Rhythmic activities of hypothalamic magnocellular neurons: Autocontrol mechanisms

Electrophysiological recordings in lactating rats show that oxytocin (OT) and vasopressin (AVP) neurons exhibit specific patterns of activities in relation to peripheral stimuli: periodic bursting firing for OT neurons during suckling, phasic firing for AVP neurons during hyperosmolarity (systemic injection of hypertonic saline). These activities are autocontrolled by OT and AVP released somato-dentritically within the hypothalamic magnocellular nuclei. In vivo, OT enhances the amplitude and frequency of bursts, an effect accompanied with an increase in basal firing rate. However, the characteristics of firing change as facilitation proceeds: the spike patterns become very irregular with clusters of spikes spaced by long silences; the firing rate is highly variable and clearly oscillates before facilitated bursts. This unstable behaviour dramatically decreases during intense tonic activation which temporarily interrupts bursting, and could therefore be a prerequisite for bursting. In vivo, the effects of AVP depend on the initial firing pattern of AVP neurons: AVP excites weakly active neurons (increasing duration of active periods and decreasing silences), inhibits highly active neurons, and does not affect neurons with intermediate phasic activity. AVP brings the entire population of AVP neurons to discharge with a medium phasic activity characterised by periods of firing and silence lasting 20–40 s, a pattern shown to optimise the release of AVP from the neurohypophysis. Each of the peptides (OT or AVP) induces an increase in intracellular Ca²⁺ concentration, specifically in the neurons containing either OT or AVP respectively. OT evokes the release of Ca²⁺ from IP3-sensitive intracellular stores. AVP induces an influx of Ca²⁺ through voltage-dependent Ca²⁺ channels of T-, L- and N-types. We postulate that the facilitatory autocontrol of OT and AVP neurons could be mediated by Ca²⁺ known to play a key role in the control of the patterns of phasic neurons.     

Cinétique de la rhizogenèse adaptative à la sécheresse. Relation avec I'évolution des paramètres hydriques et morphologiques chez le Sinapis alba, à deux niveaux d'énergie lumineuse

Kinetics of adaptive drought rhizogenesis, as related to growth and water deficits in Sinapis alba, at two irradiance levels .
The rhizogenic activity in Sinapis alba L., subjected to progressively increasing water deficit during growth, shows a sigmoidal pattern of kinetics with three phases: initiation or latent phase, rapid exponential increase, and stationary phase. This pattern remains unchanged if the level of irradiance during growth is reduced. The same number of short tuberized roots is obtained as under high irradiance, although initiation of rhizogenesis appears to be delayed by a few days. However, this delay is not observed if the rhizogenesis is analyzed as a function of the water potential of aerial organs or as a function of the soil water deficit. Since the rate of dehydration depends upon the evaporative demand, which is modulated by high or low irradiance, drought rhizogenesis seems to be directly related to water deficits in the soil-plant system. The onset of drought rhizogenesis, starting at the end of the growth period of the aerial organs, appears concomitant with the transpiration decline which is induced by water stress at both irradiance levels.     

Effect of maurotoxin, a four disulfide-bridged toxin from the chactoid scorpion Scorpio maurus, on Shaker K+ channels.

Maurotoxin is a 34-residue toxin isolated from the venom of the Tunisian chactoid scorpion Scorpio maurus palmatus and contains four disulfide bridges that are normally found in long-chain toxins of 60-70 amino acid residues, which affect voltage-gated sodium channels. However, despite the unconventional disulfide-bridge pattern of maurotoxin, the conformation of this toxin remains similar to that of other toxins acting on potassium channels. Here, we analyzed the effects of synthetic maurotoxin on voltage-gated Shaker potassium channels (ShB) expressed in Xenopus oocytes. Maurotoxin produces a strong, but reversible, inhibition of the ShB K+ current with an IC50 of 2 nM. Increasing concentrations of the toxin induce a progressively higher block at saturating concentrations. At nonsaturating concentrations of the toxin (5-20 nM), the channel block appears slightly more pronounced at threshold potentials suggesting that the toxin may have a higher affinity for the closed state of the channel. At the single channel level, the toxin does not modify the unitary current amplitude, but decreases ensemble currents by increasing the number of depolarizing epochs that failed to elicit any opening. A point mutation of Lys23 to alanine in maurotoxin produces a 1000-fold reduction in the IC50 of block by the toxin suggesting the importance of this charged residue for the interaction with the channel. Maurotoxin does not affect K+ currents carried by Kir2.3 channels in oocytes or Na+ currents carried by the alphaIIa channel expressed in CHO cells.     

Un nouveau Tachyoryctes (Mammalia,Rodentia) du bassin pliocene de Hadar (Ethiopie)

A new species of Tachyoryctes (Mammalia, Rodentia): T. pliocaenicus from the Hadar Formation (Ethiopia) ispresented in this study. It is considered here as an ancestral from T. splendens. Since it also shares characteristics with fossil Rhizomyidae (Kanisamys) from Asia, this species may represent a direct descendant of the latter and would have immigrated to East Africa before the middle Pliocene. This interpretation would agree with the attribution of the genus Tachyoryctes to the family Rhizomyidae. Moreover, the Hadar Paleoenvironment is partially reconstitued from data pertaining to the ecology of T. splendens.