Age- and dose-dependent glucose-induced increases in memory and attention in schizophrenia

Glycated Haemoglobin Is Associated With Poorer Cognitive Performance in Patients With Recent-Onset Psychosis

BACKGROUND

Glucose abnormalities and cognitive alterations are present before the onset of schizophrenia. We aimed to study whether glucose metabolism parameters are associated with cognitive functioning in recent-onset psychosis (ROP) patients while adjusting for hypothalamic-pituitary adrenal (HPA) axis measures.

METHODS

Sixty ROP outpatients and 50 healthy subjects (HS) were studied. Cognitive function was assessed with the MATRICS Consensus Cognitive Battery. Glycated haemoglobin (HbA1_c), glucose, insulin, and C-peptide levels were determined in plasma. The HOMA-insulin resistance index was calculated. Salivary samples were obtained at home on another day to assess the cortisol awakening response and cortisol levels during the day. Univariate analyses were conducted to explore the association between glucose metabolism parameters and cognitive tasks. For those parameters that were more clearly associated with the cognitive outcome, multiple linear regression analyses were conducted to adjust for covariates. Each cognitive task was considered the dependent variable. Covariates were age, sex, education level, diagnosis, antipsychotic and benzodiazepine treatment, body mass index (BMI), smoking, and HPA axis measures. Potential interactions between diagnosis and glucose parameters were tested.

RESULTS

There were no significant differences in HPA axis measures or glucose parameters, with the exception of C-peptide (that was higher in ROP patients), between groups. ROP patients had a lower performance than HS in all cognitive tasks (p < 0.01 for all tasks). Of all glucose metabolism parameters, HbA1c levels were more clearly associated with cognitive impairment in cognitive tasks dealing with executive functions and visual memory in both ROP patients and HS. Multivariate analyses found a significant negative association between HbA1c and cognitive functioning in five cognitive tasks dealing with executive functions, visual memory and attention/vigilance (a ROP diagnosis by HbA1_c negative interaction was found in this latter cognitive domain, suggesting that HBA1_c levels are associated with impaired attention only in ROP patients).

CONCLUSIONS

Our study found that HbA1_c was negatively associated with cognitive functioning in both ROP patients and HS in tasks dealing with executive functions and visual memory. In ROP patients, HbA1_c was also associated with impaired attention. These results were independent of BMI and measures of HPA axis activity.

Brain insulin resistance and altered brain glucose are related to memory impairments in schizophrenia

Memory is robustly impaired in schizophrenia (SZ) and related to functional outcome. Memory dysfunction has been shown to be related to altered brain glucose metabolism and brain insulin resistance in animal models and human studies of Alzheimer's disease. In this study, differences in brain glucose using magnetic resonance spectroscopy (MRS) and blood Extracellular Vesicle (EV) biomarkers of neuronal insulin resistance (i.e. Akt and signaling effectors) between SZ and controls were investigated, as well as whether these measures were related to memory impairments. Neuronal insulin resistance biomarkers showed a trend for being lower in SZ compared to controls, and memory measures were lower in SZ compared to controls. Occipital cortex glucose was higher in SZ compared to controls indicating lower brain glucose utilization. Linear regression analyses revealed significant relationships between neuronal insulin resistance biomarkers, memory measures, and brain glucose. More specifically, p70S6K, an insulin signaling effector, was related to verbal learning and brain MRS glucose in the SZ group. For the first time, we show that memory impairments in SZ may be related to brain glucose and brain insulin resistance. These data suggest that brain insulin resistance may play a role in the pathophysiology of learning and memory dysfunction in SZ.

Mental Work Requires Physical Energy: Self-Control Is Neither Exception nor Exceptional

The brain’s reliance on glucose as a primary fuel source is well established, but psychological models of cognitive processing that take energy supply into account remain uncommon. One exception is research on self-control depletion, where debate continues over a limited-resource model. This model argues that a transient reduction in self-control after the exertion of prior self-control is caused by the depletion of brain glucose, and that self-control processes are special, perhaps unique, in this regard. This model has been argued to be physiologically implausible in several recent reviews. This paper attempts to correct some inaccuracies that have occurred during debate over the physiological plausibility of this model. We contend that not only is such limitation of cognition by constraints on glucose supply plausible, it is well established in the neuroscience literature across several cognitive domains. Conversely, we argue that there is no evidence that self-control is special in regard to its metabolic cost. Mental processes require physical energy, and the body is limited in its ability to supply the brain with sufficient energy to fuel mental processes. This article reviews current findings in brain metabolism and seeks to resolve the current conflict in the field regarding the physiological plausibility of the self-control glucose-depletion hypothesis.

The impact of diet-based glycaemic response and glucose regulation on cognition: evidence across the lifespan

The brain has a high metabolic rate and its metabolism is almost entirely restricted to oxidative utilisation of glucose. These factors emphasise the extreme dependence of neural tissue on a stable and adequate supply of glucose. Whereas initially it was thought that only glucose deprivation (i.e. under hypoglycaemic conditions) can affect brain function, it has become apparent that low-level fluctuations in central availability can affect neural and consequently, cognitive performance. In the present paper the impact of diet-based glycaemic response and glucose regulation on cognitive processes across the lifespan will be reviewed. The data suggest that although an acute rise in blood glucose levels has some short-term improvements of cognitive function, a more stable blood glucose profile, which avoids greater peaks and troughs in circulating glucose is associated with better cognitive function and a lower risk of cognitive impairments in the longer term. Therefore, a habitual diet that secures optimal glucose delivery to the brain in the fed and fasting states should be most advantageous for the maintenance of cognitive function. Although the evidence to date is promising, it is insufficient to allow firm and evidence-based nutritional recommendations. The rise in obesity, diabetes and metabolic syndrome in recent years highlights the need for targeted dietary and lifestyle strategies to promote healthy lifestyle and brain function across the lifespan and for future generations. Consequently, there is an urgent need for hypothesis-driven, randomised controlled trials that evaluate the role of different glycaemic manipulations on cognition.

