The dopamine D3 receptor and schizophrenia: pharmacological, anatomical and genetic approaches

Reduced mammillary body volume in individuals with a schizophrenia diagnosis: an analysis of the COBRE data set

While the frontal cortices and medial temporal lobe are well associated with schizophrenia, the involvement of wider limbic areas is less clear. The mammillary bodies are important for both complex memory formation and anxiety and are implicated in several neurological disorders that present with memory impairments. However, little is known about their role in schizophrenia. Post-mortem studies have reported a loss of neurons in the mammillary bodies but there are also reports of increased mammillary body volume. The findings from in vivo MRI studies have also been mixed, but studies have typically only involved small sample sizes. To address this, we acquired mammillary body volumes from the open-source COBRE dataset, where we were able to manually measure the mammillary bodies in 72 individuals with a schizophrenia diagnosis and 74 controls. Participant age ranged from 18 to 65. We found the mammillary bodies to be smaller in the patient group, across both hemispheres, after accounting for the effects of total brain volume and gender. Hippocampal volumes, but not subiculum or total grey matter volumes, were also significantly lower in patients. Given the importance of the mammillary bodies for both memory and anxiety, this atrophy could contribute to the symptomology in schizophrenia.

Blockade of dopamine D3 receptor in ventral tegmental area attenuating contextual fear memory

The abnormal fear memory will lead to the onset of stress disorders, such as post-traumatic stress disorder (PTSD) and so on. Therefore, the intervention in the formation of abnormal fear memory will provide a new strategy for the prevention and treatment of PTSD. In our previous studies, we found that blockade of dopamine D3 receptor (DRD3) with highly selective antagonist YQA14 or knockout of DRD3 was able to attenuate the expression or retrieval of fear memory in PTSD animal models. However, the neurobiological mechanism of regulation of DRD3 in fear is unclear. In the present research, we clarified that DRD3 was expressed in the dopaminergic (DAergic) neurons in the ventral tegmental area (VTA). Then, we identified that microinjection of YQA14 (1 μg/0.2 μl/side) in VTA before the aversive stimuli in the training session or during days subsequent to the shock significantly meliorated the freezing behaviors in the inescapable electric foot-shock model. At last, using fiber photometry system, we found that microinjection of YQA14 in VTA promoted the dopamine neurotransmitter release in the basolateral amygdala (BLA), and pre-training YQA14 infusion in VTA lowered the increase of dopamine (DA) in BLA induced by shock during the training session or by context during the retrieval session. All above the results demonstrated that YQA14 attenuated the fear learning through the blockade of DRD3 in VTA decreasing the excitability of the projection to BLA. This study may provide new mechanisms and potential intervention targets for stress disorders with abnormal fear memory.

Current concepts in treating mild cognitive impairment in Parkinson's disease

Impairment in various aspects of cognition is recognized as an important non-motor symptom of Parkinson's disease (PD). Mild cognitive impairment in PD (PD-MCI) is common in non-demented PD patients and is often associated with severity of motor symptoms, disease duration and increasing age. Further, PD-MCI can have a significant negative effect on performance of daily life activities and may be a harbinger of development of PD dementia. Thus, there is significant interest in developing therapeutic strategies to ameliorate cognitive deficits in PD and improve cognitive functioning of PD patients. However, due to significant questions that remain regarding the pathophysiology of cognitive dysfunction in PD, remediation of cognitive dysfunction in PD has proven difficult. In this paper, we will focus on PD-MCI and will review some of the current therapeutic approaches being taken to try to improve cognitive functioning in patients with PD-MCI.

Brain Derived Neurotrophic Factor and Cognitive Dysfunction in the Schizophrenia-Bipolar Spectrum: A Systematic Review and Meta-Analysis

BACKGROUND

Cognitive impairment is a core feature of disorders on the schizophrenia-bipolar spectrum, i.e., schizophrenia, bipolar disorder, and schizoaffective disorder. Brain-derived neurotrophic factor (BDNF) has been proposed to be a biomarker of cognitive impairment in these disorders as it plays a critical role in neuroplasticity and proposed to mediate some of the psychotropic effects of medication. However, despite numerous studies investigating the association between circulating BDNF and these disorders, no solid conclusions have been drawn regarding its involvement in cognitive impairment.

