Final answer:
Monoamine oxidase inhibitors (MAOIs) such as phenelzine and tranylcypromine block the intragranular uptake of norepinephrine by inhibiting the enzyme responsible for neurotransmitter breakdown.
Step-by-step explanation:
The drug that blocks intragranular uptake of norepinephrine (NE) is known as a monoamine oxidase inhibitor (MAOI). These drugs inhibit the activity of monoamine oxidase, an enzyme responsible for the breakdown of neurotransmitters like NE, dopamine, and serotonin within the neuron. This inhibition results in an increase in the levels of these neurotransmitters available in the synaptic cleft. MAOIs were among the first types of antidepressants developed. However, because of their potential toxicity and severe food and drug interactions, they are now rarely used. Examples of MAOIs include phenelzine, tranylcypromine, and isocarboxazid. While MAOIs can be effective, their use is generally restricted to cases where other treatments have failed due to the associated risks.