Neural shutdown under stress: an evolutionary perspective on spreading depolarization

Dietary potassium and cold acclimation additively increase cold tolerance in Drosophila melanogaster

In the cold, chill susceptible insects lose the ability to regulate ionic and osmotic gradients. This leads to hemolymph hyperkalemia that drives a debilitating loss of cell membrane polarization, triggering cell death pathways and causing organismal injury. Biotic and abiotic factors can modulate insect cold tolerance by impacting the ability to mitigate or prevent this cascade of events. In the present study, we test the combined and isolated effects of dietary manipulations and thermal acclimation on cold tolerance in fruit flies. Specifically, we acclimated adult Drosophila melanogaster to 15 or 25 °C and fed them either a K+-loaded diet or a control diet. We then tested the ability of these flies to recover from and survive a cold exposure, as well as their capacity to protect transmembrane K+ gradients, and intracellular Na+ concentration. As predicted, cold-exposed flies experienced hemolymph hyperkalemia and cold-acclimated flies had improved cold tolerance due to an improved maintenance of the hemolymph K+ concentration at low temperature. Feeding on a high-K+ diet improved cold tolerance additively, but paradoxically reduced the ability to maintain extracellular K+ concentrations. Cold-acclimation and K+-feeding additively increased the intracellular K+ concentration, aiding in maintenance of the transmembrane K+ gradient during cold exposure despite cold-induced hemolymph hyperkalemia. There was no effect of acclimation or diet on intracellular Na+ concentration. These findings suggest intracellular K+ loading and reduced muscle membrane K+ sensitivity as mechanisms through which cold-acclimated and K+-fed flies are able to tolerate hemolymph hyperkalemia.

The freeze-avoiding mountain pine beetle (Dendroctonus ponderosae) survives prolonged exposure to stressful cold by mitigating ionoregulatory collapse

Insect performance is linked to environmental temperature, and surviving through winter represents a key challenge for temperate, alpine and polar species. To overwinter, insects have adapted a range of strategies to become truly cold hardy. However, although the mechanisms underlying the ability to avoid or tolerate freezing have been well studied, little attention has been given to the challenge of maintaining ion homeostasis at frigid temperatures in these species, despite this limiting cold tolerance for insects susceptible to mild chilling. Here, we investigated how prolonged exposure to temperatures just above the supercooling point affects ion balance in freeze-avoidant mountain pine beetle (Dendroctonus ponderosae) larvae in autumn, mid-winter and spring, and related it to organismal recovery times and survival. Hemolymph ion balance was gradually disrupted during the first day of exposure, characterized by hyperkalemia and hyponatremia, after which a plateau was reached and maintained for the rest of the 7-day experiment. The degree of ionoregulatory collapse correlated strongly with recovery times, which followed a similar asymptotical progression. Mortality increased slightly during extensive cold exposures, where hemolymph K+ concentration was highest, and a sigmoidal relationship was found between survival and hyperkalemia. Thus, the cold tolerance of the freeze-avoiding larvae of D. ponderosae appears limited by the ability to prevent ionoregulatory collapse in a manner similar to that of chill-susceptible insects, albeit at much lower temperatures. Based on these results, we propose that a prerequisite for the evolution of insect freeze avoidance may be a convergent or ancestral ability to maintain ion homeostasis during extreme cold stress.

Thermal limits of survival and reproduction depend on stress duration: A case study of Drosophila suzukii

Studies of ectotherm responses to heat extremes often rely on assessing absolute critical limits for heat coma or death (CTmax), however, such single parameter metrics ignore the importance of stress exposure duration. Furthermore, population persistence may be affected at temperatures considerably below CTmax through decreased reproductive output. Here we investigate the relationship between tolerance duration and severity of heat stress across three ecologically relevant life-history traits (productivity, coma and mortality) using the global agricultural pest Drosophila suzukii. For the first time, we show that for sublethal reproductive traits, tolerance duration decreases exponentially with increasing temperature (R2 > 0.97), thereby extending the Thermal Death Time framework recently developed for mortality and coma. Using field micro-environmental temperatures, we show how thermal stress can lead to considerable reproductive loss at temperatures with limited heat mortality highlighting the importance of including limits to reproductive performance in ecological studies of heat stress vulnerability.

