Optical deconstruction of parkinsonian neural circuitry

Sodium dynamics: another key to astroglial excitability?

Astroglial excitability is largely mediated by fluctuations in intracellular ion concentrations. In addition to generally acknowledged Ca²⁺ excitability of astroglia, recent studies have demonstrated that neuronal activity triggers transient increases in the cytosolic Na⁺ concentration ([Na⁺](i)) in perisynaptic astrocytes. These [Na⁺](i) transients are controlled by multiple Na⁺-permeable channels and Na⁺-dependent transporters; spatiotemporally organized [Na⁺](i) dynamics in turn regulate diverse astroglial homeostatic responses such as metabolic/signaling utilization of lactate and glutamate, transmembrane transport of neurotransmitters and K⁺ buffering. In particular, near-membrane [Na⁺](i) transients determine the rate and the direction of the transmembrane transport of GABA and Ca²⁺. We discuss here the role of Na⁺ in the regulation of various systems that mediate fast bidirectional communication between neurones and glia at the single synapse level.

Intermittent neural synchronization in Parkinson's disease

Motor symptoms of Parkinson's disease are related to the excessive synchronized oscillatory activity in the beta frequency band (around 20Hz) in the basal ganglia and other parts of the brain. This review explores the dynamics and potential mechanisms of these oscillations employing ideas and methods from nonlinear dynamics. We present extensive experimental documentation of the relevance of synchronized oscillations to motor behavior in Parkinson's disease, and we discuss the intermittent character of this synchronization. The reader is introduced to novel time-series analysis techniques aimed at the detection of the fine temporal structure of intermittent phase locking observed in the brains of parkinsonian patients. Modeling studies of brain networks are reviewed, which may describe the observed intermittent synchrony, and we discuss what these studies reveal about brain dynamics in Parkinson's disease. The parkinsonian brain appears to exist on the boundary between phase-locked and nonsynchronous dynamics. Such a situation may be beneficial in the healthy state, as it may allow for easy formation and dissociation of transient patterns of synchronous activity which are required for normal motor behavior. Dopaminergic degeneration in Parkinson's disease may shift the brain networks closer to this boundary, which would still permit some motor behavior while accounting for the associated motor deficits. Understanding the mechanisms of the intermittent synchrony in Parkinson's disease is also important for biomedical engineering since efficient control strategies for suppression of pathological synchrony through deep brain stimulation require knowledge of the dynamics of the processes subjected to control.

"Connectomic surgery": diffusion tensor imaging (DTI) tractography as a targeting modality for surgical modulation of neural networks

Deep brain stimulation (DBS) is being used to treat a growing number of neurological disorders. Until recently, DBS has been thought to act mainly by suppressing local neuronal activity, essentially producing a functional lesion. Numerous studies are now demonstrating that DBS has widespread network effects mediated by white matter pathways. The new science of connectomics aims to map the connectivity between brain regions in health and disease. Targeting DBS specifically to pathways which exhibit pathological connectivity could greatly expand the possibilities for treating brain diseases. This brief review examines the current state of brain imaging for visualization of these networks and describes how DBS might be used to restore normal connectivity in pathological states.

Temporal convergence of dynamic cell assemblies in the striato-pallidal network

The basal ganglia (BG) have been hypothesized to implement a reinforcement learning algorithm. However, it is not clear how information is processed along this network, thus enabling it to perform its functional role. Here we present three different encoding schemes of visual cues associated with rewarding, neutral, and aversive outcomes by BG neuronal populations. We studied the response profile and dynamical behavior of two populations of projection neurons [striatal medium spiny neurons (MSNs), and neurons in the external segment of the globus pallidus (GPe)], and one neuromodulator group [striatal tonically active neurons (TANs)] from behaving monkeys. MSNs and GPe neurons displayed sustained average activity to cue presentation. The population average response of MSNs was composed of three distinct response groups that were temporally differentiated and fired in serial episodes along the trial. In the GPe, the average sustained response was composed of two response groups that were primarily differentiated by their immediate change in firing rate direction. However, unlike MSNs, neurons in both GPe response groups displayed prolonged and temporally overlapping persistent activity. The putamen TANs stereotyped response was characterized by a single transient response group. Finally, the MSN and GPe response groups reorganized at the outcome epoch, as different task events were reflected in different response groups. Our results strengthen the functional separation between BG neuromodulators and main axis neurons. Furthermore, they reveal dynamically changing cell assemblies in the striatal network of behaving primates. Finally, they support the functional convergence of the MSN response groups onto GPe cells.

