Brain-derived neurotrophic factor controls dopamine D3 receptor expression: implications for neurodevelopmental psychiatric disorders

Interaction of Brain-Derived Neurotrophic Factor Val66Met genotype and history of stress in regulation of prepulse inhibition in mice

The Brain-Derived Neurotrophic Factor (BDNF) Val66Met polymorphism results in reduced activity-dependent BDNF release and has been implicated in schizophrenia. However, effects of the polymorphism on functional dopaminergic and N-methyl-d-aspartate (NMDA) receptor-associated activity remain unclear. We used prepulse inhibition, a measure of sensorimotor gating which is disrupted in schizophrenia, and assessed the effects of acute treatment with the dopamine receptor agonist, apomorphine (APO), and the NMDA receptor antagonist, MK-801. We used adult humanized hBDNF^Val66Met 'knockin' mice which express either the Val/Val, Val/Met or Met/Met genotype. An interaction of BDNF with stress was modelled by chronic young-adult treatment with corticosterone (CORT). At 1 or 3mg/kg, APO had no effect in Val/Val mice but significantly reduced PPI at the 100ms inter-stimulus interval (ISI) in Val/Met and Met/Met mice. However, after CORT pretreatment, APO significantly reduced PPI in all genotypes similarly. At 0.1 or 0.25mg/kg, MK-801 significantly disrupted PPI at the 100ms ISI independent of genotype or CORT pretreatment. There were differential effects of APO and MK-801 on PPI at the 30ms ISI and startle between the genotypes, irrespective of CORT pretreatment. These results show that the BDNF Val66Met Val/Met and Met/Met genotypes are more sensitive than the Val/Val genotype to the effect of APO on PPI. A history of stress, here modelled by chronic CORT administration, increases effects of APO in Val/Val mice.

Long-term effects of iloperidone on cerebral dopamine receptor subtypes

The atypical antipsychotic drug iloperidone has high affinity for a wide range of neurotransmitter receptors, including dopaminergic (DA), serotonergic, and adrenergic receptors. We examined the long-term effects of multiple doses of iloperidone on DA D1 , D2 , D3 , and D4 receptor subtypes. Sprague-Dawley adult rats (n = 8/group) received daily intraperitoneal injections of iloperiodone (0.5, 1, or 5 mg/kg) or vehicle for 4 weeks. Receptor autoradiography quantified the levels of DA receptors in medial prefrontal cortex (MPC), dorsolateral frontal cortex (DFC), caudate putamen (CPu), nucleus accumbens (NAc), and hippocampus (HIP). Four weeks of iloperidone treatment at 5 mg/kg significantly increased D1 receptors in NAc (36%) and CPu (38%). Iloperidone (1.5 and 5 mg/kg) dose-dependently increased D2 receptors in MPC (37 and 47%) and HIP (32 and 40%). Only the high dose of iloperidone (5 mg/kg) increased D2 receptors in NAc (39%) and CPu (38%). Repeated treatment with iloperidone (1.5 and 5 mg/kg) increased D4 receptors in the NAc (39 and 78%), CPu (42 and 83%) and HIP (54 and 72%). The three doses of iloperidone failed to alter D3 receptors in the brain regions examined in this study. These results suggest that iloperidone exerts region- and dose-specific effects on forebrain DA receptor subtypes, which may contribute to its therapeutic benefits in improving the positive and negative symptoms of schizophrenia with minimal extrapyramidal side effects.

Selective α-Synuclein Knockdown in Monoamine Neurons by Intranasal Oligonucleotide Delivery: Potential Therapy for Parkinson's Disease

Progressive neuronal death in brainstem nuclei and widespread accumulation of α-synuclein are neuropathological hallmarks of Parkinson's disease (PD). Reduction of α-synuclein levels is therefore a potential therapy for PD. However, because α-synuclein is essential for neuronal development and function, α-synuclein elimination would dramatically impact brain function. We previously developed conjugated small interfering RNA (siRNA) sequences that selectively target serotonin (5-HT) or norepinephrine (NE) neurons after intranasal administration. Here, we used this strategy to conjugate inhibitory oligonucleotides, siRNA and antisense oligonucleotide (ASO), with the triple monoamine reuptake inhibitor indatraline (IND), to selectively reduce α-synuclein expression in the brainstem monoamine nuclei of mice after intranasal delivery. Following internalization of the conjugated oligonucleotides in monoamine neurons, reduced levels of endogenous α-synuclein mRNA and protein were found in substantia nigra pars compacta (SNc), ventral tegmental area (VTA), dorsal raphe nucleus (DR), and locus coeruleus (LC). α-Synuclein knockdown by ∼20%-40% did not cause monoaminergic neurodegeneration and enhanced forebrain dopamine (DA) and 5-HT release. Conversely, a modest human α-synuclein overexpression in DA neurons markedly reduced striatal DA release. These results indicate that α-synuclein negatively regulates monoamine neurotransmission and set the stage for the testing of non-viral inhibitory oligonucleotides as disease-modifying agents in α-synuclein models of PD.

