Stochastic resonance at early visual cortex during figure orientation discrimination using transcranial magnetic stimulation.

Visual noise usually reduces the visibility of stimuli. However, very low contrast or subliminal visual noise can sometimes enhance the visibility of low-contrast stimuli. It has been suggested that this enhancement occurs at the visual cortex. The aims of this study are to clarify the role of the early visual cortex (V1/V2) in the enhancement effect and to clarify the relationship of the SR characteristics among different experiments. Noise was added directly to the visual cortex by using transcranial magnetic stimulation (TMS) with randomly varying intensity. The location on the scalp and the timing (stimulus onset asynchrony, SOA) of TMS were specifically adjusted to target the early visual cortex. Contrast thresholds for figure orientation discrimination were measured as a function of TMS noise intensity. With increasing TMS noise intensity the contrast threshold for figure discrimination first decreased (enhancement) and then increased (impairment). These effects were clearly dependent both on scalp location and timing (SOA). The optimum SOA was around 60 ms, while the optimum location varied across participants. Outside the optimum location and SOA values, no TMS effects were found. The enhancement effect can be accounted for by the stochastic resonance (SR) theory based on a threshold device. In addition, we reveal similarity in characteristics of the SR phenomenon between different experiments.

Development of an efficient gene-targeting system for elucidating infection mechanisms of the fungal pathogen Trichosporon asahii.

Trichosporon asahii is a pathogenic fungus that causes severe, deep-seated fungal infections in neutropenic patients. Elucidating the infection mechanisms of T. asahii based on genetic studies requires a specific gene-targeting system. Here, we established an efficient gene-targeting system in a highly pathogenic T. asahii strain identified using the silkworm infection model. By comparing the pathogenicity of T. asahii clinical isolates in a silkworm infection model, T. asahii MPU129 was identified as a highly pathogenic strain. Using an Agrobacterium tumefaciens-mediated gene transfer system, we obtained a T. asahii MPU129 mutant lacking the ku70 gene, which encodes the Ku70 protein involved in the non-homologous end-joining repair of DNA double-strand breaks. The ku70 gene-deficient mutant showed higher gene-targeting efficiency than the wild-type strain for constructing a mutant lacking the cnb1 gene, which encodes the beta-subunit of calcineurin. The cnb1 gene-deficient mutant showed reduced pathogenicity against silkworms compared with the parental strain. These results suggest that an efficient gene-targeting system in a highly pathogenic T. asahii strain is a useful tool for elucidating the molecular mechanisms of T. asahii infection.

A novel silkworm infection model with fluorescence imaging using transgenic Trichosporon asahii expressing eGFP.

Trichosporon asahii is a pathogenic fungus that causes deep mycosis in patients with neutropenia. Establishing an experimental animal model for quantitatively evaluating pathogenicity and developing a genetic recombination technology will help to elucidate the infection mechanism of T. asahii and promote the development of antifungal drugs. Here we established a silkworm infection model with a transgenic T. asahii strain expressing eGFP. Injecting T. asahii into silkworms eventually killed the silkworms. Moreover, the administration of antifungal agents, such as amphotericin B, fluconazole, and voriconazole, prolonged the survival time of silkworms infected with T. asahii. A transgenic T. asahii strain expressing eGFP was obtained using a gene recombination method with Agrobacterium tumefaciens. The T. asahii strain expressing eGFP showed hyphal formation in the silkworm hemolymph. Both hyphal growth and the inhibition of hyphal growth by the administration of antifungal agents were quantitatively estimated by monitoring fluorescence. Our findings suggest that a silkworm infection model using T. asahii expressing eGFP is useful for evaluating both the pathogenicity of T. asahii and the efficacy of antifungal drugs.

Comparison of the localization of tetrodotoxin between wild pufferfish Takifugu rubripes juveniles and hatchery-reared juveniles with tetrodotoxin administration.

