Obesity and Brain Positron Emission Tomography

Obesity, an increasingly common problem in modern societies, results from energy intake chronically exceeding energy expenditure. This imbalance of energy can be triggered by the internal state of the caloric equation (homeostasis) and non-homeostatic factors, such as social, cultural, psychological, environmental factors or food itself. Nowadays, positron emission tomography (PET) radiopharmaceuticals have been examined to understand the cerebral control of food intake in humans. Using ¹⁵O-H₂ PET, changes in regional cerebral blood flow (rCBF) coupled to neuronal activity were reported in states of fasting, satiation after feeding, and sensory stimulation. In addition, rCBF in obese subjects showed a greater increase in insula, the primary gustatory cortex. ¹⁸F-fluorodeoxyglucose PET showed higher metabolic activity in postcentral gyrus of the parietal cortex and lower in prefrontal cortex and anterior cingulate cortex in obese subjects. In addition, dopamine receptor (DR) PET demonstrated lower DR availability in obese subjects, which might lead to overeating to compensate. Brain PET has been utilized to reveal the connectivity between obesity and brain. This could improve understanding of obesity and help develop a new treatment for obesity.

Brain imaging of neurovascular dysfunction in Alzheimer's disease

Neurovascular dysfunction, including blood-brain barrier (BBB) breakdown and cerebral blood flow (CBF) dysregulation and reduction, are increasingly recognized to contribute to Alzheimer's disease (AD). The spatial and temporal relationships between different pathophysiological events during preclinical stages of AD, including cerebrovascular dysfunction and pathology, amyloid and tau pathology, and brain structural and functional changes remain, however, still unclear. Recent advances in neuroimaging techniques, i.e., magnetic resonance imaging (MRI) and positron emission tomography (PET), offer new possibilities to understand how the human brain works in health and disease. This includes methods to detect subtle regional changes in the cerebrovascular system integrity. Here, we focus on the neurovascular imaging techniques to evaluate regional BBB permeability (dynamic contrast-enhanced MRI), regional CBF changes (arterial spin labeling- and functional-MRI), vascular pathology (structural MRI), and cerebral metabolism (PET) in the living human brain, and examine how they can inform about neurovascular dysfunction and vascular pathophysiology in dementia and AD. Altogether, these neuroimaging approaches will continue to elucidate the spatio-temporal progression of vascular and neurodegenerative processes in dementia and AD and how they relate to each other.

MRI of blood flow of the human retina

This study reports a high-resolution MRI approach to image basal blood flow and hypercapnia-induced blood-flow changes in the unanesthetized human retina on a 3-T MRI scanner. Pseudocontinuous arterial spin labeling technique with static tissue suppression was implemented to minimize movement artifacts and improve blood-flow sensitivity. Turbo spin-echo acquisition was used to achieve high spatial resolution free of susceptibility artifacts. The size, shape, and position of a custom-made receive radiofrequency coil were optimized for sensitivity in the posterior retina. Synchronized eye blink and respiration to the end of each data readout minimized eye movement and physiological fluctuation. Robust high-contrast blood-flow MRI of the unanesthetized human retina was obtained at 500 × 800 μm(2) in-plane resolution. Blood flow in the posterior retina was 93 ± 31 mL/(100 mL min) (mean ± standard deviation, N = 5). Hypercapnic inhalation (5% CO(2)) increased blood flow by 12 ± 4% relative to air (P < 0.01, N = 5). This study demonstrates the feasibility of blood-flow MRI of the retina of unanesthetized human. Because blood flow is tightly coupled to metabolic function under normal conditions and it is often perturbed in diseases, this approach could provide unique insights into retinal physiology and serve as an objective imaging biomarker for disease staging and testing of novel therapeutic strategies. This approach could open up new avenue of retinal research.

Probing fast neuronal interaction using fMRI

The paired-stimuli paradigms were used to study cross-modal neural interactions between the visual and auditory systems in human. It was found that the primary visual cortex was actively involved in the 'illusory double-flash' phenomenon. The similar paradigms were used to study the influence of MR scanner noise on human visual activities from the perspective of neural interaction. The results reveal that the gradient acoustic noise does interfere with the neural behavior in human visual cortex.

