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

Mifepristone decreases nicotine intake in dependent and non-dependent adult rats

BACKGROUND

Addiction to tobacco and nicotine products has adverse health effects and afflicts more than a billion people worldwide. Therefore, there is an urgent need for new treatments to reduce tobacco and nicotine use. Glucocorticoid receptor blockade shows promise as a novel treatment for drug abuse and stress-related disorders.

AIM

These studies aim to investigate whether glucocorticoid receptor blockade with mifepristone diminishes the reinforcing properties of nicotine in rats with intermittent or daily long access to nicotine.

METHODS

The rats self-administered 0.06 mg/kg/inf of nicotine for 6 h per day, with either intermittent or daily access for 4 weeks before treatment with mifepristone. Daily nicotine self-administration models regular smoking, while intermittent nicotine self-administration models occasional smoking. To determine whether the rats were dependent, they were treated with the nicotinic acetylcholine receptor antagonist mecamylamine, and somatic signs were recorded.

RESULTS

The rats with intermittent access to nicotine had a higher level of nicotine intake per session than those with daily access but only the rats with daily access to nicotine showed signs of physical dependence. Furthermore, mecamylamine increased nicotine intake during the first hour of access in rats with daily access but not in those with intermittent access. Mifepristone decreased total nicotine intake in rats with intermittent and daily access to nicotine. Moreover, mifepristone decreased the distance traveled and rearing in the open field test and operant responding for food pellets.

CONCLUSION

These findings indicate that mifepristone decreases nicotine intake but this effect may be partially attributed to the sedative effects of mifepristone.

Brain stimulation rewarding experience attenuates neonatal clomipramine-induced adulthood anxiety by reversal of pathological changes in the amygdala

Major depressive disorder (MDD) is associated with enhanced anxiety and reduced reward processing leading to impaired cognitive flexibility. These pathological changes during depression are accompanied by dysfunctional hypothalamic-pituitary-adrenal (HPA) axis and its impaired regulation by the amygdala. Notably, the electrical stimulation of brain reward areas produces an antidepressant effect in both MDD patients and animal models of depression. However, the effects of chronic electrical self-stimulation of lateral hypothalamus - medial forebrain bundle (LH-MFB) on depression-associated anxiety and accompanying changes in plasma corticosterone levels, structural, and neurochemical alterations in the amygdala are unknown. Here, we used the neonatal clomipramine (CLI) model of depression. During adulthood, neonatal CLI and vehicle administered rats were subjected to bilateral electrode implantation at LH-MFB and trained to receive intracranial self-stimulation (ICSS) for 14 days. Rats were then tested for anhedonic and anxiety-like behaviors, followed by estimation of plasma corticosterone levels, assessment of amygdalar volumes and neuronal/glial numbers, levels of monoamines and their metabolites in the amygdala. We found that chronic ICSS of LH-MFB reverses CLI-induced anhedonia and anxiety. Interestingly, amelioration of CLI-induced enhanced anhedonia and anxiety in ICSS rats was associated with partial reversal of enhanced plasma corticosterone levels, hypertrophy of basolateral amygdala (BLA), and altered noradrenaline (NA) metabolism in the amygdalar complex. We suggest that beneficial effects of ICSS on CLI-induced anxiety at least in part mediated by the restoration of amygdalar and HPA axis functioning. Our results support the hypothesis that brain stimulation rewarding experience might be evolved as a therapeutic strategy for reversal of amygdalar dysfunction in depression.

Pituitary adenylate cyclase-activating polypeptide induces a depressive-like phenotype in rats

BACKGROUND

Major depressive disorder (MDD) is a chronic, life-threatening psychiatric condition characterized by depressed mood, psychomotor alterations, and a markedly diminished interest or pleasure in most activities known as anhedonia. Available pharmacotherapies have limited success and the need for new strategies is clear. Recent studies attribute a major role to the pituitary adenylate cyclase-activating polypeptide (PACAP) system in mediating the response to stress. PACAP knockout mice display profound alterations in depressive-like behaviors, and genetic association studies have demonstrated that genetic variants of the PACAP gene are associated with MDD. However, the effects of PACAP administration on depressive-like behaviors in rodents have not yet been systematically examined.

