As everyone from jet-lagged travelers to bleary-eyed new parents can attest, sleep is critical for basic functioning, let alone a sense of well-being. But for all that, both its mechanisms and its function still are up for grabs scientifically. Two new papers in the Oct. 18, 2007, issue of Nature and the October 2007 issue of PLoS Biology explore the mechanisms that underlie the transition between being asleep and awake.

The neurotransmitter hypocretin, which also goes by the name orexin, is one transmitter involved in sleep. A lack of hypocretin leads to narcolepsy in mammals. Hypocretin works together with monoamines such as dopamine, serotonin and epinephrine in "see-saw" circuits designed to ensure their owner is either fast asleep or wide awake. (See BioWorld Today, May 5, 2005.)

The Nature paper's authors studied the function of hypocretin using optogenetics, a method developed by co-author Karl Deisseroth and his team that allows specific sets of neurons to be activated with precise timing. For optogenetic experiments, a light-sensitive channel is engineered into neurons of interest via gene therapy - in this case, hypocretin-containing neurons in the hypothalamus of mice. The scientists then stimulate those neurons by delivering light pulses directly into the brain via optic fibers.

In the Nature paper, the researchers first showed the gene therapy itself did not affect the sleep-wake cycle of the engineered animals, and that the engineered hypocretin-neurons were stimulated by the intracranial light pulses. They then activated the hypocretin-containing neurons when mice were either in REM sleep or deeper slow-wave sleep.

Stimulation of hypocretin-neurons caused sleeping mice to wake up faster, whether they were in slow-wave or REM sleep. The effect could be blocked by a hypocretin antagonist. It also was partially blocked in hypocretin knockouts, though the authors note that the effect "seemed not to be completely blocked in . . . knockout animals, suggesting a possible role of other neurotransmitters" that also are expressed in hypocretin-containing neurons.

Luis de Lecea, associate professor of psychiatry and behavioral science at Stanford University and senior author of the Nature paper, told BioWorld Today that the hypocretin system is "a great target to consolidate sleep" - a fact which has not escaped the notice of industry. Swiss biotech company Actelion Ltd. is in the early stage clinical trials to treat insomnia with a hypocretin-receptor antagonist.

The second paper, in PLoS Biology, also by researchers from Stanford University, takes a comparative evolution approach to the same hypocretin-containing neurons, studying their function in zebrafish.

Despite the fact that its function is still a mystery, sleep is a highly conserved behavior. Birds do it, bees do it, and so do zebrafish, but the mechanisms seem to be different than in mammals.

The scientists first used immunostaining to test whether hypocretin receptors and monoamine receptors are found on the same neurons, as they are in humans. They found that hypocretin receptors do not colocalize with monoamines; nor did knocking out hypocretin receptors lead to the same effects as in mammals.

Instead, zebrafish lacking hypocretin receptors slept less overall than their wild-type cousins, and reacted to a lack of hypocretin stimulation in a way that is the opposite of mammals. While mammals without hypocretin have narcolepsy - that is, they are unable to stay awake - the zebrafish knockouts were unable to stay asleep, showing a "dramatic decrease in sleep bout length" by roughly two-thirds and an increase in the number of transitions from sleep to wakefulness.

The researchers offer the differing effects of natural light on the sleep cycles of mammals and fish as a possible explanation for their findings. While mammals can sleep just fine in daylight if they need to, light has a strong sleep-suppressing effect on zebrafish.

So they may get along fine just relying on the sun to tell them when to sleep, or not.

The authors wrote that "The need for hypocretin innervation of wake-promoting structures may have evolved later [in evolution] as the importance of the direct effects of light and melatonin on brain activity decreased, and the need to consolidate wake independently of light effects evolved."