Behavioral and physiological consequences of repeated daily intracerebroventricular injection of corticotropin-releasing factor in the rat

Functional redundancy between glucocorticoid and mineralocorticoid receptors in mature corticotropin-releasing hormone neurons protects from obesity

OBJECTIVE

Here, we aimed to investigate the role of glucocorticoid and mineralocorticoid receptors (GRs and MRs, respectively) in the regulation of energy homeostasis.

METHODS

We used three mouse models with simultaneous deletion of GRs and MRs in either forebrain neurons, the paraventricular nucleus, or corticotropin-releasing hormone (CRH) neurons and compared them with wild-type controls or isolated knockout groups. In addition to body weight, food intake, energy expenditure, insulin sensitivity, fat/lean mass distribution, and plasma corticosterone levels, we also performed transcriptomic analysis of CRH neurons and assessed their response to melanocortinergic stimulation.

RESULTS

Similar to global double-knockout models, deletion of GRs and MRs specifically in mature CRH neurons resulted in obesity. Importantly, the latter was accompanied by insulin resistance, but not increased plasma corticosterone levels. Transcriptomic analysis of these neurons revealed upregulation of several genes involved in postsynaptic signal transduction, including the Ptk2b gene, which encodes proline-rich tyrosine kinase 2. Knockout of both nuclear receptors leads to upregulation of Ptk2b in CRH neurons, which results in their diminished responsiveness to melanocortinergic stimulation.

CONCLUSIONS

Our data demonstrate the functional redundancy of GRs and MRs in CRH neurons to maintain energy homeostasis and prevent obesity. Simultaneous targeting of both receptors might represent an unprecedented approach to counteract obesity.

PVN pathways controlling energy homeostasis

Research into the control of energy balance has tended to focus on discrete brain regions, such as the brainstem, medulla, arcuate nucleus of the hypothalamus, and neocortex. Recently, a larger picture has begun to emerge in which the coordinated communication between these areas is proving to be critical to appropriate regulation of metabolism. By serving as a center for such communication, the paraventricular nucleus of the hypothalamus (PVH) is perhaps the most important brain nucleus regulating the physiological response to energetic challenges. Here we review recent advances in the understanding of the circuitry and function of the PVH.

Chronic overexpression of corticotropin-releasing factor from the central amygdala produces HPA axis hyperactivity and behavioral anxiety associated with gene-expression changes in the hippocampus and paraventricular nucleus of the hypothalamus

Environmental stress has been demonstrated to increase susceptibility for mood and anxiety disorders, and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, the primary endocrine response to stress, is often observed in these patients. HPA axis activation is initiated by corticotropin-releasing factor (CRF) from the hypothalamus, leading to the hypothesis that hypothalamic CRF overexpression contributes to HPA axis hyperactivity in psychiatric patients. In addition, elevated CRF in cerebrospinal fluid is observed in mood and anxiety disorder patients, suggesting that CRF is also being overproduced from extrahypothalamic sources such as the central amygdala (CeA) and overactivity of the amygdala in neuroimaging studies is a consistent finding in anxiety and depression patients. Due to the importance of CRF and the amygdala in the etiology of stress-sensitive psychiatric disorders, the present study sought to further dissect the impact of CRF overexpression (OE) in the amygdala on downstream behavioral, endocrine, and gene-expression changes typically associated with chronic stress. To test the hypothesis that elevated CRF output from the amygdala would reproduce HPA axis hyperactivity and behavioral symptoms of chronic stress, we developed a lentiviral vector in which 3.0kb of the CRF promoter drives overexpression of CRF (LVCRFp3.0CRF). In adult male rats, Experiment-1 examined behavioral consequences of chronic CRF overexpression from the amygdala; the dexamethasone (Dex)/CRF test was used to measure HPA axis reactivity. Experiment-2 focused on HPA axis disruptions; the dexamethasone-suppression and CRF-stimulation tests as well as the Dex/CRF test were used. In both experiments, expression of HPA-axis related transcripts were assessed.

