Facteurs influençant la fertilité des mâles deXenopus laevis D. (Batracien anoure)

Summary Under the conditions prevailing at the Station de Zoologie expérimentale (University of Geneva), the fertility of males ofXenopus laevis decreases every year in the autumn (Figure). Long-day treatment significantly increases the fertility and leads to the conclusion that the decrease is due to photoperiodic influences. Treatment with Antex (Leo) (i.e. a follicle-ripening hormone) for 3 weeks highly activates spermatogenesis at a time when the gonads normally remain inactive.

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Remerciements: Je dois ma reconnaissance à Mlle K. Ponse, Professeur d'endocrinologie, pour ses conseils et les discussions sur l'effet des différentes hormones gonadotropes; au Professeur A. Linder, Professeur de statistique mathématique, qui a bien voulu faire l'analyse statistique des croisements; et à mes collègues Mme F. Vanderhaeghe, Dr ès sc., et Mlle A. Droin, Dr ès sc., pour leurs observations et discussions. Le travail technique a été effectué avec l'appui du Fonds National Suisse pour la Recherche Scientifique N 2551.     

Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean

Climate variability in the tropical Atlantic Ocean is determined by large-scale ocean-atmosphere interactions, which particularly affect deep atmospheric convection over the ocean and surrounding continents. Apart from influences from the Pacific El Ni?o/Southern Oscillation and the North Atlantic Oscillation, the tropical Atlantic variability is thought to be dominated by two distinct ocean-atmosphere coupled modes of variability that are characterized by meridional and zonal sea-surface-temperature gradients and are mainly active on decadal and interannual timescales, respectively. Here we report evidence that the intrinsic ocean dynamics of the deep equatorial Atlantic can also affect sea surface temperature, wind and rainfall in the tropical Atlantic region and constitutes a 4.5-yr climate cycle. Specifically, vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5?yr and amplitudes of more than 10?cm?s(-1) are observed, in the deep Atlantic, to propagate their energy upwards, towards the surface. They are linked, at the sea surface, to equatorial zonal current anomalies and eastern Atlantic temperature anomalies that have amplitudes of about 6?cm?s(-1) and 0.4?°C, respectively, and are associated with distinct wind and rainfall patterns. Although deep jets are also observed in the Pacific and Indian oceans, only the Atlantic deep jets seem to oscillate on interannual timescales. Our knowledge of the persistence and regularity of these jets is limited by the availability of high-quality data. Despite this caveat, the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical Atlantic. Deep-jet generation and upward energy transmission through the Equatorial Undercurrent warrant further theoretical study.     

Switching on and off fear by distinct neuronal circuits

Switching between exploratory and defensive behaviour is fundamental to survival of many animals, but how this transition is achieved by specific neuronal circuits is not known. Here, using the converse behavioural states of fear extinction and its context-dependent renewal as a model in mice, we show that bi-directional transitions between states of high and low fear are triggered by a rapid switch in the balance of activity between two distinct populations of basal amygdala neurons. These two populations are integrated into discrete neuronal circuits differentially connected with the hippocampus and the medial prefrontal cortex. Targeted and reversible neuronal inactivation of the basal amygdala prevents behavioural changes without affecting memory or expression of behaviour. Our findings indicate that switching between distinct behavioural states can be triggered by selective activation of specific neuronal circuits integrating sensory and contextual information. These observations provide a new framework for understanding context-dependent changes of fear behaviour.