Differences in the disposition and toxicity of 1-methyl-4-phenylpyridinium in cultured rat and mouse astrocytes

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Characterizing the Neuroprotective Effects of S/B Remedy (Scutellaria baicalensis Georgi and Bupleurum scorzonerifolfium Willd) in Spinal Cord Injury

The main causes of dysfunction after a spinal cord injury (SCI) include primary and secondary injuries that occur during the first minutes, hours, to days after injury. This treatable secondary cascade provides a window of opportunity for delivering therapeutic interventions. An S/B remedy (Scutellaria baicalensis Georgi and Bupleurum scorzonerifolfium Willd) has anti-inflammatory, cytoprotective, and anticarcinogenic effects in liver or neurodegenerative diseases. The present work examined the effect of S/B on injured spinal cord neurons in cultures and in vivo. S/B effectively reduced peroxide toxicity and lipopolysaccharide stimulation in both spinal cord neuron/glial and microglial cultures with the involvement of PKC and HSP70. The effect of S/B was further conducted in contusive SCI rats. Intraperitoneal injections of S/B to SCI rats preserved spinal cord tissues and effectively attenuated microglial activation. Consistently, S/B treatment significantly improved hindlimb functions of SCI rats. In the acute stage of injury, S/B treatment markedly reduced the levels of ED1 expression and lactate and had a tendency to decrease lipid peroxidation. Taken together, we demonstrated long-term hindlimb restoration alongside histological improvements with systemic S/B remedy treatment in a clinically relevant model of contusive SCI. Our findings highlight the potential of an S/B remedy for acute therapeutic intervention after SCI.

Mitochondrial Complex I Activity is Conditioned by Supercomplex I-III2-IV Assembly in Brain Cells: Relevance for Parkinson's Disease

The assembly of complex I (CI) with complexes III (CIII) and IV (CIV) of the mitochondrial respiratory chain (MRC) to configure I-III- or I-III-IV-containing supercomplexes (SCs) regulates mitochondrial energy efficiency and reactive oxygen species (mROS) production. However, whether the occurrence of SCs impacts on CI specific activity remains unknown to our knowledge. To investigate this issue, here we determined CI activity in primary neurons and astrocytes, cultured under identical antioxidants-free medium, from two mouse strains (C57Bl/6 and CBA) and Wistar rat, i.e. three rodent species with or without the ability to assemble CIV into SCs. We found that CI activity was 6- or 1.8-fold higher in astrocytes than in neurons, respectively, from rat or CBA mouse, which can form I-III₂-IV SC; however, CI activity was similar in the cells from C57Bl/6 mouse, which does not form I-III₂-IV SC. Interestingly, CII-III activity, which was comparable in neurons and astrocytes from mice, was about 50% lower in astrocytes when compared with neurons from rat, a difference that was abolished by antioxidants- or serum-containing media. CIV and citrate synthase activities were similar under all conditions studied. Interestingly, in rat astrocytes, CI abundance in I-III₂-IV SC was negligible when compared with its abundance in I-III-containing SCs. Thus, CIV-containing SCs formation may determine CI specific activity in astrocytes, which is important to understand the mechanism for CI deficiency observed in Parkinson's disease.

Ornithine and Homocitrulline Impair Mitochondrial Function, Decrease Antioxidant Defenses and Induce Cell Death in Menadione-Stressed Rat Cortical Astrocytes: Potential Mechanisms of Neurological Dysfunction in HHH Syndrome

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is caused by deficiency of ornithine translocase leading to predominant tissue accumulation and high urinary excretion of ornithine (Orn), homocitrulline (Hcit) and ammonia. Although affected patients commonly present neurological dysfunction manifested by cognitive deficit, spastic paraplegia, pyramidal and extrapyramidal signs, stroke-like episodes, hypotonia and ataxia, its pathogenesis is still poorly known. Although astrocytes are necessary for neuronal protection. Therefore, in the present study we investigated the effects of Orn and Hcit on cell viability (propidium iodide incorporation), mitochondrial function (thiazolyl blue tetrazolium bromide-MTT-reduction and mitochondrial membrane potential-ΔΨm), antioxidant defenses (GSH) and pro-inflammatory response (NFkB, IL-1β, IL-6 and TNF-α) in unstimulated and menadione-stressed cortical astrocytes that were previously shown to be susceptible to damage by neurotoxins. We first observed that Orn decreased MTT reduction, whereas both amino acids decreased GSH levels, without altering cell viability and the pro-inflammatory factors in unstimulated astrocytes. Furthermore, Orn and Hcit decreased cell viability and ΔΨm in menadione-treated astrocytes. The present data indicate that the major compounds accumulating in HHH syndrome impair mitochondrial function and reduce cell viability and the antioxidant defenses in cultured astrocytes especially when stressed by menadione. It is presumed that these mechanisms may be involved in the neuropathology of this disease.

