Allopurinol augmentation for poorly responsive schizophrenia

Adenosine A2A receptor in schizophrenia: an in vivo brain PET imaging study

Adenosine A_2A receptors are highly enriched in the basal ganglia system, a region that is functionally implicated in schizophrenia. Preclinical evidence suggests a cross-regulation between adenosine A_2A and dopamine D₂ receptors in this region and that it is linked to the sensitization of the dopamine system. However, the relationship between A_2A receptor availability and schizophrenia has not been directly examined in vivo in patients with this disorder. To investigate, using positron emission tomography (PET), the availability of A_2A receptors in patients diagnosed with schizophrenia in comparison to matched healthy controls. A_2A receptor availability was measured using the PET tracer [¹¹C]SCH442416. Twelve male patients with chronic schizophrenia were compared to 13 matched healthy subjects. All patients were medicated with antipsychotics and none presented with any motor or extrapyramidal symptoms. Binding potential (BP_ND), a ratio measure between specific and non-specific tracer uptake, were compared between the groups for the caudate, putamen, accumbens and globus pallidum. There was no differences between A_2A receptor binding potential (BP_ND) of schizophrenia patients in the caudate (p = 0.16), putamen (p = 0.86), accumbens (p = 0.44) and globus pallidum (p = 0.09) to that of matched healthy subjects. There was also no significant correlation between [11C]SCH442416 binding and severity of psychotic symptoms (p = 0.2 to 0.82) or antipsychotic dosage (p = 0.13 to 0.34). By showing that A2A receptor availability in medicated patients with chronic male schizophrenia is not different than in healthy controls, this study does not support the primary role of this receptor in the pathogenesis of schizophrenia.

Allopurinol for fibromyalgia pain in adults: A randomized controlled trial

BACKGROUND

Allopurinol is a potent inhibitor of the enzyme xanthine oxidase used in the treatment of hyperuricemia and gout. Because it is well known that purines exert multiple affects on pain transmission, we hypothesized that the inhibition of xanthine oxidase by allopurinol could be a valid strategy to treat pain in humans. This study aimed to compare the analgesic efficacy of oral allopurinol versus placebo as an adjuvant therapy in patients displaying fibromyalgia.

METHODS

This randomized, double-blinded, placebo-controlled study included 60 women with the diagnosis of fibromyalgia. Patients were randomly assigned to receive either oral allopurinol 300 mg (n = 31) or placebo (n = 29) twice daily during 30 days. The patients were submitted to evaluation for pain sensitivity, anxiety, depression, and functional status before treatment, and 15 and 30 days thereafter.

RESULTS

Oral administration of allopurinol 300 mg twice daily was ineffective in improving pain scores measured by several tools up to 30 days of treatment (P > 0.05). Additionally, no significant effects of allopurinol over anxiety, depressive symptoms, and functional status of fibromyalgia patients were observed in the present study.

CONCLUSIONS

Although previous findings indicated that allopurinol could present intrinsic analgesic effects in both animals and humans, this study showed no benefit of the use of oral allopurinol as an adjuvant strategy during 30 days in women displaying fibromyalgia. However, considering previous promising results, new prospective studies are still valid to further investigate allopurinol and more selective purine derivatives in the management of pain syndromes.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Multiple Adenosine-Dopamine (A2A-D2 Like) Heteroreceptor Complexes in the Brain and Their Role in Schizophrenia

In the 1980s and 1990s, the concept was introduced that molecular integration in the Central Nervous System could develop through allosteric receptor–receptor interactions in heteroreceptor complexes presents in neurons. A number of adenosine–dopamine heteroreceptor complexes were identified that lead to the A_2A-D₂ heteromer hypothesis of schizophrenia. The hypothesis is based on strong antagonistic A_2A-D₂ receptor–receptor interactions and their presence in the ventral striato-pallidal GABA anti-reward neurons leading to reduction of positive symptoms. Other types of adenosine A_2A heteroreceptor complexes are also discussed in relation to this disease, such as A_2A-D₃ and A_2A-D₄ heteroreceptor complexes as well as higher order A_2A-D₂-mGluR5 and A_2A-D₂-Sigma1R heteroreceptor complexes. The A_2A receptor protomer can likely modulate the function of the D₄ receptors of relevance for understanding cognitive dysfunction in schizophrenia. A_2A-D₂-mGluR5 complex is of interest since upon A_2A/mGluR5 coactivation they appear to synergize in producing strong inhibition of the D2 receptor protomer. For understanding the future of the schizophrenia treatment, the vulnerability of the current A_2A-D₂like receptor complexes will be tested in animal models of schizophrenia. A_2A-D₂-Simag1R complexes hold the highest promise through Sigma1R enhancement of inhibition of D2R function. In line with this work, Lara proposed a highly relevant role of adenosine for neurobiology of schizophrenia.