OBJECTIVES

The current systematic review and meta-analysis aims to examine blood BDNF levels and cognitive dysfunction in patients on the schizophrenia-bipolar spectrum as well as to evaluate whether circulating BDNF measurements can act as a biomarker for cognitive dysfunction.

METHODS

Studies were identified by searching Embase and Medline databases for English language articles published in peer-reviewed journals between 2000 January and 2021 June according to the PRISMA guidelines. A total of 815 articles were identified of which 32 met the inclusion criteria for the systematic review - reporting on comparisons between blood BDNF levels and cognitive functions of schizophrenia or bipolar disorder patients versus healthy controls (no studies involving schizoaffective patients were specifically obtained for the time being). Twenty-four of these studies (19 with schizophrenia and 5 with bipolar disorder patients) were eligible to be included in the meta-analysis.

RESULTS

Our findings indicated that circulating BDNF levels were significantly reduced in patients experiencing an acute episode of schizophrenia or bipolar disorder compared to healthy controls. Cognitive function was also found to be significantly worse in patients, however, correlations between BDNF levels and cognitive impairment were not always detected. Interventions, especially pharmacotherapy seemed to improve certain aspects of cognition and increase circulating BDNF levels.

CONCLUSION

Circulating BDNF alone does not seem to be a valid biomarker of cognitive dysfunction in patients with disorders on the schizophrenia-bipolar spectrum, owing to several confounding factors. Changes of the circulating levels of BDNF should be evaluated in a wider context of other stress-, immune-, and inflammatory-related factors.

Antipsychotics: Mechanisms underlying clinical response and side-effects and novel treatment approaches based on pathophysiology

Antipsychotic drugs are central to the treatment of schizophrenia and other psychotic disorders but are ineffective for some patients and associated with side-effects and nonadherence in others. We review the in vitro, pre-clinical, clinical and molecular imaging evidence on the mode of action of antipsychotics and their side-effects. This identifies the key role of striatal dopamine D2 receptor blockade for clinical response, but also for endocrine and motor side-effects, indicating a therapeutic window for D2 blockade. We consider how partial D2/3 receptor agonists fit within this framework, and the role of off-target effects of antipsychotics, particularly at serotonergic, histaminergic, cholinergic, and adrenergic receptors for efficacy and side-effects such as weight gain, sedation and dysphoria. We review the neurobiology of schizophrenia relevant to the mode of action of antipsychotics, and for the identification of new treatment targets. This shows elevated striatal dopamine synthesis and release capacity in dorsal regions of the striatum underlies the positive symptoms of psychosis and suggests reduced dopamine release in cortical regions contributes to cognitive and negative symptoms. Current drugs act downstream of the major dopamine abnormalities in schizophrenia, and potentially worsen cortical dopamine function. We consider new approaches including targeting dopamine synthesis and storage, autoreceptors, and trace amine receptors, and the cannabinoid, muscarinic, GABAergic and glutamatergic regulation of dopamine neurons, as well as post-synaptic modulation through phosphodiesterase inhibitors. Finally, we consider treatments for cognitive and negative symptoms such dopamine agonists, nicotinic agents and AMPA modulators before discussing immunological approaches which may be disease modifying. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Early Antipsychotic Treatment in Juvenile Rats Elicits Long-Term Alterations to the Dopamine Neurotransmitter System