The evolutionary meaning of migraine

INTRODUCTION

Migraine's astonishing prevalence and preserved genetic background contrast with the definition of a disease and the biological meaning of experiencing recurrent, severe headache attacks is still puzzling.

METHODS

To provide a comprehensive explanation of the migraine evolutionary meaning, we review (i) the putative role of the autonomic nervous system in migraine attacks, (ii) the inter-ictal autonomic, functional, and metabolic signature of migraine patients, (iii) the bio-behavioral perspective of pain, and (iv) the allostatic perception of migraine chronification.

RESULTS

Migraineurs have inter-ictal cortical hyperexcitability and metabolic dysfunction that predisposes to brain energetic imbalance. Multiple precipitating factors may lead to brain energy consumption over the migraine attack generation threshold. In response, the brain engenders adaptive, evolutionary conserved, autonomic-behavior responses through the antidromic activation of the trigeminovascular system. The sickness behavior and severe pain experienced during migraine attacks result in avoiding mental and physical activity, allowing brain energy restoration. Chronic exposure to stressors may result in an allostatic overload, leading to maladaptive chronic activation of these responses. In this bio-behavioral perspective, the chronification of migraine should be envisioned as a pathological process, whereas the migraine itself should not.

CONCLUSION

Migraine has an evolutionary (Darwinian) meaning.

Rapid cold hardening modifies ion regulation to delay anoxia-induced spreading depolarization in the CNS of the locust

Insects experience different kinds of environmental stresses that can impair neural performance, leading to spreading depolarization (SD) of nerve cells and neural shutdown underlying coma. SD is associated with a sudden loss of ion, notably K+, homeostasis in the central nervous system. The sensitivity of an insect's nervous system to stress (e.g., anoxia) can be modulated by acute pre-treatment. Rapid cold hardening (RCH) is a form of preconditioning, in which a brief exposure to low temperature can enhance the stress tolerance of insects. We used a pharmacological approach to investigate whether RCH affects anoxia-induced SD in the locust, Locusta migratoria, via one or more of the following homeostatic mechanisms: (1) Na+/K+-ATPase (NKA), (2) Na+/K+/2Cl- co-transporter (NKCC), and (3) voltage-gated K+ (Kv) channels. We also assessed abundance and phosphorylation of NKCC using immunoblotting. We found that inhibition of NKA or Kv channels delayed the onset of anoxia-induced SD in both control and RCH preparations. However, NKCC inhibition preferentially abrogated the effect of RCH. Additionally, we observed a higher abundance of NKCC in RCH preps but no statistical difference in its phosphorylation level, indicating the involvement of NKCC expression or degradation as part of the RCH mechanism.

Low Glycolysis Is Neuroprotective during Anoxic Spreading Depolarization (SD) and Reoxygenation in Locusts

Migratory locusts enter a reversible hypometabolic coma to survive environmental anoxia, wherein the cessation of CNS activity is driven by spreading depolarization (SD). While glycolysis is recognized as a crucial anaerobic energy source contributing to animal anoxia tolerance, its influence on the anoxic SD trajectory and recovery outcomes remains poorly understood. We investigated the effects of varying glycolytic capacity on adult female locust anoxic SD parameters, using glucose or the glycolytic inhibitors 2-deoxy-d-glucose (2DG) or monosodium iodoacetate (MIA). Surprisingly, 2DG treatment shared similarities with glucose yet had opposite effects compared with MIA. Specifically, although SD onset was not affected, both glucose and 2DG expedited the recovery of CNS electrical activity during reoxygenation, whereas MIA delayed it. Additionally, glucose and MIA, but not 2DG, increased tissue damage and neural cell death following anoxia-reoxygenation. Notably, glucose-induced injuries were associated with heightened CO2 output during the early phase of reoxygenation. Conversely, 2DG resulted in a bimodal response, initially dampening CO2 output and gradually increasing it throughout the recovery period. Given the discrepancies between effects of 2DG and MIA, the current results require cautious interpretations. Nonetheless, our findings present evidence that glycolysis is not a critical metabolic component in either anoxic SD onset or recovery and that heightened glycolysis during reoxygenation may exacerbate CNS injuries. Furthermore, we suggest that locust anoxic recovery is not solely dependent on energy availability, and the regulation of metabolic flux during early reoxygenation may constitute a strategy to mitigate damage.

