Dopamine D1 and D2 receptor-mediated acute and long-lasting behavioral effects of glial cell line-derived neurotrophic factor administered into the striatum

Finding an Optimal Level of GDNF Overexpression: Insights from Dopamine Cycling

The application of glial cell line-derive neurotrophic factor (GDNF) to cell cultures and animal models has demonstrated positive effects upon dopaminergic neuronal survival and development, function, restoration, and protection. On this basis, recombinant GDNF protein has been trialled in the treatment of late-stage human Parkinson's disease patients with only limited success that is likely due to a lack of viable receptor targets in an advanced state of neurodegeneration. The latest research points to more refined approaches of modulating GDNF signalling and an optimal quantity and spatial regulation of GDNF can be extrapolated using regulation of dopamine as a proxy measure. The basic research literature on dopaminergic effects of GDNF in animal models is reviewed, concluding that a twofold increase in natively expressing cells increases dopamine turnover and maximises neuroprotective and beneficial motor effects whilst minimising hyperdopaminergia and other side-effects. Methodological considerations for measurement of dopamine levels and neuroanatomical distinctions are made between populations of dopamine neurons and their respective effects upon movement and behaviour that will inform future research into this still-relevant growth factor.

Astrocyte-specific DJ-1 overexpression protects against rotenone-induced neurotoxicity in a rat model of Parkinson's disease

DJ-1 is a redox-sensitive protein with several putative functions important in mitochondrial physiology, protein transcription, proteasome regulation, and chaperone activity. High levels of DJ-1 immunoreactivity are reported in astrocytes surrounding pathology associated with idiopathic Parkinson's disease, possibly reflecting the glial response to oxidative damage. Previous studies showed that astrocytic over-expression of DJ-1 in vitro prevented oxidative stress and mitochondrial dysfunction in primary neurons. Based on these observations, we developed a pseudotyped lentiviral gene transfer vector with specific tropism for CNS astrocytes in vivo to overexpress human DJ-1 protein in astroglial cells. Following vector delivery to the substantia nigra and striatum of adult Lewis rats, the DJ-1 transgene was expressed robustly and specifically within astrocytes. There was no observable transgene expression in neurons or other glial cell types. Three weeks after vector infusion, animals were exposed to rotenone to induce Parkinson's disease-like pathology, including loss of dopaminergic neurons, accumulation of endogenous α-synuclein, and neuroinflammation. Animals over-expressing hDJ-1 in astrocytes were protected from rotenone-induced neurodegeneration, and displayed a marked reduction in neuronal oxidative stress and microglial activation. In addition, α-synuclein accumulation and phosphorylation were decreased within substantia nigra dopaminergic neurons in DJ-1-transduced animals, and expression of LAMP-2A, a marker of chaperone mediated autophagy, was increased. Together, these data indicate that astrocyte-specific overexpression of hDJ-1 protects neighboring neurons against multiple pathologic features of Parkinson's disease and provides the first direct evidence in vivo of a cell non-autonomous neuroprotective function of astroglial DJ-1.

An analysis of the rewarding and aversive associative properties of nicotine in the neonatal quinpirole model: Effects on glial cell line-derived neurotrophic factor (GDNF)

This study analyzed the associative properties of nicotine in a conditioned place preference (CPP) paradigm in adolescent rats neonatally treated with quinpirole (NQ) or saline (NS). NQ produces dopamine D₂ receptor supersensitivity that persists throughout the animal's lifetime, and therefore has relevance towards schizophrenia. In two experiments, rats were ip administered quinpirole (1mg/kg) or saline from postnatal day (P)1-21. After an initial preference test at P42-43, animals were conditioned for eight consecutive days with saline or nicotine (0.6mg/kg free base) in Experiment 1 or saline or nicotine (1.8mg/kg free base) in Experiment 2. In addition, there were NQ and NS groups in each experiment given the antipsychotic haloperidol (0.05mg/kg) or clozapine (2.5mg/kg) before nicotine conditioning. A drug free post-conditioning test was administered at P52. At P53, the nucleus accumbens (NAc) was analyzed for glial cell-line derived neurotrophic factor (GDNF). Results revealed that NQ enhanced nicotine CPP, but blunted the aversive properties of nicotine. Haloperidol was more effective than clozapine at blocking nicotine CPP in Experiment 1, but neither antipsychotic affected nicotine conditioned place aversion in Experiment 2. NQ increased accumbal GDNF which was sensitized in NQ rats conditioned to nicotine in Experiment 1, but the aversive dose of nicotine reduced GDNF in NQ animals in Experiment 2. Both antipsychotics in combination with the aversive dose of nicotine decreased accumbal GDNF. In sum, increased D₂ receptor sensitivity influenced the associative properties and GDNF response to nicotine which has implications towards pharmacological targets for smoking cessation in schizophrenia.