Neural targets for relieving parkinsonian rigidity and bradykinesia with pallidal deep brain stimulation

Clinical evidence has suggested that subtle changes in deep brain stimulation (DBS) settings can have differential effects on bradykinesia and rigidity in patients with Parkinson's disease. In this study, we first investigated the degree of improvement in bradykinesia and rigidity during targeted globus pallidus DBS in three 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated rhesus macaques. Behavioral outcomes of DBS were then coupled with detailed, subject-specific computational models of neurons in the globus pallidus internus (GPi), globus pallidus externus (GPe), and internal capsule (IC) to determine which neuronal pathways when modulated with high-frequency electrical stimulation best correlate with improvement in motor symptoms. The modeling results support the hypothesis that multiple neuronal pathways can underlie the therapeutic effect of DBS on parkinsonian bradykinesia and rigidity. Across all three subjects, improvements in rigidity correlated most strongly with spread of neuronal activation into IC, driving a small percentage of fibers within this tract (<10% on average). The most robust effect on bradykinesia resulted from stimulating a combination of sensorimotor axonal projections within the GP, specifically at the site of the medial medullary lamina. Thus the beneficial effects of pallidal DBS for parkinsonian symptoms may occur from multiple targets within and near the target nucleus.

Sound-driven synaptic inhibition in primary visual cortex

Multimodal objects and events activate many sensory cortical areas simultaneously. This is possibly reflected in reciprocal modulations of neuronal activity, even at the level of primary cortical areas. However, the synaptic character of these interareal interactions, and their impact on synaptic and behavioral sensory responses are unclear. Here, we found that activation of auditory cortex by a noise burst drove local GABAergic inhibition on supragranular pyramids of the mouse primary visual cortex, via cortico-cortical connections. This inhibition was generated by sound-driven excitation of a limited number of cells in infragranular visual cortical neurons. Consequently, visually driven synaptic and spike responses were reduced upon bimodal stimulation. Also, acoustic stimulation suppressed conditioned behavioral responses to a dim flash, an effect that was prevented by acute blockade of GABAergic transmission in visual cortex. Thus, auditory cortex activation by salient stimuli degrades potentially distracting sensory processing in visual cortex by recruiting local, translaminar, inhibitory circuits.

Theoretical principles underlying optical stimulation of a channelrhodopsin-2 positive pyramidal neuron

Optogenetics is an emerging field of neuromodulation that permits scaled, millisecond temporal control of the membrane dynamics of genetically targeted cells using light. Optogenetic technology has revolutionized neuroscience research; however, numerous biophysical questions remain on the optical and neuronal factors impacting the modulation of neural activity with photon-sensitive ion channels. To begin to address such questions, we developed a computational tool to explore the underlying principles of optogenetic neural stimulation. This "light-neuron" model consists of theoretical representations of the light dynamics generated by a fiber optic in brain tissue, coupled to a multicompartment cable model of a cortical pyramidal neuron embedded with channelrhodopsin-2 (ChR2) membrane dynamics. Simulations revealed that the large energies required to generate an action potential are primarily due to the limited conductivity of ChR2, and that the major determinants of stimulation threshold are the surface area of illuminated cell membrane and proximity to the light source. Our results predict that the activation threshold is sensitive to many of the properties of ChR2 (density, conductivity, and kinetics), tissue medium (scattering and absorbance), and the fiber-optic light source (diameter and numerical aperture). We also illustrate the impact of redistributing the ChR2 expression density (uniform vs. nonuniform) on the activation threshold. The model system developed in this study represents a scientific instrument to characterize the effects of optogenetic neuromodulation, as well as an engineering design tool to help guide future development of optogenetic technology.

Optogenetic investigation of neural circuits underlying brain disease in animal models

Optogenetic tools have provided a new way to establish causal relationships between brain activity and behaviour in health and disease. Although no animal model captures human disease precisely, behaviours that recapitulate disease symptoms may be elicited and modulated by optogenetic methods, including behaviours that are relevant to anxiety, fear, depression, addiction, autism and parkinsonism. The rapid proliferation of optogenetic reagents together with the swift advancement of strategies for implementation has created new opportunities for causal and precise dissection of the circuits underlying brain diseases in animal models.

