Effect of unilateral vestibular stimulation on histamine release from the hypothalamus of rats in vivo.

1. We investigated the effect of unilateral vestibular stimulation on histamine release from the anterior hypothalamic area of urethan-anesthetized rats in vivo, using a brain microdialysis method coupled with high-performance liquid chromatography fluorometry. 2. The histamine release was increased to approximately 180% of the basal release by the electrical stimulation of the inner ear with 1 Hz, 500 microA, and 200 ms for 20 min. This effect was dependent on the current intensity. 3. Activation of the unilateral horizontal semicircular canal by middle ear irrigation for 15 min with 45 degrees C water increased the histamine release to approximately 200% of the basal release. 4. Irrigation of the middle ear with ice water for 15 min increased the histamine release to approximately 190% of the basal release. 5. The histamine release was not changed by the irrigation of the middle ear with 37 degrees C water and the irrigation of the auricle with ice water, which suggests that neither somatosensory stimulation to the middle ear nor nonspecific cold stress affects the histamine release. 6. All these findings suggest that the sensory mismatch signals induced by caloric stimulation and unilateral electrical vestibular stimulation activate the histaminergic neuron system in the brain.