Study of Brain Function and Bioenergetics using fMRI and In Vivo MRS at High Fields

The greatest merit of magnetic resonance (MR) methodology applied to medicine is its capabilities of measuring a variety of physiological parameters in vivo. MR imaging (MRI) with unique imaging contrasts can provide vital information which tightly links to brain functions at both normal and diseased states. In contrast, in vivo MR spectroscopy (MRS) is capable of determining metabolites, bioenergetics and chemical reaction rates in brain noninvasively. These capabilities are further enhanced at high/ultrahigh magnetic fields because of significant gain in MR sensitivity and improvements in the spectral resolution of MRS and imaging contrasts. However, MR research also faces many technical challenges which have attracted many scientists from interdisciplinary research backgrounds to find the optimal solutions. Recent progresses in this research field have showed great promise of MRI/MRS for studying brain function, physiology, and neurochemistry. This talk will discuss the developed MR technologies and their applications in brain study at high fields.

Self-reported fatigue common among optimally treated HIV patients: no correlation with cerebral FDG-PET scanning abnormalities

OBJECTIVE

It was the aim of this study to determine the prevalence and severity of fatigue among optimally treated HIV patients and to investigate the potential association with systemic inflammation and abnormalities of the distribution of cerebral glucose metabolism.

METHODS

A cohort of HIV patients (n = 95), known to be HIV positive for 5 years, on anti-retroviral therapy for a minimum of 3 years and with CD4 counts above 0.2 x 10(9) cells/l, completed a validated fatigue inventory, and plasma was analysed for pro-inflammatory markers including tumour necrosis factor-alpha, interleukin 6 and soluble urokinase receptor (suPAR) levels. The distribution of the regional cerebral metabolic rate of glucose was measured in a sub-group of patients suffering from severe fatigue (n = 9) and a group with no fatigue (n = 7) using fluorine-18-fluorodeoxyglucose positron emission tomography (FDG-PET) scanning.

RESULTS

Fifteen percent suffered from severe fatigue, but no association with pro-inflammatory markers was found. About 50% of the FDG-PET-scanned patients showed minor abnormalities in the relative cerebral metabolic rate of glucose. These abnormalities were not associated with fatigue but tended to correlate with a short HIV history (p = 0.058), a low CD4 nadir (p = 0.082) and elevated tumour necrosis factor-alpha levels (p = 0.074).

CONCLUSION

Fatigue is common among optimally treated HIV patients. FDG-PET-described signs of imminent neurodegeneration among HIV patients who had a low CD4 nadir may illustrate an aspect of HIV neuropathogenicity.

Role of the cerebellum in implicit motor skill learning: a PET study

To depict neural substrates of implicit motor learning, regional cerebral blood flow was measured using positron emission tomography (PET) in 13 volunteers in the rest condition and during performance of a unimanual two-ball rotation task. Subjects rotated two balls in a single hand; a slow rotation (0.5 Hz) was followed by two sessions requiring as rapid rotation as possible. The process was repeated four times by a single hand (Block 1) and then by the opposite hand (Block 2). One group of volunteers began with the right hand (n = 7), and the other with the left (n = 6). Performance was assessed by both quickness and efficiency of movements. The former was assessed with the maximum number of rotation per unit time, and the latter with the electromyographic activity under constant speed of the movement. Both showed learning transfer from the right hand to the left hand. Activation of cerebrum and cerebellum varied according to hand. Activation common to both hands occurred in the bilateral dorsal premotor cortex and parasagittal cerebellum, right inferior frontal gyms, left lateral cerebellum and thalamus, supplementary motor area, and cerebellar vermis. The left lateral cerebellum showed the most prominent activation on the first trial of the novel task, and hence may be related the early phase of learning, or "what to do" learning. Left parasagittal cerebellum activity diminished with training both in first and second blocks, correlating inversely with task performance. This region may therefore be involved in later learning or "how to do" learning. The activity of these regions was less prominent with prior training than without it. Thus the left cerebellar hemisphere may be related to learning transfer across hands.