Excitotoxic lesions of the bed nucleus of the stria terminalis (BNST) attenuate the effects of repeated stress on weight gain: evidence for the recruitment of BNST activity by repeated, but not acute, stress

Exposure to repeated stress can lead to diverse and widespread behavioral consequences, including reduction in food and water intake and subsequent diminution in weight gain. Many reports have suggested that repeated stress substantially alters the neurochemistry, morphology and physiology of neurons within the bed nucleus of the stria terminalis (BNST). Here we investigate the role of the BNST in mediating the reduced weight gain observed during repeated stress. Rats exposed to a one-week variate stress paradigm exhibited a reduction in weight gain over the course of the 7-day paradigm. Excitotoxic lesions to a subregion of the anterolateral BNST containing the oval nucleus had no effects early in the 7-day paradigm, but significantly attenuated the effects of repeated stress on weight gain by the last day of stress. These data suggest that at least two mechanisms mediate the effects of stress on body weight gain, and that when stressor exposure becomes repeated, the BNST is recruited, worsening the symptoms of stressor exposure.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Brain-derived neurotrophic factor and hypothalamic-pituitary-adrenal axis adaptation processes in a depressive-like state induced by chronic restraint stress

Depression is potentially life-threatening. The most important neuroendocrine abnormality in this disorder is hypothalamo-pituitary-adrenocortical (HPA) axis hyperactivity. Recent findings suggest that all depression treatments may boost the neurotrophin production especially brain-derived neurotrophic factor (BDNF). Moreover, BDNF is highly involved in the regulation of HPA axis activity. The aim of this study was to determine the impact of chronic stress (restraint 3h/day for 3 weeks) on animal behavior and HPA axis activity in parallel with hippocampus, hypothalamus and pituitary BDNF levels. Chronic stress induced changes in anxiety (light/dark box test) and anhedonic states (sucrose preference test) and in depressive-like behavior (forced swimming test); general locomotor activity and body temperature were modified and animal body weight gain was reduced by 17%. HPA axis activity was highly modified by chronic stress, since basal levels of mRNA and peptide hypothalamic contents in CRH and AVP and plasma concentrations in ACTH and corticosterone were significantly increased. The HPA axis response to novel acute stress was also modified in chronically stressed rats, suggesting adaptive mechanisms. Basal BDNF contents were increased in the hippocampus, hypothalamus and pituitary in chronically stressed rats and the BDNF response to novel acute stress was also modified. This multiparametric study showed that chronic restraint stress induced a depressive-like state that was sustained by mechanisms associated with BDNF regulation.

The central nervous system as target for antihypertensive actions of a proline-rich peptide from Bothrops jararaca venom

Pyroglutamyl proline-rich oligopeptides, present in the venom of the pit viper Bothrops jararaca (Bj-PROs), are the first described naturally occurring inhibitors of the angiotensin I-converting enzyme (ACE). The inhibition of ACE by the decapeptide Bj-PRO-10c (<ENWPHPQIPP) and other Bj-PROs was classically used to explain the pharmacological effects of these venom peptides in mammals resulting in a decrease of blood pressure. Recent studies, however, suggest that ACE inhibition alone is not sufficient for explaining the antihypertensive actions exerted by these peptides. In this study, we show that intracerebroventricular injection of Bj-PRO-10c induced a significant reduction of mean arterial pressure (MAP) together with a decrease of heart rate (HR) in spontaneously hypertensive rats, indicating that Bj-PRO-10c may act on the central nervous system. In agreement with its supposed neuronal action, this peptide dose-dependently evoked elevations of intracellular calcium concentration ([Ca(2+)](i)) in primary culture from postnatal rat brain. The N-terminal sequence of the peptide was not essential for induction of calcium fluxes, while any changes of C-terminal Pro or Ile residues affected Bj-PRO-10c's activity. Using calcium imaging by confocal microscopy and fluorescence imaging plate reader analysis, we have characterized Bj-PRO-10c-induced [Ca(2+)](i) transients in rat brain cells as being independent from bradykinin-mediated effects and ACE inhibition. Bj-PRO-10c induced pertussis toxin-sensitive G(i/o)-protein activity mediated through a yet unknown receptor, influx and liberation ofcalcium from intracellular stores, as well as reduction of intracellular cAMP levels. Bj-PRO-10c promoted glutamate and GABA release that may be responsible for its antihypertensive activity and its effect on HR.