Induction of a Proinflammatory Response in Cortical Astrocytes by the Major Metabolites Accumulating in HMG-CoA Lyase Deficiency: the Role of ERK Signaling Pathway in Cytokine Release

3-Hydroxy-3-methylglutaric aciduria (HMGA) is an inherited metabolic disorder caused by 3-hydroxy-3-methylglutaryl-CoA lyase deficiency. It is biochemically characterized by predominant tissue accumulation and high urinary excretion of 3-hydroxy-3-methylglutarate (HMG) and 3-methylglutarate (MGA). Affected patients commonly present acute symptoms during metabolic decompensation, including vomiting, seizures, and lethargy/coma accompanied by metabolic acidosis and hypoketotic hypoglycemia. Although neurological manifestations are common, the pathogenesis of brain injury in this disease is poorly known. Astrocytes are important for neuronal protection and are susceptible to damage by neurotoxins. In the present study, we investigated the effects of HMG and MGA on important parameters of redox homeostasis and cytokine production in cortical cultured astrocytes. The role of the metabolites on astrocyte mitochondrial function (thiazolyl blue tetrazolium bromide (MTT) reduction) and viability (propidium iodide incorporation) was also studied. Both organic acids decreased astrocytic mitochondrial function and the concentrations of reduced glutathione without altering cell viability. In contrast, they increased reactive species formation (2'-7'-dichlorofluorescein diacetate (DCFHDA) oxidation), as well as IL-1β, IL-6, and TNF α release through the ERK signaling pathway. Taken together, the data indicate that the principal compounds accumulating in HMGA induce a proinflammatory response in cultured astrocytes that may possibly be involved in the neuropathology of this disease.

Cytoprotective and anti-inflammatory effects of PAL31 overexpression in glial cells

Background

Acute spinal cord injury (SCI) leads to a series of reactive changes and causes severe neurological deficits. A pronounced inflammation contributes to secondary pathology after SCI. Astroglia respond to SCI by proliferating, migrating, and altering phenotype. The impact of reactive gliosis on the pathogenesis of SCI is not fully understood. Our previous study has identified an inflammatory modulating protein, proliferation related acidic leucine-rich protein (PAL31) which is upregulated in the microglia/macrophage of injured cords. Because PAL31 participates in cell cycle progression and reactive astroglia often appears in the injured cord, we aim to examine whether PAL31 is involved in glial modulation after injury.

Results

Enhanced PAL31 expression was shown not only in microglia/macrophages but also in spinal astroglia after SCI. Cell culture study reveal that overexpression of PAL31 in mixed glial cells or in C6 astroglia significantly reduced LPS/IFNγ stimulation. Further, enhanced PAL31 expression in C6 astroglia protected cells from H₂O₂ toxicity; however, this did not affect its proliferative activity. The inhibiting effect of PAL31 on LPS/IFNγ stimulation was observed in glia or C6 after co-culture with neuronal cells. The results demonstrated that the overexpressed PAL31 in glial cells protected neuronal damages through inhibiting NF-kB signaling and iNOS.

Conclusions

Our data suggest that PAL31upregulation might be beneficial after spinal cord injury. Reactive gliosis might become a good target for future therapeutic interventions.

Primary cultured astrocytes from old rats are capable to activate the Nrf2 response against MPP+ toxicity after tBHQ pretreatment

Astrocytes are key players for brain physiology, protecting neurons by releasing antioxidant enzymes; however, they are also susceptible to damage by neurotoxins. Nuclear factor erythroid-derived 2-like 2 (Nrf2) is a central regulator of the antioxidant response, and therefore, pharmacologic inducers are often used to activate this transcription factor to induce cellular protection. To date, it still remains unknown if cells from aged animals are capable of developing this response. Therefore, the purpose of this work was to determine if cortical astrocytes derived from old rats are able to respond to tertbuthyl-hydroquinene (tBHQ) pretreatment and stimulate the Nrf2-antioxidant response pathway to induce an antioxidant strategy against MPP+ toxicity, one of the most used molecules to model Parkinson's disease. Our results show that, although astrocytes from adult and old rats were more susceptible to MPP+ toxicity than astrocytes from newborn rats, when pretreated with tertbuthyl-hydroquinene, they were able to transactivate Nrf2, increasing antioxidant enzymes and developing cellular protection. These results are discussed in terms of the doses used to create protective responses.

