Transcranial direct current stimulation improves motor function in rats with 6-hydroxydopamine-induced Parkinsonism.

Transcranial direct current stimulation (tDCS) is increasingly being used for Parkinson's disease (PD); however, the evaluation of its clinical impact remains complex owing to the heterogeneity of patients and treatments. Therefore, we used a unilateral 6-hydroxydopamine-induced PD rat model to investigate whether anodal tDCS of the primary motor cortex (M1) alleviates PD motor deficits. Before tDCS treatment, unilateral PD rats preferentially used the forelimb ipsilateral to the lesion in the exploratory cylinder test and showed reduced locomotor activity in the open field test. In addition, PD-related clumsy forelimb movements during treadmill walking were detected using deep learning-based video analysis (DeepLabCut). When the 5-day tDCS treatment began, the forelimb-use asymmetry was ameliorated gradually, and locomotor activity increased to pre-lesion levels. tDCS treatment also normalized unnatural forelimb movement during walking and restored a balanced gait. However, these therapeutic effects were rapidly lost or gradually disappeared when the tDCS treatment was terminated. Histological analysis at the end of the experiment revealed that the animals had moderately advanced PD, with 40-50% of dopamine neurons and fibers preserved on the injured side compared with those on the intact side. Although it remains a challenge to elucidate the neural mechanisms of the transient improvement in motor function induced by tDCS, the results of this study provide evidence that tDCS of the M1 produces positive behavioral outcomes in PD animals and provides the basis for further clinical research examining the application of tDCS in patients with PD.

Cortical direct current stimulation improves signal transmission between the motor cortices of rats.

Transcranial direct current (DC) stimulation is a noninvasive brain stimulation technique that is now widely used to improve motor and cognitive function. The neuromodulatory effects of DC is considered to extend to nearby as well as remote brain areas from the site of stimulation because of current flowing into the brain and/or signal transmission in neuronal networks. However, the effects of DC on cortico-cortical neuronal transmission are not well known. In the present study, we focused on signal transmission from the primary (M1) to secondary (M2) motor cortex of rats. Intra-cortical microstimulation (ICMS) was applied to the M1 under DC conditions, and changes in synaptic activity in the M2 were examined using current-source density analyses. The synaptic input to the M2 superficial layers was enhanced during DC stimulation, while the synaptic input to the M2 deeper layers was increased after DC stimulation. These results suggest that DC stimulation improves cortico-cortical neuronal transmission from M1 to M2, and that the effectiveness of DC may be different among different projection neuron types in the M1.

Encoding of social exploration by neural ensembles in the insular cortex.

The insular cortex (IC) participates in diverse complex brain functions, including social function, yet their cellular bases remain to be fully understood. Using microendoscopic calcium imaging of the agranular insular cortex (AI) in mice interacting with freely moving and restrained social targets, we identified 2 subsets of AI neurons-a larger fraction of "Social-ON" cells and a smaller fraction of "Social-OFF" cells-that change their activity in opposite directions during social exploration. Social-ON cells included those that represented social investigation independent of location and consisted of multiple subsets, each of which was preferentially active during exploration under a particular behavioral state or with a particular target of physical contact. These results uncover a previously unknown function of AI neurons that may act to monitor the ongoing status of social exploration while an animal interacts with unfamiliar conspecifics.

An Implantable Cranial Window Using a Collagen Membrane for Chronic Voltage-Sensitive Dye Imaging.

Incorporating optical methods into implantable neural sensing devices is a challenging approach for brain-machine interfacing. Specifically, voltage-sensitive dye (VSD) imaging is a powerful tool enabling visualization of the network activity of thousands of neurons at high spatiotemporal resolution. However, VSD imaging usually requires removal of the dura mater for dye staining, and thereafter the exposed cortex needs to be protected using an optically transparent artificial dura. This is a major disadvantage that limits repeated VSD imaging over the long term. To address this issue, we propose to use an atelocollagen membrane as the dura substitute. We fabricated a small cranial chamber device, which is a tubular structure equipped with a collagen membrane at one end of the tube. We implanted the device into rats and monitored neural activity in the frontal cortex 1 week following surgery. The results indicate that the collagen membrane was chemically transparent, allowing VSD staining across the membrane material. The membrane was also optically transparent enough to pass light; forelimb-evoked neural activity was successfully visualized through the artificial dura. Because of its ideal chemical and optical manipulation capability, this collagen membrane may be widely applicable in various implantable neural sensors.

