Adrenal steroid regulation of neuropeptide Y (NPY) mRNA: differences between dentate hilus and locus coeruleus and arcuate nucleus

Long term effects of stress on hippocampal function: Emphasis on early life stress paradigms and potential involvement of neuropeptide Y

The brain is both central in orchestrating the response to stress, and, a very sensitive target when such response is not controlled. In fact, stress has long been associated with the onset and/or exacerbation of several neuropsychiatric disorders such as anxiety, depression, and drug addiction. The hippocampus is a key brain region involved in the response to stress, not only due to its anatomical connections with the hypothalamic-pituitary-adrenal axis but also as a major target of stress mediators. The hippocampal dentate gyrus (DG)-CA3 circuit, composed of DG granule cells axons (mossy fibers) synapsing onto CA3 pyramidal cells, plays an essential role in memory encoding and retrieval, functions that are vulnerable to stress. Although naturally excitatory, this circuit is under the inhibitory control of GABAergic interneurons that maintain the excitation/inhibition balance. One subgroup of such interneurons produces neuropeptide Y (NPY), which has emerged as a promising endogenous stress "resilience molecule" due to its anxiolytic and anti-epileptic properties. Here we examine existing evidence that reveals a potential role for hilar NPY+ interneurons in mediating stress-induced changes in hippocampal function. We will focus specifically on rodent models of early life stress (ELS), defined as adverse conditions during the early postnatal period that can have profound consequences for neurodevelopment. Collectively, these findings suggest that the long-lasting effects of ELS might stem from the loss of GABAergic NPY+ cells, which then can lead to reduced inhibition in the DG-CA3 pathway. Such change might then lead to hyperexcitability and concomitant hippocampal-dependent behavioral deficits.

Estradiol regulates effects of hindbrain activator 5-aminoimidazole-4-carboxamide-riboside administration on hypothalamic adenosine 5'-monophosphate-activated protein kinase activity and metabolic neurotransmitter mRNA and protein expression

Hindbrain adenosine 5'-monophosphate-activated protein kinase (AMPK) activation alters hypothalamic neuronal genomic activity in an estradiol (E)-dependent manner. This study examines the premise that E regulates metabolic effector neuron reactivity to hindbrain AMPK. Paraventricular (PVH), arcuate (ARH), and ventromedial (VMH) nuclei were micropunched from brains of E- or oil (O)-implanted ovariectomized female rats that had been injected, into the fourth ventricle, with the AMPK activator 5-aminoimidazole-4-carboxamide-riboside (AICAR; A) or saline (S) and analyzed by quantitative polymerase chain reaction and Western blotting for neurotransmitter mRNA and protein expression. PVH corticotrophin-releasing hormone gene and protein profiles were decreased in O/A and E/A animals. ARH pro-opiomelanocortin (POMC) mRNA and protein were both elevated in O/A but were diminished or unchanged, respectively, in E/A animals; ARH neuropeptide Y (NPY) transcription was inhibited in O/A and E/A animals, but neuropeptide content was augmented in E/A only. VMH SF-1 mRNA and protein were reduced in O and E animals. AICAR did not alter AMPK protein in any structure but elevated PVH (↑E), did not alter ARH, and decreased VMH (↓O,↓E) pAMPK. Results demonstrate hypothalamic metabolic neurotransmitter and AMPK reactivity to hindbrain AMPK activation, including E-dependent adjustments in POMC and NPY transcription and protein expression. Dissimilar POMC (↑O vs. ↔E) and NPY (↓O vs. ↑E) neuropeptide responses to caudal fourth ventricle AICAR indicate E regulation of hindbrain AMPK signaling and/or target receptivity, implying that ARH-controlled metabolic responses may differ in the presence vs. absence of E. Evidence for variable changes in hypothalamic AMPK activity resulting from hindbrain sensor manipulation suggests that individual (or region-based groups of) AMPK-expressing neuron populations are uniquely impacted by hindbrain AMPK.

Poststress block of kappa opioid receptors rescues long-term potentiation of inhibitory synapses and prevents reinstatement of cocaine seeking

BACKGROUND

Dopaminergic neurons in the ventral tegmental area of the brain are an important site of convergence of drugs and stress. We previously identified a form of long-term potentiation of gamma-aminobutyric acid (GABA)ergic synapses on these neurons (LTPGABA). Our studies have shown that exposure to acute stress blocks this LTP and that reversal of the block of LTPGABA is correlated with prevention of stress-induced reinstatement of cocaine-seeking behavior.

METHODS

Sprague-Dawley rats were subjected to cold-water swim stress. Midbrain slices were prepared following stress, and whole-cell patch clamp recordings of inhibitory postsynaptic currents were performed from ventral tegmental area dopamine neurons. Antagonists of glucocorticoid receptors and kappa opioid receptors (κORs) were administered at varying time points after stress. Additionally, the ability of a kappa antagonist administered following stress to block forced swim stress-induced reinstatement of cocaine self-administration was tested.

RESULTS

We found that an acute stressor blocks LTPGABA for 5 days after stress through a transient activation of glucocorticoid receptors and more lasting contribution of κORs. Even pharmacological block of κORs beginning 4 days after stress has occurred reversed the block of LTPGABA. Administration of a κORs antagonist following stress prevents reinstatement of cocaine-seeking behavior.