OBJECTIVES

The present study investigated the effects of central administration of PACAP in rats on depressive-like behaviors, using well-established animal models that represent some of the endophenotypes of depression.

METHODS

We used intracranial self-stimulation (ICSS) to assess the brain reward function, saccharin preference test to assess anhedonia, social interaction to assess social withdrawal, and forced swim test (FST) to assess behavioral despair.

RESULTS

PACAP raised the current threshold for ICSS, elevation blocked by the PACAP antagonist PACAP(6-38). PACAP reduced the preference for a sweet saccharin solution and reduced the time the rats spent interacting with a novel animal. Interestingly, PACAP administration did not affect immobility in the FST.

CONCLUSIONS

Our results demonstrate a role for the central PACAP/PAC1R system in the regulation of depressive-like behaviors and suggest that hyperactivity of the PACAP/PAC1R system may contribute to the pathophysiology of depression, particularly the associated anhedonic symptomatology and social dysfunction.

Expression and interaction of TNF-α and VEGF in chronic stress-induced depressive rats

The incidence of depression increases annually but the pathogenesis is not yet fully understood. The aim of the present study was to explore the expression and interaction of tumor necrosis factor-α (TNF-α) and vascular endothelial growth factor (VEGF) in chronic stress-induced depressive rats. A total of 20 adult healthy Sprague Dawley rats (180-220 g) were randomly divided into the control and experimental depression groups. The depression model was established with a chronic stress method, and the success of model construction was assessed through weigh measurements and the sugar consumption and open-field tests. The expression of TNF-α and VEGF was detected using the reverse transcription quantitative polymerase chain reaction (RT-qPCR), western blotting and immunohistochemistry. Compared with the control group, the weight of the rats in the experimental group was found to be reduced (P<0.05). The open-field test showed significant differences in the horizontal and vertical motion of the rats between the two groups, and the rats in the experimental group exhibited a significantly reduced ability to adapt to a new environment (P<0.05). Furthermore, the sensitivity of the rats in the experimental group to reward stimulation was decreased. The relative mRNA expression levels of TNF-α and VEGF in the hippocampus of the experimental group were lower than those in the control group, and western blot analysis revealed that the protein expression of VEGF and TNF-α was reduced in the experimental group. Neurons of the experimental group exhibited reduced immunohistochemical staining compared with neurons from the normal hippocampus in the control group. In conclusion, the present study investigated the association between the occurrence of depression and TNF-α and VEGF at the mRNA and protein levels using RT-qPCR, western blotting, immunohistochemistry and animal behavior experiments. The results provide a fundamental basis for follow-up clinical research.

Sex differences in sensitivity to the depressive-like effects of the kappa opioid receptor agonist U-50488 in rats

BACKGROUND

Dynorphin, an endogenous ligand at kappa opioid receptors (KORs), produces depressive-like effects and contributes to addictive behavior in male nonhuman primates and rodents. Although comorbidity of depression and addiction is greater in women than men, the role of KORs in female motivated behavior is unknown.

METHODS

In adult Sprague-Dawley rats, we used intracranial self-stimulation to measure effects of the KOR agonist (±)-trans-U-50488 methanesulfonate salt (U-50488) (.0-10.0 mg/kg) on brain stimulation reward in gonadally intact and castrated males and in females at estrous cycle stages associated with low and high estrogen levels. Pharmacokinetic studies of U-50488 in plasma and brain were conducted. Immunohistochemistry was used to identify sex-dependent expression of U-50488-induced c-Fos in brain.

