Upon excitation, synaptic vesicles release neurotransmitter into the synapse where the neurotransmitters are then free to distribute and associate and disassociate with receptors in a concentration dependent basis. Association with post-synapticreceptors results in neurotransmission by activating ion-channels, enzymes, second-messenger systems or protein-synthesis.
Association with pre-synaptic auto-receptors often results in modulation of neurotransmission in the form of feed-back loops. Association with pre-synaptic hetero-receptors can result in either stimulation or inhibition of neurotransmission.
If norepinephrine is released into the synapse and interacts with a receptor that modulates the neurotransmission of serotonin, the receptor is a:
Extra info: Because the norepinephrine is effecting another type of neuron (serotonin) this is an example of a hetero-receptor.