Potassium-selective microelectrode revealed difference in threshold potassium concentration for cortical spreading depression in female and male rat brain

From Frog Muscle to Brain Neurons: Joys and Sorrows in Neuroscience

One element, potassium, can be identified as the connecting link in the research of Czech neurophysiologist Prof. František Vyskočil. It accompanied him from the first student experiments on the frog muscle (Solandt effect) via sodium-potassium pump and quantum and non-quantum release of neurotransmitters (e.g. acetylcholine) to the most appreciated work on the reversible leakage of K+ from brain neurons during the Leao´s spreading cortical depression, often preceding migraine. He used a wide range of methods at the systemic, cellular and genetic levels. The electrophysiology and biochemistry of nerve-muscle contacts and synapses in the muscles and brain led to a range of interesting findings and discoveries on normal, denervated and hibernating laboratory mammals and in tissue cultures. Among others, he co-discovered the facilitating effects of catecholamines (adrenaline in particular) by end-plate synchronization of individual evoked quanta. This helps to understand the general effectiveness of nerve-muscle performance during actual stress. After the transition of the Czech Republic to capitalism, together with Dr. Josef Zicha from our Institute, he was an avid promoter of scientometry as an objective system of estimating a scientist´s success in basic research (journal Vesmír, 69: 644-645, 1990 in Czech).

Spreading Depolarizations Contribute to the Acute Behavior Deficits Associated With a Mild Traumatic Brain Injury in Mice

Concussions or mild traumatic brain injuries (mTBIs) are often described and diagnosed by the acute signs and symptoms of neurological dysfunction including weakness, dizziness, disorientation, headaches, and altered mental state. The cellular and physiological mechanisms of neurological dysfunction and acute symptoms are unclear. Spreading depolarizations (SDs) occur after severe TBIs and have recently been identified in closed-skull mouse models of mTBIs. SDs are massive waves of complete depolarization that result in suppression of cortical activity for multiple minutes. Despite the clear disruption of brain physiology after SDs, the role of SDs in the acute neurological dysfunction and acute behavioral deficits following mTBIs remains unclear. We used a closed-skull mouse model of mTBI and a series of behavioral tasks collectively scored as the neurological severity score (NSS) to assess acute behavior. Our results indicate that mTBIs are associated with significant behavioral deficits in the open field and NSS tasks relative to sham-condition animals. The behavioral deficits associated with the mTBI recovered within 3 h. We show here that the presence of mTBI-induced bilateral SDs were significantly associated with the acute behavioral deficits. To identify the role of SDs in the acute behavioral deficits, we used exogenous potassium and optogenetic approaches to induce SDs in the absence of the mTBI. Bilateral SDs alone were associated with similar behavioral deficits in the open field and NSS tasks. Collectively, these studies demonstrate that bilateral SDs are linked to the acute behavioral deficits associated with mTBIs.

Migraine in older adults

Migraine is an evolving, and sometimes lifelong disorder. The prevalence of episodic migraine peaks among individuals aged in their late 30s, implying a tendency for the disorder to remit with increasing age thereafter, whereas chronic migraine is more likely to persist into later life. Diagnosis and treatment of migraine in older adults, defined as individuals aged 60 years or older, is rendered more complex by increasing probabilities of atypical clinical features and comorbidities, with patients' comorbidities sometimes limiting their therapeutic options. However, the changing clinical presentation of migraine over an individual's lifespan is not well characterised. The neurobiological basis of remission in older adults remains unclear, although vascular, neuronal, and hormonal changes are likely to be involved. Long-term longitudinal studies of individuals with migraine would be particularly informative, with the potential not only to suggest new research directions, but also to lead to the identification of novel therapeutic agents. Although several novel migraine medications are becoming available, their effectiveness, tolerability, and safety often remain uncertain in older adults, who have commonly been excluded from the evaluation of these agents in randomised controlled trials, or who constitute only a small proportion of study populations. There is a need to recognise these limitations in the available evidence, and the specific, and often unmet, clinical needs of older adults with migraine, not least because older adults constitute an increasing proportion of populations worldwide.

Estrogen modulation of cortical spreading depression

BACKGROUND AND AIMS

Cortical spreading depression (CSD), a transient neuronal and glial depolarization that propagates slowly across the cerebral cortex, is the putative electrophysiological event underlying migraine aura and a headache trigger. Migraine is three times more prevalent in women than men, linked to circulating female hormones. High estrogen levels or estrogen withdrawal may be a migraine trigger for many women. We, therefore, aimed to examine whether sex, gonadectomy, and female hormone supplementation and withdrawal affect the susceptibility to CSD.

