Neocortical Axon Arbors Trade-off Material and Conduction Delay Conservation

The brain contains a complex network of axons rapidly communicating information between billions of synaptically connected neurons. The morphology of individual axons, therefore, defines the course of information flow within the brain. More than a century ago, Ramón y Cajal proposed that conservation laws to save material (wire) length and limit conduction delay regulate the design of individual axon arbors in cerebral cortex. Yet the spatial and temporal communication costs of single neocortical axons remain undefined. Here, using reconstructions of in vivo labelled excitatory spiny cell and inhibitory basket cell intracortical axons combined with a variety of graph optimization algorithms, we empirically investigated Cajal''s conservation laws in cerebral cortex for whole three-dimensional (3D) axon arbors, to our knowledge the first study of its kind. We found intracortical axons were significantly longer than optimal. The temporal cost of cortical axons was also suboptimal though far superior to wire-minimized arbors. We discovered that cortical axon branching appears to promote a low temporal dispersion of axonal latencies and a tight relationship between cortical distance and axonal latency. In addition, inhibitory basket cell axonal latencies may occur within a much narrower temporal window than excitatory spiny cell axons, which may help boost signal detection. Thus, to optimize neuronal network communication we find that a modest excess of axonal wire is traded-off to enhance arbor temporal economy and precision. Our results offer insight into the principles of brain organization and communication in and development of grey matter, where temporal precision is a crucial prerequisite for coincidence detection, synchronization and rapid network oscillations.     

Morpho-functional study of the brain of animals with different types of individual resistance to hypoxia

The experiments on white rats have shown that animals with distinct tolerance to hypoxia were characterized by individual metabolic changes in phylogenetically different brain structures. Adaptation to hypoxia in animals with high tolerance was associated with metabolic changes in the reticular formation and in animals with low tolerance with changes in the cerebral cortex. The experiments have shown that white rats with distinct individual tolerance to hypoxia are characterized by an inherent level of plastic metabolism in different brain structures. A correlation between brain tissue metabolism and individual tolerance of animals to hypoxia is suggested.     

Developmental Switch in Neurovascular Coupling in the Immature Rodent Barrel Cortex

Neurovascular coupling (NVC) in the adult central nervous system (CNS) is a mechanism that provides regions of the brain with more oxygen and glucose upon increased levels of neural activation. Hemodynamic changes that go along with neural activation evoke a blood oxygen level-dependent (BOLD) signal in functional magnetic resonance imaging (fMRI) that can be used to study brain activity non-invasively. A correct correlation of the BOLD signal to neural activity is pivotal to understand this signal in neuronal development, health and disease. However, the function of NVC during development is largely unknown. The rodent whisker-to-barrel cortex is an experimentally well established model to study neurovascular interdependences. Using extracellular multi-electrode recordings and laser-Doppler-flowmetry (LDF) we show in the murine barrel cortex of postnatal day 7 (P7) and P30 mice in vivo that NVC undergoes a physiological shift during the first month of life. In the mature CNS it is well accepted that cortical sensory processing results in a rise in regional cerebral blood flow (rCBF). We show in P7 animals that rCBF decreases during prolonged multi-whisker stimulation and goes along with multi unit activity (MUA) fatigue. In contrast at P30, MUA remains stable during repetitive stimulation and is associated with an increase in rCBF. Further we characterize in both age groups the responses in NVC to single sensory stimuli. We suggest that the observed shift in NVC is an important process in cortical development that may be of high relevance for the correct interpretation of brain activity e.g. in fMRI studies of the immature central nervous system (CNS).     

The Role of Neurotransmitters and Proteins in the Genetic-Functional Organization of the Animal Brain

It was shown that animals that differ in behavioral characteristics (August and Wistar rats) also differ in neurotransmitter and protein metabolism, which can be considered as tests that adequately reflect the functional condition of the central nervous system. These differences are expressed at the level of both subcortical structures (the hippocampus and caudate nucleus) and various morphofunctional types of the sensorimotor cortex neurons (layers III and V). Studies on genetically different animals strains have revealed metabolic features that allow the determination of individual behavioral features and an estimation of individual brain structures in these processes.     

