Time resolved 3-dimensional recording of redox ratio during spreading depression in gerbil brain

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

Spreading depression (SD) is a transient wave of near-complete neuronal and glial depolarization associated with massive transmembrane ionic and water shifts. It is evolutionarily conserved in the central nervous systems of a wide variety of species from locust to human. The depolarization spreads slowly at a rate of only millimeters per minute by way of grey matter contiguity, irrespective of functional or vascular divisions, and lasts up to a minute in otherwise normal tissue. As such, SD is a radically different breed of electrophysiological activity compared with everyday neural activity, such as action potentials and synaptic transmission. Seventy years after its discovery by Leão, the mechanisms of SD and its profound metabolic and hemodynamic effects are still debated. What we did learn of consequence, however, is that SD plays a central role in the pathophysiology of a number of diseases including migraine, ischemic stroke, intracranial hemorrhage, and traumatic brain injury. An intriguing overlap among them is that they are all neurovascular disorders. Therefore, the interplay between neurons and vascular elements is critical for our understanding of the impact of this homeostatic breakdown in patients. The challenges of translating experimental data into human pathophysiology notwithstanding, this review provides a detailed account of bidirectional interactions between brain parenchyma and the cerebral vasculature during SD and puts this in the context of neurovascular diseases.

Shedding light on mitochondrial function by real time monitoring of NADH fluorescence: I. Basic methodology and animal studies

Normal mitochondrial function in the process of metabolic energy production is a key factor in maintaining cellular activities. Many pathological conditions in animals, as well as in patients, are directly or indirectly related to dysfunction of the mitochondria. Monitoring the mitochondrial activity by measuring the autofluorescence of NADH has been the most practical approach since the 1950s. This review presents the principles and technological aspects, as well as typical results, accumulated in our laboratory since the early 1970s. We were able to apply the fiber-optic-based NADH fluorometry to many organs monitored in vivo under various pathophysiological conditions in animals. These studies were the basis for the development of clinical monitoring devices as presented in accompanying article. The encouraging experimental results in animals stimulated us to apply the same technology in patients after technological adaptations as described in the accompanying article. Our medical device was approved for clinical use by the FDA.

Imaging mitochondrial redox potential and its possible link to tumor metastatic potential

Cellular redox states can regulate cell metabolism, growth, differentiation, motility, apoptosis, signaling pathways, and gene expressions etc. A growing body of literature suggest the importance of redox status for cancer progression. While most studies on redox state were done on cells and tissue lysates, it is important to understand the role of redox state in a tissue in vivo/ex vivo and image its heterogeneity. Redox scanning is a clinical-translatable method for imaging tissue mitochondrial redox potential with a submillimeter resolution. Redox scanning data in mouse models of human cancers demonstrate a correlation between mitochondrial redox state and tumor metastatic potential. I will discuss the significance of this correlation and possible directions for future research.

Cortical spreading depression impairs oxygen delivery and metabolism in mice

Cortical spreading depression (CSD) is associated with severe hypoperfusion in mice. Using minimally invasive multimodal optical imaging, we show that severe flow reductions during and after spreading depression are associated with a steep decline in cerebral metabolic rate of oxygen. Concurrent severe hemoglobin desaturation suggests that the oxygen metabolism becomes at least in part supply limited, and the decrease in cortical blood volume implicates vasoconstriction as the mechanism. In support of oxygen supply-demand mismatch, cortical nicotinamide adenine dinucleotide (NADH) fluorescence increases during spreading depression for at least 5 minutes, particularly away from parenchymal arterioles. However, modeling of tissue oxygen delivery shows that cerebral metabolic rate of oxygen drops more than predicted by a purely supply-limited model, raising the possibility of a concurrent reduction in oxygen demand during spreading depression. Importantly, a subsequent spreading depression triggered within 15 minutes evokes a monophasic flow increase superimposed on the oligemic baseline, which markedly differs from the response to the preceding spreading depression triggered in naive cortex. Altogether, these data suggest that CSD is associated with long-lasting oxygen supply-demand mismatch linked to severe vasoconstriction in mice.

