Abstract

Nucleus accumbens receives afferents from the hippocampus, the amygdala and the prefrontal cortex and projects to the several motor structures such as the globus pallidus and the substantia nigra. The dopaminergic(DA) system of the ventral tegmental area innervates the hippocampus, the amygdala and the prefrontal cortex as well as the nucleus accumbens. So the limbiccotical afferents and the DA terminals arising in the vental tegmental area are overlapped in the nucleus accumbens. Several hypothses of the behavioral function of the mesoaccumbens DA system were proposed. Wise(1982) and Bozarth(1991) proposed that the DA system is activated by rewards. By their reward hypotheses, mesoaccumbens DA system is involved in the rewarding effects of electrical brain stimulation, psychomotor stimulants and opiates as well as the natural rewards such as feeding, drinking, and sexual behavior. Reward hypothesis has undergone revision. The revised reward hypotheses propose that mesoaccubens DA system is activeted by conditined incentive stimuli as well as the primary reinfocers after conditioned stimuli are associated with primary stimuli. Phillips et al.(1991) argued that the system is not activated by rewards, but rather by the stimuli that precede and predict the delivery of rewards. And they proposed the system is involved in the incentive learning. Hypotheses of neuromodulatory functions were proposed by Mogenson & Yim(1991), Cador et al.(1991), Salamone(1991, 1992), Scheel-Krüger & Willner(1991). They believe that the DA system plays a crucial role in the regulation of the transfer of information from its inputs to the output structures of the striatum. Schultz et al.(1993) found that midbrain DA neurons can be activated both by primary food and fluid rewards and by conditioned incentive stimuli predicting reward, so they proposed midbrain DA neurons are involved in basic attentional and motivational processes. Similarly Robinson & Berridge(1993) argued that midbrain DA projections mediate a process termed "incentive salience attribution". Many experimental results supporting several hypotheses will be considered and summarized. And the function of the mesoaccumbens DA system will be synthesized.