No abstract

SUMMARY The primary objectives of this study were to determine (1) whether rapid cold-hardening (RCH) preserves reproductive behaviors during modest cooling,(2) whether increased mating success at a lower temperature comes at the cost of decreased performance at a higher temperature and (3) whether RCH is associated with an elevated metabolic rate. Drosophila melanogaster(Diptera: Drosphilidae) were rapidly cold-hardened by a 2-h exposure to 16°C prior to experiments. A temperature decrease of only 7°C(23°C to 16°C) prevented half (11/22) of the control pairs of D. melanogaster from engaging in any courtship activity. By contrast, most RCH pairs courted (17/20). Additionally, the 7°C transfer prevented mating in every pair of control flies, whereas more than half (11/20) of the RCH pairs mated. There was no evidence of impaired courtship or mating performance when RCH pairs were tested at 23°C. Finally, RCH is apparently not an energy-demanding process because no increase in the metabolic rate was detected during its induction. Overall, these data demonstrate that RCH serves to constantly fine-tune an insect's physiological state to match slight changes in environmental temperature. Furthermore, the RCH response is not restricted to cryoprotection and survival in the cold but also preserves more subtle behaviors, such as courtship, at moderate to high temperatures throughout the year.

As with other tissues, exposing the mammalian CNS to nonlethal heat stress (i.e., thermal preconditioning) increases levels of heat-shock proteins (Hsps) such as Hsp70 and enhances the viability of neurons under subsequent stress. Using a medullary slice preparation from a neonatal mouse, including the site of the neural network that generates respiratory rhythm (the pre-Bötzinger complex), we show that thermal preconditioning has an additional fundamental effect, protection of synaptic function. Relative to 30 degrees C baseline, initial thermal stress (40 degrees C) greatly increased the frequency of synaptic currents recorded without pharmacological manipulation by approximately 17-fold (p < 0.01) and of miniature postsynaptic currents (mPSCs) elicited by GABA (20-fold) glutamate (10-fold), and glycine (36-fold). Thermal preconditioning (15 min at 40 degrees C) eliminated the increase in frequency of overall synaptic transmission during acute thermal stress and greatly attenuated the frequency increases of GABAergic, glutamatergic, and glycinergic mPSCs (for each, p < 0.05). Moreover, without thermal preconditioning, incubation of slices in solution containing inducible Hsp70 (Hsp72) mimicked the effect of thermal preconditioning on the stress-induced release of neurotransmitter. That preconditioning and exogenous Hsp72 can affect and preserve normal physiological function has important therapeutic implications.

Although laboratory studies demonstrate that cooling to ecologically relevant temperatures and/or at ecologically relevant rates induces rapid coldhardening (RCH) in a variety of insects, little is known of the induction of RCH in nature. In the present study, caged Drosophila melanogaster (1 -2 days posteclosion) from a colony established with flies collected locally are placed in a field setting (i.e. in approximately 4-cm deep leaf litter beneath an apple tree in Mount Pleasant, Michigan) during late afternoon (18.00 h EST; 05.00 h GMT). As the cage temperature falls from 22.1 ± 0.8 to 10.1 ± 0.1 °C between 18.00 and 06.00 h, the proportion of flies surviving a transfer to -6 °C for 1 h increases from 10.0 ± 6.2% to 68.1 ± 7.2%. When obtained from field cages, and then cooled from 23 °C at approximately 0.33 °C min −1 , more female flies remain behaviourally responsive (clinging to surfaces, exhibiting an active righting response, and/or climbing) at temperatures of 8 -12 °C (24.00 h samples) or 7 -12 °C (06.00 h samples), than do those sampled from cages kept in an incubator (23 °C). Field cooling reduces chill coma temperature from 8.7 ± 0.2 °C at 06.00 h to 7.1 ± 0.2 °C at 24.00 h, and to 6.6 ± 0.2 °C at 06.00 h. These data demonstrate that, in a recently collected culture of D. melanogaster , natural changes in microenvironmental temperature induce RCH that can benefit the organism at temperatures encountered in nature.

Little is known about the nature of injury due to cold shock, or its prevention by rapid cold-hardening, in insects. To understand these phenomena better at the system level, physiological and behavioural comparisons were made between control, cold shock injured, and rapidly cold-hardened flesh flies, Sarcophaga crassipalpis Macquart (Diptera, Sarcophagidae). Cold shock impaired the proboscis extension reflex in response to 0.125,0.5 and 1 . 0~ sucrose solutions. Cold shock-injured flies were unable to groom effectively and spent only 12.5% of the first 5min following general dust application producing ineffectual leg movements. In contrast, control and rapidly coldhardened flies exhibited normal grooming behaviour spending 92.4% and 94.1 % of the first 5 min following generalized dust application grooming. Cold shock also decreased the mean resting membrane potential of tergotrochanteral muscle fibres from -65.9 mV in control flies to -4 1.6 mV. Conduction velocities of the three motor neurone populations innervating the tergotrochanteral muscle were all significantly lower in cold-shocked flies than in control or rapidly cold-hardened flies. Finally, cold shock impaired neuromuscular transmission as evidenced by a lack of evoked end plate potentials.