Is the development of orientation selectivity instructed by activity?

Is the development of orientation selectivity in visual cortex instructed by the patterns of neural activity of input neurons? We review evidence as to the role of activity, review models of activity-instructed development, and discuss how these models can be tested. The models can explain the normal development of simple cells with binocularly matched orientation preferences, the effects of monocular deprivation and reverse suture on the orientation map, and the development of a full intracortical circuit sufficient to explain mature response properties including the contrast-invariance of orientation tuning. Existing experiments are consistent with the models, in that (a) selective blockade of ON-center ganglion cells, which will degrade or eliminate the information predicted to drive development of orientation selectivity, in fact prevents development of orientation selectivity; and (b) the spontaneous activities of inputs serving the two eyes are correlated in the lateral geniculate nucleus at appropriate developmental times, as was predicted to be required to achieve binocular matching of preferred orientations. However, definitive tests remain to be done to firmly establish the instructive rather than simply permissive role of activity and determine whether the retinotopically and center type-specific patterns of activity predicted by the models actually exist. We conclude by critically examining alternative scenarios for the development of orientation selectivity and maps, including the idea that maps are genetically prespecified.     

Is the K permeability of the resting membrane controlled by the excitable K channel?

To test whether or not the potassium permeability of the resting membrane is controlled by the excitable K channels (delayed rectifier), we examined changes in the Na and K permeability ratio, PNa/PK, of the squid axon before and after the excitable K channels were blocked. The blockage of the K channels was accomplished by three independent methods: internal application of tetraethylammonium, internal application of 4-aminopyridine plus Cs, and prolong internal perfusion of NaF solution. The permeability ratio was determined using two different methods: the conventional electrophysiological method and a new method based on the measurements of the hyperpolarizing effect of Na removal. We found that blocking the K channels did not cause a proportional decrease in the K permeability of the resting membrane, suggesting that the semipermeable property of the resting membrane is not determined by the excitable K channels.     

Phase transitions in nuclei and chromatin. Is nuclear volume controlled by the chromatin or by the nuclear matrix?

Changes in the volume of rat liver nuclei have been monitored as a function of modifications in ionic environment (from 0 to 20 mM), temperature (from 4 to 37 degrees C), and pH (from 1 to 8). An abrupt reduction of nuclear volume occurred with increasing ion concentration, this contraction being more pronounced with bivalent (either Ca2+ or Mg2+) than with monovalent (either Na+ or K+) cations. The lowering of pH produced a similar effect. Parallel changes in chromatin structure took place at the same time as phase-like transitions. Atomic absorption spectroscopy allowed determination of free and nuclei-bound ions, pointing to the presence of a sizeable number of free binding sites for chromatin-DNA even within intact nuclei. DNA-phosphate sites appear to be neutralized by ions strictly according to the size of the electric charge and polyelectrolyte theory. Partial digestion (by micrococcal nuclease) or simple breaks (by chemical carcinogens) of the chromatin-DNA fiber caused respectively elimination or reduction of the abrupt volume changes in the intact nuclei. The apparent role of chromatin structure versus nuclear matrix in determining the shape and volume of intact nuclei is briefly discussed.     

Is hexokinase present in the basal lateral membranes of rat kidney proximal tubular epithelial cells?

The possible presence of hexokinase in basal lateral membranes from rat kidney proximal tubules was investigated. Basal lateral membranes were obtained from homogenates of rat kidney cortex by differential centrifugation and free flow electrophoresis. They were further purified by density gradient centrifugation. Hexokinase activity was measured as the phosphorylation of D-[U14C]glucose. Throughout the purification of the membranes, the specific activity of hexokinase decreased while that of (Na+ + K+)-ATPase increased. Hexokinase activity in all fractions could be quantitatively accounted for in terms of cytosolic and mitochondrial enzyme contributions. It is concluded that there is no hexokinase activity in basal lateral membranes from rat kidney.     

Is calpain activity regulated by membranes and autolysis or by calcium and calpastatin?

Although the Ca(2+)-dependent proteinase (calpain) system has been found in every vertebrate cell that has been examined for its presence and has been detected in Drosophila and parasites, the physiological function(s) of this system remains unclear. Calpain activity has been associated with cleavages that alter regulation of various enzyme activities, with remodeling or disassembly of the cell cytoskeleton, and with cleavages of hormone receptors. The mechanism regulating activity of the calpain system in vivo also is unknown. It has been proposed that binding of the calpains to phospholipid in a cell membrane lowers the Ca2+ concentration, [Ca2+], required for the calpains to autolyze, and that autolysis converts an inactive proenzyme into an active protease. Recent studies, however, show that the calpains bind to specific proteins and not to phospholipids, and that binding to cell membranes does not affect the [Ca2+] required for autolysis. It seems likely that calpain activity is regulated by binding of Ca2+ to specific sites on the calpain molecule, with binding to each site eliciting a response (proteolytic activity, calpastatin binding, etc.) specific for that site. Regulation must also involve an, as yet, undiscovered mechanism that increases the affinity of the Ca(2+)-binding sites for Ca2+.     