Striatal changes underlie MPEP-mediated suppression of the acquisition and expression of pramipexole-induced place preference in an alpha-synuclein rat model of Parkinson's disease

Impulsive-compulsive disorders in Parkinson's disease patients have been described as behavioural or substance addictions including pathological gambling or compulsive medication use of dopamine replacement therapy. A substantial gap remains in the understanding of these disorders. We previously demonstrated that the rewarding effect of the D2/D3 agonist pramipexole was enhanced after repeated exposure to L-dopa and alpha-synuclein mediated dopaminergic nigral loss with specific transcriptional signatures suggesting a key involvement of the glutamatergic pathway. Here, we further investigate the therapeutic potential of metabotropic glutamate receptor 5 antagonism in Parkinson's disease/dopamine replacement therapy related bias of reward-mediated associative learning. We identified protein changes underlying the striatal remodelling associated with the pramipexole-induced conditioned place preference. Acquisition and expression of the pramipexole-induced conditioned place preference were abolished by the metabotropic glutamate receptor 5 antagonist 2-methyl-6-phenylethynyl (pyridine) (conditioned place preference scores obtained with pramipexole conditioning were reduced by 12.5% and 125.8% when 2-methyl-6-phenylethynyl (pyridine) was co-administrated with pramipexole or after the pramipexole conditioning, respectively). Up-regulation of the metabotropic glutamate receptor 5 was found in the dorsomedial-striatum and nucleus accumbens core. Activation of these two brain sub-regions was also highlighted through FosB immunohistochemistry. Convergent molecular and pharmacological data further suggests metabotropic glutamate receptor 5 as a promising therapeutic target for the management of Parkinson's disease/dopamine replacement therapy related reward bias.

The transfection of BDNF to dopamine neurons potentiates the effect of dopamine D3 receptor agonist recovering the striatal innervation, dendritic spines and motor behavior in an aged rat model of Parkinson's disease

The progressive degeneration of the dopamine neurons of the pars compacta of substantia nigra and the consequent loss of the dopamine innervation of the striatum leads to the impairment of motor behavior in Parkinson's disease. Accordingly, an efficient therapy of the disease should protect and regenerate the dopamine neurons of the substantia nigra and the dopamine innervation of the striatum. Nigral neurons express Brain Derived Neurotropic Factor (BDNF) and dopamine D3 receptors, both of which protect the dopamine neurons. The chronic activation of dopamine D3 receptors by their agonists, in addition, restores, in part, the dopamine innervation of the striatum. Here we explored whether the over-expression of BDNF by dopamine neurons potentiates the effect of the activation of D3 receptors restoring nigrostriatal innervation. Twelve-month old Wistar rats were unilaterally injected with 6-hydroxydopamine into the striatum. Five months later, rats were treated with the D3 agonist 7-hydroxy-N,N-di-n-propy1-2-aminotetralin (7-OH-DPAT) administered i.p. during 4½ months via osmotic pumps and the BDNF gene transfection into nigral cells using the neurotensin-polyplex nanovector (a non-viral transfection) that selectively transfect the dopamine neurons via the high-affinity neurotensin receptor expressed by these neurons. Two months after the withdrawal of 7-OH-DPAT when rats were aged (24 months old), immunohistochemistry assays were made. The over-expression of BDNF in rats receiving the D3 agonist normalized gait and motor coordination; in addition, it eliminated the muscle rigidity produced by the loss of dopamine. The recovery of motor behavior was associated with the recovery of the nigral neurons, the dopamine innervation of the striatum and of the number of dendritic spines of the striatal neurons. Thus, the over-expression of BDNF in dopamine neurons associated with the chronic activation of the D3 receptors appears to be a promising strategy for restoring dopamine neurons in Parkinson's disease.

Long-term effects of cariprazine exposure on dopamine receptor subtypes

INTRODUCTION

All clinically effective antipsychotics are known to act on the dopaminergic system, and previous studies have demonstrated that repeated treatment with antipsychotics produced region-specific changes in dopamine receptor levels. Cariprazine is a dopamine D₃ and D₂ receptor partial agonist with preferential binding to D₃ receptors. We examined the effects of chronic cariprazine administration on dopamine receptor levels.

METHODS

Rats were administered either vehicle or cariprazine (0.06, 0.2, or 0.6 mg/kg) for 28 days. Dopamine receptor levels were quantitated using autoradiographic assays on brain tissue sections from the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), caudate putamen (CPu), hippocampus (HIPP), olfactory tubercle (OT), and islands of Calleja (ICj).

RESULTS

Chronic treatment with cariprazine did not alter D₁ receptor levels in any brain region tested. Cariprazine increased D₂ receptor levels in mPFC (27%-43%), NAc (40%-45%), medial (41%-53%) and lateral (52%-63%) CPu, and HIPP (38%). Cariprazine dose-dependently upregulated D₃ receptor levels in ICj (32%-57%), OT (27%-67%), and NAc shell (31%-48%). Repeated cariprazine treatment increased D₄ receptor in NAc (53%-82%), medial (54%-98%) and lateral (58%-74%) CPu, and HIPP (38%-98%).

CONCLUSION

Similar to other antipsychotics, cariprazine upregulated D₂ and D₄ receptor levels in various brain regions. Cariprazine was unique among antipsychotics in increasing D₃ receptor levels, which may support its unique psychopharmacologic properties.

