Effects of the adenosine A2A receptor antagonist on cognitive dysfunction in Parkinson's disease

Purinergic receptors in cognitive disturbances

Purinergic receptors (Rs) of the ATP/ADP, UTP/UDP (P2X, P2Y) and adenosine (A1, A2A)-sensitive classes broadly interfere with cognitive processes both under quasi normal and disease conditions. During neurodegenerative illnesses, high concentrations of ATP are released from the damaged neuronal and non-neuronal cells of the brain; then, this ATP is enzymatically degraded to adenosine. Thus, the primary injury in neurodegenerative diseases appears to be caused by various protein aggregates on which a superimposed damage mediated by especially P2X7 and A2AR activation develops; this can be efficiently prevented by small molecular antagonists in animal models of the above diseases, or are mitigated in the respective knockout mice. Dementia is a leading symptom in Alzheimer's disease (AD), and accompanies Parkinson's disease (PD) and Huntington's disease (HD), especially in the advanced states of these illnesses. Animal experimentation suggests that P2X7 and A2ARs are also involved in a number of psychiatric diseases, such as major depressive disorder (MDD), obsessive compulsive behavior, and attention deficit hyperactivity disorder. In conclusion, small molecular antagonists of purinergic receptors are expected to supply us in the future with pharmaceuticals which are able to combat in a range of neurological/psychiatric diseases the accompanying cognitive deterioration.

Pharmacological Management of Apathy in Dementia

Apathy is a highly prevalent symptom of dementia. Despite its association with faster cognitive and functional decline, decreased quality of life and increased mortality, no therapies are currently approved to treat apathy. The objective of this review was to summarize the drugs that have been studied for apathy treatment in patients with dementia (specifically Alzheimer's disease [AD], Huntington's disease [HD] and Parkinson's disease [PD] dementia; dementia with Lewy bodies [DLB]; vascular dementia [VaD]; and frontotemporal dementia [FTD]) based on their putative mechanisms of action. A search for relevant studies was performed using ClinicalTrials.gov and PubMed. Eligible studies were randomized controlled trials that were available in English and included at least one drug intervention and an apathy measure scale. A total of 52 studies that included patients with AD (n = 33 studies), PD (n = 5), HD (n = 1), DLB (n = 1), FTD (n = 3), VaD (n = 1), VaD and AD (n = 4), VaD and mixed dementia (n = 1), and AD, VaD and mixed dementia (n = 3) were eligible for inclusion. These studies showed that methylphenidate, olanzapine, cholinesterase inhibitors, choline alphoscerate, citalopram, memantine, and mibampator are the only beneficial drugs in AD-related apathy. For PD-related apathy, only methylphenidate, rotigotine and rivastigmine showed benefits. Regarding FTD- and DLB-related apathy, initial studies with agomelatine and rivastigmine showed benefits, respectively. As for HD- and only-VaD-related apathy, no drugs demonstrated benefits. With regards to mixed populations, memantine, galantamine and gingko biloba showed effects on apathy in the AD plus VaD populations and nimodipine in the VaD plus mixed dementia populations. Of the drugs with positive results, some are already prescribed to patients with dementia to target other symptoms, some have characteristics-such as medical contraindications (e.g., cardiovascular) and adverse effects (e.g., gastrointestinal disturbances)-that limit their clinical use and some require further study. Future studies should investigate apathy as a primary outcome, making use of appropriate sample sizes and study durations to ensure durability of results. There should also be a consensus on using scales with high test/retest and interrater reliabilities to limit the inconsistencies between clinical trials. In conclusion, there are currently no US FDA-approved drugs that target apathy in dementia, so there is an ongoing need for the development of such drugs.

Blockade of adenosine A2A receptor alleviates cognitive dysfunction after chronic exposure to intermittent hypoxia in mice

