Norepinephrine-induced CRH and AVP gene transcription within the hypothalamus: differential regulation by corticosterone

Knockdown of endoplasmic reticulum chaperone BiP leads to the death of parvocellular AVP/CRH neurons in mice

Arginine vasopressin (AVP) is expressed in both magnocellular (magnAVP) and parvocellular AVP (parvAVP) neurons of the paraventricular nucleus, and AVP colocalizes with corticotropin-releasing hormone (CRH) only in the parvocellular neurons. The immunoglobulin heavy chain binding protein (BiP) is a major endoplasmic reticulum (ER) chaperone which regulates the unfolded protein response under ER stress. We previously demonstrated that knockdown of BiP in magnAVP neurons exacerbated ER stress, which resulted in the autophagy-associated cell death of magnAVP neurons. Using the same approach, in the present study we examined the role of BiP in mouse parvAVP/CRH neurons. Our data demonstrate that BiP is expressed in mouse parvAVP/CRH neurons under nonstress conditions and is upregulated in proportion to the increase in CRH expression after adrenalectomy. For BiP knockdown in parvAVP/CRH neurons, we utilized a viral approach in combination with shRNA interference. Knockdown of BiP expression induced ER stress in parvAVP/CRH neurons, as reflected by the expression of C/EBP homologous protein. Furthermore, BiP knockdown led to the loss of parvAVP/CRH neurons after 4 weeks. In summary, our results demonstrate that BiP plays a pivotal role in parvAVP/CRH neurons, which function as neuroendocrine cells producing a large number of secretory proteins.

Regulation of corticotropin-releasing hormone neuronal network activity by noradrenergic stress signals

Noradrenaline is a neurotransmitter released in response to homeostatic challenge and activates the hypothalamic-pituitary-adrenal axis via stimulation of corticotropin-releasing hormone (CRH) neurons. Here we investigated the mechanism through which noradrenaline regulates activity within the CRH neuronal network. Using a combination of in vitro GCaMP6f Ca²⁺ imaging and electrophysiology, we show that noradrenaline induces a robust increase in excitability in a proportion of CRH neurons with many neurons displaying a bursting mode of activity. Noradrenaline-induced activation required α₁ -adrenoceptors and L-type voltage-gated Ca²⁺ channels, but not GABA/glutamate synaptic transmission or sodium action potentials. Exposure of mice to elevated corticosterone levels was able to suppress noradrenaline-induced activation. These results provide further insight into the mechanisms by which noradrenaline regulates CRH neural network activity and hence stress responses. KEY POINTS: GCaMP6f Ca²⁺ imaging and on-cell patch-clamp recordings reveal that corticotropin-releasing hormone neurons are activated by noradrenaline with many neurons displaying a bursting mode of activity. Noradrenaline-induced activation requires α₁ -adrenoceptors. Noradrenaline-induced Ca²⁺ elevations persist after blocking GABA_A , AMPA, NMDA receptors and voltage-gated Na⁺ channels. Noradrenaline-induced Ca²⁺ elevations require L-type voltage-gated Ca²⁺ channels. Corticosterone suppresses noradrenaline-induced excitation.

Astrocytes Amplify Neuronal Dendritic Volume Transmission Stimulated by Norepinephrine

In addition to their support role in neurotransmitter and ion buffering, astrocytes directly regulate neurotransmission at synapses via local bidirectional signaling with neurons. Here, we reveal a form of neuronal-astrocytic signaling that transmits retrograde dendritic signals to distal upstream neurons in order to activate recurrent synaptic circuits. Norepinephrine activates α₁ adrenoreceptors in hypothalamic corticotropin-releasing hormone (CRH) neurons to stimulate dendritic release, which triggers an astrocytic calcium response and release of ATP; ATP stimulates action potentials in upstream glutamate and GABA neurons to activate recurrent excitatory and inhibitory synaptic circuits to the CRH neurons. Thus, norepinephrine activates a retrograde signaling mechanism in CRH neurons that engages astrocytes in order to extend dendritic volume transmission to reach distal presynaptic glutamate and GABA neurons, thereby amplifying volume transmission mediated by dendritic release.

Effects of acute kidney dysfunction on hypothalamic arginine vasopressin synthesis in transgenic rats

Acute loss of kidney function is a critical internal stressor. Arginine vasopressin (AVP) present in the parvocellular division of the paraventricular nucleus (PVN) plays a key role in the regulation of stress responses. However, hypothalamic AVP dynamics during acute kidney dysfunction remain unclear. In this study, we investigated the effects of bilateral nephrectomy on AVP, using a transgenic rat line that expressed the AVP-enhanced green fluorescent protein (eGFP). The eGFP fluorescent intensities in the PVN were dramatically increased after bilateral nephrectomy. The mRNA levels of eGFP, AVP, and corticotrophin-releasing hormone in the PVN were dramatically increased after bilateral nephrectomy. Bilateral nephrectomy also increased the levels of Fos-like immunoreactive cells in brainstem neurons. These results indicate that bilateral nephrectomy upregulates the AVP-eGFP synthesis. Further studies are needed to identify the neural and/or humoral factors that activate AVP synthesis and regulate neuronal circuits during acute kidney dysfunction.

