Brain-derived neurotrophic factor controls dopamine D3 receptor expression: therapeutic implications in Parkinson's disease

Irisin: A Multifaceted Hormone Bridging Exercise and Disease Pathophysiology

The fibronectin domain-containing protein 5 (FNDC5), or irisin, is an adipo-myokine hormone produced during exercise, which shows therapeutic potential for conditions like metabolic disorders, osteoporosis, sarcopenia, obesity, type 2 diabetes, and neurodegenerative diseases, including Alzheimer's disease (AD). This review explores its potential across various pathophysiological processes that are often considered independent. Elevated in healthy states but reduced in diseases, irisin improves muscle-adipose communication, insulin sensitivity, and metabolic balance by enhancing mitochondrial function and reducing oxidative stress. It promotes osteogenesis and mitigates bone loss in osteoporosis and sarcopenia. Irisin exhibits anti-inflammatory effects by inhibiting NF-κB signaling and countering insulin resistance. In the brain, it reduces amyloid-β toxicity, inflammation, and oxidative stress, enhancing brain-derived neurotrophic factor (BDNF) signaling, which improves cognition and synaptic health in AD models. It also regulates dopamine pathways, potentially alleviating neuropsychiatric symptoms like depression and apathy. By linking physical activity to systemic health, irisin emphasizes its role in the muscle-bone-brain axis. Its multifaceted benefits highlight its potential as a therapeutic target for AD and related disorders, with applications in prevention, in treatment, and as a complement to exercise strategies.

BDNF Therapeutic Mechanisms in Neuropsychiatric Disorders

Brain-derived neurotrophic factor (BDNF) is the most abundant neurotrophin in the adult brain and functions as both a primary neurotrophic signal and a neuromodulator. It serves essential roles in neuronal development, maintenance, transmission, and plasticity, thereby influencing aging, cognition, and behavior. Accumulating evidence associates reduced central and peripheral BDNF levels with various neuropsychiatric disorders, supporting its potential utilization as a biomarker of central pathologies. Subsequently, extensive research has been conducted to evaluate restoring, or otherwise augmenting, BDNF transmission as a potential therapeutic approach. Promising results were indeed observed for genetic BDNF upregulation or exogenous administration using a multitude of murine models of neurological and psychiatric diseases. However, varying mechanisms have been proposed to underlie the observed therapeutic effects, and many findings indicate the engagement of disease-specific and other non-specific mechanisms. This is because BDNF essentially affects all aspects of neuronal cellular function through tropomyosin receptor kinase B (TrkB) receptor signaling, the disruptions of which vary between brain regions across different pathologies leading to diversified consequences on cognition and behavior. Herein, we review the neurophysiology of BDNF transmission and signaling and classify the converging and diverging molecular mechanisms underlying its therapeutic potentials in neuropsychiatric disorders. These include neuroprotection, synaptic maintenance, immunomodulation, plasticity facilitation, secondary neuromodulation, and preservation of neurovascular unit integrity and cellular viability. Lastly, we discuss several findings suggesting BDNF as a common mediator of the therapeutic actions of centrally acting pharmacological agents used in the treatment of neurological and psychiatric illness.

Dopamine-transporter heterozygous rats carrying maternal wild-type allele are more vulnerable to the development of compulsive behavior

Compulsivity is defined as an unstoppable tendency towards repetitive and habitual actions, which are reiterated despite negative consequences. Polydipsia is induced preclinically by intermittent reward leading rodents to ingest large amounts of fluids. We focused on the role of dopamine transporter (DAT) and inheritance factors in compulsive behavior. Our sample consisted of DAT heterozygous (HET) rats with different genetic inheritance (MAT-HET, born from WT-dams x KO-fathers; MIX-HET, born from HET-dams x KO-fathers). As controls, we used both wild-type (WT) rats and their socially-isolated (WTi) siblings. We ran the schedule-induced polydipsia (SIP) protocol, to induce compulsive behavior; then the Y-maze and marble-burying tests, to verify its actual development. Only MAT-HET (who inherited the functional DAT allele from the WT mother) is vulnerable to developing compulsive behavior. MAT-HET rats drank increasingly more water during SIP and showed significant perseverance in the Y-maze test and exhibited compulsive actions in the marble-burying test. Interestingly, compulsive behaviors of MAT-HET rats correlate with expression ex-vivo of different genes in different areas. Regarding the prefrontal cortex (PFC), D2R correlates with Y-maze "perseverance" in addition to BDNF; considering the amygdala (AMY), both D3R and OXTR correlate with SIP "licks". Indeed, compulsivity may be linked to D2R and BDNF in PFC, while extreme anxiety in MAT-HET rats may be associated with D3R and OXTR in the amygdala. These results confirm some similarities between MAT-HET and DAT-KO subjects and link the epigenetic context of the DAT gene to the development of compulsive behavior. This article is protected by copyright. All rights reserved.

Irisin Acts via the PGC-1α and BDNF Pathway to Improve Depression-like Behavior

Depression is the most prevalent psychiatric disorder experienced by the world's population. Mechanisms associated with depression-like behavior have not been fully investigated. Among the therapeutic solution for depression, exercise is considered an important regulator attenuating depressive neuropathology. Exercise has been reported to boost the secretion of myokines such as irisin and myostatin in skeletal muscles. Myokines secreted during exercise are involved in various cellular responses including the endocrine and autocrine systems. Especially, irisin as a cleaved version of fibronectin domain-containing protein 5 has multiple functions such as white fat-browning, energy expenditure increase, anti-inflammatory effects, and mitochondrial function improvement in both systemic circulation and central nervous system. Furthermore, irisin activates energy metabolism-related signaling peroxisome proliferator-activated receptor-gamma coactivator-1 alpha and memory formation-related signaling brain-derived neurotrophic factor involved in depression. However, the role and mechanism of irisin in depression disorder is not obvious until now. Here, we review recent evidences regarding the therapeutic effect of irisin in depression disorder. We suggest that irisin is a key molecule that suppresses several neuropathological mechanisms involved in depression.

A novel dopamine D3R agonist SK609 with norepinephrine transporter inhibition promotes improvement in cognitive task performance in rodent and non-human primate models of Parkinson's disease