Constitutive activities and inverse agonism in dopamine receptors

The concept of activation in the absence of agonists has been demonstrated for many GPCRs and is now solidified as one of the principal aspects of GPCR signaling. In this chapter, we review how dopamine receptors demonstrate this ability. Although difficult to prove in vivo due to the presence of endogenous dopamine and lack of subtype-selective inverse agonists and "pure" antagonists (neutral ligands), in vitro assays such as measuring intracellular cAMP, [(35)S]GTPγS binding, and [(3)H]thymidine incorporation have uncovered the constitutive activation of D1- and D2-class receptors. Nevertheless, because of limited and inconsistent findings, the existence of constitutive activity for D2-class receptors is currently not well established. Mutagenesis studies have shown that basal signaling, notably by D1-class receptors, is governed by the collective contributions of transmembrane domains and extracellular/intracellular loops, such as the third extracellular loop, the third intracellular loop, and C-terminal tail. Furthermore, constitutive activities of D1-class receptors are subjected to regulation by kinases. Among the dopamine receptor family, the D5 receptor subtype exhibits a higher basal signaling and bears resemblance to constitutively active mutant forms of GPCRs. The presence of its constitutive activity in vivo and its pathophysiological relevance, with a brief mention of other subtypes, are also discussed.

Brain-derived neurotrophic factor as a potential biomarker of cognitive recovery in schizophrenia

Brain-derived neurotrophic factor (BDNF) has been proposed as a biomarker of schizophrenia and, more specifically, as a biomarker of cognitive recovery. Evidence collected in this review indicates that BDNF is relevant in the pathophysiology of schizophrenia and could play a role as a marker of clinical response. BDNF has been shown to play a positive role as a marker in antipsychotic treatment, and it has been demonstrated that typical antipsychotics decrease BDNF levels while atypical antipsychotics maintain or increase serum BDNF levels. Furthermore, BDNF levels have been associated with severe cognitive impairments in patients with schizophrenia. Consequently, BDNF has been proposed as a candidate target of strategies to aid the cognitive recovery process. There is some evidence suggesting that BDNF could be mediating neurobiological processes underlying cognitive recovery. Thus, serum BDNF levels seem to be involved in some synaptic plasticity and neurotransmission processes. Additionally, serum BDNF levels significantly increased in schizophrenia subjects after neuroplasticity-based cognitive training. If positive replications of those findings are published in the future then serum BDNF levels could be definitely postulated as a peripheral biomarker for the effects of intensive cognitive training or any sort of cognitive recovery in schizophrenia. All in all, the current consideration of BDNF as a biomarker of cognitive recovery in schizophrenia is promising but still premature.

BDNF Val66Met and spontaneous dyskinesias in non-clinical psychosis

BACKGROUND

Evidence indicating that symptoms of non-clinical psychosis (NCP) occur in 6-8% of the general population suggests that psychosis may occur across a continuum. Although a number of studies have examined environmental contributors, to date there have been few investigations of biological/genetic factors in this integral population. A recent study observed spontaneous dyskinetic movements (reflecting an innervated striatal system) in individuals reporting NCP. The present investigation is designed to replicate this finding and determine if brain-derived neurotrophic factor (BDNF) (implicated in striatal dopamine function) is associated with dyskinesias.

METHOD

A total of 68 young-adult participants reporting High and Low-NCP were assessed for dyskinetic movements using a sensitive instrumental measure of force variability. Saliva from the participants was genotyped for val66met (rs6265), a common functional polymorphism of the BDNF gene (the Met allele is associated with lower activity-dependent release of BDNF).

RESULTS

Participants in the High-NCP group showed significantly elevated levels of force variability. Met allele carriers exhibited significantly higher levels of force variability when compared with the Val homozygotes. Logistic regression indicated that the odds of membership in the High-NCP group were significantly higher given the presence of dyskinesias (OR=2.32; CI: 1.25-4.28).

CONCLUSION

Findings of elevated force variability suggest that individuals with NCP exhibit subtle signs of striatal vulnerability, reflected more dramatically as jerking and hyperkinetic movements in patients with formal psychosis. The results are consistent with a larger literature implicating BDNF as a critical factor underlying abnormal movements, and suggest that specific candidate genes underlie putative markers across a psychosis continuum.

A Review of Brain-derived Neurotrophic Factor as a Candidate Biomarker in Schizophrenia

Brain-derived neurotrophic factor (BDNF), a neurotrophin known to be responsible for development, regeneration, survival and maintenance of neurons has been implicated in the pathophysiology of schizophrenia. This review seeks to complement previous reviews on biological roles of BDNF and summarizes evidence on the involvement of BDNF in the pathophysiology of schizophrenia with an emphasis on clinical relevance. The expressions of BDNF were altered in patients with schizophrenia and were found to be correlated with psychotic symptomatology. Antipsychotics appeared to have differential effects on expression of BDNF but did not restore BDNF expression of patients with schizophrenia to normal levels. In addition, evidence suggests that BDNF is involved in the major neurotransmitter systems and is associated with disruptions in brain structure, neurodevelopmental process, cognitive function, metabolic and immune systems commonly associated with schizophrenia. Besides that, BDNF has been demonstrated to be tightly regulated with estrogen which has also been previously implicated in schizophrenia. Evidence gathered in this review confirms the relevance of BDNF in the pathophysiology of schizophrenia and the potential utility of BDNF as a suitable biomarker for diagnostic and prognostic purposes for disease outcome and other co-morbidities. However, further investigations are warranted to examine the specificity of BDNF in schizophrenia compared to other neurodegenerative disorders and other neuropsychiatric illness. Longitudinal prospective studies will also be of added advantage for evaluation of prognostic utility of BDNF in schizophrenia.