Comparative effects of bone marrow mesenchymal stem cells on lipopolysaccharide-induced microglial activation

After injury to the CNS, microglia are rapidly activated and concentrated and trigger inflammatory reaction at the sites of injury. Bone marrow mesenchymal stem cells (BMMSC) represent attractive cell sources for treating CNS injury. Although anti-inflammatory and paracrine effects of grafted BMMSC have been shown, direct modulation of BMMSC on microglia in situ remains unclear. The present work employs in vitro transwell assay to characterize the effects of BMMSC on LPS-stimulated microglia. BMMSC are cultivated in serum and serum-free (sf) conditions, namely, BMMSC and BMMSC-sf. Both cultures express major surface markers specific for mesenchymal stem cells. However, the BMMSC-sf exhibit sphere-like structure with reduced expression of two adherent cell markers, CD29 and CD90. Compared to BMMSC-sf, BMMSC are fibroblast like and have faster differentiation potential into neural-like cells. Furthermore, BMMSC release significant levels of TIMP-1 and VEGF, regardless of being alone or in coculture. The downregulated MMP-9 mRNA may be caused by TIMP-1 secretion from BMMSC. Our cell culture system provides a powerful tool for investigating the molecular and cellular changes in microglia-BMMSC cocultures.

Head-twitch response in rodents induced by the hallucinogen 2,5-dimethoxy-4-iodoamphetamine: a comprehensive history, a re-evaluation of mechanisms, and its utility as a model

Two primary animal models persist for assessing hallucinogenic potential of novel compounds and for examining the pharmacological and neurobiological substrates underlying the actions of classical hallucinogens, the two-lever drug discrimination procedure and the drug-induced head-twitch response (HTR) in rodents. The substituted amphetamine hallucinogen, serotonin 2 (5-HT(2) ) receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI) has emerged as the most popular pharmacological tool used in HTR studies of hallucinogens. Synthesizing classic, recent, and relatively overlooked findings, addressing ostensibly conflicting observations, and considering contemporary theories in receptor and behavioural pharmacology, this review provides an up-to-date and comprehensive synopsis of DOI and the HTR model, from neural mechanisms to utility for understanding psychiatric diseases. Also presented is support for the argument that, although both the two-lever drug discrimination and the HTR models in rodents are useful for uncovering receptors, interacting proteins, intracellular signalling pathways, and neurochemical processes affected by DOI and related classical hallucinogens, results from both models suggest they are not reporting hallucinogenic experiences in animals.

Silymarin protects spinal cord and cortical cells against oxidative stress and lipopolysaccharide stimulation

Contusive spinal cord injury (SCI) is a devastating event which leads to a loss of neurological function below the level of injury. A secondary degenerative process is initiated following acute SCI. This secondary cascade provides opportunities for the delivery of therapeutic interventions. Silymarin, a widely used "liver herb", is frequently used for the protection against various hepatobiliary problems. However, the effectiveness of silymarin in central nervous system (CNS), especially in spinal cord, is not firmly established. The present work evaluates the effects of silymarin and its major constituent, silybin, on oxidative stress and lipopolysaccharide (LPS) stimulation in primary neuronal/glial cell cultures and in vivo. Silymarin or silybin inhibited glial cell proliferation in a concentration-dependent manner. Furthermore, it protected glial cells against peroxide-induced reactive oxygen species (ROS) formation, ATP depletion, and cell damage. Interestingly, the inhibition of peroxide-induced ROS by silybin could be partially attenuated by inhibitors of NFκB or protein kinase C (PKC), suggesting an involvement of NFκB and PKC signaling pathways. In mixed neuronal/glial cell cultures from cerebral cortex or spinal cord, silymarin or silybin effectively attenuated peroxide-induced ROS formation, with silymarin being more effective than silybin, implicating other constituents of silymarin that may be involved. Consistently, silymarin reduced LPS-induced injures in spinal neuronal/glial cell cultures. In vivo, intrathecal administration of silymarin immediately after eliciting contusive SCI effectively improved hindlimb locomotor behavior in the rats. Taken together, silymarin or silybin shows promise in protecting the CNS cells from toxin- or injury-induced damages and might be used to treat head- or spinal cord-injuries related to free radical assault.

