Involvement of nitric oxide in nigrostriatal dopaminergic system degeneration: a neurochemical study

Phytochemicals targeting nitric oxide signaling in neurodegenerative diseases

Neurodegenerative diseases are a set of diseases in which slow and progressive neuronal loss occurs. Nitric oxide (NO) as a neurotransmitter performs key roles in the stimulation and blockade of various inflammatory processes. Although physiological NO is necessary for protection against a variety of pathogens, reactive oxygen species-mediated oxidative stress induces inflammatory cascades and apoptosis. Activation of glial cells particularly astrocytes and microglia induce overproduction of NO, resulting in neuroinflammation and neurodegenerative disorders. Hence, inhibiting the overproduction of NO is a beneficial therapeutic approach for numerous neuroinflammatory conditions. Several compounds have been explored for the management of neurodegenerative disorders, but they have minor symptomatic benefits and several adverse effects. Phytochemicals have currently gained more consideration owing to their ability to reduce the overproduction of NO in neurodegenerative disorders. Furthermore, phytochemicals are generally considered to be safe and beneficial. The mechanisms of NO generation and their implications in neurodegenerative disorders are explored in this review article, as well as several newly discovered phytochemicals that might have NO inhibitory activity. The current review could aid in the discovery of new anti-neuroinflammatory drugs that can suppress NO generation, particularly during neuroinflammatory and neurodegenerative conditions.

Re-routing Metabolism by the Mitochondrial Pyruvate Carrier Inhibitor MSDC-0160 Attenuates Neurodegeneration in a Rat Model of Parkinson's Disease

A growing body of evidence supports the idea that mitochondrial dysfunction might represent a key feature of Parkinson's disease (PD). Central regulators of energy production, mitochondria, are also involved in several other essential functions such as cell death pathways and neuroinflammation which make them a potential therapeutic target for PD management. Interestingly, recent studies related to PD have reported a neuroprotective effect of targeting mitochondrial pyruvate carrier (MPC) by the insulin sensitizer MSDC-0160. As the sole point of entry of pyruvate into the mitochondrial matrix, MPC plays a crucial role in energetic metabolism which is impacted in PD. This study therefore aimed at providing insights into the mechanisms underlying the neuroprotective effect of MSDC-0160. We investigated behavioral, cellular, and metabolic impact of chronic MSDC-0160 treatment in unilateral 6-OHDA PD rats. We evaluated mitochondrially related processes through the expression of pivotal mitochondrial enzymes in dorsal striatal biopsies and the level of metabolites in serum samples using nuclear magnetic resonance spectroscopy (NMR)-based metabolomics. MSDC-0160 treatment in unilateral 6-OHDA rats improved motor behavior, decreased dopaminergic denervation, and reduced mTOR activity and neuroinflammation. Concomitantly, MSDC-0160 administration strongly modified energy metabolism as revealed by increased ketogenesis, beta oxidation, and glutamate oxidation to satisfy energy needs and maintain energy homeostasis. MSDC-0160 exerts its neuroprotective effect through reorganization of multiple pathways connected to energy metabolism.

Re-routing Metabolism by the Mitochondrial Pyruvate Carrier Inhibitor MSDC-0160 Attenuates Neurodegeneration in a Rat Model of Parkinson's Disease

A growing body of evidence supports the idea that mitochondrial dysfunction might represent a key feature of Parkinson's disease (PD). Central regulators of energy production, mitochondria, are also involved in several other essential functions such as cell death pathways and neuroinflammation which make them a potential therapeutic target for PD management. Interestingly, recent studies related to PD have reported a neuroprotective effect of targeting mitochondrial pyruvate carrier (MPC) by the insulin sensitizer MSDC-0160. As the sole point of entry of pyruvate into the mitochondrial matrix, MPC plays a crucial role in energetic metabolism which is impacted in PD. This study therefore aimed at providing insights into the mechanisms underlying the neuroprotective effect of MSDC-0160. We investigated behavioral, cellular, and metabolic impact of chronic MSDC-0160 treatment in unilateral 6-OHDA PD rats. We evaluated mitochondrially related processes through the expression of pivotal mitochondrial enzymes in dorsal striatal biopsies and the level of metabolites in serum samples using nuclear magnetic resonance spectroscopy (NMR)-based metabolomics. MSDC-0160 treatment in unilateral 6-OHDA rats improved motor behavior, decreased dopaminergic denervation, and reduced mTOR activity and neuroinflammation. Concomitantly, MSDC-0160 administration strongly modified energy metabolism as revealed by increased ketogenesis, beta oxidation, and glutamate oxidation to satisfy energy needs and maintain energy homeostasis. MSDC-0160 exerts its neuroprotective effect through reorganization of multiple pathways connected to energy metabolism.

