SEROTONIN AND 8-OH-DPAT REDUCE EXCITATORY TRANSMISSION IN RAT HIPPOCAMPAL AREA CA1 VIA REDUCTION IN PRESUMED PRESYNAPTIC CA2+ ENTRY

The effect of 5-HT and its 1A receptor agonist 8-hydroxy-2-(di-n-propy lamino)tetralin (8-OH-DPAT) on excitatory transmission in CA1 pyramida l cells was studied. Using concentrations of 5-HT within a range of 10 -50 mu M we observed no change in excitatory postsynaptic potentials ( EPSPs) in CA1 cells evoked by Schaffer collateral stimulation. However , at higher concentrations, greater than or equal to 100 mu M, 5-HT ca used a significant decrease (30-40%) in EPSP/Cs, an effect that was al so mimicked by 50 mu M 8-OH-DPAT. A presumed presynaptic Ca2+ entry wa s measured in stratum radiatum following repetitive stimulation of the Schaffer collaterals with all excitatory synaptic transmission blocke d. Both 5-HT and 8-OH-DPAT reduced this Ca2+ entry. These results sugg est that 5-HT acts at presynaptic 5-HT1A receptors to reduce Ca2+ entr y and thereby glutamatergic synaptic transmission.