Stress-induced thermotolerance of ventilatory motor pattern generation in the locust, Locusta migratoria

Comparative transcriptome analysis of the rice leaf folder (Cnaphalocrocis medinalis) to heat acclimation

Background

The rice leaf folder Cnaphalocrocis medinalis Güenée is a serious insect pest of rice in Asia. This pest occurs in summer, and it is sensitive to high temperature. However, the larvae exhibit heat acclimation/adaptation. To understand the underlying mechanisms, we established a heat-acclimated strain via multigenerational selection at 39 °C. After heat shock at 41 °C for 1 h, the transcriptomes of the heat-acclimated (S-39) and unacclimated (S-27) larvae were sequenced, using the unacclimated larvae without exposure to 41 °C as the control.

Results

Five generations of selection at 39 °C led larvae to acclimate to this heat stress. Exposure to 41 °C induced 1160 differentially expressed genes (DEGs) between the heat-acclimated and unacclimated larvae. Both the heat-acclimated and unacclimated larvae responded to heat stress via upregulating genes related to sensory organ development and structural constituent of eye lens, whereas the unacclimated larvae also upregulated genes related to structural constituent of cuticle. Compared to unacclimated larvae, heat-acclimated larvae downregulated oxidoreductase activity-related genes when encountering heat shock. Both the acclimated and unacclimated larvae adjusted the longevity regulating, protein processing in endoplasmic reticulum, antigen processing and presentation, MAPK and estrogen signaling pathway to responsed to heat stress. Additionally, the unacclimated larvae also adjusted the spliceosome pathway, whereas the heat-acclimated larvae adjusted the biosynthesis of unsaturated fatty acids pathway when encountering heat stress. Although the heat-acclimated and unacclimated larvae upregulated expression of heat shock protein genes under heat stress including HSP70, HSP27 and CRYAB, their biosynthesis, metabolism and detoxification-related genes expressed differentially.

Conclusions

The rice leaf folder larvae could acclimate to a high temperature via multigenerational heat selection. The heat-acclimated larvae induced more DEGs to response to heat shock than the unacclimated larvae. The changes in transcript level of genes were related to heat acclimation of larvae, especially these genes in sensory organ development, structural constituent of eye lens, and oxidoreductase activity. The DEGs between heat-acclimated and unacclimated larvae after heat shock were enriched in the biosynthesis and metabolism pathways. These results are helpful to understand the molecular mechanism underlying heat acclimation of insects.

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.

Differential activation of serotonergic neurons during short- and long-term gregarization of desert locusts

Serotonin is a neurochemical with evolutionarily conserved roles in orchestrating nervous system function and behavioural plasticity. A dramatic example is the rapid transformation of desert locusts from cryptic asocial animals into gregarious crop pests that occurs when drought forces them to accumulate on dwindling resources, triggering a profound alteration of behaviour within just a few hours. The onset of crowding induces a surge in serotonin within their thoracic ganglia that is sufficient and necessary to induce the switch from solitarious to gregarious behaviour. To identify the neurons responsible, we have analysed how acute exposure to three gregarizing stimuli--crowding, touching the hind legs or seeing and smelling other locusts--and prolonged group living affect the expression of serotonin in individual neurons in the thoracic ganglia. Quantitative analysis of cell body immunofluorescence revealed three classes of neurons with distinct expressional responses. All ganglia contained neurons that responded to multiple gregarizing stimuli with increased expression. A second class showed increased expression only in response to intense visual and olfactory stimuli from conspecifics. Prolonged group living affected a third and entirely different set of neurons, revealing a two-tiered role of the serotonergic system as both initiator and substrate of socially induced plasticity. This demonstrates the critical importance of ontogenetic time for understanding the function of serotonin in the reorganization of behaviour.

