Metabolic interactions between glutamatergic and dopaminergic neurotransmitter systems are mediated through D(1) dopamine receptors

Memantine and SKF82958 but not an enriched environment modulate naloxone-precipitated morphine abstinence syndrome without affecting hippocampal tPA mRNA levels in rats

There is accumulating evidence supporting the involvement of tissue-plasminogen activator (tPA) in the mechanisms underlying the effects of morphine and an enriched environment. This study was designed to investigate possible interactive roles of the glutamatergic and the dopaminergic systems regarding hippocampal tPA in the neurobiology of morphine dependence. For this purpose, Wistar albino rats, housed in either a standard- (SE) or an enriched environment (EE) were implanted subcutaneously with morphine (150 mg base) or placebo pellets. Behavioral and somatic signs of morphine abstinence precipitated by an opioid-receptor antagonist naloxone (1 mg/kg, i.p.) 72 h after the pellet implantation were observed individually for 15 min in all groups. Memantine (10 mg/kg i.p.), an antagonist of N-methyl-D-aspartic acid class of glutamatergic receptor-subtype decreased teeth-chattering, ptosis, diarrhea and the loss of body weight. SKF82958 (1 mg/kg, i.p.), a dopamine D1-receptor agonist decreased jumping and ptosis but increased rearing and loss of body weight. On the other hand, co-administration of SKF82958 with memantine prevented some of their effects that occur when administered alone at the same doses. Furthermore, the EE did not change the intensity of morphine abstinence. The level of hippocampal tPA mRNA was found to be lower in the SE morphine abstinence group than in the placebo group and close to the EE morphine abstinence group, whereas there was no significant alteration of its level in the memantine or SKF82958 groups. These findings suggest that the interaction between the glutamatergic and the dopaminergic systems may be an important component of the neurobiology of morphine dependence, and the role of tPA in this interaction should be further investigated.

Increased glutamate and glutamine levels and their relationship to astrocytes and dopaminergic transmissions in the brains of adults with autism

Increased excitatory neuronal tones have been implicated in autism, but its mechanism remains elusive. The amplified glutamate signals may arise from enhanced glutamatergic circuits, which can be affected by astrocyte activation and suppressive signaling of dopamine neurotransmission. We tested this hypothesis using magnetic resonance spectroscopy and positron emission tomography scan with ¹¹C-SCH23390 for dopamine D1 receptors in the anterior cingulate cortex (ACC). We enrolled 18 male adults with high-functioning autism and 20 typically developed (TD) male subjects. The autism group showed elevated glutamate, glutamine, and myo-inositol (mI) levels compared with the TD group (p = 0.045, p = 0.044, p = 0.030, respectively) and a positive correlation between glutamine and mI levels in the ACC (r = 0.54, p = 0.020). In autism and TD groups, ACC D1 receptor radioligand binding was negatively correlated with ACC glutamine levels (r =  - 0.55, p = 0.022; r =  - 0.58, p = 0.008, respectively). The enhanced glutamate-glutamine metabolism might be due to astroglial activation and the consequent reinforcement of glutamine synthesis in autistic brains. Glutamine synthesis could underly the physiological inhibitory control of dopaminergic D1 receptor signals. Our findings suggest a high neuron excitation-inhibition ratio with astrocytic activation in the etiology of autism.

Dopamine D1 Receptors Regulate Spines in Striatal Direct-Pathway and Indirect-Pathway Neurons

BACKGROUND

Dopamine transmission is involved in the maintenance of the structural plasticity of direct-pathway and indirect-pathway striatal projection neurons (d-SPNs and i-SPNs, respectively). The lack of dopamine in Parkinson's disease produces synaptic remodeling in both types of SPNs, reducing the length of the dendritic arbor and spine density and increasing the intrinsic excitability. Meanwhile, the elevation of dopamine levels by levodopa recovers these alterations selectively in i-SPNs. However, little is known about the specific role of the D1 receptor (D1R) in these alterations.

METHODS

To explore the specific role of D1R in the synaptic remodeling of SPNs, we used knockout D1R mice (D1R^-/- ) and wild-type mice crossed with drd2-enhanced green fluorescent protein (eGFP) to identify d-SPNs and i-SPNs. Corticostriatal slices were used for reconstruction of the dendritic arbors after Lucifer yellow intracellular injection and for whole-cell recordings in naïve and parkinsonian mice treated with saline or levodopa.

RESULTS

The genetic inactivation of D1R reduces the length of the dendritic tree and the spine density in all SPNs, although more so in d-SPNs, which also increases their spiking. In parkinsonian D1R^-/- mice, the spine density decreases in i-SPNs, and this spine loss recovers after chronic levodopa.

