Sensory transduction in an insect mechanoreceptor: Linear and nonlinear properties

Mechanotransduction in the femoral tactile spine of the cockroach, Periplaneta americana, was examined as a function of displacement of the spine axially in its socket. Linear behaviour was analyzed by measurement of the frequency response function between displacement and action potential output using sinusoidal stimulation and random noise stimulation. The measured frequency response functions can be well fitted by a relationship which is a fractional power of complex frequency. This power was close to 0.5 for all experiments. To distinguish between the effects of nonlinearity and of inherent variability, the averaged responses of the preparation to repeated sequences of pseudorandom noise were compared to those from experiments in which continuous pseudorandom noise were used. The lack of sensitivity of the coherence function to these two methods of measurement suggests that mechanical stimuli are encoded into action potentials with a large signal-to-noise ratio. The low value of the coherence function which is characteristics of insect mechanoreceptors is therefore due to the strong non-linearity of their responses. To investigate the nonlinear properties of transduction, the second-order frequency response function of the tactile spine was measured for random noise stimulation experiments. Two models of the transduction process were considered in which a linear element with memory was cascaded with a nonlinear element without memory in the two possible configurations. Comparison of the experimental second-order frequency response functions with predictions based upon these two models and the measured first-order frequency response suggests that the transduction mechanism can be modelled by a linear element, which may be associated with the viscoelastic properties of the dendritic tubular body, and a zeromemory nonlinearity, which is most likely to be rectification by the dendritic membrane.     

The reaction of menadione with haemoglobin. Mechanism and effect of superoxide dismutase.

1. Menadione was found to react with both the haem groups and the beta-93 thiol groups of haemoglobin. 2. It oxidized the haem groups of oxyhaemoglobin, giving mainly methaemoglobin and a smaller amount of haemichrome. The reaction rate was decrease in the presence of catalase and markedly accelerated in the presence of superoxide dismutase. It is proposed that the overall reaction involves the initial reversible formation of methaemoglobin and the semiquinone, and that the effect of superoxide dismutase is to prevent the reverse reaction, by removing superoxide and hene O2-. E.s.r. evidence for the information of the semi-quinone and its reactions is presented. 3. The reaction of menadione with the beta-93 thiol groups of haemoglobin appeared to be similar to that with other thiols, forming the 3-thioether derivative of menadione, but it was also accompanied by reduction of methaemoglobin. This reduction was prevented by superoxide dismutase, but appeared to be caused by the semiquinone radical, which was produced as an intermediate. 4. Reduced glutathione functioned only to a limited extent as a scavenger of the menadione semiquinone. Its main reaction was directly with menadione to form the thioether. Ascorbate was a more efficient scavenger, and accelerated the oxidation of oxyhaemoglobin by menadione. 5. The significance of these findings in relation to menadione-induced erythrocyte haemolysis is discussed.     

Androgen receptor in nuclei of rat testis.

Testis nuclei of hypophysectomized rats selectively accumulate labeled testosterone and 5alpha-dihydrotestosterone following the injection of tritiated testosterone in vivo. Testosterone and 5alpha-dihydrotestosterone are bound to macromolecules in nuclei and can be extracted with 0.5 M KCl. Accumulation of protein bound radioactive androgens in nuclei of isolated seminiferous tubules is similar to that of whole testis. The relative amounts of testosterone and dihydrotestosterone in purified nuclei were similar to the relative amounts bound to cytoplasmic receptors, suggesting that cytoplasmic androgen-receptor complexes may be transported into the nuclei. Binding of labeled androgen is saturable and inhibited by prior injection of unlabeled testosterone or cyproterone acetate. Nuclear binding sites are destroyed by the proteolytic enzyme pronase, but not by DNase. Like the cytoplasmic androgen-receptor complexes in rat testis, nuclear androgen-protein complexes are heat labile and dissociate slowly at 0 degrees C. androgens fail to accumulate in testis nuclei of the Stanley-Gumbreck androgen insensitive rat, a species lacking cytoplasmic androgen receptors in testis and other androgen target tissues.     

High affinity GDP binding sites on brown adipose tissue mitochondria of genetically obese ‘fa/fa’ rats

The number of high affinity [³H]GDP binding sites in brown adipose tissue mitochondria is normal in obese ( f a / f a ) rats in contrast to the reduced number of low affinity GDP binding sites. Adrenalectomy corrected the loss of low affinity binding sites in fa/fa rats but had no effect on the number of high affinity sites in either lean or obese rats. Equilibrium dialysis was used to show the presence of both high and low affinity binding sites on the purified 32 kdalton protein.     

Uncertainty in research about key invasion characteristics limits the evaluation of exotic perennial grasses in natural systems in New South Wales,Australia

Exotic perennial grasses (EPGs) pose a significant risk to native communities globally. With over 2,200 species in Australia, understanding which characteristics enable high threat invasions, and comparing between functionally similar EPGs, can help prioritise species management. We developed a framework of risk and used the literature to rank 21 EPGs considered a threat to plant communities in New South Wales, while also evaluating the reliability of information currently available. Characteristics were scored within five broad categories that distinguish invasiveness: Arrival, Establishment, Persistence, Impact and Distribution. These included aspects of reproductive biology, competitive ability and environmental tolerance. The risk assessment was effective in assessing key characteristics of invasion. EPGs with an economic benefit (trade‐off species) were more likely to have reliable research and frequently ranked as high‐risk invaders in natural habitats due to the overlap of characteristics important in invasion with those considered important in agriculture. Lack of formal scientific research hindered assessment for some species, and some traits had been poorly assessed in the literature. High uncertainty was associated with key characteristics for Establishment, Persistence and Impact. Uncertainty in key characteristics revealed a need for improved integration of less formal research validated by more formal scientific research. This may lead to more informed decisions in the management of EPGs in native habitats and assist in early control of EPGs not yet assessed.