La place de la scintiphotographie dans les études physiopathologiques

Summary Scintillation camera allows several metabolic investigations to be carried out routinely in the course of physiopathological studies. Examples are presented, including experimental tumour visualization, liver uptake of tracers, visualization of the isotope renogram, and distribution of labelled albumin with an illustration of the disturbances produced by lethal irradiation.     

Conservation of total synaptic weight through balanced synaptic depression and potentiation

Memory is believed to depend on activity-dependent changes in the strength of synapses. In part, this view is based on evidence that the efficacy of synapses can be enhanced or depressed depending on the timing of pre- and postsynaptic activity. However, when such plastic synapses are incorporated into neural network models, stability problems may develop because the potentiation or depression of synapses increases the likelihood that they will be further strengthened or weakened. Here we report biological evidence for a homeostatic mechanism that reconciles the apparently opposite requirements of plasticity and stability. We show that, in intercalated neurons of the amygdala, activity-dependent potentiation or depression of particular glutamatergic inputs leads to opposite changes in the strength of inputs ending at other dendritic sites. As a result, little change in total synaptic weight occurs, even though the relative strength of inputs is modified. Furthermore, hetero- but not homosynaptic alterations are blocked by intracellular dialysis of drugs that prevent Ca2+ release from intracellular stores. Thus, in intercalated neurons at least, inverse heterosynaptic plasticity tends to compensate for homosynaptic long-term potentiation and depression, thus stabilizing total synaptic weight.     

Sur des œstrogènes dérivés du triphényléthylène

Summary A series of derivatives of triphenylethylene have been tested and some of them found to possess interesting oestrogenic properties; the relations between chemical constitution and oestrogenic activity have been discussed.     

Carbon loss by deciduous trees in a CO2-rich ancient polar environment

Fossils demonstrate that deciduous forests covered the polar regions for much of the past 250 million years when the climate was warm and atmospheric CO2 high. But the evolutionary significance of their deciduous character has remained a matter of conjecture for almost a century. The leading hypothesis argues that it was an adaptation to photoperiod, allowing the avoidance of carbon losses by respiration from a canopy of leaves unable to photosynthesize in the darkness of warm polar winters. Here we test this proposal with experiments using 'living fossil' tree species grown in a simulated polar climate with and without CO2 enrichment. We show that the quantity of carbon lost annually by shedding a deciduous canopy is significantly greater than that lost by evergreen trees through wintertime respiration and leaf litter production, irrespective of growth CO2 concentration. Scaling up our experimental observations indicates that the greater expense of being deciduous persists in mature forests, even up to latitudes of 83 degrees N, where the duration of the polar winter exceeds five months. We therefore reject the carbon-loss hypothesis as an explanation for the deciduous nature of polar forests.     

Benthic community structure in two adjacent streams in Yellowstone National Park five years after the 1988 wildfires

Physical characteristics, benthic macroinvertebrates, and periphyton assemblages in two adjacent headwater streams in Yellowstone National Park were evaluated five years after the 1988 wildfires. The catchment of one stream was burned by wildfire (burned stream) while the other catchment was unburned (unburned stream). Physical measures revealed channel alteration in the burned stream relative to the unburned stream Periphyton biomass was lower in the burned than the unburned stream (29.2 vs. 50.5 g/m 2 AFDM, respectively), further demonstrating the unstable physical conditions of that system. Kendall's coefficient of concordance (an index of similarity) between diatom assemblages was 0.22, indicating distinct assemblage composition between streams. Navicula permitis Hust. was the most abundant diatom in the burned stream while Hannaea arcus (Ehr.) Patr. was dominant in the unburned stream. Macroinvertebrate taxa richness, density, and biomass were all greater in the unburned stream, although Chironomidae was the most abundant taxon in both streams. Results suggest the removal of terrestrial/riparian vegetation by wildfire can directly influence benthic assemblages by altering the inherent disturbance regime of the physical habitat templet.     

Cooperative dimeric and tetrameric clam haemoglobins are novel assemblages of myoglobin folds

Cooperative functioning of many protein systems depends on communication between different subunits of those systems. Perhaps the best understood cooperative protein system is the vertebrate haemoglobin tetramer, in which the subunits share a similar tertiary structure (the myoglobin fold) with each other and with myoglobins and haemoglobins from at least four different animal phyla and leguminous plants. Blood clams have cooperative tetrameric haemoglobin. In view of previous reports concerning the role of dimers in the vertebrate tetramer, the clam haemoglobins represent a very interesting model system. We report here the low-resolution three-dimensional crystal structures of the dimeric and tetrameric cooperative haemoglobins from the blood clam Scapharca inaequivalvis. We find that clam haemoglobins are made of myoglobin-like subunits but their assembly to form dimers and tetramers is quite different from that of vertebrate haemoglobin. The arrangement of the subunits provides a simple structural explanation for haem-haem interaction in the dimer and tetramer.     

Climate sensitivity constrained by CO2 concentrations over the past 420 million years

A firm understanding of the relationship between atmospheric carbon dioxide concentration and temperature is critical for interpreting past climate change and for predicting future climate change. A recent synthesis suggests that the increase in global-mean surface temperature in response to a doubling of the atmospheric carbon dioxide concentration, termed 'climate sensitivity', is between 1.5 and 6.2 degrees C (5-95 per cent likelihood range), but some evidence is inconsistent with this range. Moreover, most estimates of climate sensitivity are based on records of climate change over the past few decades to thousands of years, when carbon dioxide concentrations and global temperatures were similar to or lower than today, so such calculations tend to underestimate the magnitude of large climate-change events and may not be applicable to climate change under warmer conditions in the future. Here we estimate long-term equilibrium climate sensitivity by modelling carbon dioxide concentrations over the past 420 million years and comparing our calculations with a proxy record. Our estimates are broadly consistent with estimates based on short-term climate records, and indicate that a weak radiative forcing by carbon dioxide is highly unlikely on multi-million-year timescales. We conclude that a climate sensitivity greater than 1.5 degrees C has probably been a robust feature of the Earth's climate system over the past 420 million years, regardless of temporal scaling.