development and maintenance of substance use disorders/addiction, positive reinforcement, neuropharmacological effects of drugs, tolerance and withdrawal

1. With reference to associative learning theories and studies, explain positive reinforcement in relation to the development of substance use disorders and addiction. How do the neuropharmacological effects of most drugs produce this phenomenon?

2. After repeated substance use many users report diminishing enjoyment from drug intake and severe discomfort after stopping drug use. Explain the neurological changes responsible for these phenomena and how this supports continued drug intake via associative learning mechanisms.