Maternal care, hippocampal glucocorticoid receptors, and hypothalamic-pituitary-adrenal responses to stress

Stress and the aging hippocampus

The "glucocorticoid cascade hypothesis" of hippocampal aging has stimulated a great deal of research into the neuroendocrine aspects of aging and the role of glucocorticoids, in particular. Besides strengthening the methods for investigating the aging brain, this research has revealed that the interactions between glucocorticoids and hippocampal neurons are far more complicated than originally envisioned and involve the participation of neurotransmitter systems, particularly the excitatory amino acids, as well as calcium ions and neurotrophins. New information has provided insights into the role of early experience in determining individual differences in brain and body aging by setting the reactivity of the hypothalamopituitary-adrenal axis and the autonomic nervous system. As a result of this research and advances in neuroscience and the study of aging, we now have a far more sophisticated view of the interactions among genes, early development, and environmental influences, as well as a greater appreciation of events at the cellular and molecular levels which protect neurons, and a greater appreciation of pathways of neuronal damage and destruction. While documenting the ultimate vulnerability of the brain to stressful challenges and to the aging process, the net result of this research has highlighted the resilience of the brain and offered new hope for treatment strategies for promoting the health of the aging brain.

Commentary: is maternal stimulation the mediator of the handling effect in infancy?

In a recent report, a series of studies is described showing that individual differences in rat maternal licking and grooming are correlated with their offsprings' later adrenal response to a stressor (Liu et al., 1997). Pups that received more maternal stimulation in infancy had lower ACTH, corticosterone, and CRH mRNA, while they had greater amounts of GR mRNA. Liu et al. also compared maternal behavior in litters where rat pups were handled daily to maternal behavior of nonhandled litters. They found that mothers of handled pups licked and groomed their young significantly more often than did mothers of control litters. In their discussion, Liu et al. proposed that their data support Levine's (1975) thesis that handling effects in infancy are mediated via the mother. Levine had proposed that handling of pups will modify the mother--pup interaction, thereby causing a change in maternal behavior. The purpose of this commentary is to summarize data obtained with mice and rabbits which, in part, support and, in part, challenge the conclusions of Liu et al.

Ultrasonic vocalizations by infant laboratory mice: a preliminary spectrographic characterization under different conditions

During the first 2 to 3 weeks of life, isolated neonatal mice emit ultrasonic vocalizations, with various conditions such as hypothermia or olfactory or tactile stimulation eliciting this behavior. Although it is known that pup vocalizations stimulate prompt expression of maternal behavior, the communicative role of infant ultrasonic calls is still a matter of investigation. A fine-grained spectrographic analysis of ultrasonic calls emitted by pups exposed to different conditions was performed. Forty 8-day-old outbred CD-1 mice (Mus musculus) were isolated from their mothers and littermates and randomly exposed to one of the following conditions: (a) odor from the nest, (b) social isolation, (c) low temperature-isolation, (d) tactile stimulation, or (e) odor from a conspecific adult male. Upon consideration of the spectrogram typology and emission frequency interval, it appears that the conditions under which vocalizations are emitted influence the sound characteristics of call production.

Genetic dissection of glucocorticoid receptor function in mice

Upon hormone binding, the activated glucocorticoid receptor (GR) functions as a transcription factor via different modes of action to control gene expression. Recent gene-targeting studies in mice provide new insight into the role of GR in vivo and are helping decipher the molecular mechanisms underlying its actions.

Blunted cortisol response to a psychosocial stressor in schizophrenia

Schizophrenia is considered a neurodevelopmental disorder in which vulnerability to stress may be a contributing factor. Coping is an important psychological component of stress processing, and the hypothalamic-pituitary-adrenal system (HPA system) is one of the biological components of stress adaptation. Disturbances of either of these components may make schizophrenic patients more vulnerable to develop a psychosis under stressful circumstances. In this study, 10 schizophrenic men were compared with 10 healthy male controls in their response to a psychosocial stressor, consisting of a public-speaking task. Heart rate was monitored as a measure of autonomic arousal. HPA responses were assessed by measuring salivary cortisol. Coping skills were measured by using the Utrecht Coping List and the Ways of Coping Checklist. The stress of speaking in public increased the heart rate in both patients and controls; however, a significant cortisol response was found in the controls, but not in the schizophrenic patients. The patients used more passive and avoidant coping strategies than controls. The findings provide support for the notion that schizophrenic patients have an impaired ability to adapt, both psychologically and biologically, to their environment.

