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.