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°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.

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Summary Ten hyperinsulinaemic ponies divided into conditioned (N = 5) and rested (N = 5) groups were evaluated for their insulin and glucose response following oral glucose administration at Weeks 0, 2, 4, and 6. All ponies received a controlled intake of a pelleted ration during the study. In both groups body weight had decreased from baseline by Week 4 and remained low. After 2 weeks of exercise, ponies in the conditioned group had significantly decreased insulin and glucose indices, including peak insulin response, area under the insulin curve from 0 to 210 min (TIS), and the TIS value: area under the glucose curve from 0 to 210 min. By Week 4 of conditioning, although the insulin and glucose indices continued to decrease in the exercised ponies, there was no significant difference between the groups. Over the first 6 weeks of the study all ponies improved their insulin sensitivity accompanied by a loss of body weight. The conditioned ponies were further evaluated during deconditioning at Weeks 8, 10 and 12. The improved insulin sensitivity was maintained during deconditioning.

Both immunity and reproduction are thought to be energetically costly and therefore likely to make trade-offs with one another. To assess whether increasing immune system activity results in a decline in egg production, the immune system in the cricket Gryllus texensis is activated over a period of 12 days with regular injections of lipopolysaccharide (LPS) derived from Serratia marcescens , and the number of eggs laid during this time counted. Egg quality is also assessed by measuring total protein of eggs laid, fertilization and hatching success, and the weight of individual eggs laid after the series of injections. Indirect evidence suggests that LPS induces an immune response in G. texensis . However, the number of eggs produced is not affected. There is also no effect of repeated LPS injections on female weight, egg protein content, or fertilization and hatching success. Taken together, these results suggest that with food and water provided ad libitum , females can protect many aspects of fitness in the face of increased immune system activity. However, there is some evidence to suggest that large (100 g) doses of LPS lead to reduced female longevity, and also in egg weight that could affect offspring success. Although the possibility exists that the decline in lifespan and egg weight after high-dose injections reflects a trade-off between reproduction and immune investment, another possibility is that these doses yield nonspecific effects, or that the high-dose induces an overwhelming immune response that leads to self-damage.

To maximize fitness, the rate of offspring production should be sensitive to factors that predict the likelihood of parental survival. We predicted that acutely activating the immune system in the cricket Gryllus texensis Cade and Otte, 2000, signaling the possibility of decreased life expectancy, would lead to an immediate increase in reproductive effort. We found that lifetime fecundity varied among individual crickets and that female crickets laid more eggs in moist sand than in moist cotton, suggesting that females have the capacity to increase oviposition rates in response to substrate conditions. However, we found that exposing female crickets to a potentially lethal pathogen, Serratia marcescens Bizio, led to an increase in egg laying only when substrate conditions were preferable or at doses approaching the LD 50 .Résumé : Afin de maximiser la fitness, le taux de production des rejetons doit être sensible aux facteurs qui permettent de prévoir la probabilité de survie des parents. Nous avons prédit qu'une activation aiguë du système immunitaire chez le grillon (Gryllus texensis Cade et Otte, 2000), qui signale la possibilité d'une diminution de l'espérance de vie, devrait se traduire par une augmentation immédiate de l'effort reproductif. La fécondité durant la vie totale varie d'un grillon individuel à un autre et les grillons femelles pondent un plus grand nombre d'oeufs dans du sable humide que dans du coton humide, ce qui indique que les femelles ont la capacité d'augmenter leur taux de ponte en fonction des conditions du substrat. Cependant, l'exposition des grillons femelles à un pathogène potentiellement létal, Serratia marcescens Bizio, n'entraîne d'augmentation de la ponte que lorsque les conditions de substrat sont de choix ou lorsque les doses s'approchent de LD 50 .[Traduit par la Rédaction] Shoemaker et al. 418