1,721,013 research outputs found
Neurosteroids differentially modulate fast and slow interictal discharges in the hippocampal CA3 area
No full textAllotetrahydrodeoxycorticosterone (THDOC)
belongs to a class of pregnane neurosteroidal compounds
that enhance brain inhibition by interacting directly with
GABAA signaling, mainly through an increase in tonic inhibitory
current. Here, we addressed the role of THDOC in the
modulation of interictal- and ictal-like activity and associated
high-frequency oscillations (HFOs, 80–500 Hz; ripples:
80–200 Hz, fast ripples: 250–500 Hz) recorded in vitro in the
rat piriform cortex, a highly excitable brain structure that
is implicated in seizure generation and maintenance. We
found that THDOC: (i) increased the duration of interictal
discharges in the anterior piriform cortex while decreasing
ictal discharge duration in both anterior and posterior piriform
cortices; (ii) reduced the occurrence of HFOs associated
to both interictal and ictal discharges; and (iii)
prolonged the duration of 4-aminopyridine-induced, glutamatergic
independent synchronous field potentials that
are known to mainly result from the activation of GABAA
receptors. Our results indicate that THDOC can modulate
epileptiform synchronization in the piriform cortex presumably
by potentiating GABAA receptor-mediated signaling.
This evidence supports the view that neurosteroids regulate
neuronal excitability and thus control the occurrence of seizure
Climate change and the formation of nickel laterite deposits
No full textOne of the major controls on nickel laterite deposit formation is climate. A comparison of climatic data for regions where ultramafic rocks are exposed shows that areas currently conducive to nickel laterite formation receive >1000 mm/yr precipitation, have cold month mean (CMM) temperatures between 15 and 27 °C, and warm month mean (WMM) temperatures between 22 and 31 °C. Using a compilation of paleoclimatic data and the defined climatic window of nickel laterite deposit formation allows an assessment of the timing and duration of paleo–nickel laterite formation in ancient terrains. To illustrate the potential of this technique, we show that the formation of the Çalda? nickel laterite deposit of western Turkey most likely initiated when the ultramafic protolith of Cretaceous age was exposed in the Early Eocene, and that the majority of the deposit formed during the Eocene. In contrast, the formation of the Bitincke deposit, south Albania, was mostly complete by the Early Eocene, after which CMM temperatures dropped below the optimum for laterite formation
Weathering of Tethyan Ophiolites and the formation of silicate and oxide Ni Laterites in the Late Cretaceous
No full textThe caldag Ni laterite deposit in western Turkey and the Devolli deposit in southern Albania are both overlain by Eocene sediments, and are formed by the intense weathering of ophiolites obducted during the Cretaceous. Although the deposits formed from similar protoliths and in similar climatic regimes, they display distinct mineralogical and morphological characteristics. The weathering process at caldag resulted in an oxide dominated laterite, whereas supergene formation of secondary silicates at Devolli resulted in a deposit with both silicate and oxide components. Changes in texture and mineralogy are related tothe weathering history and post-formation transportation mechanisms. The combination of a serpentinite protolith, a high water table and low topography, in association with an aggressive weathering environment in a tropical climate resulted in the formation of an oxide-dominated deposit at caldag. In contrast, a moderate topography and relatively seasonal climate were required to produce the silicate Devolli laterite
The Hakkari nonsulfide Zn-Pb deposit in the context of other Nonsulfide Zn-Pb deposits in the Tethyan Metallogenetic Belt of Turkey.
No full textThe Hakkari nonsulfide zinc deposit is situated close to the southeastern border of Turkey. Here both sulfide and nonsulfide Zn≫Pb ores are hosted in carbonate rocks of the Jurassic Cudi Group with features typical of carbonate-hosted supergene nonsulfide zinc mineralization. The regional strike extent of the mineralized district is at least 60 km. The age of the supergene deposit has not been determined, but it is probable that the main weathering happened during Upper Tertiary, possibly between Upper Miocene and Lower Pliocene. The Hakkari mineralization can be compared to other carbonate-hosted Zn–Pb deposits in Turkey, and an interpretation
made of its geological setting. The zinc mineral association at Hakkari typically comprises smithsonite and hemimorphite, which apparently replace both sulfide minerals and carbonate host rock. Two generations of smithsonite are present: the first is relatively massive, the second occurs as concretions in cavities as a final filling of remnant porosity. Some zinc is also hosted within Fe–Mn- (hydr)oxides. Lead is present in cerussite, but also as partially oxidized galena. Lead can also occur in Mn-(hydr)oxides (max 30% PbO). The features of the supergene mineralization suggest that the Hakkari deposit belongs both to the “direct replacement” and the “wall-rock replacement” types of nonsulfide ores. Mineralization varies in style from tabular bodies of variable thickness (b0.5 to 13 m) to cross-cutting breccia zones and disseminated ore minerals in pore spaces and fracture planes. At Hakkari a As–Sb–Tl(≫Hg) geochemical association has been detected, which may point to primary sulfide mineralization, quite different from typical MVT
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