Rapid cold hardening and octopamine modulate chill tolerance in Locusta migratoria

Temperature has profound effects on the neural function and behaviour of insects. When exposed to low temperature, chill-susceptible insects enter chill coma, a reversible state of neuromuscular paralysis. Despite the popularity of studying the effects of low temperature on insects, we know little about the physiological mechanisms controlling the entry to, and recovery from, chill coma. Spreading depolarization (SD) is a phenomenon that causes a neural shutdown in the central nervous system (CNS) and it is associated with a loss of K⁺ homeostasis in the CNS. Here, we investigated the effects of rapid cold hardening (RCH) on chill tolerance of the migratory locust. With an implanted thermocouple in the thorax, we determined the temperature associated with a loss of responsiveness (i.e. the critical thermal minimum - CT_min) in intact male adult locusts. In parallel experiments, we recorded field potential (FP) in the metathoracic ganglion (MTG) of semi-intact preparations to determine the temperature that would induce neural shutdown. We found that SD in the CNS causes a loss of coordinated movement immediately prior to chill coma and RCH reduces the temperature that evokes neural shutdown. Additionally, we investigated a role for octopamine (OA) in the locust chill tolerance and found that OA reduces the CT_min and mimics the effects of prior stress (anoxia) in locust.