Hypotheses Derived from Morphological Data: When and How They Are Useful

SYNOPSIS. This paper discusses the validity of hypotheses basedon morphological data,and distinguishes between hypotheses,which are testable, and speculation, which is not. Specificexamples from the mammalian auditory system are examined: arecently evolved, highly derived character (enlarged middleears in desert rodents); and an older, more general character(the inner and outer hair cells of the mammalian organ of Corti).It is concluded that morphologically-based hypotheses are powerfuland important when accompanied by experimental data.     

Microvessels Isolated from Rat Brain: Localization of Astrocyte Processes by Immunohistochemical Techniques

Microvascular networks were isolated from rat telencephalon by density gradient centrifugation. The vessels prepared were characterized morphologically at the light and electron microscope level and immunohistochemically by the localization of glutamine synthetase and glial fibrillary acidic protein, two proteins found almost exclusively in astrocytes. The vast majority of the vessels prepared contained more than just endothelial cells surrounded by a basement membrane. Many arterioles were found still retaining their smooth muscle cells. Pericytes were found in association with most of the venules and many of the capillaries. Astrocyte processes remained attached to most of the microvessels. These results show that vessels prepared from rat brain still maintain most of their complex intercellular contacts and must be viewed as a heterogeneous network of cells.     

Questions Left Unanswered: How the Brain Responds to Missing Information

It sometimes happens that when someone asks a question, the addressee does not give an adequate answer, for instance by leaving out part of the required information. The person who posed the question may wonder why the information was omitted, and engage in extensive processing to find out what the partial answer actually means. The present study looks at the neural correlates of the pragmatic processes invoked by partial answers to questions. Two experiments are presented in which participants read mini-dialogues while their Event-Related brain Potentials (ERPs) are being measured. In both experiments, violating the dependency between questions and answers was found to lead to an increase in the amplitude of the P600 component. We interpret these P600-effects as reflecting the increased effort in creating a coherent representation of what is communicated. This effortful processing might include the computation of what the dialogue participant meant to communicate by withholding information. Our study is one of few investigating language processing in conversation, be it that our participants were ‘eavesdroppers’ instead of real interactants. Our results contribute to the as of yet small range of pragmatic phenomena that modulate the processes underlying the P600 component, and suggest that people immediately attempt to regain cohesion if a question-answer dependency is violated in an ongoing conversation.     

Mental Processes and the Brain During Dreams.

This article examines the different theoretical approaches to dreaming and compares them with recent data from brain-mapping studies. Two lines of investigation were considered: a neurobiological and a cognitive approach. Both lines of investigation can be usefully integrated into recent research using the techniques of brain mapping. Two aspects of particular interest are discussed: (a) The pattern of limbic and paralimbic activation in rapid eye movement (REM) and non-REM could explain some differences of oneiric hallucination during different stages of sleep, and (b) the deactivation of the heteromodal cortex could explain the loss of reality testing and the absence of self-consciousness during dreams. The complex nature of the dreaming phenomenon makes it necessary to distinguish clearly between mental representation and the underlying neurobiological changes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lactate and pH in the Brain: Association and Dissociation in Different Pathophysiological States

Brain tissue pH and lactate content were measured in rats under three different experimental conditions, namely: during complete global cerebral ischemia; after reversible near-complete cerebral ischemia; and in experimental brain tumors. At the end of the experiments brains were frozen with liquid nitrogen. A series of 20-microns thick coronal sections was prepared in a cryostat and then used for the regional determination of tissue pH (umbelliferone technique) and tissue lactate (bioluminescent technique). In addition, tissue samples were taken for the quantitative measurement of brain lactate (enzymatic fluorometric technique). The relationship between lactate content and tissue pH was different for each of the three experimental models studied: only after short-term global cerebral ischemia did an increase in the lactate content correlate with a decrease in tissue pH (r = 0.94; p less than 0.001). A highly significant increase in the lactate content (p less than 0.001) was accompanied by physiological pH values (6.96 +/- 0.08 in comparison to 6.97 +/- 0.04 in controls) during recirculation after transient cerebral ischemia and in brain tumors even by an alkaline pH shift. In view of these observations the term "lactacidosis" should not be used without measuring both the lactate content and the pH. The observed dissociation between pH and lactate is due to the fact that both parameters are regulated independently. During anaerobiosis the main source of proton production is ATP hydrolysis rather than glycolysis. It is, therefore, suggested that the terms "acidosis" and "lactosis" should be used instead of "lactacidosis."     

When and How Much to Reproduce: The Trade-Off Between Power and Efficiency

A compromise between speed and efficiency of energy conversionsgives the maximum power of useful energy conversion at intermediateefficiencies. Organisms are selected to maximize the power ofenergy conversions to a useful form. However, most species havevery little capacity to vary the efficiency of their energyconversions in response to variation in the intensity of theenvironmental power supply. Plants can respond slowly to horizontalvariation in the availability of energy by growth. The trade-offbetween power and efficiency which is dependent on the compromisebetween speed and efficiency of energy conversions does seemto apply to the relative efficiencies of successive speciesin a sere of secondary terrestrial plant succession. The applicationof the power trade-off to species in a sere predicts the commongeneral properties of growth and reproduction in succession.The power trade-off may also help to explain reproductive patternsin animals that differ in the concentration of their food supply.The thermodynamic basis of causation provided by the power trade-offcould be a valuable tool for connecting evol utionary ecologywith community and ecosystem studies.     

