Modulation of nigrostriatal dopaminergic transmission by antisense oligodeoxynucleotide against brain-derived neurotrophic factor

Bdnf gene is a downstream target of Nurr1 transcription factor in rat midbrain neurons in vitro

The transcription factor Nurr1 is essential for the generation of midbrain dopaminergic neurons (mDA). Only a few Nurr1-regulated genes have so far been identified and it remains unclear how Nurr1 influences the development and function of dopaminergic neurons. To identify novel Nurr1 target genes we have used genome-wide expression profiling in rat midbrain primary cultures, enriched in dopaminergic neurons, following up-regulation of Nurr1 expression by depolarization. In this study we demonstrate that following depolarization the hyperexpression of Nurr1 and the brain derived neurotrophic factor (BDNF) are phospholipase C- and protein kinase C-dependent. We show that Bdnf, which encodes a neurotrophin involved also in the phenotypic maturation of mDA neurons, is a novel Nurr1 target gene. By RNA interference experiments we show that a decreased Nurr1 expression is followed by tyrosine hydroxylase and BDNF mRNA and protein down-regulation. Reporter gene assay experiments performed on midbrain primary cultures using four Bdnf promoter constructs show that Bdnf is a direct target gene of Nurr1. Taken together, our findings suggest that Nurr1 might also influence the development and the function of midbrain dopaminergic neurons via direct regulation of Bdnf expression.

Rotenone produces opposite effects upon mouse striatal dopamine function as a result of environmental temperature

Rotenone is a commonly used pesticide that can function as an environmental neurotoxin. Rotenone is a known mitochondrial complex I inhibitor which can lead to oxidative stress and results in dopaminergic cell death. Another environmental factor known to exacerbate oxidative stress and result in striatal dopaminergic cell death is elevated environmental temperature. In this study we evaluated the effects of a single injection of various doses of rotenone (0.65, 1.3 and 2.6 mg/kg) on striatal dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations in CD-1 mice and compared this with a single injection of two doses of methamphetamine (MA - 10 or 20 mg/kg), a known striatal DA depleting agent, as administered to mice maintained at 21 degrees C (Experiment 1). These results were then compared to striatal DA and DOPAC concentrations of mice treated with rotenone (1.3 or 2.6 mg/kg) or MA (10 or 20 mg/kg) administered to mice maintained at 28 degrees C (Experiment 2). A single injection of rotenone to mice maintained at 21 degrees C resulted in a significant increase in DA and decrease in DOPAC concentrations for all doses tested compared to controls, whereas a single injection of MA at the same temperature resulted in a significant decrease in DA and no change in DOPAC concentrations. At a temperature of 28 degrees C, a single injection of rotenone resulted in a significant decrease in both DA and DOPAC concentrations similar to that seen with the MA-treated mice. Collectively, these results indicate that rotenone interacts with environmental temperature to produce opposite effects upon striatal DA concentrations -- significantly increasing striatal DA when administered at 21 degrees C and significantly decreasing striatal DA when administered at 28 degrees C, while producing similar decreases in striatal DOPAC under both temperatures.

Inhibiting BDNF expression by antisense oligonucleotide infusion causes loss of nigral dopaminergic neurons

Brain derived neurotrophic factor (BDNF) expression is significantly reduced in the Parkinson's disease substantia nigra. This neurotrophin has potent affects on dopaminergic neuron survival protecting them from the neurotoxins MPTP and 6-hydroxydopamine (6-OHDA) commonly used to create animal models of Parkinson's disease and also promoting dopaminergic axonal sprouting. In this study, we demonstrate that an antisense oligonucleotide infusion (200 nM for 28 days) to prevent BDNF production in the substantia nigra of rats mimics many features of the classical animal models of Parkinson's disease. 62% of antisense treated rats rotate (P < or = 0.05) in response to dopaminergic receptor stimulation by apomorphine. 40% of substantia nigra pars compacta tyrosine hydroxylase immunoreactive neurons are lost (P < or = 0.00001) and dopamine uptake site density measured by (3)H-mazindol autoradiography is reduced by 34% (P < or = 0.005). Loss of haematoxylin and eosin stained nigral neurons is significant (P < or = 0.0001) but less extensive (34%). These observations indicate that loss of BDNF expression leads both to down regulation of the dopaminergic phenotype and to dopaminergic neuronal death. Therefore, reduced BDNF mRNA expression in Parkinson's disease substantia nigra may contribute directly to the death of nigral dopaminergic neurons and the development of Parkinson's disease.

