Plastic alteration of vestibulo-cardiovascular reflex induced by 2 weeks of 3-G load in conscious rats

Galvanic vestibular stimulation-induced activation of C1 neurons in medulla oblongata protects against acute lung injury

Autonomic nerves, including the sympathetic and parasympathetic nerves, control the immune system along with their physiological functions. On the peripheral side, the interaction between the splenic sympathetic nerves and immune cells is important for the anti-inflammatory effects. However, the central mechanism underlying these anti-inflammatory effects remains unclear. C1 neurons respond to stressors and subsequently determine the outflow of the autonomic nervous system. We have previously shown that C1 neurons protect against acute kidney injury and found a signaling connection between peripheral vestibular organs and C1 neurons. Thus, we hypothesized that hypergravity load or galvanic vestibular stimulation (GVS) might protect against acute lung injury. We showed that C1 neurons are histologically and functionally activated by stimulating the peripheral vestibular organs. Protection against acute lung injury that was induced by a 2 G load disappeared due to vestibular lesions or the deletion of C1 neurons. This GVS-induced protective effect was also eliminated by the deletion of the C1 neurons. Furthermore, GVS increased splenic sympathetic nerve activity in conscious mice, and splenic sympathetic denervation abolished the GVS-induced protection against acute lung injury. Therefore, the activated pathway between C1 neurons and splenic sympathetic nerves is indispensable for GVS-induced protection against acute lung injury.

Changes in metabolism and vestibular function depend on gravitational load in mice

The vestibular system is known to participate in controlling posture and metabolism. Different gravitational environments, including microgravity or hypergravity, cause plastic alteration of the vestibular system, and plasticity is important for adaptation to a novel gravitational environment. However, it is unclear whether the degree of change in vestibular-related physiological function depends on gravitational loading. To examine this, we used a hypergravity environment including 1.33 G, 1.67 G, and 2 G for 29 days. We found that a gravitational threshold induces physiological changes, including vestibular-related posture control and metabolism in mice. Body mass did not return to the preloading level in 1.67 G and 2 G mice. A significant drop in food intake, observed on the first day of hypergravity load, disappeared in all mice after longer exposure. However, a reduction in water intake was sustained in 2 G mice but not 1.33 G and 1.67 G mice. Body temperature did not return to the preloading level in 2 G mice by the final day. A decrease in the skill of the righting reflex was observed in 2 G mice but not 1.33 G and 1.67 G mice. In conclusion, this study showed that hypergravity-induced changes in metabolism and vestibular function depended on the amount of gravitational loading. The 2 G load affected vestibular-related posture control and metabolism considerably, compared with 1.33 G and 1.67 G loads.NEW & NOTEWORTHY It is unclear whether the degree of change in vestibular-related physiological function depends on gravitational loading. Present study showed that exposure to hypergravity-induced degrees of change in metabolism and vestibular function depended on the gravitational loading. The response of body mass depended on the gravitational loading size. Especially in 2 G environment, water intake, body temperature, and vestibular function were influenced. These changes could involve plastic alteration of vestibular-related autonomic and motor functions.

Hypergravity load-induced hyperglycemia occurs due to hypothermia and increased plasma corticosterone level in mice

Hypothermia has been observed during hypergravity load in mice and rats. This response is beneficial for maintaining blood glucose level, although food intake decreases. However, saving glucose is not enough to maintain blood glucose level during hypergravity load. In this study, we examined the contribution of humoral factors related to glycolysis in maintaining blood glucose level in a 2 G environment. Increased plasma corticosterone levels were observed in mice with intact peripheral vestibular organs, but not in mice with vestibular lesions. Plasma glucagon levels did not change, and decrease in plasma adrenaline levels was observed in mice with intact peripheral vestibular organs. Accordingly, it is possible that increase in plasma corticosterone level and hypothermia contribute to prevent hypoglycemia in a 2 G environment.

VGLUT2-expressing neurons in the vestibular nuclear complex mediate gravitational stress-induced hypothermia in mice

The vestibular system, which is essential for maintaining balance, contributes to the sympathetic response. Although this response is involved in hypergravity load-induced hypothermia in mice, the underlying mechanism remains unknown. This study showed that hypergravity (2g) decreased plasma catecholamines, which resulted in hypoactivity of the interscapular brown adipose tissue (iBAT). Hypothermia induced by 2g load was significantly suppressed by administration of beta-adrenergic receptor agonists, suggesting the involvement of decrease in iBAT activity through sympathoinhibition. Bilateral chemogenetic activation of vesicular glutamate transporter 2 (VGLUT2)-expressing neurons in the vestibular nuclear complex (VNC) induced hypothermia. The VGLUT2-expressing neurons contributed to 2g load-induced hypothermia, since their deletion suppressed hypothermia. Although activation of vesicular gamma-aminobutyric acid transporter-expressing neurons in the VNC induced slight hypothermia instead of hyperthermia, their deletion did not affect 2g load-induced hypothermia. Thus, we concluded that 2g load-induced hypothermia resulted from sympathoinhibition via the activation of VGLUT2-expressing neurons in the VNC.

