In vivo intracellular analysis of rat dentate granule cells.

In vivo intracellular recordings and dentate EEG were obtained in urethanized-curarized rats. Granule cells (GCs) were identified by antidromic activation as well as by intracellular staining with Lucifer yellow (LY). GCs fired spikes which, in 43.6% of the cases, had brief post-hyperpolarization. Slow spikes were recorded at hyperpolarized levels during the rebound of hyperpolarizing pulses or during inhibitory postsynaptic potentials (IPSPs). Medial septal nucleus or perforant pathway stimulation evoked an excitatory postsynaptic potential (EPSP)-IPSP sequence. During theta (theta) rhythm, the membrane potential of 80.3% of the GCs showed rhythmic sine-like waves of up to 15 mV at a theta frequency which were phase-locked with extracellular theta. GCs were classified into 3 types: type 1 (67.1%) showed intracellular theta and rhythmic firing; type 2 (13.2%) revealed intracellular theta and random firing, but spikes tended to occur at a preferred phase of the dentate theta; and type 3 (19.7%) had neither intracellular theta nor rhythmic firing. Intracellular theta amplitude was wider during injection of the hyperpolarizing current and narrower during depolarizing ones, indicating that rhythmic EPSPs contribute to theta genesis. Intracellular theta was unaffected by Cl- or Cs+ diffusion, suggesting that IPSP is not essential to theta genesis.