Chronic intranigral administration of brain-derived neurotrophic factor produces striatal dopaminergic hypofunction in unlesioned adult rats and fails to attenuate the decline of striatal dopaminergic function following medial forebrain bundle transection

Potential Therapeutic Approach using Aromatic l-amino Acid Decarboxylase and Glial-derived Neurotrophic Factor Therapy Targeting Putamen in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative illness characterized by specific loss of dopaminergic neurons, resulting in impaired motor movement. Its prevalence is twice as compared to the previous 25 years and affects more than 10 million individuals. Lack of treatment still uses levodopa and other options as disease management measures. Treatment shifts to gene therapy (GT), which utilizes direct delivery of specific genes at the targeted area. Therefore, the use of aromatic L-amino acid decarboxylase (AADC) and glial-derived neurotrophic factor (GDNF) therapy achieves an effective control to treat PD. Patients diagnosed with PD may experience improved therapeutic outcomes by reducing the frequency of drug administration while utilizing provasin and AADC as dopaminergic protective therapy. Enhancing the enzymatic activity of tyrosine hydroxylase (TH), glucocorticoid hormone (GCH), and AADC in the striatum would be useful for external L-DOPA to restore the dopamine (DA) level. Increased expression of glutamic acid decarboxylase (GAD) in the subthalamic nucleus (STN) may also be beneficial in PD. Targeting GDNF therapy specifically to the putaminal region is clinically sound and beneficial in protecting the dopaminergic neurons. Furthermore, preclinical and clinical studies supported the role of GDNF in exhibiting its neuroprotective effect in neurological disorders. Another Ret receptor, which belongs to the tyrosine kinase family, is expressed in dopaminergic neurons and sounds to play a vital role in inhibiting the advancement of PD. GDNF binding on those receptors results in the formation of a receptor-ligand complex. On the other hand, venous delivery of recombinant GDNF by liposome-based and encapsulated cellular approaches enables the secure and effective distribution of neurotrophic factors into the putamen and parenchyma. The current review emphasized the rate of GT target GDNF and AADC therapy, along with the corresponding empirical evidence.

Neurotrophins as Therapeutic Agents for Parkinson’s Disease; New Chances From Focused Ultrasound?

Magnetic Resonance–guided Focused Ultrasound (MRgFUS) represents an effective micro-lesioning approach to target pharmaco-resistant tremor, mostly in patients afflicted by essential tremor (ET) and/or Parkinson’s disease (PD). So far, experimental protocols are verifying the clinical extension to other facets of the movement disorder galaxy (i.e., internal pallidus for disabling dyskinesias). Aside from those neurosurgical options, one of the most intriguing opportunities of this technique relies on its capability to remedy the impermeability of blood–brain barrier (BBB). Temporary BBB opening through low-intensity focused ultrasound turned out to be safe and feasible in patients with PD, Alzheimer’s disease, and amyotrophic lateral sclerosis. As a mere consequence of the procedures, some groups described even reversible but significant mild cognitive amelioration, up to hippocampal neurogenesis partially associated to the increased of endogenous brain-derived neurotrophic factor (BDNF). A further development elevates MRgFUS to the status of therapeutic tool for drug delivery of putative neurorestorative therapies. Since 2012, FUS-assisted intravenous administration of BDNF or neurturin allowed hippocampal or striatal delivery. Experimental studies emphasized synergistic modalities. In a rodent model for Huntington’s disease, engineered liposomes can carry glial cell line–derived neurotrophic factor (GDNF) plasmid DNA (GDNFp) to form a GDNFp-liposome (GDNFp-LPs) complex through pulsed FUS exposures with microbubbles; in a subacute MPTP-PD model, the combination of intravenous administration of neurotrophic factors (either through protein or gene delivery) plus FUS did curb nigrostriatal degeneration. Here, we explore these arguments, focusing on the current, translational application of neurotrophins in neurodegenerative diseases.

Gene Therapy for Parkinson's Disease, An Update

The current mainstay treatment of Parkinson's disease (PD) consists of dopamine replacement therapy which, in addition to causing several side effects, does not delay disease progression. The field of gene therapy offers a potential means to improve current therapy. The present review gives an update of the present status of gene therapy for PD. Both non-disease and disease modifying transgenes have been tested for PD gene therapy in animal and human studies. Non-disease modifying treatments targeting dopamine or GABA synthesis have been successful and promising at improving PD symptomatology in randomized clinical studies, but substantial testing remains before these can be implemented in the standard clinical treatment repertoire. As for disease modifying targets that theoretically offer the possibility of slowing the progression of disease, several neurotrophic factors show encouraging results in preclinical models (e.g., neurturin, GDNF, BDNF, CDNF, VEGF-A). However, so far, clinical trials have only tested neurturin, and, unfortunately, no trial has been able to meet its primary endpoint. Future clinical trials with neurotrophic factors clearly deserve to be conducted, considering the still enticing goal of actually slowing the disease process of PD. As alternative types of gene therapy, opto- and chemogenetics might also find future use in PD treatment and novel genome-editing technology could also potentially be applied as individualized gene therapy for genetic types of PD.

Cell-Mediated Delivery of Brain-Derived Neurotrophic Factor Enhances Dopamine Levels in an Mpp+ Rat Model of Substantia Nigra Degeneration

Brain-derived neurotrophic factor (BDNF) promotes the survival of fetal mesencephalic dopaminergic cells and protects dopaminergic neurons against the toxicity of MPP+ in vitro. Supranigral implantation of fibroblasts genetically engineered to secrete BDNF attenuates the loss of substantia nigra pars compacta (SNc) dopaminergic neurons associated with striatal infusion of MPP+ in the adult rat. Using this MPP+ rat model of nigral degeneration, we evaluated the neurochemical effects of supranigral, cell-mediated delivery of BDNF on substantia nigra (SN) dopamine (DA) content and turnover. Genetically engineered BDNF-secreting fibroblasts (~12 ng BDNF/24 h) were implanted dorsal to the SN 7 days prior to striatal MPP+ administration. The present results demonstrate that BDNF-secreting fibroblasts, as compared to control fibroblasts, enhance SN DA levels ipsilateral as well as contralateral to the graft without altering DA turnover. This augmentation of DA levels suggests that local neurotrophic factor delivery by genetically engineered cells may provide a therapeutic strategy for preventing neuronal death or enhancing neuronal function in neurodegenerative diseases characterized by dopaminergic neuronal dysfunction, such as Parkinson's disease.

Striatal Infarction Elicits Secondary Extrafocal MRI Changes in Ipsilateral Substantia Nigra

Focal ischemia may induce pathological alterations in brain areas distant from the primary lesion. In animal models, exofocal neuron death in the ipsilateral midbrain has been described after occlusion of the middle cerebral artery (MCA). Using sequential magnetic resonance imaging (T2- and diffusion-weighted) at 3 Tesla, we investigated acute ischemic stroke patients on days 1, 2, 6, 8, and 10 after stroke onset. Sixteen consecutive patients who had suffered a stroke involving the caudate nucleus and/or putamen of either hemisphere were recruited into the study. Four additional patients with strokes sparing the caudate nucleus and putamen but encompassing at least one-third of the MCA territory served as controls. Ischemic lesions involving striatal structures resulted in hyperintense lesions in ipsilateral midbrain that emerged between days 6 and 10 after stroke and were not present on the initial scans. In contrast, none of the control stroke patients developed secondary midbrain lesions. Hyperintense lesions in the pyramidal tract or the brain stem caused by degeneration of the corticospinal tract could be clearly distinguished from these secondary midbrain gray matter lesions and were detectable from day 2 after ischemia. Co-registration of high-resolution images with a digitized anatomic atlas revealed localization of secondary lesions primarily in the substantia nigra pars compacta. Apparent diffusion coefficient (ADC) values in the secondary lesions showed a delayed sharp decline through day 10. Normalization of ADC values was observed at late measurements. Taken together, our study demonstrates that striatal infarction elicits delayed degenerative changes in ipsilateral substantia nigra pars compacta.

