Antioxidant properties of bromocriptine, a dopamine agonist

Therapeutic Potential of Dopamine and Related Drugs as Anti-Inflammatories and Antioxidants in Neuronal and Non-Neuronal Pathologies

Dopamine (DA), its derivatives, and dopaminergic drugs are compounds widely used in the management of diseases related to the nervous system. However, DA receptors have been identified in nonneuronal tissues, which has been related to their therapeutic potential in pathologies such as sepsis or septic shock, blood pressure, renal failure, diabetes, and obesity, among others. In addition, DA and dopaminergic drugs have shown anti-inflammatory and antioxidant properties in different kinds of cells.

AIM

To compile the mechanism of action of DA and the main dopaminergic drugs and show the findings that support the therapeutic potential of these molecules for the treatment of neurological and non-neurological diseases considering their antioxidant and anti-inflammatory actions.

METHOD

We performed a review article. An exhaustive search for information was carried out in specialized databases such as PubMed, PubChem, ProQuest, EBSCO, Scopus, Science Direct, Web of Science, Bookshelf, DrugBank, Livertox, and Clinical Trials.

RESULTS

We showed that DA and dopaminergic drugs have emerged for the management of neuronal and nonneuronal diseases with important therapeutic potential as anti-inflammatories and antioxidants.

CONCLUSIONS

DA and DA derivatives can be an attractive treatment strategy and a promising approach to slowing the progression of disorders through repositioning.

Amelioration of oxidative stress utilizing nanoemulsion loaded with bromocriptine and glutathione for the management of Parkinson's disease

Parkinson's disease (PD) is triggered by the formation of free radicals in dopaminergic neurons, which results in oxidative stress-induced neurodegeneration. The objective of the work was to relieve oxidative stress by employing intranasal delivery of Bromocriptine Mesylate (BRM) and Glutathione (GSH) loaded nanoemulsion for the better management of PD. The depth of permeation of the nanoemulsion was assessed through confocal laser scanning microscopy (CLSM) which revealed higher nanoemulsion permeation in contrast to suspension. Biocompatibility of nanoemulsion was confirmed by nasal cilio toxicity study. The DPPH study showed that the nanoemulsion had significant antioxidant activity. Biochemical estimation studies in Wistar rats were carried out in order to determine the effect of nanoemulsion on oxidative stress. The levels of GSH, superoxide dismutase (SOD), and catalase (CAT) were significantly enhanced; and the level of thiobarbituric acid reactive substances (TBARS) was significantly reduced after the intranasal administration of nanoemulsion in the haloperidol-induced model of PD. Furthermore, the levels of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) were also determined which reduced significantly after the administration of nanoemulsion. The oxidative stress levels were lowered with nanoemulsion, showing the combined antioxidant capability of BRM and GSH. The neuroprotective effect of the prepared nanoemulsion was confirmed by histopathological studies. Pharmacokinetic study revealed a higher concentration of BRM and GSH in the brain of Wistar rats after intranasal administration of nanoemulsion with a higher Brain/Plasma ratio. A higher value of AUC_(0-8) of nanoemulsion in the brain after intranasal administration revealed that BRM and GSH remained in the brain for a longer period due to sustained release from nanoemulsion. According to the findings, BRM and GSH loaded nanoemulsion has the potential to provide a combined and synergistic anti-oxidant effect for efficient management of PD.

CCRD based development of bromocriptine and glutathione nanoemulsion tailored ultrasonically for the combined anti-parkinson effect

Bromocriptine Mesylate (BRM) acts as a dopamine receptor agonist along with antioxidant effect and is utilized in the treatment of Parkinson's disease (PD). Glutathione (GSH) is a thiol- reducing agent having antioxidant properties in the brain. Replenishment of GSH inside the brain can play a major role in the management of PD. Both BRM and GSH suffer from low oral bioavailability and poor absorption. The objective of the present study was to develop BRM and GSH loaded nanoemulsion for the combined and synergistic effect delivered through the intranasal route for the better and effective management of PD. After extensive screening experiments, Capmul PG-8 NF was selected as oil, polyethylene glycol (PEG) 400 as a surfactant and propylene glycol as co-surfactant. Ultrasonication technique was employed for the fabrication of nanoemulsion. Central composite rotatable design (CCRD) was used to obtain the best formulation by optimization. Oil (%), S_mix (%), and sonication time (second) were chosen as independent variables for the optimization. Particle size, PDI, zeta potential, % transmittance, pH, refractive index, viscosity and conductivity of the optimized nanoemulsion were found to be 80.71 ± 2.75 nm, 0.217 ± 0.009, -12.60 ± 0.10 mV, 96.00 ± 3.05 %, 6.48 ± 0.28, 1.36 ± 0.03, 30.12 ± 0.10 mPas and 214.28 ± 2.79 μS/cm respectively. Surface morphology demonstrated that nanoemulsion possessed spherical and globular nature of the particle which showed 3.4 times and 1.5 times enhancement in drug permeation in the case of BRM and GSH respectively as compared to suspension. MTT assay done on neuro-2a cell lines revealed that nanoemulsion was safe for intranasal delivery. Behavioural studies were carried out to prove the efficacy of optimized nanoemulsion in PD using forced swimming test, locomotor activity test, catalepsy test, rota-rod test, and akinesia test in Wistar rats. The outcomes of the behavioural studies revealed that BRM and GSH loaded nanoemulsion treatment showed significant improvement in behavioural activities of PD (haloperidol-induced) rats after intranasal administration. This study concluded that BRM and GSH loaded nanoemulsion could be promising for the combined and synergistic anti-parkinson effect for the effective management of PD.

Repurposing of Bromocriptine for Cancer Therapy

Bromocriptine is an ergot alkaloid and dopamine D₂ receptor agonist used to treat Parkinson's disease, acromegaly, hyperprolactinemia, and galactorrhea, and more recently diabetes mellitus. The drug is also active against pituitary hormone-dependent tumors (prolactinomas and growth-hormone producing adenomas). We investigated, whether bromocriptine also inhibits hormone-independent and multidrug-resistant (MDR) tumors. We found that bromocriptine was cytotoxic towards drug-sensitive CCRF-CEM, multidrug-resistant CEM/ADR5000 leukemic cells as well as wild-type or multidrug-resistant ABCB5-transfected HEK293 cell lines, but not sensitive or BCRP-transfected multidrug-resistant MDA-MB-231 breast cancer cells. Bromocriptine strongly bound to NF-κB pathway proteins as shown by molecular docking and interacted more strongly with DNA-bound NF-κB than free NF-κB, indicating that bromocriptine may inhibit NF-κB binding to DNA. Furthermore, bromocriptine decreased NF-κB activity by a SEAP-driven NF-κB reporter cell assay. The expression of MDR-conferring ABC-transporters (ABCB1, ABCB5, ABCC1, and ABCG2) and other resistance-mediating factors (EGFR, mutated TP53, and IκB) did not correlate with cellular response to bromocriptine in a panel of 60 NCI cell lines. There was no correlation between cellular response to bromocriptine and anticancer drugs usually involved in MDR (e.g., anthracyclines, Vinca alkaloids, taxanes, epipodophyllotoxins, and others). COMPARE analysis of microarray-based mRNA expression in these cell lines revealed that genes from various functional groups such as ribosomal proteins, transcription, translation, DNA repair, DNA damage, protein folding, mitochondrial respiratory chain, and chemokines correlated with cellular response to bromocriptine. Our results indicate that bromocriptine inhibited drug-resistant tumor cells with different resistance mechanisms in a hormone-independent manner. As refractory and otherwise drug-resistant tumors represent a major challenge to successful cancer chemotherapy, bromocriptine may be considered for repurposing in cancer therapy.