Transcription factor Six2 mediates the protection of GDNF on 6-OHDA lesioned dopaminergic neurons by regulating Smurf1 expression

Glial cell line-derived neurotrophic factor (GDNF) has strong neuroprotective and neurorestorative effects on dopaminergic (DA) neurons in the substantia nigra (SN); however, the underlying molecular mechanisms remain to be fully elucidated. In this study, we found that the expression level of transcription factor Six2 was increased in damaged DA neurons after GDNF rescue in vivo and in vitro. Knockdown of Six2 resulted in decreased cell viability and increased the apoptosis of damaged DA neurons after GDNF treatment in vitro. In contrast, Six2 overexpression increased cell viability and decreased cell apoptosis. Furthermore, genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) indicated that Six2 directly bound to the promoter CAGCTG sequence of smad ubiquitylation regulatory factor 1 (Smurf1). ChIP-quantitative polymerase chain reaction (qPCR) analysis showed that Smurf1 expression was significantly upregulated after GDNF rescue. Moreover, knockdown of Six2 decreased Smurf1 expression, whereas overexpression of Six2 increased Smurf1 expression in damaged DA neurons after GDNF rescue. Meanwhile, knockdown and overexpression of Smurf1 increased and decreased p53 expression, respectively. Taken together, our results from in vitro and in vivo analysis indicate that Six2 mediates the protective effects of GDNF on damaged DA neurons by regulating Smurf1 expression, which could be useful in identifying potential drug targets for injured DA neurons.

Reversal of dopamine neurons and locomotor ability degeneration in aged rats with smilagenin

The purpose of this paper is to study the effect of smilagenin (SMI) (PYM50028), a sapogenin compound originally identified from Chinese medicinal herb, on dopamine neurons and locomotor ability in aged rats. Experiments were carried out on young and aged Sprague-Dawley rats, which were daily administered with either SMI (18mg/kg/day) or vehicle (0.5% sodium carboxymethycellulose [CMCNa]). Open-field and rotarod performance tests revealed that behavioral ability was impaired in aged rats and was improved by oral administration of smilagenin. Immunohistochemical data showed that tyrosine hydroxylase (TH)-positive neuron numbers in the substantia nigra pars compacta (unbiased stereological counting) were altered with aging and were increased by smilagenin treatment. Likewise, the dopamine receptor density and the striatal dopamine transporter (DAT) density ((125)I-2β-carbomethoxy-3β-(4-iodophenyl)-N-(3-fluoropropyl) nortropane [(125)I-FP-CIT] autoradiography) were significantly lowered in aged rats as compared to young rats, and treatment with smilagenin significantly elevated the dopamine receptor and DAT density in aged rats. Furthermore, smilagenin enhances glial cell-derived neurotrophic factor (GDNF) release both in the striatum and midbrain. These results indicate a possible role of smilagenin in the treatment of age-related extrapyramidal disorders.

Glial cell line-derived neurotrophic factor partially ameliorates motor symptoms without slowing neurodegeneration in mice with respiratory chain-deficient dopamine neurons

Degeneration of midbrain dopamine neurons causes the striatal dopamine deficiency responsible for the hallmark motor symptoms of Parkinson's disease (PD). Intraparenchymal delivery of neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), is a possible future therapeutic approach. In animal PD models, GDNF can both ameliorate neurodegeneration and promote recovery of the dopamine system following a toxic insult. However, clinical studies have generated mixed results, and GDNF has not been efficacious in genetic animal models based on α-synuclein overexpression. We have tested the response to GDNF in a genetic mouse PD model with progressive degeneration of dopamine neurons caused by mitochondrial impairment. We find that GDNF, delivered to the striatum by either an adeno-associated virus or via miniosmotic pumps, partially alleviates the progressive motor symptoms without modifying the rate of neurodegeneration. These behavioral changes are accompanied by increased levels of dopamine in the midbrain, but not in striatum. At high levels, GDNF may instead reduce striatal dopamine levels. These results demonstrate the therapeutic potential of GDNF in a progressively impaired dopamine system.