Biphasic direct current shift, haemoglobin desaturation and neurovascular uncoupling in cortical spreading depression

Cortical spreading depression is a propagating wave of depolarization that plays important roles in migraine, stroke, subarachnoid haemorrhage and brain injury. Cortical spreading depression is associated with profound vascular changes that may be a significant factor in the clinical response to cortical spreading depression events. We used a combination of optical intrinsic signal imaging, electro-physiology, potassium sensitive electrodes and spectroscopy to investigate neurovascular changes associated with cortical spreading depression in the mouse. We identified two distinct phases of altered neurovascular function, one during the propagating cortical spreading depression wave and a second much longer phase after passage of the wave. The direct current shift associated with the cortical spreading depression wave was accompanied by marked arterial constriction and desaturation of cortical haemoglobin. After recovery from the initial cortical spreading depression wave, we observed a second phase of prolonged, negative direct current shift, arterial constriction and haemoglobin desaturation, lasting at least an hour. Persistent disruption of neurovascular coupling was demonstrated by a loss of coherence between electro-physiological activity and perfusion. Extracellular potassium concentration increased during the cortical spreading depression wave, but recovered and remained at baseline after passage of the wave, consistent with different mechanisms underlying the first and second phases of neurovascular dysfunction. These findings indicate that cortical spreading depression is associated with a multiphasic alteration in neurovascular function, including a novel second direct current shift accompanied by arterial constriction and decrease in tissue oxygen supply, that is temporally and mechanistically distinct from the initial propagated cortical spreading depression wave. Vascular/metabolic uncoupling with cortical spreading depression may have important clinical consequences, and the different phases of dysfunction may represent separate therapeutic targets in the disorders where cortical spreading depression occurs.

Intrinsic fluorescence and redox changes associated with apoptosis of primary human epithelial cells

Apoptosis plays a key role in the development and maintenance of human tissues. This process has been studied traditionally in cells that are stained with exogenous fluorophores. These approaches affect cell viability, and thus are ill-suited for in vivo applications. We present an imaging approach that can identify apoptotic cells in living cell populations based on detection and quantification of distinct changes in the intensity and localization of cellular autofluorescence. Specifically, we acquire NAD(P)H, FAD, and redox ratio autofluorescence images of primary keratinocytes following 1, 9, 14, and 18 h of treatment with cisplatin, a known apoptosis-inducing chemotherapy agent. We find that intense autofluorescence combined with a low redox fluorescence ratio is progressively confined to a gradually smaller perinuclear cytoplasmic region with cisplatin treatment. Studies with exogenous nuclear fluorophores demonstrate that these autofluorescence changes occur at early stages of apoptosis. Additional costaining experiments suggest that this strongly autofluorescent, highly metabolically active perinuclear ring represents a subpopulation of mitochondria that are mobilized in response to the apoptotic stimulus and may provide the energy required to execute the final apoptotic steps. Thus, autofluorescence localization changes could serve as a sensitive, noninvasive indicator of early apoptosis in vivo.

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa stimulation. Autofluorescence responses reflecting oxidative metabolism demonstrated a fast increase with a delay of 0.1 s. The sign-reversed speckle contrast reflecting CBF started to rise with a delay of 0.6 s and reached its maximum 1.4 s after the stimulation offset. The fractional signal changes were 2.0% in AF and 9.7% in LSI. Pixelwise modelling revealed that CBF maps spread over an area up to 2.5-times larger than metabolic maps. The results provide evidence that the increase in cerebral oxidative metabolism in response to sensory stimulation is considerably faster and more localized than the CBF response. This suggests that future developments in functional imaging concentrating on the metabolic response promise an increased spatial resolution.

Carbocyanine labeled LDL for optical imaging of tumors

RATIONALE AND OBJECTIVES

The purpose of this study was to define and characterize carbocyanine labeled low-density lipoprotein (LDL) to be used in the optical imaging of LDL receptor (LDLr)-overexpressing tumor models.

