Spreading Depolarization Evoked by Oxygen-Glucose Deprivation or by Temperature Change in Frog and Rat Brain Slices

Abstract

As a result of anoxia or abrupt change in temperature spreading depolarizations (SD) occur in the grey matter of the higher brain, promoting neuronal injury. SD in mammals and insects induces a sudden shutdown of the central nervous system (CNS) gray matter, but it is unclear if SD is generated in the cerebral cortex (CC) of lower vertebrates. In this study, the effects of oxygen glucose deprivation (OGD, which simulates ischemia) or abrupt temperature change was studied on live coronal brain slices from Northern Leopard frog (Rana pipiens) and Sprague Dawley rat (Rattus norvegicus) brain slices to compare SD susceptibility. We hypothesized that this cold-blooded vertebrate generates SD, reflecting the need for a shutdown capability, but that the frog brain is less susceptible to anoxia or temperature change because of its variable environment. We used light transmittance imaging (LT) during exposure to various solutions/temperatures to examine whether frog CC can generate SD and if so, its propensity to initiate and propagate compared to the rat. The frog CC generated delayed OGD-SD at 12±2.6 min (mean ± SE) at 35oC (n=8) compared to rat slices at 1.6±0.34 min at 35oC (n=4). The frog CC generated ouabain-SD at 5.6±0.3 min at 35oC (n=12) compared to 4.8 min on average in rat neocortex as shown previously in our lab. At 26oC, frog slices generated SD induced by 100 M ouabain (a Na+/K+pump inhibitor) at 9.3±0.5 min (n=12) compared to rat slices at 6.6±0.2 min (n=10). Unlike in rat, SD was not evoked in frog CC by elevating bath [K+]o to 26 mM at any of the tested temperatures. A 35 to 40⁰C ramp generated SD at 10.8±1.1 min (n=6) in frogs and 5.5±0.5 min (n=16) in rats. A 35 to 4.7⁰C ramp generated SD in the rat at 2.5±0.6 min (n=8) but did not elicit SD in the frog. Our findings show that the capacity to generate SD in the cerebral cortex evolved early in vertebrate evolution. However, the frog CC is less sensitive to SD shutdown evoked by simulated ischemia or by sudden temperature change. This appears appropriate to the amphibian environment where oxygen content of the water and ambient temperature vary considerably.