Dissociation in the effects of neonatal maternal separations on maternal care and the offspring's HPA and fear responses in rats

The development of the hypothalamic-pituitary-adrenal (HPA) response to stress is influenced by the early mother-infant relationship. In rats, early handling (brief daily mother-offspring separations) attenuates the adult offspring's HPA and fear responses compared to both nonhandling (no separations) and maternal separation (prolonged daily separations). It has been proposed that variation in the amount of maternal care mediates these effects of neonatal manipulations on the adult offspring's stress and fear responses. Here we tested this hypothesis by assessing maternal care and the adult offspring's HPA and fear responses in Lister hooded rats which were subjected to either early handling (EH) or maternal separation (MS) from postnatal day 1-13, or were left completely undisturbed (nonhandled, NH) throughout this period. Both EH and MS induced a more active nursing style and elevated levels of maternal care compared to NH. Total levels of maternal care were indistinguishable between EH and MS, but diurnal distribution differed. MS dams showed elevated levels of maternal care following the 4-h separation period, thereby fully compensating for the amount of maternal care provided by EH dams during the time MS dams were separated from their pups. However, while EH resulted in reduced HPA and fear responses in the adult offspring compared to NH, MS and NH offspring did not differ. Our findings therefore demonstrate dissociation in the effects of EH and MS on maternal care and on the stress and fear responses in the offspring. This indicates that maternal care cannot be the sole mediator of these effects.     

Neonatal interventions differently affect maternal care quality and have sexually dimorphic developmental effects on corticosterone secretion

Neonatal handling (H) and maternal separation (MS) both induce changes in maternal care, but the contribution of these changes to the behavioral and neurochemical outcomes of the offspring remains unclear, as studies often find opposite results concerning the frequency of maternal behaviors, particularly in the MS paradigm. In this study, behavior displayed by H, MS and non-handled (NH) Wistar rat dams were observed during the first 10 days after birth. A tentative assessment of the quality of maternal care was made, using a previously reported score that reflects behavior fragmentation and inconsistency. Central oxytocin levels and hippocampal synaptic plasticity markers were also evaluated in dams, immediately after litter weaning. In adulthood, male and female offspring were subjected to a contextual stress-induced corticosterone challenge to provide further information on the impact of early interventions on neuroendocrine parameters. We found that while both H and MS interventions induced an increase in the amount of pup-directed behavior, MS dams displayed a more fragmented and inconsistent pattern of care, reflecting poorer maternal care quality. Interestingly, an increase in oxytocin levels was observed only in H dams. While H offspring did not differ from NH, MS males and females showed marked differences in corticosterone secretion compared to controls. Our results suggest that briefly removing the pups from the nest alters maternal care quantity but not quality and increases central oxytocin, while long separations appear to increase low quality maternal care and change neuroendocrine responses in adult offspring in a sex-specific manner.     

Early deprivation, but not maternal separation, attenuates rise in corticosterone levels after exposure to a novel environment in both juvenile and adult female rats

Separation from the maternal nest alters the hypothalamic-pituitary-adrenal (HPA) axis stress response in adult male rats, but little research has addressed how separation affects female rats. The following experiments investigated how early maternal separation from postnatal day (PND) 2 to 14 affected stress-induced corticosterone and ACTH after exposure to an open field in juvenile and adult female rats. Female rats were separated for 5 h daily from mother and littermates (early deprivation: ED), separated from mother but not littermates (maternal separation: MS), or animal facility reared (AFR). Male siblings were left with the mother rat during separation. Female rats were exposed to an open field arena either during the juvenile period (PND 30) or during adulthood (PND 80-100). Results show that ED juvenile female rats showed a lower corticosterone stress response than MS and AFR female rats when measured at 5 min post-stress, but no difference at 20 or 60 min post-stress. In adulthood, ED female rats showed comparable elevations of corticosterone as MS and AFR rats at 5 min post-stress but lower elevations at 20 min. In terms of behavior, there were no significant effects of early experience. However, in adulthood, ED and MS rats tended to show a decreased proportion of inner grid crossings of the open field compared to AFR rats, suggesting a tendency for increased anxiety in these two separation groups.     