RESULTS

U-50488 dose-dependently increased the frequency of stimulation (threshold) required to maintain intracranial self-stimulation responding in male and female rats, a depressive-like effect. However, females were significantly less sensitive than males to the threshold-increasing effects of U-50488, independent of estrous cycle stage in females or gonadectomy in males. Although initial plasma concentrations of U-50488 were higher in females, there were no sex differences in brain concentrations. Sex differences in U-50488-induced c-Fos activation were observed in corticotropin releasing factor-containing neurons of the paraventricular nucleus of the hypothalamus and primarily in non-corticotropin releasing factor-containing neurons of the bed nucleus of the stria terminalis.

CONCLUSIONS

These data suggest that the role of KORs in motivated behavior of rats is sex-dependent, which has important ramifications for the study and treatment of mood-related disorders, including depression and drug addiction in people.

Does stress elicit depression? Evidence from clinical and preclinical studies

Exposure to stressful situations may induce or deteriorate an already existing depression. Stress-related depression can be elicited at an adolescent/adult age but evidence also shows that early adverse experiences even at the fetal stage may predispose the offspring for later development of depression. The hypothalamus-pituitary-adrenal axis (HPA-axis) plays a key role in regulating the stress response and dysregulation in the system has been linked to depression both in humans and in animal models. This chapter critically reviews clinical and preclinical findings that may explain how stress can cause depression, including HPA-axis changes and alterations beyond the HPA-axis. As stress does not elicit depression in the majority of the population, this motivated research to focus on understanding the biology underlying resilient versus sensitive subjects. Animal models of depression have contributed to a deeper understanding of these mechanisms. Findings from these models will be presented.

Involvements of stress hormones in the restraint-induced conditioned place preference

The conditioned place preference (CPP) paradigm is widely used when examining the reinforcing effects of drugs. Some previous studies have shown that an acute stressor, such as restraint could also induce CPP. Although the modulating effects of stress hormones on various forms of learning are well known, the finding that a stressor has a potentially direct role in the reinforcement mechanism is novel. This study focused on the function of stress hormones in restraint-induced CPP in Wistar rats administered agonist or antagonist of 2 critical stress hormones prior to conditioning. Results showed that peripheral applications of corticosterone (CORT, 1, 3, 5, and 10 mg/kg, subcutaneously) failed to induce CPP. Furthermore, a glucocorticoid (GC) antagonist (mifepristone, 10, 40, or 100 mg/kg, sc) failed to block the restraint-induced CPP. Intracerebroventricular injection of a selective corticotropin-releasing factor receptor 1 (CRFR1) antagonist antalarmin (1 μg/5 μl), on the contrary, completely blocked the restraint-induced CPP. We concluded that CRFR1 plays an essential role in the neural mechanism of restraint-induced CPP. Negative feedback of CORT from peripheral sources may not be involved in this phenomenon.

Methamphetamine use: a comprehensive review of molecular, preclinical and clinical findings

Methamphetamine (MA) is a highly addictive psychostimulant drug that principally affects the monoamine neurotransmitter systems of the brain and results in feelings of alertness, increased energy and euphoria. The drug is particularly popular with young adults, due to its wide availability, relatively low cost, and long duration of psychoactive effects. Extended use of MA is associated with many health problems that are not limited to the central nervous system, and contribute to increased morbidity and mortality in drug users. Numerous studies, using complementary techniques, have provided evidence that chronic MA use is associated with substantial neurotoxicity and cognitive impairment. These pathological effects of the drug, combined with the addictive properties of MA, contribute to a spectrum of psychosocial issues that include medical and legal problems, at-risk behaviors and high societal costs, such as public health consequences, loss of family support and housing instability. Treatment options include pharmacological, psychological or combination therapies. The present review summarizes the key findings in the literature spanning from molecular through to clinical effects.

