What does "spike" mean here? Is it used as a verb in this sentence?
Yes, that's correct. "Spike" is used here as a verb to mean to increase sharply and then decrease, or to reach a peak. The speaker says "dopamine levels spike in the reward system hot spots" to mean that "dopamine levels reach a peak" in those areas. Ex: Nurse! Come quickly. The patient's heart rate monitor just spiked suddenly. Ex: The company's sales spiked in the third quarter, then fell suddenly.
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