Twice daily long maternal separations in Wistar rats decreases anxiety-like behaviour in females but does not affect males

Prolonged daily separations of rat pups from their mother have been reported to increase anxiety-like behaviour in adult offspring. However, there are an increasing number of studies not showing this. It has been proposed that the effect of long maternal separation (LMS) is partly due to the disruption of maternal care caused by the separations. The aim of the present study was to investigate whether increasing the number of daily separations would produce more robust effects in the adult offspring on anxiety-like behaviour in the defensive withdrawal test, and on spontaneous motor activity. Since previous studies of LMS have revealed sex differences in behaviour, we included both males and females. In our separation paradigm we subjected rat pups to either two daily 3h maternal separations during the first 2 weeks postpartum (LMS), two daily 15 min maternal separations (brief maternal separations, BMS) during the same time period to control for the effects of handling, or to normal husbandry conditions. As adults we found no effects of this LMS paradigm in male rats, although BMS males showed a tendency toward decreased anxiety-like behaviour. In contrast, LMS females showed a decrease in anxiety-like behaviour. We also found significant sex differences that were most prominent in the LMS group, indicating that females are more sensitive to our maternal separation paradigm. The present study suggests that increasing the number of maternal separations does not increase anxiety-like behaviour in neither male nor female Wistar rats.     

Long-lasting changes in behavioural and neuroendocrine indices in the rat following neonatal maternal separation: gender-dependent effects

Neonatal maternal separation (MS) has been used to model long-term changes in neurochemistry and behaviour associated with exposure to early-life stress. This study characterises changes in behavioural and neuroendocrine parameters following MS. On postnatal days (PND) 3-15, male and female Long-Evans rats underwent 3 h daily MS. Non-handled (NH) control offspring remained with the dams. Starting at PND 90, behaviour was assessed at weekly intervals in the elevated plus-maze, elevated T-maze, and locomotor activity boxes, and body weight monitored throughout. At the end of the study, adrenals were weighed and blood collected for analysis of plasma corticosterone and adrenocorticotropic hormone (ACTH) under basal conditions and following restraint stress. As adults, MS weighed more than NH animals. Activity on the open arms of the plus-maze was similar between MS and NH animals. In the T-maze, MS males had shorter emergence latencies than their NH counterparts. Spontaneous ambulation in a novel environment was significantly higher in MS than in NH animals, and males exhibited overall lower activity than females. Basal plasma corticosterone was lower in MS than in NH females, but no rearing condition difference was observed following restraint stress. Females had higher corticosterone and ACTH levels than males, whereas adrenal glands of MS animals weighed less than those of NH controls. The MS paradigm caused long-term gender dependent effects on behaviour and HPA axis status. The consistent gender differences confirm and expand existing results showing altered anxiety and stress reactivity in male and female rats.     

Alterations in diurnal cortisol rhythm and acoustic startle response in nonhuman primates with adverse rearing.

BACKGROUND: Early adverse experiences represent risk factors for the development of anxiety and mood disorders. Studies in nonhuman primates have largely focused on the impact of protracted maternal and social deprivation, but such intense manipulations also result in severe social and emotional deficits very difficult to remediate. This study attempts to model more subtle developmental perturbations that may increase the vulnerability for anxiety/mood disorders but lack the severe deficits associated with motherless rearing. METHODS: We investigated the consequences of repeated maternal separations between 3 to 6 months of age on rhesus monkeys' hypothalamic-pituitary-adrenal (HPA) axis function and acoustic startle reactivity. RESULTS: Repetitive maternal separation led to increased cortisol reactivity to the separation protocol in female infants and alterations in mother-infant interaction. It also resulted in a flattened diurnal rhythm of cortisol secretion and increased acoustic startle reactivity at later ages. CONCLUSIONS: Macaques with adverse rearing exhibited short-term and long-term alterations in HPA axis function and increased acoustic startle response comparable with changes associated with mood/anxiety disorders. The magnitude of HPA axis reactivity to the separations and the alterations in mother-infant relationship detected during the separation protocol predicted some of the alterations in HPA axis and emotionality exhibited later in life.     

