Development neurobiology of the stress response: multilevel regulation of corticotropin-releasing hormone function

Chronic hM4Di-DREADD-Mediated Chemogenetic Inhibition of Forebrain Excitatory Neurons in Postnatal or Juvenile Life Does Not Alter Adult Mood-Related Behavior

G-protein-coupled receptors (GPCRs) coupled to G_i signaling, in particular downstream of monoaminergic neurotransmission, are posited to play a key role during developmental epochs (postnatal and juvenile) in shaping the emergence of adult anxiodepressive behaviors and sensorimotor gating. To address the role of G_i signaling in these developmental windows, we used a CaMKIIα-tTA::TRE hM4Di bigenic mouse line to express the hM4Di-DREADD (designer receptor exclusively activated by designer drugs) in forebrain excitatory neurons and enhanced G_i signaling via chronic administration of the DREADD agonist, clozapine-N-oxide (CNO) in the postnatal window (postnatal days 2–14) or the juvenile window (postnatal days 28–40). We confirmed that the expression of the HA-tagged hM4Di-DREADD was restricted to CaMKIIα-positive neurons in the forebrain, and that the administration of CNO in postnatal or juvenile windows evoked inhibition in forebrain circuits of the hippocampus and cortex, as indicated by a decline in expression of the neuronal activity marker c-Fos. hM4Di-DREADD-mediated inhibition of CaMKIIα-positive forebrain excitatory neurons in postnatal or juvenile life did not impact the weight profile of mouse pups, and also did not influence the normal ontogeny of sensory reflexes. Further, postnatal or juvenile hM4Di-DREADD-mediated inhibition of CaMKIIα-positive forebrain excitatory neurons did not alter anxiety- or despair-like behaviors in adulthood and did not impact sensorimotor gating. Collectively, these results indicate that chemogenetic induction of G_i signaling in CaMKIIα-positive forebrain excitatory neurons in postnatal and juvenile temporal windows does not appear to impinge on the programming of anxiodepressive behaviors in adulthood.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Early life trauma and attachment: immediate and enduring effects on neurobehavioral and stress axis development

Over half a century of converging clinical and animal research indicates that early life experiences induce enduring neuroplasticity of the HPA-axis and the developing brain. This experience-induced neuroplasticity is due to alterations in the frequency and intensity of stimulation of pups' sensory systems (i.e., olfactory, somatosensory, gustatory) embedded in mother-infant interactions. This stimulation provides "hidden regulators" of pups' behavioral, physiological, and neural responses that have both immediate and enduring consequences, including those involving the stress response. While variation in stimulation can produce individual differences and adaptive behaviors, pathological early life experiences can induce maladaptive behaviors, initiate a pathway to pathology, and increase risk for later-life psychopathologies, such as mood and affective disorders, suggesting that infant-attachment relationships program later-life neurobehavioral function. Recent evidence suggests that the effects of maternal presence or absence during this sensory stimulation provide a major modulatory role in neural and endocrine system responses, which have minimal impact on pups' immediate neurobehavior but a robust impact on neurobehavioral development. This concept is reviewed here using two complementary rodent models of infant trauma within attachment: infant paired-odor-shock conditioning (mimicking maternal odor attachment learning) and rearing with an abusive mother that converge in producing a similar behavioral phenotype in later-life including depressive-like behavior as well as disrupted HPA-axis and amygdala function. The importance of maternal social presence on pups' immediate and enduring brain and behavior suggests unique processing of sensory stimuli in early life that could provide insight into the development of novel strategies for prevention and therapeutic interventions for trauma experienced with the abusive caregiver.

Gestational stress influences cognition and behavior

The developmental origins of disease or fetal programming model predicts that early exposures to threat or adverse conditions have lifelong consequences that result in harmful outcomes for health. The vast majority of the studies in support of the programming model in human beings are retrospective and most rely on surrogate measures of early experience such as birth weight or preterm birth. Recently, a small number of prospective studies have been reported that have documented the developmental consequences of exposures to stressful intrauterine conditions. These studies of gestational stress have clearly shown that fetal exposures to psychosocial and/or biological markers of adversity have significant and largely negative consequences for fetal, infant and child neurological development. Fetal exposure to stress, especially early in gestation, results in delayed fetal maturation and impaired cognitive performance during infancy and results in decreased brain volume in areas associated with learning and memory in children. The accumulating evidence supports the conclusion that fetal exposure to stress profoundly influences the nervous system, with consequences that persist into childhood and perhaps beyond.