Enhanced expression of glycine N-methyltransferase by adenovirus-mediated gene transfer in CNS culture is neuroprotective

Glycine N-methyltransferase (GNMT) is the most abundant hepatic methyltransferase and plays important roles in regulating methyl group metabolism. In the central nervous system, GNMT expression is low and its function has not been revealed. The present study examines the effect of GNMT overexpression by adenovirus-mediated transfer in cortical mixed neuron-glial cultures. Infection of adenovirus encoding green fluorescence protein to cultures demonstrates high preference for non-neuronal cells. Optimal GNMT overexpression in cultures by adenoviral GNMT (Ad-GNMT) infection not only induces protein kinase C phosphorylation, but also increases neuronal/oligodendroglial survival. Furthermore, these Ad-GNMT-infected cultures are significantly resistant to H(2)O(2) toxicity and lipopolysaccharide stimulation. Conditioned media from Ad-GNMT-infected microglia also significantly enhance neuronal survival. Taken together, enhanced GNMT expression in mixed neuronal-glial cultures is neuroprotective, most likely mediated through non-neuronal cells.

Neurotrophic and neurotoxic effects of nitric oxide on fetal midbrain cultures

There is evidence suggesting that nitric oxide (NO) may play an important role in dopamine (DA) cell death. Thus, the aim of this study was to investigate the effects of NO on apoptosis and functionality of DA neurones and glial cells. The experiments were carried out in neuronal-enriched midbrain cultures treated with the NO donor diethylamine-nitric oxide complexed sodium (DEA-NO). DEA-NO, at doses of 25 and 50 microM, exerted neurotrophic effects on dopamine cells, increasing the number of tyrosine hydroxylase positive (TH(+)) cells, TH(+) neurite processes, DA levels and [(3)H]DA uptake. A dose of 25 microM DEA-NO protected DA cells from apoptosis. In addition, it induced de novo TH synthesis and increased intracellular reduced glutathione (GSH) levels, indicating a possible neuroprotective role for GSH. However, in doses ranging from 200 to 400 microM, DEA-NO decreased TH(+) cells, DA levels, [(3)H]DA uptake and the number of mature oligodendrocytes (O1(+) cells). No changes in either the amount or morphology of astrocytes and glial progenitors were detected. A dose- and time-dependent increase in apoptotic cells in the DEA-NO-treated culture was also observed, with a concomitant increase in the proapoptotic Bax protein levels and a reduction in the ratio between Bcl-xL and Bcl-xS proteins. In addition, DEA-NO induced a dose- and time-dependent increase in necrotic cells. 1H-[1,2,4]oxadiazolo[4, 3a]quinoxaline-1-one (ODQ, 0.5 microM), a selective guanylate cyclase inhibitor, did not revert the NO-induced effect on [(3)H]DA uptake. Glia-conditioned medium, obtained from fetal midbrain astrocyte cultures, totally protected neuronal-enriched midbrain cultures from NO-induced apoptosis and rescued [(3)H]DA uptake and TH(+) cell number. In conclusion, our results show that low NO concentrations have neurotrophic effects on DA cells via a cGMP-independent mechanism that may implicate up-regulation of GSH. On the other hand, higher levels of NO induce cell death in both dopamine neurones and mature oligodendrocytes that is totally reverted by soluble factors released from glia.

Oxidative stress induced by MPTP and MPP(+): selective vulnerability of cultured mouse astrocytes

Oxidative stress has been implicated in the pathogenesis of Parkinson's disease. In the present study, reactive oxygen species (ROS) formation and antioxidant enzyme superoxide dismutase (SOD) activities were examined in cultured cortical, striatal and mesencephalic mouse astrocytes after 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium (MPP(+)) treatment. Linear regression analysis showed that control mesencephalic (slope coefficient=0.01) astrocytes had a three-fold (F-test, p<0.05) greater rate of change in ROS production when compared to cortical (0.003) or striatal (0.003) astrocytes. However, when treated with 500 microM MPTP for 120 min, mesencephalic and striatal astrocytes demonstrated a decreased and increased rate of change in ROS production respectively. On the other hand, when treated with 10 microM MPP(+), a significant increase in the rate of change in ROS formation was observed in both mesencephalic and striatal astrocytes, with mesencephalic astrocytes producing a four-fold greater increase when compared to striatal astrocytes. Cortical astrocytes did not show any significant changes in ROS production when treated with MPTP or MPP(+). When astrocytes were treated with MPTP over a 24 h period, striatal astrocytes demonstrated significant increases in SOD activity to 12 h, followed by a return towards control levels after 8 h treatment. In contrast, mesencephalic astrocytes showed trends for a decrease in SOD production as well as a significant decrease in ATP levels by 24 h MPTP treatment. The present results suggested that mesencephalic astrocytes are more vulnerable to oxidative stress when compared to striatal astrocytes, given their greater rates of ROS production at basal and MPP(+) conditions. Striatal astrocytes, on the other hand, may have a more protective capacity against oxidative stress by producing greater SOD activities.