Mild cognitive impairment is present in a number of neurodegenerative disorders including Parkinson's disease (PD). Mild cognitive impairment in PD (PD-MCI) often manifests as deficits in executive functioning, attention, and spatial and working memory. Clinical studies have suggested that the development of mild cognitive impairment may be an early symptom of PD and may even precede the onset of motor impairment by several years. Dysfunction in several neurotransmitter systems, including dopamine (DA), norepinephrine (NE), may be involved in PD-MCI, making it difficult to treat pharmacologically. In addition, many agents used to treat motor impairment in PD may exacerbate cognitive impairment. Thus, there is a significant unmet need to develop therapeutics that can treat both motor and cognitive impairments in PD. We have recently developed SK609, a selective, G-protein biased signaling agonist of dopamine D3 receptors. SK609 was successfully used to treat motor impairment and reduce levodopa-induced dyskinesia in a rodent model of PD. Further characterization of SK609 suggested that it is a selective norepinephrine transporter (NET) inhibitor with the ability to increase both DA and NE levels in the prefrontal cortex. Pharmacokinetic analysis of SK609 under systemic administration demonstrated 98% oral bioavailability and high brain distribution in striatum, hippocampus and prefrontal cortex. To evaluate the effects of SK609 on cognitive deficits of potential relevance to PD-MCI, we used unilateral 6-hydroxydopamine (6-OHDA) lesioned rats and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated cynomolgus macaques, with deficits in performance in a sustained attention and an object retrieval task, respectively. SK609 dose dependently improved the performance of 6-OHDA-lesioned rats, with peak performance achieved using a 4 mg/kg dose. This improvement was predominantly due to a significant reduction in the number of misses and false alarm errors, contributing to an increase in sustained attention. In MPTP-lesioned monkeys, this same dose also improved performance in an object retrieval task, significantly reducing cognitive errors (barrier reaches) and motor errors (fine motor dexterity problems). These data demonstrate that SK609 with its unique pharmacological effects on modulating both DA and NE can ameliorate cognitive impairment in PD models and may provide a therapeutic option to treat both motor and cognitive impairment in PD patients.

Severity of Dyskinesia and D3R Signaling Changes Induced by L-DOPA Treatment of Hemiparkinsonian Rats Are Features Inherent to the Treated Subjects

Extensive damage to nigrostriatal dopaminergic neurons leads to Parkinson’s disease (PD). To date, the most effective treatment has been administration of levodopa (L-DOPA) to increase dopaminergic tone. This treatment leads to responses that vary widely among patients, from predominantly beneficial effects to the induction of disabling, abnormal movements (L-DOPA induced dyskinesia (LID)). Similarly, experimental studies have shown animals with widely different degrees of LID severity. In this study, unilateral injections of 6-hydroxydopamine (6-OHDA) in the medial forebrain bundle (MFB) produced more than 90% depletion of dopamine in both the striatum and the substantia nigra reticulata (SNr) of rats. Population analysis showed that dopamine depletion levels were clustered in a single population. In contrast, analysis of abnormal involuntary movements (AIMs) induced by L-DOPA treatment of 6-OHDA-lesioned animals yielded two populations: one with mild LID, and the other with severe LID, which are also related to different therapeutic responses. We examined whether the severity of LID correlated with changes in dopamine 3 receptor (D3R) signaling because of the following: (a) D3R expression and the induction of LID are strongly correlated; and (b) dopaminergic denervation induces a qualitative change in D3R signaling in the SNr. We found that the effects of D3R activation on cAMP accumulation and depolarization-induced [³H]-gamma-aminobutyric acid ([³H]-GABA) release were switched. L-DOPA treatment normalized the denervation-induced changes in animals with mild LID. The D3R activation caused depression of both dopamine 1 receptor (D1R)-induced increases in cAMP production and depolarization-induced [³H]-GABA release, which were reversed to their pre-denervation state. In animals with severe LID, none of the denervation-induced changes were reversed. The finding that in the absence of identifiable differences in 6-OHDA and L-DOPA treatment, two populations of animals with different D3R signaling and LIDs severity implies that mechanisms intrinsic to the treated subject determine the segregation.

The Role of Irisin in Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by progressive memory dysfunction, oxidative stress, and presence of senile plaques formed by amyloid beta (Aβ) accumulation in the brain. AD is one of the most important causes of morbidity and mortality worldwide. AD has a variety of risk factors, including environmental factors, metabolic dysfunction, and genetic background. Recent research has highlighted the relationship between AD and systemic metabolic changes such as glucose and lipid imbalance and insulin resistance. Irisin, a myokine closely linked to exercise, has been associated with glucose metabolism, insulin sensitivity, and fat browning. Recent studies have suggested that irisin is involved in the process in central nervous system (CNS) such as neurogenesis and has reported the effects of irisin on AD as one of the neurodegenerative disease. Here, we review the roles of irisin with respect to AD and suggest that irisin highlight therapeutic important roles in AD. Thus, we propose that irisin could be a potential future target for ameliorating AD pathology and preventing AD onset.

Blind Spot for Sedentarism: Redefining the Diseasome of Physical Inactivity in View of Circadian System and the Irisin/BDNF Axis

Introduction: The term "diseasome of physical inactivity" was coined by Pedersen to explain clustering of chronic diseases linked to physical inactivity. Accordingly, physical inactivity per se contributes to the accumulation of visceral fat, which, generates chronic low-grade systemic inflammation, contributes to emergence of chronic, non-communicable diseases. Diversity of these disorders posits the possible involvement of a supraphysiological system. Methods: Hypothesis driven literature search and deductive reasoning was used to review relevant literature and formulate a novel theory. Results: We have identified the circadian system, omnipresent in virtually every cell, as a possible vehicle for brain muscle crosstalk, explaining some aspects of the diseasome of physical inactivity This system is hierarchically organized, with the suprachiasmatic nucleus (SCN) being the master clock that entrains to the dark/light cycle and synchronizes subsidiary molecular clocks in the periphery. Insufficient photic entrainment also causes chronic disease evolution. The recently identified irisin, was shown to induce brain-derived neurotrophic factor (BDNF) production in several brain areas. BDNF assumes significant role in gating light's influence in the retinohypothalamic synapse, by having a permissive effect on glutamate signal transduction underlying photic entrainment. Conclusions: Here we provide theoretical evidence to support the hypothesis that irisin may facilitate photic entrainment of the SCN, via BDNF. By this irisin opens up possible pathways for peripheral non-photic entrainment signals to exert influence on the master clock that is otherwise resistant to these. Furthermore, we suggest that intertwining processes of circadian, redox, inflammatory, and myokine systems lay underneath the diseasome of physical inactivity.

The Alteration of Irisin-Brain-Derived Neurotrophic Factor Axis Parallels Severity of Distress Disorder in Bronchial Asthma Patients

Distress disorder (a collective term for generalized anxiety disorder and major depressive disorder) is a well-known co-morbidity of bronchial asthma. The irisin-brain-derived neurotrophic factor (BDNF) axis is a pathway that influences several neurobehavioral mechanisms involved in the pathogenesis of distress disorder. Thus, the aim of the present study was to quantify the serum irisin and BDNF concentrations in order to investigate the possible link between the irisin/BDNF axis and distress disorder in an asthma patient cohort. Data of 167 therapy-controlled asthma patients were analyzed. Demographic, anthropometric, and anamnestic data were collected, routine laboratory parameters supplemented with serum irisin and BDNF levels were determined, pulmonary function test was performed using whole-body plethysmography, and quality of life was quantified by means of the St. George's Respiratory Questionnaire (SGRQ). Correlation analysis as well as simple and multiple linear regression were used to assess the relationship between the irisin level and the Impacts score of SGRQ, which latter is indicative of the presence and severity of distress disorder. We have found a significant, positive linear relationship between the Impacts score and the reciprocal of irisin level. This association was stronger in patients whose BDNF level was higher, and it was weaker (and statistically non-significant) in patients whose BDNF level was lower. Our results indicate that higher serum irisin level together with higher serum BDNF level are associated with milder (or no) distress disorder. This finding suggests that alteration of the irisin/BDNF axis influences the presence and severity of distress disorder in asthma patients.

