Oral noribogaine shows high brain uptake and anti-withdrawal effects not associated with place preference in rodents

This study investigated the effects of noribogaine, the principal metabolite of the drug ibogaine, on substance-related disorders. In the first experiment, mice chronically treated with morphine were subjected to naloxone-precipitated withdrawal two hours after oral administration of noribogaine. Oral noribogaine dose dependently decreased the global opiate withdrawal score by up to 88% of vehicle control with an ED50 of 13 mg/kg. In the second experiment, blood and brain levels of noribogaine showed a high brain penetration and a brain/blood ratio of 7±1 across all doses tested. In a third experiment, rats given oral noribogaine up to 100 mg/kg were tested for abuse liability using a standard biased conditioned place paradigm. Noribogaine-treated rats did not display place preference, suggesting that noribogaine is not perceived as a hedonic stimulus in rodents. Retrospective review of published studies assessing the efficacy of ibogaine on morphine withdrawal shows that the most likely cause of the discrepancies in the literature is the different routes of administration and time of testing following ibogaine administration. These results suggest that the metabolite noribogaine rather than the parent compound mediates the effects of ibogaine on blocking naloxone-precipitated withdrawal. Noribogaine may hold promise as a non-addicting alternative to standard opiate replacement therapies to transition patients to opiate abstinence.

Neuronal NOS and cyclooxygenase-2 contribute to DNA damage in a mouse model of Parkinson disease

DNA damage is a proposed pathogenic factor in neurodegenerative disorders such as Parkinson disease. To probe the underpinning mechanism of such neuronal perturbation, we sought to produce an experimental model of DNA damage. We thus first assessed DNA damage by in situ nick translation and emulsion autoradiography in the mouse brain after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 4 x 20 mg/kg, ip, every 2 h), a neurotoxin known to produce a model of Parkinson disease. Here we show that DNA strand breaks occur in vivo in this mouse model of Parkinson disease with kinetics and a topography that parallel the degeneration of substantia nigra neurons, as assessed by FluoroJade labeling. Previously, nitric oxide synthase and cyclooxygenase-2 (Cox-2) were found to modulate MPTP-induced dopaminergic neuronal death. We thus assessed the contribution of these enzymes to DNA damage in mice lacking neuronal nitric oxide synthase (nNOS), inducible nitric oxide synthase (iNOS), or Cox-2. We found that the lack of Cox-2 and nNOS activities but not of iNOS activity attenuated MPTP-related DNA damage. We also found that not only nuclear, but also mitochondrial, DNA is a target for the MPTP insult. These results suggest that the loss of genomic integrity can be triggered by the concerted actions of nNOS and Cox-2 and provide further support to the view that DNA damage may contribute to the neurodegenerative process in Parkinson disease.

Alpha-synuclein overexpression increases cytosolic catecholamine concentration

Dysregulation of dopamine homeostasis and elevation of the cytosolic level of the transmitter have been suggested to underlie the vulnerability of catecholaminergic neurons in Parkinson's disease. Because several known mutations in alpha-synuclein or overexpression of the wild-type (WT) protein causes familial forms of Parkinson's disease, we investigated possible links between alpha-synuclein pathogenesis and dopamine homeostasis. Chromaffin cells isolated from transgenic mice that overexpress A30P alpha-synuclein displayed significantly increased cytosolic catecholamine levels as measured by intracellular patch electrochemistry, whereas cells overexpressing the WT protein and those from knock-out animals were not different from controls. Likewise, catechol concentrations were higher in L-DOPA-treated PC12 cells overexpressing A30P or A53T compared with those expressing WT alpha-synuclein, although the ability of cells to maintain a low cytosolic dopamine level after L-DOPA challenge was markedly inhibited by either protein. We also found that incubation with low-micromolar concentrations of WT, A30P, or A53T alpha-synuclein inhibited ATP-dependent maintenance of pH gradients in isolated chromaffin vesicles and that the WT protein was significantly less potent in inducing the proton leakage. In summary, we demonstrate that overexpression of different types of alpha-synuclein disrupts vesicular pH and leads to a marked increase in the levels of cytosolic catechol species, an effect that may in turn trigger cellular oxyradical damage. Although multiple molecular mechanisms may be responsible for the perturbation of cytosolic catecholamine homeostasis, this study provides critical evidence about how alpha-synuclein might exert its cytotoxicity and selectively damage catecholaminergic cells.