CONCLUSIONS

D1R is essential for the maintenance of spine plasticity in d-SPNs but also affects i-SPNs, indicating an important crosstalk between these 2 types of neurons. © 2020 International Parkinson and Movement Disorder Society.

The Role of Gene Expression Changes in ceRNA Network Underlying Ossification of Ligamentum Flavum Development

We aimed at exploring the role of gene expression changes regulated by non-coding RNAs in ossification of ligamentum flavum (OLF). Three microarray datasets, including long non-coding RNA (lncRNA)/mRNA expression profile (GSE106253), circular RNA (circRNA) expression profile (GSE106255), and microRNA (miRNA) expression profile (GSE106256), were downloaded from the public Gene Expression Omnibus repository. The differentially expressed (DE) mRNAs, lncRNAs, miRNAs, and circRNAs in OLF tissues were analyzed, compared with normal tissues. Two competing endogenous RNA (ceRNA) networks with lncRNA-miRNA-mRNA and circRNA-miRNA-mRNA interactions were constructed, separately. Random walk with the restart model was applied to calculate the correlations of mRNAs with the published OLF-related genes. The top 50 mRNAs were subjected to function enrichment analysis and active small-molecule prediction. Total 2323 DE mRNAs, 1168 lncRNAs, 336 circRNAs, and 29 miRNAs were identified based on the microarray datasets. The LncRNA-related ceRNA network was constructed with 614 lncRNA-miRNA, 494 miRNA-mRNA, and 2099 lncRNA-mRNA interaction pairs; the circRNA-related ceRNA network was constructed with 153 circRNA-miRNA, 190 miRNA-mRNA, and 210 circRNA-mRNA interaction pairs. There were 17 OLF-related genes retrieved from previous literature, such as NPPS, COL6A1, and COL11A2, among which COL6A1 was the overlapped gene with mRNAs in the ceRNA network. Subsequently, top 50 mRNAs that closely correlated with COL6A1 in the ceRNA network were captured and these genes were closely related with the collagen catabolic process, regulation of cell growth, and neuronal action potential. DRD1 and COL6A1 were predicted to be the targets by small active molecule drugs. The collagen catabolic process may be implicated in OLF development. COL6A1 and DRD1 may be the candidate targets for OLF. However, further validations were needed.

Epigenetic Effects Induced by Methamphetamine and Methamphetamine-Dependent Oxidative Stress

Methamphetamine is a widely abused drug, which possesses neurotoxic activity and powerful addictive effects. Understanding methamphetamine toxicity is key beyond the field of drug abuse since it allows getting an insight into the molecular mechanisms which operate in a variety of neuropsychiatric disorders. In fact, key alterations produced by methamphetamine involve dopamine neurotransmission in a way, which is reminiscent of spontaneous neurodegeneration and psychiatric schizophrenia. Thus, understanding the molecular mechanisms operated by methamphetamine represents a wide window to understand both the addicted brain and a variety of neuropsychiatric disorders. This overlapping, which is already present when looking at the molecular and cellular events promoted immediately after methamphetamine intake, becomes impressive when plastic changes induced in the brain of methamphetamine-addicted patients are considered. Thus, the present manuscript is an attempt to encompass all the molecular events starting at the presynaptic dopamine terminals to reach the nucleus of postsynaptic neurons to explain how specific neurotransmitters and signaling cascades produce persistent genetic modifications, which shift neuronal phenotype and induce behavioral alterations. A special emphasis is posed on disclosing those early and delayed molecular events, which translate an altered neurotransmitter function into epigenetic events, which are derived from the translation of postsynaptic noncanonical signaling into altered gene regulation. All epigenetic effects are considered in light of their persistent changes induced in the postsynaptic neurons including sensitization and desensitization, priming, and shift of neuronal phenotype.

L-DOPA Reverses the Increased Free Amino Acids Tissue Levels Induced by Dopamine Depletion and Rises GABA and Tyrosine in the Striatum