Chikara Abe, Yusuke Yamaoka et al. show that chemogenetic activation of VGLUT2-expressing neurons in the vestibular nuclear complex induces hypothermia, while their deletion suppresses hypergravity load-induced hypothermia in mice. These findings suggest an important role for these glutamatergic neurons in thermoregulation.

Understanding vestibular-related physiological functions could provide clues on adapting to a new gravitational environment

The peripheral vestibular organs are sensors for linear acceleration (gravity and head tilt) and rotation. Further, they regulate various body functions, including body stability, ocular movement, autonomic nerve activity, arterial pressure, body temperature, and muscle and bone metabolism. The gravitational environment influences these functions given the highly plastic responsiveness of the vestibular system. This review demonstrates that hypergravity or microgravity induces changes in vestibular-related physiological functions, including arterial pressure, muscle and bone metabolism, feeding behavior, and body temperature. Hopefully, this review contributes to understanding how human beings can adapt to a new gravitational environment, including the moon and Mars, in future.

Hypergravity-induced plastic alteration of the vestibulo-sympathetic reflex involves decrease in responsiveness of CAMK2-expressing neurons in the vestibular nuclear complex

The vestibular system contributes to not only eye movement and posture but also the sympathetic response. Plastic alteration of the vestibulo-sympathetic reflex is induced by hypergravity load; however, the mechanism remains unknown. Here, we examined 2 g-induced changing in responsiveness of CAMK2-expressing neurons in the vestibular nucleus complex using optogenetic tools. The excitatory photostimulation of the CAMK2-expressing neurons in the unilateral vestibular nuclear complex induced body tilt to the contralateral side, while inhibitory photostimulation showed the opposite response. Photoactivation of either cell body or the axonal terminal in the rostral ventrolateral medulla showed sympathoexcitation followed by the pressor response. Furthermore, this response was significantly attenuated (49.8 ± 4%) after the 1st day of 2 g loading, and this value was further reduced by the 5th day (22.4 ± 3%), suggesting that 2 g-induced attenuation of the vestibulo-sympathetic reflex involves at least decrease in responsiveness of CAMK2-expressing neurons in the vestibular nuclear complex.

Effects of hypergravity on the gene expression of the hypothalamic feeding-related neuropeptides in mice via vestibular inputs

The effects of hypergravity on the gene expression of the hypothalamic feeding-related neuropeptides in sham-operated (Sham) and vestibular-lesioned (VL) mice were examined by in situ hybridization histochemistry. Corticotrophin-releasing hormone (CRH) in the paraventricular nucleus was increased significantly in Sham but not in VL mice after 3 days of exposure to a 2 g environment compared with a 1 g environment. Significant decreases in pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript and significant increases in neuropeptide Y, agouti-related protein in the arcuate nucleus and orexin in the lateral hypothalamic area were observed in both Sham and VL mice. After 2 weeks of exposure, CRH and POMC were increased significantly in Sham but not in VL mice. After 8 weeks of exposure, the hypothalamic feeding-related neuropeptides were comparable between Sham and VL mice. These results suggest that the hypothalamic feeding-related neuropeptides may be affected during the exposed duration of hypergravity via vestibular inputs.

Comparison among ultrasonic, electrical apparatus, and toxic chemicals for vestibular lesion in mice

BACKGROUND

The vestibular lesion (VL) is required to examine the physiological function of the vestibular system in animals. Toxic chemicals or electrical apparatus have been used for the VL, however, they are not ideal as they have low specificity, and can result in unintended damage, and systemic toxic effect.

NEW METHOD

Localized vibration-induced VL, using an ultrasonicator, is expected to overcome the problems associated with chemical and electrical lesions. Thus, we examined the effect of the ultrasonication on the VL from the aspects of both the physiological function and histology in the present study.