Na+-K+-ATPase trafficking induced by heat shock pretreatment correlates with increased resistance to anoxia in locusts

The sensitivity of insect nervous systems to anoxia can be modulated genetically and pharmacologically, but the cellular mechanisms responsible are poorly understood. We examined the effect of a heat shock pretreatment (HS) on the sensitivity of the locust (Locusta migratoria) nervous system to anoxia induced by water immersion. Prior HS made locusts more resistant to anoxia by increasing the time taken to enter a coma and by reducing the time taken to recover the ability to stand. Anoxic comas were accompanied by surges of extracellular potassium ions in the neuropile of the metathoracic ganglion, and HS reduced the time taken for clearance of excess extracellular potassium ions. This could not be attributed to a decrease in the activity of protein kinase G, which was increased by HS. In homogenates of the metathoracic ganglion, HS had only a mild effect on the activity of Na(+)-K(+)-ATPase. However, we demonstrated that HS caused a threefold increase in the immunofluorescent localization of the α-subunit of Na(+)-K(+)-ATPase in metathoracic neuronal plasma membranes relative to background labeling of the nucleus. We conclude that HS induced trafficking of Na(+)-K(+)-ATPase into neuronal plasma membranes and suggest that this was at least partially responsible for the increased resistance to anoxia and the increased rate of recovery of neural function after a disturbance of K(+) homeostasis.

Coordinated ventilation and spiracle activity produce unidirectional airflow in the hissing cockroach, Gromphadorhina portentosa

Insects exchange respiratory gases via an extensive network of tracheal vessels that open to the surface of the body through spiracular valves. Although gas exchange is known to increase with the opening of these spiracles, it is not clear how this event relates to gas flow through the tracheal system. We examined the relationship between respiratory airflow and spiracle activity in a ventilating insect, the hissing cockroach, Gromphadorhina portentosa, to better understand the complexity of insect respiratory function. Using simultaneous video recordings of multiple spiracular valves, we found that abdominal spiracles open and close in unison during periods of ventilation. Additionally, independent recordings of CO2 release from the abdominal and thoracic regions and observations of hyperoxic tracer gas movement indicate that air is drawn into the thoracic spiracles and expelled from the abdominal spiracles. Our video recordings suggest that this unidirectional flow is driven by abdominal contractions that occur when the abdominal spiracles open. The spiracles then close as the abdomen relaxes and fills with air from the thorax. Therefore, the respiratory system of the hissing cockroach functions as a unidirectional pump through the coordinated action of the spiracles and abdominal musculature. This mechanism may be employed by a broad diversity of large insects that respire by active ventilation.

What's hot: the enhancing effects of thermal stress on long-term memory formation in Lymnaea stagnalis

The pond snail, Lymnaea stagnalis, naturally inhabits slow flowing, shallow and stagnant environments in the northern temperate zone. Consequently, it will experience wide temperature fluctuations dependent on prevailing weather conditions. We hypothesize that periods of warming act as a thermal stressor to alter memory formation. Snails were exposed to an acute 1h period of 30°C pond water and we determined how memory formation following operant conditioning of aerial respiration was affected. In snails used here (the Dutch strain), a single 0.5h training session (TS) results in intermediate-term (3h) but not long-term memory (LTM). Applying the thermal stressor during training caused memory enhancement (i.e. LTM lasting 24 h). However, the breathing rate also increased in warm water, which might explain the enhanced memory. Therefore, we applied the thermal stressor (1h at 30°C) up to 4h before or 1h after training. This did not alter baseline breathing rate during the period when snails would experience training. However, the thermal stressor weather experienced prior to or following the single TS, resulted in an enhanced memory that persisted up to 48h (i.e. LTM). We conclude that memory enhancement is due to the stress associated with the thermal stimulus.

Protein expression following heat shock in the nervous system of Locusta migratoria

There is a thermal range for the operation of neural circuits beyond which nervous system function is compromised. Locusta migratoria is native to the semiarid regions of the world and provides an excellent model for studying neural phenomena. In this organism previous exposure to sublethal high temperatures (heat shock, HS) can protect neuronal function against future hyperthermia but, unlike many organisms, the profound physiological adaptations are not accompanied by a robust increase of Hsp70 transcript or protein in the nervous system. We compared Hsp70 increase following HS in the tissues of isolated and gregarious locusts to investigate the effect of population density. We also localized Hsp70 in the metathoracic ganglion (MTG) of gregarious locusts to determine if HS affects Hsp70 in specific cell types that could be masked in whole ganglion assays. Our study indicated no evidence of a consistent change in Hsp70 level in the MTG of isolated locusts following HS. Also, Hsp70 was mainly localized in perineurium, neural membranes and glia and prior HS had no effect on its density or distribution. Finally, we applied 2-D gels to study the proteomic profile of MTG in gregarious locusts following HS; although these experiments showed some changes in the level of ATP-synthase β isoforms, the overall amount of this protein was found unchanged following HS. We conclude that the constitutive level of Hsps in the tissues of locusts is high. Also the thermoprotective effect of HS on the nervous system might be mediated by post-translational modifications or protein trafficking.