Central injections of CRF reinstate cocaine seeking in rats after postinjection delays of up to 3 h: an influence of time and environmental context

RATIONALE

In drug addicts, the induction of drug craving is not always associated with an immediate opportunity to take drugs again. It is, therefore, important to study how delays in opportunity for drug seeking affect the time-course of relapse. Intracerebroventricular (i.c.v.) injection of corticotropin-releasing factor (CRF) is a stressor that reinstates heroin and alcohol seeking in rats, when administered just before tests for reinstatement. It is not known whether CRF reinstates cocaine seeking; moreover, the effect of delaying testing for reinstatement, after CRF injection, has not been studied.

OBJECTIVES

To determine whether i.c.v. CRF induces reinstatement of cocaine seeking after postinjection delays of up to 3 h and to determine whether the context in which a delay is experienced influences the time-course of CRF-induced reinstatement.

METHODS

Rats self-administered cocaine (1.0 mg/kg per infusion) for 8-10 days. Subsequently, responding for drug was extinguished and testing for reinstatement by i.c.v. CRF (0.5 microg) was conducted. Animals were tested after postinjection delays of up to 3 h; the delays were experienced either in the self-administration (SA) chamber or home cage (HC).

RESULTS

When delays were spent in the SA chambers, CRF induced reinstatement in all delay groups. When delays were spent in the HC, CRF did not induce reinstatement after a 2-h delay.

CONCLUSIONS

We argue that the effects we observed are consistent with a contextual conditioning account of reinstatement, whereby CRF that was experienced in the context of the SA chamber served to elicit a conditioned excitatory response developed to that context during training.

The physiological and behavioral effects of subchronic intracisternal administration of TGF-β in rats: comparison with the effects of CRF

We studied the physiological and behavioral effects of subchronic intracisternal administration of transforming growth factor-beta (TGF-beta) for 7 days. Subchronic intracisternal administration of TGF-beta significantly inhibited the increase in body weight of rats but did not affect food intake. In the measurement of locomotor activity after the final intracisternal administration on day 7, the total count for 1.5 h increased significantly in the TGF-beta group compared with the vehicle group. However, that for 10 h was not different between both groups. Furthermore, significant elevations in oxygen consumption were observed in the TGF-beta group during both light and dark phase. Subchronic TGF-beta treatment induced a significant decrease in the number of total leukocytes and lymphocytes and the relative weight of the thymus, and a significant increase in brown adipose tissue weight. Corticotropin-releasing factor (CRF) is the primary neuroendocrine factor released in response to stress. Subchronic treatment with CRF, as a positive control, significantly affected body weight, food intake, oxygen consumption, total leukocyte and lymphocyte counts, and thymus and adrenal weight. Subchronic TGF-beta administration partially mimicked the stress responses, implicating a role for TGF-beta in the brain in stress.

The Darwinian concept of stress: benefits of allostasis and costs of allostatic load and the trade-offs in health and disease

Why do we get the stress-related diseases we do? Why do some people have flare ups of autoimmune disease, whereas others suffer from melancholic depression during a stressful period in their life? In the present review possible explanations will be given by using different levels of analysis. First, we explain in evolutionary terms why different organisms adopt different behavioral strategies to cope with stress. It has become clear that natural selection maintains a balance of different traits preserving genes for high aggression (Hawks) and low aggression (Doves) within a population. The existence of these personality types (Hawks-Doves) is widespread in the animal kingdom, not only between males and females but also within the same gender across species. Second, proximate (causal) explanations are given for the different stress responses and how they work. Hawks and Doves differ in underlying physiology and these differences are associated with their respective behavioral strategies; for example, bold Hawks preferentially adopt the fight-flight response when establishing a new territory or defending an existing territory, while cautious Doves show the freeze-hide response to adapt to threats in their environment. Thus, adaptive processes that actively maintain stability through change (allostasis) depend on the personality type and the associated stress responses. Third, we describe how the expression of the various stress responses can result in specific benefits to the organism. Fourth, we discuss how the benefits of allostasis and the costs of adaptation (allostatic load) lead to different trade-offs in health and disease, thereby reinforcing a Darwinian concept of stress. Collectively, this provides some explanation of why individuals may differ in their vulnerability to different stress-related diseases and how this relates to the range of personality types, especially aggressive Hawks and non-aggressive Doves in a population. A conceptual framework is presented showing that Hawks, due to inefficient management of mediators of allostasis, are more likely to be violent, to develop impulse control disorders, hypertension, cardiac arrhythmias, sudden death, atypical depression, chronic fatigue states and inflammation. In contrast, Doves, due to the greater release of mediators of allostasis (surplus), are more susceptible to anxiety disorders, metabolic syndromes, melancholic depression, psychotic states and infection.