The peacefulness gene promotes aggression in Drosophila

Natural aggressiveness is commonly observed in all animal species, and is displayed frequently when animals compete for food, territory and mating. Aggression is an innate behaviour, and is influenced by both environmental and genetic factors. However, the genetics of aggression remains largely unclear. In this study, we identify the peacefulness (pfs) gene as a novel player in the control of male-male aggression in Drosophila. Mutations in pfs decreased intermale aggressiveness, but did not affect locomotor activity, olfactory avoidance response and sexual behaviours. pfs encodes for the evolutionarily conserved molybdenum cofactor (MoCo) synthesis 1 protein (Mocs1), which catalyzes the first step in the MoCo biosynthesis pathway. Neuronal-specific knockdown of pfs decreased aggressiveness. By contrast, overexpression of pfs greatly increased aggressiveness. Knocking down Cinnamon (Cin) catalyzing the final step in the MoCo synthesis pathway, caused a pfs-like aggression phenotype. In humans, inhibition of MoCo-dependent enzymes displays anti-aggressive effects. Thus, the control of aggression by Pfs-dependent MoCo pathways may be conserved throughout evolution.

Allopurinol as Adjuvant Therapy in Poorly Responsive or Treatment Refractory Schizophrenia

Objective:

To review the available literature evaluating the effectiveness of allopurinol for poorly responsive or treatment refractory schizophrenia.

Data Sources:

Searches of MEDLINE (1966–October 2006), the Cochrane Library, and International Pharmaceutical Abstracts (1970–October 2006) were conducted using the terms allopurinol and schizophrenia. Limits were set to select studies conducted in humans.

Study Selection and Data Extraction:

All articles identified from the data sources were evaluated. All case reports or clinical trials located were included in the review.

Data Synthesis:

Dopamine has been implicated for many years in the pathophysiology of schizophrenia, and the typical antipsychotics, via blockade of dopaminergic neurotransmission, have provided relief for patients with positive symptoms. However, because dopamine blockade does not relieve all symptoms of schizophrenia, it is now evident that many neurotransmitters may be involved in the pathogenesis of schizophrenia. Therefore, atypical antipsychotics, which target multiple neurotransmitters, have emerged as first-line therapies. An evolving body of evidence also supports a purinergic hypothesis for schizophrenia. Increased adenosinergic transmission is thought to reduce the affinity of dopamine agonists for dopamine receptors. Allopurinol, a xanthine oxidase inhibitor, may increase circulating pools of adenosine and may ultimately have antipsychotic and anxiolytic effects. Growing evidence for use of allopurinol as adjunctive therapy has been reported in both case reports and small clinical trials.

Conclusions:

Clinical trials show that adjuvant allopurinol may provide benefit to patients who are poorly responsive to current treatments for schizophrenia. Allopurinol is well tolerated by most patients. However, larger, randomized clinical trials need to be performed to determine the magnitude of this benefit, whether allopurinol should be routinely used as adjuvant therapy to antipsychotics, and which patient population is most likely to benefit from allopurinol use. For patients with limited options, allopurinol in doses of 300 mg once or twice daily may improve psychotic symptoms, especially refractory positive symptoms.

Paliperidone regulates intracellular redox system in rat brain: Role of purine mechanism

OBJECTIVE

The treatment of schizophrenia is multifactorial, with antipsychotic medications comprising a major part of treatment. Paliperidone is a newly commercialized antipsychotic whose formulation includes the principal active metabolite risperidone, 9-hydroxyrisperidone. Ever since the relationship between schizophrenia and oxidative stress was first demonstrated, many studies have been conducted in order to probe the potential protective effects of antipsychotic drugs on the oxidant-antioxidant system and lipid peroxidation. The basic aim of this study is to determine the effects of the newly marketed drug paliperidone on the activities of the enzymes adenosine deaminase (ADA), xanthine oxidase (XO), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) as well as on malondialdehyde (MDA) and nitric oxide (NO) levels in rat brain tissues.

METHODS

Twenty male Sprague-Dawley rats were used for the study, which were divided into two equal groups. The first was the control group (n = 10) and the second was the paliperidone group (n = 10). Saline was administered once daily for 14 days in the control group. In the paliperidone group, paliperidone was administered once daily with a dose of 1 mg/kg for 14 days. All rats were sacrificed at the end of the fourteenth day. Brain samples were collected and then analyzed.

RESULTS

Our results demonstrated that paliperidone significantly decreased the activities of ADA (P = 0.015), XO (P = 0.0001), and CAT (P = 0.004) while insignificantly increasing the activity of SOD (P = 0.49), MDA (P = 0.71), and NO (P = 0.26) levels in rat brain tissues. In addition, paliperidone insignificantly decreased the activity of GSH-Px (P = 0.30) compared to the control group in rat brain tissues.

DISCUSSION

In conclusion, the data obtained in this study suggest that paliperidone can positively alter antioxidant status and, accordingly, can offer positive outcomes in the treatment of schizophrenia by reducing activity in the enzymes ADA and XO, which are associated with purine metabolism. We believe that such a comprehensive approach used with other antipsychotic drugs warrants further study.