3D Reconstruction and Standardization of the Rat Vibrissal Cortex for Precise Registration of Single Neuron Morphology

The three-dimensional (3D) structure of neural circuits is commonly studied by reconstructing individual or small groups of neurons in separate preparations. Investigation of structural organization principles or quantification of dendritic and axonal innervation thus requires integration of many reconstructed morphologies into a common reference frame. Here we present a standardized 3D model of the rat vibrissal cortex and introduce an automated registration tool that allows for precise placement of single neuron reconstructions. We (1) developed an automated image processing pipeline to reconstruct 3D anatomical landmarks, i.e., the barrels in Layer 4, the pia and white matter surfaces and the blood vessel pattern from high-resolution images, (2) quantified these landmarks in 12 different rats, (3) generated an average 3D model of the vibrissal cortex and (4) used rigid transformations and stepwise linear scaling to register 94 neuron morphologies, reconstructed from in vivo stainings, to the standardized cortex model. We find that anatomical landmarks vary substantially across the vibrissal cortex within an individual rat. In contrast, the 3D layout of the entire vibrissal cortex remains remarkably preserved across animals. This allows for precise registration of individual neuron reconstructions with approximately 30 µm accuracy. Our approach could be used to reconstruct and standardize other anatomically defined brain areas and may ultimately lead to a precise digital reference atlas of the rat brain.     

Direct Detection of Ascorbyl Radical in Experimental Brain Injury: Microdialysis and an Electron Spin Resonance Spectroscopic Study

Abstract: To examine the role played by free radicals in brain injury, we performed experiments to detect radicals in the frontal cortex of rats, using electron spin resonance (ESR) and microdialysis. A dialysis probe was inserted into the frontal cortex, and spin adducts in perfusates were immediately detected by ESR. We obtained a relatively stable doublet signal, with parameters of g = 2.0057 and aH = 0.17 mT. This signal corresponded with that of the ascorbyl radical. Ascorbyl radical in the perfusate collected from the frontal cortex was augmented by microinjection of H₂O₂ and FeCl₂ adjacent to the dialysis probe. When the rats were challenged with cold-induced brain injury, ascorbyl radical and lactate dehydrogenase (LDH) level in the perfusate increased significantly. Pretreatment with superoxide dismutase and catalase attenuated the increase in ascorbyl radical and LDH level induced by the cold injury. Infusion of FeCl₂ dissolved in perfusate caused a pronounced increase in ascorbyl radical and LDH level after the cold injury. We conclude that the direct detection of free radical formation further supports the hypothesis that free radicals play an important role in traumatic brain injury. Our findings also indicate that combined microdialysis with ESR spectroscopy is a useful in vivo method for monitoring free radical production in the brain.     

Model of Familiarity Discrimination in the Perirhinal Cortex

Much evidence indicates that recognition memory involves two separable processes, recollection and familiarity discrimination, with familiarity discrimination being dependent on the perirhinal cortex of the temporal lobe. Here, we describe a new neural network model designed to mimic the response patterns of perirhinal neurons that signal information concerning the novelty or familiarity of stimuli. The model achieves very fast and accurate familiarity discrimination while employing biologically plausible parameters and Hebbian learning rules. The fact that the activity patterns of the model's simulated neurons are closely similar to those of neurons recorded from the primate perirhinal cortex indicates that this brain region could discriminate familiarity using principles akin to those of the model. If so, the capacity of the model establishes that the perirhinal cortex alone may discriminate the familiarity of many more stimuli than current neural network models indicate could be recalled (recollected) by all the remaining areas of the cerebral cortex. This efficiency and speed of detecting novelty provides an evolutionary advantage, thereby providing a reason for the existence of a familiarity discrimination network in addition to networks used for recollection.     

Restoring Susceptibility Induced MRI Signal Loss in Rat Brain at 9.4 T: A Step towards Whole Brain Functional Connectivity Imaging

The aural cavity magnetic susceptibility artifact leads to significant echo planar imaging (EPI) signal dropout in rat deep brain that limits acquisition of functional connectivity fcMRI data. In this study, we provide a method that recovers much of the EPI signal in deep brain. Needle puncture introduction of a liquid-phase fluorocarbon into the middle ear allows acquisition of rat fcMRI data without signal dropout. We demonstrate that with seeds chosen from previously unavailable areas, including the amygdala and the insular cortex, we are able to acquire large scale networks, including the limbic system. This tool allows EPI-based neuroscience and pharmaceutical research in rat brain using fcMRI that was previously not feasible.     