A neurophysiological limit and its biogeographic correlations: cold-induced spreading depolarization in tropical butterflies

The physiology of insects is directly influenced by environmental temperature, and thermal tolerance is therefore intrinsically linked to their thermal niche and distribution. Understanding the mechanisms that limit insect thermal tolerance is crucial to predicting biogeography and range shifts. Recent studies on locusts and flies suggest that the critical thermal minimum (CTmin) follows a loss of CNS function via a spreading depolarization. We hypothesized that other insect taxa share this phenomenon. Here, we investigate whether spreading depolarization events occur in butterflies exposed to cold. Supporting our hypothesis, we found that exposure to stressful cold induced spreading depolarization in all 12 species tested. This reinforces the idea that spreading depolarization is a common mechanism underlying the insect CTmin. Furthermore, our results highlight how CNS function is tuned to match the environment of a species. Further research into the physiology underlying spreading depolarization will likely elucidate key mechanisms determining insect thermal tolerance and ecology.

A cold and quiet brain: mechanisms of insect CNS arrest at low temperatures

Exposure to cold causes insects to enter a chill coma at species-specific temperatures and such temperature sensitivity contributes to geographic distribution and phenology. Coma results from abrupt spreading depolarization (SD) of neural tissue in the integrative centers of the CNS. SD abolishes neuronal signaling and the operation of neural circuits, like an off switch for the CNS. Turning off the CNS by allowing ion gradients to collapse will conserve energy and may offset negative consequences of temporary immobility. SD is modified by prior experience via rapid cold hardening (RCH) or cold acclimation which alter properties of K_v channels, Na⁺/K⁺-ATPase and Na⁺/K⁺/2Cl^- cotransporter. The stress hormone octopamine mediates RCH. Future progress depends on developing a more complete understanding of ion homeostasis in and of the insect CNS.

An Isotonic Model of Neuron Swelling Based on Co-Transport of Salt and Water

Neurons spend most of their energy building ion gradients across the cell membrane. During energy deprivation the neurons swell, and the concomitant mixing of their ions is commonly assumed to lead toward a Donnan equilibrium, at which the concentration gradients of all permeant ion species have the same Nernst potential. This Donnan equilibrium, however, is not isotonic, as the total concentration of solute will be greater inside than outside the neurons. The present theoretical paper, in contrast, proposes that neurons follow a path along which they swell quasi-isotonically by co-transporting water and ions. The final neuronal volume on the path is taken that at which the concentration of impermeant anions in the shrinking extracellular space equals that inside the swelling neurons. At this final state, which is also a Donnan equilibrium, all permeant ions can mix completely, and their Nernst potentials vanish. This final state is isotonic and electro-neutral, as are all intermediate states along this path. The path is in principle reversible, and maximizes the work of mixing.

Plasticity in Na+/K+-ATPase thermal kinetics drives variation in the temperature of cold-induced neural shutdown of adult Drosophila melanogaster

Most insects can acclimate to changes in their thermal environment and counteract temperature effects on neuromuscular function. At the critical thermal minimum, a spreading depolarization (SD) event silences central neurons, but the temperature at which this event occurs can be altered through acclimation. SD is triggered by an inability to maintain ion homeostasis in the extracellular space in the brain and is characterized by a rapid surge in extracellular K+ concentration, implicating ion pump and channel function. Here, we focused on the role of the Na+/K+-ATPase specifically in lowering the SD temperature in cold-acclimated Drosophila melanogaster. After first confirming cold acclimation altered SD onset, we investigated the dependency of the SD event on Na+/K+-ATPase activity by injecting the inhibitor ouabain into the head of the flies to induce SD over a range of temperatures. Latency to SD followed the pattern of a thermal performance curve, but cold acclimation resulted in a left-shift of the curve to an extent similar to its effect on the SD temperature. With Na+/K+-ATPase activity assays and immunoblots, we found that cold-acclimated flies have ion pumps that are less sensitive to temperature, but do not differ in their overall abundance in the brain. Combined, these findings suggest a key role for plasticity in Na+/K+-ATPase thermal sensitivity in maintaining central nervous system function in the cold, and more broadly highlight that a single ion pump can be an important determinant of whether insects can respond to their environment to remain active at low temperatures.