Effect of long-term blockade of CRF(1) receptors on exploratory behaviour, monoamines and transcription factor AP-2

Corticotropin-releasing factor (CRF) holds a central role in reactions to various environmental stimuli. In the present study, the administration of a selective nonpeptide CRF(1) receptor antagonist, CP-154,526, for 6 days, exerted an anxiolytic effect in the elevated zero-maze (EZM) test. CP-154,526 did not affect behaviour in the exploration box when administered acutely, but increased exploration when administered for 5 days, contingently with daily behavioural testing. This effect, although of lesser magnitude, was also present in animals with neurotoxin DSP-4-induced selective denervation of locus coeruleus (LC) projections. When drug administration and behavioural testing were noncontingent in a 2-week administration schedule, CP-154,526 blocked the habituation-induced increase in exploration. This suggests that drug-environment interaction is an important component in the manifestation of the anxiolytic-like effects of CRF(1) receptor blockade. Long-term administration of CP-154,526 had a decreasing effect on noradrenaline (NA) metabolism in the frontal cortex. No manipulation influenced the levels of the transcription factor AP-2 isoforms in the LC area. AP-2 levels correlated positively with 3-methoxy-4-hydroxyphenylglycol (MHPG) in the frontal cortex of vehicle-treated animals. There was a negative correlation between the NA levels in the hippocampus and AP-2 isoforms in the LC area of naive animals. In contrast, in vehicle-treated animals, this correlation was positive. Treatment with CP-154,526, however, made the associations between LC AP-2 levels and hippocampal NA content negative, as was the case in the naive animals. This suggests that CRF(1) receptor blockade counteracts certain mechanisms of habituation, possibly by reducing the LC activity.

The effects of repeated intra-amygdala CRF injections on rat behavior and HPA axis function after stress

Patients diagnosed with certain anxiety disorders or depression show symptoms of a dysregulated HPA-axis secondary to increased release of corticotropin releasing factor (CRF). Male Wistar rats were injected with CRF (100 ng/microL) in the basolateral amygdala (BLA) for 5 days. Measurement of behavior was performed on the elevated plus maze and open field test. Behavioral and neuroendocrine response to restraint stress was also evaluated. Chronic treatment of CRF resulted in a significant increase in grooming after restraint stress in the Open Field test. Basal plasma corticosterone concentrations were significantly lower in the CRF-injected rats. These animals also showed greater and longer increase in corticosterone levels following the restraint stress than controls, but had comparable ACTH responses to restraint stress. Our results indicate that chronic administration of CRF into the basolateral amygdala may promote stress-induced grooming behavior in rats. In addition the data suggests that increased CRF in the amygdala may contribute to the dysregulation of corticosterone secretion. These findings may have important implications for patients suffering from psychiatric illnesses such as posttraumatic stress disorder and depression that are characterized by abnormalities in cortisol release.

Glucocorticoid maintains pulsatile aecretion of luteinizing hormone under infectious stress condition

We have previously shown that TNF-alpha, a major proinflammatory cytokine, suppressed hypothalamic GnRH pulse generator activity and that this inhibitory effect was enhanced by alpha-helical CRH, a CRH receptor antagonist. The present study was conducted to elucidate the involvement of glucocorticoid (GC) in modulating LH pulses under infectious stress condition. Adrenalectomy (ADX) markedly enhanced the suppressive effect of TNF-alpha (1 micro g), injected iv, on LH pulses in ovariectomized (OVX) rats. Pretreatment with a sc injection of corticosterone (10 mg) almost completely restored LH pulses after TNF-alpha injection in OVX/ADX animals. Injection of TNF-alpha increased the number of c-Fos-immunoreactive cells in the supraoptic nucleus (SON), the dorsomedial hypothalamic nucleus (DMH), and the parvocellular region of the paraventricular nucleus (PVN), which was more prominent in OVX/ADX than OVX animals except in the DMH. Pretreatment with corticosterone decreased the number of Fos-immunoreactive cells in the PVN and SON but not in the DMH. These results suggest that GC has a potent protective effect on LH pulsatility under conditions of infectious stress, the mechanism of which involves at least the suppression of the excitability of PVN and SON neurons. In addition, the DMH does not seem to mediate the central action of GC, though it may play an important role in inducing pathophysiological reactions to invasive stress.

