When cooled to their critical thermal minimum (CT min), these insects lose the ability to perform coordinated movements and shortly after enter a paralytic state known as chill coma ( Mellanby, 1939 Hazell and Bale, 2011 MacMillan and Sinclair, 2011 Overgaard and MacMillan, 2017). Most insects are chill susceptible, meaning that adverse effects of cold on physiology begin to manifest at temperatures above those that cause any freezing ( Bale, 1996 Sinclair, 1999 Overgaard and MacMillan, 2017). Temperature directly influences insect performance as temperature is lowered, most biological processes are slowed ( Lee, 2012). Combined, these findings suggest a key role for plasticity in Na +/K +-ATPase thermal sensitivity in maintaining central nervous system function in the cold, and more broadly highlight that a single ion pump can be an important determinant of whether insects can respond to their environment to remain active at low temperatures. With Na +/K +-ATPase activity assays and immunoblots, we found that cold-acclimated flies have ion pumps that are less sensitive to temperature, but do not differ in their overall abundance in the brain. Latency to SD followed the pattern of a thermal performance curve, but cold acclimation resulted in a left-shift of the curve to an extent similar to its effect on the SD temperature. After first confirming cold acclimation altered SD onset, we investigated the dependency of the SD event on Na +/K +-ATPase activity by injecting the inhibitor ouabain into the head of the flies to induce SD over a range of temperatures. Here, we focused on the role of the Na +/K +-ATPase specifically in lowering the SD temperature in cold-acclimated Drosophila melanogaster.
SD is triggered by an inability to maintain ion homeostasis in the extracellular space in the brain and is characterized by a rapid surge in extracellular K + concentration, implicating ion pump and channel function. At the critical thermal minimum, a spreading depolarization (SD) event silences central neurons, but the temperature at which this event occurs can be altered through acclimation. Most insects can acclimate to changes in their thermal environment and counteract temperature effects on neuromuscular function.
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