Muscle membrane potential and insect chill coma

Chill-susceptible insects enter a reversible paralytic state, termed chill coma, at mild low temperatures. Chill coma is caused by neuromuscular impairment, allegedly triggered by cold-induced depolarization of muscle resting membrane potential (V-m). We used five Drosophila species that vary in cold tolerance (chill coma temperature spanning similar to 11 degrees C) and repeatedly measured muscle V-m during a downward temperature ramp (20 to -3 degrees C). Cold-tolerant species were able to defend their V-m down to lower temperatures, which is not explained by species-specific differences in initial V-m at 20 degrees C, but by cold-tolerant drosophilids defending V-m across a broad range of temperatures. We found support for a previously suggested 'critical threshold' of V-m, related to chill coma, in three of the five species. Interestingly, the cold-tolerant Drosophila species may enter coma as a result of processes unrelated to muscle depolarization as their V-m was not significantly depolarized at their chill coma temperatures.