Serum uric acid levels and different phases of illness in bipolar I patients treated with lithium

Recent findings support the role of purinergic system dysfunction in the pathophysiology of bipolar disorder (BD). The present study aimed to evaluate the pattern of serum uric acid levels in a sample of 98 BD I patients followed-up prospectively in a naturalistic study and treated with lithium monotherapy or in association with other mood stabilizers (valproate or carbamazepine), in relation to different phases of illness and to pharmacological treatment. The results showed that uric acid levels were significantly higher in patients suffering from a manic/mixed episode, than in those euthymic or during a depressive phase. Further, these levels were related to the Clinical Global Impression-Bipolar Version (CGI-BP) scale score for the severity of manic symptoms. A positive correlation was found also with male sex and with serum lithium levels. These findings suggest that a dysregulation of the purinergic system may occur during manic/mixed episodes, and they support a possible role of serum uric acid levels as a state-dependent marker of BD manic phases.

Adenosine hypothesis in schizophrenia and bipolar disorder: a systematic review and meta-analysis of randomized controlled trial of adjuvant purinergic modulators

OBJECTIVE

Adenosine has been reported to interact with dopamine and glutamate of which are currently central pathophysiology of schizophrenia. Further, there have been emerging reports that patients with bipolar disorder (BD) have pathophysiological changes of the purinergic system. Thus, we performed a systematic review and meta-analysis of adenosine modulators in these disorders.

METHOD

We searched PubMed, EMBASE, the Cochrane Library databases, CINAHL, and PsycINFO up to April 25, 2013. Randomized controlled trials comparing adenosine modulator adjuvant therapy with placebo in patients with schizophrenia and BD were included. Primary outcome measures were Positive and Negative Syndrome Scale (PANSS) and Young Mania Rating Scales (YMRS). The risk ratio, 95% confidence interval, and standardized mean differences (SMD) were used.

RESULTS

Nine studies, including six studies in schizophrenia (total n=457) and three studies in BD (total n=289) were included. Overall, adenosine modulators were superior to placebo in PANSS total scores (SMD=-1.07, p=0.01) and positive and general but not negative symptom subscale scores in schizophrenia. Individually, allopurinol failed to show its superiority to placebo in all primary outcome measures in schizophrenia. In BD, data from pooled adenosine modulators indicated significant reduction of YMRS scores in comparison to placebo (SMD=-0.39, p=0.004).

CONCLUSIONS

Our results suggest that adenosine modulator adjuvant therapy is more beneficial in overall psychopathology (especially positive symptoms) in schizophrenia and in treating mania episodes of BD in comparison to placebo. The limited sample size of available studies suggests that more research should be done to evaluate both efficacy and tolerability of these medications.

Reduced striatal ecto-nucleotidase activity in schizophrenia patients supports the "adenosine hypothesis"

Schizophrenia (SZ) is a major chronic neuropsychiatric disorder characterized by a hyperdopaminergic state. The hypoadenosinergic hypothesis proposes that reduced extracellular adenosine levels contribute to dopamine D2 receptor hyperactivity. ATP, through the action of ecto-nucleotidases, constitutes a main source of extracellular adenosine. In the present study, we examined the activity of ecto-nucleotidases (NTPDases, ecto-5'-nucleotidase, and alkaline phosphatase) in the postmortem putamen of SZ patients (n = 13) compared with aged-matched controls (n = 10). We firstly demonstrated, by means of artificial postmortem delay experiments, that ecto-nucleotidase activity in human brains was stable up to 24 h, indicating the reliability of this tissue for these enzyme determinations. Remarkably, NTPDase-attributable activity (both ATPase and ADPase) was found to be reduced in SZ patients, while ecto-5'-nucleotidase and alkaline phosphatase activity remained unchanged. In the present study, we also describe the localization of these ecto-enzymes in human putamen control samples, showing differential expression in blood vessels, neurons, and glial cells. In conclusion, reduced striatal NTPDase activity may contribute to the pathophysiology of SZ, and it represents a potential mechanism of adenosine signalling impairment in this illness.

Associations between purine metabolites and monoamine neurotransmitters in first-episode psychosis

