Suckling is a persistent stimulus to the adrenocortical system of the rat

Activation of the central but not the medial and cortical amygdala during anticipation for daily nursing in the rabbit

Rabbits have remarkable nursing behavior: after parturition, does visit daily their pups for nursing only once with circadian periodicity. Before the nursing events, they present increased activity and arousal, which shift according to the timing of scheduled nursing, either during the day or night. Brain areas related to maternal behavior and neuroendocrine cells for milk secretion are also entrained. The daily return of the doe for nursing at approximately the same hour suggests a motivational drive with circadian periodicity. Previously, we reported the activation of the mesolimbic system at the time of nursing, but not 12 h before that. Aiming at a better understanding of the mechanism of this anticipatory behavior, we explored the participation of the limbic regions of the amygdala and the bed nucleus of the stria terminalis, as well as the possible activation of the hypothalamic-pituitaryadrenal axis, specifically the corticotropin-releasing factor cells in the hypothalamic paraventricular nucleus of does at different times before and after nursing. The medial and cortical amygdala, the bed nucleus of the stria terminalis, and corticotropin cells showed activation only after nursing. However, the central amygdala was also activated before nursing. We conclude that the medial and the cortical amygdala form part of the afferent olfactory pathway for entrainment, and the central amygdala participates in the anticipatory motivational circuit of the control of periodic nursing. The lack of activation of corticotropin cells before nursing is consistent with the possible harmful effects of the doe's high glucocorticoid levels on the developing pups.

Perceived predation risk predicts glucocorticoid hormones, but not reproductive success in a colonial rodent

The Cort-Adaptation hypothesis suggests that elevated glucocorticoids (GCs) can facilitate an adaptive response to environmental and physiological challenges. Most previous studies have focused on avian species, which may limit their generalizability to mammals, where lactation is known to be a major physiological challenge. Furthermore, the effect of predation risk on GC levels has not been tested in the Cort-Adaptation hypothesis. We sought to test this hypothesis in a colonial prey species, black-tailed prairie dogs (Cynomys ludovicianus). We predicted that individuals located near fewer neighboring conspecifics would perceive an increased risk of predation and, in turn, have increased GCs (measured through hair cortisol concentration (HCC)) and reduced annual reproductive success compared to more centrally located individuals. We also investigated other putative influences on HCC: age, lactation status, body condition, and season of hair growth. Levels of vigilance behavior were higher for those with fewer neighboring conspecifics, suggesting variation in perceived risk of predation. Further, the risk of predation appeared to represent a chronic, detrimental stressor as evidenced by a significant increase in HCC for prairie dogs with fewer neighbors. Lactation status and season also influenced HCC. We found support for the Cort-Adaptation hypothesis where increased HCC during the reproductive season correlated with whether a female produced a litter, but not litter size, suggesting a minimum threshold of GCs is required for successful reproduction in this species. Our work illustrates that HCC may operate as an indicator of perceived predation risk, but care should be taken to consider the variety of factors influencing GC homeostasis, in particular lactation, when drawing conclusions using HCC as a marker of long-term stress.

Lactation is not required for maintaining maternal care and active coping responses in chronically stressed postpartum rats: Interactions between nursing demand and chronic variable stress

Women who do not breastfeed or discontinue breastfeeding early are more likely to develop postpartum depression (PPD) and stress is a significant risk factor for depression, including PPD. Using a rat model, we investigated whether the absence of nursing would increase the susceptibility to chronic stress-related behavioral and neural changes during the postpartum period. Adult female rats underwent thelectomy (thel; removal of teats), sham surgery, or no surgery (control) and were paired with males for breeding. All litters were rotated twice daily until postpartum day (PD) 26. Sham rats served as surrogates for thel litters, yielding a higher nursing demand for sham rats. Concurrently, rats received either no stress or chronic variable stress until PD 25. Rats were observed for maternal behaviors and tested in a series of tasks including open field, sucrose preference, and forced swim. We used immunohistochemistry (IHC) for doublecortin (DCX; to label immature neurons) or for mineralocorticoid receptor (MR). Contrary to our expectations, non-nursing thel rats were resistant to the effects of stress in all dependent measures. Our data indicate that even in chronic adverse conditions, nursing is not required for maintaining stable care to offspring or active coping responses in an acutely stressful task. We discuss the possible role of offspring contact and consider future directions for biomedical and clinical research. In rats with high nursing demand, however, chronic stress increased immobility, hippocampal neurogenesis, and MR expression (largely in opposition to the effects of stress in rats with typical nursing demand). We discuss these patterns in the context of energetics and allostatic load. This research highlights the complexity in relationships between stress, nursing, and neurobehavioral outcomes in the postpartum period and underscores the need for additional biomedical and clinical research geared toward optimizing treatments and interventions for women with PPD, regardless of breastfeeding status. SIGNIFICANCE STATEMENT: The goal of this research was to determine how the absence of nursing and higher nursing demand impact stress-coping behaviors and neural changes associated with chronic stress in order to disentangle the complex interplay of factors that contribute to psychological illness during the postpartum period.

Glucocorticoids and activity in free-living arctic ground squirrels: Interrelationships between weather, body condition, and reproduction

The dynamic relationship between glucocorticoids and behavior are not well understood in wild mammals. We investigated how weather, body condition, and reproduction interact to affect cortisol levels and activity patterns in a free-living population of arctic ground squirrels (Urocitellus parryii). As a proxy for foraging and escape behaviors, collar-mounted accelerometers and light loggers were used to measure above-ground activity levels and the amount of time squirrels spent below the surface, respectively. Fecal cortisol metabolites (FCMs) were quantified to assess glucocorticoid secretion in squirrels. Male and female squirrels differed in above-ground activity levels and time spent below-ground across the active season, with males being most active during mating and females most active during lactation. We also found that female, but not male, squirrels exhibited seasonal variation in FCM levels, with concentrations highest during mid-lactation and lowest after the lactation period. In female squirrels, the seasonal relationships between breeding stage, activity, and FCM levels were also consistent with changes in maternal investment and the preparative role that glucocorticoids are hypothesized to play in energy mobilization. Body condition was not associated with FCM levels in squirrels. As predicted, deteriorating weather also influenced FCM levels and activity patterns in squirrels. FCM concentrations were affected by an interaction between temperature and wind speed when seasonal temperatures were lowest. In addition, above-ground activity, but not time spent below-ground, positively correlated with FCM levels. These results suggest that, although ground squirrels avoid inclement weather by remaining below-ground, activation of the stress axis may stimulate foraging activity.