Socially-induced variation in physiological mediators of parental care in a colonial bird

Social facilitation of reproduction occurs in humans and animals, and may represent one of the bases of reproduction in groups. However, its underlying physiological mechanisms remain largely unexplored. Here, we found in a colonial bird, the Adélie penguin (Pygoscelis adeliae), that the number of parental interactions (nest relief ceremonies) performed by breeding individuals on the colony was positively related to prolactin levels in other breeding individuals exposed to these interactions (i.e. focal individuals). As prolactin is typically involved in the expression of parental behaviour in birds, this suggests that parental interactions by conspecifics represent social cues that might increase parental motivation in focal individuals. Moreover, parental interactions were not related to corticosterone levels in focal individuals, suggesting that these social cues were not stressful for penguins. However, social stimulation still had a cost for focal individuals, as it was negatively related to their antioxidant defences (a component of self-maintenance). As social stimulation was also positively related to prolactin levels, this highlights the fact that social stimulation acts on the trade-off between reproduction and self-maintenance. For the first time, the results of the current study shed light on the physiological factors potentially underlying social facilitation of parental care. Importantly, they suggest that, even though social facilitation of parental care may increase breeding performance, it can also negatively affect other fitness components.

Therapeutics in the Neurorehabilitation of Parkinson's Disease

Parkinson's disease (PD) affects 1 percent of the population over the age of 65. The number of people with this disorder is steadily rising. Therapy for PD remains primarily pharmacologic, with medications that target the depleted dopaminergic system being the mainstay of therapy. Surgical therapies, both ablative and stimulatory, are in creasingly being used for patients with more advanced disease and/or complications of drug therapy. Experimental therapies aimed at restoring dopaminergic function and protecting dopaminergic cells are being studied. Alternate neurotransmitter systems are being evaluated as potential targets for therapy. Complete treatment of patients with PD utilizes education, physical therapy, support groups, and medication. When a comprehensive approach is used, PD is treatable and manageable.

Bromocriptine Mesylate Attenuates Amyotrophic Lateral Sclerosis: A Phase 2a, Randomized, Double-Blind, Placebo-Controlled Research in Japanese Patients

OBJECTIVE

Bromocriptine mesylate (BRC), a dopamine D2 receptor agonist has been shown to confer neuroprotection, sustained motor function and slowed disease progression in mouse models of amyotrophic lateral sclerosis (ALS) Here we report a first in human trial in ALS.

DESIGN

A multicenter, Riluzole add-on, randomized, double-blind, placebo controlled 102-week extension BRC clinical trial.

METHODS

The trial was conducted between January 2009 and March 2012 on 36 Japanese ALS patients. A 12-week treatment with Riluzole observational period was followed by combined treatment (Riluzole + BRC; n = 29 or Riluzole + placebo; n = 7). The dosing commenced at 1.25 mg/day increasing in steps at two weeks intervals to a maximum of 15 mg/day. The efficacy of BRC was evaluated by comparing BRC and placebo groups upon completion of stepwise dosing at 14 weeks 2 points (1st endpoint) and upon completion or discontinuation of the study (2nd endpoint) of the dosing.

RESULTS

Statistics analyses revealed a marginal BRC treatment efficacy with P≦20%to placebo by 1st and 2nd endpoint analysis. In the 1st endpoint analysis, BRC group was significantly effective on the scores of ALSAQ40-communicaton (P = 1.2%), eating and drinking (P = 2.2%), ALSFRS-R total (P = 17.6%), grip strength (P = 19.8%) compared to the placebo group. In the 2nd endpoint analysis, differences between the scores of Limb Norris Scale (P = 18.3%), ALSAQ40-communication (P = 11.9%), eating and drinking (P = 13.6%), and neck forward-bent test (P = 15.4%) of BRC group were detected between the two groups. There was no significant difference between the treatment groups for adverse events or serious drug reactions incidence.

CONCLUSIONS

BRC sustains motoneuronal function at least in part through BRC treatment. Further analysis involving a Phase 2b or 3 clinical trial is required but BRC currently shows promise for ALS treatment.

TRIAL REGISTRATION

UMIN Clinical Trials UMIN000008527.

Effect of bromocriptine alginate nanocomposite (BANC) on a transgenic Drosophila model of Parkinson's disease

The effect of bromocriptine, a dopamine agonist, administered in the form of bromocriptine alginate nanocomposite (BANC) was studied on Parkinson's disease (PD) model flies. The synthesized BANC was subject to characterization and, at a final concentration of 0.5, 1.0 and 1.5 µM, was mixed in diet. The PD flies were allowed to feed on it for 24 days. A significant dose-dependent delay in the loss of climbing activity and activity pattern was observed in PD flies exposed to 0.5, 1.0 and 1.5 µM BANC. The PD flies exposed to BANC also showed a significant reduction in lipid peroxidation and glutathione-S-transferase activity, and an increase in glutathione content. However, no gross morphological changes were observed in the brains of PD flies compared with controls. The results suggest that BANC is effective in reducing the PD symptoms in these transgenic flies.

Summary: The results suggest that the bromocriptine alginate nanocomposite is potent in reducing the symptoms of Parkinson's disease in a transgenic fly model of the disease.

Divergent long-term consequences of chronic treatment with haloperidol, risperidone, and bromocriptine on traumatic brain injury-induced cognitive deficits

Antipsychotic drugs (APDs) are provided in the clinic to manage traumatic brain injury (TBI)-induced agitation and aggression. Experimental TBI studies consistently show that daily administration of the APDs, haloperidol (HAL) and risperidone (RISP), hinder recovery. However, it is unknown how long the adverse effects remain after cessation of treatment. To elucidate this clinically relevant issue, anesthetized male rats were randomly assigned to four TBI (controlled cortical impact) and four sham groups administered HAL (0.5 mg/kg), RISP (0.45 mg/kg), bromocriptine (BRO; 5.0 mg/kg, included as a control for D2 receptor action), or vehicle (VEH; 1 mL/kg) 24 h after surgery and once-daily for 19 days. Motor and cognitive recovery was assessed on days 1-5 and 14-19, respectively, and again at 1 and 3 months after drug withdrawal. No overall group differences were observed for motor function among the TBI groups, although the HAL group showed a greater beam-walk deficit on day 5 versus the VEH and BRO groups. Cognitive recovery was significantly impaired in the HAL and RISP groups during the treatment phase versus VEH and BRO. Further, BRO was superior to VEH (p=0.0042). At 1 month, both groups that received APDs continued to exhibit significant cognitive impairment versus VEH and BRO; at 3 months, only the HAL group was impaired. Moreover, the HAL, RISP, and VEH groups continued to be cognitively deficient versus BRO, which also reduced cortical damage. These data replicate previous reports that HAL and RISP impede cognitive recovery after TBI and expand the literature by revealing that the deleterious effects persist for 3 months after drug discontinuation. BRO conferred cognitive benefits when administered concomitantly with behavioral testing, thus replicating previous findings, and also after cessation demonstrating enduring efficacy.

Synergistic inhibition of lipid peroxidation by vitamin E and a dopamine agonist, cabergoline

We examined antioxidant activity of cabergoline, a dopamine agonist, during the aerobic oxidation of phosphatidylcholine liposomes at 37 degrees C. Cabergoline retarded the oxidation initiated with a lipid-soluble initiator significantly better than that with a water-soluble initiator, suggesting that cabergoline locates in the lipid layer of liposomal membranes. Cabergoline inhibited the oxidation of liposomal membranes synergistically with endogenous antioxidants such as ascorbic acid, ubiquinol-10 and vitamin E, and vitamin E was the most efficient synergist. These results suggest that cabergoline may have a neuroprotective effect on the substantia nigra of Parkinsonian patients because of its synergistic antioxidant activity with vitamin E as well as its action on dopamine receptor.