A functional fast scan cyclic voltammetry assay to characterize dopamine D2 and D3 autoreceptors in the mouse striatum

Dopamine D2 and D3 autoreceptors are located on pre-synaptic terminals and are known to control the release and synthesis of dopamine. Dopamine D3 receptors have a fairly restricted pattern of expression in the mammalian brain. Their localization in the nucleus accumbens core and shell is of particular interest because of their association with the rewarding properties of drugs of abuse. Using background subtracted fast scan cyclic voltammetry, we investigated the effects of dopamine D2 and D3 agonists on electrically stimulated dopamine release and uptake rates in the mouse caudate-putamen and nucleus accumbens core and shell. The dopamine D2 agonists (-)-quinpirole hydrochloride and 5,6,7,8-Tetrahydro-6-(2-propen-1-yl)-4H-thiazolo[4,5-d]azepin-2-amine dihydrochloride (B-HT 920) had the same dopamine release inhibition effects on caudate-putamen and nucleus accumbens (core and shell) based on their EC(50) and efficacies. This suggests that the dopamine D2 autoreceptor functionality is comparable in all three striatal regions investigated. The dopamine D3 agonists (4aR,10bR)-3,4a,4,10b-Tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride ((+)-PD 128907) and (+/-)-7-Hydroxy-2-dipropylaminotetralin hydrobromide (7-OH-DPAT) had a significantly greater effect on dopamine release inhibition in the nucleus accumbens shell than in caudate-putamen. This study confirms that, the dopamine D3 autoreceptor functionality is greater in the nucleus accumbens shell followed by the nucleus accumbens core, with the caudate-putamen having the least. Neither dopamine D2 nor D3 agonists affected the uptake rates in nucleus accumbens but concentrations greater than 0.3 muM lowered the uptake rate in caudate-putamen. To validate our method of evaluating dopamine D2 and D3 autoreceptors, sulpiride (D2 antagonist) and nafadotride (D3 antagonist) were used to reverse the effects of the dopamine agonists to approximately 100% of the pre-agonist dopamine release concentration. Finally, these results demonstrate a functional voltammetric assay that characterizes dopamine D2-like agonist as either D2- or D3-preferring agonists by taking advantage of the unique receptor density within the striatum.

Differential regulation of psychostimulant-induced gene expression of brain derived neurotrophic factor and the immediate-early gene Arc in the juvenile and adult brain

Psychostimulant drugs are widely used in children for the treatment of attention-deficit/hyperactivity disorder. Recent animal studies have suggested that exposure to these agents in early life could be detrimental to brain development. Here, for the first time, the effect of methylphenidate (MPH) and D-amphetamine (AMPH) on the expression of two key genes for neuronal development and plasticity, brain-derived neurotrophic factor (bdnf) and the effector immediate early gene activity-regulated, cytoskeletal-associated protein (Arc), was examined in both juvenile and adult rats. Both MPH [2 mg/kg, intraperitoneal (i.p.)] and AMPH (0.5 mg/kg, i.p.) induced marked decreases of bdnf mRNA in hippocampal and cortical brain regions of juveniles, whereas effects in adults were significantly less (hippocampus) or opposite (frontal cortex). In comparison, Arc mRNA was decreased (hippocampus and parietal cortex), largely unaffected (frontal cortex) or increased (striatum) in juveniles, whereas in adults, Arc mRNA increased in most brain regions. MPH-induced locomotion was also measured, and showed a much smaller increase in juveniles than in adults. In summary, our data show that the effects of MPH and AMPH on expression of the neurodevelopmentally important genes, bdnf and Arc, differ markedly in juvenile and adult rats, with juveniles showing evidence of brain region-specific decreases in both genes. These age-dependent effects on gene expression may be linked with the reported long-term harmful effects of psychostimulants in animal models.

Differential regional and dose-related effects of asenapine on dopamine receptor subtypes

RATIONALE

The novel psychopharmacologic agent, asenapine, has high affinity for a range of receptors including the dopaminergic receptors.

OBJECTIVE

We examined the long-term effects of multiple doses of asenapine on dopamine receptor subtypes: D(1)-like (D(1) and D(5)), D(2), D(3), and D(4).

METHODS

Rats were given asenapine 0.03, 0.1, or 0.3 mg/kg (subcutaneously, twice daily) or vehicle for 4 weeks. Receptor binding was determined by autoradiography from brain sections collected from the medial prefrontal cortex (mPFC), dorsolateral frontal cortex, caudate putamen (CPu), nucleus accumbens (NAc), and hippocampus (HIP).

RESULTS

Four weeks of asenapine at 0.3 mg/kg significantly (P < 0.05) increased D(1)-like binding in the mPFC (by 26%), NAc (59%), and CPu (55%). Asenapine (0.1 and 0.3 mg/kg) also increased D(2) binding in mPFC (43% and 55%, respectively). All doses of asenapine dose-dependently increased D(2) binding in HIP (by 32%, 45%, and 63%, respectively). In contrast, only 0.3 mg/kg of asenapine significantly (P < 0.05) increased D(2) binding in the NAc (32%) and CPu (41%). Repeated treatment with 0.1 and 0.3 mg/kg of asenapine increased D(4) binding in the NAc (36% and 71%), CPu (27% and 70%), and HIP (48% and 77%). However, asenapine, at the doses tested, did not significantly alter D(3) binding in the brain regions examined in this study.

