Chihiro Awazu

Vestibular system plays a significant role in arterial pressure control during head-up tilt in young subjects.

We recently demonstrated that galvanic vestibular stimulation (GVS) obscured the vestibulo-cardiovascular reflex in rats during gravitational change. Here, we used GVS to examine the role of the vestibular-mediated arterial pressure (AP) control during a 60 degrees head-up tilt (HUT) in young and aged subjects (19-24 years old, 12 males and 3 females for young subjects, 63-91 years old, 5 males and 5 females for aged subjects). In young subjects, the AP did not change during posture transition from the supine position to HUT when GVS was not applied. When GVS was applied, AP immediately and significantly decreased by 17+/-2 mmHg upon HUT. When they were exposed to lower-body negative pressure (LBNP) in the supine position, the degree of footward fluid shift induced was the same as that induced by HUT. LBNP elicited only a footward fluid shift without alteration of vestibular input, while HUT elicited both. LBNP decreased the AP significantly, and the decrease was similar to that observed in the HUT with GVS. Thus, GVS modified the AP responses in young subjects during HUT but not during LBNP. In contrast, in aged subjects, the AP decreased during HUT regardless of whether GVS was applied. The vestibular system plays an important role in initial AP control during posture transition in young subjects. However, this function might be impaired in aged subjects.

Roles of the vestibular system in controlling arterial pressure in conscious rats during a short period of microgravity

In order to evaluate the roles of the vestibular system in controlling arterial pressure (AP) during exposure to a short period of microgravity (microG), the AP was measured in conscious free-moving rats having intact vestibular systems and those having vestibular lesions (FM-Intact and FM-VL groups, respectively). During free drop-induced microG, the AP increased in the FM-Intact group; it was 38+/-4 mmHg more than the AP observed during 1G. However, the increase in AP was significantly lower in the FM-VL group (20+/-2 mmHg). Further, to examine the sudden effect of a body floating in the midair in response to the AP during exposure to muG a body stabilizer was placed on the back of rats having intact vestibular systems and those having vestibular lesions (STAB-Intact and STAB-VL groups, respectively). The increase in the AP was significantly depressed in the STAB-Intact group; when compared with that in the FM-Intact group, but the increase was still significant (27+/-2 mmHg). On the other hand, the increase in the AP was completely eliminated in the STAB-VL group (7+/-5 mmHg). These results indicate that the AP increases during exposure to muG in conscious rats, and the vestibular system and body stability are significantly involved in this response.

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.

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.