An agonist is a chemical or a drug that binds to receptors in the brain and causes a reaction.
A receptor is the part of a nerve that receives and reads chemical signals. It then transmits the information to the brain and nervous system using electrical signals. Agonists attach to receptors and stimulate them which causes a response. Agonists can occur naturally in the body as hormones and neurotransmitters (endogenous agonists) or come from exterior sources like drugs and toxins (exogenous agonists). Examples of endogenous agonists are dopamine and serotonin.
Examples of exogenous agonists are poisonous nerve gases (soman and serin), choline (building block of acetylcholine, which is the most common type of neurotransmitter), physostigmine, nicotine (from tobacco), muscarine (found in hallucinogens), Black Widow Spider venom, and some pesticides like sevendust.