Asymmetric propagation of spreading depression along the anteroposterior axis of the cerebral cortex in mice

Distinct Activity of Endocannabinoid-Hydrolyzing Enzymes MAGL and FAAH in Key Regions of Peripheral and Central Nervous System Implicated in Migraine

In migraine pain, cannabis has a promising analgesic action, which, however, is associated with side psychotropic effects. To overcome these adverse effects of exogenous cannabinoids, we propose migraine pain relief via activation of the endogenous cannabinoid system (ECS) by inhibiting enzymes degrading endocannabinoids. To provide a functional platform for such purpose in the peripheral and central parts of the rat nociceptive system relevant to migraine, we measured by activity-based protein profiling (ABPP) the activity of the main endocannabinoid-hydrolases, monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH). We found that in trigeminal ganglia, the MAGL activity was nine-fold higher than that of FAAH. MAGL activity exceeded FAAH activity also in DRG, spinal cord and brainstem. However, activities of MAGL and FAAH were comparably high in the cerebellum and cerebral cortex implicated in migraine aura. MAGL and FAAH activities were identified and blocked by the selective and potent inhibitors JJKK-048/KML29 and JZP327A, respectively. The high MAGL activity in trigeminal ganglia implicated in the generation of nociceptive signals suggests this part of ECS as a priority target for blocking peripheral mechanisms of migraine pain. In the CNS, both MAGL and FAAH represent potential targets for attenuation of migraine-related enhanced cortical excitability and pain transmission.

Optogenetic induction of cortical spreading depression in anesthetized and freely behaving mice

Cortical spreading depression, which plays an important role in multiple neurological disorders, has been studied primarily with experimental models that use highly invasive methods. We developed a relatively non-invasive optogenetic model to induce cortical spreading depression by transcranial stimulation of channelrhodopsin-2 ion channels expressed in cortical layer 5 neurons. Light-evoked cortical spreading depression in anesthetized and freely behaving mice was studied with intracortical DC-potentials, multi-unit activity and/or non-invasive laser Doppler flowmetry, and optical intrinsic signal imaging. In anesthetized mice, cortical spreading depression induction thresholds and propagation rates were similar for invasive (DC-potential) and non-invasive (laser Doppler flowmetry) recording paradigms. Cortical spreading depression-related vascular and parenchymal optical intrinsic signal changes were similar to those evoked with KCl. In freely behaving mice, DC-potential and multi-unit activity recordings combined with laser Doppler flowmetry revealed cortical spreading depression characteristics comparable to those under anesthesia, except for a shorter cortical spreading depression duration. Cortical spreading depression resulted in a short increase followed by prolonged reduction of spontaneous active behavior. Motor function, as assessed by wire grip tests, was transiently and unilaterally suppressed following a cortical spreading depression. Optogenetic cortical spreading depression induction has significant advantages over current models in that multiple cortical spreading depression events can be elicited in a non-invasive and cell type-selective fashion.

Aquaporin-4 regulates the velocity and frequency of cortical spreading depression in mice

The astrocyte water channel aquaporin-4 (AQP4) regulates extracellular space (ECS) K(+) concentration ([K(+)]e) and volume dynamics following neuronal activation. Here, we investigated how AQP4-mediated changes in [K(+)]e and ECS volume affect the velocity, frequency, and amplitude of cortical spreading depression (CSD) depolarizations produced by surface KCl application in wild-type (AQP4(+/+)) and AQP4-deficient (AQP4(-/-)) mice. In contrast to initial expectations, both the velocity and the frequency of CSD were significantly reduced in AQP4(-/-) mice when compared with AQP4(+/+) mice, by 22% and 32%, respectively. Measurement of [K(+)]e with K(+)-selective microelectrodes demonstrated an increase to ∼35 mM during spreading depolarizations in both AQP4(+/+) and AQP4(-/-) mice, but the rates of [K(+)]e increase (3.5 vs. 1.5 mM/s) and reuptake (t1/2 33 vs. 61 s) were significantly reduced in AQP4(-/-) mice. ECS volume fraction measured by tetramethylammonium iontophoresis was greatly reduced during depolarizations from 0.18 to 0.053 in AQP4(+/+) mice, and 0.23 to 0.063 in AQP4(-/-) mice. Analysis of the experimental data using a mathematical model of CSD propagation suggested that the reduced velocity of CSD depolarizations in AQP4(-/-) mice was primarily a consequence of the slowed increase in [K(+)]e during neuronal depolarization. These results demonstrate that AQP4 effects on [K(+)]e and ECS volume dynamics accelerate CSD propagation.