Rhythmic behaviour and pattern-generating circuits in the locust: key concepts and recent updates

There is growing recognition that rhythmic activity patterns are widespread in our brain and play an important role in all aspects of the functioning of our nervous system, from sensory integration to central processing and motor control. The study of the unique properties that enable central circuits to generate their rhythmic output in the absence of any patterned, sensory or descending, inputs, has been very rewarding in the relatively simple invertebrate preparations. The locust, specifically, is a remarkable example of an organism in which central pattern generator (CPG) networks have been suggested and studied in practically all aspects of their behaviour. Here we present an updated overview of the various rhythmic behaviours in the locust and aspects of their neural control. We focus on the fundamental concepts of multifunctional neuronal circuits, neural centre interactions and neuromodulation of CPG networks. We are certain that the very broad and solid knowledge base of locust rhythmic behaviour and pattern-generating circuits will continue to expand and further contribute to our understanding of the principles behind the functioning of the nervous system and, indeed, the brain.

Coma in response to environmental stress in the locust: a model for cortical spreading depression

Spreading depression (SD) is an interesting and important phenomenon due to its role in mammalian pathologies such as migraine, seizures, and stroke. Until recently investigations of the mechanisms involved in SD have mostly utilized mammalian cortical tissue, however we have discovered that SD-like events occur in the CNS of an invertebrate model, Locusta migratoria. Locusts enter comas in response to stress during which neural and muscular systems shut down until the stress is removed, and this is believed to be an adaptive strategy to survive extreme environmental conditions. During stress-induced comas SD-like events occur in the locust metathoracic ganglion (MTG) that closely resemble cortical SD (CSD) in many respects, including mechanism of induction, extracellular potassium ion changes, and propagation in areas equivalent to mammalian grey matter. In this review we describe the generation of comas and the associated SD-like events in the locust, provide a description of the similarities to CSD, and show how they can be manipulated both by stress preconditioning and pharmacologically. We also suggest that locust SD-like events are adaptive by conserving energy and preventing cellular damage, and we provide a model for the mechanism of SD onset and recovery in the locust nervous system.

Inhibition of protein kinase G activity protects neonatal mouse respiratory network from hyperthermic and hypoxic stress

In spite of considerable research attention focused on clarifying the mechanisms by which the mammalian respiratory rhythm is generated, little attention has been given to examining how this neuronal circuit can be protected from heat stress. Hyperthermia has a profound effect on neuronal circuits including the circuit that generates breathing in mammals. As temperature of the brainstem increases, respiratory frequency concomitantly rises. If temperature continues to increase respiratory arrest (apnea) and death can occur. Previous research has implicated protein kinase G (PKG) activity in regulating neuronal thermosensitivity of neuronal circuits in invertebrates. Here we examine if pharmacological manipulation of PKG activity in a brainstem slice preparation could alter the thermosensitivity of the fictive neonatal mouse respiratory rhythm. We report a striking effect following alteration of PKG activity in the brainstem such that slices treated with the PKG inhibitor KT5823 recovered fictive respiratory rhythm generation significantly faster than control slices and slices treated with a PKG activator (8-Br-cGMP). Furthermore, slices treated with 8-Br-cGMP arrested fictive respiration at a significantly lower temperature than all other treatment groups. In a separate set of experiments we examined if altered PKG activity could regulate the response of slices to hypoxia by altering the protective switch to fictive gasping. Slices treated with 8-Br-cGMP did not switch to the fictive gasp-like pattern following exposure to hypoxia whereas slices treated with KT5823 did display fictive gasping. We propose that PKG activity inversely regulates the amount of stress the neonatal mammalian respiratory rhythm can endure.

