Diurnal rhythms in neural activation in the mesolimbic reward system: critical role of the medial prefrontal cortex.

Previous evidence suggests a circadian modulation of drug-seeking behavior and responsiveness to drugs of abuse. To identify potential mechanisms for rhythmicity in reward, a marker of neural activation (cFos) was examined across the day in the mesolimbic reward system. Rats were perfused at six times during the day [zeitgeber times (ZTs): 2, 6, 10, 14, 18, and 22], and brains were analysed for cFos and tyrosine hydroxylase (TH)-immunoreactive (IR) cells. Rhythmic expression of cFos was observed in the nucleus accumbens (NAc) core and shell, in the medial prefrontal cortex (mPFC), and in TH-IR and non-TH-IR cells in the ventral tegmental area (VTA), with peak expression during the late night and nadirs during the late day. No significant rhythmicity was observed in the basolateral amgydala or the dentate gyrus. As the mPFC provides excitatory input to both the NAc and VTA, this region was hypothesised to be a key mediator of rhythmic neural activation in the mesolimbic system. Hence, the effects of excitotoxic mPFC lesions on diurnal rhythms in cFos immunoreactivity at previously observed peak (ZT18) and nadir (ZT10) times were examined in the NAc and VTA. mPFC lesions encompassing the prelimbic and infralimbic subregions attenuated peak cFos immunoreactivity in the NAc, eliminating the diurnal rhythm, but had no effect on VTA rhythms. These results suggest that rhythmic neural activation in the mesolimbic system may contribute to diurnal rhythms in reward-related behaviors, and indicate that the mPFC plays a critical role in mediating rhythmic neural activation in the NAc.