Evaluation of acute anodal direct current stimulation-induced effects on somatosensory-evoked responses in the rat.

Transcranial direct current stimulation (tDCS) is a non-invasive tool used to treat brain disorders. The DC electric field is thought to modulate neuronal excitability and it has been reported to exert effects within the localized treatment area under the electrode, as well as in diffuse brain regions extending beyond the electrode. However, the manner in which tDCS influences neural transmission in the cortex and modulates neural activity in distant interconnected cortical regions remains unclear. Thus, the present study investigated the effects of anodal DCS (aDCS) on the forelimb-evoked sensory response that initially appears in the primary sensorimotor cortex (S1-M1) and then propagates to the secondary motor cortex (M2). When aDCS application was confined to the S1-M1 region, local field potential (LFP) recordings and voltage-sensitive dye (VSD) imaging revealed that the forelimb-evoked response in the S1-M1 was clearly enhanced. In contrast, the neural response in the M2 remained almost unchanged. On the other hand, aDCS application confined to the M2 increased the forelimb-evoked response in the M2 but not the S1-M1. Taken together, these results suggest that, when applied to the cortex, the aDCS may have intrinsic local effects, influencing afferent neural activity immediately underneath the stimulation site. Thus, the present results indicate that aDCS has less influence on neural activity in distant cortical areas interconnected to the stimulation site than at the stimulation site itself. Therefore, the findings do not support the idea of DCS remote activation via cortico-cortical connections, at least between the S1-M1 and M2 regions in rats.

High-order motor cortex in rats receives somatosensory inputs from the primary motor cortex via cortico-cortical pathways.

The motor cortex of rats contains two forelimb motor areas; the caudal forelimb area (CFA) and the rostral forelimb area (RFA). Although the RFA is thought to correspond to the premotor and/or supplementary motor cortices of primates, which are higher-order motor areas that receive somatosensory inputs, it is unknown whether the RFA of rats receives somatosensory inputs in the same manner. To investigate this issue, voltage-sensitive dye (VSD) imaging was used to assess the motor cortex in rats following a brief electrical stimulation of the forelimb. This procedure was followed by intracortical microstimulation (ICMS) mapping to identify the motor representations in the imaged cortex. The combined use of VSD imaging and ICMS revealed that both the CFA and RFA received excitatory synaptic inputs after forelimb stimulation. Further evaluation of the sensory input pathway to the RFA revealed that the forelimb-evoked RFA response was abolished either by the pharmacological inactivation of the CFA or a cortical transection between the CFA and RFA. These results suggest that forelimb-related sensory inputs would be transmitted to the RFA from the CFA via the cortico-cortical pathway. Thus, the present findings imply that sensory information processed in the RFA may be used for the generation of coordinated forelimb movements, which would be similar to the function of the higher-order motor cortex in primates.

Neural Activity during Voluntary Movements in Each Body Representation of the Intracortical Microstimulation-Derived Map in the Macaque Motor Cortex.

In order to accurately interpret experimental data using the topographic body map identified by conventional intracortical microstimulation (ICMS), it is important to know how neurons in each division of the map respond during voluntary movements. Here we systematically investigated neuronal responses in each body representation of the ICMS map during a reach-grasp-retrieval task that involves the movements of multiple body parts. The topographic body map in the primary motor cortex (M1) generally corresponds to functional divisions of voluntary movements; neurons at the recording sites in each body representation with movement thresholds of 10 µA or less were differentially activated during the task, and the timing of responses was consistent with the movements of the body part represented. Moreover, neurons in the digit representation responded differently for the different types of grasping. In addition, the present study showed that neural activity depends on the ICMS current threshold required to elicit body movements and the location of the recording on the cortical surface. In the ventral premotor cortex (PMv), no correlation was found between the response properties of neurons and the body representation in the ICMS map. Neural responses specific to forelimb movements were often observed in the rostral part of PMv, including the lateral bank of the lower arcuate limb, in which ICMS up to 100 µA evoked no detectable movement. These results indicate that the physiological significance of the ICMS-derived maps is different between, and even within, areas M1 and PMv.

