SSRIs, SNRIs, and DNRIs increase neurotransmitter concentration in the synapse by inhibiting transporter reuptake.

“all of these medications are changing the way we move neurotransmitters from the synapse which is between two neurons back into one neuron and what they do is they generally do reuptake inhibition meaning that they're just inhibiting a transporter that's taking the neurotransmitter out of the synapse so they're just putting more neurotransmitter in the synapse that's the way these medications work”