Obstructive sleep apnea-hypopnea syndrome (OSAHS) is widely known for its multiple systems damage, especially neurocognitive deficits in children. Since their discovery, adenosine A_2A receptors (A_2ARs) have been considered as key elements in signaling pathways mediating neurodegenerative diseases such as Huntington's and Alzheimer's, as well as cognitive function regulation. Herein, we investigated A_2AR role in cognitive impairment induced by chronic intermittent hypoxia (CIH). Mice were exposed to CIH 7 h every day for 4 weeks, and intraperitoneally injected with A_2AR agonist CGS21680 or A_2AR antagonist SCH58261 half an hour before IH exposure daily. The 8-arm radial arm maze was utilized to assess spatial memory after CIH exposures.To validate findings using pharmacology, the impact of intermittent hypoxia was investigated in A_2AR knockout mice. CIH-induced memory dysfunction was manifested by increased error rates in the radial arm maze test. The behavioral changes were associated with hippocampal pathology, neuronal apoptosis, and synaptic plasticity impairment. The stimulation of adenosine A_2AR exacerbated memory impairment with more serious neuropathological damage, attenuated long-term potentiation (LTP), syntaxin down-regulation, and increased BDNF protein. Moreover, apoptosis-promoting protein cleaved caspase-3 was upregulated while anti-apoptotic protein Bcl-2 was downregulated. Consistent with these findings, A_2AR inhibition with SCH58261 and A_2AR deletion exhibited the opposite result. Overall, these findings suggest that A_2AR plays a critical role in CIH-induced impairment of learning and memory by accelerating hippocampal neuronal apoptosis and reducing synaptic plasticity. Blockade of adenosine A_2A receptor alleviates cognitive dysfunction after chronic exposure to intermittent hypoxia in mice.

Insight into the Role of the STriatal-Enriched Protein Tyrosine Phosphatase (STEP) in A2A Receptor-Mediated Effects in the Central Nervous System

The STriatal-Enriched protein tyrosine phosphatase STEP is a brain-specific tyrosine phosphatase that plays a pivotal role in the mechanisms of learning and memory, and it has been demonstrated to be involved in several neuropsychiatric diseases. Recently, we found a functional interaction between STEP and adenosine A_2A receptor (A_2AR), a subtype of the adenosine receptor family widely expressed in the central nervous system, where it regulates motor behavior and cognition, and plays a role in cell survival and neurodegeneration. Specifically, we demonstrated the involvement of STEP in A_2AR-mediated cocaine effects in the striatum and, more recently, we found that in the rat striatum and hippocampus, as well as in a neuroblastoma cell line, the overexpression of the A_2AR, or its stimulation, results in an increase in STEP activity. In the present article we will discuss the functional implication of this interaction, trying to examine the possible mechanisms involved in this relation between STEP and A_2ARs.

The role of glutamate mGlu5 and adenosine A2a receptor interactions in regulating working memory performance and persistent cocaine seeking in rats

Cocaine use disorder (CUD) is associated with neurobehavioral deficits that are resistant to current treatments. While craving and high rates of relapse are prominent features of CUD, persistent cognitive impairments are common and linked to poorer treatment outcomes. Here we sought to develop an animal model to study post-cocaine changes in drug seeking and working memory, and to evaluate 'therapeutic' effects of combined glutamate mGlu5 and adenosine A2a receptor blockade. As mGlu5 antagonists reduce drug seeking, and A2a blockade ameliorates working memory impairment, we hypothesized that mGlu5 + A2a antagonist cocktail would reduce both cocaine relapse and post-cocaine working memory deficits. Adult male Sprague-Dawley rats were first trained and tested in an operant delayed match-to-sample (DMS) task to establish the working memory baseline, followed by 6 days of limited and 12 days of extended access cocaine self-administration. Chronic cocaine reduced working memory performance (abstinence day 30-40) and produced robust time-dependent cocaine seeking at 45-, but not 120-days of abstinence. Systemic administration of A2a antagonist KW-6002 (0.125 and 1 mg/kg) failed to rescue post-cocaine working memory deficit. It also failed to reverse working memory impairment produced by mGlu5 NAM MTEP (1 mg/kg). Finally, KW-6002 prevented the ability of MTEP to reduce cocaine seeking and increased locomotor behavior. Thus, despite mGlu5 and A2a being exclusively co-localized in the striatum and showing behavioral synergism towards reducing cocaine effects in some studies, our findings advocate against the use of mGlu5 + A2a antagonist cocktail as it may further compromise cognitive deficits and augment drug craving in CUD.

Istradefylline to Treat Patients with Parkinson's Disease Experiencing "Off" Episodes: A Comprehensive Review

Parkinson’s disease (PD) is a common neurodegenerative disorder that leads to significant morbidity and disability. PD is caused by a loss of dopaminergic, cholinergic, serotonergic, and noradrenergic neurons in the central nervous system (CNS), and peripherally; the syndromic parkinsonism symptoms of movement disorder, gait disorder, rigidity and tremor are mostly driven by the loss of these neurons in the basal ganglia. Unfortunately, a significant proportion of patients taking levodopa, the standard of care treatment for PD, will begin to experience a decrease in effectiveness at varying times. These periods, referred to as “off episodes”, are characterized by increased symptoms and have a detrimental effect on quality of life and disability. Istradefylline, a novel adenosine A2A receptor antagonist, is indicated as a treatment addition to levodopa/carbidopa in patients experiencing “off episodes”. It promotes dopaminergic activity by antagonizing adenosine in the basal ganglia. This review will discuss istradefylline as a treatment for PD patients with off episodes.