Neurotensin-polyplex-mediated brain-derived neurotrophic factor gene delivery into nigral dopamine neurons prevents nigrostriatal degeneration in a rat model of early Parkinson's disease

Background

The neurotrophin Brain-Derived Neurotrophic Factor (BDNF) influences nigral dopaminergic neurons via autocrine and paracrine mechanisms. The reduction of BDNF expression in Parkinson’s disease substantia nigra (SN) might contribute to the death of dopaminergic neurons because inhibiting BDNF expression in the SN causes parkinsonism in the rat. This study aimed to demonstrate that increasing BDNF expression in dopaminergic neurons of rats with one week of 6-hydroxydopamine lesion recovers from parkinsonism. The plasmids phDAT-BDNF-flag and phDAT-EGFP, coding for enhanced green fluorescent protein, were transfected using neurotensin (NTS)-polyplex, which enables delivery of genes into the dopaminergic neurons via neurotensin-receptor type 1 (NTSR1) internalization.

Results

Two weeks after transfections, RT-PCR and immunofluorescence techniques showed that the residual dopaminergic neurons retain NTSR1 expression and susceptibility to be transfected by the NTS-polyplex. phDAT-BDNF-flag transfection did not increase dopaminergic neurons, but caused 7-fold increase in dopamine fibers within the SN and 5-fold increase in innervation and dopamine levels in the striatum. These neurotrophic effects were accompanied by a significant improvement in motor behavior.

Conclusions

NTS-polyplex-mediated BDNF overexpression in dopaminergic neurons has proven to be effective to remit hemiparkinsonism in the rat. This BDNF gene therapy might be helpful in the early stage of Parkinson’s disease.

Selective neuronal loss in ischemic stroke and cerebrovascular disease

As a sequel of brain ischemia, selective neuronal loss (SNL)-as opposed to pannecrosis (i.e. infarction)-is attracting growing interest, particularly because it is now detectable in vivo. In acute stroke, SNL may affect the salvaged penumbra and hamper functional recovery following reperfusion. Rodent occlusion models can generate SNL predominantly in the striatum or cortex, showing that it can affect behavior for weeks despite normal magnetic resonance imaging. In humans, SNL in the salvaged penumbra has been documented in vivo mainly using positron emission tomography and (11)C-flumazenil, a neuronal tracer validated against immunohistochemistry in rodent stroke models. Cortical SNL has also been documented using this approach in chronic carotid disease in association with misery perfusion and behavioral deficits, suggesting that it can result from chronic or unstable hemodynamic compromise. Given these consequences, SNL may constitute a novel therapeutic target. Selective neuronal loss may also develop at sites remote from infarcts, representing secondary 'exofocal' phenomena akin to degeneration, potentially related to poststroke behavioral or mood impairments again amenable to therapy. Further work should aim to better characterize the time course, behavioral consequences-including the impact on neurological recovery and contribution to vascular cognitive impairment-association with possible causal processes such as microglial activation, and preventability of SNL.

Transcranial near-infrared laser therapy applied to promote clinical recovery in acute and chronic neurodegenerative diseases

One of the most promising methods to treat neurodegeneration is noninvasive transcranial near-infrared laser therapy (NILT), which appears to promote acute neuroprotection by stimulating mitochondrial function, thereby increasing cellular energy production. NILT may also promote chronic neuronal function restoration via trophic factor-mediated plasticity changes or possibly neurogenesis. Clearly, NILT is a treatment that confers neuroprotection or neurorestoration using pleiotropic mechanisms. The most advanced application of NILT is for acute ischemic stroke based upon extensive preclinical and clinical studies. In laboratory settings, NILT is also being developed to treat traumatic brain injury, Alzheimer's disease and Parkinson's disease. There is some intriguing data in the literature that suggests that NILT may be a method to promote clinical improvement in neurodegenerative diseases where there is a common mechanistic component, mitochondrial dysfunction and energy impairment. This article will analyze and review data supporting the continued development of NILT to treat neurodegenerative diseases.

GDNF delivery for Parkinson's disease

The mainstays of Parkinson's disease (PD) treatment remain symptomatic, including initial dopamine replacement and subsequent deep brain stimulation, however, neither of these approaches is neuroprotective. Neurotrophic factors - proteins that activate cell signalling pathways regulating neuronal survival, differentiation, growth and regeneration - represent an alternative for treating dopaminergic neurons in PD but are difficult to administer clinically because they do not pass through the blood-brain barrier. Glial cell line-derived neurotrophic factor (GDNF) has potent neurotrophic effects particularly but not exclusively on dopaminergic neurons; in animal models of PD, it has consistently demonstrated both neuroprotective and neuroregenerative effects when provided continuously, either by means of a viral vector or through continuous infusion either into the cerebral ventricles (ICV) or directly into the denervated putamen. This led to a human PD study in which GDNF was administered by monthly bolus intracerebroventricular injections, however, no clinical benefit resulted, probably because of the limited penetration to the target brain areas, and instead significant side effects occurred. In an open-label study of continuous intraputamenal GDNF infusion in five patients (one unilaterally and four bilaterally), we reported excellent tolerance, few side effects and clinical benefit evident within three months of the commencement of treatment. The clinical improvement was sustained and progressive, and by 24-months patients demonstrated a 57 and 63% improvement in their off-medication motor and activities of daily living UPDRS subscores, respectively, with clear benefit in dyskinesias. The benefit was associated with a significant increase in putamenal 18F-dopa uptake on positron emission tomography (PET), and in one patient coming to autopsy after 43 months of unilateral infusion there was evident increased tyrosine hydroxylase immunopositive nerve fibres in the infused putamen. A second open trial in 10 patients using unilateral intraputamenal GDNF infusions has also demonstrated a greater than 30% bilateral benefit in both on- and off-medication scores at 24 weeks. Based on our 6-month results, a randomized controlled clinical trial was conducted to confirm the open-label results, however, GDNF infusion over 6-months did not confer the predetermined level of clinical benefit to patients with PD despite increased 18F-dopa uptake surrounding the catheter tip. It is possible that technical differences between this trial and the positive open label studies contributed to this negative outcome.

A new and simple transection knife for study of neurodegeneration and neuroregeneration in animal model

BACKGROUND

The purpose of this study was to design and make a simple, inexpensive brain knife that could produce consistent results following transection in animal model.

MATERIALS AND METHODS

After testing various materials including commercially available products, microelectrode recording needles as used in deep brain stimulation (DBS) surgery were selected as ideal candidates. They were modified to serve as type of wire-knife for the purposes of study. For this study, the major pathway for dopaminergic neuron from substantia nigra to striatum was selected for transection. A total of 40 Sprague-Dawley rats were assigned to 8 groups; normal, 1-4, 6, 8, and 10 weeks post-transection. Degree of cell death was determined and surviving neurons were counted by means of fluorescent microscopic examination, immunohistochemistry involving tyrosine hydroxylase (TH)-immunoreactive staining, and mapping to verify complete transection.

RESULTS

Compared to control, percentage of remaining neurons in each group was 61.3, 36.8, 29.9, 5.1, 5.9, 7.0%, respectively. Completeness of lesion was correlated with the absence of TH-immunoreactivity in the striatum.

