What was the first drug that was known to have an effect on a neurotransmitter?

Question

please site your sources

Answer 1

Depends on how you mean. The first drug that was known to effect a neurotransmitter specifically was nicotine. It didn't affect the neurotransmitter (Acetylcholine) itself, but it stimulated the acetylcholine receptor class that now bears the name nicotinic receptor. This was instrumental in the discovery of neurotransmitters and neurotransmitter receptors.

In short, it was the first drug identified that emulated a neurotransmitter, and immediately led to the discovery of the first neurotransmitter - acetylcholine.