Schizophrenia (SZ) is a biochemically complex disorder characterized by widespread defects in multiple metabolic pathways whose dynamic interactions, until recently, have been difficult to examine. Rather, evidence for these alterations has been collected piecemeal, limiting the potential to inform our understanding of the interactions amongst relevant biochemical pathways. We herein review perturbations in purine and neurotransmitter metabolism observed in early SZ using a metabolomic approach. Purine catabolism is an underappreciated, but important component of the homeostatic response of mitochondria to oxidant stress. We have observed a homeostatic imbalance of purine catabolism in first-episode neuroleptic-naïve patients with SZ (FENNS). Precursor and product relationships within purine pathways are tightly correlated. Although some of these correlations persist across disease or medication status, others appear to be lost among FENNS suggesting that steady formation of the antioxidant uric acid (UA) via purine catabolism is altered early in the course of illness. As is the case for within-pathway correlations, there are also significant cross-pathway correlations between respective purine and tryptophan (TRP) pathway metabolites. By contrast, purine metabolites show significant cross-pathway correlation only with tyrosine, and not with its metabolites. Furthermore, several purine metabolites (UA, guanosine, or xanthine) are each significantly correlated with 5-hydroxyindoleacetic acid (5-HIAA) in healthy controls, but not in FENNS at baseline or 4-week after antipsychotic treatment. Taken together, the above findings suggest that purine catabolism strongly associates with the TRP pathways leading to serotonin (5-hydroxytryptamine, 5-HT) and kynurenine metabolites. The lack of a significant correlation between purine metabolites and 5-HIAA, suggests alterations in key 5-HT pathways that may both be modified by and contribute to oxidative stress via purine catabolism in FENNS.

Adenosine and autism: a spectrum of opportunities

In rodents, insufficient adenosine produces behavioral and physiological symptoms consistent with several comorbidities of autism. In rodents and humans, stimuli postulated to increase adenosine can ameliorate these comorbidities. Because adenosine is a broad homeostatic regulator of cell function and nervous system activity, increasing adenosine's influence might be a new therapeutic target for autism with multiple beneficial effects. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Adenosine augmentation ameliorates psychotic and cognitive endophenotypes of schizophrenia

An emerging theory of schizophrenia postulates that hypofunction of adenosine signaling may contribute to its pathophysiology. This study was designed to test the "adenosine hypothesis" of schizophrenia and to evaluate focal adenosine-based strategies for therapy. We found that augmentation of adenosine by pharmacologic inhibition of adenosine kinase (ADK), the key enzyme of adenosine clearance, exerted antipsychotic-like activity in mice. Further, overexpression of ADK in transgenic mice was associated with attentional impairments linked to schizophrenia. We observed that the striatal adenosine A2A receptor links adenosine tone and psychomotor response to amphetamine, an indicator of dopaminergic signaling. Finally, intrastriatal implants of engineered adenosine-releasing cells restored the locomotor response to amphetamine in mice overexpressing ADK, whereas the same grafts placed proximal to the hippocampus of transgenic mice reversed their working memory deficit. This functional double dissociation between striatal and hippocampal adenosine demonstrated in Adk transgenic mice highlights the independent contributions of these two interconnected brain regions in the pathophysiology of schizophrenia and thus provides the rationale for developing local adenosine augmentation therapies for the treatment of schizophrenia.

Alteration of the pro-oxidant xanthine oxidase (XO) in the thalamus and occipital cortex of patients with schizophrenia

OBJECTIVES

Mounting evidence shows that oxidative stress (OS) and the purine/adenosine system play a key role in the pathophysiology of schizophrenia. Lately, our group pointed out that not only antioxidants, but also the prooxidant system plays an important role in neuro-psychiatric disorders. Xanthine oxidase (XO) is an enzyme of special interest in this context, since it acts as a prooxidant, but its main product is a vastly important antioxidant, uric acid (UA). Furthermore, XO plays major part in the purine/adenosine metabolism, which has been hypothesised to play a role in schizophrenia as well.

METHODS

We examined the activity of XO in the striato-cortico-limbic system of schizophrenic patients (SP) and controls using a commercially available activity assay.

RESULTS

We found decreased activity of XO in the occipital cortex and thalamus of patients with psychosis. Furthermore, XO shows a significant positive correlation with chlorpromazine equivalents in the putamen and the temporal cortex.

CONCLUSIONS

Nevertheless, our results might suggest a downregulation of cellular defence mechanisms in schizophrenia in several brain regions, which could account for neuronal alterations which have been described before. This demonstrates that more research is needed to fully understand the role of the complex enzyme XO in the pathophysiology of schizophrenia.

Abnormalities in thalamic neurophysiology in schizophrenia: could psychosis be a result of potassium channel dysfunction?

Psychosis in schizophrenia is associated with source-monitoring deficits whereby self-initiated behaviors become attributed to outside sources. One of the proposed functions of the thalamus is to adjust sensory responsiveness in accordance with the behavioral contextual cues. The thalamus is markedly affected in schizophrenia, and thalamic dysfunction may here result in reduced ability to adjust sensory responsiveness to ongoing behavior. One of the ways in which the thalamus accomplishes the adjustment of sensory processing is by a neurophysiological shift to post-inhibitory burst firing mode prior to and during certain exploratory actions. Reduced amount of thalamic burst firing may result from increased neuronal excitability secondary to a reported potassium channel dysfunction in schizophrenia. Pharmacological agents that reduce the excitability of thalamic cells and thereby promote burst firing by and large tend to have antipsychotic effects.

