Muscarine

Muscarine is an alkaloid found in mushrooms of the genera Amanita, Clitocybe, and Inocybe, with chemical formula C₅H₁₅NO₃. Muscarine was believed to be responsible for the hallucinogenic properties of the fungus Amanita muscaria, but the compound is only present in 2 parts per million. Ibotenic acid and muscimol were later found to be responsible for the hallucinogenic effects. Muscarine is poisonous, causing gastrointestinal problems and diarrhea. The lethal dose is estimated to be between 40 and 180mg. One of the receptor subtypes of acetylcholine was named after muscarine, called the muscarinic receptor.

Chemical structure:

          H
         /
 H    H O
  \    \|
H--C    C    H
  / \ /   \ /
 H   C     C--H H H
    / \   /     |/
   H   O-C   H--C   H
        / \    /   /
       H   C--N+--C--H
          /|  |    \
         H H  C--H  H
             / \
            H   H

Sources:
http://www.life.uiuc.edu/plantbio/363/lecture40.html
http://www.erowid.org/plants/amanitas/references/journal/1979_mochtar_afghanistan1.shtml
http://www.emedicine.com/ped/topic2863.htm