Fornix deep brain stimulation circuit effect is dependent on major excitatory transmission via the nucleus accumbens

Efficacy and Safety of Bilateral Deep Brain Stimulation (DBS) for Severe Alzheimer's Disease: A Comparative Analysis of Fornix Versus Basal Ganglia of Meynert

BACKGROUND

Deep brain stimulation (DBS) is a novel therapy for severe Alzheimer's disease (AD). However, there is an ongoing debate regarding the optimal target for DBS, particularly the fornix and the basal ganglia of Meynert (NBM).

OBJECTIVE

This study aimed to investigate the safety and efficacy of DBS for severe AD and to compare the fornix and the NBM as potential targets.

METHODS

We conducted a prospective, nonrandomized clinical study involving 20 patients with severe AD (MMSE score 0 to 10, CDR level 3) from January 2015 to August 2022, comprising 12 males and eight females, with a mean age of 59.05 ± 6.45 years. All patients underwent DBS treatment, among which 14 received bilateral fornix implantation, while six received bilateral implantation in the NBM. Electrical stimulation commenced 1 month postoperatively. We assessed the patients before surgery, followed by evaluations at 1 month, 3 months, 6 months, and 12 months poststimulation. Primary outcome measures focused on changes in cognitive function, assessed using the MMSE, MoCA, ADAS-Cog, and CDR scales. Secondary measures encompassed quality of life, caregiver burden, neuropsychiatric symptoms, and sleep disturbances, evaluated through the BI, FAQ, FIM, ZBI, NPI, HAMA, HAMD, and PSQI scales.

RESULTS

All patients tolerated DBS well, with no serious adverse effects reported. Early on, DBS significantly improved cognitive function and quality of life. Long-term benefits include the improvement of neuropsychiatric symptoms and sleep disorders and the alleviation of caregiver burden. Comparison between DBS targeting the NBM and fornix revealed no significant differences in overall scale scores. However, upon deeper analysis, NBM-DBS exhibited a more pronounced improvement in neuropsychiatric symptoms, particularly in NPI scores.

CONCLUSION

DBS is a potential therapeutic approach for severe AD, capable of improving patients' cognitive function, quality of life, and neuropsychiatric symptoms. Notably, NBM-DBS showed distinct advantages in ameliorating neuropsychiatric symptoms, providing valuable insights for clinically selecting the optimal DBS target.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT03115814.

Neuronal Network Activation Induced by Forniceal Deep Brain Stimulation in Mice

Background: The fimbria-fornix is a nerve fiber bundle that connects various structures of the limbic system in the brain and plays a key role in cognition. It has become a major target of deep brain stimulation (DBS) to treat memory impairment in both dementia patients and animal models of neurological diseases. Previously, we have reported the beneficial memory effects of chronic forniceal DBS in mouse models of intellectual disability disorders. In Rett syndrome and CDKL5 deficiency disorder models, DBS strengthens hippocampal synaptic plasticity, reduces dentate inhibitory transmission or increases adult hippocampal neurogenesis that aids memory. However, the underlying neuronal circuitry mechanisms remain unknown. This study we explored the neural network circuits involved in forniceal DBS treatment. Methods: We used acute forniceal DBS-induced expression of c-Fos, an activity-dependent neuronal marker, to map the brain structures functionally connected to the fornix. We also evaluated the mouse behavior of locomotion, anxiety, and fear memory after acute forniceal DBS treatment. Results: Acute forniceal DBS induces robust activation of multiple structures in the limbic system. DBS-induced neuronal activation extends beyond hippocampal formation and includes brain structures not directly innervated by the fornix. Conclusions: Acute forniceal DBS activates multiple limbic structures associated with emotion and memory. The neural circuits revealed here help elucidate the neural network effect and pave the way for further research on the mechanism by which forniceal DBS induces benefits on cognitive impairments.

Deep brain stimulation mitigates memory deficits in a rodent model of traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) is a major life-threatening event. In addition to neurological deficits, it can lead to long-term impairments in attention and memory. Deep brain stimulation (DBS) is an established therapy for movement disorders that has been recently investigated for memory improvement in various disorders. In models of TBI, stimulation delivered to different brain targets has been administered to rodents long after the injury with the objective of treating motor deficits, coordination and memory impairment.

OBJECTIVE

To test the hypothesis that DBS administered soon after TBI may prevent the development of memory deficits and exert neuroprotective effects.

METHODS

Male rats were implanted with DBS electrodes in the anterior nucleus of the thalamus (ANT) one week prior to lateral fluid percussion injury (FPI). Immediately after TBI, animals received active or sham stimulation for 6 h. Four days later, they were assessed in a novel object/novel location recognition test (NOR/NLR) and a Barnes maze paradigm. After the experiments, hippocampal cells were counted. Separate groups of animals were sacrificed at different timepoints after TBI to measure cytokines and brain derived neurotrophic factor (BDNF). In a second set of experiments, TBI-exposed animals receiving active or sham stimulation were injected with the tropomyosin receptor kinase B (TrkB) antagonist ANA-12, followed by behavioural testing.

