Neurobiological Changes Induced by NAcb-DBS in HAB Mice

Alteration of challenge-induced neuronal activation patterns
The NAcb receives projections from the main monoaminergic nuclei including the raphe nuclei and the locus coeruleus and from regions associated with locomotion, emotion, and memory, including the globus pallidus, the amygdala, the cortex, and the HPC (Nauto and Domesick, 1984). In turn, NAcb projects to pallidal and nigral complexes, to cortical areas such as the medial prefrontal cortex, and to the thalamic and hypothalamic regions. Several of these brain areas have been implicated in the pathophysiology of depression and/or in the processing of antidepressant effects (Krishnan and Nestler, 2008). Furthermore, many of these areas are positioned along the superolateral arm of the medial forebrain bundle, a structure related to the reward circuitry, likely to be stimulated by most of, if not all, the different electrode placements (eg the anterior limb of the internal capsule, the subgenual cingulate gyrus, the NAcb) used in the treatment of TRD patients (Coenen et al, 2011). Regarding potential mechanisms of action identified so far, DBS preferentially modulates network fibers passing the electrode, while local effects of DBS on somatodendritc structures are minor (McIntyre and Grill, 1999; McIntyre et al, 2004; Nowak and Bullier, 1998a, 1998b). Specifically, direct inhibition of the electrode target area by muscimol injections or radiofrequency lesions does not seem to resemble effects of DBS on anxiety/depression networks (Hamani et al, 2010; Rodriguez-Romaguera et al, 2012). To identify distant parts of circuitries affected by NAcb-DBS at the present stimulation conditions, which may underlie the antidepressant effect observed in HAB mice, we used c-Fos mapping in specific brain areas (Singewald, 2007). The focus was laid upon those brain areas that have been previously shown to be associated with therapeutic modulation of enhanced depression-like behavior (Muigg et al, 2007; Sah et al, 2012; Winter et al, 2011).
One candidate area is the HPC, a highly stress-sensitive key brain structure dysregulated in depression (Floresco et al, 2001; Kingwell, 2010) in terms of reduced volume and dysfunctional activation under emotional challenge (Kempton et al, 2011; Lee et al, 2007; Milne et al, 2012; Tan et al, 2012; also see Disner et al (2011)). Interestingly, HAB rats (Muigg et al, 2007; Salomé et al, 2004), and more recently HAB mice (Muigg et al, 2009; Sah et al, 2012), display hypoactivation of the dentate gyrus (DG) by stress challenge. Here, we observed that NAcb-DBS enhanced the c-Fos induction in response to FST stress, suggesting that (i) neuronal DG activity is restored in HAB mice, and (ii) DG activity is strongly correlated with depression-like behavior. To our knowledge, so far changes in DG/HPC activity have not been reported in TRD patients undergoing DBS, while activity of the HPC is enhanced in addicted patients undergoing NAcb-DBS (Heldmann et al, 2012).
In addition, the c-Fos response was enhanced in the OFC and the lateral habenula, but attenuated in the prelimbic cortex following FST. In line with our results, pERK expression is enhanced in prefrontal regions, including the OFC, following ventral striatum/NAcb-DBS, suggesting functional connectivity between these spatially distinct structures (Rodriguez-Romaguera et al, 2012). In further support of our findings, McCracken and Grace (2007, 2009) propose an antidromic activation of NAcb-input fibers descending from the OFC by NAcb-DBS, resulting in the modulation of activity within the OFC and, thus, potentially affecting disturbed communication between prefrontal areas, limbic areas, and the OFC in an ultimately beneficial way.

NAcb-DBS enhanced the number of immature neurons in the DG of the HPC
Whereas stress as a triggering factor for depression attenuates adult neurogenesis in the HPC, antidepressant treatment enhances proliferation and survival in the hippocampal neurogenic niche and has been proposed to be at least partially required for antidepressant efficacy (for a review see Samuels and Hen, 2011). Given that HAB mice display reduced adult hippocampal neurogenesis in comparison with NABs (Sah et al, 2012), we also investigated whether the blunted neurogenesis of HABs would be affected by NAcb-DBS. Indeed, the number of DCX-positive cells was enhanced in the DG in NAcb-DBS-treated HAB mice in comparison with NAcb-sham controls. This finding points towards enhanced adult hippocampal neurogenesis, as DCX is exclusively expressed in neuronal precursors and not fully differentiated neurons reflecting the developmental stages of neuroblast 1 and 2 cells as well as immature neurons (for a review see Encinas et al, 2006). Similarly, DBS of the anterior thalamic nucleus slightly increases the number of neuroblast 1 cells in the HPC, while the symmetric division of amplifying neuronal progenitors is its main effect (Encinas et al, 2006, 2011; Toda et al, 2008). To gain insight into the effect of NAcb-DBS on different developmental stages of newly born cells, BrdU staining was used. Interestingly, only when BrdU was injected before, but not during NAcb-DBS, an enhanced number of BrdU-positive cells was found. These results suggest that the higher number of DCX-positive neurons reflects a cell cohort, which was actually born before the NAcb-DBS intervention and whose survival was then increased by NAcb-DBS. This is supported by a recent study showing that a single DBS session of the entorhinal cortex increases the survival rate of cells born up to 10 days before DBS in the DG (Stone et al, 2011). In this context, it is speculated that 1- to 3-week-old progenitors are especially sensitive to life events that promote survival (eg environmental enrichment) or deteriorate it (eg through stressful condition; Zhao et al, 2008). However, future work will need to verify whether these cells contribute to the reduction of depressive symptoms.
In summary, the data presented here demonstrate that NAcb-DBS selectively rescued the enhanced depression- and anxiety-related behaviors in an SSRI-resistant psychopathological mouse model of high trait depression/anxiety without affecting these behaviors in normal depression/anxiety NAB controls. The specific changes that occurred in challenge-induced neuronal activation suggest that the beneficial effects of NAcb-DBS are mediated via a distributed network that includes the HPC, and cortical and thalamic areas. Furthermore, enhanced adult neurogenesis following repeated NAcb-DBS indicates that long-term alterations may also be an important part of the mechanism(s) of NAcb-DBS, leading to a rescue of exaggerated anxiety- and depression-like symptoms.