Phasic Dopamine Transmission Reflects Initiation Vigor and Exerted Effort in an Action- and Region-Specific Manner

Initiating a reward-seeking behavior involves deciding on an action, how fast to initiate the action (initiation vigor), as well as how much effort to exert. These processes are thought to involve the mesolimbic dopamine system. Dopamine levels in the ventral striatum rise before initiating a reliably reinforced behavior. However, it is unknown whether dopamine is similarly involved with unreinforced actions (inactive lever presses, premature food port entries, insufficient number of active lever presses). Furthermore, does the dopamine response when initiating an action reflect specific aspects of motivated behavior, such as initiation vigor and exerted effort? Here, we analyzed voltammetry recordings of dopamine levels in the nucleus accumbens (NAcc) core and shell in rats working for food under a progressive ratio reinforcement schedule. We examined dopamine levels when rats initiated distinct actions (active lever presses, inactive lever presses, food port entries) that were temporally separated from cue- and reward-evoked dopamine release. Active lever pressing bouts were preceded by elevated dopamine release in the NAcc shell, as well as in the NAcc core, although only when rats exhibited high initiation vigor. Dopamine levels were transiently reduced in the NAcc core following an unreinforced food port entry and were unchanged throughout the NAcc when initiating inactive lever presses. The effort exerted and vigor to initiate a bout of active lever presses were signaled by dopamine transmission in the NAcc core, but not in the NAcc shell. These results demonstrate that the dopamine response when initiating a behavior is both region- and action-specific.

Exogenous activation of the mesolimbic dopamine system facilitates motivated behavior. However, a direct relationship has not been established between endogenous phasic dopamine transmission and measures of motivation, such as the vigor to initiate an action and the effort exerted in a bout of activity. The present work demonstrates that the dopamine response when initiating an action depends both upon where dopamine is released and what action is performed. Furthermore, dopamine reflects measures of motivated behavior selectively within the nucleus accumbens core.