Effects of early life social stress on endocrinology,maternal behavior,and lactation in rats

Exposure to early life stress is a predictor of mental health disorders, and two common forms of early life stress are social conflict and impaired maternal care, which are predominant features of postpartum mood disorders. Exposure of lactating female rats to a novel male intruder involves robust social conflict and induces deficits in maternal care towards the F1 offspring. This exposure is an early life social stressor for female F1 pups that induces inefficient lactation associated with central changes in oxytocin (OXT), prolactin (PRL), and arginine vasopressin (AVP) gene expression in adult F1 females.     

Adolescent exposure to oxytocin,but not the selective oxytocin receptor agonist TGOT,increases social behavior and plasma oxytocin in adulthood

There are indications that exposing adolescent rodents to oxytocin (OT) may have positive “trait-changing” effects resulting in increased sociability and decreased anxiety that last well beyond acute drug exposure and into adulthood. Such findings may have relevance to the utility of OT in producing sustained beneficial effects in human psychiatric conditions. The present study further examined these effects using an intermittent regime of OT exposure in adolescence, and using Long Evans rats, that are generally more sensitive to the acute prosocial effects of OT. As OT has substantial affinity for the vasopressin V1a receptor (V1aR) in addition to the oxytocin receptor (OTR), we examined whether a more selective peptidergic OTR agonist – [Thr4, Gly7]-oxytocin (TGOT) – would have similar lasting effects on behavior. Male Long Evans rats received OT or TGOT (0.5–1 mg/kg, intraperitoneal), once every three days, for a total of 10 doses during adolescence (postnatal day (PND) 28–55). Social and anxiety-related behaviors were assessed during acute administration as well as later in adulthood (from PND 70 onwards). OT produced greater acute behavioral effects than TGOT, including an inhibition of social play and reduced rearing, most likely reflecting primary sedative effects. In adulthood, OT but not TGOT pretreated rats displayed lasting increases in social interaction, accompanied by an enduring increase in plasma OT. These findings confirm lasting behavioral and neuroendocrine effects of adolescent OT exposure. However, the absence of such effects with TGOT suggests possible involvement of the V1aR as well as the OTR in this example of developmental neuroplasticity.     

Transgenerational effects of prenatal bisphenol A on social recognition

Bisphenol A (BPA) is a man-made endocrine disrupting compound used to manufacture polycarbonate plastics. It is found in plastic bottles, canned food linings, thermal receipts and other commonly used items. Over 93% of people have detectable BPA levels in their urine. Epidemiological studies report correlations between BPA levels during pregnancy and activity, anxiety, and depression in children. We fed female mice control or BPA-containing diets that produced plasma BPA concentrations similar to concentrations in humans. Females were mated and at birth, pups were fostered to control dams to limit BPA exposure to gestation in the first generation. Sibling pairs were bred to the third generation with no further BPA exposure. First (F1) and third (F3) generation juveniles were tested for social recognition and in the open field. Adult F3 mice were tested for olfactory discrimination. In both generations, BPA exposed juvenile mice displayed higher levels of investigation than controls in a social recognition task. In F3 BPA exposed mice, dishabituation to a novel female was impaired. In the open field, no differences were noted in F1 mice, while in F3, BPA lineage mice were more active than controls. No impairments were detected in F3 mice, all were able to discriminate different male urine pools and urine from water. No sex differences were found in any task. These results demonstrate that BPA exposure during gestation has long lasting, transgenerational effects on social recognition and activity in mice. These findings show that BPA exposure has transgenerational actions on behavior and have implications for human neurodevelopmental behavioral disorders.     

Interaction effects of corticosterone and experience on aggressive behavior in the green anole lizard