BDNF over-expression induces striatal serotonin fiber sprouting and increases the susceptibility to l-DOPA-induced dyskinesia in 6-OHDA-lesioned rats

In addition to its role in neuronal survival, the brain neurotrophic factor (BDNF) has been shown to influence serotonin transmission and synaptic plasticity, events strongly implicated in the appearance of l-DOPA-induced dyskinesia (LID), a motor complication occurring in parkinsonian patients after long-term treatment with the dopamine precursor. In order to evaluate a possible influence of BDNF in the appearance of LID, 6-OHDA-lesioned rats received a striatal injection of different concentrations of an adeno-associated viral (AAV) vector over-expressing either BDNF or GFP, as control vector. Eight weeks later, animals started to receive a daily treatment with l-DOPA (4-6mg/kg plus benserazide 4-6mg/kg, s.c.) or saline, and dyskinesias, as well as l-DOPA-induced rotations, were evaluated at several time-points. Moreover, molecular changes in striatal D1 receptor-dependent cAMP/PKA and ERK/mTORC signaling pathways, as well as, sprouting of striatal serotonin axons, were measured. Results showed that the AAV-BDNF vector injection induced striatal over-expression of BDNF, as well as striatal and pallidal serotonin axon hyperinnervation. Moreover, rats that over-expressed BDNF were more prone to develop LID and l-DOPA-induced rotations, compared to the GFP-treated control group. Finally, rats that over-expressed BDNF showed increased levels of striatal D1R-dependent signaling phospho-proteins in response to l-DOPA administration. This study suggests that BDNF over-expression, by inducing changes in pre-synaptic serotonin axonal trophism, is able to exacerbate maladaptive responses to l-DOPA administration.

Alteration of the irisin-brain-derived neurotrophic factor axis contributes to disturbance of mood in COPD patients

COPD is accompanied by limited physical activity, worse quality of life, and increased prevalence of depression. A possible link between COPD and depression may be irisin, a myokine, expression of which in the skeletal muscle and brain positively correlates with physical activity. Irisin enhances the synthesis of brain-derived neurotrophic factor (BDNF), a neurotrophin involved in reward-related processes. Thus, we hypothesized that mood disturbances accompanying COPD are reflected by the changes in the irisin-BDNF axis. Case history, routine laboratory parameters, serum irisin and BDNF levels, pulmonary function, and disease-specific quality of life, measured by St George's Respiratory Questionnaire (SGRQ), were determined in a cohort of COPD patients (n=74). Simple and then multiple linear regression were used to evaluate the data. We found that mood disturbances are associated with lower serum irisin levels (SGRQ's Impacts score and reciprocal of irisin showed a strong positive association; β: 419.97; 95% confidence interval [CI]: 204.31, 635.63; P<0.001). This association was even stronger among patients in the lower 50% of BDNF levels (β: 434.11; 95% CI: 166.17, 702.05; P=0.002), while it became weaker for patients in the higher 50% of BDNF concentrations (β: 373.49; 95% CI: -74.91, 821.88; P=0.1). These results suggest that irisin exerts beneficial effect on mood in COPD patients, possibly by inducing the expression of BDNF in brain areas associated with reward-related processes involved in by depression. Future interventional studies targeting the irisin-BDNF axis (eg, endurance training) are needed to further support this notion.

In Vitro Modulation of TrkB Receptor Signaling upon Sequential Delivery of Curcumin-DHA Loaded Carriers Towards Promoting Neuronal Survival

PURPOSE

To in vitro investigate the capacity of carrier-free and lipid-nanoparticle (NP)-encapsulated phytochemical compounds to prevent neuronal damage through neurotrophin potentiating activities. Delivery of molecules promoting the neurotrophin receptor signaling in the central nervous system (CNS) present ongoing interest for combination therapy development.

METHODS

Super-resolution Stimulated Emission Depletion (STED) microscopy imaging and flow cytometry analysis were employed to study the expression of the neurotrophin TrkB receptor in a neuronal cell model, which is highly responsive to binding of brain-derived neurotrophic factor (BDNF). Dual drug-loaded nanoparticle formulations, prepared by self-assembly of lyotropic lipids and PEGylated amphiphile derivatives, were delivered to differentiated human neuroblastoma SH-SY5Y cells subjected to degenerative conditions.

RESULTS

The expression of BDNF in the intra and extracellular domains was quantified by ELISA and flow cytometry after sequential treatment of the degenerating SH-SY5Y cells by neurotherapeutic formulations. Flow cytometry was also used to assess the phosphorylation of the transcription factor cAMP response element-binding protein (CREB) in the intracellular domain as a result of the treatment by nanoformulations.

CONCLUSION

Over time, dual drug formulations (curcumin and docosahexaenoic acid (DHA)) promoted the neuronal survival and repair processes through enhanced BDNF secretion and increased phosphorylation of CREB as compared to untreated degenerating cells.

Neurotrophin Signaling and Stem Cells-Implications for Neurodegenerative Diseases and Stem Cell Therapy

Neurotrophins (NTs) are members of a neuronal growth factor protein family whose action is mediated by the tropomyosin receptor kinase (TRK) receptor family receptors and the p75 NT receptor (p75NTR), a member of the tumor necrosis factor (TNF) receptor family. Although NTs were first discovered in neurons, recent studies have suggested that NTs and their receptors are expressed in various types of stem cells mediating pivotal signaling events in stem cell biology. The concept of stem cell therapy has already attracted much attention as a potential strategy for the treatment of neurodegenerative diseases (NDs). Strikingly, NTs, proNTs, and their receptors are gaining interest as key regulators of stem cells differentiation, survival, self-renewal, plasticity, and migration. In this review, we elaborate the recent progress in understanding of NTs and their action on various stem cells. First, we provide current knowledge of NTs, proNTs, and their receptor isoforms and signaling pathways. Subsequently, we describe recent advances in the understanding of NT activities in various stem cells and their role in NDs, particularly Alzheimer's disease (AD) and Parkinson's disease (PD). Finally, we compile the implications of NTs and stem cells from a clinical perspective and discuss the challenges with regard to transplantation therapy for treatment of AD and PD.