CONCLUSION

Our model seems to provide complete cell death in early period after transection with consistent results. Thus, this type of brain knife can be very handy, without any extra cost, in any research model involving transection of fiber bundle for studies on neurodegeneration and neuroregeneration.

Substantia nigra compacta neurons that innervate the reticular thalamic nucleus in the rat also project to striatum or globus pallidus: Implications for abnormal motor behavior

The projections of the substantia nigra pars compacta (SNc) to the reticular thalamic nucleus (RTn) were assessed by measuring dopamine content and counting tyrosine hydroxylase positive (TH (+)) cells in rats with unilateral lesions induced by 6-hydroxydopamine (6-OHDA), and by using a fluorescent tract-tracing technique in rats without lesions. Injection of 6-OHDA in the RTn reduced dopamine content and the number of TH (+) cells in the SNc by about 50%. Branching of SNc was suggested by the finding that 6-OHDA deposited in the RTn significantly reduced dopamine in the striatum and globus pallidus. Moreover, injections of 6-OHDA into either the striatum or the globus pallidus significantly reduced dopamine content in the RTn. Fluorescent tracers injected into the RTn labeled TH (+) cells in the SNc. A high proportion of these TH (+) cells was double labeled when tracers were also injected into either the globus pallidus or striatum. Other experiments showed that systemic injection of apomorphine or methamphetamine induced turning behavior in rats with local deposits of 6-OHDA in either the RTn or the studied basal ganglia nuclei. The extensive dopaminergic branching suggests that the abnormal motor behavior of rats with 6-OHDA deposits in the RTn may be caused by dopaminergic denervation of more than one structure. The fact that lesion of a single dopaminergic neuron can reduce dopamine transmission in more than one structure is probably important in generating the manifestations of Parkinson's disease.

New therapeutic approaches to Parkinson's disease including neural transplants

Parkinson's disease (PD) is a common neurodegenerative disorder of the brain and typically presents with a disorder of movement. The core pathological event underlying the condition is the loss of the dopaminergic nigrostriatal pathway with the formation of alpha-synuclein positive Lewy bodies. As a result, drugs that target the degenerating dopaminergic network within the brain work well at least in the early stages of the disease. Unfortunately, with time these therapies fail and produce their own unique side-effect profile, and this, coupled with the more diffuse pathological and clinical findings in advancing disease, has led to a search for more effective therapies. In this review, the authors will briefly discuss the emerging new drug therapies in PD before concentrating on a more detailed discussion on the state of cell therapies to cure PD.

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.

Brain-derived neurotrophic factor as a drug target for CNS disorders

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family of trophic factors. BDNF is widely and abundantly expressed in the CNS and is available to some peripheral nervous system neurons that uptake the neurotrophin produced by peripheral tissues. BDNF promotes survival and differentiation of certain neuronal populations during development. In adulthood, BDNF can modulate neuronal synaptic strength and has been implicated in hippocampal mechanisms of learning and memory and spinal mechanisms for pain. Several CNS disorders are associated with a decrease in trophic support. As BDNF and its high affinity receptor are abundant throughout the whole CNS, and BDNF is a potent neuroprotective agent, this trophic factor is a good candidate for therapeutic treatment of some of CNS disorders. This review aims to correlate the features of some CNS disorders (Parkinson's disease, Alzheimer's disease, depression, epilepsy and chronic pain) to changes in BDNF expression in the brain. The cellular and molecular mechanism by which BDNF might be a therapeutic strategy are critically examined.

Ontogenic cell death in the nigrostriatal system

Like most neural systems, dopamine neurons of the substantia nigra undergo apoptotic natural cell death during development. In rodents, this occurs largely postnatally and is biphasic with an initial major peak just after birth and a second minor peak on postnatal day 14. As envisioned by classic neurotrophic theory, this event is regulated by interactions with the target of these neurons, the striatum, because a developmental target lesion results in an augmented natural cell death event with fewer nigral dopamine neurons surviving into adulthood. Until recently, the striatal target-derived neurotrophic factors providing developmental support of dopamine neurons were unknown, but there is now growing evidence that glial-cell-line-derived neurotrophic factor (GDNF) serves as a physiologic limiting neurotrophic factor for these neurons during the first phase of natural cell death. During this phase, intrastriatal injection of GDNF diminishes the natural cell death event and neutralizing antibodies augment it. Sustained overexpression of GDNF in the striatum throughout development in a unique double transgenic mouse model results in an increased number of dopamine neurons surviving the first phase of natural cell death. However, this increase does not persist into adulthood. Therefore, other factors or mechanisms must play important roles in the determination of the mature number of nigral dopamine neurons. Further elucidation of these mechanisms will be important in the development of neuroprotective and cell replacement therapies for Parkinson's disease.

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.

Neurotrophins

Nerve growth factor was the first identified protein with anti-apoptotic activity on neurons. This prototypic neurotrophic factor, together with the three structurally and functionally related growth factors brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and neurotrophin-4/5 (NT4/5), forms the neurotrophin protein family. Target T cells for neurotrophins include many neurons affected by neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and peripheral polyneuropathies. In addition, the neurotrophins act on neurons affected by other neurological and psychiatric pathologies including ischemia, epilepsy, depression and eating disorders. Work with cell cultures and animal models provided solid support for the hypothesis that neurotrophins prevent neuronal death. While no evidence exists that a lack of neurotrophins underlies the etiology of any neurodegenerative disease, these studies have spurred on hopes that neurotrophins might be useful symptomatic-therapeutic agents. However first clinical trials led to variable results and severe side effects were observed. For future therapeutic use of the neurotrophins it is therefore crucial to expand our knowledge about their physiological functions as well as their pharmacokinetic properties. A major challenge is to develop methods for their application in effective doses and in a precisely timed and localized fashion.

Long-term high-frequency electro-acupuncture stimulation prevents neuronal degeneration and up-regulates BDNF mRNA in the substantia nigra and ventral tegmental area following medial forebrain bundle axotomy

Electroacupuncture (EA) has been used in China for many years to treat Parkinson's disease (PD) with reportedly effective results. However, the physiological and biological mechanism behind its effectiveness is still unknown. In the present study, different frequencies of chronic EA stimulation (0, 2, 100 Hz) were tested in a partially lesioned rat model of PD which was induced by transection of the medial forebrain bundle (MFB). After 24 sessions of EA stimulation (28 days after MFB transection), dopaminergic neurons in the ventral midbrain were examined by immunohistochemical staining, and brain-derived neurotrophic factor (BDNF) mRNA levels in ventral midbrain were measured by in situ hybridization. The results show a marked decrease of dopaminergic neurons on the lesioned side of the substantia nigra (SN) comparing with the unlesioned side. Zero Hz and 2 Hz EA stimulation had no effect on the disappearance of dopaminergic neurons. However, after 100 Hz EA, about 60% of the tyrosine hydroxylase (TH)-positive neurons remained on the lesioned side of the SN. In addition, levels of BDNF mRNA in the SN and ventral tegmental area (VTA) of the lesioned side were significantly increased in the 100 Hz EA group, but unchanged in the 0 and 2 Hz groups. Our results suggest that long-term high-frequency EA is effective in halting the degeneration of dopaminergic neurons in the SN and up-regulating the levels of BDNF mRNA in the subfields of the ventral midbrain. Activation of endogenous neurotrophins by EA may be involved in the regeneration of the injured dopaminergic neurons, which may underlie the effectiveness of EA in the treatment of PD.