Neurobehavioral consequences of stressor exposure in rodent models of epilepsy

Both normal, non-epileptic as well as seizure-prone rodents exhibit a spectrum of anxiogenic-like behaviors in response to stressor exposure. Comparative analysis reveals that the same set of emotionality dependent measures is sensitive to both stress reactivity in normal rodents as well as stress hyperreactivity typically seen in seizure-prone rodents. A variety of unconditioned, exploratory tasks reflect global sensitivity to stressor exposure in the form of behavioral inhibition of locomotor output. Moreover, well chosen stressors can trigger de novo seizures with or without a history of seizure incidence. Seizures may be elicited in response to stressful environmental stimuli such as noxious noises, tail suspension handling, or home cage disturbance. Stress reactivity studies in rodents with a genetic predisposition to seizures have yielded important clues regarding brain substrates that mediate seizure ontogeny and modulate ictogenesis. Brains of seizure susceptible rodents reflect elevated content of the stress-related neuropeptide, corticotropin-releasing factor (CRF) in several nuclei relative to non-susceptible controls and neutralization of brain CRF attenuates seizure sensitivity. Findings outlined in this review support a diathesis-stress hypothesis in which behavioral- and neuro-pathologies of genetically seizure susceptible rodents arise in part due to multifaceted hyperreactivity to noxious environmental stimuli.

Dopamine receptor mechanisms mediate corticotropin-releasing factor-induced long-term potentiation in the rat amygdala following cocaine withdrawal

Corticotropin-releasing factor (CRF) in the amygdala is involved in stress responses. Moreover, dopaminergic neurotransmission in the brain reward system including the amygdala plays a significant role in the pathology of cocaine addiction. The present study analysed CRF-induced synaptic plasticity, its pharmacological sensitivity and interactions with the dopamine (DA) system in the basolateral to lateral capsula central amygdala (lcCeA) pathway after a 2-week withdrawal from repeated cocaine administration. A physiologically relevant CRF concentration (25 nm) induced long-term potentiation (LTP) that was enhanced after cocaine withdrawal. In saline-treated rats, CRF-induced LTP was mediated through N-methyl-d-aspartate (NMDA) receptors, L-type voltage-gated calcium channels (L-VGCCs) and CRF(1) receptors. However, in cocaine-withdrawn animals, activation of CRF(1) and CRF(2) receptors was found to enhance LTP. This enhanced CRF-induced LTP after cocaine withdrawal was mediated through endogenous activation of both D1- and D2-like receptors. Furthermore, expression of the D1 receptor (D1R) but not the D2R, D3R, D4R or D5R was significantly increased after cocaine withdrawal. CRF(1) but not CRF(2) protein expression was increased, suggesting that elevated levels of these proteins contributed to the enhancement of CRF-induced LTP during cocaine withdrawal. CRF interactions with the DA system in the amygdala may represent a fundamental neurochemical and cellular mechanism linking stress to cocaine-induced neuronal plasticity.

The central corticotropin releasing factor system during development and adulthood

Corticotropin releasing factor (CRH) has been shown to contribute critically to molecular and neuroendocrine responses to stress during both adulthood and development. This peptide and its receptors are expressed in the hypothalamus, as well as in limbic brain areas including amygdala and hippocampus. This is consistent with roles for CRH in mediating the influence of stress on emotional behavior and cognitive function. The expression of CRH and of its receptors in hypothalamus, amygdala and hippocampus is age-dependent, and is modulated by stress throughout life (including the first postnatal weeks). Uniquely during development, the cardinal influence of maternal care on the central stress response governs the levels of central CRH expression, and may alter the 'set-point' of CRH-gene sensitivity to stress in a lasting manner.