Perturbations in the cerebral levels of various amino acids are associated with neurological disorders, and previous studies have suggested that such alterations have a role in the motor and non-motor symptoms of Parkinson's disease. However, the direct effects of chronic L-DOPA treatment, that produces dyskinesia, on neural tissue amino acid concentrations have not been explored in detail. To evaluate whether striatal amino acid concentrations are altered in peak dose dyskinesia, 6-hydroxydopamine (6-OHDA)-lesioned hemiparkinsonian mice were treated chronically with L-DOPA and tissue amino acid concentrations were assessed by HPLC analysis. These experiments revealed that neither 6-OHDA nor L-DOPA treatment are able to alter glutamate in the striatum. However, glutamine increases after 6-OHDA and returns back to normal levels with L-DOPA treatment, suggesting increased striatal glutamatergic transmission with lack of dopamine. In addition, glycine and taurine levels are increased following dopamine denervation and restored to normal levels by L-DOPA. Interestingly, dyskinetic animals showed increased levels of GABA and tyrosine, while aspartate striatal tissue levels are not altered. Overall, our results indicate that chronic L-DOPA treatment, besides normalizing the altered levels of some amino acids after 6-OHDA, robustly increases striatal GABA and tyrosine levels which may in turn contribute to the development of L-DOPA-induced dyskinesia.

Amphetamine-related drugs neurotoxicity in humans and in experimental animals: Main mechanisms

Amphetamine-related drugs, such as 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (METH), are popular recreational psychostimulants. Several preclinical studies have demonstrated that, besides having the potential for abuse, amphetamine-related drugs may also elicit neurotoxic and neuroinflammatory effects. The neurotoxic potentials of MDMA and METH to dopaminergic and serotonergic neurons have been clearly demonstrated in both rodents and non-human primates. This review summarizes the species-specific cellular and molecular mechanisms involved in MDMA and METH-mediated neurotoxic and neuroinflammatory effects, along with the most important behavioral changes elicited by these substances in experimental animals and humans. Emphasis is placed on the neuropsychological and neurological consequences associated with the neuronal damage. Moreover, we point out the gap in our knowledge and the need for developing appropriate therapeutic strategies to manage the neurological problems associated with amphetamine-related drug abuse.

Compensatory glutamine metabolism promotes glioblastoma resistance to mTOR inhibitor treatment

The mechanistic target of rapamycin (mTOR) is hyperactivated in many types of cancer, rendering it a compelling drug target; however, the impact of mTOR inhibition on metabolic reprogramming in cancer is incompletely understood. Here, by integrating metabolic and functional studies in glioblastoma multiforme (GBM) cell lines, preclinical models, and clinical samples, we demonstrate that the compensatory upregulation of glutamine metabolism promotes resistance to mTOR kinase inhibitors. Metabolomic studies in GBM cells revealed that glutaminase (GLS) and glutamate levels are elevated following mTOR kinase inhibitor treatment. Moreover, these mTOR inhibitor-dependent metabolic alterations were confirmed in a GBM xenograft model. Expression of GLS following mTOR inhibitor treatment promoted GBM survival in an α-ketoglutarate-dependent (αKG-dependent) manner. Combined genetic and/or pharmacological inhibition of mTOR kinase and GLS resulted in massive synergistic tumor cell death and growth inhibition in tumor-bearing mice. These results highlight a critical role for compensatory glutamine metabolism in promoting mTOR inhibitor resistance and suggest that rational combination therapy has the potential to suppress resistance.

Methamphetamine causes degeneration of dopamine cell bodies and terminals of the nigrostriatal pathway evidenced by silver staining

Methamphetamine is a widely abused illicit drug. Recent epidemiological studies showed that methamphetamine increases the risk for developing Parkinson's disease (PD) in agreement with animal studies showing dopaminergic neurotoxicity. We examined the effect of repeated low and medium doses vs single high dose of methamphetamine on degeneration of dopaminergic terminals and cell bodies. Mice were given methamphetamine in one of the following paradigms: three injections of 5 or 10 mg/kg at 3 h intervals or a single 30 mg/kg injection. The integrity of dopaminergic fibers and cell bodies was assessed at different time points after methamphetamine by tyrosine hydroxylase immunohistochemistry and silver staining. The 3 × 10 protocol yielded the highest loss of striatal dopaminergic terminals, followed by the 3 × 5 and 1 × 30. Some degenerating axons could be followed from the striatum to the substantia nigra pars compacta (SNpc). All protocols induced similar significant degeneration of dopaminergic neurons in the SNpc, evidenced by amino-cupric-silver-stained dopaminergic neurons. These neurons died by necrosis and apoptosis. Methamphetamine also killed striatal neurons. By using D1-Tmt/D2-GFP BAC transgenic mice, we observed that degenerating striatal neurons were equally distributed between direct and indirect medium spiny neurons. Despite the reduced number of dopaminergic neurons in the SNpc at 30 days after treatment, there was a partial time-dependent recovery of dopamine terminals beginning 3 days after treatment. Locomotor activity and motor coordination were robustly decreased 1-3 days after treatment, but recovered at later times along with dopaminergic terminals. These data provide direct evidence that methamphetamine causes long-lasting loss/degeneration of dopaminergic cell bodies in the SNpc, along with destruction of dopaminergic terminals in the striatum.