CONCLUSIONS

A brief stressor produces changes in the reward circuitry lasting several days. Our findings reveal roles for glucocorticoid receptors and κORs as mediators of the lasting effects of stress on synaptic plasticity. κORs antagonists reverse the neuroadaptations underlying stress-induced drug-seeking behavior and may be useful in the treatment of cocaine addiction.

The role of neuropeptide Y in energy homeostasis

When administered into the brain, NPY acts at Y1 and Y5 receptors to increase food intake. The response occurs with a short latency and is quite robust, such that exogenous NPY is generally considered to be the most potent of a growing list of orexigenic compounds that act in the brain. The role of endogenous NPY is not so straightforward, however. Evidence from diverse types of experiments suggests that rather than initiating behavioral eating per se, endogenous NPY elicits autonomic responses that prepare the individual to better cope with consuming a calorically large meal.

An animal model of eating disorders associated with stressful experience in early life

Experience of childhood abuse is prevalent among patients with eating disorders, and dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis is implicated in its pathophysiology. Neonatal maternal separation is considered as an animal model of stressful experience early in life. Many of studies have demonstrated its impact both on the activity of HPA axis and the development of psycho-emotional disorders later in life. In this paper, a series of our researches on developing an animal model of eating disorders is reviewed. An animal model of neonatal maternal separation was used; Sprague-Dawley pups were separated from dam daily for 180 min during the first 2 weeks of life (MS) or undisturbed. Anxiety-/depression-like behaviors were observed in MS rats at the age of two months with decreased serotonergic activity in the hippocampus and the raphe. Post-weaning social isolation promoted food intake and weight gain of adolescent MS pups, with impacts on anxiety-like behaviors. Sustained hyperphagia was observed in the MS pups subjected to a fasting/refeeding cycle repeatedly during adolescence, with increased plasma corticosterone levels. Anhedonia, major symptom of depression, to palatable food was observed in adolescent MS pups with blunted response of the mesolimbic dopaminergic activity to stress. Results suggest that neonatal maternal separation lead to the development of eating disorders when it is challenged with social or metabolic stressors later in life, in which dysfunctions in the HPA axis and the brain monoaminergic systems may play important roles.

Corticotropin-releasing hormone (CRH) transgenic mice display hyperphagia with increased Agouti-related protein mRNA in the hypothalamic arcuate nucleus

Although glucocorticoid-induced hyperphagia is observed in the patients with glucocorticoid treatment or Cushing's syndrome, its molecular mechanism is not clear. We thus explored the expression of neuropeptide mRNAs in the hypothalamus related to appetite regulation in CRH over-expressing transgenic mice (CRH-Tg), a model of Cushing's syndrome. We measured food intake, body weight (including body fat weight) and plasma corticosterone levels in CRH-Tg and their wild-type littermates (WT) at 6 and 14 weeks old. We also examined neuropeptide Y (NPY), proopiomelanocortin (POMC) and Agouti-related protein (AgRP) mRNAs in the arcuate nucleus (ARC) using in situ hybridization. Circulating corticosterone levels in CRH-Tg were markedly elevated at both 6 and 14 weeks old. Body fat weight in CRH-Tg was significantly increased at 14 weeks old, which is considered as an effect of chronic glucocorticoid excess. At both 6 and 14 weeks old, CRH-Tg mice showed significant hyperphagia compared with WT (14w old: WT 3.9±0.1, CRH-Tg 5.1±0.7 g/day, p<0.05). Unexpectedly, NPY mRNA levels in CRH-Tg were significantly decreased at 14 weeks old (WT: 1571.5±111.2, CRH-Tg: 949.1±139.3 dpm/mg, p<0.05), and there were no differences in POMC mRNA levels between CRH-Tg and WT. On the other hand, AgRP mRNA levels in CRH-Tg were significantly increased compared with WT at both ages (14w old: WT 365.6±88.6, CRH-Tg 660.1±87.2 dpm/ mg, p<0.05). These results suggest that glucocorticoid-induced hyperphagia is associated with increased hypothalamic AgRP. Our results also indicate that hypothalamic NPY does not have an essential role in the increased food intake during glucocorticoid excess.

Neuropeptide Y in normal eating and in genetic and dietary-induced obesity

Neuropeptide Y (NPY) is one the most potent orexigenic peptides found in the brain. It stimulates food intake with a preferential effect on carbohydrate intake. It decreases latency to eat, increases motivation to eat and delays satiety by augmenting meal size. The effects on feeding are mediated through at least two receptors, the Y1 and Y5 receptors. The NPY system for feeding regulation is mostly located in the hypothalamus. It is formed of the arcuate nucleus (ARC), where the peptide is synthesized, and the paraventricular (PVN), dorsomedial (DMN) and ventromedial (VMN) nuclei and perifornical area where it is active. This activity is modulated by the hindbrain and limbic structures. It is dependent on energy availability, e.g. upregulation with food deprivation or restriction, and return to baseline with refeeding. It is also sensitive to diet composition with variable effects of carbohydrates and fats. Leptin signalling and glucose sensing which are directly linked to diet type are the most important factors involved in its regulation. Absence of leptin signalling in obesity models due to gene mutation either at the receptor level, as in the Zucker rat, the Koletsky rat or the db/db mouse, or at the peptide level, as in ob/ob mouse, is associated with increased mRNA abundance, peptide content and/or release in the ARC or PVN. Other genetic obesity models, such as the Otsuka-Long-Evans-Tokushima Fatty rat, the agouti mouse or the tubby mouse, are characterized by a diminution in NPY expression in the ARC nucleus and by a significant increase in the DMN. Further studies are necessary to determine the exact role of NPY in these latter models. Long-term exposure to high-fat or high-energy palatable diets leads to the development of adiposity and is associated with a decrease in hypothalamic NPY content or expression, consistent with the existence of a counter-regulatory mechanism to diminish energy intake and limit obesity development. On the other hand, an overactive NPY system (increased mRNA expression in the ARC associated with an upregulation of the receptors) is characteristic of rats or rodent strains sensitive to dietary-induced obesity. Finally, NPY appears to play an important role in body weight and feeding regulation, and while it does not constitute the only target for drug treatment of obesity, it may nevertheless provide a useful target in conjunction with others.