Prescription of antipsychotic drugs (APDs) to children has substantially increased in recent years. Whilst current investigations into potential long-term effects have uncovered some alterations to adult behaviours, further investigations into potential changes to neurotransmitter systems are required. The current study investigated potential long-term changes to the adult dopamine (DA) system following aripiprazole, olanzapine and risperidone treatment in female and male juvenile rats. Levels of tyrosine hydroxylase (TH), phosphorylated-TH (p-TH), dopamine active transporter (DAT), and D₁ and D₂ receptors were measured via Western blot and/or receptor autoradiography. Aripiprazole decreased TH and D₁ receptor levels in the ventral tegmental area (VTA) and p-TH levels in the prefrontal cortex (PFC) of females, whilst TH levels decreased in the PFC of males. Olanzapine decreased PFC p-TH levels and increased D₂ receptor expression in the PFC and nucleus accumbens (NAc) in females only. Additionally, risperidone treatment increased D₁ receptor levels in the hippocampus of females, whilst, in males, p-TH levels increased in the PFC and hippocampus, D₁ receptor expression decreased in the NAc, and DAT levels decreased in the caudate putamen (CPu), and elevated in the VTA. These results suggest that early treatment with various APDs can cause different long-term alterations in the adult brain, across both treatment groups and genders.

The dopamine dysfunction in schizophrenia revisited: new insights into topography and course

Schizophrenia has long been associated with an imbalance in dopamine (DA) neurotransmission, and brain imaging has played an important role in advancing our knowledge and providing evidence for the dopaminergic abnormalities. This chapter reviews the evidence for DA dysfunction in different brain regions in schizophrenia, in particular striatal, extrastriatal, and prefrontal regions, with emphasis on recently published findings. As opposed to the traditional view that most striatal dopaminergic excess, associated with the positive symptoms of schizophrenia, involves the dopaminergic mesolimbic pathway, recent evidence points to the nigrostriatal pathway as the area of highest dysregulation. Furthermore, evidence from translational research suggests that dopaminergic excess may be present in the prodromal phase, and may by itself, as suggested by the phenotype observed in transgenic mice with developmental overexpression of dorso-striatal D(2) receptors, be an early pathogenic condition, leading to irreversible cortical dysfunction.

A double-blind, randomized, placebo-controlled study of the dopamine D₃ receptor antagonist ABT-925 in patients with acute schizophrenia

There is substantial preclinical and clinical evidence to suggest a potential role for the dopamine D₃ receptor in the treatment of schizophrenia. ABT-925 is a selective dopamine D₃ receptor antagonist with an approximately 100-fold higher in vitro affinity for dopamine D₃ versus D₂ receptors. This double-blind, randomized, placebo-controlled, escalating-dose, parallel-group study assessed the efficacy and safety of ABT-925 in the treatment of patients with acute exacerbation of schizophrenia. One hundred fifty-five patients were assessed over a 6-week double-blind treatment period (placebo: n = 48; ABT-925 50 mg once daily [QD]: n = 53; ABT-925 150 mg QD: n = 54). The primary efficacy measure was mean change from baseline to final evaluation on the Positive and Negative Syndrome Scale total score. Secondary measures of efficacy and pharmacokinetic parameters were also assessed. Safety assessments included adverse event monitoring, laboratory tests, vital signs, movement rating scales, and electrocardiogram measures. No statistically significant treatment effect was observed with ABT-925 50 mg QD or 150 mg QD compared with placebo on primary or secondary efficacy end points. Pharmacokinetic parameter estimates increased with dose in a linear fashion. ABT-925 50 mg QD and 150 mg QD were generally well tolerated, with adverse event profiles similar to that of placebo. Findings from a concurrent positron emission tomography study among healthy volunteers suggest that the ABT-925 doses used in this study may not have been sufficient to adequately occupy D₃ receptors, thereby underscoring the importance of pharmacodynamic markers, such as PET, in determining appropriate compound doses before embarking on studies in a target population.

Dissociable control of impulsivity in rats by dopamine d2/3 receptors in the core and shell subregions of the nucleus accumbens

