Final answer:
The psychological dependence and reward associated with drugs of abuse is mainly due to the modulation of the endogenous opioid system by psychoactive drugs.
Step-by-step explanation:
The psychological dependence and reward of drugs of abuse is thought to be primarily due to the modulation of the endogenous opioid system. Psychoactive drugs, which include drugs of abuse, generally produce their effects by affecting brain chemistry and can act either as agonists, enhancing the activity of neurotransmitters, or as antagonists, reducing neurotransmitter activity.
Specifically, drugs that affect the endogenous opioid system, such as opiate drugs, act as powerful analgesics and are known for creating significant psychological dependence due to their influence on mood, perception, and behavior.