Genome-Wide Analysis of Glucocorticoid-Responsive Transcripts in the Hypothalamic Paraventricular Region of Male Rats

Glucocorticoids (GCs) are essential for stress adaptation, acting centrally and in the periphery. Corticotropin-releasing factor (CRF), a major regulator of adrenal GC synthesis, is produced in the paraventricular nucleus of the hypothalamus (PVH), which contains multiple neuroendocrine and preautonomic neurons. GCs may be involved in diverse regulatory mechanisms in the PVH, but the target genes of GCs are largely unexplored except for the CRF gene (Crh), a well-known target for GC negative feedback. Using a genome-wide RNA-sequencing analysis, we identified transcripts that changed in response to either high-dose corticosterone (Cort) exposure for 12 days (12-day high Cort), corticoid deprivation for 7 days (7-day ADX), or acute Cort administration. Among others, canonical GC target genes were upregulated prominently by 12-day high Cort. Crh was upregulated or downregulated most prominently by either 7-day ADX or 12-day high Cort, emphasizing the recognized feedback effects of GC on the hypothalamic-pituitary-adrenal (HPA) axis. Concomitant changes in vasopressin and apelin receptor gene expression are likely to contribute to HPA repression. In keeping with the pleotropic cellular actions of GCs, 7-day ADX downregulated numerous genes of a broad functional spectrum. The transcriptome response signature differed markedly between acute Cort injection and 12-day high Cort. Remarkably, six immediate early genes were upregulated 1 hour after Cort injection, which was confirmed by quantitative reverse transcription PCR and semiquantitative in situ hybridization. This study may provide a useful database for studying the regulatory mechanisms of GC-dependent gene expression and repression in the PVH.

Biomolecular aspects of depression: A retrospective analysis

OBJECTIVE

The effects of psychological stress, oxidative stress, and chronic low grade inflammation on the neuro-immune connection have been implicated in the pathogenesis of depression. Thus, in the recent past, there has been a growing effort in determining the mechanism of this pathogenesis. While attempting to map out, this mechanism researchers and clinicians have searched for clinically relevant biomarkers for use in the diagnosis and for the assessment of those suffering from depression. In this study, we have performed a retrospective analysis of biomarkers with clinically relevant potentials, including peripheral catecholamines, chemokines, cytokines, and neurotransmitters.

METHODS

The retrospective analysis was performed on data collected over a six-year period of time (July 2009 to July 2015), gathered from patients (N=1399; Mage=42, SD=13; 71% female, 29% male) who submitted samples with complaints of feeling hopeless, worthless, isolated, alone, general sadness, overwhelmed, and/or a lack of interest in things they once enjoyed. The data collected consisted of quantitative values of urinary catecholamines and neurotransmitters (peripheral dopamine, epinephrine, histamine, kynurenic acid, norepinephrine, β-PEA, and serotonin), salivary hormones (peripheral cortisol and melatonin), and peripheral blood mononuclear cell secreted cytokines and chemokines (Interleukins 1β, 6, 8, 10, MCP-1, GCSF, and TNFα). Statistical and clinical significance was assessed by comparison with a control group (N=2395; Mage=42, SD=13; 70% female, 30% male), calculating the percent mean difference, p value, and effect size (Cohen's ɗ) for each parameter between groups.

RESULTS

The findings of this study suggested that, in a model of general depression, there is a dysregulation in the enzymatic production and degradation of catecholamines, neurotransmitters, hormones, and immunological proteins. A cycle of interaction was found between all of these biomolecules, where an increase or decrease in one marker could result in a stimulatory or inhibitory effect on others. The mechanism of this was proposed to occur through the interaction of psychological stress, inflammation, and oxidative stress pathways. All of these biomolecules were found to be significantly altered in the general depression group and are key components of the interaction between the neurological and immunological systems.

CONCLUSIONS

This study serves to further elucidate the role of biomolecules in the regulation of affective disorders, such as depression. Resulting in providing a network of clinically relevant biomarkers to objectively assess and monitor general depression.

Interoceptive modulation of neuroendocrine, emotional, and hypophagic responses to stress

Periods of caloric deficit substantially attenuate many centrally mediated responses to acute stress, including neural drive to the hypothalamic-pituitary-adrenal (HPA) axis, anxiety-like behavior, and stress-induced suppression of food intake (i.e., stress hypophagia). It is posited that this stress response plasticity supports food foraging and promotes intake during periods of negative energy balance, even in the face of other internal or external threats, thereby increasing the likelihood that energy stores are repleted. The mechanisms by which caloric deficit alters central stress responses, however, remain unclear. The caudal brainstem contains two distinct populations of stress-recruited neurons [i.e., noradrenergic neurons of the A2 cell group that co-express prolactin-releasing peptide (PrRP+ A2 neurons), and glucagon-like peptide 1 (GLP-1) neurons] that also are responsive to interoceptive feedback about feeding and metabolic status. A2/PrRP and GLP-1 neurons have been implicated anatomically and functionally in the central control of the HPA axis, anxiety-like behavior, and stress hypophagia. The current review summarizes a growing body of evidence that caloric deficits attenuate physiological and behavioral responses to acute stress as a consequence of reduced recruitment of PrRP+ A2 and hindbrain GLP-1 neurons, accompanied by reduced signaling to their brainstem, hypothalamic, and limbic forebrain targets.

