Knockdown of the glucocorticoid receptor alters functional integration of newborn neurons in the adult hippocampus and impairs fear-motivated behavior.

Glucocorticoids (GCs) secreted after stress reduce adult hippocampal neurogenesis, a process that has been implicated in cognitive aspects of psychopathology, amongst others. Yet, the exact role of the GC receptor (GR), a key mediator of GC action, in regulating adult neurogenesis is largely unknown. Here, we show that GR knockdown, selectively in newborn cells of the hippocampal neurogenic niche, accelerates their neuronal differentiation and migration. Strikingly, GR knockdown induced ectopic positioning of a subset of the new granule cells, altered their dendritic complexity and increased their number of mature dendritic spines and mossy fiber boutons. Consistent with the increase in synaptic contacts, cells with GR knockdown exhibit increased basal excitability parallel to impaired contextual freezing during fear conditioning. Together, our data demonstrate a key role for the GR in newborn hippocampal cells in mediating their synaptic connectivity and structural as well as functional integration into mature hippocampal circuits involved in fear memory consolidation.

Stress and estrous cycle affect strategy but not performance of female C57BL/6J mice.

Stress induces a switch in learning strategies of male C57BL/6J mice from predominantly spatial to more stimulus-response learning. To study generalization of these findings over sex, we investigated female C57BL/6J mice at three phases of the estrous cycle under non stress and acute (10 min) restraint stress conditions. On a circular hole board (CHB) task, about half of the naive female mice used spatial and stimulus-response strategies to solve the task. Under stress, female mice favored spatial over stimulus-response strategies, with 100% of female mice in the estrus phase. Performance expressed as latency to solve the task is only improved in stressed female mice in the estrus phase. We conclude that the use of learning strategies is influenced by sex and this difference between sexes is aggravated by acute stress.

Sex differences in fear memory and extinction of mice with forebrain-specific disruption of the mineralocorticoid receptor.

Previous studies showed that the mineralocorticoid receptor (MR) is needed for behavioral flexibility in a fear conditioning paradigm. Female mice with forebrain-specific deletion of the MR gene (MR(CaMKCre) ) were unable to show extinction of contextual fear, and could not discriminate between cue and context fear unlike control mice. In the present study, male and female (MR(CaMKCre) ) mice and control littermates were used to study sex-specific fear conditioning, memory performance and extinction. The fear conditioning paradigm assessed both context- and cue-related fear within one experimental procedure. We observed that at the end of the conditioning all mice acquired the fear-motivated response. During the first minutes of the memory test, both male and female MR(CaMKCre) mice remembered and feared the context more than the control mice. Furthermore, female MR(CaMKCre) mice were not able to extinguish this memory even on the second day of memory testing. The female mutants also could not discriminate between cue (more freezing) and context periods (less freezing). In contrast, male MR(CaMKCre) mice and the controls showed extinction and were capable to discriminate, although the MR(CaMKCre) mice needed more time before they started extinction. These findings further support the relevance of MR for behavioral flexibility and extinction of fear-motivated behavior. In conclusion, the loss of MR in the forebrain results in large differences in emotional and cognitive behaviors between female and male mice, which suggests a role of this receptor in the female prevalence of stress- and anxiety-regulated disorders.

Stress or no stress: mineralocorticoid receptors in the forebrain regulate behavioral adaptation.

Corticosteroid effects on cognitive abilities during behavioral adaptation to stress are mediated by two types of receptors. While the glucocorticoid receptor (GR) is mainly involved in the consolidation of memory, the mineralocorticoid receptor (MR) mediates appraisal and initial responses to novelty. Recent findings in humans and mice suggest that under stress, the MR might be involved in the use of different learning strategies. Here, we used male mice lacking the MR in the forebrain (MR(CaMKCre)), which were subjected to 5-10 min acute restraint stress, followed 30 min later by training trials on the circular hole board. Mice had to locate an exit hole using extra- and intra-maze cues. We assessed performance and the use of spatial and stimulus-response strategies. Non-stressed MR(CaMKCre) mice showed delayed learning as compared to control littermates. Prior stress impaired performance in controls, but did not further deteriorate learning in MR(CaMKCre) mice. When stressed, 20-30% of both MR(CaMKCre) and control mice switched from a spatial to a stimulus-response strategy, which rescued performance in MR(CaMKCre) mice. Furthermore, MR(CaMKCre) mice showed increased GR mRNA expression in all CA areas of the hippocampus and an altered basal and stress-induced corticosterone secretion, which supports their role in the modulation of neuroendocrine activity. In conclusion, our data provide evidence for the critical role of MR in the fast formation of spatial memory. In the absence of forebrain MR spatial learning performance was under basal circumstances impaired, while after stress further deterioration of performance was rescued by switching behavior increasingly to a stimulus-response strategy.