Detrimental effects of chronic hypothalamic-pituitary-adrenal axis activation. From obesity to memory deficits

Increasing evidence suggests that the detrimental effects of glucocorticoid (GC) hypersecretion occur by activation of the hypothalamic-pituitary-adrenal (HPA) axis in several human pathologies, including obesity, Alzheimer's disease, AIDS dementia, and depression. The different patterns of response by the HPA axis during chronic activation are an important consideration in selecting an animal model to assess HPA axis function in a particular disorder. This article will discuss how chronic HPA axis activation and GC hypersecretion affect hippocampal function and contribute to the development of obesity. In the brain, the hippocampus has the highest concentration of GC receptors. Chronic stress or corticosterone treatment induces neuropathological alterations, such as dendritic atrophy in hippocampal neurons, which are paralleled by cognitive deficits. Excitatory amino acid (EAA) neurotransmission has been implicated in chronic HPA axis activation. EAAs play a major role in neuroendocrine regulation. Hippocampal dendritic atrophy may involve alterations in EAA transporter function, and decreased EAA transporter function may also contribute to chronic HPA axis activation. Understanding the molecular mechanisms of HPA axis activation will likely advance the development of therapeutic interventions for conditions in which GC levels are chronically elevated.

Differential sensitivity to ACTH, but not stress, in two sources of outbred Sprague-Dawley rats

Adrenocorticotropic hormone (ACTH) is the major regulator of adrenocortical steroidogenesis in mammals. By comparing the sensitivity to ACTH of isolated adrenocortical cells from two sources of the same strain (Sprague-Dawley, SD) of outbred rats, we have identified a source of rat with low sensitivity to ACTH in vitro. Cells isolated from Holtzman SD rats had a high sensitivity to ACTH (minimal effective concentration 50 pg/ml), whereas Taconic SD rats had a low sensitivity (minimal effective concentration 250 pg/ml; maximal steroidogenesis < 50% of Holtzman cells). The responsiveness to analogues of cyclic adenosine monophosphate and cholesterol was also significantly lower in Taconic SD rats. Taconic adrenals were smaller, had significantly more mitochondria per cell, but approximately 20% less total lipid droplet volume per cell. There was no difference in latency to ACTH in vitro; however, steroidogenesis plateaued in Taconic cells after 25 min, while Holtzman cells secreted corticosterone almost linearly for at least 120 min. By contrast, the cyclic adenosine monophosphate secretion increased at the same rate for at least 120 min in cells from both sources. There were no differences between cells from the two sources in immunoreactive steroidogenic enzyme content. In vivo, the magnitude of the ACTH and corticosterone responses to two types of stress were similar in both sources. The thymus glands of Holtzman rats were significantly larger than those of Taconic rats. It is concluded that: (1) reduced sensitivity to ACTH in vitro in Taconic SD rats results from differences in the later stages of the steroidogenic pathway; (2) factors in addition to ACTH are required for maximal steroidogenesis in Taconic SD rats: (3) a comparison of the steroidogenic pathways in adrenal cells from these two sources of outbred rats should be useful in further delineating the relative importance of putative intracellular signalling mechanisms involved in initiation and maintenance of steroidogenesis, and (4) these data suggest that different sources of the same strain of rats sufficiently diverge over time to become separate strains ('substrains'). Overreliance on a single source of laboratory rodent may obscure natural variability in endocrine responses to stress and provide a misleading indication of homogeneity of responses.

Psychoneuroendocrinology of depression. Hypothalamic-pituitary-adrenal axis

Among the more consistent observations in patients with major depression is dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis presenting as elevation of basal cortisol, dexamethasone-mediated negative feedback resistance, increased cerebrospinal fluid levels of corticotropin-releasing factor (CRF), and a blunted adrenocorticotropic hormone (ACTH) response to challenge with exogenous CRF. These features appear to be state, rather than trait markers, and are normalized upon successful treatment. These pathophysiologic adaptations may arise from defects in central drive to the neuroendocrine hypothalamus, disruption of normal adrenocortical hormone receptor function or a modification of HPA axis function at any level. Functional assessment of the HPA axis is thought to provide a window into central nervous system operation that may be of diagnostic value in this and other affective disorders regardless of whether CRF and glucocorticoids are directly involved in the origin of major depression or merely exacerbate the consequences of other primary defects.

