Context-independent expression of spatial code in hippocampus.

The hippocampus plays a crucial role in the formation and retrieval of spatial memory across mammals and episodic memory in humans. Episodic and spatial memories can be retrieved irrespective of the subject's awake behavioral state and independently of its actual spatial context. However, the nature of hippocampal network activity during such out-context retrieval has not been described so far. Theoretically, context-independent spatial memory retrieval suggests a shift of the hippocampal spatial representations from coding the current spatial context to coding the remembered environment. In this study we show in rats that the CA3 neuronal population can switch spontaneously across representations and transiently activate another stored familiar spatial pattern without direct external sensory cuing. This phenomenon qualitatively differs from the well-described sharp wave-related pattern reactivations during immobility. Here, it occurs under the theta oscillatory state during active exploration and reflects the preceding experience of sudden environmental change. The respective out-context coding spikes appeared later in the theta cycle than the in-context ones. Finally, the experience also induced the emergence of population vectors with a co-expression of both codes segregated into different phases of the theta cycle.

Impact of aging on mitochondrial respiration in various organs.

Mitochondria are considered central regulator of the aging process; however, majority of studies dealing with the impact of age on mitochondrial oxygen consumption focused on skeletal muscle concluding (although not uniformly) a general declining trend with advancing age. In addition, gender related differences in mitochondrial respiration have not been satisfactorily described yet. The aim of the present study was to evaluate mitochondrial oxygen consumption in various organs of aging male and female Fischer 344 rats at the ages of 6, 12 and 24 months. Mitochondrial respiration of homogenized (skeletal muscle, left and right heart ventricle, hippocampus, cerebellum, kidney cortex), gently mechanically permeabilized (liver) tissue or intact cells (platelets) was determined using high-resolution respirometry (oxygraphs O2k, Oroboros, Austria). The pattern of age-related changes differed in each tissue: in the skeletal muscle and kidney cortex of both sexes and in female heart, parameters of mitochondrial respiration significantly declined with age. Resting respiration of intact platelets displayed an increasing trend and it did not correlate with skeletal muscle respiratory states. In the heart of male rats and brain tissues of both sexes, respiratory states remained relatively stable over analyzed age categories with few exceptions of lower mitochondrial oxygen consumption at the age of 24 months. In the liver, OXPHOS capacity was higher in females than in males with either no difference between the ages of 6 and 24 months or even significant increase at the age of 24 months in the male rats. In conclusion, the results of our study indicate that the concept of general pattern of age-dependent decline in mitochondrial oxygen consumption across different organs and tissues could be misleading. Also, the statement of higher mitochondrial respiration in females seems to be conflicting, since the gender-related differences may vary with the tissue studied, combination of substrates used and might be better detectable at younger ages than in old animals.

Water maze performance, exploratory activity, inhibitory avoidance and hippocampal plasticity in aged superior and inferior learners.

In 28- to 30-month-old rats, in vitro short-term and long-term potentiation (STP and LTP) were measured in area CA1 of the hippocampus in seven superior and seven inferior learners, that were selected from a pool of 40 rats based on water maze escape performance over a period of 9 days. The aim was to examine whether levels of STP and LTP could account for group differences in learning of water maze escape, spatial preference and wall (thigmotaxis)-avoidance and in short-term retention of an inhibitory avoidance task. There was no significant group difference in open-field exploration, i.e. the number of rearings. In contrast to expectation, the superior and inferior learners did not differ significantly from each other in levels of STP and LTP. However, variability in escape and spatial learning, but not thigmotaxis-avoidance learning, was significantly predicted by variability in STP and LTP in the superior group. Also, open-field exploratory rearings were significantly correlated with STP and LTP as well as with maze escape learning in the superior group. The results show that, in the aged superior group, levels of CA1 STP and LTP coincided with residual water maze escape and spatial preference learning as well as open-field exploration, i.e. behavioural expressions known to be related to hippocampal functioning, but not with learning to avoid thigmotaxis in the maze. The lack of such correlations in the inferior group may be due to the severe impairment in escape and spatial preference learning and/or the influence of yet unknown third variables on these relationships.