Ontogeny of steroid receptor coactivators in the hippocampus and their role in regulating postnatal HPA axis function

The function and regulation of the hypothalamic-pituitary-adrenal (HPA) axis during ontogeny differs markedly from the situation in adult animals. Postnatally mice undergo a so-called stress hypo-responsive period, which is characterized by a relative inability of mild stressors to induce a marked corticosterone response. Steroid receptor coactivators (SRCs) have been shown to influence the function of the HPA axis in adult animals by interacting with steroid receptors as the mineralocorticoid and the glucocorticoid receptor. Here we test the hypothesis that expression changes of the three identified SRC genes (SRC1, SRC2 and SRC3) correlate with differences in HPA axis activity during postnatal development. First, we mapped the ontogeny of the three SRCs during postnatal development in the hippocampus. We found a time- and region-specific regulation of gene expression, which was specific for each SRC. However, there was no relation between the age-dependent stress system activity and the expression levels of the SRCs. Further, we studied the acute regulation of the three SRCs following maternal deprivation in 9-day-old wild-type or CRH receptor type 1 (CRHr1) knockout mice. Under these conditions, no differential expression of any of the tested SRCs could be detected. Thus, while it seems likely that their varying abundance throughout postnatal life affects steroid receptor function in the different hippocampal subregions, acute changes of HPA axis activity or reactivity are not mediated by hippocampal changes in expression of this coactivator family.

Corticotropin-releasing factor receptors differentially regulate stress-induced tau phosphorylation

Hyperphosphorylation of the microtubule-associated protein tau is a key event in the development of Alzheimer's disease (AD) neuropathology. Acute stress can induce hippocampal tau phosphorylation (tau-P) in rodents, but the mechanisms and pathogenic relevance of this response are unclear. Here, we find that hippocampal tau-P elicited by an acute emotional stressor, restraint, was not affected by preventing the stress-induced rise in glucocorticoids but was blocked by genetic or pharmacologic disruption of signaling through the type 1 corticotropin-releasing factor receptor (CRFR1). Conversely, these responses were exaggerated in CRFR2-deficient mice. Parallel CRFR dependence was seen in the stress-induced activation of specific tau kinases. Repeated stress exposure elicited cumulative effects on tau-P and its sequestration in an insoluble, and potentially pathogenic, form. These findings support differential regulatory roles for CRFRs in an AD-relevant form of neuronal plasticity and may link datasets documenting alterations in the CRF signaling system in AD and implicating chronic stress as a risk factor in age-related neurological disorders.

Neural, endocrine and electroencephalographic hyperreactivity to human contact: a diathesis-stress model of seizure susceptibility in El mice

The El mouse strain provides a non-induced model of idiopathic, multifactorial epilepsy in which seizures are elicited in response to stressful environmental stimuli such as tail suspension handling. In the present studies, genetically seizure susceptible El and non-susceptible ddY control mice were exposed to tail suspension, foot-shock and social stressors in order to test the hypothesis that neural and physiological responses to such stimuli would be exaggerated in the El strain. The first experiment assessed neural cell density, stress neuropeptide (corticotropin releasing factor--CRF) levels, and plasma corticosterone activation in El and ddY mice in an unhandled control condition or following exposure to tail suspension or foot-shock stressors. The second experiment assessed brain electroencephalographic activity using telemetrically monitored skull surface electrodes in El and ddY mice exposed to tail suspension or social interaction stressors. Assessment of El mouse brains revealed higher cell counts in amygdala and elevated CRF peptide content in the paraventricular thalamic nucleus relative to ddY controls. El mice exhibited significantly elevated plasma corticosterone levels 60 min following exposure to tail suspension and foot-shock stressors relative to ddY controls. Finally, El mice exhibited significantly elevated brain electroencephalographic (1-4 Hz) activity in response to tail suspension, but not social interaction, relative to ddY controls. These results indicate that potentiated neural, endocrine and physiological activation arises in the El strain following exposure to a known seizure trigger stimulus, involuntary tail suspension handling. The findings support a diathesis-stress hypothesis in which genetically seizure susceptible El mice exhibit a multifaceted hyperreactivity to noxious environmental stimuli.