Finding the right thermal limit: a framework to reconcile ecological, physiological and methodological aspects of CTmax in ectotherms

Upper thermal limits (CTmax) are frequently used to parameterize the fundamental niche of ectothermic animals and to infer biogeographical distribution limits under current and future climate scenarios. However, there is considerable debate associated with the methodological, ecological and physiological definitions of CTmax. The recent (re)introduction of the thermal death time (TDT) model has reconciled some of these issues and now offers a solid mathematical foundation to model CTmax by considering both intensity and duration of thermal stress. Nevertheless, the physiological origin and boundaries of this temperature–duration model remain unexplored. Supported by empirical data, we here outline a reconciling framework that integrates the TDT model, which operates at stressful temperatures, with the classic thermal performance curve (TPC) that typically describes biological functions at permissive temperatures. Further, we discuss how the TDT model is founded on a balance between disruptive and regenerative biological processes that ultimately defines a critical boundary temperature (Tc) separating the TDT and TPC models. Collectively, this framework allows inclusion of both repair and accumulation of heat stress, and therefore also offers a consistent conceptual approach to understand the impact of high temperature under fluctuating thermal conditions. Further, this reconciling framework allows improved experimental designs to understand the physiological underpinnings and ecological consequences of ectotherm heat tolerance.

Rapid cold hardening increases axonal Na+/K+-ATPase activity and enhances performance of a visual motion detection circuit in Locusta migratoria

Rapid cold hardening (RCH) is a type of phenotypic plasticity that delays the occurrence of chill coma in insects. Chill coma is mediated by a spreading depolarization of neurons and glia in the CNS, triggered by a failure of ion homeostasis. We used biochemical and electrophysiological approaches in the locust, Locusta migratoria, to test the hypothesis that the protection afforded by RCH is mediated by activation of the Na+/K+-ATPase (NKA) in neural tissue. RCH did not affect NKA activity measured in a biochemical assay of homogenized thoracic ganglia. However, RCH hyperpolarized the axon of a visual interneuron (DCMD) and increased the amplitude of an activity-dependent hyperpolarization (ADH) shown previously to be blocked by ouabain. RCH also improved performance of the visual circuitry presynaptic to DCMD to minimize habituation and increase excitability. We conclude that RCH enhances in situ NKA activity in the nervous system but also affects other neuronal properties that promote visual processing in locusts.

Knockdown of a Cyclic Nucleotide-Gated Ion Channel Impairs Locomotor Activity and Recovery From Hypoxia in Adult Drosophila melanogaster

Cyclic guanosine monophosphate (cGMP) modulates the speed of recovery from anoxia in adult Drosophila and mediates hypoxia-related behaviors in larvae. Cyclic nucleotide-gated channels (CNG) and cGMP-activated protein kinase (PKG) are two cGMP downstream targets. PKG is involved in behavioral tolerance to hypoxia and anoxia in adults, however little is known about a role for CNG channels. We used a CNGL (CNG-like) mutant with reduced CNGL transcripts to investigate the contribution of CNGL to the hypoxia response. CNGL mutants had reduced locomotor activity under normoxia. A shorter distance travelled in a standard locomotor assay was due to a slower walking speed and more frequent stops. In control flies, hypoxia immediately reduced path length per minute. Flies took 30–40 min in normoxia for >90% recovery of path length per minute from 15 min hypoxia. CNGL mutants had impaired recovery from hypoxia; 40 min for ∼10% recovery of walking speed. The effects of CNGL mutation on locomotor activity and recovery from hypoxia were recapitulated by pan-neuronal CNGL knockdown. Genetic manipulation to increase cGMP in the CNGL mutants increased locomotor activity under normoxia and eliminated the impairment of recovery from hypoxia. We conclude that CNGL channels and cGMP signaling are involved in the control of locomotor activity and the hypoxic response of adult Drosophila.

