A systematic review of MRI studies examining the relationship between physical fitness and activity and the white matter of the ageing brain.

Higher levels of physical fitness or activity (PFA) have been shown to have beneficial effects on cognitive function and grey matter volumes in older adults. However, the relationship between PFA and the brain's white matter (WM) is not yet well established. Here, we aim to provide a comprehensive and systematic review of magnetic resonance imaging studies examining the effects of PFA on the WM of the ageing brain. Twenty-nine studies were included in the review: eleven examined WM volume, fourteen WM lesions, and nine WM microstructure. While many studies found that higher levels of PFA were associated with greater WM volumes, reduced volume or severity of WM lesions, or improved measures of WM microstructure, a number of negative findings have also been published. Meta-analyses of global measures of WM volume and WM lesion volume yielded significant, but small, effect sizes. Overall, we found evidence for cautious support of links between PFA and WM structure, and highlighted key areas for future research including the extent to which the relationship between PFA and WM structure is anatomically specific, the influence of possible confounding factors, and the relationship between PFA, WM and cognition.

Increased burst-firing of ventral tegmental area dopaminergic neurons in D-amino acid oxidase knockout mice in vivo.

d-Amino acid oxidase (DAO) degrades the N-methyl-d-aspartate (NMDA) receptor co-agonist d-serine, and is implicated in schizophrenia as a risk gene and therapeutic target. In schizophrenia, the critical neurochemical abnormality affects dopamine, but to date there is little evidence that DAO impacts on the dopamine system. To address this issue, we measured the electrophysiological properties of dopaminergic (DA) and non-DA neurons in the ventral tegmental area (VTA) of anaesthetised DAO knockout (DAO(-/-) ) and DAO heterozygote (DAO(+/-) ) mice as compared with their wild-type (DAO(+/+) ) littermates. Genotype was confirmed at the protein level by western blotting and immunohistochemistry. One hundred and thirty-nine VTA neurons were recorded in total, and juxtacellular labelling of a subset revealed that neurons immunopositive for tyrosine hydroxylase had DA-like electrophysiological properties that were distinct from those of neurons that were tyrosine hydroxylase-immunonegative. In DAO(-/-) mice, approximately twice as many DA-like neurons fired in a bursting pattern than in DAO(+/-) or DAO(+/+) mice, but other electrophysiological properties did not differ between genotypes. In contrast, non-DA-like neurons had a lower firing rate in DAO(-/-) mice than in DAO(+/-) or DAO(+/+) mice. These data provide the first direct evidence that DAO modulates VTA DA neuron activity, which is of interest for understanding both the glutamatergic regulation of dopamine function and the therapeutic potential of DAO inhibitors. The increased DA neuron burst-firing probably reflects increased availability of d-serine at VTA NMDA receptors, but the site, mechanism and mediation of the effect requires further investigation, and may include both direct and indirect processes.

Subthalamic nucleus shows opposite functional connectivity pattern in Huntington's and Parkinson's disease.

Huntington's and Parkinson's disease are two movement disorders representing mainly opposite states of the basal ganglia inhibitory function. Despite being an integral part of the cortico-subcortico-cortical circuitry, the subthalamic nucleus function has been studied at the level of detail required to isolate its signal only through invasive studies in Huntington's and Parkinson's disease. Here, we tested whether the subthalamic nucleus exhibited opposite functional signatures in early Huntington's and Parkinson's disease. We included both movement disorders in the same whole-brain imaging study, and leveraged ultra-high-field 7T MRI to achieve the very fine resolution needed to investigate the smallest of the basal ganglia nuclei. Eleven of the 12 Huntington's disease carriers were recruited at a premanifest stage, while 16 of the 18 Parkinson's disease patients only exhibited unilateral motor symptoms (15 were at Stage I of Hoehn and Yahr off medication). Our group comparison interaction analyses, including 24 healthy controls, revealed a differential effect of Huntington's and Parkinson's disease on the functional connectivity at rest of the subthalamic nucleus within the sensorimotor network, i.e. an opposite effect compared with their respective age-matched healthy control groups. This differential impact in the subthalamic nucleus included an area precisely corresponding to the deep brain stimulation 'sweet spot'-the area with maximum overall efficacy-in Parkinson's disease. Importantly, the severity of deviation away from controls' resting-state values in the subthalamic nucleus was associated with the severity of motor and cognitive symptoms in both diseases, despite functional connectivity going in opposite directions in each disorder. We also observed an altered, opposite impact of Huntington's and Parkinson's disease on functional connectivity within the sensorimotor cortex, once again with relevant associations with clinical symptoms. The high resolution offered by the 7T scanner has thus made it possible to explore the complex interplay between the disease effects and their contribution on the subthalamic nucleus, and sensorimotor cortex. Taken altogether, these findings reveal for the first time non-invasively in humans a differential, clinically meaningful impact of the pathophysiological process of these two movement disorders on the overall sensorimotor functional connection of the subthalamic nucleus and sensorimotor cortex.