Glucose improves object-location binding in visual-spatial working memory

RATIONALE

There is evidence that glucose temporarily enhances cognition and that processes dependent on the hippocampus may be particularly sensitive. As the hippocampus plays a key role in binding processes, we examined the influence of glucose on memory for object-location bindings.

OBJECTIVE

This study aims to study how glucose modifies performance on an object-location memory task, a task that draws heavily on hippocampal function.

METHODS

Thirty-one participants received 30 g glucose or placebo in a single 1-h session. After seeing between 3 and 10 objects (words or shapes) at different locations in a 9 × 9 matrix, participants attempted to immediately reproduce the display on a blank 9 × 9 matrix. Blood glucose was measured before drink ingestion, mid-way through the session, and at the end of the session.

RESULTS

Glucose significantly improves object-location binding (d = 1.08) and location memory (d = 0.83), but not object memory (d = 0.51). Increasing working memory load impairs object memory and object-location binding, and word-location binding is more successful than shape-location binding, but the glucose improvement is robust across all difficulty manipulations. Within the glucose group, higher levels of circulating glucose are correlated with better binding memory and remembering the locations of successfully recalled objects.

CONCLUSIONS

The glucose improvements identified are consistent with a facilitative impact on hippocampal function. The findings are discussed in the context of the relationship between cognitive processes, hippocampal function, and the implications for glucose's mode of action.

Severe disturbance of glucose metabolism in peripheral blood mononuclear cells of schizophrenia patients: a targeted metabolomic study

Background

Schizophrenia is a widespread and debilitating mental disorder. However, the underlying molecular mechanism of schizophrenia remains largely unknown and no objective laboratory tests are available to diagnose this disorder. The aim of the present study was to characterize the alternations of glucose metabolites and identify potential diagnostic biomarkers for schizophrenia.

Methods

Gas chromatography/mass spectrometry based targeted metabolomic method was used to quantify the levels of 13 glucose metabolites in peripheral blood mononuclear cells (PBMCs) derived from healthy controls, schizophrenia and major depression subjects (n = 55 for each group).

Results

The majority (84.6%) of glucose metabolites were significantly disturbed in schizophrenia subjects, while only two (15.4%) glucose metabolites were differently expressed in depression subjects relative to healthy controls in both training set (n = 35/group) and test set (n = 20/group). Antipsychotics had only a subtle effect on glucose metabolism pathway. Moreover, ribose 5-phosphate in PBMCs showed a high diagnostic performance for first-episode drug-naïve schizophrenia subjects.

Conclusion

These findings suggested disturbance of glucose metabolism may be implicated in onset of schizophrenia and could aid in development of diagnostic tool for this disorder.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0540-y) contains supplementary material, which is available to authorized users.

Coupling physical exercise with dietary glucose supplement for treating cognitive impairment in schizophrenia: a theoretical model and future directions

AIMS

Metabolic dysregulation may disrupt the complex neuroprotective mechanisms essential for brain health. Recent studies have pointed out the possible aetiological role of metabolic dysregulation in the onset of schizophrenia and the associated cognitive impairment. In this paper, we aimed to generate a theoretical model of how a combination of physical exercise and dietary glucose supplement may help to alleviate cognitive impairment in schizophrenia.

METHODS

Literature on metabolic dysregulation, especially insulin resistance, in relation to the onset of schizophrenia and the associated cognitive impairment is reviewed. The cognitive enhancement effects of physical exercise and dietary glucose supplement are then summarised. Finally, we propose a theoretical model based on the concerted effects of physical exercise and glucose supplement.

RESULTS

In general, the joint action of physical exercise and dietary glucose supplement could up-regulate glucose and insulin transport into the brain, as well as augmenting the release of insulin growth factor-1 and brain-derived neurotrophic factor. Physical exercise and glucose supplement could enhance energy supply and neuroplasticity in brain, subsequently leading to potential cognitive enhancement in schizophrenia. However, glucose supplement is not suitable for patients with abnormal metabolic profile.

CONCLUSIONS

The combination of physical exercise and glucose supplement has potential therapeutic values in treating cognitive impairment in schizophrenia. Further research is necessary to investigate the optimal patterns of exercise and doses of glucose for treating cognitive impairment in schizophrenia.

Drug discovery based on genetic and metabolic findings in schizophrenia

Recent progress in the genetics of schizophrenia provides the rationale for re-evaluating causative factors and therapeutic strategies for this disease. Here, we review the major candidate susceptibility genes and relate the aberrant function of these genes to defective regulation of energy metabolism in the schizophrenic brain. Disturbances in energy metabolism potentially lead to neurodevelopmental deficits, impaired function of the mature nervous system and failure to maintain neurites/dendrites and synaptic connections. Current antipsychotic drugs do not specifically address these underlying deficits; therefore, a new generation of more effective medications is urgently needed. Novel targets for future drug discovery are identified in this review. The coordinated application of structure-based drug design, systems biology and research on model organisms may greatly facilitate the search for next-generation antipsychotic drugs.

