Gene expression of aromatic L-amino acid decarboxylase in cultured rat glial cells

Kinetic Properties and Pharmacological Modulation of High- and Low-Affinity Dopamine Transport in Striatal Astrocytes of Adult Rats

Astrocytes actively participate in neurotransmitter homeostasis by bidirectional communication with neuronal cells, a concept named the tripartite synapse, yet their role in dopamine (DA) homeostasis remains understudied. In the present study, we investigated the kinetic and molecular mechanisms of DA transport in cultured striatal astrocytes of adult rats. Kinetic uptake experiments were performed using radiolabeled [3H]-DA, whereas mRNA expression of the dopamine, norepinephrine, organic cation and plasma membrane monoamine transporters (DAT, NET, OCTs and PMAT) and DA receptors D1 and D2 was determined by qPCR. Additionally, astrocyte cultures were subjected to a 24 h treatment with the DA receptor agonist apomorphine, the DA receptor antagonist haloperidol and the DA precursor L-DOPA. [3H]-DA uptake exhibited temperature, concentration and sodium dependence, with potent inhibition by desipramine, nortriptyline and decynium-22, suggesting the involvement of multiple transporters. qPCR revealed prominent mRNA expression of the NET, the PMAT and OCT1, alongside lower levels of mRNA for OCT2, OCT3 and the DAT. Notably, apomorphine significantly altered NET, PMAT and D1 mRNA expression, while haloperidol and L-DOPA had a modest impact. Our findings demonstrate that striatal astrocytes aid in DA clearance by multiple transporters, which are influenced by dopaminergic drugs. Our study enhances the understanding of regional DA uptake, paving the way for targeted therapeutic interventions in dopaminergic disorders.

Transporters involved in adult rat cortical astrocyte dopamine uptake: Kinetics, expression and pharmacological modulation

Astrocytes, glial cells in the central nervous system, perform a multitude of homeostatic functions and are in constant bidirectional communication with neuronal cells, a concept named the tripartite synapse; however, their role in the dopamine homeostasis remains unexplored. The aim of this study was to clarify the pharmacological and molecular characteristics of dopamine transport in cultured cortical astrocytes of adult rats. In addition, we were interested in the expression of mRNA of dopamine transporters as well as dopamine receptors D1 and D2 and in the effect of dopaminergic drugs on the expression of these transporters and receptors. We have found that astrocytes possess both Na+-dependent and Na+-independent transporters. Uptake of radiolabelled dopamine was time-, temperature- and concentration-dependent and was inhibited by decynium-22, a plasma membrane monoamine transporter inhibitor, tricyclic antidepressants desipramine and nortriptyline, both inhibitors of the norepinephrine transporter. Results of transporter mRNA expression indicate that the main transporters involved in cortical astrocyte dopamine uptake are the norepinephrine transporter and plasma membrane monoamine transporter. Both dopamine receptor subtypes were identified in cortical astrocyte cultures. Twenty-four-hour treatment of astrocyte cultures with apomorphine, a D1/D2 agonist, induced upregulation of D1 receptor, norepinephrine transporter and plasma membrane monoamine transporter, whereas the latter was downregulated by haloperidol and L-DOPA. Astrocytes take up dopamine by multiple transporters and express dopamine receptors, which are sensitive to dopaminergic drugs. The findings of this study could open a promising area of research for the fine-tuning of existing therapeutic strategies.

Cortical and Striatal Astrocytes of Neonatal Rats Display Distinct Molecular and Pharmacological Characteristics of Dopamine Uptake

Astrocytes are crucial in the regulation of neurotransmitter homeostasis, and while their involvement in the dopamine (DA) tripartite synapse is acknowledged, it necessitates a more comprehensive investigation. In the present study, experiments were conducted on primary astrocyte cultures from the striatum and cortex of neonatal rats. The pharmacological intricacies of DA uptake, including dependence on time, temperature, and concentration, were investigated using radiolabelled [3H]-DA. The mRNA expression of transporters DAT, NET, PMAT, and OCTs was evaluated by qPCR. Notably, astrocytes from both brain regions exhibited prominent mRNA expression of NET and PMAT, with comparatively lower expression of DAT and OCTs. The inhibition of DA uptake by the DAT inhibitor, GBR12909, and NET inhibitors, desipramine and nortriptyline, impeded DA uptake in striatal astrocytes more than in cortical astrocytes. The mRNA expression of NET and PMAT was significantly upregulated in cortical astrocytes in response to the DA receptor agonist apomorphine, while only the mRNA expression of NET exhibited changes in striatal astrocytes. Haloperidol, a DA receptor antagonist, and L-DOPA, a DA precursor, did not induce significant alterations in transporter mRNA expression. These findings underscore the intricate and region-specific mechanisms governing DA uptake in astrocytes, emphasizing the need for continued exploration to unravel the nuanced dynamics of astrocytic involvement in the DA tripartite synapse.

Interactions Between the Serotonergic and Other Neurotransmitter Systems in the Basal Ganglia: Role in Parkinson's Disease and Adverse Effects of L-DOPA

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra. However, other non-dopaminergic neuronal systems such as the serotonergic system are also involved. Serotonergic dysfunction is associated with non-motor symptoms and complications, including anxiety, depression, dementia, and sleep disturbances. This pathology reduces patient quality of life. Interaction between the serotonergic and other neurotransmitters systems such as dopamine, noradrenaline, glutamate, and GABA controls the activity of striatal neurons and are particularly interesting for understanding the pathophysiology of PD. Moreover, serotonergic dysfunction also causes motor symptoms. Interestingly, serotonergic neurons play an important role in the effects of L-DOPA in advanced PD stages. Serotonergic terminals can convert L-DOPA to dopamine, which mediates dopamine release as a "false" transmitter. The lack of any autoregulatory feedback control in serotonergic neurons to regulate L-DOPA-derived dopamine release contributes to the appearance of L-DOPA-induced dyskinesia (LID). This mechanism may also be involved in the development of graft-induced dyskinesias (GID), possibly due to the inclusion of serotonin neurons in the grafted tissue. Consistent with this, the administration of serotonergic agonists suppressed LID. In this review article, we summarize the interactions between the serotonergic and other systems. We also discuss the role of the serotonergic system in LID and if therapeutic approaches specifically targeting this system may constitute an effective strategy in PD.

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

Heterogenic Distribution of Aromatic L-Amino Acid Decarboxylase Neurons in the Rat Spinal Cord

Aromatic L-amino acid decarboxylase (AADC) is an essential enzyme in the synthesis of serotonin, dopamine, and certain trace amines and is present in a variety of organs including the brain and spinal cord. It is previously reported that in mammalian spinal cord AADC cells (called D-cells) were largely confined to a region around the central canal and that they do not produce monoamines. To date, there has not been a detailed description of their distribution and morphology in mammals. In the present study this issue is systematically investigated using immunohistochemistry. We have found that AADC cells in the rat spinal cord are both more numerous and more widely distributed than previously reported. In the gray matter, AADC neurons immunolabeled for NeuN were not only found in the region around the central canal but also in the dorsal horn, intermediate zone, and ventral horn. In the white matter a large number of glial cells were AADC-immunopositive in different spinal segments and the vast majority of these cells expressed oligodendrocyte and radial glial phenotypes. Additionally, a small number of AADC neurons labeled for NeuN were found in the white matter along the ventral median fissure. The shapes and sizes of AADC neurons varied according to their location. For example, throughout cervical and lumbar segments AADC neurons in the intermediate zone and ventral horn tended to be rather large and weakly immunolabeled, whereas those in comparable regions of sacrocaudal segments were smaller and more densely immunolabeled. The diverse morphological characteristics of the AADC cells suggests that they could be further divided into several subtypes. These results indicate that AADC cells are heterogeneously distributed in the rat spinal cord and they may exert different functions in different physiological and pathological situations.

