Local field potentials identify features of cortico-hippocampal communication impacted by stroke and environmental enrichment therapy.

Objective. Cognitive and memory impairments are common sequelae after stroke, yet how middle cerebral artery (MCA) stroke chronically affects the neural activity of the hippocampus, a brain region critical for memory but remote from the stroke epicenter, is poorly understood. Environmental enrichment (EE) improves cognition following stroke; however, the electrophysiology that underlies this behavioral intervention is still elusive.Approach.We recorded extracellular local field potentials simultaneously from sensorimotor cortex and hippocampus in rats during urethane anesthesia following MCA occlusion and subsequent EE treatment.Main results.We found that MCA stroke significantly impacted the electrophysiology in the hippocampus, in particular it disrupted characteristics of sharp-wave associated ripples (SPW-Rs) altered brain state, and disrupted phase amplitude coupling (PAC) within the hippocampus and between the cortex and hippocampus. Importantly, we show that EE mitigates stroke-induced changes to SPW-R characteristics but does not restore hippocampal brain state or PAC.Significance.These results begin to uncover the complex interaction between cognitive deficit following stroke and EE treatment, providing a testbed to assess different strategies for therapeutics following stroke.

Experimental cortical stroke induces aberrant increase of sharp-wave-associated ripples in the hippocampus and disrupts cortico-hippocampal communication.

The functional consequences of ischemic stroke in the remote brain regions are not well characterized. The current study sought to determine changes in hippocampal oscillatory activity that may underlie the cognitive impairment observed following distal middle cerebral artery occlusion (dMCAO) without causing hippocampal structural damage. Local field potentials were recorded from the dorsal hippocampus and cortex in urethane-anesthetized rats with multichannel silicon probes during dMCAO and reperfusion, or mild ischemia induced by bilateral common carotid artery occlusion (CCAO). Bilateral change of brain state was evidenced by reduced theta/delta amplitude ratio and shortened high theta duration following acute dMCAO but not CCAO. An aberrant increase in the occurrence of sharp-wave-associated ripples (150-250 Hz), crucial for memory consolidation, was only detected after dMCAO reperfusion, coinciding with an increased occurrence of high-frequency discharges (250-450 Hz). dMCAO also significantly affected the modulation of gamma amplitude in the cortex coupled to hippocampal theta phase, although both hippocampal theta and gamma power were temporarily decreased during dMCAO. Our results suggest that MCAO may disrupt the balance between excitatory and inhibitory circuits in the hippocampus and alter the function of cortico-hippocampal network, providing a novel insight in how cortical stroke affects function in remote brain regions.

Hemodynamic and Light-Scattering Changes of Rat Spinal Cord and Primary Somatosensory Cortex in Response to Innocuous and Noxious Stimuli.

Neuroimaging technologies with an exceptional spatial resolution and noninvasiveness have become a powerful tool for assessing neural activity in both animals and humans. However, the effectiveness of neuroimaging for pain remains unclear partly because the neurovascular coupling during pain processing is not completely characterized. Our current work aims to unravel patterns of neurovascular parameters in pain processing. A novel fiber-optic method was used to acquire absolute values of regional oxy- (HbO) and deoxy-hemoglobin concentrations, oxygen saturation rates (SO2), and the light-scattering coefficients from the spinal cord and primary somatosensory cortex (SI) in 10 rats. Brief mechanical and electrical stimuli (ranging from innocuous to noxious intensities) as well as a long-lasting noxious stimulus (formalin injection) were applied to the hindlimb under pentobarbital anesthesia. Interhemispheric comparisons in the spinal cord and SI were used to confirm functional activation during sensory processing. We found that all neurovascular parameters showed stimulation-induced changes; however, patterns of changes varied with regions and stimuli. Particularly, transient increases in HbO and SO2 were more reliably attributed to brief stimuli, whereas a sustained decrease in SO2 was more reliably attributed to formalin. Only the ipsilateral SI showed delayed responses to brief stimuli. In conclusion, innocuous and noxious stimuli induced significant neurovascular responses at critical centers (e.g., the spinal cord and SI) along the somatosensory pathway; however, there was no single response pattern (as measured by amplitude, duration, lateralization, decrease or increase) that was able to consistently differentiate noxious stimuli. Our results strongly suggested that the neurovascular response patterns differ between brief and long-lasting noxious stimuli, and can also differ between the spinal cord and SI. Therefore, a use of multiple-parameter strategy tailored by stimulus modality (brief or long-lasting) as well as region-dependent characteristics may be more effective in detecting pain using neuroimaging technologies.