METHODS

To determine CSD susceptibility, we recorded the frequency of CSDs triggered during 2-h topical KCl application in intact or gonadectomized female and male rats, without or with estradiol or progesterone supplementation via daily intraperitoneal injections. Estrogen or progesterone treatment followed by withdrawal was studied in a separate cohort. To take the first step towards identifying potential mechanisms, we studied glutamate and GABAA receptor binding using autoradiography.

RESULTS

The CSD frequency in intact female rats was higher than intact male and ovariectomized rats. We did not detect a change in CSD frequency during different stages of the estrous cycle in intact females. Daily estrogen injections for three weeks did not change CSD frequency. However, one-week estrogen withdrawal after two weeks of treatment significantly increased CSD frequency compared with the vehicle group in gonadectomized females. The same protocol of estrogen treatment and withdrawal was ineffective in gonadectomized males. In contrast to estrogen, daily progesterone injections for three weeks elevated CSD susceptibility, and one-week withdrawal after two weeks of treatment partially normalized this effect. Autoradiography did not reveal significant changes in glutamate or GABAA receptor binding density after estrogen treatment and withdrawal.

CONCLUSIONS

These data suggest that females are more susceptible to CSD, and sexual dimorphism is abrogated by gonadectomy. Moreover, estrogen withdrawal after prolonged daily treatment enhances CSD susceptibility. These findings may have implications for estrogen-withdrawal migraine, although the latter tends to be without aura.

In vivo imaging of nitric oxide in the male rat brain exposed to a shock wave

While numerous studies have suggested the involvement of cerebrovascular dysfunction in the pathobiology of blast-induced traumatic brain injury (bTBI), its exact mechanisms and how they affect the outcome of bTBI are not fully understood. Our previous study showed the occurrence of cortical spreading depolarization (CSD) and subsequent long-lasting oligemia/hypoxemia in the rat brain exposed to a laser-induced shock wave (LISW). We hypothesized that this hemodynamic abnormality is associated with shock wave-induced generation of nitric oxide (NO). In this study, to verify this hypothesis, we used an NO-sensitive fluorescence probe, diaminofluorescein-2 diacetate (DAF-2 DA), for real-time in vivo imaging of male Sprague-Dawley rats' brain exposed to a mild-impulse LISW. We observed the most intense fluorescence, indicative of NO production, along the pial arteriolar walls during the period of 10-30 min post-exposure, parallel with CSD occurrence. This post-exposure period also coincided with the early phase of hemodynamic abnormalities. While the changes in arteriolar wall fluorescence measured in rats receiving pharmacological NO synthase inhibition by nitro-L-arginine methyl ester (L-NAME) 24 h before exposure showed a temporal profile similar to that of changes observed in LISW-exposed rats with CSD, their intensity level was considerably lower; this suggests partial involvement of NOS in shock wave-induced NO production. To the best of our knowledge, this is the first real-time in vivo imaging of NO in rat brain, confirming the involvement of NO in shock-wave-induced hemodynamic impairments. Finally, we have outlined the limitations of this study and our future research directions.

Diverse mechanisms regulating brain energy supply at the capillary level

Neural information processing depends critically on the brain's energy supply, which is provided in the form of glucose and oxygen in the blood. Regulation of this supply occurs by smooth muscle and contractile pericytes adjusting the diameter of arterioles and capillaries, respectively. Controversies exist over the relative importance of capillary and arteriolar level control, whether enzymatically generated signals or K⁺ ions are the dominant controller of cerebral blood flow, and the involvement of capillary endothelial cells. Here, we try to synthesise the relevant recent data into a coherent view of how brain energy supply is controlled and suggest approaches to answering key questions.

Applications of ion level nanosensors for neuroscience research

Ion activities are tightly associated with brain physiology, such as intracranial cell membrane potential, neural activity and neuropathology. Thus, monitoring the ion levels in the brain is of great significance in neuroscience research. Recently, nanosensors have emerged as powerful tools for monitoring brain ion levels and dynamics. With controllable structures and functions, nanosensors have been intensively used for monitoring neural activity and cell function and can be used in disease diagnosis. Here, we summarize the recent advances in the design and application of ion level nanosensors at different physiological levels, aiming to draw a connection of the interrelated intracranial ion activities. Furthermore, perspectives on the rationally designed ion level nanosensors in understanding the brain functions are highlighted.