K+ homeostasis and central pattern generation in the metathoracic ganglion of the locust

Stress-induced arrest of ventilatory motor pattern generation is tightly correlated with an abrupt increase in extracellular potassium concentration ([K+]o) within the metathoracic neuropil of the locust, Locusta migratoria. Na+/K+-ATPase inhibition with ouabain elicits repetitive surges of [K+]o that coincide with arrest and recovery of motor activity. Here we show that ouabain induces repetitive [K+]o events in a concentration-dependent manner. 10(-5)M, 10(-4)M, and 10(-3)M ouabain was bath-applied in semi-intact locust preparations. 10(-4)M and 10(-3)M ouabain reliably induced repetitive [K+]o events whereas 10(-5)M ouabain had no significant effect. In comparison to 10(-4)M ouabain, 10(-3)M ouabain increased the number and hastened the time to onset of repetitive [K+]o waves, prolonged [K+]o event duration, increased resting [K+]o, and diminished the absolute value of [K+]o waves. Recovery of motor patterning following [K+]o events was less likely in 10(-3)M ouabain. In addition, we show that K+ channel inhibition using TEA suppressed the onset and decreased the amplitude of ouabain-induced repetitive [K+]o waves. Our results demonstrate that ventilatory circuit function in the locust CNS is dependent on the balance between mechanisms of [K+] accumulation and [K+] clearance. We suggest that with an imbalance in favour of accumulation the system tends towards a bistable state with transitions mediated by positive feedback involving voltage-dependent K+ channels.

Loss of potassium homeostasis underlies hyperthermic conduction failure in control and preconditioned locusts

At extreme temperature, neurons cease to function appropriately. Prior exposure to a heat stress (heat shock [HS]) can extend the temperature range for action potential conduction in the axon, but how this occurs is not well understood. Here we use electrophysiological recordings from the axon of a locust visual interneuron, the descending contralateral movement detector (DCMD), to examine what physiological changes result in conduction failure and what modifications allow for the observed plasticity following HS. We show that at high temperature, conduction failure in the DCMD occurred preferentially where the axon passes through the thoracic ganglia rather than in the connective. Although the membrane potential hyperpolarized with increasing temperature, we observed a modest depolarization (3-6 mV) in the period preceding the failure. Prior to the conduction block, action potential amplitude decreased and half-width increased. Both of these failure-associated effects were attenuated following HS. Extracellular potassium concentration ([K+]o) increased sharply at failure and the failure event could be mimicked by the application of high [K+]o. Surges in [K+]o were muted following HS, suggesting that HS may act to stabilize ion distribution. Indeed, experimentally increased [K+]o lowered failure temperature significantly more in control animals than in HS animals and experimentally maintained [K+]o was found to be protective. We suggest that the more attenuated effects of failure on the membrane properties of the DCMD axon in HS animals is consistent with a decrease in the disruptive nature of the [K+]o-dependent failure event following HS and thus represents an adaptive mechanism to cope with thermal stress.

cGMP-dependent protein kinase as a modifier of behaviour

The importance of cGMP-dependent protein kinase (PKG) to the modulation of behavioural phenotypes has become increasingly clear in recent decades. The effects of PKG on behaviour have been studied in diverse taxa from perspectives as varied as ethology, evolution, genetics and neuropharmacology. The genetic variation of the Drosophila melanogaster gene, foraging (for), has provided a fertile model for examining natural variation in a single major gene influencing behaviour. Concurrent studies in other invertebrates and mammals suggest that PKG is an important signalling molecule with varied influences on behaviour and a large degree of pleiotropy and plasticity. Comparing these cross-taxa effects suggests that there are several potentially overlapping behavioural modalities in which PKG signalling acts to influence behaviours which include feeding, learning, stress and biological rhythms. More in-depth comparative analyses across taxa of the similarities and differences of the influence of PKG on behaviour may provide powerful mechanistic explications of the evolution of behaviour.

Stress preconditioning of spreading depression in the locust CNS

Cortical spreading depression (CSD) is closely associated with important pathologies including stroke, seizures and migraine. The mechanisms underlying SD in its various forms are still incompletely understood. Here we describe SD-like events in an invertebrate model, the ventilatory central pattern generator (CPG) of locusts. Using K(+) -sensitive microelectrodes, we measured extracellular K(+) concentration ([K(+)](o)) in the metathoracic neuropile of the CPG while monitoring CPG output electromyographically from muscle 161 in the second abdominal segment to investigate the role K(+) in failure of neural circuit operation induced by various stressors. Failure of ventilation in response to different stressors (hyperthermia, anoxia, ATP depletion, Na(+)/K(+) ATPase impairment, K(+) injection) was associated with a disturbance of CNS ion homeostasis that shares the characteristics of CSD and SD-like events in vertebrates. Hyperthermic failure was preconditioned by prior heat shock (3 h, 45 degrees C) and induced-thermotolerance was associated with an increase in the rate of clearance of extracellular K(+) that was not linked to changes in ATP levels or total Na(+)/K(+) ATPase activity. Our findings suggest that SD-like events in locusts are adaptive to terminate neural network operation and conserve energy during stress and that they can be preconditioned by experience. We propose that they share mechanisms with CSD in mammals suggesting a common evolutionary origin.