Maternal separation induces alterations of serotonergic system in different aged rats

Maternal separation (MS) induces profound behavioral and neurochemical dysregulations in adult rodents. In the present longitudinal study, we investigated the effects of repeated (4 h/day) maternal separation during postnatal days 1–21 on serotonergic synthesis and activity in the prefrontal cortex (PFC), nucleus accumbens (NAc) and hippocampus of juvenile (post-natal day 21, PND 21), adolescent (PND 35) and early adult (PND 56) male Wistar rats. We found that MS increased 5-HT levels in the PFC of juvenile rats, and although MS increased 5-HT and 5-HIAA levels in the NAc of adolescent rats, the ratio between 5-HIAA and 5-HT decreased in the PFC. In addition, MS-treated adult rats showed increased levels of 5-HT in the PFC as well as 5-HT and 5-HIAA in the NAc. These data provided evidence that MS leads to profound and age-specific changes in serotonergic synthesis and activity in rodents. Our study also found that there are U-shaped and inverted U-shaped patterns for serotonergic synthesis and serotonergic activity from younger rats to adults, respectively. Together, our findings support the use of maternal separation as an animal model for studying the neurobiological pathogenesis of neurodevelopmental diseases.     

Fasting-induced increases of arcuate NPY mRNA and plasma corticosterone are blunted in the rat experienced neonatal maternal separation

This study was conducted to examine the effects of neonatal maternal separation on the hypothalamic expression of feeding peptides in later life. Pups in maternal separation (MS) groups were separated from their dam for 3 h daily from postnatal day (PND) 1-14, while pups in non-handled (NH) groups were left undisturbed. Rats were sacrificed on PND 60 to examine the gene expression of neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) in the hypothalamic arcuate nucleus by mRNA in situ hybridization. Half of the rats from each group were food-deprived for 48 h before sacrifice. POMC mRNA expression increased in the free fed MS group compared with the free fed NH group. Food deprivation significantly decreased the arcuate POMC mRNA level in both groups. Body weight gain, basal levels of plasma corticosterone, leptin, and arcuate NPY mRNA were not modulated by experience of neonatal maternal separation. However, fasting-induced increases of plasma corticosterone and arcuate NPY expression were blunted in MS rats. These results suggest that neonatal maternal separation may increase the basal expression level of arcuate POMC mRNA, while inhibit the fasting-induced expression of arcuate NPY mRNA, later in life. Lastly, the altered expression of arcuate NPY mRNA, but not of arcuate POMC mRNA, appeared to be related with altered activity of the hypothalamic-pituitary-adrenal gland axis in offspring by neonatal maternal separation.     

Foster litters prevent hypothalamic-pituitary-adrenal axis sensitization mediated by neonatal maternal separation

Neonatal maternal separation of rat pups has been shown to produce long-term increases in hypothalamic-pituitary-adrenal (HPA) axis responsiveness, elevated levels of hypothalamic corticotropin releasing factor (CRF) mRNA in the hypothalamic paraventricular nucleus (PVN), and enhanced anxiety-like behavior. These effects appear to be at least partially mediated by subtle disruptions in the quality of maternal-pup interactions. This hypothesis was tested by providing half the dams with foster litters during the maternal separation paradigm, so that in those litters, only the pups and not the dams were experiencing a period of separation. The separation protocol took place daily from PND2-14 for either 15 min (HMS15, handled) or 180 min (HMS180, maternal separation). During the period of separation dams were either transferred to adjacent cages without any pups present (HMS15, HMS180) or to cages containing an age-matched foster litter (HMS15F, HMS180F). As adults, the HMS180 progeny exhibited the expected increased expression of CRF mRNA in the PVN, stress hyper-responsiveness to airpuff startle and evidence of impaired feedback both in the CORT response, as well as in response to the dexamethasone suppression test. The HMS180F rats, however, appeared to be resistant to these effects of maternal separation as they demonstrated CRF mRNA levels intermediate between HMS15 and HMS180 rats. Their stress responses and feedback regulation of the HPA axis was comparable to that of the HMS15 rats. GR mRNA was elevated in the cortex of HMS180F rats. Overall, these studies support the thesis that the long-term effects of neonatal maternal separation may largely result from alterations in the quality of maternal care rather than from direct effects of the separation per se on the pups.     