RESULTS

Rats exposed to TBI given DBS had an improvement in several variables of the Barnes maze, but no significant improvements in NOR/NLR compared to Sham DBS TBI animals or non-implanted controls. Animals receiving stimulation had a significant increase in BDNF levels, as well as in hippocampal cell counts in the hilus, CA3 and CA1 regions. DBS failed to normalize the increased levels of TNFα and the proinflammatory cytokine IL1β in the perilesional cortex and the hippocampus of the TBI-exposed animals. Pharmacological experiments revealed that ANA-12 administered alongside DBS did not counter the memory improvement observed in ANT stimulated animals.

CONCLUSIONS

DBS delivered immediately after TBI mitigated memory deficits, increased the expression of BDNF and the number of hippocampal cells in rats. Mechanisms for these effects were not related to an anti-inflammatory effect or mediated via TrkB receptors.

Utilizing diffusion tensor imaging as an image biomarker in exploring the therapeutic efficacy of forniceal deep brain stimulation in a mice model of Alzheimer's disease

Objective.With prolonged life expectancy, the incidence of memory deficits, especially in Alzheimer's disease (AD), has increased. Although multiple treatments have been evaluated, no promising treatment has been found to date. Deep brain stimulation (DBS) of the fornix area was explored as a possible treatment because the fornix is intimately connected to memory-related areas that are vulnerable in AD; however, a proper imaging biomarker for assessing the therapeutic efficiency of forniceal DBS in AD has not been established.Approach.This study assessed the efficacy and safety of DBS by estimating the optimal intersection volume between the volume of tissue activated and the fornix. Utilizing a gold-electroplating process, the microelectrode's surface area on the neural probe was increased, enhancing charge transfer performance within potential water window limits. Bilateral fornix implantation was conducted in triple-transgenic AD mice (3 × Tg-AD) and wild-type mice (strain: B6129SF1/J), with forniceal DBS administered exclusively to 3 × Tg-AD mice in the DBS-on group. Behavioral tasks, diffusion tensor imaging (DTI), and immunohistochemistry (IHC) were performed in all mice to assess the therapeutic efficacy of forniceal DBS.Main results.The results illustrated that memory deficits and increased anxiety-like behavior in 3 × Tg-AD mice were rescued by forniceal DBS. Furthermore, forniceal DBS positively altered DTI indices, such as increasing fractional anisotropy (FA) and decreasing mean diffusivity (MD), together with reducing microglial cell and astrocyte counts, suggesting a potential causal relationship between revised FA/MD and reduced cell counts in the anterior cingulate cortex, hippocampus, fornix, amygdala, and entorhinal cortex of 3 × Tg-AD mice following forniceal DBS.Significance.The efficacy of forniceal DBS in AD can be indicated by alterations in DTI-based biomarkers reflecting the decreased activation of glial cells, suggesting reduced neural inflammation as evidenced by improvements in memory and anxiety-like behavior.

Techniques for Measurement of Serotonin: Implications in Neuropsychiatric Disorders and Advances in Absolute Value Recording Methods

Serotonin (5-HT) is a monoamine neurotransmitter in the peripheral, enteric, and central nervous systems (CNS). Within the CNS, serotonin is principally involved in mood regulation and reward-seeking behaviors. It is a critical regulator in CNS pathologies such as major depressive disorder, addiction, and schizophrenia. Consequently, in vivo serotonin measurements within the CNS have emerged as one of many promising approaches to investigating the pathogenesis, progression, and treatment of these and other neuropsychiatric conditions. These techniques vary in methods, ranging from analyte sampling with microdialysis to voltammetry. Provided this diversity in approach, inherent differences between techniques are inevitable. These include biosensor size, temporal/spatial resolution, and absolute value measurement capabilities, all of which must be considered to fit the prospective researcher's needs. In this review, we summarize currently available methods for the measurement of serotonin, including novel voltammetric absolute value measurement techniques. We also detail serotonin's role in various neuropsychiatric conditions, highlighting the role of phasic and tonic serotonergic neuronal firing within each where relevant. Lastly, we briefly review the present clinical application of these techniques and discuss the potential of a closed-loop monitoring and neuromodulation system utilizing deep brain stimulation (DBS).

Forniceal deep brain stimulation in a mouse model of Rett syndrome increases neurogenesis and hippocampal memory beyond the treatment period

BACKGROUND

Rett syndrome (RTT), caused by mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MeCP2), severely impairs learning and memory. We previously showed that forniceal deep brain stimulation (DBS) stimulates hippocampal neurogenesis with concomitant improvements in hippocampal-dependent learning and memory in a mouse model of RTT.

OBJECTIVES

To determine the duration of DBS benefits; characterize DBS effects on hippocampal neurogenesis; and determine whether DBS influences MECP2 genotype and survival of newborn dentate granular cells (DGCs) in RTT mice.

METHODS

Chronic DBS was delivered through an electrode implanted in the fimbria-fornix. We tested separate cohorts of mice in contextual and cued fear memory at different time points after DBS. We then examined neurogenesis, DGC apoptosis, and the expression of brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) after DBS by immunohistochemistry.

RESULTS

After two weeks of forniceal DBS, memory improvements lasted between 6 and 9 weeks. Repeating DBS every 6 weeks was sufficient to maintain the improvement. Forniceal DBS stimulated the birth of more MeCP2-positive than MeCP2-negative DGCs and had no effect on DGC survival. It also increased the expression of BDNF but not VEGF in the RTT mouse dentate gyrus.

