In vivo release of adenosine from cat basal ganglia—studies with a push pull cannula

The release of newly synthesized [³H]adenosine has been studied in vivo in cat caudate nucleus and substantia nigra, using a push pull cannula. In the presence of [³H]adenosine as precursor, spontaneously released [³H]adenosine was easily detectable in superfusates of the push pull cannula. In the caudate nucleus, potassium and veratridine caused a marked and reversible increase in [³H]adenosine release. The effect of veratridine was completely blocked by tetrodotoxin (TTX) although TTX had no action by itself. Ouabain as well as glutamate, also markedly increased the release of [³H]adenosine.The specific 5′ nucleotidase inhibitor α,β-methylene ADP, did not alter the increase in the amount of [³H]adenosine obtained by veratridine, although it diminished the spontaneous release of [³H]adenosine by about 20%.Push pull cannulae were also implanted simultaneously into the caudate nucleus and substantia nigra. Potassium applied into the caudate nucleus increased the local release of adenosine but did not change that observed in the substantia nigra. When potassium was applied into the substantia nigra, it also increased the local release of adenosine but did not change that observed in the caudate nucleus.The results are discussed in term of the possible existence of “purinergic neurons” and of the relation between the adenosine release and central nervous activity.     

Notch signalling is needed to maintain, but not to initiate, the formation of prosensory patches in the chick inner ear

Notch signalling is well-known to mediate lateral inhibition in inner ear sensory patches, so as to generate a balanced mixture of sensory hair cells and supporting cells. Recently, however, we have found that ectopic Notch activity at an early stage can induce the formation of ectopic sensory patches. This suggests that Notch activity may have two different functions in normal ear development, acting first to promote the formation of the prosensory patches, and then later to regulate hair-cell production within the patches. The Notch ligand Serrate1 (Jag1 in mouse and humans) is expressed in the patches from an early stage and may provide Notch activation during the prosensory phase. Here, we test whether Notch signalling is actually required for prosensory patch development. When we block Notch activation in the chick embryo using the gamma-secretase inhibitor DAPT, we see a complete loss of prosensory epithelial cells in the anterior otocyst, where they are diverted into a neuroblast fate via failure of Delta1-dependent lateral inhibition. The cells of the posterior prosensory patch remain epithelial, but expression of Sox2 and Bmp4 is drastically reduced. Expression of Serrate1 here is initially almost normal, but subsequently regresses. The patches of sensory hair cells that eventually develop are few and small. We suggest that, in normal development, factors other than Notch activity initiate Serrate1 expression. Serrate1, by activating Notch, then drives the expression of Sox2 and Bmp4, as well as expression of the Serrate1 gene itself. The positive feedback maintains Notch activation and thereby preserves and perhaps extends the prosensory state, leading eventually to the development of normal sensory patches.     

Habitat Use,Roost Selection and Conservation of Bats in Tsingy De Bemaraha National Park,Madagascar

Although the land mammals of Madagascar have been the subject of many studies, the island’s bats have yet to feature prominently on the research or conservation agenda. In this study we used mist nets, acoustic sampling and cave surveys to assess habitat use, seasonality and roost selection. Four microchiropteran species (Triaenops rufus, T. furculus, Miniopterus manavi and Myotis goudoti) appeared to be strongly associated with the forest interior based on trapping, but analysis of time-expanded echolocation recordings revealed that T. rufus and M.␣manavi were frequently recorded in forest edges and clearings. Bat activity was significantly lower inside the forest than at the interface between agricultural land and forest. The caves visited most often by tourists were low in bat abundance and species richness. Anjohikinakina Cave, which was visited infrequently by people, was used by five species and contained between 54% (winter) and 99% (summer) of bats counted in 16 caves and is a site of national importance for bat conservation. Hipposideros commersoni was only netted in our study area during October and may be a migrant to the site or present but inactive during the austral winter. The forest surrounding the caves is therefore important because it provides cover for emerging bats and a potential source of invertebrate prey whilst the forest edge is important to foraging bats.     

Carbon economy in walnut seedlings during the acquisition of autotrophy studied by long-term labelling with 13CO2

A closed growth chamber was designed to study the acquisition of autotrophy by seedlings of walnut ( Juglans regia L. cv. Lara) in controlled conditions (22°C, 12-h photoperiod) during the first two months of growth. The chamber consisted of two airtight compartments, in which continuous gas exchange was measured on the aerial and subterranean parts of several batches of tree seedlings. Long-term labelling with ¹³CO₂ was used in the chamber to study the import, distribution, and respiratory losses of photoassimilates (autotrophic carbon) in relation to the partitioning and use of reserves of the maternal seed (heterotrophic carbon). The carbon economy of walnut seedlings was estimated by measurements of gas exchange, carbon content, and ¹³C/¹²C isotopic ratio in dry matter and respiratory CO₂. The seedlings were entirely heterotrophic for energy and structural growth during the first 21 days after sowing. From day 22, photosynthesis appeared. At day 29, autotrophic carbon accounted for 25% and 30% of respiration in the root and shoot respectively; these proportions increased to 45% and 65% at day 54. The autotrophic carbon was incorporated into the dry matter of the shoot from day 32 but only after day 40 into the dry matter of the taproot. From day 32, the total contribution of heterotrophic carbon decreased regularly, and until day 43, it was essentially used for root growth. Thereafter, the contribution of heterotrophic carbon was negligible, and at day 54 the walnut seedlings were entirely autotrophic.     