MATERIALS AND METHODS

1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was used to label LDL (DiI-LDL). Scatchard plots were generated to determine the maximum binding capacity B(max) and dissociation constants K(D) of DiI-LDL in B16 melanoma (B16) and hepatoblastoma G(2) (HepG(2)) cell lines. Selective uptake of DiI-LDL into both tumor cells and corresponding subcutaneous tumors in mice were demonstrated by confocal microscopy and three-dimensional Cryo-imaging, respectively.

RESULTS

The labeling efficiency of DiI-LDL was 61 ng DiI/microg LDL protein (34 mol DiI/mol LDL protein). B(max) and K(D) for B16 cells were 6.311 ng LDL/mg cell protein and 60.38 microg protein/mL (117 nM), respectively. B(max) and K(D) were 7.573 ng LDL/mg cell protein and 26.79 microg protein/mL (52 nM) for HepG(2) cells, respectively. Confocal microscopic images showed specific uptake of DiI-LDL throughout the cytoplasm in the B16/HepG(2) cells. Cryo-imaging demonstrated preferential accumulations of DiI-LDL in the viable tumor regions of both B16 and HepG(2) tumors compared with their adjacent normal tissues and corresponding necrotic tumor regions. In addition, uptake of DiI-LDL by the HepG(2) tumor was much higher than that of the B16 tumor, consistent with the fact that the probe binding affinity for LDLrs of HepG(2) cells is 2.3 times that of B16 cells.

CONCLUSION

This study suggested that carbocyanine labeled LDL could be used for optical imaging of tumors overexpressing LDLr.

Short-term plasticity visualized with flavoprotein autofluorescence in the somatosensory cortex of anaesthetized rats

In the present study, short-term plasticity of somatosensory neural responses was investigated using flavoprotein autofluorescence imaging in rats anaesthetized with urethane (1.5 g/kg, i.p.) Somatosensory neural activity was elicited by vibratory skin stimulation (50 Hz for 1 s) applied on the surface of the left plantar hindpaw. Changes in green autofluorescence (lambda = 500-550 nm) in blue light (lambda = 450-490 nm) were elicited in the right somatosensory cortex. The normalised maximal fluorescence responses (deltaF/F) was 2.0 +/- 0.1% (n = 40). After tetanic cortical stimulation (TS), applied at a depth of 1.5-2.0 mm from the cortical surface, the responses elicited by peripheral stimulation were significantly potentiated in both peak amplitude and size of the responsive area (both P < 0.02; Wilcoxon signed rank test). This potentiation was clearly observed in the recording session started 5 min after the cessation of TS, and returned to the control level within 30 min. However, depression of the responses was observed after TS applied at a depth of 0.5 mm. TS-induced changes in supragranular field potentials in cortical slices showed a similar dependence on the depth of the stimulated sites. When TS was applied on the ipsilateral somatosensory cortex, marked potentiation of the ipsilateral responses and slight potentiation of the contralateral responses to peripheral stimulation were observed after TS, suggesting the involvement of commissural fibers in the changes in the somatosensory brain maps. The present study clearly demonstrates that functional brain imaging using flavoprotein autofluorescence is a useful technique for investigating neural plasticity in vivo.

Dynamic imaging of somatosensory cortical activity in the rat visualized by flavoprotein autofluorescence