(13)C NMR spectroscopy applications to brain energy metabolism

(13)C nuclear magnetic resonance (NMR) spectroscopy is the method of choice for studying brain metabolism. Indeed, the most convincing data obtained to decipher metabolic exchanges between neurons and astrocytes have been obtained using this technique, thus illustrating its power. It may be difficult for non-specialists, however, to grasp thefull implication of data presented in articles written by spectroscopists. The aim of the review is, therefore, to provide a fundamental understanding of this topic to facilitate the non-specialists in their reading of this literature. In the first part of this review, we present the metabolic fate of (13)C-labeled substrates in the brain in a detailed way, including an overview of some general neurochemical principles. We also address and compare the various spectroscopic strategies that can be used to study brain metabolism. Then, we provide an overview of the (13)C NMR experiments performed to analyze both intracellular and intercellular metabolic fluxes. More particularly, the role of lactate as a potential energy substrate for neurons is discussed in the light of (13)C NMR data. Finally, new perspectives and applications offered by (13)C hyperpolarization are described.

Cocaine potentiates MDMA-induced oxidative stress but not dopaminergic neurotoxicity in mice: implications for the pathogenesis of free radical-induced neurodegenerative disorders

RATIONALE

The drugs of abuse 3,4-methylenedioxymethamphetamine (MDMA; "ecstasy") and cocaine both increase the generation of free radicals, and in the case of MDMA, this increase in oxidative stress is involved in the dopaminergic neurotoxicity produced by the drug in mice. Oxidative stress processes are also involved in the pathogenesis of several neurodegenerative diseases.

OBJECTIVES

We aimed to determine the consequences of the combined administration of MDMA and cocaine on oxidative stress and dopaminergic neurotoxicity.

METHODS

Mice received MDMA (20 mg/kg, i.p.; two doses separated by 3 h) followed by cocaine 1, 3, 6, or 24 h after the second MDMA dose. Mice were killed between 1 h and 7 days after cocaine injection.

RESULTS

MDMA decreased dopamine transporter density and dopamine concentration 7 days later. Cocaine did not alter this neurotoxicity. MDMA produced an increase in the concentration of 2,3-dihydroxybenzoic acid in striatal microdialysis samples and an increase in lipid peroxidation in the striatum which were potentiated by cocaine. MDMA and cocaine given together also increased nitrate and 3-nitrotyrosine levels compared with either drug given alone. On the other hand, MDMA increased superoxide dismutase activity and decreased catalase activity, changes which were prevented by cocaine administration. In addition, cocaine administration produced an increase in glutathione peroxidase (GPx) activity in both saline-treated and MDMA-treated mice.

CONCLUSIONS

Cocaine potentiates MDMA-induced oxidative stress but does not produce an increase in the neurotoxicity produced by MDMA, and this lack of potentiation may involve an increase in GPx activity.

The role of dopamine receptors in the neurotoxicity of methamphetamine

Methamphetamine is a synthetic drug consumed by millions of users despite its neurotoxic effects in the brain, leading to loss of dopaminergic fibres and cell bodies. Moreover, clinical reports suggest that methamphetamine abusers are predisposed to Parkinson's disease. Therefore, it is important to elucidate the mechanisms involved in methamphetamine-induced neurotoxicity. Dopamine receptors may be a plausible target to prevent this neurotoxicity. Genetic inactivation of dopamine D1 or D2 receptors protects against the loss of dopaminergic fibres in the striatum and loss of dopaminergic neurons in the substantia nigra. Protection by D1 receptor inactivation is due to blockade of hypothermia, reduced dopamine content and turnover and increased stored vesicular dopamine in D1R(-/-) mice. However, the neuroprotective impact of D2 receptor inactivation is partially dependent on an effect on body temperature, as well as on the blockade of dopamine reuptake by decreased dopamine transporter activity, which results in reduced intracytosolic dopamine levels in D2R(-/-) mice.