Hippocampus-dependent retrieval and hippocampus-independent extinction of place avoidance navigation, and stress-induced out-of-context activation of a memory revealed by reversible lesion experiments in rats.

The use of reversible lesion techniques in memory research was pioneered in the laboratory of Jan Bures and Olga Buresova. We use the occasion of Jan's 75th birthday to briefly review the experimental utility of this approach. Two experiments from our current research are reported in which reversible lesioning methods are used to ask otherwise experimentally untenable questions about memory retrieval. The first experiment used intra-hippocampal injections of tetrodotoxin to temporarily inactivate the hippocampus during retrieval of a well-learned place avoidance navigation memory. This revealed that the hippocampus is necessary for place avoidance retrieval but that the extinction of place avoidance can occur independently of retrieving the memory and intact hippocampal function. The second experiment used KCl-induced cortical spreading depression in an interhippocampal transfer paradigm to demonstrate that a Y-maze memory that is learned by only one cortical hemisphere can be made to transfer to the other hemisphere by forcing the rat to swim, a unique stressful experience that occurred in a different apparatus, different behavioral context, and involved different behaviors than the Y-maze training. This demonstrates, we believe for the first time behaviorally, that memories can be activated outside of the behavioral context of their acquisition and expression in rats.

Facilitation of learning and modulation of frontal cortex acetylcholine by ventral pallidal injection of heparin glucosaminoglycan.

We examined the effects of heparin on learning and frontal cortex acetylcholine parameters following injection of the glucosaminoglycan into the ventral pallidum. In Experiment 1, possible mnemoactive effects of intrapallidal heparin injection were assessed. Rats with chronically implanted cannulae were administered heparin (0.1, 1.0, 10 ng) or vehicle (0.5 microl) and were tested on a one-trial step-through avoidance task. Two retention tests were carried out in each animal, one at 1.5 h after training to measure short-term memory and another at 24 h to measure long-term memory. Post-trial intrapallidal injection of 1.0 ng heparin improved both short- and long-term retention of the task, whereas the lower and the higher dose of the glucosaminoglycan had no effect. When the effective dose of heparin was injected 5 h, rather than immediately after training, it no longer facilitated long-term retention of the conditioned avoidance response. In Experiment 2, the effects of ventral pallidal heparin injection on frontal cortex acetylcholine and choline concentrations were investigated with in vivo microdialysis in anaesthetized rats. Heparin, administered in the dose of 1.0 ng, which was effective in facilitating avoidance performance, produced a delayed increase in cortical acetylcholine levels ipsi- and contralaterally to the side of intrabasalis injection, resembling the known neurochemical effects obtained for another glycosaminoglycan, chondroitin sulfate, which recently was shown to facilitate inhibitory avoidance learning and to increase frontal cortex acetylcholine. The present findings indicate that heparin, like other extracellular matrix proteoglycans, can exert beneficial effects on memory and strengthen the presumptive relationship between such promnestic effects of proteoglycans and basal forebrain cholinergic mechanisms. The data are discussed with respect to the presumed roles of matrix molecules in extrasynaptic volume transmission and in the 'cross-talk' between synapses.

[Multiple lymphomatous polyposis of the gastrointestinal tract]. / Mnohotná lymfomatózní polypóza gastrointestinálního traktu.

We studied three patients in whom histology revealed multiple lymphomatous polyposis of the gastrointestinal tract. It is a distinctive type of primary gastrointestinal lymphoma characterized by polypoid accumulations of lymphoma tissue involving long segments of the gastrointestinal tract. This lymphoma consists of a diffuse proliferation of small round lymphocytes and small cleaved cells and tends to an extraintestinal dissemination. Clinical behavior of this entity is more aggressive than that of the same primary nodal lymphoma. We discuss the evolution of opinions of this entity.

Greenland Ice Sheet Surface Properties and Ice Dynamics from ERS-1 SAR Imagery.

C-band synthetic aperture radar (SAR) imagery from the European Space Agency's ERS-1 satellite reveals the basic zonation of the surface of the Greenland Ice Sheet. The zones have backscatter signatures related to the structure of the snowpack, which varies with the balance of accumulation and melt at various elevations. The boundaries of zones can be accurately located with the use of this high-resolution imagery. The images also reveal a large flow feature in northeast Greenland that is similar to ice streams in Antarctica and may play a major role in the discharge of ice from the ice sheet.