We used autofluorescence of mitochondrial flavoproteins to image cortical neural activity in the rat. Green autofluorescence in blue light was examined in slices obtained from rat cerebral cortex. About half of the basal autofluorescence was modulated by the presence or absence of O2 or glucose in the medium. Repetitive electrical stimulation at 20 Hz for 1 s produced a localized fluorescence increase in the slices. The amplitude of the increase was 27 +/- 2 % (mean +/- S.D., n = 35). Tetrodotoxin or diphenyleneiodonium, an inhibitor of flavoproteins, blocked the autofluorescence responses. The autofluorescence responses were not observed in slices perfused with calcium-, glucose- or O2-free medium. In the primary somatosensory cortex of rats anaesthetized with urethane (1.5 g kg-1, I.P.), an activity-dependent increase in autofluorescence of 20 +/- 4 % (n = 6) was observed after electrical cortical stimulation at 100 Hz for 1 s, and an increase of 2.6 +/- 0.5 % (n = 33) after vibratory skin stimulation at 50 Hz for 1 s applied to the plantar hindpaw. These responses were large enough to allow visualization of the neural activity without having to average a number of trials. The distribution of the fluorescence responses after electrical or vibratory skin stimulation was comparable to that of the cortical field potentials in the same rats. The fluorescence responses were followed by an increase in arterial blood flow. The former were resistant to an inhibitor of nitric oxide synthase, while the latter was inhibited. Thus, activity-dependent changes in the autofluorescence of flavoproteins are useful for functional brain imaging in vivo.

Three-dimensional redox image of the normal gerbil brain

Regional differences in the redox ratio were studied in the gerbil brain. Brains were frozen using an in situ funnel-freezing method, and sliced coronally for scanning of mitochondrial redox imaging. The relative local redox ratio of nicotinamide-adenosine dinucleotide to its reduced form was calculated from fluorescence signals of intrinsic fluorochromes, i.e. reduced nicotinamide-adenosine dinucleotide and flavoproteins, using a high resolution fluorometer developed in our laboratory. Twelve consecutive coronal images were obtained from each of 10 gerbils. The mean value of the regional redox ratio in both the cerebral and cerebellar gray matter were found to be significantly lower than that in the cerebral and cerebellar white matter (P < 0.01, Mann-Whitney test). Local differences in the redox ratio were also found among subregions of gray matter. The redox ratio in the globus pallidus was significantly higher than values in other subregions of gray matter (P < 0.01, Mann-Whitney test) We postulate that a high concentration of the reduced form of pyridine nucleotide is maintained to provide redox energy for rapid turnover of ATP in the areas of high energy consumption.

JUN, FOS, KROX, and CREB transcription factor proteins in the rat cortex: basal expression and induction by spreading depression and epileptic seizures

The expression of the nuclear c-JUN, JUN B, JUN D, c-FOS, FOS B, KROX-24, and CREB transcription factors was investigated in the cortex of adult rats by immunocytochemistry. The expression patterns were studied in untreated rats and up to 24 hours following topical application of 1 M KCl to the cortical surface (KCl) or i.v. injection of bicuculline (BIC). Topical KCl induced cortical spreading depression and systemic injection of bicuculline evoked generalized tonic-clonic seizures. In untreated rats, JUN B, c-FOS, and FOS B were expressed in a small number of neurons in the piriform, perirhinal, entorhinal, and insular cortex and in layers II, III, and VI of all neocortical areas. In contrast, c-JUN, JUN D, and KROX-24 were expressed in all cortical layers but with different intensities of immunoreactivity (IR): c-JUN-IR was generally weak and predominantly present in layers II, III, and VI. JUN D-IR was equally strong in all layers. KROX-24 showed a prominent expression in layers II, IV, and VI. The CREB protein exhibited a slight preponderance in layer II and piriform cortex. Following KCl or BIC, a strong induction was seen for c-FOS, JUN B, and KROX-24, whereas c-JUN, JUN D, and FOS B showed only a moderate increase compared to basal levels. Changes of CREB-IR could not be detected. The localization of induced JUN, FOS, and KROX proteins reflected the pattern of labelling in untreated animals but demonstrated a higher intensity of labelling and an increased number of immunoreactive nuclei. The intensity and persistence of IR as well as the number of labelled cells following BIC exceeded those following KCl. Following BIC, increased levels of FOS B and JUN D were still present after 24 hours. Counterstaining with cresyl-violet and GFAP, a marker for astrocytes, revealed that JUN, FOS, and KROX proteins were expressed in neurons but not in glial cell populations. The present data demonstrate that CREB, JUN, FOS, and KROX transcription factors exhibit a layer-specific expression in the cerebral cortex with only slight area-specific differences both in untreated rats and following stimulation with KCl and BIC. The expression of transcription factor proteins indicate complex molecular genetic changes in cortical neurons due to pathophysiological events such as seizure activity and spreading depression.