RESULTS

and Comparison with Existing Method(s) Complete VL, which was evaluated by deterioration of swimming skills, righting reflex, and body stability, was induced using an ultrasonicator or electrical apparatus. Histological evaluation shows that hair cell layers in the saccule and utricle were completely destroyed in both methods Furthermore, significant drop in body mass was observed in VL. However, abscess at the cranial base was observed in VL induced by the electrical apparatus in ICR mice. Complete chemically-induced VL was observed in C57BL/6J but not ICR mice, and systemic leakage of the toxic chemicals (arsenic) was not detectable even 1day after surgery.

CONCLUSIONS

Compared to the electrical apparatus, the ultrasonicator is useful for inducing VL in ICR and C57BL/6J mice, as it results in less non-specific damage. Toxic chemicals can be used for inducing VL in C57BL/6J mice; however, this method does not ensure complete disruption of the hair cells in the saccule and utricle.

Glutamate and GABA in Vestibulo-Sympathetic Pathway Neurons

The vestibulo-sympathetic reflex (VSR) actively modulates blood pressure during changes in posture. This reflex allows humans to stand up and quadrupeds to rear or climb without a precipitous decline in cerebral perfusion. The VSR pathway conveys signals from the vestibular end organs to the caudal vestibular nuclei. These cells, in turn, project to pre-sympathetic neurons in the rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively). The present study assessed glutamate- and GABA-related immunofluorescence associated with central vestibular neurons of the VSR pathway in rats. Retrograde FluoroGold tract tracing was used to label vestibular neurons with projections to RVLM or CVLM, and sinusoidal galvanic vestibular stimulation (GVS) was employed to activate these pathways. Central vestibular neurons of the VSR were identified by co-localization of FluoroGold and cFos protein, which accumulates in some vestibular neurons following galvanic stimulation. Triple-label immunofluorescence was used to co-localize glutamate- or GABA- labeling in the identified VSR pathway neurons. Most activated projection neurons displayed intense glutamate immunofluorescence, suggestive of glutamatergic neurotransmission. To support this, anterograde tracer was injected into the caudal vestibular nuclei. Vestibular axons and terminals in RVLM and CVLM co-localized the anterograde tracer and vesicular glutamate transporter-2 signals. Other retrogradely-labeled cFos-positive neurons displayed intense GABA immunofluorescence. VSR pathway neurons of both phenotypes were present in the caudal medial and spinal vestibular nuclei, and projected to both RVLM and CVLM. As a group, however, triple-labeled vestibular cells with intense glutamate immunofluorescence were located more rostrally in the vestibular nuclei than the GABAergic neurons. Only the GABAergic VSR pathway neurons showed a target preference, projecting predominantly to CVLM. These data provide the first demonstration of two disparate chemoanatomic VSR pathways.

Feasibility of a Short-Arm Centrifuge for Mouse Hypergravity Experiments

To elucidate the pure impact of microgravity on small mammals despite uncontrolled factors that exist in the International Space Station, it is necessary to construct a 1 g environment in space. The Japan Aerospace Exploration Agency has developed a novel mouse habitat cage unit that can be installed in the Cell Biology Experiment Facility in the Kibo module of the International Space Station. The Cell Biology Experiment Facility has a short-arm centrifuge to produce artificial 1 g gravity in space for mouse experiments. However, the gravitational gradient formed inside the rearing cage is larger when the radius of gyration is shorter; this may have some impact on mice. Accordingly, biological responses to hypergravity induced by a short-arm centrifuge were examined and compared with those induced by a long-arm centrifuge. Hypergravity induced a significant Fos expression in the central nervous system, a suppression of body mass growth, an acute and transient reduction in food intake, and impaired vestibulomotor coordination. There was no difference in these responses between mice raised in a short-arm centrifuge and those in a long-arm centrifuge. These results demonstrate the feasibility of using a short-arm centrifuge for mouse experiments.

Vestibulo-sympathetic responses

Evidence accumulated over 30 years, from experiments on animals and human subjects, has conclusively demonstrated that inputs from the vestibular otolith organs contribute to the control of blood pressure during movement and changes in posture. This review considers the effects of gravity on the body axis, and the consequences of postural changes on blood distribution in the body. It then separately considers findings collected in experiments on animals and human subjects demonstrating that the vestibular system regulates blood distribution in the body during movement. Vestibulosympathetic reflexes differ from responses triggered by unloading of cardiovascular receptors such as baroreceptors and cardiopulmonary receptors, as they can be elicited before a change in blood distribution occurs in the body. Dissimilarities in the expression of vestibulosympathetic reflexes in humans and animals are also described. In particular, there is evidence from experiments in animals, but not humans, that vestibulosympathetic reflexes are patterned, and differ between body regions. Results from neurophysiological and neuroanatomical studies in animals are discussed that identify the neurons that mediate vestibulosympathetic responses, which include cells in the caudal aspect of the vestibular nucleus complex, interneurons in the lateral medullary reticular formation, and bulbospinal neurons in the rostral ventrolateral medulla. Recent findings showing that cognition can modify the gain of vestibulosympathetic responses are also presented, and neural pathways that could mediate adaptive plasticity in the responses are proposed, including connections of the posterior cerebellar vermis with the vestibular nuclei and brainstem nuclei that regulate blood pressure.