Cocaine enhances resistance to extinction of responding for brain-stimulation reward in adult prenatally stressed rats

The present experiment assessed whether prenatal stress (PS) can alter the ability of acute and chronic cocaine administration to increase and decrease the rewarding effectiveness of the medial forebrain bundle (MFB) using intracranial self-stimulation (ICSS), and also whether PS can affect the extinction of the MFB stimulation response. Adult male offspring of female rats that received PS or no PS (nPS) were implanted with MFB stimulating electrodes, and were then tested in ICSS paradigms. In both nPS and PS offspring, acute cocaine injection decreased ICSS thresholds dose-dependently. However, the threshold-lowering effects at any dose were not significantly different between groups. There was also no group-difference in the threshold-elevating effects of chronic cocaine administration. Nevertheless, chronically drug-administered PS rats exhibited a resistance to the extinguishing of the response for brain-stimulation reward when acutely treated with cocaine, as compared to extinction without cocaine treatment. The results suggest that PS may weaken the ability for response inhibition under cocaine loading in male adult offspring.

Caveats of chronic exogenous corticosterone treatments in adolescent rats and effects on anxiety-like and depressive behavior and hypothalamic-pituitary-adrenal (HPA) axis function

BACKGROUND

Administration of exogenous corticosterone is an effective preclinical model of depression, but its use has involved primarily adult rodents. Using two different procedures of administration drawn from the literature, we explored the possibility of exogenous corticosterone models in adolescence, a time of heightened risk for mood disorders in humans.

METHODS

In experiment 1, rats were injected with 40 mg/kg corticosterone or vehicle from postnatal days 30 to 45 and compared with no injection controls on behavior in the elevated plus maze (EPM) and the forced swim test (FST). Experiment 2 consisted of three treatments administered to rats from postnatal days 30 to 45 or as adults (days 70 to 85): either corticosterone (400 μg/ml) administered in the drinking water along with 2.5% ethanol, 2.5% ethanol or water only. In addition to testing on EPM, blood samples after the FST were obtained to measure plasma corticosterone. Analysis of variance (ANOVA) and alpha level of P < 0.05 were used to determine statistical significance.

RESULTS

In experiment 1, corticosterone treatment of adolescent rats increased anxiety in the EPM and decreased immobility in the FST compared to no injection control rats. However, vehicle injected rats were similar to corticosterone injected rats, suggesting that adolescent rats may be highly vulnerable to stress of injection. In experiment 2, the intake of treated water, and thus doses delivered, differed for adolescents and adults, but there were no effects of treatment on behavior in the EPM or FST. Rats that had ingested corticosterone had reduced corticosterone release after the FST. Ethanol vehicle also affected corticosterone release compared to those ingesting water only, but differently for adolescents than for adults.

CONCLUSIONS

The results indicate that several challenges must be overcome before the exogenous corticosterone model can be used effectively in adolescents.

Behavioral and neurobiological consequences of prolonged glucocorticoid exposure in rats: relevance to depression

Stress is a critical environmental trigger for the development of clinical depression, yet little is known about the specific neurobiological mechanisms by which stress influences the development of depressive symptomatology. Animal models provide an efficient way to study the etiology of human disorders such as depression, and a number of preclinical models have been developed to assess the link between stress, glucocorticoids, and depressive behavior. These mode ls typically make use of repeated exposure to physical or psychological stressors in rodents or other small laboratory animals. This review focuses primarily on a recently developed preclinical model of depression that uses exogenous administration of the stress hormone corticosterone (CORT) in rodents instead of exposure to physical or psychological stressors. Repeated CORT administration in rats or mice produces reliable behavioral and neurobiological alterations that parallel many of the core symptoms and neurobiological changes associated with human depression. This provides an opportunity to study behavior and neurobiology in the same animal, so that the neurobiological factors that underlie specific symptoms can be identified. Taken together, these findings suggest that exogenous CORT administration is a useful method for studying the relationship between stress, glucocorticoids, and depression. Further study with this model may provide important new data regarding the neurobiological bases of depression.