Bromocriptine inhibits adipogenesis and lipogenesis by agonistic action on α2-adrenergic receptor in 3T3-L1 adipocyte cells

The primary goals of the present study were to investigate the inhibitory effects of bromocriptine (BC) on adipogenesis and lipogenesis in 3T3-L1 adipocyte cells as well as to elucidate its molecular mechanism of action. Adipogenic and lipogenic capacity of BC-treated cells was evaluated by oil red-O staining, triglyceride content assay, real-time RT-PCR and immunoblotting. To determine the mechanism responsible for the anti-obesity effect of BC, we applied two methods. Firstly, we knocked down dopamine D2 receptor (D2R) up to 50% using siRNA. Secondly, we blocked the activity of α2-adrenergic receptor (α2-AR) by yohimbine treatment and monitored its effects on adipogenic and lipogenic events in 3T3-L1 cells. BC decreased the expression levels of adipogenic activators, including Pparα, Pparγ, and Cebpα, as well as major lipogenic target genes, including Me1, Acc1, 6Pgd, Fasn, and Prkaa1. Moreover, BC markedly reduced intracellular nitric oxide formation in a dose-dependent manner and expression of pro-inflammatory genes, Tnfα and Il6, which reflects attenuated pro-inflammatory responses. Further, upon treatment with BC, D2R-deficient cells displayed a significant decrease in lipogenic activity compared to control cells, whereas yohimbine-treated cells exhibited no reduction in lipogenic activity. BC can effectively attenuate adipogenesis and lipogenesis in 3T3-L1 cells by downregulating the expression of lipogenic genes and proteins. Our current experimental data collectively establish that the anti-obesity effects of BC are not D2R-dependent but result from the action of α2-AR in 3T3-L1 adipocytes.

Recent advances on the neuroprotective potential of antioxidants in experimental models of Parkinson's disease

Parkinson’s disease (PD), a neurodegenerative movement disorder of the central nervous system (CNS) is characterized by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta region of the midbrain. Although the etiology of PD is not completely understood and is believed to be multifactorial, oxidative stress and mitochondrial dysfunction are widely considered major consequences, which provide important clues to the disease mechanisms. Studies have explored the role of free radicals and oxidative stress that contributes to the cascade of events leading to dopamine cell degeneration in PD. In general, in-built protective mechanisms consisting of enzymatic and non-enzymatic antioxidants in the CNS play decisive roles in preventing neuronal cell loss due to free radicals. But the ability to produce these antioxidants decreases with aging. Therefore, antioxidant therapy alone or in combination with current treatment methods may represent an attractive strategy for treating or preventing the neurodegeneration seen in PD. Here we summarize the recent discoveries of potential antioxidant compounds for modulating free radical mediated oxidative stress leading to neurotoxicity in PD.

Bromocriptine methylate suppresses glial inflammation and moderates disease progression in a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterized by a selective loss of upper and lower motor neurons. Since oxidative stress plays a crucial role in the progression of motor neuron loss observed in ALS, anti-oxidative agents could be an important therapeutic means for the ALS treatment. We have previously developed a drug screening system allowing the identification of small chemical compounds that upregulate endogenous neuronal apoptosis inhibitory protein (NAIP), an oxidative stress-induced cell death suppressor. Using this system, we identified the dopamine D2 receptor agonist bromocriptine (BRC) as one of NAIP-upregulating compounds. In this study, to prove the efficacy of BRC in ALS, we conducted a set of preclinical studies using a transgenic ALS mouse model carrying the H46R mutation in the human Cu/Zn superoxide dismutase (SOD1) gene ALS(SOD1(H46R)) by the post-onset administration of BRC. ALS(SOD1(H46R)) mice receiving BRC showed sustained motor functions and modest prolonged survival after onset. Further, BRC treatment delayed anterior horn cell loss, and reduced the number of reactive astrocytes and the level of inflammatory factors such as inducible nitric oxide synthase (iNOS) and tumor necrosis factor (TNF)-α in the spinal cord of late symptomatic mice. In vitro study showed the reduced level of extracellular TNF-α after lipopolysaccharide (LPS) exposure in BRC-treated mouse astrocytes. BRC-treated ALS(SOD1(H46R)) mice also showed a reduced level of oxidative damage in the spinal cord. Notably, BRC treatment resulted in an upregulation of anti-oxidative stress genes, activating transcription factor 3 (ATF3) and heme oxygenase-1 (HO-1), and the generation of a glutathione (GSH) in SH-SY5Y cultured neuronal cells in a dopamine receptor-independent manner. These results imply that BRC protects motor neurons from the oxidative injury via suppression of astrogliosis in the spinal cord of ALS(SOD1(H46R)) mice. Thus, BRC might be a promising therapeutic agent for the treatment of ALS.

Dopamine receptors and Parkinson's disease

Parkinson's disease (PD) is a progressive extrapyramidal motor disorder. Pathologically, this disease is characterized by the selective dopaminergic (DAergic) neuronal degeneration in the substantia nigra. Correcting the DA deficiency in PD with levodopa (L-dopa) significantly attenuates the motor symptoms; however, its effectiveness often declines, and L-dopa-related adverse effects emerge after long-term treatment. Nowadays, DA receptor agonists are useful medication even regarded as first choice to delay the starting of L-dopa therapy. In advanced stage of PD, they are also used as adjunct therapy together with L-dopa. DA receptor agonists act by stimulation of presynaptic and postsynaptic DA receptors. Despite the usefulness, they could be causative drugs for valvulopathy and nonmotor complication such as DA dysregulation syndrome (DDS). In this paper, physiological characteristics of DA receptor familyare discussed. We also discuss the validity, benefits, and specific adverse effects of pharmaceutical DA receptor agonist.

Bromocriptine, an ergot alkaloid, inhibits excitatory amino acid release mediated by glutamate transporter reversal

Bromocriptine, a dopamine D(2) receptor agonist, has widely been used for patients with Parkinson's disease. The aim of the present study was to investigate the effect of bromocriptine on glutamate transporter. Since the astroglial glutamate transporter GLT-1 (EAAT2) is the predominant isoform in the forebrain, we generated EAAT2-expressing human embryonic kidney cells and immortalized mouse astrocytes. In the present studies, we observed a GLT-1-immunoreactive band and significant Na(+)-dependent d-[(3)H] aspartate uptake. Furthermore, the glutamate transporter inhibitors, dl-threo-beta-benzyloxyaspartic acid (TBOA) and dihydrokainate (DHK), displayed a dose-dependent reduction of d-[(3)H] aspartate uptake in both types of cells. In contrast, cells exposed to either chemical anoxia or high KCl elicited a marked release of d-[(3)H] aspartate, and the release was inhibited by TBOA and DHK, implying the contribution of glutamate transporter reversal. Interestingly, we found that bromocriptine dose-dependently inhibits d-[(3)H] aspartate release elicited by chemical anoxia or high KCl, while no changes occurred in the uptake. The inhibitory action of bromocriptine was not affected by sulpiride, a dopamine D(2) receptor antagonist. On the other hand, bromocriptine had no effect on swelling-induced d-[(3)H] aspartate release, which is mediated by volume-regulated anion channels. In vivo studies revealed that bromocriptine suppresses the excessive elevation of glutamate levels in gerbils subjected to transient forebrain ischemia in a manner similar to DHK. Taken together, these results provide evidence that bromocriptine inhibits excitatory amino acid release via reversed operation of GLT-1 without altering forward transport.

