Ionotropic Receptors (Ligand-Gated Channels)

A ligand is a drug or neurotransmitter. A receptor is a protein on the surface of neurons and other target cells that has a pocket where ligands bind. When a ligand binds, the receptor protein changes shape and initiates one of two responses.

There are two categories of neurotransmitter receptors, one for each type of response. Both are similar in that they bind to an extracellular signaling molecule released from a neuron (i.e. they bind to neurotransmitters) and that they change shape when the neurotransmitter is bound.

Where they differ is in what happens when the ligand binds. If a channel is opened, allowing ions to pass, the receptor is called a ligand-gated channel or an ionotropic^[1] receptor.

If a ligand-bound receptor changes the intracellular biochemistry of the target (i.e. its metabolism), it is called a metabotropic receptor. Metabotropic receptors will be discussed in the next section.

The term ionotropic derives from ions, which can pass through its channel, combined with the Greek word τρόπος (trópos), “a turn”, describing the shape change which is initiated by receptor binding.

Agonists are drugs which act in the same way as the naturally occuring neurotransmitter. In some cases, agonists bind more strongly to the receptor than even the naturally occuring neurotransmitter.

Antagonists are drugs which partially or completely block the response of the naturally occuring neurotransmitter. In a novel, the antagonist is trying to stop the main character from carrying out an action. It’s the same for receptor antagonists; they get in the way of what the neurotransmitter is trying to do.