If an effect of binding the neurotransmitter depends on the receptor type, not the neurotransmitter itself, then why don't we have just one neurotransmitter and many different receptors?
$\begingroup$ biology.stackexchange.com/questions/35532/… is likely relevant, though there are also places where multiple neurotransmitters are used in close proximity where they wouldn't be interchangeable. $\endgroup$– Bryan Krause ♦Feb 8, 2022 at 15:35
$\begingroup$ The question "why do we have multiple neurotransmitters/receptors" is indeed interesting imo, but how well a ligand binds to a receptor depends on its molecular geometry. Some neurotransmitters might bind fairly well to multiple receptors, but globally to each receptor its neurotransmitter. $\endgroup$– David CianFeb 15, 2022 at 12:58