Models of repair mechanisms for future treatment modalities of Parkinson's disease

Parkinson's disease is one of the most likely neurological disorders to be fully treatable by drugs and new therapeutic modalities. The age-dependent and multifactorial nature of its pathogenesis allows for many strategies of intervention and repair. Most data indicate that the selectively vulnerable dopaminergic neurons in the substantia nigra of patients that have developed Parkinson's disease can be modified by protective and reparative therapies. First, the oxidative stress, protein abnormalities, and cellular inclusions typically seen could be dealt with by anti-oxidants, trophic factors, and proteolytic enhancements. Secondly, if the delay of degeneration is not sufficient, then immature dopamine neurons can be placed in the parkinsonian brain by transplantation. Such neurons can be derived from stem cell sources or even stimulated to repair from endogenous stem cells. Novel molecular and cellular treatments provide new tools to prevent and alleviate Parkinson's disease.

Gene therapy for Parkinson's disease

Significant progress has been made in the field of gene therapy for Parkinson's disease (PD). Successful vehicles for gene transfer into the central nervous system have been developed and clinical efficacy and safety have both been shown in various animal models of PD. Further optimisation of dosing, timing and location of gene therapy delivery as well as the ability to regulate and prolong gene expression will be important for the commencement of human trials. Current gene therapy models for PD have focused on two treatment strategies. One is the replacement of biosynthetic enzymes for dopamine synthesis and the second strategy is the addition of neurotrophic factors for protection and restoration of dopaminergic neurones. Concepts of neuroprotection and restoration of the nigrostriatal pathway will become important themes for future genetic treatment strategies for PD and may include, in addition to neurotrophic factors, genes to prevent apoptosis or detoxify free radical species. This review will highlight the recent literature on gene therapy for PD and summarise general approaches to gene therapy.

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.

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.

Neurotrophic factors in Alzheimer's and Parkinson's disease brain

The biomedical literature on the subject of neurotrophic growth factors has expanded prodigiously. This essay reviews neurotrophic factors (NTF) and their receptors in Alzheimer's disease (AD) and Parkinson's disease (PD) brain and recent updates on receptor signaling. The hypotheses for specific NTF involvement in neurodegenerative diseases in human and as potential therapy are based mainly on experimental animal and in vitro models. There are wide gaps in information on regional synthesis and cell contents of NTFs and their receptors in human brain. Observations on AD brain indicate increases in NGF and decreases in BDNF in surviving neurons of hippocampus and certain neocortical regions and decreases in TrkA in cortex and nucleus basalis. In PD brain, the few data available indicate decreases in neuronal content of GDNF and bFGF in surviving substantia nigra dopaminergic neurons. There are very few data regarding age-dependent effects on NTFs and on their receptors in human brain. Since NTFs in neurons are subject to retrograde and, in at least some cases, to anterograde transport from and to target neurons, their effects may be related to synthesis in local or remote sites or to changes in axoplasmic transport. Also, certain NTFs and their receptors are found to be expressed in activated glia. Thus, comparative in situ data for transcription levels and protein contents for NTFs and their receptors in both sites of neuronal origin and termination in human brain are needed to understand their potential roles in treating human diseases.

Cell delivery to the central nervous system

A dysfunctional central nervous system (CNS) resulting from neurological disorders and diseases impacts all of humanity. The outcome presents a staggering health care issue with a tremendous potential for developing interventive therapies. The delivery of therapeutic molecules to the CNS has been hampered by the presence of the blood-brain barrier (BBB). To circumvent this barrier, putative therapeutic molecules have been delivered to the CNS by such methods as pumps/osmotic pumps, osmotic opening of the BBB, sustained polymer release systems and cell delivery via site-specific transplantation of cells. This review presents an overview of some of the CNS delivery technologies with special emphasis on transplantation of cells with and without the use of polymer encapsulation technology.

Upregulation of BDNF mRNA and trkB mRNA in the nigrostriatal system and in the lesion site following unilateral transection of the medial forebrain bundle

We have performed unilateral transection of the medial forebrain bundle (MFB) and studied BDNF mRNA and trkB mRNA levels at different postlesion times in the nigrostriatal system by means of in situ hybridization. BDNF mRNA levels were transiently induced in the substantia nigra pars compacta at 1 day postaxotomy. The disposition of BDNF mRNA expressing cells at this postlesion time in substantia nigra mimicked that of the dopaminergic neurons expressing the mRNA for the dopamine transporter. TrkB mRNA levels remained unaltered in the ventral mesencephalon at the different postlesion times examined-1 to 14 days. In contrast, trkB mRNA levels were significantly induced in the striatum at the longer postlesion time examined-14 days-when all neurodegenerative events are completed. It is becoming apparent that nigral BDNF mRNA levels are anterogradely transported to its target tissue in striatum. However, following axotomy, the lesion site represents a second potential target for BDNF action. Consequently, we also analyzed the pattern of mRNA expression for BDNF and trkB at the lesion site where dopaminergic axons are disconnected. There, we found notable inductions of both BDNF mRNA and trkB mRNA levels at 4 days postaxotomy. BDNF mRNA expressing cells were confined at the site of axotomy, which coincided precisely to that showing induction of trkB mRNA. Altogether, our results anticipate promising trophic roles of BNDF in the injured nigrostriatal system.

trkB messenger RNA expression in normal human brain and in the substantia nigra of parkinsonian patients: an in situ hybridization study

trkB is a high-affinity receptor for brain-derived neurotrophic factor, a neurotrophin acting on numerous cells, including dopaminergic neurons. Yet, little is known of its expression in the human brain. We report an in situ hybridization analysis of trkB messenger RNA, encoding the catalytic form of the receptor, in the human brain post mortem. Its expression was found to be widespread but heterogeneous among all the cerebral structures studied, the highest level being found in the cerebral cortex and the cerebellum. A strong but less intense staining was observed in the striatum, nucleus basalis of Meynert, hippocampus, tegmental pedonculopontinus nucleus and substantia nigra pars compacta. Combined immunohistochemistry for tyrosine hydroxylase and in situ hybridization for trkB messenger RNA showed that within the substantia nigra pars compacta a major proportion of dopaminergic neurons expressed trkB messenger RNA. Furthermore, we compared trkB messenger RNA expression in the mesencephalon of six control subjects and five patients with Parkinson's disease, a neurodegenerative disorder characterized by a severe loss of dopaminergic neurons. Despite the fact that the number of trkB messenger RNA-containing neurons was dramatically reduced in the substantia nigra pars compacta and ventral tegmental area of patients with Parkinson's disease, the level of trkB messenger RNA was unchanged in the remaining neurons in diseased brains. These results suggests that trkB is not involved in the process of neuronal death in Parkinson's disease. Furthermore, expression of brain-derived neurotrophic factor high-affinity receptor in patients could allow this neurotrophin to be used to prevent degeneration of surviving neurons at early stages of the disease.

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

Brain-derived neurotrophic factor (BDNF) promotes the differentiation and growth of developing dopamine (DA) neurons and supports the survival of mature DA cells in culture. However, the neurotrophic role of endogenous BDNF in the adult DA system in vivo has not been well established. To investigate the hypothesis that blockade of endogenous BDNF expression results in DA dysregulation, we used an 18-mer antisense oligodeoxynucleotide (ODN) targeted to the first ATG codon of the BDNF transcript. The biological activity of the antisense ODN was initially tested in vitro. In cultured dopaminergic MES 23.5 cells, antisense BDNF (20 microM) effectively reduced BDNF protein expression and cell survival. Furthermore, in primary embryonic mesencephalic cultures, antisense BDNF reduced the number of tyrosine hydroxylase positive neurons and inhibited [3H]DA uptake in a time- and dose-dependent manner. The specificity of the antisense molecule was confirmed by comparing its effects with those of a control ODN having the same base composition but in scrambled sequence. In rats, two days following an intranigral or intrastriatal injection of antisense BDNF (0.5 microg), we observed a two-fold and five-fold increase in nigral DA levels, respectively, but no change in striatal DA content. Seven days after an intrastriatal antisense BDNF injection, DA levels were elevated in the striatum apparently due to decreased DA turnover. These observations suggest that inhibition of endogenous BDNF expression tends to augment rather than inhibit nigrostriatal DA transmission. Thus, the biological effects of endogenous BDNF on the nigrostriatal DA system in the adult organism merits further investigation.

