Narcolepsy Forum Posting

Posted by PHILIP on April 27, 2000 at 11:25:52:

brain : Sweet dreams are made of these

DAVID ADAM

Nerve cells that help determine why you have a hard day’s night while I’m only sleeping are identified this week. The discovery of sleep-promoting neurons in slices of rat brain could help us to understand and treat sleep disorders.

A brain region called the ‘ventrolateral preoptic nucleus’ (VLPO) wakes up when we snooze - two thirds of its neurons fire during sleep. Now Michel Mühlethaler at the Centre Medical Universitaire, Geneva, Switzerland, and colleagues report that two out of every three neurons in the VLPO show striking similarities, and that hormones produced when we are awake inhibit their activity. They conclude in Nature1, that these cells must help us to sleep.

This finding could prove crucial for future exploration of the land of Nod. "These cells represent an extremely homogenous population," Mühlethaler says. "One can thus visualize what a sleep-promoting neuron looks like. This opens the door for the in vitro study of the mechanisms which lead to sleep."

John Huguenard, a neurologist at Stanford University, California, agrees that the discovery is significant. But he points out that almost a third of the newly identified neurons behave differently from the others - they are excited by the neurotransmitter ‘serotonin’. "This indicates an underlying heterogeneity, which may complicate the development of a simple understanding of sleep disorders and their treatment," Huguenard cautions.

The VLPO neurons are wired to the parts of the brain that release ‘wakeful’ hormones. The hormone supply is cut off when the neurons fire. But the same hormones stop the nerve cells from firing. This, Mühlethaler’s group says could explain how we drop off - and why some people don’t.

On the threshold of sleep, the group suggests, an initial burst of VLPO cell activity reduces the wakeful hormone supply. This allows more VLPO neurons to fire, further reducing the hormone release and bringing on the zs.

"There is a bit of a chicken and egg issue here," says Huguenard. "[If] increased activity in the VLPO neurons in some way leads to decreased activity in the brainstem neurons, [then] how can the VLPO neurons activate themselves to initiate this process, if they are inhibited?"

In other words, what causes the initial nervous surge that triggers the loop and turns one wink into forty? The correct temperature might do this, the team suggests, or perhaps nervous signals from the eyes and chemical messages from the body.

"These questions will need to be answered before it can be known whether the VLPO neurons are responsible for sleep onset per se," Huguenard concludes.

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Gallopin, T. et al. Identification of sleep-promoting neurons in vitro. Nature 404, 992-995 (2000)

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