Overexpression of corticotropin-releasing hormone in transgenic mice and chronic stress-like autonomic and physiological alterations

To gain a greater insight into the relationship between hyperactivity of the corticotropin-releasing hormone (CRH) system and autonomic and physiological changes associated with chronic stress, we developed a transgenic mouse model of central CRH overproduction. The extent of central and peripheral CRH overexpression, and the amount of bioactive CRH in the hypothalamus were determined in two lines of CRH-overexpressing (CRH-OE) mice. Furthermore, 24 h patterns of body temperature, heart rate, and activity were assessed using radiotelemetry, as well as cumulative water and food consumption and body weight gain over a 7-day period. CRH-OE mice showed increased amounts of CRH peptide and mRNA only in the central nervous system. Despite the presence of the same CRH transgene in their genome, only in one of the two established lines of CRH-OE mice (line 2122, but not 2123) was overexpression of CRH associated with increased levels of bioactive CRH in the hypothalamus, increased body temperature and heart rate (predominantly during the light (inactive) phase of the diurnal cycle), decreased heart rate variability during the dark (active) phase, and increased food and water consumption, when compared with littermate wildtype mice. Because line 2122 of the CRH transgenic mice showed chronic stress-like neuroendocrine and autonomic changes, these mice appear to represent a valid animal model for chronic stress and might be valuable in the research on the consequences of CRH excess in situations of chronic stress.

Diverse actions of ovarian steroids in the serotonin neural system

All of the serotonin-producing neurons of the mammalian brain are located in 10 nuclei in the mid- and hindbrain regions. The cells of the rostal nuclei project to almost every area of the forebrain and regulate diverse neural processes from higher order functions in the prefrontal cortex such as integrative cognition and memory, to limbic system control of arousal and mood, to diencephalic functions such as pituitary hormone secretion, satiety, and sexual behavior. The more caudal serotonin neurons project to the spinal cord and interact with numerous autonomic and sensory systems. All of these neural functions are sensitive to the presence or absence of the ovarian hormones, estrogen and progesterone. We have shown that serotonin neurons in nonhuman primates contain estrogen receptor beta and progestin receptors. Thus, they are targets for ovarian steroids which in turn modify gene expression. Any change in serotoninergic neural function could be manifested by a change in any of the projection target systems and in this manner, serotonin neurons integrate steroid hormone information and partially transduce their action in the CNS. This article reviews the work conducted in this laboratory on the actions of estrogens and progestins in the serotonin neural system of nonhuman primates. Comparisons to results obtained in other laboratory animal models are made when available and limited clinical data are referenced. The ability of estrogens and progestins to alter the function of the serotonin neural system at various levels provides a cellular mechanism whereby ovarian hormones can impact cognition, mood or arousal, hormone secretion, pain, and other neural circuits.

Do centrally administered neuropeptides access cognate receptors?: an analysis in the central corticotropin-releasing factor system

To determine the extent to which centrally administered corticotropin-releasing factor (CRF) activates neurons that express CRF receptors (CRF-Rs), we followed the kinetics and distribution (relative to those of CRF-Rs) of Fos induction seen in response to intracerebroventricular (icv) injection of the peptide (1-10 microg). CRF provoked widespread Fos expression: its strength was dose-related, it peaked at 2 hr after injection, and it was antagonized in a dose-dependent manner by coinjection of CRF-R antagonists. The activation pattern closely mimicked the distribution of CRF-R1 mRNA, in including widespread Fos induction throughout the cortical mantle, in cell groups involved in sensory information processing, and in the cerebellum and several of its major afferents and targets. Dual labeling revealed extensive correspondence of CRF-stimulated Fos-immunoreactivity (Fos-ir) and CRF-R1 mRNA at these and other loci. Unique sites of CRF-R2 expression were relatively unresponsive to CRF but were more so after icv administration of urocortin (UCN), a new mammalian CRF-related peptide. Both CRF and UCN elicited activational responses in cell groups that are involved in central autonomic control but that express neither CRF-R, including the central amygdaloid and paraventricular hypothalamic nuclei, and brainstem catecholaminergic cell groups. The results support an ability of CRF-related peptides in the ventricular system to access receptor-expressing cells directly but leave open questions as to the basis for the recruitment of central autonomic structures, many of which have been identified as stress-related sites of CRF action.