Bone loss during simulated weightlessness: a biomechanical and mineralization study in the rat model.

BACKGROUND: Astronauts exposed to weightlessness for extended periods experience significant decreases in bone mineral density. The clinical implications of this demineralization are not entirely clear, and the biomechanics involved are not completely understood. HYPOTHESIS: Local (rather than global) measurements of geometry and calcium concentration effectively predict femur strength in adult rats exposed to a hind-limb suspension model of weightlessness. METHODS: Female Fischer rats (6-mo-old) were divided into groups of control and hind-limb-suspended animals. Animals were sacrificed after 2 or 4 wk of hind-limb suspension, and both femurs removed from each animal. The 3-point bending strength and total bone mineralization were determined for one femur from each animal, and the mid-shaft cross-sectional geometrical properties and distribution of calcium were determined for the contralateral femur. RESULTS: Although suspension led to significant decreases in total bone mineralization, the concentration of calcium at the anterior periosteal surface was unaffected. Total bone percent mineralization was not well correlated with structural properties, but bone geometrical properties (particularly cross-sectional moment of inertia and length) correlated strongly with ultimate bending strength (r2 = 0.81). Differences in bone geometry due to suspension were consistent with a distribution of bone material closer to the axis of the femur. CONCLUSIONS: Structural properties of bone are predicted well by bone geometry and poorly by total bone percent mineralization. Decreased bone mechanical strength in this model of weightlessness is primarily due to a distribution of bone material nearer the axis of the bone.

-Fos expression in female hamster brain following sexual and aggressive behaviors.

The goal of these experiments was to use c-Fos immunocytochemistry to determine areas of the female hamster brain that are active during lordosis and aggression. Ovariectomized hamsters were given (i) estradiol and progesterone, plus a lordosis test, (ii) estradiol and progesterone, but no lordosis test, (iii) oil, plus an aggressive behavior test, or (iv) oil, but no behavior test. Results showed that following lordosis, there was increased c-Fos expression in the medial bed nucleus of the stria terminalis, medial accumbens, medial preoptic nucleus, paraventricular nucleus and medial amygdala. Following a single aggression test, c-Fos was significantly increased only within the medial amygdala. There was no effect of lordosis or aggression on c-Fos expression within the lateral or central ventromedial hypothalamus, suprachiasmatic nucleus or dorsal midbrain central gray. In a second experiment, ovariectomized female hamsters were given (i) repeated aggressive experience, (ii) a single aggression test or (iii) no aggression test. Because some females were not aggressive towards males, they became a separate group post hoc. The number of cells expressing c-Fos was higher in the medial preoptic nucleus and medial amygdala of females given a single aggressive test and in non-aggressive females vs control females. Females given prior aggressive experience showed higher c-Fos expression only in the medial preoptic nucleus. These results demonstrate that increased neural activation in several forebrain nuclei is seen after sexual or aggressive behaviors in female hamsters. However, because the pattern of c-Fos staining in the non-aggressive females was similar to the pattern in aggressive females, this questions previous conclusions regarding the behavioral specificity of these effects and suggests instead that such activation is common to social interactions in general.