Cocaine withdrawal enhances long-term potentiation induced by corticotropin-releasing factor at central amygdala glutamatergic synapses via CRF1, NMDA receptors and PKA

Cocaine addiction is an enduring, relapsing, behavioural disorder in which stressors reinstate cocaine-seeking even after prolonged abstinence. Evidence suggests that the 'anxiety-like' behaviour and stress associated with protracted withdrawal may be mediated by increased corticotropin-releasing factor (CRF) in the central nucleus of the amygdala (CeA), a part of the limbic circuitry engaged in the coding and transmission of stimulus-reward associations. In the present study we describe a long-lasting potentiation of glutamatergic transmission induced at lateral amygdala (LA)-to-CeA synapses by rat/human CRF. After 2 weeks of withdrawal from repeated intermittent exposure to cocaine, CRF-induced long-term potentiation (LTP) was greatly enhanced compared to the respective saline control group while, after short-term withdrawal (24 h), there was no significant difference between the two treatment groups, indicating alterations in CRF systems during protracted withdrawal from chronic cocaine. After prolonged withdrawal, CRF-induced LTP was dependent on activation of CRF2, CaV2.3 (R-type) calcium channels and intracellular signalling through protein kinase C in both saline- and cocaine-treated groups. The enhanced CRF-induced LTP after 2 weeks of withdrawal was mediated through augmented CRF1 receptor function, associated with an increased signalling through protein kinase A, and required N-methyl-D-aspartate (NMDA) receptors. Accordingly, single-cell recordings revealed a significantly increased NMDA/AMPA ratio after prolonged withdrawal from the cocaine treatment. These results support a role for CRF1 receptor antagonists as plausible treatment options during withdrawal from chronic cocaine and suggest Ca(V)2.3 blockers as potential candidates for pharmaceutical modulation of CRF systems.

Hippocampal neuroplasticity induced by early-life stress: functional and molecular aspects

Whereas genetic factors contribute crucially to brain function, early-life events, including stress, exert long-lasting influence on neuronal function. Here, we focus on the hippocampus as the target of these early-life events because of its crucial role in learning and memory. Using a novel immature-rodent model, we describe the deleterious consequences of chronic early-life 'psychological' stress on hippocampus-dependent cognitive tasks. We review the cellular mechanisms involved and discuss the roles of stress-mediating molecules, including corticotropin releasing hormone, in the process by which stress impacts the structure and function of hippocampal neurons.

Neuroplasticity of the hypothalamic-pituitary-adrenal axis early in life requires recurrent recruitment of stress-regulating brain regions

An eloquent example of experience-induced neuroplasticity involves the enduring effects of daily "handling" of rat pups on the expression of genes regulating hormonal and behavioral responses to stress. Handling-evoked augmentation of maternal care of pups induces long-lasting reduction of hypothalamic corticotropin releasing hormone (CRH) expression and upregulates hippocampal glucocorticoid receptor levels. These changes promote a lifelong attenuation of hormonal stress responses. We have found previously that handling-evoked downregulation of CRH expression occurs already by postnatal day 9, implicating it as an early step in this experience-induced neuroplasticity. Here, we investigated the neuronal pathways and cellular mechanisms involved. CRH mRNA expression in hypothalamic paraventricular nucleus (PVN) diminished after daily handling but not after handling once only, indicating that "recurrent" handling was required for this effect. Return of handled pups to their cage provoked a burst of nurturing behavior in dams that, in turn, induced transient, coordinate Fos expression in selected regions of the pups' brains. These included central nucleus of the amygdala (ACe) and bed nucleus of the stria terminals (BnST), regions that are afferent to PVN and influence CRH expression there. Whereas handling once sufficed to evoke Fos expression within ACe and BnST, expression in thalamic paraventricular nucleus, a region involved in storing and processing stress-related experience, required recurrent handling. Fos induction in all three regions elicited reduced transcription factor phosphorylation, followed by attenuated activation of CRH gene transcription within the PVN. These studies provide a neurobiological foundation for the profound neuroplasticity of stress-related genes evoked by early-life experience.