Maturation but not survival of dopaminergic nigrostriatal neurons is affected in developing and aging BDNF-deficient mice

Brain-derived neurotrophic factor (BDNF) promotes survival of injured dopaminergic nigrostriatal neurons of the adult rodent substantia nigra pars compacta, as well their development in vitro. BDNF deficiency may play a role in Parkinson's disease, as the surviving dopaminergic nigrostriatal neurons have reduced levels of BDNF, and a BDNF gene polymorphism is present in a subpopulation of patients. Here, we investigated whether a lack of BDNF in early postnatal BDNF-/- mice or a chronic 50% reduction in BDNF levels in aging BDNF+/- mice would affect the survival of the dopaminergic nigrostriatal neurons. In general terms, BDNF-/- and BDNF+/- mice had morphologically and quantitatively normal nigrostriatal neurons at any time between postnatal day 14 (P14) and 18 months, when compared to their wild-type littermates. BDNF-/- mice (P14 and P21 only) had fewer dopaminergic dendrites in the substantia nigra, suggesting that BDNF plays a role in phenotypic maturation, but not in neuronal birth or survival. BDNF-/- mice also had aberrant tyrosine hydroxylase (TH) positive cell bodies in the pars reticulata. During adulthood and aging, BDNF+/- mice performed equally well as their wild-type littermates in tests of motor coordination, and both showed aging-related decreases in the size of the dopaminergic neurons as well as in motor coordination. These results suggest that chronic deficits in BDNF alone do not affect survival or function of dopaminergic nigrostriatal neurons during aging or potentially even in Parkinson's disease.

Factors promoting survival of mesencephalic dopaminergic neurons

Growth factors promoting survival of mesencephalic dopaminergic neurons are discussed in the context of their requirement during development and adulthood. The expression of growth factors should be detectable in the nigrostriatal system during critical periods of development, i.e., during the period of ontogenetic cell death and synaptogenesis and during neurite extension and neurotransmitter synthesis. Growth factors discussed include members of the family of glial-cell-line-derived neurotrophic factors (GDNF), neurotrophins, transforming growth factors beta, and low molecular compounds mimicking growth factor activities. To date, the available data support the notion that GDNF is a highly promising candidate, although GDNF-null mice lack a dopaminergic phenotype. There remains a possibility that endogenous dopaminotrophic factors remain to be discovered.

Age-related changes in nigrostriatal dopaminergic function are accentuated in +/− brain-derived neurotrophic factor mice

The effects of a deletion for the brain derived neurotrophic factor (BDNF) allele (+/- BDNF) upon age-related changes in nigrostriatal dopaminergic (NSDA) function were assessed. Behavioral (beam crossing and spontaneous activity) and neurochemical (potassium-stimulated dopamine release from superfused striatum) measures were compared among Young (4-5 month), Middle (11-13 month) and Aged (19-21 month) +/- BDNF and their wild type littermate control (+/+ BDNF) mice. No statistically significant differences were obtained between +/+ and +/- BDNF mice at the Young age sampling period for any of the behavioral or neurochemical measures. Behavioral and neurochemical responses indices of NSDA function begin to diverge between +/+ and +/- Middle age BDNF mice and maximal differences were observed at the Aged period. For both movement and stereotypy times, scores obtained from +/+ mice were significantly decreased compared with +/- BDNF mice at the Aged period and center time scores of +/+ mice were decreased at both the Middle and Aged periods compared with +/- BDNF mice. Neurochemically, potassium-stimulated DA release of +/+ mice was significantly greater than +/- BDNF mice with maximal differences obtained at the Aged period. These results demonstrate marked differences in age-related changes of NSDA function between +/+ and +/- BDNF mice and suggest that the deletion of one allele for BDNF may make these mice more susceptible to age-related declines in NSDA function.