Bromocriptine treatment in a murine parkinson's model: Ultrastructural evaluation after dopaminergic deafferentation

The objective of this article was to identify the effects of bromocriptine on the ultrastructure of the caudate nucleus in rats with a 6-hydroxidopamine (6-OHDA) unilateral lesion of the substantia nigra pars compacta. Eighteen Wistar male rats were stereotactically lesioned with 6-OHDA (n=12), or sham lesioned (n=6). Two days after rotational behavior was tested, and 2 days later, 6 rats were treated with 0.3 mg/Kg bromocriptine orally for a month and 6 rats were kept for the same time without treatment. The neuropile of the sham operated and bromocriptine-treated rats was well preserved contrary to the non-bromocriptine-treated rats. Also, it was found that there was a significant difference in the number of synaptic endings with edema in caudate of bromocriptine-treated rats compared with non-treated rats; however, the size of the synaptic endings were different to those found in the sham lesioned rats. Also, as in the sham lesioned group, the bromocriptines showed more synaptic contacts with dendritic spines contrasting to the non-treated group. The results suggest that bromocriptine possesses antioxidant properties because it decreased the ultrastructural alterations after 6-OHDA lesion.

Protective effect of bromocriptine against BH4-induced Cath.a cell death involving up-regulation of antioxidant enzymes

Previously, we suggested that tetrahydrobiopterin (BH4), an obligatory cofactor for dopamine synthesis, as an intrinsic contributor to dopaminergic neuron vulnerability. The BH4 toxicity is observed in dopamine-producing cells, including Cath.a cells, but not in non-dopaminergic cells. Furthermore, the dopaminergic cell death induced by BH4 is apoptotic in nature and involves oxidative stress, similar to that observed in Parkinson's disease. Accordingly, various antioxidants have been found to protect dopaminergic cells from BH4. This study was undertaken to evaluate protective effects of the dopamine receptor agonist bromocriptine on BH4-induced Cath.a cell death, because bromocriptine has been reported to be an antioxidant with a neuroprotective activity. In the presence of bromocriptine, the increase in LDH activity and mitochondrial cytochrome c release induced by BH4 were significantly abolished. This cytoprotective effect was phosphatidylinositol 3-kinase (PI3K)/Akt pathway-dependent. In addition, bromocriptine was found to up-regulate the expressions of nuclear factor-E2-related factor-2 and antioxidant enzymes including NAD(P)H quinone oxidoreductase 1. Our findings show that bromocriptine stimulates antioxidant defense mechanisms in Cath.a cells and suggest a potential use of bromocriptine as a neuroprotectant.

Antioxidant effects of some drugs on ethanol-induced ulcers

The aim of this work was to investigate the antioxidant potential of some commonly used drugs (bromocriptine, haloperidol and azithromycin) on alcohol-induced ulcers in the rat. The following parameters were determined: content of reduced glutathione, activities of catalase, xanthine oxidase, glutathione reductase, glutathione peroxidase, peroxidase, and lipid peroxidation intensity. A battery of biochemical assays were used and the resulting data was statistically analyzed. Alcohol stress caused gastric ulcerations and hemorrhages and changed all the examined parameters except glutathione peroxidase activity. All drugs reduced the ulcer index and hemorrhages, with azithromycin showing the strongest effects. The drugs in combination with alcohol showed different effects on biochemical parameters. Our results indicate that the gastroprotective effects of the investigated drugs on experimental lesions induced by 100% ethanol could not be correlated with their antioxidative properties.

Effects of various drugs on alcohol-induced oxidative stress in the liver

The major aim of this work was to investigate how alcohol-induced oxidative stress in combined chemotherapy changes the metabolic function of the liver in experimental animals. This research was conducted to establish how bromocriptine, haloperidol and azithromycin, applied to the experimental model, affected the antioxidative status of the liver. The following parameters were determined: reduced glutathione, activities of glutathione peroxidase, glutathione reductase, peroxidase, catalase, xanthine oxidase and lipid peroxidation intensity. Alanine transaminase was measured in serum. Alcohol stress (AO group) reduced glutathione and the activity of xanthine oxidase and glutathione peroxidase, but increased catalase and alanine transaminase activity. The best protective effect was achieved with the bromocriptine (AB1 group), while other groups had similar effects on the studied parameters.

Contrasting effects of bromocriptine on learning of a partially baited radial arm maze task in the presence and absence of restraint stress

RATIONALE

Severe, traumatic stress or repeated exposure to stress can result in long-term deleterious effects, including hippocampal cell atrophy and death, which, in turn, result in memory impairments and behavioural abnormalities. The dopaminergic D(2) receptor agonist, bromocriptine, has been shown to modulate learning, and chronic stress is associated with dopaminergic dysfunction.

OBJECTIVES

In the present study, we evaluated the effects of bromocriptine in the presence or absence of restraint stress.

MATERIALS AND METHODS

Adult male Wistar rats were subjected to restraint stress for 21 days (6 h/day) followed by bromocriptine treatment, and learning was assessed in the partially baited radial arm maze task. In a separate group of animals, the effects of bromocriptine per se was evaluated. Dopamine levels were estimated by high-performance liquid chromatography with electrochemical detection.

RESULTS

Stressed rats showed impairment in both acquisition and retention of the radial arm maze task, and bromocriptine treatment after stress showed a reversal of stress-induced impairment. Interestingly, in the absence of stress, bromocriptine exhibited dose-dependent differential effects on learning. While rats treated with bromocriptine 5 mg/kg, i.p., demonstrated impairment in learning, the bromocriptine 10 mg/kg and vehicle-treated groups did not differ from normal controls. To understand the neurochemical basis for the effects of bromocriptine, dopamine levels were estimated. The stress-induced decrease in dopamine levels in the hippocampus and frontal cortex were restored by bromocriptine treatment. In contrast, bromocriptine alone (5 mg/kg, i.p.) decreased dopamine levels in the frontal cortex and striatum.

CONCLUSIONS

Our study shows that amelioration of stress-induced learning impairment correlates with restoration of dopamine levels by bromocriptine treatment.

Proteomic analysis of striatal proteins in the rat model of l-DOPA-induced dyskinesia

L-DOPA-induced dyskinesia (LID) is among the motor complications that arise in Parkinson's disease (PD) patients after a prolonged treatment with L-DOPA. To this day, transcriptome analysis has been performed in a rat model of LID [Neurobiol. Dis., 17 (2004), 219] but information regarding the proteome is still lacking. In the present study, we investigated the changes occurring at the protein level in striatal samples obtained from the unilaterally 6-hydroxydopamine-lesion rat model of PD treated with saline, L-DOPA or bromocriptine using two-dimensional difference gel electrophoresis and mass spectrometry (MS). Rats treated with L-DOPA were allocated to two groups based on the presence or absence of LID. Among the 2000 spots compared for statistical difference, 67 spots were significantly changed in abundance and identified using matrix-assisted laser desorption/ionization time-of-flight MS, atmospheric pressure matrix-assisted laser desorption/ionization and HPLC coupled tandem MS (LC/MS/MS). Out of these 67 proteins, LID significantly changed the expression level of five proteins: alphabeta-crystalin, gamma-enolase, guanidoacetate methyltransferase, vinculin, and proteasome alpha-2 subunit. Complementary techniques such as western immunoblotting and immunohistochemistry were performed to investigate the validity of the data obtained using the proteomic approach. In conclusion, this study provides new insights into the protein changes occurring in LID.

