Nitric oxide (NO) is a biological messenger synthesized by three main isoforms of NO synthase (NOS): neuronal (nNOS, constitutive calcium dependent), endothelial (eNOS, constitutive, calcium dependent) and inducible (iNOS, calcium independent). NOS is distributed in the brain either in circumscribed neuronal sets or in sparse interneurons. Within the laterodorsal tegmentum (LDT), pedunculopontine tegmentum and dorsal raphe nucleus, NOS-containing neurons overlap neurons grouped according to their contribution to steep mechanisms. The main target for NO is the soluble guanylate cyclase that triggers an overproduction of cyclic guanosine monophosphate. NO in neurons of the pontine tegmentum facilitates steep (particularly rapid-eye-movement steep), and NO contained within the LDT intervenes in modulating the discharge of the neurons through an auto-inhibitory process involving the co-synthesized neurotransmitters. Moreover, NO synthesized within cholinergic neurons of the basal forebrain, while under control of the LDT, may modulate the spectral components of the EEG instead of the amounts of different steep states. Finally, impairment of NO production (e.g. neurodegeneration, iNOS induction) has identifiable effects, including ageing, neuropathologies and parasitaemia. (c) 2004 Published by Elsevier Ltd.

Extensive evidences now suggest that an association between inducible nitric oxide synthase and oxidative stress takes place during aging. Since the part played by inducible nitric oxide synthase in the sleep impairments associated with aging still remains unexplored, we compared its involvement in old rats (20–24 months) versus adult ones (3–5 months) using polygraphic, biochemical, voltammetric and immunohistochemical techniques. The experiments were conducted either in basal condition or after a systemic injection of selected inducible nitric oxide synthase inhibitors. We found that 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (10mg/kg, i.p.) or aminoguanidine (400mg/kg, i.p.) was capable to suppress rapid-eye-movement sleep and induce a delayed enhancement in slow-wave sleep in old rats. These effects did not occur in adult animals. Within the frontal cortex, the laterodorsal tegmentum and dorsal raphe nuclei, the basal inducible nitric oxide synthase activity was 85–200% higher in old rats than in adult ones. In contrast, the neuronal nitric oxide synthase activity did not vary in both groups. 2-Amino-5,6-dihydro-6-methyl-4H-1,3-thiazine administration significantly reduced inducible nitric oxide synthase activity (70–80% according to the brain areas) independently of age, but significantly decreased the cortical nitric oxide release in old rats. Finally, in frontal cortex and dorsal raphe immunohistochemical analysis showed inducible nitric oxide synthase-positive cells again only in old animals. These data support the idea that nitric oxide produced by inducible nitric oxide synthase plays a role in the triggering and maintenance of rapid-eye-movement sleep during aging.

Nitric oxide (NO) is a biological messenger synthesized by three main isoforms of NO synthase (NOS): neuronal (nNOS, constitutive calcium dependent), endothelial (eNOS, constitutive, calcium dependent) and inducible (iNOS, calcium independent). NOS is distributed in the brain either in circumscribed neuronal sets or in sparse interneurons. Within the laterodorsal tegmentum (LDT), pedunculopontine tegmentum and dorsal raphe nucleus, NOS-containing neurons overlap neurons grouped according to their contribution to sleep mechanisms. The main target for NO is the soluble guanylate cyclase that triggers an overproduction of cyclic guanosine monophosphate. NO in neurons of the pontine tegmentum facilitates sleep (particularly rapid-eye-movement sleep), and NO contained within the LDT intervenes in modulating the discharge of the neurons through an auto-inhibitory process involving the co-synthesized neurotransmitters. Moreover, NO synthesized within cholinergic neurons of the basal forebrain, while under control of the LDT, may modulate the spectral components of the EEG instead of the amounts of different sleep states. Finally, impairment of NO production (e.g. neurodegeneration, iNOS induction) has identifiable effects, including ageing, neuropathologies and parasitaemia.

