Forward Masking Additivity and Auditory Compression at Low and High Frequencies

The additivity of nonsimultaneous masking can be used as a measure of nonlinearity in the auditory system. For example, two equally effective forward maskers should produce 3 dB of additional masking when they are combined, assuming linearity with respect to intensity. A combined effect greater than this (excess masking) indicates compression. In the present experiments, the signal was a 10-ms pure tone presented 20 ms after a 200-ms narrowband noise masker and/or immediately after a 20-ms narrowband noise masker. The signal frequency was 250, 500, or 4000 Hz. The signal threshold produced by combining two equally effective maskers was measured. At all three frequencies, little excess masking was observed for a signal 10 dB above absolute threshold, indicating linear additivity (no compression). At signal levels 30 dB above absolute threshold, excess masking was observed at all three frequencies. The estimated compression exponents were 0.29 at 250 Hz, 0.34 at 500 Hz, and 0.17 at 4000 Hz. In contrast with physiological studies on other mammals, the present results provide evidence for substantial compression at low frequencies in humans

Multidrug resistance protein MdtM adds to the repertoire of antiporters involved in alkaline pH homeostasis in <em>Escherichia coli</em>

BACKGROUND: In neutralophilic bacteria, monovalent metal cation/H(+) antiporters play a key role in pH homeostasis. In Escherichia coli, only four antiporters (NhaA, NhaB, MdfA and ChaA) are identified to function in maintenance of a stable cytoplasmic pH under conditions of alkaline stress. We hypothesised that the multidrug resistance protein MdtM, a recently characterised homologue of MdfA and a member of the major facilitator superfamily, also functions in alkaline pH homeostasis. RESULTS: Assays that compared the growth of an E. coli ΔmdtM deletion mutant transformed with a plasmid encoding wild-type MdtM or the dysfunctional MdtM D22A mutant at different external alkaline pH values (ranging from pH 8.5 to 10) revealed a potential contribution by MdtM to alkaline pH tolerance, but only when millimolar concentrations of sodium or potassium was present in the growth medium. Fluorescence-based activity assays using inverted vesicles generated from transformants of antiporter-deficient (ΔnhaA, ΔnhaB, ΔchaA) E. coli TO114 cells defined MdtM as a low-affinity antiporter that catalysed electrogenic exchange of Na(+), K(+), Rb(+) or Li(+) for H(+). The K(+)/H(+) antiport reaction had a pH optimum at 9.0, whereas the Na(+)/H(+) exchange activity was optimum at pH 9.25. Measurement of internal cellular pH confirmed MdtM as contributing to maintenance of a stable cytoplasmic pH, acid relative to the external pH, under conditions of alkaline stress. CONCLUSIONS: Taken together, the results support a role for MdtM in alkaline pH tolerance. MdtM can therefore be added to the currently limited list of antiporters known to function in pH homeostasis in the model organism E. coli

Genetic Inactivation of European Sea Bass (Dicentrarchus labrax L.) Eggs Using UV-Irradiation: Observations and Perspectives

International audienceAndrogenesis is a form of uniparental reproduction leading to progenies inheriting only the paternal set of chromosomes. Ithas been achieved with variable success in a number of freshwater species and can be attained by artificial fertilization ofgenetically inactivated eggs following exposure to gamma (c), X-ray or UV irradiation (haploid androgenesis) and byrestoration of diploidy by suppression of mitosis using a pressure or thermal shock. The conditions for the geneticinactivation of the maternal genome in the European sea bass (Dicentrarchus labraxL.) were explored using differentcombinations of UV irradiation levels and durations. UV treatments significantly affected embryo survival and generated awide range of developmental abnormalities. Despite the wide range of UV doses tested (from 7.2 to 720 mJ.cm22), only onedose (60 mJ.cm22.min21with 1 min irradiation) resulted in a small percentage (14%) of haploid larvae at hatching in theinitial trials as verified by flow cytometry. Microsatellite marker analyses of three further batches of larvae produced by usingthis UV treatment showed a majority of larvae with variable levels of paternal and maternal contributions and only one larvadisplaying pure paternal inheritance. The results are discussed also in the context of an assessment of the UV-absorbancecharacteristics of egg extracts in this species that revealed the presence of gadusol, a compound structurally related tomycosporine-like amino acids (MAAs) with known UV-screening properties