Pericytes Make Spinal Cord Breathless after Injury

Traumatic spinal cord injury is a devastating condition that leads to significant neurological deficits and reduced quality of life. Therapeutic interventions after spinal cord lesions are designed to address multiple aspects of the secondary damage. However, the lack of detailed knowledge about the cellular and molecular changes that occur after spinal cord injury restricts the design of effective treatments. Li and colleagues using a rat model of spinal cord injury and in vivo microscopy reveal that pericytes play a key role in the regulation of capillary tone and blood flow in the spinal cord below the site of the lesion. Strikingly, inhibition of specific proteins expressed by pericytes after spinal cord injury diminished hypoxia and improved motor function and locomotion of the injured rats. This work highlights a novel central cellular population that might be pharmacologically targeted in patients with spinal cord trauma. The emerging knowledge from this research may provide new approaches for the treatment of spinal cord injury.

L-DOPA Uptake in Astrocytic Endfeet Enwrapping Blood Vessels in Rat Brain

Astrocyte endfeet surround brain blood vessels and can play a role in the delivery of therapeutic drugs for Parkinson's disease. However, there is no previous evidence of the presence of LAT transporter for L-DOPA in brain astrocytes except in culture. Using systemic L-DOPA administration and a combination of patch clamp, histochemistry and confocal microscopy we found that L-DOPA is accumulated mainly in astrocyte cell bodies, astrocytic endfeet surrounding blood vessels, and pericytes. In brain slices: (1) astrocytes were exposed to ASP⁺, a fluorescent monoamine analog of MPP⁺; (2) ASP⁺ taken up by astrocytes was colocalized with L-DOPA fluorescence in (3) glial somata and in the endfeet attached to blood vessels; (4) these astrocytes have an electrogenic transporter current elicited by ASP⁺, but intriguingly not by L-DOPA, suggesting a different pathway for monoamines and L-DOPA via astrocytic membrane. (5) The pattern of monoamine oxidase (MAO type B) allocation in pericytes and astrocytic endfeet was similar to that of L-DOPA accumulation. We conclude that astrocytes control L-DOPA uptake and metabolism and, therefore, may play a key role in regulating brain dopamine level during dopamine-associated diseases. These data also suggest that different transporter mechanisms may exist for monoamines and L-DOPA.

A new perspective on the treatment of aromatic L-amino acid decarboxylase deficiency

The final step in production of the neurotransmitters dopamine and serotonin is catalyzed by aromatic l-amino acid decarboxylase (AADC). AADC deficiency is a debilitating genetic condition that results in a deficit in these neurotransmitters, and manifests in infancy as a severe movement disorder with developmental delay. Response to current treatments is often disappointing. We have reviewed the literature to look for improvements to the current treatment strategy and also for new directions for AADC deficiency treatment. There may be differences in the mode of action, side-effect risk and effectiveness between different dopamine agonists and monoamine oxidase inhibitors currently used for AADC deficiency treatment. The range of these drugs used requires re-evaluation as some may have greater efficacy than others. Pyridoxal 5'-phosphate, the AADC cofactor may stabilize AADC and could increase AADC activity. Pyridoxal 5'-phosphate could have advantages as a treatment instead of pyridoxine. Atypical neuroleptics and peripheral AADC inhibitors both increase AADC activity in vivo and could be a future direction for AADC deficiency treatment and related conditions. Parkinson's disease gene therapy to deliver and express the human AADC gene in striatum is being tested in humans. Consequently gene therapy for AADC deficiency could be a realistic aim however an animal model of AADC deficiency is important for further progression.

Dopamine release modifies intracellular calcium levels in tyrosine hydroxylase-transfected C6 cells

Glioma cell line C6, transfected with tyrosine hydroxylase (TH) cDNA under the control of the glial fibrillary acid protein promoter (C6-THA cells), elicited a reduction in the apomorphine-induced turning behavior when they are implanted in Parkinson's disease models. Nevertheless, dopamine (Da) release has not been explicitly demonstrated nor has a possible mechanism of release been implicated. In this study, the in vitro Da release by C6 and C6-THA cells after chemical stimulation with KCl or glutamate was quantified using HPLC. Modifications in intracellular calcium levels in response to KCl stimulation and participation of Da receptor-mediated feedback in calcium regulation were also studied using FLUO 3 as a calcium concentration indicator. C6-THA cells release dopamine in basal conditions, and increase its release after KCl or glutamic acid stimulation. In a fraction of C6 and C6-THA cells, a transient intracellular calcium increase was observed after KCl stimulation, but C6-THA cells demonstrated a faster rate of calcium removal. C6 cells express mRNA from all five subtypes of Da receptors as demonstrated by real time PCR. D1 receptors were most abundant in C6 cells and its expression was further increased in C6-THA cells. Blocking D1-like receptors in C6-THA cells with the specific antagonist drug SCH-23390 induced a decrease in intracellular calcium removal rate, resembling non-manipulated C6 cells' calcium clearance. Da release by C6-THA cells could be related to calcium dependent mechanisms. Furthermore, production of Da by C6-THA cells seems to upregulate the expression of D1 receptors' mRNA.

Multiple genes and factors associated with bipolar disorder converge on growth factor and stress activated kinase pathways controlling translation initiation: implications for oligodendrocyte viability

Famine and viral infection, as well as interferon therapy have been reported to increase the risk of developing bipolar disorder. In addition, almost 100 polymorphic genes have been associated with this disease. Several form most of the components of a phosphatidyl-inositol signalling/AKT1 survival pathway (PIK3C3, PIP5K2A, PLCG1, SYNJ1, IMPA2, AKT1, GSK3B, TCF4) which is activated by growth factors (BDNF, NRG1) and also by NMDA receptors (GRIN1, GRIN2A, GRIN2B). Various other protein products of genes associated with bipolar disorder either bind to or are affected by phosphatidyl-inositol phosphate products of this pathway (ADBRK2, HIP1R, KCNQ2, RGS4, WFS1), are associated with its constituent elements (BCR, DUSP6, FAT, GNAZ) or are downstream targets of this signalling cascade (DPYSL2, DRD3, GAD1, G6PD, GCH1, KCNQ2, NOS3, SLC6A3, SLC6A4, SST, TH, TIMELESS). A further pathway relates to endoplasmic reticulum-stress (HSPA5, XBP1), caused by problems in protein glycosylation (ALG9), growth factor receptor sorting (PIK3C3, HIP1R, SYBL1), or aberrant calcium homoeostasis (WFS1). Key processes relating to these pathways appear to be under circadian control (ARNTL, CLOCK, PER3, TIMELESS). DISC1 can also be linked to many of these pathways. The growth factor pathway promotes protein synthesis, while the endoplasmic reticulum stress pathway, and other stress pathways activated by viruses and cytokines (IL1B, TNF, Interferons), oxidative stress or starvation, all factors associated with bipolar disorder risk, shuts down protein synthesis via control of the EIF2 alpha and beta translation initiation complex. For unknown reasons, oligodendrocytes appear to be particularly prone to defects in the translation initiation complex (EIF2B) and the convergence of these environmental and genomic signalling pathways on this area might well explain their vulnerability in bipolar disorder.