Periodic maternal separation decreases hippocampal neurogenesis without affecting basal corticosterone during the stress hyporesponsive period, but alters HPA axis and coping behavior in adulthood

Although not directly evaluated, the early rise of glucocorticoid (GC) levels, as occur after exposure to adverse early life experience, are assumed to affect hippocampal ontogeny by altering the hippocampus negative feedback on adult HPA axis. To test whether hippocampal ontogeny is affected by early exposure to stress we estimated the survival of recently formed hippocampal granule cells in rat pups subjected to periodic maternal separation (180 min/day; MS180) from postnatal days (PND) 1 to 14. Accordingly, MS180 pups injected with bromodeoxyuridine (BrdU, 50 mg/kg, ip) at PND 5 showed decreased density of doublecortin (DCX) positive BrdU-labeled cells at PND 15. MS180 and AFR pups showed similar corticosterone (CORT) basal levels between PND 3 and 12, whereas adult MS180 rats presented with higher CORT levels than AFR adults. Nonetheless, both AFR and MS180 pups and adults showed similar transient increments of CORT levels in response to stress. In addition, MS180 had no effect on the adult anxiety-like behavior evaluated in the elevated plus maze, but evoked a passive coping strategy in the forced swimming test. The data show that the decrease in hippocampal neurogenesis is an early onset phenomenon, and suggests that adverse experiences alter hippocampal ontogeny without chronic elevation of GC levels.     

Variations of maternal care alter offspring levels of behavioural defensiveness in adulthood: evidence for a threshold model

Natural variations of maternal care in the rat influence the development of neuronal systems that regulate defensive responses to threat. Thus, as adults, rats that received higher levels of maternal licking/grooming (LG) in infancy display dramatic reductions in burying in the shock-probe test, relative to offspring of low LG mothers. We sought to replicate that finding and determine whether maternal care similarly influences offspring responses to social threat, using the resident-intruder test. We also examined whether maternal LG influences offspring behaviour along a continuum by comparing defensive responses of offspring of mid LG mothers to those of offspring of high LG and low LG mothers. A final goal was to assess whether the reductions in adult offspring reactivity to threat that typically follow corticosterone (CORT) administration to dams across lactation are mediated through CORT-induced changes in maternal care. Adult offspring of high LG mothers spent less time burying the shock-probe, relative to offspring of mid and low LG mothers, whereas offspring of CORT-treated mothers did not differ from any group. Similarly, offspring of high LG (but not CORT-treated) mothers displayed fewer defensive responses in the resident-intruder test. Thus, only natural variations of maternal care were associated with individual differences in offspring reactivity to threat. Furthermore, because offspring of mid and low LG mothers displayed equivalent levels of defensive responding in both tests, it appears that a critical threshold of maternal LG is necessary to alter the developmental trajectory of neural systems mediating defensive behaviours.     

Cholecystokinin tetrapeptide effects on HPA axis function and elevated plus maze behaviour in maternally separated and handled rats

Deficits in the function of the hypothalamic-pituitary-adrenal (HPA) axis have been suggested to predispose to the development of depression and anxiety disorders. This is mirrored in the animal model "Maternal Separation (MS)" where the stress of repeated separation of rat pups from the dam during early postnatal development results in long lasting alterations in HPA axis function. Cholecystokinin increases serum concentrations of stress axis hormones and might be involved in the dam-pup interaction in rats. Therefore, we hypothesized that adult animals, which had been separated daily (postnatal days (PND) 2-14) for 180 min (MS180) would differ in HPA axis responsiveness to an intravenous challenge dose of cholecystokinin tetrapeptide (CCK-4) compared to handled rats, separated for 15 min daily. The study explored the effects of intravenous CCK-4 on elevated plus maze behaviour and HPA axis hormones. MS180 animals displayed reduced general activity but unaltered levels of open arm activity in the elevated plus maze. CCK-4 administration elevated general activity in the handled rats, while leaving MS180 rats unaffected. MS180 rats had increased baseline CRF mRNA expression in the paraventricular nucleus of the hypothalamus. When CRF mRNA was assessed in chronically catheter implanted and single housed rats, lower levels were found in the paraventricular nucleus of MS180 animals compared to handled animals and this parameter was not affected by CCK-4 treatment. Adrenocorticotropin concentrations in serum were equal in MS180 and handled rats and unaffected by CCK-4. Corticosterone serum concentrations were lower in saline treated MS180 rats compared to saline treated handled rats. CCK-4 injection raised serum corticosterone in MS180 rats to levels equal to the handled rats, while leaving handled rats unaffected. We suggest that the lower levels of hypothalamic CRF mRNA and serum corticosterone concentrations in MS180 rats might be due to the experimental set-up with chronic venous catheter implants and single housing. In conclusion, this study supports the hypothesis of elevated CCK sensitivity in separated rats as measured by corticosterone changes thus adding to the existing literature reporting early life stress having long-term impact on HPA axis function.     