Previous research has identified the nucleus accumbens (NAcb) as an important brain region underlying inter-individual variation in impulsive behavior. Such variation has been linked to decreased dopamine (DA) D2/3 receptor availability in the ventral striatum of rats exhibiting spontaneously high levels of impulsivity on a 5-choice serial reaction time (5-CSRT) test of sustained visual attention. This study investigated the involvement of DA D2/3 receptors in the NAcb core (NAcbC) and the NAcb shell (NAcbS) in impulsivity. We investigated the effects of a DA D2/3 receptor antagonist (nafadotride) and a DA D2/3 partial agonist (aripiprazole) infused directly into either the NAcbC or NAcbS of rats selected for high (HI) and low (LI) impulsivity on the 5-CSRT task. Nafadotride increased significantly the level of impulsivity when infused into the NAcbS, but decreased impulsivity when infused into the NAcbC of HI rats. By contrast, intra-NAcb microinfusions of aripiprazole did not affect impulsivity. Systemic administration of nafadotride had no effect on impulsive behavior but increased the number of omissions and correct response latencies, whereas systemic injections of aripiprazole decreased impulsive and perseverative behavior, and increased the number of omissions and correct response latencies. These findings indicate an opponent modulation of impulsive behavior by DA D2/3 receptors in the NAcbS and NAcbC. Such divergent roles may have relevance for the etiology and treatment of clinical disorders of behavioral control, including attention-deficit hyperactivity disorder and drug addiction.

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.

Hybrid approach for the design of highly affine and selective dopamine D3 receptor ligands using privileged scaffolds of biogenic amine GPCR ligands

A series of compounds containing privileged scaffolds of the known histamine H(1) receptor antagonists cetirizine, mianserin, ketotifen, loratadine, and bamipine were synthesized for further optimization as ligands for the related biogenic amine binding dopamine D(3) receptor. A pharmacological screening was carried out at dopamine D(2) and D(3) receptors. In the preliminary testing various ligands have shown moderate to high affinities for dopamine D(3)receptors, for example, N-(4-{4-[benzyl(phenyl)amino]piperidin-1-yl}butylnaphthalen-2-carboxamide (19a) (hD(3)K(i)=0.3 nM; hD(2)K(i)=703 nM), leading to a selectivity ratio of 2343.

No association between the Ser9Gly polymorphism of the dopamine D3 receptor gene and schizophrenia in a Spanish sample

This study aims to further evaluate the controversial association between the Ser9Gly polymorphism in codon 9 of the D3 dopamine receptor gene (DRD3) and schizophrenia in psychiatric inpatients acutely hospitalized in two general hospitals in Madrid, Spain. The Ser9Gly polymorphism of the DRD3 was examined in 178 schizophrenic patients, 286 patients with other psychiatric diagnoses, and 132 controls recruited. Genotype frequencies were in Hardy-Weinberg equilibrium. No association was found between schizophrenia and the Ser9Gly polymorphism of the D3 dopamine receptor gene.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

Lack of functional D2 receptors prevents the effects of the D3-preferring agonist (+)-PD 128907 on dialysate dopamine levels

Substantial pharmacological evidence is consistent with an inhibitory effect of D3 receptor activation on dopamine (DA) release. Although receptor selectivity of the ligands employed in initial studies has been questioned, studies employing new, more selective, compounds continue to support an involvement of this receptor subtype in regulating extracellular dopamine levels in the dorsal striatum and nucleus accumbens. Consistent with this hypothesis, microdialysis studies have shown that the dose-effect curve for (+)-PD 128907, a moderately selective D3 agonist, is shifted to the right in D3 knock out mice. The present microdialysis studies sought to further examine the role of D2 vs. D3 receptors in mediating (+)-PD 128907-evoked alterations in basal and depolarization-evoked DA levels. Dialysate DA levels were determined in D2 knock out mice and wild type littermate controls following both systemic and local administration of (+)-PD 128907. In view of regional differences in D3 receptor localization, studies were conducted in the nucleus accumbens, a D3 receptor rich area, and in the dorsal striatum, a region with low D3 receptor abundance. Systemic or reverse dialysis of (+)-PD 128907 into the nucleus accumbens significantly decreased basal and depolarization-evoked DA levels in wild type mice. A similar effect was observed in the dorsal striatum. Regardless of the route of administration, (+)-PD 128907 was ineffective in modulating DA levels in either brain region of D2 knock out mice. These data contrast with previous results in D3 knock out mice and indicate that the D2 receptor is necessary for the inhibition of presynaptic DA neurotransmission produced by a preferential D3 agonist. Based on the documented physical interaction of D2 and D3 receptors in heterologous expression systems, we put forth a hypothesis that reconciles the seemingly paradoxical results of this and previous microdialysis studies.