Aggressive encounters are accompanied by a release of stress hormone, and this corticosterone (CORT) secretion could influence aggressive behavior in subsequent encounters. We investigated the modulating effects of CORT on aggressive behavior in the context of a 5-day social experience in male green anole lizards. In Experiment 1, we measured plasma CORT levels in animals that were exposed for different times to aggressive males. In Experiment 2, using metyrapone, a CORT synthesis blocker, we tested whether CORT secretion in response to the aggressive stimulus plays a role in experience-dependent facilitation of aggressive behavior. We hypothesized that aggressive encounters would increase plasma CORT levels, and that blocking CORT synthesis with metyrapone treatment during the aggressive encounter would cause an animal to become more aggressive. We also tested whether blocking CORT would interfere with the influence of 5-day social experience on animals' behavior in a subsequent aggressive encounter. Animals that were exposed to another male showed higher plasma CORT levels immediately after the 10 min encounter than animals exposed to the non-social video, and this high level was maintained through day 5. Within the aggressive video groups, in Experiment 2, there was a distinctly different pattern in displays depending on drug condition: vehicle-injected animals showed gradual increases followed by decreases in aggressive behavioral responses to the video as the five days proceeded (habituation), while animals injected with metyrapone started out with high aggressive behavior and did not decrease behavioral responses at later trials (no habituation). Finally, when tested with a novel conspecific on day 6, animals previously injected with metyrapone showed no higher aggression than did animals previously injected with vehicle and exposed to the aggressive video. These results suggest that blocking CORT synthesis during the exposure to the aggressive video induced animals to remain aggressive toward the repetitive stimulus without habituating, while not becoming more aggressive than controls toward a novel challenger.     

Changes in masculine sexual behavior, corticosterone and testosterone in response to acute and chronic stress in male rats

Chronic exposure to stressors increases HPA axis activity and concomitantly reduces HPG axis activity. This antagonistic relationship between both these axes has been proposed to underlie the inhibition of reproductive function due to stress. Sexual behavior in males may be the most vulnerable aspect of male reproduction to acute and chronic stress and it has been suggested that alterations in sexual behavior during stress are due to the antagonistic relationship between testosterone and corticosteroids. However, only in a few studies has a correlation between the levels of testosterone and corticosterone, and sexual behavior been made. In this study, we evaluated the effects of different stressors, applied both acute and chronically, on masculine sexual behavior and whether or not these effects on sexual behavior are accompanied by changes in plasma levels of corticosterone and testosterone. Additionally, we evaluated the effect of testosterone treatment on the effects of stress on sexual behavior. Sexually experienced male rats were exposed to one of the following stressors: immobilization (IMB), electric foot shocks (EFS) or immersion in cold water (ICW). Sexual behavior and plasma levels of testosterone and corticosterone were assessed on days 1, 5, 10, 15, and 20 of stress. In a second experiment, males were castrated, treated with 3 different doses of testosterone propionate (TP) and exposed to ICW for 20 consecutive days. Sexual behavior was assessed on days 1, 5, 10, 15, and 20 and steroids were evaluated on day 20. Parameters of masculine sexual behavior were modified depending on the characteristics of each stressor. Mount, intromission and ejaculation latencies increased significantly, the number of mounts increased, and ejaculations decreased significantly in males exposed to EFS and to ICW but not in males exposed to IMB. Associated with these effects, testosterone decreased in the EFS and ICW groups on days 1, 15, and 20. However, corticosterone increased only in males exposed to ICW. In castrated males, TP treatment failed to block the effects of stress by ICW on sexual behavior and corticosterone. These results indicate that the effects of stress on sexual behavior depend on the characteristics of each stressor, and these effects, as well as the decrease in testosterone are not necessarily associated with the increase in corticosterone. The fact that testosterone treatment did not prevent the effects of stress on sexual behavior suggests that other mediators could be involved in the alterations of sexual behavior caused by stress.     

The effects of adrenalectomy and corticosterone replacement on maternal behavior in the postpartum rat

It is well known that the hypothalamic-pituitary-adrenal (HPA) axis is activated during stress. Recent work suggests it is also implicated in the regulation of "normal" behaviors. The present studies investigated the effects of adrenalectomy and of varying glucocorticoid concentrations on adult maternal behavior in primiparous rats. In two studies, rats in late pregnancy were adrenalectomized or given sham surgeries and were tested for maternal behavior. In the first study, primiparous rats were given 0, 25, 100, 300, or 500 microg/ml of corticosterone in their drinking water. In the second study, primiparous rats were given either control or corticosterone time-release pellets. Blood samples were taken to ensure that rats demonstrated levels of corticosterone in blood that were relative to doses received. In studies one and two, primiparous adrenalectomized rats showed slightly, but significantly, lower levels of some maternal behaviors, including licking and time in nest, than primiparous sham rats. Primiparous rats given higher doses of corticosterone replacement showed higher levels of these maternal behaviors than primiparous rats given lower doses of corticosterone. In conclusion, adrenalectomy decreases, but does not abolish, maternal behavior. Corticosterone replacement reverses these effects. Corticosterone is not necessary for the initiation or maintenance of maternal behavior but plays a role in the modulation of ongoing maternal behavior.     