Bromocriptine reduces steatosis in obese rodent models

BACKGROUND/AIMS

Obesity is a risk factor for glucose intolerance, steatosis, and oxidative stress, characteristics of nonalcoholic fatty liver disease. Bromocriptine may have anti-obesity, insulin-sensitizing, lipolytic, and antioxidant properties. We, therefore, hypothesized that bromocriptine would improve markers of nonalcoholic fatty liver disease in obese rodent models.

METHODS

We performed a randomized, controlled experiment in genetically obese fatty Zucker rats and diet-induced obese rats to assess for behavioral and peripheral anti-obesity actions of bromocriptine (10mg/kg) that would improve nonalcoholic fatty liver disease.

RESULTS

Behaviorally, food intake decreased and locomotor activity increased in bromocriptine-treated fatty Zucker and dietary-induced obese rats. Peripherally, liver triglycerides were significantly reduced and hepatic manganese superoxide dismutase significantly increased in bromocriptine-treated fatty Zucker and diet-induced obese rats compared to controls. Blood glucose was significantly lower in bromocriptine-treated Zucker rats compared to fatty controls and was no different than that of lean controls.

CONCLUSIONS

Improvements in obesigenic behaviors, glucose tolerance, hepatic lipid accumulation, and mitochondrial oxidative stress observed in genetically obese and diet-induced obese rodents indicate that bromocriptine may be promising as a broad-based therapy for nonalcoholic fatty liver disease.

Effect of dopamine on rat diaphragm apoptosis and muscle performance

The purpose of this study was to determine whether dopamine (DA) decreases diaphragm apoptosis and attenuates the decline in diaphragmatic contractile performance associated with repetitive isometric contraction using an in vitro diaphragm preparation. Strenuous diaphragm contractions produce free radicals and muscle apoptosis. Dopamine is a free radical scavenger and, at higher concentrations, increases muscle contractility by simulating beta2-adrenoreceptors. A total of 47 male Sprague-Dawley rats weighing 330-450 g were used in a prospective, randomized, controlled in vitro study. Following animal anaesthetization, diaphragms were excised, and muscle strips prepared and placed in a temperature-controlled isolated tissue bath containing Krebs-Ringer solution (KR) or KR plus 100 microm DA. The solutions were equilibrated with oxygen (O2) at 10, 21 or 95% and 5% carbon dioxide, with the balance being nitrogen. Diaphragm isometric twitch and subtetanic contractions were measured intermittently over 65 min. The diaphragms were then removed and, using a nuclear differential dye uptake method, the percentages of normal, apoptotic and necrotic nuclei were determined using fluorescent microscopy. There were significantly fewer apoptotic nuclei in the DA group diaphragms than in the KR-only group diaphragms in 10 and 21% O2 following either twitch or subtetanic contractions. Dopamine at 100 microm produced only modest increases in muscle performance in both 10 and 21% O2. The attenuation of apoptosis by DA was markedly greater than the effect of DA on muscle performance. Dopamine decreased diaphragmatic apoptosis, perhaps by preventing the activation of intricate apoptotic pathways, stimulating antiapoptotic mechanisms and/or scavenging free radicals.

A dopamine D4 receptor antagonist attenuates ischemia-induced neuronal cell damage via upregulation of neuronal apoptosis inhibitory protein

Neuronal apoptosis inhibitory protein (NAIP/BIRC1), the inhibitor of apoptosis protein (IAP) family member, suppresses neuronal cell death induced by a variety of insults, including cell death from ischemia and stroke. The goal of the present study was to develop an efficient method for identification of compounds with the ability to upregulate endogenous NAIP and to determine the effects on these compounds on the cellular response to ischemia. A novel NAIP-enzyme-linked immunosorbent assay (ELISA)-based in vitro drug-screening system is established. Use of this system identified an antagonist of dopamine D4 receptor, termed L-745,870, with a potent NAIP upregulatory effect. L-745,870-mediated NAIP upregulation in neuronal and nonneuronal cultured cells resulted in decreased vulnerability to oxidative stress-induced apoptosis. Reducing NAIP expression via RNA interference techniques resulted in prevention of L-745,870-mediated protection from oxidative stress. Further, systemic administration of L-745,870 attenuated ischemia-induced damage of the hippocampal CA1 neurons and upregulated NAIP expression in the rescued hippocampal CA1 neurons in a gerbil model. These data suggest that the NAIP upregulating compound, L-745,870, has therapeutic potential in acute ischemic disorders and that our NAIP-ELISA-based drug screening may facilitate the discovery of novel neuroprotective compounds.

Bromocriptine reduces lipid peroxidation and enhances spatial learning and hippocampal neuron survival in a rodent model of focal brain trauma

Oxidative stress is a significant contributor to the secondary sequelae of traumatic brain injury (TBI), and may mediate subsequent neurobehavioral deficits and histopathology. The present study examined the neuroprotective effects of bromocriptine (BRO), a dopamine D2 receptor agonist with significant antioxidant properties, on cognition, histopathology, and lipid peroxidation in a rodent model of focal brain trauma. BRO (5 mg/kg) or a comparable volume of vehicle (VEH) was administered intraperitoneally 15 min prior to cortical impact or sham injury. In experiment 1, spatial learning was assessed in an established water maze task on post-surgery days 14-18, followed by quantification of hippocampal cell survival and cortical lesion volume at 4 weeks. In experiment 2, rats were sacrificed 1 hr post-surgery, and malondialdehyde (MDA), the end product of lipid peroxidation, was measured in the frontal cortex, striatum, and substantia nigra using a thiobarbituric acid reactive substances assay. The TBI+BRO group was significantly more adept at locating a hidden platform in the water maze compared to the TBI+VEH group and also exhibited a greater percentage of surviving CA3 hippocampal neurons. TBI increased MDA in all examined regions of the VEH-treated, but not BRO-treated group versus SHAMs. MDA was significantly decreased in both the striatum (4.22 +/- 0.52 versus 5.60 +/- 0.44 nmol per mg/tissue +/- SEM) and substantia nigra (4.18 +/- 0.35 versus 7.76 +/- 2.05) of the TBI+BRO versus TBI+VEH groups, respectively, while only a trend toward decreased MDA was observed in the frontal cortex (5.44 +/- 0.44 versus 6.96 +/- 0.77). These findings suggest that TBI-induced oxidative stress is attenuated by acute BRO treatment, which may, in part, explain the benefit in cognitive and histological outcome.

Dopamine D2 and D3 receptor agonists limit oligodendrocyte injury caused by glutamate oxidative stress and oxygen/glucose deprivation

Dopamine receptor activation is thought to contribute adversely to several neuropathological disorders, including Parkinson's disease and schizophrenia. In addition, dopamine may have a neuroprotective role: dopamine receptor agonists are reported to protect nerve cells by virtue of their antioxidant properties as well as by receptor-mediated mechanisms. White matter injury can also be a significant factor in neurological disorders. Using real-time RT-PCR, we show that differentiated rat cortical oligodendrocytes express dopamine D2 receptor and D3 receptor mRNA. Oligodendrocytes were vulnerable to oxidative glutamate toxicity and to oxygen/glucose deprivation injury. Agonists for dopamine D2 and D3 receptors provided significant protection of oligodendrocytes against these two forms of injury, and the protective effect was diminished by D2 and D3 antagonists. Levels of oligodendrocyte D2 receptor and D3 receptor protein, as measured by Western blotting, appeared to increase following combined oxygen and glucose deprivation. Our results suggest that dopamine D2 and D3 receptor activation may play an important role in oligodendrocyte protection against oxidative glutamate toxicity and oxygen-glucose deprivation injury.

