Imaging Spreading Depolarization in the Brainstem

Abstract

Within two minutes of global ischemia or focal stroke, a sudden loss of neuronal and glial membrane potential induces anoxic depolarization (AD) that propagates across gray matter of the cerebral hemispheres at 2-5 mm/min. It induces terminal neuronal damage, forming the initial ischemic core. In healthy gray matter, a milder version termed spreading depression (SD) does not cause tissue damage and generates the migraine aura, often a marching sensory deficit preceding the pain. AD and SD have been well studied in the cerebral and cerebellar cortices, but not the in the brainstem. We induced AD in coronal brain slices of mouse using oxygen/glucose deprived (OGD) saline. SD was induced by briefly elevating the extracellular K+ concentration. AD or SD propagation was imaged as a moving front of elevated light transmittance (LT) in cerebral and cerebellar cortices. Most ventral brainstem areas did not support AD or SD but in the dorsal brainstem some LT mini-fronts were observed in the superficial superior colliculus (SC), edges of inferior colliculus (IC), periaqueductal gray (PAG), tegmental nucleus (TN) and solitary nucleus (SolN). Their AD/SD characteristics were compared to those in the ‘higher’ brain regions. Although time of onset was not different, the present AD and SD propagated more slowly in brainstem gray matter. A non-specific glutamate receptor antagonist kynurenic acid (KYNA) successfully blocked the SD but not AD in PAG, TN and SolN. Two-photon laser scanning microscopy (2-PLSM) of live YFP+ mice brain slices showed that pyramidal neurons in ‘higher’ hippocamal CA1 irreversibly swelled and formed dendritic beads while neurons in the ‘lower’ mesencephalic trigeminal nucleus (Mes) of the midbrain-pons did not significantly swell or display any sign of injury. Finally, dendritic beading was induced in intact mouse neocortex and hippocampus by cardiac perfusion with OGD saline or ice-cold saline. However, dendrites in the brainstem from the same mice showed no obvious beading. Taken together, our study supports the concept that most brainstem regions are comparatively resistant to AD/SD compared to the ‘higher’ regions of cerebral and cerebellar cortices.