The rat medial frontal cortex controls pace, but not breakpoint, in a progressive ratio licking task

The medial frontal cortex (MFC) is crucial for selecting actions and evaluating their outcomes. Outcome monitoring may be triggered by the rostral part of MFC, which contains neurons that are modulated by reward consumption and is necessary for the expression of relative reward value. Here, we examined if the MFC further has a role in the control of instrumental licking. We used a progressive ratio licking task in which rats had to make increasing numbers of licks to receive liquid sucrose rewards. We determined what measures of progressive ratio performance are sensitive to value by testing rats with rewards containing 0-16% sucrose. We found some measures (e.g. breakpoint, number of licking bouts) were sensitive to sucrose concentration and others (e.g. response rate, duration of licking bouts) were not. Then, we examined the effects of reversibly inactivating rostral (medial orbital) and caudal (prelimbic) parts of the MFC. We were surprised to find that inactivation had no effects on measures associated with value (e.g. breakpoint). Instead, inactivation altered behavioral measures associated with the pace of task performance (response rate and time to break). These effects depended on where inactivations were made. Response rates increased and the time to break decreased when the caudal prelimbic area was inactivated. By contrast, response rates decreased and the time to break increased when the rostral medial orbital cortex was inactivated. Our findings suggest that the medial frontal cortex has a role in maintaining task engagement, but not in the motivational control of action, in the progressive ratio licking task.