Ce travail est le fruit d’une collaboration pluri-catégorielle (professeur d’école, formateurs IUFM, universitaire) et pluridisciplinaire (philosophie, sémantique logique, didactique et épistémologie de la biologie). L’analyse d’une séquence d’enseignement sur le développement des grenouilles («Comment les œufs de grenouille se transforment-ils en jeunes grenouilles ?») nous permet d’exposer puis de mettre en œuvre nos références théoriques et méthodologiques respectives. Les paradoxes sémantiques correspondent à l'existence de plusieurs sens d’un même terme pourtant scientifique. Ils mettent la pensée devant une contradiction à résoudre en se référant aux objets étudiés et à l’approche épistémologique qui leur donne sens. Ainsi la logique, qui amorce le dialogue argumentatif, est-elle contrainte de se déplacer sur le terrain épistémologique. Les principaux paradoxes sémantiques étudiés ont porté sur les termes suivants : grenouilles (individus ou espèce ?), œufs et se transformer (grandir/rapetisser, morphologie/fonctions, ontogenèse/phylogenèse).

Variations occurring in cortical nitric oxide (NO) release were analysed with a voltametric method in rats (i) placed in control conditions, (ii) while being paradoxical sleep deprived (PSD), or (iii) recovering from a PSD. Activities of neuronal (nNOS) and inducible (iNOS) NO-synthases as well as nNOS expression were also determined in several brain regions. In baseline conditions, circadian variations in nNOS expression and activity were maximal during the dark period and minimal during the light one for all the structures analysed (frontal cortex, pons and medulla). In the same way, cortical NO release occurred through a circadian rhythm exhibiting maxima and minima during dark and light periods, respectively. In the same experimental conditions, iNOS activity did not exhibit time-dependent changes. The correlative changes observed in baseline conditions between NO release, nNOS expression and activity within the frontal cortex were disrupted during PSD and subsequent recovery. Still again, iNOS activity remained unchanged. Results obtained point out that the tight coupling existing in control conditions between nNOS expression-activity and NO release is disrupted by a PSD and remains affected during the subsequent 24 h recovery. Their significance is discussed.

Changes in sleep-wake states and nitric oxide release were examined in aged rats versus young-adult ones. Sleep-wake recordings and nitric oxide measurements were taken from animals chronically equipped with polygraphic and voltametric electrodes. Animals were examined in baseline conditions and in response to a 24-hour paradoxical sleep deprivation. In aged rats, basal amount of paradoxical sleep is decreased during the light phase versus young-adult animals. After paradoxical sleep deprivation, a paradoxical sleep rebound occurs with an amount and intensity that are less marked in aged animals than in young-adult rats. The amplitude of the circadian distribution for wakefulness, slow-wave sleep and paradoxical sleep amounts is reduced with age. Finally, delta-slow-wave sleep and theta-paradoxical sleep power spectra are attenuated either in baseline conditions or after paradoxical sleep deprivation in aged animals. It is also reported that cortical nitric oxide release exhibits a circadian rhythm with higher amplitude in aged rats than in young-adult ones. However, after paradoxical sleep deprivation, a limited overproduction of nitric oxide is obtained compared with young-adult ones. These results, evidencing the dynamics of the nitric oxide changes occurring in relation to the sleep-wake cycle, point out the homeostatic paradoxical sleep regulation as an age-dependent process in which the nitric oxide molecule is possibly involved.

Le mythe de la guêpe maçonne, et nos réactions à son égard, illustrent les contradictions logiques à l'origine des observations et interprétations et montrent ainsi les limites des observations et interprétations scientifiques, telles que celles qui sont proposées à nos élèves, et qui diffèrent si peu de ce que nous critiquons dans le fonctionnement du mythe. L'interaction entre nos représentations sociales et nos capacités et difficultés à observer et interpréter est soulignée.

Certain ascidian species, colonial or solitary, contain calcareous spicules in the tunic, in internal tissues or both. The minerology of these bioaccumulatios has seldom been investigated. Many kinds of ascidian spicules were analysed using several physical and chemical techniques, and their crystallochemical characteristics compared in a taxonomic review. Fluorine was always present regardless of the type of calcium salt found in the spicules. Comparative data from other marine invertebrates are given. For the first time, the presence of calcium oxalate crystals is demonstrated in ascidians

