Final answer:
Laboratory animals with unlimited self-administration of cocaine become addicted due to its effects on the dopamine system and rewarding properties, leading to dependence and withdrawal symptoms upon cessation.
Step-by-step explanation:
When laboratory animals are given unlimited opportunities to self-administer cocaine injections, they will become addicted to cocaine. This addiction results from cocaine's potent effect on the brain's reward pathway. Cocaine operates by blocking the reuptake of dopamine, a neurotransmitter that contributes to feelings of pleasure and reward. Due to this action, cocaine is highly reinforcing, leading to the development of addiction and subsequent dependence. With continued use, animals will often exhibit behaviors characteristic of addiction, such as compulsive use despite negative consequences. When the use is stopped, withdrawal symptoms are likely to occur, which may include depression, decreased pleasure, and subjective fatigue, all indicative of a physical and psychological dependence.