The mysterious trace amines: protean neuromodulators of synaptic transmission in mammalian brain

The trace amines are a structurally related group of amines and their isomers synthesized in mammalian brain and peripheral nervous tissues. They are closely associated metabolically with the dopamine, noradrenaline and serotonin neurotransmitter systems in mammalian brain. Like dopamine, noradrenaline and serotonin the trace amines have been implicated in a vast array of human disorders of affect and cognition. The trace amines are unique as they are present in trace concentrations, exhibit high rates of metabolism and are distributed heterogeneously in mammalian brain. While some are synthesized in their parent amine neurotransmitter systems, there is also evidence to suggest other trace amines may comprise their own independent neurotransmitter systems. A substantial body of evidence suggests that the trace amines may play very significant roles in the coordination of biogenic amine-based synaptic physiology. At high concentrations, they have well-characterized presynaptic "amphetamine-like" effects on catecholamine and indolamine release, reuptake and biosynthesis; at lower concentrations, they possess postsynaptic modulatory effects that potentiate the activity of other neurotransmitters, particularly dopamine and serotonin. The trace amines also possess electrophysiological effects that are in opposition to these neurotransmitters, indicating to some researchers the existence of receptors specific for the trace amines. While binding sites or receptors for a few of the trace amines have been advanced, the absence of cloned receptor protein has impeded significant development of their detailed mechanistic roles in the coordination of catecholamine and indolamine synaptic physiology. The recent discovery and characterization of a family of mammalian G protein-coupled receptors responsive to trace amines such as beta-phenylethylamine, tyramine, and octopamine, including socially ingested psychotropic drugs such as amphetamine, 3,4-methylenedioxymethamphetamine, N,N-dimethyltryptamine, and lysergic acid diethylamide, have revitalized the field of scientific studies investigating trace amine synaptic physiology, and its association with major human disorders of affect and cognition.

Transplantation of autologous sympathetic neurons as a potential strategy to restore metabolic functions of the damaged nigrostriatal dopamine nerve terminals in Parkinson's disease

Grafting of catecholamine-producing cells can be a possible therapeutic strategy for attenuating motor symptoms in Parkinson's disease (PD). The potential of autologous sympathetic neurons has been investigated as a donor for cell therapy of PD. The clinical trials of autotransplantation of sympathetic ganglion cells in PD have revealed that the grafts increase the duration of L-DOPA (L-dihydroxy phenyl alanine)-induced beneficial effects, and that the graft-mediated effect is detectable during a follow-up period of at least 1 year postgrafting. In an in vitro analysis of the ability of human sympathetic neurons to release catecholamines, although DA was not detectable under basal conditions, DA levels were significantly increased upon exposure to exogenous L-DOPA. Furthermore, animal experiments with xenografting of human sympathetic ganglionic neurons in the DA-denervated striatum of rats demonstrated that a significant increase in striatal DA levels is noted after systemic L-DOPA treatment, and that the DA levels remain high for longer periods of time in the grafted rats than in control animals with sham surgery. The L-DOPA-induced rise of striatal DA levels was significantly attenuated when given reserpine pretreatment. This suggests that DA derived from exogenously administered L-DOPA is subjected to, at least in part, vesicular storage in grafted sympathetic neurons. Histological examinations indeed showed that the grafts express aromatic-L-amino acid decarboxylase and vesicular monoamine transporter-2, both of which are important molecules for the synthesis and the storage of DA, respectively. Taken together, grafted sympathetic neurons can provide a site for both the conversion of exogenous L-DOPA to DA and the storage of the synthesized DA in the DA-denervated striatum. This might be an explanation for a mechanism by which sympathetic neuron autografts can increase the duration of L-DOPA effects in PD patients. This review article summarizes the clinical effect of transplantation of autologous sympathetic neurons in PD and discusses the underlying mechanism for the effect based on experimental evidence previously obtained.

Brain transplantation of human neural stem cells transduced with tyrosine hydroxylase and GTP cyclohydrolase 1 provides functional improvement in animal models of Parkinson disease

Parkinson disease is a neurodegenerative disease characterized by loss of midbrain dopaminergic neurons resulting in movement disorder. Neural stem cells (NSC) of the CNS have recently aroused a great deal of interest, not only because of their importance in basic research of neural development, but also for their therapeutic potential in neurological disorders. We have recently generated an immortalized human NSC cell line, HB1.F3, via retrovirus-mediated v-myc transfer. This line is capable of self-renewal, is multipotent, and expresses cell specific markers for NSC, ATP-binding cassettes transporter (ABCG2) and nestin. Next, we co-transduced the F3 NSC line with genes encoding tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GTPCH1) in order to generate dopamine-producing NSC. The F3.TH.GTPCH human NSC line expresses TH and GTPCH phenotypes as determined by RT-PCR, western blotting and immunocytochemistry, and shows a 800 to 2000-fold increase in production of L-dihydroxyphenyl alanine in HPLC analysis. A marked improvement in amphetamine-induced turning behavior was observed in parkinsonian rats implanted with F3.TH.GTPCH cells, but not in control rats receiving F3 NSC. In the animals showing functional improvement, a large number of TH-positive F3.TH.GTPCH NSC were found at injection sites. These results indicate that human NSC, genetically transduced with TH and GTPCH1 genes, have great potential in clinical utility for cell replacement therapy in patients suffering from Parkinson disease.

L-DOPA treatment from the viewpoint of neuroprotection. Possible mechanism of specific and progressive dopaminergic neuronal death in Parkinson's disease

With regard to the mechanism of selective dopaminergic neuronal death, experimental results of studies on the neurotoxicity of MPTP and rotenone indicate that degeneration of dopamine neurons is closely related to mitochondrial dysfunction, inflammatory process and oxidative stress, particularly with regard to the generation of quinones as dopamine neuron-specific oxidative stress. Thus, it is now clear that the presence of high levels of discompartmentalized free dopamine in dopaminergic neurons may explain the specific vulnerability of dopaminergic neurons through the generation of highly toxic quinones.

Behavioral correction of Parkinsonian rats following the transplantation of immortalized fibroblasts genetically modified with TH and GCH genes

Eukaryotic plasmid vectors encoding the tyrosine hydroxylase (TH) gene and GTP cyclohydrolase-1 (GCH) gene were constructed and introduced into immortalized fibroblasts obtained from SV40 large antigen (LT(AG)) transformed rat primary fibroblasts. TH and GCH positive clones were selected and identified by immunohistochemistry and RT-PCR, respectively. Hemi-parkinsonian rats created using 6-hydroxydopamine (6-OHDA) were used to assess the therapeutic effect created by the co-implantation of immortalized fibroblasts genetically modified by TH or GCH genes. Animal behavior was significantly improved two weeks following implantation and behavioral correction was maintained for over 14 weeks. Behavioral improvement was paralleled by exogenous TH gene expression, identified by TH immunohistochemistry and RT-PCR analyses. The transplanted cells survived for at least 38 weeks as demonstrated by fibronectin immunohistochemical staining. Tumor formation or host reaction was not seen, although TH expression was negative for 20 weeks after the implantation. This work demonstrates that the co-transplantation of immortalized fibroblasts genetically modified by TH and GCH genes may be developed as a valuable approach to the treatment of Parkinson's disease.

