Cocaine addiction in mice

Animal models can provide useful insights into the effects of cocaine addiction on the human brain. In such studies, scientists place an injection needle directly into areas of the dopaminergic pathway in a mouse’s brain. The needle is attached to a lever, which releases cocaine into the brain whenever the mouse pushes it. Mice quickly learn that pushing the lever leads to a pleasurable feeling.

A scientist decided to study how two different sets of environmental conditions might affect mice’s addiction to cocaine. One group of cages was very bare—it contained only the lever and minimal food and water. The other group of cages was enriched—it contained sweetened water, plenty of food, and a wheel for the mouse to exercise. In addition, the scientist had half the mice in each cage condition reared in isolation, and the other half reared in a typical family. The scientist first trained the mice to push the lever to receive a cocaine injection and then measured how many milligrams of cocaine the mice self-administered over a two-week period. Table $1$‍ outlines average self-administered doses of cocaine (in milligrams) for each condition.