The effects of hypergravity on vestibular epithelia and behaviour of the rat

Researchers examined the vestibules of rats and hamsters raised in a centrifuge to determine the histology of the peripheral sensory system, vestibule-induced ocular reflexes, and vestibular-controlled behavior. Actin and tubulin structures were compared in animals raised in hypergravity and normal gravity. Air righting and swimming also were compared.     

The effects of support-proprioceptive deprivation on visual-manual tracking and vestibular function

In order to determine the effects of support and proprioceptive afferentation on the characteristics of visual-manual tracking (VMT), we used a model of weightlessness—horizontal dry immersion. Altogether 30 subjects who stayed in the immersion bath from 5 to 7 days were examined to evaluate the accuracy of the VMT in tasks to pursue the jerky (saccadically) and smooth (linear, pendular and circular) movement of a point visual stimulus. Examinations were performed before, during and after immersion using electrooculography (to record eye movements) and a joystick (to record hand movements) with a biological visual feedback—one of the two visible stimuli on the screen matched the current angle of the joystick handle. Computerized visual stimulation programs were presented to subjects using virtual-reality glasses. We analyzed the time, amplitude and velocity characteristics of the visual and manual tracking (VT and MT respectively), including the efficiency ratio (eVT and eMT) and the gain (gVT and gMT) as the respective ratios of the amplitudes and velocities of the eyes/hand movements to the stimulus movement. eVT was significantly reduced in comparison to the baseline all the time, while the subject lay in the immersion bath and until R+4 day after immersion. eMT decreased significantly only on I-1 and I-3 days of immersion. gVT significantly differed from the baseline only on I-3 and I-6 days of immersion and R+1 day after immersion. We found no significant changes in gMT. Evaluations of the vestibular function (VF) were performed before and after immersion using videooculography. We analyzed the static torsional otolith-cervical-ocular reflex (OCOR), the dynamical vestibular-cervical-ocular reactions (VCOR), spontaneous eye movements (SpEM), and the accuracy of the perception of the subjective visual vertical (SVV). After immersion, 47% of all subjects had a significant reduction of OCOR with a simultaneous significant increase of VCOR on 37% of subjects, as well as significant changes in the accuracy of the perception of the SVV, which correlated with changes in OCOR. We found a correlation between characteristics of the VT and MT and between the characteristics of the VF and VT, but we found no correlation between VF and MT. We discovered that removal of the support and minimization of the proprioceptive afferentation has a greater impact upon the accuracy of the VT than the accuracy of the MT.     

Effects of gravity, hypergravity and microgravity on vestibular neurones of the crab

Recordings were made from identified balancing system interneurones using implanted electrodes in crabs oscillated at 0.3 Hz during bouts of Parabolic flight. Repeatable non stabilized patterns of response firing were seen in head up and head down interneurones. During the hypergravity phases, the ratio of firing frequencies in the two directional categories of interneurones was altered showing that hypergravity produced effects normally seen during tilting of the crab, implying greater bending of the sensory thread hairs. During microgravity, firing levels remained low and constant or changed slowly towards initial firing levels.     

The effects of movement velocity during squatting on energy expenditure and substrate utilization in whole-body vibration

The purpose of this study was to examine whether and how cycle time duration affects energy expenditure and substrate utilization during whole-body vibration (WBV). Nine men performed 3 squatting exercises in execution frequency cycles of 6, 4, and 2 seconds to 90 degrees knee flexion with vibration (Vb+) (frequency was set at 30 Hz and the amplitude of vibration was 4 mm) and without vibration (Vb-) during 3 minutes, each with an additional load of 30% of the subject's body weight. A 2-way analysis of variance for VO2 revealed a significant vibration condition main effect (p < 0.001) and a cycle time duration effect (p < 0.001). When differences were analyzed by Fisher's LSD test, cycle time duration of 2 seconds was significantly different from 4 and 6 seconds, both in Vb+ and Vb-. Total energy expenditure (EE(tot)), carbohydrate oxidation rate (EE(cho)), and fat oxidation rate (EE(fat)) demonstrated a significant vibration condition main effect (EE(tot): p < 0.01; EE(cho): p < 0.001; EE(fat): p < 0.001) and cycle time duration main effect (EE(tot) and EE(cho): p < 0.001; EE(fat): p < 0.01). EE(tot), EE(cho), and EE(fat) post hoc comparisons indicated that values for the 2-second test significantly differed from 4 and 6 seconds when compared in the same vibration condition. VO2 and EE values were greater in Vb+ than in Vb- conditions with the same cycle time duration. Our study confirms that squatting at a greater frequency helps to maximize energy expenditure during exercise with or without vibration. Therefore, cycle time duration must be controlled when vibration exercise is prescribed.     