The effect of tryptophan depletion on brain activation measured by functional magnetic resonance imaging during the Stroop test in healthy subjects

We investigated the role of serotonin in cognitive activation of the frontal cortex. The serotonergic system was affected by the administration of an amino acids mixture without tryptophan (tryptophan depletion). In a placebo-controlled double-blind cross-over study with 20 healthy volunteers, we tested the hypothesis that a tryptophan (serotonin) decrease affects the activation of prefrontal cortex by the Stroop test. Cognitive brain activation was evaluated by functional magnetic resonance imaging (fMRI). Tryptophan depletion decreased the plasma tryptophan level up to 90 % for five hours after the tryptophan-free drink had been consumed when compared with the same mixture with tryptophan (p?0.0001). Tryptophan depletion did not affect the Stroop test performance. We compared fMRI activation in both conditions (tryptophan depletion and placebo) with plasma tryptophan levels as the covariates. The tryptophan depletion increased the activation (fMRI signal) in the bilateral mediofrontal cortex, anterior cingulate and left dorsolateral prefrontal cortex. The present findings allow the postulate that serotonergic medial forebrain and cingulum bundle pathways play a role in the activity of cortical structures involved in Stroop test processing.     

Administration of Histidine to Female Rats Induces Changes in Oxidative Status in Cortex and Hippocampus of the Offspring

Histidinemia is an inherited metabolic disorder biochemically characterized by high concentrations of histidine in biological fluids. Usually affected patients are asymptomatic although some individuals have mental retardation and speech disorders. Considering the high prevalence of histidinemia and the scarce information on the effects of maternal histidinemia on their progeny, we investigated various parameters of oxidative stress in brain cortex and hippocampus of the offspring from female rats that received histidine (0.5 mg/g of body weight) in the course of pregnancy and lactation. At 21 days of age we found a significant increase of thiobarbituric acid reactive substances (TBARS), 2′,7′-dihydrodichlorofluorescein oxidation, superoxide dismutase (SOD) activity, catalase (CAT) activity, total sulfhydryls and glutathione (GSH) content in cerebral cortex and hippocampus. We also verified that at 60 days of age, GSH, SOD and total sulfhydryls returned to normal levels in brain cortex, while the other parameters decreased in the same structure. In the hippocampus, at 60 days of age GSH returned to normal levels, CAT persisted elevated and the other parameters decreased. These results indicate that histidine administration to female rats can induce oxidative stress in the brain from the offspring, which partially recovers 40 days after breastfeeding stopped.     

The role of neuromediators and proteins in genetic and functional organization of animal brain

It was shown that animals that differ in behavioral characteristics (August and Wistar rats) also differ in neurotransmitter and protein metabolism, which can be considered as tests that adequately reflect the functional condition of the central nervous system. These differences are expressed at the level of both subcortical structures (hippocampus and caudate nucleus) and various morphofunctional types of the sensorimotor cortex neurons (layers III and V). Studies on genetically different animals strains have revealed metabolic features that allow determination of individual behavioral features and estimation of individual brain structures in these processes.     

Determination of cholecystokinin tetrapeptide and cholecystokinin octapeptide sulfate in different rat brain regions by high-pressure liquid chromatography with electrochemical detection

A method for the determination of cholecystokinins in biological material, based on high-pressure liquid chromatography with direct electrochemical detection (HPLC-EC), is described. Using this method, the levels of cholecystokinin tetrapeptide and octapeptide sulfate in rat brain cortex, hippocampus, striatum, and brain stem were measured and found to be comparable to those reported using radioimmunoassay methods. We show that HPLC-EC is sensitive enough to accurately determine neuropeptides in brain tissue without prior derivatization and is therefore, due to its simplicity, an attractive alternative to existing methods.     

