Final answer:
Cocaine blocks dopamine reuptake and post-synaptic cells down-regulate dopamine receptors in response to the increase in dopamine levels.
Step-by-step explanation:
When cocaine is present, it blocks the reuptake of dopamine from the synapse into the presynaptic cell, which leads to higher levels of dopamine in the synapse. In response to this increase, the post-synaptic cells down-regulate or decrease the number of dopamine receptors available.
This down-regulation by post-synaptic cells is a mechanism to compensate for the excess dopamine caused by cocaine, and it can lead to a decreased sensitivity to dopamine over time. This can contribute to the development of tolerance and addiction.