Role of Nitric Oxide in Neurodegeneration: Function, Regulation, and Inhibition

Reactive nitrogen species (RNS) and reactive oxygen species (ROS), collectively known as reactive oxygen and nitrogen species (RONS), are the products of normal cellular metabolism and interact with several vital biomolecules including nucleic acid, proteins, and membrane lipids and alter their function in an irreversible manner which can lead to cell death. There is an imperative role for oxidative stress in the pathogenesis of cognitive impairments and the development and progression of neural injury. Elevated production of higher amounts of nitric oxide (NO) takes place in numerous pathological conditions, such as neurodegenerative diseases, inflammation, and ischemia, which occur concurrently with elevated nitrosative/oxidative stress. The enzyme nitric oxide synthase (NOS) is responsible for the generation of NO in different cells by conversion of L-arginine (Arg) to L-citrulline. Therefore, the NO signaling pathway represents a viable therapeutic target. Naturally occurring polyphenols targeting the NO signaling pathway can be of major importance in the field of neurodegeneration and related complications. Here, we comprehensively review the importance of NO and its production in the human body and afterwards highlight the importance of various natural products along with their mechanisms against various neurodegenerative diseases involving their effect on NO production.

The role of nitric oxide in brain disorders: Autism spectrum disorder and other psychiatric, neurological, and neurodegenerative disorders

Nitric oxide (NO) is a multifunctional signalling molecule and a neurotransmitter that plays an important role in physiological and pathophysiological processes. In physiological conditions, NO regulates cell survival, differentiation and proliferation of neurons. It also regulates synaptic activity, plasticity and vesicle trafficking. NO affects cellular signalling through protein S-nitrosylation, the NO-mediated posttranslational modification of cysteine thiols (SNO). SNO can affect protein activity, protein-protein interaction and protein localization. Numerous studies have shown that excessive NO and SNO can lead to nitrosative stress in the nervous system, contributing to neuropathology. In this review, we summarize the role of NO and SNO in the progression of neurodevelopmental, psychiatric and neurodegenerative disorders, with special attention to autism spectrum disorder (ASD). We provide mechanistic insights into the contribution of NO in diverse brain disorders. Finally, we suggest that pharmacological agents that can inhibit or augment the production of NO as well as new approaches to modulate the formation of SNO-proteins can serve as a promising approach for the treatment of diverse brain disorders.

Animal models of early-stage Parkinson's disease and acute dopamine deficiency to study compensatory neurodegenerative mechanisms

Parkinson's disease is a common neurodegenerative disease characterized by a widely variety of motor and non-motor symptoms. While the motor deficits are only visible following a severe dopamine depletion, neurodegenerative process and some non-motor symptoms are manifested years before the motor deficits. Importantly, chronic degeneration of dopaminergic neurons leads to the development of compensatory mechanisms that play roles in the progression of the disease and the response to anti-parkinsonian therapies. The identification of these mechanisms will be of great importance for improving our understanding of factors with important contributions to the disease course and the underlying adaptive process. To date, most of the data obtained from animal models reflect the late, chronic, dopamine-depleted states, when compensatory mechanisms have already been established. Thus, adequate animal models with which researchers are able to dissect early- and late-phase mechanisms are necessary. Here, we reviewed the literature related to animal models of early-stage PD and pharmacological treatments capable of inducing acute dopamine impairments and/or depletion, such as reserpine, haloperidol and tetrodotoxin. We highlighted the advantages, limitations and the future prospective uses of these models, as well as their applications in the identification of novel agents for treating this neurological disorder.

