Exercise Reduces Effects of 5-HT2CR Agonist Figure 6. Six weeks of voluntary wheel running decreases 5-HT2C mRNA expression in the medial dorsal striatum. (A) The region of the striatum as shown by Paxinos and Watson (published in the Rat Brain in Stereotaxic Coordinates, 4th ed., Copyright Elsevier (1998). (B) Representative autoradiographic coronal section through the region of the striatum (Bregma 1.70 mm) in a sedentary rat processed with in situ hybridization for 5-HT2CR messenger ribonucleic acid (mRNA). (C) Representative autoradiographic coronal section through the region of the striatum (Bregma 1.70 mm) in a physically active rat (Run) processed with in situ hybridization for 5-HT2CR mRNA. (D) Relative levels of 5-HT2C receptor (mRNA) in the medial and lateral dorsal striatum (DS) of sedentary rats or rats allowed voluntary access to running wheels for 6 weeks (Run). Values represent mean integrated density 6 SEM. * p#0.05 relative to respective sedentary group. doi:10.1371/journal.pone.0046118.g006 behavior [66,67]. The role of the 5-HT2CR in the CeA in anxiety, however, remains relatively unknown. 5-HT2CRs are expressed throughout the amygdala complex, but 5-HT2CR mRNA (Figure 5), as well as receptor density [68], appears to be greatest in the lateral amygdala. Moreover, activation of 5-HT2CR in the CeA has no effect on the expression of some types of anxiety-like behaviors [38,41]. Instead, it appears that anxiety-like effects of 5- HT activity in the CeA are more likely mediated by 5-HT1A receptors [69]. Further work is necessary to determine if the observed reduction of 5-HT2CR mRNA levels in the CeA of physically active rats contributes to a behavioral effect of exercise. In addition to the BLA, the current data implicate the 5-HT2CR in the DS as a target for the antidepressant effects of exercise. Hypoactivity of striatal dopaminergic neurotransmission is hy- pothesized to occur in depression [70,71]. Consistent with this idea, the shuttle box escape deficit produced by uncontrollable stress (an established animal model of depression) can be thought of as a failure of a rapid instrumental learning process [50] which requires dopamine in the DS for optimal acquisition [72]. Importantly, extracellular dopamine in the DS can be reduced by DS 5-HT2CR activation [73]. It is therefore possible that the reduced expression and function of 5-HT2CR in the DS of physically active, relative to sedentary, rats observed in the current study could alleviate depressive symptoms by restoring dopamine transmission in the DS. Indeed, exercise also prevents the shuttle box escape deficit produced by uncontrollable stress [8,74]. Although cannulae in the current study were aimed at the border between the medial and lateral DS, it is likely that 5- HT2CR activation interfered with escape behavior through action in the medial portion of the DS. 5-HT2CR are expressed more heavily in the medial, relative to the lateral, DS ([75] and Figure 6). Moreover, it is widely accepted that distinct sub-regions of the DS mediate different aspects of instrumental learning. The early stages of instrumental learning are modulated by the medial DS, whereas the lateral DS contributes to the later stages of instrumental learning (for review see [76,77]). The fact that the acquisition of the escape contingency by rats injected with the low dose of CP- 809101 occurs rapidly (within the first 5 trials), seems to indicate a role for the medial DS (see also [50]). Finally, voluntary exercise reduced 5-HT2CR mRNA in specifically the medial, and not the lateral, DS. These data support a role for 5-HT2CR activation in the medial DS in interference with shuttle box escape behavior and aversively-motivated instrumental learning in general. It is important to note that the 5-HT2CR is known to undergo post-translational modifications. Increasing evidence suggests that the editing of 5-HT2CR mRNA can lead to the expression of multiple 5-HT2CR isoforms that have different G-protein activity and affinities for 5-HT [78,79], altered constitutive activity [80,81], as well as intracellular effects [82]. These editing-induced changes in 5-HT2CR function have been speculated to play a critical role in the etiology of anxiety and depression [83,84,85,86]. Additionally, 5-HT2CR mRNA editing changes seem to occur after perturbations of 5-HT levels. Specifically, PLOS ONE | www.plosone.org 8 September 2012 | Volume 7 | Issue 9 | e46118