The activity of the Striatal-enriched protein tyrosine phosphatase in neuronal cells is modulated by adenosine A2A receptor

We recently demonstrated that a tonic activation of adenosine A_2A receptors (A_2A Rs) is required for cocaine-induced synaptic depression and increase in the activity of STriatal-Enriched protein tyrosine Phosphatase (STEP). In this study, we elaborated on the relationship between A_2A R and STEP using genetic, pharmacological, and cellular tools. We found that the activities of protein tyrosine phosphatases (PTPs), and in particular of STEP, are significantly increased in the striatum and hippocampus of a transgenic rat strain over-expressing the neuronal A_2A R (NSEA_2A ) with respect to wild-type (WT) rats. Moreover the selective A_2A R agonist 4-[2-[[6-Amino-9-(N-ethyl-β-d-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride up-regulates PTPs and STEP activities in WT but not in NSEA_2A rats, while the selective A_2A R antagonist 4-(-2-[7-amino-2-{2-furyl}{1,2,4}triazolo{2,3-a} {1,3,5}triazin-5-yl-amino]ethyl)phenol restores the tyrosine phosphatase activities in NSEA_2A , having no effects in WT rats. In addition, while cocaine induced the activation of PTP and STEP in WT rats, it failed to increase phosphatase activity in NSEA_2A rats. A_2A Rs modulate STEP activity also in the SH-SY5Y neuroblastoma cell line, where a calcium-dependent calcineurin/PP1 pathway was found to play a major role. In summary, the present study identified a novel interaction between A_2A R and STEP that could have important clinical implications, since STEP has emerged as key regulator of signaling pathways involved in neurodegenerative and neuropsychiatric diseases and A_2A Rs are considered a promising target for the development of therapeutic strategies for different diseases of the central nervous system. Read the Editorial Highlight for this article on page 270.

The Impact of Purinergic System Enzymes on Noncommunicable, Neurological, and Degenerative Diseases

Evidences show that purinergic signaling is involved in processes associated with health and disease, including noncommunicable, neurological, and degenerative diseases. These diseases strike from children to elderly and are generally characterized by progressive deterioration of cells, eventually leading to tissue or organ degeneration. These pathological conditions can be associated with disturbance in the signaling mediated by nucleotides and nucleosides of adenine, in expression or activity of extracellular ectonucleotidases and in activation of P2X and P2Y receptors. Among the best known of these diseases are atherosclerosis, hypertension, cancer, epilepsy, Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). The currently available treatments present limited effectiveness and are mostly palliative. This review aims to present the role of purinergic signaling highlighting the ectonucleotidases E-NTPDase, E-NPP, E-5'-nucleotidase, and adenosine deaminase in noncommunicable, neurological, and degenerative diseases associated with the cardiovascular and central nervous systems and cancer. In conclusion, changes in the activity of ectonucleotidases were verified in all reviewed diseases. Although the role of ectonucleotidases still remains to be further investigated, evidences reviewed here can contribute to a better understanding of the molecular mechanisms of highly complex diseases, which majorly impact on patients' quality of life.

The Corticostriatal Adenosine A2A Receptor Controls Maintenance and Retrieval of Spatial Working Memory

BACKGROUND

Working memory (WM) taps into multiple executive processes including encoding, maintenance, and retrieval of information, but the molecular and circuit modulation of these WM processes remains undefined due to the lack of methods to control G protein-coupled receptor signaling with temporal resolution of seconds.

METHODS

By coupling optogenetic control of the adenosine A_2A receptor (A_2AR) signaling, the Cre-loxP-mediated focal A_2AR knockdown with a delayed non-match-to-place (DNMTP) task, we investigated the effect of optogenetic activation and focal knockdown of A_2ARs in the dorsomedial striatum (n = 8 to 14 per group) and medial prefrontal cortex (n = 16 to 22 per group) on distinct executive processes of spatial WM. We also evaluated the therapeutic effect of the A_2AR antagonist KW6002 on delayed match-to-sample/place tasks in 6 normal and 6 MPTP-treated cynomolgus monkeys.

RESULTS

Optogenetic activation of striatopallidal A_2ARs in the dorsomedial striatum selectively at the delay and choice (not sample) phases impaired DNMTP performance. Optogenetic activation of A_2ARs in the medial prefrontal cortex selectively at the delay (not sample or choice) phase improved DNMTP performance. The corticostriatal A_2AR control of spatial WM was specific for a novel but not well-trained DNMTP task. Focal dorsomedial striatum A_2AR knockdown or KW6002 improved DNMTP performance in mice. Last, KW6002 improved spatial WM in delayed match-to-sample and delayed match-to-place tasks of normal and dopamine-depleted cynomolgus monkeys.