The Effects of Crocin on 6-OHDA-Induced Oxidative/Nitrosative Damage and Motor Behaviour in Hemiparkinsonian Rats

BACKGROUND

Crocin is considered to prevent oxidative stress-related diseases, such as ischemia and Alzheimer's. The aim of the present investigation was to evaluate the effects of crocin on motor behaviour and 6-OHDA-induced oxidative/nitrosative damage to the striatum in an experimental model of Parkinson's disease.

METHODS

Left medial forebrain bundle was lesioned by microinjection of 6-OHDA (16μg in 0.2% ascorbate-saline). Crocin (30 and 60 mg/kg) was injected intraperitoneally three days before surgery until six weeks. Rotational behaviour and biochemical analysis were used to evaluate the effect of crocin in a unilateral 6-OHDA-induced model of Parkinson's disease.

RESULTS

The contralateral rotations induced by apomorphine in 6-OHDA lesioned group were highly significant (P < 0.001) as compared to the sham group. Moreover, chronic administration of crocin at doses of 30 and 60 mg/kg over six weeks did not change the rotations. The TBARS and nitrite levels in the striatum were also significantly (P < 0.05) increased in lesioned group. Treatment with crocin at a dose of 60 mg/kg significantly decreased the nitrite levels (P < 0.05) in the striatum.

CONCLUSION

Crocin at a dose of 60 mg/kg could be effective in preventing the nitrosative damage in the striatum. Further investigations using higher doses of crocin is suggested to get the full neuroprotective effects of crocin in Parkinson's disease.

Neuronal Nitric Oxide Synthase Negatively Regulates Zinc-Induced Nigrostriatal Dopaminergic Neurodegeneration

The study aimed to investigate the role of NO and neuronal NO synthase (nNOS) in Zn-induced neurodegeneration. Animals were treated with zinc sulfate (20 mg/kg), twice a week, for 2-12 weeks along with control. In a few sets, animals were also treated with/without a NO donor, sodium nitroprusside (SNP), or S-nitroso-N-acetyl penicillamine (SNAP) for 12 weeks. Moreover, human neuroblastoma (SH-SY-5Y) cells were also employed to investigate the role of nNOS in Zn-induced toxicity in in vitro in the presence/absence of nNOS inhibitor, 7-nitroindazole (7-NI). Zn caused time-dependent reduction in nitrite content and total/nNOS activity/expression. SNP/SNAP discernibly alleviated Zn-induced neurobehavioral impairments, dopaminergic neurodegeneration, tyrosine hydroxylase (TH) expression, and striatal dopamine depletion. NO donors also salvage from Zn-induced increase in lipid peroxidation (LPO), mitochondrial cytochrome c release, and caspase-3 activation. While Zn elevated LPO content, it attenuated nitrite content, nNOS activity, and glutathione level along with the expression of TH and nNOS in SH-SY-5Y cells. 7-NI further augmented Zn-induced changes in the cell viability, oxidative stress, and expression of TH and nNOS. The results obtained thus demonstrate that Zn inhibits nNOS that partially contributes to an increase in oxidative stress, which subsequently leads to the nigrostriatal dopaminergic neurodegeneration.

The preferential nNOS inhibitor 7-nitroindazole and the non-selective one N(G)-nitro-L-arginine methyl ester administered alone or jointly with L-DOPA differentially affect motor behavior and monoamine metabolism in sham-operated and 6-OHDA-lesioned rats