Mineralocorticoid receptor antagonism prevents hedonic deficits induced by a chronic sodium appetite

Our laboratory has reported that manipulations that provoke a robust sodium appetite (e.g., sodium depletion, deoxycorticosterone acetate) decrease lateral hypothalamic self-stimulation (LHSS) reward if rats are denied access to hypertonic saline solutions. The following studies investigated the interaction between chronic sodium appetite and the renin-angiotensin-aldosterone system on LHSS reward. In Experiment 1, animals treated with the diuretic furosemide (20 mg/kg) when denied access to saline exhibited an increase in the current required to produce 50% of the maximum LHSS response rate (ECu50) 48 hr after extracellular volume depletion. Furosemide-depleted rats that were allowed to drink 0.3 M saline after depletion, or that were treated with the selective mineralocorticoid receptor (MR) antagonist spironolactone, which significantly reduced sodium appetite, did not show ECu50 changes. In Experiment 2 chronic intracerebroventricular administration of the selective MR antagonist RU 28318 (10 microg/microl/hr) prevented decreases in the ECu50 induced by deoxycorticosterone acetate-no salt treatment. We conclude that an unresolved sodium appetite will reduce responding for rewards and that experimental manipulations that reduce sodium appetite (e.g., access to saline or blockade of MR) decrease hedonic deficits.

Repeated paroxetine treatment reverses anhedonia induced in rats by chronic mild stress or dexamethasone

The present study was designed to assess the effect of dexamethasone, a synthetic glucocorticoid receptor agonist, in the sucrose preference test in rats. Rats treated acutely with dexamethasone (5-10 mg/kg) showed a significant decrease in sucrose preference (anhedonia) in comparison to vehicle treated rats, although 1 mg/kg dexamethasone did not alter the sucrose preference. Daily paroxetine treatment (10 g/kg, i.p., 14 days) reversed the anhedonic effect of acute dexamethasone (5 mg/kg), while causing no increased sucrose preference in rats that received dexamethasone vehicle. The paroxetine vehicle treated rats showed anhedonia even 14 days after acute dexamethasone administration. Paroxetine (10 mk/kg, i.p. for 28 days) also reversed anhedonia induced by chronic mild stress (8 weeks). In conclusion, acute dexamethasone induced an enduring anhedonic state that was reversed by repeated paroxetine treatment. Thus, the present study adds new data to the evidence supporting an important role for glucocorticoid in depression.

Corticosterone impairs insulin-stimulated translocation of GLUT4 in the rat hippocampus

BACKGROUND

Exposure to stress levels of glucocorticoids produces physiological responses that are characteristic of type 2 diabetes, such as peripheral insulin resistance and impairment in insulin-stimulated trafficking of glucose transporter 4 (GLUT4) in muscle and fat. In the central nervous system, stress produces neuroanatomical and neurochemical changes in the hippocampus that are associated with cognitive impairments.

METHODS

In view of these observations, the current studies examined the effects of short-term (1 week) exposure of stress levels of glucocorticoids upon insulin receptor (IR) expression and signaling, including GLUT4 translocation, in the rat hippocampus.

RESULTS

One week of corticosterone (CORT) treatment produced insulin resistance in response to peripheral glucose challenge. In the hippocampus, IR expression was unchanged in CORT-treated rats as compared with vehicle-treated rats. However, insulin-stimulated phosphorylation of the IR, total Akt levels and total GLUT4 levels were reduced in CORT-treated rats when compared to controls. In addition, insulin-stimulated translocation of hippocampal GLUT4 to the plasma membrane was completely abolished in CORT-treated rats.