The Scalp Confounds Near-Infrared Signal from Rat Brain Following Innocuous and Noxious Stimulation.

Functional near-infrared imaging (fNIRI) is a non-invasive, low-cost and highly portable technique for assessing brain activity and functions. Both clinical and experimental evidence suggest that fNIRI is able to assess brain activity at associated regions during pain processing, indicating a strong possibility of using fNIRI-derived brain activity pattern as a biomarker for pain. However, it remains unclear how, especially in small animals, the scalp influences fNIRI signal in pain processing. Previously, we have shown that the use of a multi-channel system improves the spatial resolution of fNIRI in rats (without the scalp) during pain processing. Our current work is to investigate a scalp effect by comparing with new data from rats with the scalp during innocuous or noxious stimulation (n = 6). Results showed remarkable stimulus-dependent differences between the no-scalp and intact-scalp groups. In conclusion, the scalp confounded the fNIRI signal in pain processing likely via an autonomic mechanism; the scalp effect should be a critical factor in image reconstruction and data interpretation.

Perturbation of Brain Oscillations after Ischemic Stroke: A Potential Biomarker for Post-Stroke Function and Therapy.

Brain waves resonate from the generators of electrical current and propagate across brain regions with oscillation frequencies ranging from 0.05 to 500 Hz. The commonly observed oscillatory waves recorded by an electroencephalogram (EEG) in normal adult humans can be grouped into five main categories according to the frequency and amplitude, namely δ (1-4 Hz, 20-200 µV), θ (4-8 Hz, 10 µV), α (8-12 Hz, 20-200 µV), ß (12-30 Hz, 5-10 µV), and γ (30-80 Hz, low amplitude). Emerging evidence from experimental and human studies suggests that groups of function and behavior seem to be specifically associated with the presence of each oscillation band, although the complex relationship between oscillation frequency and function, as well as the interaction between brain oscillations, are far from clear. Changes of brain oscillation patterns have long been implicated in the diseases of the central nervous system including ischemic stroke, in which the reduction of cerebral blood flow as well as the progression of tissue damage have direct spatiotemporal effects on the power of several oscillatory bands and their interactions. This review summarizes the current knowledge in behavior and function associated with each brain oscillation, and also in the specific changes in brain electrical activities that correspond to the molecular events and functional alterations observed after experimental and human stroke. We provide the basis of the generations of brain oscillations and potential cellular and molecular mechanisms underlying stroke-induced perturbation. We will also discuss the implications of using brain oscillation patterns as biomarkers for the prediction of stroke outcome and therapeutic efficacy.

Cerebrovascular responses of the rat brain to noxious stimuli as examined by functional near-infrared whole brain imaging.

While near-infrared (NIR) spectroscopy has been increasingly used to detect stimulated brain activities with an advantage of dissociating regional oxy- and deoxyhemoglobin concentrations simultaneously, it has not been utilized much in pain research. Here, we investigated and demonstrated the feasibility of using this technique to obtain whole brain hemodynamics in rats and speculated on the functional relevance of the NIR-based hemodynamic signals during pain processing. NIR signals were emitted and collected using a 26-optodes array on rat's dorsal skull surface after the removal of skin. Following the subcutaneous injection of formalin (50 µl, 3%) into a hindpaw, several isolable brain regions showed hemodynamic changes, including the anterior cingulate cortex, primary/secondary somatosensory cortexes, thalamus, and periaqueductal gray (n = 6). Time courses of hemodynamic changes in respective regions matched with the well-documented biphasic excitatory response. Surprisingly, an atypical pattern (i.e., a decrease in oxyhemoglobin concentration with a concomitant increase in deoxyhemoglobin concentration) was seen in phase II. In a separate group of rats with innocuous brush and noxious pinch of the same area (n = 11), results confirmed that the atypical pattern occurred more likely in the presence of nociception than nonpainful stimulation, suggesting it as a physiological substrate when the brain processes pain. In conclusion, the NIR whole brain imaging provides a useful alternative to study pain in vivo using small-animal models. Our results support the notion that neurovascular response patterns depend on stimuli, bringing attention to the interpretation of vascular-based neuroimaging data in studies of pain.

Inhibition of spinal cord dorsal horn neuronal activity by electrical stimulation of the cerebellar cortex.

