RESULTS

HAB Mice Displayed Behavioral Insensitivity to SSRIs
To investigate whether the HAB mouse model mimics features of treatment resistance, we first examined the efficacy of different classes of antidepressant drugs in reducing the enhanced depression-like behavior displayed by HAB mice in comparison with NAB mice. Reproducing our previous results (Sah et al, 2012), the immobility displayed in the FST was enhanced in HAB animals compared with their NAB counterparts. Chronic treatment with one of three SSRIs did not change the high immobility scores of male HAB mice in the FST (F4,31=156.8, P<0.001; Figure 1). We observed that treatment resistance seemed to be restricted to drugs targeting solely the serotonergic system, as both the selective noradrenaline re-uptake inhibitor reboxetine and the tricyclic antidepressant desipramine significantly reduced the depression-like behavior of HAB mice assessed in the FST (F2,22=8.761, P=0.002) (Supplementary Figure S1).

NAcb-DBS Reduced Depression-Like Behavior in HAB, but not NAB Mice
We next examined whether NAcb-DBS would be able to alter the enhanced depression-like behavior of SSRI-insensitive HAB mice. A single session of NAcb-DBS had no effect and resulted in unaltered time spent immobile compared with NAcb-sham HAB animals both in the FST (t=0.576, P=0.577; Figure 3a) and TST (t=−0.580, P=0.573; Figure 3b). In contrast, HAB mice receiving repeated (7 × ) NAcb-DBS displayed a reduced amount of time spent immobile both in the FST (t=3.021, P=0.007; Figure 3a) and the TST (t=3.001, P=0.008; Figure 3b), indicating a robust antidepressant effect. Next, we investigated whether NAcb-DBS could affect the depression-like behavior of NAB mice, which similar to non-depressed human subjects (Barr et al, 1997; Gelfin et al, 1998) do not respond to pharmacological interventions (Sah et al, 2012). Indeed, neither single (t=0.949, P=0.356) nor repeated (t=0.302, P=0.767) NAcb-DBS altered the depression-like behavior of NAB mice (Supplementary Figure S2A), suggesting that repeated NAcb-DBS is only effective in deranged, that is, pathophysiological, systems using the present stimulation conditions.

NAcb-DBS Reduced Anxiety-Related Behavior in HAB, but not NAB Mice
Given that HAB mice are characterized by enhanced anxiety-related behavior (Kromer et al, 2005), we also studied possible effects of NAcb-DBS on emotionality in HABs. In comparison with NAcb-sham HAB controls, four repetitions of NAcb-DBS greatly reduced the latency to eat a preferred food offered in the center of the testing area of the novelty suppressed feeding paradigm (t=3.064, P=0.007; Figure 3c), indicating a clear anxiolytic effect in HAB mice. In contrast, there was no significant anxiolytic effect of repeated NAcb-DBS as compared with NAcb-sham in NAB animals (t=1.684, P=0.111; Supplementary Figure S2B).

NAcb-DBS did not Influence Locomotor Activity of HAB Mice
To exclude the possibility that the observed behavioral changes following repeated NAcb-DBS were affected by unspecific alterations in general locomotion, locomotor activity of animals was studied in an open field (see scheme in Figure 2a). Following five DBS stimulations, the distance traveled in the open field by HAB mice did not differ within the first 5 min (t=1.410, P=0.178), reflecting the testing period of the TST and FST, or during the total 10 min testing time (t=0.007, P=0.994; Table 1).

NAcb-DBS Enhanced Adult Hippocampal Neurogenesis in HAB Mice
To examine possible underlying mechanisms of successful NAcb-DBS, we next studied whether repeated NAcb-DBS would increase adult hippocampal neurogenesis in HAB mice, as this has been previously shown for various other antidepressant interventions, including pharmacotherapy and electroconvulsive seizures (for a review see Samuels and Hen, 2011). Indeed, in HAB mice, DCX immunohistochemistry revealed an enhanced number of immature neurons in the dentate gyrus of the HPC following repeated NAcb-DBS as compared with NAcb-sham conditions (t=2.706, P=0.015; Figures 4a and c or t=2.914, P=0.001; Figures 4e and g). In addition, BrdU was injected either before NAcb-DBS or during NAcb-DBS (see Figure 2a, Experiments 3 and 4) to reveal putatively different effects of NAcb-DBS at different developmental stages of newly born cells. It was observed that the number of BrdU-positive cells was increased following NAcb-DBS treatment as compared with NAcb-Sham condition when BrdU was injected before NAcb-DBS (t=2.361, P=0.030; Figures 4b and d), but not during NAcb-DBS (t=0.268, P=0.792; Figures 4f and h).

NAcb-DBS Modulated Challenge-Induced Neuronal Activation Patterns in HAB Mice
An overview of c-Fos expression in the analyzed brain areas and P-values are given in Table 2. Repeated NAcb-DBS modulated stress-induced neuronal activity in four out of seven investigated areas. In comparison with their NAcb-sham controls, in NAcb-DBS-treated HAB mice the numbers of c-Fos-positive cells were significantly enhanced in the lateral OFC, lateral habenula, and dentate gyrus of the HPC, while c-Fos induction was reduced in the prelimbic cortex only (Figure 5).