NeuroscienceDec 21, 2013

Brain cell activity in the body’s internal clock and nearby areas during sleep and wakefulness in mice

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Apr 5, 2026

Abstract

A total of 79 suprachiasmatic nucleus (SCN) neurons were identified and categorized into three distinct types based on their discharge activity during the wake-sleep cycle.

Type I neurons exhibited regular discharge activity and were primarily located in the ventral SCN, while type II neurons displayed irregular activity and were found in the dorsal SCN.
Type III neurons, which were mainly located in the lateral SCN, showed quiescent activity during waking and irregular discharge during sleep.
The type I and II neurons exhibited long-duration action potentials, whereas type III neurons displayed either short-duration or long-duration action potentials.
Type I neurons discharged as single isolated spikes, while type II and III neurons could fire as single spikes, clusters, or bursts.
Most type I and II neurons demonstrated an increased discharge rate in response to light exposure, indicating their potential role in light-mediated signaling.
The study suggests a heterogeneous population of neurons in the SCN and surrounding regions, implying varying functions in regulating circadian rhythms and sleep-wake cycles.

Simplified

The suprachiasmatic nucleus (SCN) of the mammalian hypothalamus contains a circadian clock for timing of diverse neuronal, endocrine, and behavioral rhythms, such as the cycle of sleep and wakefulness. Using extracellular single unit recordings, we have determined, for the first time, the discharge activity of individual SCN neurons during the complete wake-sleep cycle in non-anesthetized, head restrained mice. SCN neurons (n=79) were divided into three types according to their regular (type I; n=38) or irregular (type II; n=19) discharge activity throughout the wake-sleep cycle or their quiescent activity during waking and irregular discharge activity during sleep (type III; n=22). The type I and II neurons displayed a long-duration action potential, while the type III neurons displayed either a short-duration or long-duration action potential. The type I neurons discharged exclusively as single isolated spikes, whereas the type II and III neurons fired as single isolated spikes, clusters, or bursts. The type I and II neurons showed wake-active, wake/paradoxical (or rapid eye movement) sleep-active, or state-unrelated activity profiles and were, respectively, mainly located in the ventral or dorsal region of the SCN. In contrast, the type III neurons displayed sleep-active discharge profiles and were mainly located in the lateral region of the SCN. The majority of type I and II neurons tested showed an increase in discharge rate following application of light to the animal's eyes. Of the 289 extra-SCN neurons recorded, those displaying sleep-active discharge profiles were mainly located dorsal to the SCN, whereas those displaying wake-active discharge profiles were mainly located lateral or dorsolateral to the SCN. This study shows heterogeneity of mouse SCN and surrounding anterior hypothalamic neurons and suggests differences in their topographic organization and roles in mammalian circadian rhythms and the regulation of sleep and wakefulness.

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Brain cell activity in the body’s internal clock and nearby areas during sleep and wakefulness in mice

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