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The action of serotonin in the rat hippocampal slice preparation.

1. Intracellular activity was recorded from neurones in the CA1 pyramidal layer of slices of rat hippocampus maintained in vitro. 2. Application of 5‐HT in a droplet or via ionophoresis produced a 3‐5 mV hyperpolarization associated with a 30% decrease in input resistance. 3. The response to 5‐HT was minimal with a drop concentration of 1 microM and maximal with 100 microM. The responses appeared to be blocked by methysergide applied in the superfusion medium. 4. The responses to 5‐HT were minimal when the drug was applied in the apical dendritic region and maximal when it was applied near the soma. 5. 5‐HT produced no substantial changes in e.p.s.p.s evoked by stimulation of the Schaffer collateral‐commissural system or in i.p.s.p.s which were occasionally encountered following stimuli to the stratum radiatum. 6. The responses to 5‐HT are true post‐synaptic responses and are not indirect effects since they are present in a Ca2+‐deficient Mg2+‐enriched medium which blocks synaptic transmission. 7. The responses to 5‐HT were not dependent on extracellular Cl‐ concentration. 8. These experiments indicate that 5‐HT produces its effects in the rat hippocampus by activating K+ channels.