Relevance of stress and female sex hormones for emotion and cognition.

There are clear sex differences in incidence and onset of stress-related and other psychiatric disorders in humans. Yet, rodent models for psychiatric disorders are predominantly based on male animals. The strongest argument for not using female rodents is their estrous cycle and the fluctuating sex hormones per phase which multiplies the number of animals to be tested. Here, we will discuss studies focused on sex differences in emotionality and cognitive abilities in experimental conditions with and without stress. First, female sex hormones such as estrogens and progesterone affect emotions and cognition, contributing to sex differences in behavior. Second, females respond differently to stress than males which might be related to the phase of the estrous cycle. For example, female rats and mice express less anxiety than males in a novel environment. Proestrus females are less anxious than females in the other estrous phases. Third, males perform in spatial tasks superior to females. However, while stress impairs spatial memory in males, females improve their spatial abilities, depending on the task and kind of stressor. We conclude that the differences in emotion, cognition and responses to stress between males and females over the different phases of the estrous cycle should be used in animal models for stress-related psychiatric disorders.

Recovery from disrupted ultradian glucocorticoid rhythmicity reveals a dissociation between hormonal and behavioural stress responsiveness.

Ultradian release of glucocorticoids is thought to be essential for homeostasis and health. Furthermore, deviation from this pulsatile release pattern is considered to compromise resilience to stress-related disease, even after hormone levels have normalised. In the present study, we investigate how constant exposure to different concentrations of corticosterone affects diurnal and ultradian pulsatility. The rate of recovery in pulsatile hypothalamic-pituitary-adrenal (HPA) activity after withdrawal of exogenous corticosterone is also examined. Finally, the behavioural and neuroendocrine responsiveness to an audiogenic stressor is studied. Adrenally intact male rats were subcutaneously implanted with vehicle, 40% or 100% corticosterone pellets for 7 days. The continuous release of corticosterone from these implants abolished diurnal and ultradian corticosterone variation, as measured with high-frequency automated blood sampling. Pellet removal on post-surgery day 8 allowed rapid recovery of endogenous rhythms in animals previously exposed to daily average concentrations (40%) but not after exposure to high concentrations (100%) of corticosterone. Behavioural and neuroendocrine responsiveness to stress was distinctly different between the treatment groups. Audiogenic stimulation 1 day after pellet removal resulted in a similar corticosterone response in animals previously exposed to 40% corticosterone or vehicle. The 40% pellet group, however, showed less and shorter behavioural activity (i.e. locomotion, risk assessment) to noise stress compared to 100% corticosterone and vehicle-treated animals. In conclusion, unlike the animals impanted with 100% corticosterone, we find that basal HPA axis activity in the 40% group, which had mean daily levels of circulating corticosterone in the physiological range, rapidly reverts to the characteristic pulsatile pattern of corticosterone secretion. Upon reinstatement of the ultradian rhythm, and despite the fact that these animals did not differ from controls in their response to noise stress, they did show substantial changes in their behavioural response to stress.

Ontogeny of the HPA axis of the CD1 mouse following 24 h maternal deprivation at pnd 3.

One of the striking characteristics of the developing neuroendocrine system of rats and mice is the stress hypo-responsive period (SHRP), i.e. low basal corticosterone secretion and the inability to increase corticosterone in response to mild stressors during the first 2 weeks of life. However, immediately after 24 h of deprivation from maternal care the response of the hypothalamic-pituitary-adrenal (HPA) axis to mild stressors is enhanced. This study examines in CD1 mouse pups the recovery pattern of markers of HPA axis (re)activity from maternal deprivation (once for 24 h from postnatal days (pnds) 3 to 4). As expected, deprivation induced a profound corticosterone response to novelty immediately after deprivation. In contrast, 1 day after reunion with the mother (pnd 5), this effect was abolished, lasting for at least 3 days. Basal corticosterone remained even below control levels. Corticotropin-releasing hormone (CRH) mRNA expression in the hypothalamic paraventricular nucleus (PVN) was suppressed for 2 days, exceeded control levels at pnds 7 and 8, and subsequently followed the gradual decline observed in controls until pnd 12. Delayed and rather short-lasting changes were found for adrenocorticotropic hormone (low at pnd 5), and glucocorticoid receptor mRNA expression (decreased in the PVN at pnd 4, and in the hippocampal CA1 area at pnd 5). Hippocampal mineralocorticoid receptor mRNA expression was unaffected. From pnds 9 to 13, both deprived and control pups gradually emerged from the SHRP in a similar temporal pattern. In conclusion, maternal deprivation at pnd 3 augments hypo-responsiveness of corticosterone secretion to mild stress for several days, but does not affect the duration of the SHRP. Whether CRH and glucocorticoid receptor changes are cause or consequence remains to be established.