Glutamate and GABA in Appetite Regulation

Appetite is regulated by a coordinated interplay between gut, adipose tissue, and brain. A primary site for the regulation of appetite is the hypothalamus where interaction between orexigenic neurons, expressing Neuropeptide Y/Agouti-related protein, and anorexigenic neurons, expressing Pro-opiomelanocortin cocaine/Amphetamine-related transcript, controls energy homeostasis. Within the hypothalamus, several peripheral signals have been shown to modulate the activity of these neurons, including the orexigenic peptide ghrelin and the anorexigenic hormones insulin and leptin. In addition to the accumulated knowledge on neuropeptide signaling, presence and function of amino acid neurotransmitters in key hypothalamic neurons brought a new light into appetite regulation. Therefore, the principal aim of this review will be to describe the current knowledge of the role of amino acid neurotransmitters in the mechanism of neuronal activation during appetite regulation and the associated neuronal-astrocytic metabolic coupling mechanisms. Glutamate and GABA dominate synaptic transmission in the hypothalamus and administration of their receptors agonists into hypothalamic nuclei stimulates feeding. By using (13)C High-Resolution Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy based analysis, the Cerdán group has shown that increased neuronal firing in mice hypothalamus, as triggered by appetite during the feeding-fasting paradigm, may stimulate the use of lactate as neuronal fuel leading to increased astrocytic glucose consumption and glycolysis. Moreover, fasted mice showed increased hypothalamic [2-(13)C]GABA content, which may be explained by the existence of GABAergic neurons in key appetite regulation hypothalamic nuclei. Interestingly, increased [2-(13)C]GABA concentration in the hypothalamus of fasted animals appears to result mainly from reduction in GABA metabolizing pathways, rather than increased GABA synthesis by augmented activity of the glutamate-glutamine-GABA cycle.

Marine Ο-3 polyunsaturated fatty acids induce sex-specific changes in reinforcer-controlled behaviour and neurotransmitter metabolism in a spontaneously hypertensive rat model of ADHD

Background

Previous reports suggest that omega-3 (n-3) polyunsaturated fatty acids (PUFA) supplements may reduce ADHD-like behaviour. Our aim was to investigate potential effects of n-3 PUFA supplementation in an animal model of ADHD.

Methods

We used spontaneously hypertensive rats (SHR). SHR dams were given n-3 PUFA (EPA and DHA)-enriched feed (n-6/n-3 of 1:2.7) during pregnancy, with their offspring continuing on this diet until sacrificed. The SHR controls and Wistar Kyoto (WKY) control rats were given control-feed (n-6/n-3 of 7:1). During postnatal days (PND) 25–50, offspring were tested for reinforcement-dependent attention, impulsivity and hyperactivity as well as spontaneous locomotion. The animals were then sacrificed at PND 55–60 and their neostriata were analysed for monoamine and amino acid neurotransmitters with high performance liquid chromatography.

Results

n-3 PUFA supplementation significantly enhanced reinforcement-controlled attention and reduced lever-directed hyperactivity and impulsiveness in SHR males whereas the opposite or no effects were observed in females. Analysis of neostriata from the same animals showed significantly enhanced dopamine and serotonin turnover ratios in the male SHRs, whereas female SHRs showed no change, except for an intermediate increase in serotonin catabolism. In contrast, both male and female SHRs showed n-3 PUFA-induced reduction in non-reinforced spontaneous locomotion, and sex-independent changes in glycine levels and glutamate turnover.

Conclusions

Feeding n-3 PUFAs to the ADHD model rats induced sex-specific changes in reinforcement-motivated behaviour and a sex-independent change in non-reinforcement-associated behaviour, which correlated with changes in presynaptic striatal monoamine and amino acid signalling, respectively. Thus, dietary n-3 PUFAs may partly ameliorate ADHD-like behaviour by reinforcement-induced mechanisms in males and partly via reinforcement-insensitive mechanisms in both sexes.

Anterior cingulate cortex γ-aminobutyric acid in depressed adolescents: relationship to anhedonia

CONTEXT

Anhedonia, a core symptom of major depressive disorder (MDD) and highly variable among adolescents with MDD, may involve alterations in the major inhibitory amino acid neurotransmitter system of γ-aminobutyric acid (GABA).

OBJECTIVE

To test whether anterior cingulate cortex (ACC) GABA levels, measured by proton magnetic resonance spectroscopy, are decreased in adolescents with MDD. The associations of GABA alterations with the presence and severity of anhedonia were explored.

DESIGN

Case-control, cross-sectional study using single-voxel proton magnetic resonance spectroscopy at 3 T.

SETTING

Two clinical research divisions at 2 teaching hospitals.

PARTICIPANTS

Twenty psychotropic medication-free adolescents with MDD (10 anhedonic, 12 female, aged 12-19 years) with episode duration of 8 weeks or more and 21 control subjects group matched for sex and age.

MAIN OUTCOME MEASURES

Anterior cingulate cortex GABA levels expressed as ratios relative to unsuppressed voxel tissue water (w) and anhedonia scores expressed as a continuous variable.

