Exposure to heat shock affects thermosensitivity of the locust flight system

Heat shock response and homeostatic plasticity

Heat shock response and homeostatic plasticity are mechanisms that afford functional stability to cells in the face of stress. Each mechanism has been investigated independently, but the link between the two has not been extensively explored. We explore this link. The heat shock response enables cells to adapt to stresses such as high temperature, metabolic stress and reduced oxygen levels. This mechanism results from the production of heat shock proteins (HSPs) which maintain normal cellular functions by counteracting the misfolding of cellular proteins. Homeostatic plasticity enables neurons and their target cells to maintain their activity levels around their respective set points in the face of stress or disturbances. This mechanism results from the recruitment of adaptations at synaptic inputs, or at voltage-gated ion channels. In this perspective, we argue that heat shock triggers homeostatic plasticity through the production of HSPs. We also suggest that homeostatic plasticity is a form of neuroprotection.

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.

Phase maintenance in a rhythmic motor pattern during temperature changes in vivo

Central-pattern-generating neural circuits function reliably throughout an animal's life, despite constant molecular turnover and environmental perturbations. Fluctuations in temperature pose a problem to the nervous systems of poikilotherms because their body temperature follows the ambient temperature, thus affecting the temperature-dependent dynamics of various subcellular components that constitute neuronal circuits. In the crustacean stomatogastric nervous system, the pyloric circuit produces a triphasic rhythm comprising the output of the pyloric dilator, lateral pyloric, and pyloric constrictor neurons. In vitro, the phase relationships of these neurons are maintained over a fourfold change in pyloric frequency as temperature increases from 7°C to 23°C. To determine whether these temperature effects are also found in intact crabs, in the presence of sensory feedback and neuromodulator-rich environments, we measured the temperature dependence of the pyloric frequency and phases in vivo by implanting extracellular electrodes into Cancer borealis and Cancer pagurus and shifting tank water temperature from 11°C to 26°C. Pyloric frequency in the intact crab increased significantly with temperature (Q10 = 2-2.5), while pyloric phases were generally conserved. For a subset of the C. borealis experiments, animals were subsequently dissected and the stomatogastric ganglion subjected to a similar temperature ramp in vitro. We found that the maximal frequency attained at high temperatures in vivo is lower than it is under in vitro conditions. Our results demonstrate that, over a wide temperature range, the phases of the pyloric rhythm in vivo are generally preserved, but that the frequency range is more restricted than it is in vitro.

Robustness of a rhythmic circuit to short- and long-term temperature changes

Recent computational and experimental work has shown that similar network performance can result from variable sets of synaptic and intrinsic properties. Because temperature is a global perturbation that differentially influences every biological process within the nervous system, one might therefore expect that individual animals would respond differently to temperature. Nonetheless, the phase relationships of the pyloric rhythm of the stomatogastric ganglion (STG) of the crab, Cancer borealis, are remarkably invariant between 7 and 23°C (Tang et al., 2010). Here, we report that, when isolated STG preparations were exposed to more extreme temperature ranges, their networks became nonrhythmic, or "crashed", in a reversible fashion. Animals were acclimated for at least 3 weeks at 7, 11, or 19°C. When networks from the acclimated animals were perturbed by acute physiologically relevant temperature ramps (11-23°C), the network frequency and phase relationships were independent of the acclimation group. At high acute temperatures (>23°C), circuits from the cold-acclimated animals produced less-regular pyloric rhythms than those from warm-acclimated animals. At high acute temperatures, phase relationships between pyloric neurons were more variable from animal to animal than at moderate acute temperatures, suggesting that individual differences across animals in intrinsic circuit parameters are revealed at high temperatures. This shows that individual and variable neuronal circuits can behave similarly in normal conditions, but their behavior may diverge when confronted with extreme external perturbations.

