Striatal GDNF administration increases tyrosine hydroxylase phosphorylation in the rat striatum and substantia nigra

Phosphodiesterase inhibition and Gucy2C activation enhance tyrosine hydroxylase Ser40 phosphorylation and improve 6-hydroxydopamine-induced motor deficits

BACKGROUND

Parkinson's disease is characterized by a progressive loss of dopaminergic neurons in the nigrostriatal pathway, leading to dopamine deficiency and motor impairments. Current treatments, such as L-DOPA, provide symptomatic relief but result in off-target effects and diminished efficacy over time. This study explores an alternative approach by investigating the activation of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. Specifically, we explore the effects of phosphodiesterase (PDE) inhibition and guanylate cyclase-C (GUCY2C) activation on tyrosine hydroxylase Ser40 phosphorylation and their impact on motor behavior in a 6-hydroxydopamine (6-OHDA) Parkinson's disease model.

RESULTS

Our findings demonstrate that increasing cyclic nucleotide levels through PDE inhibition and GUCY2C activation significantly enhances tyrosine hydroxylase Ser40 phosphorylation. In a Pitx3-deficient mouse model, which mimics the loss of dopaminergic neurons seen in Parkinson's disease, Ser40 phosphorylation remained manipulable despite reduced tyrosine hydroxylase protein levels. Moreover, we observed no evidence of tyrosine hydroxylase degradation due to Ser40 phosphorylation, challenging previous reports. Furthermore, both PDE inhibition and GUCY2C activation resulted in improved motor behavior in the 6-OHDA Parkinson's disease mouse model, highlighting the potential therapeutic benefits of these approaches.

CONCLUSIONS

This study underscores the therapeutic potential of enhancing tyrosine hydroxylase Ser40 phosphorylation to improve motor function in Parkinson's disease. Both PDE inhibition and GUCY2C activation represent promising non-invasive strategies to modulate endogenous dopamine biosynthesis and address motor deficits. These findings suggest that targeting cyclic nucleotide pathways could lead to novel therapeutic approaches, either as standalone treatments or in combination with existing therapies like L-DOPA, aiming to provide more durable symptom relief and potentially mitigate neurodegeneration in Parkinson's disease.

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

Oxidative stress induced by sustained supraphysiological intrastriatal GDNF delivery is prevented by dose regulation

Despite its established neuroprotective effect on dopaminergic neurons and encouraging phase I results, intraputaminal GDNF administration failed to demonstrate significant clinical benefits in Parkinson's disease patients. Different human GDNF doses were delivered in the striatum of rats with a progressive 6-hydroxydopamine lesion using a sensitive doxycycline-regulated AAV vector. GDNF treatment was applied either continuously or intermittently (2 weeks on/2 weeks off) during 17 weeks. Stable reduction of motor impairments as well as increased number of dopaminergic neurons and striatal innervation were obtained with a GDNF dose equivalent to 3- and 10-fold the rat endogenous level. In contrast, a 20-fold increased GDNF level only temporarily provided motor benefits and neurons were not spared. Strikingly, oxidized DNA in the substantia nigra increased by 50% with 20-fold, but not 3-fold GDNF treatment. In addition, only low-dose GDNF allowed to preserve dopaminergic neuron cell size. Finally, aberrant dopaminergic fiber sprouting was observed with 20-fold GDNF but not at lower doses. Intermittent 20-fold GDNF treatment allowed to avoid toxicity and spare dopaminergic neurons but did not restore their cell size. Our data suggest that maintaining GDNF concentration under a threshold generating oxidative stress is a pre-requisite to obtain significant symptomatic relief and neuroprotection.

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.

Differential expression of RET and GDNF family receptor, GFR-α1, between striatum and substantia nigra following nigrostriatal lesion: a case for diminished GDNF-signaling

Although glial cell line-derived neurotrophic factor (GDNF) showed efficacy in preclinical and early clinical studies to alleviate parkinsonian signs in Parkinson's disease (PD), later trials did not meet primary endpoints, giving pause to consider further investigation. While GDNF dose and delivery methods may have contributed to diminished efficacy, one crucial aspect of these clinical studies is that GDNF treatment across all studies began ∼8 years after PD diagnosis; a time point representing several years after near 100% depletion of nigrostriatal dopamine markers in striatum and at least 50% in substantia nigra (SN), and is later than the timing of GDNF treatment in preclinical studies. With nigrostriatal terminal loss exceeding 70% at PD diagnosis, we utilized hemi-parkinsonian rats to determine if expression of GDNF family receptor, GFR-α1, and receptor tyrosine kinase, RET, differed between striatum and SN at 1 and 4 weeks following a 6-hydroxydopamine (6-OHDA) lesion. Whereas GDNF expression changed minimally, GFR-α1 expression decreased progressively in striatum and in tyrosine hydroxylase positive (TH+) cells in SN, correlating with reduced TH cell number. However, in nigral astrocytes, GFR-α1 expression increased. RET expression decreased maximally in striatum by 1 week, whereas in the SN, a transient bilateral increase occurred that returned to control levels by 4 weeks. Expression of brain-derived neurotrophic factor (BDNF) or its receptor, TrkB, were unchanged throughout lesion progression. Together, these results reveal that differential GFR-α1 and RET expression between the striatum and SN, and cell-specific differences in GFR-α1 expression in SN, occur during nigrostriatal neuron loss. Targeting loss of GDNF receptors appears critical to enhance GDNF therapeutic efficacy against nigrostriatal neuron loss.

Significance Statement

Although preclinical evidence supports that GDNF provides neuroprotection and improves locomotor function in preclinical studies, clinical data supporting its efficacy to alleviate motor impairment in Parkinson's disease patients remains uncertain. Using the established 6-OHDA hemi-parkinsonian rat model, we determined whether expression of its cognate receptors, GFR-α1 and RET, were differentially affected between striatum and substantia nigra in a timeline study. In striatum, there was early and significant loss of RET, but a gradual, progressive loss of GFR-α1. In contrast, RET transiently increased in lesioned substantia nigra, but GFR-α1 progressively decreased only in nigrostriatal neurons and correlated with TH cell loss. Our results indicate that direct availability of GFR-α1 may be a critical element that determines GDNF efficacy following striatal delivery.

Highlights

GDNF expression was minimally affected by nigrostriatal lesionGDNF family receptor, GFR-α1, progressively decreased in striatum and in TH neurons in SN.GFR-α1 expression decreased along with TH neurons as lesion progressedGFR-α1 increased bilaterally in GFAP+ cells suggesting an inherent response to offset TH neuron lossRET expression was severely reduced in striatum, whereas it increased in SN early after lesion induction.

Long-term exposure to GDNF induces dephosphorylation of Ret, AKT, and ERK1/2, and is ineffective at protecting midbrain dopaminergic neurons in cellular models of Parkinson's disease

Glial cell line-derived neurotrophic factor (GDNF) promotes differentiation, proliferation, and survival in different cell types, including dopaminergic neurons. Thus, GDNF has been proposed as a promising neuroprotective therapy in Parkinson's disease. Although findings from cellular and animal models of Parkinson's disease were encouraging, results emerging from clinical trials were not as good as expected, probably due to the inappropriate administration protocols. Despite the growing information on GDNF action mechanisms, many aspects of its pharmacological effects are still unclear and data from different studies are still contradictory. Considering that GDNF action mechanisms are mediated by its receptor tyrosine kinase Ret, which activates PI3K/AKT and MAPK/ERK signaling pathways, we aimed to investigate Ret activation and its effect over both signaling pathways in midbrain cell cultures treated with GDNF at different doses (0.3, 1, and 10 ng/ml) and times (15 min, 24 h, 24 h (7 days), and 7 continuous days). The results showed that short-term or acute (15 min, 24 h, and 24 h (7 days)) GDNF treatment in rat midbrain neurons increases Tyrosine hydroxylase (TH) expression and the phosphorylation levels of Ret (Tyr 1062), AKT (Ser 473), ERK1/2 (Thr202/Tyr204), S6 (Ser 235/236), and GSK3-β (Ser 9). However, the phosphorylation level of these kinases, TH expression, and dopamine uptake, decreased below basal levels after long-term or prolonged treatment with 1 and 10 ng/ml GDNF (7 continuous days). Our data suggest that long-term GDNF treatment inactivates the receptor by an unknown mechanism, affecting its neuroprotective capacity against degeneration caused by 6-OHDA or rotenone, while short-term exposure to GDNF promoted dopaminergic cell survival. These findings highlight the need to find new and more effective long-acting therapeutic approaches for disorders in which GDNF plays a beneficial role, including Parkinson's disease. In this regard, it is necessary to propose new GDNF treatment guidelines to regulate and control its long-term expression levels and optimize the clinical use of this trophic factor in patients with Parkinson's disease.