Prenatal LPS induces sickness behaviour and decreases maternal and predatory behaviours after an LPS challenge

Purpose: The influence of a challenge dose of lipopolysaccharide (LPS) on the behavioural selection between maternal (MB) and predatory behaviours (PB) of female rats prenatally treated with the same endotoxin or saline solution (F1 generation) were studied.Material and methods: Thus, in adult age, these female rats were mated and, at lactation days 5 or 6, the following groups were formed: (1) LPS + LPS group-female rats prenatally treated with LPS and received an LPS challenge dose; (2) S + LPS group-female rats prenatally treated with saline solution and received a challenge LPS dose (3) S + S group-females rats prenatally treated with saline which received a saline injection. MB, PB to cockroaches, exploratory behaviour, periaqueductal grey (PAG) expression of the astrocytic biomarker glial fibrillary acidic protein (GFAP), and corticosterone and TNF-alpha serum levels were evaluated.Results: Showed that: (1) relative to the S + S group, the LPS + S group showed decreased MB and slightly increased PB, without inducing sickness behaviour; (2) the LPS + LPS group showed decreased MB but few effects on PB; (3) there was increased sickness behaviour associated with increased TNF-alpha serum levels in the LPS + LPS group; (4) a significant increase in GFAP expression was observed in both LPS groups, which was greater in the LPS + LPS group and (5) no differences in the corticosterone of all groups.Conclusions: Prenatal LPS impaired the switch from MB to PB in female rats of the LPS + LPS group by increased sickness behaviour as well as an increase in plasmatic TNF-alpha levels inducing PAG astrogliosis.

Effects of single parenthood on mothers' behavior, morphology, and endocrine function in the biparental California mouse

Motherhood is energetically costly for mammals and is associated with pronounced changes in mothers' physiology, morphology and behavior. In ~5% of mammals, fathers assist their mates with rearing offspring and can enhance offspring survival and development. Although these beneficial consequences of paternal care can be mediated by direct effects on offspring, they might also be mediated indirectly, through beneficial effects on mothers. We tested the hypothesis that fathers in the monogamous, biparental California mouse (Peromyscus californicus) reduce the burden of parental care on their mates, and therefore, that females rearing offspring with and without assistance from their mates will show differences in endocrinology, morphology and behavior, as well as in the survival and development of their pups. We found that pups' survival and development in the lab did not differ between those raised by a single mother and those reared by both mother and father. Single mothers spent more time in feeding behaviors than paired mothers. Both single and paired mothers had higher lean mass and/or lower fat mass and showed more anxiety-like behavior in open-field tests and tail-suspension tests, compared to non-breeding females. Single mothers had higher body-mass-corrected liver and heart masses, but lower ovarian and uterine masses, than paired mothers and/or non-breeding females. Mass of the gastrointestinal tract did not differ between single and paired mothers, but single mothers had heavier gastrointestinal tract compared to non-breeding females. Single motherhood also induced a flattened diel corticosterone rhythm and a blunted corticosterone response to stress, compared to non-breeding conditions. These findings suggest that the absence of a mate induces morphological and endocrine changes in mothers, which might result from increased energetic demands of pup care and could potentially help maintain normal survival and development of pups.

Brain-Derived Neurotrophic Factor in the Nucleus Accumbens Mediates Individual Differences in Behavioral Responses to a Natural, Social Reward

BDNF-oxytocin interactions in the brain are implicated in mammalian maternal behavior. We found that BDNF gene expression is increased in the hippocampus of rat mothers that show increased pup licking/grooming (high LG mothers) compared to low LG mothers. High LG mothers also showed increased BDNF protein levels in the nucleus accumbens (nAcc). Immunoneutralization of BDNF in the nAcc eliminated the differences in pup LG between high and low LG mothers. Oxytocin antagonist in the ventral hippocampus significantly decreased the frequency of maternal LG behavior. Oxytocin antagonist significantly prevented the oxytocin-induced BDNF gene expression in primary hippocampal cell cultures. We suggest that oxytocin-induced regulation of BDNF in the nAcc provides a neuroendocrine basis for both individual differences in maternal behavior and resilience to the stress of reproduction in female mammals.

Cell signaling dependence of rapid glucocorticoid-induced endocannabinoid synthesis in hypothalamic neuroendocrine cells

Glucocorticoids induce a rapid synthesis of endocannabinoid in hypothalamic neuroendocrine cells by activation of a putative membrane receptor. Somato-dendritically released endocannabinoid acts as a retrograde messenger to suppress excitatory synaptic inputs to corticotropin-releasing hormone-, oxytocin-, and vasopressin-secreting cells. The non-genomic signaling mechanism responsible for rapid endocannabinoid synthesis by glucocorticoids has yet to be fully characterized. Here we manipulated cell signaling molecules pharmacologically using an intracellular approach to elucidate the signaling pathway activated by the membrane glucocorticoid receptor in hypothalamic neuroendocrine cells. We found that rapid glucocorticoid-induced endocannabinoid synthesis in magnocellular neuroendocrine cells requires the sequential activation of multiple kinases, phospholipase C, and intracellular calcium mobilization. While there remain gaps in our understanding, our findings reveal many of the critical players in the rapid glucocorticoid signaling that culminates in the retrograde endocannabinoid modulation of excitatory synaptic transmission.

The epigenetics of the hypothalamic-pituitary-adrenal axis in fetal development

The hypothalamic-pituitary-adrenal (HPA) axis is an important hormonal mechanism of the human body and is extremely programmable during embryonic and fetal development. Analyzing its development in this period is the key to understanding in fact how vulnerabilities of congenital diseases occur and any other changes in the phenotypic and histophysiological aspects of the fetus. The environment in which the mother is exposed during the gestational period can influence this axis. Knowing this, our objective was to analyze in recent research the possible impact of epigenetic programming on the HPA axis and its consequences for fetal development. This review brought together articles from two databases: ScienceDirect and PUBMED researched based on key words such as "epigenetics, HPA axis, cardiovascular disease, and circulatory problems" where it demonstrated full relevance in experimental and scientific settings. A total of 101 articles were selected following the criteria established by the researchers. Thus, it was possible to verify that the development of the HPA axis is directly related to changes that occur in the cardiovascular system, to the cerebral growth and other systems depending on the influence that it receives in the period of fetal formation.

Neuroendocrinology and Adaptive Physiology of Maternal Care

Parental care is critical for offspring survival in many species. In mammals, parental care is primarily provided through maternal care, due to obligate pregnancy and lactation constraints, although some species also show paternal and alloparental care. These behaviors are driven by specialized neural circuits that receive sensory, cortical, and hormonal input to generate a coordinated and timely change in behavior, and sustain that behavior through activation of reward pathways. Importantly, the hormonal changes associated with pregnancy and lactation also act to coordinate a broad range of physiological changes to support the mother and enable her to adapt to the demands of these states. This chapter will review the neural pathways that regulate maternal behavior, the hormonal changes that occur during pregnancy and lactation, and how these two facets merge together to promote both young-directed maternal responses (including nursing and grooming) and young-related responses (including maternal aggression and other physiological adaptions to support the development of and caring for young). We conclude by examining how experimental animal work has translated into knowledge of human parenting, particularly in regards to maternal mental health issues.