Aging Impairs Cerebrovascular Reactivity at Preserved Resting Cerebral Arteriolar Tone and Vascular Density in the Laboratory Rat

The age-related (mal)adaptive modifications of the cerebral microvascular system have been implicated in cognitive impairment and worse outcomes after ischemic stroke. The magnitude of the hyperemic response to spreading depolarization (SD), a recognized principle of ischemic lesion development has also been found to be reduced by aging. Here, we set out to investigate whether the SD-coupled reactivity of the pial arterioles is subject to aging, and whether concomitant vascular rarefaction may contribute to the age-related insufficiency of the cerebral blood flow (CBF) response. CBF was assessed with laser-speckle contrast analysis (LASCA), and the tone adjustment of pial arterioles was followed with intrinsic optical signal (IOS) imaging at green light illumination through a closed cranial window created over the parietal cortex of isoflurane-anesthetized young (2 months old) and old (18 months old) male Sprague-Dawley rats. Global forebrain ischemia and later reperfusion were induced by the bilateral occlusion and later release of both common carotid arteries. SDs were elicited repeatedly with topical 1M KCl. Pial vascular density was measured in green IOS images of the brain surface, while the density and resting diameter of the cortical penetrating vasculature was estimated with micro-computed tomography of paraformaldehyde-fixed cortical samples. Whilst pial arteriolar dilation in response to SD or ischemia induction were found reduced in the old rat brain, the density and resting diameter of pial cortical vessels, and the degree of SD-related oligemia emerged as variables unaffected by age in our experiments. Spatial flow distribution analysis identified an age-related shift to a greater representation of higher flow ranges in the reperfused cortex. According to our data, impairment of functional arteriolar dilation, at preserved vascular density and resting vascular tone, may be implicated in the age-related deficit of the CBF response to SD, and possibly in the reduced efficacy of neurovascular coupling in the aging brain. SD has been recognized as a potent pathophysiological contributor to ischemic lesion expansion, in part because of the insufficiency of the associated CBF response. Therefore, the age-related impairment of cerebral vasoreactivity as shown here is suggested to contribute to the age-related acceleration of ischemic lesion development.

Calcium-dependent molecular fMRI using a magnetic nanosensor

Calcium ions are ubiquitous signalling molecules in all multicellular organisms, where they mediate diverse aspects of intracellular and extracellular communication over widely varying temporal and spatial scales ¹ . Though techniques to map calcium-related activity at a high resolution by optical means are well established, there is currently no reliable method to measure calcium dynamics over large volumes in intact tissue ² . Here, we address this need by introducing a family of magnetic calcium-responsive nanoparticles (MaCaReNas) that can be detected by magnetic resonance imaging (MRI). MaCaReNas respond within seconds to [Ca²⁺] changes in the 0.1-1.0 mM range, suitable for monitoring extracellular calcium signalling processes in the brain. We show that the probes permit the repeated detection of brain activation in response to diverse stimuli in vivo. MaCaReNas thus provide a tool for calcium-activity mapping in deep tissue and offer a precedent for the development of further nanoparticle-based sensors for dynamic molecular imaging with MRI.

17-β-Estradiol induces spreading depression and pain behavior in alert female rats

AIMS

Test the putative contribution of 17-β-estradiol in the development of spreading depression (SD) events and head pain in awake, non-restrained rats.

MAIN METHODS

Female, Sprague-Dawley rats were intact or underwent ovariectomy followed one week later by surgery to place electrodes onto the dura to detect epidural electroencephalographic activity (dEEG). dEEG activity was recorded two days later for 12 hours after systemic administration of 17-β-estradiol (180 μg/kg, i.p.). A separate set of rats were observed for changes in exploratory, ambulatory, fine, and rearing behaviors; periorbital allodynia was also assessed.

KEY FINDINGS

A bolus of 17-β-estradiol significantly elevated serum estrogen levels, increased SD episodes over a 12-hour recording period and decreased rearing behaviors in ovariectomized rats. Pre-administration of ICI 182,780, an estrogen receptor antagonist, blocked 17-β-estradiol-evoked SD events and pain behaviors; similar results were observed when the antimigraine therapeutic sumatriptan was used.

SIGNIFICANCE

These data indicate that an estrogen receptor-mediated mechanism contributes to SD events in ovariectomized rats and pain behaviors in both ovariectomized -and intact- rats. This suggests that estrogen plays a different role in each phenomenon of migraine where intense fluctuations in concentration may influence SD susceptibility. This is the first study to relate estrogen peaks to SD development and pain behaviors in awake, freely moving female rats, establishing a framework for future preclinical migraine studies.