The proteomic to biology inference, a frequently overlooked concern in the interpretation of proteomic data: a plea for functional validation

Proteomics will celebrate its 20th year in 2014. In this relatively short period of time, it has invaded most areas of biology and its use will probably continue to spread in the future. These two decades have seen a considerable increase in the speed and sensitivity of protein identification and characterization, even from complex samples. Indeed, what was a challenge twenty years ago is now little more than a daily routine. Although not completely over, the technological challenge now makes room to another challenge, which is the best possible appraisal and exploitation of proteomic data to draw the best possible conclusions from a biological point of view. The point developed in this paper is that proteomic data are almost always fragmentary. This means in turn that although better than an mRNA level, a protein level is often insufficient to draw a valid conclusion from a biological point of view, especially in a world where PTMs play such an important role. This means in turn that transformation of proteomic data into biological data requires an important intermediate layer of functional validation, i.e. not merely the confirmation of protein abundance changes by other methods, but a functional appraisal of the biological consequences of the protein level changes highlighted by the proteomic screens.

Immune system and glucose metabolism interaction in schizophrenia: a chicken-egg dilemma

Impaired glucose metabolism and the development of metabolic syndrome contribute to a reduction in the average life expectancy of individuals with schizophrenia. It is unclear whether this association simply reflects an unhealthy lifestyle or whether weight gain and impaired glucose tolerance in patients with schizophrenia are directly attributable to the side effects of atypical antipsychotic medications or disease-inherent derangements. In addition, numerous previous studies have highlighted alterations in the immune system of patients with schizophrenia. Increased concentrations of interleukin (IL)-1, IL-6, and transforming growth factor-beta (TGF-β) appear to be state markers, whereas IL-12, interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), and soluble IL-2 receptor (sIL-2R) appear to be trait markers of schizophrenia. Moreover, the mononuclear phagocyte system (MPS) and microglial activation are involved in the early course of the disease. This review illustrates a "chicken-egg dilemma", as it is currently unclear whether impaired cerebral glucose utilization leads to secondary disturbances in peripheral glucose metabolism, an increased risk of cardiovascular complications, and accompanying pro-inflammatory changes in patients with schizophrenia or whether immune mechanisms may be involved in the initial pathogenesis of schizophrenia, which leads to disturbances in glucose metabolism such as metabolic syndrome. Alternatively, shared underlying factors may be responsible for the co-occurrence of immune system and glucose metabolism disturbances in schizophrenia.

Memory processing and the glucose facilitation effect: The effects of stimulus difficulty and memory load

Previous research has consistently found enhancement of memory after the ingestion of a glucose containing drink. The aims of the present study were to specify more precisely the nature of this facilitation by examining the cognitive demand hypothesis. This hypothesis predicts greater glucose induced facilitation on tasks that require significant mental effort. In two experiments, both employing an unrelated sample design, participants consumed either 25 g of glucose or a control solution. In experiment 1, participants first studied low and high imagery word-pairs and memory was assessed 1-, 7- and 14-days later by cued recall. Overall, glucose enhanced both encoding and consolidation processes only for the more difficult low imagery pairs. In experiment 2, the degree of mental effort in a verbal memory task was manipulated in two ways: (1) by varying the phonological similarity of the words; and (2) by varying the length of word lists. Glucose was found to enhance memory only for longer word lists. These data are consistent with the idea that glucose is especially effective in demanding memory tasks, but place some limits on the forms of difficulty that are susceptible to enhancement.

The Influence of Fat Co-administration on the Glucose Memory Facilitation Effect

Memory for a list of 20 words can be enhanced when learning is preceded by consumption of 25 g of glucose, compared with consumption of an equally sweet aspartame solution. The present study examined whether memory performance is also enhanced when glucose is administered in conjunction with another food constituent, in particular fat. Four groups of healthy young participants were tested under one of four conditions: (a) glucose + full-fat yoghurt; (b) glucose + fat-free yoghurt; (c) aspartame + full-fat yoghurt; (d) aspartame + fat-free yoghurt. The groups were compared on measures of blood glucose and cognitive performance. Participants receiving a glucose drink in conjunction with a fat-free yoghurt displayed higher blood glucose levels (BGL) and better performance on short- and long-delay recall of the word list compared with (a) individuals who consumed the glucose drink in conjunction with a full-fat yoghurt and (b) individuals who consumed the aspartame drink. The glycaemic data indicated that the presence of fat slows down glucose absorption. The findings suggest that only foods with a relatively fast glucose absorption rate are able to significantly enhance the encoding and long-term retention of novel memory materials in healthy young adults.

The application of selective reaction monitoring confirms dysregulation of glycolysis in a preclinical model of schizophrenia

BACKGROUND

Establishing preclinical models is essential for novel drug discovery in schizophrenia. Most existing models are characterized by abnormalities in behavioral readouts, which are informative, but do not necessarily translate to the symptoms of the human disease. Therefore, there is a necessity of characterizing the preclinical models from a molecular point of view. Selective reaction monitoring (SRM) has already shown promise in preclinical and clinical studies for multiplex measurement of diagnostic, prognostic and treatment-related biomarkers.

METHODS

We have established an SRM assay for multiplex analysis of 7 enzymes of the glycolysis pathway which is already known to be affected in human schizophrenia and in the widely-used acute PCP rat model of schizophrenia. The selected enzymes were hexokinase 1 (Hk1), aldolase C (Aldoc), triosephosphate isomerase (Tpi1), glyceraldehyde-3-phosphate dehydrogenase (Gapdh), phosphoglycerate mutase 1 (Pgam1), phosphoglycerate kinase 1 (Pgk1) and enolase 2 (Eno2). The levels of these enzymes were analyzed using SRM in frontal cortex from brain tissue of PCP treated rats.

