Neuronal sources of theta rhythm in the entorhinal cortex of the rat. II. Phase relations between unit discharges and theta field potentials.

The discharge patterns and layer distribution of entorhinal cortex (EC) units were investigated in paralysed and locally anesthetized rats injected with physostigmine in order to induce theta (theta) rhythm. Entorhinal unit activity and field potentials were recorded simultaneously with the same micropipette. Hippocampal CA1 theta rhythm was used as reference. Statistical analysis included auto- and cross-correlations and interval histograms. Results showed: a. the existence of rhythmic and non-rhythmic cells, both tending to fire in a constant phase relationship with theta rhythm; b. in all EC subdivisions, most rhythmic cells were located in superficial cell layers (II-III); c. on the average, rhythmic cells from the medial EC fired synchronously; d. non-rhythmic cells tended also to fire synchronously but with an opposite phase relationship with respect to rhythmic neurons. Although a complex organization in the rhythmicity of EC units is revealed, it is concluded that the neuronal sources of theta activity in the EC are located in superficial cell layers, and it is strongly suggested that the EC output through the perforant path may rhythmically modulate the discharge pattern of hippocampal pyramidal and dentate granule cells.