Distribution of corticotropin-releasing factor neurons in the mouse brain: a study using corticotropin-releasing factor-modified yellow fluorescent protein knock-in mouse

We examined the morphological features of corticotropin-releasing factor (CRF) neurons in a mouse line in which modified yellow fluorescent protein (Venus) was expressed under the CRF promoter. We previously generated the CRF-Venus knock-in mouse, in which Venus is inserted into the CRF gene locus by homologous recombination. In the present study, the neomycin phosphotransferase gene (Neo), driven by the pgk-1 promoter, was deleted from the CRF-Venus mouse genome, and a CRF-Venus∆Neo mouse was generated. Venus expression is much more prominent in the CRF-Venus∆Neo mouse when compared to the CRF-Venus mouse. In addition, most Venus-expressing neurons co-express CRF mRNA. Venus-expressing neurons constitute a discrete population of neuroendocrine neurons in the paraventricular nucleus of the hypothalamus (PVH) that project to the median eminence. Venus-expressing neurons were also found in brain regions outside the neuroendocrine PVH, including the olfactory bulb, the piriform cortex (Pir), the extended amygdala, the hippocampus, the neocortices, Barrington's nucleus, the midbrain/pontine dorsal tegmentum, the periaqueductal gray, and the inferior olivary nucleus (IO). Venus-expressing perikarya co-expressing CRF mRNA could be observed clearly even in regions where CRF-immunoreactive perikarya could hardly be identified. We demonstrated that the CRF neurons contain glutamate in the Pir and IO, while they contain gamma-aminobutyric acid in the neocortex, the bed nucleus of the stria terminalis, the hippocampus, and the amygdala. A population of CRF neurons was demonstrated to be cholinergic in the midbrain tegmentum. The CRF-Venus∆Neo mouse may be useful for studying the structural and functional properties of CRF neurons in the mouse brain.

Chronic alcohol exposure differentially affects activation of female locus coeruleus neurons and the subcellular distribution of corticotropin releasing factor receptors

Understanding the neurobiological bases for sex differences in alcohol dependence is needed to help guide the development of individualized therapies for alcohol abuse disorders. In the present study, alcohol-induced adaptations in (1) anxiety-like behavior, (2) patterns of c-Fos activation and (3) subcellular distribution of corticotropin releasing factor receptor in locus coeruleus (LC) neurons was investigated in male and female Sprague-Dawley rats that were chronically exposed to ethanol using a liquid diet. Results confirm and extend reports by others showing that chronic ethanol exposure produces an anxiogenic-like response in both male and female subjects. Ethanol-induced sex differences were observed with increased c-Fos expression in LC neurons of female ethanol-treated subjects compared to controls or male subjects. Results also reveal sex differences in the subcellular distribution of the CRFr in LC-noradrenergic neurons with female subjects exposed to ethanol exhibiting a higher frequency of plasmalemmal CRFrs. These adaptations have implications for LC neuronal activity and its neural targets across the sexes. Considering the important role of the LC in ethanol-induced activation of the hypothalamo-pituitary-adrenal (HPA) axis, the present results indicate important sex differences in feed-forward regulation of the HPA axis that may render alcohol dependent females more vulnerable to subsequent stress exposure.

Visualization of corticotropin-releasing factor neurons by fluorescent proteins in the mouse brain and characterization of labeled neurons in the paraventricular nucleus of the hypothalamus

Corticotropin-releasing factor (CRF) is the key regulator of the hypothalamic-pituitary-adrenal axis. CRF neurons cannot be distinguished morphologically from other neuroendocrine neurons in the paraventricular nucleus of the hypothalamus (PVH) without immunostaining. Thus, we generated a knock-in mouse that expresses modified yellow fluorescent protein (Venus) in CRF neurons (CRF-Venus), and yet its expression is driven by the CRF promoter and responds to changes in the interior milieu. In CRF-Venus, Venus-expressing neurons were distributed in brain regions harboring CRF neurons, including the PVH. The majority of Venus-expressing neurons overlapped with CRF-expressing neurons in the PVH, but many neurons expressed only Venus or CRF in a physiological glucocorticoid condition. After glucocorticoid deprivation, however, Venus expression intensified, and most Venus neurons coexpressed CRF. Conversely, Venus expression was suppressed by excess glucocorticoids. Expression of copeptin, a peptide encoded within the vasopressin gene, was induced in PVH-Venus neurons by glucocorticoid deprivation and suppressed by glucocorticoid administration. Thus, Venus neurons recapitulated glucocorticoid-dependent vasopressin expression in PVH-CRF neurons. Noradrenaline increased the frequency of glutamate-dependent excitatory postsynaptic currents recorded from Venus-expressing neurons in the voltage clamp mode. In addition, the CRF-iCre knock-in mouse was crossed with a CAG-CAT-EGFP reporter mouse to yield the Tg(CAG-CAT-EGFP/wt);CRF(iCre/wt) (EGFP/CRF-iCre) mouse, in which enhanced green fluorescent protein (EGFP) is driven by the CAG promoter. EGFP was expressed more constitutively in the PVH of EGFP/CRF-iCre mice. Thus, CRF-Venus may have an advantage for monitoring dynamic changes in CRF neurons and CRF networks in different glucocorticoid states.