Evoked dopamine overflow is augmented in the striatum of calcitriol treated rats

Calcitriol, the active metabolite of vitamin D, has been shown to have significant effects on the brain. These actions include reducing the severity of some central nervous system lesions, possibly by upregulating trophic factors such as glial cell line-derived neurotrophic factor (GDNF). GDNF has substantial effects on the nigrostriatal dopamine (DA) system of young adult, aged and lesioned animals. Thus, the administration of calcitriol may lead to significant effects on nigrostriatal DA neuron functioning. The present experiments were designed to examine the ability of calcitriol to alter striatal DA release, and striatal and nigral tissue levels of DA. Male Fischer-344 rats were administered vehicle or calcitriol (0.3, 1.0, or 3.0 μg/kg, s.c.) once daily for eight consecutive days. Three weeks later in vivo microdialysis experiments were conducted to measure basal and stimulus evoked overflow of DA from the striatum. Basal levels of extracellular DA were not significantly affected by the calcitriol treatments. However, the 1.0 and 3.0 μg/kg doses of calcitriol led to increases in both potassium and amphetamine evoked overflow of striatal DA. Although post-mortem tissue levels of striatal DA were not altered by the calcitriol injections, nigral tissue levels of DA and its main metabolites were increased by both the 1.0 and 3.0 μg/kg doses of calcitriol. In a separate group of animals GDNF levels were augmented in the striatum and substantia nigra after eight consecutive daily injections of calcitriol. These results suggest that systemically administered calcitriol can upregulate dopaminergic release processes in the striatum and DA levels in the substantia nigra. Increases in the levels of endogenous GDNF following calcitriol treatment may in part be responsible for these changes. The ability of calcitriol to lead to augmented DA release in the striatum suggests that calcitriol may be beneficial in disease processes involving dopaminergic dysfunction.

Neurturin protects against 6-hydroxydopamine-induced reductions in evoked dopamine overflow in rat striatum

Neurturin (NTN), a member of the glial cell line-derived neurotrophic factor (GDNF) family, has substantial effects on normal and lesioned nigrostriatal dopamine systems. However, its ability to protect against toxin-induced loss of striatal dopamine release has not been previously reported. The goal of the present study was to determine if NTN could protect against 6-hydroxydopamine (6-OHDA)-induced reductions in striatal dopamine overflow and tissue levels of dopamine and to compare the effects of NTN with those of GDNF. Male Fischer-344 rats were given a single injection of vehicle, or 5 microg NTN or GDNF, into the right striatum. The following day the animals were given a single injection of 12 microg 6-OHDA into the striatum at the same site where the trophic factor was injected. Microdialysis experiments conducted three weeks later indicated that the 6-OHDA decreased basal levels of dopamine and metabolites in the lesioned striatum compared to the contralateral striatum, and NTN was able to partially protect against the 6-OHDA-induced reductions. Injection of NTN one day prior to 6-OHDA also led to significant protection against loss of both potassium- and amphetamine-evoked overflow of dopamine. The NTN treatments partially protected against 6-OHDA-induced reductions in striatal tissue levels of dopamine and completely protected against loss of nigral dopamine content. The protective effects of NTN were similar in magnitude to those of GDNF. These results support that within the experimental parameters used in this study, NTN is as effective as GDNF in protecting against the dopamine-depleting effects of intrastriatal 6-OHDA.

Neurturin effects on nigrostriatal dopamine release and content: comparison with GDNF

Neurturin (NTN) is a member of the glial cell line-derived neurotrophic factor (GDNF) family; and, while GDNF has been shown to increase dopamine (DA) release in normal animals, the ability of NTN to alter DA release has not been previously reported. The purpose of the present study was to determine if NTN could alter striatal DA release, and to compare the effects of NTN to GDNF. Male Fischer-344 rats were given a single injection of vehicle or 5 microg NTN or GDNF into the right substantia nigra. Three weeks later microdialysis experiments were conducted to assess striatal DA release. Basal extracellular levels of striatal DA were not affected by either NTN or GDNF. However, both NTN and GDNF led to increases in amphetamine-evoked overflow of DA from the ipsilateral striatum, and there was a trend for potassium-evoked overflow to be augmented. Postmortem tissue levels of DA were decreased by approximately 20% in the striatum, and increased by approximately 100% in the substantia nigra, on the ipsilateral side of the brain compared to the contralateral side following both NTN and GDNF injection. Thus, NTN, like GDNF, can augment striatal DA release, and the magnitude of the NTN effects are similar to those of GDNF.

