Autoreceptor
An autoreceptor is a specific molecule that sends messages to the axon terminal regarding the amount of the neurotransmitter that has been released.
Autoreceptors are located in membranes that exist in presynaptic neuron terminals. An autoreceptor is only sensitive to the specific neurotransmitter or hormone that is released by the neuron in the presynaptic membrane in which the autoreceptor sits. They also control some internal cell processes such as synthesis and the release of neurotransmitters.