A powerful psychedelic derived from the fly agaric mushroom. The online medical dictionary at graylab.ac.uk has this to say about muscimol:
Chemical Name: 5-(aminomethyl)-3-isoxazolol.
Neurotoxic isoxazole isolated from amanita muscaria and a. Phalloides and also obtained by decarboxylation of ibotenic acid. It is a potent agonist at gaba-a receptors and is used mainly as an experimental tool in animal and tissue studies.
Pharmacological action: gaba agonist.
If you've taken neuroscience courses or LSD, maybe you understood that bit. If not, here's what Michael Pemulis, a fictional high school tennis star in David Foster Wallace's "Infinite Jest", has to say to the younger boys under his care in an impromptu dorm meeting:
The organopsychedelic muscimole (sic), an isoxazole-alkaloid derived from Amanita muscaria, a.k.a. the fly agaric mushroom-- by no means to be confused with phalloides or verna or certain other kill-you-dead species of North America's Amanita genus-- goes by the moniker 5-(aminomethyl)-3-isoxazolol, requires about like maybe ten to twenty oral mg. per ingestion, making it two to three times as potent as psilocybin, and frequently results in ... a kind of sleep-like trance with visions, elation, sensations of physical lightness and increased strength, and favorable distortions in body-image.
I would guess that Infinite Jest sells rather better than the medical dictionaries do.
And here's a pretty picture of muscimol, for all of you who know how to build this sort of thing:
| | NH2
/ \\ /