FNDC5/irisin, a molecular target for boosting reward-related learning and motivation

Interventions focusing on the prevention and treatment of chronic non-communicable diseases are on rise. In the current article, we propose that dysfunction of the mesocortico-limbic reward system contributes to the emergence of the WHO-identified risk behaviors (tobacco use, unhealthy diet, physical inactivity and harmful use of alcohol), behaviors that underlie the evolution of major non-communicable diseases (e.g. cardiovascular diseases, cancer, diabetes and chronic respiratory diseases). Given that dopaminergic neurons of the mesocortico-limbic system are tightly associated with reward-related processes and motivation, their dysfunction may fundamentally influence behavior. While nicotine and alcohol alter dopamine neuron function by influencing some receptors, mesocortico-limbic system dysfunction was associated with elevation of metabolic set-point leading to hedonic over-eating. Although there is some empirical evidence, precise molecular mechanism for linking physical inactivity and mesocortico-limbic dysfunction per se seems to be missing; identification of which may contribute to higher success rates for interventions targeting lifestyle changes pertaining to physical activity. In the current article, we compile evidence in support of a link between exercise and the mesocortico-limbic system by elucidating interactions on the axis of muscle - irisin - brain derived neurotrophic factor (BDNF) - and dopaminergic function of the midbrain. Irisin is a contraction-regulated myokine formed primarily in skeletal muscle but also in the brain. Irisin stirred considerable interest, when its ability to induce browning of white adipose tissue parallel to increasing thermogenesis was discovered. Furthermore, it may also play a role in the regulation of behavior given it readily enters the central nervous system, where it induces BDNF expression in several brain areas linked to reward processing, e.g. the ventral tegmental area and the hippocampus. BDNF is a neurotropic factor that increases neuronal dopamine content, modulates dopamine release relevant for neuronal plasticity and increased neuronal survival as well as learning and memory. Further linking BDNF to dopaminergic function is BDNF's ability to activate tropomyosin-related kinase B receptor that shares signalization with presynaptic dopamine-3 receptors in the ventral tegmental area. Summarizing, we propose that the skeletal muscle derived irisin may be the link between physical activity and reward-related processes and motivation. Moreover alteration of this axis may contribute to sedentary lifestyle and subsequent non-communicable diseases. Preclinical and clinical experimental models to test this hypothesis are also proposed.

D3 dopamine receptor-preferring [11C]PHNO PET imaging in Parkinson patients with dyskinesia

OBJECTIVE

To investigate whether levodopa-induced dyskinesias (LID) are associated with D3 overexpression in levodopa-treated humans with Parkinson disease (PD).

METHODS

In this case-control study, we used PET with the D3-preferring radioligand [(11)C]-(+)-PHNO to estimate D2/3 receptor binding in patients with levodopa-treated PD with LID (n = 12) and without LID (n = 12), and healthy control subjects matched for age, sex, education, and mental status (n = 18).

RESULTS

Compared to nondyskinetic patients, those with LID showed heightened [(11)C]-(+)-PHNO binding in the D3-rich globus pallidus. Both PD groups also showed higher binding than controls in the sensorimotor division of the striatum. In contrast, D2/3 binding in the ventral striatum was lower in patients with LID than without, possibly reflecting higher dopamine levels.

CONCLUSIONS

Dopaminergic abnormalities contributing to LID may include elevated D2/3 binding in globus pallidus, perhaps reflecting D3 receptor upregulation. The findings support therapeutic strategies that target and diminish activity at D3 to prevent LID.

Brain-derived neurotrophic factor serum levels correlate with cognitive performance in Parkinson's disease patients with mild cognitive impairment

Brain-derived neurotrophic factor (BDNF) is a trophic factor regulating cell survival and synaptic plasticity. Recent findings indicate that BDNF could be a potential regulatory factor for cognitive functioning in normal and/or neuropathological conditions. With regard to neurological disorders, recent data suggest that individuals with Parkinson’s disease (PD) may be affected by cognitive deficits and that they have altered BDNF production. Therefore, the hypothesis can be advanced that BDNF levels are associated with the cognitive state of these patients. With this in mind, the present study was aimed at exploring the relationship between BDNF serum levels and cognitive functioning in PD patients with mild cognitive impairment (MCI). Thirteen PD patients with MCI were included in the study. They were administered an extensive neuropsychological test battery that investigated executive, episodic memory, attention, visual-spatial and language domains. A single score was obtained for each cognitive domain by averaging z-scores on tests belonging to that specific domain. BDNF serum levels were measured by enzyme-linked immunoassay (ELISA). Pearson’s correlation analyses were performed between BDNF serum levels and cognitive performance. Results showed a significant positive correlation between BDNF serum levels and both attention (p < 0.05) and executive (p < 0.05) domains. Moreover, in the executive domain we found a significant correlation between BDNF levels and scores on tests assessing working memory and self-monitoring/inhibition. These preliminary data suggest that BDNF serum levels are associated with cognitive state in PD patients with MCI. Given the role of BDNF in regulating synaptic plasticity, the present findings give further support to the hypothesis that this trophic factor may be a potential biomarker for evaluating cognitive changes in PD and other neurological syndromes associated with cognitive decline.

Structural plasticity in mesencephalic dopaminergic neurons produced by drugs of abuse: critical role of BDNF and dopamine

Mesencephalic dopaminergic neurons were suggested to be a critical physiopathology substrate for addiction disorders. Among neuroadaptive processes to addictive drugs, structural plasticity has attracted attention. While structural plasticity occurs at both pre- and post-synaptic levels in the mesolimbic dopaminergic system, the present review focuses only on dopaminergic neurons. Exposures to addictive drugs determine two opposite structural responses, hypothrophic plasticity produced by opioids and cannabinoids (in particular during the early withdrawal phase) and hypertrophic plasticity, mostly driven by psychostimulants and nicotine. In vitro and in vivo studies identified BDNF and extracellular dopamine as two critical factors in determining structural plasticity, the two molecules sharing similar intracellular pathways involved in cell soma and dendrite growth, the MEK-ERK1/2 and the PI3K-Akt-mTOR, via preferential activation of TrkB and dopamine D3 receptors, respectively. At present information regarding specific structural changes associated to the various stages of the addiction cycle is incomplete. Encouraging neuroimaging data in humans indirectly support the preclinical evidence of hypotrophic and hypertrophic effects, suggesting a possible differential engagement of dopamine neurons in parallel and partially converging circuits controlling motivation, stress, and emotions.

Adolescent social isolation influences cognitive function in adult rats

Adolescence is a critical period for neurodevelopment. Evidence from animal studies suggests that isolated rearing can exert negative effects on behavioral and brain development. The present study aimed to investigate the effects of adolescent social isolation on latent inhibition and brain-derived neurotrophic factor levels in the forebrain of adult rats. Male Wistar rats were randomly divided into adolescent isolation (isolated housing, 38–51 days of age) and social groups. Latent inhibition was tested at adulthood. Brain-derived neurotrophic factor levels were measured in the medial prefrontal cortex and nucleus accumbens by an enzyme-linked immunosorbent assay. Adolescent social isolation impaired latent inhibition and increased brain-derived neurotrophic factor levels in the medial prefrontal cortex of young adult rats. These data suggest that adolescent social isolation has a profound effect on cognitive function and neurotrophin levels in adult rats and may be used as an animal model of neurodevelopmental disorders.

What is the role of the D3 receptor in addiction? A mini review of PET studies with [(11)C]-(+)-PHNO

The chronic use of drugs, including psychostimulants such as cocaine and amphetamine, has been associated with low D2/3 dopamine receptor availability, which in turn has been linked to poor clinical outcome. In contrast, recent studies focused on the D3 receptor (a member of the D2-like receptor family) suggest that chronic exposure to stimulant drugs can up-regulate this receptor subtype, which, in preclinical models, is linked to dopamine system sensitization - a process hypothesized to contribute to relapse in addiction. In this mini review we present recent human data suggesting that the D3 receptor may contribute to core features of addiction, and discuss the usefulness of the PET imaging probe [(11)C]-(+)-PHNO in investigating this question.