CONCLUSIONS

The A_2ARs in striatopallidal and medial prefrontal cortex neurons exert distinctive control of WM maintenance and retrieval to achieve cognitive stability and flexibility. The procognitive effect of KW6002 in nonhuman primates provides the preclinical data to translate A_2AR antagonists for improving cognitive impairments in Parkinson's disease.

Targeting Adenosine Signaling in Parkinson's Disease: From Pharmacological to Non-pharmacological Approaches

Parkinson's disease (PD) is one of the most prevalent neurodegenerative disease displaying negative impacts on both the health and social ability of patients and considerable economical costs. The classical anti-parkinsonian drugs based in dopaminergic replacement are the standard treatment, but several motor side effects emerge during long-term use. This mini-review presents the rationale to several efforts from pre-clinical and clinical studies using adenosine receptor antagonists as a non-dopaminergic therapy. As several studies have indicated that the monotherapy with adenosine receptor antagonists reaches limited efficacy, the usage as a co-adjuvant appeared to be a promising strategy. The formulation of multi-targeted drugs, using adenosine receptor antagonists and other neurotransmitter systems than the dopaminergic one as targets, have been receiving attention since Parkinson's disease presents a complex biological impact. While pharmacological approaches to cure or ameliorate the conditions of PD are the leading strategy in this area, emerging positive aspects have arisen from non-pharmacological approaches and adenosine function inhibition appears to improve both strategies.

How does adenosine control neuronal dysfunction and neurodegeneration?

The adenosine modulation system mostly operates through inhibitory A₁ (A₁ R) and facilitatory A_2A receptors (A_2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A₁ R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: high-frequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A_2A R; A_2A R switch off A₁ R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A₁ R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A₁ R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A_2A R, probably to bolster adaptive changes, but this heightens brain damage since A_2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A_2A R, whereas astrocytic and microglia A_2A R might control the spread of damage. The A_2A R signaling mechanisms are largely unknown since A_2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A₁ R preconditioning and preventing excessive A_2A R function might afford maximal neuroprotection. The main physiological role of the adenosine modulation system is to sharp the salience of information encoding through a combined action of adenosine A_2A receptors (A_2A R) in the synapse undergoing an alteration of synaptic efficiency with an increased inhibitory action of A₁ R in all surrounding synapses. Brain insults trigger an up-regulation of A_2A R in an attempt to bolster adaptive plasticity together with adenosine release and A₁ R desensitization; this favors synaptotocity (increased A_2A R) and decreases the hurdle to undergo degeneration (decreased A₁ R). Maximal neuroprotection is expected to result from a combined A_2A R blockade and increased A₁ R activation. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

An evaluation of istradefylline treatment on Parkinsonian motor and cognitive deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated macaque models

Istradefylline (KW-6002), an adenosine A2A receptor antagonist, is used adjunct with optimal doses of L-3,4-dihydroxyphenylalanine (l-DOPA) to extend on-time in Parkinson's disease (PD) patients experiencing motor fluctuations. Clinical application of istradefylline for the management of other l-DOPA-induced complications, both motor and non-motor related (i.e. dyskinesia and cognitive impairments), remains to be determined. In this study, acute effects of istradefylline (60-100 mg/kg) alone, or with optimal and sub-optimal doses of l-DOPA, were evaluated in two monkey models of PD (i) the gold-standard 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated macaque model of parkinsonian and dyskinetic motor symptoms and (ii) the chronic low dose (CLD) MPTP-treated macaque model of cognitive (working memory and attentional) deficits. Behavioural analyses in l-DOPA-primed MPTP-treated macaques showed that istradefylline alone specifically alleviated postural deficits. When combined with an optimal l-DOPA treatment dose, istradefylline increased on-time, enhanced therapeutic effects on bradykinesia and locomotion, but exacerbated dyskinesia. Istradefylline treatment at specific doses with sub-optimal l-DOPA specifically alleviated bradykinesia. Cognitive assessments in CLD MPTP-treated macaques showed that the attentional and working memory deficits caused by l-DOPA were lowered after istradefylline administration. Taken together, these data support a broader clinical use of istradefylline as an adjunct treatment in PD, where specific treatment combinations can be utilised to manage various l-DOPA-induced complications, which importantly, maintain a desired anti-parkinsonian response.