Neurotrophins prevent death and differentially affect tyrosine hydroxylase of adult rat nigrostriatal neurons in vivo

Brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) promote survival of mesencephalic dopaminergic neurons in vitro and affect normal and damaged ones in vivo. Here, these neurotrophins had markedly different potencies to prevent the death of axotomized nigrostriatal dopaminergic neurons when infused close to the rostral end of the nigral nucleus of adult rats (NT-4 > BDNF > NT-3; nerve growth factor or NGF without effect). With a high dose of BDNF (30 micrograms/day) complete protection was achieved in the rostral but not caudal nigral regions, consistent with its poor diffusion characteristics in brain tissue. Measurements of tyrosine hydroxylase immunoreactivity suggest that BDNF and NT-4 (presumably through their TrkB receptor) reduce the synthesis of this rate-limiting enzyme for dopamine synthesis in rescued as well as in normal neurons. In sharp contrast, survival-promoting doses of NT-3 (presumably through its TrkC receptor) maintained normal levels of tyrosine hydroxylase immunoreactivity in the rescued nigrostriatal neurons. These results suggest that for these adult central nervous system neurons, some neurotrophic factors are predominantly involved in facilitating cell survival, whereas others are more involved in regulating neurotransmitter function.

Glial cell line-derived neurotrophic factor attenuates the excitotoxin-induced behavioral and neurochemical deficits in a rodent model of Huntington's disease

The present study determined the effects of intraventricularly administered glial cell line-derived neurotrophic factor on the behavioral and neurochemical sequelae of unilateral excitotoxic lesions of the striatum. Distinct asymmetrical rotational behavior in response to peripheral administration of amphetamine (5 mg/kg) was noted one and two weeks following injections of quinolinic acid (200 nmol) into two sites in the left striatum. In rats given a single intraventricular injection of glial cell line-derived neurotrophic factor (10-1000 micrograms) 30 min before the toxin, amphetamine-induced rotational behavior was significantly attenuated. Analysis of Nissl-stained coronal sections showed marked neuronal loss in the striatum ipsilateral to the quinolinic acid injections, which was at least partially prevented by glial cell line-derived neurotrophic factor D1 and D2 dopamine binding sites in the striatum, the majority of which are localized to subpopulations of GABAergic neurons, were decreased to a similar extent by quinolinic acid. Moreover, the reduction was attenuated by glial cell line-derived neurotrophic factor treatment to a similar degree, suggesting that the two subpopulations of GABAergic striatal output neurons are equally vulnerable to excitotoxic damage. Concomitant changes in neurotransmitter function as a result of the lesion were also observed: [3H]GABA uptake into striatal target tissues (globus pallidus and substantia nigra) was considerably reduced in the lesioned compared to the contralateral unlesioned tissues, as were [3H]choline and [3H]dopamine uptake into striatal synaptosomes. Similarly, striatal choline acetyltransferase activity was decreased by the lesion. Decrements in neuropeptide levels of similar magnitude were evident ipsilateral to the lesion; substance P, met-enkephalin and dynorphin A contents in the globus pallidus and substantia nigra were significantly reduced. Striatal somatostatin and neuropeptide Y levels were not altered. All of the neurochemical deficits induced by striatal quinolinic acid lesions were attenuated by intraventricular delivery of glial cell line-derived neurotrophic factor. Continuous intraventricular infusion of this trophic factor (10 micrograms/day) over a two-week period did not afford notable improvement compared to the single injection of 10 micrograms. In contrast, continuous infusion of brain-derived neurotrophic factor (10 micrograms/day) directly into the striatum did not affect any of the neurochemical parameters studied. However, neurotrophin-3 (10 micrograms/day) delivery into the striatum significantly increased [3H]GABA uptake, but only modestly affected [3H]choline uptake. The results indicate that glial cell line-derived neurotrophic factor counteracts neuronal damage induced by a striatal excitotoxic insult and support its potential use as a treatment for central nervous system disorders that may be a consequence of excitotoxic processes, such as Huntington's disease.

Glial cell line-derived neurotrophic factor attenuates behavioural deficits and regulates nigrostriatal dopaminergic and peptidergic markers in 6-hydroxydopamine-lesioned adult rats: comparison of intraventricular and intranigral delivery

The effects of intranigrally- or intraventricularly-administered glial cell line-derived neurotrophic factor were tested on low dose (0.05 mg/kg) apomorphine-induced rotations and tyrosine hydroxylase activity in the substantia nigra and striatum of stable 6-hydroxydopamine-lesioned rats. In addition, we determined if 6-hydroxydopamine lesions in the absence or presence of treatment affected neuropeptide (substance P, met-enkephalin, dynorphin) content in the striatum. Glial cell line-derived neurotrophic factor, when administered intranigrally, prevented apomorphine-induced rotational behaviour for 11 weeks following a single injection. In comparison, intraventricularly-administered glial cell line-derived neurotrophic factor produced a transient reduction in rotational behaviour that lasted for two to three weeks following a single injection. We also show that rotational behaviour is reduced following each subsequent intraventricular injection of glial cell line-derived neurotrophic factor given every six weeks, a time-point when baseline rotation deficits were re-established. Intranigrally- or intraventricularly-administered glial cell line-derived neurotrophic factor significantly reduced weight gain in all 6-hydroxydopamine-lesioned rats in this study. Following behavioural analysis where a confirmed improvement of behaviour was established, tissues were dissected for neurochemical analysis. In lesioned rats with intranigral injections of administered glial cell line-derived neurotrophic factor, significant increases of nigral, but not striatal tyrosine hydroxylase activity were measured. Additionally, 6-hydroxydopamine lesions significantly increased striatal dynorphin (61-139%) and met-enkephalin (81-139%), but not substance P levels. In these rats, intranigrally-administered glial cell line-derived neurotrophic factor injections reversed lesion-induced increases in nigral dynorphin A levels and increased nigral dopamine levels, but did not alter nigral met-enkephalin or substance P levels nor striatal dopamine levels. In lesioned rats with intraventricular injections of glial cell line-derived neurotrophic factor, tyrosine hydroxylase ispilateral to the lesion was increased in the substantia nigra, but not in the striatum. Intraventricularly-administered glial cell line-derived neurotrophic factor did not reverse lesion-induced increases in nigral dynorphin A or met-enkephalin levels nor did glial cell line-derived neurotrophic factor affect substance P levels in the striatum. These results suggest that in an animal model of Parkinson's disease, the neurotrophic factor glial cell line-derived neurotrophic factor reverses behavioural consequences of 6-hydroxydopamine administration, an effect that may involve both dopaminergic and peptidergic neurotransmission.