In vivo Large-Scale Cortical Mapping Using Channelrhodopsin-2 Stimulation in Transgenic Mice Reveals Asymmetric and Reciprocal Relationships between Cortical Areas

We have mapped intracortical activity in vivo independent of sensory input using arbitrary point channelrhodopsin-2 (ChR2) stimulation and regional voltage sensitive dye imaging in B6.Cg-Tg (Thy1-COP4/EYFP)18Gfng/J transgenic mice. Photostimulation of subsets of deep layer pyramidal neurons within forelimb, barrel, or visual primary sensory cortex led to downstream cortical maps that were dependent on synaptic transmission and were similar to peripheral sensory stimulation. ChR2-evoked maps confirmed homotopic connections between hemispheres and intracortical sensory and motor cortex connections. This ability of optogentically activated subpopulations of neurons to drive appropriate downstream maps suggests that mechanisms exist to allow prototypical cortical maps to self-assemble from the stimulation of neuronal subsets. Using this principle of map self-assembly, we employed ChR2 point stimulation to map connections between cortical areas that are not selectively activated by peripheral sensory stimulation or behavior. Representing the functional cortical regions as network nodes, we identified asymmetrical connection weights in individual nodes and identified the parietal association area as a network hub. Furthermore, we found that the strength of reciprocal intracortical connections between primary and secondary sensory areas are unequal, with connections from primary to secondary sensory areas being stronger than the reciprocal.

CMOS image sensor integrated with micro-LED and multielectrode arrays for the patterned photostimulation and multichannel recording of neuronal tissue

We developed a complementary metal oxide semiconductor (CMOS) integrated device for optogenetic applications. This device can interface via neuronal tissue with three functional modalities: imaging, optical stimulation and electrical recording. The CMOS image sensor was fabricated on 0.35 μm standard CMOS process with built-in control circuits for an on-chip blue light-emitting diode (LED) array. The effective imaging area was 2.0 × 1.8 mm². The pixel array was composed of 7.5 × 7.5 μm² 3-transistor active pixel sensors (APSs). The LED array had 10 × 8 micro-LEDs measuring 192 × 225 μm². We integrated the device with a commercial multichannel recording system to make electrical recordings.

Multiscale autoregressive identification of neuroelectrophysiological systems

Electrical signals between connected neural nuclei are difficult to model because of the complexity and high number of paths within the brain. Simple parametric models are therefore often used. A multiscale version of the autoregressive with exogenous input (MS-ARX) model has recently been developed which allows selection of the optimal amount of filtering and decimation depending on the signal-to-noise ratio and degree of predictability. In this paper, we apply the MS-ARX model to cortical electroencephalograms and subthalamic local field potentials simultaneously recorded from anesthetized rodent brains. We demonstrate that the MS-ARX model produces better predictions than traditional ARX modeling. We also adapt the MS-ARX results to show differences in internuclei predictability between normal rats and rats with 6OHDA-induced parkinsonism, indicating that this method may have broad applicability to other neuroelectrophysiological studies.

Improved sequence learning with subthalamic nucleus deep brain stimulation: evidence for treatment-specific network modulation

We used a network approach to study the effects of anti-parkinsonian treatment on motor sequence learning in humans. Eight Parkinson's disease (PD) patients with bilateral subthalamic nucleus (STN) deep brain stimulation underwent H(2)(15)O positron emission tomography (PET) imaging to measure regional cerebral blood flow (rCBF) while they performed kinematically matched sequence learning and movement tasks at baseline and during stimulation. Network analysis revealed a significant learning-related spatial covariance pattern characterized by consistent increases in subject expression during stimulation (p = 0.008, permutation test). The network was associated with increased activity in the lateral cerebellum, dorsal premotor cortex, and parahippocampal gyrus, with covarying reductions in the supplementary motor area (SMA) and orbitofrontal cortex. Stimulation-mediated increases in network activity correlated with concurrent improvement in learning performance (p < 0.02). To determine whether similar changes occurred during dopaminergic pharmacotherapy, we studied the subjects during an intravenous levodopa infusion titrated to achieve a motor response equivalent to stimulation. Despite consistent improvement in motor ratings during infusion, levodopa did not alter learning performance or network activity. Analysis of learning-related rCBF in network regions revealed improvement in baseline abnormalities with STN stimulation but not levodopa. These effects were most pronounced in the SMA. In this region, a consistent rCBF response to stimulation was observed across subjects and trials (p = 0.01), although the levodopa response was not significant. These findings link the cognitive treatment response in PD to changes in the activity of a specific cerebello-premotor cortical network. Selective modulation of overactive SMA-STN projection pathways may underlie the improvement in learning found with stimulation.