The cerebellum plays a major role in not only modulating motor activity, but also contributing to other functions, including nociception. The intermediate hemisphere of the cerebellum receives sensory input from the limbs. With the extensive connection between the cerebellum to brain-stem structures and cerebral cortex, it is possible that the cerebellum may facilitate the descending system to modulate spinal dorsal horn activity. This study provided the first evidence to support this hypothesis. Thirty-one wide-dynamic-range neurons from the left lumbar and 27 from the right lumbar spinal dorsal horn were recorded in response to graded mechanical stimulation (brush, pressure, and pinch) at the hind paws. Electrical stimulation of the cerebellar cortex of the left intermediate hemisphere significantly reduced spinal cord dorsal horn neuron-evoked responses bilaterally in response to peripheral high-intensity mechanical stimuli. It is concluded that the cerebellum may play a potential antinociceptive role, probably through activating descending inhibitory pathways indirectly.

Septal stimulation inhibits spinal cord dorsal horn neuronal activity.

Deep brain stimulation (DBS) has been used for relieving chronic pain in patients that have been through other existing options. The septum has been one of the targets for such treatment. The purpose of this study was to determine the inhibitory effect of electrical stimulation in the medial septum diagonal band of broca (MSDB) on neuronal activity in the spinal cord of rats anesthetized with sodium pentobarbital. While unilaterally stimulating the MSDB, wide dynamic range neurons in the lumbar region of the spinal cord were recorded in response to graded mechanical stimulation of the hind paws (brush, pressure, and pinch). Stimulation was at 1, 5, 10, and 20V, at 100Hz, and 0.1ms duration. Significant bilateral reduction was observed in response to pressure (ipsilaterally: 0.90±0.05, 0.48±0.06*, 0.55±0.05*, 0.40±0.05*; and contralaterally: 0.70±0.06*, 0.59±0.08*, 0.75±0.05*, 0.49±0.07*) and pinch (ipsilaterally: 0.89±0.08, 0.46±0.05*, 0.54±0.04*, 0.50±0.05*; and contralaterally: 0.78±0.05, 0.61±0.07*, 0.64±0.04*, 0.53±0.06*). Data were expressed as a fraction of control. Significant changes were also found in responses to brush in certain groups (ipsilaterally: 1.08±0.08, 0.72±0.06*, 1.00±0.12, 0.65±0.06*; and contralaterally: 0.93±0.05, 0.77±0.07*, 0.98±0.05, 0.84±0.07). Further analysis suggested that 5V was adequate for achieving optimal inhibition. It is concluded that the MSDB can be used as alternative target for DBS in the treatment of pain.

Simultaneous absolute measures of glabrous skin hemodynamic and light-scattering change in response to formalin injection in rats.

Subcutaneous injection of formalin is a well-known model to study the nature of inflammatory pain. One of the cardinal signs of inflammation is redness, as a result of increased blood perfusion. We used an optical technology, light reflectance spectroscopy, to noninvasively obtain absolute measures of cutaneous hemodynamic components, including the concentrations of oxy- ([HbO]), deoxy- ([Hb]), total-hemoglobin ([HbT]), oxygen saturation (SO(2)), and the reduced light-scattering coefficient (µs'). The objective is to assess the effect of formalin-induced skin inflammation on the aforementioned parameters. Six rats were injected with formalin (50 µl, 3%) into left hind paw under pentobarbital anesthesia. Our results indicate prolonged increases in [HbO], [HbT], and SO(2) post injection only in the ipsilateral side. No statistically significant changes in [Hb] and µ(s)' occurred in either side. The arterial blood influx tends to be the major attribute of local hyperemia during inflammation. Thereby, [HbO] appears to be superior to [Hb] in measuring inflammation. In conclusion, the needle-probe-based light reflectance can be a feasible means to obtaining absolute measures of skin hemodynamic and light-scattering parameters when studying inflammatory pain.

Quantification of light reflectance spectroscopy and its application: determination of hemodynamics on the rat spinal cord and brain induced by electrical stimulation.

Two quantification methods for light reflectance spectroscopy (LRS) were developed and validated to determine absolute and relative values of hemodynamic parameters and light scattering, followed by a specific application using in vivo animal experiments. A single-channel LRS system consisted of a light source, CCD-array detector, and a computer along with a bifurcated, 2-mm-diameter optical probe; this system was utilized to perform laboratory tissue phantoms for validation of the algorithms. In the animal study, a multi-channel, multisite approach was used to measure several reflectance spectra from rat brain and spinal cord on both the ipsi-lateral and contra-lateral sides, using thin 800-µm-diameter optic probes. The neuro-hemodynamic changes were induced by 10-V electrical stimulation in rat hind paw. The LRS data of the animals were analyzed using both absolute and relative methods. The results show that the relative method is computation-efficient and offers a quick estimation of changes in oxy-hemoglobin concentration for real-time monitoring. The absolute quantification method, on the other hand, provides us with an accurate computational tool to calculate absolute values of oxy-, deoxy-, total hemoglobin concentrations, and light scattering coefficients. We also observe that the hemodynamic responses in rat spinal cord were delayed with a few seconds and have an overall broader full width at half maximum, as compared to those from rat somatosensory cortex. LRS as a measurement system provides a robust method for studying local hemodynamic changes and a potential technique to investigate hemo-neural mechanisms in pain processing.