To reveal the accumulation profile of tetrodotoxin (TTX) in pufferfish Takifugu rubripes juveniles, we compared the localization of TTX in various tissues among wild juveniles and hatchery-reared juveniles with or without TTX administration using immunohistochemical technique with anti-TTX monoclonal antibody. Immuno-positive reaction was observed in hepatic tissue, basal cell of skin and olfactory, olfactory epithelium, optic nerve and brain (optic tectum, cerebellum, medulla oblongata) of wild juveniles (body length: BL, 4.7-9.4 cm). TTX was detected in the same tissues as wild juveniles and epithelial cell layer of intestine of hatchery-reared juveniles (BL, 5.0-5.3 cm) to which TTX was orally administrated. No positive reaction was observed from the tissues of hatchery-reared juveniles without TTX administration. These results suggest that orally administrated TTX to the non-toxic cultured juveniles is accumulated in the same manner of wild juveniles. In addition, our study revealed that pufferfish accumulates TTX in the central nervous system.

Iridium-catalyzed borylation of benzene with diboron. Theoretical elucidation of catalytic cycle including unusual iridium(v) intermediate.

Iridium-catalyzed borylation of benzene with diboron was theoretically investigated with the DFT method, where an iridium(I) boryl complex, Ir(Beg)(NN) 1, and an iridium(III) tris(boryl) complex, Ir(Beg)(3)(NN) 14, (eg (ethyleneglycolato) = -OCH(2)CH(2)O-, NN = HN=CHCH=NH (diim) or 2,2'-bipyridine (bpy)) were adopted as models of active species and B(2)(eg)(2) was adopted as a model of bis(pinacolato)diboron (pinacolato = -OCMe(2)CMe(2)O-). Oxidative addition of a benzene C-H sigma-bond to 1 takes place with an activation barrier (E(a)) of 11.2 kcal/mol, followed by reductive elimination of phenylborane, Ph-Beg, from Ir(Beg)(H)(Ph)(diim) with an activation barrier of 15.6 kcal/mol. Though the oxidative addition and the reductive elimination occur with moderate activation barriers, B(2)(eg)(2) much more easily reacts with 1 to afford 14 than does benzene, of which the activation barrier is very small (2.9 kcal/mol). Oxidative addition of the benzene C-H sigma-bond to 14 occurs with a moderate activation barrier of 24.2 kcal/mol to afford an unusual seven-coordinate iridium(V) complex, Ir(H)(Ph)(Beg)(3)(bpy) 16. From this complex, phenylborane Ph-Beg is produced through the reductive elimination with concomitant formation of IrH(Beg)(2)(bpy) 17, where the activation barrier is 4.9 kcal/mol. Complex 17 further reacts with diboron to form Ir(H)(Beg)(4)(bpy) (E(a) = 8.0 kcal/mol), followed by the reductive elimination of borane H-Beg (E(a) = 2.6 kcal/mol) to regenerate Ir(Beg)(3)(bpy), when diboron exists in excess in the reaction solution. After consumption of diboron, IrH(Beg)(2)(bpy) reacts with borane, H-Beg, to form Ir(H)(2)(Beg)(3) (E(a) = 21.3 kcal/mol) followed by the reductive elimination of H(2), to regenerate Ir(Beg)(3)(bpy) with concomitant formation of H(2). Formation of the iridium(III) tris(boryl) complex 14 from IrCl(diim) and diboron was also theoretically investigated; IrCl(diim) undergoes two steps of oxidative addition of diboron to afford a seven-coordinate iridium(V) complex, IrCl(Beg)(4)(NN), from which the reductive elimination of Cl-Beg takes place easily to afford 14. From these results, it should be clearly concluded that the iridium(III) tris(boryl) complex is an active species and an unusual iridium(V) species is involved as a key intermediate in the reaction. Detailed discussion is presented on the full catalytic cycle and the importance of a seven-coordinate iridium(V) intermediate.