Metabolism of 3-nitrotyrosine induces apoptotic death in dopaminergic cells

Intrastriatal injection of 3-nitrotyrosine, which is a biomarker for nitrating oxidants, provokes dopaminergic neuronal death in rats by unknown mechanisms. Herein, we show that extracellular 3-nitrotyrosine is transported via the l-aromatic amino acid transporter in nondopaminergic NT2 cells, whereas in dopaminergic PC12 cells, it is transported by both the l-aromatic amino acid and the dopamine transporters. In both cell lines, 3-nitrotyrosine is a substrate for tyrosine tubulin ligase, resulting in its incorporation into the C terminus of alpha-tubulin. In NT2 cells, incorporation of 3-nitrotyrosine into alpha-tubulin induces a progressive, reversible reorganization of the microtubule architecture. In PC12 cells, 3-nitrotyrosine decreases intracellular dopamine levels and is metabolized by the concerted action of the aromatic amino acid decarboxylase and monoamine oxidase. Intracellular levels of 133 micromol of 3-nitrotyrosine per mole of tyrosine did not alter NT2 viability but induced PC12 apoptosis. The cell death was reversed by caspases and aromatic amino acid decarboxylase and monoamine oxidase inhibitors. 3-Nitrotyrosine induced loss of tyrosine hydroxylase-positive primary rat neurons, which was also prevented by an aromatic amino acid decarboxylase inhibitor. These findings provide a novel mechanism by which products generated by reactive nitrogen species induce dopaminergic neuron death and thus may contribute to the selective neurodegeneration in Parkinson's disease.

Toxin-induced models of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease that appears essentially as a sporadic condition. It results mainly from the death of dopaminergic neurons in the substantia nigra. PD etiology remains mysterious, whereas its pathogenesis begins to be understood as a multifactorial cascade of deleterious factors. Most insights into PD pathogenesis come from investigations performed in experimental models of PD, especially those produced by neurotoxins. Although a host of natural and synthetic molecules do exert deleterious effects on dopaminergic neurons, only a handful are used in living laboratory animals to recapitulate some of the hallmarks of PD. In this review, we discuss what we believe are the four most popular parkinsonian neurotoxins, namely 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, and paraquat. The main goal is to provide an updated summary of the main characteristics of each of these four neurotoxins. However, we also try to provide the reader with an idea about the various strengths and the weaknesses of these neurotoxic models.

Evolution and cell biology of dopamine receptors in vertebrates

Dopamine, one of main modulatory neurotransmitters of the nervous system acts on target cells through two classes of G protein-coupled receptors, D1 and D2. The two dopamine receptor classes display different structures, interact with different regulatory partners (including heterotrimeric G proteins) and, accordingly, have independent evolutionary origins. In vertebrates, each of these receptor classes comprises several subtypes, generated by two steps of gene duplications, early in vertebrate evolution. In the D1 receptor class, the D1A, D1B, D1C and D1D subtypes, and in the D2 class, the D2, D3 and D4 receptor subtypes have been conserved in most vertebrate groups. This conservation has been driven by the acquisition, by each receptor subtype, of a small number of specific properties, which were selected for adaptive purpose in vertebrates. Among these properties, affinity for dopamine, the natural ligand, intrinsic receptor activity, and agonist-induced desensitization clearly distinguish the receptor subtypes. In addition, each dopamine receptor subtype is addressed to a specific location within neuronal networks, although detailed information is lacking for several receptor subtypes. Receptors localization at diverse subcellular places in neurons may also differ from one subtype to another, resulting in different ways of regulating cell signalisation. One challenge for future research on dopamine and its receptors would be to identify the nature of the protein partners and the molecular mechanisms involved in localizing receptors to the neuronal plasma membrane. In this respect, the evolutionary approach we have undertaken suggests that, due to gene duplications, a reasonable degree of freedom exists in the tight organisation of dopamine receptors in neurons. This "evolvability" of dopamine systems has been instrumental to adapt the vertebrate species to nearly all the possible environments.

Mutations in the lipid-binding domain of alpha-synuclein confer overlapping, yet distinct, functional properties in the regulation of dopamine transporter activity

Alpha-synuclein and its missense mutants (A30P, A53T) have been linked to the genesis of idiopathic and rare familial forms of Parkinson's disease, respectively. Here we show that, similar to the wild-type alpha-synuclein, the A30P mutant forms a strong complex with the human dopamine transporter (hDAT), through direct protein:protein interactions between the nonamyloid beta component (NAC) domain of the A30P mutant and the last 22 aminoacyl residues of the carboxy-terminal tail of hDAT. The A30P mutant negatively modulates hDAT functional activity and to a greater extent than wild-type alpha-synuclein, with reduced uptake of extracellular dopamine and dopamine-mediated, hDAT-dependent cytotoxicity. By contrast, the A53T mutant neither forms a strong protein:protein complex with hDAT nor modulates dopamine uptake by hDAT, and dopamine-mediated, hDAT-dependent cytotoxicity is higher than with either wild-type or the A30P variant of alpha-synuclein, but not significantly different from that of cells expressing hDAT alone. Confocal microscopy shows substantial overlap in colocalization of all three alpha-synuclein variants with hDAT, with only minor differences. Although the complex formation with hDAT occurs through the NAC domain of the alpha-synuclein variants, it is the familial Parkinson's disease-linked missense mutations present in the amino-terminal lipid binding domain of the alpha-synuclein variants that dictate the extent of the regulation of hDAT function. These studies highlight previously unknown properties of the A30P and the A53T mutants of alpha-synuclein with respect to the modulation of hDAT activity and/or regulation, and its subsequent functional outcome, which are uniquely distinct.