Spaceflight and brain plasticity: Spaceflight effects on regional expression of neurotransmitter systems and neurotrophic factors encoding genes

The critical problem of space exploration is the effect of long-term space travel on brain functioning. Current information concerning the effects of actual spaceflight on the brain was obtained on rats and mice flown on five missions of Soviet/Russian biosatellites, NASA Neurolab Mission STS90, and International Space Station (ISS). The review provides converging lines of evidence that: 1) long-term spaceflight affects both principle regulators of brain neuroplasticity - neurotransmitters (5-HT and DA) and neurotrophic factors (CDNF, GDNF but not BDNF); 2) 5-HT- (5-HT_2A receptor and MAO A) and especially DA-related genes (TH, MAO A, COMT, D1 receptor, CDNF and GDNF) belong to the risk neurogenes; 3) brain response to spaceflight is region-specific. Substantia nigra, striatum and hypothalamus are highly sensitive to the long-term spaceflight: in these brain areas spaceflight decreased the expression of both DA-related and neurotrophic factors genes. Since DA system is involved in the regulation of movement and cognition the data discussed in the review could explain dysfunction of locomotion and behavior of astronauts and direct further investigations to the DA system.

Activation of AMPK/aPKCζ/CREB pathway by metformin is associated with upregulation of GDNF and dopamine

Parkinson's disease (PD) is the second most common neurodegenerative disease, which is characterized by the progressive loss of dopaminergic neurons in the substantia nigra, leading to a decrease in striatal dopamine. There is no antiparkinsonian therapy that offers a true disease-modifying treatment till date and there is an urgent need for a safe and effective neuroprotective or neurorestorative therapy. Our previous study demonstrated that metformin upregulated dopamine in the mouse brain and provided significant neuroprotection in animal model of PD. Therefore, we designed this study to investigate the molecular mechanism underlying such pharmacological effect of metformin. Herein, we found that metformin enhanced the phosphorylation of tyrosine hydroxylase (TH) which was accompanied by increase in brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and activation of their downstream signaling pathways in the mouse brain and SH-SY5Y cells. We further investigated the role of the neurotrophic factors in the activation of TH and observed that both BDNF and GDNF-induction were essential for metformin-induced TH activation. We found that the AMPK/aPKCζ/CREB pathway was essential for metformin-induced GDNF upregulation and TH activation. Thus, this study reveals the TH-activating property of metformin in the brain via induction of neurotrophic factors along with the signaling mechanism. These results potentiate the candidacy of metformin not only as a neuroprotective agent, but also as restorative therapy for the treatment of PD.

Treatment of Parkinson’s disease using focused ultrasound with GDNF retrovirus-loaded microbubbles to open the blood–brain barrier

This study aims to prepare ultrasound-targeted glial cell-derived neurotrophic factor (GDNF) retrovirus-loaded microbubbles (M pLXSN-GDNF) to verify the properties of the microbubbles and to study the therapeutic effect of the GDNF retrovirus-loaded microbubbles combined with ultrasound (U) to open the blood–brain barrier (BBB) in a Parkinson’s disease (PD) model in rats, allowing the retrovirus to pass through the BBB and transfect neurons in the substantia nigra of the midbrain, thereby increasing the expression of GDNF. The results of western blot analysis revealed significant differences between U + MpLXSN-EGFP, U + M + pLXSN-GDNF, and M pLXSN-GDNF (P < 0.05) groups. After 8 weeks of treatment, the evaluation of the effect of increased GDNF expression on behavioral deficits in PD model rats was conducted. The rotation symptom was significantly improved in the U + MpLXSN-GDNF group, and the difference before and after treatment was significant (P < 0.05). Also, the content of dopamine and the number of tyrosine hydroxylase-positive (dopaminergic) neurons were found to be higher in the brain of PD rats in the U + M pLXSN-GDNF group than in the control groups. Ultrasound combined with GDNF retrovirus-loaded microbubbles can enhance the transfection efficiency of neurons in vivo and highly express the exogenous GDNF gene to play a therapeutic role in PD model rats.

Acute normobaric hypoxia does not affect the simultaneous exercise-induced increase in circulating BDNF and GDNF in young healthy men: A feasibility study

Physical exercise has a neuromodulatory effect on the central nervous system (CNS) partially by modifying expression of neuropeptides produced and secreted by neurons and glial cells, among which the best examined are brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF). Because both neurotrophins can cross the brain-blood barrier (BBB), their blood levels indirectly reflect their production in the CNS. Moreover, both neuropeptides are involved in modulation of dopaminergic and serotoninergic system function. Because limited information is available on the effects of exercise to volition exhaustion and acute hypoxia on CNS, BDNF and GDNF formation, the aims of the present study were to verify whether 1) acute exercise to exhaustion in addition to neurons also activates glial cells and 2) additional exposure to acute normobaric moderate hypoxia affects their function. In this feasibility study we measured blood concentrations of BDNF, GDNF, and neuropeptides considered as biomarkers of brain damage (bFGF, NGF, S100B, GFAP) in seven sedentary healthy young men who performed a graded exercise test to volitional exhaustion on a cycle ergometer under normoxic (N) and hypoxic conditions: 2,000 m (H2; FiO2 = 16.6%) and 3,000 m altitude (H3; FiO2 = 14.7%). In all conditions serum concentrations of both BDNF and GDNF increased immediately after cessation of exercise (p<0.01). There was no effect of condition or interaction (condition x time of measurement) and exercise on any of the brain damage biomarkers: bFGF, NGF, S100B, GFAP. Moreover, in N (0<0.01) and H3 (p<0.05) exercise caused elevated serum 5-HT concentration. The results suggest that a graded effort to volitional exhaustion in normoxia, as well as hypoxia, simultaneously activates both neurons and astrocytes. Considering that s100B, GFAP, bFGF, and NGF (produced mainly by astrocytes) are markers of brain damage, it can be assumed that a maximum effort in both conditions is safe for the CNS.

GFR-α1 Expression in Substantia Nigra Increases Bilaterally Following Unilateral Striatal GDNF in Aged Rats and Attenuates Nigral Tyrosine Hydroxylase Loss Following 6-OHDA Nigrostriatal Lesion

Glial cell line-derived neurotrophic factor (GDNF) improved motor function in Parkinson's disease (PD) patients in Phase I clinical trials, and these effects persisted months after GDNF discontinuation. Conversely, phase II clinical trials reported no significant effects on motor improvement vs placebo. The disease duration and the quantity, infusion approach, and duration of GDNF delivery may affect GDNF efficacy in PD treatment. However, identifying mechanisms activated by GDNF that affect nigrostriatal function may reveal additional avenues to partially restore nigrostriatal function. In PD and aging models, GDNF affects tyrosine hydroxylase (TH) expression or phosphorylation in substantia nigra (SN), long after a single GDNF injection in striatum. In aged rats, the GDNF family receptor, GFR-α1, increases TH expression and phosphorylation in SN. To determine if GFR-α1 could be a mechanistic link in long-term GDNF impact, we conducted two studies; first to determine if a single unilateral striatal delivery of GDNF affected GFR-α1 and TH over time (1 day, 1 week, and 4 weeks) in the striatum or SN in aged rats, and second, to determine if soluble GFR-α1 could mitigate TH loss following 6-hydroxydopamine (6-OHDA) lesion. In aged rats, GDNF bilaterally increased ser31 TH phosphorylation and GFR-α1 expression in SN at 1 day and 4 weeks after GDNF, respectively. In striatum, GFR-α1 expression decreased 1 week after GDNF, only on the GDNF-injected side. In 6-OHDA-lesioned rats, recombinant soluble GFR-α1 mitigated nigral, but not striatal, TH protein loss following 6-OHDA. Together, these results show GDNF has immediate and long-term impact on dopamine regulation in the SN, which includes a gradual increase in GFR-α1 expression that may sustain TH expression and dopamine function therein.