Disruption of the Blood–Brain Barrier and Neuronal Cell Death in Cingulate Cortex, Dentate Gyrus, Thalamus, and Hypothalamus in a Rat Model of Gulf-War Syndrome

We investigated the effects of a combined exposure to restraint stress and low doses of chemicals pyridostigmine bromide (PB), N, N-diethyl-m-toluamide (DEET), and permethrin in adult male rats, a model of Gulf-War syndrome. Animals were exposed daily to one of the following for 28 days: (i) a combination of stress and chemicals (PB, 1.3 mg/kg/day; DEET, 40 mg/kg/day; and permethrin, 0.13 mg/kg/day); (ii) stress and vehicle; (iii) chemicals alone; and (iv) vehicle alone. All animals were evaluated for: (i) the disruption of the blood-brain barrier (BBB) using intravenous horseradish peroxidase (HRP) injections and endothelial barrier antigen (EBA) immunostaining; (ii) neuronal cell death using H&E staining, silver staining, and glial fibrillary acidic protein (GFAP) immunostaining; and (iii) acetylcholinesterase (AChE) activity and m2-muscarinic acetylcholine receptors (m2-AChR). Animals subjected to stress and chemicals exhibited both disruption of the BBB and neuronal cell death in the cingulate cortex, the dentate gyrus, the thalamus, and the hypothalamus. Other regions of the brain, although they demonstrated some neuronal cell death, did not exhibit disruption of the BBB. The neuropathological changes in the above four brain regions were highly conspicuous and revealed by a large number of HRP-positive neurons (21-40% of total neurons), a decreased EBA immunostaining (42-51% reduction), a decreased number of surviving neurons (27-40% reduction), the presence of dying neurons (4-10% of total neurons), and an increased GFAP immunostaining (45-51% increase). These changes were also associated with decreased forebrain AChE activity and m2-AchR (19-25% reduction). In contrast, in animals exposed to stress and vehicle or chemicals alone, the above indices were mostly comparable to that of animals exposed to vehicle alone. Thus, a combined exposure to stress and low doses of PB, DEET, and permethrin leads to significant brain injury. The various neurological symptoms reported by Gulf-War veterans could be linked to this kind of brain injury incurred during the war.

The neonatal glucocorticoid treatment-produced long-term changes of the pituitary-adrenal function and brain corticosteroid receptors in rats

Two distinct periods of sensitivity to elevated glucocorticoid hormone levels during postnatal development of the pituitary-adrenal axis were studied. Wistar rats were injected subcutaneously (s.c.) with cortisol (1 mg/kg) on postnatal days 1-5 or 14-18. The steroid treatment during the first postnatal week resulted in a decrease of the morning basal and stress-induced plasma corticosterone levels in 30 day-old male rats, as well as in rats that were injected with cortisol on the third postnatal week. Stress-induced corticosterone levels in 90-day old cortisol-treated rats were determined in blood samples drawn from the tail vein before the restraint stress, immediately after the 20-min long stress, then 60 and 180 min afterwards. Only the rats treated with cortisol during the third week showed a prolonged stress-induced corticosterone secretion, with the highest corticosterone level in 180 min after the restraint stress. The early neonatal cortisol treatment had no effect on (3)H-corticosterone binding in all studied brain areas of the 90-day old rats. The rats treated with cortisol at the 14-17th postnatal days showed a significantly lower (3)H-corticosterone binding in the frontal cortex, hippocampus, and hypothalamus. These findings suggest that the third week of life in rats is more sensitive to elevated levels of corticosterone than the first one. The high level of glucocorticoids at this period has long-term effects on the efficiency of the negative feedback mechanisms provided by hypothalamus-pituitary-adrenal axis.

Rapid phosphorylation of the CRE binding protein precedes stress-induced activation of the corticotropin releasing hormone gene in medial parvocellular hypothalamic neurons of the immature rat