RESULTS

Univariate analyses showed statistically significant altered levels of Tpi1 and alteration of Hk1, Aldoc, Pgam1 and Gapdh with borderline significance in PCP rats compared to controls. Most interestingly, multivariate analysis which considered the levels of all 7 enzymes simultaneously resulted in generation of a bi-dimensional chart that can distinguish the PCP rats from the controls.

CONCLUSIONS

This study not only supports PCP treated rats as a useful preclinical model of schizophrenia, but it also establishes that SRM mass spectrometry could be used in the development of multiplex classification tools for complex psychiatric disorders such as schizophrenia.

The Impact of Age and Task Domain on Cognitive Performance: A Meta-Analytic Review of the Glucose Facilitation Effect

Considerable research has been devoted to the issue of whether the ingestion of glucose enhances cognitive performance in both younger and older adults. However, it has proven difficult to draw firm conclusions from this literature due to the diversity of methodologies that have been used, and it is therefore still unclear what factors might moderate the facilitative effect. The present study investigates methodological variations and their impact on the magnitude of the glucose enhancement effect. In particular, age group (young vs. old) and task domain (memory vs. nonmemory tasks) were considered as moderator variables. In addition, differences in experimental design, glucose dose and fasting regime were examined. The effect size d was extracted from studies comparing cognitive performance after the ingestion of either a glucose or control solution. Subsequent meta-analyses were conducted (using the procedures outlined by Hedges & Olkin, 1985) to examine potential moderators. A moderate overall effect size was found (d = 0.56), which clearly demonstrated the value of glucose ingestion as a cognitive enhancer. However, this effect size was not representative of the 104 individual effect sizes reported. Secondary analyses found larger effect sizes for memory compared to nonmemory tasks. Unexpectedly, no evidence was provided for greater beneficial effects of glucose on cognitive performance for older compared to younger adults. The impact of other methodological variables and the need for further exploration in an elderly population are discussed.

Proteomics as a tool for understanding schizophrenia

Schizophrenia is likely to be a multifactorial disorder, consequence of alterations in gene and protein expression since the neurodevelopment that together to environmental factors will trigger the establishment of the disease. In the post-genomic era, proteomics has emerged as a promising strategy for revealing disease and treatment biomarkers as well as a tool for the comprehension of the mechanisms of schizophrenia pathobiology. Here, there is a discussion of the potential pathways and structures that are compromised in schizophrenia according to proteomic findings while studying five distinct brain regions of post-mortem tissue from schizophrenia patients and controls. Proteins involved in energy metabolism, calcium homeostasis, myelinization, and cytoskeleton have been recurrently found to be differentially expressed in schizophrenia brains. These findings may encourage new studies on the understanding of schizophrenia biochemical pathways and even new potential drug targets.

Oxidative stress in schizophrenia: pathogenetic and therapeutic implications

Over a century, a wide-ranging variety of pathophysiological models and causal hypotheses have been conceptualized for schizophrenia. One among these is the role for free radical-mediated pathology in schizophrenia, indicating impaired antioxidant defense system (AODS) and presence of oxidative stress in patients with schizophrenia. For the past two decades, the whole investigative domain of AODS and oxidative stress has broadened to include the wider AODS components, direct central nervous system assays of AODS, chemical imaging studies, proteomics, genetics of AODS, and, of importance to sufferers of schizophrenia, antioxidant therapeutics. These are some of the perspectives that are reviewed by several articles in this Forum. Overall, there has been growing recognition of the importance of oxidative stress in the pathophysiology of schizophrenia and in treatment-related side effects. The totality of the evidence from biochemistry, metabolomics, proteomics, genetics, and in vivo brain imaging points to the presence of multifarious abnormalities in the AODS and redox signaling in schizophrenia.

The role of energy metabolism dysfunction and oxidative stress in schizophrenia revealed by proteomics

Schizophrenia is a psychiatric illness that affects approximately 30 million people worldwide. Converging lines of evidence suggest that mitochondrial function may be compromised in this disorder, and this can lead to perturbations in calcium buffering, oxidative phosphorylation, increased production of reactive oxygen species, and apoptotic factors, which can, in turn, affect neuronal processes such as neurotransmitter synthesis and synaptic plasticity. Proteomics studies in brain and peripheral tissues of schizophrenia patients have provided considerable evidence and identified biomarker fingerprints corresponding to such pathways. Here we review the results of these studies with a focus on the biomarker pattern depicting alterations in energy metabolism and oxidative stress in this debilitating illness.

Metabolic agents that enhance ATP can improve cognitive functioning: a review of the evidence for glucose, oxygen, pyruvate, creatine, and L-carnitine

Over the past four or five decades, there has been increasing interest in the neurochemical regulation of cognition. This field received considerable attention in the 1980s, with the identification of possible cognition enhancing agents or "smart drugs". Even though many of the optimistic claims for some agents have proven premature, evidence suggests that several metabolic agents may prove to be effective in improving and preserving cognitive performance and may lead to better cognitive aging through the lifespan. Aging is characterized by a progressive deterioration in physiological functions and metabolic processes. There are a number of agents with the potential to improve metabolic activity. Research is now beginning to identify these various agents and delineate their potential usefulness for improving cognition in health and disease. This review provides a brief overview of the metabolic agents glucose, oxygen, pyruvate, creatine, and L-carnitine and their beneficial effects on cognitive function. These agents are directly responsible for generating ATP (adenosine triphosphate) the main cellular currency of energy. The brain is the most metabolically active organ in the body and as such is particularly vulnerable to disruption of energy resources. Therefore interventions that sustain adenosine triphosphate (ATP) levels may have importance for improving neuronal dysfunction and loss. Moreover, recently, it has been observed that environmental conditions and diet can affect transgenerational gene expression via epigenetic mechanisms. Metabolic agents might play a role in regulation of nutritional epigenetic effects. In summary, the reviewed metabolic agents represent a promising strategy for improving cognitive function and possibly slowing or preventing cognitive decline.