Migraine Aura, Transient Ischemic Attacks, Stroke, and Dying of the Brain Share the Same Key Pathophysiological Process in Neurons Driven by Gibbs–Donnan Forces, Namely Spreading Depolarization

Neuronal cytotoxic edema is the morphological correlate of the near-complete neuronal battery breakdown called spreading depolarization, or conversely, spreading depolarization is the electrophysiological correlate of the initial, still reversible phase of neuronal cytotoxic edema. Cytotoxic edema and spreading depolarization are thus different modalities of the same process, which represents a metastable universal reference state in the gray matter of the brain close to Gibbs–Donnan equilibrium. Different but merging sections of the spreading-depolarization continuum from short duration waves to intermediate duration waves to terminal waves occur in a plethora of clinical conditions, including migraine aura, ischemic stroke, traumatic brain injury, aneurysmal subarachnoid hemorrhage (aSAH) and delayed cerebral ischemia (DCI), spontaneous intracerebral hemorrhage, subdural hematoma, development of brain death, and the dying process during cardio circulatory arrest. Thus, spreading depolarization represents a prime and simultaneously the most neglected pathophysiological process in acute neurology. Aristides Leão postulated as early as the 1940s that the pathophysiological process in neurons underlying migraine aura is of the same nature as the pathophysiological process in neurons that occurs in response to cerebral circulatory arrest, because he assumed that spreading depolarization occurs in both conditions. With this in mind, it is not surprising that patients with migraine with aura have about a twofold increased risk of stroke, as some spreading depolarizations leading to the patient percept of migraine aura could be caused by cerebral ischemia. However, it is in the nature of spreading depolarization that it can have different etiologies and not all spreading depolarizations arise because of ischemia. Spreading depolarization is observed as a negative direct current (DC) shift and associated with different changes in spontaneous brain activity in the alternating current (AC) band of the electrocorticogram. These are non-spreading depression and spreading activity depression and epileptiform activity. The same spreading depolarization wave may be associated with different activity changes in adjacent brain regions. Here, we review the basal mechanism underlying spreading depolarization and the associated activity changes. Using original recordings in animals and patients, we illustrate that the associated changes in spontaneous activity are by no means trivial, but pose unsolved mechanistic puzzles and require proper scientific analysis.

Rapid cold hardening delays the onset of anoxia-induced coma via an octopaminergic pathway in Locusta migratoria

Rapid cold hardening (RCH) is a short-term hormesis that occurs in many invertebrate species, especially in insects. Although RCH is best known as enhancing cold tolerance, it can also enhance anoxic tolerance. When exposed to prolonged anoxia, insects enter a reversible coma, which is associated with spreading depolarization (SD) in the central nervous system (CNS). In this study, we investigated the effects of RCH and octopamine (OA) on anoxia-induced SD in L. migratoria. OA is an insect stress hormone that has roles in many physiological processes. Thus, we hypothesized that OA is involved in the mechanism of RCH. First, we found that RCH affects the K⁺ sensitivity of the locust blood brain barrier (BBB) in a way similar to the previously described effects of OA. Next, using SD as an indicator of anoxia-induced coma, we took a pharmacological approach to investigate the effects of OA and epinastine (EP), an octopaminergic receptor (OctR) antagonist. We found that OA mimics, whereas EP blocks, the effect of RCH on anoxia-induced SD. This study demonstrates that OA is involved in the mechanism of RCH in delaying the onset of anoxia-induced locust coma and contributes to determining the mechanism of RCH that modulates insect stress tolerances.

The Critical Role of Spreading Depolarizations in Early Brain Injury: Consensus and Contention

BACKGROUND

When a patient arrives in the emergency department following a stroke, a traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug available to help protect their jeopardized neurons. One crucial reason is that we have not identified the molecular mechanisms leading to electrical failure, neuronal swelling, and blood vessel constriction in newly injured gray matter. All three result from a process termed spreading depolarization (SD). Because we only partially understand SD, we lack molecular targets and biomarkers to help neurons survive after losing their blood flow and then undergoing recurrent SD.

METHODS

In this review, we introduce SD as a single or recurring event, generated in gray matter following lost blood flow, which compromises the Na⁺/K⁺ pump. Electrical recovery from each SD event requires so much energy that neurons often die over minutes and hours following initial injury, independent of extracellular glutamate.

RESULTS

We discuss how SD has been investigated with various pitfalls in numerous experimental preparations, how overtaxing the Na⁺/K⁺ ATPase elicits SD. Elevated K⁺ or glutamate are unlikely natural activators of SD. We then turn to the properties of SD itself, focusing on its initiation and propagation as well as on computer modeling.

