EFFECTS OF VESTIBULAR STIMULATION ON ACETYLCHOLINE-RELEASE FROM RAT HIPPOCAMPUS - AN IN-VIVO MICRODIALYSIS STUDY

1. The effects of electrical stimulation to the round window of the inner ear and caloric vestibular stimulation on the in vivo release of acetylcholine (ACh) from rat hippocampus were investigated, using brain microdialysis coupled with high performance liquid chromatography-electrochemical detection. 2. Hippocampal ACh release was increased to 152% of the basal release by 1-Hz, 200-ms, and 500-mu A electrical stimulation to the round window. This effect was dependent on current intensity. 3. Microinjection of 6,7-dinitroquinoxaline-2,3-dione (DNQX) (10 nmol/0.2 mu l), an antagonist of the non-N-methyl-D-aspartate (non-NMDA) type of glutamate receptor, into the medial vestibular nucleus blocked the increase of hippocampal ACh release by electrical stimulation to the round window. In contrast, microinjection of DNQX ( 10 nmol/0.2 mu l) into the ventral cochlear nucleus did not inhibit the increase of hippocampal ACh release by electrical stimulation to the round window. 4. Electrical stimulation of the round window increased histamine release from the medial septum, which is the origin of the hippocampal cholinergic terminals. However, the depletion of neuronal histamine by alpha-fluoromethylhistidine (100 mg/kg ip) did not suppress the vestibular-evoked release of hippocampal ACh. 5. Although caloric stimulation with water at 37 degrees C did not affect the hippocampal ACh release, the release was increased to 131% of the basal release by caloric stimulation with hot water at 45 degrees C and was also increased to 160% of the basal release by caloric stimulation with ice water, suggesting that the changes in activities of vestibular afferents increased the hippocampal ACh release. 6. These findings suggest that the vestibular information activated the hippocampal cholinergic neuron system independently of the histaminergic system. The septo-hippocampal cholinergic system may be involved in the central processing of vestibular information.