Vision and migraine

BACKGROUND

Migraine, a common brain disorder, disrupts vision more than any other motor or sensory function. The possible visual aura symptoms vary from occasional small flashes of light to complex visual hallucinations, the stereotyped teichopsia being the most typical pattern. It is unclear as to why aura occurs serendipitously, sometimes preceding, but also occurring after the headache, and why aura can present with multiple phenotypes.

METHODS

To better understand the nature of visual disturbances in migraine, 4 aspects must be considered: What are the visual perceptions in migraine; why vision is affected in migraine; the role of cortical spreading depression (CSD); how does vision could affect migraine. Evidence supporting each of these topics is reviewed.

RESULTS

CSD travels at a similar pace as the march of symptoms in the visual field. Functional neuroimaging studies show spreading changes compatible with CSD regardless of aura. Computerized models reproducing the CSD march on the visual cortex predict a sensory experience compatible with naturally occurring visual auras. Rather than spreading in all directions, these models suggest that CSD moves preferentially in one direction. Migraine-preventive drugs increase the CSD threshold and reduce CSD velocity. Blind migraineurs may present atypical visual aura, with more colors, shorter duration, different shapes, and atypical symptoms, such as auditory experiences.

CONCLUSIONS

CSD is the underlying phenomenon in migraine with and without aura. In migraine without aura, CSD probably does not run over silent areas of the cortex, but rather does not reach symptomatology threshold. Normal vision is important in migraine, as lack of sight may change the visual experience during migraine aura, probably due to cortical reorganization and changes in local susceptibility to CSD.

Direct, live imaging of cortical spreading depression and anoxic depolarisation using a fluorescent, voltage-sensitive dye

Perilesion depolarisations, whether transient anoxic depolarisation (AD) or spreading depression (SD), occur in stroke models and in patients with acute brain ischaemia, but their contribution to lesion progression remains unclear. As these phenomena correspond to waves of cellular depolarisation, we have developed a technique for their live imaging with a fluorescent voltage-sensitive (VS) dye (RH-1838). Method development and validation were performed in two different preparations: chicken retina, to avoid any vascular interference; and cranial window exposing the cortical surface of anaesthetised rats. Spreading depression was produced by high-K medium, and AD by complete terminal ischaemia in rats. After dye loading, the preparation was illuminated at its excitation wavelength and fluorescence changes were recorded sequentially with a charge-coupled device camera. No light was recorded when the VS dye was omitted, ruling out the contribution of any endogenous fluorophore. With both preparations, the changes in VS dye fluorescence with SD were analogous to those of the DC (direct current) potential recorded with glass electrodes. Although some blood quenching of the emitted light was identified, the VS dye signatures of SD had a good signal-to-noise ratio and were reproducible. The changes in VS dye fluorescence associated with AD were more complex because of additional interferents, especially transient brain swelling with subsequent shrinkage. However, the kinetics of the AD-associated changes in VS dye fluorescence was also analogous to that of the DC potential. In conclusion, this method provides the imaging equivalent of electrical extracellular DC potential recording, with the SD and AD negative shifts translating directly to fluorescence increase.

Lasting accelerative effects of 1 Hz and 20 Hz electrical stimulation on cortical spreading depression: relevance for clinical applications of brain stimulation

Clinical applications of brain stimulation have been increasing during the last decade; however, the mechanisms of action remain unknown. One proposed mechanism of action is that repetitive stimulation modulates cortical excitability. Herein, we explore the question of whether repetitive electric stimulation increases cortical excitability as indexed by the cortical spreading depression. Twenty-four Wistar rats were divided into three groups according to the treatment: sham, 1-Hz and 20-Hz stimulation. Stimulation was applied to the left frontal cortex through a pair of epidurally implanted silver-wire electrodes. The cortical spreading depression-features were analysed at three time points (one day before, one day after and 2 weeks after treatment) in both the stimulated and unstimulated hemisphere. A 3 x 2 x 3 factorial anova with repeated measures showed significant differences in the main effect of time (P < 0.0001), hemisphere (P = 0.0002) and stimulation group (P = 0.008). The interaction between time vs. hemisphere vs. stimulation group was also significant (P < 0.0001). Posthoc analysis demonstrated that 1-Hz and 20-Hz repetitive electrical stimulation significantly increased the velocity of cortical spreading depression in the stimulated hemisphere. Furthermore, 20-Hz stimulation showed a greater effect on cortical spreading depression compared to 1-Hz stimulation. The results show that 1-Hz and 20-Hz repetitive electrical stimulation results in an increase in cortical spreading depression velocity that is associated with the frequency and the hemisphere of stimulation. Furthermore, the effects are found to be long lasting. We believe that these findings have strong relevance to support the clinical application of therapies involving electrical stimulation for diseases of reduced cortical excitability.