Glial cell line-derived neurotrophic factor induces the dopaminergic and cholinergic phenotype and increases locomotor activity in aged Fischer 344 rats

Glial cell line-derived neurotrophic factor has been shown to affect dopaminergic and cholinergic neuron markers and functions in young rats. However, it is not known if the response to exogenous glial cell line-derived neurotrophic factor is augmented during normal aging. Thus, the effects of chronic intraventricular infusions of glial cell line-derived neurotrophic factor were determined in young adult (three-months-old) and aged (24-months-old) Fischer 344 (F344) male rats. The effects of glial cell line-derived neurotrophic factor were compared to the effects of the neurotrophin nerve growth factor. Growth factors were administered at a dose of 10 mg/day for 14 days. Locomotor activity and weight changes were also examined in all rats. Aged F344 rats showed significantly reduced (by 75-80%) locomotor activity compared to young rats. In glial cell line-derived neurotrophic factor-treated aged and young rats there was significantly increased (242% and 149%, respectively) locomotor activity measured at seven days. There was also a significant increase in locomotor activity measured 14 days after the start of infusion. Both glial cell line-derived neurotrophic factor and nerve growth factor reduced weight gain by 10% in young and old F344 rats. Two weeks following the start of nerve growth factor or glial cell line-derived neurotrophic factor administration the brains were used for neurochemical analyses. Glial cell line-derived neurotrophic factor significantly increased tyrosine hydroxylase activity in the substantia nigra and striatum of aged rats and in the substantia nigra of young rats. Nerve growth factor treatment did not significantly affect tyrosine hydroxylase activity. However, glial cell line-derived neurotrophic factor and nerve growth factor increased choline acetyltransferase activity in the septum, hippocampus, striatum and cortex of aged rats and in the hippocampus and striatum of young rats to a comparable degree. These findings indicate that specific dopaminergic and cholinergic neuron populations remain responsive to glial cell line-derived neurotrophic factor during the life span of the rat and may be involved in maintaining phenotypic expression within multiple neuronal populations. Additionally, the glial cell line-derived neurotrophic factor-induced up-regulation of brain neurotransmitter systems may be responsible for increased locomotor activity in F344 rats.

Gene therapeutic strategies for neuroprotection: implications for Parkinson's disease

Gene transfer methodologies are being explored as strategies to restore and preserve neuronal function in Parkinson's Disease. This technology represents a new therapeutic modality, holding promise for continuous and localized delivery, of neuroprotective molecules. Two primary approaches for gene transfer have emerged: in vivo and ex vivo. Recent advances in the construction and characterization of gene transfer vectors have generated more efficient vehicles to deliver and express candidate therapeutic genes. Direct gene transfer into the CNS can be achieved with replication-deficient viral vectors of several types: adenovirus, adeno-associated virus, and herpes simplex virus. These vector systems are being evaluated in models of Parkinson's disease. Strategies to deliver genes include those that either augment dopamine biosynthesis or attenuate loss of dopaminergic neurons. A discussion of the various approaches is detailed.

Application of ex vivo gene therapy in the treatment of Parkinson's disease

Ex vivo gene therapy approaches hold great promise for the treatment of neurodegenerative diseases where there is currently no cure or adequate treatment for affected individuals. In this review we have focused on the use of ex vivo gene transfer techniques in Parkinson's disease models; however, the issues and approaches outlined are applicable to other neurodegenerative disorders. In utilizing the ex vivo strategy two considerations are critical for delivery of therapeutic levels of transgene product to the target: (i) the vector system and (ii) the cell type for grafting. We describe herein different vector systems that are currently available and briefly review the various cell types that have been transduced and grafted into the striatum of animals with experimental Parkinson's disease. The strategies for application of gene therapy techniques to a treatment for Parkinson's disease have expanded beyond the classical dopamine replacement toward the use of neurotrophic factors in enhancing cell function or preventing cell death. In addition, we explore the utility of CNS-derived neural progenitors as alternative cell types for ex vivo gene therapy in an animal model of Parkinson's disease.

GDNF reduces drug-induced rotational behavior after medial forebrain bundle transection by a mechanism not involving striatal dopamine

Parkinson's disease (PD) is characterized by the progressive loss of the substantia nigra (SN) dopaminergic neurons projecting to the striatum. Neurotrophic factors may have the potential to prevent or slow down the degenerative process occurring in PD. To that end, we examined whether low amounts of glial cell line-derived neurotrophic factor (GDNF) continuously released from polymer-encapsulated genetically engineered cells are able to prevent the loss of tyrosine hydroxylase immunoreactivity (TH-IR) in SN neurons and ameliorate the amphetamine-induced rotational asymmetry in rats that have been subjected to a unilateral medial forebrain bundle (MFB) axotomy. Baby hamster kidney (BHK) cells transfected with the cDNA for GDNF were encapsulated in a polymer fiber and implanted unilaterally at a location lateral to the MFB and rostral to the SN. ELISA assays before implantation show that the capsules release approximately 5 ng of GDNF/capsule per day. One week later, the MFB was axotomized unilaterally ipsilateral to the capsule placement. Seven days later, the animals were tested for amphetamine-induced rotational asymmetry and killed. The striatum was excised and analyzed either for catecholamine content or TH-IR, while the SN was immunostained for the presence of TH-IR. GDNF did not prevent the loss of dopamine in the striatum. However, GDNF significantly rescued TH-IR neurons in the SN pars compacta. Furthermore, GDNF also significantly reduced the number of turns per minute ipsilateral to the lesion under the influence of amphetamine. Improvement of rotational behavior in the absence of dopaminergic striatal reinnervation may reflect neuronal plasticity in the SN, as suggested by the dendritic sprouting observed in animals receiving GDNF. These results illustrate that the continuous release of low levels of GDNF close to the SN is capable of protecting the nigral dopaminergic neurons from an axotomy-induced lesion and significantly improving pharmacological rotational behavior by a mechanism other than dopaminergic striatal reinnervation.

Antidepressant-like effect of brain-derived neurotrophic factor (BDNF)

Previous studies have shown that infusion of brain-derived neurotrophic factor (BDNF) into the midbrain, near the PAG and dorsal/median raphe nuclei, produced analgesia and increased activity in monoaminergic systems. Alterations in monoaminergic activity have also been implicated in the pathogenesis and treatment of depression. The present studies examined the ability of centrally administered BDNF to produce antidepressant-like activity in two animal models of depression, learned helplessness following exposure to inescapable shock and the forced swim test. In the learned helplessness paradigm, vehicle-infused rats pre-exposed to inescapable shock (veh/shock) showed severe impairments in escape behavior during subsequent conditioned avoidance trials, including a 47% decrease in the number of escapes and a 5 fold increase in escape latency, as compared to vehicle-infused rats which received no pre-shock treatment (veh/no shock). Midbrain BDNF infusion (12-24 micrograms/day) reversed these deficits, and in fact, BDNF-infused rats pre-exposed to inescapable shock (BDNF/shock) showed escape latencies similar to veh/no shock and BDNF/no shock rats. In the forced swim test, BDNF infusion decreased the immobility time by 70% as compared to vehicle-infused controls. Non-specific increases in activity could not account for these effects since general locomotor activity of BDNF- and vehicle-infused animals was not different. These findings demonstrate an antidepressant-like property of BDNF in two animal models of depression, which may be mediated by increased activity in monoaminergic systems.

Pharmacology of neurotrophic factors

The field of neurotrophic factor pharmacology emerged during the past decade with the discovery that these proteins can counteract neuronal atrophy and death in the adult nervous system. These concepts are being tested in clinical trials. Therapeutic use of neurotrophic proteins seems practical for diseases of the peripheral nervous system (PNS), where they can be given by systemic administration. For diseases of the CNS, special administration strategies will have to be developed to deliver the neurotrophic factors into the brain. The development of small molecule mimetics represents an alternative approach that is actively pursued to provide brain-penetrant neurotrophics.

