Background Recognition of stress and mobilization of adequate “fight-or-flight” responses is key for survival and health. Previous studies have shown that exposure of Caenorhabditis elegans to pathogens or toxins simultaneously stimulates cellular stress and detoxification responses and aversive behavior. However, whether a coordinated regulation exists between cytoprotective stress responses and behavioral defenses remains unclear. Results Here, we show that exposure of C. elegans to high concentrations of naturally attractive food-derived odors, benzaldehyde and diacetyl, induces toxicity and food avoidance behavior. Benzaldehyde preconditioning activates systemic cytoprotective stress responses involving DAF-16/FOXO, SKN-1/Nrf2, and Hsp90 in non-neuronal cells, which confer both physiological (increased survival) and behavioral tolerance (reduced food avoidance) to benzaldehyde exposure. Benzaldehyde preconditioning also elicits behavioral cross-tolerance to the structurally similar methyl-salicylate, but not to the structurally unrelated diacetyl. In contrast, diacetyl preconditioning augments diacetyl avoidance, weakens physiological diacetyl tolerance, and does not induce apparent molecular defenses. The inter-tissue connection between cellular and behavioral defenses is mediated by JNK-like stress-activated protein kinases and the neuropeptide Y receptor NPR-1. Reinforcement of the stressful experiences using spaced training forms stable stress-specific memories. Memory retrieval by the olfactory cues leads to avoidance of food contaminated by diacetyl and context-dependent behavioral decision to avoid benzaldehyde only if there is an alternative, food-indicative odor. Conclusions Our study reveals a regulatory link between conserved cytoprotective stress responses and behavioral avoidance, which underlies “fight-or-flight” responses and facilitates self-protection in real and anticipated stresses. These findings imply that variations in the efficiency of physiological protection during past episodes of stress might shape current behavioral decisions. Graphical abstract

Stress exposure early in life is implicated in various behavioural and somatic diseases. experiences during the critical perinatal period form permanent, imprinted memories promoting adult survival.Although imprinting is widely recognized to dictate behaviour, whether it actuates specific transcriptional responses at the cellular level is unknown. Here we report that in response to early life stresses, Caenorhabditis elegans nematodes form an imprinted cellular defense memory. We show that exposing newly-born worms to toxic antimycin A and paraquat, respectively, stimulates the expression of toxin-specific cytoprotective reporters. Toxin exposure also induces avoidance of the toxincontaining bacterial lawn. in contrast, adult worms do not exhibit aversive behaviour towards stressassociated bacterial sensory cues. However, the mere re-encounter with the same cues reactivates the previously induced cytoprotective reporters. Learned adult defenses require memory formation during the L1 larval stage and do not appear to confer increased protection against the toxin. Thus, exposure of C. elegans to toxic stresses in the critical period elicits adaptive behavioural and cytoprotective responses, which do not form imprinted aversive behaviour, but imprint a cytoprotective memory. Our findings identify a novel form of imprinting and suggest that imprinted molecular defenses might underlie various pathophysiological alterations related to early life stress.Associative learning ensures rapid, efficient adaptation to already experienced, re-emerging conditions 1,2 . Re-encountering sensory cues associated with a relevant past experience retrieves the memory and elicits a complex response corresponding to the past incident. Associative memories are generally transient. In contrast, a peculiar learning process takes place early in life during a specific time window, called the critical or sensitive period and gives rise to especially persistent memories. Hence the name, imprinting, which was coined by Konrad Lorenz who observed that newly hatched birds created a strong bond with the first moving object seen 3 . Besides visual cues, olfactory memories driving adult behaviours have been recognized in several vertebrate species, including the homing of salmons to reproduce in the creek they were born 4 and preference for odours associated perinatally with food in mammals 5,6 . Hence, imprinting serves as the biological basis of secure, long lasting attachment to qualities essential for individual and/or species' survival. In further support of the profound, life-long impact of imprinted memories, a growing body of evidence shows that facing adversity in the critical period is connected to different cognitive and affective disorders and are accompanied by brain epigenetic, structural and endocrine alterations 7-9 . On the other hand, early life stress predispose to increased telomere erosion, metabolic and cardiovascular diseases in adulthood ultimately affecting healthy lifespan 10-12 . Some of the open questions emerging from th...

19Background 20 Protection of organismal integrity involve physiological stress responses and behavioral 21 defenses. Recent studies in the roundworm Caenorhabditis elegans have shown that pathogen 22 and toxin exposure simultaneously stimulate cellular stress and detoxification responses and 23 aversive behavior. However, whether a coordinate regulation exists between cellular and 24 neurobehavioral defenses remains unclear. 25 Results 26Here we show that exposure of C. elegans to high concentrations of naturally attractive food-27 derived odors, benzaldehyde and diacetyl, induces toxicity and aversive behavior. 28 Benzaldehyde preconditioning activates systemic cytoprotective stress responses involving 29 DAF-16/FOXO, SKN-1/Nrf and Hsp90 in somatic cells, which confer behavioral tolerance to 30 benzaldehyde and cross-tolerance to the structurally similar methyl-salicylate, but not to the 31 structurally unrelated diacetyl. In contrast, diacetyl preconditioning augments diacetyl 32 avoidance and does not induce apparent molecular defenses. Reinforcement of the experiences 33 using massed training forms relevant associative memories. Memory retrieval by the odor 34 olfactory cues leads to avoidance of food contaminated by diacetyl and context-dependent 35 behavioral decision to avoid benzaldehyde only if there is an alternative, food-indicative odor. 36 Conclusions 37 Our findings reveal a regulatory link between physiological stress responses and learned 38 behavior which facilitates self-protection in real and anticipated stresses. The potential 39 conservation of this somato-neuronal connection might have relevance in maladaptive avoidant 40 human behaviors.41 3 Background 42Adequate, coordinated responses of multicellular organisms are key to adapt to and 43 overcome fundamental alterations of the environment (1-3). These responses originate from 44 intracellular molecular defenses, such as the oxidative, xenobiotic, metabolic and proteotoxic 45 stress responses, which guard homeostasis and confer cytoprotection against the respective 46 stresses, promoting physiological adaptation, fitness and longevity at the organismal level (4). 47Adaptation also involve complex behavioral responses orchestrated by the neuroendocrine 48 system (5-7). For instance, sensory cues representing danger evoke aversive behavior as a result 49 of perception of multiple sensory stimuli, neuronal processing and decision making both in 50 humans and in other species (8-10). In some cases, the neural impulse of perceived danger is 51 so intense that the organism decides to avoid co-occurring cues representing life-sustaining 52 qualities such as food (6, 11). Besides external sensory cues, decision making is modulated by 53 neural context like arousal, motivation, and reward (12, 13). Importantly, behavioral decisions 54 are also influenced by sensory cues that evoke associative memories of past events (14). 55Moreover, exaggerated, inadequate avoidant behavior is characteristic to human anxiety 56 disorders such as phobias (11)...

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