CONCLUSION

Improvements in learning and memory from forniceal DBS in RTT mice extends well beyond the treatment period and can be maintained by repeated DBS. Stimulation of BDNF expression correlates with improvements in hippocampal neurogenesis and memory benefits.

Deep brain stimulation of fornix in Alzheimer's disease: From basic research to clinical practice

Alzheimer's disease (AD) is one of the most common progressive neurodegenerative diseases associated with the degradation of memory and cognitive ability. Current pharmacotherapies show little therapeutic effect in AD treatment and still cannot prevent the pathological progression of AD. Deep brain stimulation (DBS) has shown to enhance memory in morbid obese, epilepsy and traumatic brain injury patients, and cognition in Parkinson's disease (PD) patients deteriorates during DBS off. Some relevant animal studies and clinical trials have been carried out to discuss the DBS treatment for AD. Reviewing the fornix trials, no unified conclusion has been reached about the clinical benefits of DBS in AD, and the dementia ratings scale has not been effectively improved in the long term. However, some patients have presented promising results, such as improved glucose metabolism, increased connectivity in cognition-related brain regions and even elevated cognitive function rating scale scores. The fornix plays an important regulatory role in memory, attention, and emotion through its complex fibre projection to cognition-related structures, making it a promising target for DBS for AD treatment. Moreover, the current stereotaxic technique and various evaluation methods have provided references for the operator to select accurate stimulation points. Related adverse events and relatively higher costs in DBS have been emphasized. In this article, we summarize and update the research progression on fornix DBS in AD and seek to provide a reliable reference for subsequent experimental studies on DBS treatment of AD.

Intracranial Self-stimulation of the Medial Forebrain Bundle Ameliorates Memory Disturbances and Pathological Hallmarks in an Alzheimer's Disease Model by Intracerebral Administration of Amyloid-β in Rats

No curative or fully effective treatments are currently available for Alzheimer's disease (AD), the most common form of dementia. Electrical stimulation of deep brain areas has been proposed as a novel neuromodulatory therapeutic approach. Previous research from our lab demonstrates that intracranial self-stimulation (ICSS) targeting medial forebrain bundle (MFB) facilitates explicit and implicit learning and memory in rats with age or lesion-related memory impairment. At a molecular level, MFB-ICSS modulates the expression of plasticity and neuroprotection-related genes in memory-related brain areas. On this basis, we suggest that MFB could be a promising stimulation target for AD treatment. In this study, we aimed to assess the effects of MFB-ICSS on both explicit memory as well as the levels of neuropathological markers ptau and drebrin (DBN) in memory-related areas, in an AD rat model obtained by Aβ icv-injection. A total of 36 male rats were trained in the Morris water maze on days 26-30 after Aβ injection and tested on day 33. Results demonstrate that this Aβ model displayed spatial memory impairment in the retention test, accompanied by changes in the levels of DBN and ptau in lateral entorhinal cortex and hippocampus, resembling pathological alterations in early AD. Administration of MFB-ICSS treatment consisting of 5 post-training sessions to AD rats managed to reverse the memory deficits as well as the alteration in ptau and DBN levels. Thus, this paper reports both cognitive and molecular effects of a post-training reinforcing deep brain stimulation procedure in a sporadic AD model for the first time.

Deep brain stimulation of fornix for memory improvement in Alzheimer's disease: A critical review

Memory reflects the brain function in encoding, storage and retrieval of the data or information, which is a fundamental ability for any live organism. The development of approaches to improve memory attracts much attention due to the underlying mechanistic insight and therapeutic potential to treat neurodegenerative diseases with memory loss, such as Alzheimer's disease (AD). Deep brain stimulation (DBS), a reversible, adjustable, and non-ablative therapy, has been shown to be safe and effective in many clinical trials for neurodegenerative and neuropsychiatric disorders. Among all potential regions with access to invasive electrodes, fornix is considered as it is the major afferent and efferent connection of the hippocampus known to be closely associated with learning and memory. Indeed, clinical trials have demonstrated that fornix DBS globally improved cognitive function in a subset of patients with AD, indicating fornix can serve as a potential target for neurosurgical intervention in treating memory impairment in AD. The present review aims to provide a better understanding of recent progresses in the application of fornix DBS for ameliorating memory impairments in AD patients.

Defining a Path Toward the Use of Fast-Scan Cyclic Voltammetry in Human Studies

Fast Scan Cyclic Voltammetry (FSCV) has been used for decades as a neurochemical tool for in vivo detection of phasic changes in electroactive neurotransmitters in animal models. Recently, multiple research groups have initiated human neurochemical studies using FSCV or demonstrated interest in bringing FSCV into clinical use. However, there remain technical challenges that limit clinical implementation of FSCV by creating barriers to appropriate scientific rigor and patient safety. In order to progress with clinical FSCV, these limitations must be first addressed through (1) appropriate pre-clinical studies to ensure accurate measurement of neurotransmitters and (2) the application of a risk management framework to assess patient safety. The intent of this work is to bring awareness of the current issues associated with FSCV to the scientific, engineering, and clinical communities and encourage them to seek solutions or alternatives that ensure data accuracy, rigor and reproducibility, and patient safety.

