Sleep-waking discharge of basal forebrain projection neurons in cats

We have previously described a population of neurons in the magnocellular basal forebrain which have selectively elevated discharge rates during slow-wave sleep compared to waking; we postulate that these sleep-active neurons are a component of a basal forebrain sleep-promoting system. The purpose of the present experiment was to determine if sleep-active neurons contribute axons to recently described basal forebrain projection pathways. In cats prepared for chronic single unit and EEG-sleep recordings, stimulating electrodes were placed in the mesencephalic reticular formation, and the external capsule and anterior cingulate bundle, fiber bundles known to contain axons of basal forebrain projection neurons. Fifty-nine neurons were antidromically driven; differences in antidromic response latencies were related to sleep-waking discharge profiles. Of the cells with short antidromic latencies (less than 5 msec), the majority (9 of 12) had high discharge rates during waking and low rates during slow-wave sleep. Cells with long antidromic latencies had either very low discharge rates (less than 1 spike/sec) across all states, or had elevated discharge rates in slow-wave sleep. Sleep-active neurons were antidromically driven from external capsule (n = 9), anterior cingulate bundle (n = 9), or mesencephalic reticular formation (n = 5). Projection sleep-active neurons were recorded in the substantia innominata, ventral to the globus pallidus and medial to the central nucleus of the amygdala. Our study found that identified basal forebrain projection neurons in cats exhibit a variety of sleep-waking discharge patterns and conduction velocities. Sleep-active neurons were found to have slowly conducting axons, and to be a source of both ascending and descending projections.