Influence of neonatal rearing conditions on stress-induced adrenocorticotropin responses and norepinepherine release in the hypothalamic paraventricular nucleus.

Postnatal rearing conditions influence the development of hypothalamic-pituitary-adrenal (HPA) responses to stress in the rat. Thus, postnatal handling dampens HPA responsivity to stress, while prolonged periods of maternal separation have the opposite effect. HPA responses to stress are initiated by the release of corticotropin-releasing factor and/or arginine vasopressin from the neurones of the paraventricular nucleus of the hypothalamus (PVNh). A major source of input to the PVNh arises from brainstem noradrenergic neurones with signalling occurring via alpha1 adrenoreceptors. We examined the noradrenergic response to stress in the PVNh in adult animals exposed to daily periods of handling or maternal separation over the first 2 weeks of life using microdialysis in conscious animals. Maternal separation increased, while handling greatly decreased and norepinepherine responses to restraint stress in the PVNh as compared to non-handled controls; the same pattern was observed for plasma adrenocorticotropic hormone (ACTH) responses to stress. Rearing condition did not affect either alpha1 or alpha2 receptor levels in the PVNh. However, alpha2 receptor binding levels in the noradrenergic cell body regions of the locus coeruleus and the n. tractus solitarius were significantly increased in handled animals. These alpha2 receptors are principally located on noradrenergic neurones (i.e. autoreceptors) and inhibit noradrenaline release at terminal sites. The effects on alpha2 receptor levels could serve as a mechanism for the differences in stress-induced noradrenaline levels in the PVNh and in HPA activity among handled vs non-handled and maternal separation animals. Thus, early life events may serve to influence the differentiation of noradrenergic neurones and thus alter HPA responses stress in adulthood.

The effects of early rearing environment on the development of GABAA and central benzodiazepine receptor levels and novelty-induced fearfulness in the rat.

We compared the effects of handling or maternal separation from the day following birth until postnatal day 14 on behavioral responses to novelty and on GABAA and central benzodiazepine (CBZ) receptor levels in the rat. As adults, handled animals showed reduced startle responsivity, increased exploration in a novel open field, and decreased novelty-induced suppression of feeding relative to the handled (H) and/or maternal separation (MS) groups. As compared with handled animals, both nonhandled (NH) and MS animals displayed: (1) reduced GABAA receptor levels in the locus coeruleus (LC) and the n. tractus solitarius (NTS); (2) reduced CBZ receptor sites in the central and lateral n. of the amygdala, the frontal cortex, and in the LC and NTS; and (3) reduced levels of the mRNA for the gamma 2 subunit of the GABAA receptor complex, which confers high affinity BZ binding, in the amygdaloid nuclei as well as in the LC and NTS. Both the amygdala and the ascending noradrenergic systems have been considered as critical sites for the anxiolytic effects of benzodiazepines. These data suggest that early life events influence the development of the GABAA receptor system, thus altering the expression of fearfulness in adulthood.

Variations in maternal care in infancy regulate the development of stress reactivity.

Naturally occurring variations in maternal care in early postnatal life are associated with the development of individual differences in behavioral and hypothalamic-pituitary-adrenal responses to stress in the rat. These effects appear to be mediated by the influence of maternal licking/grooming on the development of central systems that serve to activate (corticotropin-releasing factor) or inhibit (gamma-aminobutyric acid) the expression of behavioral and endocrine responses to stress through effects on forebrain noradrenergic systems. Importantly, individual differences in maternal care are transmitted from mother to daughter, providing a mechanism for the behavioral transmission of individual differences in stress reactivity across generations.

The role of corticotropin-releasing factor--norepinephrine systems in mediating the effects of early experience on the development of behavioral and endocrine responses to stress.

Naturally occurring variations in maternal care in early postnatal life are associated with the development of individual differences in behavioral and hypothalamic-pituitary-adrenal responses to stress in the rat. These effects appear to be mediated by the influence of maternal licking and grooming on the development of central corticotropin-releasing factor (CRF) systems, which regulate the expression of behavioral, endocrine, and autonomic responses to stress through activation of forebrain noradrenergic systems. These findings provide a neurobiologic basis for the observed relationship between early life events and health in adulthood. In more recent studies, we explored the behavioral transmission of individual differences in stress reactivity, and thus, vulnerability to stress-induced illness, across generations.