Neuroprotection by Pergolide Against Levodopa-Induced Cytotoxicity of Neural Stem Cells

Neural stem cells (NSCs) are currently considered very hopeful candidates for cell replacement therapy in neurodegenerative pathologies such as Parkinson's disease (PD), but like embryonic neural tissue transplantation, levodopa medication may still be required to improve symptoms even after cell transplantation. The issues of whether levodopa induces cytotoxicity and apoptosis of NSCs following transplantation, as well as the means to prevent these processes from occurring remain to be elucidated. In this study, the possible cytotoxicity of levodopa at different doses on C17.2 neural stem cells and subsequent neuroprotection by pergolide were investigated. The cell viability was determined by the MTT assay. Cell proliferation was assayed by BrdU labeling, while apoptosis was detected by Annexin-V-FLUOS staining and flow cytometry. Levels of p53, Bax, Bcl-2, NFkB, cytochrome c, caspase-3 as well as cleavage of caspase-3 were measured by western blotting. We found levodopa induced a concentration- and time-dependent decrease in cell viability and proliferation. Apoptotic cells were observed at different stages, specifically 12 and 24 h following exposure to levodopa (200 microM). Elevated p53, Bax, cytochrome c, caspase-3 and active fragments of caspase-3 protein were observed in the cells exposed to levodopa. These alterations were partly inhibited by pergolide, a dopamine receptor agonist, while Bcl-2 and NFkB p65 levels remained constant at the various time-points in all the groups examined. These observations indicate that levodopa at high concentrations (> or = 200 microM) was neurotoxic to C17.2 neural stem cells via inhibition of DNA synthesis and cell proliferation. Activation of the mitochondria-dependent pathway and caspase-3 protease may contribute to the mechanism by which levodopa induces apoptosis. Pergolide, an anti-Parkinson drug, has a neuroprotective effect and partly blocks levodopa-induced cytotoxicity.

Neuroprotective and neurotrophic effect of apomorphine in the striatal 6-OHDA-lesion rat model of Parkinson's disease

We investigated the possible neuroprotective effect of the dopamine (DA) receptor agonist R-apomorphine (R-APO) within the striatal 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease. In one group of rats, R-APO administration (10 mg/kg/day, s.c.) started 15 min before 6-OHDA-injection. In a second group, R-APO administration started 24 h after lesion induction. Both groups received R-APO chronically for 11 days. Testing was carried out 2 weeks post-lesioning. R-APO treatment, whether started before or after the lesion induction, significantly reduced both the amphetamine-induced ipsiversive rotation and the size of the lesion at the level of the substantia nigra. Moreover, the dopamine cell shape and size resembled that observed in intact animals. R-APO treatment had no effect on the number of cells in the substantia nigra of intact rats, but significantly increased the number of cells in the ventral tegmental area (VTA), suggesting selective neurotrophic properties of R-APO in this region. R-APO treatment significantly attenuated the 6-OHDA-induced striatal DA depletion and DOPAC/DA ratios were normalized. Finally, an acute injection of 10 mg/kg R-APO was unable to scavenge 6-OHDA or MPP(+)-induced hydroxyl radicals as determined with the in vivo salicylate trapping technique. These data provide further evidence of the neurorescuing properties of R-APO. At least at the dose used in this study, this effect possibly occurs via mechanisms other than scavenging of hydroxyl radicals. In intact rats, we also show neurotrophic effects of the R-APO treatment. These seem to be limited to the VTA.

Dopamine D2-like receptor agonist bromocriptine protects against ischemia/reperfusion injury in rat kidney

BACKGROUND

Dopamine, via activation of D1-like and D2-like receptors, plays an important role in the regulation of renal sodium excretion. Recently, we demonstrated that dopamine D2-like receptor agonist (bromocriptine) stimulates p44/42 mitogen-activated protein kinases (MAPKs) and Na+,K(+)ATPase (NKA) activity in proximal tubular epithelial cells. Since both these parameters are compromised in ischemia/reperfusion (I/R) injury to the kidney, we investigated whether bromocriptine protects against the injury.

METHODS

In this study we used unilateral rat model of renal I/R injury. The Sprague-Dawley rats were divided into vehicle and bromocriptine groups. The vehicle and bromocriptine group was treated with vehicle and bromocriptine (500 microg/kg intravenously), respectively, 15 minutes before the induction of unilateral ischemia followed by 24- or 48-hour reperfusion. At the end of 24 or 48 hours the animals were sacrificed to collect control and ischemic kidney cortices, in which necrosis, apoptosis, NKA activity, NKA alpha1 subunit expression, and p44/42 MAPK phosphorylation were measured.

RESULTS

We found extensive necrosis, apoptosis, and decreased NKA activity (with no change in alpha1 subunit) in the ischemic kidney cortex compared to the nonischemic cortex from the vehicle-treated rats as early as 24 hours post-reperfusion. In contrast, I/R injury-induced necrotic, apoptotic, and decrease in NKA activity were absent in the outer cortex of bromocriptine-treated rats after 24 or 48 hours. Interestingly, we detected significantly higher phosphorylation of p44/42 MAPKs in control and ischemic kidneys of bromocriptine-treated rats compared to those of vehicle-treated rats.

CONCLUSION

Therefore, bromocriptine, a D1-like receptor agonist, may protect against I/R injury to proximal tubules of the kidney, via p44/42 MAPK activation.

Antioxidative activities of novel diphenylalkyl piperazine derivatives with high affinities for the dopamine transporter

A new series of diphenylalkyl piperazine derivatives with high affinities for the dopamine transporter (DAT), which were modified at both the diphenylalkyl moiety and the phenyl ring in the phenylamino moiety of 1-[4,4-bis(4-fluorophenyl)butyl]-4-[2-hydroxy-3-(phenylamino)propyl]piperazine 1, was evaluated for their inhibitory activities against auto-oxidative lipid peroxidation in canine brain homogenates. Some of these were approximately equivalent in activity to alpha-tocopherol as a potent antioxidant with IC(50) values of low micromolar order, and the 4-hydroxyphenyl derivative 11 showed the most potent antioxidative activity with an IC(50) value of 0.32 microM, exhibiting approximately 5-fold more potent activity than alpha-tocopherol. The structure-activity relationship (SAR) studies of the antioxidative activity of these derivatives are presented.

Arguments for the use of dopamine receptor agonists in clinical and preclinical Parkinson's disease

On the basis of experimental studies which have demonstrated deleterious effects of L-DOPA (L-3,4-dihydroxyphenylalanine) in vivo and in vitro, it has been suggested that L-DOPA itself may contribute to the progression of Parkinson's disease. This hypothesis is, for many clinicians, the rationale for postponing the employment of and reducing the applied dosage of L-DOPA and for beginning therapy with dopamine receptor agonists or the monoamine oxidase type B (MAO-B) inhibitor selegiline. Furthermore, clinical studies have demonstrated that early treatment with dopamine receptor agonists is associated with a lower incidence of motor fluctuations and dyskinesia. Dopamine receptor agonists exert their symptomatic effect by directly activating dopamine receptors, bypassing the presynaptic synthesis of dopamine and the degenerating nigro-striatal dopaminergic system. They can thus also be of benefit late in the therapy of the disorder. In addition, the pharmacological profile of dopamine receptor agonists suggests a possible neuroprotective effect. This paper reviews briefly the pharmacology of dopamine receptor agonists and basic knowledge concerning the dopamine receptor stimulation which underlies their therapeutic effect. Preclinical approaches for demonstrating neuroprotective effects and their clinical relevance are also discussed.