Abnormalities in the tricarboxylic acid (TCA) cycle in the brains of schizophrenia patients

Images of brain metabolism and measurements of activities of components of the electron transport chain support earlier studies that suggest that brain glucose oxidation is inherently abnormal in a significant proportion of persons with schizophrenia. Therefore, we measured the activities of enzymes of the tricarboxylic (TCA) cycle in dorsolateral-prefrontal-cortex from schizophrenia patients (N=13) and non-psychiatric disease controls (N=13): the pyruvate dehydrogenase complex (PDHC), citrate synthase (CS), aconitase, isocitrate dehydrogenase (ICDH), the alpha-ketoglutarate dehydrogenase complex (KGDHC), succinate thiokinase (STH), succinate dehydrogenase (SDH), fumarase and malate dehydrogenase (MDH). Activities of aconitase (18.4%, p<0.05), KGDHC (26%) and STH (28.2%, p<0.05), enzymes in the first half of the TCA cycle, were lower, but SDH (18.3%, p<0.05) and MDH (34%, p<0.005), enzymes in the second half, were higher than controls. PDHC, CS, ICDH and fumarase activities were unchanged. There were no significant correlations between enzymes of TCA cycle and cognitive function, age or choline acetyl transferase activity, except for aconitase activity which decreased slightly with age (r=0.55, p=003). The increased activities of dehydrogenases in the second half of the TCA cycle may reflect a compensatory response to reduced activities of enzymes in the first half. Such alterations in the components of TCA cycle are adequate to alter the rate of brain metabolism. These results are consistent with the imaging studies of hypometabolism in schizophrenia. They suggest that deficiencies in mitochondrial enzymes can be associated with mental disease that takes the form of schizophrenia.

Glucose enhancement of human memory: a comprehensive research review of the glucose memory facilitation effect

The brain relies upon glucose as its primary fuel. In recent years, a rich literature has developed from both human and animal studies indicating that increases in circulating blood glucose can facilitate cognitive functioning. This phenomenon has been termed the 'glucose memory facilitation effect'. The purpose of this review is to discuss a number of salient studies which have investigated the influence of glucose ingestion on neurocognitive performance in individuals with (a) compromised neurocognitive capacity, as well as (b) normally functioning individuals (with a focus on research conducted with human participants). The proposed neurocognitive mechanisms purported to underlie the modulatory effect of glucose on neurocognitive performance will also be considered. Many theories have focussed upon the hippocampus, given that this brain region is heavily implicated in learning and memory. Further, it will be suggested that glucose is a possible mechanism underlying the phenomenon that enhanced memory performance is typically observed for emotionally laden stimuli.

S100B Serum Levels in Schizophrenia Are Presumably Related to Visceral Obesity and Insulin Resistance

Elevated blood levels of S100B in schizophrenia have so far been mainly attributed to glial pathology, as S100B is produced by astro- and oligodendroglial cells and is thought to act as a neurotrophic factor with effects on synaptogenesis, dopaminergic and glutamatergic neutrotransmission. However, adipocytes are another important source of S100B since the concentration of S100B in adipose tissue is as high as in nervous tissue. Insulin is downregulating S100B in adipocytes, astrocyte cultures and rat brain. As reviewed in this paper, our recent studies suggest that overweight, visceral obesity, and peripheral/cerebral insulin resistance may be pivotal for at least part of the elevated S100B serum levels in schizophrenia. In the context of this recently identified framework of metabolic disturbances accompanying S100B elevation in schizophrenia, it rather has to be attributed to systemic alterations in glucose metabolism than to be considered a surrogate marker for astrocyte-specific pathologies.

Prefrontal cortex shotgun proteome analysis reveals altered calcium homeostasis and immune system imbalance in schizophrenia

Schizophrenia is a complex disease, likely to be caused by a combination of serial alterations in a number of genes and environmental factors. The dorsolateral prefrontal cortex (Brodmann's Area 46) is involved in schizophrenia and executes high-level functions such as working memory, differentiation of conflicting thoughts, determination of right and wrong concepts and attitudes, correct social behavior and personality expression. Global proteomic analysis of post-mortem dorsolateral prefrontal cortex samples from schizophrenia patients and non-schizophrenic individuals was performed using stable isotope labeling and shotgun proteomics. The analysis resulted in the identification of 1,261 proteins, 84 of which showed statistically significant differential expression, reinforcing previous data supporting the involvement of the immune system, calcium homeostasis, cytoskeleton assembly, and energy metabolism in schizophrenia. In addition a number of new potential markers were found that may contribute to the understanding of the pathogenesis of this complex disease.

