Monoamine neurotransmitters (a group including noradrenaline and serotonin / 5-HT) are disrupted in depressed states. This allows targeting by drugs, allowing the modulation of their release and response.
Monoamine oxidase inhibitors block the break down of monoamine neurotransmitters, allowing them to build up in the pre-synaptic terminal. This results in higher levels being released and therefore more receptors being activated on the post-synaptic terminal.
Tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs) block the uptake of monoamines form the synaptic cleft, leaving the neurotransmitter within the synapse for longer, allowing signalling to have a longer duration. Tricyclic antidepressants block the reuptake of noradrenaline, serotonin, and potentially dopamine. SSRIs are much more selective, only blocking the reuptake of serotonin (5-HT). This selectivity gives SSRIs fewer side effects.