Physiology and gene regulation of the brain NPY Y1 receptor

Neuropeptide Y (NPY) is one of the most prominent and abundant neuropeptides in the mammalian brain where it interacts with a family of G-protein coupled receptors, including the Y(1) receptor subtype (Y(1)R). NPY-Y(1)R signalling plays a prominent role in the regulation of several behavioural and physiological functions including feeding behaviour and energy balance, sexual hormone secretion, stress response, emotional behaviour, neuronal excitability and ethanol drinking. Y(1)R expression is regulated by neuronal activity and peripheral hormones. The Y(1)R gene has been isolated from rodents and humans and it contains multiple regulatory elements that may participate in the regulation of its expression. Y(1)R expression in the hypothalamus is modulated by changes in energetic balance induced by a wide variety of conditions (fasting, pregnancy, hyperglycaemic challenge, hypophagia, diet induced obesity). Estrogens up-regulate responsiveness to NPY to stimulate preovulatory GnRH and gonadotropin surges by increasing Y(1)R gene expression both in the hypothalamus and the pituitary. Y(1)R expression is modulated by different kinds of brain insults, such as stress and seizure activity, and alteration in its expression may contribute to antidepressant action. Chronic modulation of GABA(A) receptor function by benzodiazepines or neuroactive steroids also affects Y(1)R expression in the amygdala, suggesting that a functional interaction between the GABA(A) receptor and Y(1)R mediated signalling may contribute to the regulation of emotional behaviour. In this paper, we review the state of the art concerning Y(1)R function and gene expression, including our personal contribution to many of the subjects mentioned above.

Fasting-induced increases of arcuate NPY mRNA and plasma corticosterone are blunted in the rat experienced neonatal maternal separation

This study was conducted to examine the effects of neonatal maternal separation on the hypothalamic expression of feeding peptides in later life. Pups in maternal separation (MS) groups were separated from their dam for 3 h daily from postnatal day (PND) 1-14, while pups in non-handled (NH) groups were left undisturbed. Rats were sacrificed on PND 60 to examine the gene expression of neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) in the hypothalamic arcuate nucleus by mRNA in situ hybridization. Half of the rats from each group were food-deprived for 48 h before sacrifice. POMC mRNA expression increased in the free fed MS group compared with the free fed NH group. Food deprivation significantly decreased the arcuate POMC mRNA level in both groups. Body weight gain, basal levels of plasma corticosterone, leptin, and arcuate NPY mRNA were not modulated by experience of neonatal maternal separation. However, fasting-induced increases of plasma corticosterone and arcuate NPY expression were blunted in MS rats. These results suggest that neonatal maternal separation may increase the basal expression level of arcuate POMC mRNA, while inhibit the fasting-induced expression of arcuate NPY mRNA, later in life. Lastly, the altered expression of arcuate NPY mRNA, but not of arcuate POMC mRNA, appeared to be related with altered activity of the hypothalamic-pituitary-adrenal gland axis in offspring by neonatal maternal separation.

Proopiomelanocortin-deficient mice are hypersensitive to the adverse metabolic effects of glucocorticoids

Congenital lack of proopiomelanocortin (POMC) causes obesity and glucocorticoid deficiency. The responses of Pomc-/- and wild-type mice to the administration of corticosterone were compared. In study 1, mice were given corticosterone-supplemented water (CORT) for 10 days, resulting in plasma CORT levels within the physiological range, with partial suppression of hypothalamic corticotropin-releasing hormone expression to a similar degree between genotypes. Body weight, fat mass, and food intake increased in CORT-treated Pomc-/- but not wild-type mice. CORT increased plasma insulin levels 50-fold in Pomc-/- versus 14-fold in wild-type mice (P < 0.01) and increased hypothalamic agouti-related protein (AgRP) expression by more than 200% in Pomc-/- versus 40% in wild type (P < 0.05). In study 2, mice were given CORT from weaning, and Pomc-/- but not wild-type mice developed hyperglycemia, ketonuria, and hepatic steatosis by 8-12 weeks. Thus, Pomc-/- mice are hypersensitive to the adverse metabolic effects of glucocorticoids. Additionally, as the levels of plasma CORT achieved, especially in study 1, were not grossly supraphysiological, we conclude that glucocorticoid deficiency may afford Pomc-/- mice some protection from the full adverse consequences of melanocortin deficiency. This may occur through a mechanism involving the suppression of AgRP by the hypoadrenal state.

