Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Mar 13, 2020Physiological reviews

Ion Channels That Regulate Daily Rhythms in the Brain’s Biological Clock System

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Abstract

The suprachiasmatic nucleus (SCN) in mammals regulates circadian rhythms through changes in neuronal excitability, with higher firing rates during the day and lower rates at night.

SCN neurons exhibit periodic changes in excitability that correspond to the day-night cycle.
During daytime, increased sodium and calcium currents enhance neuronal firing frequency.
At night, elevated potassium currents contribute to decreased neuronal firing and hyperpolarization.
Multiple neuromodulators are involved in modulating action potential activity and rhythmic patterns in SCN neurons.
These mechanisms support the synchronization and adaptability of the circadian timekeeping system.

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Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components:) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Naand Cacurrents, as well as several Kcurrents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different Kcurrents, including Ca-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code. 1 2 + 2+ + + 2+

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Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

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