Roles of the locus coeruleus and adrenergic receptors in brain-mediated hypothalamic-pituitary-adrenal axis responses to intracerebroventricular alcohol

BACKGROUND

Alcohol activates the hypothalamic-pituitary-adrenal (HPA) axis through its actions in both the periphery and the central nervous system (CNS). The studies presented here were designed to test the CNS-specific noradrenergic mechanisms by which alcohol stimulates HPA activity in the male rat.

METHODS

We used an experimental paradigm in which a small, nontoxic amount (5 μl) of alcohol was slowly microinfused intracerebroventricularly (icv). Alcohol was administered icv to animals with lesions of the locus coeruleus (LC) or in animals pretreated with α- or β-adrenergic receptor antagonists. Hormonal HPA activation was determined by measuring secretion of the pituitary stress hormone adrenocorticotropin (ACTH). Neuronal activation was determined by quantification of the expression of the transcription factor c-fos (Fos).

RESULTS

As expected, icv alcohol stimulated ACTH secretion from the pituitary and Fos expression in the paraventricular nucleus of the hypothalamus (PVN). Bilateral electrolytic LC lesions blocked the ability of icv alcohol to stimulate ACTH secretion. Pretreatment with icv propranolol increased basal ACTH secretion levels, but icv alcohol did not increase this effect. Propranolol also blunted icv alcohol-induced PVN Fos expression. A low dose of phenoxybenzamine, an α-adrenergic receptor antagonist, did not affect the ability of icv alcohol to stimulate ACTH release. However, a higher dose of the drug was able to block the ACTH response to icv alcohol. Despite this, phenoxybenzamine did not inhibit alcohol-induced Fos expression. Icv pretreatment with corynanthine, a selective α-1 adrenergic receptor antagonist, modestly raised basal ACTH levels and blocked the icv alcohol-induced secretion of this hormone.

CONCLUSIONS

These results indicate that the LC and norepinephrine play important roles in HPA activation caused by icv alcohol administration, but that the specific adrenergic receptor subtypes involved in this phenomenon still need to be identified.

Regulatory mechanism of hypothalamo-pituitary-adrenal (HPA) axis and neuronal changes after adrenalectomy in type 2 diabetes

Diabetes, especially type 2, is closely associated with hypothalamo-pituitary-adrenal (HPA) axis regulation. Short-term effects of adrenalectomy (ADX) in type 2 diabetes are well characterized; however, there have been few reports on the long-term effects of ADX in genetically engineered type 2 diabetes and the neuroendocrine system. We performed bilateral ADX in Zucker Lean Control rats (ZLC; ADX-ZLC), Zucker Diabetic Fatty rats (ZDF; ADX-ZDF), and sham control rats to evaluate how the HPA axis would be regulated in long-term corticosterone deficient type 2 diabetic animals. We evaluated arginine vasopressin (AVP), glucocorticoid receptor (GR), and corticotropin-releasing hormone (CRH) expression with immunohistochemistry (IHC), immunofluorescence, real-time PCR, and Western blot analysis in each treatment group 7 weeks post ADX to assess HPA axis regulatory patterns in connection with type 2 diabetes. Additionally, mRNA expression of AVP and CRH receptors (V1aR, V1bR, CRHR1, and CRHR2) was also measured and adrenocorticotropin hormone (ACTH) immunoreactivity was surveyed by IHC to add to data regarding the regulatory mechanism. AVP and CRH protein expression levels increased after ADX in the hypothalamus of diabetic rats based on IHC results; however, we found that the subtypes of each receptor may be regulated differently in ADX groups compared to sham groups. Immunoreactivity of ACTH in the pituitary gland was enhanced in ADX groups and GR expression levels in the hypothalamic paraventricular nuclei (PVN) remained high, as determined by IHC as well as Western blot analysis. Without the negative feedback system of corticosterone, CRH is highly enhanced and may primarily combine with CRHR1 to stimulate negative feedback through ACTH in the pituitary gland in type 2 diabetic rats with long-term ADX. Although the negative feedback signal was not transmitted appropriately following long-term ADX with type 2 diabetes, a high GR protein level was maintained as in type 2 diabetes. The long-termed lack of corticosterone in the blood stream is a very important factor for normal regulation of the HPA axis even in diabetic animals. From the data, we can conclude that the stimulated HPA axis regulation in the developing type 2 diabetic animals following long-term adrenalectomy has remained elevated rather than diminished. Therefore, the current study may provide useful information to better understand patients suffering from both type 2 diabetes and Addison's disease.