Adaptive Modifications of Maternal Hypothalamic-Pituitary-Adrenal Axis Activity during Lactation and Salsolinol as a New Player in this Phenomenon

Both basal and stress-induced secretory activities of the hypothalamic-pituitary-adrenal (HPA) axis are distinctly modified in lactating females. On the one hand, it aims to meet the physiological demands of the mother, and on the other hand, the appropriate and stable plasma cortisol level is one of the essential factors for the proper offspring development. Specific adaptations of HPA axis activity to lactation have been extensively studied in several animal species and humans, providing interesting data on the HPA axis plasticity mechanism. However, most of the data related to this phenomenon are derived from studies in rats. The purpose of this review is to highlight these adaptations, with a particular emphasis on stress reaction and differences that occur between species. Existing data on breastfeeding women are also included in several aspects. Finally, data from the experiments in sheep are presented, indicating a new regulatory factor of the HPA axis-salsolinol-which typical role was revealed in lactation. It is suggested that this dopamine derivative is involved in both maintaining basal and suppressing stress-induced HPA axis activities in lactating dams.

Combined effect of gestational stress and postpartum stress on maternal care in rats

Variations in maternal care in the rat influence the development of individual differences in behavioral and endocrine responses to stress. This study aimed to examine the interaction between intragastric intubation during late gestation and postpartum stress, induced by pup separation, on maternal behavior and on dams' emotional state and HPA axis function. Rats received intragastric intubation of water on days 12-20 of gestation or remained untreated in their home cage (naïve dams). Pup separation was used as a model of postpartum stress. The procedure consisted of a daily separation of the dam from its litter for 3h from PND 3 until PND 15. Pup separation was carried out in both naïve and intubated dams. The behavioral results indicate that the association of these two stressors significantly decreased arched-back nursing (ABN) and licking and grooming (LG), behaviors considered important parameters to discriminate the high quality of maternal care. Moreover, dams that received both stressors displayed less nest building and blanket nursing behaviors; no effect on the frequency of passive and total nursing was recorded. The analysis of single effects on ABN and LG, revealed that dams that underwent gestational stress induced by intragastric intubation displayed less LG, but ABN was overall unchanged. On the contrary, pup separation stress significantly increased ABN and LG upon reunion of naïve dams with their pups. Treatments per se or the association of both induced modest changes in plasma levels of allopregnanolone and corticosterone that likely did not influence maternal care. These data show that the association of a mild stress during gestation with an unfavorable experience after parturition had a significant impact on maternal care. This effect seems independent from HPA axis activation or from changes in emotional state; further studies would be necessary to ascertain the neural changes that could contribute to altered maternal behavior in stressed mothers. Moreover, these results suggest that the use of intragastric intubation during gestation would interfere with measures of drug-induced changes in maternal behavior and likely their consequences on the offspring.

The role of prolactin in the suppression of Crh mRNA expression during pregnancy and lactation in the mouse

Maternal stress is associated with negative health consequences for both the mother and her offspring. To prevent these adverse outcomes, activity of the hypothalamic-pituitary-adrenal (HPA) axis is attenuated during pregnancy and lactation. Although the mechanisms generating this adaptive change have not been defined fully, the anterior pituitary hormone prolactin may play a significant role. The present study investigated the role of prolactin in regulating the basal activity of the HPA axis during pregnancy and lactation in the mouse, focussing upon the corticotrophin-releasing hormone (CRH) neurones. Using in situ hybridisation, a decrease in Crh mRNA-expressing cell number in pregnant (55.6±9.0 cells per section) and lactating (97.4±4.9) mice compared to virgin controls was characterised (186.8±18.7, P<.01 Tukey-Kramer test; n=6-7 per group). Removal of the pups (24 hours) and thus the associated suckling-induced prolactin secretion, restored CRH neurone number (180.1±19.7). To specifically test the role of prolactin in suppressing Crh mRNA expression in lactation, prolactin levels were selectively manipulated in lactating mice. Lactating mice were treated with ovine prolactin (1500 μg day^-1 , osmotic minipump, s.c.; n=7) or vehicle (n=6) for 24 hours following pup removal. This was sufficient to suppress Crh mRNA expression from 108.0±13.5 to 53.7±16.7 cells per section (P<.05 Student's t-test). Additional cohorts of lactating mice were treated with bromocriptine (300 μg over 24 hours, s.c.; n=7) or vehicle (n=5) to suppress endogenous prolactin secretion; however, no change in Crh mRNA expression was detected. Thus, although prolactin was sufficient to suppress Crh mRNA expression in the paraventricular nucleus, it does not appear to be required for the ongoing regulation of the CRH neurones in lactation.

Neuroendocrine Responses to Stressors in Lactating and Nonlactating Mammals: A Literature Review

Although stress research is a popular topic of study, little is known about the neuroendocrine responses to a stressor in lactating and nonlactating humans. The purpose of this systematic review of the literature was to examine the neuroendocrine responses, specifically the glucocorticoids and catecholamines, in lactating and nonlactating animals and humans to an acute stressor. A brief overview of the physiological stress response in the human is included. Animal studies strongly suggest that lactation is associated with major changes in neuroendocrine responses to a variety of acute stressors. Neuroendocrine responses in humans to stressors are less clear due to the limited research. Future research is needed involving these responses in humans generally, as well as specifically in the patterns of neuroendocrine responses to chronic stressors in lactating and nonlactating women.

Mood, Food, and Fertility: Adaptations of the Maternal Brain

Successfully rearing young places multiple demands on the mammalian female. These are met by a wide array of alterations in maternal physiology and behavior that are coordinated with the needs of the developing young, and include adaptations in neuroendocrine systems not directly involved in maternal behavior or lactation. In this article, attenuations in the behavioral and neuroendocrine responses to stressors, the alterations in metabolic pathways facilitating both increased food intake and conservation of energy, and the changes in fertility that occur postpartum are described. The mechanisms underlying these processes as well as the factors that contribute to them and the relative contributions of these stimuli at different times postpartum are also reviewed. The induction and maintenance of the adaptations observed in the postpartum maternal brain are dependent on mother-young interaction and, in most cases, on suckling stimulation and its consequences for the hormonal profile of the mother. The peptide hormone prolactin acting on receptors within the brain makes a major contribution to changes in metabolic pathways, suppression of fertility and the attenuation of the neuroendocrine response to stress during lactation. Oxytocin is also released, both into the circulation and in some hypothalamic nuclei, in response to suckling stimulation and this hormone has been implicated in the decrease in anxiety behavior seen in the early postpartum period. The relative importance of these hormones changes across lactation and it is becoming increasingly clear that many of the adaptations to motherhood reviewed here reflect the outcome of multiple influences. © 2016 American Physiological Society. Compr Physiol 6:1493-1518, 2016.