Natural variation in the thermotolerance of neural function and behavior due to a cGMP-dependent protein kinase

Although it is acknowledged that genetic variation contributes to individual differences in thermotolerance, the specific genes and pathways involved and how they are modulated by the environment remain poorly understood. We link natural variation in the thermotolerance of neural function and behavior in Drosophila melanogaster to the foraging gene (for, which encodes a cGMP-dependent protein kinase (PKG)) as well as to its downstream target, protein phosphatase 2A (PP2A). Genetic and pharmacological manipulations revealed that reduced PKG (or PP2A) activity caused increased thermotolerance of synaptic transmission at the larval neuromuscular junction. Like synaptic transmission, feeding movements were preserved at higher temperatures in larvae with lower PKG levels. In a comparative assay, pharmacological manipulations altering thermotolerance in a central circuit of Locusta migratoria demonstrated conservation of this neuroprotective pathway. In this circuit, either the inhibition of PKG or PP2A induced robust thermotolerance of neural function. We suggest that PKG and therefore the polymorphism associated with the allelic variation in for may provide populations with natural variation in heat stress tolerance. for's function in behavior is conserved across most organisms, including ants, bees, nematodes, and mammals. PKG's role in thermotolerance may also apply to these and other species. Natural variation in thermotolerance arising from genes involved in the PKG pathway could impact the evolution of thermotolerance in natural populations.

Cytoskeletal stability and heat shock-mediated thermoprotection of central pattern generation in Locusta migratoria

Prior exposure to extreme temperatures can induce thermoprotection in migratory locusts, which is important for survival in their natural environment. An important motor activity that needs to be protected is ventilation. The mechanism underlying heat shock is not fully understood, and our goal was to test the idea that cytoskeletal stability is critical for such thermoprotection. Cytoskeletal stabilizers (concanavalin A) and destabilizers (colchicine) were bath-applied in semi-intact locust preparations in both control (C) and pre-treated heat-shocked (3 h, 45 degrees C) animals. We measured parameters of the ventilatory motor pattern during maintained high temperature (43 degrees C) and recorded the times taken for motor pattern generation to fail and then recover on returning to room temperature. We found that concanavalin A mimicked the effects of a prior heat stress in control animals by increasing time to failure and decreasing time to recovery of motor pattern generation. However, colchicine destroyed protection in heat-shocked animals by decreasing time to failure and increasing time to recovery. Our findings confirm that the cytoskeleton has a mechanistic role in preserving neural function at high temperatures, possibly through stabilizing ion channels and other integral membrane proteins (e.g. Na(+)/K(+) ATPase) and their interactions with heat shock proteins.

Octopamine mediates thermal preconditioning of the locust ventilatory central pattern generator via a cAMP/protein kinase A signaling pathway

We investigated the role of biogenic amines in generating thermoprotection of the ventilatory motor pattern circuitry in Locusta migratoria. Levels of octopamine (OA) and dopamine (DA) in the metathoracic ganglion decreased during heat stress. We measured the thermosensitivity of central pattern generation in response to a ramped increase of temperature in semi-intact preparations. OA, DA, and tyramine (TA) were either bath applied or injected into the locust hemocoel 4-8 h before testing. Neither TA nor DA modified the thermotolerance of ventilatory motor pattern generation. However, OA treatment by bath applications (10(-4) M OA) or by injections into the hemocoel (2 microg/10 microl OA) mimicked heat shock preconditioning and improved the thermotolerance of the motor pattern by increasing the failure temperature and by decreasing the time taken to recover operation after a return to room temperature. Heat shock-induced thermoprotection was eradicated in locusts preinjected with epinastine (Oct betaR antagonist). Neuropil injections of the cAMP agonist and protein kinase A (PKA) activator, Sp-cAMPs, both conferred thermoprotection in control locusts and rescued thermoprotection in epinastine-treated HS locusts. Similar injections of the PKA inhibitor Rp-cAMPs blocked the thermoprotective effect of bath-applied OA. Octopamine-mediated thermoprotection was also abolished with neuropil injections of cycloheximide or actinomycin D, indicating a requirement for transcription and translation. We conclude that OA has a crucial role in triggering protein synthesis-dependent physiological adaptations to protect CNS function during heat stress by activating a cAMP/PKA pathway.