Brain Systems for Probabilistic and Dynamic Prediction: Computational Specificity and Integration

A computational approach to functional specialization suggests that brain systems can be characterized in terms of the types of computations they perform, rather than their sensory or behavioral domains. We contrasted the neural systems associated with two computationally distinct forms of predictive model: a reinforcement-learning model of the environment obtained through experience with discrete events, and continuous dynamic forward modeling. By manipulating the precision with which each type of prediction could be used, we caused participants to shift computational strategies within a single spatial prediction task. Hence (using fMRI) we showed that activity in two brain systems (typically associated with reward learning and motor control) could be dissociated in terms of the forms of computations that were performed there, even when both systems were used to make parallel predictions of the same event. A region in parietal cortex, which was sensitive to the divergence between the predictions of the models and anatomically connected to both computational networks, is proposed to mediate integration of the two predictive modes to produce a single behavioral output.     

Processes of Proliferation and Apoptosis in the Brain of the Amur Sturgeon

Using techniques of immunoperoxidase staining of proliferating cell nuclear antigen (PCNA) and TUNEL labeling of fragmented DNA, we studied sites of proliferation and apoptosis in the myelencephalon, cerebellum, tectum opticum, thalamus, and hypothalamus of the Amur sturgeon (Acipenser schrenckii). We found that the processes of proliferation and apoptosis are maintained in the brain of 3-year-old sturgeon individuals; the ratio of these processes in different cerebral regions varied significantly. The maximum intensity of proliferative activity was found in the periventricular zone of the myelencephalon (proliferation index, on average, 21.0 ± 1.3%). This fact allows us to consider this cerebral region a most important zone were adult neurogenesis occurs in the sturgeon. In the medial reticular formation, dorsal thalamic nuclei, inner fibrous layer of the tectum, and lateral hypothalamus, the maximum numbers of apoptotic elements were found. Therefore, these zones in the brain of the sturgeon correspond, apparently, to the regions where postmitotic neuroblasts are localized. In sensory centers (tectum and nuclei of the V, VII, and X nerves), significantly varying ratios of intensities of proliferation and apoptosis were found; this is indicative of dissimilar rates of growth and differentiation in visual and chemosensory centers of the sturgeon brain. The high proliferative activity in sensory and motor cerebral centers of the sturgeon allows us to hypothesize that a neotenic pattern is preserved in these CNS regions of adult sturgeons over a long period after the embryogenesis has been completed.     

Regional Dissociation of Cyclic AMP and Inositol Phosphate Formation in Response to Thyrotropin-Releasing Hormone in the Rat Brain

The present study was undertaken to define effects of thyrotropin-releasing hormone (TRH) on formation of cyclic AMP (cAMP) and inositol phosphates (IPs) in rat brain regions. The brain of male Wistar rats was dissected into seven discrete regions, and each region was sliced. The slices were incubated in Krebs-Henseleit glucose buffer containing varying doses of TRH. TRH caused a significant and consistent increase in cAMP level, but not in formation of IPs, in the hypothalamus, striatum, and midbrain. TRH stimulated formation of IPs in the cerebellum, where the tripeptide did not change the cAMP level. In contrast, formation of neither cAMP nor IPs was affected by TRH in the cortex, hippocampus, or pons-medulla. These data suggest that TRH possesses two distinct types of brain intracellular signaling systems, which vary with brain regions.     

Ca transfer from the ER to mitochondria: When, how and why

The heterogenous subcellular distribution of a wide array of channels, pumps and exchangers allows extracellular stimuli to induce increases in cytoplasmic Ca²⁺ concentration ([Ca²⁺]_c) with highly defined spatial and temporal patterns, that in turn induce specific cellular responses (e.g. contraction, secretion, proliferation or cell death). In this extreme complexity, the role of mitochondria was considered marginal, till the direct measurement with targeted indicators allowed to appreciate that rapid and large increases of the [Ca²⁺] in the mitochondrial matrix ([Ca²⁺]_m) invariably follow the cytosolic rises. Given the low affinity of the mitochondrial Ca²⁺ transporters, the close proximity to the endoplasmic reticulum (ER) Ca²⁺-releasing channels was shown to be responsible for the prompt responsiveness of mitochondria. In this review, we will summarize the current knowledge of: i) the mitochondrial and ER Ca²⁺ channels mediating the ion transfer, ii) the structural and molecular foundations of the signaling contacts between the two organelles, iii) the functional consequences of the [Ca²⁺]_m increases, and iv) the effects of oncogene-mediated signals on mitochondrial Ca²⁺ homeostasis. Despite the rapid progress carried out in the latest years, a deeper molecular understanding is still needed to unlock the secrets of Ca²⁺ signaling machinery.     

Spreading the Signal: Dissociation of Chk1 from Chromatin

The effector kinase Chk1 plays a critical role in the DNA damage checkpoint response byphosphorylating regulators of the cell cycle machinery, resulting in an inhibition of cellcycle progression. In the presence of genotoxic stress, the PI3 kinase-like kinase ATRrapidly phosphorylates Chk1 on conserved serine residues, thereby triggering kinaseactivation through the release of an auto-inhibitory region present at its C-terminus andby regulating interactions with other proteins. Recent data have demonstrated anadditional regulatory mechanism of Chk1 functioning: Chk1 binds chromatin inunperturbed cells and dissociates from chromatin in response to DNA damage in amanner that is dependent on PIKK-mediated phosphorylation. Here, I give an overviewof these findings and discuss the implications of these data for our understanding of theexecution of the checkpoint arrest induced after the detection of DNA damage.     

The Heuristic Work of the Brain and Artificial Neural Networks

Biophysics - This paper presents two new fundamental principles of the functioning of real neural networks of the brain. These principles have inspired the design of artificial neural networks (a...