Comparison of mitochondrial respiratory chain enzyme activities in rodent astrocytes and neurones and a human astrocytoma cell line

This study has found that mitochondrial NADH-CoQ1 reductase (complex I) activity is significantly lower in C57 mice astrocytes compared with Wistar and Sprague-Dawley rat astrocytes, and a human astrocytoma cell line. In addition, complex I activity is 4-fold greater in Sprague-Dawley neurones when compared to Wistar or C57 neurones. These findings have important implications for mitochondrial studies involving rodent or human cell line systems, and in particular, indicate the importance of choosing an appropriate model when investigating the mitochondrial respiratory chain.

Nitric oxide donors protect cultured rat astrocytes from 1-methyl-4-phenylpyridinium-induced toxicity

MPP+ is thought to mediate MPTP's toxicity on dopamine neurons by inhibiting mitochondrial respiration. However, astrocytic injuries are also observed in MPTP/MPP+-treated rats. Because nitric oxide (NO.) is suggested to be cytoprotective, we examined the effects of nitroprusside (SNP), S-nitroso-N-acetylpenicillamine (SNAP), and 3-morpholinosydnonimine (SIN-1) on MPP+-induced toxicity in astrocytes. Incubation of astrocytes with MPP+ for 2 days produced a dose-dependent toxicity, including increase in lactate level and lipid peroxidation, decrease of metabolic activity and cell damage. SNP, SNAP, and SIN-1 all attenuated MPP+-induced toxicity. The same protection was not achieved with N-acetylpenicillamine or ferrocyanide, structural analogues of SNAP or SNP but devoid of NO.. Further, the effect was not attributed to the increased cGMP levels or blockade of MPP+ accumulation in astrocytes. Notably, catalase, dimethyl sulfoxide and ferricyanide, an extracellular electron acceptor, were also effective in inhibiting MPP+ damage. NO. donors and analogues were also tested against damage produced by rotenone, an irreversible complex I inhibitor. Only ferricyanide and SNP effectively protected rotenone's toxicity. These results concluded that (1) NO. may protect astrocytes from MPP+-induced free radical formation, and (2) prevention of energy depletion/free radicals production alleviate MPP+-induced toxicity.

Metabolic alterations produced by 3-nitropropionic acid in rat striata and cultured astrocytes: quantitative in vitro 1H nuclear magnetic resonance spectroscopy and biochemical characterization

Quantitative high resolution in vitro 1H nuclear magnetic resonance spectroscopy was employed to study the metabolic effects of 3-nitropropionic acid associated with aging from perchloric acid extracts of rat striata. Systemic injection of 3-nitropropionic acid in rats at a dose of 10 mg/kg/day for seven consecutive days significantly impaired energy metabolism in rats one, four and eight months of age, as evidenced by a marked elevation of succinate and lactate levels. However, a significant decrease in N-acetyl-L-aspartate level, a neuronal marker, was observed in four- and eight-month-old rats but not in one-month-old rats. This would indicate that rats at four to eight months are more susceptible to 3-nitropropionic acid than those at one month. A significant decrease in GABA level was observed in four-month-old 3-nitropropionic acid-treated rats, which is consistent with the literature that GABAergic neurons are particularly vulnerable to 3-nitropropionic acid treatment. In addition, glutamine and glutamate levels were markedly decreased at four and eight months in 3-nitropropionic acid-treated rats. Since glutamine is synthesized predominantly in glia, the observation above suggests that 3-nitropropionic acid intoxication may involve perturbation of energy metabolism, glial injury and consequent neuronal damage. Astrocytes which are essential in the metabolism of glutamate and glutamine were used to further assess 3-nitropropionic acid-induced toxicity. Glial proliferation, mitochondrial metabolism and glutamine synthetase activity were all reduced by 3-nitropropionic acid treatment with a concomitant increase, in a dose-dependent manner, of lactate levels, suggesting that 3-nitropropionic acid is also detrimental to astrocytes in vivo and thus may affect metabolic interaction between neurons and glia. These results not only imply that 3-nitropropionic acid blocks energy metabolism prior to exerting neurotoxic damage but also demonstrate that the degree of energy depletion determines the detrimental effects of 3-nitropropionic acid. In the present study, we also demonstrate that glutamate and glutamine levels as well as astrocytic functions may play pivotal roles in 3-nitropropionic acid-induced striatal lesions.