Brain vasculature and mitochondrial responses to ischemia in gerbils. I. Basic anatomical patterns and biochemical correlates

A unique blood supply to the brain, the 'Circle of Willis' (COW), exists in all mammals except for the Mongolian gerbil (Meriones unguiculatus). This system is capable of compensating for a decrease in blood supply in one of the four arteries, which may occur during pathological conditions. The posterior connection between the basilar artery and the carotid artery system have been found to be missing in most gerbils. Furthermore, in some of the animals, the anterior communication was not complete, thus leading to partial ischemia following unilateral carotid artery occlusion. Due to those peculiar characteristics, the Mongolian gerbil today has become a widespread animal model for cerebral ischemia studies. M. unguiculatus has been used in most of the studies while the level of ischemia has been evaluated by the development of neurological symptoms created by the occlusion of the carotid arteries. In the present study we investigated the vasculature structure of the commonly used gerbil, M. unguiculatus (MU-TF) and compared it to the vasculature of the Israeli gerbil, Meriones tristrami as well as to that of the Albino rat. We determined the correlation between the anatomical pattern and the biochemical responses during partial or complete ischemia and anoxia by monitoring the oxidation-reduction state of the intramitochondrial NADH using an in vivo surface fluorescence technique. The corrected fluorescence signal was found to be inversely correlated with oxygen availability and could thus be used as an indicator for the level of ischemia developed after carotid artery occlusion. This is the first time that the brain vasculature of two lines of M. tristrami (MT-HU, MT-BD) has been investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

The 21-aminosteroid antioxidant tirilazad mesylate, U-74006F, blocks cortical hypoperfusion following spreading depression

Cortical spreading depression (SD) has been implicated in the pathophysiology of classical migraine headache and cerebral ischemia. A reduction in cerebral blood flow (CBF), mimicking that seen during the aura and headache phase of migraine, is typically observed following SD in the rat. This phenomenon may also play a role in potentiating ischemic brain damage. In the present study, brief cortical exposure to 1 M KCl produced a marked suppression of EEG amplitude which persisted 20 min in the rat. Upon normalization of the EEG, cortical blood flow declined 20-30% and remained low for at least 2 h. Treatment with a 1 mg/kg i.v. dose of the 21-aminosteroid antioxidant tirilazad mesylate (U-74006F), 2 min following KCl application, completely blocked the hypoperfusion while leaving the magnitude and duration of the EEG suppression and mean arterial pressure unchanged. Tirilazad mesylate is a potent inhibitor of oxygen radical-mediated lipid peroxidation both in vitro and in vivo. Thus, based on present results, an oxygen radical hypothesis is proposed to account for the SD-induced cerebral hypoperfusion.

Intercellular signaling in glial cells: calcium waves and oscillations in response to mechanical stimulation and glutamate

Intercellular Ca2+ signaling in primary cultures of glial cells was investigated with digital fluorescence video imaging. Mechanical stimulation of a single cell induced a wave of increased [Ca2+]i that was communicated to surrounding cells. This was followed by asynchronous Ca2+ oscillations in some cells. Similar communicated Ca2+ responses occurred in the absence of extracellular Ca2+, despite an initial decrease in [Ca2+]i in the stimulated cell. Mechanical stimulation in the presence of glutamate induced a typical communicated Ca2+ wave through cells undergoing asynchronous Ca2+ oscillations in response to glutamate. The coexistence of communicated Ca2+ waves and asynchronous Ca2+ oscillations suggests distinct mechanisms for intra- and intercellular Ca2+ signaling. This intercellular signaling may coordinate cooperative glial function.