Picture this: the prince has won his way past the dragon, past the huge walls of briars. He paces slowly through the sleeping castle, toward the tower where the princess lies, in a deep, deep sleep. Finally he sees her, leans over her lovely form...
...and gently inserts a probe into her brain, letting a yellow light activate her locus coeruleus. Within moments, the princess awakes.
Now THAT'S a kiss.
Carter et al. "Mechanism for Hypocretin-mediated sleep-to-wake transitions" PNAS, 2012
I'll admit, this post isn't about sleeping beauty. Instead, it's about sleep-wake transitions, and how they might work. And the answer involves an up and coming molecule, hypocretin (aka orexin), and an area of the brain called the locus coeruleus (LC). And it involves mice, who are little sleeping beauties in their own way.
We'll start with hypocretin (or orexin*). Hypocretin is a small peptide released from the hypothalamus of the brain. It's a very recently discovered molecule (published in 1998), and has been enjoying a recent explosion in popularity, due to its interesting involvement in drug addiction and feeding behavior, and its very clear role in sleep.
You see, hypocretin controls sleep/wake cycles by mediating what we call "arousal" (which is not that, though it's that, too). Neurons that produce hypocretin are silent while you are asleep, but burst of firing and the release of hypocretin from these neurons comes immediately before wakefulness. And hypocretin is such a strong mediator of sleep/wake transitions that loss of hypocretin produces some very striking narcolepsy.
(I admit to using this video whenever I get the chance)
So we know that hypocretin controls the sleep to wake transition, but the question is...through what mechanism? Hypocretin neurons in the hypothalamus fire...and then what? Where do they go? What do they do? Neurons producing hypocretin project all over the brain, which structures matter most?
This group, one of the original groups to study hypocretin, has been on a quest to find out. They chose to focus on the locus coeruleus, a structure deep in the base of the brain that produces the neurotransmitter norepinephrine. The locus coeruleus is known to be involved in arousal, and thus in wakefulness. And we already knew that if you inject hypocretin into the locus coeruleus, you can make the neurons of the locus fire.
So this shows that hypocretin could affect sleep through the locus coeruleus, but it does not prove that this is the mechanism. To prove this, hypocretin in the locus coeruleus would have be shown to be both necessary and sufficient to induce wakefulness. And to prove this, the authors of this study turned to the most popular kid in neuroscience techniques: optogenetics.
Optogenetics is not as complicated as the word sounds. Basically, it uses a channel rhodopsin, which is a channel in a membrane that can be activated by light. When it is activated, ions flow in, the membrane depolarizes, and the cell fires. Scientists can take the genetic code for this channel and put it into the genome of something they want to express it, say, a mouse. They can put the channel under the control of another gene, so the channel is only expressed if the gene is expressed. In this case, this means that the channel was expressed either only in neurons that produce norepinephrine (in the locus coeruleus), or only in neurons that produce hypocretin (in the hypothalamus). Then, when you shine a light into that area of the brain, the channels will activate, and those neurons, and only those, will fire.
So the authors of this study first checked to make sure that stimulating hypocretin neurons in the hypothalamus produced activity in the locus coeruleus. It did. Then they had to find out if this activity was both necessary, and sufficient to produce a sleep to wake transition. To do this, they had to wait til the animal was asleep (I have to imagine this was frustrating), and then stimulated the hypocretin neurons, while at the same time inhibiting the neurons in the locus coeruleus.
What you can see here is the patterns and location of the stimulations used. Take a look at the second set of bars from the left in the bottom of the figure. You can see that when you stimulate bilaterally (on both sides of the brain, remember everything in the brain is in pairs, one nucleus on each side), and you stimulate hypocretin (which should wake an animal up), but inhibit the locus coeruleus...the animal sleeps longer, they have an increased latency from sleep to wake. This suggests that the locus coeruleus is necessary for hypocretin to produce wakefulness.
But is it sufficient? If the hypocretin/locus coeruleus activity is sufficient to wake up an animal, then if you block hypocretin signaling alone to the locus coeruleus, the animal should not wake up.
And it worked. You can see here that using a drug to block hypocretin signaling in the locus coerulus stopped the animals from waking up (compare the second set of bars from the left with the furthest right set of bars).
And finally, you can see here that as the stimulation of the locus coeruleus increases, the probabilty of an animal waking up increases dramatically. Hypocretin stimulates the locus coeruleus, and increasing that stimulation drastically increases the odds of waking the animal up.
The whole set of studies (and I've only shown you part) clearly demonstrates that hypocretin mediates the sleep to wake transition by stimulating neurons in the locus coeruleus. And the next time you wake up, thank your hypocretin...and your locus coeruleus, too.
Carter ME, Brill J, Bonnavion P, Huguenard JR, Huerta R, & de Lecea L (2012). Mechanism for Hypocretin-mediated sleep-to-wake transitions. Proceedings of the National Academy of Sciences of the United States of America, 109 (39) PMID: 22955882
*This is a classic case of two groups identifying a molecule at the same time and giving it different names. I tend to think, though hypocretin (meaning "from the hypothalamus" and "like secretin") is the more accurate term, orexin (from the Greek word for "appetite") will win out...because it's easier to say and sounds cooler. Sad, but true. Anyway, this group used 'hypocretin', and that's what I'm going with)