Systemic hypotheses for generalized cognitive deficits in schizophrenia: a new take on an old problem

The schizophrenia research community, including government, industry, and academia, has made development of procognitive treatment strategies a priority. Much current research is directed at dividing broad impairments in cognition into more delineated components that might correspond to relatively specific neural systems and serve as targets for intervention. Sometimes overlooked in this ambitious agenda is the substantial neuropsychological literature that signals a more broadly generalized dysfunction in higher order cognitive functions in this illness. In this article, we argue that a generalized cognitive deficit is at the core of the disorder, is not a methodological artifact, and deserves more focused consideration from cognitive specialists in the field. Further, we weigh evidence that this broad deficit may have systemic biological underpinnings. At the level of the central nervous system, examples of findings that might help to account for broad cognitive impairment include gray and white matter irregularities, poor signal integration by neurons and neural networks, and abnormalities in glutamate and gamma-aminobutyric acid neurotransmission. Other, more speculative hypotheses focus on even broader somatic systems, including energy metabolism and inflammatory processes. Treatment implications of systemic conceptualizations of schizophrenia are also considered.

P3b versus P3a: an event-related potential investigation of the glucose facilitation effect

The ingestion of a glucose containing drink has been shown to improve performance on a variety of cognitive tasks. There is debate, however, as to whether glucose especially benefits hippocampal memory functioning or whether it has a more global effect on attentional systems. The present study used event related potential methodology (ERPs) to investigate further glucose-mediated cognitive processes. Each participant acted as his/her own control in a repeated measures design, receiving one of two possible treatments (25 g glucose vs. placebo) in a counterbalanced order. After a two hour fasting period participants completed a visual three-stimulus oddball task. This paradigm involves an individual detecting an infrequent target stimulus randomly embedded in a train of repetitive background or standard stimuli. Detection of the target results in a large P3b ERP component (memory updating effect). The infrequent presentation of a novel and irrelevant stimulus, randomly interspersed with the target and standard stimuli, generates a P3a response (orientation of attention effect). These components were used as markers to establish whether the glucose enhancement effect was restricted to the neuro-cognitive processes related to memory. Consistent with behavioural work, glucose moderated the magnitude and latency of the P3b ERP component. However, glucose also interacted with attentional systems (P3a and an earlier P2), although this effect was non-significant. This work converges with recent fMRI findings indicating the sensitivity of the medial-temporal lobes and the pre-frontal cortex to glucose administration.

Examining encoding imprecision in spatial working memory in schizophrenia

BACKGROUND

Visuospatial working memory is not a unitary sketch pad but comprises independent dimensions of target distance and direction and at least two levels of detail (fine-grained and category level). The aim of this study was to examine these multiple aspects of encoding in patients with schizophrenia using a modified delayed response task.

METHOD

42 patients with schizophrenia and 48 healthy controls pointed, as accurately as possible from a fixed starting position, to the visual location of target stimuli presented to a touch-sensitive screen. An adaptive staircase procedure was used to equate stimulus duration for each individual. Encoding accuracy and maintenance of distance (mm) and direction ( degrees ) information was examined following a 0-second (immediate) or 4-second (unfilled) delay. Analyses utilized both absolute (unsigned) and signed data.

RESULTS

The results showed that the average duration required to detect a target was significantly longer in patients than controls. When stimulus duration was equated, (a) the absolute accuracy of distance and direction responses was not significantly different between groups at 0-second delay but was significantly reduced at 4-second delay in patients with schizophrenia, and (b) signed direction errors at 4-second delay were significantly different between groups at stimulus angles greater than 90 degrees .

CONCLUSIONS

The findings challenge previous suggestions of deficits in fine-grained encoding of spatial information in schizophrenia but confirm a difficulty maintaining both direction and distance details in working memory. Imprecision in spatial memory in schizophrenia also introduced greater bias from category level (prior) representations, especially in left hemi-space.

Acute high-dose glycine attenuates mismatch negativity (MMN) in healthy human controls

RATIONALE

Schizophrenia is commonly associated with impairments in pre-attentive change detection as represented by reduced mismatch negativity (MMN). The neurochemical basis of MMN has been linked to N-methyl-D: -aspartate (NMDA) receptor function. Glycine augments NMDA receptor function via stimulation of the glycine modulatory site of the NMDA receptor and has been shown to effectively reduce negative symptoms in schizophrenia. However, no study has investigated the possible effects of high-dose glycine on MMN. Further, the physiological consequences of administering high-dose glycine in subjects with normal NMDA receptor function are unknown.

OBJECTIVES

The aim of the present project was to investigate the acute effects of a single large dose of glycine on the human MMN in healthy subjects.

MATERIALS AND METHODS

Sixteen healthy male subjects participated in a double blind, placebo-controlled, crossover design in which each subject was tested under two acute treatment conditions separated by a 1-week washout period; placebo and 0.8 g/kg glycine. The subjects were exposed to a duration-MMN paradigm with 50-ms standard tones (91%) and 100-ms deviant tones (9%).

RESULTS

The results showed that glycine significantly attenuated duration MMN amplitude at frontal electrodes. There was no effect of glycine on MMN latencies or on amplitudes or latencies of N1, N2 and P3a.

CONCLUSIONS

These findings suggest that an acute high dosage of glycine attenuates MMN in healthy controls, raising the possibility that optimal effects of glycine and other glycine agonists may depend on the integrity of the NMDA receptor system.