CONCLUSIONS

These results indicate that asenapine has region-specific and dose-dependent effects on dopamine receptor subtypes in rat forebrain, which may contribute to asenapine's unique psychopharmacological properties.

Metabonomic and metallomic profiling in the amniotic fluid of malnourished pregnant rats

Epidemiology and studies in animal models have revealed that prenatal malnutrition is highly correlated with abnormal fetal neurodevelopment. We present here a combined metabonomic and metallomic profiling technique to associate the metabolic and trace-elemental composition variations of rat amniotic fluid (AF) in malnourished pregnant rats with the retardation of fetal rat neurodevelopment. The AF samples from three groups of pregnant Sprague-Dawley rats, which were fed either a normal diet, a low-protein diet, or "a famine diet", were subjected to GC/MS and ICP/MS combined with multivariate data analysis (MVDA). PCA scores plot of both GC/MS and ICP/MS data showed similar and unique metabolic signatures of AF in response to the different diets. Rats in the famine group released increased amounts of glycine, inositol, putrescine, and rubidium and decreased amounts of methionine, dopa, tryptophan, glutamine, zinc, cobalt, and selenium in the AF. These discriminable variations in the AF may indicate the abnormality of a number of metabolic pathways in fetal rats including the folate cycle and methionine pathway, the monoamine pathway, and tri-iodothyronine (T3) metabolism. The abnormalities may be the result of metabolites or elemental differences or a combination of both. This study demonstrates the potential of combining profiling of small-molecule metabolites and trace elements to broaden the understanding of biological variations associated with fetal neurodevelopment induced by environmental perturbation.

Brain-derived neurotrophic factor and its intracellular signaling pathways in cocaine addiction

Cocaine addiction is one of the severest health problems faced by western countries, where there is an increasing prevalence of lifelong abuse. The most challenging aspects in the treatment of cocaine addiction are craving and relapse, especially in view of the fact that, at present, there is a lack of effective pharmacological treatment for the disorder. What is required are new pharmacological approaches based on our current understanding of the neurobiological bases of drug addiction. Within the context of the behavioral and neurochemical actions of cocaine, this paper considers the contribution of brain-derived neurotrophic factor (BDNF) and its main intracellular signaling mechanisms, including mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) and phosphatidylinositol 3-kinase (PI3K), in psychostimulant addiction. Repeated cocaine administration leads to an increase in BDNF levels and enhanced activity in the intracellular pathways (PI3K and MAPK/ERK) in the reward-related brain areas, which applies especially several days following withdrawal. It has been hypothesized that these neurochemical changes contribute to the enduring synaptic plasticity that underlies sensitized responses to psychostimulants and drug-conditioned memories leading to compulsive drug use and frequent relapse after withdrawal. Nevertheless, increased BDNF levels could also have a role as a protection factor in addiction. The inhibition of the intracellular pathways, ERK and PI3K, leads to a disruption in sensitized responses and conditioned memories associated with cocaine addiction and suggests new, potential therapeutic strategies to explore in the dependence on psychostimulants.

Dopaminergic deficiency in mice with reduced levels of the dual-specificity tyrosine-phosphorylated and regulated kinase 1A, Dyrk1A(+/-)

The dual-specificity tyrosine-phosphorylated and regulated kinase 1A (DYRK1A) gene encodes a protein kinase known to play a critical role in neurodevelopment. Mice with one functional copy of Dyrk1A (Dyrk1A(+/-)) display a marked hypoactivity and altered gait dynamics in basal conditions and in novel environments. Dopamine (DA) is a key neurotransmitter in motor behavior and genetic deletion of certain genes directly related to the dopaminergic system has a strong impact on motor activity. We have studied the effects of reduced Dyrk1A expression on the function of the nigrostriatal dopaminergic system. To characterize the dopaminergic system in DYRK1A(+/-) mice, we have used behavioral, pharmacological, histological, neurochemical and neuroimaging (microPET) techniques in a multidisciplinary approach. Dyrk1A(+/-) mice exhibited decreased striatal DA levels, reduced number of DA neurons in the substantia nigra pars compacta, as well as altered behavioral responses to dopaminergic agents. Moreover, microdialysis experiments revealed attenuated striatal DA release and positron emission tomography scan display reduced forebrain activation when challenged with amphetamine, in Dyrk1A(+/-) compared with wild-type mice. These data indicate that Dyrk1A is essential for a proper function of nigrostriatal dopaminergic neurons and suggest that Dyrk1A(+/-) mice can be used to study the pathogenesis of motor disorders involving dopaminergic dysfunction.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