Dipyridamole monotherapy in schizophrenia: pilot of a novel treatment approach by modulation of purinergic signaling

BACKGROUND

Emerging data indicate the neuromodulator adenosine may play a role in the therapeutics of schizophrenia. Adenosine A(2A) receptor stimulation exerts a functional antagonism at postsynaptic D(2) receptors. Data from animal models relevant to schizophrenia support a therapeutic effect of modulating adenosinergic transmission in the ventral striatum. One previous clinical trial showed superiority of adjunctive dipyridamole, an adenosine reuptake inhibitor, compared to placebo in ameliorating positive symptoms in schizophrenia patients.

OBJECTIVES

The aim of this study was to examine the effects of dipyridamole monotherapy of 200 mg/day on positive and negative symptoms, with the goal of determining dosing for future adjunctive studies in schizophrenia.

METHODS

Twenty symptomatic schizophrenia participants were randomized to a 6-week double-blind trial comparing olanzapine (20 mg/day) to dipyridamole monotherapy (200 mg/day). Thirteen participants completed the treatment phase (eight on dipyridamole; five on olanzapine).

RESULTS

The olanzapine group showed a trend (p = 0.08) for superiority on BPRS total scores (mean ± SD: total BPRS score decreasing from 36.8 ± 2.3 at week 1, to 33.2 ± 5.5 at the end of the study). The mean total BPRS scores decreased from 36.4 ± 5.3 to 34.0 ± 7.7 in the dipyridamole group.

CONCLUSIONS

Although these pilot data do not support a significant antipsychotic effect of dipyridamole monotherapy, the results provide some evidence for examining dipyridamole (200 mg/day) as adjunct to symptomatic antipsychotic-treated schizophrenia patients.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

Adenosine hypothesis of schizophrenia--opportunities for pharmacotherapy

Pharmacotherapy of schizophrenia based on the dopamine hypothesis remains unsatisfactory for the negative and cognitive symptoms of the disease. Enhancing N-methyl-D-aspartate receptors (NMDAR) function is expected to alleviate such persistent symptoms, but successful development of novel clinically effective compounds remains challenging. Adenosine is a homeostatic bioenergetic network modulator that is able to affect complex networks synergistically at different levels (receptor-dependent pathways, biochemistry, bioenergetics, and epigenetics). By affecting brain dopamine and glutamate activities, it represents a promising candidate for reversing the functional imbalance in these neurotransmitter systems believed to underlie the genesis of schizophrenia symptoms, as well as restoring homeostasis of bioenergetics. Suggestion of an adenosine hypothesis of schizophrenia further posits that adenosinergic dysfunction might contribute to the emergence of multiple neurotransmitter dysfunctions characteristic of schizophrenia via diverse mechanisms. Given the importance of adenosine in early brain development and regulation of brain immune response, it also bears direct relevance to the aetiology of schizophrenia. Here, we provide an overview of the rationale and evidence in support of the therapeutic potential of multiple adenosinergic targets, including the high-affinity adenosine receptors (A(1)R and A(2A)R), and the regulatory enzyme adenosine kinase (ADK). Key preliminary clinical data and preclinical findings are reviewed.

Adenosine, ketogenic diet and epilepsy: the emerging therapeutic relationship between metabolism and brain activity

For many years the neuromodulator adenosine has been recognized as an endogenous anticonvulsant molecule and termed a “retaliatory metabolite.” As the core molecule of ATP, adenosine forms a unique link between cell energy and neuronal excitability. In parallel, a ketogenic (high-fat, low-carbohydrate) diet is a metabolic therapy that influences neuronal activity significantly, and ketogenic diets have been used successfully to treat medically-refractory epilepsy, particularly in children, for decades. To date the key neural mechanisms underlying the success of dietary therapy are unclear, hindering development of analogous pharmacological solutions. Similarly, adenosine receptor–based therapies for epilepsy and myriad other disorders remain elusive. In this review we explore the physiological regulation of adenosine as an anticonvulsant strategy and suggest a critical role for adenosine in the success of ketogenic diet therapy for epilepsy. While the current focus is on the regulation of adenosine, ketogenic metabolism and epilepsy, the therapeutic implications extend to acute and chronic neurological disorders as diverse as brain injury, inflammatory and neuropathic pain, autism and hyperdopaminergic disorders. Emerging evidence for broad clinical relevance of the metabolic regulation of adenosine will be discussed.

Adenosine A(2A) receptors in psychopharmacology: modulators of behavior, mood and cognition

The adenosine A(2A) receptor (A(2A)R) is in the center of a neuromodulatory network affecting a wide range of neuropsychiatric functions by interacting with and integrating several neurotransmitter systems, especially dopaminergic and glutamatergic neurotransmission. These interactions and integrations occur at multiple levels, including (1) direct receptor- receptor cross-talk at the cell membrane, (2) intracellular second messenger systems, (3) trans-synaptic actions via striatal collaterals or interneurons in the striatum, (4) and interactions at the network level of the basal ganglia. Consequently, A(2A)Rs constitute a novel target to modulate various psychiatric conditions. In the present review we will first summarize the molecular interaction of adenosine receptors with other neurotransmitter systems and then discuss the potential applications of A(2A)R agonists and antagonists in physiological and pathophysiological conditions, such as psychostimulant action, drug addiction, anxiety, depression, schizophrenia and learning and memory.

