Excretion of electrolytes in Brown Norway and Fischer 344 rats: effects of adrenalectomy and of mineralocorticoid and glucocorticoid receptor ligands.

Our previous studies showed that adrenalectomy (ADX) has surprisingly no effect on body weight and fluid intake in the Brown Norway rat strain, suggesting that mineralocorticoid receptor (MR)-mediated effects are present even in absence of corticosteroids in this strain. Moreover, glucocorticoid receptor (GR)-mediated mechanisms are more effective in Brown Norway than in Fischer 344 rats. Such functional differences in corticosteroid receptor pathways between Brown Norway and Fischer 344 rats led us to compare the effect of ADX and MR/GR-mediated actions on sodium and potassium excretion between these two rat strains. To this end, we first measured the effect of an acute high dose of aldosterone on the urinary Na+/K+ concentration ratio in intact and ADX Brown Norway and Fischer 344 rats. Second, to discriminate mineralocorticoid from glucocorticoid actions, we treated chronically ADX rats with increasing doses of aldosterone or RU28362, a pure GR agonist, in the drinking fluid. As sodium homeostasis involves salt appetite regulation, behaviour under mineralocorticoid control, we also measured saline preference in Brown Norway and Fischer 344 rats. Our data illustrate: (1) the very limited effect of ADX on body weight, food and fluid intake, diuresis, natriuresis, kaliuresis and salt appetite in Brown Norway rats, supporting the presence of MR signalling pathways in the absence of adrenal steroids in these rats; (2) the insensitivity of MR to aldosterone in intact Brown Norway rats, and the reduced sensitivity of MR to aldosterone in ADX Brown Norway rats compared with ADX Fischer 344 rats; and (3) the greater sensitivity of GR-related mechanisms to RU28362 in Brown Norway than in Fischer 344 rats in terms of body weight gain and electrolyte excretion. Considering that both MRs and GRs regulate hypothalamic-pituitary-adrenal axis processes, such functional differences in corticosteroid receptors could be at the origin, at least partly, of the strain differences in corticotropic activity/reactivity to stress previously described.

Are the semicircular canals of the European mole, Talpa europaea, adapted to a subterranean habitat?

The mechanical sensitivity of vertebrate semicircular canals is directly influenced by the canal dimensions. Three key canal parameters, whose dimensions have been shown to be critical in determining the mechanical sensitivity of semicircular canals, are the streamline length, the cross-sectional area of the canal lumen and the plane area. These parameters were measured in ten specimens of adult T. europaea and compared with the dimensions of the same parameters in R. norvegicus. The major determinant of sensitivity, canal lumen area, is significantly larger in T. europaea.

Histological characteristics of respiratory system in Brown Norway rat.

It is well known that the Brown Norway (BN) rat strain exhibits airway hyperresponsiveness to exposure to allergens or some chemicals. We investigated the histological characteristics of the trachea and lungs of this strain (10-week-old and retired animals) and compared them with those of age-matched Fischer 344 (F344) rat strain. No histological differences between two strains in tracheal epithelial cells were detected, but differences in the distribution and development of submucosal glands were clarified by the observation of serial sections cut at intervals of 100 microns. Submucosal glands of BN strain were larger in the number and better-developed than those of F344 strain, especially in the middle and lower trachea. Similar results were also obtained in scanning electron microscopic observation of resin casts. There were no significant differences between two strains in the lectin histochemical characteristics of the cytoplasm of glandular epithelial cells. No age-related changes in these morphological characteristics in the two strains were observed. These results suggest that mucin from submucosal glands is quantitatively different but qualitatively similar in the two strains. In addition, microgranuloma mainly composed of histiocytes and eosinophils was observed in the lungs of the BN strain rats.

Localisation of endothelin-1 in rat aortae, the relationship to flow and elastic tissue tears.

The aortic expression of endothelin-1 (ET-1) was examined in three species of rat using a novel en face immunohistochemical technique. A genetically hypertensive strain was compared to two normotensive strains, one of which is known to develop spontaneous lesions within the abdominal aorta. ET-1-positive staining was increased about the major aortic branch ostia and over the dorsal abdominal aortic wall in all three species indicating a flow-related expression pattern. Mitotic and hyperchromatic endothelial cells stained strongly for ET-1 as did occasional multi-nucleated endothelial cells. The aortic-lesion-prone normotensive strain developed transverse tears of the internal elastic lamina with a corresponding endothelial cell response. Endothelium at the edge of these lesions was strongly stained for ET-1 and appeared to be associated with increased leucocyte adhesion as did other strongly ET-1-stained areas in all three species. This study indicates that increased ET-1 expression is anatomically localised within the rat aorta, possibly by haemodynamic stress. This may have implications for maintaining endothelial cell confluence, aortic smooth muscle cell reparative processes and possibly eventual pathophysiological conditions such as atherosclerosis.

Structure and composition of the rodent lamina cribrosa.

To define the architecture and extracellular matrix composition of the lamina cribrosa in rodents, normal, adult pigmented rat and guinea pig eyes were frozen and sectioned for light microscopic immunohistochemistry. Antibodies specific for collagens I, III, IV and VI, laminin, elastin, and chondroitin and dermatan sulfate proteoglycans were exposed to longitudinal and cross-sections of optic nerve heads and their binding distributions observed with the avidin-biotin-peroxidase complex technique. Cross-sections of the intraocular portion of the rat optic nerve head revealed a horizontally oval shape with distinct, vertically oriented, laminar beams. The guinea pig optic nervehead cross-section was circular, with randomly oriented beams. In both animals, collagens I, III and VI were found throughout the laminar beams, along with elastin fibrils. Collagen IV and laminin antibodies deposited along laminar beam margins and within the beams, representing astrocytic and vascular endothelial cell basement membranes. Both animals showed evidence for dermatan and chondroitin sulfate-containing proteoglycans in all connective tissue structures of the nerve head. In the rat, chondroitin-4 sulfate proteoglycans appeared localized to the sclera and laminar beams. The rat and the guinea pig optic nerve head possess an identifiable lamina cribrosa with structural proteins nearly identical to that of the primate. Both animals may provide affordable alternative animal models for in vivo studies on the role of the lamina cribrosa in glaucomatous optic nerve damage.

Morphology of the cochlear nerve in Sprague-Dawley and brown Norway rats.

Cochlear nerve morphology was studied in young adult albino (Sprague-Dawley) and pigmented (Brown Norway) rats. Analysis of the material included counts of normal and degenerating fibers and of glial cell nuclei, and measurements of vascularity and of the nerves' cross-sectional areas. The median number of normal fibers in the Sprague-Dawley rats was 21,216, and, in the Brown Norway rats, it was 20,186. There were no statistically significant differences between the two strains in numbers of normal fibers, degenerating myelin sheaths, or glial cell nuclei, or in the cross-sectional areas of the nerves. The area density of blood vessels was significantly higher in nerves from the Sprague-Dawley rats. The median area density in that strain was 0.0149, while in the Brown Norway rats the median area density was 0.0105.