Photoperiod-induced plasticity of thermosensitivity and acquired thermotolerance inLocusta migratoria

The mechanisms by which different life histories affect neural circuits are largely unknown. We show that the thermosensitivity and thermotolerance of neural circuit operation are affected in a complex dynamic fashion by photoperiod, prior heat experience and the sex of the animal. We compared thermosensitivity and thermotolerance of ventilatory motor pattern generation in locusts reared under two photoperiods (12:12 and 16:8; i.e. 12 h:12 h and 16 h:8 h L:D, respectively) before and after heat shock pre-treatment (HS: 3 h, 45°C) in order to determine the effect of daylength on properties of neural function. We monitored central pattern generator (CPG) output electromyographically from muscle 161 in the second abdominal segment during ramped increases in temperature and also measured the time taken for the circuit to fail at high temperatures and the time taken to recover on return to room temperature. There were effects of photoperiod, heat pre-treatment and the sex of the animal on ventilatory rate, time-to-failure and time-to-recovery. The ventilatory motor pattern of 16:8 and 12:12 locusts responded differently to increasing and maintained high temperature stress in both control and heat shocked locusts. We found that 12:12 locusts were generally more robust than 16:8 locusts: they lived longer, they showed greater tolerance to high temperatures, and they recovered more quickly from temperature-induced circuit failure. A faster ventilatory rate in 12:12 animals at high temperatures may have accelerated evaporative cooling to mediate improved temperature tolerance.

Temperature-sensitive gating in a descending visual interneuron, DCMD

Activity in neural circuits can be modified through experience-dependent mechanisms. The effects of high temperature on a locust visual interneuron (the descending contralateral movement detector, DCMD) have previously been shown to be mitigated by prior exposure to sub-lethal, elevated temperatures (heat shock, HS). Activity in the DCMD is reduced at high temperature in naïve animals (control), whereas HS animals show a maintained spike count at all temperatures. We examined whether this finding was due to direct effects of temperature on visual processing, or whether other indirect feedback mechanisms were responsible for the observed effect in the DCMD. Activity in the DCMD was elicited using a computer-generated looming image, and the response was recorded extracellularly. The temperature of visual processing circuits contributes directly to HS-induced plasticity in the DCMD, as maintaining the brain at 25 degrees C during a thoracic temperature ramp eliminated the high frequency activity associated with HS. Removing ascending input by severing the thoracic nerve cord reduced DCMD thermosensitivity, indicating that indirect feedback mechanisms are also involved in controlling the DCMD response to increased thoracic temperature. Understanding how thermosensitive feedback within the locust affects DCMD function provides insight into critical regulatory mechanisms underlying visually-guided behaviors.

Synaptic thermoprotection in a desert-dwellingDrosophila species

Synaptic transmission is a critical mechanism for transferring information from the nervous system to the body. Environmental stress, such as extreme temperature, can disrupt synaptic transmission and result in death. Previous work on larval Drosophila has shown that prior heat-shock exposure protects synaptic transmission against failure during subsequent thermal stress. This induced thermoprotection has been ascribed to an up-regulation of the inducible heat-shock protein, Hsp70. However, the mechanisms mediating natural thermoprotection in the wild are unknown. We compared synaptic thermosensitivity between D. melanogaster and a desert species, D. arizonae. Synaptic thermosensitivity and the functional limits of the related locomotor behavior differed significantly between closely related, albeit ecologically distinct species. Locomotory behavior of wandering third instar D. arizonae larvae was less thermosensitive and the upper temperature limit of locomotory function exceeded that of D. melanogaster by 6 degrees C. Behavioral results corresponded with significantly lower synaptic thermosensitivity at the neuromuscular junction in D. arizonae. Prior heat-shock protected only D. melanogaster by increasing relative excitatory junctional potential (EJP) duration, the time required for EJP failure at 40 degrees C, and the incidence of EJP recovery following heat-induced failure. Hsp70 induction profiles following heat-shock demonstrate up-regulation of inducible Hsp70 in D. melanogaster but not in D. arizonae. However, expression of Hsp70 under control conditions is greater in D. arizonae. These results suggest that the mechanisms of natural thermoprotection involve an increase in baseline Hsp70 expression.