Mesencephalic Astrocyte-Derived Neurotrophic Factor (MANF) Elevates Stimulus-Evoked Release of Dopamine in Freely-Moving Rats

Neurotrophic factors (NTFs) hold potential as disease-modifying therapies for neurodegenerative disorders like Parkinson's disease. Glial cell line-derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), and mesencephalic astrocyte-derived neurotrophic factor (MANF) have shown neuroprotective and restorative effects on nigral dopaminergic neurons in various animal models of Parkinson's disease. To date, however, their effects on brain neurochemistry have not been compared using in vivo microdialysis. We measured extracellular concentration of dopamine and activity of dopamine neurochemistry-regulating enzymes in the nigrostriatal system of rat brain. NTFs were unilaterally injected into the striatum of intact Wistar rats. Brain microdialysis experiments were performed 1 and 3 weeks later in freely-moving animals. One week after the treatment, we observed enhanced stimulus-evoked release of dopamine in the striatum of MANF-treated rats, but not in rats treated with GDNF or CDNF. MANF also increased dopamine turnover. Although GDNF did not affect the extracellular level of dopamine, we found significantly elevated tyrosine hydroxylase (TH) and catechol-O-methyltransferase (COMT) activity and decreased monoamine oxidase A (MAO-A) activity in striatal tissue samples 1 week after GDNF injection. The results show that GDNF, CDNF, and MANF have divergent effects on dopaminergic neurotransmission, as well as on dopamine synthetizing and metabolizing enzymes. Although the cellular mechanisms remain to be clarified, knowing the biological effects of exogenously administrated NTFs in intact brain is an important step towards developing novel neurotrophic treatments for degenerative brain diseases.

Prolonged increase in ser31 tyrosine hydroxylase phosphorylation in substantia nigra following cessation of chronic methamphetamine

Methamphetamine (MA) exposure may increase the risk of motor or cognitive impairments similar to Parkinson's disease (PD) by middle age. Although damage to nigrostriatal or mesoaccumbens dopamine (DA) neurons may occur during or early after MA exposure, overt PD-like symptoms at a younger age may not manifest due to compensatory mechanisms to maintain DA neurotransmission. One possible compensatory mechanism is increased tyrosine hydroxylase (TH) phosphorylation. In the rodent PD 6-OHDA model, nigrostriatal lesion decreases TH protein in both striatum and substantia nigra (SN). However, DA loss in the SN is significantly less than that in the striatum. An increase in ser31 TH phosphorylation in the SN may increase TH activity in response to TH loss. To determine if similar compensatory mechanisms may be engaged in young mice after MA exposure, TH expression, phosphorylation, and DA tissue content were evaluated, along with dopamine transporter expression, 21 days after cessation of MA (24 mg/kg, daily, 14 days). DA tissue content was unaffected by the MA regimen in striatum, nucleus accumbens, SN, or ventral tegmental area (VTA), despite decreased TH protein in SN and VTA. In the SN, but not striatum, ser31 phosphorylation increased over 2-fold. This suggests that increased ser31 TH phosphorylation may be an inherent compensatory mechanism to attenuate DA loss against TH loss, similar to that in an established PD model. These results also indicate the somatodendritic compartments of DA neurons are more vulnerable to TH protein loss than terminal fields following MA exposure.

Aging-related limit of exercise efficacy on motor decline

Identifying lifestyle strategies and allied neurobiological mechanisms that reduce aging-related motor impairment is imperative, given the accelerating number of retirees and increased life expectancy. A physically active lifestyle prior to old age can reduce risk of debilitating motor decline. However, if exercise is initiated after motor decline has begun in the lifespan, it is unknown if aging itself may impose a limit on exercise efficacy to decelerate further aging-related motor decline. In Brown-Norway/Fischer 344 F1 hybrid (BNF) rats, locomotor activity begins to decrease in middle age (12-18 months). One mechanism of aging-related motor decline may be decreased expression of GDNF family receptor, GFRα-1, which is decreased in substantia nigra (SN) between 12 and 30 months old. Moderate exercise, beginning at 18 months old, increases nigral GFRα-1 and tyrosine hydroxylase (TH) expression within 2 months. In aged rats, replenishing aging-related loss of GFRα-1 in SN increases TH in SN alone and locomotor activity. A moderate exercise regimen was initiated in sedentary male BNF rats in a longitudinal study to evaluate if exercise could attenuate aging-related motor decline when initiated at two different ages in the latter half of the lifespan (18 or 24 months old). Motor decline was reversed in the 18-, but not 24-month-old, cohort. However, exercise efficacy in the 18-month-old group was reduced as the rats reached 27 months old. GFRα-1 expression was not increased in either cohort. These studies suggest exercise can decelerate motor decline when begun in the latter half of the lifespan, but its efficacy may be limited by age of initiation. Decreased plasticity of GFRα-1 expression following exercise may limit its efficacy to reverse motor decline.

Constitutive Ret signaling leads to long-lasting expression of amphetamine-induced place conditioning via elevation of mesolimbic dopamine

Addictive drugs enhance dopamine release in the striatum, which can lead to compulsive drug-seeking after repeated exposure. Glial cell line-derived neurotrophic factor (GDNF) is an important regulator of midbrain dopamine neurons, and may play a mechanistic role in addiction-related behaviors. To elucidate the components of GDNF-signaling that contribute to addiction-related behaviors of place preference and its extinction, we utilized two genetically modified GDNF mouse models in an amphetamine-induced conditioned place preference (CPP) paradigm and evaluated how the behavioral findings correlate with dopamine signaling in the dorsal and ventral striatum. We utilized two knock-in mouse strains to delineate contributions of GDNF and Ret signaling using MEN2B mice (constitutively active GDNF receptor Ret), and GDNF hypermorphic mice (enhanced endogenous GDNF expression). The duration of amphetamine-induced CPP was greatly enhanced in MEN2B mice, but not in the GDNF hypermorphic mice. The enhanced duration of CPP was correlated with increased tyrosine hydroxylase (TH) expression and dopamine content in the ventral striatum. Together, our results suggest that downstream components of GDNF signaling, in this case Ret, may mediate persistent drug-seeking behavior through increased TH expression and dopamine levels in the mesolimbic dopamine neurons.