Mineralocorticoid receptors in control of emotional arousal and fear memory.

The stress hormone corticosterone acts via two receptor types in the brain: the mineralocorticoid (MR) and the glucocorticoid receptor (GR). Both receptors are involved in processing of stressful events. A disbalance of MR:GR functions is thought to promote stress-related disorders. Here we studied the effect of stress on emotional and cognitive behaviors in mice with forebrain-specific inactivation of the MR gene (MR(CaMKCre), 4 months old; and control littermates). MR(CaMKCre) mice responded to prior stress (5 min of restraint) with higher arousal and less locomotor activity in an exploration task. A fear conditioning paradigm allowed assessing in one experimental procedure both context- and cue-related fear. During conditioning, MR(CaMKCre) mice expressed more cue-related freezing. During memory test, contextual freezing remained potentiated, while control mice distinguished between cue (more freezing) and context episodes (less freezing) in the second memory test. At this time, plasma corticosterone levels of MR(CaMKCre) mice were 40% higher than in controls. We conclude that control of emotional arousal and adaptive behaviors is lost in the absence of forebrain MR, and thus, anxiety-related responses are and remain augmented. We propose that such a disbalance in MR:GR functions in MR(CaMKCre) mice provides the conditions for an animal model for anxiety-related disorders.

Corticosterone facilitates extinction of fear memory in BALB/c mice but strengthens cue related fear in C57BL/6 mice.

Corticosterone, the naturally occurring glucocorticoid of rodents is secreted in response to stressors and is known for its facilitating and detrimental effects on emotional learning and memory. The large variability in the action of corticosterone on processing of emotional memories is postulated to depend on genetic background and the spatio-temporal domain in which the hormone operates. To address this hypothesis, mice of two strains with distinct corticosterone secretory patterns and behavioural phenotype (BALB/c and C57BL/6J) were treated with corticosterone (250 microg/kg, i.p.), either 5 min before or directly after acquisition in a fear conditioning task. As the paradigm allowed assessing in one experimental procedure both context- and cue-related fear behaviour, we were able to detect generalization and specificity of fear. BALB/c showed generalized strong fear memory, while C57BL/6J mice discriminated between freezing during context- and cue episodes. Corticosterone had opposite effects on fear memory depending on the strain and time of injection. Corticosterone after acquisition did not affect C57BL/6J mice, but destabilized consolidation and facilitated extinction in BALB/c. Corticosterone 5 min before acquisition strengthened stress-associated signals: BALB/c no longer showed lower fear memory, while C57BL/6J mice displayed increased fear memory and impaired extinction in cue episodes. We propose that corticosterone-induced facilitation of fear memory in C57BL/6J mice can be used to study the development of fear memories, corticosterone administration in BALB/c mice presents a model to examine treatment. We conclude that genetic background and time of corticosterone action are modifiers of fear memory with interesting translational implications for anxiety-related diseases.

Hypothalamic-pituitary-adrenal axis activity of newborn mice rapidly desensitizes to repeated maternal absence but becomes highly responsive to novelty.