Humoral activity of cord blood-derived stem/progenitor cells: implications for stem cell-based adjuvant therapy of neurodegenerative disorders

Background

Stem/progenitor cells (SPCs) demonstrate neuro-regenerative potential that is dependent upon their humoral activity by producing various trophic factors regulating cell migration, growth, and differentiation. Herein, we compared the expression of neurotrophins (NTs) and their receptors in specific umbilical cord blood (UCB) SPC populations, including lineage-negative, CD34⁺, and CD133⁺ cells, with that in unsorted, nucleated cells (NCs).

Methods and Results

The expression of NTs and their receptors was detected by QRT-PCR, western blotting, and immunofluorescent staining in UCB-derived SPC populations (i.e., NCs vs. lineage-negative, CD34⁺, and CD133⁺ cells). To better characterize, global gene expression profiles of SPCs were determined using genome-wide RNA microarray technology. Furthermore, the intracellular production of crucial neuro-regenerative NTs (i.e., BDNF and NT-3) was assessed in NCs and lineage-negative cells after incubation for 24, 48, and 72 h in both serum and serum-free conditions. We discovered significantly higher expression of NTs and NT receptors at both the mRNA and protein level in lineage-negative, CD34⁺, and CD133⁺ cells than in NCs. Global gene expression analysis revealed considerably higher expression of genes associated with the production and secretion of proteins, migration, proliferation, and differentiation in lineage-negative cells than in CD34⁺ or CD133⁺ cell populations. Notably, after short-term incubation under serum-free conditions, lineage-negative cells and NCs produced significantly higher amounts of BDNF and NT-3 than under steady-state conditions. Finally, conditioned medium (CM) from lineage-negative SPCs exerted a beneficial impact on neural cell survival and proliferation.

Conclusions

Collectively, our findings demonstrate that UCB-derived SPCs highly express NTs and their relevant receptors under steady-state conditions, NT expression is greater under stress-related conditions and that CM from SPCs favorable influence neural cell proliferation and survival. Understanding the mechanisms governing the characterization and humoral activity of subsets of SPCs may yield new therapeutic strategies that might be more effective in treating neurodegenerative disorders.

BDNF gene polymorphism, cognition and symptom severity in a Brazilian population-based sample of first-episode psychosis subjects

OBJECTIVE

To investigate the influence of brain-derived neurotrophic factor (BDNF) gene variations on cognitive performance and clinical symptomatology in first-episode psychosis (FEP).

METHODS

We performed BDNF val66met variant genotyping, cognitive testing (verbal fluency and digit spans) and assessments of symptom severity (as assessed with the PANSS) in a population-based sample of FEP patients (77 with schizophreniform psychosis and 53 with affective psychoses) and 191 neighboring healthy controls.

RESULTS

There was no difference in the proportion of Met allele carriers between FEP patients and controls, and no significant influence of BDNF genotype on cognitive test scores in either of the psychosis groups. A decreased severity of negative symptoms was found in FEP subjects that carried a Met allele, and this finding reached significance for the subgroup with affective psychoses (p < 0.01, ANOVA).

CONCLUSIONS

These results suggest that, in FEP, the BDNF gene Val66Met polymorphism does not exert a pervasive influence on cognitive functioning but may modulate the severity of negative symptoms.

From molecular to nanotechnology strategies for delivery of neurotrophins: emphasis on brain-derived neurotrophic factor (BDNF)

Neurodegenerative diseases represent a major public health problem, but beneficial clinical treatment with neurotrophic factors has not been established yet. The therapeutic use of neurotrophins has been restrained by their instability and rapid degradation in biological medium. A variety of strategies has been proposed for the administration of these leading therapeutic candidates, which are essential for the development, survival and function of human neurons. In this review, we describe the existing approaches for delivery of brain-derived neurotrophic factor (BDNF), which is the most abundant neurotrophin in the mammalian central nervous system (CNS). Biomimetic peptides of BDNF have emerged as a promising therapy against neurodegenerative disorders. Polymer-based carriers have provided sustained neurotrophin delivery, whereas lipid-based particles have contributed also to potentiation of the BDNF action. Nanotechnology offers new possibilities for the design of vehicles for neuroprotection and neuroregeneration. Recent developments in nanoscale carriers for encapsulation and transport of BDNF are highlighted.

Brain-derived neurotrophic factor Val66Met polymorphism and obsessive-compulsive symptoms in Egyptian schizophrenia patients

BACKGROUND

Brain-derived neurotrophic factor (BDNF) has been advanced as a candidate gene for schizophrenia. BDNF promote the function and growth of 5-HT neurons in the brain and modulate the synaptic plasticity of DRD3-secreting neurons in the striatum, suggesting involvement of BDNF in the mediation of obsessive-compulsive disorder.

OBJECTIVES

To test the hypothesis that the BDNF Val66Met polymorphism influence obsessive-compulsive symptoms (OCS) in schizophrenia, we examined the association between the BDNF Val66Met genotypes and OCS in a group of patients with schizophrenia.

METHODS

320 schizophrenia patients were assessed using the Yale-Brown Obsessive-Compulsive Scale (YBOCS). BDNF Val66Met polymorphism was genotyped using PCR-RFLP method, and severity of OCS were compared between the genotype groups.

RESULTS

Out of the 320 schizophrenia patients, 120 patients (37.5%) had significant OCS. There was a significant excess of valine allele in the schizophrenia with-OCS group compared to the without-OCS group. The mean YBOCS scores were significantly different among the three genotype groups. Val/Val homozygote patients had higher mean YBOCS scores compared to Val/Met genotype (p = 0.0001) as well as to the Met/Met homozygote group (p = 0.003).

CONCLUSION

Our data suggested an association between BDNF Val66Met polymorphism and OCS in Egyptian schizophrenia patients.

New approaches to therapy

L-DOPA-induced dyskinesia (LID) is a major complication of the treatment of Parkinson's disease (PD). LID comprises two major components, the priming process responsible for its onset and the expression of involuntary movements that underlies its clinical manifestation. The mechanisms responsible for these components are partially understood and their biochemical basis is being unraveled but avoidance and treatment remain an issue. In this chapter, we review what is known about the involvement of dopaminergic systems in LID and the way in which dopaminergic therapy can be used to avoid the onset of LID or to reverse or suppress involuntary movements once these have been established. The involvement of specific dopamine receptor subtypes, continuous dopaminergic stimulation (CDS) and continuous drug delivery (CDD) is reviewed. However, a major role is emerging in the avoidance and suppression of LID through the use of nondopaminergic mechanisms and we consider the present and future use of glutamatergic drugs, serotoninergic agents, adenosine antagonists and others as a means of improving therapy. There is compelling basic science supporting a role for nondopaminergic approaches to LID but at the moment the translational benefit to PD is not being achieved as predicted. There needs to be further consideration of why this is the case and how in future, both experimental models of dyskinesia and clinical trial design can be optimized to ensure success.