Neuromodulation in the Treatment of Alzheimer's Disease: Current and Emerging Approaches

Neuromodulation as a treatment strategy for psychiatric and neurological diseases has grown in popularity in recent years, with the approval of repetitive transcranial magnetic stimulation (rTMS) for the treatment of depression being one such example. These approaches offer new hope in the treatment of diseases that have proven largely intractable to traditional pharmacological approaches. For this reason, neuromodulation is increasingly being explored for the treatment of Alzheimer's disease. However, such approaches have variable, and, in many cases, very limited evidence for safety and efficacy, with most human evidence obtained in small clinical trials. Here we review work in animal models and humans with Alzheimer's disease exploring emerging neuromodulation modalities. Approaches reviewed include deep brain stimulation, transcranial magnetic stimulation, transcranial electrical stimulation, ultrasound stimulation, photobiomodulation, and visual or auditory stimulation. In doing so, we clarify the current evidence for these approaches in treating Alzheimer's disease and identify specific areas where additional work is needed to facilitate their clinical translation.

Orientation-selective and directional deep brain stimulation in swine assessed by functional MRI at 3T

Functional MRI (fMRI) has become an important tool for probing network-level effects of deep brain stimulation (DBS). Previous DBS-fMRI studies have shown that electrical stimulation of the ventrolateral (VL) thalamus can modulate sensorimotor cortices in a frequency and amplitude dependent manner. Here, we investigated, using a swine animal model, how the direction and orientation of the electric field, induced by VL-thalamus DBS, affects activity in the sensorimotor cortex. Adult swine underwent implantation of a novel 16-electrode (4 rows × 4 columns) directional DBS lead in the VL thalamus. A within-subject design was used to compare fMRI responses for (1) directional stimulation consisting of monopolar stimulation in four radial directions around the DBS lead, and (2) orientation-selective stimulation where an electric field dipole was rotated 0°−360° around a quadrangle of electrodes. Functional responses were quantified in the premotor, primary motor, and somatosensory cortices. High frequency electrical stimulation through leads implanted in the VL thalamus induced directional tuning in cortical response patterns to varying degrees depending on DBS lead position. Orientation-selective stimulation showed maximal functional response when the electric field was oriented approximately parallel to the DBS lead, which is consistent with known axonal orientations of the cortico-thalamocortical pathway. These results demonstrate that directional and orientation-selective stimulation paradigms in the VL thalamus can tune network-level modulation patterns in the sensorimotor cortex, which may have translational utility in improving functional outcomes of DBS therapy.

Modulation of Human Memory by Deep Brain Stimulation of the Entorhinal-Hippocampal Circuitry

Neurological disorders affecting human memory present a major scientific, medical, and societal challenge. Direct or indirect deep brain stimulation (DBS) of the entorhinal-hippocampal system, the brain's major memory hub, has been studied in people with epilepsy or Alzheimer's disease, intending to enhance memory performance or slow memory decline. Variability in the spatiotemporal parameters of stimulation employed to date notwithstanding, it is likely that future DBS for memory will employ closed-loop, nuanced approaches that are synergistic with native physiological processes. The potential for editing human memory-decoding, enhancing, incepting, or deleting specific memories-suggests exciting therapeutic possibilities but also raises considerable ethical concerns.

Frequency-dependent electrical stimulation of fimbria-fornix preferentially affects the mesolimbic dopamine system or prefrontal cortex

BACKGROUND

The fimbria/fornix fiber system is an essential part of the hippocampal-VTA loop, and therefore activities that are propagated through this fiber system control the activity of the mesolimbic dopamine system.

OBJECTIVES/HYPOTHESIS

We hypothesized that stimulation of the fimbria/fornix with an increasing number of electrical pulses would cause increasing activity of the mesolimbic dopamine system, which coincides with concurrent changes in neuronal activities in target regions of the mesolimbic dopaminergic system.

METHODS

Right fimbria/fornix fibers were electrically stimulated with different pulse protocols. Stimulus-induced changes in neuronal activities were visualized with BOLD-fMRI, whereas stimulus-induced release of dopamine, as measured for the activity of the mesolimbic dopamine system, was determined in the nucleus accumbens with in vivo fast-scan cyclic voltammetry.

RESULTS

Dependent on the protocol, electrical fimbria/fornix stimulation caused BOLD responses in various targets of the mesolimbic dopamine system. Stimulation in the low theta frequency range (5 Hz) triggered significant BOLD responses mainly in the hippocampal formation, infralimbic cortex, and septum. Stimulation in the beta frequency range (20 Hz) caused additional activation in the medial prefrontal cortex (mPFC), nucleus accumbens, striatum, and VTA. Stimulation in the high-gamma frequency range (100 Hz) caused further activation in the hippocampus proper and mPFC. The strong activation in the mPFC during 100 Hz stimulations depended not only on the number of pulses but also on the frequency. Thus, short bursts of 5 or 20 high-frequency pulses caused stronger activation in the mPFC than continuous 5 or 20 Hz pulses. In contrast, high-frequency burst fimbria/fornix stimulation did not further activate the mesolimbic dopamine system when compared to continuous 5 or 20 Hz pulse stimulation.

CONCLUSIONS

There exists a frequency-dependent dissociation between BOLD responses and activation of the dopaminergic system. Low frequencies were more efficient to activate the mesolimbic dopamine system, whereas high frequencies were more efficient to trigger BOLD responses in target regions of the mesolimbic dopamine system, particularly the mPFC.