Glial Cell Line-Derived Neurotrophic Factor Gene Delivery in Parkinson's Disease: A Delicate Balance between Neuroprotection, Trophic Effects, and Unwanted Compensatory Mechanisms

Glial cell line-derived neurotrophic factor (GDNF) and Neurturin (NRTN) bind to a receptor complex consisting of a member of the GDNF family receptor (GFR)-α and the Ret tyrosine kinase. Both factors were shown to protect nigro-striatal dopaminergic neurons and reduce motor symptoms when applied terminally in toxin-induced Parkinson's disease (PD) models. However, clinical trials based on intraputaminal GDNF protein administration or recombinant adeno-associated virus (rAAV)-mediated NRTN gene delivery have been disappointing. In this review, several factors that could have limited the clinical benefits are discussed. Retrograde transport of GDNF/NRTN to the dopaminergic neurons soma is thought to be necessary for NRTN/GFR-α/Ret signaling mediating the pro-survival effect. Therefore, the feasibility of treating advanced patients with neurotrophic factors is questioned by recent data showing that: (i) tyrosine hydroxylase-positive putaminal innervation has almost completely disappeared at 5 years post-diagnosis and (ii) in patients enrolled in the rAAV-NRTN trial more than 5 years post-diagnosis, NRTN was almost not transported to the substantia nigra pars compacta. In addition to its anti-apoptotic and neurotrophic properties, GDNF also interferes with dopamine homeostasis via time and dose-dependent effects such as: stimulation of dopamine neuron excitability, inhibition of dopamine transporter activity, tyrosine hydroxylase phosphorylation, and inhibition of tyrosine hydroxylase transcription. Depending on the delivery parameters, the net result of this intricate network of regulations could be either beneficial or deleterious. In conclusion, further unraveling of the mechanism of action of GDNF gene delivery in relevant animal models is still needed to optimize the clinical benefits of this new therapeutic approach. Recent developments in the design of regulated viral vectors will allow to finely adjust the GDNF dose and period of administration. Finally, new clinical studies in less advanced patients are warranted to evaluate the potential of AAV-mediated neurotrophic factors gene delivery in PD. These will be facilitated by the demonstration of the safety of rAAV administration into the human brain.

Non-human primate models of PD to test novel therapies

Non-human primate (NHP) models of Parkinson disease show many similarities with the human disease. They are very useful to test novel pharmacotherapies as reviewed here. The various NHP models of this disease are described with their characteristics including the macaque, the marmoset, and the squirrel monkey models. Lesion-induced and genetic models are described. There is no drug to slow, delay, stop, or cure Parkinson disease; available treatments are symptomatic. The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-Dopa) still remains the gold standard symptomatic treatment of Parkinson. However, involuntary movements termed L-Dopa-induced dyskinesias appear in most patients after chronic treatment and may become disabling. Dyskinesias are very difficult to manage and there is only amantadine approved providing only a modest benefit. In this respect, NHP models have been useful to seek new drug targets, since they reproduce motor complications observed in parkinsonian patients. Therapies to treat motor symptoms in NHP models are reviewed with a discussion of their translational value to humans. Disease-modifying treatments tested in NHP are reviewed as well as surgical treatments. Many biochemical changes in the brain of post-mortem Parkinson disease patients with dyskinesias are reviewed and compare well with those observed in NHP models. Non-motor symptoms can be categorized into psychiatric, autonomic, and sensory symptoms. These symptoms are present in most parkinsonian patients and are already installed many years before the pre-motor phase of the disease. The translational usefulness of NHP models of Parkinson is discussed for non-motor symptoms.

Reduced ability of calcitriol to promote augmented dopamine release in the lesioned striatum of aged rats

Parkinson's disease (PD) is a progressive and debilitating neurodegenerative disorder that affects over one million people in the United States. Previous studies, carried out in young adult rats, have shown that calcitriol, the active metabolite of vitamin D, can be neuroprotective in 6-hydroxydopamine (6-OHDA) models of PD. However, as PD usually affects older individuals, the ability of calcitriol to promote dopaminergic recovery was examined in lesioned young adult (4 month old), middle-aged (14 month old) and aged (22 month old) rats. Animals were given a single injection of 12 μg 6-OHDA into the right striatum. Four weeks later they were administered vehicle or calcitriol (1.0 μg/kg, s.c.) once a day for eight consecutive days. In vivo microdialysis experiments were carried out three weeks after the calcitriol or vehicle treatments to measure potassium and amphetamine evoked overflow of DA from both the left and right striata. In control animals treated with 6-OHDA and vehicle there were significant reductions in evoked overflow of DA on the lesioned side of the brain compared to the contralateral side. The calcitriol treatments significantly increased evoked overflow of DA from the lesioned striatum in both the young adult and middle-aged rats. However, the calcitriol treatments did not significantly augment DA overflow in the aged rats. Postmortem tissue levels of striatal DA were also increased in the young and middle-aged animals, but not in the aged animals. In the substantia nigra, the calcitriol treatments led to increased levels of DA in all three age groups. Thus, the effects of calcitriol were similar in the young adult and middle-aged animals, but in the aged animals the effects of calcitriol were diminished. These results suggest that calcitriol may help promote recovery of dopaminergic functioning in injured nigrostriatal neurons; however, the effectiveness of calcitriol may be reduced in aging.

Role of glial cell line-derived neurotrophic factor in the pathogenesis and treatment of mood disorders

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a survival factor for dopaminergic neurons, but GDNF has also been shown to promote development, differentiation, and protection of other central nervous system neurons and was thought to play an important role in various neuropsychiatric disorders. Severe mood disorders, such as primarily major depressive disorder and bipolar affective disorder, attract particular attention. These psychopathologies are characterized by structural alterations accompanied by the dysregulation of neuroprotective and neurotrophic signaling mechanisms required for the maturation, growth, and survival of neurons and glia. The main objective of this review is to summarize the recent findings and evaluate the potential role of GDNF in the pathogenesis and treatment of mood disorders. Specifically, it describes (1) the implication of GDNF in the mechanism of depression and in the effect of antidepressant drugs and mood stabilizers and (2) the interrelation between GDNF and brain neurotransmitters, playing a key role in the pathogenesis of depression. This review provides converging lines of evidence that (1) brain GDNF contributes to the mechanism underlying depressive disorders and the effect of antidepressants and mood stabilizers and (2) there is a cross-talk between GDNF and neurotransmitters representing a feedback system: GDNF-neurotransmitters and neurotransmitters-GDNF.

Evidence for an Additive Neurorestorative Effect of Simultaneously Administered CDNF and GDNF in Hemiparkinsonian Rats: Implications for Different Mechanism of Action

Parkinson's disease (PD) is a neurodegenerative disorder associated with a progressive loss of dopaminergic (DAergic) neurons of the substantia nigra (SN) and the accumulation of intracellular inclusions containing α-synuclein. Current therapies do not stop the progression of the disease, and the efficacy of these treatments wanes over time. Neurotrophic factors (NTFs) are naturally occurring proteins promoting the survival and differentiation of neurons and the maintenance of neuronal contacts. CDNF (cerebral dopamine NTF) and GDNF (glial cell line-derived NTF) are able to protect DAergic neurons against toxin-induced degeneration in experimental models of PD. Here, we report an additive neurorestorative effect of coadministration of CDNF and GDNF in the unilateral 6-hydroxydopamine (6-OHDA) lesion model of PD in rats. NTFs were given into the striatum four weeks after unilateral intrastriatal injection of 6-OHDA (20 µg). Amphetamine-induced (2.5 mg/kg, i.p.) rotational behavior was measured every two weeks. Number of tyrosine hydroxylase (TH)-positive cells from SN pars compacta (SNpc) and density of TH-positive fibers in the striatum were analyzed at 12 weeks after lesion. CDNF and GDNF alone restored the DAergic function, and one specific dose combination had an additive effect: CDNF (2.5µg) and GDNF (1µg) coadministration led to a stronger trophic effect relative to either of the single treatments alone. The additive effect may indicate different mechanism of action for the NTFs. Indeed, both NTFs activated the survival promoting PI3 kinase (PI3K)-Akt signaling pathway, but only CDNF decreased the expression level of tested endoplasmatic reticulum (ER) stress markers ATF6, glucose-regulated protein 78 (GRP78), and phosphorylation of eukaryotic initiation factor 2α subunit (eIF2α).