The birth of new neurons in the maternal brain: Hormonal regulation and functional implications

The maternal brain is remarkably plastic and exhibits multifaceted neural modifications. Neurogenesis has emerged as one of the mechanisms by which the maternal brain exhibits plasticity. This review highlights what is currently known about peripartum-associated changes in adult neurogenesis and the underlying hormonal mechanisms. We also consider the functional consequences of neurogenesis in the peripartum brain and extent to which this process may play a role in maternal care, cognitive function and postpartum mood. Finally, while most work investigating the effects of parenting on adult neurogenesis has focused on mothers, a few studies have examined fathers and these results are also discussed.

High-fat diet prevents adaptive peripartum-associated adrenal gland plasticity and anxiolysis

Maternal obesity is associated with lower basal plasma cortisol levels and increased risk of postpartum psychiatric disorders. Given that both obesity and the peripartum period are characterized by an imbalance between adrenocorticotropic hormone (ACTH) and cortisol, we hypothesized that the adrenal glands undergo peripartum-associated plasticity and that such changes would be prevented by a high-fat diet (HFD). Here, we demonstrate substantial peripartum adrenal gland plasticity in the pathways involved in cholesterol supply for steroidogenesis in female rats. In detail, the receptors involved in plasma lipid uptake, low density lipoprotein (LDL) receptor (LDLR) and scavenger receptor class B type 1 (SRB1), are elevated, intra-adrenal cholesterol stores are depleted, and a key enzyme in de novo cholesterol synthesis, hydroxymethylglutaryl coenzyme A reductase (HMGCR), is downregulated; particularly at mid-lactation. HFD prevented the lactation-associated anxiolysis, basal hypercorticism, and exaggerated the corticosterone response to ACTH. Moreover, we show that HFD prevented the downregulation of adrenal cholesterol stores and HMGCR expression, and LDLR upregulation at mid-lactation. These findings show that the adrenal gland is an important regulator of peripartum-associated HPA axis plasticity and that HFD has maladaptive consequences for the mother, partly by preventing these neuroendocrine and also behavioural changes.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

Relationship between cortisol and acute phase protein concentrations in female rabbits

Rabbit meat production in Europe is usually based on a semi-intensive system, in which lactation and gestation overlap. The demands of lactation and pregnancy are likely to be relatively stressful for female rabbits and may compromise the immune system and reproductive performance. The present study was designed to characterise circulating levels of cortisol, haptoglobin (Hp), C-reactive protein (CRP), and serum amyloid A (SAA) in non-lactating and lactating female rabbits at first and second mating, and to determine whether any relationship exists between these biomarkers and litter size. Serum cortisol concentrations were at their greatest (mean ± SEM = 39.5 ± 3.9 nmol/L) in animals at the end of lactation. However, after weaning, cortisol concentrations were not significantly different compared to nulliparous females (19.9 ± 3.6 vs. 16.3 ± 2.2 nmol/L, respectively). The highest concentrations of circulating Hp (0.14 ± 0.01 g/L) were seen in early lactating primiparous females, and lower in nulliparous females and in rabbits after weaning. In contrast, nulliparous female rabbits showed the highest plasma CRP values (13.1 ± 1.1 mg/L). No significant differences were found for SAA. Nulliparous females had smaller litter sizes than early lactating and non-lactating primiparous female rabbits. CRP and SAA showed a positive correlation (r = +0.24, P = 0.011) and were negatively related to litter size (r = -0.23, P = 0.017 and P = 0.032, respectively). Cortisol and Hp were not related to CRP, SAA, nor to litter size. These results suggest a closer association between the mechanisms that regulate release of CRP and SAA, compared to those that regulate Hp production. Thus, lactation is associated with changes in several stress biomarkers. CRP and SAA might be more useful for evaluating animal welfare and for predicting subsequent reproductive performance of female rabbits.

Tissue-specific changes in molecular clocks during the transition from pregnancy to lactation in mice

Circadian clocks regulate homeostasis and mediate responses to stressors. Lactation is one of the most energetically demanding periods of an adult female's life. Peripartum changes occur in almost every organ so the dam can support neonatal growth through milk production while homeostasis is maintained. How circadian clocks are involved in adaptation to lactation is currently unknown. The abundance and temporal pattern of core clock genes' expression were measured in suprachiasmatic nucleus, liver, and mammary from late pregnant and early lactation mice. Tissue-specific changes in molecular clocks occurred between physiological states. Amplitude and robustness of rhythms increased in suprachiasmatic nucleus and liver. Mammary rhythms of core molecular clock genes were suppressed. Attenuated rhythms appeared to be a physiological adaptation of mammary to lactation, because manipulation of timing of suckling resulting in significant differences in plasma prolactin and corticosterone had no effect on amplitude. Analysis of core clock proteins revealed that the stoichiometric relationship between positive (CLOCK) and negative (PER2) components remained 1:1 in liver but was increased to 4:1 in mammary during physiological transition. Induction of differentiation of mammary epithelial cell line HC11 with dexamethasone, insulin, and prolactin resulted in similar stoichiometric changes among positive and negative clock regulators, and prolactin induced phase shifts in HC11 Arntl expression rhythm. Data support that distinct mechanisms drive periparturient changes in mammary clock. Stoichiometric change in clock regulators occurs with gland differentiation. Suppression of mammary clock gene expression rhythms represents a physiological adaptation to suckling cues. Adaptations in mammary clock are likely needed in part to support suckling demands of neonates.

Hormonal influences on neuroimmune responses in the CNS of females

Particular reproductive stages such as lactation impose demands on the female. To cope with these demands, her physiology goes through numerous adaptations, for example, attenuation of immune and stress responses. Hormonal fluctuation during lactation exerts a strong influence, inducing neuroplasticity in the hypothalamus and extrahypothalamic regions, and diminishing the stress and inflammatory responses. Thus, hormones confer decreased vulnerability to the female brain. This mini-review focuses on the adaptations of the immune and stress response during maternity, and on the neuroprotective actions of progesterone and prolactin and their effects on inflammation. The importance of pregnancy and lactation as experimental models to study immune responses and disease is also highlighted.