Striatal dopamine output is compromised within +/- BDNF mice

We reported previously that mice lacking one brain-derived neurotrophic factor (BDNF) allele demonstrate elevated striatal dopamine (DA) concentrations but impaired behavioral responses involving the nigrostriatal dopaminergic (NSDA) system. To test the hypothesis that these elevated striatal DA concentrations are associated with perturbed NSDA functioning, we compared striatal DA output between heterozygous mutant (+/-) and wild-type littermate control (+/+) BDNF mice under conditions of an intact NSDA system, as well as following methamphetamine (MA)-induced neurotoxicity. Basal DA output from superfused CS tissue fragments did not differ between +/+ and +/- BDNF mice. Potassium (K+) stimulated DA outputs from intact striatal fragments of +/+ mice were significantly greater than that of +/- BDNF mice. Following MA treatment, K+ stimulated DA output of +/+ mice was statistically equivalent to +/- BDNF mice. Striatal DA concentrations of +/- BDNF mice were elevated, albeit not significantly, in both intact and MA-treated mice relative to +/+ mice. Following MA treatment, striatal DA concentrations were significantly decreased for both genotypes; however, the degree of DA depletion was significantly greater in +/+ mice. Analyzed collectively, these data show the differential effects exerted by a BDNF mutation upon striatal DA concentrations and output. Notably, lower striatal DA concentrations of +/+ vs. +/- BDNF mice can be contrasted with the significantly greater K+ stimulated DA output from the former. This difference was abolished following MA treatment. These results suggest that processes involved with the dynamics of DA release within the NSDA system may be compromised in +/- BDNF mutant mice.

Evaluation of nigrostriatal dopaminergic function in adult +/+ and +/- BDNF mutant mice

Deletion of a single copy of the BDNF gene has been shown to affect the nigrostriatal dopaminergic system of young adult BDNF mice. In the present report we evaluated various indices of nigrostriatal dopaminergic function between 9-month-old wild-type (+/+) and heterozygous (+/-) BDNF mutant mice. Performance in a sensorimotor beam walking task was significantly decreased in +/- mice as indicated by increased times required to traverse both a wide (21 mm) and narrow (6 mm) beam. No differences in spontaneous locomotor behavior were observed between the +/+ and +/- mice. Amphetamine-stimulated (5 mg/kg) locomotor behavior was increased to a greater degree in the +/- mice, with the number of movements performed by these mice being significantly greater than their +/+ controls. Corpus striatal dopamine concentrations were significantly greater in the +/- BDNF mice. The absence of any significant differences for dopamine concentrations within the hypothalamus and olfactory bulb of these mice, as well as an absence of any difference in striatal norepinephrine concentrations, suggested a relative specificity of these effects to the corpus striatum. Both the +/- and +/+ mice showed similar reductions in striatal dopamine concentrations in response to a neurotoxic regimen of methamphetamine (20 mg/kg). Collectively these data show increased levels of striatal dopamine concentrations associated with altered behavioral responses involving the nigrostriatal dopaminergic system within the heterozygous BDNF mutant mice.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Brain as a unique antisense environment

During the last few years, antisense oligodeoxyribonucleotides (asODN) have become a commonly used tool for blocking of gene expression in the mammalian central nervous system. Successful gene inhibition has been reported for such diverse targets as those encoding neurotransmitter receptors, neuropeptides, trophic factors, transcription factors, cytokines, transporters, ion channels, and others. This review presents a discussion of recent studies on ODN in the brain, with a focus on specific approaches taken by the researchers in this field and especially on peculiar features of this organ as a milieu for asODN action. It is concluded that from the presented literature survey no coherent view on how to rationally design ODN for brain studies has emerged.

An immortalized, type-1 astrocyte of mesencephalic origin source of a dopaminergic neurotrophic factor

Rat embryonic d 14 (E14) mesencephalic cells, 2.5% of which are glioblasts, were incubated in medium containing 10% of fetal bovine serum for 12 h and subsequently expanded in a serum-free medium using basic fibroblast growth factor (bFGF) as the mitogen. On a single occasion, after more than 15 d in culture, several islets of proliferating, glial-like cells were observed in one dish. The cells, when isolated and passaged, proliferated rapidly in either a serum-free or serum-containing growth medium. Subsequent immunocytochemical analysis showed that they stained positive for GFAP and vimentin, and negative for A2B5, O4, GalC, and MAP2. Serum-free conditioned medium (CM) prepared from these cells caused a fivefold increase in survival and promoted neuritic expansion of E14 mesencephalic dopaminergic neurons in culture. These actions are similar to those exerted by CM derived from primary, mesencephalic type-1 astrocytes. The pattern of expression of the region-selective genes; wnt-1, en-1, sis showed that 70% of the cells were heteroploid, and of these, 50% were tetraploid. No apparent decline in proliferative capacity has been observed after 25 passages. The properties of this cell line, named ventral mesencephalic cell line one (VMCL1), are consistent with those of an immortalized, type-1 astrocyte. The mesencephalic origin of the cell line, and the pattern and potency of the neurotrophic activity exerted by the CM, strongly suggest that the neurotrophic factor(s) identified are novel, and will likely be strong candidates with clinical utility for the treatment of Parkinson's disease.