Hippocampal and cortical sensory gating in rats: effects of quinpirole microinjections in nucleus accumbens core and shell

Sensory processing disturbances, as measured in the P50/sensory gating paradigm, have been linked to aberrant auditory information processing and sensory overload in schizophrenic patients. In this paradigm, the response to the second of paired-click stimuli is attenuated by an inhibitory effect of the first stimulus. Sensory gating has been observed in most healthy human subjects and normal laboratory rats. Because mesolimbic dopamine has been implicated in other filtering disturbances such as prepulse inhibition of the acoustic startle response and given the fact that amphetamine and apomorphine have been shown to disrupt gating, this study was performed to investigate the role of mesolimbic dopamine in sensory gating. The dopamine D2 receptor agonist quinpirole (10 microg/0.5 microl) was injected bilaterally in nucleus accumbens core and shell and effects on cortical and hippocampal sensory gating were investigated. Also, effects of the dopamine D2 receptor antagonist haloperidol (0.1 mg/kg, subcutaneously) as pretreatment were studied. First, quinpirole significantly reduced both the amplitude to the first click and gating as measured in the cortex and in the hippocampus. There was a tendency for the quinpirole effects on hippocampal gating to be more pronounced in rats injected in the shell. Secondly, haloperidol did not antagonize effects of quinpirole on hippocampal parameters, whereas haloperidol pretreatment fully antagonized quinpirole effects on cortical parameters. In conclusion, gating can be significantly reduced when a dopamine agonist is specifically targeted at mesolimbic dopamine D2 receptors. However, an important consideration is that the dopaminergic effects in the present study on gating are predominantly mediated by the effects on the amplitude to the first click. This has also been suggested for systemic amphetamine injections in rats and schizophrenic patients. This casts doubt on whether dopamine receptor activation affects the putative inhibitory process between the first and the second stimulus.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

Mutation and association analysis of the 5' region of the dopamine D3 receptor gene in schizophrenia patients: identification of the Ala38Thr polymorphism and suggested association between DRD3 haplotypes and schizophrenia

Although the association between the Ser9Gly polymorphism of the dopamine D3 receptor gene (DRD3) and schizophrenia has been investigated by many research groups, it is not known whether the Ser9Gly polymorphism alone or a variation in linkage disequilibrium may effect susceptibility to schizophrenia. We searched the 5' region of the DRD3 gene and found three novel polymorphisms: -712G/C, -205A/G, and Ala38Thr. The Ala38Thr polymorphism is located in the first transmembrane region and is conserved in the monkey, mouse, and rat. Case-control comparisons in 153 Japanese schizophrenia patients and 122 Japanese controls did not suggest an association between Ala38Thr and schizophrenia. However, there was a marginally significant association between the Ser9 allele of the Ser9Gly polymorphisms and schizophrenia (P = 0.02). Furthermore, there was a highly significant association between haplotypes of the -712G/C, -205A/G, and Ser9Gly polymorphisms and schizophrenia (P = 0.0007, corrected P = 0.007). These positive findings were replicated in an additional 99 Japanese schizophrenia patients and 132 controls (P = 0.04 and 0.0004, respectively). The most allelic differences of the Ser9Gly polymorphism between patient and control groups arose from the chromosome carrying specific alleles of the other three polymorphisms. This study indicates unknown variant(s) in linkage disequilibrium with the DRD3 haplotypes associated with schizophrenia.

Discriminative stimulus properties of antipsychotics

Drug discrimination methodology has been used in a number of ways to analyze the actions of novel and putative novel antipsychotics in vivo. Recent studies suggest (a) in contrast to earlier theorizing, antagonism of the low-dose d-amphetamine stimulus in rats may not be an effective screen for novel antipsychotics; (b) dopamine D2-like agonists and antagonists, some of which are putative antipsychotics, can be studied in vivo as discriminative cues, although there is a pressing need for more selective drugs that differentiate the various members of the D2 family. (c) antagonism of the cue induced by the noncompetitive NMDA antagonist MK-801, which has been proposed as a possible screen for clozapine-like compounds, may be an unreliable assay; and (d) the clozapine stimulus is probably a compound cue (a drug "mixture"), which can be used to screen for novel clozapine-like antipsychotics, although the precise receptor mechanisms involved in mediating the clozapine stimulus, and its direct relevance to the antipsychotic action of clozapine remains to be proven conclusively.

