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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss receptors and ligands.
When neurotransmitter molecules are released from a neuron, they pass the signal to the
next neuron by interacting with receptors on that neuron.
These receptors are made up of proteins embedded in the membrane of the postsynaptic cell.
There are two broad families of neurotransmitter receptors.
One type are called ionotropic receptors.
Ionotropic receptors have a site where a neurotransmitter can bind, called the binding site.
When neurotransmitter binds here, it causes a channel to open in the receptor, and ions
are permitted to flow into the neuron.
This can change the membrane potential of the neuron, and send a signal, known as the
action potential, down that neuron.
The neurotransmitter that binds is called the ligand, which is just a term for any substance
that can bind to a target protein.
Thus, ionotropic receptors are also called ligand-gated ion channels.
The other type of receptors are metabotropic receptors, also known as G-protein coupled
receptors.
Neurotransmitters also bind to these receptors, but instead of immediately opening an ion
channel, the next step after binding is the activation of an intermediate protein called
a G-protein.
The G-protein can then influence the opening of ion channels, but it can also affect enzymes
and activate intracellular signaling molecules known as second messengers, which can initiate
signaling cascades within the cell.
G-protein coupled receptors thus tend to have a slower action, but can have more widespread
effects due to their ability to influence various molecules throughout the cell.
Ligands other than neurotransmitters (such as drugs) can also bind to receptors and have
a variety of effects.
If they have the same effect as the neurotransmitter, they are known as agonists.
If they block the effects of the neurotransmitter, they are known as antagonists.
Some drugs can bind to the site the neurotransmitter binds to and have the exact opposite effect
of the neurotransmitter.
These are called inverse agonists.
Finally, drugs may bind to a site on the receptor that is separate from the site where the neurotransmitter
binds and affect the likelihood that the neurotransmitter will bind.
In this case, they are called neuromodulators, and said to have an allosteric effect.