Neuroprotective mechanisms of antiparkinsonian dopamine D2-receptor subfamily agonists

Numerous studies have shown that endogenous and/or environmental neurotoxins and oxidative stress may participate in the pathogenesis of Parkinson's disease (PD), but the detailed mechanisms are still unclear. While dopamine (DA) replacement therapy with L-DOPA (levodopa) improves PD symptoms, it does not inhibit the degeneration of DA neurons in the substantia nigra. Recently, bromocriptine, pramipexole and several other agonists of the dopamine D2-receptor subfamily (including D2, D3 and D4-subtypes) have been shown to have neuroprotective effects in parkinsonian models in vitro and in vivo. Their neuroprotective effects may be mediated directly and/or indirectly by antioxidant effects, mitochondrial stabilization or induction of the antiapoptotic Bcl-2 family.

The dopamine agonist cabergoline provides neuroprotection by activation of the glutathione system and scavenging free radicals

Free radicals are involved in the pathogenesis and/or progression of Parkinson's disease (PD). Several ergot derivative dopamine (DA) agonists have been reported to scavenge free radicals in vitro and to show a neuroprotective effect in vivo. We investigated the in vitro free radical scavenging and antioxidant activities of cabergoline, a long-acting ergot DA agonist, as well as its ability to activate glutathione (GSH), catalase (Cat) and superoxide dismutase (SOD) activating effects and its in vivo neuroprotective properties against 6-hydroxydopamine (6-OHDA) intracerebroventricularly (i.c.v.) in mice. The striatal DA turnover induced by i.c.v. injection of 6-OHDA was completely normalized by pretreatment with cabergoline. Moreover, cabergoline scavenged free radicals in vitro and significantly reduced lipid peroxidation in vitro and in vivo. Furthermore, daily administration of cabergoline to mice significantly increased striatal GSH levels by activation of RNA expressions of GSH-related enzymes, although striatal Cat and SOD activities did not change. In addition, our present results suggest that repeated administration of cabergoline attenuates both 6-OHDA-induced nigrostriatal DAergic dysfunction and DA neuronal cell death, since cabergoline also had a neuroprotective effect in the immunohistochemical experiment. In conclusion, our findings indicate that the multiple antioxidant mechanisms of cabergoline, such as activation of the GSH system and the direct free radical scavenging activity, may explain the neuroprotective effect of this ergot DA agonist.

Pergolide protects SH-SY5Y cells against neurodegeneration induced by H(2)O(2)

We found that pergolide, a dopamine D1/D2 receptor agonist used in the clinical therapy of Parkinson's disease, protects SH-SY5Y neuroblastoma cells from cell death induced by a brief pulse (15 min) of 1 mM H(2)O(2). Neuroprotection was found when pergolide was added to the culture medium either simultaneously with (EC(50)=60 nM) or 2 h before (EC(50)=40 nM) H(2)O(2) treatment. These effects were not blocked by different dopamine receptor antagonists. Our data suggest that pergolide, independently of dopamine receptor stimulation, may interfere with the early phases of the oxidative stress-induced neurotoxic process.

Antioxidative effects of fluvastatin and its metabolites against oxidative DNA damage in mammalian cultured cells

We investigated the effects of fluvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, on reactive oxygen species (ROS) and on oxidative DNA damage in vitro, as well as the effects of the main fluvastatin metabolites (M2, M3, and M4) and other inhibitors of the same enzyme, pravastatin and simvastatin. The hydroxyl radical and the superoxide anion scavenging activities of fluvastatin and its metabolites were evaluated using an electron spin resonance spectrometer. Fluvastatin and its metabolites showed superoxide anion scavenging activity in the hypoxanthine-xanthine oxidase system and a strong scavenging effect on the hydroxyl radical produced from Fenton's reaction. Protective effects of fluvastatin on ROS-induced DNA damage of CHL/IU cells were assessed using the single-cell gel electrophoresis assay. CHL/IU cells were exposed to either hydrogen peroxide or t-butylhydroperoxide. Fluvastatin and its metabolites showed protective effects on DNA damage as potent as the reference antioxidants, ascorbic acid, trolox, and probucol, though pravastatin and simvastatin did not exert clear protective effects. These observations suggest that fluvastatin and its metabolites may have radical scavenging activity and the potential to protect cells against oxidative DNA damage. Furthermore, ROS are thought to play a major role in the etiology of a wide variety of diseases such as cellular aging, inflammation, diabetes, and cancer development, so fluvastatin might reduce these risks.

Parkinson's Disease: Initial Treatment with Levodopa or Dopamine Agonists

The question of whether to use levodopa (LD) or dopamine agonists as initial therapy in Parkinson's disease has been a controversy for nearly 20 years. There are several issues relating to this treatment regimen that may effect ones decision. Review of them results in the following conclusions: LD does not cause the onset of motor fluctuations and dyskinesia; it probably relates to disease progression. Tolerance does not develop with long-term LD therapy. LD is not toxic. LD decreases mortality in Parkinson's disease. Motor fluctuations can occur with dopamine-agonist monotherapy, but the actual frequency is as yet unknown. Dopamine agonists are not neuroprotective. Clinical trials have indicated that LD remains the most potent symptomatic therapeutic agent available. Dopamine agonists do provide some symptomatic relief when used alone in early Parkinson's disease. Standard preparations of LD have the same effect on early disease as controlled release preparations. Dopamine agonists cause less dyskinesia and fluctuations. These conclusions indicate that both drugs are effective symptomatic agents with their own positive and negative aspects. There is no incorrect choice. It is reasonable to start young onset patients (younger than 50 years of age) with an agonist, because they seem to be more prone to develop motor fluctuations and dyskinesia. However, if employment is in jeopardy then LD may be needed. Because agonists cause more hallucinations, freezing, and somnolence, problems of particular relevance to the elderly (older than 70 years), then LD would be the best agent for older onset patients. In general, but particularly for those falling in between these age groups, treatment should be individualized. In this time of cost effectiveness, LD remains the least expensive of these agents.

Are dopamine receptor agonists neuroprotective in Parkinson's disease?

Dopamine receptor agonists are playing an increasingly important role in the treatment of not only patients with advanced Parkinson's disease and those with levodopa-induced motor fluctuations, but also in the early treatment of the disease. This shift has been largely due to the demonstrated levodopa-sparing effect of dopamine agonists and their putative neuroprotective effect, with evidence for the latter being based largely on experimental in vitro and in vivo studies. In this article we review the evidence for neuroprotection by the dopamine agonists pramipexole, ropinirole, pergolide, bromocriptine and apomorphine in cell cultures and animal models of injury to the substantia nigra. Most of the studies suggest that dopamine agonists may have neuroprotective effects via direct scavenging of free radicals or increasing the activities of radical-scavenging enzymes, and enhancing neurotrophic activity. However, the finding that pramipexole can normalise mitochondrial membrane potential and inhibit activity of caspase-3 in cytoplasmic hybrid cells derived from mitochondrial DNA of patients with nonfamilial Alzheimer's disease suggests an even broader implication for the neuroprotective role of dopamine agonists. Although the clinical evidence for neuroprotection by dopamine agonists is still limited, the preliminary results from several ongoing clinical trials are promising. Several longitudinal studies are currently in progress designed to demonstrate a delay or slowing of progression of Parkinson's disease using various surrogate markers of neuronal degeneration such as 18F-levodopa positron emission tomography and 123I beta-CIT (carbomethoxy-beta-4-iodophenyl-nortropane) single positron emission computed tomography. The results of these experimental and clinical studies will improve our understanding of the action of dopamine agonists and provide critical information needed for planning future therapeutic strategies for Parkinson's disease and related neurodegenerative disorders.