Neural thermal performance in porcelain crabs, genus Petrolisthes

Neurons are highly temperature sensitive. Temperature-induced nerve failure may play an important role in determining organismal thermal tolerance limits and distribution patterns. To expand our understanding of the role of neuronal thermal performance in setting thermal limits, we compared the thermal performance of neurons from five porcelain crab (genus Petrolisthes) congeners that differ in thermal habitat. In experiment 1, neuronal performance of sensory neurons was determined by extracellular recording of spontaneous action potentials during thermal ramps. Arrhenius break temperatures of action potential generation were used to calculate maximum critical temperature (CT(max)) and minimum critical temperature (CT(min)) for neuronal performance. CT(max) and CT(min) were related to habitat temperature across the five species and were found to respond to acclimation temperature. In experiment 2, we assessed the performance of neurons from Petrolisthes cinctipes acclimated at 8°, 18°, and 25°C when placed at 30°C (near the whole-organism CT(max) of this species) and demonstrated that neural performance near whole-organism CT(max) increases with increasing acclimation temperature. In experiment 3, we compared the thermal limits of sensory afferents and pacemaker efferents and found that they were correlated, although pacemaker efferents tended to have a higher CT(max) and reduced plasticity. Our final analysis, which was of transcriptomic data in cardiac tissue, leads us to hypothesize that nerve membrane K(+) conductance may underlie variation in nerve thermal tolerance.

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.

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.

The nitric oxide/cGMP pathway tunes the thermosensitivity of swimming motor patterns in Xenopus laevis tadpoles

We investigated the role of the nitric oxide (NO)/cGMP pathway in setting thresholds for failure and recovery during hyperthermic stress of the swimming central pattern generator of immobilized Xenopus tadpoles (stage 42). We recorded swimming motor patterns induced by tail skin stimulation (TS) (1 ms current pulse) or by bath application of 50 microm NMDA. Swimming rhythm frequency increased in a linear manner with increasing temperature. In the presence of the NO donor S-nitroso-N-acetylpenicillamine (SNAP), recovery from hyperthermic failure was greatly slowed, often taking longer than the duration of the experiment. Pharmacological activation of the NO/cGMP pathway using SNAP or 8-bromo-cGMP (1) decreased the duration of TS-evoked swim episodes; (2) decreased the temperature threshold for hyperthermic circuit failure; (3) decreased the temperature at which the circuit recovered; and (4) increased the time taken to recover. Pharmacological inhibition of the NO/cGMP pathway using the NO scavenger CPTIO, the nitric oxide synthase (NOS) inhibitor L-NAME or the guanylyl cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) had the opposite effects. NMDA rhythms were more resistant to hyperthermic failure than TS-evoked swim episodes, but the effects of SNAP on the temperature sensitivity of swimming evoked by NMDA were similar to those on TS-evoked swimming, suggesting that drug effects occur on central pattern-generating networks rather than sensory pathways. We conclude that the NO/cGMP pathway is involved in setting the threshold temperatures for hyperthermic failure and subsequent recovery of fictive swimming in tadpoles, and we suggest that this is part of a variable response to prevent overexcitation during abiotic stress under different environmental conditions.

Thermal activation of escape swimming in post-hatching Xenopus laevis frog larvae

Survival requires the selection of appropriate behavioural responses in the face of danger. With respect to the threat of predation, both the decision to escape and the underlying neuronal mechanisms have been extensively studied, but processes that trigger evasion of abiotic stressors, which are potentially hazardous to survival, are less well understood. Here, we document the interplay between rhythmic locomotory and 'C-start' escape swimming in Xenopus frog larvae when exposed to hyperthermic conditions. As temperature rises, swim cycle frequency increases while swim bout duration decreases, until swimming can no longer be initiated by sensory stimuli. Above a critical higher temperature, more intense sequences of spontaneous high amplitude C-start escape activity occur. Each C-start is followed by a few cycles of fast rhythmic swimming in which activity alternates between the two sides. The initial, high amplitude ventral root burst of an escape sequence propagates rostrocaudally approximately threefold faster than subsequent cycles. The high conduction velocity of this initial burst is consistent with the activation of a Mauthner neuron, one of a pair of giant reticulospinal neurons in fish and amphibians. In support of the involvement of a Mauthner neuron, unilateral lesions of the caudal hindbrain eliminated escape activity on the operated side, but activity remained on the un-operated side. Behaviourally, tadpoles responded to temperature ramps with a sequence of C-start responses in which the body arced through approximately 130 degrees in 22 ms, followed by high frequency swimming. These results suggest that high temperature activates the Mauthner neurons to trigger C-start escape behaviour.

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.

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.

