Evidence for vestibular regulation of autonomic functions in a mouse genetic model

Abstract

Physiological responses to changes in the gravitational field and body position, as well as symptoms of patients with anxiety-related disorders, have indicated an interrelationship between vestibular function and stress responses. However, the relative significance of cochlear and vestibular information in autonomic regulation remains unresolved because of the difficulties in distinguishing the relative contributions of other proprioceptive and interoceptive inputs, including vagal and somatic information. To investigate the role of cochlear and vestibular function in central and physiological responses, we have examined the effects of increased gravity in wild-type mice and mice lacking the POU homeodomain transcription factor Brn-3.1 (Brn-3bPou4f3). The only known phenotype of the Brn-3.1(-/-) mouse is related to hearing and balance functions, owing to the failure of cochlear and vestibular hair cells to differentiate properly. Here, we show that normal physiological responses to increased gravity (2G exposure), such as a dramatic drop in body temperature and concomitant circadian adjustment, were completely absent in Brn-3.1(-/-) mice. In line with the lack of autonomic responses, the massive increase in neuronal activity after 2G exposure normally detected in wild-type mice was virtually abolished in Brn-3.1(-/-) mice. Our results suggest that cochlear and vestibular hair cells are the primary regulators of autonomic responses to altered gravity and provide genetic evidence that these cells are sufficient to alter neural activity in regions involved in autonomic and neuroendocrine control.

Fig 1.

Changes in body temperature in response to altered gravitational fields. (A) Line plots during 5 days of the control 1G period demonstrating a similar mean daily body temperature and circadian rhythm between wild-type and Brn-3.1^−/− mice. (B) Line plots illustrating the changes after exposure to 2G in a wild-type and a Brn-3.1^−/− mouse. At 2G, the wild-type mouse exhibits an immediate and dramatic fall in mean daily body temperature from 36.5°C down to 30°C, and a gradual recovery. In addition, there is a loss of circadian rhythm amplitude that lasts 5–6 days. The Brn-3.1^−/− mouse shows no immediate drop in mean body temperature after 2G exposure and only a slight attenuation of circadian rhythm amplitude. The arrowheads in B show the time point where the gravity conditions were altered.

Fig 2.

Line graphs of mean daily and circadian amplitude summarizing the results of all wild-type and Brn-3.1^−/− mice in each gravitational condition (1G control period, early 2G, late 2G, and 1G recovery). The bars represent the standard error. Wild-type and Brn-3.1^−/− mice exhibit equivalent mean and circadian amplitude during the 1G baseline. During the early exposure to 2G, the wild-type mice show a significant reduction in mean daily amplitude and a loss of circadian amplitude. However, Brn-3.1^−/− mice did not exhibit a significant change in mean daily or circadian amplitude immediately after 2G exposure. No differences were found between wild-type and Brn-3.1^−/− mice after 2 weeks of chronic 2G or during the 1G recovery.

Fig 3.

Lack of c-Fos induction in the vestibular and autonomic nuclei of Brn-3.1^−/− mice after exposure to hypergravity. Shown are low-power micrographs of transverse sections from wild-type and Brn-3.1^−/− mice demonstrating the differences in c-Fos staining between the experimental groups. At 1G, no significant c-Fos labeling is seen in any brain region (A, D, and G), whereas after 2 h of 2G exposure, massive c-Fos labeling is shown in the spinal vestibular nucleus (B), specific subnuclei of the parabrachial nucleus (E), and the central nucleus of the amygdala (H). Very little c-Fos staining was seen in these regions in Brn-3.1^−/− mice after 2G exposure (C, F, and I). MVN, medial vestibular nucleus; DL, dorsal lateral parabrachial nucleus; LCA, lateral crescent area; CNA, central nucleus of amygdala. (Scale bar, 200 μm.)

Fig 4.

High-power micrographs demonstrating the nuclear localization and density of the c-Fos labeling in the paraventricular hypothalamic nucleus (A–C) and the locus coeruleus (D–F) of wild-type (A, B, D, and E) and Brn-3.1^−/− (C and F) mice after 1G (A and D) and 2G (B, C, E, and F) exposure. PVHpc, PVH–parvocellular; LC, locus coeruleus. (Scale bar, 100 μm.)