Effects of risperidone on dopamine receptor subtypes in developing rat brain

The atypical antipsychotic risperidone is often prescribed to pediatric patients with neuropsychiatric disorders, though its effects on the developing brain remain unclear. Accordingly, we studied the effects of repeated treatment of risperidone on dopamine receptors in brain regions of juvenile rat. Levels of dopamine receptors (D(1), D(2), D(3), D(4)) in forebrain regions of juvenile rats were quantified after 3 weeks of treatment with three different doses of risperidone (0.3, 1.0 and 3.0 mg/kg) and compared findings to those in adult rats treated with risperidone (3.0 mg/kg/day) previously. Risperidone (at 1.0 and 3.0 mg/kg/day) increased levels of D(1) receptors in nucleus accumbens and caudate-putamen of juvenile, but not adult rats. Conversely, all three doses of risperidone dose-dependently increased D(2) labeling in medial prefrontal cortex and hippocampus, and D(4) receptor in nucleus accumbens, caudate-putamen and hippocampus of juvenile animals as well as in adults. Only the high dose of risperidone (3.0 mg/kg) increased D(2) receptors in caudate-putamen in both juvenile and adult brain. D(3) receptors were not altered by risperidone in any brain region at any dose or age. The findings indicate dose-dependent effects of risperidone on dopamine receptors in developing animals, and that juvenile animals are more sensitive than adults to the cerebral effects of risperidone.

Association of the met66 allele of brain-derived neurotrophic factor (BDNF) with smoking

RATIONALE

It has been suggested that a susceptibility locus near the gene encoding the brain-derived neurotrophic factor (BDNF) contributes to individual differences in human addiction vulnerability. BDNF modulates several behaviors that are associated with addictive drugs, and upregulation of BDNF was found to be associated with several drugs of abuse such as amphetamine, cocaine, and nicotine. In this study, we addressed the question if a common BDNF missense variation (Val66Met) influences the risk for smoking behavior in otherwise healthy human volunteers.

MATERIALS AND METHODS

In total, 320 healthy unrelated volunteers (155 male, 165 female, mean age: 38.4 +/- 14.1 years) consisting of 43.3% never smokers, 20.9% former smokers, and 35.6% current smokers were investigated.

RESULTS

The frequency of both Met/Met genotype and Met allele was significantly increased in current and in former smokers when compared to never smokers (chi (2) = 10.856, df = 2, p = 0.004 and chi (2) = 4.350, df = 1, p = 0.045, respectively).

CONCLUSIONS

Our results suggest that humans who carry the Met allele of the BDNF missense polymorphism might be more vulnerable to initiate and also maintain smoking.

Alterations in BDNF and trkB mRNAs following acute or sensitizing cocaine treatments and withdrawal

In the present study, we used in situ hybridization to examine the influence of acute or repeated cocaine administrations and withdrawal from repeated cocaine treatment on the level of brain-derived neurotrophic factor (BDNF) and its receptor trkB mRNAs in rat brain. Cocaine (10 mg/kg i.p.) injected acutely produced locomotor hyperactivation, while repeated (single injection for 5 days) administrations of cocaine (10 mg/kg) induced a two-fold increases in the locomotor activity in rats in response to a challenge cocaine dose (10 mg/kg) on day 10, as compared to the saline-treated animals (sensitization). Cocaine treatments induced a brain-region-specific decrease in the levels of trkB mRNA. On the other hand, BDNF mRNA in the rat hippocampus was increased only in the group of rats subjected to cocaine withdrawal. Animals under cocaine withdrawal demonstrated a significant increase in the immobility time measured by the use of modified forced swimming test. Therefore, the increases in the levels of BDNF mRNA in the rat hippocampus seem to be correlated with "depressive-like" behavioral effects during withdrawal from repeated cocaine treatment. In the shell (but not in the core) of the nucleus accumbens, the levels of BDNF mRNA were significantly increased following acute and repeated cocaine treatment as well as during cocaine withdrawal, which indicates that the alterations in the neurotrophin level in the brain region important for the expression of cocaine-induced sensitization involve other mechanisms.

Secretion of brain-derived neurotrophic factor from PC12 cells in response to oxidative stress requires autocrine dopamine signaling

Expression of brain-derived neurotrophic factor (BDNF) is sensitive to changes in oxygen availability, suggesting that BDNF may be involved in adaptive responses to oxidative stress. However, it is unknown whether or not oxidative stress actually increases availability of BDNF by stimulating BDNF secretion. To approach this issue we examined BDNF release from PC12 cells, a well-established model of neurosecretion, in response to hypoxic stimuli. BDNF secretion from neuronally differentiated PC12 cells was strongly stimulated by exposure to intermittent hypoxia (IH). This response was inhibited by N-acetyl-l-cysteine, a potent scavenger of reactive oxygen species (ROS) and mimicked by exogenous ROS. IH-induced BDNF release requires activation of tetrodotoxin sensitive Na+ channels and Ca2+ influx through N- and L-type channels, as well as mobilization of internal Ca2+ stores. These results demonstrate that oxidative stress can stimulate BDNF release and that underlying mechanisms are similar to those previously described for activity-dependent BDNF secretion from neurons. Surprisingly, we also found that IH-induced secretion of BDNF was blocked by dopamine D2 receptor antagonists or by inhibition of dopamine synthesis with alpha-methyl-p-tyrosine. These data indicate that oxidative stress can stimulate BDNF release through an autocrine or paracrine loop that requires dopamine receptor activation.