The effect of aldosterone on glucose metabolism

There is an association of glucose intolerance and diabetes with primary aldosteronism, but the frequency and mechanisms are not clear. This paper reviews the possible mechanisms of impaired glucose metabolism in primary aldosteronism. Patients with primary aldosteronism can have impaired pancreatic insulin release and reduction in insulin sensitivity. These effects may be due to hypokalemia, but the evidence suggests other factors such as a direct impact of excess aldosterone on insulin action in contributing to the metabolic dysfunction. In general adrenal surgery in cases of aldosterone-producing adenoma will correct the metabolic abnormalities, but it is less sure if treatment with spironolactone in cases of idiopathic hyperplasia will correct impaired glucose tolerance.

Aldosterone impairs insulin responsiveness in U-937 human promonocytic cells via the downregulation of its own receptor

In an earlier study, we have reported an inhibition of insulin receptor (IR) mRNA levels and insulin binding by aldosterone in U-937 human promonocytic cells. In the present extension of our studies, we demonstrate that this inhibition by aldosterone had no effects on basal glucose transport or on basal thymidine incorporation into DNA, while the cell responsiveness reflected by the maximal response to insulin was decreased by 23% for glucose transport and by 31% for DNA synthesis after the aldosterone treatment. We also prove that this inhibition of the insulin response by aldosterone is mediated by a downregulation of the levels of mineralocorticoid receptors (MRs) (50% decrease) and their mRNA (50% decrease). In addition, the mineralocorticoid antagonist spironolactone reversed the decrease in MR mRNA levels elicited by aldosterone, which suggests the involvement of this receptor in the process.

Coregulator proteins and corticosteroid action in the brain

The corticosteroid hormones cortisol and corticosterone are secreted by the adrenal gland in response to stress. They have profound effects on brain function, which are mediated by the related mineralocorticoid (MR) and glucocorticoid (GR) receptors. The MR and GR are ligand-activated transcription factors and exert different, sometimes opposing effects on the brain. The balance between these two receptor activities is considered essential for appropriate corticosteroid signalling and health. An exciting recent insight in steroid biology is that the nature and magnitude of steroid receptor-mediated responses depend not only on ligand and receptor availability, but also in a critical manner on the presence of downstream mediator proteins (coregulators), such as the steroid receptor coactivators and nuclear receptor corepressors. Members of the coregulator families differ in their specific interactions with steroid receptors, as well as in their distribution throughout the brain and pituitary. The activity of these proteins can be regulated both at the expression level, and by post-translational modifications. These characteristics make coregulator proteins of outstanding interest as determinants of receptor, cell and state-dependent effects of MR and GR signalling (and steroid receptor signalling in general) in the brain.

Depression as a spreading adjustment disorder of monoaminergic neurons: a case for primary implication of the locus coeruleus

A model for the pathophysiology of depression is discussed in the context of other existing theories. The classic monoamine theory of depression suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. More recent elaborations of the classic theory also implicitly include this postulate, other theories of depression frequently prefer to depart from the monoamine-based model altogether. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity, and concomitantly, exaggerated responses to acute increases in the presynaptic firing rate and transmitter release. It is proposed that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus (LC). Dysregulation of the LC projection activities may lead in turn to dysregulation of serotonergic and dopaminergic neurotransmission. Failure of the LC function could explain the basic impairments in the processing of novel information, intensive processing of irrational beliefs, and anxiety. Concomitant impairments in the serotonergic neurotransmission may contribute to the mood changes and reduction in the mesotelencephalic dopaminergic activity to loss of motivation, and anhedonia. Dysregulation of CRF and other neuropeptides such as neuropeptide Y, galanin and substance P may reinforce the LC dysfunction and thus further weaken the adaptivity to stressful stimuli.

The effect of acute and chronic restraint on the central expression of prepro-neuropeptide Y mRNA in normotensive and hypertensive rats

Neuropeptide Y (NPY), one of the most abundant neuropeptides found in the central nervous system (CNS), has been implicated in the regulation of many autonomic functions, including cardiovascular control and the central stress response. The present study represents a detailed investigation of the effects of acute and chronic restraint stress on the expression of the mRNA encoding the NPY precursor, prepro-NPY, in the CNS of normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) using in situ hybridization histochemistry. Basal (unstressed) levels of prepro-NPY mRNA expression were found to be significantly increased in the hypothalamic arcuate nucleus of SHR compared to WKY rats, with similar levels of prepro-NPY mRNA expression found in the remaining central nuclei. Following exposure to both acute and chronic restraint, significant changes in prepro-NPY mRNA expression were found in a variety of central regions in both strains, including the arcuate nucleus and hippocampus (both strains), medial amygdala and cortex (WKY only), and dentate gyrus, nucleus of the solitary tract and ventrolateral medulla (SHR only). A comparison of the temporal response to restraint revealed that significant differences between strains existed in regions such as the arcuate nucleus, hippocampus and dentate gyrus, providing further evidence that hypertensive rats apparently have an impaired neural stress response. The present study demonstrates that exposure to restraint results in significant changes in prepro-NPY mRNA expression in specific nuclei of both WKY and SHR that are components of not only the central circuitry regulating the stress response, but also the neural network modulating autonomic function.