In CD1 mice we investigated the hypothalamic-pituitary-adrenal (HPA) axis response to maternal separation for 8 h daily from postnatal d 3 to 5. At d 3 a slow separation-induced corticosterone response developed that peaked after 8 h, and the pups became responsive to stressors. On the second and third day, the response to 8 h separation rapidly attenuated, whereas the response to novelty did not, a pattern reflected by the hypothalamic c-fos mRNA response. If maternal separation and exposure to novelty were combined, then after the third such daily exposure, the sensitivity to the stressor was further enhanced. Meanwhile, basal corticosterone and ACTH levels were persistently suppressed 16 h after pups were reunited with their mothers. To explain the HPA axis desensitization after repeated separation, we found that circulating ghrelin levels increased and glucose levels decreased after all periods of maternal separation, ruling out a role of altered metabolism. Glucocorticoid feedback was not involved either because a glucocorticoid receptor antagonist amplified the corticosterone response after the first but became ineffective after the third separation. In contrast, a mineralocorticoid receptor antagonist decreased and increased corticosterone levels after the first and third period of separation, respectively. In conclusion, the newborn's HPA axis readily desensitizes to repeated daily maternal separation, but continues to respond to novelty in a manner influenced by a central mineralocorticoid receptor- rather than glucocorticoid receptor-mediated mechanism.

Differential development of stress system (re)activity at weaning dependent on time of disruption of maternal care.

Maternal deprivation, a separation of mother and pups for 24 h in the first weeks of life has long-lasting consequences for the glucocorticoid stress system in rats. We examined in male CD1 mice whether the postnatal day (pnd) of deprivation determines the (re)activity of the stress system at weaning under basal and novelty stress conditions. Maternal deprivation was only effective when applied within the stress hypo-responsive period (SHRP) between pnds 1 and 12, but not on pnd 13. Maternal deprivation (i) early in the SHRP (pnd 3) resulted in lower hippocampal GR mRNA expression together with a prolonged corticosterone response to stress; while (ii) late in the SHRP (pnd 8) the amplitude of the ACTH response to stress was enhanced. (iii) Strikingly, the effects of the double deprivation (pnds 3 and 8) were not additive: sustained, stress non-responsive high plasma ACTH concentrations with corticosterone indistinguishable from control animals coincided with a lower expression of hippocampal MR and GR mRNA. These results present species-specific effects (mouse versus rat) of an adverse early life event on HPA axis regulation at weaning. A subsequent deprivation experience interferes with the effects of earlier deprivation. We conclude that the developmental stage of the organism determines the vulnerability for the detrimental effects of maternal deprivation and the organization of the stress system in adolescence.

Effects of maternal deprivation of CD1 mice on performance in the water maze and swim stress.

Rat pups subjected to a single 24h maternal deprivation show altered stress responsiveness and cognitive performance in the water maze in adulthood. Here we show in 6-month-old male CD1 mice (deprived 24h at postnatal day 8) an initial impairment in reversal learning: relocating the platform revealed perseverance in search for the former location. Spatial learning, long-term memory and swim-induced corticosterone responses were not affected. We conclude that reduced flexibility is a subtle long-lasting behavioural change induced by maternal deprivation.

Strain specific fear behaviour and glucocorticoid response to aversive events: modelling PTSD in mice.

"Pavlovian" fear conditioning in rodents allows studying the formation and extinction of fear memories. Male C57BL/6J but not BALB/c mice showed differential fear memory performance expressed as freezing and scanning behaviour for context and cue. Glucocorticoid stress hormones modulate the processing of fear-related stimuli. The augmented corticosterone response of BALB/c mice to conditioning and testing, therefore, might have contributed to the strain-dependent formation of fear memories. We propose that modulation of extinction processes by glucocorticoids can be relevant in modelling anxiety disorders.

Differential disinhibition of the neonatal hypothalamic- pituitary-adrenal axis in brain-specific CRH receptor 1-knockout mice.

In the adult, corticotropin-releasing hormone (CRH) is the key mediator for the behavioural and neuroendocrine response to stress. It has also been hypothesized that, during postnatal development of the stress system, CRH controls the activity of the HPA axis and mediates the effects of early disturbances, e.g. 24 h of maternal deprivation. In the current study we investigated the function of specific brain corticotropin-releasing hormone receptor type 1 (CRHR1) subpopulations in the control of the HPA axis during postnatal development under basal conditions as well as after 24 h of maternal deprivation. We used two conditional CRHR1-deficient mouse lines which lack this receptor, either specifically in forebrain and limbic structures (Cam-CRHR1) or in all neurons (Nes-CRHR1). Basal circulating corticosterone was increased in Nes-CRHR1 mice compared to controls. Corticosterone response to maternal deprivation was significantly increased in both CRHR1-deficient lines. In the paraventricular nucleus, Cam-CRHR1 animals displayed enhanced CRH and decreased vasopressin expression levels. In contrast, gene expression in Nes-CRHR1 pups was strikingly similar to that in maternally deprived control pups. Furthermore, maternal deprivation resulted in an enhanced response of Cam-CRHR1 pups in the brain, while expression levels in Nes-CRHR1 mouse pups were mostly unchanged. Our results demonstrate that brainstem and/or hypothalamic CRHR1 contribute to the suppression of basal corticosterone secretion in the neonate, while limbic and/or forebrain CRHR1 dampen the activation of the neonatal HPA axis induced by maternal deprivation.

