Abstract
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.
Highlights
The peripheral gravity sensor, located in the inner ear, consists of two components, namely, the semicircular canals and otolith organs, which detect angular and linear accelerations, respectively
Excitation of the CAMK2-expressing neurons in the vestibular nuclear complex (VNC) induced body tilt to the contralateral side, while the opposite response was observed by neural inhibition (Fig. 2a and Video S1)
The major findings of the present study are as follows: (1) CAMK2-expressing neurons in the VNC are involved in the balance function; (2) both excitation and inhibition of the CAMK2-expressing neurons in the VNC resulted in an increase in renal sympathetic nerve activity (RSNA) followed by the pressor response; (3) sympathetic and cardiovascular responses induced by the photostimulation involve the monosynaptic neural projection from the VNC to the rostral ventrolateral medulla (RVLM); and (4) exposure to hypergravity environment-induced attenuation of the vestibulo-sympathetic reflex is due at least in part to a decrease in responsiveness of CAMK2-expressing neurons in the VNC
Summary
The peripheral gravity sensor, located in the inner ear, consists of two components, namely, the semicircular canals and otolith organs, which detect angular and linear accelerations, respectively. The vestibular system is one of the plastic organs which is affected by chronic gravitational changes [14]. We demonstrated that these plastic alterations in the vestibulo-sympathetic reflex occur in rats exposed to hypergravity [5, 18, 19]. When rats were maintained in a hypergravity environment, the sympathetic and cardiovascular responses induced by vestibular stimulation, including free drop and linear acceleration, were attenuated. We clarified that attenuation of the vestibulo-sympathetic reflex is due to a decrease in head movement-related phasic inputs to the peripheral vestibular
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