CONCLUSIONS

These results demonstrate that in addition to eliciting peripheral insulin resistance, short-term CORT administration impairs insulin signaling in the rat hippocampus, effects that may contribute to the deleterious consequences of hypercortisolemic/hyperglycemic states observed in type 2 diabetes.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

The effects of adrenalectomy and corticosterone replacement on induction of maternal behavior in the virgin female rat

Maternal behavior of the sensitized virgin rat is affected by approach-avoidance systems as well as by hypothalamic-pituitary-adrenal (HPA) axis, which is also activated during stress. The present experiments investigated the effects of adrenalectomy and of varying corticosterone concentrations on the onset and expression of maternal behavior in sensitized virgin rats. In the first experiment, latency to onset of maternal behavior and time spent licking once maternal were positively related to endogenous levels of corticosterone. However, few rats showed licking. In the second experiment, virgin rats were adrenalectomized or given sham surgeries before being sensitized and being given 0, 25, 100, 300, or 500 microg/mL of corticosterone in their drinking water. In the third experiment, virgin rats were adrenalectomized or given sham surgeries and given either control or corticosterone time-release pellets after being sensitized. Maternal behavior was then tested. Adrenalectomy increased licking in the second experiment and time over pups in the third experiment. Corticosterone replacement reduced licking in the second experiment and both licking and time over pups in the third experiment. In conclusion, exogenous corticosterone had an inhibitory effect on the expression of maternal behavior in the sensitized virgin rat, unlike the facilitatory effect previously found in the postpartum rat.

Effect of different doses of corticosterone on depression-like behavior and HPA axis responses to a novel stressor

Stress is recognized to precipitate depressive illness, yet the specific relationship between stress, glucocorticoids and depression is not well understood. We have recently shown that repeated corticosterone (CORT) injections reliably increase depression-like behavior on the forced-swim test in rats, suggesting that glucocorticoids can precipitate depressive symptomatology. The purpose of this experiment was to determine the extent to which the effects of CORT on depression-like behavior depend on (1) the dose-injected and (2) the duration of treatment. Rats received either acute or repeated injections of vehicle, 10, 20 or 40 mg/kg of CORT, and then were subjected to the forced-swim test. Serum CORT levels were assessed after the 21-day injection period, and 30 and 60 min after the onset of forced-swim testing. Repeated, but not acute, CORT injections decreased body weight and increased immobility behavior in the forced-swim test in a dose-dependent manner. In addition, all doses of repeated CORT injections suppressed CORT release after the novel stress of forced-swim testing. Our results demonstrate that glucocorticoids increase depression-like behavior in rats in a dose-dependent manner and disrupt normal HPA axis function. These results support the hypothesis that high levels of cortisol contribute to the etiology of depressive symptomatology in humans.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.

Effect of lesioning the suprachiasmatic nuclei on behavioral despair in rats

The suprachiasmatic nucleus (SCN) is involved in regulating many biological rhythms. Several lines of research implicate the SCN in affective behavior. The SCN is directly involved in regulating the daily rhythms of the hypothalamo-pituitary-adrenal (HPA) axis hormones involved in stress. Bilateral lesions of the SCN disrupt both the rhythms and the basal levels of the HPA axis hormones involved in coping with stress. Moreover, stress can affect the biological rhythms regulated by the SCN, and disruption of biological rhythms in turn can cause stress. The present study assessed the effect of bilateral destruction of the SCN on behavioral despair, an animal model of depression sensitive to antidepressant treatment. The results indicate that bilateral destruction of the SCN results in reduced immobility in the second forced swimming test (FST) compared to sham controls and animals with incomplete lesions. These results indicate that bilateral destruction of the SCN has a protective effect in the induction of behavioral despair which may arise out of disruption of the secretion of the HPA axis hormones and/or of the neural connections between the SCN and the limbic structures that modulate the response to swim stress.

Corticosterone Increases Depression-Like Behavior, With Some Effects on Predator Odor-Induced Defensive Behavior, in Male and Female Rats

This experiment examined the effect of repeated corticosterone injections on anxiety and depression-like behavior in male and female rats. Rats received either corticosterone or vehicle injections for 21 consecutive days prior to behavioral testing in the forced swim, open-field, and predator odor tests. The corticosterone injections significantly increased depression-like behavior in the forced swim test in both male and female rats but had no significant effect on anxiety in the open-field test. In the predator odor test, the corticosterone injections significantly increased a subset of defensive behaviors in the male rats. These results suggest that repeated exposure to corticosterone increases depression-like behavior, with some effects on anxiety, and that male rats may be more affected than female rats by this manipulation.