Reciprocal interactions between nitrergic and dopaminergic systems play a key role in the control of motor behavior. In the present study, we performed a comparative analysis of motor behavior (locomotor activity, catalepsy, rotational behavior) and monoamine metabolism in the striatum and substantia nigra of unilaterally sham-operated and 6-OHDA-lesioned rats treated with the preferential neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI) or the non-selective one N(G)-nitro-L-arginine methyl ester (L-NAME), alone or in combination with L-DOPA. Each NOS inhibitor given alone (50mg/kg) induced a distinct catalepsy 30 min after injection but only 7-NI impaired spontaneous locomotion after 10 min. In 6-OHDA-lesioned rats, chronic L-DOPA (25mg/kg) induced 2.5-h long contralateral rotations. 7-NI (30 and 50mg/kg) markedly reduced the intensity of L-DOPA-induced contralateral rotations while extending their duration until 4.5h whereas L-NAME (50 and 100mg/kg) only tended to attenuate their intensity without affecting the duration. 7-NI but not L-NAME significantly increased endogenous tissue DA levels in the nigrostriatal system of both sham-operated and 6-OHDA-lesioned rats. In L-DOPA-treated group, 7-NI significantly enhanced the L-DOPA-derived tissue DA content in this system and decreased the level of the intracellular DA metabolite DOPAC produced by monoamine oxidase (MAO). In contrast to 7-NI, L-NAME decreased markedly DA content and did not affect DOPAC level in the ipsilateral striatum. It means that the differences in 7-NI and L-NAME-mediated modulation of L-DOPA-induced behavioral and biochemical effects resulted not only from the inhibition of NOS activity but also from differences in their ability to inhibit MAO.

Post-lesion administration of 7-NI attenuated motor and non-motor deficits in 6-OHDA induced bilaterally lesioned female rat model of Parkinson's disease

The preoperative neuroprotective effect of the 7-nitroindazole (7-NI) in 6-hydroxydopamine (6-OHDA) induced unilateral male animal models of Parkinson's disease (PD) has been widely reported. However, the therapeutic approach to PD pathology would be closely associated with the post-lesion treatment by 7-NI in 6-OHDA-induced bilateral model. Also, there is a scarcity of data on neuroprotective effect of 7-NI in PD in females. We have studied the neuroprotective effects of 7-NI in 6-OHDA-induced bilaterally lesioned female rats after short-term post-lesion treatment. Sprague-Dawley female rats with bilateral intraventricular injection of either 6-OHDA (10.5μg) (n=8-11/group) or saline (sham; n=8/group) at substantia nigra (SN) were provided with 7-NI (30mg/kg/day) intraperitoneal, once a day during the 3 consecutive days of short term treatment. 6-OHDA lesioned animals developed the motor and non-motor deficits, which were evaluated by behavioral and neuro-biochemical tests from the substantia nigra. Post-lesion administration of 7-NI reduced the motor deficits induced by 6-OHDA in the behavioral tasks such as Rota rod, open field test and forced swim test. Simultaneously, the dopamine levels were restored by 7-NI in post lesion animals up to 76% in comparison to 6-OHDA lesioned animals (23%). Furthermore, antioxidant-like effect of 7-NI was observed in lipid peroxidation, catalase, superoxide dismutase, and reduced glutathione tests. Conclusively, the present study showed that early postoperative administration of 7-NI attenuates the motor deficits induced by 6-OHDA in bilaterally lesioned female rat model of PD.

Omega-3 deficiency and neurodegeneration in the substantia nigra: involvement of increased nitric oxide production and reduced BDNF expression

BACKGROUND

Our previous study demonstrated that essential fatty acid (EFA) dietary restriction over two generations induced midbrain dopaminergic cell loss and oxidative stress in the substantia nigra (SN) but not in the striatum of young rats. In the present study we hypothesized that omega-3 deficiency until adulthood would reduce striatum's resilience, increase nitric oxide (NO) levels and the number of BDNF-expressing neurons, both potential mechanisms involved in SN neurodegeneration.

METHODS

Second generation rats were raised from gestation on control or EFA-restricted diets until young or adulthood. Lipoperoxidation, NO content, total superoxide dismutase (t-SOD) and catalase enzymatic activities were assessed in the SN and striatum. The number of tyrosine hydroxylase (TH)- and BDNF-expressing neurons was analyzed in the SN.