Association of serum uric acid levels with emotional and affective temperaments

BACKGROUND

Temperament relates to emotions and the prevailing mood or affective temperament. Uric acid (UA) is the end-product of purine metabolism and has been associated with psychological features such as high energy/drive, positive affect, achievement, good performance, higher social status and leadership.

METHODS

129 subjects (44 males, 85 females) completed with the Combined Emotional and Affective Temperaments Scale, serum UA levels and a general health questionnaire.

RESULTS

In the whole sample, serum UA levels were significantly correlated with disinhibition (r=0.36, p<0.001) and drive (r=0.25, p<0.01), but not with control, anger or any of the affective temperament scores. Among males, we found correlations at trend level (p>0.05 and <0.07) for control (r=0.27), irritable (r=0.29) and hyperthymic (r=0.27) affective temperaments. Among females, a significant correlation was found only with disinhibition (r=0.34, p=0.001). The top tertile of males (serum UA>6.0 mg/ml, n=16) had significantly higher drive (29.9+/-5.9x26.0+/-3.6, p=0.01) and higher control at trend level (21.2+/-3.1x19.3+/-2.9, p=0.054) than other males. Among women, the top tertile (serum UA>4.0 mg/ml, n=29) showed higher disinhibition scores (20.7+/-4.9x17.9+/-3.6, p<0.01) and more frequent choices of hyperthymic (8/26x6/59, p=0.023) and irritable temperaments (7/26x5/59, p=0.031) than the rest of the sample. Controlling for daily intake of meat and grains, which could lead to higher UA levels, did not change these results.

LIMITATIONS

Small sample size for males.

CONCLUSIONS

Externalized traits of temperament are associated with higher serum UA levels both in men and women.

A double-blind trial of adjunctive allopurinol for schizophrenia

OBJECTIVE

To investigate if adjunctive allopurinol reduces symptoms in schizophrenia outpatients with persistent symptoms despite adequate pharmacotherapy.

METHOD

N=59 schizophrenia outpatients were randomly assigned to receive adjunctive allopurinol 300 mg bid or identical-looking placebo for 8 weeks after a 2-week placebo run-in. Symptoms were assessed biweekly.

RESULTS

A total of n=51 patients completed the trial. Including all n=59 randomized patients, a total of 4 of 31 in the allopurinol group and 0 of 28 in the placebo group had at least a 20% reduction in total PANSS score at the final study visit (chi-square=3.88, p=.049). Among the n=51 completers, individuals in the allopurinol group rated themselves as more improved than did those in the placebo group (z=-2.24, p=.025). The allopurinol medication was well tolerated and there were not any adverse events attributed to the study medication.

CONCLUSIONS

Allopurinol may be an effective adjunctive medication for some patients with persistent schizophrenia.

Anti-nociceptive properties of the xanthine oxidase inhibitor allopurinol in mice: role of A1 adenosine receptors

BACKGROUND AND PURPOSE

Allopurinol is a potent inhibitor of the enzyme xanthine oxidase, used primarily in the treatment of hyperuricemia and gout. It is well known that purines exert multiple effects on pain transmission. We hypothesized that the inhibition of xanthine oxidase by allopurinol, thereby reducing purine degradation, could be a valid strategy to enhance purinergic activity. The aim of this study was to investigate the anti-nociceptive profile of allopurinol on chemical and thermal pain models in mice.

EXPERIMENTAL APPROACH

Mice received an intraperitoneal (i.p.) injection of vehicle (Tween 10%) or allopurinol (10-400 mg kg(-1)). Anti-nociceptive effects were measured with intraplantar capsaicin, intraplantar glutamate, tail-flick or hot-plate tests.

KEY RESULTS

Allopurinol presented dose-dependent anti-nociceptive effects in all models. The opioid antagonist naloxone did not affect these anti-nociceptive effects. The non-selective adenosine-receptor antagonist caffeine and the selective A(1) adenosine-receptor antagonist, DPCPX, but not the selective A(2A) adenosine-receptor antagonist, SCH58261, completely prevented allopurinol-induced anti-nociception. No obvious motor deficits were produced by allopurinol, at doses up to 200 mg kg(-1). Allopurinol also caused an increase in cerebrospinal fluid levels of purines, including the nucleosides adenosine and guanosine, and decreased cerebrospinal fluid concentration of uric acid.

CONCLUSIONS AND IMPLICATIONS

Allopurinol-induced anti-nociception may be related to adenosine accumulation. Allopurinol is an old and extensively used compound and seems to be well tolerated with no obvious central nervous system toxic effects at high doses. This drug may be useful to treat pain syndromes in humans.