Interaction between estradiol replacement and chronic stress on feeding behavior and on serum leptin

Exposure to stress may cause either an increase or a decrease in food intake. Behavioral and physiological responses to stress, including alterations in feeding behavior, are sexually dimorphic. This study aimed to evaluate the interaction between estradiol levels and chronic variate stress on the intake of sweet food and on serum levels of leptin, a hormone secreted by the adipose cells with a role in the regulation of body weight. Adult female Wistar rats were used. After ovariectomy, the animals received estradiol replacement (or oil) subcutaneously. Rats were then divided in controls and stressed (submitted to 30 days of variate stress). Consumption of sweet food and of serum leptin was measured. Although animals receiving estradiol replacement presented smaller weight gain, they showed an increased consumption of sweet food. Chronic variate stress decreased sweet food intake at 30, but not at 20, days of treatment. Estradiol replacement in the stressed group prevented both the reduction observed in sweet food intake and the increase in leptin levels. These results suggest that there is an interaction between chronic stress and estradiol replacement in feeding behavior concerning sweet food consumption, and this interaction may be related to altered leptin levels.

Amphetamine withdrawal does not produce a depressive-like state in rats as measured by three behavioral tests

Administration of amphetamine (AMPH) can induce symptoms of psychosis in humans and locomotor sensitization in rats; in contrast, withdrawal from a period of AMPH intake is most often associated with symptoms of human endogenous depression. The aim of this study was to determine whether AMPH withdrawal produces a depressive-like state in rats. The present study examined the effects of withdrawal from an escalating-dose AMPH schedule (ESC; three daily injections over 6 days, 1-5 mg/kg, i.p.) and an intermittent-dose AMPH schedule (INT; one daily injection over 6 days, 1.5 mg/kg, i.p.) on animals' performance in three behavioral paradigms related to depression: the Porsolt swim test, the learned helplessness assay and operant responding for sucrose on a progressive ratio schedule. ESC and INT AMPH withdrawal had no effect on any of these tests or on stress responsiveness as measured by increased plasma levels of corticosterone (CORT) and adrenocorticotropin following the swim test, although basal CORT levels were higher in AMPH-withdrawn animals compared to controls. Finally, we confirmed the presence of locomotor sensitization for both AMPH schedules after 30 days of withdrawal. Our results suggest that the ability of AMPH withdrawal to produce symptoms of depression may not be evident in all behavioral screens for depressive symptoms in the rat.

Paradoxical effects of chronic corticosterone on forced swim behaviours in aged male and female rats

The effects of chronically administered corticosterone on forced swim test and open field test behaviours were explored in aged male and female rats. Though corticosterone has typically been associated with depressive behaviours, recent data have suggested a putative antidepressive effect of corticosterone. The current study used the forced swim test as a model of antidepressant efficacy in order to explore this. Aged male and female rats received either corticosterone (20 mg/kg) or the vehicle for 10 days before testing in the forced swim test, then for an additional 3 days before testing in the open field test. On day 11, each animal was individually tested on the duration of swimming, immobile, and struggling behaviours, and on day 14, for the display of rearing and line crossing behaviours. Results revealed that corticosterone significantly increased swimming and decreased immobility behaviour in females, but failed to do so in males. Additionally, there was a main effect of corticosterone on struggling behaviour such that it decreased it in males. There were no effects of corticosterone or sex on open field test behaviours, suggesting that the present findings are not accounted for by a general effect of corticosterone on motor behaviour. Overall, the data suggest that chronically administered corticosterone possesses effects that are sex-specific, and that it may exert mildly antidepressive effects in females, but the opposite effects in males. These data are consistent with emerging evidence that corticosterone may play a paradoxical antidepressive effect.