The action of serotonin in the rat hippocampal slice preparation.

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Abstract
Intracellular activity was recorded from neurons in the CA1 pyramidal layer of slices of rat hippocampus maintained in vitro. Application of 5-HT [serotonin] in a droplet or via ionophoresis produced a 3-5 mV hyperpolarization associated with a 30% decrease in input resistance. The response to 5-HT was minimal with a drop concentration of 1 .mu.M and maximal with 100 .mu.M. The responses appeared to be blocked by methysergide applied in the superfusion medium. 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-HT produced no substantial changes in excitatory p.s.p.s. [postsynaptic potentials] evoked by stimulation of the Schaffer collateral-commissural system or in inhibitory p.s.p.s. which were occasionally encountered following stimuli to the stratum radiatum. 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. The responses to 5-HT were not dependent on extracellular Cl- concentration. 5-HT apparently produces its effects in the rat hippocampus by activating K+ channels.