Acute stress increases neuropeptide Y mRNA within the arcuate nucleus and hilus of the dentate gyrus

The effects acute restraint stress on neuropeptide Y (NPY) mRNA expression were determined within the dentate gyrus and arcuate nucleus, where the effects of adrenal steroid action were previously reported. Adult male rats were exposed to 1 h of restraint stress and then sacrificed immediately, 6 h, or 24 h later. Controls were undisturbed. Stress increased NPY mRNA levels in both the arcuate nucleus and in the hilar region of the hippocampus with different time courses. NPY mRNA increased in the arcuate at 24 h, but not earlier, as determined by film autoradiography. Single cell grain analysis was performed in the dentate gyrus hilus because the NPY mRNA was heterogeneously distributed and revealed that the number of cells expressing NPY mRNA increased 6 h after stress, returning to control levels within 24 h. These results fit with previously reported effects of adrenal steroids modulating arcuate nucleus NPY expression through the adrenal steroid Type II receptors. In the hilus where adrenal steroid Type I receptors have been reported to suppress NPY mRNA levels, the effect of stress is in the opposite direction to that of adrenal steroid action and a more complex regulation of NPY expression is indicated.

Differential regulation of neuropeptide Y mRNA expression in the arcuate nucleus and locus coeruleus by stress and antidepressants

In rats, circulating corticosterone and insulin are involved in regulation of the hypothalamic neuropeptide Y (NPY) system, which in turn, is involved in regulation of the hypothalamic-pituitary-adrenal (HPA) axis. Since the HPA axis and stress responsivity is altered in diseases such as depression, we investigated interactions between the effects of stress and antidepressant drug treatment on arcuate nucleus and locus coeruleus NPY mRNA expressions using in-situ hybridization histochemistry. After acute (2 h) and repeated immobilization (2 h daily, for 14 days), plasma concentrations of corticosterone increased, and those of insulin decreased. The expression of NPY mRNA was significantly increased in the arcuate nucleus, but was unchanged in the locus coeruleus following acute and repeated immobilization. Adrenalectomized rats with systemic corticosterone replacement (ADX+CORT), whose corticosterone concentration was maintained at approximately 50-100 ng/ml during repeated stress, showed a decrease in plasma insulin and an increase in arcuate nucleus NPY mRNA similar to that observed in sham rats, suggesting that changes in NPY mRNA levels are more closely tied to circulating insulin than to circulating corticosterone. In contrast, locus coeruleus NPY mRNA expressions in ADX+CORT rats were significantly higher than those in sham rats after repeated stress. Desmethylimipramine (DMI) treatment for 24 days did not affect basal plasma concentrations of corticosterone or insulin, or arcuate nucleus NPY mRNA expressions, but significantly decreased basal levels of locus coeruleus NPY mRNA compared to saline-treated rats. After repeated immobilization (2 h daily, for 4 days), DMI significantly reduced the stress-induced rise in locus coeruleus NPY mRNA levels, but potentiated the stress-induced rise in arcuate nucleus NPY mRNA expression. These results demonstrate that: (1) the increase in arcuate nucleus NPY mRNA expressions in stressed rats closely follows the decrease in plasma concentrations of insulin; (2) increases in NPY mRNA expressions occur in the absence of changes in plasma corticosterone; and (3) desipramine treatment potentiated the effect of stress on arcuate nucleus NPY mRNA expressions, but blocked the repeated stress-induced increase in locus coeruleus NPY mRNA expressions. Thus, NPY mRNA expression in the arcuate nucleus and the locus coeruleus is sensitive to the effects of stress and to the antidepressant drug desipramine, but the arcuate nucleus NPY system is regulated by different mechanisms than the locus coeruleus NPY system. The results provide further evidence for the importance of circulating insulin in the regulation of the arcuate nucleus NPY system.

Inhibition by aldosterone of insulin receptor mRNA levels and insulin binding in U-937 human promonocytic cells

The effect of aldosterone on insulin receptor (IR) expression was investigated in U-937 human promonocytic cells. The putative involvement of the mineralocorticoid receptor (MR) was also analysed. Aldosterone binding assays indicated the presence of MRs with high affinity and limited capacity in these cells. RNA blot assays showed that aldosterone treatment decreased the levels of the two major IR mRNAs (11 and 8.5 kb) present in these cells in a dose- and time-dependent manner. The partial reversal of such a decrease by the mineralocorticoid antagonist spironolactone suggested that MR was involved in the process. Experiments with the RNA synthesis inhibitor actinomycin D indicated that the decrease in IR mRNA content in aldosterone-treated cells was not the result of transcript destabilisation. The inhibitory action of aldosterone was not prevented by the simultaneous presence of the protein synthesis inhibitor cycloheximide, suggesting that the reduction of IR gene expression occurs as a direct response to the action of aldosterone. Furthermore, insulin binding assays showed that aldosterone decreased IR capacity but did not alter receptor affinity. In addition, the IR turnover resulted unaltered. These results provide the first evidence for an in vitro modulation of human IR expression by aldosterone.