The effects of an aerobic training intervention on cognition, grey matter volumes and white matter microstructure.

While there is strong evidence from observational studies that physical activity is associated with reduced risk of cognitive decline and dementia, the extent to which aerobic training interventions impact on cognitive health and brain structure remains subject to debate. In a pilot study of 46 healthy older adults (66.6 years ± 5.2 years, 63% female), we compared the effects of a twelve-week aerobic training programme to a waitlist control condition on cardiorespiratory fitness, cognition and magnetic resonance imaging (MRI) outcomes. Cardiorespiratory fitness was assessed by VO2 max testing. Cognitive assessments spanned executive function, memory and processing speed. Structural MRI analysis included examination of hippocampal volume, and voxel-wise assessment of grey matter volumes using voxel-based morphometry. Diffusion tensor imaging analysis of fractional anisotropy, axial diffusivity and radial diffusivity was performed using tract-based spatial statistics. While the intervention successfully increased cardiorespiratory fitness, there was no evidence that the aerobic training programme led to changes in cognitive functioning or measures of brain structure in older adults. Interventions that are longer lasting, multi-factorial, or targeted at specific high-risk populations, may yield more encouraging results.

Exercise response in Parkinson's disease: insights from a cross-sectional comparison with sedentary controls and a per-protocol analysis of a randomised controlled trial.

OBJECTIVES: To investigate the acute and adaptation cardiovascular and metabolic training responses in people with Parkinson's disease (pwP). DESIGN: (1) A cross-sectional study of exercise response of pwP compared with sedentary controls and (2) an interventional study of exercise training in pwP. SETTING: Community leisure facilities. PARTICIPANTS: pwP (n=83) and sedentary controls (n=55). INTERVENTIONS: Study 1 included participants from a two-arm-parallel single-blind phase II randomised controlled trial (RCT), that undertook a baseline maximal incremental exercise test and study 2 included those randomised to the exercise group in the RCT, who completed a 6-month weekly exercise programme (n=37). The intervention study 2 was a prescribed exercise program consisting of sessions lasting 60 min, two times a week over a 6-month period. The control group followed the same protocol which derived the same cardiorespiratory parameters, except that they were instructed to aim for a cadence of ~60 revolutions per minute and the unloaded phase lasted 3 min with an initial step of 25 W. PRIMARY AND SECONDARY OUTCOME MEASURES: Stepwise incremental exercise test to volitional exhaustion was the primary outcome measure. RESULTS: Study 1 showed higher maximum values for heart rate (HR), VO2 L/min, VCO2 L/min and ventilation L/min for the control group; respiratory exchange ratio (RER), perceived exertion and O2 pulse (VO2 L/min/HR) did not differ between groups. In study 2, for pwP who adhered to training (n=37), RER increased significantly and although there was no significant change in aerobic capacity or HR response, reduced blood pressure was found. CONCLUSIONS: An abnormal cardiovascular response to exercise was observed in pwP compared to controls. After the exercise programme, metabolic deficiencies remained for pwP. These observations add to the pathogenic understanding of PD, acknowledge an underling metabolic contribution and support that certain cardiovascular symptoms may improve as a result of this type of exercise.

D-amino acid oxidase is expressed in the ventral tegmental area and modulates cortical dopamine.

D-amino acid oxidase (DAO, DAAO) degrades the NMDA receptor co-agonist D-serine, modulating D-serine levels and thence NMDA receptor function. DAO inhibitors are under development as a therapy for schizophrenia, a disorder involving both NMDA receptor and dopaminergic dysfunction. However, a direct role for DAO in dopamine regulation has not been demonstrated. Here, we address this question in two ways. First, using in situ hybridization and immunohistochemistry, we show that DAO mRNA and immunoreactivity are present in the ventral tegmental area (VTA) of the rat, in tyrosine hydroxylase (TH)-positive and -negative neurons, and in glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes. Second, we show that injection into the VTA of sodium benzoate, a DAO inhibitor, increases frontal cortex extracellular dopamine, as measured by in vivo microdialysis and high performance liquid chromatography. Combining sodium benzoate and D-serine did not enhance this effect, and injection of D-serine alone affected dopamine metabolites but not dopamine. These data show that DAO is expressed in the VTA, and suggest that it impacts on the mesocortical dopamine system. The mechanism by which the observed effects occur, and the implications of these findings for schizophrenia therapy, require further study.

Genetic mouse models relevant to schizophrenia: taking stock and looking forward.

Genetic mouse models relevant to schizophrenia complement, and have to a large extent supplanted, pharmacological and lesion-based rat models. The main attraction is that they potentially have greater construct validity; however, they share the fundamental limitations of all animal models of psychiatric disorder, and must also be viewed in the context of the uncertain and complex genetic architecture of psychosis. Some of the key issues, including the choice of gene to target, the manner of its manipulation, gene-gene and gene-environment interactions, and phenotypic characterization, are briefly considered in this commentary, illustrated by the relevant papers reported in this special issue.