Imaging extracellular potassium dynamics in brain tissue using a potassium-sensitive nanosensor

Neuronal activity results in the release of [Formula: see text] into the extracellular space (ECS). Classically, measurements of extracellular [Formula: see text] ([Formula: see text]) are carried out using [Formula: see text]-sensitive microelectrodes, which provide a single point measurement with undefined spatial resolution. An imaging approach would enable the spatiotemporal mapping of [Formula: see text]. Here, we report on the design and characterization of a fluorescence imaging-based [Formula: see text]-sensitive nanosensor for the ECS based on dendrimer nanotechnology. Spectral characterization, sensitivity, and selectivity of the nanosensor were assessed by spectrofluorimetry, as well as in both wide-field and two-photon microscopy settings, demonstrating the nanosensor efficacy over the physiologically relevant ion concentration range. Spatial and temporal kinetics of the nanosensor responses were assessed using a localized iontophoretic [Formula: see text] application on a two-photon imaging setup. Using acute mouse brain slices, we demonstrate that the nanosensor is retained in the ECS for extended periods of time. In addition, we present a ratiometric version of the nanosensor, validate its sensitivity in brain tissue in response to elicited neuronal activity and correlate the responses to the extracellular field potential. Together, this study demonstrates the efficacy of the [Formula: see text]-sensitive nanosensor approach and validates the possibility of creating multimodal nanosensors.

Opioid receptor-dependent sex differences in synaptic plasticity in the hippocampal mossy fiber pathway of the adult rat

The mossy fiber (MF) pathway is critical to hippocampal function and influenced by gonadal hormones. Physiological data are limited, so we asked whether basal transmission and long-term potentiation (LTP) differed in slices of adult male and female rats. The results showed small sex differences in basal transmission but striking sex differences in opioid receptor sensitivity and LTP. When slices were made from females on proestrous morning, when serum levels of 17β-estradiol peak, the nonspecific opioid receptor antagonist naloxone (1 μm) enhanced MF transmission but there was no effect in males, suggesting preferential opioid receptor-dependent inhibition in females when 17β-estradiol levels are elevated. The μ-opioid receptor (MOR) antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP; 300 nm) had a similar effect but the δ-opioid receptor (DOR) antagonist naltrindole (NTI; 1 μm) did not, implicating MORs in female MF transmission. The GABAB receptor antagonist saclofen (200 μm) occluded effects of CTOP but the GABAA receptor antagonist bicuculline (10 μm) did not. For LTP, a low-frequency (LF) protocol was used because higher frequencies elicited hyperexcitability in females. Proestrous females exhibited LF-LTP but males did not, suggesting a lower threshold for synaptic plasticity when 17β-estradiol is elevated. NTI blocked LF-LTP in proestrous females, but CTOP did not. Electron microscopy revealed more DOR-labeled spines of pyramidal cells in proestrous females than males. Therefore, we suggest that increased postsynaptic DORs mediate LF-LTP in proestrous females. The results show strong MOR regulation of MF transmission only in females and identify a novel DOR-dependent form of MF LTP specific to proestrus.

Diffusing substances during spreading depolarization: analytical expressions for propagation speed, triggering, and concentration time courses

Spreading depolarization (SD) is an important phenomenon in stroke and migraine. However, the processes underlying the propagation of SD are still poorly understood, and an elementary model that is both physiological and quantitative is lacking. We show that, during the onset and propagation of SD, the concentration time courses of excitatory substances such as potassium and glutamate can be described with a reaction-diffusion equation. This equation contains four physiological parameters: (1) a concentration threshold for excitation; (2) a release rate; (3) a removal rate; and (4) an effective diffusion constant. Solving this equation yields expressions for the propagation velocity, concentration time courses, and the minimum stimulus that can trigger SD. This framework allows for analyzing experimental results in terms of these four parameters. The derived time courses are validated with measurements of potassium in rat brain tissue.

Brain-derived neurotrophic factor-estrogen interactions in the hippocampal mossy fiber pathway: implications for normal brain function and disease

The neurotrophin brain-derived neurotrophic factor (BDNF) and the steroid hormone estrogen exhibit potent effects on hippocampal neurons during development and in adulthood. BDNF and estrogen have also been implicated in the etiology of diverse types of neurological disorders or psychiatric illnesses, or have been discussed as potentially important in treatment. Although both are typically studied independently, it has been suggested that BDNF mediates several of the effects of estrogen in the hippocampus, and that these interactions play a role in the normal brain as well as disease. Here we focus on the mossy fiber (MF) pathway of the hippocampus, a critical pathway in normal hippocampal function, and a prime example of a location where numerous studies support an interaction between BDNF and estrogen in the rodent brain. We first review the temporal and spatially regulated expression of BDNF and estrogen in the MFs, as well as their receptors. Then we consider the results of studies that suggest that 17β-estradiol alters hippocampal function by its influence on BDNF expression in the MF pathway. We also address the hypothesis that estrogen influences the hippocampus by mechanisms related not only to the mature form of BDNF, acting at trkB receptors, but also by regulating the precursor, proBDNF, acting at p75NTR. We suggest that the interactions between BDNF and 17β-estradiol in the MFs are potentially important in the normal function of the hippocampus, and have implications for sex differences in functions that depend on the MFs and in diseases where MF plasticity has been suggested to play an important role, Alzheimer's disease, epilepsy and addiction.