Comparison of hypo- and hypergravity effects on skeletal muscle

Numerous observations suggest importance of gravitational loading in regulating muscle mass. Thus, centrifugation is believed to be useful for preventing muscle functional and structural losses under microgravity conditions. The aim of the present work was to evaluate plasticity of skeletal muscle exposed to chronic hypergravity.     

Microgravity and hypergravity effects on collagen biosynthesis of human dermal fibroblasts

Astronauts experiencing long periods of space flight suffer from severe loss of bone tissue, particularly in those bones that carry the body weight under normal gravity. It is assumed that the lack of mechanical load decreases connective tissue biosynthesis in bone-forming cells. To test this assumption, quantitative and qualitative aspects of collagen synthesis under microgravity, normal gravity, and hypergravity conditions were investigated by incubating human fibroblast cultures with [³H]-proline for 4, 7, 10, and 20 h during the Spacelab D2-mission in 1993. Quantitative analysis revealed an increase of collagen synthesis under microgravity conditions, being up to 143% higher than in 1 g controls. In contrast, hypergravity samples showed a decrease in collagen synthesis with increasing g, being at the 13% level at 10 g. The relative proportion of collagen in total synthesized protein showed a slight decrease with increasing g. The secretion of collagen by the cells, proline hydroxylation of individual collagen -chains, and the relative proportions of synthesized collagens I, III, and V were not affected under any of the applied conditions.Our research was supported financially by Dara GmbH Bonn (grant. no. 01QV 8866), the Deutsche Forschungsgemeinschaft (SFB A1/367) and BMFT grant. no. 01 KM 9303/8.     

The effect of hypergravity on carcinogenesis in mice

In recent years, investigators began studying the effect of hypergravity on pathological developments in the animal and human body. It was shown that a regular exposure of tail suspended rats to normal gravity diminished osteopenia and muscle atrophy. Moderate gravitational loading produced by a G-suit dramatically increased the therapeutic success rate in children with cerebral palsy. Rotation of patients with obliterating endarteritis and limb bone fractures in a short-radius centrifuge in hospitals of the city of Samara (Russia) yielded promising results. The purpose of our investigations was to investigate the effects of hypergravity on chemically induced carcinogenesis in mice. We hypothesized that gravitational loading may produce a generalized effect on the animal body and thus to enhance its nonspecific anti-tumor resistance.     

Quantitative changes in mRNA expression of glutamate receptors in the rat peripheral and central vestibular systems following hypergravity

In order to investigate the mechanisms responsible for adaptation to altered gravity, we assessed the changes in mRNA expression of glutamate receptors in vestibular ganglion cells, medial vestibular nucleus, spinal vestibular nucleus/lateral vestibular nucleus, cerebellar flocculus, and uvula/nodulus from rats exposed to hypergravity for 2 h to 1 week using real-time quantitative RT-PCR methods. The mRNA expression of GluR2 and NR1 receptors in the uvula/nodulus and NR1 receptors in the medial vestibular nucleus increased in animals exposed to 2 h of hypergravity, and it decreased gradually to the control level. The mRNA expression of GluR2 receptors in vestibular ganglion cells decreased in animals exposed to 1 week of hypergravity. Neither the metabotropic glutamate receptor 1 nor delta2 glutamate receptor in flocculus and uvula/nodulus was affected by a hypergravity load for 2 h to 1 week. It is suggested that the animals adapted to the hypergravity by enhancing the cerebellar inhibition of the vestibular nucleus neurons through activation of the NR1 and GluR2 receptors on the Purkinje cells in uvula/nodulus especially at the early phase following hypergravity. In the later phase following hypergravity, the animals adapted to the hypergravity by reducing the neurotransmission between the vestibular hair cells and the primary vestibular neurons via down-regulation of the postsynaptic GluR2 receptors in the vestibular periphery.     