Glucocorticoid dependency of surgical stress-induced FosB/DeltaFosB expression in the paraventricular and supraoptic nuclei of the rat hypothalamus

FosB is a member of the Fos family transcription factors. To determine whether FosB expression is regulated by glucocorticoids (GCs) in the hypothalamus, rats underwent sham adrenalectomy (sham-ADX) or bilateral ADX, and FosB/DeltaFosB (DeltaFosB, a truncated splice variant of FosB)-immunoreactivity (ir) was determined in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). In the parvocellular division of the PVN (paPVN) and SON, FosB/DeltaFosB-immunoreactivity (ir) increased significantly following sham-ADX compared to naive rats, which was suppressed with either corticosterone (CORT) or dexamethasone (DEX). Following ADX, the increase in FosB/DeltaFosB-ir was much more prominent than that in the sham-ADX group, and the ADX-induced robust increase was suppressed by CORT or DEX, but not by aldosterone. Stressless removal of CORT from drinking water did not induce FosB/DeltaFosB-ir in either the PVN or SON, and thus the up-regulation of FosB/DeltaFosB-ir following ADX was dependent on the systemic stress associated with surgery. In the paPVN, the majority of corticotrophin-releasing hormone (CRH) neurones co-expressed FosB/DeltaFosB-ir following ADX, whereas, in the magnocellular division of the PVN, vasopressin (AVP) and oxytocin (OXT) neurones did not express FosB/DeltaFosB-ir. In the SON, approximately 40% of the AVP neurones co-expressed FosB/DeltaFosB-ir following ADX, but the OXT neurones were devoid of FosB/DeltaFosB-ir. In concert with these results obtained in vivo, DEX suppressed the forskolin-induced increase in FosB gene promoter activity in a homologous hypothalamic cell line. These results suggest that GCs may be a potent regulator of FosB/DeltaFosB expression, which is induced by stress, in hypothalamic neuroendocrine neurones.

Regulatory mechanism of the arginine vasopressin-enhanced green fluorescent protein fusion gene expression in acute and chronic stress

Various kinds of stress cause neuroendocrine responses such as corticotropin-releasing hormone (CRH) or arginine vasopressin (AVP) release from parvocellular division of the paraventricular nucleus (PVN) and activation of the hypothalamo-pituitary adrenal (HPA) axis. We examined the effects of acute and chronic stress on the expression of the AVP-enhanced green fluorescent protein (eGFP) fusion gene in the hypothalamus, using chronic salt loading as an osmotic stimulation, intraperitoneal administration of lipopolysaccharide (LPS) as acute inflammatory stress and adjuvant arthritis (AA) as chronic inflammatory/nociceptive stress. Salt loading caused a marked increase in the eGFP gene expression and eGFP fluorescence in the supraoptic nucleus, magnocellular division of the PVN and internal layer of the median eminence (ME). Administration of LPS caused increased fluorescence in parvocellular division of the PVN and external layer of the ME. AA rats revealed an increased expression of the eGFP gene and eGFP fluorescence in both magnocellular and parvocellular divisions of the PVN and both internal and external layers of the ME. On the other hand, the levels of the CRH gene expression in parvocellular division of the PVN were significantly decreased as AA developed, though plasma concentrations of corticosterone were significantly increased. These results indicate that AVP-eGFP transgenic rats enable the detection of changes in AVP expression more easily than by using procedures such as immunohistochemistry. We propose that AVP-eGFP transgenic rats represent a useful animal model for further understanding of the physiology of AVP expression in the hypothalamo-pituitary system under various physiological conditions, including various kinds of stress.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Dexamethasone and stressor-magnitude regulation of stress-induced transcription of HPA axis secretagogues in the rat

Regulation of the production of hypothalamic-pituitary-adrenal (HPA) axis secretagogues, corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), may be differentially sensitive to the negative feedback effects of glucocorticoids. We chose to study this phenomenon by examining the ability of dexamethasone to influence CRH and AVP heteronuclear RNA (hnRNA) levels in an escapable/inescapable (ES/IS) foot-shock stress paradigm. On Day 1, adult male rats were subjected to either ES or IS foot-shock; on Day 2, saline or dexamethasone (100 microg/kg) was administered 2 h prior to the stressor. We found that ES/IS foot-shock stimulated similar robust increases in plasma adrenocorticotrophic hormone (ACTH) and corticosterone concentrations, and medial parvocellular division of the paraventricular nucleus (mpPVN) AVP and CRH hnRNA and c-fos mRNA levels in saline-treated ES/IS rats. Dexamethasone pretreatment suppressed ACTH and corticosterone levels similarly in IS and ES animals. Dexamethasone pretreatment also suppressed mpPVN CRH and AVP hnRNA levels at 30 min. However, by 120 min, the mpPVN AVP hnRNA levels in dexamethasone-treated rats were similar to those measured in the saline group. We also found that rats that received the most shocks on Day 1 had greater HPA axis activation on Day 2. We conclude that the magnitude of the foot-shock stressor, determined by learned and immediate cues, is important in determining the magnitude of the HPA response.