N-methyl-D-aspartate and TrkB receptor activation in cerebellar granule cells: an in vitro model of preconditioning to stimulate intrinsic survival pathways in neurons

Delineating the mechanisms of survival pathways that exist in neurons will provide important insight into how neurons utilize intracellular proteins as neuroprotectants against the causes of acute neurodegeneration. We have employed cultured rat cerebellar granule cells as a model for determining the mechanisms of these intraneuronal survival pathways. Glutamate has long been known to kill neurons by an N-methyl-d-aspartate (NMDA) receptor-mediated mechanism. Paradoxically, subtoxic concentrations of NMDA protect neurons against glutamate-mediated excitotoxicity. Because NMDA protects neurons in physiologic concentrations of glucose and oxygen, we refer to this phenomenon as physiologic preconditioning. One of the major mechanisms of NMDA neuroprotection involves the activation of NMDA receptors leading to the rapid release of brain-derived neurotrophic factor (BDNF). BDNF then binds to and activates its cognate receptor, receptor tyrosine kinase B (TrkB). The efficient utilization of these two receptors confers remarkable resistance against millimolar concentrations of glutamate that kill more than eighty percent of the neurons in the absence of preconditioning the neurons with a subtoxic concentration of NMDA. Exactly how the neurons mediate neuroprotection by activation of both receptors is just beginning to be understood. Both NMDA and TrkB receptors activate nuclear factor kappaB (NF-kappaB), a transcription factor known to be involved in protecting neurons against many different kinds of toxic insults. By converging on survival transcription factors, such as NF-kappaB, NMDA and TrkB receptors protect neurons. Thus, crosstalk between these very different receptors provides a rapid means of neuronal communication to upregulate survival proteins through release and transcriptional activation of messenger RNA.

Spreading depression-induced preconditioning in the mouse cortex: differential changes in the protein expression of ionotropic nicotinic acetylcholine and glutamate receptors

Preconditioning of the cerebral cortex was induced in mice by repeated cortical spreading depression (CSD), and the major ionotropic glutamate (GluRs) and nicotinic acetylcholine receptor (nAChRs) subunits were compared by quantitative immunoblotting between sham- and preconditioned cortex, 24 h after treatment. A 30% reduction in alpha-amino-3-hydroxy-5-methyl-4-iso- xazolepropionate (AMPA) GluR1 and 2 subunit immunoreactivities was observed in the preconditioned cortex (p < 0.03), but there was no significant change in the NMDA receptor subunits, NR1, NR2A and NR2B. A 12-15-fold increase in alpha7 nAChR subunit expression following in vivo CSD (p < 0.001) was by far the most remarkable change associated with preconditioning. In contrast, the alpha4 nAChR subunit was not altered. These data point to the alpha7 nAChR as a potential new target for neuroprotection because preconditioning increases consistently the tolerance of the brain to acute insults such as ischaemia. These data complement recent studies implicating alpha7 nAChR overexpression in the amelioration of chronic neuropathologies, notably Alzheimer's disease (AD).

Repetitive spreading depression induces nestin protein expression in the cortex of rats and mice. Is this upregulation initiated by N-methyl-D-aspartate receptors?

In the November issue (2001) of Neuroscience Letters, Holmin et al. (Neurosci. Lett. 314 (2001) 151) reported that the synthesis of the intermediate filament protein nestin was upregulated by potassium-induced depolarization in the rat cortex. In this letter, we provide supplementary evidence that repeated cortical spreading depression elicited by potassium induces a delayed upregulation of nestin. However, we argue against the authors' conclusion, "Nestin expression was N-methyl-D-aspartate (NMDA)-receptor dependent since dizocilpine (MK-801) treatment abolished the response" because spreading depression itself is very sensitive to NMDA-receptor block, and the drug treatment was initiated prior to potassium application to the cortex in Holmin et al.'s study.