Escalating Methamphetamine Regimen Induces Compensatory Mechanisms, Mitochondrial Biogenesis, and GDNF Expression, in Substantia Nigra

Methamphetamine (MA) produces long-lasting deficits in dopaminergic neurons in the long-term use via several neurotoxic mechanisms. The effects of MA on mitochondrial biogenesis is less studied currently. So, we evaluated the effects of repeated escalating MA regimen on transcriptional factors involved in mitochondrial biogenesis and glial-derived neurotrophic factor (GDNF) expression in substantia nigra (SN) and striatum of rat. In male Wistar rats, increasing doses of MA (1-14 mg/kg) were administrated twice a day for 14 days. At the 1st, 14th, 28th, and 60th days after MA discontinuation, we measured the PGC1α, TFAM and NRF1 mRNA levels, indicator of mitochondrial biogenesis, and GDNF expression in SN and striatum. Furthermore, we evaluated the glial fibrillary acidic protein (GFAP) and Iba1 mRNA levels, and the levels of tyrosine hydroxylase (TH) and α-synuclein (α-syn) using immunohistochemistry and real-time polymerase chain reaction (PCR). We detected increments in PGC1α and TFAM mRNA levels in SN, but not striatum, and elevations in GDNF levels in SN immediately after MA discontinuation. We also observed increases in GFAP and Iba1 mRNA levels in SN on day 1 and increases in Iba1 mRNA on days 1 and 14 in striatum. Data analysis revealed that the number of TH⁺ cells in the SN did not reduce in any time points, though TH mRNA levels was increased on day 1 after MA discontinuation in SN. These data show that repeated escalating MA induces several compensatory mechanisms, such as mitochondrial biogenesis and elevation in GDNF in SN. These mechanisms can reverse MA-induced neuroinflammation and prevent TH-immunoreactivity reduction in nigrostriatal pathway. J. Cell. Biochem. 118: 1369-1378, 2017. © 2016 Wiley Periodicals, Inc.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

A regulatable AAV vector mediating GDNF biological effects at clinically-approved sub-antimicrobial doxycycline doses

Preclinical and clinical data stress the importance of pharmacologically-controlling glial cell line-derived neurotrophic factor (GDNF) intracerebral administration to treat PD. The main challenge is finding a combination of a genetic switch and a drug which, when administered at a clinically-approved dose, reaches the brain in sufficient amounts to induce a therapeutic effect. We describe a highly-sensitive doxycycline-inducible adeno-associated virus (AAV) vector. This vector allowed for the first time a longitudinal analysis of inducible transgene expression in the brain using bioluminescence imaging. To evaluate the dose range of GDNF biological activity, the inducible AAV vector (8.0 × 10(9) viral genomes) was injected in the rat striatum at four delivery sites and increasing doxycycline doses administered orally. ERK/Akt signaling activation as well as tyrosine hydroxylase downregulation, a consequence of long-term GDNF treatment, were induced at plasmatic doxycycline concentrations of 140 and 320 ng/ml respectively, which are known not to increase antibiotic-resistant microorganisms in patients. In these conditions, GDNF covered the majority of the striatum. No behavioral abnormalities or weight loss were observed. Motor asymmetry resulting from unilateral GDNF treatment only appeared with a 2.5-fold higher vector and a 13-fold higher inducer doses. Our data suggest that using the herein-described inducible AAV vector, biological effects of GDNF can be obtained in response to sub-antimicrobial doxycycline doses.

Exercise-Mediated Increase in Nigral Tyrosine Hydroxylase Is Accompanied by Increased Nigral GFR-α1 and EAAC1 Expression in Aging Rats

Exercise may alleviate locomotor impairment in Parkinson's disease (PD) or aging. Identifying molecular responses immediately engaged by exercise in the nigrostriatal pathway and allied tissue may reveal critical targets associated with its long-term benefits. In aging, there is loss of tyrosine hydroxylase (TH) and the glial cell line-derived neurotrophic factor (GDNF) receptor, GFR-α1, in the substantia nigra (SN). Exercise can increase GDNF expression, but its effect on GFR-α1 expression is unknown. Infusion of GDNF into striatum or GFR-α1 in SN, respectively, can increase locomotor activity and TH function in SN but not striatum in aged rats. GDNF may also increase glutamate transporter expression, which attenuates TH loss in PD models. We utilized a footshock-free treadmill exercise regimen to determine the immediate impact of short-term exercise on GFR-α1 expression, dopamine regulation, glutamate transporter expression, and glutamate uptake in 18 month old male Brown-Norway/Fischer 344 F1 hybrid rats. GFR-α1 and TH expression significantly increased in SN but not striatum. This exercise regimen did not affect glutamate uptake or glutamate transporter expression in striatum. However, EAAC1 expression increased in SN. These results indicate that nigral GFR-α1 and EAAC1 expression increased in conjunction with increased nigral TH expression following short-term exercise.

Long-term controlled GDNF over-expression reduces dopamine transporter activity without affecting tyrosine hydroxylase expression in the rat mesostriatal system

The dopamine (DA) transporter (DAT) is a plasma membrane glycoprotein expressed in dopaminergic (DA-) cells that takes back DA into presynaptic neurons after its release. DAT dysfunction has been involved in different neuro-psychiatric disorders including Parkinson's disease (PD). On the other hand, numerous studies support that the glial cell line-derived neurotrophic factor (GDNF) has a protective effect on DA-cells. However, studies in rodents show that prolonged GDNF over-expression may cause a tyrosine hydroxylase (TH, the limiting enzyme in DA synthesis) decline. The evidence of TH down-regulation suggests that another player in DA handling, DAT, may also be regulated by prolonged GDNF over-expression, and the possibility that this effect is induced at GDNF expression levels lower than those inducing TH down-regulation. This issue was investigated here using intrastriatal injections of a tetracycline-inducible adeno-associated viral vector expressing human GDNF cDNA (AAV-tetON-GDNF) in rats, and doxycycline (DOX; 0.01, 0.03, 0.5 and 3mg/ml) in the drinking water during 5weeks. We found that 3mg/ml DOX promotes an increase in striatal GDNF expression of 12× basal GDNF levels and both DA uptake decrease and TH down-regulation in its native and Ser40 phosphorylated forms. However, 0.5mg/ml DOX promotes a GDNF expression increase of 3× basal GDNF levels with DA uptake decrease but not TH down-regulation. The use of western-blot under non-reducing conditions, co-immunoprecipitation and in situ proximity ligation assay revealed that the DA uptake decrease is associated with the formation of DAT dimers and an increase in DAT-α-synuclein interactions, without changes in total DAT levels or its compartmental distribution. In conclusion, at appropriate GDNF transduction levels, DA uptake is regulated through DAT protein-protein interactions without interfering with DA synthesis.

Age-related gene expression changes in substantia nigra dopamine neurons of the rat

Ageing affects most, if not all, functional systems in the body. For example, the somatic motor nervous system, responsible for initiating and regulating motor output to skeletal musculature, is vulnerable to ageing. The nigrostriatal dopamine pathway is one component of this system, with deficits in dopamine signalling contributing to major motor dysfunction, as exemplified in Parkinson's disease (PD). However, while the dopamine deficit in PD is due to degeneration of substantia nigra (SN) dopamine (DA) neurons, it is unclear whether there is sufficient loss of SN DA neurons with ageing to explain observed motor impairments. Instead, evidence suggests that age-related loss of DA neuron function may be more important than frank cell loss. To further elucidate the mechanisms of functional decline, we have investigated age-related changes in gene expression specifically in laser microdissected SN DA neurons. There were significant age-related changes in the expression of genes associated with neurotrophic factor signalling and the regulation of tyrosine hydroxylase activity. Furthermore, reduced expression of the DA neuron-associated transcription factor, Nurr1, may contribute to these changes. Together, these results suggest that altered neurotrophic signalling and tyrosine hydroxylase activity may contribute to altered DA neuron signalling and motor nervous system regulation in ageing.