Glucocorticoid replacement, but not corticotropin-releasing hormone deficiency, prevents adrenalectomy-induced anorexia in mice

There is considerable evidence that CRH can suppress food intake. As hypothalamic CRH, a main site of CRH expression, is also negatively regulated by glucocorticoids, it is unclear whether anorexia and weight loss in adrenal insufficiency are attributable to elevated CRH or to decreased glucocorticoid levels. To distinguish these possibilities, we have measured food intake and body weight in wild-type and CRH-deficient mice after sham adrenalectomy (Sham ADX) or adrenalectomy (ADX) with and without corticosterone (B) replacement. CRH deficiency neither increased basal food intake and body weight nor attenuated decreases in food intake after ADX or Sham ADX. B replacement producing plasma levels above the circadian peak completely blocked ADX-induced decreases in feeding and body weight in all mice and frequently stimulated food intake in CRH-deficient mice. Plasma levels of insulin and leptin, two other hormones involved in appetite regulation, did not differ between genotypes; however, the relationship between food intake and circulating leptin was significantly less negative at B doses that preserved appetite. B replacement levels slightly below circadian peak concentrations did not prevent hypophagia after ADX. We conclude that factors other than or in addition to CRH are more important in mediating appetite responses to adrenalectomy.

Adrenalectomy-induced neuronal degeneration

The binding of glucocorticoids to CNS receptors results in the modulation of many processes, ranging from neurotransmission to cell birth and death. It is of no surprise, therefore, that the removal of these steroids following adrenalectomy disrupts a variety of physiological functions throughout the brain. It is the aim of this review to briefly describe the findings of research examining some of these glucocorticoid-mediated CNS effects; however, as many of these areas have been reviewed extensively by others, this review will focus on the recently described phenomenon, adrenalectomy-induced hippocampal cell death.

Prevention of stress-induced morphological and cognitive consequences

Atrophy and dysfunction of the human hippocampus is a feature of aging in some individuals, and this dysfunction predicts later dementia. There is reason to believe that adrenal glucocorticoids may contribute to these changes, since the elevations of glucocorticoids in Cushing's syndrome and during normal aging are associated with atrophy of the entire hippocampal formation in humans and are linked to deficits in short-term verbal memory. We have developed a model of stress-induced atrophy of the hippocampus of rats at the cellular level, and we have been investigating underlying mechanisms in search of agents that will block the atrophy. Repeated restraint stress in rats for 3 weeks causes changes in the hippocampal formation that include suppression of 5-HT1A receptor binding and atrophy of dendrites of CA3 pyramidal neurons, as well as impairment of initial learning of a radial arm maze task. Because serotonin is released by stressors and may play a role in the actions of stress on nerve cells, we investigated the actions of agents that facilitate or inhibit serotonin reuptake. Tianeptine is known to enhance serotonin uptake, and we compared it with fluoxetine, an inhibitor of 5-HT reuptake, as well as with desipramine. Tianeptine treatment (10 mg/kg/day) prevented the stress-induced atrophy of dendrites of CA3 pycamidal neurons, whereas neither fluoxetine (10 mg/kg/day) nor desipramine (10 mg/kg/day) had any effect. Tianeptine treatment also prevented the stress-induced impairment of radial maze learning. Because corticosterone- and stress-induced atrophy of CA3 dendrites is also blocked by phenytoin, an inhibitor of excitatory amino acid release and actions, these results suggest that serotonin released by stress or corticosterone may interact pre- or post-synaptically with glutamate released by stress or corticosterone, and that the final common path may involve interactive effects between serotonin and glutamate receptors on the dendrites of CA3 neurons innervated by mossy fibers from the dentate gyrus. We discuss the implications of these findings for treating cognitive impairments and the risk for dementia in the elderly.

Elevated corticosterone is not required for the rapid induction of neuropeptide Y gene expression by an overnight fast

Fasting stimulates corticosterone (B) secretion and the expression and secretion of hypothalamic neuropeptide Y in rats. These studies tested the hypothesis that the rapid and marked fasting-induced increases in plasma B are responsible for stimulation of neuropeptide Y (NPY) gene expression. Plasma leptin and insulin were measured because they are also signals known to affect NPY messenger RNA (mRNA). Intact or adrenalectomized rats given a low fixed level of corticosterone (B replaced) were fasted for 48 h. NPY mRNA in the mediobasal hypothalamus, measured by nuclease protection assay, was elevated similarly above ad lib-fed controls in both intact and B replaced groups at 15 and 48 h after the onset of fasting. NPY immunoreactivity in the mediobasal hypothalamus increased between 3 and 48 h after onset of the fast in intact but not in B replaced groups. The fasting-induced decreases in leptin observed in intact rats at 48 h did not occur in B replaced rats. Fasting-induced decreases in insulin occurred in B replaced rats but not in intact rats. We conclude that: 1) elevated B is not required for fasting-induced increases in hypothalamic NPY gene expression; and 2) decreases in neither leptin nor insulin alone signal the changes that occur in NPY mRNA in fasted rats.