The effects of adrenalectomy and corticosterone replacement on maternal memory in postpartum rats

Hormones associated with parturition prime rats to behave maternally, although hormonal changes are not necessary for these behaviors to occur. Experience with pups after birth enhances maternal responsiveness after a period of isolation, creating a maternal memory. The purpose of this study was to determine the role of corticosterone in the formation of maternal memory. Adrenalectomy or sham surgeries were performed in late gestation with corticosterone or vehicle pellets being given to adrenalectomized rats. Pups were removed immediately following parturition, and half of the rats received 4 h of pup experience, while the other half received only brief pup experience associated with parturition. Ten days following pup experience, foster pups were given to all rats. Latency to become maternal and maternal behaviors on the first 2 days of re-exposure and the first two maternal days were recorded. Among adrenalectomized rats given corticosterone, 4-h experience with pups decreased maternal latency when compared to brief experience with pups. This maternal experience effect was not found in comparisons between adrenalectomized rats not given corticosterone. In addition, corticosterone decreased latencies regardless of pup experience. Corticosterone also increased maternal behavior upon initial exposure to foster pups. In conclusion, corticosterone enhanced maternal memory and initial maternal behavior in postpartum rats.     

The neuroscience of affiliation: forging links between basic and clinical research on neuropeptides and social behavior

Animal studies point to the role of two neuropeptides-oxytocin and vasopressin-in the regulation of affiliative behaviors including mating, pair-bond formation, maternal/parenting behavior, and attachment. These findings may have important implications for understanding and treating clinical disorders marked by social deficits and/or disrupted attachment. This review focuses on advances made to date in the effort to forge links between basic and clinical research in the area of neuropeptides and social behavior. The literature on oxytocin and its involvement in stress response, affiliation, and prosocial behavior is reviewed, and the implications of these findings for such disorders as autism as well as other social and stress-related disorders including social phobia, post-traumatic stress disorder, and some personality disorders are considered. Finally, unresolved issues and directions for future research are discussed.     

Neuropeptidergic regulation of affiliative behavior and social bonding in animals

Social relationships are essential for maintaining human mental health, yet little is known about the brain mechanisms involved in the development and maintenance of social bonds. Animal models are powerful tools for investigating the neurobiological mechanisms regulating the cognitive processes leading to the development of social relationships and for potentially extending our understanding of the human condition. In this review, we discuss the roles of the neuropeptides oxytocin and vasopressin in the regulation of social bonding as well as related social behaviors which culminate in the formation of social relationships in animal models. The formation of social bonds is a hierarchical process involving social motivation and approach, the processing of social stimuli and formation of social memories, and the social attachment itself. Oxytocin and vasopressin have been implicated in each of these processes. Specifically, these peptides facilitate social affiliation and parental nurturing behavior, are essential for social recognition in rodents, and are involved in the formation of selective mother-infant bonds in sheep and pair bonds in monogamous voles. The convergence of evidence from these animal studies makes oxytocin and vasopressin attractive candidates for the neural modulation of human social relationships as well as potential therapeutic targets for the treatment of psychiatric disorders associated with disruptions in social behavior, including autism.     

Adult rats exposed to early-life social isolation exhibit increased anxiety and conditioned fear behavior, and altered hormonal stress responses

Social isolation of rodents during development is thought to be a relevant model of early-life chronic stress. We investigated the effects of early-life social isolation on later adult fear and anxiety behavior, and on corticosterone stress responses, in male rats. On postnatal day 21, male rats were either housed in isolation or in groups of 3 for a 3 week period, after which, all rats were group-reared for an additional 2 weeks. After the 5-week treatment, adult rats were examined for conditioned fear, open field anxiety-like behavior, social interaction behavior and corticosterone responses to restraint stress. Isolates exhibited increased anxiety-like behaviors in a brightly-lit open field during the first 10 min of the test period compared to group-reared rats. Isolation-reared rats also showed increased fear behavior and reduced social contact in a social interaction test, and a transient increase in fear behavior to a conditioned stimulus that predicted foot-shock. Isolation-reared rats showed similar restraint-induced increases in plasma corticosterone as group-reared controls, but plasma corticosterone levels 2 h after restraint were significantly lower than pre-stress levels in isolates. Overall, this study shows that isolation restricted to an early part of development increases anxiety-like and fear behaviors in adulthood, and also results in depressed levels of plasma corticosterone following restraint stress.     