RESULTS

Increased NO levels were observed in the striatum of both young and adult EFA-deficient animals but not in the SN, despite a similar omega-3 depletion (~65%) in these regions. Increased lipoperoxidation and decreased catalase activity were found in both regions, while lower tSOD activity was observed only in the striatum. Fewer TH- (~40%) and BDNF-positive cells (~20%) were detected at the SN compared to the control.

CONCLUSION

The present findings demonstrate a differential effect of omega-3 deficiency on NO production in the rat's nigrostriatal system. Prolonging omega-3 depletion until adulthood impaired striatum's anti-oxidant resources and BDNF distribution in the SN, worsening dopaminergic cell degeneration.

GENERAL SIGNIFICANCE

Omega-3 deficiency can reduce the nigrostriatal system's ability to maintain homeostasis under oxidative conditions, which may enhance the risk of Parkinson's disease.

Molsidomine, a nitric oxide donor, modulates rotational behavior and monoamine metabolism in 6-OHDA lesioned rats treated chronically with L-DOPA

Some biochemical and histological studies of Parkinson's disease patients' brains and 6-OHDA-lesioned rats suggest that dopaminergic dennervation of the striatum leads to the nitrergic system hypofunction in this structure. Hence, recently the modulation of nitric oxide (NO)- soluble guanylyl cyclase-cyclic GMP signaling is considered to be a new target for the treatment of Parkinson's disease. The aim of our study was to examine the impact of chronic combined treatment with low doses of the NO donor molsidomine (2 and 4mg/kg) and L-DOPA (12.5 and 25mg/kg) on rotational behavior and monoamine metabolism in the striatum (STR) and substantia nigra (SN) of unilaterally 6-OHDA-lesioned rats. Chronic administration of molsidomine at a dose of 2mg/kg jointly with 25mg/kg of L-DOPA significantly decreased the number of contralateral rotations when compared to L-DOPA alone. Other combinations of the examined drug doses were less effective. The tissue DA levels in the ipsilateral STR and SN after the last chronic doses of molsidomine (2mg/kg) and L-DOPA (12.5 or 25mg/kg), were significantly higher than after L-DOPA alone. Chronic L-DOPA treatment alone or jointly with a lower dose of molsidomine decreased 5-HT levels and accelerated its catabolism in the examined structures. However, combination of a higher dose of molsidomine with L-DOPA (25mg/kg) did not reduce 5-HT content while its catabolism was less intensive. The obtained results show that low doses of molsidomine can modulate rotational behavior and tissue DA and 5-HT concentrations in the STR and SN of 6-OHDA-lesioned rats treated chronically with L-DOPA.

Investigations into Potential Extrasynaptic Communication between the Dopaminergic and Nitrergic Systems

Nitric oxide is unconstrained by cell membranes and can therefore act along a broad distance as a volume transmitter. Spillover of nitric oxide between neurons may have a major impact on central nervous system diseases and particularly on neurodegeneration. There is evidence whereby communication between nitrergic and dopaminergic systems plays an essential role in the control of the nigrostriatal pathway. However, there is sparse information for either the coexistence or overlap of nitric oxide and dopaminergic structures. The dual localization of immunoreactivity for nitric oxide synthase (NOS) and tyrosine hydroxylase, enzymes responsible for the synthesis of nitric oxide and dopamine, respectively, was examined in neurons of the nigrostriatal pathway in the rat brain by means of a double-immunohistochemical method and confocal laser scanning microscopy, acquired at the resolution limit. After perfusional fixation, the brains were cut and double-immunostained. A proximity analysis of tyrosine hydroxylase and NOS structures was done using binary masks generated from the respective maximum projections, using confocal laser microscopy. Unrevealed regions were determined somatodendritic positive for both NOS and tyrosine hydroxylase, within an image limit resolution at 2 μm-wide margin. The described interconnected localization of nNOS(+) and TH(+) containing neuronal fibers and cells bodies in the nigrostriatal pathway propose a close anatomical link between the two neurotransmitters.