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.

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

Variations in maternal care affect the development of individual differences in neuroendocrine responses to stress in rats. As adults, the offspring of mothers that exhibited more licking and grooming of pups during the first 10 days of life showed reduced plasma adrenocorticotropic hormone and corticosterone responses to acute stress, increased hippocampal glucocorticoid receptor messenger RNA expression, enhanced glucocorticoid feedback sensitivity, and decreased levels of hypothalamic corticotropin-releasing hormone messenger RNA. Each measure was significantly correlated with the frequency of maternal licking and grooming (all r's > -0.6). These findings suggest that maternal behavior serves to "program" hypothalamic-pituitary-adrenal responses to stress in the offspring.

The effects of different types of pre-training on the rat's retention performance in a swim-to-platform task following administration of scopolamine.

Previous research has found that centrally acting antimuscarinic drugs strongly impair the acquisition of a variety of learned behaviors in rats but have little effect on these same behaviors if training is given prior to drug treatment. We gave groups of rats different types of pre-training followed by treatment with scopolamine hydrobromide and subsequent testing on a simple swim-to-platform test. Factors such as practice in swimming without a platform to escape to, or learning to swim to a platform in a different apparatus or even to the test platform located in a different place did not protect the rats from the behavioral disruption produced by scopolamine. However, five training trials on the specific swim-to-platform task used in the retention test afforded almost complete protection against the effect of scopolamine. It appears that the protective effect of pre-training is highly specific and does not involve acquisition of some type of general rule which might survive antimuscarinic blockade.

Early environmental regulation of forebrain glucocorticoid receptor gene expression: implications for adrenocortical responses to stress.

The adrenal glucocorticoids and catecholamines comprise a frontline of defense for mammalian species under conditions which threaten homeostasis (conditions commonly referred to as stress). Glucocorticoids represent the end product of the hypothalamic-pituitary-adrenal (HPA) axis and along with the catecholamines serve to mobilize the production and distribution of energy substrates during stress. The increased secretion of pituitary-adrenal hormones in response to stress is stimulated by the release of corticotropin-releasing hormone (CRH) and/or arginine vasopressin (AVP) from neurons in the nucleus paraventricularis. In this way, a neural signal associated with the stressor is transduced into a set of endocrine and sympathetic responses. The development of the HPA response to stressful stimuli is altered by early environmental events. Animals exposed to short periods of infantile stimulation or handling show decreased HPA responsivity to stress, whereas maternal separation, physical trauma and endotoxin administration enhance HPA responsivity to stress. In all cases, these effects persist throughout the life of the animal and are accompanied by increased hypothalamic levels of the mRNAs for CRH and often AVP. The inhibitory regulation of the synthesis for these ACTH releasing factors is achieved, in part, through a negative feedback loop whereby circulating glucocorticoids act at various neural sites to decrease CRH and AVP gene expression. Such inhibitory effects are initiated via an interaction between the adrenal steroid and an intracellular receptor (either the mineralocorticoid or glucocorticoid receptor). We have found that these early environmental manipulations regulate glucocorticoid receptor gene expression in the hippocampus and frontal cortex, regions that have been strongly implicated as sites for negative-feedback regulation of CRH and AVP synthesis. When the differences in glucocorticoid receptor density are transiently reversed, so too are those in HPA responses to stress. Taken together, our findings indicate that the early postnatal environment alters the differentiation of hippocampal neurons. This effect involves an altered rate of glucocorticoid receptor gene expression, resulting in changes in the sensitivity of the system to the inhibitory effects of glucocorticoids on the synthesis of CRH and AVP in hypothalamic neurons. Changes in CRH and AVP levels, in turn, determine the responsivity of the axis to subsequent stressors; increased releasing factor production is associated with increased HPA responses to stress. Thus, the early environment can contribute substantially to the development of stable individual differences in HPA responsivity to stressful stimuli. These data provide examples of early environmental programming of neural systems. One major objective of our research is to understand how such programming occurs within the brain.