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HF-DBS is a neurosurgical technology for electrical stimulation in the brain, which has been developed since the 1870s1. In the late 1980s, HFS was first used as a potential therapeutic intervention for Parkinson's disease and other movement disorders2. In the past few decades, HF-DBS has been more and more widely used in the treatment of brain disorders which are currently untreatable by a traditional therapeutic strategy. Particularly, due to the accuracy improvement of the HFS electrode, the highly effective outcomes, and minimal side effects, the number of brain disorders treated by HF-DBS has significantly increased over the past decades3,4,5. For example, HF-DBS has been approved by the US Food and Drug Administration (FDA) for the treatment of Parkinson's disease (PD), Alzheimer's type dementia, essential tremor, and other types of movement disorders2,6,7. In PD patients, the dopaminergic medication is reduced up to 50% during HF-DBS8. In addition to the successful treatment of movement disorders, HF-DBS has also demonstrated its powerful effects in the treatment of psychiatric diseases in the clinic, and for cognitive augmentation as well2,9,10,11. It should be noted that the research of HFS for the treatment of other psychiatric disorders are in various stages, offering much promise to patients12.
Although many studies have demonstrated that a focal HFS has both local and remote effects throughout the brain13, the neurological and molecular mechanisms of the effects remain elusive2,14. In the clinic, therapeutic HF-DBS is usually applied in a long-term manner for the treatment of Parkinson's disease and chronic pain, etc. Many opinions are raised to explain the improvement generated by an HF-DBS treatment, among which one possibility that the HFS current modulates the neuronal network activity, probably by a repetitive depolarization of the axons in the vicinity of the implanted HFS electrode. Or, HF-DBS may change the discharge rate of the output neurons and the projected targets. Also, HF-DBS may lead to long-term synaptic changes, including long-term potentiation (LTP) and long-term depression (LTD), which may contribute to a symptomatic improvement. So far, it is still unclear whether HFS influences the key molecular events that regulate cellular processes such as adult neurogenesis in vivo. Several lines of studies have demonstrated that HFS in rodents could mimic similar neural responses of clinically applied DBS15,16. To understand the underlying cellular mechanisms of HF-DBS, in this study, we first set up an in vivo HFS methodology in mice in an acute (one day) or chronic (five days) manner. Secondly, we set up an activation analysis methodology to determine the alteration of the neuronal activity and neurogenesis after an HF-DBS delivery.
Given that the neuronal production from neural stem cells is abundant during the embryonic development but continues throughout adult life, the hippocampal subgranular zone is one of the major areas where the neurogenesis occurs. The process of neurogenesis is influenced by many physiological and pathological factors. In certain epileptic cases, the hippocampal neurogenesis is dramatically decreased17,18. In addition, a single electroconvulsive therapy could significantly increase the neuronal production in the dentate gyrus19. These observations suggest that the electrophysiological activity plays a critical role in the regulation of adult neurogenesis and synaptic plasticity in hippocampal neurons. Therefore, to further demonstrate the effects of HF-DBS on neuronal activity and neurogenesis, we first carry out an immunostaining assay of the immediate early gene (IEG) c-fos which is a well-known marker of short-term neuronal activity resulting from experience20. Notch1 signaling is also detected to monitor the signaling activation after the HFS delivery21,22. Moreover, we also detect the neuronal production by a BrdU labeling analysis after the HF-DBS induction in various manners, though BrdU staining can also be a marker for gliogenesis.
In the present study, two HFS methodologies are adapted to target the activation of the hippocampal DG directly and indirectly. The electrode is implanted into the DG directly or implanted into the medial perforant path (PP) which sends projections to activate the DG neurons. For the HF-DBS induction, a programmable stimulator is presented for a continuous stimulation via the fixed electrode onto the mouse head. To determine the effects of HFS on neuronal activation and neurogenesis, we detect the expression of c-fos and Notch1 by immunofluorescent staining and the number of BrdU-incorporated positive neurons in the hippocampal DG region, respectively, after the HFS treatment. Particularly, the effects of the HF-DBS on the neurogenesis in the DG are compared between an acute and a chronic stimulation manner, or between a direct and an indirect stimulation manner, respectively.