RESULTS

Compared with control subjects, adolescents with MDD had significantly decreased ACC GABA/w (t = 3.2; P < .003). When subjects with MDD were categorized based on the presence of anhedonia, only anhedonic patients had decreased GABA/w levels compared with control subjects (t = 4.08; P < .001; P(Tukey) < .001). Anterior cingulate cortex GABA/w levels were negatively correlated with anhedonia scores for the whole MDD group (r = -0.50; P = .02), as well as for the entire participant sample including the control subjects (r = -0.54; P < .001). Anterior cingulate cortex white matter was also significantly decreased in adolescents with MDD compared with controls (P = .04).

CONCLUSIONS

These findings suggest that GABA, the major inhibitory neurotransmitter in the brain, may be implicated in adolescent MDD and, more specifically, in those with anhedonia. In addition, use of a continuous rather than categorical scale of anhedonia, as in the present study, may permit greater specificity in evaluating this important clinical feature.

Nrf2 deficiency potentiates methamphetamine-induced dopaminergic axonal damage and gliosis in the striatum

Oxidative stress that correlates with damage to nigrostriatal dopaminergic neurons and reactive gliosis in the basal ganglia is a hallmark of methamphetamine (METH) toxicity. In this study, we analyzed the protective role of the transcription factor Nrf2 (nuclear factor-erythroid 2-related factor 2), a master regulator of redox homeostasis, in METH-induced neurotoxicity. We found that Nrf2 deficiency exacerbated METH-induced damage to dopamine neurons, shown by an increase in loss of tyrosine hydroxylase (TH)- and dopamine transporter (DAT)-containing fibers in striatum. Consistent with these effects, Nrf2 deficiency potentiated glial activation, indicated by increased striatal expression of markers for microglia (Mac-1 and Iba-1) and astroglia (GFAP) one day after METH administration. At the same time, Nrf2 inactivation dramatically potentiated the increase in TNFα mRNA and IL-15 protein expression in GFAP+ cells in the striatum. In sharp contrast to the potentiation of striatal damage, Nrf2 deficiency did not affect METH-induced dopaminergic neuron death or expression of glial markers or proinflammatory molecules in the substantia nigra. This study uncovers a new role for Nrf2 in protection against METH-induced inflammatory and oxidative stress and striatal degeneration.

Striatal Signaling in L-DOPA-Induced Dyskinesia: Common Mechanisms with Drug Abuse and Long Term Memory Involving D1 Dopamine Receptor Stimulation

Parkinson’s disease is a common neurodegenerative disorder caused by the degeneration of midbrain substantia nigra dopaminergic neurons that project to the striatum. Despite extensive investigation aimed at finding new therapeutic approaches, the dopamine precursor molecule, 3,4-dihydroxyphenyl-l-alanine (l-DOPA), remains the most effective and commonly used treatment. However, chronic treatment and disease progression lead to changes in the brain’s response to l-DOPA, resulting in decreased therapeutic effect and the appearance of dyskinesias. l-DOPA-induced dyskinesia (LID) interferes significantly with normal motor activity and persists unless l-DOPA dosages are reduced to below therapeutic levels. Thus, controlling LID is one of the major challenges in Parkinson’s disease therapy. LID is the result of intermittent stimulation of supersensitive D1 dopamine receptors located in the very severely denervated striatal neurons. Through increased coupling to Gα_olf, resulting in greater stimulation of adenylyl-cyclase, D1 receptors phosphorylate DARPP-32, and other protein kinase A targets. Moreover, D1 receptor stimulation activates extracellular signal-regulated kinase and triggers a signaling pathway involving mammalian target for rapamycin and modifications of histones that results in changes in translation, chromatin modification, and gene transcription. In turn, sensitization of D1 receptor signaling causes a widespread increase in the metabolic response to D1 agonists and changes in the activity of basal ganglia neurons that correlate with the severity of LID. Importantly, different studies suggest that dyskinesias may share mechanisms with drug abuse and long term memory involving D1 receptor activation. Here we review evidence implicating D1 receptor signaling in the genesis of LID, analyze mechanisms that may translate enhanced D1 signaling into dyskinetic movements, and discuss the possibility that the mechanisms underlying LID are not unique to the Parkinson’s disease brain.