Gene therapy for Parkinson's disease: current status and future potential

Parkinson's disease appears to be a good candidate for gene therapy. The primary biochemical defect associated with the disease has been clearly determined as an absence of dopamine in the caudate-putamen, and the anatomical region where the neuropathologic hallmark of the disease, death of the nigral dopamine-producing neurons, occurs, remains circumscribed. Based on the biochemical and anatomical information gathered on Parkinson's disease, different gene therapy strategies have been devised to treat it. The first, and most explored strategy so far, consists in engineering cells to produce levodopa or dopamine so they will replace dopaminergic neurotransmission. Several types of cells have been employed in these experiments, and behavioral recovery has been reported in animal models of the disease. However, this approach cannot prevent neuronal death, nor reconstruct brain circuits. Another strategy is to protect cells by transferring genes that encode neurotrophic factors. Effort is now being concentrated into this research area, and promising results have recently been reported. Finally, an additional strategy aims at generating cells with a dopaminergic phenotype so they will be capable of replacing the missing dopaminergic neurons in biochemical, anatomical and functional terms. This has the potential to become an important constituent for an effective cure. Gene therapy holds significant promise for the treatment of neurodegenerative disorders, and Parkinson's disease treatment will benefit greatly from the knowledge and information arising from gene therapy research.

A site-specific mutation of tyrosine hydroxylase reduces feedback inhibition by dopamine in genetically modified cells grafted in parkinsonian rats

Aromatic L-amino acid decarboxylase (AADC) is necessary for conversion of L-DOPA to dopamine. Therefore, AADC gene therapy has been proposed to enhance pharmacological or gene therapies delivering L-DOPA. However, addition of AADC to the grafts of genetically modified cells expressing tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1), which produce L-DOPA in parkinsonian rats, resulted in decreased production of L-DOPA and dopamine owing to feedback inhibition of TH by dopamine. End-product feedback inhibition has been shown to be mediated by the regulatory domain of TH, and site-specific mutation of serine 40 makes TH less susceptible to dopamine inhibition. Therefore, we investigated the efficacy of using TH with serine 40 mutated to leucine (mTH) in an ex vivo gene-therapy paradigm. Primary fibroblasts (PF) from Fischer 344 rats were transduced with retrovirus to express mTH or wild-type rat TH cDNA (wtTH). Both cell types were also transduced with GCH1 to provide the obligate TH cofactor, tetrahydrobiopterin. PF transfected with AADC were used as coculture and cografting partners. TH activities and L-DOPA production in culture were comparable between PFwtTHGC and PFmTHGC cells. In cocultures with PFAADC cells, PFmTHGC cells showed significant reduction in the inhibitory effect of dopamine compared with PFwtTHGC cells. In vivo microdialysis measurement showed that cografting PFAADC cells with PFmTHGC cells resulted in smaller decreases in L-DOPA and no reduction in dopamine levels compared with cografts of PFAADC cells with PFwtTHGC cells, which decreased both L-DOPA and dopamine levels. Maintenance of dopamine levels with lower levels of L-DOPA would result in more focused local delivery of dopamine and less potential side-effects arising from L-DOPA diffusion into other structures. These data support the hypothesis that mutation of serine 40 attenuates TH end-product inhibition in vivo and illustrates the importance of careful consideration of biochemical pathways and interactions between multiple genes in gene therapy.

Mechanisms of the effects of exogenous levodopa on the dopamine-denervated striatum

The efficacy of exogenous levodopa (L-DOPA) is attributed to its conversion to dopamine by the enzyme aromatic L-amino-acid decarboxylase in striatal dopaminergic terminals. However, there is controversy about the mechanisms underlying the therapeutic and adverse effects of L-DOPA after almost all striatal dopaminergic afferents have disappeared (i.e. in the later stages of Parkinson's disease). After administration of 30mg/kg or 100mg/kg of L-DOPA, rats subjected to unilateral dopaminergic denervation showed intense contraversive rotation and a high density of Fos-immunoreactive nuclei throughout the denervated striatum, with no significant induction of Fos in the intact striatum. Injection of the central aromatic L-amino-acid decarboxylase inhibitor NSD-1015 30min before and 15min after the injection of L-DOPA suppressed the rotational behavior and the striatal induction of Fos. Comparison of results obtained in rats subjected to unilateral and bilateral dopaminergic denervation indicated that the presence of contralateral dopaminergic innervation does not significantly modulate the effects of L-DOPA on the denervated striatum. Serotonergic denervation led to slight and statistically non-significant decrease in the rotational behavior and Fos expression induced by high doses of L-DOPA (100mg/kg) in the dopamine-denervated striatum, but totally suppressed the rotational behavior and Fos expression induced by low doses of L-DOPA (30mg/kg). The present data indicate that the major effects observed after administration of exogenous L-DOPA are not due to a direct action of L-DOPA on dopamine receptors, or to extrastriatal release of dopamine, but to conversion of L-DOPA to dopamine by serotonergic terminals and probably some intrastriatal cells. Given that serotonergic neurons appear to play an important role in the action of L-DOPA in the later stages of Parkinson's disease, strategies targeting the serotonergic system should be considered for the treatment of Parkinson's disease and for combating undesirable side effects of L-DOPA therapy.

The localization and functional contribution of striatal aromatic L-amino acid decarboxylase to L-3,4-dihydroxyphenylalanine decarboxylation in rodent parkinsonian models

L-3,4-Dihydroxyphenylalanine (L-dopa) is the mainstay of therapy for patients with Parkinson's disease (PD), and mediates its primary effects through conversion into dopamine by aromatic L-amino acid decarboxylase (AADC). Given the loss of AADC-containing nigrostriatal dopaminergic neurons in PD, however, the location of residual AADC that converts L-dopa into dopamine remains controversial. The first objective of this study was to establish the presence of AADC expression in striatal neurons and glia using reverse transcriptase and PCR. Transcripts for the neuronal but not nonneuronal forms of AADC were detected in striatal tissue, cultured striatal neurons, and glia. We then examined whether this striatal AADC expression represents a physiologically significant source of dopaine production. No dopamine release was detected following incubation of striatal cultures with L-dopa or transduction with adenovirus expressing tyrosine hydoxylase. Our data establish the presence of AADC expression in the striatum both in vivo and in vitro, but suggest that striatal components do not represent a primary source of L-dopa decarboxylation following nigrostriatal denervation in rats. Understanding the source and localization of AADC is important in understanding the complications of L-dopa therapy and in designing rational therapeutic strategies for PD, including cellular transplantation and gene therapy.

Phosphorylation and activation of brain aromatic L-amino acid decarboxylase by cyclic AMP-dependent protein kinase

Aromatic L-amino acid decarboxylase (AAAD), an enzyme required for the synthesis of catecholamines, indoleamines, and trace amines, is rapidly activated by cyclic AMP-dependent pathways in striatum and midbrain in vivo, suggesting enzyme phosphorylation. We now report that the catalytic subunit of cyclic AMP-dependent protein kinase (PKA) directly phosphorylated AAAD immunoprecipitated from homogenates prepared from the mouse striatum and midbrain in vitro. Under the same phosphorylation conditions, the catalytic subunit of PKA also phosphorylated a recombinant AAAD protein expressed in Escherichia coli transfected with an AAAD cDNA isolated from the bovine adrenal gland. The PKA-induced AAAD phosphorylation of immunoprecipitates from striatum and midbrain was time and concentration dependent and blocked by a specific PKA peptide inhibitor. Incubation of the catalytic subunit of PKA with striatal homogenates increased enzyme activity by approximately 20% in a time- and concentration-dependent manner. Moreover, incubation of the catalytic subunit of PKA with recombinant AAAD increased activity by approximately 70%. A direct phosphorylation of AAAD protein by PKA might underlie the cyclic AMP-induced rapid and transient activation of AAAD in vivo.

