I mentioned acetylcholine (ACh) is the star of one of my favorite neuroscience stories. So here it is:
Once upon a time, scientists were arguing whether neurological messages were transmitted chemically or electrically. Otto Loewi was in the chemical camp, and endeavored to come up with an experiment that would support his theory. One Easter night, he woke up in the middle of the night having dreamt of an elegant experiment. Immediately he wrote it down and went back to sleep. However, when he woke up he could not decipher his nighttime writing.
Thankfully, the next night he had the exact same dream. Not about to let it slip through his grasp again, he got up and went through the freezing German winter to his laboratory and conducted his experiment. You see, it was known that a frog's heart, still beating in a shallow dish despite being separated from its owner, will slow down when an electrical impulse is applied to it. If the neurological information controlling the speed of a frog's heart is transmitted chemically, then the dish should contain the chemicals released by the heart that caused it to slow down. So if you remove the shocked heart from the dish and replace it with a fresh heart, it will also slow down as if an electrical impulse had been applied to it. That was exactly what happened when Loewi did his experiment.
This is not news; all of that you can find on his Wikipedia entry
. But that's not the best part.
You see, it was key that he ran his experiment at night when the labs were not heated against a still-cold German spring. As you presumably have guessed, the chemical that causes the frog's heart to slow down is acetylcholine. However, when ACh is released by neurons, an enzyme is also released to ensure the message dissipates once it has been transmitted - acetylcholinesterase. This renders the acetylcholine inert, no longer capable of changing neuronal firing patterns (i.e. transmitting information). But like many enzymes it works much more slowly when chilled. Had he done his experiment in a nice warm lab, the ACh would have been degraded by the time he placed the fresh frog's heart in the dish, and he would not have observed the results of neurochemical transmission. While it certain engendered much agony within him when he forgot his initial dream, it turned out for the best!
That is not the best part either.
Acetylcholine does not only exist in a frog's heart. It also is used by our brains. Specifically, it becomes far, far more common in our brains when we are in REM sleep (as far as the brain is concerned, its increase is one of two main differences between being awake and being in REM). We're not entirely sure what function it serves there (at least, not last I checked, although hypotheses involving memory formation are/were popular), but it is clearly very important for the dreaming phase of sleep. So the very neurochemical whose existence he demonstrated with this experiment (and later won a Novel prize for discovering) was the one that gave him the experiment in the first place.
You see why I majored in neuroscience? ;)