Role of adrenal steroid mineralocorticoid and glucocorticoid receptors in long-term potentiation in the CA1 field of hippocampal slices

We previously demonstrated in the dentate gyrus (DG) of anesthetized and freely behaving rats that both acute as well as chronic administration of corticosterone produces a suppression in long-term potentiation (LTP). In subsequent studies we showed, again in the DG, that activation of the two types of adrenal steroid receptors (mineralocorticoid (MR) and glucocorticoid (GR)) produce biphasic effects on synaptic plasticity; activation of MR produces an enhancement while activation of GR produces a suppression in LTP. In a separate study, we further demonstrated in rats administered the specific GR agonist RU 28362 that high-frequency stimulation, which normally produces LTP, instead produced long-term depression (LTD) in these animals. In the present study we investigated the effects of MR and GR activation by adrenal steroids on synaptic plasticity of the hippocampal CA1 field, but we studied this ex vivo, in a slice preparation. The results indicate that, as in our studies in the DG, adrenal steroids produce biphasic effects: in ADX rats, aldosterone (a specific MR agonist) enhanced while RU 28362 suppressed synaptic plasticity. Unlike the in vivo preparation, however, rarely was LTD observed in the animals receiving RU 28362. Also, ADX itself did not produce noticeable effects on synaptic plasticity. The present results are in agreement with previous studies showing that elevations in corticosterone or an acute episode of experimentally induced stress in vivo causes a suppression in LTP in the hippocampal CA1 field, in vitro.

Dexamethasone rapidly increases hypothalamic neuropeptide Y secretion in adrenalectomized ob/ob mice

A single intracerebroventricular injection of dexamethasone (DEX) rapidly (within 30 min) suppresses brown adipose tissue thermogenesis and increases plasma insulin concentrations in adrenal-ectomized (ADX) ob/ob mice but not in ADX lean mice. Intracerebroventricular neuropeptide Y (NPY) administered intracerebroventricularly causes these same metabolic changes within 30 min in both ob/ob and lean ADX mice. We therefore hypothesized that DEX exerts these rapid-onset metabolic actions in ob/ob mice via a phenotype-specific enhancement of NPY secretion within the central nervous system. In support of this hypothesis, DEX (a type II glucocorticoid receptor agonist) administered intracerebroventricularly selectively lowered NPY concentrations in the whole hypothalamus of ADX ob/ob mice by 35% and in the arcuate nucleus region by approximately 70% within 30 min but not in the brain stem or hippocampus or in any of these regions of lean mice. DEX also functioned in vitro to enhance depolarization-dependent release of NPY from hypothalamic blocks of ADX ob/ob mice but not of ADX lean mice. Thus DEX acts in the hypothalamus of ob/ob mice in a phenotype-specific manner to evoke rapid transport of NPY from cell bodies within the arcuate nucleus to terminal regions including the dorsomedial and ventromedial hypothalamic regions for release.

Neuropeptide Y gene expression and receptor autoradiography in hypertensive and normotensive rat brain

Neurones containing neuropeptide Y (NPY) may participate in central cardiovascular control by tonically influencing barosensitive neurones within the nucleus tractus solitarius. The present study has employed both in situ hybridisation histochemistry and receptor autoradiography, to visualise the expression of prepro-NPY mRNA in the forebrain and to determine the NPY receptor subtype(s) in the brainstem, respectively. Prepro-NPY gene expression was visualised in the hypothalamus, cortex, dentate gyrus and lateral reticular thalamus from age-matched spontaneously hypertensive rats (SHR) and normotensive Don Ryu rats (DRY) and Wistar Kyoto rats (WKY). Quantitative densitometry revealed an increase in the NPY transcript in the arcuate nucleus of SHR rats compared to their normotensive counterparts. Autoradiography using [125I]Bolton-Hunter-NPY (BH-NPY, 15 pM) demonstrated NPY binding sites in the area postrema, the commissural nucleus tractus solitarius (cNTS) and the inferior olivary complex. NPY (1 microM) and peptide YY (1 microM), but not [Leu31,Pro34]NPY (10-100 nM), fully inhibited the binding of [125I]BH-NPY. These results indicate that NPY receptors of the Y2 subtype predominate in the dorsal vagal complex. Unilateral nodose ganglionectomy resulted in a partial loss of NPY binding sites in the commissural NTS, but not the area postrema, suggesting that a proportion of binding sites (Y2 subtype) are present on central vagal terminals. While all three rat strains appear to have the same relative proportions of NPY receptor subtypes in the brainstem, the relevance of the differential NPY gene expression in the arcuate nucleus regarding central cardiovascular control mechanisms and/or the pathogenesis of hypertension remains to be elucidated.

Effect of adrenal steroids on preproneurokinin-A gene expression in discrete regions of the rat brain