Hypergravity exposure for 14 days increases the effects of propofol in rats

BACKGROUND

It is thought that the gravitational environment of space exploration alters the effects of anesthetics; however, no evidence has as yet been reported. In the present study, we sought to provide direct evidence showing that hypergravity exposure for 14 days increases anesthetic effects and to examine the possible causes.

METHODS

Sprague-Dawley rats were raised in a 3g environment for 14 days. On the day of the experiment, rats were brought out of 3g and rested at 1g for 1 to 2 hours before IV propofol infusion (20 mg/kg, for 5 minutes). Control rats were continuously raised in a 1g environment. The effects of propofol were compared between rats raised in 1g and 3g environment by measuring time taken to induce the burst suppression in an electroencephalogram, nadir of arterial blood pressure, and time taken for the appearance of the righting response to noxious electrical stimulations. The time course of plasma propofol concentrations was also examined. Experiments were also conducted on rats with vestibular lesions to examine whether the vestibular system participated in the observed results. All values were expressed as mean ± SD.

RESULTS

In rats raised in 3g environment, the mean time to induce burst suppression in the electroencephalogram was earlier (195.7 ± 15.1 seconds, P = 0.00037), the nadir of mean arterial blood pressure was lower (75.0 ± 15.5 mm Hg, P = 0.019), and mean time for the righting response to appear was later (39.0 ± 8.4 minutes, P < 0.0001) than in rats raised in 1g environment (267.3 ± 29.4 seconds, 100.6 ± 9.1 mm Hg, and 22.0 ± 3.1 minutes, respectively). However, mean time to induce burst suppression and for the righting response to appear did not change in rats with vestibular lesions raised in 3g environment (275 ± 29.4 seconds, 108.7 ± 14.6 mm Hg, and 20.8 ± 2.8 minutes, P = 0.95, 0.73, and 0.98 vs sham-treated rats continuously raised in a 1g environment, respectively). There was no difference between groups in the time course assessment of plasma propofol concentrations.

CONCLUSION

The results provide evidence that hypergravity exposure for 14 days increases the effects of propofol. It is suggested that the results were not caused by differences in plasma propofol concentrations but by increased sensitivity, which was mediated via the vestibular system.

Interaction between vestibulo-cardiovascular reflex and arterial baroreflex during postural change in rats

To examine a cooperative role for the baroreflex and the vestibular system in controlling arterial pressure (AP) during voluntary postural change, AP was measured in freely moving conscious rats, with or without sinoaortic baroreceptor denervation (SAD) and/or peripheral vestibular lesion (VL). Voluntary rear-up induced a slight decrease in AP (-5.6 ± 0.8 mmHg), which was significantly augmented by SAD (-14.7 ± 1.0 mmHg) and further augmented by a combination of VL and SAD (-21 ± 1.0 mmHg). Thus we hypothesized that the vestibular system sensitizes the baroreflex during postural change. To test this hypothesis, open-loop baroreflex analysis was conducted on anesthetized sham-treated and VL rats. The isolated carotid sinus pressure was increased stepwise from 60 to 180 mmHg while rats were placed horizontal prone or in a 60° head-up tilt (HUT) position. HUT shifted the carotid sinus pressure-sympathetic nerve activity (SNA) relationship (neural arc) to a higher SNA, shifted the SNA-AP relationship (peripheral arc) to a lower AP, and, consequently, moved the operating point to a higher SNA while maintaining AP (from 113 ± 5 to 114 ± 5 mmHg). The HUT-induced neural arc shift was completely abolished in VL rats, whereas the peripheral arc shifted to a lower AP and the operating point moved to a lower AP (from 116 ± 3 to 84 ± 5 mmHg). These results indicate that the vestibular system elicits sympathoexcitation, shifting the baroreflex neural arc to a higher SNA and maintaining AP during HUT.

