The role of the t-SNARE SNAP-25 in action potential-dependent calcium signaling and expression in GABAergic and glutamatergic neurons

Background  

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, comprised of SNAP-25, syntaxin 1A, and VAMP-2, has been shown to be responsible for action potential (AP)-dependent, calcium-triggered release of several neurotransmitters. However, this basic fusogenic protein complex may be further specialized to suit the requirements for different neurotransmitter systems, as exemplified by neurons and neuroendocrine cells. In this study, we investigate the effects of SNAP-25 ablation on spontaneous neuronal activity and the expression of functionally distinct isoforms of this t-SNARE in GABAergic and glutamatergic neurons of the adult brain.     

Characterization of O2-CeO2 interactions using in situ Raman spectroscopy and first-principle calculations.

Interactions between O(2) and CeO(2) are examined experimentally using in situ Raman spectroscopy and theoretically using density-functional slab-model calculations. Two distinct oxygen bands appear at 825 and 1131 cm(-1), corresponding to peroxo- and superoxo-like species, respectively, when partially reduced CeO(2) is exposed to 10 % O(2). Periodic density-functional theory (DFT) calculations aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO(2). The O(2) adsorption energies on unreduced CeO(2) surfaces are endothermic (0.91相似文献   

Low temperature growth of AlGaAs by MOMBE (CBE) using trimethylamine alane

In order to gain further insight into the surface chemistry of AlGaAs growth by metalorganic molecular beam epitaxy, we have investigated the deposition behavior and material quality of AlGaAs grown at temperatures from 350 to 500°C using trimethylamine alane (TMAA), triethylgallium (TEG) and arsine (AsH₃). Though the Al incorporation rate decreases with decreasing temperature, Ga-alkyl pyrolysis, and hence Ga incorporation rate, declines more rapidly. Thus the Al content increases from X_AlAs = 0.25 at 500°C to X_AlAs = 0.57 at 350°C. Below 450°C, the Ga incorporation rate appears to be determined by the desorption of diethylgallium species, rather than interaction with adsorbed AlH₃. Similarly, carbon incorporation is enhanced by 2 orders of magnitude over this temperature range due to the increasingly inefficient pyrolysis of the Ga-C bond in TEG. Additionally, active hydrogen from the TMAA1, which normally is thought to getter the surface alkyls, is possibly less kinetically active at lower growth temperatures. Contrary to what has been observed in other growth methods, low growth temperatures produced a slight decrease in oxygen concentration. This effect is likely due to reduced interaction between Ga alkoxides (inherent in the TEG) and the atomic hydrogen blocked Al species on the growth surface. This reduction in oxygen content and increase in carbon concentration causes the room temperature PL intensity to actually increase as the temperature is reduced from 500 to 450°C. Surprisingly, the crystalline perfection as measured by ion channeling analysis is quite good, χ_min≤5%, even at growth temperatures as low as 400°C. At 350°C, the AlGaAs layers exhibit severe disorder. This disorder is indicative of insufficient Group III surface mobility, resulting in lattice site defects. The disorder also supports our conclusions of kinetically limited surface mobility of all active surface components.     

Growth of GaAs/AlGaAs HBTs by MOMBE (CBE)

In this paper, we will discuss how the unique growth chemistry of MOMBE can be used to produce high speed GaAs/AlGaAs heterojunction bipolar transistors (HBTs). The ability to grow heavily doped, well-confined layers with carbon doping from trimethylgallium (TMG) is a significant advantage for this device. However, in addition to high p-type doping, high n-type doping is also required. While elemental Sn can be used to achieve doping levels up to 1.5×10¹⁹ cm^-3, severe segregation limits its use to surface contact layers. With tetraethyltin (TESn), however, segregation does not occur and Sn doping can be used throughout the device. Using these sources along with triethylgallium (TEG), trimethylamine alane (TMAA), and AsH₃, we have fabricated Npn devices with 2 μm×10 μm emitter stripes which show gains of ≥ 20 with either ƒ_t = 55 GHz and ƒ_max = 70 GHz or ƒ_t = 70 GHz and ƒ_max = 50 GHz, depending upon the structure. These are among the best RF values reported for carbon doped HBTs grown by any method, and are the first reported for an all-gas source MOMBE process. In addition, we have fabricated a 70 transistor decision circuit whose performance at 10 Gb/s equals or exceeds that of similar circuits made from other device technologies and growth methods. These are the first integrated circuits reported from MOMBE grown material.     

High-Power GaN Electronic Devices

Gallium Nitride (GaN) and related materials (especially AlGaN) recently have attracted a lot of interest for applications in high-power electronics capable of operation at elevated temperatures and high frequencies. The AlGaInN system offers numerous advantages. These include wide bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasisubstrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices. Al_xGa_1-xN (x=0 ～.25) Schottky rectifiers were fabricated in a lateral geometry employing p⁺-implanted guard rings and rectifying contact overlap onto an SiO₂ passivation layer. The reverse breakdown voltage (V_B) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al_0.25Ga_0.75N and 6350 V for GaN, respectively, for 100-µm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of <9.67×10⁵ Vcm^?2, which is roughly a factor of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (V_B)²/R_ON, where RON is the on-state resistance, was in the range 94 to 268 MWcm^?2 for all the devices. Edge-terminated Schottky rectifiers were also fabricated on quasibulk GaN substrates grown by HVPE. For small-diameter (75?µm) Schottky contacts, Vs measured in the vertical geometry was ～700?V, with an on-state resistance (RON) of 3?mΩcm², producing a figure-of-merit V_B ²/R_ON of 162.8?MW-cm^?2. GaN p-i-n diodes were also fabricated. A direct comparison of GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher reverse breakdown voltage for the former (490?V vs. 347?V for the Schottky diodes), but lower forward turn-on voltages for the latter (～3.5?V vs. ～5?V for the p-i-n diodes). The forward I-V characteristics of the p-i-n rectifiers show behavior consistent with a multiple recombination center model. The reverse current in both types of rectifiers was dominated by surface perimeter leakage at moderate bias. Finally, all of the devices we fabricated showed negative temperature coefficients for reverse breakdown voltage, which is a clear disadvantage for elevated temperature operation. Bipolar devices are particularly interesting for high current applications such as microwave power amplifiers for radar, satellite, and communication in the l～5?GHz range, powers >l00?W, and operating temperatures >425°C. pnp Bipolar Junction Transistors and pnp Heterojunction Bipolar Transistors were demonstrated for the first time. For power microwave applications, small area self-aligned npn GaN/AlGaN HBTs were attempted. The devices showed very promising direct current characteristics.     

Dynamics and correlations in magnetic colloidal systems studied by X-ray photon correlation spectroscopy

We have studied the static and dynamic behaviour of magnetic colloidal systems (ferrofluids) by Small-Angle X-ray Scattering (SAXS) and X-ray Photon Correlation Spectroscopy (XPCS). The main features of the novel XPCS technique will be illustrated by data taken on a model system, colloidal silica spheres. We will then present the results obtained on an optically opaque suspension of magnetic colloids (maghemite) in the wave vector range from 10^-3 to 10^-2?^-1. Translational diffusion in zero field and anisotropic diffusion under external magnetic fields will be discussed. Received 9 August 1999 and Received in final form 26 July 2000