Toward a model of memory enhancement in schizophrenia: glucose administration and hippocampal function

Recognition of the need to treat cognitive deficits in schizophrenia is compelling and well established, with empirical findings and conceptual arguments related to cognitive enhancement appearing regularly in the literature. Cognitive enhancement itself, however, remains at an early stage. Biological approaches have centered on the development of antipsychotic medications that also improve cognition, but the results have so far remained modest. As a way to facilitate the development of cognitive enhancers in schizophrenia, this article focuses on adjunctive pharmacological approaches to antipsychotic medications and highlights the need for systematic explorations of relevant brain mechanisms. While numerous conceptual criteria might be employed to guide the search, we will focus on 4 points that are especially likely to be useful and which have not yet been considered together. First, the discussion will focus on deficits in a particular cognitive domain, verbal declarative memory. Second, we will review the current status of preclinical and clinical efforts to improve declarative memory using antipsychotic medications, which is the main, existing mode of treatment. Third, we will examine an example of an adjunctive intervention-glucose administration-that improves memory in animals and humans, modulates function in brain regions related to verbal declarative memory, and is highly amenable to translational research. Finally, a heuristic model will be outlined to explore how the intervention maps on to the underlying neurobiology of schizophrenia. More generally, the discussion underlines the promise of cognitive improvement in schizophrenia and the need to approach the issue in a programmatic manner.

The facilitative effects of glucose ingestion on memory retrieval in younger and older adults: is task difficulty or task domain critical?

The ingestion of a glucose-containing drink has been shown to improve cognitive performance, particularly memory functioning. However, it remains unclear as to the extent to which task domain and task difficulty moderate the glucose enhancement effect. The aim of this research was to determine whether boosts in performance are restricted to particular classes of memory (episodic v. semantic) or to tasks of considerable cognitive load. A repeated measures (25g glucose v saccharin), counterbalanced, double-blind design was used with younger and older adults. Participants performed a battery of episodic (e.g. paired associate learning) and semantic memory (e.g. category verification) tasks under low and high cognitive load. Electrophysiological measures (heart rate and galvanic skin response) of arousal and mental effort were also gathered. The results indicated that whilst glucose appeared to aid episodic remembering, cognitive load did not exaggerate the facilitative effect. For semantic memory, there was little evidence to suggest that glucose can boost semantic memory retrieval even when the load was manipulated. One exception was that glucose facilitated performance during the difficult category fluency task. Regardless, the present findings are consistent with the domain-specific account in which glucose acts primarily on the hippocampal region, which is known to support episodic memory. The possible contribution of the hippocampus in semantic memory processing is also discussed.

A Meta-Analysis of Cognitive Deficits in Adults with a Diagnosis of Schizophrenia

This review identified 1275 studies examining cognitive deficits in people with schizophrenia, published between 1990 and 2003. Data from 113 studies (4365 patients and 3429 controls) were combined in a meta-analysis carried out on the five cognitive domains of IQ, memory, language, executive function, and attention. Studies were excluded where they lacked a suitable control group or failed to present complete information. In all five cognitive domains, analysis indicated a consistent trend for patients to perform more poorly than healthy controls, with significant heterogeneity across studies. Sources of heterogeneity were analyzed and a need to ensure more appropriate composition of patient and control groups and to adopt a more refined and methodologically correct, hypothesis-driven approach was identified.

Medial temporal and prefrontal lobe activation during verbal encoding following glucose ingestion in schizophrenia: A pilot fMRI study

Verbal declarative memory is one of the most reliably impaired cognitive functions in schizophrenia. Important issues are whether the problem is reversible, and which brain regions underlie improvement. We showed previously that glucose administration improved declarative memory in patients with schizophrenia, and sought in this pilot study to identify whether glucose affects the location or degree of activation of brain regions involved in a verbal encoding task. Seven clinically stable and medicated patients with schizophrenia or schizoaffective disorder, who showed deficits on a clinical test of memory, participated in the study. Subjects served as their own controls in a double-blind, crossover protocol that consisted of two sessions about a week apart. In each session, subjects ingested a beverage flavored with lemonade that contained 50 g of glucose on one occasion, and saccharin on the other. Blood glucose was measured before and 15, 50, and 75 min after ingestion. After ingesting the beverage, they performed a verbal encoding task while undergoing brain functional magnetic resonance imaging. The results showed significantly greater activation of the left parahippocampus during novel sentence encoding in the glucose condition, compared to the saccharin condition, despite no change in memory performance. A trend towards greater activation of the left dorsolateral prefrontal cortex (p<.07) was also evident in the glucose condition. These pilot findings emphasize the sensitivity of both the medial temporal and prefrontal regions to effects of glucose administration during encoding, and are consistent with the hypothesis that these regions also participate in declarative memory improvements following glucose administration.

Short- and long-term spatial delayed response performance across the lifespan

Delayed response paradigms have been used to examine the neural basis of short and long-term memory in humans. However, limited information exists on how delayed response performance changes across the lifespan. Using a well-validated spatial delayed response (SDR) task, we examined performance at short and long delays in over 300 control participants, 7 to 80 years old. We found a significant nonlinear relation between age and short delay performance (children and older adults worse than young adults) and a significant effect of delay length across the entire lifespan (long worse than short; largest in the youngest children, diminishing nonlinearly with age). This study compares short and long-term spatial memory and suggests that the relation between these systems may alter across the lifespan.