Correlations between hypothalamus-pituitary-adrenal axis parameters depend on age and learning capacity.

Glucocorticoid hormones are released after activation of the hypothalamus-pituitary-adrenal (HPA) axis and in the brain can modulate synaptic plasticity and memory formation. Clear individual differences in spatial learning and memory in the water maze allowed classification of groups of young (3 months) and aged (24 months) male Wistar rats as superior and inferior learners. We tested 1) whether measures of HPA activity are associated with cognitive functions and aging and 2) whether correlations of these measures depend on age and learning performance. Basal ACTH, but not corticosterone, was increased in aged rats, with the stress-induced ACTH response exaggerated in aged-inferior learners. Aged-superior learners had lower expression of glucocorticoid receptor and CRH mRNA in the parvocellular paraventricular nucleus of the hypothalamus compared with all other groups. Hippocampal mineralocorticoid receptor and glucocorticoid receptor mRNAs differed modestly between groups, but steroid receptor coactivator and heat-shock-protein 90 mRNAs were not different. Strikingly, correlations between HPA axis markers were dependent on grouping animals according to learning performance or age. CRH mRNA correlated with ACTH only in aged animals. Parvocellular arginine vasopressin mRNA was negatively correlated to basal corticosterone, except in aged-inferior learners. Corticosteroid receptor mRNA expression showed a number of correlations with other HPA axis regulators specifically in superior learners. In summary, the relationships between HPA axis markers differ for subgroups of animals. These distinct interdependencies may reflect adjusted set-points of the HPA axis, resulting in adaptation (or maladaptation) to the environment and, possibly, an age-independent determination of learning ability.

Glucocorticoid receptor blockade disinhibits pituitary-adrenal activity during the stress hyporesponsive period of the mouse.

During postnatal development, mice undergo a period of reduced responsiveness of the pituitary-adrenal axis, the stress hyporesponsive period (SHRP), which is largely under control of maternal signals. The present study was designed to test the hypothesis that this quiescence in hypothalamic-pituitary-adrenal (HPA) activity is mediated by glucocorticoid feedback. For this purpose, the role of mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) in control of HPA activity was examined during the SHRP and in response to 24 h of maternal deprivation. Nondeprived or deprived (24 h) CD1 mice on postnatal d 8 were injected sc at 16 and 8 h before testing with the MR antagonist RU28318 or the GR antagonist RU38486. The results showed that, in nondeprived mice, blockade of GR rather than MR triggered a profound increase in anterior pituitary proopiomelanocortin mRNA, circulating ACTH, and corticosterone concentrations. In contrast, CRH mRNA in hypothalamus and GR mRNA in hippocampus and hypothalamus were decreased. Blockade of the GR during the deprivation period amplified the rise in corticosterone induced by maternal deprivation, whereas it reversed the deprivation effect on the other HPA markers, leading to profound increases in plasma ACTH, proopiomelanocortin mRNA expression in the anterior pituitary, CRH mRNA expression in the paraventricular nucleus, and MR mRNA expression in the hippocampus, but not in GR mRNA expression in the hippocampus and paraventricular nucleus. In conclusion, the data suggest that control of postnatal pituitary-adrenal activity during the SHRP involves GR-mediated feedback in the anterior pituitary, which is further potentiated in the absence of the mother.

Swim posture of mice does not affect performance in the water maze.

We quantified swim postures of mice in relation to their cognitive performance. After training in a water maze, young (5-6 months) and aged (14-16 months) female apolipoprotein E-knockout (apoE0/0) mice and wild type controls were video taped while swimming. Subsequently, angles of body points with the water surface were calculated. Mice with a more horizontal swim posture (young and aged apoE0/0, aged wild type mice) also showed an increased body weight. However, swim posture was not related to cognitive performance.

The dynamics of the hypothalamic-pituitary-adrenal axis during maternal deprivation.