Imaging brain regional and cortical laminar effects of selective D3 agonists and antagonists

RATIONALE

Dopamine D3 receptors (D3R) may be important therapeutic targets for both drug abuse and dyskinesias in Parkinson's disease; however, little is known about their functional circuitry.

OBJECTIVES

We wished to determine if D3R antagonists SB-277011 and PG-01037 and D3R-preferring agonist 7-OH-DPAT are D3R selective in vivo. We further wished to characterize the response to D3R drugs using whole brain imaging to identify novel D3R circuitry.

METHODS

We investigated D3R circuitry in rats using pharmacologic MRI and challenge with selective D3R antagonists and agonist at various doses to examine regional changes in cerebral blood volume (CBV). We compared regional activation patterns with D2R/D3R agonists, as well as with prior studies of mRNA expression and autoradiography.

RESULTS

D3R antagonists induced positive CBV changes and D3R agonist negative CBV changes in brain regions including nucleus accumbens, infralimbic cortex, thalamus, interpeduncular region, hypothalamus, and hippocampus (strongest in subiculum). All D3R-preferring drugs showed markedly greater responses in nucleus accumbens than in caudate/putamen consistent with D3R selectivity and contrary to what was observed with D2R agonists. At high doses of D3R agonist, functional changes were differentiated across cortical laminae, with layer V-VI yielding positive CBV changes and layer IV yielding negative CBV changes. These results are not inconsistent with differential D1R and D3R innervation in these layers respectively showed previously using post-mortem techniques.

CONCLUSIONS

MRI provides a new tool for testing the in vivo selectivity of novel D3R dopaminergic ligands where radiolabels may not be available. Further, the functional D3R circuitry strongly involves hypothalamus and subiculum as well as the limbic striatum.

Brain-derived neurotrophic factor G196A polymorphism and clinical features in Parkinson's disease

OBJECTIVES

Parkinson's disease (PD) is characterized by the dopaminergic neuronal death in substantia nigra, and genetic factors appear to be involved in the pathophysiology of this disease. Brain-derived neurotrophic factor (BDNF) is widely expressed in the central nervous system and is necessary for the survival of dopaminergic neurons in substantia nigra. G196A, a common polymorphism of the BDNF gene, not only affects cognitive and motor processes, but also is associated with various psychiatric disorders. We evaluated whether G196A polymorphism is associated with PD and/or modifies clinical manifestations in PD patients.

METHODS

We included 193 PD patients and 300 control subjects. G196A polymorphism was screened by restriction fragment length polymorphism analysis. Clinical features of each patient were examined in detail. The possible association between genotype and clinical characteristics were determined by bivariate and multivariate analyses.

RESULTS

The distribution of G196A allele and genotype frequency was similar between PD and control subjects. Clinical characteristics, including Hoehn-Yahr stage, motor symptoms, non-motor symptoms (depression, cognitive dysfunction, psychiatric dysfunctions, and sleep behavior disorder), and dyskinesias, were not associated with this polymorphism.

CONCLUSIONS

G196A polymorphism is not a risk factor for PD and does not seem to modify clinical features in PD patients studied here.

CNS inflammation and macrophage/microglial biology associated with HIV-1 infection

Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system (CNS) can result in neurological dysfunction with devastating consequences in a significant proportion of individuals with acquired immune deficiency syndrome. HIV-1 does not infect neurons directly but induces damage indirectly through the accumulation of activated macrophage/microglia (M/M) cells, some of which are infected, that release neurotoxic mediators including both cellular activation products and viral proteins. One mechanism for the accumulation of activated M/M involves the development in infected individuals of an activated peripheral blood monocyte population that traffics through the blood-brain barrier, a process that also serves to carry virus into CNS and establish local infection. A second mechanism involves the release by infected and activated M/M in the CNS of chemotactic mediators that recruit additional monocytes from the periphery. These activated M/M, some of which are infected, release a number of cytokines and small molecule mediators as well as viral proteins that act on bystander cells and in turn activate them, thus amplifying the cascade. These viral proteins and cellular products have neurotoxic properties as well, both directly and through induction of astrocyte dysfunction, which ultimately lead to neuronal injury and death. In patients effectively treated with antiretroviral therapy, frank dementia is now uncommon and has been replaced by milder forms of neurocognitive impairment, with less frequent and more focal neuropathology. This review summarizes key findings that support the critical role and mechanisms of monocyte/macrophage activation and inflammation as a major component for HIV-1 encephalitis or HIV-1 associated dementia.

Brain derived neurotrophic factor and neurotrophic factor 3 modulate neurotransmitter receptor expressions on developing spiral ganglion neurons

Cochlear spiral ganglion neurons (SGN) provide the only pathway for transmitting sound evoked activity from the hair cells to the central auditory system. Neurotrophic factor 3 (NT-3) and brain derived neurotrophic factor (BDNF) released from hair cells and supporting cells exert a profound effect on SGN survival and neural firing patterns; however, it is unclear what the effects NT-3 and BDNF have on the type of neurotransmitter receptors expressed on SGN. To address this question, the whole-cell patch clamp recording technique was used to determine what effect NT-3 and BDNF had on the function and expression of glutamate, GABA and glycine receptors (GlyR) on SGN of cochlea from postnatal C57 mouse. Receptor currents induced by the agonist of each receptor were recorded from SGN cultured with or without BDNF or NT-3. NT-3 and BDNF exerted different effects. NT-3, and to a lesser extent BDNF, enhanced the expression of GABA receptors and had comparatively little effect on glutamate receptors. Absence of BDNF and NT-3 resulted in the emergence of glycine-induced currents; however, GlyR currents were absent from the short term cultured SGN. In contrast, NT-3 and BDNF suppressed GlyR expression on SGN. These results indicate that NT-3 and BDNF exert a profound effect on the types of neurotransmitter receptors expressed on postnatal SGN, results that may have important implications for neural development and plasticity.

Differential susceptibility to ethanol and amphetamine sensitization in dopamine D3 receptor-deficient mice

RATIONALE

Dopamine D3 receptors (D3Rs) have been implicated in behavioral sensitization to various drugs of abuse, but their role in ethanol (EtOH) sensitization has not been directly examined. We used D3R knockout (D3 KO) mice to examine whether the D3R plays a permissive role in EtOH and amphetamine (AMPH) sensitization. We also investigated whether EtOH sensitization is accompanied by alterations in D3R mRNA expression or binding.

MATERIALS AND METHODS

After comparing EtOH sensitization in C57Bl/6 mice and DBA/2 mice, D3 KO, wild type (WT), and for comparison, D1 and D2 KOs received five biweekly injections of EtOH (2.2 g/kg, i.p.) or saline. Another group of D3 KOs and WT controls received six times AMPH (1.5 mg/kg, i.p.). D3R mRNA and binding were measured in EtOH-sensitized DBA/2 mice with in situ hybridization and [(125)I]-7-OH-PIPAT autoradiography, respectively.

