What are two ways drugs can increase the concentration of neurotransmitters in a synapse?

naturally, through diet or exercise or drugs, specifically receptor antagonists

Explanation:

diet/exercise - diet is self explanatory, since a healthy diet would mean that your body would be balanced, and wouldn't struggle with producing NTs

drugs - any type of drug will mimic the behavior of NTs, but receptor antagonists specifically will increase the concentration of neurotransmitters in a synapse. Antagonists would block the receptors, and thus the NTs that were intended for the blocked space, will not have anywhere to go. As a result, the NTs will stay in the synapse, and cause an increase in the concentration of NTs.

ps: you'd have more luck asking this question in biology