Velocity of body lean evoked by leg muscle vibration potentiate the effects of vestibular stimulation on posture

To investigate the vestibular and somatosensory interaction in human postural control, a galvanic vestibular stimulation of cosine bell shape resulting in a small forward or backward body lean was paired with three vibrations of both soleus muscles. The induced body lean was registered by the position of the center of foot pressure (CoP). During a quiet stance with eyes closed the vibration of both soleus muscles with frequency (of) 40 Hz, 60 Hz and 80 Hz resulted in the body lean backward with velocities related to the vibration frequencies. The vestibular galvanic stimulation with the head turned to the right caused forward or backward modification of CoP backward response to the soleus muscles vibration and peaked at 1.5-2 s following the onset of the vibration. The effect of the paired stimulation was larger than the summation of the vestibular stimulation during the quiet stance and a leg muscle vibration alone. The enhancement of the galvanic stimulation was related to the velocity of body lean induced by the leg muscle vibration. The galvanic vestibular stimulation during a faster body movement had larger effects than during a slow body lean or the quiet stance. The results suggest that velocity of a body postural movement or incoming proprioceptive signal from postural muscles potentiate the effects of simultaneous vestibular stimulations on posture.     

Analysis of spontaneous unit activity of the lateral vestibular nucleus' neurons in norm and after effects of vibration

Changes of spontaneous unit activity in the lateral vestibular nucleus of the rat following 5-, 10- and 15-day vibration (60 Hz, 2 hrs. Daily), were studied. Averaged histogramz and autocorrelograms were analysed following computerized interspike intervals. The data obtained revealed a variety of the unit activity in its direct or mediated effects exerted through several structures of the central nervous system.     

Propionyl-L-carnitine effects on postischemic recovery of heart function and substrate oxidation in the diabetic rat

Previous studies have shown that propionyl-L-carnitine (PLC) can exert cardiac antiischemic effects in models of diabetes. In the nonischemic diabetic rat heart, PLC improves ventricular function secondary to stimulation in the oxidation of glucose and palmitate. Whether this increase in the oxidation of these substrates can explain the beneficial effects of PLC in the ischemic reperfused diabetic rat heart has yet to be determined. Diabetes was induced in male Sprague-Dawley rats by an intravenous injection of streptozotocin (60 mg/kg). Treatment was initiated by supplementing the drinking water with propionyl-L-carnitine at the concentration of 1 g/L. After a 6-week treatment period, exogenous substrate oxidation and recovery of mechanical function following ischemia were determined in isolated working hearts. In aerobically perfused diabetic hearts, compared with those of controls, rates of glucose oxidation were lower, but those of palmitate oxidation were similar. Diabetes was also characterized by a pronounced decrease in heart function. Following treatment with by propionyl-L-carnitine, however, there was a marked increase in rates at which glucose and palmitate were oxidized by diabetic hearts and a significant improvement in heart performance. Postischemic recovery of function in diabetic hearts was also improved with PLC. This improvement in contractile function was accompanied by an increase in both glucose and palmitate oxidation. Our findings show that postischemic diabetic rat heart can be improved following chronic PLC treatment. This beneficial effect of propionyl-L-carnitine can be explained, in part, by an improvement in the oxidation of glucose and palmitate.     

Cervical kinematics during drinking in developing chickens.

