Somatostatin, a Presynaptic Modulator of Glutamatergic Signal in the Central Nervous System

Somatostatin is widely diffused in the central nervous system, where it participates to control the efficiency of synaptic transmission. This peptide mainly colocalizes with GABA, in inhibitory, GABA-containing interneurons from which it is actively released in a Ca2+ dependent manner upon application of depolarizing stimuli. Once released in the synaptic cleft, somatostatin acts locally, or it diffuses in the extracellular space through “volume diffusion”, a mechanism(s) of distribution which mainly operates in the cerebrospinal fluid and that assures the progression of neuronal signalling from signal-secreting sender structures towards receptor-expressing targeted neurons located extrasynaptically, in a non-synaptic, inter-neuronal form of communication. Somatostatin controls the efficiency of central glutamate transmission by either modulating presynaptically the glutamate exocytosis or by metamodulating the activity of glutamate receptors colocalized and functionally coupled with somatostatin receptors in selected subpopulations of nerve terminals. Deciphering the role of somatostatin in the mechanisms of “volume diffusion” and in the “receptor-receptor interaction” unveils new perspectives in the central role of this fine tuner of synaptic strength, paving the road to new therapeutic approaches for the cure of central disorders.

Numerical Modelling of Radiogenic Ingrowth and Diffusion of Pb in Apatite Inclusions with Variable Shape and U-Th Zonation

The fundamental premise of apatite U-Th-Pb thermochronology is that radiogenic Pb is redistributed by volume diffusion. In practice, it is often additionally assumed that crystals (1) lose radiogenic Pb to an infinite reservoir, (2) have a simple geometry and (3) are chemically homogeneous. Here we explore the significance of the latter three assumptions by numerical modelling of Pb radiogenic ingrowth and diffusion in apatite inclusions within other minerals. Our results indicate that the host minerals are likely to hamper diffusive Pb loss from the apatite inclusions by limiting the Pb flux across their boundaries, and thus the thermal histories that are reconstructed assuming a fully open boundary may be significantly inaccurate, precluding a meaningful interpretation. We also find that when apatite boundaries are flux-limited, heterogeneities in U and Th concertation within apatite have subordinate effect on bulk-grain U-Th-Pb dates and can cause intra-grain U-Th-Pb dates to increase towards the boundaries. Finally, we show that it is important to correctly account for crystal geometry when modelling intra-grain U-Th-Pb dates. We suggest that the effect of surrounding minerals on diffusive Pb loss from apatite (and loss of other radiogenic isotopes from other minerals) should be examined more closely in future research.

Studying the Stability of the K/Ar Isotopic System of Phlogopites in Conditions of High T, P: 40Ar/39Ar Dating, Laboratory Experiment, Numerical Simulation

Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produces estimates that suggest much older ages than those when these pipes were intruded. High-pressure (3 GPa) laboratory experiments enabled the authors to explore the behaviour of argon in the phlogopite structure under the conditions that correspond to the mantle, at the temperatures (from 700 to 1000 °С), far exceeding closure temperature of the K/Ar isotopic system. “Volume diffusion” remains foremost for describing the mobility of argon in phlogopite at high pressures. The mantle material age can be estimated through the dating of the phlogopites from deep-seated xenoliths of kimberlites, employing the 40Ar/39Ar method, subject to correction for a partial loss of radiogenic 40Ar when xenolith moves upwards to the Earth’s surface. The obtained data served as the basis for proposing the behaviour model of the K/Ar isotopic system of minerals in conditions of great depths (lower crust, mantle), and when transporting xenoliths in the kimberlite melt.