Heat Stress–Mediated Plasticity in a Locust Looming-Sensitive Visual Interneuron

Neural circuits are strongly affected by temperature and failure ensues at extremes. However, detrimental effects of high temperature on neural pathways can be mitigated by prior exposure to high, but sublethal temperatures (heat shock). Using the migratory locust, Locusta migratoria, we investigated the effects of heat shock on the thermosensitivity of a visual interneuron [the descending contralateral movement detector (DCMD)]. Activity in the DCMD was elicited using a looming stimulus and the response was recorded from the axon using intracellular and extracellular methods. The thoracic region was perfused with temperature-controlled saline and measurements were taken at 5° intervals starting at 25°C. Activity in DCMD was decreased in control animals with increased temperature, whereas heat-shocked animals had a potentiated response such that the peak firing frequency was increased. Significant differences were also found in the thermosensitivity of the action potential properties between control and heat-shocked animals. Heat shock also had a potentiating effect on the amplitude of the afterdepolarization. The concurrent increase in peak firing frequency and maintenance of action potential properties after heat shock could enhance the reliability with which DCMD initiates visually guided behaviors at high temperature.

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.

Effects of developmental environment on signal-preference coupling in a Hawaiian cricket

Previous work has suggested that developmental temperature influences expression of the adult male calling song of the cricket, a sexually selected mate recognition signal. The role of developmental temperature in shaping female preference functions, and thus its influence on signal-preference coupling has not been investigated. In this study, the effects of developmental temperature are examined in both males and females of the Hawaiian cricket, Laupala cerasina, to determine the degree of signal-preference matching between male song and female preference due to developmental environment. We found that rearing females in different temperature environments affected adult female acoustic preference functions in the same direction as male calling song, thereby influencing variation in adult reproductive behavior in such a way that male and female components remain coordinated. However, we further demonstrate that for male song, the effect of the rearing environment is not permanent but appears only to exert influence for a period of days. This mid-term temperature effect is distinct from the effect of short-term ambient temperature, which influences song in a matter of minutes and has been well documented. Signal-preference coordination, and sexual selection pressures due to mismatching within natural populations, likely will be influenced by nymphal developmental environments of males and females, as well as by adult singing and preference environments.

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

Ventilation is a crucial motor activity that provides organisms with an adequate circulation of respiratory gases. For animals that exist in harsh environments, an important goal is to protect ventilation under extreme conditions. Heat shock, anoxia, and cold shock are environmental stresses that have previously been shown to trigger protective responses. We used the locust to examine stress-induced thermotolerance by monitoring the ability of the central nervous system to generate ventilatory motor patterns during a subsequent heat exposure. Preparations from pre-stressed animals had an increased incidence of motor pattern recovery following heat-induced failure, however, prior stress did not alter the characteristics of the ventilatory motor pattern. During constant heat exposure at sub-lethal temperatures, we observed a protective effect of heat shock pre-treatment. Serotonin application had similar effects on motor patterns when compared to prior heat shock. These studies are consistent with previous studies that indicate prior exposure to extreme temperatures and hypoxia can protect neural operation against high temperature stress. They further suggest that the protective mechanism is a time-dependent process best revealed during prolonged exposure to extreme temperatures and is mediated by a neuromodulator such as serotonin.

Cloning and characterization of a member of the hsp70 gene family from Locusta migratoria, a highly thermotolerant insect

A complementary deoxyribonucleic acid (cDNA) and the corresponding gene segment encoding a member of the 70-kDa heat shock protein (Hsp70) family have been cloned and sequenced from Locusta migratoria, the African migratory locust. These animals are noted for their thermotolerance, which can exceed temperatures of 50 degrees C. Conceptually translated, the sequence shows a 654-residue protein with theoretical molecular weight of 71.4 kDa, which more closely resembles the mammalian Hsp70 (84-85% similarity) than Hsp70 from other insects, with approximately 75% similarity to the sequence from the fruit fly. Comparisons of cDNA and genomic sequences show that the gene contains 2 introns, a 245-bp intron located in the 5' untranslated region and a 91-bp intron in the coding region. Transcript abundance, as estimated by Northern blot analysis and reverse transcription-polymerase chain reaction, shows that heat shock treatment (45 degrees C for 3 hours) does not elevate hsp70 messenger ribonucleic acid levels in fat bodies or in neural tissues. Immunological assays of Hsp70 show that the protein is constitutively expressed, with a modest, approximately 2-fold induction after a 3-hour heat shock in fat body preparations. Although this sequence could be an hsc70 rather than an hsp70, it was the only cDNA isolated from heat-shocked tissue. Whatever the formal designation, such modest induction and constitutive expression may be ideally suited as an adaptation to the locust's chronic exposure to heat shock temperatures and the consequent demand for chaperone proteins.