Characterization of L-alpha-aminoadipic acid transport in cultured rat astrocytes

The mechanism of the selective gliotoxicity of L-alpha-aminoadipate (L-alpha AA) is thought to involve its entry into glia as a substrate for glutamate transporters or, alternatively, its ability to inhibit glial glutamate transport. To clarify the properties of L-alpha AA as a transport substrate, we explored the ionic dependence, kinetics and pharmacology of L-[3H] alpha AA uptake in rat cortical astrocytes. We observed two components of saturable L-alpha AA uptake, one Na(+)-dependent and the other Na(+)-independent. These components exhibited the characteristics of system X-AG, the widespread family of Na(+)-cotransporters of aspartate and glutamate, and system x-c, a Cl(-)-dependent glutamate/cystine exchanger, respectively. The K(m) value of Na(+)-dependent L-alpha AA uptake was 629 +/- 42 microM, and Vmax was 62 +/- 4 nmol.min-1.mg-1 protein, which was more than twice the capacity of Na(+)-dependent glutamate uptake. The kinetic parameters of Na(+)-dependent L-alpha AA uptake (K(m) of 20 +/- 2 microM, Vmax of 1.7 +/- 0.4 nmol.min-1.mg-1 protein did not differ from the values for Na(+)-independent glutamate uptake, indicating that L-alpha AA and glutamate are equally good substrates for system x-c.

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.

Expression of the extraneuronal monoamine transporter (uptake2) in human glioma cells

Tritiated methylphenylpyridinium ([3H]MPP+), a substrate of the neuronal and extraneuronal noradrenaline transporter (uptake1 and uptake2, respectively) and of the organic cation transporter (OCT1), was used to characterize the amine transport system of the established human glioma cell line SK-MG-1. Uptake of [3H]MPP+ (25 nM) into SK-MG-1 cells increased linearly with time for up to 15 min. Selective uptake1 inhibitors (e.g. (+)oxaprotiline) or omission of Na+ or Cl-ions did not affect [3H]MPP+ uptake, whereas uptake2 inhibitors such as O-methyl-isoprenaline (OMI) or corticosterone as well as depolarizing concentrations of K+ or Ba2+ strongly reduced [3H]MPP+ uptake. Initial rates of OMI(100 microM)-sensitive [3H]MPP+ uptake were saturable, with a K(m) of about 17 microM and a maximal rate of about 50 pmol/(min x mg protein). IC50 (or Ki) values for inhibition of [3H]MPP+ uptake by substrates and inhibitors of uptake2 or OCT1 were highly significantly correlated with published IC50 values for inhibition of uptake2 but not with corresponding values for inhibition of OCT1. The results presented here clearly demonstrate that human glioma cells express an uptake2 transporter. Thus, glial cells in the human central nervous system endowed with this transporter are likely to contribute to the inactivation of neuronally released noradrenaline.

1-Methyl-4-phenyl-pyridinium (MPP+) uptake does not explain the differential toxicity of MPP+ in the nigrostriatal and mesolimbic dopaminergic pathways

The present study examined the possible differential toxicity of 1-methyl-4-phenyl-pyridinium (MPP+) in the nigrostriatal and mesolimbic dopaminergic pathways in BALB/c mice, and investigated whether MPP+ uptake may account for this differential toxicity. Results indicated that chronic MPP+ infusions to the striatum (ST) and nucleus accumbens (NAc) both produced a toxicity on dopamine (DA) neurons. However, MPP+ produced a more severe DA depletion in the ST than in the NAc. Kinetic analyses from MPP+ uptake studies revealed a similar Km value in both the ST and NAc, suggesting that the affinity for MPP+ uptake is not different. The Vmax was approximately 1.6-fold higher in the ST than in the NAc. These results together suggest that chronic MPP+ infusions produce a differential toxicity on DA neurons in the nigrostriatal and mesolimbic dopaminergic pathways and that the ability of MPP+ uptake in DA terminals probably does not account for this differential toxicity.