Physiological constraints on the rapid dopaminergic modulation of striatal reward activity

While the contribution of dopaminergic (DA) neurons to associative learning is firmly established, their importance for influencing imminent behavior on short (subsecond) timescales is less clear. Mechanistically, it is thought that DA neurons drive these behavioral changes because of their ability to rapidly alter striatal spiking activity. However, due to limitations of previous approaches, the straightforward prediction that striatal spiking is rapidly influenced by physiologically relevant DA signals has not been rigorously tested. Here, we monitored changes in spiking responses in the ventral striatum while transiently reducing or increasing DA levels. Contrary to the predicted effect, neither spontaneous nor reward-evoked striatal spiking activity was strongly influenced by optogenetic manipulations, except when DA exceeded reward-matched levels. These findings challenge the view that DA plays a major role in rapidly influencing striatal activity. Finally, they suggest a need to distinguish between the modulatory functions of DA under physiological and supra-physiological conditions.