The effect of maternal stress activation on the offspring during lactation in light of vasopressin

Although it is obvious that preconceptional effects as well as stressors during pregnancy profoundly influence the progeny, the lactation period seems to be at least as important. Here we summarize how maternal stressors during the lactation period affect the offspring. As vasopressin is one of the crucial components both for stress adaptation and social behavior, special emphasis was given to this neuropeptide. We can conclude that stressing the mother does not have the same acute effect on the hypothalamo-pituitary-adrenocortical axis (as the main target of stress adaptation) of the pups as stressing the pups, but later endocrine and behavioral consequences can be similar. Vasopressin plays a role in acute and later consequences of perinatal stressor applied either to the mother or to the offspring, thereby contributing to transmitting the mothers' stress to the progeny. This mother-infant interaction does not necessarily mean a direct transmission of molecules, but rather is the result of programming the brain development through changes in maternal behavior. Thus, there is a time lag between maternal stress and stress-related changes in the offspring. The interactions are bidirectional as not only stress in the dam but also stress in the progeny has an effect on nursing.

Adaptive changes in basal and stress-induced HPA activity in lactating and post-lactating female rats

Lactation represents a period of marked adaptation of the hypothalamo-pituitary-adrenal HPA axis. We characterized basal and stress-induced HPA activity during lactation and experimental weaning using dynamic blood sampling in rats. Pulsatile and diurnal corticosterone release occurred at all reproductive stages studied (virgin; day 10 of lactation; 3 and 14 days after experimental weaning on day 10 of lactation). However, in lactating rats the diurnal peak was significantly reduced, resulting in a flattened rhythm, and three days after weaning, basal HPA activity was markedly suppressed: the number of pulses and underlying basal levels of corticosterone were reduced and the diurnal rise phase delayed. Marked changes in the HPA response to 10 min noise stress also occurred at these times: being completely absent in lactating animals, but restored and highly prolonged in early weaned animals. Injection of methylprednisolone (2 mg, iv) was used to determine whether changes in fast glucocorticoid suppression correlated with these adaptive changes. Methylprednisolone induced a rapid suppression of corticosterone in virgin animals, but this effect was markedly attenuated in lactating and early weaned animals and was accompanied by significant changes in relative expression of hippocampal glucocorticoid and mineralocorticoid receptor mRNA. All effects were reversed or partially reversed 14 days after experimental weaning. Thus, the presence of the pups has an important influence on regulation of the HPA axis, and while postpartum adaptations are reversible, acute weaning evokes marked reorganisation of basal and stress-induced HPA activity.

Many mouths to feed: the control of food intake during lactation

Providing nutrients to their developing young is perhaps the most energetically demanding task facing female mammals. In this paper we focus primarily on studies carried out in rats to describe the changes in the maternal brain that enable the dam to meet the energetic demands of her offspring. In rats, providing milk for their litter is associated with a dramatic increase in caloric intake, a reduction in energy expenditure and changes in the pattern of energy utilization as well as storage. These behavioral and physiological adaptations result, in part, from alterations in the central pathways controlling energy balance. Differences in circulating levels of metabolic hormones such as leptin, ghrelin and insulin as well as in responsiveness to these signals between lactating and nonlactating animals, contribute to the modifications in energy balance pathways seen postpartum. Suckling stimulation from the pups both directly, and through the hormonal state that it induces in the mother, plays a key role in facilitating these adaptations.

Neuroendocrinology of parental response to baby-cry

This overview attempts to synthesise current understandings of the neuroendocrine basis of parenting. The parent-infant bond is central to the human condition, contributes to risks for mood and anxiety disorders, and provides the potential for resiliency and protection against the development of psychopathology. Animal models of parenting provide compelling evidence that biological mechanisms may be studied in humans. This has led to brain imaging and endocrine system studies of human parents using baby stimuli and concerted psychological and behavioural measures. Certain brain circuits and related hormonal systems, including subcortical regions for motivation (striatum, amygdala, hypothalamus and hippocampus) and cortical regions for social cognition (anterior cingulate, insula, medial frontal and orbitofrontal cortices), appear to be involved. These brain circuits work with a range of endocrine systems to manage stress and motivate appropriate parental caring behaviour with a flexibility appropriate to the environment. Work in this field promises to link evolving models of parental brain performance with resilience, risk and treatment toward mother-infant mental health.

Effects of reproductive status on behavioral and endocrine responses to acute stress in a biparental rodent, the California mouse (Peromyscus californicus)

In several mammalian species, lactating females show blunted neural, hormonal, and behavioral responses to stressors. It is not known whether new fathers also show stress hyporesponsiveness in species in which males provide infant care. To test this possibility, we determined the effects of male and female reproductive status on stress responsiveness in the biparental, monogamous California mouse (Peromyscus californicus). Breeding (N=8 females, 8 males), nonbreeding (N=10 females, 10 males) and virgin mice (N=12 females, 9 males) were exposed to a 5-min predator-urine stressor at two time points, corresponding to the early postpartum (5-7 days postpartum) and mid/late postpartum (19-21 days postpartum) phases, and blood samples were collected immediately afterwards. Baseline blood samples were obtained 2 days prior to each stress test. Baseline plasma corticosterone (CORT) concentrations did not differ among male or female groups. CORT responses to the stressor did not differ among female reproductive groups, and all three groups showed distinct behavioral responses to predator urine. Virgin males tended to increase their CORT response from the first to the second stress test, while breeding and nonbreeding males did not. Moreover, virgin and nonbreeding males showed significant behavioral changes in response to predator urine, whereas breeding males did not. These results suggest that adrenocortical responses to a repeated stressor in male California mice may be modulated by cohabitation with a female, whereas behavioral responses to stress may be blunted by parental status.

Prolactin prevents chronic stress-induced decrease of adult hippocampal neurogenesis and promotes neuronal fate

Chronic exposure to stress results in a reduction of hippocampal neurogenesis and of hippocampal volume. We examined whether prolactin (PRL), a regulator of the stress response and stimulator of neurogenesis in the subventricular zone, influences neurogenesis in the hippocampal dentate gyrus (DG) of chronically stressed adult C57BL/6 male mice. Chronically stressed (4 h daily immobilization for 21 d) or nonstressed mice were treated with either ovine PRL or vehicle between days 1-14. BrdU was injected daily between days 1-7 to evaluate cell survival and fate, or twice on day 21 to evaluate cell proliferation. Hippocampal cell proliferation was unchanged by either stress exposure or PRL at the end of the treatments. In contrast, the number of cells in the DG that incorporated BrdU during the first phase of the experiment and survived to the end of the experiment was decreased in vehicle-treated stressed mice compared with PRL- or vehicle-treated nonstressed control mice. Stressed animals receiving PRL had significantly more BrdU-labeled cells than vehicle-treated stressed mice at this time point. Cell fate analysis revealed a higher percentage of neurons in PRL- compared with vehicle-treated stressed mice. The results demonstrate that PRL protects neurogenesis in the DG of chronically stressed mice and promotes neuronal fate.