Comparative study of brain-derived neurotrophic factor messenger RNA and protein at the cellular level suggests multiple roles in hippocampus, striatum and cortex

Brain-derived neurotrophic factor (BDNF) is important for the development and trophic support of several neuronal groups in the rat. In the present study, the distribution of BDNF messenger RNA was studied by in situ hybridization, and the cellular localization of BDNF protein was investigated with anti-peptide antibodies. Anatomical investigations were also made in animals with prolonged epileptic seizures which show an enhanced expression of BDNF messenger RNA. Major forebrain areas studied were the hippocampus, striatum and cortex. The messenger RNA coding for the putative high-affinity receptor, tyrosine kinase B, was also visualized using in situ hybridization with a probe specific for the full-length form. In the hippocampus, granule cells and pyramidal neurons expressed BDNF messenger RNA and BDNF-like immunoreactivity. Interneurons in dendritic layers did not show labelling with either method. Tyrosine kinase B messenger RNA was found within neurons in all these regions. In the medial septum-diagonal band, nucleus basalis and lateral hypothalamus, neurons with punctate cytoplasmic immunofluorescence were found, and neurons in the lateral septum were diffusely positive for BDNF. In striatum, positive labelling of medium-sized neurons was found with the antibody, whereas BDNF messenger RNA was only detectable during seizures. A laminar pattern of neuronal labelling for BDNF messenger RNA and protein was found in the neocortex. The analysis of the anatomical distribution of BDNF-producing cells suggests a number of possible cellular interactions. In the hippocampus, BDNF might act in an autocrine or paracrine manner for granule cells and pyramidal neurons, and, in addition, may serve as a signal from these principal cells to interneurons. BDNF could be a target-derived and a locally produced trophic factor for cholinergic neurons in the medial septum. The expression of BDNF in the striatum suggests that this factor could be a target-derived factor for dopaminergic neurons of substantia nigra and/or work as an autocrine/ paracrine factor within the striatum itself.

Neurotrophin-4/5 selectively protects nigral calbindin-containing neurons in rats with medial forebrain bundle transections

Three neurotrophic factors associated with the nigrostriatal dopaminergic system were tested for their trophic potential to rescue degenerating substantia nigra dopaminergic neurons in adult rats with transections of the medial forebrain bundle. Axotomy of nigral dopaminergic neurons results in a retrograde degeneration of their cell bodies. Unilateral transections resulted in a partial reduction of the number of dopaminergic neurons as identified by immunocytochemistry for tyrosine hydroxylase to approximately half of the number of neurons present on the intact contralateral substantia nigra. A similar percentage loss was found for the subpopulation of nigral neurons which contain the calcium binding protein calretinin. In contrast, the small subpopulation of neurons which contain calbindin was less sensitive to the lesion and showed only mild loss in the number of cells, which was reduced to 87% of control. Neurotrophin-4/5, transforming growth factor alpha or basic fibroblast growth factor were infused supranigrally for two weeks after transection. None of the trophic factors tested reversed the loss of tyrosine hydroxylase-positive or calretinin-positive cells. In contrast, neurotrophin-4/5, but not transforming growth factor alpha or basic fibroblast growth factor, was found to reverse the axotomy-induced loss of calbindin-positive neurons and indeed increased the number of cells to 45% above control levels. In addition, neurotrophin-4/5 elevated the number of calbindin-containing neurons in intact unlesioned animals to 15% above control. These findings suggest that neurotrophin-4/5 selectively acts on nigral calbindin neurons following medial forebrain bundle transection and prevents these cells from degenerating.

The neurotrophin NT4/5, but not NT3, enhances the efficacy of nigral grafts in a rat model of Parkinson's disease

The neurotrophins NT4/5 and NT3 have previously been shown to improve the survival and fibre outgrowth of embryonic dopaminergic neurons in vitro. In the present study we attempted to augment the efficacy of embryonic nigral grafts in vivo. This was done by directly infusing the neurotrophins intraparenchymally in close proximity to transplanted nigral tissue placed in the dopamine depleted striatum of 6-hydroxydopamine lesioned rats. Our results indicated that NT4/5, but not NT3, stimulated fibre growth from embryonic nigral cells and enhanced functional efficacy of the grafts as assessed by metamphetamine-induced rotation.

Cell-mediated delivery of brain-derived neurotrophic factor enhances dopamine levels in an MPP+ rat model of substantia nigra degeneration

Brain-derived neurotrophic factor (BDNF) promotes the survival of fetal mesencephalic dopaminergic cells and protects dopaminergic neurons against the toxicity of MPP+ in vitro. Supranigral implantation of fibroblasts genetically engineered to secrete BDNF attenuates the loss of substantia nigra pars compacta (SNc) dopaminergic neurons associated with striatal infusion of MPP+ in the adult rat. Using this MPP+ rat model of nigral degeneration, we evaluated the neurochemical effects of supranigral, cell-mediated delivery of BDNF on substantia nigra (SN) dopamine (DA) content and turnover. Genetically engineered BDNF-secreting fibroblasts (approximately 12 ng BDNF/24 h) were implanted dorsal to the SN 7 days prior to striatal MPP+ administration. The present results demonstrate that BDNF-secreting fibroblasts, as compared to control fibroblasts, enhance SN DA levels ipsilateral as well as contralateral to the graft without altering DA turnover. This augmentation of DA levels suggests that local neurotrophic factor delivery by genetically engineered cells may provide a therapeutic strategy for preventing neuronal death or enhancing neuronal function in neurodegenerative diseases characterized by dopaminergic neuronal dysfunction, such as Parkinson's disease.

GDNF induces a dystonia-like state in neonatal rats and stimulates dopamine and serotonin synthesis

To test whether glial cell line-derived neurotrophic factor (GDNF) regulates the development of nigral dopaminergic neurons in vivo, neonatal rats received bilateral injections of GDNF into the striatum. Injections at postnatal day 2 induced a unique transient behavioral pattern characterized by forelimb hyperflexure, clawed toes of all limbs, and a kinked tail. Parallel to the behavioral changes, the levels of striatal and ventral mesencephalic dopamine and serotonin were increased from 60% to 100% with a proportional increase of principal metabolite levels. GDNF increased tyrosine hydroxylase activity in the ventral mesencephalon, but did not affect striatal activity of choline acetyltransferase and GABA uptake. GDNF failed to induce sprouting of dopaminergic neurites. Our findings suggest that during development striatal GDNF regulates the capacity of dopaminergic and of serotonergic neurons for neurotransmitter production and release.

BDNF increases monoaminergic activity in rat brain following intracerebroventricular or intraparenchymal administration

We have previously demonstrated alterations in serotonin metabolism within descending pathways following infusion of brain-derived neurotrophic factor (BDNF) into the midbrain, near the periaqueductal gray and dorsal and median raphe nuclei. The aim of the present study was to extend these studies to include a comprehensive regional examination of monoamine (serotonin, dopamine and norepinephrine) and metabolite levels in discrete areas of the intact, adult rat forebrain following direct intraparenchymal midbrain BDNF infusion. We have compared neurochemical changes following midbrain infusion of BDNF to those obtained following intracerebroventricular (i.c.v.) infusion. Significant increases in levels of 5-HIAA and/or the 5-HIAA/5-HT ratio were found in all areas examined including the hippocampus, cortex, striatum, n. accumbens, substantia nigra and hypothalamus following both midbrain and i.c.v. infusion. Changes in dopaminergic activity were also observed, but displayed more regional specificity, i.e. changes were found primarily within the striatum and cortex. The two infusion sites produced similar patterns of neurochemical effects although the magnitude of the changes did vary in some areas. These results suggest that BDNF increased synthesis and/or turnover of serotonin, and to a lesser extent dopamine, in the mature rat forebrain. Furthermore, these data point to possible functional roles for BDNF in neuropsychiatric and neurodegenerative conditions which involve a dysregulation of these monoamine systems.

