Animal Models of Drug Reward
Through extensive animal research, the neurotransmitters and brain circuits that mediate drug reward, have been largely delineated; the biological basis of drug reward is exemplified by the fact that laboratory animals will press levers to receive addictive substances. When provided unlimited access, animals will consistently self-administer cocaine and amphetamine to the point of death, and the power of drug reward should not be underestimated in the clinical setting. Diverse classes of addictive drugs affect different neurotransmitter systems and produce distinct activation patterns within reward circuits. Many addictive substances (including heroin, cocaine, amphetamine, alcohol, nicotine, and marijuana) acutely increase the neurotransmitter dopamine in elements of the ventral striatum, specifically the nucleus accumbens, but this increase is most robust for psychomotor stimulants and much more modest for sedative hypnotics. Other neurotransmitter systems are also involved, including opioid peptides, GABA, glutamate, and serotonin, and play more critical roles as one moves out of the domain of psychomotor stimulants. Dopamine levels in the nucleus accumbens are also elevated during activities that lead to natural rewards, providing compelling evidence that addictive drugs tap into natural motivational circuits.