Informing brain connectivity with optogenetic functional magnetic resonance imaging

Optogenetic functional magnetic resonance imaging (ofMRI) is a novel approach that combines optogenetic control of neural circuits with high-field functional MRI. Optogenetics is a neuro-modulation technology in which light-activated trans-membrane conductance regulators are introduced into specifically targeted cell types to allow temporally precise, millisecond-scale activity modulation in vivo. By combining optogenetic control with fMRI readout, neural activity arising from specific circuit elements defined by genetic identity, cell body location, and axonal projection targets can be monitored in vivo across the whole brain. These unique features of ofMRI open new vistas for in vivo characterization of the dense plexus of neural connections according to their type and functionality.

Recombinase-driver rat lines: tools, techniques, and optogenetic application to dopamine-mediated reinforcement

Currently there is no general approach for achieving specific optogenetic control of genetically defined cell types in rats, which provide a powerful experimental system for numerous established neurophysiological and behavioral paradigms. To overcome this challenge we have generated genetically restricted recombinase-driver rat lines suitable for driving gene expression in specific cell types, expressing Cre recombinase under the control of large genomic regulatory regions (200-300 kb). Multiple tyrosine hydroxylase (Th)::Cre and choline acetyltransferase (Chat)::Cre lines were produced that exhibited specific opsin expression in targeted cell types. We additionally developed methods for utilizing optogenetic tools in freely moving rats and leveraged these technologies to clarify the causal relationship between dopamine (DA) neuron firing and positive reinforcement, observing that optical stimulation of DA neurons in the ventral tegmental area (VTA) of Th::Cre rats is sufficient to support vigorous intracranial self-stimulation (ICSS). These studies complement existing targeting approaches by extending the generalizability of optogenetics to traditionally non-genetically-tractable but vital animal models.

Explaining clinical effects of deep brain stimulation through simplified target-specific modeling of the volume of activated tissue

BACKGROUND AND PURPOSE

Although progress has been made in understanding the optimal anatomic structures as target areas for DBS, little effort has been put into modeling and predicting electromagnetic field properties of activated DBS electrodes and understanding their interactions with the adjacent tissue. Currently, DBS is performed with the patient awake to assess the effectiveness and the side effect spectrum of stimulation. This study was designed to create a robust and rather simple numeric and visual tool that provides sufficient and practical relevant information to visualize the patient's individual VAT.

MATERIALS AND METHODS

Multivariate polynomial fitting of previously obtained data from a finite-element model, based on a similar DBS system, was used. The model estimates VAT as a first-approximation sphere around the active DBS contact, using stimulation voltages and individual tissue-electrode impedances. Validation uses data from 2 patients with PD by MR imaging, DTI, fiber tractography, and postoperative CT data.

RESULTS

Our model can predict VAT for impedances between 500 and 2000 Ω with stimulation voltages up to 10 V. It is based on assumptions for monopolar DBS. Evaluation of 2 DBS cases showed a convincing correspondence between predicted VAT and neurologic (side) effects (internal capsule activation).

CONCLUSIONS

Stimulation effects during DBS can be readily explained with this simple VAT model. Its implementation in daily clinical routine might help in understanding the types of tissues activated during DBS. This technique might have the potential to facilitate DBS implantations with the patient under general anesthesia while yielding acceptable clinical effectiveness.

Deep brain stimulation and the role of astrocytes

Deep brain stimulation (DBS) has emerged as a powerful surgical therapy for the management of treatment-resistant movement disorders, epilepsy and neuropsychiatric disorders. Although DBS may be clinically effective in many cases, its mode of action is still elusive. It is unclear which neural cell types are involved in the mechanism of DBS, and how high-frequency stimulation of these cells may lead to alleviation of the clinical symptoms. Neurons have commonly been a main focus in the many theories explaining the working mechanism of DBS. Recent data, however, demonstrates that astrocytes may be active players in the DBS mechanism of action. In this review article, we will discuss the potential role of reactive and neurogenic astrocytes (neural progenitors) in DBS.

Deep brain stimulation for movement disorders

Stereotactic technique and the introduction of deep brain stimulation (DBS) can be considered two milestones in the field of surgical neuromodulation. At present the role of DBS in the treatment of clinically and epidemiologically relevant movement disorders is widely accepted and DBS procedures are performed in many clinical centers worldwide. Here we review the current state of the art of DBS treatment for the most common movement disorders: Parkinson’s disease, essential tremor, and dystonia. In this review, we give a brief description of the candidate patient selection criteria, the different anatomical targets for each of these condition, and the expected outcomes as well as possible side effects.