Clioquinol rescues Parkinsonism and dementia phenotypes of the tau knockout mouse

Iron accumulation and tau protein deposition are pathological features of Alzheimer's (AD) and Parkinson's diseases (PD). Soluble tau protein is lower in affected regions of these diseases, and we previously reported that tau knockout mice display motor and cognitive behavioral abnormities, brain atrophy, neuronal death in substantia nigra, and iron accumulation in the brain that all emerged between 6 and 12 months of age. This argues for a loss of tau function in AD and PD. We also showed that treatment with the moderate iron chelator, clioquinol (CQ) restored iron levels and prevented neuronal atrophy and attendant behavioral decline in 12-month old tau KO mice when commenced prior to the onset of deterioration (6 months). However, therapies for AD and PD will need to treat the disease once it is already manifest. So, in the current study, we tested whether CQ could also rescue the phenotype of mice with a developed phenotype. We found that 5-month treatment of symptomatic (13 months old) tau KO mice with CQ increased nigral tyrosine hydroxylase phosphorylation (which induces activity) and reversed the motor deficits. Treatment also reversed cognitive deficits and raised BDNF levels in the hippocampus, which was accompanied by attenuated brain atrophy, and reduced iron content in the brain. These data raise the possibility that lowering brain iron levels in symptomatic patients could reverse neuronal atrophy and improve brain function, possibly by elevating neurotrophins.

Tricyclic antidepressant treatment evokes regional changes in neurotrophic factors over time within the intact and degenerating nigrostriatal system

In addition to alleviating depression, trophic responses produced by antidepressants may regulate neural plasticity in the diseased brain, which not only provides symptomatic benefit but also potentially slows the rate of disease progression in Parkinson's disease (PD). Recent in vitro and in vivo data provide evidence that neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) may be key mediators of the therapeutic response to antidepressants. As such, we conducted a cross-sectional time-course study to determine whether antidepressant-mediated changes in neurotrophic factors occur in relevant brain regions in response to amitriptyline (AMI) treatment before and after intrastriatal 6-hydroxydopamine (6OHDA). Adult male Wistar rats were divided into seven cohorts and given daily injections (i.p.) of AMI (5mg/kg) or saline throughout the duration of the study. In parallel, various cohorts of intact or parkinsonian animals were sacrificed at specific time points to determine the impact of AMI treatment on trophic factor levels in the intact and degenerating nigrostriatal system. The left and right hemispheres of the substantia nigra, striatum, frontal cortex, piriform cortex, hippocampus, and anterior cingulate cortex were dissected, and BDNF and GDNF levels were measured with ELISA. Results show that chronic AMI treatment elicits effects in multiple brain regions and differentially regulates levels of BDNF and GDNF depending on the region. Additionally, AMI halts the progressive degeneration of dopamine (DA) neurons elicited by an intrastriatal 6-OHDA lesion. Taken together, these results suggest that AMI treatment elicits significant trophic changes important to DA neuron survival within both the intact and degenerating nigrostriatal system.

Dynamic changes in dopamine neuron function after DNSP-11 treatment: effects in vivo and increased ERK 1/2 phosphorylation in vitro

Glial cell-line derived neurotrophic factor (GDNF) has demonstrated robust effects on dopamine (DA) neuron function and survival. A post-translational processing model of the human GDNF proprotein theorizes the formation of smaller, amidated peptide(s) from the proregion that exhibit neurobiological function, including an 11-amino-acid peptide named dopamine neuron stimulating peptide-11 (DNSP-11). A single treatment of DNSP-11 was delivered to the substantia nigra in the rat to investigate effects on DA-neuron function. Four weeks after treatment, potassium (K+) and D-amphetamine evoked DA release were studied in the striatum using microdialysis. There were no significant changes in DA-release after DNSP-11 treatment determined by microdialysis. Dopamine release was further examined in discrete regions of the striatum using high-speed chronoamperometry at 1-, 2-, and 4-weeks after DNSP-11 treatment. Two weeks after DNSP-11 treatment, potassium-evoked DA release was increased in specific subregions of the striatum. However, spontaneous locomotor activity was unchanged by DNSP-11 treatment. In addition, we show that a single treatment of DNSP-11 in the MN9D dopaminergic neuronal cell line results in phosphorylation of ERK1/2, which suggests a novel cellular mechanism responsible for increases in DA function.

ser31 Tyrosine hydroxylase phosphorylation parallels differences in dopamine recovery in nigrostriatal pathway following 6-OHDA lesion

Compensatory mechanisms in dopamine (DA) signaling have long been proposed to delay onset of locomotor symptoms during Parkinson's disease progression until ~ 80% loss of striatal DA occurs. Increased striatal dopamine turnover has been proposed to be a part of this compensatory response, but may occur after locomotor symptoms. Increased tyrosine hydroxylase (TH) activity has also been proposed as a mechanism, but the impact of TH protein loss upon site-specific TH phosphorylation in conjunction with the impact on DA tissue content is not known. The tissue content of DA was determined against TH protein loss in the striatum and substantia nigra (SN) following 6-hydroxydopamine lesion in the medial forebrain bundle in young Sprague-Dawley male rats. Although DA predictably decreased in both regions following 6-hydroxydopamine, there was a significant difference in DA loss between the striatum (75%) and SN (40%), despite similar TH protein loss. Paradoxically, there was a significant decrease in DA against remaining TH protein in striatum, but a significant increase in DA against remaining TH in SN. In the SN, increased DA per remaining TH protein was matched by increased ser31, but not ser40, TH phosphorylation. In striatum, both ser31 and ser40 phosphorylation decreased, reflecting decreased DA per TH. However, in control nigral and striatal tissue, only ser31 phosphorylation correlated with DA per TH protein. Combined, these results suggest that the phosphorylation of ser31 in the SN may be a mechanism to increase DA biosynthesis against TH protein loss in an in vivo model of Parkinson's disease. Properties of dopamine biosynthesis were evaluated in the 6-OHDA model of Parkinson's disease by studying the impact of tyrosine hydroxylase (TH) protein loss on its own phosphorylation and dopamine (DA) tissue content in rat nigrostriatal pathway. A dichotomous response was observed between striatum and substantia nigra in that dopamine per remaining TH decreased in striatum, but increased in substantia nigra. Phosphorylation at ser31 reflected these differences, indicating that ser31 phosphorylation may be critical to maintain dopamine with progressive TH protein loss. Drawings are from slides purchased from Motifolio (http://motifolio.com/).

The impact of adult vitamin D deficiency on behaviour and brain function in male Sprague-Dawley rats

Background

Vitamin D deficiency is common in the adult population, and this has been linked to depression and cognitive outcomes in clinical populations. The aim of this study was to investigate the effects of adult vitamin D (AVD) deficiency on behavioural tasks of relevance to neuropsychiatric disorders in male Sprague-Dawley rats.

Methods

Ten-week old male Sprague-Dawley rats were fed a control or vitamin D deficient diet for 6 weeks prior to, and during behavioural testing. We first examined a range of behavioural domains including locomotion, exploration, anxiety, social behaviour, learned helplessness, sensorimotor gating, and nociception. We then assessed locomotor response to the psychomimetic drugs, amphetamine and MK-801. Attention and vigilance were assessed using the 5 choice serial reaction time task (5C-SRT) and the 5 choice continuous performance task (5C-CPT) and, in a separate cohort, working memory was assessed using the delay match to sample (DMTS) task. We also examined excitatory and inhibitory neurotransmitters in prefrontal cortex and striatum.

Results

AVD-deficient rats were deficient in vitamin D₃ (<10 nM) and had normal calcium and phosphate levels after 8–10 weeks on the diet. Overall, AVD deficiency was not associated with an altered phenotype across the range of behavioural domains tested. On the 5C-SRT AVD-deficient rats made more premature responses and more head entries during longer inter-trial intervals (ITI) than control rats. On the 5C-CPT AVD-deficient rats took longer to make false alarm (FA) responses than control rats. AVD-deficient rats had increases in baseline GABA levels and the ratio of DOPAC/HVA within the striatum.

Conclusions

AVD-deficient rats exhibited no major impairments in any of the behavioural domains tested. Impairments in premature responses in AVD-deficient rats may indicate that these animals have specific alterations in striatal systems governing compulsive or reward-seeking behaviour.