Possible involvement of corticosterone and serotonin in antidepressant and antianxiety effects of chromium picolinate in chronic unpredictable mild stress induced depression and anxiety in rats

In the present study, we investigated the effects of chromium picolinate (CrP) on behavioural and biochemical parameters in chronic unpredictable mild stress (CUMS) induced depression and anxiety in rats. The normal and stressed male Swiss albino rats were administered CrP (8 and 16 μg/mL in drinking water), they received stressors for seven days (each day one stressor) and this cycle was repeated three times for 21 days. On 22nd day, behaviour assessments followed by biochemical estimations were conducted. The results showed that treatment of CrP produced significant antidepressant effect, which has been evidenced by decrease in immobility time in modified forced swimming test (FST) in chronic unpredictable mild stress (CUMS) induced depression in rats. In elevated plus maze (EPM), CrP (16 μg/mL) showed significant reduction in time spent in open arm. CrP (8 μg/mL and 16 μg/mL) also showed significant decrease in number of entries in open arm that shows antianxiety effect of CrP in CUMS rats. It was also found that CrP (8 and 16 μg/mL) significantly increased 5-HT concentration in the discrete regions of brain (cortex and cerebellum). On the other hand, the plasma corticosterone level was significantly decreased with CrP (16 μg/mL). The results suggested that increase in the concentration of 5-HT and decrease in plasma corticosterone levels could be responsible for improvement in symptoms of depression and anxiety in CUMS induced depression and anxiety in rats.     

Interaction between isolation rearing and social development on exploratory behavior in male rats

The effect of isolation on exploratory behavior has been shown to differ depending on the developmental stages of male rats. However, there has been little systematic comparison of the frequencies and the patterns of exploratory behavior across the developmental stages. The present study assessed the frequencies of exploration using the emergence test and exploratory patterns in the open-field test in three developmental stages of male rats: juvenile, post-puberty, and adult. A lower propensity for exploration was observed in rats isolated during the juvenile stage, as assessed by increased latency and decreased duration of exploratory behaviors compared to pair-reared rats, and this tendency was maintained in adulthood. Altered patterns of exploratory behavior were demonstrated both in rats isolated in adulthood, who showed an increased active pattern, and those pair-reared following puberty, who shifted to a more passive pattern. However, rats isolated during the juvenile stage did not change their exploratory patterns following puberty. These results suggest that the changes in the exploratory pattern, which can be observed in adulthood, are associated with the emergence of adult-like dominance relationships. Juvenile-isolated rats did not show these changes following puberty, suggesting the importance of social interaction as juveniles for the ontogenetic emergence of behavioral flexibility implicated in the regulation of exploratory patterns.     

Effects of neonatal handling on social memory, social interaction, and number of oxytocin and vasopressin neurons in rats

Early-life environmental events can induce profound long-lasting changes in several behavioral and neuroendocrine systems. The neonatal handling procedure, which involves repeated brief maternal separations followed by experimental manipulations, reduces stress responses and sexual behavior in adult rats. The purpose of this study was to analyze the effects of neonatal handling on social behaviors of male and female rats in adulthood, as manifest by the results of social memory and social interaction tests. The number of oxytocin (OT) and vasopressin (VP) neurons in the paraventricular (PVN) and supraoptic (SON) nuclei of hypothalamus were also analyzed. The results did not demonstrate impairment of social memory. Notwithstanding, handling did reduce social investigative interaction and increase aggressive behavior in males, but did not do so in females. Furthermore, in both males and females, handling was linked with reduced number of OT-neurons in the parvocellular region of the PVN, while no differences were detected in the magnocellular PVN or the SON. On the other hand, handled males exhibited increased number of VP-neurons in the magnocellular zone of the PVN. We may conclude that the repeated brief maternal separations can reduce affiliative social behavior in adult male rats. Moreover, the disruption of the mother-infant relationship caused by the handling procedure induced long-lasting morphological changes in critical neuroendocrine areas that are involved in social bonding in mammals.     