Optogenetic activation of LiGluR-expressing astrocytes evokes anion channel-mediated glutamate release

Increases in astrocyte Ca(2+) have been suggested to evoke gliotransmitter release, however, the mechanism of release, the identity of such transmitter(s), and even whether and when such release occurs, are controversial, largely due to the lack of a method for selective and reproducible stimulation of electrically silent astrocytes. Here we show that photoactivation of the light-gated Ca(2+)-permeable ionotropic GluR6 glutamate receptor (LiGluR), and to a lesser extent the new Ca(2+)-translocating channelrhodopsin CatCh, evokes more reliable Ca(2+) elevation than the mutant channelrhodopsin 2, ChR2(H134R) in cultured cortical astrocytes. We used evanescent-field excitation for near-membrane Ca(2+) imaging, and epifluorescence to activate and inactivate LiGluR. By alternating activation and inactivation light pulses, the LiGluR-evoked Ca(2+) rises could be graded in amplitude and duration. The optical stimulation of LiGluR-expressing astrocytes evoked probabilistic glutamate-mediated signalling to adjacent LiGluR-non-expressing astrocytes. This astrocyte-to-astrocyte signalling was insensitive to the inactivation of vesicular release, hemichannels and glutamate-transporters, and sensitive to anion channel blockers. Our results show that LiGluR is a powerful tool to selectively and reproducibly activate astrocytes.

Two-photon optogenetics

The use of optogenetics, the technology that combines genetic and optical methods to monitor and control the activity of specific cell populations, is now widely adopted in neuroscience. The development of optogenetic tools, such as natural photosensitive ion channels and pumps or calcium- and voltage-sensitive proteins, has been growing tremendously during the past 10 years, thanks to the improvement of their performances in terms of facilitating light stimulation. To this aim, efficient illumination methods are also needed. The most common way to photostimulate optogenetic tools has been, so far, widefield illumination with visible light. However, the necessity of addressing the complexity of brain architecture has recently imposed switching to the use of two-photon excitation, which provides a better spatial specificity and deeper penetration in scattering tissue. Two-photon excitation is still challenging, due to intrinsic characteristics of optogenetic tools (e.g., the low conductivity of light-sensitive channels), and efficient illumination methods are therefore essential for advancing in this domain. Here, we present a review on the existing two-photon optical approaches for photoactivation of optogenetic tools, and future perspectives for the widespread implementation of these techniques.

A comprehensive concept of optogenetics

Fundamental questions that neuroscientists have previously approached with classical biochemical and electrophysiological techniques can now be addressed using optogenetics. The term optogenetics reflects the key program of this emerging field, namely, combining optical and genetic techniques. With the already impressively successful application of light-driven actuator proteins such as microbial opsins to interact with intact neural circuits, optogenetics rose to a key technology over the past few years. While spearheaded by tools to control membrane voltage, the more general concept of optogenetics includes the use of a variety of genetically encoded probes for physiological parameters ranging from membrane voltage and calcium concentration to metabolism. Here, we provide a comprehensive overview of the state of the art in this rapidly growing discipline and attempt to sketch some of its future prospects and challenges.

Parkinson's disease therapeutics: new developments and challenges since the introduction of levodopa

The demonstration that dopamine loss is the key pathological feature of Parkinson's disease (PD), and the subsequent introduction of levodopa have revolutionalized the field of PD therapeutics. This review will discuss the significant progress that has been made in the development of new pharmacological and surgical tools to treat PD motor symptoms since this major breakthrough in the 1960s. However, we will also highlight some of the challenges the field of PD therapeutics has been struggling with during the past decades. The lack of neuroprotective therapies and the limited treatment strategies for the nonmotor symptoms of the disease (ie, cognitive impairments, autonomic dysfunctions, psychiatric disorders, etc.) are among the most pressing issues to be addressed in the years to come. It appears that the combination of early PD nonmotor symptoms with imaging of the nigrostriatal dopaminergic system offers a promising path toward the identification of PD biomarkers, which, once characterized, will set the stage for efficient use of neuroprotective agents that could slow down and alter the course of the disease.

