Final answer:
Excessive dopamine in the synapse initiates down-regulation of dopamine receptors in the post-synaptic cell to decrease cell sensitivity.
Step-by-step explanation:
If too much dopamine is present in the synapse, and excessive stimulation persists, down-regulation of the dopamine receptors in the post-synaptic cell begins. This is a protective homeostatic response aimed at decreasing the cell's sensitivity to dopamine and involves reducing the number of dopamine receptors available to bind to the neurotransmitter. This phenomenon occurs as a response to the prolonged increase in dopamine concentration within the synapse, which can be due to drugs that block dopamine reuptake, such as cocaine and amphetamines, or due to dysfunctions in the regulation of neurotransmitter release.