Development of head neck motion patterns is studied in drinking chickens to examine (1) general motion principles, (2) ontogenetic changes in these patterns, and (3) whether pattern changes are due to scaling effects during growth. Behavioral patterns are analyzed by high speed filming, radiography, and calculation of rotation patterns for each joint during all movement patterns. Flexibility and variability are great, but representative kinematic patterns are selected for immersion, upstroke, and tip-up phases. Five principles were found that control cervical motion. Two principles maximize rotation efficiency: the geometric and lever arm principles. Two trajectory compensating principles occur; one controls compensation for overflexion, and the other corrects curved into straight trajectories of head motion. One principle occurs that minimizes rotation force if large forces tend to develop in one joint. This principle results in a characteristic cervical motion pattern ("bike chain" pattern). There are three developmental periods: (1) hatchlings (2) chickens 1 to 4 weeks old (1-4W), and (3) older than 4 weeks. Each period is characterized by different kinematic patterns. In 1-4W chicks, the rotation force is minimized. In older stages, the cervical joints rotate according to geometric and lever arm principles. The totally different motion pattern in hatchlings results from a different behavioral reaction to water and the influence of large centrifugal forces. Transitions in cervical motion patterns are connected to effects of scaling, primarily changes in head and body weights. Changes in motion patterns are not related to changes in anatomical characters such as flexion extremes and relative length of each vertebra since these are similar in all stages.     

Effects of hypergravity exposure on the developing central nervous system: possible involvement of thyroid hormone

The present study examined the effects of hypergravity exposure on the developing brain and specifically explored the possibility that these effects are mediated by altered thyroid status. Thirty-four timed-pregnant Sprague-Dawley rats were exposed to continuous centrifugation at 1.5 G (HG) from gestational Day 11 until one of three key developmental points: postnatal Day (P) 6, P15, or P21 (10 pups/dam: 5 males/5 females). During the 32-day centrifugation, stationary controls (SC, n = 25 dams) were housed in the same room as HG animals. Neonatal body, forebrain, and cerebellum mass and neonatal and maternal thyroid status were assessed at each time point. The body mass of centrifuged neonates was comparatively lower at each time point. The mass of the forebrain and the mass of the cerebellum were maximally reduced in hypergravity-exposed neonates at P6 by 15.9% and 25.6%, respectively. Analysis of neonatal plasma suggested a transient hypothyroid status, as indicated by increased thyroid stimulating hormone (TSH) level (38.6%) at P6, while maternal plasma TSH levels were maximally elevated at P15 (38.9%). Neither neonatal nor maternal plasma TH levels were altered, suggesting a moderate hypothyroid condition. Thus, continuous exposure of the developing rats to hypergravity during the embryonic and neonatal periods has a highly significant effect on the developing forebrain and cerebellum and neonatal thyroid status (P < 0.05, Bonferroni corrected). These data are consistent with the hypothesized role of the thyroid hormone in mediating the effect of hypergravity in the developing central nervous system and begin to define the role of TH in the overall response of the developing organism to altered gravity.     

The vestibulo-ocular reflex of hypergravity rats

The vertebrate vestibular system detects linear (otolith organs) and angular (semicircular canals) acceleration. The function of the otolith system is twofold, 1: perception of linear acceleration of the head, and 2: assessment of the spatial orientation of the head relative to the vector of gravity. Because of the latter function, a change of gravity will affect the vestibular input which, in turn, may have a wide range of serious physiological effects, for instance on ocular reflexes. The function of the vestibulo-ocular reflex (VOR) is to stabilize the visual image on the retina. Measurement of this VOR provides a method to investigate the (processing within the) vestibular system. Discrimination between gravity and linear acceleration, caused by movement of the head, is not possible. Therefore, information from the otolith system must be constantly compared with additional information from other sensory systems in order to solve the inherent ambiguity between tilt and translation. In this processing, cues from the semicircular canals also play a role. During parabolic flight, experiments can be performed at altered gravity levels for brief periods of time. On earth, the only effective possibility to manipulate gravity for longer periods of time is a centrifuge. Together with experiments in weightlessness during orbital flight, these methods form useful tools to investigate the influence of gravity on physiology. In our laboratory, rats have been kept inside a centrifuge at 2.5 g during their entire life-span (i.e. including gestation).