RESULTS

C57Bl/6 mice expressed EtOH sensitization albeit to a lesser extent than DBA/2 mice. Compared to WT mice, D3 KOs were resistant to EtOH sensitization. The behavioral profile of D3 KOs was more similar to D1 KOs than D2 KOs, which also failed to develop EtOH sensitization. However, D3 KOs developed AMPH sensitization normally. EtOH sensitization was not accompanied by changes in either D3R mRNA or D3R binding in the islands of Calleja, nucleus accumbens, dorsal striatum, or cerebellum.

CONCLUSIONS

These results suggest a necessary role for the D3R in EtOH but not AMPH sensitization, possibly through postreceptor intracellular mechanisms. Results also suggest that different neurochemical mechanisms underlie sensitization to different drugs of abuse.

Decreased binding of the D3 dopamine receptor-preferring ligand [11C]-(+)-PHNO in drug-naive Parkinson's disease

The D(3) dopamine (DA) receptor is a member of the D(2)-like DA receptor family. While the D(2) receptor is abundant especially in motor-regions of the striatum, the D(3) receptor shows a relative abundance in limbic regions and globus pallidus. This receptor is of current interest in neurology because of its potential involvement in psychiatric and motor complications in Parkinson's disease and the possibility that dopamine D(3)-preferring agonist therapy might delay progression of the disorder. Preclinical data indicate that striatal levels of the D(3) (but not the D(2)) DA receptor are decreased following lesion of nigrostriatal DA neurons; at present, there are no in vivo data on this receptor subtype in Parkinson's disease. The objective of this positron emission tomography study was to compare [(11)C]-(+)-PHNO (D(3) versus D(2) preferring) and [(11)C]raclopride (D(3) = D(2)) binding in brain of non-depressed, non-demented, dopaminergic drug-naïve patients with early-stage Parkinson's disease (n = 10), relative to matched-controls (n = 9). Parkinson's disease was associated with a trend for bilaterally decreased [(11)C]-(+)-PHNO (but not [(11)C]raclopride) binding in the D(3)-rich ventral striatum (-11%, P = 0.07) and significantly decreased binding in globus pallidus (-42%, P = 0.02). In contrast, in the primarily D(2)-populated putamen, both [(11)C]-(+)-PHNO (25%, P = 0.02) and [(11)C]raclopride (25%, P < 0.01) binding were similarly increased, especially on the side contra-lateral to the symptoms. In the midbrain, presumably containing D(3) receptors localized to the substantia nigra, [(11)C]-(+)-PHNO binding was normal. Decreased [(11)C]-(+)-PHNO to [(11)C]raclopride ratio correlated with motor deficits and lowered-mood (P < 0.02). Our imaging data suggest that brain DA neuron loss in the human causes region-specific differential changes in DA D(2) and D(3) receptors with D(3) receptor 'downregulation' possibly related to some motor and mood problems in Parkinson disease. D(3) receptor levels might be a determinant vulnerability factor underlying side-effects associated with treatment; hence, these initial findings provide valuable baseline information to understand the role of D(3) receptors in response to Parkinson's disease medication.

Investigating the role of the brain-derived neurotrophic factor (BDNF) val66met variant in obsessive-compulsive disorder (OCD)

Although evidence from family studies suggest that genetic factors play an important role in mediating obsessive-compulsive disorder (OCD), results from genetic case-control association analyses have been inconsistent. Discrepant findings may be attributed to the lack of phenotypic resolution, and population stratification. The aim of the present study was to investigate the role that the val66met variant within the gene encoding brain-derived neurotrophic factor (BDNF) may play in mediating the development of selected OCD subtypes accounting for the aforementioned confounding factors. One hundred and twelve OCD subjects and 140 controls were selected from the South African Afrikaner population. A significant association was observed in the male subgroup, with the met66 allele implicated as the risk allele in the development of OCD. This allele was also found to be associated with an earlier age at onset of OCD in males. On the other hand, the val66val genotype was associated with more severe OCD in the female population. No evidence of population stratification was observed in Afrikaner control subjects. These preliminary results point towards genetically distinct characteristics of OCD mediated by dysfunctions in BDNF. The present investigation forms part of ongoing research to elucidate the genetic components involved in the aetiology of OCD and OCD-related characteristics.

Memantine upregulates BDNF and prevents dopamine deficits in SIV-infected macaques: a novel pharmacological action of memantine

N-methyl-D-aspartate (NMDA) receptor activation is involved in the pathogenetic cascades of neurodegenerative disorders including human immunodeficiency virus (HIV) dementia. Memantine, an uncompetitive NMDA receptor antagonist, which has been recently approved for the treatment of Alzheimer's disease, is being discussed as a potential adjunctive therapeutic substance for HIV dementia. We used simian immunodeficiency virus-infected rhesus macaques to assess the effects of memantine on brain dysfunction and brain pathology within 3-5 months after initial infection during early asymptomatic stage of disease. We had shown previously that within this time frame, marked changes were evident in the dopaminergic systems. Memantine was administered two weeks post infection, at peak viremia, in order to prevent early NMDA receptor activation due to immune mediators. We found that memantine prevented onset of dopamine deficits in the brains of SIV-infected macaques, without affecting early brain pathology or peripheral course of infection. Memantine specifically upregulated mRNA and protein expression of the neurotrophic factor brain-derived neurotrophic factor (BDNF), suggesting that the protective effect of memantine on dopamine function may be mechanistically remote from NMDA receptor antagonism. This novel pharmacological action of memantine may also be relevant for other neurodegenerative disorders and supports the involvement of neurotrophic factors in adult brain neuroprotection.

In vivo evidence of D3 dopamine receptor sensitization in parkinsonian primates and rodents with l-DOPA-induced dyskinesias

A growing body of evidence indicates a role for D(3) receptors in l-DOPA-induced dyskinesias. This involvement could be amenable to non-invasive in vivo analysis using functional neuroimaging. With this goal, we examined the hemodynamic response to the dopamine D(3)-preferring agonist 7-hydroxy-N,N-di-n-propyl-2 aminotetralin (7-OHDPAT) in naïve, parkinsonian and l-DOPA-treated, dyskinetic rodents and primates using pharmacological MRI (phMRI) and relative cerebral blood volume (rCBV) mapping. Administration of 7-OHDPAT induced minor negative changes of rCBV in the basal ganglia in naïve and parkinsonian animals. Remarkably, the hemodynamic response was reversed (increased rCBV) in the striatum of parkinsonian animals rendered dyskinetic by repeated l-DOPA treatment. Such increase in rCBV is consistent with D(1) receptor-like signaling occurring in response to D(3) stimulation, demonstrates a dysregulation of dopamine receptor function in dyskinesia and provides a potentially novel means for the characterization and treatment of l-DOPA-induced dyskinesia in patients.