An in situ hybridization histochemical procedure was developed to monitor the cellular distributions of the three major alternatively spliced alpha, beta and gamma species of mRNA encoding neurokinin molecules of the CNS. Two oligodeoxyribonucleotide probes were synthesized corresponding to common sequences of the alpha, beta, and gamma NK-A mRNA. The first experiment used rats that were sham-operated (Sham), adrenalectomized (ADX), and ADX rats treated with corticosterone (ADX+CORT). Intense labelling was observed within the habenula (Hb), while strong labelling was detected within the olfactory tubercle (OT), the lateral olfactory tubercle (LOT), the horizontal diagonal band of Broca (HDB), the bed nucleus of the stria terminalis (BNST), and the dorsal and ventral part of the caudate putamen (d-CP, v-CP). Moderate labelling of a number of cells was observed within the medial preoptic area (mPOA), the postero-dorsal part of the medial amygdala (MePD), and the dorsal and ventral part of the premamillary nucleus of the hypothalamus (PM-D; PM-V). ADX decreased NK-A mRNA in OT, LOT, HDB, BNST, CP compared to sham-operated rats, whereas CORT replacement elevated NK-A mRNA to above sham levels in OT, LOT, HDB, BNST and CP. There was no effect of ADX or CORT in Hb, while smaller, and often non-significant, effects of ADX and CORT replacement were found in other areas. Since there are two types of adrenal steroid receptors in brain, we next investigated the effects of agonists for type I and type II adrenal steroid receptors. ADX rats were given either aldosterone (ALDO, 10 micrograms/ml/h, Alzet minipumps) or RU 28362 (10 micrograms/ml/h, Alzet minipumps) for 8 days. Type I receptor activation by ALDO partially reversed the ADX-induced decrease in NK-A mRNA, whereas type II steroid receptor activation by RU 28362 restored the decrease caused by ADX and caused an elevation of NK-A mRNA above sham levels in OT. These findings show that adrenal steroids regulate NK-A gene expression through both type I and type II receptors in a number of brain areas. The results are consistent with a role for adrenal steroids and neurokinins in the regulation of body fluid homeostasis.

Regulation of angiotensinogen gene expression in the rat forebrain by adrenal steroids and relation to salt appetite

The renin-angiotensin system (RAS) is present in the brain where it participates in regulation of fluid-electrolyte homeostasis and possibly plays a role in arousal of salt appetite. In the present studies using quantitative in situ hybridization histochemistry we examined the level of the angiotensinogen (ANG) mRNA in the forebrain areas associated with fluid-electrolyte balance in adrenalectomized (ADX) rats and ADX rats supplemented either with selective glucocorticoid type II receptor agonist RU 28362 or with the selective type I receptor agonist, aldosterone (ALDO). RU 28362 and ALDO were administered for 7 days via Alzet 2001 osmotic minipumps at the rates of 10 micrograms/microliters/h and 1 microgram/microliter/h, respectively. Following adrenalectomy, rats were maintained on a standard rat chow, water and 3% NaCl ad lib. In situ hybridization was performed either with a synthetic [33P]- or [32P]-3' end-labeled oligonucleotide probe and the level of ANG mRNA was detected by grain counting over a single cell or by quantitative film autoradiography, respectively. Seven days post ADX the ANG mRNA level in all studied forebrain areas -septum-diagonal band of Broca (SEPT/DBB), the areas immediately adjacent to the organum vasculosum of the lamina terminalis (OVLT), the median preoptic nucleus (MnPO), and the medial preoptic area (mPOA)-of ADX rats decreased by 50-60%. ALDO treatment, which did prevent ADX-induced saline ingestion, did not prevent this decrease. However, supplementation with RU 28362 maintained normal levels of ANG mRNA in all the above regions of the brain. Thus the expression of the ANG gene in the studied areas of rat forebrain is predominantly under the control of the adrenal glucocorticoids via the type II receptor and not regulated by an ALDO dose that stabilizes natriuresis from the kidney.

Effects of chronic social stress on tyrosine hydroxylase mRNA and protein levels

Males housed in mixed sex groups quickly form dominance hierarchies; subordinates can be further subdivided into stress responsive subordinates (SRS) and non-responsive subordinates (NRS) based on corticosterone responses to a novel stressor. Tyrosine hydroxylase (TH) mRNA levels measured with in situ hybridization were elevated in locus coeruleus (LC) of NRS compared to singly or pair-housed controls; NRS also had higher TH levels than dominants. TH protein levels determined by immunoautoradiography were also higher in LC of NRS and SRS versus pair-housed controls.

Effects of adrenal steroid manipulations and repeated restraint stress on dynorphin mRNA levels and excitatory amino acid receptor binding in hippocampus

Adrenal steroid and stress effects were determined in hippocampus on levels of dynorphin (DYN) mRNA, expressed in dentate gyrus, and excitatory amino acid receptors, measured in Ammon's horn and dentate gyrus. Adrenalectomy (ADX) decreased DYN mRNA levels in dentate gyrus and replacement with aldosterone (ALDO), a specific type I adrenal steroid receptor agonist, prevented the decrease. Ru28362, a specific type II receptor agonist, had no effect. Likewise, kainate receptor binding to the stratum lucidum and hilus region of dorsal hippocampus was decreased after ADX and this decrease was prevented by ALDO but not by Ru28362 treatment. Similar though smaller effects were found for CNQX binding to AMPA receptors but only in the dentate gyrus molecular or infra- and supragranular layers. Although corticosterone (CORT) treatment of intact rats (40 mg/kg for 3 weeks) elevated DYN mRNA levels in dentate gyrus, up to 14 days of daily restraint stress (1 or 6 h/day) had no significant effect. Neither CORT treatment nor repeated restraint stress altered NMDA and non-NMDA glutamate receptors in hippocampus. The results of this study showing ADX-induced decreases of DYN mRNA and CNQX binding in dentate gyrus and decreased kainate binding in mossy fiber terminal regions are consistent with morphological evidence showing that adrenal steroids maintain normal integrity and structure of dentate gyrus neurons and do so via type I adrenal steroid receptors. These same parameters are apparently not sensitive to chronic restraint stress although the effects of other stressors must be examined.