When insects are cooled, they initially lose their ability to perform coordinated movements at their critical thermal minima (CTmin). At a slightly lower temperature, they enter a state of complete paralysis (chill coma onset temperature - CCO) and if they are returned to permissive temperatures they regain function after a recovery period which is termed chill coma recovery time (CCRT). These three phenotypes (CTmin, CCO, and CCRT) are all popular measures of insect cold tolerance and it is therefore important to characterize the physiological processes that are responsible for these phenotypes. In the present study we measured extracellular field potentials in the central nervous system (CNS) and muscle membrane potential (Vm) during cooling and recovery in three Drosophila species that have different cold tolerances. With these measurements we assess the role of the CNS and muscle Vm in setting the lower thermal limits (CTmin and CCO) and in delaying chill coma recovery (CCRT). The experiments suggest that entry into chill coma is primarily caused by the onset of a spreading depolarization in the CNS for all three species. In the two most cold-sensitive species we observed that the loss of CNS function was followed closely by a depolarization of muscle Vm which is known to compromise muscle function. When flies are returned to benign temperature after a cold exposure we observe a rapid recovery of CNS function, but functional recovery was delayed by a slower recovery of muscle polarization. Thus, we demonstrate the primacy of different physiological systems (CNS vs. muscle) as determinants of the most commonly used cold tolerance measures for insects (CTmin vs. CCRT).
Keywords: CT(min); Cold tolerance; Drosophila; Membrane potential; Spreading depolarization.
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