Potential therapeutic interest of adenosine A2A receptors in psychiatric disorders

The interest on targeting adenosine A(2A) receptors in the realm of psychiatric diseases first arose based on their tight physical and functional interaction with dopamine D(2) receptors. However, the role of central A(2A) receptors is now viewed as much broader than just controlling D(2) receptor function. Thus, there is currently a major interest in the ability of A(2A) receptors to control synaptic plasticity at glutamatergic synapses. This is due to a combined ability of A(2A) receptors to facilitate the release of glutamate and the activation of NMDA receptors. Therefore, A(2A) receptors are now conceived as a normalizing device promoting adequate adaptive responses in neuronal circuits, a role similar to that fulfilled, in essence, by dopamine. This makes A(2A) receptors particularly attractive targets to manage psychiatric disorders since adenosine may act as go-between glutamate and dopamine, two of the key players in mood processing. Furthermore, A(2A) receptors also control glia function and brain metabolic adaptation, two other emerging mechanisms to understand abnormal processing of mood, and A(2A) receptors are important players in controlling the demise of neurodegeneration, considered an amplificatory loop in psychiatric disorders. Current data only provide an indirect confirmation of this putative role of A(2A) receptors, based on the effects of caffeine (an antagonist of both A(1) and A(2A) receptors) in psychiatric disorders. However, the introduction of A(2A) receptors antagonists in clinics as anti-parkinsonian agents is hoped to bolster our knowledge on the role of A(2A) receptors in mood disorders in the near future.

A placebo controlled study of the propentofylline added to risperidone in chronic schizophrenia

Impaired activity of the purinergic system is a plausible common factor that could be responsible for many aspects of schizophrenia. Based on purinegic hypothesis of schizophrenia, pharmacological treatments enhancing adenosine activity could be effective treatment in schizophrenia. Propentofylline is a novel xantine derivative which is being developed for treatment of degenerative and vascular dementia. It enhances extracellular adenosine level via inhibition of adenosine uptake. The purpose of the present investigation was to assess the efficacy of propentofylline as an adjuvant agent in the treatment of chronic schizophrenia in an 8-week double blind and placebo controlled trial. Eligible participants in this study were 50 patients with chronic schizophrenia. All patients were inpatients and were in the active phase of the illness, and met DSM-IV-TR criteria for schizophrenia. Patients were allocated in a random fashion, 25 to risperidone 6 mg/day plus propentofylline 900 mg/day (300 mg TDS) and 25 to risperidone 6 mg/day plus placebo. The principal measure of the outcome was Positive and Negative Syndrome Scale (PANSS). Although both protocols significantly decreased the score of the positive, negative and general psychopathological symptoms over the trial period, the combination of risperidone and propentofylline showed a significant superiority over risperidone alone in the treatment of positive symptoms, general psychopathology symptoms as well as PANSS total scores. The means Extrapyramidal Symptoms Rating Scale for the placebo group were higher than in the propentofylline group over the trial. However, the differences were not significant. The present study indicates propentofylline as a potential adjunctive treatment strategy for chronic schizophrenia. Nevertheless, results of larger controlled trials are needed, before recommendation for a broad clinical application can be made.

Treatment-resistant schizophrenia

This article opens with a brief history of pharmacologic treatment of schizophrenia. It then discusses the definition and treatment of treatment-resistant schizophrenia, with particular attention to clinical, biological and neuroimaging correlates, as well as the best treatment options, including the use of clozapine in patients who meet the definition of treatment-resistant schizophrenia.

Increased serum adenosine deaminase activity in schizophrenic receiving antipsychotic treatment

Adenosine is an important modulator of the nervous system that has been implicated in the pathophysiology of schizophrenia. We studied peripheral adenosine metabolism by determining the activity of serum adenosine deaminase, which converts adenosine into inosine, and 5'-nucleotidase, which converts AMP into adenosine, in 26 DSM-IV male schizophrenic patients under antipsychotic monotherapy and 26 healthy volunteers balanced for age and race. Schizophrenic patients treated either with typical antipsychotics or clozapine showed increased serum adenosine deaminase activity compared to controls (controls=18.96+/-4.61 U/l; typical=25.09+/-10.98 U/l; clozapine=30.32+/-10.83 U/l; p<0.05, ANOVA) and 5'-nucleotidase activity was also increased in patients on clozapine. After adjusting for confounding factors, adenosine deaminase, but not 5'-nucleotidase, alterations remained significant particularly in the clozapine group. This result suggests that either altered adenosine metabolism is present in schizophrenic patients or is influenced by treatment with antipsychotics, particularly clozapine.