Neonatal maternal separation reduces hippocampal mossy fiber density in adult Long Evans rats

Neonatal maternal separation of rat pups leads to a stable stress hyper-responsive phenotype characterized by increased basal levels of corticotropin releasing factor (CRF) mRNA in the hypothalamic and extra-hypothalamic nuclei, increased hypothalamic CRF release, and enhanced adrenocorticotrophin hormone (ACTH) and corticosterone (CORT) responses to psychological stressors. Stress and exposure to glucocorticoids either early in life or in adulthood have been associated with hippocampal atrophy and impairments in learning and memory. In this study, male Long Evans rat pups were exposed to daily 3-h (HMS180) or 15-min (HMS15) periods of maternal separation on postnatal days (PND) 2–14 or normal animal facility rearing. Maternal separation and subsequent reunion with the dam resulted in elevated plasma CORT levels versus HMS15 animals at PND7, a time when rat pups are normally hyporesponsive to stressors and show limited pituitary–adrenal responses. As adults, HMS180 rats exhibited elevated indices of anxiety, startle-induced pituitary–adrenal hyper-responsiveness, and slight, but significant impairment on acquisition in the Morris water maze task. In addition, HMS180 rats exhibited decreased mossy fiber density in the stratum oriens region of the hippocampus as measured by Timm’s staining, but no change in volume of the dentate gyrus. These changes may be the result of neonatal exposure to elevated glucocorticoids and/or changes in other signaling systems in response to maternal separation. Overall the results suggest that repeated, daily, 3-h maternal separations during critical periods of hippocampal development can disrupt hippocampal cytoarchitecture in a stable manner. The resulting change in morphology may contribute to the subtle, but consistent learning deficit and overall stress hyper-responsive phenotype observed in these animals.     

Prenatal Social Stress in the Rat Programmes Neuroendocrine and Behavioural Responses to Stress in the Adult Offspring: Sex‐Specific Effects

Stress exposure during pregnancy can ‘programme’ adult behaviour and hypothalamic‐pituitary‐adrenal (HPA) axis stress responsiveness. In the present study, we utilised an ethologically relevant social stressor to model the type of stress that pregnant women may experience. We investigated the effects of social defeat by a resident lactating rat over 5 days during the last week of pregnancy on the pregnant intruder rat HPA axis, and on HPA responsivity to stress and anxiety‐related behaviour in the adult offspring of the socially‐defeated intruder rats. HPA axis responses after social defeat were attenuated in the pregnant rats compared to virgin females. In the adult offspring, systemic interleukin (IL)‐1β or restraint increased adrenocorticotrophic hormone and corticosterone secretion in male and female control rats; however, in prenatally stressed (PNS) offspring, HPA responses were greatly enhanced and peak hormone responses to IL‐1β were greater in females versus males. Male PNS rats displayed increased anxiety behaviour on the elevated plus maze; however, despite marked changes in anxiety behaviour across the oestrous cycle, there were no differences between female control and PNS rats. Investigation of possible mechanisms showed mineralocorticoid mRNA levels were reduced in the hippocampus of male and female PNS offspring, whereas glucocorticoid receptor mRNA expression was modestly reduced in the CA2 hippocampal subfield in female PNS rats only. Corticotropin‐releasing hormone mRNA and glucocorticoid receptor mRNA expression in the central amygdala was greater in PNS males and females compared to controls. The data obtained in the present study indicate that prenatal social stress differentially programmes anxiety behaviour and HPA axis responses to stress in male and female offspring. Attenuated glucocorticoid feedback mechanisms in the limbic system may underlie HPA axis hyper‐reactivity to stress in PNS offspring.     