Rostral anterior cingulate glutamate predicts response to subcallosal deep brain stimulation for resistant depression

BACKGROUND

Deep brain stimulation (DBS) of the subcallosal cingulate (SCC) provided benefit for treatment-resistant depression (TRD) in open-label studies but failed in a recent randomized sham-controlled trial. Informed patient selection, based on reliable biomarkers, is needed to optimize outcome. We investigated if rostral anterior cingulate (rACC) glutamate/glutamine concentration could serve as a potential biomarker of response.

METHODS

Sixteen adults with TRD (Major Depression; MDD = 14; Bipolar Depression; BD =2) underwent proton magnetic resonance spectroscopy using a short-echo proton spectroscopy with a voxel placed in the rACC, prior to DBS. Improvement in depression was assessed using the 17-item Hamilton Rating Scale for Depression (HDRS). Glutamate and glutamine concentrations at baseline in the rACC were examined in relation to clinical outcomes at six months.

RESULTS

Lower baseline glutamate predicted significant reduction in HDRS scores in all TRD patients (p = 0.018), and predicted both HDRS reduction (p = 0.002) and 6-month response outcome in MDD-TRD patients (p = 0.013). Neither baseline glutamine nor glutamine/glutamate ratio significantly related to outcome or symptom improvement.

LIMITATIONS

Our study was limited by sample size, though it is large for a DBS study. We measured from a single voxel in the brain, so we cannot be certain our findings are specific to the rACC.

CONCLUSIONS

These preliminary results suggest that baseline rACC-glutamate concentration could serve as a response-predictive biomarker for SCC-DBS, particularly in patients with resistant major depression. If our findings are replicated and validated, rACC-glutamate may provide a basis to prospectively select TRD patients to improve likelihood of response to SCC-DBS.

Fornix Stimulation Induces Metabolic Activity and Dopaminergic Response in the Nucleus Accumbens

The Papez circuit, including the fornix white matter bundle, is a well-known neural network that is involved in multiple limbic functions such as memory and emotional expression. We previously reported a large-animal study of deep brain stimulation (DBS) in the fornix that found stimulation-induced hemodynamic responses in both the medial limbic and corticolimbic circuits on functional resonance imaging (fMRI) and evoked dopamine responses in the nucleus accumbens (NAc), as measured by fast-scan cyclic voltammetry (FSCV). The effects of DBS on the fornix are challenging to analyze, given its structural complexity and connection to multiple neuronal networks. In this study, we extend our earlier work to a rodent model wherein we characterize regional brain activity changes resulting from fornix stimulation using fludeoxyglucose (¹⁸F-FDG) micro positron emission tomography (PET) and monitor neurochemical changes using FSCV with pharmacological confirmation. Both global functional changes and local changes were measured in a rodent model of fornix DBS. Functional brain activity was measured by micro-PET, and the neurochemical changes in local areas were monitored by FSCV. Micro-PET images revealed increased glucose metabolism within the medial limbic and corticolimbic circuits. Neurotransmitter efflux induced by fornix DBS was monitored at NAc by FSCV and identified by specific neurotransmitter reuptake inhibitors. We found a significant increase in the metabolic activity in several key regions of the medial limbic circuits and dopamine efflux in the NAc following fornix stimulation. These results suggest that electrical stimulation of the fornix modulates the activity of brain memory circuits, including the hippocampus and NAc within the dopaminergic pathway.

A circuit view of deep brain stimulation in Alzheimer's disease and the possible mechanisms

Alzheimer's disease (AD) is characterized by chronic progressive cognitive deterioration frequently accompanied by psychopathological symptoms, including changes in personality and social isolation, which severely reduce quality of life. Currently, no viable therapies or present-day drugs developed for the treatment of AD symptoms are able to slow or reverse AD progression or prevent the advance of neurodegeneration. As such, non-drug alternatives are currently being tested, including deep brain stimulation (DBS). DBS is an established therapy for several neurological and psychiatric indications, such as movement disorders. Studies assessing DBS for other disorders have also found improvements in cognitive function, providing the impetus for clinical trials on DBS for AD. Targets of DBS in AD clinical trials and animal model studies include the fornix, entorhinal cortex (EC), nucleus basalis of Meynert (NBM), and vertical limb of diagonal band (VDB). However, there is still no comprehensive theory explaining the effects of DBS on AD symptoms or a consensus on which targets provide optimal benefits. This article reviews the anatomy of memory circuits related to AD, as well as studies on DBS rescue of AD in these circuits and the possible therapeutic mechanisms.

The Relationship Between Dopamine Neurotransmitter Dynamics and the Blood-Oxygen-Level-Dependent (BOLD) Signal: A Review of Pharmacological Functional Magnetic Resonance Imaging

Functional magnetic resonance imaging (fMRI) is widely used in investigations of normal cognition and brain disease and in various clinical applications. Pharmacological fMRI (pharma-fMRI) is a relatively new application, which is being used to elucidate the effects and mechanisms of pharmacological modulation of brain activity. Characterizing the effects of neuropharmacological agents on regional brain activity using fMRI is challenging because drugs modulate neuronal function in a wide variety of ways, including through receptor agonist, antagonist, and neurotransmitter reuptake blocker events. Here we review current knowledge on neurotransmitter-mediated blood-oxygen-level dependent (BOLD) fMRI mechanisms as well as recently updated methodologies aimed at more fully describing the effects of neuropharmacologic agents on the BOLD signal. We limit our discussion to dopaminergic signaling as a useful lens through which to analyze and interpret neurochemical-mediated changes in the hemodynamic BOLD response. We also discuss the need for future studies that use multi-modal approaches to expand the understanding and application of pharma-fMRI.