Anoxia induces thermotolerance in the locust flight system

Heat shock and anoxia are environmental stresses that are known to trigger similar cellular responses. In this study, we used the locust to examine stress cross-tolerance by investigating the consequences of a prior anoxic stress on the effects of a subsequent high-temperature stress. Anoxic stress and heat shock induced thermotolerance by increasing the ability of intact locusts to survive normally lethal temperatures. To determine whether induced thermotolerance observed in the intact animal was correlated with electrophysiological changes, we measured whole-cell K+ currents and action potentials from locust neurons. K+ currents recorded from thoracic neuron somata were reduced after anoxic stress and decreased with increases in temperature. Prior anoxic stress and heat shock increased the upper temperature limit for generation of an action potential during a subsequent heat stress. Although anoxia induced thermotolerance in the locust flight system, a prior heat shock did not protect locusts from a subsequent anoxic stress. To determine whether changes in bioenergetic status were implicated in whole-animal cross-tolerance, phosphagen levels and rates of mitochondrial respiration were assayed. Heat shock alone had no effect on bioenergetic status. Prior heat shock allowed rapid recovery after normally lethal heat stress but afforded no protection after a subsequent anoxic stress. Heat shock also afforded no protection against disruption of bioenergetic status after a subsequent exercise stress. These metabolite studies are consistent with the electrophysiological data that demonstrate that a prior exposure to anoxia can have protective effects against high-temperature stress but that heat shock does not induce tolerance to anoxia.

Heat shock-induced thermoprotection of action potentials in the locust flight system

There is increasing evidence that heat shock (HS) has long-term effects on electrophysiological properties of neurons and synapses. Prior HS protects neural circuitry from a subsequent heat stress but little is known about the mechanisms that mediate this plasticity and induce thermotolerance. Exposure of Locusta migratoria to HS conditions of 45 degrees C for 3 h results in thermotolerance to hitherto lethal temperatures. Locust flight motor patterns were recorded during tethered flight at room temperature, before and after HS. In addition, intracellular action potentials (APs) were recorded from control and HS motoneurons in a semi-intact preparation during a heat stress. HS did not alter the timing of representative depressor or elevator muscle activity, nor did it affect the ability of the locust to generate a steering motor pattern in response to a stimulus. However, HS did increase the duration of APs recorded from neuropil segments of depressor motoneurons. Increases in AP duration were associated with protection of AP generation against failure at subsequent elevated temperatures. Failure of AP generation at high temperatures was preceded by a concomitant burst of APs and depolarization of the membrane. The protective effects of HS were mimicked by pharmacological blockade of I(K+) with tetraethylammonium (TEA). Taken together, these findings are consistent with a hypothesis that HS protects neuronal survival and function via K+ channel modulation.

Identified nerve cells and insect behavior

Studies of insect identified neurons over the past 25 years have provided some of the very best data on sensorimotor integration; tracing information flow from sensory to motor networks. General principles have emerged that have increased the sophistication with which we now understand both sensory processing and motor control. Two overarching themes have emerged from studies of identified sensory interneurons. First, within a species, there are profound differences in neuronal organization associated with both the sex and the social experience of the individual. Second, single neurons exhibit some surprisingly rich examples of computational sophistication in terms of (a) temporal dynamics (coding superimposed upon circadian and shorter-term rhythms), and also (b) what Kenneth Roeder called "neural parsimony": that optimal information can be encoded, and complex acts of sensorimotor coordination can be mediated, by small ensembles of cells. Insect motor systems have proven to be relatively complex, and so studies of their organization typically have not yielded completely defined circuits as are known from some other invertebrates. However, several important findings have emerged. Analysis of neuronal oscillators for rhythmic behavior have delineated a profound influence of sensory feedback on interneuronal circuits: they are not only modulated by feedback, but may be substantially reconfigured. Additionally, insect motor circuits provide potent examples of neuronal restructuring during an organism's lifetime, as well as insights on how circuits have been modified across evolutionary time. Several areas where future advances seem likely to occur include: molecular genetic analyses, neuroecological syntheses, and neuroinformatics--the use of digital resources to organize databases with information on identified nerve cells and behavior.