Evaluation of L-DOPA biotransformation during repeated L-DOPA infusion into the striatum in freely-moving young and old rats

The aim of this study was to assess changes in L-3, 4-dihydroxyphenylalanine (L-DOPA) biotransformation in response to two-pulse infusion of L-DOPA into the striatum of freely-moving young (3-4 month) and old (21-26 month) male Wistar rats. In addition, the effects of L-DOPA infusion on the vesicular dopamine (DA) store in young rats were also studied. Both L-DOPA-induced DA overflow and uptake of the perfused L-DOPA by the striatum were used to study L-DOPA biotransformation during microdialysis. High potassium-induced DA depletion was performed to assess the dynamics of the vesicular DA store following L-DOPA infusion. Concentric microdialysis probes were stereotaxically implanted in the lateral striatum of rats of both age groups and microdialysis was begun 24 h later. All rats received 2x20 min infusions of 3 mgr L-DOPA separated by an interval of 60 min. In the striatum of both groups, L-DOPA-induced DA overflow and uptake of exogenous L-DOPA were both significantly enhanced during the second infusion compared to the first. In young rats, when a 20-min infusion of 3 mgr L-DOPA was given between 2x20 min infusions of 100 mM potassium, no increased DA release was seen at the second high potassium challenge compared with the first. Our results suggest that the enhancement of DA overflow induced by the second L-DOPA infusion is, at least partially, due to an increase in L-DOPA biotransformation, and not simply to an enlarged DA pool. In contrast to the in vitro results, our own in vivo results show that L-DOPA utilization in the aging striatum does not deteriorate with age.

Astrocyte line SVG-TH grafted in a rat model of Parkinson's disease

The present review describes gene transfer into the brain using extraneuronal cells with an ex vivo approach. The mild immunological reactions in the central nervous system to grafts provided the rationale and empirical basis for brain-transplantation, to replace dying cells, of potential clinical relevance. Fetal human astrocytes were genetically engineered to express tyrosine hydroxylase, the rate-limiting enzyme for the synthesis of catecholamines. These cells were also found to produce constitutively and secrete GDNF and interleukins. Therefore, these cells may prove as a drug-delivery system for the treatment of neurological degenerative conditions such as Parkinson's disease (PD). The field of neuronal reconstruction has reached a critical threshold and there is a need to evaluate the variables that will become critical as the field matures. One of the needs is to characterize the neurochemical alterations in the microenvironment in the context of grafted-host connectivity. This review discusses the functional effects of the pharmacologically-active construct, which consists of astrocytes producing L-DOPA and GDNF. The striatum in PD that lacks the dopaminergic projection from the substantia nigra metabolizes and releases dopamine differently from normal tissue and may react to different factors released by the grafted cells. Moreover, neurochemicals of the host tissue may effect grafted cells as well. An understanding of the way in which these neurochemicals are abnormal in PD and their role in the grafted brain is critical to the improvement of reconstructive strategies using cellular therapeutic strategies.

Dopa-producing astrocytes generated by adenoviral transduction of human tyrosine hydroxylase gene: in vitro study and transplantation to hemiparkinsonian model rats

Astrocytes secreting a large amount of 3,4-dihydroxyphenylalanine (dopa) were generated by adenoviral transduction of the human tyrosine hydroxylase (TH) gene. After characterizing in vitro, the effect of transplantation of these astrocytes to the striatum of hemiparkinsonian model rats was investigated. Subconfluent cortical astrocytes were infected by replication-defect adenovirus type 5 carrying the human TH-1 gene or the LacZ reporter gene under the promoter of the glial fibrillary acidic protein (AdexGFAP-HTH-1, AdexGFAP-NL-LacZ). Dopa secretion was not evident at 3 days after the transduction of the HTH-1 gene but it increased from 7 days up to at least 4 months. The secretion was substrate (tyrosine)-dependent, and was enhanced by loading tetrahydrobioputerin (BH4) concentration-dependently. One-third of the hemiparkinsonian model rats, that were transplanted the HTH-1 gene-transduced astrocytes or introduced the direct injection of the viral vector to the striatum, showed a reduction of methamphetamine-induced rotations for at least 6 weeks. Apomorphine-induced rotation was decreased to the 50% level of the control's, but the reduction was obtained equally by the transplantation of HTH-1 gene-transduced or LacZ reporter gene-transduced astrocytes, or by the introduction of HTH-1 or LacZ gene carrying adenovirus. Treatment with FK506 for 3 weeks improved the late-phase apomorphine-induced rotations following the introduction of the HTH-1 gene carrying adenovirus. Histological examination revealed that, in animals that showed a reduction of methamphetamine-rotation, the TH positive astrocytes-like cells were distributed widely in the host striatum for at least 4 weeks. The number of TH positive astrocytes-like cells and their immunoreactivity decreased after 6 weeks when OX-41 positive microglias/macrophages were infiltrated. Data indicate that the adenoviral transduction of the human TH gene to astrocytes and its introduction to the striatum is a promising approach for the treatment of Parkinson's disease. However, the further technical improvements are required to optimize the adenoviral gene delivery, such as the control of viral toxicity and the regulation of the immune response.

Microdialysis study of the effects of the antiparkinsonian drug budipine on L-DOPA-induced release of dopamine and 5-hydroxytryptamine by rat substantia nigra and corpus striatum

The purpose of this study was to determine if systemic treatment with the antiparkinsonian drug budipine was capable of influencing the release of dopamine newly synthesised from L-DOPA in the substantia nigra and corpus striatum of the monoamine-depleted rat. Dual probe microdialysis was therefore employed in freely moving animals pretreated with reserpine (4 mg/kg i.p. 18-20 h earlier) and alpha-methyl-p-tyrosine (200 mg/kg i.p. 45 min earlier). Budipine (10 mg/kg i.p.) alone evoked a small but significant increase in basal dopamine efflux in nigra, though not in striatum, but did not affect the spontaneous outputs of DOPAC, 5-HT, or 5-HIAA in either structure. A threshold amount of L-DOPA (25 mg/kg i.p.) stimulated the release of dopamine, DOPAC, and 5-HT (but not 5-HIAA), both in nigra and striatum. The L-DOPA-induced releases of dopamine and DOPAC were greatly accentuated by pretreatment with budipine (10 mg/kg i.p. 45 min earlier), which delayed rather than potentiated the nigral and striatal effluxes of 5-HT. A higher dose of L-DOPA (100 mg/kg i.p.) did not significantly raise the outputs of dopamine or 5-HT, but greatly magnified that of DOPAC. In these experiments, pretreatment with budipine (10 mg/kg i.p.) facilitated the formation of DOPAC from L-DOPA, without increasing the extracellular concentration of dopamine. We conclude from these findings that budipine, at a therapeutically relevant dose, potentiates the release of dopamine newly synthesised from L-DOPA from either end of the nigrostriatal dopamine axis. This effect of budipine could be related to the drug's recently described ability to increase the activity of the converting enzyme, aromatic L-amino acid decarboxylase, and could explain the clinical efficacy of budipine as an adjunct to L-DOPA therapy of Parkinson's disease in man. The significance of 5-HT release to the antiparkinsonian L-DOPA, and the delay in this release caused by budipine, remain to be established.