Brain glutamine synthesis requires neuronal-born aspartate as amino donor for glial glutamate formation

The glutamate-glutamine cycle faces a drain of glutamate by oxidation, which is balanced by the anaplerotic synthesis of glutamate and glutamine in astrocytes. De novo synthesis of glutamate by astrocytes requires an amino group whose origin is unknown. The deficiency in Aralar/AGC1, the main mitochondrial carrier for aspartate-glutamate expressed in brain, results in a drastic fall in brain glutamine production but a modest decrease in brain glutamate levels, which is not due to decreases in neuronal or synaptosomal glutamate content. In vivo (13)C nuclear magnetic resonance labeling with (13)C(2)acetate or (1-(13)C) glucose showed that the drop in brain glutamine is due to a failure in glial glutamate synthesis. Aralar deficiency induces a decrease in aspartate content, an increase in lactate production, and lactate-to-pyruvate ratio in cultured neurons but not in cultured astrocytes, indicating that Aralar is only functional in neurons. We find that aspartate, but not other amino acids, increases glutamate synthesis in both control and aralar-deficient astrocytes, mainly by serving as amino donor. These findings suggest the existence of a neuron-to-astrocyte aspartate transcellular pathway required for astrocyte glutamate synthesis and subsequent glutamine formation. This pathway may provide a mechanism to transfer neuronal-born redox equivalents to mitochondria in astrocytes.

Genetic inactivation of pleiotrophin triggers amphetamine-induced cell loss in the substantia nigra and enhances amphetamine neurotoxicity in the striatum

Pleiotrophin (PTN) is a neurotrophic factor with important effects in survival and differentiation of dopaminergic neurons that has been suggested to play important roles in drug of abuse-induced neurotoxicity. To test this hypothesis, we have studied the effects of amphetamine (10 mg/kg, four times, every 2 h) on the nigrostriatal pathway of PTN genetically deficient (PTN-/-) mice. We found that amphetamine causes a significantly enhanced loss of dopaminergic terminals in the striatum of PTN-/- mice compared to wild type (WT+/+) mice. In addition, we found a significant decrease ( approximately 20%) of tyrosine hydroxylase (TH)-positive neurons only in the substantia nigra of amphetamine-treated PTN-/- mice, whereas this area of WT+/+ animals remained unaffected after amphetamine treatment. This effect was accompanied by enhanced amphetamine-induced astrocytosis in the substantia nigra of PTN-/- mice. Interestingly, we found a significant decrease in the phosphorylation levels of p42 extracellular-signal regulated kinase (ERK2) in both saline- and amphetamine-treated PTN-/- mice, whereas phosphorylation of p44 ERK (ERK1) was almost abolished in the striatum of PTN-/- mice compared to WT+/+ mice, suggesting that basal deficiencies in the phosphorylation levels of ERK1/2 could underlie the higher vulnerability of PTN-/- mice to amphetamine-induced neurotoxic effects. The data suggest an important role of PTN in the protection of nigrostriatal pathways against amphetamine insult.

Restoration of amphetamine-induced locomotor sensitization in dopamine D1 receptor-deficient mice

RATIONALE AND OBJECTIVES

Amphetamine-induced sensitization is thought to involve dopamine D(1) receptors. Using mice lacking dopamine D(1) receptors (D (1) (-/-) ), we found that they exhibited blunted sensitization to low doses of amphetamine, while others using different treatment and testing regimens reported inconsistent results. We investigated whether experimental variables, alteration in gene expression or cholinergic input played a role in amphetamine-induced responses.

METHODS

D (1) (-/-) and wild-type (D (1) (+/+) ) mice pretreated with amphetamine (1 mg/kg, 3-7 days) or various doses of nicotine (chronically but intermittently) were challenged with amphetamine (0.7 and/or 1 mg/kg) after short and long abstinence periods. Expression of brain-derived neurotrophic factor (BDNF) and phosphorylated c-AMP response element binding protein (p-CREB) genes were measured under basal conditions and after acute or repeated amphetamine treatments.

RESULTS

D (1) (-/-) mice failed to exhibit amphetamine-induced sensitization following short-term treatments and long abstinence periods, but expressed sensitization following prolonged amphetamine treatment or a shorter abstinence period. Basal expression of p-CREB (but not BDNF) was higher in D (1) (-/-) than D (1) (+/+) mice and was reduced after amphetamine treatment. Prolonged nicotine pretreatment augmented locomotor responses to amphetamine in both genotypes and restored sensitization in D (1) (-/-) mice.

CONCLUSIONS

D(1) receptors were necessary for induction, but may not be necessary for expression of amphetamine-induced sensitization at low doses. The manifestation of amphetamine sensitization depended on the duration of treatment and length of the withdrawal period. Cholinergic-nicotinic stimulation restored amphetamine-induced sensitization in D (1) (-/-) mice. Enhanced basal expression of p-CREB in D (1) (-/-) mice may represent an adaptive mechanism related to lack of D(1) receptors.