Immediate and enduring effects of neonatal isolation on maternal behavior in rats

Previously, we showed that neonatal isolation (1-h isolation/day from dam, litter, and nest on PND 2–9) facilitates cocaine self-administration and increases extracellular dopamine responses in ventral striatum after stimulant administration in adulthood. Recent studies suggest that enduring alterations in neurobehavioral responses associated with early life manipulations reflect changes in maternal behavior. Thus, we sought to determine if neonatal isolation alters maternal care and if dams with neonatal isolation experience as pups showed differential maternal care towards their pups. In Experiment 1, litters were assigned to one of three conditions: neonatal isolation, handled (5-min separation of dam from litter), or non-handled (no separation). Maternal behaviors were rated on PND 2–9 for 60-min immediately following reunion of mother and litter. In Experiment 2, female rats with or without neonatal isolation experience were assigned to either the neonatal isolation or non-handled litter condition and maternal behaviors rated. Dams of isolated and handled litters spent more time licking pups and less time picking up pups to put outside the nest than dams of non-handled litters. Further, dams of isolated and handled vs. non-handled litters showed less non-maternal behaviors of burrowing and grooming. Neonatal isolation-experienced dams with isolated litters failed to increase pup-licking and decrease non-maternal behaviors. Rather, these dams picked up pups to place outside the nest more than non-handled-experienced dams. Neonatal isolation alters maternal behavior that, in turn, may shape neurobehavioral responses of offspring including effects on maternal care. Such changes may reflect epigenetic effects resulting from changes in maternal behavior.     

Neonatal factors influence adult stroke outcome

Neonatal environment can have important, life-long influences on stress-reactivity and hypothalamic-pituitary-adrenal (HPA) axis regulation. In rodents, brief mother-infant separations have been shown to improve efficiency of the HPA axis, decrease stress-reactivity, and decrease age-related declines in cognitive function. Here, we provide evidence that there are potential costs associated with improved HPA axis regulation, including increased sensitivity to cerebral inflammation and glucocorticoid-mediated neuronal death following stroke. Specifically, brief mother-infant separation decreases the initial corticosteroid response to experimental stroke in adult mice, but increases post-stroke pro-inflammatory cytokine expression, edema, and infarct volume compared to ischemic controls. Brief maternal separation also compromises functional recovery and long-term survival following stroke. In addition, adrenalectomy reverses the effects of brief maternal separation on stroke outcome when corticosterone is replaced at baseline, but not ischemic, concentrations; thus, neonatally separated mice are more sensitized as adults to the detrimental effects of elevated corticosterone during ischemia. Taken together, these data provide the first direct evidence that neonatal environment can substantially influence adult cerebrovascular health.     

Maternal separation alters ICSS responding in adult male and female rats, but morphine and naltrexone have little affect on that behavior

Prior research has provided evidence that the early postnatal environment can have long lasting effects on both the physiology and behavior of offspring. This is modeled in rats by using a maternal separation paradigm in which pups are separated from their mother for a few hours daily during their first two postnatal weeks. While this model has been used extensively to study stress effects and anxiety, less research has been done to examine how these separations affect measures of reward and reinforcement in adulthood. The current study investigated the impact of maternal separation (MS) on intracranial self-stimulation (ICSS) maintained responding in male and female offspring, and the effects of morphine (0.3-3.0 mg/kg) and naltrexone (0.1-10 mg/kg) on that responding. Rearing condition (MS or non-handled, NH) significantly altered response rates during acquisition in both sexes, with NH offspring exhibiting the highest rates. Group differences in baseline responding on a progressive ratio (PR-2) schedule of reinforcement were evident only in females, with MS females having response rates 50% lower than NH females. Neither morphine nor naltrexone differentially affected either rearing group. Sex impacted NH offspring: males acquired responding more readily, but females had higher response rates and breakpoints during all other phases of the experiment. In MS offspring, no sex differences were observed during acquisition, but during all other phases males had higher response rates and breakpoints than females. These results indicate that maternal separation during the first two postnatal weeks can have long-term effects on responding for ICSS, but these effects do not appear tied to endogenous opioid systems in the lateral hypothalamus.     