GM1 ganglioside restores dopaminergic neurochemical and morphological markers in aged rats

The monosialoganglioside GM1 exerts neurotrophic-like activity in vitro and in vivo. In particular, it improves cholinergic neuron morphology and chemistry and learning abilities of cognitively impaired aged rats and young animals with cholinergic lesions, and restores neurochemical, pharmacological, morphological and behavioral parameters in animal models of Parkinson's disease. Our studies present evidence that GM1 reverses dopaminergic deficits in the nigrostriatal neurons of aged rats. GM1 administered to aged Sprague-Dawley rats for 30 days reversed the decreased activity of tyrosine hydroxylase in the midbrain and striatum, elevated the reduced protein content and mRNA levels of the enzyme in the midbrain, and reversed the decrements of dopamine and 3,4-dihydroxyphenylacetic acid content in both the midbrain and striatum. Tyrosine hydroxylase activity of the midbrain, but not of the striatum, remained elevated for 15 days after discontinuing GM1. The count profiles of tyrosine hydroxylase-immunopositive neurons, the size of tyrosine hydroxylase-immunopositive neurons and the number of tyrosine hydroxylase-immunopositive fibers were decreased in the substantia nigra pars compacta and the ventral tegmental area of aged rats. GM1 corrected the morphology of dopaminergic neurons in the substantia nigra pars compacta and partially improved it in the ventral tegmental area. These findings support the notion that the aged striatal dopaminergic neurons respond to GM1, and strengthen the utility of using this compound for combating age-associated neuronal deficits.

Comparison between the decrease of dopamine transporter and that of L-DOPA uptake for detection of early to advanced stage of Parkinson's disease in animal models

Early diagnosis of Parkinson's disease (PD) is important for the potential application of neuroprotective therapies. The purpose of this study was to assess the detection of the early changes of PD by either imaging the dopamine transporter (DAT) or uptake of L-3,4-dihydroxyphenylalanine (L-DOPA). An early to advanced stage model of PD was induced in rats by stereotaxic injection of 1-10 microg 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta. Using adjacent sections of the same animals, the binding of [I-125]beta-CIT, which labels DAT and the uptake of [C-14]L-DOPA, were evaluated 4 weeks after induction of the lesion. Any decrease in dopaminergic neurons was evaluated by in situ hybridization histochemistry (ISH) by detection of DAT mRNA-positive neurons. In addition, the expression levels of DAT, dopa decarboxylase (DDC), and vesicular monoamine transporter (VMAT2) in each neuron were studied with ISH. Our results show a decrease in both [I-125]beta-CIT binding and [C-14]L-DOPA uptake in parallel with a decrease in DA neurons from early to advanced stage models of PD. The decrease in [C-14]L-DOPA uptake was smaller than that in [I-125]beta-CIT binding in the same animal (P < 0.0001). Expression levels of DAT, DDC, and VMAT2 mRNAs were also decreased with the progression of the disease. Although ISH failed to detect the origin of the discrepancy between [I-125]beta-CIT and [C-14]L-DOPA levels, it was concluded that [C-14]L-DOPA levels underestimated the decrease of dopaminergic neurons and that [I-125]beta-CIT levels more precisely reflected the decrease.

Parallel modulation of striatal dopamine synthetic enzymes by second messenger pathways

The activity of tyrosine hydroxylase and aromatic L-amino acid decarboxylase in the striatum and their mRNA content in the midbrain were assayed in mice following the intracerebroventricular injection of forskolin or phorbol-12,13-myristic acid (PMA). Control and 1-methyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned animals were studied. Both forskolin and PMA induced a rapid and transient increase of tyrosine hydroxylase and aromatic L-amino acid decarboxylase activity in the striatum that lasted less than 45 and 60 min, respectively. A second belated increase of striatal tyrosine hydroxylase and aromatic L-amino acid decarboxylase activities was seen only after forskolin, and it was accompanied by a rise of tyrosine hydroxylase and aromatic L-amino acid decarboxylase mRNA in the midbrain. In the MPTP-lesioned mouse, the rise of tyrosine hydroxylase and aromatic L-amino acid decarboxylase following forskolin appeared exaggerated, while the response to PMA was not. These studies suggest that tyrosine hydroxylase and aromatic L-amino acid decarboxylase of striatum can be modulated in parallel by protein kinase A and protein kinase C, and that exaggerated responsiveness to protein kinase A is observed in the partially denervated striatum.

Dopamine turnover and metabolism in the striatum of parkinsonian rats grafted with genetically-modified human astrocytes

The potential of a novel therapeutic approach for treating Parkinson's disease, which involves the transplantation of a transfected human astrocyte cell line SVG-TH, that stably expresses the rate-limiting enzyme for dopamine production, tyrosine hydroxylase, was examined. SVG-TH and untransfected parent cells were grafted into the diseased striatum of rats in which Parkinson's disease had been induced by the administration of 6-hydroxydopamine. The in situ production and spillover of 3,4-dihydroxyphenylalanine (the precursor of dopamine), dopamine and their metabolites in the striatal extracellular fluid of the grafted rats was determined in conscious animals using the microdialysis technique and a high pressure liquid chromatography apparatus. Alleviation of symptoms of Parkinson's disease (abnormal movements) was evaluated by rotation tests. Upon transplantation of the SVG-TH cells into the striatum of the parkinsonian rats, the levels of dopamine in extracellular fluid of the striatum reached those of the normal rats, and correlated well with the improvement (74%) in their rotating behaviour (behavioural deficit). The levels of the two main dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, were low in the lesioned rats, even after SVG-TH transplantation. An alternative route of metabolism of dopamine may occur in the transplanted striatum, since the dopamine metabolite, 3-O-methoxy-4-hydroxy-phenylethylamine, appeared, which indicates activity of catechol-O-methyl transferase. Upon blockade of L-aromatic-amino acid decarboxylase, 3,4-dihydroxyphenylalanine accumulated in extracellular fluid of the 6-hydroxydopamine-lesioned and SVG-TH-grafted rats, which indicated that these cells produced active tyrosine hydroxylase in vivo. These findings indicate the potential of treating Parkinson's disease by the intrabrain grafting of human astrocyte cells transfected with the rate limiting enzyme for dopamine production.

Reduced striatal tyrosine hydroxylase activity is not accompanied by change in responsiveness of dopaminergic receptors following chronic treatment with deprenyl

Deprenyl is the only selective monoamine oxidase B (MAO-B) inhibitor that is in clinical use for the treatment of Parkinson's disease. Our previous studies showed that chronic treatment of rats with low (MAO-B selective) doses of deprenyl inhibited dopamine (DA) re-uptake and enhanced DA release in the striatum. These changes could affect DA synthesis rate by activation of negative feedback loops. Chronic deprenyl treatment has also been suggested to cause down-regulation of release-modulating DA receptors. The effects of chronic and acute treatment with deprenyl on ex vivo striatal tyrosine hydroxylase activity were therefore studied, by determination of steady-state tissue level of DOPA following administration of NSD-1015 (100 mg/kg i.p.). In addition, we assessed changes in the in vivo sensitivity of dopaminergic receptors from the reduction in DOPA extracellular level after systemic apomorphine administration (2.5 mg/kg s.c.), following elevation of microdialysate DOPA by systemic or local aromatic amino acid decarboxylase inhibition with NSD-1015. Chronic treatment with deprenyl (0.25 mg/kg s.c. daily for 21 days) caused a significant reduction in tyrosine hydroxylase activity to 60% of control, with no change in the apomorphine-induced reduction of microdialysate DOPA and DOPAC. The reduction in tyrosine hydroxylase activity is compatible with our previous results showing an increase in striatal DA extracellular level following chronic treatment with deprenyl. The increased extracellular striatal DA level could reduce tyrosine hydroxylase activity through activation of a negative feedback loop, by activation of either presynaptic or postsynaptic DA receptors.