Synaptic dysfunction in schizophrenia

Schizophrenia alters basic brain processes of perception, emotion, and judgment to cause hallucinations, delusions, thought disorder, and cognitive deficits. Unlike neurodegeneration diseases that have irreversible neuronal degeneration and death, schizophrenia lacks agreeable pathological hallmarks, which makes it one of the least understood psychiatric disorders. With identification of schizophrenia susceptibility genes, recent studies have begun to shed light on underlying pathological mechanisms. Schizophrenia is believed to result from problems during neural development that lead to improper function of synaptic transmission and plasticity, and in agreement, many of the susceptibility genes encode proteins critical for neural development. Some, however, are also expressed at high levels in adult brain. Here, we will review evidence for altered neurotransmission at glutamatergic, GABAergic, dopaminergic, and cholinergic synapses in schizophrenia and discuss roles of susceptibility genes in neural development as well as in synaptic plasticity and how their malfunction may contribute to pathogenic mechanisms of schizophrenia. We propose that mouse models with precise temporal and spatial control of mutation or overexpression would be useful to delineate schizophrenia pathogenic mechanisms.

Optogenetic neuromodulation

The recent development of optogenetics, a revolutionary research tool in neuroscience, portends an evolution of current clinical neuromodulation tools. A form of gene therapy, optogenetics makes possible highly precise spatial and temporal control of specific neuronal populations. This technique has already provided several new insights relevant to clinical neuroscience, from the physiological substrate of functional magnetic resonance imaging to the mechanism of deep brain stimulation in Parkinson's disease. The increased precision of optogenetic techniques also raises the possibility of eventual human use. Translational efforts have begun in primates, with success reported from multiple labs in rhesus macaques. These developments will remain of ongoing interest to neurologists and neurosurgeons.

Diffusion tensor imaging and neuromodulation: DTI as key technology for deep brain stimulation

Diffusion tensor imaging (DTI) is more than just a useful adjunct to invasive techniques like optogenetics which recently have tremendously influenced our understanding of the mechanisms of deep brain stimulation (DBS). In combination with other technologies, DTI helps us to understand which parts of the brain tissue are connected to others and which ones are truly influenced with neuromodulation. The complex interaction of DBS with the surrounding tissues-scrutinized with DTI-allows to create testable hypotheses that can explain network interactions. Those interactions are vital for our understanding of the net effects of neuromodulation. This work naturally was first done in the field of movement disorder surgery, where a lot of experience regarding therapeutic effects and only a short latency between initiation of neuromodulation and alleviation of symptoms exist. This chapter shows the journey over the past 10 years with first applications in DBS toward current research in affect regulating network balances and their therapeutic alterations with the neuromodulation technology.

Therapeutic high-frequency stimulation of the subthalamic nucleus in Parkinson's disease produces global increases in cerebral blood flow

Chronic, high-frequency electrical stimulation of the subthalamic nuclei (STNs) has become an effective and widely used therapy in Parkinson's disease (PD), but the therapeutic mechanism is not understood. Stimulation of the STN is believed to reorganize neurophysiological activity patterns within the basal ganglia, whereas local field effects extending to tracts adjacent to the STN are viewed as sources of nontherapeutic side effects. This study is part of a larger project investigating the effects of STN stimulation on speech and regional cerebral blood flow (CBF) in human subjects with PD. While generating measures of global CBF (gCBF) to normalize regional CBF values for a subsequent combined analysis of regional CBF and speech data, we observed a third effect of this therapy: a gCBF increase. This effect was present across three estimates of gCBF ranging from values based on the highest activity voxels to those based on all voxels. The magnitude of the gCBF increase was related to the subject's duration of PD. It is not clear whether this CBF effect has a therapeutic role, but the impact of deep brain stimulation on cerebrovascular control warrants study from neuroscience, pathophysiological, and therapeutic perspectives.

Optical dissection of neural circuits responsible for Drosophila larval locomotion with halorhodopsin

Halorhodopsin (NpHR), a light-driven microbial chloride pump, enables silencing of neuronal function with superb temporal and spatial resolution. Here, we generated a transgenic line of Drosophila that drives expression of NpHR under control of the Gal4/UAS system. Then, we used it to dissect the functional properties of neural circuits that regulate larval peristalsis, a continuous wave of muscular contraction from posterior to anterior segments. We first demonstrate the effectiveness of NpHR by showing that global and continuous NpHR-mediated optical inhibition of motor neurons or sensory feedback neurons induce the same behavioral responses in crawling larvae to those elicited when the function of these neurons are inhibited by Shibire^ts, namely complete paralyses or slowed locomotion, respectively. We then applied transient and/or focused light stimuli to inhibit the activity of motor neurons in a more temporally and spatially restricted manner and studied the effects of the optical inhibition on peristalsis. When a brief light stimulus (1–10 sec) was applied to a crawling larva, the wave of muscular contraction stopped transiently but resumed from the halted position when the light was turned off. Similarly, when a focused light stimulus was applied to inhibit motor neurons in one or a few segments which were about to be activated in a dissected larva undergoing fictive locomotion, the propagation of muscular constriction paused during the light stimulus but resumed from the halted position when the inhibition (>5 sec) was removed. These results suggest that (1) Firing of motor neurons at the forefront of the wave is required for the wave to proceed to more anterior segments, and (2) The information about the phase of the wave, namely which segment is active at a given time, can be memorized in the neural circuits for several seconds.