Pain After Spinal Cord Injury Is Associated With Abnormal Presynaptic Inhibition in the Posterior Nucleus of the Thalamus

Pain after spinal cord injury (SCI-Pain) is one of the most debilitating sequelae of spinal cord injury, characterized as relentless, excruciating pain that is largely refractory to treatments. Although it is generally agreed that SCI-Pain results from maladaptive plasticity in the pain processing pathway that includes the spinothalamic tract and somatosensory thalamus, the specific mechanisms underlying the development and maintenance of such pain are yet unclear. However, accumulating evidence suggests that SCI-Pain may be causally related to abnormal thalamic disinhibition, leading to hyperactivity in the posterior thalamic nucleus (PO), a higher-order nucleus involved in somatosensory and pain processing. We previously described several presynaptic mechanisms by which activity in PO is regulated, including the regulation of GABAergic as well as glutamatergic release by presynaptic metabotropic gamma-aminobutyric acid (GABA_B) receptors. Using acute slices from a mouse model of SCI-Pain, we tested whether such mechanisms are affected by SCI-Pain. We reveal 2 abnormal changes in presynaptic signaling in the SCI-Pain condition. The substantial tonic activation of presynaptic GABA_B receptors on GABAergic projections to PO-characteristic of normal animals-was absent in mice with SCI-Pain. Also absent in mice with SCI-Pain was the normal presynaptic regulation of glutamatergic projections to the PO by GABA_B receptors. The loss of these regulatory presynaptic mechanisms in SCI-Pain may be an element of maladaptive plasticity leading to PO hyperexcitability and behavioral pain, and may suggest targets for development of novel treatments.

PERSPECTIVE

This report presents synaptic mechanisms that may underlie the development and maintenance of SCI-Pain. Because of the difficulty in treating SCI-Pain, a better understanding of the underlying neurobiological mechanisms is critical, and may allow development of better treatment modalities.

Neurostimulation Devices for the Treatment of Neurologic Disorders

Rapid advancements in neurostimulation technologies are providing relief to an unprecedented number of patients affected by debilitating neurologic and psychiatric disorders. Neurostimulation therapies include invasive and noninvasive approaches that involve the application of electrical stimulation to drive neural function within a circuit. This review focuses on established invasive electrical stimulation systems used clinically to induce therapeutic neuromodulation of dysfunctional neural circuitry. These implantable neurostimulation systems target specific deep subcortical, cortical, spinal, cranial, and peripheral nerve structures to modulate neuronal activity, providing therapeutic effects for a myriad of neuropsychiatric disorders. Recent advances in neurotechnologies and neuroimaging, along with an increased understanding of neurocircuitry, are factors contributing to the rapid rise in the use of neurostimulation therapies to treat an increasingly wide range of neurologic and psychiatric disorders. Electrical stimulation technologies are evolving after remaining fairly stagnant for the past 30 years, moving toward potential closed-loop therapeutic control systems with the ability to deliver stimulation with higher spatial resolution to provide continuous customized neuromodulation for optimal clinical outcomes. Even so, there is still much to be learned about disease pathogenesis of these neurodegenerative and psychiatric disorders and the latent mechanisms of neurostimulation that provide therapeutic relief. This review provides an overview of the increasingly common stimulation systems, their clinical indications, and enabling technologies.

Correction of metal-induced susceptibility artifacts for functional MRI during deep brain stimulation

Functional magnetic resonance imaging (fMRI) is an emerging tool for investigating brain activation associated with, or modulated by, deep brain stimulation (DBS). However, DBS-fMRI generally suffers from severe susceptibility to artifacts in regions near the metallic stimulation electrodes, as well as near tissue/air boundaries of the brain. These result in strong intensity and geometric distortions along the phase-encoding (PE) (i.e., blipped) direction in gradient-echo echo-planar imaging (GE-EPI). Distortion presents a major challenge to conducting reliable data analysis and in interpreting the findings. A recent study showed that the point spread function (PSF) mapping-based reverse gradient approach has a potential to correct for distortions not only in spin-echo EPI, but also in GE-EPI acquired in both the forward and reverse PE directions. In this study, we adapted that approach in order to minimize severe metal-induced susceptibility artifacts for DBS-fMRI, and to evaluate the performance of the approach in a phantom study and a large animal DBS-fMRI study. The method combines the distortion-corrected GE-EPI pair with geometrically different intensity distortions due to the opposing encoding directions. The results demonstrate that the approach can minimize susceptibility artifacts that appear around the metallic electrodes, as well as in the regions near the tissue/air boundaries in the brain. We also demonstrated that an accurate geometric correction is important in improving BOLD contrast in the group dataset, especially in regions where strong susceptibility artifacts appear.