Unusual radar echoes from the greenland ice sheet.

Airborne radar images of part of the Greenland ice sheet reveal icy terrain whose radar properties are unique among radar-studied terrestrial surfaces but resemble those of Jupiter's icy Galilean satellites. The 5.6- and 24-centimeter-wavelength echoes from the Greenland percolation zone, like the 3.5- and 13-centimeter-wavelength echoes from the icy satellites, are extremely intense and have anomalous circular and linear polarization ratios. However, the detailed subsurface configurations of the Galilean satellite regoliths, where heterogeneities are the product of prolonged meteoroid bombardment, are unlikely to resemble that within the Greenland percolation zone, where heterogeneities are the product of seasonal melting and refreezing.

Effects of water and ice layers on the scattering properties of diffuse reflectors.

Measurements were made of the angular distribution of power scattered from a diffuse reflector illuminated by a laser beam directed normal to the surface of the reflector. Experiments were performed on dry, wet, and ice-covered planar targets. They revealed that the diffuse component of scattered power from a wet or icecovered target is reduced by an amount proportional to the inverse of the square of the index of refraction of the layer, which is consistent with simple theory. Backscattered radiation from a water-or ice-covered target was found to be enhanced compared with that from a dry target in the region about a cone centered on the line normal to the target. The half-angles of the cones for dry, water-covered, and ice-covered targets were 2.5, 12.5, and 30 degrees , respectively. The large half-angles of the covered targets may be due to multiple reflections within the layer. Small air bubbles in the ice and the roughness of the ice surface may be responsible for the particularly large increase in half-angle of the ice-covered target.

Rheology of glacier ice.

A new method for calculating the stress field in bounded ice shelves is used to compare strain rate and deviatoric stress on the Ross Ice Shelf, Antarctica. The analysis shows that strain rate (per second) increases as the third power of deviatoric stress (in newtons per square meter), with a constant of proportionality equal to 2.3 x 10(-25).

Effect of a pause on action potentials of the working ventricular myocardium of human embryos.

The effect of a period of rest (Tp) lasting from 5 to 120 s on the action potentials (AP) of the ventricular myocardium of 9- to 11-week human embryos was studied. The result was shortening of the AP proportional to the duration of the pause and it was accompanied by a shift of the AP plateau phase to more negative membrane voltages. Shortening of the AP during the pause was more pronounced in solutions with a half extracellular calcium concentration. Recovery from the effect of the pause took place significantly more slowly in solutions with the lower extracellular calcium concentration than in the presence of a normal concentration.

Action potentials of the rabbit, guinea pig, dog and albino rat working ventricular myocardium under interpolated extrasystole conditions during postnatal ontogenesis.

Experiments were carried out on the working myocardium of the right heart ventricle of newborn and adult rabbits, guinea-pigs, dogs and albino rats. In the dog, the guinea-pig and the rabbit, after ten action potentials (AP) elicited with 1 Hz frequency we always interpolated an extrasystole at an interval (TE) of 100-900 ms. In albino rats we used a basic frequency of 2 Hz and a TE of 30-370 ms from the last regular AP. Using glass microelectrodes, we recorded the extrasystolic AP (EAP) and the next subsequent AP (2AP). The results were evaluated by constructing graphs of the correlations of the duration of the plateau phase (D0) to TE and of the duration of repolarization to -60 mV level (D60) to the TE. In the myocardium of newborn rabbits, guinea-pigs and dogs, with short TE both D0 and D60 of the EAP are shorter than in the steady state (SS), while for the 2AP the same parameters are influenced only a little. As the TE lengthens, the EAP gradually acquire a length corresponding more to the SS. With TE longer than half the duration of the cycle in the steady state the EAP return to normal, while the 2AP become shorter. The effect of extrasystole on the rat EAP and 2AP diminished with advancing age. In the myocardium of adult rabbits and adult guinea-pigs, and slightly in the myocardium of adult dogs and newborn rats, we observed that the duration of the EAP, with certain TE, was greater than in the steady state.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acetylcholine and adrenaline on ventricular action potentials of the human embryonic myocardium.

