High frequency (200 Hz) oscillations and firing patterns in the basolateral amygdala and dorsal endopiriform nucleus of the behaving rat

The known repertoire of rhythms in the amygdala and paleocortex includes a range of oscillations from slow waves (<1 Hz) to fast gamma (40-100 Hz). In the present report, we show approximately 200 Hz oscillations in the basolateral nucleus of the amygdala (BL) and the adjacent dorsal endopiriform nucleus (EPN) of the behaving rat. Microwire techniques were applied for recording single units and field activity from these structures and EEG from the dorsal or temporal CA1 subfields of the hippocampus. Units from both EPN and BL exhibited similar irregular firing patterns with bursts. The mean firing rates in EPN were <1 Hz, whereas units in the BL fired in a range of <1-17 Hz. Neuronal activity in both BL and EPN was phase-locked with high-frequency field oscillations (HFO, approximately 200 Hz). Amygdaloid/EPN HFO displayed on average lower numbers of cycles and smaller amplitudes than hippocampal ripples. Neuronal firing and HFO in the BL and EPN were state dependent with a maximal occurrence during slow-wave sleep (SWS), being lower during waking and paradoxical sleep. Cross-correlation between hippocampal ripples and EPN or BL units and field HFO did not reveal any synchrony. These data suggest common principles of temporal coding in BL and EPN in certain behavioural states via short scale population synchrony though they convey signals of different modalities.