The ventral tegmental area modulates intracortical microstimulation (ICMS)-evoked M1 activity in a time-dependent manner.

Intracortical microstimulation (ICMS)-evoked neural activity combined with ventral tegmental area (VTA) stimulation was studied in rat primary motor cortex (M1). We used voltage-sensitive dye (VSD) imaging to analyze the spatiotemporal dynamics of M1 activity following VTA-M1 paired stimulation. VTA stimulation was preceded by M1 ICMS at inter-stimulus intervals (ISIs) of 15-350ms. VSD imaging showed an excitatory-inhibitory sequence of neural activity after composing VTA stimulus- and ICMS-induced M1 neural activity. To evaluate the net ICMS M1 response, the optical response to unpaired VTA stimulation was subtracted from the VTA-M1 paired response. This revealed that the net ICMS-evoked M1 neural activity was inhibited when the ISI was 30-50ms, but highly facilitated when the ISI was 100-350ms. These results suggest that VTA modulates M1 excitability in the order of tens to hundreds of milliseconds and might directly affect the motor command generation process in the M1.

A transparent epidural electrode array for use in conjunction with optical imaging.

BACKGROUND: Combining optical imaging and direct cortical stimulation can be a powerful technique for high-resolution functional mapping of the cortex. However, stimulating electrodes often obstruct the field of view, resulting in a lack of functional images behind the electrodes. NEW METHOD: To overcome this problem, we developed a transparent electrode array with 16-contact grids for epidural cortical stimulation. Using a commercially available indium tin oxide (ITO)-coated polyethylene terephthalate (PET) sheet, the electrode array was fabricated using a photolithography process. Because a complete circuit pattern, including the electrode contact itself, was formed in the transparent metal ITO layer, our electrode array was completely transparent and therefore could be used with optical imaging. RESULTS: Cortical stimulation was performed using the transparent electrode array. Evoked neural activity was successfully monitored through the array using a voltage-sensitive dye and optical imaging. The newly developed electrode array made it possible to detect optical signals from directly below the stimulating electrode. The electrode array could also be used for epidural recording of somatosensory evoked potentials. COMPARISON WITH EXISTING METHODS: A variety of surface electrodes for cortical recording and stimulation exist. However, this study aimed to make electrodes as transparent as possible. We provided a simple and low-cost fabrication process for producing the transparent electrode arrays. CONCLUSIONS: Our transparent epidural electrode can be used for both stimulation and recordings without interfering with the detection of optical signals from the underlying cortex.

Voltage-sensitive dye imaging of primary motor cortex activity produced by ventral tegmental area stimulation.

The primary motor cortex (M1) receives dopaminergic projections from the ventral tegmental area (VTA) through the mesocortical dopamine pathway. However, few studies have focused on changes in M1 neuronal activity caused by VTA activation. To address this issue, we used voltage-sensitive dye imaging (VSD) to reveal the spatiotemporal dynamics of M1 activity induced by single-pulse stimulation of VTA in anesthetized rats. VSD imaging showed that brief electrical stimulation of unilateral VTA elicited a short-latency excitatory-inhibitory sequence of neuronal activity not only in the ipsilateral but also in the contralateral M1. The contralateral M1 response was not affected by pharmacological blockade of ipsilateral M1 activity, but it was completely abolished by corpus callosum transection. Although the VTA-evoked neuronal activity extended throughout the entire M1, we found the most prominent activity in the forelimb area of M1. The 6-OHDA-lesioned VTA failed to evoke M1 activity. Furthermore, both excitatory and inhibitory intact VTA-induced activity was entirely extinguished by blocking glutamate receptors in the target M1. When intracortical microstimulation of M1 was paired with VTA stimulation, the evoked forelimb muscle activity was facilitated or inhibited, depending on the interval between the two stimuli. These findings suggest that VTA neurons directly modulate the excitability of M1 neurons via fast glutamate signaling and, consequently, may control the last cortical stage of motor command processing.