Regional differences in neuronal responses to 5-HT: intracellular studies in hippocampal slices.

1. Intracellular recordings were obtained from cells in the regions CA1, CA3 and the dentate gyrus (DG) of rat hippocampal slices. 2. Topical application of 5-HT induced a 2-10 mV hyperpolarization in all cells tested. 3. The hyperpolarization was accompanied by a marked reduction in input resistance in CA1 and DG cells, but by a much smaller resistance change in CA3 cells. 4. These effects of 5-HT were not abolished by tetrodotoxin. 5. It is suggested that 5-HT causes an increase in K+ conductance in regions CA1 and DG and that this results in hyperpolarization. A different mechanism might be activated by 5-HT in cells of the CA3 region.