They don't. Antidepressants don't actually contain norepinephrine (ne), but several can affect ne reuptake into the neuronal synapse. The result is more ne available in the synapse, one theory of antidepressants' effectiveness. Antidepressants with this kind of activity include tricyclics, effexor, pristiq, cymbalta, savella, (milnacipran) and fetzima. They also affect serotonin reuptake. Wellbutrin affects ne ; da.
None. Nothing marketed as an antidepressant contains norepinephrine. This would dissipate almost immediately when taken into the body ; have no effect on depression. However, a number of antidepressants increase the amount or effect of existing norepinephrine at nerve synapses. These include tricyclics, MAO inhibitors, Effexor (venlafaxine), Remeron (mirtazapine) (mirtazepine), Cymbalta (duloxetine), ; others.