Involvement of adenosine in the neurobiology of schizophrenia and its therapeutic implications

Based on the neuromodulatory and homeostatic actions of adenosine, adenosine dysfunction may contribute to the neurobiological and clinical features of schizophrenia. The present model of adenosine dysfunction in schizophrenia takes into consideration the dopamine and glutamate hypotheses, since adenosine exerts neuromodulatory roles on these systems, and proposes that adenosine plays a role in the inhibitory deficit found in schizophrenia. Given the role of adenosine activation of adenosine A1 receptor (A1R) in mediating neurotoxicity in early stages of brain development, pre- and peri-natal complications leading to excessive adenosine release could induce primary brain changes (i.e., first hit). These events would lead to an adenosine inhibitory deficit through a partial loss of A1R that may emerge as reduced control of dopamine activity and increased vulnerability to excitotoxic glutamate action in the mature brain (i.e., second hit). Adenosine dysfunction is reasonably compatible with symptoms, gray and white matter abnormalities, progressive brain loss, pre- and peri-natal risk factors, age of onset, response to current treatments, impaired sensory gating and increased smoking in schizophrenia. Pharmacological treatments enhancing adenosine activity could be effective for symptom control and for alleviating deterioration in the course of the illness. Accordingly, allopurinol, which may indirectly increase adenosine, has been effective and well tolerated in the treatment of schizophrenia. Since much of the evidence for the adenosine hypothesis is preliminary and theoretical, further investigation in the field is warranted.

Beneficial antipsychotic effects of allopurinol as add-on therapy for schizophrenia: a double blind, randomized and placebo controlled trial

There is a large amount of data showing that adenosine plays a role opposite to dopamine in the brain. Adenosine agonists and antagonists produce behavioral effects similar to dopamine antagonists and dopamine agonists, respectively. Allopurinol, a well-known hypouricemic drug that inhibits xantine oxidase, has been used as an add-on drug in the treatment of poorly responsive schizophrenic patients. Indeed, the neuropsychiatric effects of allopurinol in schizophrenia have been suggested to be secondary to its inhibitory effect of purine degradation, enhancing adenosinergic activity. The purpose of the present investigation was to assess the efficacy of allopurinol as an adjuvant agent in the treatment of chronic schizophrenia in an 8-week double blind and placebo controlled trial. Eligible participations in the study were 46 patients with schizophrenia. All patients were inpatients and were in the active phase of the illness, and met DSM-IV criteria for chronic schizophrenia. Patients were allocated in a random fashion, 23 to haloperidol 15 mg/day plus allopurinol 300 mg/day and 23 to haloperidol 15 mg/day plus placebo. Although both protocols significantly decreased the score of the positive, negative and general psychopathological symptoms over the trial period, the combination of haloperidol and allopurinol showed a significant superiority over haloperidol alone in the treatment of positive symptoms, general psychopathology symptoms as well as PANSS total scores. The means of Extrapyramidal Symptoms Rating Scale for the placebo group were higher than in the allopurinol group over the trial, and the differences were significant in weeks 6 and 8. A significant difference was observed between the overall mean biperiden dosages in two groups. The results of this study suggest that allopurinol may be an effective adjuvant agent in the management of patients with chronic schizophrenia. Nevertheless, results of larger controlled trials are needed, before recommendations for a broad clinical application can be made.

Allopurinol for the treatment of aggressive behaviour in patients with dementia

Aggressive behaviour is commonly observed in patients with dementia, and current pharmacological treatments are still deficient in terms of efficacy and tolerability. Allopurinol is an inhibitor of the enzyme xanthine oxidase, with previously suggested anti-aggressive effects. After successful treatment of aggression in two patients, we performed a case-series study with allopurinol 300 mg a day orally for 6 weeks (increasing 300 mg every 2 weeks if the response was less than 50%) in six patients with dementia associated with prominent aggressive behaviour who failed to respond to two previous treatment strategies. Five patients were considerably responsive to allopurinol (four with 300 mg within 2 weeks and one with 600 mg), apparently without side-effects, which is in accordance with its well-established safety and tolerability profile. The observed therapeutic effect of allopurinol might be due to the inhibition of the enzyme xanthine oxidase, possibly decreasing production of oxygen-free radicals or promoting the accumulation of purines. Controlled studies are warranted to confirm these preliminary observations.

Inhibitory deficit in schizophrenia is not necessarily a GABAergic deficit

1. Current evidence strongly supports the idea of an inhibitory deficit as a central pathophysiological mechanism in schizophrenia. This deficit has been well documented in sensory gating and paired-pulse studies and may be related to decreases in inhibitory interneurons found in schizophrenic patients. 2. The GABAergic system has been repeatedly postulated to mediate this deficit, but the findings are controversial, at least in some areas, and mostly negative regarding treatment with drugs enhancing GABAergic activity. Therefore, the scope of mediators of this inhibitory deficit should be widened and the neuromodulator adenosine is proposed as a candidate to be further studied. 3. A state of adenosinergic hypoactivity in schizophrenia is compatible not only with the inhibitory deficit but also with symptoms, clinical response to antipsychotics, impaired sensory gating, deteriorating course, increased smoking, and sleep alterations reported in schizophrenia. 4. It is concluded that although the GABAergic system should be further studied, especially in sensory gating model in humans, emphasis on other inhibitory mechanisms may prove useful and provide more effective treatment.