Surprising Silence from the SCN

The firing of neurons within the suprachiasmatic nuclei (SCN) of the brain is considered the driving force for controlling the circadian rhythms through out the mammalian body. For many years it was believed that a simple pattern of rapid firing from SCN neurons during the day and lower rates at night encoded the body's physiological response to night and day, and subsequent sleep patterns. In 2009 two electorphysiologists from Manchester University, Mino Belle and Hugh Piggins, collaborated with Mathematicians from the University of Michigan to explore in more detail the unusual electrophysiological activity of SCN neurons.

Forger and Diekman from the University of Michigan created a mathematical model that suggested bursts of electrical activity at dawn and dusk, with a peculiar "depolarized rest state" during the day. This theory was met with initial skepticism until researchers at Manchester University looked more closely at the sub-types of neurons in the SCN. They distinguished between two distinct, and very different types of cells in the SCN; "clock cells" expressing Per-1 neurons and non-Per-1 cells. They made whole-cell recordings using the PatchStar micromanipulator to measure electrical activity through-out the day from both types of cells in the SCN. Previous experiments did not make the distinction between these two cells types, and therefore showed misleading patterns of firing activity across the day and at night.

The team found that action potential (AP) firing in the Per-1 neuron's, the time keeping cells, matched very closely the pattern of activity predicted by Forger's team; the neurons remained in a state of almost silent depolarization throughout most of the day with very little membrane activity. There were peaks of AP discharge at dawn and dusk which indicates a more complex mechanism for controlling circadian rhythm. These surprising findings highlight the benefits of cross-discipline collaboration and raises importance questions to be addressed in future research.

Science 9 October 2009:

Vol. 326 no. 5950 pp. 281-284

DOI: 10.1126/science.1169657

Daily Electrical Silencing in the Mammalian Circadian Clock

Mino D. C. Belle, Casey O. Diekman, Daniel B. Forger and Hugh D. Piggins,