CONCLUSIONS

Finally, we summarize points of consensus and contention among the authors as well as where SD research may be heading. In an accompanying review, we critique the role of the glutamate excitotoxicity theory, how it has shaped SD research, and its questionable importance to the study of early brain injury as compared with SD theory.

Hypoxia Tolerance Declines with Age in the Absence of Methionine Sulfoxide Reductase (MSR) in Drosophila melanogaster

Unlike the mammalian brain, Drosophila melanogaster can tolerate several hours of hypoxia without any tissue injury by entering a protective coma known as spreading depression. However, when oxygen is reintroduced, there is an increased production of reactive oxygen species (ROS) that causes oxidative damage. Methionine sulfoxide reductase (MSR) acts to restore functionality to oxidized methionine residues. In the present study, we have characterized in vivo effects of MSR deficiency on hypoxia tolerance throughout the lifespan of Drosophila. Flies subjected to sudden hypoxia that lacked MSR activity exhibited a longer recovery time and a reduced ability to survive hypoxic/re-oxygenation stress as they approached senescence. However, when hypoxia was induced slowly, MSR deficient flies recovered significantly quicker throughout their entire adult lifespan. In addition, the wildtype and MSR deficient flies had nearly 100% survival rates throughout their lifespan. Neuroprotective signaling mediated by decreased apoptotic pathway activation, as well as gene reprogramming and metabolic downregulation are possible reasons for why MSR deficient flies have faster recovery time and a higher survival rate upon slow induction of spreading depression. Our data are the first to suggest important roles of MSR and longevity pathways in hypoxia tolerance exhibited by Drosophila.

Motor patterning, ion regulation and spreading depolarization during CNS shutdown induced by experimental anoxia in Locusta migratoria

Anoxia induces a reversible coma in insects. Coma onset is triggered by the arrest of mechanisms responsible for maintaining membrane ion homeostasis in the CNS, resulting in a wave of neuronal and glial depolarization known as spreading depolarization (SD). Different methods of anoxia influence the behavioural response but their effects on SD are unknown. We investigated the effects of CO₂, N₂, and H₂O on the characteristics of coma induction and recovery in Locusta migratoria. Water immersion delayed coma onset and recovery, likely due to involvement of the tracheal system and the nature of asphyxiation but otherwise resembled N₂ delivery. The main difference between N₂ and CO₂ was that CO₂ hastened onset of neural failure and SD and delayed recovery. In the CNS, this was associated with CO₂ inducing an abrupt and immediate decrease of interstitial pH and increase of extracellular [K⁺]. Recording of the transperineurial potential showed that SD propagation and a postanoxic negativity (PAN) were similar with both gases. The PAN increased with ouabain treatment, likely due to removal of the counteracting electrogenic effect of Na⁺/K⁺-ATPase, and was inhibited by bafilomycin, a proton pump inhibitor, suggesting that it was generated by the electrogenic effect of a Vacuolar-type ATPase (VA). Muscle fibres depolarized by ~20 mV, which happened more rapidly with CO₂ compared with N₂. Wing muscle motoneurons depolarized nearly completely in two stages, with CO₂ causing more rapid onset and slower recovery than N₂. Other parameters of SD onset and recovery were similar with the two gases. Electrical resistance across the ganglion sheath increased during anoxia and at SD onset. We provisionally attribute this to cell swelling reducing the dimensions of the interstitial pathway from neuropil to the bathing saline. Neuronal membrane resistance decreased abruptly at SD onset indicating opening of an unidentified membrane conductance. Consideration of the intracellular recording relative to the saline suggests that the apical membrane of perineurial glia depolarizes prior to neuron depolarization. We propose that SD is triggered by events at the perineurial sheath and then propagates laterally and more deeply into the neuropil. We conclude that the fundamental nature of SD is not dependent on the method of anoxia however the timing of onset and recovery are influenced; water immersion is complicated by the tracheal system and CO₂ delivery has more rapid and longer lasting effects, associated with severe interstitial acidosis.