The mechanisms of the molecular and neuroendocrine responses to stress in the immature rat have been a focus of intense investigation. A principal regulator of the these responses in both mature and developing rat is the neuropeptide corticotropin releasing hormone (CRH), and levels of hypothalamic CRH mRNA are enhanced by stress. In vitro, transcription of the CRH gene is governed by binding of the phosphorylated form of cAMP responsive element binding protein (pCREB) to the promoter. Here we tested the hypothesis that rapid, stress-induced CRH transcription occurred during the first two postnatal weeks, and is associated with pCREB expression. The time-course of induction of unedited, heteronuclear CRH RNA (CRH hnRNA) was examined in hypothalamic paraventricular nucleus (PVN) of immature rats subjected to both modest and strong acute stressors using in situ hybridization; pCREB abundance was determined in individual neurons in specific PVN sub-nuclei using immunocytochemistry and unbiased quantitative analysis. CRH hnRNA signal was negligible in PVN of immature rats sacrificed under stress-free conditions, but was readily detectable within 2 min, and peaked at 15 min, in PVN of stressed animals. Enhanced pCREB immunoreactivity was evident within 2 min of stress onset, and was enhanced specifically in stress-responsive, CRH-expressing medial parvocellular neurons. These data support the notion that, already during early postnatal life, stress induces rapid CREB phosphorylation, interaction of pCREB-containing transcription complexes with the CRE element of the CRH gene promoter, and initiation of CRH hnRNA production in stress-responsive neurons of rat PVN.

Neurobiology of the stress response early in life: evolution of a concept and the role of corticotropin releasing hormone

Over the last few decades, concepts regarding the presence of hormonal and molecular responses to stress during the first postnatal weeks in the rat and the role of the neuropeptide corticotropin releasing hormone (CRH) in these processes, have been evolving. CRH has been shown to contribute critically to molecular and neuroendocrine responses to stress during development. In turn the expression of this neuropeptide in both hypothalamus and amygdala is differentially modulated by single and recurrent stress, and is determined also by the type of stress (eg, psychological or physiological). A likely transcriptional regulatory factor for modulating CRH gene expression, the cAMP responsive element binding protein CREB, is phosphorylated (activated) in the developing hypothalamus within seconds of stress onset, preceding the transcription of the CRH gene and initiating the activation of stress-induced cellular and neuroendocrine cascades. Finally, early life stress may permanently modify the hypothalamic pituitary adrenal axis and the response to further stressful stimuli, and recent data suggest that CRH may play an integral role in the mechanisms of these long-term changes.

Altered regulation of gene and protein expression of hypothalamic-pituitary-adrenal axis components in an immature rat model of chronic stress

Chronic stress early in postnatal life influences hormonal and behavioural responses to stress persistently, but the mechanisms and molecular cascades that are involved in this process have not been clarified. To approach these issues, a chronic stress paradigm for the neonatal rat, using limited bedding material to alter the cage environment, was devised. In 9-day-old rats subjected to this chronic stress for 1 week, significant and striking changes in the expression and release patterns of key molecules that govern the neuroendocrine stress responses were observed. The presence of sustained stress was evident from enhanced activation of peripheral elements of the neuroendocrine stress response, i.e. increased basal plasma corticosterone concentrations, high adrenal weight and decreased body weight. Central regulatory elements of the neuroendocrine stress response were perturbed, including reduced expression of hypothalamic corticotropin-releasing hormone that, surprisingly, was accompanied by reduced glucocorticoid receptor expression. Thus, the effects of chronic sustained stress in the neonatal rat on the hypothalamic-pituitary-adrenal axis included substantial changes in the expression and activity of major regulators of this axis. Importantly, the changes induced by this chronic stress differed substantially from those related to acute or recurrent stress, providing a novel model for studying the long-term effects of chronic, early life stress on neuroendocrine functions throughout life.

Down-regulation of hypothalamic corticotropin-releasing hormone messenger ribonucleic acid (mRNA) precedes early-life experience-induced changes in hippocampal glucocorticoid receptor mRNA

Early-life experiences, including maternal interaction, profoundly influence hormonal stress responses during adulthood. In rats, daily handling during a critical neonatal period leads to a significant and permanent modulation of key molecules that govern hormonal secretion in response to stress. Thus, hippocampal glucocorticoid receptor (GR) expression is increased, whereas hypothalamic CRH-messenger RNA (mRNA) levels and stress-induced glucocorticoid release are reduced in adult rats handled early in life. Recent studies have highlighted the role of augmented maternal sensory input to handled rats as a key determinant of these changes. However, the molecular mechanisms, and particularly the critical, early events leading from enhanced sensory experience to long-lasting modulation of GR and CRH gene expression, remain largely unresolved. To elucidate the critical primary genes governing this molecular cascade, we determined the sequence of changes in GR-mRNA levels and in hypothalamic and amygdala CRH-mRNA expression at three developmental ages, and the temporal relationship between each of these changes and the emergence of reduced hormonal stress-responses. Down-regulation of hypothalamic CRH-mRNA levels in daily-handled rats was evident already by postnatal day 9, and was sustained through postnatal days 23 and 45, i.e. beyond puberty. In contrast, handling-related up-regulation of hippocampal GR-mRNA expression emerged subsequent to the 23rd postnatal day, i.e. much later than changes in hypothalamic CRH expression. The hormonal stress response of handled rats was reduced starting before postnatal day 23. These findings indicate that early, rapid, and persistent changes of hypothalamic CRH gene expression may play a critical role in the mechanism(s) by which early-life experience influences the hormonal stress-response long-term.