Levodopa therapy from the neuroprotection viewpoint

There are many reports indicating the existence of free radical adducts in the Parkinson's disease (PD) brain. However, levodopa may have two aspects in its characteristics, one is that levodopa is a source of free radicals in the pathogenesis of PD, and the other is that it is the precursor of the most efficient transmitter in dopaminergic neurons from the viewpoint of therapy. Dopamine (DA) supplemented by levodopa acts as a neural transmitter and as a dopaminergic receptor-mediated trophic agent. From studies using rodent models of parkinsonism, physical exercise potentially induces neurotrophins in the brain and protects or enhances the regeneration of nigrostriatal neurons. In our preliminary experiment, the potential effects of physical exercise on motor and psychological symptoms were observed, suggesting that physical exercise may, to a certain extent, modulate pathological conditions in the brain. On the basis of the above we conclude that DA supplementation in PD potentially increases the levels of neurotrophins in a dopaminergic receptor-mediated manner, and secondarily enhances mobility (physical exercise). In general, levodopa demand increases with disease progression, and physical therapy is more important in advanced stages. To obtain good outcomes of physical exercise, it is essential to maintain a good medical control, particularly at the advanced stages of PD. From these results, it can be speculated that levodopa exerts some neuroprotective effects as long as the dose is not excessively high.

Overexpression of glial cell line-derived neurotrophic factor using a lentiviral vector induces time- and dose-dependent downregulation of tyrosine hydroxylase in the intact nigrostriatal dopamine system

The effects of continuous glial cell line-derived neurotrophic factor (GDNF) overexpression in the intact nigrostriatal dopamine (DA) system was studied using recombinant lentiviral (rLV) vector delivery of GDNF to the striatum or substantia nigra (SN) in the rat. Intrastriatal delivery of rLV-GDNF resulted in significant overexpression of GDNF in the striatum (2-4 ng/mg tissue) and anterograde transport of GDNF protein to the SN. Striatal rLV-GDNF delivery initially induced an increase in DA turnover (1-6 weeks), accompanied by significant contralateral turning in response to amphetamine, suggesting an enhancement of the DA system on the injected side. Starting 6 weeks after continuous GDNF delivery, we observed a selective downregulation of tyrosine hydroxylase (TH) protein (approximately 70%) that was maintained until the end of the experiment (24 weeks). A similar effect was observed when rLV-GDNF was injected into the SN. The magnitude of TH downregulation was related to the level of GDNF expression and was most pronounced in animals in which the striatal GDNF level exceeded 0.7 ng/mg tissue. The decreased TH protein levels were associated with similar reductions in the in vitro TH enzyme activity (approximately 70%); however, in vivo L-3,4-dihydroxyphenylalanine production rate and DA tissue levels were maintained at normal levels. The results indicate that downregulation of TH protein reflects a compensatory effect in response to continuous GDNF stimulation of the DA neurons mediated by a combination of overactivity at the DA synapse and a direct GDNF-induced action on TH gene expression. This compensatory mechanism is proposed to maintain long-term DA neuron function within the normal range.

Astrocyte delivery of glial cell line-derived neurotrophic factor in a mouse model of Parkinson's disease

Primary astrocytes were genetically modified ex vivo to express recombinant glial cell line-derived neurotrophic factor (GDNF) and subsequently were tested for their ability to provide neuroprotection to dopaminergic neurons in a 6-hydroxydopamine (6-OHDA) mouse model of Parkinson's disease. A replication-defective retrovirus was constructed, which contained the rat GDNF sequence and a sequence encoding a beta-galactosidase (beta-gal)/neomycin phosphotransferase fusion protein, linked via an internal ribosomal entry site. Murine astrocytes transduced with this vector secreted GDNF into the culture media at the rate of 115 +/- 34 pg/24 h/10(5) cells and expressed cytoplasmic beta-gal, whereas control nontransduced astrocytes were negative for GDNF production and cytoplasmic beta-gal expression. Mice that received implants of GDNF-producing astrocytes into the striatum or nigra displayed elevated levels of GDNF compared to mice that received control nontransduced astrocytes. In addition, tissue content of GDNF was increased bilaterally and in brain regions both proximal and distal to the graft, even though astrocyte migration away from the graft site did not occur. Importantly, GDNF-producing astrocytes provided marked neuroprotection of nigral dopaminergic perikarya, and partial protection of striatal dopaminergic fibers, when implanted into the midbrain 6 days prior to a retrograde 6-OHDA lesion, as assessed by tyrosine hydroxylase immunohistochemistry. Similarly, GDNF-producing astrocytes prevented the acquisition of amphetamine-induced rotational behavior in 6-OHDA-treated mice and completely prevented dopamine depletion within the substantia nigra, as assessed by high-performance liquid chromatography. These results indicate that continuous exposure to low levels of GDNF provided by transgenic astrocytes provides marked neuroprotection of nigral dopaminergic neurons. (c)2002 Elsevier Science (USA).