Dopamine-regulated adrenocorticotropic hormone secretion in lactating rats: functional plasticity of melanotropes

Pro-opiomelanocortin (POMC) is processed to adrenocorticotropic hormone (ACTH) and beta-lipotropin in corticotropes of the anterior lobe, and to alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin in melanotropes of the intermediate lobe (IL) of the pituitary gland. While ACTH secretion is predominantly under the stimulatory influence of the hypothalamic factors, hormone secretion of the IL is tonically inhibited by neuroendocrine dopamine (NEDA) neurons. Lobe-specific POMC processing is not absolute. For example, D(2) type DA receptor (D2R)-deficient mice have elevated plasma ACTH levels, although it is known that corticotropes do not express D2R(s). Moreover, observations that suckling does not influence alpha-MSH release, while it induces an increase in plasma ACTH is unexplained. The aim of the present study was to investigate the involvement of the NEDA system in the regulation of ACTH secretion and the participation of the IL in ACTH production in lactating rats. Untreated and estradiol (E(2))-substituted ovariectomized (OVX) females were also studied. The concentration of ACTH in the IL was higher in lactating rats than in OVX rats, while the opposite change in alpha-MSH level of the IL was observed. DA levels in the IL and the neural lobe were lower in lactating rats than in OVX rats. Suckling-induced ACTH response was eliminated by pretreatment with the DA receptor agonist, bromocriptine (BRC). Inhibition of DA biosynthesis by alpha-methyl-p-tyrosine (alphaMpT) and blockade of D2R by domperidone (DOM) elevated plasma ACTH levels, but did not influence plasma alpha-MSH levels in lactating rats. The same drugs had opposite effects in OVX and OVX + E(2) animals. In lactating mothers, BRC was able to block ACTH responses induced by both alphaMpT and DOM. Surgical denervation of the IL elevated basal plasma levels of ACTH. Taken together, these data indicate that melanotropes synthesize ACTH during lactation and its release from these cells is regulated by NEDA neurons.

Characterization of maternal motivation in the lactating rat: Contrasts between early and late postpartum responses

We previously assessed the motivational properties of pups relative to those of cocaine in parturient female rats (dams) across the postpartum period and demonstrated that the larger subset of dams in early postpartum (PPD8) preferred the pup-associated chamber, whereas the majority of dams tested in late postpartum (PPD16) preferred the cocaine-associated chamber [Mattson, B.J., Williams, S., Rosenblatt, J.S., Morrell, J.I. 2001. Comparison of two positive reinforcing stimuli: pups and cocaine throughout the postpartum period. Behav. Neurosci., 115, 683-694; Seip, K.M., Morrell, J.I. 2007. Increasing the incentive salience of cocaine challenges preference for pup- over cocaine-associated stimuli during early postpartum: place preference and locomotor analyses in the lactating female rat. Psychopharmacology 194, 309-319]. The present study uses a dual-choice conditioned place preference to ask how the progression of the postpartum period, including natural pup development, influences maternal motivation for pups. Preferences for cued chambers associated with pups that were age-matched to the postpartum stage of the dam in contrast to a stimulus with little incentive salience were higher during the early than the late postpartum, suggesting that the incentive salience of pups diminishes as the postpartum period progresses. Preferences of the early postpartum dams deprived of pups for 15 min, 2, 6, 12 or 22 hrs prior to conditioning and testing did not differ statistically but there was a trend of more pup preference after 22 hr deprivation; pup age was not an important factor in early postpartum. In marked contrast, late postpartum dams only exhibited robust pup-associated place preference when they were conditioned with young (4-7 day-old) pups or after a 22 hr period of deprivation from contemporaneous pups. Together these results suggest that both forces are at work in the mother-pup dyad, changes in the pups as they develop and changes in the physiological and endocrine state of the female as she progresses through the postpartum period.

Adaptive responses of the maternal hypothalamic-pituitary-adrenal axis during pregnancy and lactation

Over the past 40 years, it has been recognised that the maternal hypothalamic-pituitary-adrenal (HPA) axis undergoes adaptations through pregnancy and lactation that might contribute to avoidance of adverse effects of stress on the mother and offspring. The extent of the global adaptations in the HPA axis has been revealed and the underlying mechanisms investigated within the last 20 years. Both basal, including the circadian rhythm, and stress-induced adrenocorticotrophic hormone and glucocorticoid secretory patterns are altered. Throughout most of pregnancy, and in lactation, these changes predominantly reflect reduced drive by the corticotropin-releasing factor (CRF) neurones in the parvocellular paraventricular nucleus (pPVN). An accompanying profound attenuation of HPA axis responses to a wide variety of psychological and physical stressors emerges after mid-pregnancy and persists until the end of lactation. Central to this suppression of stress responsiveness is reduced activation of the pPVN CRF neurones. This is consequent on the reduced effectiveness of the stimulation of brainstem afferents to these CRF neurones (for physical stressors) and of altered processing by limbic structures (for emotional stressors). The mechanism of reduced CRF neurone responses to physical stressors in pregnancy is the suppression of noradrenaline release in the PVN by an up-regulated endogenous opioid mechanism, which is induced by neuroactive steroid produced from progesterone. By contrast, in lactation suckling the young provides a neural stimulus that dampens the HPA axis circadian rhythm and reduces stress responses. Reduced noradrenergic input activity is involved in reduced stress responses in lactation, although central prolactin action also appears important. Such adaptations limit the adverse effects of excess glucocorticoid exposure on the foetus(es) and facilitate appropriate metabolic and immune responses.

Contact with infants modulates anxiety-generated c-fos activity in the brains of postpartum rats

The postpartum period is associated with many behavioral changes, including a reduction in anxiety, which is thought to be necessary for mothers' ability to appropriately care for infants. In laboratory rats, this reduction in anxiety requires recent contact with pups, but areas of the brain where infant contact influences neural activity to reduce anxiety are mostly unknown. We examined c-fos expression in lactating rats whose pups were removed for 4h to increase mothers' anxiety, or not removed to maintain low anxiety in mothers, followed by exposure to the anxiogenic stimuli of either brief handling or handling followed by exposure to an elevated plus maze. Control animals had their litters removed or not, but no further stimulation. A large number of neural sites traditionally implicated in regulating anxiety in male rats were examined, and similar to what is found in male rats, most showed increased Fos expression after handling and/or elevated plus-maze exposure. Litter presence before testing affected Fos expression due to handling or elevated plus-maze exposure only in the ventral bed nucleus of the stria terminalis, dorsal and ventral preoptic area, ventromedial hypothalamus, lateral habenula, and supramammillary nucleus. Contrary to expectations, prior litter presence was associated with more Fos expression in most of these sites after handling and/or elevated plus-maze stimulation, and only after such stimulation. These sites may be of particular importance for how sensory inputs from infants modulate anxiety and other mood states during the postpartum period.