Effects of prolactin deficiency during the early postnatal period on the development of maternal behavior in female rats: Mother's milk makes the difference

During early life, prolactin (PRL) ingested by the pups through the milk participates in the development of neuroendocrine, immunological and reproductive systems. The present study tested whether a deficiency in PRL in the dam's milk during early lactation affected the offspring in terms of the maternal responsiveness in the sensitization paradigm and behavioral response to a novel environment in the offspring. Thus, lactating rats were injected (sc) on postnatal days (PND) 2–5 with bromocriptine (125 μg/day), bromocriptine + ovine PRL (125 μg + 300 μg/day), or vehicle. As juveniles (at PND 24) or adults (PND 90–100), one female from each litter was exposed to 5 foster pups continuously for 8 days and their maternal responsiveness was recorded. Female offspring were also tested in an open field arena. Adult, but not juvenile, female offspring of bromocriptine-treated mothers showed an increased latency to become maternal, in comparison to latencies displayed by the offspring of control mothers. Furthermore, the proportion of adult, but not juvenile, offspring of bromocriptine-treated mothers that became maternal was lower than that showed by the offspring of vehicle-treated mothers. In comparison to female offspring of vehicle-treated mothers, female offspring of bromocriptine-treated mothers spent less time hovering over the pups (as juvenile females), body licking (as both juvenile and adult females), and in close proximity to pups (as adult females) during the maternal behavior test. Simultaneous administration of ovine PRL and bromocriptine reversed almost all the negative effects of bromocriptine. These data suggest that maternally-derived PRL participates during the early postnatal period in the development of neural systems that underlie the control of maternal behavior.     

Male rats with the testicular feminization mutation of the androgen receptor display elevated anxiety-related behavior and corticosterone response to mild stress

Testosterone influences the hypothalamic–pituitary–adrenal axis, anxiety-related behavior, and sensorimotor gating in rodents, but little is known about the role of the androgen receptor (AR) in mediating these influences. We compared levels of the stress hormone corticosterone at baseline and following exposure to a novel object in an open field in wild type (wt) male and female rats, and male rats with the testicular feminization mutation (Tfm) of the AR, which disables its function. Basal corticosterone was equivalent in all groups, but exposure to a novel object in an open field elicited a greater increase in corticosterone in Tfm males and wt females than in wt males. Tfm males also showed increased behavioral indices of anxiety compared to wt males and females in the test. Analysis of the immediate early gene c-Fos expression after exposure to a novel object revealed greater activation in Tfm males than wt males in some regions (medial preoptic area) and lesser activation in others (dentate gyrus, posterodorsal medial amygdala). No differences were found in a measure of sensorimotor gating (prepulse inhibition of the acoustic startle response), although Tfm males had an increased acoustic startle response compared to wt males and females. These findings demonstrate that ARs play a role in regulating anxiety-related behaviors, as well as corticosterone responses and neural activation following exposure to a mild stressor in rats.     

Effects of pair-housing after social defeat experience on elevated plus-maze behavior in rats

Social defeat experience in male rats causes an increase in anxiety-like behavior in the elevated plus-maze. Some researchers have suggested that housing rats socially following social defeat attenuates and/or prevents an increase in anxiety-like behavior. However, many other studies have shown that individual housing per se enhances anxiety-like behavior even in the absence of social defeat. In the present study, we assessed the relative contributions of the experience of social defeat and housing conditions on animals’ performance in the elevated plus-maze. Rats were assigned to one of the following four groups: defeat/individual housing, defeat/pair-housing, non-defeat/individual housing, and non-defeat/pair-housing. The elevated plus-maze test was conducted 2 weeks after the defeat experience. Our results demonstrated that the defeat/individual housing group spent less time than the other groups in the open arms: moreover, there were no differences between the other three groups. These results confirm the claim that the group-housing of rats prevents an increase in anxiety-like behavior caused by defeat.     

Chronic enhancement of brain oxytocin levels causes enduring anti-aggressive and pro-social explorative behavioral effects in male rats

Oxytocin (OXT) has been implicated in the regulation of social behaviors, including intermale offensive aggression. Recently, we showed that acute enhancement of brain OXT levels markedly suppressed offensive aggression and increased social exploration in resident rats confronted with an intruder in their home territory. Moreover, a different responsivity to the exogenous OXTergic manipulation was observed among individuals based on their baseline aggression. In this study we aimed at evaluating the behavioral response to chronically enhancing or attenuating central OXT levels, and at scrutinizing whether the trait-aggression moderates the treatment-induced behavioral changes. To this end, resident male wild-type Groningen rats were continuously (via osmotic minipumps) intracerebroventricularly infused with synthetic OXT or a selective OXT receptor (OXTR) antagonist for 7 days. Changes in behavior were assessed performing a resident–intruder test before and at the end of the treatment period, as well as after 7 days of withdrawal. Chronic infusion of OXT was found to selectively suppress aggression and enhance social exploration. Chronic blockage of OXTRs instead increased introductory aggressive behavior (i.e. lateral threat), yet without affecting the total duration of the aggression. The magnitude of the anti-aggressive changes correlated positively with the level of baseline aggression. Interestingly, OXT-induced behavioral changes persisted 7 days after cessation of the treatment. In conclusion, these findings provide further evidence that enhanced functional activity of the central OXTergic system decreases social offensive aggression while it increases social explorative behavior. The data also indicate that chronically enhancing brain OXT levels may cause enduring anti-aggressive and pro-social explorative behavioral effects.     

