In vivo intracellular correlates of hippocampal formation theta-on and theta-off cells.

Using urethane-anesthetized rat, intracellular recordings were made in hippocampal formation cells classified according to previously established criteria as either theta-on or theta-off, in order to further define the electrophysiological characteristics of these cells. Four cells classified as phasic theta-off cells had short duration spikes (less than 1 ms), high input resistances (54-61 M omega) and large fast afterhyperpolarizations (6-10 mV), thus sharing some of the properties of identified hippocampal interneurons. Phasic theta-off cells also exhibited rhythmic membrane potential oscillations (MPOs) ranging from 4 to 10 mV in amplitude during the simultaneous occurrence of extracellular theta field activity, but not during the occurrence of large amplitude irregular field activity (LIA). The MPOs of phasic theta-off cells were the same frequency as and were highly coherent with the extracellular theta field activity. In all four phasic theta-off cells the positive peak of the MPO was in phase with the positive peak of the local theta field activity. At the onset of extracellular theta field activity above 4-5 Hz, the membrane potentials of phasic theta-off cells showed a 5-10-mV hyperpolarizing shift, accompanied by MPOs without spike discharges. As theta frequency slowed down there was a return to baseline membrane potential levels and spike discharges occurred near the positive peak of the MPOs. The seven cells classified as phasic theta-on had longer duration spikes (greater than 1 ms), lower input resistances (22-36 M omega) and small (approx. 1.0 mV) fast afterhyperpolarizations, thus sharing some of the properties of hippocampal projection cells.(ABSTRACT TRUNCATED AT 250 WORDS)