Insects Provide Unique Systems to Investigate How Early-Life Experience Alters the Brain and Behavior

Early-life experiences have strong and long-lasting consequences for behavior in a surprising diversity of animals. Determining which environmental inputs cause behavioral change, how this information becomes neurobiologically encoded, and the functional consequences of these changes remain fundamental puzzles relevant to diverse fields from evolutionary biology to the health sciences. Here we explore how insects provide unique opportunities for comparative study of developmental behavioral plasticity. Insects have sophisticated behavior and cognitive abilities, and they are frequently studied in their natural environments, which provides an ecological and adaptive perspective that is often more limited in lab-based vertebrate models. A range of cues, from relatively simple cues like temperature to complex social information, influence insect behavior. This variety provides experimentally tractable opportunities to study diverse neural plasticity mechanisms. Insects also have a wide range of neurodevelopmental trajectories while sharing many developmental plasticity mechanisms with vertebrates. In addition, some insects retain only subsets of their juvenile neuronal population in adulthood, narrowing the targets for detailed study of cellular plasticity mechanisms. Insects and vertebrates share many of the same knowledge gaps pertaining to developmental behavioral plasticity. Combined with the extensive study of insect behavior under natural conditions and their experimental tractability, insect systems may be uniquely qualified to address some of the biggest unanswered questions in this field.

Pathophysiological Bases of Comorbidity in Migraine

Despite that it is commonly accepted that migraine is a disorder of the nervous system with a prominent genetic basis, it is comorbid with a plethora of medical conditions. Several studies have found bidirectional comorbidity between migraine and different disorders including neurological, psychiatric, cardio- and cerebrovascular, gastrointestinal, metaboloendocrine, and immunological conditions. Each of these has its own genetic load and shares some common characteristics with migraine. The bidirectional mechanisms that are likely to underlie this extensive comorbidity between migraine and other diseases are manifold. Comorbid pathologies can induce and promote thalamocortical network dysexcitability, multi-organ transient or persistent pro-inflammatory state, and disproportionate energetic needs in a variable combination, which in turn may be causative mechanisms of the activation of an ample defensive system with includes the trigeminovascular system in conjunction with the neuroendocrine hypothalamic system. This strategy is designed to maintain brain homeostasis by regulating homeostatic needs, such as normal subcortico-cortical excitability, energy balance, osmoregulation, and emotional response. In this light, the treatment of migraine should always involves a multidisciplinary approach, aimed at identifying and, if necessary, eliminating possible risk and comorbidity factors.

Thermal acclimation alters Na+/K+-ATPase activity in a tissue-specific manner in Drosophila melanogaster

Insects, like the model species Drosophila melanogaster, lose neuromuscular function and enter a state of paralysis (chill coma) at a population- and species-specific low temperature threshold that is decreased by cold acclimation. Entry into this coma is related to a spreading depolarization in the central nervous system, while recovery involves restoration of electrochemical gradients across muscle cell membranes. The Na⁺/K⁺-ATPase helps maintain ion balance and membrane potential in both the brain and hemolymph (surrounding muscles), and changes in thermal tolerance traits have therefore been hypothesized to be closely linked to variation in the expression and/or activity of this pump in multiple tissues. Here, we tested this hypothesis by measuring activity and thermal sensitivity of the Na⁺/K⁺-ATPase at the tagma-specific level (head, thorax and abdomen) in warm- (25 °C) and cold-acclimated (15 °C) flies by measuring Na⁺/K⁺-ATPase activity at 15, 20, and 25 °C. We relate differences in pump activity to differences in chill coma temperature, spreading depolarization temperature, and thermal dependence of muscle cell polarization. Differences in pump activity and thermal sensitivity induced by cold acclimation varied in a tissue-specific manner: While thermal sensitivity remained unchanged, cold-acclimated flies had decreased Na⁺/K⁺-ATPase activity in the thorax (mainly muscle) and head (mainly composed of brain). We argue that these changes may assist in maintenance of K⁺ homeostasis and membrane potential across muscle membranes, and discuss how reduced Na⁺/K⁺-ATPase activity in the brain may counterintuitively help insects delay coma onset in the cold.

