor occupancy were detected at the level of the pituitary gland. Clinical outcomes, in particular a full extra pyramidal tolerability, were similar. In this sample of neuroleptic-refractory schizophrenic patients, olanzapine and clozapine showed a different pattern of occupancy of D2-like receptor despite a common lack of extrapyramidal side-effects.

D3 dopamine receptor mRNA expression in lymphocytes: a peripheral marker for schizophrenia?

BACKGROUND

Identification of schizophrenia, a common neuropsychiatric disorder, is based on clinical examination. An easily measurable peripheral marker, which may enable a more rapid and more accurate diagnosis, is not available. A possible candidate is the D3 dopamine receptor on lymphocytes.

OBJECTIVE

The D3 receptor is investigated for its clinical significance as a marker for diagnosing schizophrenia.

METHODS

From eight schizophrenic patients and eight controls lymphocyte RNA was isolated. A semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) was carried out and the intensities of the specific D3 dopamine receptor bands of patients and controls were compared.

RESULTS

No difference could be seen between the intensities of the bands from patients and controls.

CONCLUSION

An aberrant D3 dopamine receptor mRNA expression in lymphocytes of schizophrenics could not be demonstrated. This might be caused by down-regulation of D3 receptor production by antipsychotic drug treatment. At present, the D3 receptor seems to have no diagnostic value in schizophrenia.

Preferential alterations in the mesolimbic dopamine pathway of heterozygous reeler mice: an emerging animal-based model of schizophrenia

Based on a number of neuroanatomical and behavioural similarities, recent evidence suggests that heterozygous reeler mice, haploinsufficient for reelin expression, represent a useful model of psychosis vulnerability. As brain mesolimbic dopamine pathways have been proposed to be associated with the pathophysiology of psychotic disorders, we thought it would be of interest to examine whether these animals present disturbances in the mesolimbic dopamine system. To this end we studied by immunocytochemical, in situ hybridization procedures and receptor autoradiography, several markers of the mesotelencephalic dopamine pathway in heterozygous reeler mice and controls. We report that heterozygous reeler mice exhibit a reduction in the number of tyrosine hydroxylase-immunoreactive cell bodies and tyrosine hydroxylase mRNA levels in the ventral tegmental area, as well as a reduction of tyrosine hydroxylase and dopamine transporter immunoreactivity in the dopamine terminal fields of the limbic striatum. In these areas we also observed a reduction of dopamine D2 receptor mRNA. Finally, a marked increase in D3 receptor mRNA levels was observed concomitant with a significant increase in D3 binding sites. On the contrary, the nigrostriatal pathway did not show any significant alteration in heterozygous reeler mice with regards to the dopaminergic markers examined in substantia nigra cell bodies and dorsal striatum dopamine terminal fields. These results suggest a specific link between reelin-related neuronal pathology and dopamine involvement in the pathophysiology of psychotic disorders.

Neuropsychiatry of the basal ganglia

This review aims to relate recent findings describing the role and neural connectivity of the basal ganglia to the clinical neuropsychiatry of basal ganglia movement disorders and to the role of basal ganglia disturbances in "psychiatric"' states. Articles relating to the relevant topics were initially collected through MEDLINE and papers relating to the clinical conditions discussed were also reviewed. The anatomy and connections of the basal ganglia indicate that these structures are important links between parts of the brain that have classically been considered to be related to emotional functioning and brain regions previously considered to have largely motor functions. The basal ganglia have a role in the development and integration of psychomotor behaviours, involving motor functions, memory and attentional mechanisms, and reward processes.

Substance abuse and suicidality in schizophrenia: a common risk factor linked to impulsivity

Lifetime substance abuse comorbidity is frequent in schizophrenic patients, but the clinical correlates remain unclear. We have explored the chronological relations between substance abuse and course of schizophrenia, and compared several clinical characteristics and personality dimensions in 50 schizophrenic patients with or without lifetime substance abuse or dependence. Abuse occurred mainly after the first prodromal symptoms and just before the first psychotic episode. Substance-abusing patients were not different from non-substance-abusing patients on the Chapman Physical Anhedonia Scale, PANSS total score, negative subscore or depression item, CGI, treatment response and demographic variables. In contrast, substance-abusing patients had higher scores on the Barratt Impulsivity Scale (total, cognitive and non-planning scores) and had attempted suicide more often. In patients with schizophrenia, as in the general population, substance abuse or dependence appears associated with higher impulsivity and suicidality. High impulsivity could facilitate substance abuse as a maladaptive behavior in response to prodromal symptoms, precipitating the onset of a characterized psychosis.

D2 but not D3 receptors are elevated after 9 or 11 months chronic haloperidol treatment: influence of withdrawal period

Previous postmortem studies have identified divergent alterations in D2 and D3 receptors in schizophrenia but those results cannot be interpreted without further understanding of whether antipsychotic regulation of the D3 receptor is different from that of the D2 receptor. Depot parenteral administration of haloperidol decanoate was utilized to achieve consistent high levels in rat brain for 9 months with 2-month withdrawal or 11 months with 48-h withdrawal and compared to vehicle control and acute haloperidol (48-h) treatment groups. Autoradiographic means for measuring levels of D2 ([(3)H]-spiperone) and D3 receptors ([(125)I]trans 7-OH-PIPAT) and of D3 mRNA by in situ hybridization histochemistry in rat caudate-putamen, nucleus accumbens, islands of Calleja, and olfactory tubercle determined that there were significant group differences for regulation of D2 receptor. Chronic haloperidol for 9 or 11 months elevated D2 but not D3 receptors or D3 mRNA in all regions measured. Acute haloperidol treatment had no significant effects for any measure. Treatment for 9 months with a 2-month withdrawal resulted in a persistent increase in D2 receptors that was greater than that observed in the 11 months with 48-h withdrawal. This effect was most noticeable in the olfactory tubercle. These data confirm previous findings that short- or long-term haloperidol treatment leads to elevations in D2 but not D3 receptors or D3 mRNA, and long-term withdrawal from chronic haloperidol does not lead to elevations in D3 receptors or D3 mRNA. This suggests that an elevation in D3 receptors identified at postmortem in schizophrenics withdrawn from antipsychotics is not the result of the previous drug history [Gurevich et al. (1997) Arch Gen Psychiatry 54:225-232].

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinson's disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

Cortical involvement in acquisition and extinction of trace eyeblink conditioning

Previous studies have implicated 2 cortical regions interconnected with the hippocampal formation, the retrosplenial cortex (RSC) and the medial prefrontal cortex (mPFC), as loci important for the acquisition of hippocampally dependent trace eyeblink conditioning. These loci have also been proposed to serve as long-term storage sites of task critical information. This study used lesions made prior to training to investigate the roles of the RSC, as well as the caudal and rostral subdivisions of the mPFC, in the acquisition and subsequent extinction of trace eyeblink conditioning in the rabbit. The caudal mPFC and rostral mPFC were shown to be critical for acquisition and extinction of the conditioned reflex, respectively. The data indicate that the RSC is not critical for acquisition or extinction of the trace conditioned reflex.

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

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