Synaptic contributions to theta rhythm genesis in rat CA1-CA3 hippocampal pyramidal neurons in vivo.

The effects of intracellular Cl- diffusion and hyperpolarizing current pulses on inhibitory postsynaptic potentials (IPSPs) and the transmembrane theta rhythm of CA1-CA3 pyramidal neurons were tested in urethanized and curarized rats. Cl- diffusion and hyperpolarizing currents decreased the amplitude of IPSPs evoked by fornix stimulation without modifying the theta rhythm amplitude and phase. The membrane conductance was typically 22-46% higher at the positive than negative intracellular theta peaks. Results indicate that in curarized rats excitatory postsynaptic potentials were the main components of intracellular theta without an important participation of IPSPs in theta rhythm genesis.