Trophic and protective actions of brain-derived neurotrophic factor on striatal DARPP-32-containing neurons in vitro

We have examined the effects of either brain-derived neurotrophic factor (BDNF), the BB-isoform of platelet-derived growth factor (PDGF-BB), or a combination of these growth factors on the survival and the morphological development of embryonic striatal neurons grown under serum-free culture conditions. Striatal neurons were identified using immunocytochemistry for "dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein with a molecular weight of 32 kilodalton" (DARPP-32). BDNF and PDGF-BB promoted the survival of DARPP-32-positive neurons, with the magnitude of their effects being comparable. A combination of these growth factors exerted no significant additive effects on cell survival. BDNF stimulated morphological differentiation of DARPP-32-containing neurons by increasing the length of neurites, the number of branching points on the neurites, and the soma area. By contrast, PDGF-BB increased the neurite length and the cell body area, but not the number of branching points. BDNF also protected striatal neurons from excitotoxicity induced by N-methyl-D-aspartate, whereas PDGF-BB had no effect under the same treatment conditions as those for BDNF. Thus, BDNF is trophic for striatal DARPP-32-containing neurons in vitro by enhancing the survival, morphological differentiation and resistance to excitotoxicity, and its mechanisms of action are probably different from those of PDGF-BB.

Morphological and functional effects of intranigrally administered GDNF in normal rhesus monkeys

Effects of a single injection of either 150 micrograms human recombinant glial cell line-derived neurotrophic factor (rGDNF) or vehicle into the right substantia nigra were analyzed in 12 normal adult female rhesus monkeys. The studies included evaluating whole animal behavior, electrochemical recordings of striatal dopamine release, neurochemical determinations of basal ganglia and nigral monoamine levels, and immunohistochemical staining of the nigrostriatal dopamine system. The behavioral effects over the 3-week observation period following trophic factor administration were small, with blinded observers unable to distinguish between GDNF- and vehicle-treated animals. Quantitative measurements did show that five of six trophic factor recipients experienced some weight loss and four of the six GDNF recipients displayed small, but significant, increases in daytime activity levels. In vivo electrochemical recordings in the ipsilateral caudate and putamen 3 weeks after GDNF administration revealed increased potassium-evoked release of dopamine in trophic factor recipients. In a second series of animals killed at the same time, dopamine levels in the substantia nigra and ventral tegmental area of GDNF recipients were significantly increased, with ipsilateral values more than 200% higher than contralateral and control levels. Levels of the dopamine metabolite HVA were significantly elevated in the substantia nigra, ventral tegmental area, and caudate nucleus ipsilateral to the trophic factor injection. There was a trend toward increased HVA levels in the ipsilateral putamen, nucleus accumbens, and globus pallidus in GDNF-treated animals, but the ratios of HVA to dopamine were not significantly different between vehicle- and GDNF-treated recipients. Although some tissue damage from the delivery of concentrated trophic factor was evident, dopamine neurons remained in an adjacent to the injection site. In the substantia nigra ipsilateral to GDNF administration, dopamine-neuron perikaryal size was significantly increased, along with a significant increase in tyrosine hydroxylase-positive axons and dendrites. We conclude that, in the adult rhesus monkey, a single intranigral GDNF injection induces a significant upregulation of mesencephalic dopamine neurons which lasts for weeks.

Neurokinin-3 receptors modulate dopamine cell function and alter the effects of 6-hydroxydopamine

Neurokinin-3 receptor expression within rat midbrain dopamine neurons was demonstrated using a combination of in situ hybridization and receptor autoradiographic techniques. Continuous intranigral infusion of the neurokinin-3 receptor agonist senktide selectively increased striatal dopamine metabolism over a period of several days, followed by apparent development of tolerance. In contrast, in animals with moderate unilateral 6-hydroxydopamine-induced lesions of nigrostriatal dopamine cells, intranigral senktide infusion increased dopamine turnover in the surviving dopamine neurons and reduced functional dopamine asymmetry (reflected by spontaneous rotations) over the 2-week period tested. Thus nigral neurokinin receptors can modulate normal dopamine cell activity and may provide a therapeutic target in the treatment of Parkinson's disease.

GDNF protects nigral dopamine neurons against 6-hydroxydopamine in vivo

Glial cell line-derived neurotrophic factor (GDNF), a novel member of the TGF-beta superfamily, has been shown to promote the survival and morphological differentiation of fetal dopamine neurons in culture and increase dopamine levels and metabolism in adult rats. Since several other trophic factors are able to rescue specific populations of mature CNS neurons following injury, the present study was designed to investigate a possible neuroprotective role by GDNF for midbrain dopamine neurons in rats exposed to the neurotoxin 6-hydroxydopamine (6-OHDA). Prior to surgery, young adult male Fisher 344 rats were divided into the following groups (n = 7-8/group): (1) intranigral saline + intranigral 6-OHDA; (2) intranigral GDNF + intranigral 6-OHDA; (3) intranigral saline + intrastriatal 6-OHDA; and (4) intranigral GDNF + intrastriatal 6-OHDA. The saline treated groups received a single 2 microliters intranigral injection of phosphate buffered saline (PBS) while the GDNF treated rats received 10 micrograms/2 microliters GDNF in PBS. Twenty-four hours later, the animals received a unilateral 4 micrograms/microliters 6-OHDA infusion either into the substantia nigra or striatum. The rats were sacrificed two weeks postsurgery and the brains processed for tyrosine hydroxylase (TH) immunocytochemistry. Representative TH immunoreactive (TH-IR) sections were also counterstained with hematoxylin and eosin to determine the total number of neurons remaining in the substantia nigra pars compacta and ventral tegmental area. In the nigral lesion groups, there was significantly less loss of TH-IR neurons in the substantia nigra pars compacta of GDNF (47% survival) vs. PBS (9% survival) treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative study of cell death in the substantia nigra following a mechanical lesion of the medial forebrain bundle

An extruding wire knife was used to give adult male CFHB rats a minimally traumatic unilateral mechanical lesion of the medial forebrain bundle. In addition, some rats received bilateral intrastriatal injections of one of three fluorescent retrograde tracers either eight days before or eight days after the lesion. Injections made after the lesion revealed that about half of the animals had complete lesions of the nigrostriatal tract, while the other half were incompletely lesioned, the mean proportion of non-axotomized neurons being 23%. Over the 10 weeks following the lesions, the number of tyrosine hydroxylase-immunoreactive cells in the lesioned substantia nigra fell linearly, reaching a mean of 29% of that of the control substantia nigra. In the animals which were completely lesioned, neuronal survival at 10 weeks varied between 6 and 12%. That the disappearance of tyrosine hydroxylase-immunoreactive neurons was due to cell death rather than the loss of tyrosine hydroxylase itself was confirmed by labelling the cells with Fluoro Gold before axotomy; the tracer was seen in survival neurons, microglia and in a few involuted neurons which continued to be tyrosine hydroxylase-immunoreactive. This percentage of neurons surviving axotomy corresponds to the proportion of substantia nigra neurons which project to the contralateral striatum, and these neurons were in the region of the substantia nigra from which the contralateral projection originated. It is concluded that following mechanical transection of the nigrostriatal tract, all truly axotomized substantia nigra neurons die over a period of about 10 weeks.