Projection neurons of the lateral amygdaloid nucleus are virtually silent throughout the sleep--waking cycle.

1. Amygdala neurons were recorded extracellularly during the sleep-waking cycle in chronically implanted cats. Neurons were identified as projection cells when they could be antidromically invaded from the perirhinal and/or entorhinal cortices. 2. In contrast with other nuclei of the amygdala, few spontaneously active neurons were encountered in the lateral nucleus. However, when hunting stimuli were applied to the parahippocampal cortices, we noticed the presence of numerous projection cells that would have otherwise remained undetected because they had little or no spontaneous activity. 3. In the states of waking, slow-wave sleep, and paradoxical sleep, the discharge rate of antidromically invaded neurons averaged 0.09 +/- 0.07 Hz (mean +/- SE) with 82% of cells firing at < 0.01 Hz in all states. However, they transiently increased their firing rate when cats were presented complex sensory stimuli, which apparently were specific to each cell. In contrast to projection cells, spontaneously active neurons of the lateral nucleus that could not be backfired from the parahippocampal cortices had an average firing rate of 4.34 +/- 1.15 Hz with 38% of cells firing at > or = 6 Hz in at least one state. 4. These results on the extremely low firing rates of identified projection cells suggest that previous extracellular studies of lateral amygdaloid neurons were biased toward a class of spontaneously active cells which probably includes local-circuit cells.