Alaboratory rat is injected with amphetamines and heroin during the course of an experiment to determine how the animal responds. findings will show that the activity level of the neurons in the animal's nucleus

Increases

Explanation:

The nucleus accumbens plays an important role in the reward circuit of the body. It controls and regulates two essential neurotransmitters: dopamine and serotonin.

Dopamine, stimulates desire, and serotonin, affects satiety and inhibition, and these are some of the reactions from a "drug rush" initiated by heroin and amphetamines.

Basically, the best way to study the effect of said drugs is to check the first area that they are going to target, and since they are stimulants, then study the increment of activity in said area (the nucleus accumbens)  

true, i don't really know what you're asking but from the info you provided im guessing the answer is true.