Addiction to all four major classes of abused drugs -- psychostimulants, opiates, ethanol and nicotine -- has been linked to increased dopamine transmission in the same parts of the brain associated with normal reward processing. Cues that predict both normal reward and effects of cocaine or alcohol also make dopamine nerve cells fire as do the experiences they recall. That had led to idea that drug addiction must take over normal reward-memory nerve pathways.
No, "has" is correct, because it belongs to "addiction": "Addiction ... has been linked to...". The plural nouns that stand between "addiction" and "has" do not change that.
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