Restriction of rear-up-behavior-induced attenuation of vestibulo-cardiovascular reflex in rats

Previously, we have demonstrated that the vestibulo-cardiovascular reflex was attenuated in rats reared in a 3G environment for 14 days. Because continuous galvanic vestibular stimulation preserved the vestibulo-cardiovascular reflex in rats at 3G, this attenuation might be attributable to a reduction in the phasic input to the vestibular system. The present study shows that the head movements of rats were significantly suppressed in the 3G environment. Therefore, we hypothesized that the attenuation of the vestibulo-cardiovascular reflex is induced by the reduced vestibular phasic input caused by the restriction of rear-up behavior. To examine this hypothesis, the pressor responses to linear acceleration were measured in rats reared in a low-roof cage. The linear-acceleration-induced pressor response was significantly suppressed in these rats. The suppressive effect of the low-roof cage was similar to that of 3G. There was no difference in the air-jet-induced pressor response among three groups (rats reared in a usual 1G environment, rats reared in the low-roof cage, and rats reared in the 3G environment), suggesting that the sensitivity of the vestibulo-cardiovascular reflex was selectively suppressed. These results indicate that a reduction in the vestibular phasic input acts to attenuate the vestibulo-cardiovascular reflex.

Vestibular-mediated increase in central serotonin plays an important role in hypergravity-induced hypophagia in rats

Exposure to a hypergravity environment induces acute transient hypophagia, which is partially restored by a vestibular lesion (VL), suggesting that the vestibular system is involved in the afferent pathway of hypergravity-induced hypophagia. When rats were placed in a 3-G environment for 14 days, Fos-containing cells increased in the paraventricular hypothalamic nucleus, the central nucleus of the amygdala, the medial vestibular nucleus, the raphe nucleus, the nucleus of the solitary tract, and the area postrema. The increase in Fos expression was completely abolished or significantly suppressed by VL. Therefore, these regions may be critical for the initiation and integration of hypophagia. Because the vestibular nucleus contains serotonergic neurons and because serotonin (5-HT) is a key neurotransmitter in hypophagia, with possible involvement in motion sickness, we hypothesized that central 5-HT increases during hypergravity and induces hypophagia. To examine this proposition, the 5-HT concentrations in the cerebrospinal fluid were measured when rats were reared in a 3-G environment for 14 days. The 5-HT concentrations increased in the hypergravity environment, and these increases were completely abolished in rats with VL. Furthermore, a 5-HT(2A) antagonist (ketanserin) significantly reduced 3-G (120 min) load-induced Fos expression in the medial vestibular nucleus, and chronically administered ketanserin ameliorated hypergravity-induced hypophagia. These results indicate that hypergravity induces an increase in central 5-HT via the vestibular input and that this increase plays a significant role in hypergravity-induced hypophagia. The 5-HT(2A) receptor is involved in the signal transduction of hypergravity stress in the vestibular nucleus.

Galvanic vestibular stimulation counteracts hypergravity-induced plastic alteration of vestibulo-cardiovascular reflex in rats

Recent data from our laboratory demonstrated that, when rats are raised in a hypergravity environment, the sensitivity of the vestibulo-cardiovascular reflex decreases. In a hypergravity environment, static input to the vestibular system is increased; however, because of decreased daily activity, phasic input to the vestibular system may decrease. This decrease may induce use-dependent plasticity of the vestibulo-cardiovascular reflex. Accordingly, we hypothesized that galvanic vestibular stimulation (GVS) may compensate the decrease in phasic input to the vestibular system, thereby preserving the vestibulo-cardiovascular reflex. To examine this hypothesis, we measured horizontal and vertical movements of rats under 1-G or 3-G environments as an index of the phasic input to the vestibular system. We then raised rats in a 3-G environment with or without GVS for 6 days and measured the pressor response to linear acceleration to examine the sensitivity of the vestibulo-cardiovascular reflex. The horizontal and vertical movement of 3-G rats was significantly less than that of 1-G rats. The pressor response to forward acceleration was also significantly lower in 3-G rats (23 +/- 1 mmHg in 1-G rats vs. 12 +/- 1 mmHg in 3-G rats). The pressor response was preserved in 3-G rats with GVS (20 +/- 1 mmHg). GVS stimulated Fos expression in the medial vestibular nucleus. These results suggest that GVS stimulated vestibular primary neurons and prevent hypergravity-induced decrease in sensitivity of the vestibulo-cardiovascular reflex.