The pathogenesis of clinical depression: stressor- and cytokine-induced alterations of neuroplasticity

Stressful events promote neurochemical changes that may be involved in the provocation of depressive disorder. In addition to neuroendocrine substrates (e.g. corticotropin releasing hormone, and corticoids) and central neurotransmitters (serotonin and GABA), alterations of neuronal plasticity or even neuronal survival may play a role in depression. Indeed, depression and chronic stressor exposure typically reduce levels of growth factors, including brain-derived neurotrophic factor and anti-apoptotic factors (e.g. bcl-2), as well as impair processes of neuronal branching and neurogenesis. Although such effects may result from elevated corticoids, they may also stem from activation of the inflammatory immune system, particularly the immune signaling cytokines. In fact, several proinflammatory cytokines, such as interleukin-1, tumor necrosis factor-alpha and interferon-gamma, influence neuronal functioning through processes involving apoptosis, excitotoxicity, oxidative stress and metabolic derangement. Support for the involvement of cytokines in depression comes from studies showing their elevation in severe depressive illness and following stressor exposure, and that cytokine immunotherapy (e.g. interferon-alpha) elicited depressive symptoms that were amenable to antidepressant treatment. It is suggested that stressors and cytokines share a common ability to impair neuronal plasticity and at the same time altering neurotransmission, ultimately contributing to depression. Thus, depressive illness may be considered a disorder of neuroplasticity as well as one of neurochemical imbalances, and cytokines may act as mediators of both aspects of this illness.

The BDNF Val66Met polymorphism has a gender specific influence on planning ability in Parkinson's disease

Parkinson's disease (PD) patients show a range of cognitive deficits,which may relate to abnormalities in dopaminergic transmission in fronto-striatal circuitry. In this study, we have investigated the impact of brainderived neurotrophic factor (BDNF) val66met polymorphisms on performance of the Tower of London (TOL) test of planning by PD patients. This polymorphism significantly influences BDNF secretion in the CNS, and BDNF is known to influence dopaminergic neurons and cognitive processes. Patients with PD totalling 291 who had undergone detailed motor and cognitive assessments as part of a population-based study of PD were genotyped for the BDNF val66met polymorphism. The impact of this polymorphism on cognitive ability was determined using multivariate analysis to adjust for possible confounding variables. Patients with low rates of BDNF secretion (met alleles) performed significantly better at the TOL task than those with high rates of secretion (val alleles). Furthermore, subgroup analyses revealed that the effect is most apparent in women and among patients with prior dopaminergic exposure. We speculate that BDNF may interact with dopaminergic transmission and dopamine receptor stimulation in the frontostriatal circuitry, with subsequent consequences on cognition in Parkinson's disease.

Towards an immuno-precipitated neurodevelopmental animal model of schizophrenia

Epidemiological studies have indicated an association between maternal bacterial and viral infections during pregnancy and the higher incidence of schizophrenia in the resultant offspring post-puberty. One hypothesis asserts that the reported epidemiological link is mediated by prenatal activation of the foetal immune system in response to the elevation of maternal cytokine level due to infection. Here, we report that pregnant mouse dams receiving a single exposure to the cytokine-releasing agent, polyriboinosinic-polyribocytidilic acid (PolyI:C; at 2.5, 5.0, or 10.0 mg/kg) on gestation day 9 produced offspring that subsequently exhibited multiple schizophrenia-related behavioural deficits in adulthood, in comparison to offspring from vehicle injected or non-injected control dams. The efficacy of the PolyI:C challenge to induce cytokine responses in naïve non-pregnant adult female mice and in foetal brain tissue when injected to pregnant mice were further ascertained in separate subjects: (i) a dose-dependent elevation of interleukin-10 was detected in the adult female mice at 1 and 6h post-injection, (ii) 12 h following prenatal PolyI:C challenge, the foetal levels of interleukin-1beta were elevated. The spectrum of abnormalities included impairments in exploratory behaviour, prepulse inhibition, latent inhibition, the US-pre-exposure effect, spatial working memory; and enhancement in the locomotor response to systemic amphetamine (2.5 mg/kg, i.p.) as well as in discrimination reversal learning. The neuropsychological parallels between prenatal PolyI:C treatment in mice and psychosis in humans, demonstrated here, leads us to conclude that prenatal PolyI:C treatment represents one of the most powerful environmental-developmental models of schizophrenia to date. The uniqueness of this model lies in its epidemiological and immunological relevance. It is, sui generis, ideally suited for the investigation of the neuropsychoimmunological mechanisms implicated in the developmental aetiology and disease processes of schizophrenia.

Expanding the 'central dogma': the regulatory role of nonprotein coding genes and implications for the genetic liability to schizophrenia

It is now evident that nonprotein coding RNA (ncRNA) plays a critical role in regulating the timing and rate of protein translation. The potential importance of ncRNAs is suggested by the observation that the complexity of an organism is poorly correlated with its number of protein coding genes, yet highly correlated with its number of ncRNA genes, and that in the human genome only a small fraction (2-3%) of genetic transcripts are actually translated into proteins. In this review, we discuss several examples of known RNA mechanisms for the regulation of protein synthesis. We then discuss the possibility that ncRNA regulation of schizophrenia risk genes may underlie the diverse findings of genetic linkage studies including that protein-altering gene polymorphisms are not generally found in schizophrenia. Thus, inadequate or mistimed expression of a functional protein may occur either due to mutation or other dysfunction of the DNA coding base pair sequence, leading to a dysfunctional protein, or due to post-transcriptional events such as abnormal ncRNA regulation of a normal gene. One or more 'schizophrenia disease genes' may turn out to include abnormal transcriptional units that code for RNA regulators of protein coding gene expression or to be proximal to such units, rather than to be abnormalities in the protein coding gene itself. Understanding the genetics of schizophrenia and other complex neuropsychiatric disorders might very well include consideration of RNA and epigenetic regulation of protein expression in addition to polymorphisms of the protein coding gene.

