The action of 5-hydroxytryptamine on neuronal membranes and synaptic transmission in area CA1 of the hippocampus in vitro.

Cell membrane properties and synaptic transmission were examined by intracellular recording from the CA1 region of hippocampal slices during application of 5-hydroxytryptamine (5-HT). Seventeen cells (50%) showed an inhibitory response to 5-HT. The cells were hyperpolarized with decreased spontaneous activity and increased membrane conductance. The increase may be in potassium or chloride conductance. Synaptic transmission was not involved since Mg2+, Co2+ or Mn2+ did not block the response. 5-HT inhibited dendritic EPSPs by local postsynaptic effect. Four cells (11%) showed an excitatory response. They were depolarized while the spontaneous activity increased and the conductance decreased. The decrease may be in the potassium or chloride conductance. Two cells (6%) showed a mixed type of response with an early excitation and late inhibition. This respons may be a mixture of the two other responses or associated with a third type of 5-HT receptor. Eleven cells (33%) did not respond to 5-HT, but only in 3 of these 5-HT was applied in the soma region. In conclusion, 5-HT may inhibit dendritic excitatory synapses and decrease recurrent inhibition in hippocampus.