Maternal experience inhibits the production of immature neurons in the hippocampus during the postpartum period through elevations in adrenal steroids

Motherhood is accompanied by alterations in numerous nonreproductive behaviors, including learning and memory, as well as anxiety and stress regulation. These functions have been linked to adult neurogenesis in the hippocampus, but the effect of maternal experience on this brain region has not been completely explored. To determine whether the production of new hippocampal granule cells is altered during the postpartum period, we examined the number of proliferating cells and their progeny in the dentate gyrus of primiparous female rats at various time points during the postpartum period while they were caring for their offspring, as well as after weaning. Additionally, we investigated whether cell proliferation in the postpartum female is affected by the presence of offspring and nursing-induced increases in glucocorticoids. Analysis of the number of BrdU-labeled cells revealed that cell proliferation in the dentate gyrus was suppressed in lactating postpartum females until the time of weaning. This effect was temporary; a difference was detectable at 1 week after BrdU-labeling, when the majority of cells expressed a marker of immature and mature granule neurons (TuJ1) but not at 2 weeks, when most cells expressed a marker of mature neurons (NeuN). The decrease in cell proliferation was dependent on elevated basal glucocorticoid levels associated with lactation; removal of nursing pups reduced basal corticosterone levels and prevented the decrease in the number of BrdU-labeled cells. Moreover, preventing increased basal corticosterone levels by means of adrenalectomy and low-dose corticosterone replacement eliminated the reduction in cell proliferation. These findings indicate that offspring interactions inhibit adult neurogenesis through changes in adrenal steroids, and further suggest a potential mechanism for alterations in hippocampal function during the postpartum period.

Neuroendocrine actions and regulation of hypothalamic neuropeptide Y during lactation

The expression of neuropeptide Y (NPY) and its co-messenger, agouti-related peptide (AgRP), in arcuate neurons of the hypothalamus is increased during lactation in rats. Our research has been addressing the questions of the physiological actions of these peptides during lactation and the physiological signals associated with lactation that result in increased expression of their genes. Our studies indicate that NPY and AgRP exert pleiotropic actions during lactation that help integrate neuroendocrine regulation of energy balance with controls over anterior and posterior pituitary hormone secretion. Further, reciprocal signaling to the NPY/AgRP system by leptin and ghrelin is responsible for the changes in expression of these hypothalamic peptides in lactating animals, and thus, may contribute to regulation of food intake and the various neuroendocrine adaptations of lactation.

[Oxytocin and maternal stress during the post-partum period]

Oxytocin is required for lactation by promoting milk expulsion. Oxytocin has also been reported to exert a positive role in social attachment. The postpartum period has been shown to be crucial for maternal behavior initiation, and required self-trust reinforcement. However, this period is also remarkable for the high risk exposure of either psychic or physical stress. A negative impact on young mother is suspected, both in the short, medium or long term, which can even be deleterious for child-mother relationships. During lactation in female rats and sheep, oxytocin production has been proved to decrease stress-induced hormonal changes and later consequences. In human beings, only the first hour after breast-feeding seems to protect against physical or psychic stress. Oxytocin improves the stress-induced response by reducing the ACTH and cortisol secretion thus representing a potential therapeutic pathway in post-partum pathologies such as depression. Thus, this review of recent literature about oxytocin and stress during post-partum period, leads to the assumption that oxytocin, at the moment of installation of breastfeeding, acts not only on the physiological condition, but also on the psychic condition of the mother.

Activation of the hypothalamo-pituitary-adrenal axis by isolation and restraint stress during lactation in ewes: effect of the presence of the lamb and suckling

We investigated the effect of the presence and absence of lambs and suckling by lambs to attenuate activation of the hypothalamo-pituitary-adrenal (HPA) axis to isolation and restraint stress in lactating sheep. In experiment 1, blood samples were collected every 10 min from nonlactating (n = 5) and lactating (n = 5) ewes for 4 h before and during stress. In experiment 2, ewes (n = 6) were allocated to 1) nonlactating, 2) lactating with lambs absent, 3) lactating with lambs present but unable to suckle, and 4) lactating with lambs present and able to suckle. Blood samples were collected over 8 h with no stress (control day) and for 4 h before and 4 h during stress (stress day). In experiment 1, the mean (+/-SEM) cortisol concentrations increased significantly (P < 0.05) in nonlactating ewes during stress but did not change in lactating ewes. In experiment 2, cortisol did not vary on the control day or pretreatment of the stress day but increased (P < 0.05) during stress in all groups except lactating ewes with lambs present and able to suckle. The greatest cortisol response occurred in nonlactating ewes followed by lactating ewes with lambs absent and lactating ewes with lambs present but unable to suckle. During stress, the ACTH concentrations increased (P < 0.05) in nonlactating ewes and lactating ewes with lambs absent but not in lactating ewes with lambs present. We conclude that the activity of the HPA axis during isolation and restraint is reduced in lactating ewes and that the presence of lambs increases this level of attenuation.

Deoxyribonucleic acid microarray analysis of gene expression pattern in the arcuate nucleus/ventromedial nucleus of hypothalamus during lactation

Lactation is characterized by extreme hyperphagia and negative energy balance resulting from a large energy drain due to milk production and by a suppression of cyclic ovarian function. Increases in neuropeptide Y and agouti-related protein and a decrease in proopiomelanocortin expression in the arcuate nucleus of hypothalamus (ARH) may contribute to the hyperphagia to maintain energy balance and to the suppression of LH secretion associated with lactation. However, little is known about the full extent of neuroendocrine changes in the ARH that may contribute to the various adaptations occurring during lactation. To address this issue, we used Affymetrix microarray to acquire a reliable profile of the lactation-induced transcriptional changes in micropunches containing the ARH and a portion of the ventromedial nucleus of the hypothalamus. Using high stringency criteria, 12 genes were identified as being differentially regulated during lactation, and an additional 10 genes and three transcribed sequences were identified using moderate stringency criteria. Changes in neuropeptide Y, enkephalin, tyrosine hydroxylase, and dynorphin, genes previously shown to be differentially regulated during lactation, provide validation for the microarray analysis. New genes identified as being differentially expressed include those related to neurotransmission, growth factors, signal transduction, and structure remodeling. These data identify new genes in ARH/ventromedial nucleus of the hypothalamus that may play an important role in the adaptations of lactation related to hyperphagia, milk production, and the suppression of cyclic reproductive function and may contribute to elucidating a framework for integrating changes in energy intake with the regulation of reproductive function during lactation.