Plasma leptin and adiposity during antipsychotic treatment of schizophrenia

Alterations in plasma leptin have been reported in schizophrenia patients treated with antipsychotics, suggesting the hypothesis that impairments in leptin secretion or signaling might play a role in antipsychotic-induced weight gain. Plasma leptin was measured in 72 schizophrenia patients chronically treated with olanzapine (n=27), risperidone (n=24) or typical antipsychotics (n=21) and 124 healthy adult control subjects. ANCOVA was used to test effects of adiposity (body mass index kg/m2; BMI), subject group (treated patients vs untreated controls), and treatment group (specific medication groups and untreated controls) on plasma leptin concentrations. Additional analyses were performed in a subset of patients and controls individually matched for BMI to further assess group differences in plasma leptin independent of adiposity. BMI strongly predicted plasma leptin concentrations in the overall sample. In addition, a significant three-way interaction between BMI, subject group, and gender was observed. In the individually BMI-matched sample, modestly reduced plasma leptin levels (effect size 0.4 SD) were observed in treated patients in comparison to the BMI-matched healthy controls, with both groups including males and females. However, no differences in plasma leptin levels were observed in the matched sample when separately comparing male patients vs untreated male controls and female patients vs untreated female controls. Plasma leptin in chronically treated patients with schizophrenia is strongly predicted by adiposity, similar to untreated healthy individuals despite adequate power to detect a difference. The results argue against a role for defective leptin secretion or sensitivity in the weight gain induced by antipsychotic medications.

Evidence for linkage between regulatory enzymes in glycolysis and schizophrenia in a multiplex sample

Observations of impaired glucose regulation in schizophrenia are long-standing, although their pathological and etiological significance is uncertain. One approach to the issue that minimizes environmental variables (e.g., medication and diet) is to determine whether genes related to glucose regulation show genetic linkage to schizophrenia. We examined the potential role of glucose metabolism in schizophrenia through a genome scan of affection status in schizophrenia and an empirical method for deriving P-values. Data were utilized from the NIMH Genetics Initiative for Schizophrenia dataset, which comprises a total sample consisting of 71 pedigrees containing 218 nuclear families and 987 individuals. A genome scan with 459 markers spaced at an average of 10 cM intervals was conducted using the linkage analysis program Genehunter separately for European- and African-American groups. Enzymes that regulate glycolysis were identified and the genes regulating these enzymes were located through the Online Mendelian Inheritance in Man (OMIM) website. The focus in this study was on genes located near previously reported schizophrenia susceptibility regions. The genome-wide significance of these genes to schizophrenia was assessed using permutation testing. When results were adjusted for multiple testing within and across ethnic groups, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 2 (PFKFB2; chromosome 1q32.2) achieved genome-wide significance (P = 0.04). In addition, hexokinase 3 (HK3; chromosome 5q35.3) was also suggestive of linkage (P = 0.09). For the European-American sample, PFKFB2 (1q32.2), hexokinase 3 (HK3; 5q35.3), and pyruvate kinase 3 (PK3; chromosome 15q23) achieved significance at the 0.05 level. None of the genes showed significance in the African-American sample. Our results provide further support for the view that genes that regulate glucose metabolism may also influence susceptibility to schizophrenia. More generally, they support the view that relationships between glucose dysregulation and schizophrenia are inherent to the disorder, and are not merely epiphenomena related to medication or other treatment factors.

Glucose effects on cognition in schizophrenia

Clozapine has been shown to improve verbal declarative memory and other cognitive functions in chronic schizophrenia. This raises the possibility that additional adjunctive manipulations might improve memory further. In this study, we hypothesized that glucose, which improves memory in a variety of conditions, including schizophrenia, would improve memory more than saccharin in a group of patients stabilized on clozapine. Twelve outpatients with schizophrenia who received treatment with clozapine participated in a double-blind, counterbalanced, crossover study. Subjects received beverages containing either glucose or saccharin on one occasion, and then the other beverage about a week later. Fifteen minutes after ingesting the beverage, subjects received a brief battery of neuropsychological tests to assess verbal declarative memory, attention, and executive functions. Blood glucose levels were assessed at baseline, and at 15 and 50 min after beverage ingestion. The main findings were that retention of a list of words was improved in the glucose condition, while performance on a complex test of sustained vigilance declined after glucose ingestion. These findings provide evidence that glucose improves declarative memory in patients with schizophrenia who were treated with clozapine, and underscore the possibility of developing effective protocols to reduce cognitive dysfunctions in the disorder. They also highlight the need to explore the extent to which glucose modulates nonmemory cognitive functions such as attention, and to understand more generally how glucose availability and regulation influence cognition.

Glucose/mitochondria in neurological conditions

Impairments of glucose and mitochondrial function are important causes of brain dysfunction and therefore of brain disease. Abnormalities have been found in association with disease of the nervous system in most of the components of glucose/mitochondrial metabolism. In many, molecular genetic abnormalities have been defined. Brain glucose oxidation is abnormal in common diseases of the nervous system, including Alzheimer disease and other dementias, Parkinson disease, delirium, probably schizophrenia and other psychoses, and of course cerebrovascular disease. Defects in a single component and even a single mutation can be associated with different clinical phenotypes. The same clinical phenotype can result from different genotypes. The complex relationship between biological abnormality in brain glucose utilization and clinical disorder is similar to that in other disorders that have been intensively studied at the genetic level. Genes for components of the pathways of brain glucose oxidation are good candidate genes for disease of the brain. Preliminary data support the proposal that treatments to normalize abnormalities in brain glucose oxidation may benefit many patients with common brain diseases.

Cognitive effects of insulin in the central nervous system

Evidence has been accumulating recently that the hormone insulin may modulate cognitive activity by acting in the central nervous system. Initially derived from the observation that insulin and insulin receptors are found in specific brain areas, this evidence also includes cognitive assessments of humans in insulin-deficient and insulin-resistant disease states and experimental manipulation of rodent models. Additional support is derived from in vivo and in vitro systems that are used to investigate the neurophysiological basis of learning and memory. This article is a brief review of the literature that suggests a connection between insulin and memory and draws together some of the findings relevant to possible physiological mechanisms for this cognitive effect.