Histaminergic modulation of hippocampal acetylcholine release in vivo.

In order to elucidate the modulatory role of the histaminergic neural system in the cholinergic neural system, the acetylcholine release from the CA1-CA3 region in the hippocampus of anesthetized rats was studied by an in vivo microdialysis method coupled with HPLC-electrochemical detection. The mean value for the basal acetylcholine release was 0.98 +/- 0.04 pmol/20 min. The acetylcholine release was increased to 172% of the basal level when an electrical stimulation at 200 microA was applied to the tuberomammillary nucleus. An administration of alpha-fluoromethylhistidine (100 mg/kg i.p.) blocked the electrically evoked release of histamine both from the septal-diagonal band complex and the hippocampus, and abolished the electrically evoked release of acetylcholine from the hippocampus. Zolantidine (5 mg/kg i.p.) attenuated the increase in the electrically stimulated acetylcholine release, but pyrilamine (5 mg/kg i.p.) did not attenuate the increase in the acetylcholine release. These drugs showed no significant effect on the basal acetylcholine release. An administration of (R)-alpha-methylhistamine (5 mg/kg i.p.) caused a decrease in the acetylcholine release to 48.7% of the basal level, whereas thioperamide (5 mg/kg i.p.) caused an increase in the acetylcholine release 60 min after the injection. These results suggest that the histaminergic system may contribute to the modulation of the activity of the septohippocampal cholinergic system, mainly through H2 receptors.