Differential long-term effects of social stress during adolescence on anxiety in Wistar and wild-type rats

Severe and chronic stress may interfere with adolescent neuronal plasticity that turns the juvenile brain into an adult brain increasing the vulnerability to develop anxiety disorders. It is well-known from adult stress research that there is a large individual differentiation in stress vulnerability. The current study is aimed at the individual resilience and vulnerability to adolescent social stress. Two strains of rats that differ in social behavioral skills were subjected to social stress during adolescence. In three experiments we studied short and long term effects of adolescent social stress using a water conflict test in different contexts. Wistar rats which had been socially defeated on postnatal days 45 and 46 showed, following water deprivation, a strong decrease in the total amount of water consumed and time spent drinking when tested 2 days and 3 weeks later in the context where they received the defeat experience. Also a strong increase in drinking latency was noticed in the context of the previous defeat. No differences in these parameters were found between defeated and non-defeated wild-type rats. The results of the water conflict test in an environment where no association with the previous defeat experience was present showed that the adolescent social stress did not induce a generalized anxiety.In conclusion, the water conflict test is a useful tool to measure the influence of social defeat on the motivation to obtain resources under conditions with different stimulus properties. In addition, our data suggest the importance of the strain used in adolescent stress experiments. The fact that Wistar rats showed a strong association with the context at adulthood whereas no effect was observed in the wild-type rats shows that victim characteristics are important determining factors for the long term effects of adolescent social stress.     

外源性皮质酮对雄性根田鼠交配行为的作用

本文研究了皮质酮对雄性根田鼠交配行为的作用。实验个体分别注射0.10 ug/ g体重、0. 60 ug / g体重和1.00 ug/ g体重剂量的皮质酮,1 h后测定每只雄性根田鼠的交配行为。结果显示,3 个处理组动物具有射精能力的个体比率及其爬跨、抽动和射精潜伏期与对照组动物相比均无显著差异,爬跨和抽动频次也无显著变化。不同处理组个体的血浆睾酮含量无显著差异。因此,皮质酮没有影响雄性根田鼠的交配行为和性激素的分泌。该结果提示,哺乳动物在急性应激条件下所分泌的皮质酮可能不参与对性行为的调控。     

Modulation of prolactin but not corticosterone responses to stress in relation to parental effort in a long-lived bird

We tested the hypothesis that parental effort modulates the magnitude of corticosterone and prolactin responses to stress in a long-lived bird, the Black-legged kittiwake (Rissa tridactyla). To do so, we compared corticosterone and prolactin responses to capture/restraint stress between chick-rearing birds and failed breeders (no parental effort). We predicted that (1) the increase in plasma corticosterone levels in response to stress should be lower in chick-rearing birds, (2) the decrease in plasma prolactin levels in response to stress should be lower in chick-rearing birds, and (3) as both sexes care for the chick, there should be no sex difference in the hormonal response to stress. Baseline plasma corticosterone and prolactin levels were higher in chick-rearing birds and were not influenced by body condition. Failed breeders were in better condition than chick-rearing individuals. Corticosterone response to stress was unaffected by parental effort as both chick-rearing and failed birds exhibited a robust corticosterone increase. Prolactin response to stress was however clearly influenced by parental effort: chick-rearing birds showed a modest 9% prolactin decrease whereas in failed birds prolactin concentrations fell by 41%. Body condition did not influence hormonal responses to stress. When facing stressful condition, breeding kittiwakes attenuate their prolactin response to stress while enhancing their secretion of corticosterone. Increasing corticosterone secretion triggers foraging efforts and diminishes nest attendance whereas an attenuation of prolactin response to stress maintains parental behavior. We suggest that this hormonal mechanism facilitates a flexible time-budget that has been interpreted as a buffer against environmental variability.