PRESYNAPTIC INHIBITORY SEROTONIN AUTORECEPTORS IN THE HUMAN HIPPOCAMPUS

Hippocampal slices were prepared from brain tissue of patients undergo ing neurosurgery for epilepsy. The slices were incubated with H-3-sero tonin (H-3-5-HT) and then superfused with physiological salt solution containing 6-nitroquipazine. Tritium overflow was evoked either electr ically (3 Hz) or by K+ 25 mM. The electrically evoked overflow of trit ium was almost abolished by tetrodotoxin or by omission of Ca2+ ions. 5-HT 0.1 and 1 mu M reduced the evoked overflow by 38 and 55%, respect ively. The effect of 5-HT 1 mu M was abolished by the 5-HT1/2 receptor antagonist methiothepin 1 mu M, which, by itself, increased the evoke d overflow by 59%. Tritium overflow evoked by high K+ in slices superf used with medium containing tetrodotoxin was reduced by 5-HT 1 mu M by 49%. This effect was markedly attenuated by methiothepin 0.32 mu M, w hich, by itself, tended to increase the evoked overflow. The results s how that the serotoninergic neurones of the human hippocampus are endo wed with presynaptic inhibitory autoreceptors.