Corticotropin-releasing factor in the basolateral amygdala enhances memory consolidation via an interaction with the beta-adrenoceptor-cAMP pathway: dependence on glucocorticoid receptor activation

Extensive evidence indicates that stress hormone effects on the consolidation of emotionally influenced memory involve noradrenergic activation of the basolateral complex of the amygdala (BLA). The present experiments examined whether corticotropin-releasing factor (CRF) modulates memory consolidation via an interaction with the beta-adrenoceptor-cAMP system in the BLA. In a first experiment, male Sprague Dawley rats received bilateral infusions of the CRF-binding protein ligand inhibitor CRF(6-33) into the BLA either alone or together with the CRF receptor antagonist alpha-helical CRF(9-41) immediately after inhibitory avoidance training. CRF(6-33) induced dose-dependent enhancement of 48 h retention latencies, which was blocked by coadministration of alpha-helical CRF(9-41), suggesting that CRF(6-33) enhances memory consolidation by displacing CRF from its binding protein, thereby increasing "free" endogenous CRF concentrations. In a second experiment, intra-BLA infusions of atenolol (beta-adrenoceptor antagonist) and Rp-cAMPS (cAMP inhibitor), but not prazosin (alpha(1)-adrenoceptor antagonist), blocked CRF(6-33)-induced retention enhancement. In a third experiment, the CRF receptor antagonist alpha-helical CRF(9-41) administered into the BLA immediately after training attenuated the dose-response effects of concurrent intra-BLA infusions of clenbuterol (beta-adrenoceptor agonist). In contrast, alpha-helical CRF(9-41) did not alter retention enhancement induced by posttraining intra-BLA infusions of either cirazoline (alpha(1)-adrenoceptor agonist) or 8-br-cAMP (cAMP analog). These findings suggest that CRF facilitates the memory-modulatory effects of noradrenergic stimulation in the BLA via an interaction with the beta-adrenoceptor-cAMP cascade, at a locus between the membrane-bound beta-adrenoceptor and the intracellular cAMP formation site. Moreover, consistent with evidence that glucocorticoids enhance memory consolidation via a similar interaction with the beta-adrenoceptor-cAMP cascade, a last experiment found that the CRF and glucocorticoid systems within the BLA interact in influencing beta-adrenoceptor-cAMP effects on memory consolidation.

Maternal attenuation of hypothalamic paraventricular nucleus norepinephrine switches avoidance learning to preference learning in preweanling rat pups

Infant rats learn to prefer stimuli paired with pain, presumably due to the importance of learning to prefer the caregiver to receive protection and food. With maturity, a more 'adult-like' learning system emerges that includes the amygdala and avoidance/fear learning. The attachment and 'adult-like' systems appear to co-exist in older pups with maternal presence engaging the attachment system by lowering corticosterone (CORT). Specifically, odor-shock conditioning (11 odor-0.5 mA shock trials) in 12-day-old pups results in an odor aversion, although an odor preference is learned if the mother is present during conditioning. Here, we propose a mechanism to explain pups ability to 'switch' between the dual learning systems by exploring the effect of maternal presence on hypothalamic paraventricular nucleus (PVN) neural activity, norepinephrine (NE) levels and learning. Maternal presence attenuates both PVN neural activity and PVN NE levels during odor-shock conditioning. Intra-PVN NE receptor antagonist infusion blocked the odor aversion learning with maternal absence, while intra-PVN NE receptor agonist infusion permitted odor aversion learning with maternal presence. These data suggest maternal control over pup learning acts through attenuation of PVN NE to reduce the CORT required for pup odor aversion learning. Moreover, these data also represent pups' continued maternal dependence for nursing, while enabling aversion learning outside the nest to prepare for pups future independent living.

Catecholaminergic control of mitogen-activated protein kinase signaling in paraventricular neuroendocrine neurons in vivo and in vitro: a proposed role during glycemic challenges

