Dopamine (DA) neurons originating in the substantia nigra and ventral tegmental area project to a variety of forebrain structures thereby forming a complex neuromodulatory system that is essential for numerous motor, cognitive and motivational processes. DA neurons can operate in distinct temporal modes, i. e. they display responses across varying timescales including fast phasic changes of DA release on a seconds time scale and slower phasic changes in a minutes to hour range. Furthermore, tonic levels of DA provide a DA receptor 'tone'. Here we briefly highlight findings indicating that temporally distinct responses of DA neurons in the prefrontal cortex and striatum are related to different kinds of information and may serve dissociable behavioural functions. Furthermore we consider how DA neuronal activity and temporarily distinct patterns of DA release are regulated and summarize some relevant findings on the basic neuroanatomy of ascending DA systems. A better understanding of the specific functions of phasic and tonic DA signals will be critical for determining the role of DA in behaviour in detail.
(c) Georg Thieme Verlag KG Stuttgart-New York.