Hyperkalaemia, not apoptosis, accurately predicts insect chilling injury

There is a growing appreciation that insect distribution and abundance are associated with the limits of thermal tolerance, but the physiology underlying thermal tolerance remains poorly understood. Many insects, like the migratory locust (Locusta migratoria), suffer a loss of ion and water balance leading to hyperkalaemia (high extracellular [K⁺]) in the cold that indirectly causes cell death. Cells can die in several ways under stress, and how they die is of critical importance to identifying and understanding the nature of thermal adaptation. Whether apoptotic or necrotic cell death pathways are responsible for low-temperature injury is unclear. Here, we use a caspase-3 specific assay to indirectly quantify apoptotic cell death in three locust tissues (muscle, nerves and midgut) following prolonged chilling and recovery from an injury-inducing cold exposure. Furthermore, we obtain matching measurements of injury, extracellular [K⁺] and muscle caspase-3 activity in individual locusts to gain further insight into the mechanistic nature of chilling injury. We found a significant increase in muscle caspase-3 activity, but no such increase was observed in either nervous or gut tissue from the same animals, suggesting that chill injury primarily relates to muscle cell death. Levels of chilling injury measured at the whole animal level, however, were strongly correlated with the degree of haemolymph hyperkalaemia, and not apoptosis. These results support the notion that cold-induced ion balance disruption triggers cell death but also that apoptosis is not the main form of cell damage driving low-temperature injury.

Inhibition of ATP-sensitive potassium channels exacerbates anoxic coma in Locusta migratoria

We demonstrate the involvement of ATP-sensitive K+ (KATP) channels during recovery from spreading depolarization (SD) induced via anoxic coma in locusts. KATP inhibition using glybenclamide impaired ion homeostasis across the blood-brain barrier, resulting in a longer time to recovery of transperineurial potential following SD. Comparison with ouabain indicates that the effects of glybenclamide are not mediated by the Na+/K+-ATPase but are a result of KATP channel inhibition.

Neural dysfunction correlates with heat coma and CTmax in Drosophila but does not set the boundaries for heat stress survival

When heated, insects lose coordinated movement followed by the onset of heat coma (critical thermal maximum, CTmax). These traits are popular measures to quantify interspecific and intraspecific differences in insect heat tolerance, and CTmax correlates well with current species distributions of insects, including Drosophila Here, we examined the function of the central nervous system (CNS) in five species of Drosophila with different heat tolerances, while they were exposed to either constant high temperature or a gradually increasing temperature (ramp). Tolerant species were able to preserve CNS function at higher temperatures and for longer durations than sensitive species, and similar differences were found for the behavioural indices (loss of coordination and onset of heat coma). Furthermore, the timing and temperature (constant and ramp exposure, respectively) for loss of coordination or complete coma coincided with the occurrence of spreading depolarisation (SD) events in the CNS. These SD events disrupt neurological function and silence the CNS, suggesting that CNS failure is the primary cause of impaired coordination and heat coma. Heat mortality occurs soon after heat coma in insects; to examine whether CNS failure could also be the proximal cause of heat death, we used selective heating of the head (CNS) and abdomen (visceral tissues). When comparing the temperature causing 50% mortality (LT50) of each body part versus that of the whole animal, we found that the head was not particularly heat sensitive compared with the abdomen. Accordingly, it is unlikely that nervous failure is the principal/proximate cause of heat mortality in Drosophila.

Role of adenosine in functional recovery following anoxic coma in Locusta migratoria

When exposed to prolonged anoxia insects enter a reversible coma during which neural and muscular systems temporarily shut down. Nervous system shut down is a result of spreading depolarization throughout neurons and glial cells. Upon return to normoxia, recovery occurs following the restoration of ion gradients. However, there is a delay in the functional recovery of synaptic transmission following membrane repolarization. In mammals, the build-up of extracellular adenosine following spreading depolarization contributes to this delay. Adenosine accumulation is a marker of metabolic stress and it has many downstream effects through the activation of adenosine receptors, including the inhibition of cAMP production. Here we demonstrate that adenosine lengthens the time to functional recovery following anoxic coma in locusts. Caffeine, used as an adenosine receptor antagonist, decreased the time to recovery in intact animals and lengthened the time to recovery in semi-intact animals. A cAMP inhibitor, NKH 477, delayed recovery time in male animals. Our results show that the rate of recovery in insect systems is affected by the presence of adenosine.