Significance and limits in the use of predawn leaf water potential for tree irrigation

Research in estimating the water status of crops is increasingly based on plant responses to water stress. Several indicators can now be used to estimate this response, the most widely available of which is leaf water potential (Ψ_LWP) as measured with a pressure chamber. For many annual crops, the predawn leaf water potential (Ψ_PLWP), assumed to represent the mean soil water potential next to the roots, is closely correlated to the relative transpiration rate, RT. A similar correlation also holds for young fruit trees grown in containers. However, exceptions to this rule are common when soil water content is markedly heterogeneous. Two experimental conditions were chosen to assess the validity of this correlation for heterogeneous soil water content: 1) young walnut trees in split-root containers. The heterogeneity was created by two unequal compartments (20% and 80% of total volume), of which only the smaller was irrigated and kept at a moisture content higher than field capacity (permanent drainage). 2) adult walnut trees in an orchard. In this case, soil water heterogeneity was achieved by limiting the amount of localised irrigation (20% of the irrigated control) which was applied every evening. Values of sap flux and of minimum and predawn leaf water potentials with homogeneous and heterogeneous soil water content were compared for trees grown in the orchard and in containers. In spite of intense drought reflected by very low RT or stem water potential, Ψ_PLWP of trees under heterogeneous moisture conditions remained high (between -0.2 and -0.4 MPa) both in the orchard and in containers. These values were higher than those usually considered critical under homogeneous soil conditions. A semi-quantitative model, based on the application of Ohm's analogy to split-root conditions, is proposed to explain the apparently conflicting results in the literature on the relation between Ψ_PLWP and soil water potential. This revised version was published online in August 2006 with corrections to the Cover Date.     

Morphological and molecular changes in the inner hair cell region of the rat cochlea after amikacin treatment

This study investigates the morphological and molecular changes that occur in the inner hair cell area of the rat cochlea following aminoglycoside treatment. Rats were injected daily with 500 mg/kg of amikacin between postnatal day 9 (PND9) and PND16. Cochleae were examined at PND16 to PND120 using both scanning and transmission electron microscopy and molecular fluorescent labeling. The inner hair cells showed obvious signs of apoptosis in response to amikacin treatment and most of them were missing by one week after the end of the aminoglycoside exposure period. Concomitantly, the epithelium became scarred as the surrounding supporting cells expanded and filled the space vacated by the missing IHCs. The mid-basolateral region of these modified supporting cells was surrounded by many afferent and efferent terminals. However, these cells expressed neither calbindin nor SNAP25, proteins that are both expressed by IHCs in the normal, untreated organ of Corti in the rat. In addition, these supporting cells remained attached to the basal lamina by a thin cytoplasmic process. The supporting cells surrounding the inner hair cells therefore appear unable to convert directly into inner hair cells following aminoglycoside induced hair-cell loss but may be able to provide trophic support for the remaining afferent and efferent neurites.     

Generalized Münch coupling between sugar and water fluxes for modelling carbon allocation as affected by water status

A model of within-plant carbon allocation is proposed which makes a generalized use of the Münch mechanism to integrate carbon and water functions and their involvement in growth limitations. The plant is envisioned as a branched network of resistive pathways (phloem and xylem) with nodal organs acting as sources and sinks for sucrose. Four elementary organs (leaf, stem, fruit, root) are described with their particular sink functions and hydraulic attributes. Given the rates of photosynthesis and transpiration and the hydraulic properties of the network as inputs, the model calculates the internal fluxes of water and sucrose. Xylem water potential (Psi), phloem sucrose concentration (C) and turgor pressure (P) are calculated everywhere in the network accounting for osmotic equilibrium between apoplasm and symplasm and coupled functioning of xylem and phloem. The fluxes of phloem and xylem saps are driven by the gradients of P and Psi, respectively. The fruit growth rate is assumed as turgor pressure dependent. To demonstrate its ability to address within-plant competition, the model is run with a simple-branched structure gathering three leaves, eight stem segments, three competing growing fruits and one root. The model was programmed with P-Spice, a software specifically designed for simulating electrical circuits but easily adaptable to physiology. Simulations of internal water fluxes, sucrose concentrations and fruit growth rates are given for different conditions of soil water availability and hydraulic resistances (sensitivity analysis). The discussion focuses on the potential interest of this approach in functional--structural plant models to address water stress-induced effects.     

Two contrasting roles for Notch activity in chick inner ear development: specification of prosensory patches and lateral inhibition of hair-cell differentiation

Lateral inhibition mediated by Notch is thought to generate the mosaic of hair cells and supporting cells in the inner ear, but the effects of the activated Notch protein itself have never been directly tested. We have explored the role of Notch signalling by transiently overexpressing activated Notch (NICD) in the chick otocyst. We saw two contrasting consequences, depending on the time and site of gene misexpression: (1) inhibition of hair-cell differentiation within a sensory patch; and (2) induction of ectopic sensory patches. We infer that Notch signalling has at least two functions during inner ear development. Initially, Notch activity can drive cells to adopt a prosensory character, defining future sensory patches. Subsequently, Notch signalling within each such patch mediates lateral inhibition, restricting the proportion of cells that differentiate as hair cells so as to generate the fine-grained mixture of hair cells and supporting cells.