Describe and explain one presynaptic and one postsynaptic mechanism
that pharmaceuticals can use to modify synaptic transmission.

Explanation:

Presynaptic mechanism: one way to modify the synaptic transmission before it happens is affecting transmitters release, one example is calcium channel blockers (for example, verapamil), this action won't allow transmitters out, therefore, there won't be a synaptic transmission.Postsynaptic mechanism: one method to modify synaptic transmission after it happens is blocking the transmitter receptors, one example of this is antipsychotic drugs that block dopamine receptors.

I added some images with another examples for both cases. I hope you find this information interesting! Good luck!