The vestibular system is integral in regulating plastic alterations in the pressor response to free drop mediated by the nonvestibular system

Microgravity resulting from free drop elicits a pressor response that involves both vestibular and nonvestibular pathways. In rats reared under a 3G environment for 2 weeks, plastic alterations in both vestibular- and nonvestibular-mediated responses are induced; specifically, the pressor responses involving both pathways are reduced [C. Abe, K. Tanaka, C. Awazu, H. Chen, H. Morita, Plastic alteration of vestibulo-cardiovascular reflex induced by 2 weeks of 3-G load in conscious rats, Exp. Brain Res. 181 (2007) 639-646]. It is currently unknown whether plastic alterations in the nonvestibular system depend on the vestibular system. To examine this topic, the pressor response to free drop was compared between rats with and without vestibular lesion (VL) reared under 1G or 3G environments. The pressor response to free drop was 34+/-3mmHg in vestibular intact rats reared under 1G, and was significantly attenuated in rats reared under a 3G environment for 2 weeks (13+/-3mmHg); however, the pressor response was similar between VL-1G (18+/-3mmHg) and VL-3G (19+/-3mmHg) rats. Therefore, the 3G environment induced plastic alterations in the pressor response to free drop mediated by both the vestibular and nonvestibular systems, and the vestibular system is indispensable for induction of the plastic alteration of the nonvestibular-meidated pressor response to free drop.

Impairment of vestibular-mediated cardiovascular response and motor coordination in rats born and reared under hypergravity

It is well known that environmental stimulation is important for the proper development of sensory function. The vestibular system senses gravitational acceleration and then alters cardiovascular and motor functions through reflex pathways. The development of vestibular-mediated cardiovascular and motor functions may depend on the gravitational environment present at birth and during subsequent growth. To examine this hypothesis, arterial pressure (AP) and renal sympathetic nerve activity (RSNA) were monitored during horizontal linear acceleration and performance in a motor coordination task in rats born and reared in 1-G or 2-G environments. Linear acceleration of +/-1 G increased AP and RSNA. These responses were attenuated in rats with a vestibular lesion, suggesting that the vestibular system mediated AP and RSNA responses. These responses were also attenuated in rats born in a 2-G environment. AP and RSNA responses were partially restored in these rats when the hypergravity load was removed, and the rats were maintained in a 1-G environment for 1 wk. The AP response to compressed air, which is mediated independently of the vestibular system, did not change in the 2-G environment. Motor coordination was also impaired in the 2-G environment and remained impaired even after 1 wk of unloading. These results indicate that hypergravity impaired both the vestibulo-cardiovascular reflex and motor coordination. The vestibulo-cardiovascular reflex was only impaired temporarily and partially recovered following 1 wk of unloading. In contrast, motor coordination did not return to normal in response to unloading.

Strong galvanic vestibular stimulation obscures arterial pressure response to gravitational change in conscious rats

Galvanic vestibular stimulation (GVS) is known to create an imbalance in the vestibular inputs; thus it is possible that the simultaneously applied GVS obscures adequate gravity-based inputs to the vestibular organs or modifies an input-output relationship of the vestibular system and then impairs the vestibular-mediated response. To examine this, arterial pressure (AP) response to gravitational change was examined in conscious rats with and without GVS. Free drop-induced microgravity and centrifugation-induced hypergravity were employed to elicit vestibular-mediated AP response. GVS itself induced pressor response in an intensity-dependent manner. This pressor response was completely abolished by vestibular lesion, suggesting that the GVS-induced response was mediated by the vestibular system. The pressor response to microgravity (35 +/- 3 mmHg) was significantly reduced by simultaneously applied GVS (19 +/- 1 mmHg), and pressor response to 3-G load was also significantly reduced by GVS. However, GVS had no effect on air jet-induced pressor response. The effects of GVS on pressor response to gravitational change were qualitatively and quantitatively similar to that caused by the vestibular lesion, effects of which were demonstrated in our previous studies (Gotoh TM, Fujiki N, Matsuda T, Gao S, Morita H. Am J Physiol Regul Integr Comp Physiol 286: R25-R30, 2004; Matsuda T, Gotoh TM, Tanaka K, Gao S, Morita H. Brain Res 1028: 140-147, 2004; Tanaka K, Gotoh TM, Awazu C, Morita H. Neurosci Lett 397: 40-43, 2006). These results indicate that GVS reduced the vestibular-mediated pressor response to gravitational change but has no effect on the non-vestibular-mediated pressor response. Thus GVS might be employed for the acute interruption of the AP response to gravitational change.