Molecular mechanism in activation of glutathione system by ropinirole, a selective dopamine D2 agonist

We have previously reported that ropinirole, a non-ergot dopamine agonist, has neuroprotective effects against 6-hydroxydopamine in mice based on in vivo antioxidant properties such as the glutathione (GSH)-activating effect. In the present study, we determined that the effects of ropinirole on the level of expression of GSH-related enzyme mRNA, these enzymes were shown to regulate GSH contents in the brain. This study focused on the mechanism of GSH enhancement by ropinirole. Striatal GSH contents were significantly increased by 7-day daily administration of ropinirole. Furthermore, the expression levels of gamma-glutamylcysteine synthetase (gamma-GCS), glutathione peroxidase (GPx), glutathione reductase (GR) and glutathione S-transferase (GST) mRNA increased following daily injections of ropinirole for 7 days. In addition, ropinirole treatment for 7 days suppressed auto-oxidation in mouse striatal homogenates, in contrast to the vehicle treatment. In conclusion, ropinirole was able to suppress auto-oxidation, most probably by increasing GSH levels due to an increase of GSH synthesis. In addition, it is likely that auto-oxidation was also suppressed by the activation of GSH-regulating enzymes such as GPx, GR, and GST in the mouse striatum. Thus, our results indicate that the GSH-activating effect of ropinirole may render this dopamine agonist beneficial as a neuroprotective drug.

The activation of dopamine D4 receptors inhibits oxidative stress-induced nerve cell death

Oxidative stress is thought to be the cause of nerve cell death in many CNS pathologies, including ischemia, trauma, and neurodegenerative disease. Glutamate kills nerve cells that lack ionotropic glutamate receptors via the inhibition of the cystine-glutamate antiporter x(c)(-), resulting in the inhibition of cystine uptake, the loss of glutathione, and the initiation of an oxidative stress cell death pathway. A number of catecholamines were found to block this pathway. Specifically, dopamine and related ligands inhibit glutamate-induced cell death in both clonal nerve cell lines and rat cortical neurons. The protective effects of dopamine, apomorphine, and apocodeine, but not epinephrine and norepinephrine, are antagonized by dopamine D4 antagonists. A dopamine D4 agonist also protects, and this protective effect is inhibited by U101958, a dopamine D4 antagonist. Although the protective effects of some of the catecholamines are correlated with their antioxidant activities, there is no correlation between the protective and antioxidant activities of several other ligands. Normally, glutamate causes an increase in reactive oxygen species (ROS) and intracellular Ca(2+). Apomorphine partially inhibits glutamate-induced ROS production and blocks the opening of cGMP-operated Ca(2+) channels that lead to Ca(2+) elevation in the late part of the cell death pathway. These data suggest that the protective effects of apomorphine on oxidative stress-induced cell death are, at least in part, mediated by dopamine D4 receptors via the regulation of cGMP-operated Ca(2+) channels.

Neuroprotective effects of D3 dopamine receptor agonists

The effects of D(3) receptor activation are unresolved at this time, but may have practical implications in the treatment of Parkinson's disease (PD). As a result of assessing the neuroprotective effects of the direct-acting D(3) preferring dopamine (DA) agonist pramipexole (PPX), we have observed that drugs which psossess D(3) affinity increase the production of a DA neurotrophic factor in tissue culture. This molecule is increased by treatment with PPX, is constitutively produced by DA neurons in culture, and possesses a molecular weight of approximately 35kDa. It is hypothesized that this molecule may be the so-called DA autotrophic factor referred to by many authors over the past two decades. Interestingly, the protein is oxidant-labile and, therefore, D(3) agonists which increase its production and also possess antioxidant capacity would provide unique neuroprotective benefits to patients with PD. However, many questions remain. Although the data supporting this notion are strong, it is clear that other unknown characteristics of DA agonists, including increased production of anti-apoptotic proteins, are also involved. This manuscript will review this concept in the context of tissue culture strategies of neuroprotection. Although no conclusion can be made at this time, it is clear that direct comparisons of the neuroprotective effects of direct-acting DA agonists in mesencephalic culture can provide considerable insight into the mechanistic actions of anti-dopaminergic drugs.

Potent, hydroxyl radical-scavenging effect of apomorphine with iron and dopamine perfusion in rat striatum

In dopaminergic neurons, free radicals are likely produced via dopamine metabolism by monoamine oxidase or via its auto-oxidation, a process facilitated by transition metals. In this study we examined the effect and possible mechanisms of apomorphine to reduce iron- and dopamine-induced 2,3-dihydroxybenzoic acid (2,3-DHBA) formation by microdialysis. We have shown that (1) FeSO(4).7H(2)O reduced both the release of dopamine and the output of dihydroxyphenylacetic acid (DOPAC); (2) apomorphine may reduce FeSO(4).7H(2)O-induced increases of 2,3-DHBA formation; (3) apomorphine has substantially reduced DOPAC output in early phase and blocked dopamine-induced increase of 2,3-DHBA levels. It is concluded that apomorphine is a potent hydroxyl radical scavenger in vivo, especially for the dopamine formation.

Reactive oxygen species in cell signaling

Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. When cellular production of ROS overwhelms its antioxidant capacity, damage to cellular macromolecules such as lipids, protein, and DNA may ensue. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of human diseases including those of the lung. Recent studies have also implicated ROS that are generated by specialized plasma membrane oxidases in normal physiological signaling by growth factors and cytokines. In this review, we examine the evidence for ligand-induced generation of ROS, its cellular sources, and the signaling pathways that are activated. Emerging concepts on the mechanisms of signal transduction by ROS that involve alterations in cellular redox state and oxidative modifications of proteins are also discussed.

The dopamine agonist pramipexole scavenges hydroxyl free radicals induced by striatal application of 6-hydroxydopamine in rats: an in vivo microdialysis study

Hydroxyl free radical production seems to play an important role in the pathogenesis of Parkinson's disease. In the present study, we investigated the dopamine agonists pramipexole and pergolide as well as the nitrone compound S-PBN (N-tert-butyl-alpha-(2-sulfophenyl)nitrone) to reduce hydroxyl radical formation. Microdialysis experiments were carried out in non-anaesthetized Wistar rats. Salicylate was incorporated into the perfusion fluid to measure indirectly hydroxyl radicals indicated by 2,3-dihydroxybenzoic acid (2,3-DHBA). Local perfusion with 0.2 or 2 nmol/2 microl/min 6-hydroxydopamine (6-OHDA) via the microdialysis probe significantly increased 2,3-DHBA levels 14-fold and 47-fold, respectively. Systemic application of either pergolide (0.05 mg/kg) or pramipexole (1 mg/kg) failed to significantly reduce 6-OHDA-induced hydroxyl radical production. In contrast, a 40 min pretreatment with pramipexole (2 and 10 nmol/2 microl/min via the probe) before onset of 6-OHDA perfusion, significantly attenuated 2, 3-DHBA levels compared with vehicle controls. S-PBN pretreatment (2 nmol/2 microl/min) was not effective to reduce 2,3-DHBA levels. In conclusion, pramipexole was able to reduce hydroxyl radical levels induced by 6-OHDA in vivo after local application. This property of pramipexole may be beneficial under conditions of enhanced hydroxyl radical formation in parkinsonian brains and may add to its well known dopamine D(2)-like receptor agonistic effects.