A close contact between the dam and the litter is essential for the normal development of the hypothalamic-pituitary-adrenal (HPA) axis in rats and mice. Maternal signals, as licking and feeding, have been shown to sustain the HPA axis of the pups in a hypo-responsive state. Disruption of this mother-pup interaction by 24 h of maternal deprivation activates the otherwise quiescent stress system of the neonates, resulting in an enhanced adrenal sensitivity to adrenocorticotropic hormone (ACTH) and a decreased expression of central HPA markers, such as corticotropin-releasing hormone (CRH). However, the dynamics of these central and peripheral changes over the 24h period are largely unknown. In this study, we examined the time course of some of the central and peripheral indices of HPA activity during 24 h of maternal deprivation. We measured corticosterone and ACTH in the blood as well as CRH, mineralocorticoid and glucocorticoid receptor expression in the brain. Our results demonstrate that each of the components of the HPA axis responds to maternal deprivation at different time points following removal of the mother and with a very specific time course. The main activation of the HPA axis occurred between 4 h and 8 h of maternal absence. By contrast, during the second half of the deprivation period, negativefeedback mechanisms restrained the further increase in ACTH and corticosterone release. We conclude that maternal deprivation triggers a cascade of sequential changes at the various levels of the stress system, and that measuring only one aspect of the system at one time point does not accurately reflect the dynamic alterations of the HPA axis.

Regulation of the developing hypothalamic-pituitary-adrenal axis in corticotropin releasing hormone receptor 1-deficient mice.

During postnatal development, mice undergo a so-called stress hyporesponsive period, which is characterized by low basal corticosterone levels and the inability of mild stressors to induce a corticosterone response. The stress hyporesponsiveness is in part regulated by maternal factors. Twenty-four hours of deprivation results in an activation of basal and stress-induced corticosterone and a down-regulation of corticotropin releasing hormone (CRH), mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) expression in the brain. It has been hypothesized that the CRH receptor 1 (CRHr1) may play an important regulatory role during development by mediating the effects of maternal deprivation. Using CRHr1-deficient mice we examined the role of this receptor on the maternal deprivation effects and in regulating the expression of hypothalamic-pituitary-adrenal axis-related genes. We could demonstrate that the CRHr1 is essential for the activation of the corticosterone response following maternal deprivation, most likely due to the lack of the receptor in the pituitary. Furthermore, we could show that the CRHr1 is regulating the expression of CRH and MRs. In contrast, effects of maternal deprivation during postnatal development on GRs are not mediated by this receptor.

The postnatal development of the hypothalamic-pituitary-adrenal axis in the mouse.

The main characteristic of the postnatal development of the stress system in the rat is the so-called stress hypo-responsive period (SHRP). Lasting from postnatal day (pnd) 4 to pnd 14, this period is characterized by very low basal corticosterone levels and an inability of mild stressors to induce an enhanced ACTH and corticosterone release. During the last years, the mouse has become a generally used animal in stress research, also due to the wide availability of genetically modified mouse strains. However, very few data are available on the ontogeny of the stress system in the mouse. This study therefore describes the postnatal ontogeny of peripheral and central aspects of the hypothalamic-pituitary-adrenal (HPA) axis in the mouse. We measured ACTH and corticosterone in blood and CRH, urocortin 3 (UCN3), mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) transcripts in the brain at postnatal days 1, 2, 4, 6, 9, 12, 14 and 16. Our results show that we can subdivide the postnatal development of the HPA axis in the mouse in two phases. The first phase corresponds to the SHRP in the rat and lasts from right after birth (pnd 1) until pnd 12. Basal corticosterone levels were low and novelty exposure did not enhance corticosterone or ACTH levels. This period is further characterized by a high expression of CRH in the paraventricular nucleus (PVN) of the hypothalamus. Expression levels of GR in the hippocampus and UCN3 in the perifornical area are low at birth but increase significantly during the SHRP, both reaching the highest expression level at pnd 12. In the second phase, the mice have developed past the SHRP and were now exhibiting enhanced corticosterone basal levels and a response of ACTH and corticosterone to mild novelty stress. CRH expression was decreased significantly, while expression of UCN3 and GR remained high, with a small decrease at pnd 16. The expression of MR in the hippocampus was very dynamic throughout the postnatal development of the HPA axis and changed in a time and subregion specific manner. These results demonstrate for the first time the correlation between the postnatal endocrine development of the mouse and gene expression changes of central regulators of HPA axis function.