Cortical metabolism in pyruvate dehydrogenase deficiency revealed by ex vivo multiplet C NMR of the adult mouse brain

The pyruvate dehydrogenase complex (PDC), required for complete glucose oxidation, is essential for brain development. Although PDC deficiency is associated with a severe clinical syndrome, little is known about its effects on either substrate oxidation or synthesis of key metabolites such as glutamate and glutamine. Computational simulations of brain metabolism indicated that a 25% reduction in flux through PDC and a corresponding increase in flux from an alternative source of acetyl-CoA would substantially alter the ¹³C NMR spectrum obtained from brain tissue. Therefore, we evaluated metabolism of [1,6-¹³C₂]glucose (oxidized by both neurons and glia) and [1,2-¹³C₂]acetate (an energy source that bypasses PDC) in the cerebral cortex of adult mice mildly and selectively deficient in brain PDC activity, a viable model that recapitulates the human disorder. Intravenous infusions were performed in conscious mice and extracts of brain tissue were studied by ¹³C NMR. We hypothesized that mice deficient in PDC must increase the proportion of energy derived from acetate metabolism in the brain. Unexpectedly, the distribution of ¹³C in glutamate and glutamine, a measure of the relative flux of acetate and glucose into the citric acid cycle, was not altered. The ¹³C labeling pattern in glutamate differed significantly from glutamine, indicating preferential oxidation of [1,2-¹³C]acetate relative to [1,6-¹³C]glucose by a readily discernible metabolic domain of the brain of both normal and mutant mice, presumably glia. These findings illustrate that metabolic compartmentation is preserved in the PDC-deficient cerebral cortex, probably reflecting intact neuron–glia metabolic interactions, and that a reduction in brain PDC activity sufficient to induce cerebral dysgenesis during development does not appreciably disrupt energy metabolism in the mature brain.     

Negatively calcium-modulated membrane guanylate cyclase signaling system in the rat olfactory bulb

The mechanism by which the individual odor signals are translated into the perception of smell in the brain is unknown. The signal processing occurs in the olfactory system which has three major components: olfactory neuroepithelium, olfactory bulb, and olfactory cortex. The neuroepithelial layer is composed of ciliated sensory neurons interspersed among supportive cells. The sensory neurons are the sites of odor transduction, a process that converts the odor signal into an electrical signal. The electrical signal is subsequently received by the neurons of the olfactory bulb, which process the signal and then relay it to the olfactory cortex in the brain. Apart from information about certain biochemical steps of odor transduction, there is almost no knowledge about the means by which the olfactory bulb and cortical neurons process this information. Through biochemical, functional, and immunohistochemical approaches, this study shows the presence of a Ca(2+)-modulated membrane guanylate cyclase (mGC) transduction system in the bulb portion of the olfactory system. The mGC is ROS-GC1. This is coexpressed with its specific modulator, guanylate cyclase activating protein type 1 (GCAP1), in the mitral cells. Thus, a new facet of the Ca(2+)-modulated GCAP1--ROS-GC1 signaling system, which, until now, was believed to be unique to phototransduction, has been revealed. The findings suggest a novel role for this system in the polarization and depolarization phenomena of mitral cells and also contradict the existing belief that no mGC besides GC-D exists in the olfactory neurons.     

Increased Amygdala and Visual Cortex Activity and Functional Connectivity towards Stimulus Novelty Is Associated with State Anxiety

Novel stimuli often require a rapid reallocation of sensory processing resources to determine the significance of the event, and the appropriate behavioral response. Both the amygdala and the visual cortex are central elements of the neural circuitry responding to novelty, demonstrating increased activity to new as compared to highly familiarized stimuli. Further, these brain areas are intimately connected, and thus the amygdala may be a key region for directing sensory processing resources to novel events. Although knowledge regarding the neurocircuit of novelty detection is gradually increasing, we still lack a basic understanding of the conditions that are necessary and sufficient for novelty-specific responses in human amygdala and the visual cortices, and if these brain areas interact during detection of novelty. In the present study, we investigated the response of amygdala and the visual cortex to novelty, by comparing functional MRI activity between 1^st and 2^nd time presentation of a series of emotional faces in an event-related task. We observed a significant decrease in amygdala and visual cortex activity already after a single stimulus exposure. Interestingly, this decrease in responsiveness was less for subjects with a high score on state anxiety. Further, novel faces stimuli were associated with a relative increase in the functional coupling between the amygdala and the inferior occipital gyrus (BA 18). Thus, we suggest that amygdala is involved in fast sensory boosting that may be important for attention reallocation to novel events, and that the strength of this response depends on individual state anxiety.     