Early postnatal stress confers enduring vulnerability to limbic epileptogenesis

PURPOSE: Early life stress has enduring behavioral and neuroendocrine effects, particularly in hippocampus and amygdala. This may be relevant to mesial temporal lobe epilepsy (MTLE) that arises from these structures. In rats, we tested the hypothesis that early postnatal stress, in the form of maternal separation (MS), creates vulnerability to limbic epileptogenesis in adult life. METHODS: On postnatal days 2-14, we exposed male and female nonepileptic rats to either MS for 180 min/day, or early handling (EH) and brief separation (15 min/day). At 7 weeks of age, rats of both genders exposed to MS displayed significantly increased anxiety, as evidenced by reduced time spent in the open arms of the elevated plus maze compared with EH rats. For epileptogenesis experiments, separate cohorts of rats, similarly exposed to either early life MS or EH, were implanted with bipolar electrodes into the left amygdala and one week later rapid electrical kindling performed until fully kindled (five Class V seizures, Racine scale). RESULTS: In females, fewer stimulations were required following MS than EH to reach the fully kindled state (39.6 +/- 6.4 vs. 67.1 +/- 9.4; p < 0.0001); no differences were observed in males (MS: 49.1 +/- 5.1; EH: 53.7 +/- 6.6 stimulations). DISCUSSION: We conclude that, while postnatal MS stress increases anxiety in both genders, this early life stressor results in persisting vulnerability to limbic epileptogenesis only in females. This has implications for human MTLE and its psychiatric comorbidities, suggesting a common causation model and the involvement of gender-specific factors such as sex hormones.     

Maternal separation enhances neuronal activation and cardiovascular responses to acute stress in borderline hypertensive rats

There is much evidence suggesting early life events, such has handling or repeated separations from the nest, can have a long-term effect on the biological and behavioral development of rats. The current study examined the effect of repeated maternal separation (MS) on the behavioral, cardiovascular, and neurobiological responses to stress in subjects vulnerable to environmental stressors as adults. Borderline hypertensive rats (BHR), which are the first generation offspring of spontaneously hyperternsive and Wistar-Kyoto rats, were separated from the dams for 3h per day from postnatal day 1 through 14. Non-separated controls remained in the home cage. When allowed to explore the open field chamber for 60 min as adults, MS subjects had significantly greater locomotor activity compared to controls. All subjects were exposed to 30 min of restraint stress during which time mean arterial pressure (MAP) and heart rate (HR) were measured. Although both groups had comparable increases in MAP, MS animals displayed significantly higher HR throughout the stress period. Finally, MS subjects had significantly more stress-induced Fos positive cells, an estimate of neuronal activation, in the central nucleus of the amygdala (CeA), paraventricular nucleus of the hypothalamus (PVN), and the bed nucleus of the stria terminalis (BNST), each of which plays an important role in organizing the biobehavioral response to stress. These results suggest that maternal separation can further enhance stress reactivity in this model and may represent a useful approach for studying the relationship between early life events and future vulnerability to stressful situations.     

Effects of foraging demand on maternal behaviour and adult offspring anxiety and stress response in C57BL/6 mice

It has been proposed that developmental plasticity of anxiety and stress responses in rodents is mediated by environment-dependent variations in maternal behaviour, but recent evidence indicates that other factors must be involved. To examine this further, we exposed lactating C57BL/6 mice to environmental conditions that imposed high- (HFD), low- (LFD) or variable-foraging demand (VFD) from postnatal day 1-13, depending on the amount and predictability of food supply. While nest attendance was unaffected by treatment, both HFD and VFD-dams showed increased active maternal care compared to LFD-dams. Anxiety in adult male and female offspring was examined on an elevated-O-maze (EZM) and in the open-field test, while hypothalamo-pituitary-adrenal (HPA) reactivity to 20min novelty/isolation stress was determined based on changes in plasma corticosterone (CORT) levels. There were no persistent treatment effects on the offspring's CORT response to novelty/isolation stress. However VFD-males, but not HFD-males, behaved more anxious than LFD males. Their reduced activities throughout the behavioural tests are indicative of a more passive coping style. Conversely, VFD-females, but not HFD-females, behaved less anxious than LFD-females. Our results demonstrate (1) that maternal behaviour in C57BL/6 mice is sensitive to specific characteristics of the environment, (2) that even subtle environment-dependent variations in maternal behaviour can have persistent effects on the offspring's behavioural phenotype, (3) that other factors besides active maternal care must have contributed to these effects, and (4) that male and female offspring may be differentially sensitive to early maternal and/or environmental cues.