Neuroprotection in the rat Parkinson model by intrastriatal GDNF gene transfer using a lentiviral vector

We used a recombinant lentiviral vector (rLV) for gene delivery of GDNF to the striatum, and assessed its neuroprotective effects in the intrastriatal 6-hydroxydopamine (6-OHDA) lesion model. The level of GDNF expression obtained with the rLV-GDNF vector was dose-related and ranged between 0.9-9.3 ng/mg tissue in the transduced striatum, as determined by ELISA, and 0.2-3.0 ng/mg tissue were detected in the ipsilateral substantia nigra (SN), due to anterograde transport of the GDNF protein. GDNF expression was apparent at 4 days and maintained for > 8 months after injection. Striatal delivery of rLV-GDNF efficiently protected the nigral dopamine (DA) neurons and their projection, against the 6-OHDA lesion (65-77% of intact side). Sprouting of the lesioned axons was observed along the nigrostriatal pathway, precisely corresponding to the areas containing anterogradely transported GDNF.

Delayed infusion of GDNF promotes recovery of motor function in the partial lesion model of Parkinson's disease

Here we studied the effects of glial cell line-derived neurotrophic factor (GDNF) in a rat model that represents the symptomatic stages of Parkinson's disease. GDNF was infused starting 2 weeks after an intrastriatal 6-hydroxydopamine (6-OHDA) lesion in order to halt the ongoing degeneration of the nigrostriatal dopaminergic neurons. GDNF or vehicle was infused in the striatum or the lateral ventricle via an osmotic minipump over a total 4-week period (2-6 weeks postlesion). Motor function was evaluated by the stepping, paw reaching and drug-induced motor asymmetry tests before the pump infusion was initiated, and was repeated once during (5 weeks postlesion) and twice after the withdrawal of the minipumps (7 and 11 weeks postlesion). We found that within two weeks following the lesion approximately 40% of the nigral TH-positive neurons were lost. In the vehicle infusion groups there was an additional 20% cell loss between 2 and 12 weeks after the lesion. This latter cell loss occurred mainly in the caudal part of the SN whereas the cell loss in the rostral SN was almost complete within the first two weeks. Ventricular GDNF infusion completely blocked the late degenerating neurons in the caudal SN and had long lasting behavioural effects on the stepping test and amphetamine rotation, extending to 6 weeks after withdrawal of the factor. Striatal infusion affected the motor behaviour transiently during the infusion period but the motor performance of these animals returned to baseline upon cessation of the GDNF delivery, and the delayed nigral cell loss was marginally affected. We conclude that intraventricular GDNF can successfully block the already initiated degenerative process in the substantia nigra, and that the effects achieved via the striatal route, when GDNF is given acutely after the lesion, diminish as the fibre terminal degeneration proceeds.

Preservation of a functional nigrostriatal dopamine pathway by GDNF in the intrastriatal 6-OHDA lesion model depends on the site of administration of the trophic factor

Here we studied whether glial cell line-derived neurotrophic factor (GDNF), given as a single bolus injection before an intrastriatal 6-hydroxydopamine (6-OHDA) lesion, can protect the nigrostriatal dopamine neurons against the toxin-induced damage and preserve normal motor functions in the lesioned animals. GDNF or vehicle was injected in the striatum (25 microg), substantia nigra (25 microg) or lateral ventricle (50 microg) 6 h before the 6-OHDA lesion (20 microg/3 microL). Motor function was evaluated by the stepping and drug-induced motor asymmetry tests. Lesioned animals given vehicle alone showed a clear ipsilateral-side bias in response to amphetamine (13 turns/min), a moderate contralateral-side bias to apomorphine (4.5 turns/min) and a moderate to severe stepping deficit on the contralateral forepaw (three to four steps, as compared with 11-13 steps on the unimpaired side). Injection of GDNF into the striatum had a significant protective effect both on nigrostriatal function (1-2 turns/min in the rotation tests and seven to eight steps in the stepping test), and the integrity of the nigrostriatal pathway, seen as a protection of both the cell bodies in the substantia nigra and the dopamine innervation in the striatum. Injection of GDNF in the nigra had a protective effect on the nigral cell bodies, but not the striatal innervation, and failed to provide any functional benefit. In contrast, intranigral GDNF had deleterious effects on both the striatal TH-positive fibre density and on drug-induced rotation tests. Intraventricular injection had no effect. We conclude that preservation of normal motor functions in the intrastriatal 6-OHDA lesion model requires protection of striatal terminal innervation, and that this can be achieved by intrastriatal, but not nigral or intraventricular, administration of GDNF.