Modulation of dopamine transporter function by alpha-synuclein is altered by impairment of cell adhesion and by induction of oxidative stress

Human alpha-synuclein accumulates in dopaminergic neurons as intraneuronal inclusions, Lewy bodies, which are characteristic of idiopathic Parkinson's disease (PD). Here, we suggest that modulation of the functional activity of the dopamine transporter (DAT) by alpha-synuclein may be a key factor in the preferential degeneration of mesencephalic dopamine (DA)-synthesizing neurons in PD. In cotransfected Ltk-, HEK 293, and SK-N-MC cells, alpha-synuclein induced a 35% decrease in [3H]DA uptake. Biotinylated DAT levels were decreased by 40% in cotransfected cells relative to cells expressing only DAT. DAT was colocalized with alpha-synuclein in mesencephalic neurons and cotransfected Ltk- cells. Coimmunoprecipitation studies showed the existence of a complex between alpha-synuclein and DAT, in specific rat brain regions and cotransfected cells, through specific amino acid motifs of both proteins. The attenuation of DAT function by alpha-synuclein was cytoprotective, because DA-mediated oxidative stress and cell death were reduced in cotransfected cells. The neurotoxin MPP+ (1-methyl-4-phenylpyridinium), oxidative stress, or impairment of cell adhesion ablated the alpha-synuclein-mediated inhibition of DAT activity, which caused increased uptake of DA and increased biotinylated DAT levels, in both mesencephalic neurons and cotransfected cells. These studies suggest a novel normative role for alpha-synuclein in regulating DA synaptic availability and homeostasis, which is relevant to the pathophysiology of PD.

Autocrine activation of adenosine A1 receptors blocks D1A but not D1B dopamine receptor desensitization

Adenosine is known to modulate dopamine responses in several brain areas. Here, we show that tonic activation of adenosine receptors is able to impede desensitization of D1 dopamine receptors. As measured by cAMP accumulation in transfected COS-7 cells, long-term exposure to dopamine agonists promoted desensitization of D1B receptor but not that of D1A receptor. The inability of D1A receptor to desensitize was a result of the adenosine present in culture medium acting through activation of adenosine A1 receptors. Cell incubation with either adenosine deaminase, CGS-15943, a generic adenosine receptor antagonist, or the A1 antagonist DPCPX restored the long-term desensitization time-course of D1A receptors. In Ltk cells stably expressing A1 adenosine receptors and D1A dopamine receptors, pre-treatment of cells with R(-)-PIA, a full A1 receptor agonist, did not significantly inhibit the acute increase in cAMP levels induced by D1 receptor agonists, but blocked desensitization of D1A receptors. However, simultaneous activation of A1 and D1A receptors promoted a delayed D1A receptor desensitization. This suggests that functional interaction between A1 and D1A receptors may depend on the activation kinetics of components regulating D1 receptor responses, acting differentially on D1A and D1B receptors.

Intracellular retention of the two isoforms of the D2 dopamine receptor promotes endoplasmic reticulum disruption

The dopamine D2 receptor exists as a long (D2a) and a short (D2b) isoform generated by alternative splicing of the corresponding transcript, which modifies the length of the third cytoplasmic loop implicated in heterotrimeric G-protein-coupling. Anatomical data suggested that this segment regulates the intracellular traffic and localization of the receptor. To directly address this question we used a combination of tagging procedures and immunocytochemical techniques to detect each of the two D2 receptor isoforms. Surprisingly, most of the newly synthesized receptors accumulate in large intracellular compartments, the plasma membrane being only weakly labeled, without significant difference between the two receptor isoforms. Double labeling experiments showed that this localization corresponded neither to endosomal compartments nor to the Golgi apparatus. The D2 receptor is mostly retained in the endoplasmic reticulum (ER), the long isoform more efficiently than the short one. It is accompanied by a striking vacuolization of the ER, roughly proportional to the expression levels of the two receptor isoforms. This phenomenon is partly overcome by treatment with pertussis toxin. In addition, an intrinsic activity of the D2 receptor isoforms is revealed by [35S]-GTPγS binding and cAMP assay, which suggested that expression of weakly but constitutively active D2 receptors promotes activation of heterotrimeric G protein inside the secretory pathway. This mechanism may participate in the regulation of the cellular traffic of the D2 receptors isoforms.