A role for Sv2c in basal ganglia functions

SV2C is an isoform of the synaptic vesicle 2 protein family that exhibits a particular pattern of brain expression with enriched expression in several basal ganglia nuclei. In the present study, we have investigated SV2C implication in both normal and pathological basal ganglia functioning with a peculiar attention to dopamine neuron containing regions. In SV2C-/- mice, the expression of tyrosine hydroxylase mRNA in midbrain dopaminergic neurons was largely and significantly increased and enkephalin mRNA expression was significantly decreased in the caudate-putamen and accumbens nucleus. The expression of SV2C was studied in two models of dopaminergic denervation (6-OHDA- and MPTP-induced lesions). In dopamine-depleted animals, SV2C mRNA expression was significant increased in the striatum. In order to further understand the role of SV2C, we performed behavioral experiments on SV2C-/- mice and on knock-down mice receiving an injection of adeno-associated virus expressing SV2C miRNA specifically in the ventral midbrain. These modifications of SV2C expression had little or no impact on behavior in open field and elevated plus maze. However, even if complete loss of SV2C had no impact on conditioned place preference induced by cocaine, the specific knock-down of SV2C expression in the dopaminergic neurons completely abolished the development of a CPP while the reaction to an acute drug injection remains similar in these mice compared to control mice. These results showed that SV2C, a poorly functionally characterized protein is strongly involved in normal operation of the basal ganglia network and could be also involved in system adaptation in basal ganglia pathological conditions.

A synthetic five amino acid propeptide increases dopamine neuron differentiation and neurochemical function

A major consequence of Parkinson's disease (PD) involves the loss of dopaminergic neurons in the substantia nigra (SN) and a subsequent loss of dopamine (DA) in the striatum. We have shown that glial cell line-derived neurotrophic factor (GDNF) shows robust restorative and protective effects for DA neurons in rats, non-human primates and possibly in humans. Despite GDNF's therapeutic potential, its clinical value has been questioned due to its limited diffusion to target areas from its large size and chemical structure. Several comparatively smaller peptides are thought to be generated from the prosequence. A five amino-acid peptide, dopamine neuron stimulating peptide-5 (DNSP-5), has been proposed to demonstrate biological activity relevant to neurodegenerative disease. We tested the in vitro effects of DNSP-5 in primary dopaminergic neurons dissected from the ventral mesencephalon of E14 Sprague Dawley rat fetuses. Cells were treated with several doses (0.03, 0.1, 1.0, 10.0 ng/mL) of GDNF, DNSP-5, or an equivalent volume of citrate buffer (vehicle). Morphological features of tyrosine hydroxylase positive neurons were quantified for each dose. DNSP-5 significantly increased (p < 0.001) all differentiation parameters compared to citrate vehicle (at one or more dose). For in vivo studies, a unilateral DNSP-5 treatment (30 μg) was administered directly to the SN. Microdialysis in the ipsilateral striatum was performed 28 days after treatment to determine extracellular levels of DA and its primary metabolites (3,4-dihydroxyphenylacetic acid and homovanillic acid). A single treatment significantly increased (~66%) extracellular DA levels compared to vehicle, while DA metabolites were unchanged. Finally, the protective effects of DNSP-5 against staurosporine-induced cytotoxicity were investigated in a neuronal cell line showing substantial protection by DNSP-5. Altogether, these studies strongly indicate biological activity of DNSP-5 and suggest that DNSP-5 has neurotrophic-like properties that may be relevant to the treatment of neurodegenerative diseases like PD.

Nigral GFRα1 infusion in aged rats increases locomotor activity, nigral tyrosine hydroxylase, and dopamine content in synchronicity

Delivery of exogenous glial cell line-derived neurotrophic factor (GDNF) increases locomotor activity in rodent models of aging and Parkinson's disease in conjunction with increased dopamine (DA) tissue content in substantia nigra (SN). Striatal GDNF infusion also increases expression of GDNF's cognate receptor, GFRα1, and tyrosine hydroxylase (TH) ser31 phosphorylation in the SN of aged rats long after elevated GDNF is no longer detectable. In aging, expression of soluble GFRα1 in the SN decreases in association with decreased TH expression, TH ser31 phosphorylation, DA tissue content, and locomotor activity. Thus, we hypothesized that, in aged rats, replenishing soluble GFRα1 in SN could reverse these deficits and increase locomotor activity. We determined that the quantity of soluble GFRα1 in young adult rat SN is ~3.6 ng. To replenish age-related loss, which is ~30 %, we infused 1 ng soluble GFRα1 bilaterally into SN of aged male rats and observed increased locomotor activity compared to vehicle-infused rats up to 4 days following infusion, with maximal effects on day 3. Five days after infusion, however, neither locomotor activity nor nigrostriatal neurochemical measures were significantly different between groups. In a separate cohort of male rats, nigral, but not striatal, DA, TH, and TH ser31 phosphorylation were increased 3 days following unilateral infusion of 1 ng soluble GFRα1into SN. Therefore, in aged male rats, the transient increase in locomotor activity induced by replenishing age-related loss of soluble GFRα1is temporally matched with increased nigral dopaminergic function. Thus, expression of soluble GFRα1 in SN may be a key component in locomotor activity regulation through its influence over TH regulation and DA biosynthesis.

Glial cell-line derived neurotrophic factor (GDNF) replacement attenuates motor impairments and nigrostriatal dopamine deficits in 12-month-old mice with a partial deletion of GDNF

Glial cell-line derived neurotrophic factor (GDNF) has been established as a growth factor for the survival and maintenance of dopamine (DA) neurons. In phase I clinical trials, GDNF treatment in Parkinson's disease patients led to improved motor function and GDNF has been found to be down regulated in Parkinson's disease patients. Studies using GDNF heterozygous (Gdnf(+/-)) mice have demonstrated that a partial reduction of GDNF leads to an age-related accelerated decline in nigrostriatal DA system- and motor-function and increased neuro-inflammation and oxidative stress in the substantia nigra (SN). Therefore, the purpose of the current studies was to determine if GDNF replacement restores motor function and functional markers within the nigrostriatal DA system in middle-aged Gdnf(+/-) mice. At 11months of age, male Gdnf(+/-) and wildtype (WT) mice underwent bilateral intra-striatal injections of GDNF (10μg) or vehicle. Locomotor activity was assessed weekly 1-4weeks after treatment. Four weeks after treatment, their brains were processed for analysis of GDNF levels and various DAergic and oxidative stress markers. An intrastriatal injection of GDNF increased motor activity in Gdnf(+/-) mice to levels comparable to WT mice (1week after injection) and this effect was maintained through the 4-week time point. This increase in locomotion was accompanied by a 40% increase in striatal GDNF protein levels and SN GDNF expression in Gdnf(+/-) mice. Additionally, GDNF treatment significantly increased the number of tyrosine hydroxylase (TH)-positive neurons in the SN of middle-aged Gdnf(+/-) mice, but not WT mice, which was coupled with reduced oxidative stress in the SN. These studies further support that long-term changes related to the dysfunction of the nigrostriatal pathway are influenced by GDNF expression and add that this dysfunction appears to be responsive to GDNF treatment. Additionally, these studies suggest that long-term GDNF depletion alters the biological and behavioral responses to GDNF treatment.