7-Nitroindazole down-regulates dopamine/DARPP-32 signaling in neostriatal neurons in a rat model of Parkinson's disease

Neuronal nitric oxide synthase (nNOS) is involved in the regulation of diverse intracellular messenger systems in the brain. Nitric Oxide (NO) contributes to inducing signaling cascades that involve a complex pattern of phosphorylation of DARPP-32 (in Thr-34), which controls the phosphoproteins involved in neuronal activation. However, the role of NO in the pathophysiology of Parkinson's disease (PD) and its effect in striatal neurons have been scarcely explored. In the present work, we investigate the effects of a nitric oxide synthase (NOS) inhibitor, 7-nitroindazole (7-NI) in the nigrostriatal pathway of striatal 6-hydroxydopamine (6-OHDA) lesioned rats. Our quantitative histological findings show that treatment with 7-NI significantly reduced 6-OHDA-induced dopaminergic damage in the dorsolateral striatum and Substantia Nigra pars compacta (SNpc). Moreover, 6-OHDA lesioned rats show a significant increase of nNOS(+) and Phospho-Thr34-DARPP-32(+) cells, accompanied by a consequent decrease of total DARPP-32(+) cells, which suggests an imbalance of NO activity in the DA-depleted striatum, which is also reflected in behavioral studies. Importantly, these effects are reverted in the group treated with 7-NI. These results show a clear link between the state of phosphorylation of DARPP-32 and parkinsonism, which is regulated by nNOS. This new evidence suggests a prominent role for nitric oxide in the neurotransmitter balance within the basal ganglia in the pathophysiology of experimental parkinsonism.

Nitric oxide in neurodegeneration: potential benefits of non-steroidal anti-inflammatories

The cellular messenger nitric oxide (NO) has been linked to neurodegenerative disorders due to the increased expression of the enzymes that catalyze its synthesis in postmortem tissues derived from sufferers of these diseases. Nitrated proteins have also been detected in these samples, revealing that NO is biologically active in regions damaged during neurodegeneration. Modulation of NO levels has been reported not only in the neurons of the central nervous system, but also in the glial cells (microglia and astroglia) activated during the neuroinflammatory response. Neuroinflammation has been found in some neurodegenerative conditions, and inhibition of these neuroinflammatory signals has been shown to delay the progress of such disorders. Thus NO and the pathways triggering its release are emerging as an important research focus in the search for strategies to prevent, halt or cure neurodegenerative diseases.

Critical role of nitric oxide on nicotine-induced hyperactivation of dopaminergic nigrostriatal system: Electrophysiological and neurochemical evidence in rats

Nicotine, the main psychoactive ingredient in tobacco, stimulates dopamine (DA) function, increasing DA neuronal activity and DA release. DA is involved in both motor control and in the rewarding and reinforcing effects of nicotine; however, the complete understanding of its molecular mechanisms is yet to be attained. Substantial evidence indicates that the reinforcing properties of drugs of abuse, including nicotine, can be affected by the nitric oxide (NO) system, which may act by modulating central dopaminergic function. In this study, using single cell recordings in vivo coupled with microiontophoresis and microdialysis in freely moving animals, the role of NO signaling on the hyperactivation elicited by nicotine of the nigrostriatal system was investigated in rats. Nicotine induced a dose-dependent increase of the firing activity of the substantia nigra pars compacta (SNc) DA neurons and DA and 3,4-dihydroxyphenylacetic acid (DOPAC) release in the striatum. Pharmacological manipulation of the NO system did not produce any change under basal condition in terms of neuronal discharge and DA release. In contrast, pretreatments with two NO synthase (NOS) inhibitors, N-omega-nitro-l-arginine methyl ester (l-NAME) and 7-nitroindazole (7-NI) were both capable of blocking the nicotine-induced increase of SNc DA neuron activity and DA striatal levels. The effects of nicotine in l-NAME and 7-NI-pretreated rats were partially restored when rats were pretreated with the NO donor molsidomine. These results further support the evidence of an important role played by NO on modulation of dopaminergic function and drug addiction, thus revealing new pharmacological possibilities in the treatment of nicotine dependence and other DA dysfunctions.