Non-rhythmical hippocampal units, theta rhythm and afferent stimulation

Abstract

Physostigmine induced theta (.theta.) rhythm and unit activity were recorded from the 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 .theta. 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 .theta. 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. In the presence of .theta. about 1/2 of the Type 2 cells revealed a degree of phase-locking with the rhythm. This finding suggests a causal relationship between both phenomena. Cross-correlograms between Type 1 and 2 discharges, when positive, showed a symmetrical periodicity suggesting that Type 2 cells might function as a hippocampal output. The activity of Type 2 pairs, positively cross-correlated in nearly 2/3 of the cases, revealed excitatory interactions. Hippocampal afferent stimulation reset .theta. activity in phase, modified the temporal relationships between cells and with the .theta. rhythm, and changed the Type 2 discharge pattern. .theta.-Related Type 2 cells carry information representing .theta. rhythmicity. Phase-relationships between Type 2 cells and .theta.; and their modifications by hippocampal afferent activity may be necessary to establish time-relationships with other brain structures.