Cortical Spreading Depression Can Be Triggered by Sensory Stimulation in Primed Wild Type Mouse Brain: a Mechanistic Insight to Migraine Aura Generation

Abstract Background: Unlike the spontaneously appearing aura in migraineurs, experimentally, cortical spreading depression (CSD), the neurophysiological correlate of aura is induced by non-physiological stimuli. Consequently, neural mechanisms involved in spontaneous CSD generation, which may provide insight how migraine starts in an otherwise healthy brain, remains largely unclear. We hypothesized that CSD can be physiologically induced by sensory stimulation in primed mouse brain. Methods: Cortex was made susceptible to CSD with partial inhibition of Na+/K+-ATPase by epidural application of a low dose of Na+/K+-ATPase blocker ouabain that does not induce repetitive CSDs or by knocking-down α2 subunit of Na+/K+-ATPase, which is crucial for K+ and glutamate re-uptake by astrocytes, with shRNA. Stimulation-induced CSDs and extracellular K+ changes were monitored in vivo electrophysiologically or with a K+-sensitive fluoroprobe (IPG-4). Results: After priming with ouabain, photic stimulation increased the CSD incidence compared with non-stimulated animals (44.0 vs. 4.9%, p<0.001). Whisker stimulation was less effective (14.9 vs. 2.4%, p=0.02). Knocking-down Na+/K+-ATPase (50% decrease in mRNA) lowered the CSD threshold in all mice tested but triggered stimulus-induced CSDs in 14.3% and 16.7% of mice with photic and whisker stimulation, respectively. Confirming Na+/K+-ATPase hypofunction, extracellular K+ significantly rose during stimulation after subthreshold ouabain or shRNA treatment unlike controls. In line with higher CSD susceptibility, K+ rise was more prominent after ouabain. To gain insight to preventive mechanisms reducing the incidence of stimulus-induced CSDs, we applied an A1-receptor (DPCPX) or GABA-A (bicuculine) antagonist over the occipital cortex, because adenosine formed during stimulation or inhibitory interneuron activity can reduce CSD susceptibility. DPCPX induced CSDs or CSD-like small-DC shifts during photic stimulation, whereas bicuculine was not effective. Conclusions: Our findings indicate that normal brain is well protected against CSD generation. For CSD to be ignited under physiological conditions, priming and predisposing factors are required as seen in migraine patients. Intense sensory stimulation has the potential to trigger a CSD when co-existing conditions can bring extracellular K+ and glutamate concentrations over threshold via reduced uptake of K+ and glutamate (e.g. inefficient fueling of α2-Na+/K+-ATPase due to reduced glycogen breakdown) or facilitated glutamate release (e.g. reduced presynaptic adenosinergic inhibition).