Chronic stress exposure influences local cerebral blood flow in the rat hippocampus

To examine the influence of chronic stress on the brain, we measured local cerebral blood flow in the hippocampus of rats which had been exposed to chronic stress by the hydrogen clearance method in the freely moving status. Rats were exposed, once a day for 12 weeks, to stress of a 15-min immersion in cold water at 4 degrees C (the stress group) or slightly handled for about 1 min (the control group). Local cerebral blood flow values in the hippocampus, which were measured after a 12-week recovery period, were lower in rats in the stress group than those of rats in the control group only in the dark cycle, but not in the light cycle. Accordingly, local cerebral blood flow in the hippocampus of rats in the stress group did not have a daily fluctuation, i.e. lower in the light cycle and higher in the dark cycle, as was shown in rats in the control group. There were no significant changes in motor activity in rats in the stress group as compared to those in the control group. Severe structural damages were observed in the CA2 and CA3 cell fields of the hippocampus of rats in the stress group. We found that an increase in local cerebral blood flow in the hippocampus in the dark cycle was blunted following chronic stress exposure, suggesting that chronic stress exposure caused hippocampal neurons to be less responsive to environmental stimuli derived from motor activity during the dark cycle.

The contribution of functional imaging techniques to our understanding of rheumatic pain

The main cerebral components of the human pain matrix have been defined using functional imaging techniques. The experience of pain is likely to be elaborated as a result of parallel processing within this matrix. There is not, therefore, a single pain center. The determinants of pain are as likely to be determined by top-down as by bottom-up processes. The precise function of the different components of the matrix are just beginning to be defined. There appear to be important adaptive responses in the forebrain components of the matrix during arthritic pain. Endogenous opioid peptides are strong candidates for the modulation of some of these responses. More extensive and sequential behavioral and functional imaging studies are required to establish the contribution these adaptive responses make to the perception of pain.

Insulin-induced hypoglycemia does not impair the surge of luteinizing hormone secretion in the proestrous rat

To know whether insulin-induced hypoglycemia (IIH), which has been shown to inhibit pulsatile luteinizing hormone (LH) secretion, also affects the surge of LH secretion, adult female rats were injected with insulin (5 units/rat) or saline intravenously at 1300 or 1600 h on the day of proestrus and serum concentrations of LH and blood glucose were determined during the period from 1100 to 2100 h. The injection of insulin at neither 1300 nor 1600 h affected the surge of LH secretion, but it significantly decreased the blood glucose. Together with our recent hypothesis that the pulsatile and surge of LH secretion are controlled by separate gonadotropin-releasing hormone (GnRH) mechanisms, we conclude that the GnRH surge generator is less sensitive to IIH than the GnRH pulse generator.

On the characteristics of functional magnetic resonance imaging of the brain

In this review we discuss various recent topics that characterize functional magnetic resonance imaging (fMRI). These topics include a brief description of MRI image acquisition, how to cope with noise or signal fluctuation, the basis of fMRI signal changes, and the relation of MRI signal to neuronal events. Several observations of fMRI that show good correlation to the neurofunction are referred to. Temporal characteristics of fMRI signals and examples of how the feature of real time measurement is utilized are then described. The question of spatial resolution of fMRI, which must be dictated by the vascular structure serving the functional system, is discussed based on various fMRI observations. Finally, the advantage of fMRI mapping is shown in a few examples. Reviewing the vast number of recent fMRI application that have now been reported is beyond the scope of this article.

Functional neuroimaging studies of depression: the anatomy of melancholia

Functional brain imaging techniques, which permit noninvasive measures of neurophysiology and neuroreceptor binding, are powerful and sensitive tools for research aimed at elucidating the pathophysiology of major depression. The application of these technologies in depression research has produced several studies of resting cerebral blood flow (BF) and glucose metabolism in subjects imaged during various phases of illness and treatment. This review examines these data and the principles relevant to their interpretation and discusses the insights they provide into the anatomical correlates of depression. Within the anatomical networks implicated in emotional processing by other types of evidence, these BF and metabolic data demonstrate that major depression is associated with reversible, mood state-dependent, neurophysiological abnormalities in some structures and irreversible, trait-like abnormalities in other structures. In some of the regions in which trait-like abnormalities appear, abnormal metabolic activity appears at least partly related to the anatomical abnormalities identified in magnetic resonance imaging (MRI) studies of depression.

Activation of cerebellar climbing fibers increases cerebellar blood flow: role of glutamate receptors, nitric oxide, and cGMP

BACKGROUND

The mechanisms regulating the cerebellar microcirculation during neural activity are poorly understood. One of the major neural inputs to the cerebellar cortex is the climbing fiber (CF), a pathway that uses excitatory amino acids, including glutamate, as a transmitter. We studied whether CF activation increases cerebellar blood flow (BFcrb) and, if so, we investigated the role of glutamate receptors, nitric oxide (NO) and cGMP, in the response.