Enhancement of short-term synaptic plasticity by prior environmental stress

All chemical synapses can rapidly up- or downregulate the strength of their connections to reshape the postsynaptic signal, thereby stressing the informational importance of specific neural pathways. It is also true that an organism's environment can exert a powerful influence on all aspects of neural circuitry. We investigated the effect of a prior high-temperature stress on the short-term plasticity of a neuromuscular synapse in the hindleg tibial extensor muscle of Locusta migratoria. We found that the prior stress acted to precondition the synapse by increasing the upper temperature limit for synaptic transmission during a subsequent stressful exposure. As well, preexposure to a stressful high-temperature environment increased short-term facilitation of excitatory junction potentials concurrent with a decrease in excitatory junction potential amplitude and a reduction in its temporal parameters. We conclude that a stressful environment can modify synaptic physiological properties resulting in an enhancement of short-term plasticity of the synapse.

Heat-shock-induced thermoprotection of hindleg motor control in the locust

Functional neuromuscular connections are critical for appropriate behavioural responses, but can be negatively affected by increases in temperature. We investigated the effects of heat shock on the thermosensitivity of a neuromuscular pathway to the hindleg tibial extensor muscle of Locusta migratoria. We found that exposure to heat shock induced thermoprotection of both neuromuscular transmission and extensor muscle contraction by (i) increasing the upper temperature limit for failure, (ii) improving recovery following heat-induced failure and (iii) stabilizing excitatory junction potential amplitude and duration and extensor muscle contraction force at high temperatures. Furthermore, the heat-shock-induced thermoprotection of extensor muscle contraction was not attributable to a protective effect on intrinsic components of muscle contraction. Finally, the use of jumping as a locomotor strategy to avoid capture, a behavioural response dependent upon functionally competent neuromuscular connections at the hindleg tibial extensor muscle, became less sensitive to temperature following heat shock. We conclude that the natural stress response of the locust stabilizes neuromuscular signalling during temperature stress, and that this can underlie a thermoprotection of muscle contraction force and thus alter the thermosensitivity of an escape behaviour critical for survival.

Neuroprotection at Drosophila synapses conferred by prior heat shock

Synapses are critical sites of information transfer in the nervous system, and it is important that their functionality be maintained under stressful conditions to prevent communication breakdown. Here we show that synaptic transmission at the Drosophila larval neuromuscular junction is protected by prior exposure to heat shock that strongly induces expression of heat shock proteins, in particular hsp70. Using a macropatch electrode to record synaptic activity at individual, visualized boutons, we found that prior heat shock sustains synaptic performance at high test temperatures through pre- and postsynaptic alterations. After heat shock, nerve impulses release more quantal units at high temperatures and exhibit fewer failures of release (presynaptic modification), whereas the amplitude of quantal currents remains more constant than does that in nonheat-shocked controls (postsynaptic modification). The time course of these physiological changes is similar to that of elevated hsp70. Thus, stress-induced neuroprotective mechanisms maintain function at synapses by modifying their properties.

Long-term effects of prior heat shock on neuronal potassium currents recorded in a novel insect ganglion slice preparation

Brief exposure to high temperatures (heat shock) induces long-lasting adaptive changes in the molecular biology of protein interactions and behavior of poikilotherms. However, little is known about heat shock effects on neuronal properties. To investigate how heat shock affects neuronal properties we developed an insect ganglion slice from locusts. The functional integrity of neuronal circuits in slices was demonstrated by recordings from rhythmically active respiratory neurons and by the ability to induce rhythmic population activity with octopamine. Under these "functional" in vitro conditions we recorded outward potassium currents from neurons of the ventral midline of the A1 metathoracic neuromere. In control neurons, voltage steps to 40 mV from a holding potential of -60 mV evoked in control neurons potassium currents with a peak current of 10.0 +/- 2.5 nA and a large steady state current of 8.5 +/- 2.6 nA, which was still activated from a holding potential of -40 mV. After heat shock most of the outward current inactivated rapidly (peak amplitude: 8.4 +/- 2.4 nA; steady state: 3.6 +/- 2.0 nA). This current was inactivated at a holding potential of -40 mV. The response to temperature changes was also significantly different. After changing the temperature from 38 to 42 degrees C the amplitude of the peak and steady-state current was significantly lower in neurons obtained from heat-shocked animals than those obtained from controls. Our study indicates that not only heat shock can alter neuronal properties, but also that it is possible to investigate ion currents in insect ganglion slices.