Contributions of dorsal root reflex and axonal reflex to formalin-induced inflammation.

The dorsal root reflex (DRR) and the axonal reflex (AR) are antidromic activities in primary afferents and are involved in neurogenic inflammation. DRRs and/or ARs lead to release of neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP). CGRP causes blood vessels to dilate leading to an increase in blood perfusion, whereas SP causes plasma extravasation, leading to edema. Both DRR and AR can be evoked by noxious stimuli. The goal of this study was to determine the role of DRR and AR in neurogenic inflammation by examining the blood perfusion (BP) change in hindpaws in response to formalin injection (an acute inflammatory agent). Laser Doppler images were collected simultaneously in both hindpaws in anesthetized rats to determine the level of BP. Local lidocaine was applied to the left sciatic nerve to block both orthodromic signals and antidromic DRRs without affecting ARs. All rats then received a subcutaneous formalin injection to the left hindpaw. Our results showed that (1) the mean BP of the left paw increased significantly following formalin injection, with or without lidocaine; (2) application of lidocaine in the left sciatic nerve alone significantly increased BP ipsilaterally; (3) formalin injection following lidocaine application significantly increased BP more than the group without lidocaine; and (4) there was delayed significant BP increase in the right (contralateral) hindpaw following formalin injection with or without lidocaine. It is concluded that ARs play a more important role than DRRs in formalin-induced neurogenic inflammation.

Biphasic effects of the anterior cingulate cortex stimulation on glabrous skin blood flow in rats.

A growing body of evidence indicates that the anterior cingulate cortex (ACC) is associated with sensory, cognition and emotion processing. We have shown that electrical stimulation of rat ACC depressed the spinal cord dorsal horn neuron activity in response to noxious stimuli, possibly through a release of GABA. GABA may elicit dorsal root reflexes (DRRs) to induce peripheral vasodilatation. On the other hand, the ACC may also regulate autonomic flow via the lateral hypothalamus (LH). The goal of this work was to investigate the role of ACC in regulating autonomic activity. A laser Doppler imager was used to continuously monitor rat glabrous skin blood perfusion in both hind paws, while a simultaneous heart rate (HR) and DRRs were recorded to assess contributions of sympathetic flow and sensory afferent to the ACC-induced vascular change. Twenty-three rats were divided into three groups: a unilateral electrolytic LH lesion group (n = 6), a sham lesion group (n = 9), and a control group (neither lesion nor stimulation, n = 8). ACC stimulation induced a biphasic systemic vascular response, with an initial transient cutaneous vasoconstriction followed by a prolonged vasodilatation. Unilateral LH lesion did not alter this biphasic response. A short-term tachycardia occurred in response to the ACC stimulation, but did not correlate with the prolonged vasodilatation. No significant change in DRRs was found (in 35 fibers). ACC stimulation induced a biphasic vascular response in the skin. Data are consistent with sympathetic contribution. However, other mechanisms should also be involved.

A combined wireless neural stimulating and recording system for study of pain processing.

Clinical studies have shown that spinal or cortical neurostimulation could significantly improve pain relief. The currently available stimulators, however, are used only to generate specific electrical signals without the knowledge of physiologically responses caused from the stimulation. We thus propose a new system that can adaptively generate the optimized stimulating signals base on the correlated neuron activities. This new method could significantly improve the efficiency of neurostimulation for pain relief. We have developed an integrated wireless recording and stimulating system to transmit the neuronal signals and to activate the stimulator over the wireless link. A wearable prototype has been developed consisting of amplifiers, wireless modules and a microcontroller remotely controlled by a Labview program in a computer to generate desired stimulating pulses. The components were assembled on a board with a size of 2.5 cm x 5 cm to be carried by a rat. To validate our system, lumbar spinal cord dorsal horn neuron activities of anesthetized rats have been recorded in responses to various types of peripheral graded mechanical stimuli. The stimulation at the periaqueductal gray and anterior cingulate cortex with different combinations of electrical parameters showed a comparable inhibition of spinal cord dorsal horns activities in response to the mechanical stimuli. The Labview program was also used to monitor the neuronal activities and automatically activate the stimulator with designated pulses. Our wireless system has provided an opportunity for further study of pain processing with closed-loop stimulation paradigm in a potential new pain relief method.