Frequency selectivity and dopamine-dependence of plasticity at glutamatergic synapses in the subthalamic nucleus

In Parkinson's disease, subthalamic nucleus (STN) neurons burst fire with increased periodicity and synchrony. This may entail abnormal release of glutamate, the major source of which in STN is cortical afferents. Indeed, the cortico-subthalamic pathway is implicated in the emergence of excessive oscillations, which are reduced, as are symptoms, by dopamine-replacement therapy or deep brain stimulation (DBS) targeted to STN. Here we hypothesize that glutamatergic synapses in the STN may be differentially modulated by low-frequency stimulation (LFS) and high-frequency stimulation (HFS), the latter mimicking deep brain stimulation. Recordings of evoked and spontaneous excitatory post synaptic currents (EPSCs) were made from STN neurons in brain slices obtained from dopamine-intact and chronically dopamine-depleted adult rats. HFS had no significant effect on evoked (e) EPSC amplitude in dopamine-intact slices (104.4±8.0%) but depressed eEPSCs in dopamine-depleted slices (67.8±6.2%). Conversely, LFS potentiated eEPSCs in dopamine-intact slices (126.4±8.1%) but not in dopamine-depleted slices (106.7±10.0%). Analyses of paired-pulse ratio, coefficient of variation, and spontaneous EPSCs suggest that the depression and potentiation have a presynaptic locus of expression. These results indicate that the synaptic efficacy in dopamine-intact tissue is enhanced by LFS. Furthermore, the synaptic efficacy in dopamine-depleted tissue is depressed by HFS. Therefore the therapeutic effects of DBS in Parkinson's disease appear mediated, in part, by glutamatergic cortico-subthalamic synaptic depression and implicate dopamine-dependent increases in the weight of glutamate synapses, which would facilitate the transfer of pathological oscillations from the cortex.

The optogenetic (r)evolution

Optogenetics is a rapidly evolving field of technology that allows optical control of genetically targeted biological systems at high temporal and spatial resolution. By heterologous expression of light-sensitive microbial membrane proteins, opsins, cell type-specific depolarization or silencing can be optically induced on a millisecond time scale. What started in a petri dish is applicable today to more complex systems, ranging from the dissection of brain circuitries in vitro to behavioral analyses in freely moving animals. Persistent technical improvement has focused on the identification of new opsins, suitable for optogenetic purposes and genetic engineering of existing ones. Optical stimulation can be combined with various readouts defined by the desired resolution of the experimental setup. Although recent developments in optogenetics have largely focused on neuroscience it has lately been extended to other targets, including stem cell research and regenerative medicine. Further development of optogenetic approaches will not only highly increase our insight into health and disease states but might also pave the way for a future use in therapeutic applications.

Stimulation on demand: closing the loop on deep brain stimulation

High-frequency open-loop deep brain stimulation (DBS) has been used to alleviate Parkinson's symptoms for almost 20 years. In this issue of Neuron, Rosin et al. present a closed-loop real-time approach that improves DBS and shines light on the etiology of motor symptoms in Parkinson's disease.

Integrated device for combined optical neuromodulation and electrical recording for chronic in vivo applications

Studying brain function and its local circuit dynamics requires neural interfaces that can record and stimulate the brain with high spatiotemporal resolution. Optogenetics, a technique that genetically targets specific neurons to express light-sensitive channel proteins, provides the capability to control central nervous system neuronal activity in mammals with millisecond time precision. This technique enables precise optical stimulation of neurons and simultaneous monitoring of neural response by electrophysiological means, both in the vicinity of and distant to the stimulation site. We previously demonstrated, in vitro, the dual capability (optical delivery and electrical recording) while testing a novel hybrid device (optrode-MEA), which incorporates a tapered coaxial optical electrode (optrode) and a 100 element microelectrode array (MEA). Here we report a fully chronic implant of a new version of this device in ChR2-expressing rats, and demonstrate its use in freely moving animals over periods up to 8 months. In its present configuration, we show the device delivering optical excitation to a single cortical site while mapping the neural response from the surrounding 30 channels of the 6 × 6 element MEA, thereby enabling recording of optically modulated single-unit and local field potential activity across several millimeters of the neocortical landscape.