Paraventricular hypothalamic (PVH) corticotropin-releasing hormone (CRH) neuroendocrine neurons mount neurosecretory and transcriptional responses to glycemic challenges [intravenous 2-deoxyglucose (2-DG) or insulin]. Although these responses require signals from intact afferents originating from hindbrain CA (catecholaminergic) neurons, the identity of these signals and the mechanisms by which they are transduced by PVH neurons during glycemic challenge remain unclear. Here, we tested whether the prototypical catecholamine, norepinephrine (NE), can reproduce PVH neuroendocrine responses to glycemic challenge. Because these responses include phosphorylation of p44/42 mitogen-activated protein (MAP) kinases [extracellular signal-regulated kinases 1/2 (ERK1/2)], we also determined whether NE activates ERK1/2 in PVH neurons and, if so, by what mechanism. We show that systemic insulin and 2-DG, and PVH-targeted NE microinjections, rapidly elevated PVH phospho-ERK1/2 levels. NE increased Crh and c-fos expression, together with circulating ACTH/corticosterone. However, because injections also increased c-Fos mRNA in other brain regions, we used hypothalamic slices maintained in vitro to clarify whether NE activates PVH neurons without contribution of inputs from distal regions. In slices, bath-applied NE triggered robust phospho-ERK1/2 immunoreactivity in PVH (including CRH) neurons, which attenuated markedly in the presence of the alpha1 adrenoceptor antagonist, prazosin, or the MAP kinase kinase (MEK) inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene). Therefore, at a systems level, local PVH delivery of NE is sufficient to account for hindbrain activation of CRH neuroendocrine neurons during glycemic challenge. At a cellular level, these data provide the first demonstration that MAP kinase signaling cascades (MEK-->ERK) are intracellular transducers of noradrenergic signals in CRH neurons, and implicate this transduction mechanism as an important component of central neuroendocrine responses during glycemic challenge.

Activation of stress‐related hypothalamic neuropeptide gene expression during morphine withdrawal

Morphine withdrawal results in serious affective and somatic symptoms including activation of the hypothalamo-pituitary-adrenocortical (HPA) axis. To reveal secretory, activational and transcriptional changes in the hypothalamus of morphine-dependent rats during naloxone precipitated opioid withdrawal, we measured corticosterone secretion, c-Fos induction and heteronuclear (hn)RNA levels of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) in naïve and morphine dependent animals injected with saline or 5 mg/kg naloxone. Naloxone precipitated morphine withdrawal resulted in a significant increase in corticosterone secretion and induction of neuronal activation in the hypothalamic paraventricular nucleus (PVH) 2 h after challenge. Using probes complementary to intronic sequences of genes encoding neuropeptides in parvocellular neurosecretory neurons of the PVH, we found robust increases in CRH and AVP hnRNAs in morphine dependent rats during naloxone precipitated withdrawal. Naïve rats and animals that were implanted with morphine pellets for 8 days did not display significant up-regulation of ongoing neuropeptide expression in the parvocellular compartment of the PVH. In addition to hypophyseotropic neurons, naloxone precipitated withdrawal resulted in a marked activation in autonomic-related projection neurons in PVH and in the magnocellular neurons in the PVH and supraoptic nuclei. These activations however were not associated with induction of CRH or AVP hnRNAs.

Role of glucocorticoids and cAMP-mediated repression in limiting corticotropin-releasing hormone transcription during stress

The role of glucocorticoids and the repressor isoform of cAMP response element (CRE) modulator (CREM), inducible cAMP early repressor (ICER), in limiting corticotropin-releasing hormone (CRH) transcription during restraint stress were examined in both intact and adrenalectomized rats receiving glucocorticoid replacement. CRH primary transcript, measured by intronic in situ hybridization, increased after 30 min of restraint and returned to basal levels by 90 min, despite the persistent stressor. The decline was independent of circulating glucocorticoids, because adrenalectomized rats displayed an identical pattern. ICER mRNA in the hypothalamic paraventricular nucleus (PVN) increased after 30 min and remained elevated for up to 4 h in a glucocorticoid-independent manner. Western blot and electrophoretic mobility shift assay analyses showed increases in endogenous ICER in the PVN of rats subjected to restraint stress for 3 h. Chromatin immunoprecipitation assays showed the recruitment of CREM by the CRH CRE in conjunction with decreases in RNA polymerase II (Pol II) binding in the PVN region of rats restrained for 3 h. These data show that stress-induced glucocorticoids do not mediate the limitation of CRH transcription. Furthermore, the ability of CREM to bind the CRH CRE and the time relationship between elevated CREM and reduced Pol II recruitment by the CRH promoter suggest that inhibitory isoforms of CREM induced during stress contribute to the decline in CRH gene transcription during persistent stimulation.

Glucocorticoid regulation of peptide genes in neuroendocrine CRH neurons: a complexity beyond negative feedback

This review will examine our current knowledge of a fundamental property of CRH neuroendocrine neurons: how the major endpoint of the HPA axis--adrenal glucocorticoids--interacts with the mechanisms controlling the expression of the genes that encode ACTH secretogogues. A great deal of work over the past 25 years has led to the notion that this question has an ostensibly simple answer: glucocorticoids inhibit peptide gene expression using "negative feedback" at the CRH neuron and elsewhere. However, closely examining how glucocorticoids act in different physiological circumstances reveals a much more complex set of answers, particularly if we consider how the processes that control peptide synthesis and release are coupled. Out of this examination emerges a more flexible and complex framework for examining the integrative mechanisms controlling the CRH neuron. Although we will mostly focus on the Crh gene, relevant aspects of the vasopressin (Avp) and pro-enkephalin (pEnk) gene regulatory mechanisms will also be discussed.