The geochemical and geochronological implications of nanoscale trace-element clusters in rutile

The geochemical analysis of trace elements in rutile (e.g., Pb, U, and Zr) is routinely used to extract information on the nature and timing of geological events. However, the mobility of trace elements can affect age and temperature determinations, with the controlling mechanisms for mobility still debated. To further this debate, we use laser-ablation–inductively coupled plasma–mass spectrometry and atom probe tomography to characterize the micro- to nanoscale distribution of trace elements in rutile sourced from the Capricorn orogen, Western Australia. At the >20 µm scale, there is no significant trace-element variation in single grains, and a concordant U-Pb crystallization age of 1872 ± 6 Ma (2σ) shows no evidence of isotopic disturbance. At the nanoscale, clusters as much as 20 nm in size and enriched in trace elements (Al, Cr, Pb, and V) are observed. The 207Pb/206Pb ratio of 0.176 ± 0.040 (2σ) obtained from clusters indicates that they formed after crystallization, potentially during regional metamorphism. We interpret the clusters to have formed by the entrapment of mobile trace elements in transient sites of radiation damage during upper amphibolite facies metamorphism. The entrapment would affect the activation energy for volume diffusion of elements present in the cluster. The low number and density of clusters provides constraints on the time over which clusters formed, indicating that peak metamorphic temperatures are short-lived, <10 m.y. events. Our results indicate that the use of trace elements to estimate volume diffusion in rutile is more complex than assuming a homogeneous medium.

Ultrasonic Assisted Sintering Using Heat Converted from Mechanical Energy

A new sintering method, namely ultrasonic assisted sintering (UAS), has been proposed using mechanical heat converted from high frequency motion between particles. Pure aluminum specimens with diameter of 5 mm and thickness of ~2 mm have been successfully sintered in two seconds. Based on the thermodynamic analysis, the underlying heating mechanism is quantitatively interpreted, which involves high-frequency interparticle friction and plastic deformation driven by ultrasonic squeezing. Consequently, temperature rises rapidly at a speed of about 300 K/s, and the maximum temperature reaches up to 0.9 times of melting point of the aluminum during UAS. The sintered specimens have a high density of dislocations, under the combined effects of dislocations and undulating stress field, volume diffusion coefficient for sintering increases by several orders of magnitude, therefore, rapid densification can be accomplished in seconds. In addition, the sintered aluminum has ultrahigh nanohardness (~1.13 GPa), which can be attributed to the hierarchical structure formed during UAS process.

Two-Dimensional Indium Selenide for Sulphur Vapour Sensing Applications

Surface-to-volume ratio in two-dimensional (2D) materials highlights among their characteristics as an inherent and intrinsic advantage taking into account their strong sensitivity to surface effects. For this reason, we have proposed in this work micromechanically exfoliated 2D nanosheets of InSe as an optical vapour sensor. As a proof of concept, we used 2-mercaptoethanol as the chemical analyte in vapour phase to monitor the change of the InSe photoluminescence (PL) before and after exposure to the analyte. For short vapour exposure times (at low analyte concentration), we found a PL enhancement of InSe nanosheets attributed to the surface localization of Se defects. For long vapour exposure times (or higher concentrations) a PL reduction is observed, probably due to the diffusion of molecules within the nanosheet. These results confirm the capability of 2D InSe as a photoluminescent sensor of vapours, because of its sensitivity to surface passivation or volume diffusion of molecules.

Micro-analytical studies of discontinuous precipitation in Fe-13.5 at.% Zn alloy

For the first time, the analytical electron microscopy has been used to determine the solute concentration profiles left behind the moving reaction front (RF) of the discontinuous precipitation (DP) reaction in a Fe-13.5 at.% Zn alloy. These profiles have been converted into grain boundary diffusivity (sδDb) values, using Cahn’s diffusion equation in its original form and the data of the growth rate of the discontinuous precipitates obtained from independent measurements. This approach has essentially removed existing difference in comparison to sδDb values obtained from Cahn′s simplified and Petermann–Hornbogen models relevant for the global approach to the DP. Simultaneously, the local values of sδDb have been up to 8–10 orders of magnitude higher than the data for volume diffusion coefficients and much greater than for diffusion at the stationary grain boundaries of Zn in pure Fe. This is clear indication that the rate controlling factor for DP reaction in the Fe-13 at.% Zn alloy is diffusion at the moving RF.