Answer: Autoreceptors are transmembrane proteins that are expressed presynaptically, and act to limit the amount of neurotransmitter being released.
An autoreceptor is a class of neurotransmitter receptor. Most receptors are proteins that span the cell membrane, having an extracellular part that responds to neurotransmitters in the extracellular space, and an intracellular part that is responsible for sending the signal to the cell (signal transduction). Most autoreceptors are inhibitory, and act in a negative feedback role by becoming active when the amount of neurotransmitter release is too high. They can be expressed anywhere on the cell body, but are often conceived of existing on the terminals.
As a generalization, most autoreceptors are metabotropic, meaning they affect the cell through a complex cascade of intracellular signaling molecules called second messengers (compare with an ionotropic receptor, which allows ions to pass into and out of the cell after agonist binding.)
Some of the common classes of autoreceptors include the following:
Dopamine D2 short receptors, which are located on dopamine cells. They have a higher affinity than the other dopamine receptors. They are coupled to the Gi/o protein, which decreases release probability by inhibiting production of cyclic adenosine monophosphate.
Group II glutamate receptors include mGluR2 and mGluR3, both of which are metabotropic and decrease glutamate release through inhibition of the cAMP signaling pathway through coupling with Gi/o proteins.
GABAB may serve as at autoreceptor at GABAergic synapses. GABAB is negatively coupled to Ca channels, so GABAB decreases neurotransmitter release by decreasing vesicle fusion. GABAB also opens K channels, which hyperpolarizes cells, making them less likely to release neurotransmitter.
5-HT1A and the 5-HT5A serotonin receptors can be autoreceptors expressed presynaptically.