HappyMutations:

Neurotransmitter (NTs) are called “chemical messengers” because they are chemical compounds that are used to carry along a signal from neuronal cells to other cells. Hormones are also chemical messengers, but they are biosynthesized in endocrine glands, and when they are released from them they need to travel via the circulatory system before reaching their target cells which are relatively quite distant. Not only endogenous chemicals can bind to Rs. Exogenous ones -i.e., drugs- can also bind them. Drugs that bind to Rs can be either agonists (binding on and activating their target Rs, which causes intracellular responses), antagonists (binding on the Rs rather sloppily, thus not activating them and instead disallowing agonists form binding on them).

There also exist partial agonists, which do activate the Rs they bind, but triggering a partial, not a full, effect. They could also be considered, in a sense, as antagonists, since they antagonize full agonists, thus inhibiting their potential full effect of the Rs.

A neuronal cell may synthesise and use more than one type of neurotransmitter (NT). However, each R can only accept one NT; hence Rs are called according to the NT they can bind. Rs are not only classified in types but also in subtypes. That being said, certain ligands are selective to only specific subtypes; other ligands are less selective.

Receptor (R) effects can be excitatory (inducing further action potentials), inhibitory (block further APs), or dependent on the neuronal cell they reside on. For example, Glutamatergic Rs are generally excitatory, GABAergic Rs are generally inhibitory; Cholinergic Rs on the frog’s skeletal muscle induces depolarization, while on the cardiac muscle it induces hyperpolarization.