METHODS

The CF were activated by harmaline administration (40 mg/kg, i.p.) in halothane-anesthetized rats with a cranial window placed over the cerebellar vermis. BFcrb was monitored by a laser-Doppler probe, and arterial pressure and blood gases were controlled.

RESULTS

With Ringer superfusion, harmaline produced sustained increases in BFcrb that peaked 20 minutes after administration (+115 +/- 13%; n=6; P<.05). The increases in BFcrb were substantially reduced by superfusion with tetrodotoxin (10 micromol/L; -91 +/- 5%; n=5; P<.05 from Ringer). The response was also attenuated by the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor inhibitor 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxaline (100 micromol/L; -70 +/- 6%; P<.05; n=5), but not by the N-methyl-D-aspartate receptor blocker 2-amino-5-phosphonopentanoic acid (500 micromol/L; P>.05; n=5). The response was attenuated by the nonselective NO synthase (NOS) inhibitor nitro-L-arginine (1 mmol/L; -73 +/- 5%; n=6) or by 7-NI (50 mg, i.p.; -71 +/- 5%; n=5), a relatively selective neuronal NOS inhibitor. The soluble guanylyl cyclase inhibitor 1H-1,2,4oxadiazolo[4,3-a]quinoxalin-1-one (100 micromol/L) attenuated the response to harmaline (-73 +/- 5; P<.05; n=6) but not to superfusion with adenosine (P>.05; n=5) or 8-bromo-cGMP (P>.05; n=5).

CONCLUSIONS

Activation of the CF system increases BFcrb. The response depends on activation of glutamate receptors and is in large part mediated by NO via stimulation of soluble guanylyl cyclase. Glutamate receptors NO and cGMP are important factors in the mechanisms of functional hyperemia in cerebellar cortex.

Going beyond the information given: the relation of illusory visual motion to brain activity

There are many instances in which human subjects perceive a component which is not physically present in a visual stimulus. To study the cerebral activity which correlates with the perception of such an illusory component, we chose Enigma, a static figure in which many subjects perceive illusory motion. By using the technique of positron emission tomography (PET) we recorded the relative regional cerebral blood flow (rCBF) in the brain of 13 subjects while they viewed it and reported seeing the illusory motion. We found that, when subjects perceived illusory motion, the increases in rCBF took place in regions of the brain closely related to, and perhaps identical with, area V5, as defined by the site of rCBF change that took place when the same subjects viewed a physically moving stimulus. In addition, there was activity in other cortical areas outside the visual cortex, not present when the subjects had been viewing objective motion. This suggests that the generation of illusory motion depends not only on a highly specific visual area but also on relative contributions from other parts of the brain that are not activated to the same extent when humans perceive objective motion.

Cortical and subcortical localization of response to pain in man using positron emission tomography

A quantitative study of the regional cerebral responses to non-painful and painful thermal stimuli in six normal volunteers has been done by monitoring serial measurements of regional blood flow measured by positron emission tomography (PET). In comparison to a baseline of warm stimulation no statistically significant changes in blood flow were seen in relation to increasing non-painful heat. However, highly significant increases in blood flow were seen in response to painful heat in comparison to non-painful heat. These changes were in the contralateral cingulate cortex, thalamus and lenticular nucleus. These findings are discussed in relation to previous physiological observations of responses to nociceptive stimuli in man and primates.

Diurnal variation of cerebral blood flow in rat hippocampus

We measured local cerebral blood flow over 24 hours in 10 unanesthetized, freely moving rats to determine whether blood flow in the hippocampus fluctuated as a function of time of day. We measured hydrogen clearance at 1-hour intervals using a polyurethane-coated platinum electrode with a 1-mm bare tip implanted in the dorsal hippocampus. Individual rats displayed a wide range of local cerebral blood flow values (from 30 to 100 ml/min/100 g tissue) in a day. In seven of the 10 rats, the overall mean hippocampal blood flow for the dark cycle (7 PM-5 AM) was significantly (p less than 0.001, 0.01, or 0.05) greater than that for the light cycle (6 AM-6 PM), showing an average increase of 20%. Further, the maximum mean hippocampal blood flow at 11 PM in all 10 rats was 42% greater than the minimum at noon. Our study demonstrates for the first time that local cerebral blood flow in the hippocampus shows diurnal variation.