MIND THE TIMESCALE: SUSTAINED MESOLIMBIC DOPAMINE REPORTS RECENT REWARD RATE TO SHAPE TASK ENGAGEMENT

Dopamine signalling unfolds across multiple timescales. While much research has focused on the role of fast, phasic transients in shaping adaptive behaviour, the function of slower, sustained dopamine remains less well understood. In fact, it is unclear whether sustained dopamine simply reflects time-averaged transients, or provides distinct information that shapes behaviour. We designed three experiments to test whether slow dopamine signals 1)are more than a running average of transients, 2)report reward rate, and 3)shape task engagement. We expressed the dopamine sensor dLight1.1 in the Nucleus Accumbens, and the opsin ChrimsonR bilaterally in ventral tegmental area (VTA) dopamine neurons of DAT-Cre mice (N=14, 6 females). First, we varied dopamine transient rates by having mice nose-poke for water rewards available at high vs low rates (VI10 or VI20s). We then recreated matched stimulus-induced transient rates using 25Hz optogenetic VTA stimulation. High water delivery rates elevated sustained dopamine levels, whereas optogenetic stimulation failed to do so, indicating that sustained dopamine is distinct from the long-run average of fast transients. Next, mice performed a 2-armed bandit task in which reward rate was manipulated across blocks by varying either reward probability pairings (0.9/0.5 or 0.5/0.1) or trial frequency via inter-trial interval length (2-4s or 7-9s). Both manipulations had a positive effect on sustained dopamine levels. Higher sustained dopamine was associated with greater task engagement, and sustained 5Hz VTA dopamine stimulation causally increased engagement. Together, these findings indicate that sustained dopamine provides distinct information from fast transients, reporting recent average reward rate and shaping behavioural engagement.