Do early-life events permanently alter behavioral and hormonal responses to stressors?

Early-life stimulation (e.g., brief handling) attenuates the behavioral and neuroendocrine responses to stressors encountered in adulthood, particularly with respect to activation of hypothalamic-pituitary-adrenal (HPA) activity. In contrast, if neonates were subjected to a more severe stressor, such as protracted separation from the dam or exposure to an endotoxin, then the adult response to a stressor was exaggerated. These early-life experiences program HPA functioning, including negative feedback derived from stimulation of hippocampal glucocorticoid receptors, and corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) coexpression in PVN neurons, to modify the response to subsequent stressor experiences. The persistent variations of HPA activity observed in handled/stimulated animals may stem from alterations in dam-pup interactions (e.g. increased arched-back feeding, licking, grooming). In addition genetic makeup is critical in determining stress reactivity. For instance, BALB/cByJ mice are more reactive to stressors than C57BL/6ByJ mice, exhibiting greater HPA hormonal alterations and behavioral disturbances. BALB/cByJ also fail to acquire a spatial learning response in a Morris water-maze paradigm, which has been shown to be correlated with hippocampal cell loss associated with aging. Early-life handling of BALB/cByJ mice prevented these performance deficits and attenuated the hypersecretion of ACTH and corticosterone elicited by stressors. The stressor reactivity may have been related to maternal and genetic factors. When BALB/cByJ mice were raised by a C57BL/6ByJ dam, the excessive stress-elicited HPA activity was reduced, as were the behavioral impairments. However, cross-fostering the more resilient C57BL/6ByJ mice to a BALB/cByJ dam failed to elicit the behavioral disturbances. It is suggested that genetic factors may influence dam-pup interactive styles and may thus proactively influence the response to subsequent stressors among vulnerable animals. In contrast, in relatively hardy animals the early-life manipulations may have less obvious effects.

Anterior pituitary response to stress: time-related changes and adaptation

A wide array of physical and psychological stressors alter the secretion of anterior pituitary hormones. However, both the qualitative and the quantitative features of the stressors as well as its duration markedly influence the final endocrine response. In addition, among all anterior pituitary hormones, only ACTH and prolactin levels appear to reflect the intensity of the stress experienced by the animals. Although physical stressors show a somewhat specific neuroendocrine profile, the response of the pituitary-adrenal (PA) and sympathomedulloadrenal axes are common to almost all stressors. After an initial stimulatory effect of stress, an inhibition of all anterior pituitary hormones, except ACTH, can be found provided the stressor is intense enough. The mechanisms responsible for this biphasic response to stress are likely to be located at sites above the pituitary. When the animals are repeatedly exposed to the same stressor, some behavioural and physiological consequences of stress exposure are reduced, suggesting that the animals become adapted to the stimulus. This process has been also termed habituation. Among all the pituitary hormones, only ACTH and prolactin levels are reduced as a consequence of repeated exposure to the same (homotypic) stressor, although some negative results have been reported. However, it has been recently reported that subtle changes in the characteristics of the stressors or in their regularity can greatly influence adaptation, and these factors might explain failure to find adaptation of ACTH and prolactin in some works. Habituation of ACTH and prolactin, when observed, appears to be specific for the chronically applied stressor so that the potentiality of the PA axis and prolactin to respond to a novel (heterotypic) stressor can be preserved. In the case of the PA axis, an intact or potentiated response to a novel stressor is observed in spite of presumably negative feedback exerted by daily stress-induced glucocorticoid release and the high resting levels of glucocorticoids. This phenomenon has been termed as facilitation and can be unmasked alternating stress. Although with the exception of the PA axis, developmental aspects of anterior pituitary response to stress have been poorly studied, available data suggest that dramatic changes occur in some hormones during weaning, with some, but less profound, change thereafter. Responsiveness to stressors appears to mature with age, but developmental patterns differ among the various anterior pituitary hormones.