Acute pressure changes in the brain are correlated with MR elastography stiffness measurements: initial feasibility in an in vivo large animal model

PURPOSE

The homeostasis of intracranial pressure (ICP) is of paramount importance for maintaining normal brain function. A noninvasive technique capable of making direct measurements of ICP currently does not exist. MR elastography (MRE) is capable of noninvasively measuring brain tissue stiffness in vivo, and may act as a surrogate to measure ICP. The objective of this study was to investigate the impact of changing ICP on brain stiffness using MRE in a swine model.

METHODS

Baseline MRE measurements were obtained, and then catheters were surgically placed into the left and right lateral ventricles of three animals. ICP was systematically increased over the range of 0 to 55 millimeters mercury (mmHg), and stiffness measurements were made using brain MRE at vibration frequencies of 60 hertz (Hz), 90 Hz, 120 Hz, and 150 Hz.

RESULTS

A significant linear correlation between stiffness and ICP in the cross-subject comparison was observed for all tested vibrational frequencies (P ≤ 0.01). The 120 Hz (0.030 ± 0.004 kilopascal (kPa)/mmHg, P < 0.0001) and 150 Hz (0.031 ± 0.008 kPa/mmHg, P = 0.01) vibrational frequencies had nearly identical slopes, which were approximately two- to three-fold higher than the 90 Hz (0.017 ± 0.002 kPa/mmHg, P < 0.0001) and 60 Hz (0.009 ± 0.002 kPa/mmHg, P = 0.001) slopes, respectively.

CONCLUSION

In this study, MRE demonstrated the potential for noninvasive measurement of changes in ICP. Magn Reson Med 79:1043-1051, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Subcellular probes for neurochemical recording from multiple brain sites

Dysregulation of neurochemicals, in particular, dopamine, is epitomized in numerous debilitating disorders that impair normal movement and mood aspects of our everyday behavior. Neurochemical transmission is a neuron-specific process, and further exhibits region-specific signaling in the brain. Tools are needed to monitor the heterogeneous spatiotemporal dynamics of dopamine neurotransmission without compromising the physiological processes of the neuronal environment. We developed neurochemical probes that are ten times smaller than any existing dopamine sensor, based on the size of the entire implanted shaft and its sensing tip. The microfabricated probe occupies a spatial footprint (9 μm) coordinate with the average size of individual neuronal cells (∼10 μm). These cellular-scale probes were shown to reduce inflammatory response of the implanted brain tissue environment. The probes are further configured in the form of a microarray to permit electrochemical sampling of dopamine and other neurotransmitters at unprecedented spatial densities and distributions. Dopamine recording was performed concurrently from up to 16 sites in the striatum of rats, revealing a remarkable spatiotemporal contrast in dopamine transmission as well as site-specific pharmacological modulation. Collectively, the reported platform endeavors to enable high density mapping of the chemical messengers fundamentally involved in neuronal communication through the use of minimally invasive probes that help preserve the neuronal viability of the implant environment.

Currents of memory: recent progress, translational challenges, and ethical considerations in fornix deep brain stimulation trials for Alzheimer's disease

The serendipitous discovery of triggered autobiographical memories and eventual memory improvement in an obese patient who received fornix deep brain stimulation in 2008 paved the way for several phase I and phase II clinical trials focused on the safety and efficacy of this potential intervention for people with Alzheimer's disease. In this article, we summarize clinical trials and case reports on fornix deep brain stimulation for Alzheimer's disease and review experiments on animal models evaluating the physiological or behavioral effects of this intervention. Based on information from these reports and studies, we identify potential translational challenges of this approach and determine practical and ethical considerations for clinical trials, focusing on issues regarding selection criteria, trial design, and outcome evaluation. Based on initial results suggesting greater benefit for those with milder disease stage, we find it essential that participant expectations are carefully managed to avoid treatment disenchantment and/or frustration from participants and caregivers. Finally, we urge for collaboration between centers to establish proper clinical standards and to promote better trial results comparison.

Functional Circuitry Effect of Ventral Tegmental Area Deep Brain Stimulation: Imaging and Neurochemical Evidence of Mesocortical and Mesolimbic Pathway Modulation

Background: The ventral tegmental area (VTA), containing mesolimbic and mesocortical dopaminergic neurons, is implicated in processes involving reward, addiction, reinforcement, and learning, which are associated with a variety of neuropsychiatric disorders. Electrical stimulation of the VTA or the medial forebrain bundle and its projection target the nucleus accumbens (NAc) is reported to improve depressive symptoms in patients affected by severe, treatment-resistant major depressive disorder (MDD) and depressive-like symptoms in animal models of depression. Here we sought to determine the neuromodulatory effects of VTA deep brain stimulation (DBS) in a normal large animal model (swine) by combining neurochemical measurements with functional magnetic resonance imaging (fMRI). Methods: Animals (n = 8 swine) were implanted with a unilateral DBS electrode targeting the VTA. During stimulation (130 Hz frequency, 0.25 ms pulse width, and 3 V amplitude), fMRI was performed. Following fMRI, fast-scan cyclic voltammetry in combination with carbon fiber microelectrodes was performed to quantify VTA-DBS-evoked dopamine release in the ipsilateral NAc. In a subset of swine, the blood oxygen level-dependent (BOLD) percent change evoked by stimulation was performed at increasing voltages (1, 2, and 3 V). Results: A significant increase in VTA-DBS-evoked BOLD signal was found in the following regions: the ipsilateral dorsolateral prefrontal cortex, anterior and posterior cingulate, insula, premotor cortex, primary somatosensory cortex, and striatum. A decrease in the BOLD signal was also observed in the contralateral parahippocampal cortex, dorsolateral and anterior prefrontal cortex, insula, inferior temporal gyrus, and primary somatosensory cortex (Bonferroni-corrected < 0.001). During neurochemical measurements, stimulation time-locked changes in dopamine release were recorded in the NAc, confirming that mesolimbic dopaminergic neurons were stimulated by DBS. In the parametric study, BOLD signal changes were positively correlated with stimulation amplitude. Conclusions: In this study, the modulation of the neural circuitry associated with VTA-DBS was characterized in a large animal. Our findings suggest that VTA-DBS could affect the activity of neural systems and brain regions implicated in reward, mood regulation, and in the pathophysiology of MDD. In addition, we showed that a combination of fMRI and electrochemically-based neurochemical detection platform is an effective investigative tool for elucidating the circuitry involved in VTA-DBS.