METABOLISM OF AMINO ACIDS AND AMMONIA IN RAT BRAIN CORTEX SLICES IN VITRO: A POSSIBLE ROLE OF AMMONIA IN BRAIN FUNCTION

Abstract— (1) The sum of the values of total (tissue + medium) amino acid-N of glutamate, glutamine, γ-aminobutyrate, and aspartate (referred to as the glutamate system) and of ammonia-N of incubated rat brain cortex slices is approximately constant under a variety of metabolic conditions (presence or absence of glucose or of oxygen or in the presence of metabolic inhibitors such as aminooxyacetate, malonate, methionine sulfoximine, fluoroacetate, ouabain, 2:4 dinitrophenol, or Amytal). Fluctuations in the value of one constituent are compensated by fluctuations in the values of other constituents. The same applies to infant rat brain cortex slices and to rat brain synaptosome preparations. It is suggested that the constancy of the glutamate-ammonia system implies a coupling of neurons and glia in such a manner that glutamate released from the neurons during excitation is taken up by the glia and there converted to glutamine. The glutamine is returned to the neurons where it is hydrolysed to glutamate and ammonia. The glia, on this view, exercise an important buffering effect on the extracellular content of the excitatory amino acid, glutamate, and possibly on that of other functionally active amino acids emanating from the neurons. (2) The magnitude of the glutamate-ammonia system in the infant rat brain cortex is about 43% of that in the adult. It is suggested that, with maturity, the development of the glutamate-ammonia system is linked with the development of the citric acid cycle of operations. (3) The ammonia in the system is tightly linked to the activity of the ATP-controlled glutamine synthetase. (4) Proteolytic ammonia and amino acids are formed, during the incubation, to values that seem to be independent of a wide variety of metabolic conditions. The total value is approximately 10 μmol/g in the first h of incubation. (5) As the ammonium ion is necessary for the return of glutamate to the neuron in the form of glutamine, it is inferred that the ion plays a functional role in the nervous system by helping to maintain the steady state of glutamate in the neuron.     

Reactions to Media Violence: It’s in the Brain of the Beholder

Media portraying violence is part of daily exposures. The extent to which violent media exposure impacts brain and behavior has been debated. Yet there is not enough experimental data to inform this debate. We hypothesize that reaction to violent media is critically dependent on personality/trait differences between viewers, where those with the propensity for physical assault will respond to the media differently than controls. The source of the variability, we further hypothesize, is reflected in autonomic response and brain functioning that differentiate those with aggression tendencies from others. To test this hypothesis we pre-selected a group of aggressive individuals and non-aggressive controls from the normal healthy population; we documented brain, blood-pressure, and behavioral responses during resting baseline and while the groups were watching media violence and emotional media that did not portray violence. Positron Emission Tomography was used with [¹⁸F]fluoro-deoxyglucose (FDG) to image brain metabolic activity, a marker of brain function, during rest and during film viewing while blood-pressure and mood ratings were intermittently collected. Results pointed to robust resting baseline differences between groups. Aggressive individuals had lower relative glucose metabolism in the medial orbitofrontal cortex correlating with poor self-control and greater glucose metabolism in other regions of the default-mode network (DMN) where precuneus correlated with negative emotionality. These brain results were similar while watching the violent media, during which aggressive viewers reported being more Inspired and Determined and less Upset and Nervous, and also showed a progressive decline in systolic blood-pressure compared to controls. Furthermore, the blood-pressure and brain activation in orbitofrontal cortex and precuneus were differentially coupled between the groups. These results demonstrate that individual differences in trait aggression strongly couple with brain, behavioral, and autonomic reactivity to media violence which should factor into debates about the impact of media violence on the public.     