Cerebral activation by fasting induces lactate accumulation in the hypothalamus

Carbon-13 ((13)C) high-resolution magic angle spinning (HR-MAS) spectroscopy was used to investigate the neuroglial coupling mechanisms underlying appetite regulation in the brain of C57BL/6J mice metabolizing [1-(13)C]glucose. Control fed or overnight fasted mice received [1-(13)C]glucose (20 micromol/g intraperitoneally [i.p.]), 15 min prior to brain fixation by focused microwaves. The hypothalamic region was dissected from the rest of the brain and (13)C HR-MAS spectra were obtained from both biopsies. Fasting resulted in a significant increase in hypothalamic [3-(13)C]lactate and [2-(13)C]gamma-aminobutyric acid (GABA) relative to the remaining brain. Administration of the orexigenic peptide ghrelin (0.3 nmol/g i.p.) did not increase hypothalamic [3-(13)C]lactate or [2-(13)C]GABA, suggesting that ghrelin signaling is not sufficient to elicit all the metabolic consequences of hypothalamic activation by fasting. Our results indicate that the hypothalamic regulation of appetite involves, in addition to the well-known neuropeptide signaling, increased neuroglial lactate shuttling and augmented GABA concentrations.

Selective vulnerability in striosomes and in the nigrostriatal dopaminergic pathway after methamphetamine administration : early loss of TH in striosomes after methamphetamine

Methamphetamine (METH), a commonly abused psychostimulant, causes dopamine neurotoxicity in humans, rodents, and nonhuman primates. This study examined the selective neuroanatomical pattern of dopaminergic neurotoxicity induced by METH in the mouse striatum. We examined the effect of METH on tyrosine hydroxylase (TH) and dopamine transporter (DAT) immunoreactivity in the different compartments of the striatum and in the nucleus accumbens. The levels of dopamine and its metabolites, 3,4-dihidroxyphenylacetic acid and homovanillic acid, as well as serotonin (5-HT) and its metabolite, 5-hydroxyindolacetic acid, were also quantified in the striatum. Mice were given three injections of METH (4 mg/kg, i.p.) at 3 h intervals and sacrificed 7 days later. This repeated METH injection induced a hyperthermic response and a decrease in striatal concentrations of dopamine and its metabolites without affecting 5-HT concentrations. In addition, the drug caused a reduction in TH- and DAT-immunoreactivity when compared to saline-treated animals. Interestingly, there was a significantly greater loss of TH- and DAT-immunoreactivity in striosomes than in the matrix. The predominant loss of dopaminergic terminals in the striosomes occurred along the rostrocaudal axis of the striatum. In contrast, METH did not decrease TH- or DAT-immunoreactivity in the nucleus accumbens. These results provide the first evidence that compartments of the mouse striatum, striosomes and matrix, and mesolimbic and nigrostriatal pathways have different vulnerability to METH. This pattern is similar to that observed with other neurotoxins such as MPTP, the most widely used model of Parkinson’s disease, in early Huntington’s disease and hypoxic/ischemic injury, suggesting that these conditions might share mechanisms of neurotoxicity.

The turnover of the H3 deuterons from (2-13C) glutamate and (2-13C) glutamine reveals subcellular trafficking in the brain of partially deuterated rats

We investigated by 13C NMR the turnover of the H3 deuterons of (2-13C) glutamate and (2-13C) glutamine in the brain of partially deuterated rats. Adult animals (150-200 g) fed ad libitum received 50% 2H2O or tap water 9 days before infusing (1-13C) glucose or (2-13C) acetate for 5, 10, 15, 30, 60, or 90 min. The brains were then funnel-frozen and acid extracts were prepared and analyzed by high-resolution 13C NMR. The deuteration of one or the two H3 hydrogens of (2-13C) glutamate or glutamine resulted in single (-0.07 ppm) or double (-0.14 ppm) isotopic shifts upfield of the corresponding C2 perprotonated resonance, demonstrating two sequential deuteration steps. The faster monodeuteration generated 3R or 3S (2-13C, 3-2H) glutamate or glutamine through the alternate activities of cerebral aconitase or isocitrate dehydrogenase, respectively. The slower process produced bideuterated (2-13C, 3,3'-2H2) glutamate or glutamine through the consecutive activity of both enzymes. The kinetics of deuteration was fitted to a Michaelis-Menten model including the apparent K(m)' and Vmax' values for the observed deuterations. Our results revealed different kinetic constants for the alternate and consecutive deuterations, suggesting that these processes were caused by the different cytosolic or mitochondrial isoforms of aconitase and isocitrate dehydrogenase, respectively. The deuterations of (2-13C) glutamate or glutamine followed also different kinetics from (1-13C) glucose or (2-13C) acetate, revealing distinct deuteration environments in the neuronal or glial compartments.