Biosciences Division, SRI International, Menlo Park, CA 94025, USA.
The presence of large-amplitude, slow waves in the EEG is a primary characteristic that distinguishes cerebral activity during sleep from that which occurs during wakefulness. Although sleep-active neurons have been identified in other brain areas, neurons that are specifically activated during slow-wave sleep have not previously been described in the cerebral cortex. We have identified a population of cells in the cortex that is activated during sleep in three mammalian species. These cortical neurons are a subset of GABAergic interneurons that express neuronal NOS (nNOS). Because Fos expression in these sleep-active, nNOS-immunoreactive (nNOS-ir) neurons parallels changes in the intensity of slow-wave activity in the EEG, and these neurons are innvervated by neurotransmitter systems previously implicated in sleep/wake control, cortical nNOS-ir neurons may be part of the neurobiological substrate that underlies homeostatic sleep regulation.