SUMMARY and General discussion
DBS of the nucleus accumbens and ventral striatum/ventral capsule has proven to be an effective treatment for patients with OCD (Denys et al.2010;Gabriels et al.2003;Greenberg et al.2010). DBS has a rapid effect on anxiety and mood followed by changes in compulsive behaviour within weeks to months (Denys et al.2010). Yet, there remain many questions regarding the optimal target for DBS, the neurobiological mechanisms underlying the rapid effects of DBS and the specific impact of DBS on the different symptoms of OCD. In this thesis, we report a series of experiments in rodents addressing these questions. The aim of the thesis was twofold: (1) to examine the mechanism of action of DBS, i.e. the neurobiological underpinnings such as changes in monoamine levels and neurogenesis, and (2) to examine the differential effect of DBS on anxiety and compulsivity, both core symptoms of OCD.

8.2 SUMMARY of main findings
The mechanism of action of DBS
There is compelling evidence that the serotonergic system plays a major role in the treatment of OCD (chapter 2). Pharmacotherapeutic studies have shown that SSRIs are more effective in achieving clinical response compared to placebo, and that atypical antipsychotics, which have a high affinity for 5-HT2A, and 5-HT2C receptors, may augment the effect of SSRIs. However, these findings do not necessarily reflect the existence of a neurobiological abnormality in the central serotonergic system in OCD. At this moment, direct evidence for an abnormality in the serotonergic system in the pathophysiology of OCD is still scarce. Hence, it is open to discussion whether the serotonergic system plays a primary role in the pathophysiology of OCD or is secondarily involved due to the efficacy of SSRIs. SSRIs may modulate serotonin via an intact serotonergic system to compensate for the underlying pathogenesis that is related to another neurotransmitter system. Several reviews have summarized the growing evidence that dopamine might be involved in OCD as well (Denys et al.2004c;Koo et al.2010). For example, atypical antipsychotics with potent 5-HT2A, 5-HT2C and D2 antagonistic properties have antiobsessional properties as additional therapy to SSRIs. Denys et al. found that the combination of quetiapine and fluvoxamine may cause a synergistic dopamine increase in the prefrontal cortex and thalamus (Denys et al.2004a). Moreover, SPECT and PET studies in OCD patients hint to decreased D2 receptor binding in the ventral striatum (Denys et al.2004b), dopamine transporter binding abnormalities in the basal ganglia (Kim et al.2007;van der Wee et al.2004) and indicate that successful SSRI treatment is associated with normalization of the dopamine function (Kim et al.2007;Pogarell et al.2005). Therefore, future research should focus much more on the interaction of the serotonergic system with other neurochemical systems, such as dopamine in relation to OCD.

In chapter 3 and 4 we report the effect of DBS in the NAc on monoamine levels locally and in the prefrontal cortex. Using freely moving animals we found that DBS of the NAc core has no effect on local in vivo release of dopamine, serotonin or noradrenaline. However, we found rapid increases in the release of dopamine and serotonin to a maximum of 177% and 127% in the mPFC and an increase up to 171% and 166% for dopamine and noradrenaline in the OFC following onset of stimulation in the NAc core. Our study suggests that activation of monoamine release in the prefrontal areas rather than local effects in the NAc core are involved in the early behavioural effects of DBS in the NAc. 
In chapter 5 we found no effect of DBS of the NAc on neurogenesis in the dentate gyrus (DG) during stimulation or 5 days following stimulation. Our results contrast with other studies, showing profound effects of high frequency stimulation on neurogenesis in the DG after stimulation in the anterior nucleus of the thalamus, subthalamic nucleus and the entorhinal cortex (Encinas et al.2011;Khaindrava et al.2011;Stone et al.2011;Toda et al.2008). The lack of effect could be due to an absence of projections of the NAc to the DG or to acute nature of the stimulation. Our preliminary results suggest that hippocampal neurogenesis is not involved in the therapeutic effects of DBS in the NAc.

The effect of DBS on anxiety and compulsivity
In chapter 6 we explored the effect of DBS in five different brain areas (NAc core and shell, BNST, IC and CAU) on unconditioned and conditioned anxiety. We found different anxiolytic effects of stimulation in the five target areas. Stimulation of the CAU decreased both conditioned and unconditioned anxiety, while stimulation of the IC uniquely reduced conditioned anxiety. Remarkably, neither NAc nor BNST stimulation affected conditioned or unconditioned anxiety. These findings suggest that (1) DBS may have a differential effect on unconditioned and conditioned anxiety depending on the stimulation area, and that (2) high frequency stimulation of the IC exclusively reduces conditioned anxiety.  Anxiolytic effects of DBS in OCD patients may not be induced by stimulation of the NAc, but rather by the IC. 

The effect of DBS on compulsive grooming of the sapap3 mutant mouse, an animal model for OCD, was examined in chapter 7. Sapap3 mutant mice were bilaterally stimulated at 3 brain areas, the NAc, IC and BNST for 2 hours. Stimulation of the BNST had no effect on compulsive grooming behaviour or locomotor activity. Stimulation of the NAc and the IC did not alter the percentage of time spent grooming but did increase the number of times grooming is initiated (i.e. number of bouts). This resulted in the IC specifically, in a significant decrease of the duration of bouts, implying that IC stimulated sapap3 mutant mice initiated the grooming behaviour more often but that the grooming bout was aborted quicker. Stimulation of the NAc but not the IC increased the distance moved and the movement velocity of the sapap3 mutant mice. These results in combination with earlier findings of an anxiolytic effect of IC stimulation on conditioned anxiety suggest an important role of the IC in the working mechanisms of DBS.