Atypical antipsychotic-like effects of the dopamine D3 receptor agonist, (+)-PD 128,907

Anti-schizophrenia agents with improved efficacy and side-effect profiles are required. A dopamine D3 receptor agonist, R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3- b]-1,4-oxazin-9-ol HCl ((+)-PD 128,907), displayed an atypical antipsychotic profile comparable to that of clozapine. (+)-PD 128,907 blocked stereotypy produced by dizocilpine (MK-801) at 12-fold lower doses than those affecting apomorphine-induced stereotypes in mice and did not produce catalepsy. These effects of (+)-PD 128,907 were stereospecific and were blocked by a D3 antagonist. These data suggest a role for D3 receptors in antipsychotic drug action.

Differential effect of haloperidol on release of neurotensin in extrapyramidal and limbic systems

The effect of the antipsychotic drug haloperidol on extracellular neurotensin-like immunoreactivity was investigated by microdialysis and compared with the time-dependent response of tissue neurotensin-like immunoreactivity content in brain structures containing dopamine nerve cell bodies and terminals. A single administration of haloperidol (1 mg/kg) increased the extracellular neurotensin-like immunoreactivity levels in nucleus accumbens as measured by microdialysis, but decreased its extracellular concentration in the caudate regions surrounding the probe. The same treatment increased the tissue content of neurotensin-like immunoreactivity in both the nucleus accumbens core and all caudate regions examined within 24 h after the injection. Interestingly, although the neurotensin-like immunoreactivity concentration in the substantia nigra was not altered by the haloperidol treatment, neurotensin-like immunoreactivity levels decreased significantly in the ventral tegmental area. These findings suggest that varied neurotensin systems are associated with nigrostriatal and mesolimbic dopamine pathways and these systems have different responses to haloperidol. The changes in the release of neurotensin may contribute to altered caudate and accumbens neurotensin-like immunoreactivity tissue content induced by haloperidol treatment, but other factors, such as variation in synthesis also likely influence these effects. Differential actions of haloperidol on neurotensin release might be due to regional differences in dopamine or sigma receptor subtypes associated with the neurotensin-containing neurons.

Assessment of association of D3 dopamine receptor MscI polymorphism with schizophrenia: analysis of symptom ratings, family history, age at onset, and movement disorders

Several studies have reported an association between schizophrenia and homozygosity for the MscI restriction site in exon 1 of the D3 dopamine receptor gene, but other studies have failed to find this association. Recent reports have suggested that the association is most salient in male patients with a family history of schizophrenia. We examined this restriction site in a group of schizophrenic patients (n = 84) and in normal controls (n = 77). Patients were subdivided according to demographic and clinical features, particular attention being paid to movement disorders. No significant difference in allelic or genotypic distribution was seen between the two groups. No association was seen between homozygosity and a positive family history, age at onset of illness, clinical subtype, negative symptom score, or movement disorder scores.

A search for association between schizophrenia and dopamine-related alleles

Dopamine receptor dysfunction and altered tyrosine hydroxylase activity have both been implicated in the pathophysiology of schizophrenia. Schizophrenic patients and control subjects were examined for allele frequencies in the tyrosine hydroxylase and dopamine D2 and D4 receptor genes. No significant differences of allele or genotype frequencies were found between the two groups after adjustment for multiple comparisons. Neither were any significant relationships observed between allele frequencies and a number of clinical variables within the schizophrenic subsample. When no adjustment was made for multiple testing a few significant tendencies were obtained which warrant further research in extended patient and control materials. The results are compatible with the view that the tyrosine hydroxylase, dopamine receptor D2 and D4 gene polymorphisms examined are not of major importance in the aetiology or pathophysiology of schizophrenia.