Modulation of vestibular function and receptor expression by experimental hypergravity in a rat model

Motion sickness develops because of a mismatch or conflict between the information gathered from the visual input, somatosensory input, and the vestibular system. Altered gravity can cause motion sickness while traveling in space. In this study, we aimed to establish a rat model of hypergravity-stimulated motion sickness using hypergravity equipment and investigate changes in serotonin and histamine receptors. Overall, 21 rats were allocated to three experimental groups (4G-24 h, 4G-2 weeks, and 4G-4weeks) and one control group, with seven rats in each group. We performed western blotting and immunohistochemistry analysis to determine the protein levels and expression, respectively, of serotonin and histamine receptors in the vestibular nuclei (VN). Rats in the hypergravity group showed a significant reduction in vestibulo ocular reflex (VOR) gain compared to those in the control group. Decreased VOR gains were restored to the normal range on 3-4 days after stopping the hypergravity stimulation. Hypergravity stimulation significantly increased the expression levels of 5-HT1B and 5-HT2A receptors in the VN. Furthermore, histamine receptors (H1 and H2) were significantly increased in the VN after hypergravity stimulation. These findings indicate that altered gravity influences the vestibular system by inducing both functional behavioral changes and molecular alterations in receptor expression.