External incentives and internal states guide goal-directed behavior via the differential recruitment of the nucleus accumbens and the medial prefrontal cortex.
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Goal-directed behavior is governed by internal physiological states and external incentives present in the environment (e.g. hunger and food). While the role of the mesocorticolimbic dopamine (DA) system in behavior guided by environmental incentives has been well studied, the effect of relevant physiological states on the function of this system is less understood. The current study examined the role of the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAcc) in the kind of food-reinforced behaviors known to be sensitive to the internal state produced by food deprivation conditions. Operant lever-press reinforced on fixed ratio 1 (FR1) and progressive ratio (PR) schedules was tested after temporary inactivation of, or DA receptor blockade in, the prelimbic mPFC or NAcc core of rats with differing levels of food deprivation (0, 12 and 36-h). Food deprivation increased PR breakpoints, as well as the number of lever-presses emitted on the FR1 schedule. Both temporary inactivation and DA blockade of NAcc reduced breakpoints across deprivation conditions, while temporary inactivation and DA blockade of mPFC reduced breakpoints only in food-deprived rats. Neither manipulation of mPFC and NAcc had any effect on behavior reinforced on the FR1 schedule. Thus, mPFC and NAcc were differentially relevant to the behaviors tested-NAcc was recruited when the behavioral cost per reinforcer was rising or high regardless of food deprivation conditions, while mPFC was recruited when food-deprived animals behaved through periods of sparse reinforcement density in order to maximize available gain.