Optetrode: a multichannel readout for optogenetic control in freely moving mice

Recent advances in optogenetics have improved the precision with which defined circuit elements can be controlled optically in freely moving mammals; in particular, recombinase-dependent opsin viruses, used with a growing pool of transgenic mice expressing recombinases, allow manipulation of specific cell types. However, although optogenetic control has allowed neural circuits to be manipulated in increasingly powerful ways, combining optogenetic stimulation with simultaneous multichannel electrophysiological readout of isolated units in freely moving mice remains a challenge. We designed and validated the optetrode, a device that allows for colocalized multi-tetrode electrophysiological recording and optical stimulation in freely moving mice. Optetrode manufacture employs a unique optical fiber-centric coaxial design approach that yields a lightweight (2 g), compact and robust device that is suitable for behaving mice. This low-cost device is easy to construct (2.5 h to build without specialized equipment). We found that the drive design produced stable high-quality recordings and continued to do so for at least 6 weeks following implantation. We validated the optetrode by quantifying, for the first time, the response of cells in the medial prefrontal cortex to local optical excitation and inhibition, probing multiple different genetically defined classes of cells in the mouse during open field exploration.

Expanding applications of deep brain stimulation: a potential therapeutic role in obesity and addiction management

BACKGROUND

The indications for deep brain stimulation (DBS) are expanding, and the feasibility and efficacy of this surgical procedure in various neurologic and neuropsychiatric disorders continue to be tested. This review attempts to provide background and rationale for applying this therapeutic option to obesity and addiction. We review neural targets currently under clinical investigation for DBS—the hypothalamus and nucleus accumbens—in conditions such as cluster headache and obsessive-compulsive disorder. These brain regions have also been strongly implicated in obesity and addiction. These disorders are frequently refractory, with very high rates of weight regain or relapse, respectively, despite the best available treatments.

METHODS

We performed a structured literature review of the animal studies of DBS, which revealed attenuation of food intake, increased metabolism, or decreased drug seeking. We also review the available radiologic evidence in humans, implicating the hypothalamus and nucleus in obesity and addiction.

RESULTS

The available evidence of the promise of DBS in these conditions combined with significant medical need, support pursuing pilot studies and clinical trials of DBS in order to decrease the risk of dietary and drug relapse.

CONCLUSIONS

Well-designed pilot studies and clinical trials enrolling carefully selected patients with obesity or addiction should be initiated.

Optogenetics and thermogenetics: technologies for controlling the activity of targeted cells within intact neural circuits

In recent years, interest has grown in the ability to manipulate, in a temporally precise fashion, the electrical activity of specific neurons embedded within densely wired brain circuits, in order to reveal how specific neurons subserve behaviors and neural computations, and to open up new horizons on the clinical treatment of brain disorders. Technologies that enable temporally precise control of electrical activity of specific neurons, and not these neurons' neighbors-whose cell bodies or processes might be just tens to hundreds of nanometers away-must involve two components. First, they require as a trigger a transient pulse of energy that supports the temporal precision of the control. Second, they require a molecular sensitizer that can be expressed in specific neurons and which renders those neurons specifically responsive to the triggering energy delivered. Optogenetic tools, such as microbial opsins, can be used to activate or silence neural activity with brief pulses of light. Thermogenetic tools, such as thermosensitive TRP channels, can be used to drive neural activity downstream of increases or decreases in temperature. We here discuss the principles underlying the operation of these two recently developed, but widely used, toolboxes, as well as the directions being taken in the use and improvement of these toolboxes.

Cholinergic modulation of synaptic integration and dendritic excitability in the striatum

Modulatory interneurons such as, the cholinergic interneuron, are always a perplexing subject to study. Far from clear-cut distinctions such as excitatory or inhibitory, modulating interneurons can have many, often contradictory effects. The striatum is one of the most densely expressing brain areas for cholinergic markers, and actylcholine (ACh) plays an important role in regulating synaptic transmission and cellular excitability. Every cell type in the striatum has receptors for ACh. Yet even for a given cell type, ACh affecting different receptors can have seemingly opposing roles. This review highlights relevant effects of ACh on medium spiny neurons (MSNs) of the striatum and suggests how its many effects may work in concert to modulate MSN firing properties.