Is stomatal conductance in a tomato crop controlled by soil or atmosphere?

Summary The effects of soil water deficits and air vapour pressure deficits on stomatal conductance of tomato leaves were analysed separately under field conditions in central Portugal. Three conditions were created: low soil and air humidity (A), high soil and air humidity (B) and low soil but high air humidity (C). The results show that the effect of air vapour pressure deficit on stomatal behaviour is more important than the effect of soil water deficit when the predawn leaf water potential is above –0.4 MPa.     

The development of ammonoid septa: An epithelial invagination process controlled by morphogens?

Suture lines are important in ammonoid taxonomy. Their complex morphologies, caused by iterated invaginations of the posterior body wall, can be explained using a mechano‐chemical model inspired by modem developmental models for epithelial folding in kidneys, lungs, teeth, mammary glands and other organs. A morphogen, organized to form a regular spacing pattern by reaction‐diffusion dynamics or similar processes, induces changes in cell shape and/or rate of proliferation, causing invagination and the formation of lobes. Interactions between mechanical and chemical effects, combined with expanding size due to overall growth, produce a “fractal”; pattern of smaller (secondary) invaginations superimposed on larger (primary) ones. The pattern of increasing sutural complexity that is observed in ammonoid evolution may be a simple heterochronic effect, that allowed iterated invagination to be extended to higher and higher levels of folding. In contrast with some earlier theories, the present model is based on the view that the development of ammonoid septa must have been under relatively strict genetic control.     

Is Neuronal Injury Caused by Hypoglycemic Coma of the Necrotic or Apoptotic Type?

In this study, we explored if a 30 minute period of hypoglycemic coma yields damage which shows some features associated with apoptosis. To that end, we induced insulin-hypoglycemic coma of 30 min duration, and studied brain tissues after the coma period, and after recovery period of 30 min, 3 h, and 6 h. Histopathological data confirmed neuronal damage in all of the vulnerable neuronal populations. Release of cytochrome c (cyt c), assessed by Western Blot, was observed in the neocortex and caudoputamen after 3 and 6 h of recovery. In these regions, the caspase-like activity increased above control after 6 h of recovery. By laser-scanning confocal microscopy, a clear expression of Bax was observed after 30 min of coma in the superficial layers of the neocortex, reaching a peak after 30 min of recovery. Punctuate immunolabeling surrounding nuclei in soma and dendrites in cortical pyramidal neurons likely represents mitochondria, which suggests that Bax protein assembled at the surface of mitochondria in vulnerable neocortical neurons. It is concluded that although previous morphological data have suggested that cells die by necrosis, neuronal damage after hypoglycemic coma shows some features of apoptosis.     

Is nectar reabsorption restricted by the stalk cells of floral and extrafloral nectary trichomes?

Reabsorption is a phase of nectar dynamics that occurs concurrently with secretion; it has been described in floral nectaries that exude nectar through stomata or unicellular trichomes, but has not yet been recorded in extrafloral glands. Apparently, nectar reabsorption does not occur in multicellular secretory trichomes (MST) due to the presence of lipophilic impregnations – which resemble Casparian strips – in the anticlinal walls of the stalk cells. It has been assumed that these impregnations restrict solute movement within MST to occur unidirectionally and exclusively by the symplast, thereby preventing nectar reflux toward the underlying nectary tissues. We hypothesised that reabsorption is absent in nectaries possessing MST. The fluorochrome lucifer yellow (LYCH) was applied to standing nectar of two floral and extrafloral glands of distantly related species, and then emission spectra from nectary sections were systematically analysed using confocal microscopy. Passive uptake of LYCH via the stalk cells to the nectary tissues occurred in all MST examined. Moreover, we present evidence of nectar reabsorption in extrafloral nectaries, demonstrating that LYCH passed the stalk cells of MST, although it did not reach the deepest nectary tissues. Identical (control) experiments performed with neutral red (NR) demonstrated no uptake of this stain by actively secreting MST, whereas diffusion of NR did occur in plasmolysed MST of floral nectaries at the post‐secretory phase, indicating that nectar reabsorption by MST is governed by stalk cell physiology. Interestingly, non‐secretory trichomes failed to reabsorb nectar. The role of various nectary components is discussed in relation to the control of nectar reabsorption by secretory trichomes.     

Is the “soluble” phase of cells structured?

Evidence suggesting that small molecular-weight precursors as well as polymers such as proteins and RNA, are not homogeneously distributed in the "soluble" phase of the cell, and that there may be, on one hand, hitherto unidentified barriers to normal diffusion within the cell and, on the other, channels through which certain molecular species may move in the cell sap much faster than their diffusion coefficients will permit, is discussed. Some implications of such barriers and channels, if they exist, are stated.     