Measuring stress responses in postpartum mothers: perspectives from studies in human and animal populations

Reduced hypothalamic-pituitary-adrenal (HPA) responses to stress during the last week of pregnancy and lactation have been consistently observed in rat studies. Several contributing factors have been proposed for this phenomenon in lactation, including the suckling stimulus from the pups, hormones (oxytocin and prolactin) and opioids, a decrease in the ability of noradrenaline to potentiate hypothalamic responses and changes in pituitary responsiveness to ACTH secretagogues (AVP and CRF). In contrast to this vast literature using the rat model, only few studies have addressed this issue in the human population. The consensus is that women engaging in breastfeeding activities exhibit reduced anxiety, although the reductions in neuroendocrine and autonomic responses to stressors are variable, in part because of the different nature of the stressors used. Further work is required to investigate how additional factors, such as maternal parity or emotional salience of the stressor can affect stress responsiveness in postpartum women. Here, we review first the findings regarding stress responsiveness during lactation in both rat and human studies, and then discuss potential research avenues and methodological issues that could be the lead to future research protocols in human subjects. Knowing the reciprocal relationship in the mother-infant dyad, it is clear that investigation of the mechanisms regulating stress responses and mental health in postpartum mothers can only be beneficial to the development of the infant.

Breastfeeding and Maternal Stress Response and Health

This article reviews findings on the maternal stress and health effects of lactation. Several significant associations have emerged. Compared with not breastfeeding, breastfeeding is associated with increased parasympathetic nervous system modulation, greater vascular stress response, lower perceived stress levels, and fewer depressive symptoms. Breastfeeding exclusively is associated with an attenuated initial sympathetic cardiac nervous system response to some laboratory stressors. Bottle-feeding is associated with increased sympathetic and decreased parasympathetic cardiac control. The act of breastfeeding is associated with decreased neuroendocrine response to stressors and decreased negative mood. Finally, breastfeeding is associated with enhanced physical and mental health compared with non-breastfeeding.

Inhibition of uncoupling protein expression during lactation: role of leptin

Uncoupling proteins (UCPs) are mitochondrial proteins that play a role in regulation of energy expenditure by uncoupling respiration from ATP synthesis. Lactation is a physiological condition characterized by negative energy balance due to the loss of energy sources to the production of milk. The objective of the current study was to investigate whether UCP mRNA and protein expressions were altered during lactation compared with those after 48 h of fasting. Lactation significantly reduced serum leptin levels, and removal of pups for 48 h increased serum leptin to higher levels than those observed in control rats. Compared with control rats, mRNA expression of UCP1 and UCP3 in brown adipose tissue (BAT) was dramatically reduced during lactation and fasting. The reduction in mRNAs was reflected by a lowered UCP1 protein level, and to some extent, UCP3 protein. Treatment of lactating rats with exogenous leptin (3 mg/kg) or removal of pups for 48 h completely reversed the down-regulation of UCP1 and UCP3 mRNA expression in BAT, and pup removal led to a recovery of protein expression. In contrast to BAT, UCP3 expression in skeletal muscle was increased in fasted rats and decreased during lactation. Similar changes were observed in serum free fatty acid levels. These changes are consistent with the idea that the utilization of free fatty acids as a fuel source is spared during lactation. As in BAT, leptin treatment and removal of pups were able to restore changes in mRNA expression of UCP3 in skeletal muscle during lactation. The present results suggest that the inhibition of leptin secretion during lactation is involved in the down-regulation of UCP expression in BAT and skeletal muscle, which, in turn, is responsible for the decrease in metabolic fuel oxidation and thermogenesis.

Social support and oxytocin interact to suppress cortisol and subjective responses to psychosocial stress

BACKGROUND

The presence of social support has been associated with decreased stress responsiveness. Recent animal studies suggest that the neuropeptide oxytocin is implicated both in prosocial behavior and in the central nervous control of neuroendocrine responses to stress. This study was designed to determine the effects of social support and oxytocin on cortisol, mood, and anxiety responses to psychosocial stress in humans.

METHODS

In a placebo-controlled, double-blind study, 37 healthy men were exposed to the Trier Social Stress Test. All participants were randomly assigned to receive intranasal oxytocin (24 IU) or placebo 50 min before stress, and either social support from their best friend during the preparation period or no social support.

RESULTS

Salivary free cortisol levels were suppressed by social support in response to stress. Comparisons of pre- and poststress anxiety levels revealed an anxiolytic effect of oxytocin. More importantly, the combination of oxytocin and social support exhibited the lowest cortisol concentrations as well as increased calmness and decreased anxiety during stress.

CONCLUSIONS

Oxytocin seems to enhance the buffering effect of social support on stress responsiveness. These results concur with data from animal research suggesting an important role of oxytocin as an underlying biological mechanism for stress-protective effects of positive social interactions.

Pups presence eliminates the stress hyporesponsiveness of early lactating females to a psychological stress representing a threat to the pups

Blunted neuroendocrine responses to stress are reported in lactating females after exposure to various stressors. However, many of the stimuli used in these studies have little ethological relevance for maternal protection of the litter in a threatening environment. The question that arises is whether the relevance of the stressor to the infant is critical in the 'gating' of the neuroendocrine response. We hypothesized that the presence of pups with their mothers at the time of exposure to an intruder or a predator odour is an effective way to increase the emotional salience of the psychological stressor, thus eliminating the stress hyporesponsiveness in lactating females. We first compared neuroendocrine responses [corticotropin-releasing factor (CRF) mRNA in the paraventricular nucleus of the hypothalamus (PVN) and central nucleus of the amygdala (CeA), plasma adrenocorticotropic hormone (ACTH) and corticosterone] between early (EL, PPD3-5), late (LL, PPD 15) lactating and virgin (V) females to a male intruder in the home cage. We next investigated the effect of pups' presence at the time of stressor exposure on the magnitude of the hormonal response to a male intruder in the home cage or to a predator odour (fox urine) in a novel environment. In the male intruder paradigm, levels of CRF mRNA expression in the PVN and CeA were lower in LL compared to EL or V females and plasma ACTH and corticosterone secretion was not as elevated in LL compared to EL females. Aggression towards the intruder was high in EL females in the presence of their pups and a positive correlation was found with the integrated ACTH response. Aggression rapidly declined after pup separation (2.5 h or 48 h) or in LL nursing females. In EL females, the presence of the pups with their mothers (EL + pups) at the time of stress significantly increased plasma ACTH and corticosterone responses to either male intruder or predator odour compared to EL females without their pups for 2.5 h or 48 h (EL - pups). Plasma ACTH response to fox urine in EL + pups females was comparable to that of virgin females, suggesting that increasing the salience of emotionally relevant stimuli by keeping the pups present in the cage could eliminate the hyporesponsiveness detected for EL females without their pups. These studies indicate the critical role of the pups in modulating the maternal response to stressors that represent a threat for the litter. We hypothesize that the amygdala, because of its ability to process olfactory stimuli and stimuli with affective properties, might play an essential role in 'gating' the neuroendocrine response to stress during lactation.