Activation of specific neuronal circuits by corticotropin releasing hormone as indicated by c-fos expression and glucose metabolism

The neuropeptide corticotropin releasing hormone (CRH) is the central nervous system (CNS) transducer of stressful stimuli. Endogenous CRH is released from neuronal terminals in several central nervous system regions-for example, amygdala and hypothalamus-during stress, and exogenous CRH administration mimics stress-related behaviors and hormonal patterns. However, whereas the role of endogenous CRH as a stress neuromodulator has been established, recent findings suggest that the peptide also functions to influence cognitive, emotional, and neuroimmune functions by modulating neuronal communication in a number of circuits. Although anatomic and pharmacologic approaches have provided evidence for this wider spectrum of CRH actions, the discrete regions and specific circuits activated by CRH have not been fully elucidated. In this article, the authors report on the use of two complementary methods to discern specific regions and cell groups activated by the administration of CRH. Glucose metabolism analysis provided quantitative measures of CRH-induced activation, but at a regional resolution; expression of the immediate early gene c-fos permitted a single cell resolution, but underestimated the neuroanatomic extent of CRH-induced activation. Overlapping regions activated using both methods delineated discrete cortical, limbic. and motor pathways. Importantly, cell groups activated by CRH included those possessing either or both members of the CRH receptor family, suggesting that both receptors may mediate the effects of the endogenous ligand. In summary, CRH activates a broad but selective array of neuronal structures belonging to cortical, limbic, and motor circuits. These findings indicate that stress-related release of this peptide may contribute to a spectrum of important modulations of CNS function.

Chronic corticosterone enhances the rewarding effect of hypothalamic self-stimulation in rats

Excessive levels of glucocorticoids have been implicated in the etiology of affective disorders in humans, and in a range of behavioral deficits in animals. In the present study, we used an established regimen of corticosterone administration (40 mg/kg, for 21 days) to determine its effects upon responding for rewarding brain stimulation in rats. After chronic treatment, subjects exhibited an unexpected but significantly increased sensitivity to the rewarding effects of brain stimulation. These results suggest that chronic, high levels of corticosterone are unlikely to cause anhedonia in rodents.

Co-localization of corticotropin-releasing hormone with glutamate decarboxylase and calcium-binding proteins in infant rat neocortical interneurons

Corticotropin releasing hormone (CRH) has been localized to interneurons of the mammalian cerebral cortex, but these neurons have not been fully characterized. The present study determined the extent of co-localization of CRH with glutamate decarboxylase (GAD) and calcium-binding proteins in the infant rat neocortex using immunocytochemistry. CRH-immunoreactive (ir) neurons were classified into two major groups. The first group was larger and consisted of densely CRH-immunostained small bipolar cells, predominantly localized to layers II and III. The second group of CRH-ir cells was lightly labeled and included multipolar neurons mainly found in deep cortical layers. Co-localization studies indicated that the vast majority of CRH-ir neurons, including both bipolar and multipolar types, was co-immunolabeled for GAD-65 and GAD-67. Most multipolar, but only some bipolar, CRH-ir neurons also contained parvalbumin, while CRH-ir neurons rarely contained calbindin or calretinin. These results indicate that virtually all CRH-ir neurons in the rat cerebral cortex are GABAergic. Furthermore, since parvalbumin is expressed by cortical basket and chandelier cells, the co-localization of CRH and parvalbumin suggests that some cortical CRH-ir neurons may belong to these two cell types.