Using the glass microelectrode technique we measured the effect of adrenaline (ADR, 10(-5) mol . l-1) and acetylcholine (ACh, 10(-5) mol . l-1) on action potentials (AP) of the ventricular myocardium of 8- to 11-week-old human embryos. ACh prolonged the AP (by 20%), without affecting voltage of the plateau phase. ADR markedly elevated the plateau phase (by 8 mV) and prolonged the AP (by up to 40% of the initial duration). The ACh effect attained the maximum in 5-7 min and was frequency dependent. The ADR effect reached the maximum in 1-2 min. Neither ACh nor ADR affected the resting membrane potential value. The effect of ACh was blocked by atropine (10(-6) mol . l-1), showing that it is mediated by way of M-receptors. Propranolol (6.10(-6) mol . l-1) only partly blocked the effects of ADR. The results show that the human ventricular myocardium is already sensitive to the action of ACh and ADR before autonomic innervation of the heart develops.

An analysis of the postnatal development of the action potential repolarization process in the working ventricular myocardium of albino rats (effect of tea, frequency, verapamil and adrenaline).

Using the glass microelectrodes techniques, we analysed the causes of the shortening of the ventricular myocardial action potentials (AP) of albino rats during postnatal development. Delayed outward currents were blocked with tetraethylammonium (TEA) in 20 mmol.l-1 concentration. TEA led to prolongation of action potentials and to accentuation of frequency sensitivity in all the given age groups, but a block of TEA-sensitive currents nevertheless does not permit the conclusion that the cause of postnatal AP shortening is due to an increase in the intensity of outward currents. Slow inward current (Isi) were blocked with verapamil (2.10(-5) mol.l-1), which markedly shortened the myocardial AP of newborn rats; with advancing age its effect diminished and shifted to the negative membrane voltage level. The Isi was stimulated by adrenaline (10(-5) mol.l-1), which markedly prolonged the AP of newborn animals; with advancing age its effect rapidly diminished and was virtually undetectable in 10-day-old animals. The results indicate that postnatal AP prolongation is caused by a drop in the Isi rate rather than by the growth of outward repolarizing currents.

Postnatal development of electrophysiological manifestations of the working ventricular myocardium of albino rats.

To elucidate the postnatal development of electrogenesis in the rat ventricular myocardium, we investigated specimens of the right ventricles of rats of different ages (newborn, 2 days, 5 days, 10 days and adult) by means of glass microelectrodes and a programmable stimulator. The resting membrane potential rose from -82 mV (newborn) to -89.6 mV (adult). During the first 10 postnatal days the plateau phase of the action potential (AP) shortened from 46 ms to 16 ms; in adulthood its duration was 9.6 ms. An increase in the stimulation frequency from 2 Hz to 10 Hz led to pronounced shortening of the myocardial AP of newborn and 2-day-old rats; with advancing age the influence of a mounting frequency diminished and the myocardium of adult rats was frequency-insensitive. The first AP evoked after a rest interval in the myocardium of newborn animals was prolonged proportionally to the logarithm of the duration of the interval; the AP of adult rats was unaffected by the length of the interval. During the rest interval diastolic depolarization appeared on the myocardium of newborn rats; with advancing age it slowed down and the decrease in membrane voltage was smaller. In the adult rats myocardium there was no diastolic depolarization.

Basic electrophysiological parameters and frequency sensitivity of the ventricular myocardium of human embryos.

The basic electrophysiological manifestations of the ventricular myocardium of twelve 7- to 12-week human embryos were measured with a glass electrode and a programmed stimulation technique. The resting membrane potential value was 79.37 +/- 0.34 mV and the overshoot 32.7 +/- 0.57 mV; the action potential (AP) duration at 1 Hz stimulation frequency was 120.0 +/- 5.7 ms at AP plateau phase levels and 258 +/- 17 ms at the level corresponding to 95% repolarization. The duration of the AP was a function of the stimulation frequency. i.e. it altered in correlation to the stimulation programme fully developed frequency sensitivity). In stimulation with different frequencies the duration of the steady state AP was in an inverse relation to the stimulation frequency, the maximum changes being found in the terminal repolarization zone. An interpolated extrasystole mainly affected the duration of the plateau phase.