Colocalization of (3H)-adenosine accumulation and GABA immunoreactivity in the chicken and rabbit retinas

Using combined autoradiography and immunohistochemistry, we have compared (3H)-adenosine accumulation and GABA immunoreactivity in the chicken and rabbit retinas. Colocalization of the two markers was observed in a subset of amacrine cells and in certain cell bodies in the ganglion cell layer in both species and in a few horizontal cells in the chicken retina. Cells that contained only (3H)-adenosine or GABA were also seen. The degree of colocalization differed greatly between the two species. The results demonstrate a morphological relationship between the adenosine and GABA systems and provides information on the possible anatomical substrates underlying at least some types of functional interactions.     

Molecular features characterizing non-peptide selectivity to the human B1 and B2 bradykinin receptors

Bradykinin is produced in response to inflammation, trauma, burns, shock, allergy and some cardiovascular diseases. Actions of this peptide are mediated through two different G-protein coupled receptors, named B1 and B2 that have different pharmacological characteristics. The former is up-regulated during inflammation episodes or tissue trauma whereas, the latter is constitutively expressed in a variety of cell types. In a previous work we have characterized the molecular features that explain the observed structure-activity results for both receptors by means of molecular modeling studies, using diverse ligands for both receptors. These results were summarized in the form of two different pharmacophores that provided new insights to be used for the design of novel molecules with antagonistic profile. In the present work, we compare these pharmacophores to understand the features that characterize ligand selectivity to the two bradykinin receptors. The study shows that most of the residues involved in the binding pocket are similar in both receptors and consequently are the pharmacophores obtained. The main difference between the two pharmacophores remains on point #5 that involves a polar moiety for the B1 receptor and an aromatic ring for the B2 receptor. Accordingly, analysis of the prospective bound conformation of several non-selective small molecule ligands of the bradykinin receptors permits to conclude that fulfilment of point#5 is a requirement to produce selective ligands. However, the study also shows that this is a necessary condition only, since ligands need also to be bulky enough to avoid binding to these receptors in diverse poses. These results provide new insights for a better understanding of the molecular features that ligands are required to exhibit to be selective bradykinin ligands.     

Oestradiol changes the dielectric structure of bilayer membranes

The addition of the hormone Oestradiol to Phosphatidylcholine-Cholesterol membrane changes the frequency dependence of the membrane impedance. It increases severalfold the electrical admittance of the polar regions and consequently provides a conducting shunt from the hydrocarbon region to the aqueous phase.     

The role of steroids in reproduction in female elasmobranchs and reptiles

Adequate evidence exists to suggest the importance of temporal changes in steroid hormone ratios in the normal reproductive/vitellogenin cycle in oviparous and viviparous elasmobranchs and reptiles. In oviparous species, where the cycle is relatively short, secretion of gonadal hormones is synchronous; thus inhibitory actions of progesterone (P) on hepatic or reproductive tract functions would be offset by stimulatory actions of estradiol (E), resulting in appropriate vitellogenin secretion and reproductive tract development. In viviparous species, temporal asynchrony of E and P secretion occurs, and the actions of the individual hormones can be more easily dissected out. Thus, during gestation, where P is the dominant hormone, antagonistic or stimulatory actions of E may be prevented, and the inhibitory action of P on vitellogensis dominant. Hence vitellogenesis is limited to the follicular phase and eggs are retained.

Although the elasmobranch and reptilian species discussed here do not form a continuum through phylogenesis, but rather are extant forms of a particular line of evolution, it is possible to extrapolate from these observations to the probable endocrine interactions in a species as viviparity evolves from oviparity. The theoretical intermediate stage would involve; (a) egg retention, (b) extension of the luteal phase and increased P secretion and (c) resulting in E/P asynchrony and potential expression of “independent” P action, egg retention and yolk suppression.     

Diet Switching by Mammalian Herbivores in Response to Exotic Grass Invasion

Invasion by exotic grasses is a severe threat to the integrity of grassland ecosystems all over the world. Because grasslands are typically grazed by livestock and wildlife, the invasion is a community process modulated by herbivory. We hypothesized that the invasion of native South American grasslands by Eragrostis plana Nees, an exotic tussock-forming grass from Africa, could be deterred by grazing if grazers switched dietary preferences and included the invasive grass as a large proportion of their diets. Bos taurus (heifers) and Ovis aries (ewes) grazed plots with varying degrees of invasion by E. plana in a replicated manipulative experiment. Animal positions and species grazed were observed every minute in 45-min grazing session. Proportion of bites and steps in and out of E. plana tussocks were measured and used to calculate several indices of selectivity. Both heifers and ewes exhibited increasing probability of grazing E. plana as the proportion of area covered by tussocks increased, but they behaved differently. In agreement with expectations based on the allometry of dietary preferences and morphology, ewes consumed a low proportion of E. plana, except in areas that had more than 90% E. plana cover. Heifers consumed proportionally more E. plana than ewes. Contrary to our hypothesis, herbivores did not exhibit dietary switching towards the invasive grass. Moreover, they exhibited avoidance of the invasive grass and preference for short-statured native species, both of which should tend to enhance invasion. Unless invasive plants are highly palatable to livestock, the effect of grazing to deter the invasion is limited, due to the inherent avoidance of the invasive grass by the main grazers in the ecosystem, particularly sheep.