A Correspondence between Individual Differences in the Brain's Intrinsic Functional Architecture and the Content and Form of Self-Generated Thoughts

Although neural activity often reflects the processing of external inputs, intrinsic fluctuations in activity have been observed throughout the brain. These may relate to patterns of self-generated thought that can occur while not performing goal-driven tasks. To understand the relationship between self-generated mental activity and intrinsic neural fluctuations, we developed the New York Cognition Questionnaire (NYC-Q) to assess the content and form of an individual''s experiences during the acquisition of resting-state fMRI data. The data were collected as a part of the Nathan Kline Rockland Enhanced sample. We decomposed NYC-Q scores using exploratory factor analysis and found that self-reported thoughts clustered into distinct dimensions of content (future related, past related, positive, negative, and social) and form (words, images, and specificity). We used these components to perform an individual difference analysis exploring how differences in the types of self-generated thoughts relate to whole brain measures of intrinsic brain activity (fractional amplitude of low frequency fluctuations, regional homogeneity, and degree centrality). We found patterns of self-generated thoughts related to changes that were distributed across a wide range of cortical areas. For example, individuals who reported greater imagery exhibited greater low frequency fluctuations in a region of perigenual cingulate cortex, a region that is known to participate in the so-called default-mode network. We also found certain forms of thought were associated with other areas, such as primary visual cortex, the insula, and the cerebellum. For example, individuals who reported greater future thought exhibited less homogeneous neural fluctuations in a region of lateral occipital cortex, a result that is consistent with the claim that particular types of self-generated thought depend on processes that are decoupled from sensory processes. These data provide evidence that self-generated thought is a heterogeneous category of experience and that studying its content can be helpful in understanding brain dynamics.     

Modelling of cortical and thalamic 600 Hz activity by means of oscillatory networks

The purpose of this study is to investigate information processing in the primary somatosensory system with the help of oscillatory network modelling. Specifically, we consider interactions in the oscillatory 600 Hz activity between the thalamus and the cortical Brodmann areas 3b and 1. This type of cortical activity occurs after electrical stimulation of peripheral nerves such as the median nerve. Our measurements consist of simultaneous 31-channel MEG and 32-channel EEG recordings and individual 3D MRI data. We perform source localization by means of a multi-dipole model. The dipole activation time courses are then modelled by a set of coupled oscillators, described by linear second-order ordinary delay differential equations (DDEs). In particular, a new model for the thalamic activity is included in the oscillatory network. The parameters of the DDE system are successfully fitted to the data by a nonlinear evolutionary optimization method. To activate the oscillatory network, an individual input function is used, based on measurements of the propagated stimulation signal at the biceps. A significant feedback from the cortex to the thalamus could be detected by comparing the network modelling with and without feedback connections. Our finding in humans is supported by earlier animal studies. We conclude that this type of rhythmic brain activity can be modelled by oscillatory networks in order to disentangle feed forward and feedback information transfer.     

Chronic metabolic sequelae of traumatic brain injury: prolonged suppression of somatosensory activation

Injuries to the brain acutely disrupt normal metabolic function and may deactivate functional circuits. It is unknown whether these metabolic abnormalities improve over time. We used 2-deoxyglucose (2-DG) autoradiographic image-averaging to assess local cerebral glucose utilization (lCMR(Glc)) of the rat brain 2 mo after moderate (1.7-2.1 atm) fluid-percussion traumatic brain injury (FPI). Four animal groups (n = 5 each) were studied: sham-injured rats with and without stimulation of the vibrissae-barrel field ipsilateral to injury; and animals with prior FPI, with or without this stimulation. In sham-injured rats, resting lCMR(Glc) was normal, and vibrissae stimulation produced right-sided metabolic activation of the ventrolateral thalamic and somatosensory-cortical projection areas. In rats with prior injury, lCMR(Glc) contralateral to injury was normal, but lCMR(Glc) of the ipsilateral forebrain was depressed by approximately 38-45% compared with shams. Whisker stimulation in rats with prior trauma failed to induce metabolic activation of either cortex or thalamus. Image-mapping of histological material obtained in the same injury model was undertaken to assess the possible influence of injury-induced regional brain atrophy on computed lCMR(Glc); an effect was found only in the lateral cortex at the trauma epicenter. Our results show that, 2 mo after trauma, resting cerebral metabolic perturbations persist, and the whisker-barrel somatosensory circuit shows no signs of functional recovery.