A single dose of metyrapone caused long-term dysregulation of the hypothalamic–pituitary–adrenal axis in the rat

There is evidence that metyrapone (MET), apart from its inhibition of 11-beta steroid hydroxylation, may exert some stress-like effects in the brain, including the activation of the hypothalamic-pituitary-adrenal (HPA) axis and the induction of c-fos. Since a single exposure to some stressors has been found to exert long-term effects on the HPA axis, we hypothesized that a single dose of MET (200 mg/kg, s.c.) could exert even stronger effects, due to the combination of its stressful properties with the lack of constrain of the HPA axis by glucocorticoids. Whereas the inhibitory effect of the drug on corticosterone secretion lasted less than 24 h, its stimulatory effect on the HPA axis could be seen for at least 2 days after the injection. Surprisingly, on day 8, an exacerbated HPA response to immobilization stress was observed in MET rats, despite complete normalization of resting levels of HPA hormones. At this time it was also observed, under basal conditions, increased levels of mRNA for CRH and arginin-vasopressin in the parvocellular region of the paraventricular nucleus of the hypothalamus (pPVN), along with reduced mRNA for glucocorticoid receptors in dentate gyrus and hippocampus CA1, but not in pPVN or medial prefrontal cortex. These data suggest that a single MET administration can exert a marked and long-lasting dysregulation of both resting and stress-induced activity of the HPA axis. Thus, attention should be paid to these properties when using the drug to study the functional role of glucocorticoids.

Control of food intake through regulation of cAMP

The 3', 5'-cyclic adenosine monophosphate (cAMP) is a classic second messenger that is intimately involved in the regulation of food intake at the hypothalamus. cAMP can mediate the orexigenic and anorectic effects of various peripheral hormones or neuropeptides in a region-specific and neuron-specific manner. The importance of cAMP is particularly highlighted in a series of findings about cAMP transducing the anorectic signals of leptin and alpha-msh. This chapter provides an overview of several studies on how regulation of food intake takes place with cAMP as the second messenger in the hypothalamus.

Neuropeptides in anxiety modulation

This review is focused on the involvement of neuropeptides in the modulation of physiological and pathological anxiety. Neuropeptides play a major role as endogenous modulators of complex behaviours, including anxiety-related behaviour and psychopathology, particularly due to their high number and diversity, the dynamics of release patterns in distinct brain areas and the multiple and variable modes of interneuronal communication they are involved in. Manipulations of central neuropeptidergic systems to reveal their role in anxiety (and often comorbid depression-like behaviour) include a broad spectrum of loss-of-function and gain-of-function approaches. This article concentrates on those neuropeptides for which an involvement as endogenous anxiolytic or anxiogenic modulators is well established by such complementary approaches. Particular attention is paid to corticotropin-releasing hormone (CRH) and vasopressin (AVP) which, closely linked to stress, neuroendocrine regulation, social behaviour and learning/memory, play critical roles in the regulation of anxiety-related behaviour of rodents. Provided that their neurobiology, neuroendocrinology and molecular-genetic background are well characterized, these and other neuropeptidergic systems may be promising targets for future anxiolytic strategies.

Differential effects of forced swim-stress on the corticotropin-releasing hormone and vasopressin gene transcription in the parvocellular division of the paraventricular nucleus of rat hypothalamus

Corticotropin-releasing hormone (CRH) and vasopressin (AVP) colocalize in the parvocellular division of the paraventricular nucleus of the hypothalamus (PVN). We examined the effect of forced swim-stress on the CRH and AVP primary transcript (hnRNA) levels in the rat PVN by semi-quantitative in situ hybridization. CRH hnRNA increased markedly following 10-min swim-stress and returned to the basal level by 2 h. AVP hnRNA in the parvocellular division of the PVN, where AVP colocalizes with CRH, did not change significantly immediately after the swim-stress, but it did increase significantly 2 h after the stress. Pretreatment with dexamethasone abolished the increases in CRH and AVP hnRNA levels after the swim-stress. The present results demonstrate the differential effects of forced swim-stress on the CRH and AVP gene transcription in the parvocellular PVN, confirming the diverse response of the dual peptide-containing system in the face of acute stressful events.

Heart failure and the brain: new perspectives

Despite recent therapeutic advances, the prognosis for patients with heart failure remains dismal. Unchecked neurohumoral excitation is a critical element in the progressive clinical deterioration associated with the heart failure syndrome, and its peripheral manifestations have become the principal targets for intervention. The link between peripheral systems activated in heart failure and the central nervous system as a source of neurohumoral drive has therefore come under close scrutiny. In this context, the forebrain and particularly the paraventricular nucleus of the hypothalamus have emerged as sites that sense humoral signals generated peripherally in response to the stresses of heart failure and contribute to the altered volume regulation and augmented sympathetic drive that characterize the heart failure syndrome. This brief review summarizes recent studies from our laboratory supporting the concept that the forebrain plays a critical role in the pathogenesis of ischemia-induced heart failure and suggesting that the forebrain contribution must be considered in designing therapeutic strategies. Forebrain signaling by neuroactive products of the renin-angiotensin system and the immune system are emphasized.