Dopamine, depression and antidepressants

Abstract The relationship between depression and dopamine deficiency in the mesolimbic pathway has been hypothesized for many years. The experimental studies with animal models of depression and the human studies implicate the role of the dopamine system in depression. Not only do dopaminergic receptor agonists, but also antagonists such as olanzapine exhibit antidepressant effects associated with standard antidepressants in patients with treatment-resistant depression. This paradoxical result suggests that further investigations are necessary to understand the role played by dopamine in depression.

Induction of neurogenesis in the adult rat subventricular zone and neostriatum following dopamine D3 receptor stimulation

Discrete regions of the adult CNS, including the subventricular zone (SVZ), do retain the capacity for neurogenesis. These progenitor cells may represent a potential new source of cells for replacement therapies in neuroregenerative diseases. An understanding of the microenvironmental signals regulating neurogenesis in the adult brain would facilitate the development of such therapeutic approaches. A particularly strong expression of dopamine D(3) receptor mRNA occurs in the proliferative SVZ during prenatal and early postnatal ontogeny. Although its expression diminishes following development, a restricted D(3) receptor expression persists in this region through adulthood, coincident with continued proliferation in this region. Here, we demonstrate a two-fold induction of cell proliferation (BrdU incorporation) in the SVZ and rostral migratory stream of the adult Sprague-Dawley rat brain following intrasubventricular administration of the dopamine D(3) receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) for 2 weeks. The number of BrdU-positive cells was elevated ten-fold from very low baseline levels in the neighbouring neostriatum, another region known to express D(3) receptors. These striatal BrdU-positive cells appeared within 3 days following intracerebral infusion of 7-OH-DPAT and were distributed homogeneously throughout the striatum following systemic administration. This suggests that these cells originate from resident progenitor cells rather than the SVZ. Dopamine D(3) receptor activation may serve as a proneuronal differentiation signal as 60-70% of the new cells had neuronal markers following 7-OH-DPAT infusion. These results suggest that the dopamine D(3) receptor may be a good drug target for cell replacement strategies, particularly because of the fact that its expression is almost exclusively limited to the nervous system.

Dopamine D2 agonist priming in intact and dopamine-lesioned rats

Receptor priming is a recently discovered phenomenon by which receptor agonists produce abrupt and long-lived supersensitization of receptors. Induction of dopamine (DA) D2 receptor supersensitivity by the agonist quinpirole was discovered approximately 15 years ago, and was found to occur consistently if rats were treated repeatedly at daily or weekly or monthly intervals with low or high doses of quinpirole. In this review we summarize and discuss some of the major studies that underlie DA D2 receptor supersensitivity, describe behavioral processes that are known to be altered by DA D2 receptor supersensitivity, and discuss the importance of DA innervation on expression of enhanced behaviors. DA D2 receptor supersensitivity represents one of the neural mechanisms implicated in psychiatric disorders. Also, DA D2 receptor supersensitivity and increased DA D3 receptor expression are associated with motor dyskinesias, as in L-DOPA-treated Parkinson's disease patients. An understanding of receptor priming, a knowledge of the types of behavioral expression associated with DA D2 receptor supersensitivity, and an understanding of mechanisms associated with receptor supersensitization, can lead to improvements in the treatments of psychiatric and neurological disorders.

Brain sites of movement disorder: genetic and environmental agents in neurodevelopmental perturbations

In assessing and assimilating the neurodevelopmental basis of the so-called movement disorders it is probably useful to establish certain concepts that will modulate both the variation and selection of affliction, mechanisms-processes and diversity of disease states. Both genetic, developmental and degenerative aberrations are to be encompassed within such an approach, as well as all deviations from the necessary components of behaviour that are generally understood to incorporate "normal" functioning. In the present treatise, both conditions of hyperactivity/hypoactivity, akinesia and bradykinesia together with a constellation of other symptoms and syndromes are considered in conjunction with the neuropharmacological and brain morphological alterations that may or may not accompany them, e.g. following neonatal denervation. As a case in point, the neuroanatomical and neurochemical points of interaction in Attention Deficit and Hyperactivity disorder (ADHD) are examined with reference to both the perinatal metallic and organic environment and genetic backgrounds. The role of apoptosis, as opposed to necrosis, in cell death during brain development necessitates careful considerations of the current explosion of evidence for brain nerve growth factors, neurotrophins and cytokines, and the processes regulating their appearance, release and fate. Some of these processes may possess putative inherited characteristics, like alpha-synuclein, others may to greater or lesser extents be endogenous or semi-endogenous (in food), like the tetrahydroisoquinolines, others exogenous until inhaled or injested through environmental accident, like heavy metals, e.g. mercury. Another central concept of neurodevelopment is cellular plasticity, thereby underlining the essential involvement of glutamate systems and N-methyl-D-aspartate receptor configurations. Finally, an essential assimilation of brain development in disease must delineate the relative merits of inherited as opposed to environmental risks not only for the commonly-regarded movement disorders, like Parkinson's disease, Huntington's disease and epilepsy, but also for afflictions bearing strong elements of psychosocial tragedy, like ADHD, autism and Savantism.