Comprehensive profiling of dopamine regulation in substantia nigra and ventral tegmental area

Dopamine is a vigorously studied neurotransmitter in the CNS. Indeed, its involvement in locomotor activity and reward-related behaviour has fostered five decades of inquiry into the molecular deficiencies associated with dopamine regulation. The majority of these inquiries of dopamine regulation in the brain focus upon the molecular basis for its regulation in the terminal field regions of the nigrostriatal and mesoaccumbens pathways; striatum and nucleus accumbens. Furthermore, such studies have concentrated on analysis of dopamine tissue content with normalization to only wet tissue weight. Investigation of the proteins that regulate dopamine, such as tyrosine hydroxylase (TH) protein, TH phosphorylation, dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) protein often do not include analysis of dopamine tissue content in the same sample. The ability to analyze both dopamine tissue content and its regulating proteins (including post-translational modifications) not only gives inherent power to interpreting the relationship of dopamine with the protein level and function of TH, DAT, or VMAT2, but also extends sample economy. This translates into less cost, and yet produces insights into the molecular regulation of dopamine in virtually any paradigm of the investigators' choice. We focus the analyses in the midbrain. Although the SN and VTA are typically neglected in most studies of dopamine regulation, these nuclei are easily dissected with practice. A comprehensive readout of dopamine tissue content and TH, DAT, or VMAT2 can be conducted. There is burgeoning literature on the impact of dopamine function in the SN and VTA on behavior, and the impingements of exogenous substances or disease processes therein (1-5). Furthermore, compounds such as growth factors have a profound effect on dopamine and dopamine-regulating proteins, to a comparatively greater extent in the SN or VTA (6-8). Therefore, this methodology is presented for reference to laboratories that want to extend their inquiries on how specific treatments modulate behaviour and dopamine regulation. Here, a multi-step method is presented for the analyses of dopamine tissue content, the protein levels of TH, DAT, or VMAT2, and TH phosphorylation from the substantia nigra and VTA from rodent midbrain. The analysis of TH phosphorylation can yield significant insights into not only how TH activity is regulated, but also the signaling cascades affected in the somatodendritic nuclei in a given paradigm. We will illustrate the dissection technique to segregate these two nuclei and the sample processing of dissected tissue that produces a profile revealing molecular mechanisms of dopamine regulation in vivo, specific for each nuclei (Figure 1).

Gene therapy for the treatment of Parkinson's disease: the nature of the biologics expands the future indications

The pharmaceutical industry's development of therapeutic medications for the treatment of Parkinson's disease (PD) endures, as a result of the continuing need for better agents, and the increased clinical demand due to the aging population. Each new drug offers advantages and disadvantages to patients when compared to other medical offerings or surgical options. Deep brain stimulation (DBS) has become a standard surgical remedy for the effective treatment of select patients with PD, for whom most drug regimens have failed or become refractory. Similar to DBS as a surgical option, gene therapy for the treatment of PD is evolving as a future option. In the four different PD gene therapy approaches that have reached clinical trials investigators have documented an excellent safety profile associated with the stereotactic delivery, viral vectors and doses utilized, and transgenes expressed. In this article, we review the clinically relevant gene therapy strategies for the treatment of PD, concentrating on the published preclinical and clinical results, and the likely mechanisms involved. Based on these presentations, we advance an analysis of how the nature of the gene therapy used may eventually expand the scope and utility for the management of PD.

Schizophrenia: do all roads lead to dopamine or is this where they start? Evidence from two epidemiologically informed developmental rodent models

The idea that there is some sort of abnormality in dopamine (DA) signalling is one of the more enduring hypotheses in schizophrenia research. Opinion leaders have published recent perspectives on the aetiology of this disorder with provocative titles such as 'Risk factors for schizophrenia--all roads lead to dopamine' or 'The dopamine hypothesis of schizophrenia--the final common pathway'. Perhaps, the other most enduring idea about schizophrenia is that it is a neurodevelopmental disorder. Those of us that model schizophrenia developmental risk-factor epidemiology in animals in an attempt to understand how this may translate to abnormal brain function have consistently shown that as adults these animals display behavioural, cognitive and pharmacological abnormalities consistent with aberrant DA signalling. The burning question remains how can in utero exposure to specific (environmental) insults induce persistent abnormalities in DA signalling in the adult? In this review, we summarize convergent evidence from two well-described developmental animal models, namely maternal immune activation and developmental vitamin D deficiency that begin to address this question. The adult offspring resulting from these two models consistently reveal locomotor abnormalities in response to DA-releasing or -blocking drugs. Additionally, as adults these animals have DA-related attentional and/or sensorimotor gating deficits. These findings are consistent with many other developmental animal models. However, the authors of this perspective have recently refocused their attention on very early aspects of DA ontogeny and describe reductions in genes that induce or specify dopaminergic phenotype in the embryonic brain and early changes in DA turnover suggesting that the origins of these behavioural abnormalities in adults may be traced to early alterations in DA ontogeny. Whether the convergent findings from these two models can be extended to other developmental animal models for this disease is at present unknown as such early brain alterations are rarely examined. Although it is premature to conclude that such mechanisms could be operating in other developmental animal models for schizophrenia, our convergent data have led us to propose that rather than all roads leading to DA, perhaps, this may be where they start.

Dichotomy of tyrosine hydroxylase and dopamine regulation between somatodendritic and terminal field areas of nigrostriatal and mesoaccumbens pathways

Measures of dopamine-regulating proteins in somatodendritic regions are often used only as static indicators of neuron viability, overlooking the possible impact of somatodendritic dopamine (DA) signaling on behavior and the potential autonomy of DA regulation between somatodendritic and terminal field compartments. DA reuptake capacity is less in somatodendritic regions, possibly placing a greater burden on de novo DA biosynthesis within this compartment to maintain DA signaling. Therefore, regulation of tyrosine hydroxylase (TH) activity may be particularly critical for somatodendritic DA signaling. Phosphorylation of TH at ser31 or ser40 can increase activity, but their impact on L-DOPA biosynthesis in vivo is unknown. Thus, determining their relationship with L-DOPA tissue content could reveal a mechanism by which DA signaling is normally maintained. In Brown-Norway Fischer 344 F₁ hybrid rats, we quantified TH phosphorylation versus L-DOPA accumulation. After inhibition of aromatic acid decarboxylase, L-DOPA tissue content per recovered TH protein was greatest in NAc, matched by differences in ser31, but not ser40, phosphorylation. The L-DOPA per catecholamine and DA turnover ratios were significantly greater in SN and VTA, suggesting greater reliance on de novo DA biosynthesis therein. These compartmental differences reflected an overall autonomy of DA regulation, as seen by decreased DA content in SN and VTA, but not in striatum or NAc, following short-term DA biosynthesis inhibition from local infusion of the TH inhibitor α-methyl-p-tyrosine, as well as in the long-term process of aging. Such data suggest ser31 phosphorylation plays a significant role in regulating TH activity in vivo, particularly in somatodendritic regions, which may have a greater reliance on de novo DA biosynthesis. Thus, to the extent that somatodendritic DA release affects behavior, TH regulation in the midbrain may be critical for DA bioavailability to influence behavior.

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.

Biphasic dopamine regulation in mesoaccumbens pathway in response to non-contingent binge and escalating methamphetamine regimens in the Wistar rat

RATIONALE

Methamphetamine (MA) increases extracellular dopamine (DA) and at chronic high doses induces toxicity as indicated by decreased expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT). Notably, rats will self-administer MA in escalating quantities producing such toxicity. However, the impact of MA at sub-toxic doses on DA regulation is not well established.

OBJECTIVE

The temporal dynamics of DA regulation following cessation of sub-toxic escalating and binge doses of non-contingent MA were investigated as changes therein may be associated with escalation of MA intake.

MATERIALS AND METHODS

MA was administered 3×/day using an established 14-day escalating-dose regimen (0.1-4.0 mg/kg) or a single-day binge-style administration (3 × 4 mg/kg). DA tissue content, DA turnover, TH protein, TH phosphorylation, DAT, and vesicular monoamine transporter 2 were measured in nigrostriatal and mesoaccumbens pathways 48 h and 2 weeks after MA cessation.

RESULTS

Changes in striatal DA regulation were limited to increased DA turnover. However, in the mesoaccumbens pathway, escalating MA had biphasic effects. DA was increased in ventral tegmental area (VTA) and decreased in nucleus accumbens at 48 h post-MA while the reverse was seen at 2 weeks. These changes were matched by similar changes in TH protein and, in the VTA, by changes in DAT.

CONCLUSION

Escalation of MA intake produces both transient and long-lasting effects upon DA, TH, and DAT in the mesoaccumbens pathway. The eventual decrease of DA in the VTA is speculated to contribute to craving for MA and, thus, may be associated with MA escalation and resulting dopaminergic toxicity.