Non-rhythmical hippocampal units, theta rhythm and afferent stimulation

Physostigmine induced theta (θ) rhythm and unit activity were recorded from dorsal hippocampus in immobilized locally anesthetized rats. Two functionally different types of cells were identified, Type 1 which fired in rhythmical bursts phase-locked with γ cycles, and Type 2 which were non-rhythmical. Most of the units (80%) were Type 2 and either had bursting (40%) or non-bursting discharge patterns. Correlations between the θ rhythm and Type 2, Type 1 and Type 2, and between pairs of Type 2 cells were studied. Modifications of correlations by afferent stimulation were also analyzed. The principal findings was that in the presence of θ about half of the Type 2 cells revealed a degree of phase-locking with the rhythm. This finding suggests a caused relationship between both phenomena. Crosscorrelograms between Type 1 and 2 discharges, when positive, showed a symmetrical periodicity suggesting that Type 2 cells might function as an hippocampal output. The activity of Type 2 pairs, positively crossconelated in nearly two thirds of the cases, revealed excitatory interactions. Hippocampal afferent stimulation reset θ activity in phase, modified the temporal relationships between cells and with the θ rhythm, and changed the Type 2 discharge pattern. The above results indicate that θ-related Type 2 cells carry information representing θ rhythmicity. Phase-relationships between Type 2 cells and θ, and their modifications by hippocampal afferent activity may be necessary to establish time-relationships with other brain structures.