L-Dopa and dopamine-producing gene cassettes for gene therapy approaches to Parkinson's disease

As an aid in the development of vector systems for use in gene therapy paradigms of central nervous system disorders such as Parkinson's disease, we have developed L-Dopa or dopamine-producing gene cassettes. Specifically, a human tyrosine hydroxylase cDNA (HTH-2) was rendered constitutively active by truncation of the N-terminal regulatory domain (tHTH). In addition, a bicistronic construct capable of directing the production of dopamine was created by inserting an internal ribosome entry site downstream of tHTH followed by the coding sequences of aromatic amino acid decarboxylase. All three constructs generated immunoreactive peptides of the predicted size, were enzymatically active, and produced L-Dopa (HTH-2, tHTH) or dopamine (bicistronic construct) following transient transfection of COS-7 cells. These constructs, in conjunction with viral or nonviral expression systems, may be efficacious in gene therapy approaches to Parkinson's disease.

Tyrosine hydroxylase and aromatic L-amino acid decarboxylase in mesencephalic cultures after MPP+: the consequences of treatment with GM1 ganglioside

Rat embryonic mesencephalic cultures were treated with the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+), and GM1 ganglioside added after the toxin. Twelve days after a 24-h exposure to MPP+, there was a significant decrement in tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AAAD) activities. In addition, TH mRNA was decreased, whereas AAAD mRNA was no different from control cultures. Adding GM1 to control unlesioned cultures had no effect on TH activity or mRNA. In contrast, GM1 modestly increased both the activity and mRNA for AAAD. In the MPP+-treated cultures, GM1 induced a partial recovery of TH and AAAD activity and increased mRNA for both above unlesioned control levels. Our studies demonstrate that GM1 upregulates the synthetic enzymes for dopamine in MPP+-lesioned embryonic mesencephalic cultures, and suggest that TH and AAAD respond differentially to the neurotoxin insult.

Modulation of tyrosine hydroxylase and aromatic L-amino acid decarboxylase after inhibiting monoamine oxidase-A

After acute administration of the monoamine oxidase inhibitor clorgyline there is a reduction of aromatic L-amino acid decarboxylase and tyrosine hydroxylase activity in the mouse striatum. Similar responses were seen after administering the non-selective monoamine oxidase inhibitor pargyline and high, but not low, doses of the selective monoamine oxidase-B inhibitor deprenyl. Changes of tyrosine hydroxylase activity were observed only when subsaturated concentrations of the pteridine cofactor were used for the assay. The monoamine oxidase inhibitors altered the abundance of aromatic L-amino acid decarboxylase and tyrosine hydroxylase mRNA in the midbrain. Pargyline and high doses of deprenyl increased, aromatic L-amino acid decarboxylase mRNA, while clorgyline initially decreased and then increased it. All three compounds caused an early decrease of tyrosine hydroxylase mRNA. The acidic metabolites of dopamine appeared most affected by pargyline and clorgyline, supporting the notion that deamination of striatal dopamine in rodents is primarily by monoamine oxidase-A. Our results suggest, that striatal tyrosine hydroxylase and aromatic L-amino acid decarboxylase are apparently modulated via different mechanisms in response to perturbation of dopamine metabolism.

Aromatic L-amino acid decarboxylase: a neglected and misunderstood enzyme

Classically, aromatic L-amino acid decarboxylase (AADC) has been regarded as an unregulated, rather uninteresting enzyme. In this review, we describe advances made during the past 10 years, demonstrating that AADC is regulated both pre- and post-translation. The significance of such regulatory mechanisms is poorly understood at present, but the presence of tissue specific control of expression raises the real possibility of AADC being involved in processes other than neuro-transmitter synthesis. We further discuss clinical and physiological situations in which such regulatory mechanisms may be important, including the intriguing possibility of AADC gene regulation being linked to that of factors thought to have a role in apoptosis and its prevention.

Tissue-specific expression of the nonneuronal promoter of the aromatic L-amino acid decarboxylase gene is regulated by hepatocyte nuclear factor 1

The rat aromatic l-amino acid decarboxylase (AADC) gene contains alternative promoters which direct expression of neuronal and nonneuronal mRNAs that differ only in their 5'-untranslated regions (UTRs). We have analyzed the expression of the nonneuronal promoter of the rat AADC gene in the kidney epithelial cell line LLC-PK1 and in cells which do not express the nonneuronal form of AADC by transient transfection. These studies revealed that the first 1.1 kilobases of the nonneuronal promoter, including the nonneuronal-specific 5'-UTR (Exon 1), contains sufficient information to direct tissue-specific expression. Serial deletions of this promoter localized the cis-active element to a region between -52 and -28 base pairs upstream of the nonneuronal transcription start site. An A/T-rich sequence, within this region which we have termed KL-1, was found to bind a kidney and liver-specific factor by DNase footprint analysis and was capable of directing tissue-specific expression from a heterologous promoter. Moreover, when the KL-1 sequence was mutated in the context of the entire promoter sequence, all transcriptional activity was abolished. DNA sequence comparison revealed that the KL-1 fragment is highly homologous to the binding site for hepatocyte nuclear factor-1 (HNF-1). Mobility shift studies utilizing an antibody to HNF-1 demonstrated binding of HNF-1 to the KL-1 fragment and cotransfection of HNF-1 cDNA into cells which do not express the nonneuronal form of AADC resulted in activation of transfected AADC nonneuronal promoter constructs. These results strongly suggest that the transcription factor which regulates the tissue-specific expression of the nonneuronal form of AADC mRNA is HNF-1.

Characterization of L-DOPA transport in cultured rat and mouse astrocytes

The present work studied the transport of L-DOPA in cultured rat and mouse astrocytes. Results indicated that the uptake of L-[14C]DOPA in both rat and mouse astrocytes was Na+ independent and temperature sensitive. It was mediated by a carrier-mediated mechanism with Km values of 36 and 60.3 microM, and V(max) values of 2.4 and 1.9 nmol/min/mg protein for rat and mouse cells, respectively. L-DOPA uptake was potently inhibited by aromatic or branched-chain amino acids. Interestingly, the release of intracellular L-[14C]DOPA was also trans stimulated by competitors that affect L-DOPA uptake. The 14C recovered in the 2-min uptake process was identified as L-[14C]DOPA. However, [14C]dopamine (DA) was also detected in both rat and mouse astrocytes after 30 min L-DOPA incubation, indicating the existence of aromatic L-amino acid decarboxylase (AADC). Taken together, L-DOPA was uphill transported into rat and mouse astrocytes by a (Na+) -independent exchanger with preference for aromatic and branched-chain amino acids. Rat astrocytes possessed higher affinity for L-DOPA uptake than mouse astrocytes. Both cells synthesized DA from exogenous L-DOPA. Thus, astrocytes serve not only as a temporary storage site for L-DOPA but also as a DA-producing machinery. These results may be of particular importance to parkinsonian patients under L-DOPA medication.