Heterotrophic activity in the bottom few cm of annual sea ice in the Canadian Arctic was measured throughout the spring bloom of ice algae, using tritium-labelled thymidine and glucose. Experiments with chloramphenicol and cyclohexamide indicated that thymidine assimilation was due to procaryotic microbes but that about half of the glucose assimilation was due to eucaryotic organisms. Glucose and thymidine assimilation rates increased with salinity, from 10 ppt to 30 ppt. Thymidine assimilation rates increased from 1.16 to 4.94·10−21mol·cell−1·h−1 during the latter half of the algal bloom, while the exponential growth rate of the in situ populations decreased from 0.058 to 0.025 d−1. Bacterial production and specific growth rates calculated from thymidine assimilation were 149mgC·m−2 and 0.25 d−1 or less respectively over the 50 day observation period, compared with net primary production of 5,500 mgC·m−2. Thymidine assimilation rates suggested that about half of the bacterial production may be consumed or lost from the ice during the bloom.

Changes in intracellular distribution of recent (labelled) photosynthate during light-dark incubations were consistent with overnight consumption of low molecular weight compounds and carbohydrate, but continued protein synthesis, in sea ice algae from Resolute Passage, Canada, during April and May, 1986. Synthesis of labelled protein at night varied with preceding light availability but was always less than 30% of the daytime rate. Comparisons of labelled photosynthate dynamics against oxygen consumption and net changes of particulate matter composition and concentration showed that soluble polysaccharide was not the only major metabolic substrate, and that much of the carbon lost overnight was not recent, labelled photosynthate. Total net lipid synthesis was greatly underestimated by labelled photosynthate allocation. However, overnight consumption of the labelled lipid was directly proportional to photon flux density in the preceding light period, suggesting a short-term energy storage function for the small labelled portion of the total lipid pool. Arctic ice algae appear to incorporate only limited amounts of recent photosynthate into their large lipid pools even over a full 24 h photoperiod.

The dynamics of bacterial populations in annual sea ice were measured throughout the vernal bloom of ice algae near Resolute in the Canadian Arctic. The maximum concentration of bacteria was 6.0·1011 cells·m−2 (about 2.0·1010 cells·l−1) and average cell volume was 0.473 μm3 in the lower 4 cm of the ice sheet. On average, 37% of the bacteria were epiphytic and were most commonly attached (70%) to the dominant alga,Nitzschia frigida (58% of total algal numbers). Bacterial population dynamics appeared exponential, and specific growth rates were higher in the early season (0.058 day−1), when algal biomass was increasing, than in the later season (0.0247 day−1), when algal biomass was declining. The proportion of epiphytes and the average number of epiphytes per alga increased significantly (P<0.05) through the course of the algal bloom. The net production of bacteria was 67.1 mgC·m−2 throughout the algal bloom period, of which 45.5 mgC·m−2 occurred during the phase of declining algal biomass. Net algal production was 1942 mgC·m−2. Sea ice bacteria (both arctic and antarctic) are more abundant than expected on the basis of relationships between bacterioplankton and chlorophyll concentrations in temperate waters, but ice bacteria biomass and net production are nonetheless small compared with the ice algal blooms that presumably support them.

Biochemical composition of the sea ice microbial community was measured in populations of different light histories in the Canadian Arctic (Resolute, N.W.T.). The average composition of the particulate organic matter [soluble and insoluble polysaccharide, particulate protein, intracellular free amino acids (IFAA), lipid, and chlorophyll a] was within the published range for microalgae, but lipid was a relatively large (31–59%) and protein a small (20–24%) part of the total. Protein and IFAA pools apparently comprised about 50% of the particulate organic nitrogen, of which 6–10% was in the IFAA pool. Over the entire spring growth season, the net synthesis of protein, IFAA, and Chl a (relative to total cell carbon) decreased with increasing light while relative synthesis of lipid and soluble polysaccharide increased, consistent with patterns of short‐term photosynthate allocation. In the early growth season patterns of synthesis were relatively insensitive to light, and rates of lipid synthesis were large for all light histories. Photosynthate allocation in 24‐h incubations greatly underestimated actual rates of net lipid synthesis and probably overestimated protein synthesis. Microalgae of cold, low‐light environments can display rates of lipid synthesis much larger than rates normally encountered in microalgae without displaying a corresponding pattern of shorter term photosynthate allocation.