Production of brain-derived neurotrophic factor by mononuclear cells of patients with multiple sclerosis treated with glatiramer acetate, interferon-beta 1a, and high doses of immunoglobulins

Sixty, relapsing remitting (RR) multiple sclerosis (MS) patients, who underwent treatment with glatiramer acetate (GA), interferon (IFN)-beta 1a, and immunoglobulins (Igs) (20 per treatment group), were assessed for levels of brain-derived neurotrophic factor (BDNF) in the supernatants of unstimulated and stimulated peripheral blood mononuclear cells (PBMCs) in the first year of treatment. Phytohemagglutinin (PHA), anti-OKT3 antibody, myelin basic protein (MPB) and GA were used as stimuli. Cytokine responses by ELISPOT and lymphoproliferative responses were also assessed. The GA-treated MS patient group showed a progressive increase in BDNF levels, from baseline to month three; thereafter, the levels remained stable and significantly greater compared with baseline and controls (ANOVA=P<0.001). IFN-beta 1a had no effect on BDNF production, whereas Igs induced a slight decrease (ANOVA=P<0.04). ELISPOT analysis revealed a significant decrease of IFN-gamma, an increase of interleukin (IL)-4 and IL-5 in GA-treated MS patients, and an increase of IL-10 in patients treated with IFN-beta 1a and GA. No significant correlation was found between BDNF secretion in the supernatants of PBMCs and cytokine response, lesional load, and measures of atrophy. Increased BDNF production related to GA treatment can have implications for understanding the mechanism of action of this immunomodulatory agent, in light of evidence suggesting its effects in promoting neuroprotective immunity in MS patients; however, a clinically measurable effect, especially in terms of an impact on actual disease progression, remains to be established.

Dopamine D2-like receptors and the antidepressant response

Converging lines of evidence suggest a role for the mesolimbic dopamine system in the response to somatic antidepressant therapies. Here, we review evidence suggesting that antidepressant treatments of different types share the effect of increasing the sensitivity of dopamine D2-like receptors in the nucleus accumbens, clinical studies suggesting that activation of these receptors has antidepressant efficacy, as well as relevant imaging and genetic data on the role of this system in the antidepressant response. We then attempt to reconcile this data with evidence of a common target of antidepressant drugs in the cyclic adenosine monophosphate (cAMP) response element binding protein-brain-derived neurotrophic factor (CREB-BDNF) pathway in a model that suggests potential directions for future inquiry.

Dose-response analysis of locomotor activity and stereotypy in dopamine D3 receptor mutant mice following acute amphetamine

Accumulating evidence suggests that dopamine D3 receptor (D3R) stimulation is inhibitory to spontaneous and psychostimulant-induced locomotion through opposition of concurrent D1R and D2R-mediated signaling. To evaluate this model, we used homozygous D3R mutant mice and wild-type controls to investigate the role of the D3R in locomotor activity and stereotypy stimulated by acute amphetamine (AMPH) (0.2, 2.5, 5.0, 10.0 mg/kg). At the lowest dose tested (0.2 mg/kg), neither D3R mutant mice nor wild-type mice exhibited measurable change in locomotor activity or stereotypy relative to their respective saline-treated controls. D3R mutant mice exhibited a significantly greater increase in locomotor activity, but not stereotypy, relative to wild-type mice in response to treatment with AMPH 2.5 mg/kg. AMPH-induced locomotor activity and stereotypy were similar in both wild-type and D3R mutant mice at both the 5.0 and 10 mg/kg AMPH doses. These findings provide further support for an inhibitory role for the D3R in AMPH-induced locomotor activity, and demonstrate a more limited role for the D3R in modulating AMPH-induced stereotypy.

Brain-derived neurotrophic factor (BDNF) Val66Met polymorphism in schizophrenia is associated with age at onset and symptoms

Brain-derived neurotrophic factor (BDNF) is a neurotrophic factor that promotes several functions of neurons and modulates neurotransmissions. It has been reported that there are alterations of BDNF levels in schizophrenic brains and that BDNF gene expressional changes would be responsible for the etiology of schizophrenia. Recent studies have shown that a variation of BDNF gene (Val66Met polymorphism) affects the function of neurons, and is associated with several neurological and psychiatrical disorders. We investigated the relationship between BDNF Val66Met polymorphism and the onset age as well as levels of clinical symptoms in 159 of chronic schizophrenia in-patients diagnosed by DSM-IV. The mean onset ages were 27.5+/-9.5 for BDNF Val/Val, 25.5+/-7.4 for BDNF Val/Met and 22.9+/-6.0 for BDNF Met/Met and this polymorphism was significantly associated with age at onset (P=0.023). The mean Brief Psychiatric Rating Scale scores (BPRS) were significantly different among those three groups (P=0.003). No significant differences were demonstrated comparing the BDNF genotype distributions of positive and negative family history (P=0.21). Our investigation indicates that the BDNF gene Val66Met polymorphism is related to the onset age of schizophrenia and the levels of clinical symptoms that remain after long-term antipsychotic treatment.

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 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.

Habit Learning and the Genetics of the Dopamine D₃ Receptor: Evidence From Patients With Schizophrenia and Healthy Controls

In this study, the authors investigated the relationship between the Ser9Gly (SG) polymorphism of the dopamine D3 receptor (DRD3) and striatal habit learning in healthy controls and patients with schizophrenia. Participants were given the weather prediction task, during which probabilistic cue-response associations were learned for tarot cards and weather outcomes (rain or sunshine). In both healthy controls and patients with schizophrenia, participants with Ser9Ser (SS) genotype did not learn during the early phase of the task (1-50 trials), whereas participants with SG genotype did so. During the late phase of the task (51-100 trials), both participants with SS and SG genotype exhibited significant learning. Learning rate was normal in patients with schizophrenia. These results suggest that the DRD3 variant containing glycine is associated with more efficient striatal habit learning in healthy controls and patients with schizophrenia.

Neurodegenerative alterations in the nigrostriatal system of trkB hypomorphic mice

Brain-derived neurotrophic factor (BDNF) acts through the neurotrophin receptor TrkB and promotes survival and differentiation of dopaminergic ventral mesencephalic neurons. To further evaluate the role of TrkB in the nigrostriatal pathway, we studied neurotrophin levels, dopamine metabolism, and morphology of dopaminergic neurons of the substantia nigra (SN-DA) in young adult hypomorphic trkB mice (trkBfbz/fbz), which express only approximately 25% of wild type levels of TrkB. Tyrosine hydroxylase immunostaining revealed altered morphology of SN-DA neurons in trkBfbz/fbz when compared to wild type mice, in particular a significant enlargement of nuclear size. Cell counts revealed a pronounced loss of SN-DA neurons in these mice. Measurement of monoamine levels by high performance liquid chromatography (HPLC) showed that dopamine (DA) levels in the target field (striatum) were significantly elevated in trkBfbz/fbz compared to trkB+/fbz and wild type mice (P < 0.05), without altering DA turnover. Likewise, enzyme-linked immunosorbent assay (ELISA) for neurotrophic factors measurement showed that BDNF levels were increased in the striatum (P < 0.01) and frontal cortex (P < 0.005) of trkBfbz/fbz mice, but not in the SN when compared to trkB+/fbz and wild type mice. These data suggest that elevated neurotransmitter and neurotrophic factor levels might be a compensatory mechanism following dopaminergic cell loss in the SN. Thus, TrkB-activation seems essential for the maintenance of the nigrostriatal dopaminergic system.

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.