Aromatic L-amino acid decarboxylase: biological characterization and functional role

1. Aromatic L-amino acid decarboxylase is the enzyme responsible for the decarboxylation step in both the catecholamine and the indolamine synthetic pathways. Immunological and molecular biological studies suggest that it is a single enzyme with one catalytic site but with different locations for attachment of the substrates. The enzyme is widely distributed in the brain and in peripheral tissues. 2. Recent investigations have shown that the enzyme is regulated by short term mechanisms that may involve activation of adenyl cyclase or protein kinase C. In addition, a long-term mechanism of activation by altered gene expression has also been suggested.

L-dihydroxyphenylalanine and its decarboxylase: new ideas on their neuroregulatory roles

Recent experimental reports concerning L-dihydroxyphenylalanine (L-DOPA) and aromatic L-amino acid decarboxylase (AADC, L-DOPA decarboxylase) are reviewed in this article. Both in vitro and in vivo data now suggest that L-DOPA is an endogenous neuroactive compound that is released from neurons and acts as a neurotransmitter or neuromodulator in the brain. Administration of exogenous L-DOPA affects dopamine receptor status, AADC activity, and mitochondrial oxidation in experimental animals. The type and severity of these effects depend on the duration of the treatment. These findings may partly explain the limited efficacy of L-DOPA therapy in Parkinson's disease (PD). AADC also plays a controlling role in the central nervous system, being a regulatory enzyme in the synthesis of a putative neuromodulator 2-phenylethylamine and other trace amines. Recent experimental findings on AADC activity and localisation are of importance because they suggest that striatal [18F]DOPA uptake used as an indicator of PD progression in positron emission tomography (PET) studies is likely to overestimate nigrostriatal integrity in advanced PD. Possible new PET tracers of presynaptic dopaminergic function are discussed in this context.

An HSV-1 vector expressing tyrosine hydroxylase causes production and release of L-dopa from cultured rat striatal cells

In this report we demonstrate that a defective herpes simplex virus type one (HSV-1) vector can express enzymatically active tyrosine hydroxylase in cultured striatal cells that are thereby converted into L-DOPA-producing cells. A human tyrosine hydroxylase cDNA (form II) was inserted into an HSV-1 vector (pHSVth) and packaged into virus particles using an HSV-1 strain 17 mutant in the immediate early 3 gene (either ts K or D30EBA) as helper virus. Cultured fibroblasts were infected with pHSVth and 1 day later tyrosine hydroxylase immunoreactivity and tyrosine hydroxylase enzyme activity were observed. The tyrosine hydroxylase enzyme activity directed the production of L-DOPA. pHSVth infection of striatal cells in dissociated cell culture resulted in expression of tyrosine hydroxylase RNA and tyrosine hydroxylase immunoreactivity. Release of L-DOPA and low levels of dopamine were observed from cells in pHSVth-infected striatal cultures. Expression of tyrosine hydroxylase and release of catecholamines were maintained for at least 1 week after infection.

Aromatic L-amino acid decarboxylase modulation and Parkinson's disease

Aromatic L-amino acid decarboxylase (AAAD) is the second enzyme in the sequence leading to the synthesis of the catecholamines and serotonin, and it is the rate-limiting enzyme for the synthesis of the trace amines. In the striatum AAAD activity is increased by neuronal firing and diminished or enhanced by activation or blocking dopamine (DA) D1 or D2 receptors, respectively. At least two biochemical mechanisms appear responsible for modulation, short-term involving second messengers and possible phosphorylation, and long-term involving protein synthesis. In Parkinson's disease AAAD is the rate-controlling enzyme for the synthesis of DA when L-DOPA is administered and any change of AAAD activity could have clinical consequences. Indeed, the "on-off phenomenon" where there are fluctuations between off-periods of marked akinesia over several hours with on-periods of improved motility may be related to oscillating or poorly modulated AAAD activity and conversion of L-DOPA to DA. Studies are presented demonstrating how AAAD activity can be enhanced in an animal model of Parkinson's disease and how rapid fluctuations of AAAD can be provoked via second messenger system activation.

Long-term behavioral recovery in parkinsonian rats by an HSV vector expressing tyrosine hydroxylase

One therapeutic approach to treating Parkinson's disease is to convert endogenous striatal cells into levo-3,4-dihydroxyphenylalanine (L-dopa)-producing cells. A defective herpes simplex virus type 1 vector expressing human tyrosine hydroxylase was delivered into the partially denervated striatum of 6-hydroxydopamine-lesioned rats, used as a model of Parkinson's disease. Efficient behavioral and biochemical recovery was maintained for 1 year after gene transfer. Biochemical recovery included increases in both striatal tyrosine hydroxylase enzyme activity and in extracellular dopamine concentrations. Persistence of human tyrosine hydroxylase was revealed by expression of RNA and immunoreactivity.

Induction of aromatic L-amino acid decarboxylase mRNA by interleukin-1 beta and prostaglandin E2 in PC12 cells

Aromatic 1-amino acid decarboxylase (AADC) is involved in the synthesis of the putative neurotransmitters dopamine (DA), norepinephrine (NA) and 5-hydroxytryptamine (5-HT). We report here that the gene expression of AADC can be regulated by interleukin (IL) 1-beta and prostaglandin (PG) E2 in PC12 cells. The cells were treated with different doses of IL 1-beta and PGE2 for 3 days. Slot blot hybridization was performed to detect AADC mRNA and Western immunoblot to detect AADC protein. The cDNA probe for rat AADC was generated by the PCR method. IL 1-beta and PGE2 produced a dose- and time-dependent up-regulation in AADC mRNA levels (up to 200% of the control values) which was followed by a stable increase in AADC protein. The data further support the suggestion that AADC is a regulated enzyme and that the regulation occurs at the level of gene expression. Because IL-1 is synthesized, and acts locally, within the brain to influence neuronal and glial functions, it has been proposed to be a mediator with both beneficial and detrimental responses to inflammation and injury. The regulation of AADC by IL-1 may indicate a possible involvement for AADC in neuronal injury and recovery. Since IL-1 promotes PGE2 formation, its effects may be occurring by increasing level of PGE2.

The effects of monoamine oxidase B inhibition on dopamine metabolism in rats with nigro-striatal lesions

The purpose of this study was to examine whether monoamine oxidase type B (MAO-B) has a role in striatal dopamine metabolism in animals with a unilateral lesion of the medial forebrain bundle, and whether 2-phenylethylamine (PE) could have a role in amplification of dopamine (DA) responses in DA depleted striatum. Inhibition of MAO-B did not alter DA metabolism in lesioned striata. PE accumulation decreased with loss of DA as long as there was no DA dysfunction. In lesioned striata with dysfunction of DA transmission at the synaptic level, PE accumulation increased, suggesting a compensatory increase in PE synthesis. This increase in PE levels does not appear to be mediated by an increase in the total striatal aromatic L-amino acid decarboxylase (AADC) activity. We conclude that inhibition of MAO-B has no effect on DA metabolism in the hemi-parkinsonian rat striatum and that PE could be involved in the antiparkinsonian action of MAO-B inhibitors.