Anterior thalamic unit discharge profiles and coherence with hippocampal theta rhythm

The anterior thalamus (ATh) is a key structure of the limbic system and serves a direct role in spatial memory. We examined the discharge properties of neurons of the anterior thalamus during states of the hippocampal electroencephalogram (theta and non-theta states). Units were recorded in the anteroventral (AV, n=96), the anterodorsal (AD, n=44) and the anteromedial (AM, n=48) nuclei of the thalamus. The majority of theta-related cells fired at higher rates in the presence than absence of theta (theta-on cells); while a small percentage (∼13%) discharge at reduced rates with theta (theta-off cells). Theta-off cells were found in AD and AM but not in AV. Mean discharge rates for theta-on cells during control and theta conditions were 6.0±0.52 and 14.48±0.96 Hz for AV cells; 4.43±1.25 and 10.05±1.28 Hz for AD cells, and 2.60±0.3 and 6.42±0.9 Hz for AM cells. Approximately 40% of AV cells, 21.9% of AD units, and 5.7% of AM cells discharged rhythmically, synchronous with the theta rhythm. A subpopulation of ATh cells fired slightly rhythmicity, but with activity strongly phase-locked to EEG oscillations in the crosscorrelogram, indicating a modulation at theta frequency. Cells were classified as: rhythmic (R), non-rhythmic (N), and intermediate (I) based on quantitative criteria. About 75% of theta-on cells (i.e. R and I cells) showed significant coherence with theta. These cells were distributed throughout the extent of the anterior thalamus. The present findings of theta rhythmic cells in the anterior thalamus, together with previous demonstrations of ‘theta’ cells in other structures of Papez’s circuit, suggests that a theta rhythmic signal may reverberate throughout the circuit, possibly involved in memory processing functions of this limbic network.