Regulation of glucocorticoid receptors and Na−K ATPase activity by hydrocortisone in proximal tubular epithelial cells

Summary The effect of hydrocortisone (HC) in modulating glucocorticoid receptors (GR) and sodium-potassium adenosine triphosphatase (Na−K ATPase) activity was studied in primary cultures of immunoisolated murine proximal tubular epithelial cells (PTEC). Utilizing monoclonal antibody against stage-specific embryonic antigen-1, a homogeneous population of PTEC was obtained in high yield. The cells were cultured to confluence and further treated for 48 h in serum-free growth medium containing no HC (control); 50 nM HC; or 50 nM HC plus 20 nM of the antiglucocorticoid, RU 38486. PTEC treated with 50 nM HC had 56% of GR binding and 160% Na−K ATPase activity as compared to controls (P<0.01). GR binding was abolished by incubation in RU 38486 whereas Na−K ATPase fell below control values (P<0.05). Brief incubations of HC-treated PTEC with 0.5 mM ouabain resulted in a fall in GR binding without a change in Na−K ATPase activity. These data indicate that in PTEC, HC regulates GR binding and they suggest that stimulation of Na−K ATPase activity is a direct biological response to this receptor-hormone interaction. Thus, primary cultures of immunoaffinity-isolated PTEC offer a good model system for investigating the molecular basis underlying the regulation of GR binding and postreceptor events influenced by glucocorticoids.     

Is competition between mesocarp cells of peach fruits affected by the percentage of wild species genome?

The number of cells and the mean cell volume in the mesocarps of fruits from peach genotypes with different percentages of the genome of Prunus davidiana, a wild, related, species, were evaluated. The mesocarp mass varied greatly between the four groups of genotypes. The mean cell volume and the number of cells were negatively correlated within each group. This correlation can be interpreted as a relationship of competition between cells. In order to describe the type of competition in the different groups, we tried to adjust a model of competition for resources proposed by Lescourret and Génard (Ecoscience 10:334–341, 2003). To estimate the values of the three parameters of the model for the different groups, we applied model selection. Within nested models, we identified a single best model with six parameter values. This model was roughly accurate, but it allowed us to describe the general relationship for each group. The parameter values revealed a strong and under-compensating density-dependence effect for all groups. The percentage of P. davidiana genome appeared to influence the maximal number of cells and the strength of the competition, but no effect was found on the maximal mean volume of cells.     

Can the grey mould disease of the grape-vine be controlled by yeast?

Botrytis cinerea has been found to be highly pathogenic to 'Chardonnay' and 'Pinot noir' cultivars of the grape-vine producing the characteristic grey mould symptoms within 7 days of inoculation to the vitro-plants. The yeast Pichia anomala (strain FY-102), isolated from apple skin, was found to be antagonistic to B. cinerea as it completely inhibited the appearance of the grey mould symptoms when grown together. The yeast was responsible for morphological changes such as coagulation and leakage of the cytoplasm of B. cinerea. The pathogen, when applied together with P. anomala, failed to bring about the grey mould symptoms on the grape-vine, suggesting that the yeast could control the expression of this disease. An account of the interaction between B. cinerea and P. anomala, as well as the sequences of the complete ITS region of the ribosomal DNA of the yeast are described here.     

Relationship between dough-raising activity of baker's yeast and the fraction of ‘vital’ cells as determined by methylene blue staining or by dielectrophoresis

Three methods of comparing the vitility of baker's yeast (Saccharomyces cerevisiae) were examined, and found to correlated well. The rate of dough-raising, the degree of visible staining by a reducible dye (methylene blue), and the rate of dielectrophoretic collection as the cells were subject to a nonuniform ac field all appear to agree well as indices of cell viability. The simplicity of the dielectrophoretic technique suggests its further use in cellular assays.     

Is cholinergic activity of the caudate nucleus involved in memory?

A review was made of experiments dealing with the involvement of cholinergic activity of the caudate nucleus in memory processes. Injections of acetylcholine-receptor blockers or of neurotoxins against cholinergic interneurons into the striatum produce marked impairments in acquisition and retention of instrumental tasks while injections of acetylcholine or choline into the caudate produce the opposite effect. However, after a period of overtraining cholinergic blockade or interference with neural activity of the caudate does not produce significant deficits in retention. It is concluded that striatal cholinergic activity is critically involved in memory of recent events and that long-term memory is mediated by different neurochemical systems outside the caudate nucleus.     

Is sociality driven by the costs of dispersal or the benefits of philopatry? A role for kin-discrimination mechanisms

The role of ecological constraints in promoting sociality is currently much debated. Using a direct-fitness approach, we show this role to depend on the kin-discrimination mechanisms underlying social interactions. Altruism cannot evolve under spatially based discrimination, unless ecological constraints prevent complete dispersal. Increasing constraints enhances both the proportion of philopatric (and thereby altruistic) individuals and the level of altruistic investments conceded in pairwise interactions. Familiarity-based discrimination, by contrast, allows philopatry and altruism to evolve at significant levels even in the absence of ecological constraints. Increasing constraints further enhances the proportion of philopatric (and thereby altruistic) individuals but not the level of altruism conceded. Ecological constraints are thus more likely to affect social evolution in species in which restricted cognitive abilities, large group size, and/or limited period of associative learning force investments to be made on the basis of spatial cues.