Functional circuit mapping of striatal output nuclei using simultaneous deep brain stimulation and fMRI

The substantia nigra pars reticulata (SNr) and external globus pallidus (GPe) constitute the two major output targets of the rodent striatum. Both the SNr and GPe converge upon thalamic relay nuclei (directly or indirectly, respectively), and are traditionally modeled as functionally antagonistic relay inputs. However, recent anatomical and functional studies have identified unanticipated circuit connectivity in both the SNr and GPe, demonstrating their potential as far more than relay nuclei. In the present study, we employed simultaneous deep brain stimulation and functional magnetic resonance imaging (DBS-fMRI) with cerebral blood volume (CBV) measurements to functionally and unbiasedly map the circuit- and network level connectivity of the SNr and GPe. Sprague-Dawley rats were implanted with a custom-made MR-compatible stimulating electrode in the right SNr (n=6) or GPe (n=7). SNr- and GPe-DBS, conducted across a wide range of stimulation frequencies, revealed a number of surprising evoked responses, including unexpected CBV decreases within the striatum during DBS at either target, as well as GPe-DBS-evoked positive modulation of frontal cortex. Functional connectivity MRI revealed global modulation of neural networks during DBS at either target, sensitive to stimulation frequency and readily reversed following cessation of stimulation. This work thus contributes to a growing literature demonstrating extensive and unanticipated functional connectivity among basal ganglia nuclei.

Monitoring In Vivo Changes in Tonic Extracellular Dopamine Level by Charge-Balancing Multiple Waveform Fast-Scan Cyclic Voltammetry

Dopamine (DA) modulates central neuronal activity through both phasic (second to second) and tonic (minutes to hours) terminal release. Conventional fast-scan cyclic voltammetry (FSCV), in combination with carbon fiber microelectrodes, has been used to measure phasic DA release in vivo by adopting a background subtraction procedure to remove background capacitive currents. However, measuring tonic changes in DA concentrations using conventional FSCV has been difficult because background capacitive currents are inherently unstable over long recording periods. To measure tonic changes in DA concentrations over several hours, we applied a novel charge-balancing multiple waveform FSCV (CBM-FSCV), combined with a dual background subtraction technique, to minimize temporal variations in background capacitive currents. Using this method, in vitro, charge variations from a reference time point were nearly zero for 48 h, whereas with conventional background subtraction, charge variations progressively increased. CBM-FSCV also demonstrated a high selectivity against 3,4-dihydroxyphenylacetic acid and ascorbic acid, two major chemical interferents in the brain, yielding a sensitivity of 85.40 ± 14.30 nA/μM and limit of detection of 5.8 ± 0.9 nM for DA while maintaining selectivity. Recorded in vivo by CBM-FSCV, pharmacological inhibition of DA reuptake (nomifensine) resulted in a 235 ± 60 nM increase in tonic extracellular DA concentrations, while inhibition of DA synthesis (α-methyl-dl-tyrosine) resulted in a 72.5 ± 4.8 nM decrease in DA concentrations over a 2 h period. This study showed that CBM-FSCV may serve as a unique voltammetric technique to monitor relatively slow changes in tonic extracellular DA concentrations in vivo over a prolonged time period.

Anterior Thalamic Deep Brain Stimulation: Functional Activation Patterns in a Large Animal Model

BACKGROUND

Deep brain stimulation (DBS) of the anterior thalamic nucleus (ATN) exerts its effects by modulating neural circuits involved in seizures. However, these networks remain incompletely characterized.

OBJECTIVE

Investigate the effects of ATN DBS on network activity in a large animal model using 3-T fMRI.

METHODS

Anesthetized swine underwent ATN DBS using stimulation parameters applied in the Stimulation of the Anterior Thalamus for the Treatment of Epilepsy (SANTE) trial. Stimulation amplitude, frequency, and temporal paradigm were varied and the resulting blood oxygen level-dependent signal was measured.

RESULTS

ATN DBS resulted in activation within temporal, prefrontal, and sensorimotor cortex. An amplitude-dependent increase in cluster volume was observed at 60 Hz and 145 Hz stimulation.

CONCLUSION

ATN DBS in swine induced parameter-dependent activation in cortical regions including but not limited to the Papez circuit. These findings may hold clinical implications for treatment of epilepsy in patients with temporal or extratemporal seizure foci.