Corticosterone release in response to repeated, short episodes of neonatal isolation: evidence of sensitization

Repeated isolation of neonatal rats produces persistent changes in physiology and behavior. In Experiment 1, we examined changes in plasma corticosterone (CORT) levels as a possible mechanism for the effects of isolation. Pups that were isolated from their mother and the nest for 1 h per day on postnatal days (PND) 2-9 were compared to control litters of pups that were either nonhandled or handled but not isolated. On PND 2, compared to nonhandled pups, handled pups had elevated CORT levels that returned to baseline levels within 30 to 60 min of return to the home cage. No significant elevation of CORT levels were found in handled pups on PND 9. The CORT levels of isolated pups were over twice those of nonhandled pups on PND 2 and four times those of nonhandled pups on PND 9. In Experiment 2, we investigated whether the increased CORT release in response to isolation on PND 9 was the result of the pups' treatment on the previous six days as against an effect of maturation. Plasma CORT levels were measured in rat pups that were either isolated, handled or nonhandled on PNDs 2-8 during the conditions of isolation, handling and nonhandling on PND 9. There were no differences among the groups in basal plasma levels of CORT. Handling on PND 9 did not result in elevated CORT levels in any of the groups. All three groups showed a significant increase in plasma CORT levels after isolation on PND 9. However, the CORT response to isolation of pups previously isolated on PND 2-8 were significantly higher than pups that were either handled or nonhandled on PNDs 2-8. Thus, daily episodes of isolation potentiate the hypothalamic-pituitary-adrenal response to stress.

Differential effects of neonatal handling on anxiety, corticosterone response to stress, and hippocampal glucocorticoid and serotonin (5-HT)2A receptors in Lewis rats

Neonatal handling (during the first 3 weeks of age) has been reported by others to diminish the hypothalamo-pituitary-adrenal (HPA) responsivity to stress in adult Long Evans rats, an effect involving a serotonin (5-HT)2A receptor-mediated increase in glucocorticoid receptor (GR) gene expression in the frontal cortex and the hippocampus. In addition, handled animals may also display enduring reductions in anxiety-related behaviours, including in the elevated plus-maze. We have thus analysed the aforementioned neuroendocrine and behavioural consequences of neonatal stress in male and female adult Lewis rats, a strain characterised by its high anxiety and its hyporesponsive HPA axis. Plasma corticosterone, but not behavioural, responses to an elevated plus-maze test were decreased in handled rats. Besides, hippocampal mineralocorticoid receptor (MR) and GR binding capacities were not different between handled and non-handled Lewis rats, an observation which could be extended to our adult Long Evans rats. Lastly, neither hippocampal nor cortical 5-HT2A receptor binding capacities in adult Lewis rats were affected by prior handling. In keeping with the failure to detect early handling-induced increases in hippocampal GR binding in 3-week old Lewis and Long Evans rats, the present study reinforces past findings indicating that environmental and genetic factors are crucial variables in the neonatal handling paradigm.

Maternal care during infancy regulates the development of neural systems mediating the expression of fearfulness in the rat

The mothers of infant rats show individual differences in the frequency of licking/grooming and arched-back nursing (LG-ABN) of pups that contribute to the development of individual differences in behavioral responses to stress. As adults, the offspring of mothers that exhibited high levels of LG-ABN showed substantially reduced behavioral fearfulness in response to novelty compared with the offspring of low LG-ABN mothers. In addition, the adult offspring of the high LG-ABN mothers showed significantly (i) increased central benzodiazepine receptor density in the central, lateral, and basolateral nuclei of the amygdala as well as in the locus ceruleus, (ii) increased alpha2 adrenoreceptor density in the locus ceruleus, and (iii) decreased corticotropin-releasing hormone (CRH) receptor density in the locus ceruleus. The expression of fear and anxiety is regulated by a neural circuitry that includes the activation of ascending noradrenergic projections from the locus ceruleus to the forebrain structures. Considering the importance of the amygdala, notably the anxiogenic influence of CRH projections from the amygdala to the locus ceruleus, as well as the anxiolytic actions of benzodiazepines, for the expression of behavioral responses to stress, these findings suggest that maternal care during infancy serves to "program" behavioral responses to stress in the offspring by altering the development of the neural systems that mediate fearfulness.