Acute changes in dopamine metabolism in the medio-basal hypothalamus following adrenalectomy

Persistent depressive state after chronic stress in rats is accompanied by HPA axis dysregulation and reduced prefrontal dopaminergic neurotransmission

Exposure to stress is thought to play an important role in the etiology of depression. Dysregulation of the hypothalamo-pituitary-adrenal (HPA) axis characterized by glucocorticoid negative feedback resistance is frequently observed in human depressives. Additionally, dysfunctions of the dopaminergic and serotonergic systems in the prefrontal cortex (PFC) are thought to be involved in the development of a depressive state. In rats, chronic stress induces a behaviorally depressive state, concomitant with dysregulation of the HPA axis and reductions in dopaminergic and serotonergic transmissions in the PFC. Considering that dysregulation of the HPA axis is associated with relapse and persistency of depression, it is possible that the chronic stress-induced depressive state persists during long-term rest after its exposure. In the present study, we examined this possibility in rats and found that the behaviorally depressive state in the rotarod test, negative feedback resistance in the dexamethasone suppression test, and a decrease in the extracellular concentration of dopamine but not serotonin in the PFC persisted for 3 months following a 4-week stress session. These results suggest that dysregulation of the HPA system and reduced dopaminergic transmission in the PFC underlies persistent behavioral depression following chronic stress.

Dexamethasone induces different wheel running activity than corticosterone through vasopressin release from the suprachiasmatic nucleus

During the analysis of wheel running activity, we found that corticosterone (1 mg/100 g BW) injection decreased wheel activity, while dexamethasone (0.1 mg/100 g) increased the activity. To clarify the functional differences between corticosterone and dexamethasone, we measured Arg-vasopressin (AVP) release from the suprachiasmatic nucleus (SCN) slice culture in vitro and AVP coding mRNA in the SCN in vivo. The corticosterone (0.2 and 2 microg/ml, final concentration in medium) decreased the AVP release, while it increased by dexamethasone (0.2 and 2 microg/ml). An AVP mRNA in the SCN was decreased by both corticosterone (1 mg/100 g) and dexamethasone (0.1 mg/100 g). The differences in wheel activity by corticosterone and dexamethasone are discussed from the changes of AVP in the SCN.

Chronic stress attenuates glucocorticoid negative feedback: involvement of the prefrontal cortex and hippocampus

Disruption of the glucocorticoid negative feedback system is observed in approximate one half of human depressives, and a similar condition is induced in animals by chronic stress. This disruption is thought to involve down-regulation of glucocorticoid receptors (GRs) in the feedback sites of the brain. However, the responsible site of the brain has not been well elucidated. Here we examined the effects of chronic stress induced by water immersion and restraint (2 h/day) for 4 weeks followed by recovery for 10 days on the GR levels in the prefrontal cortex (PFC), hippocampus, and hypothalamus of rats using a Western immunoblot technique. In the PFC, the cytosolic GR levels were decreased, but the nuclear GR levels were not changed. In the hippocampus, the levels of cytosolic and nuclear GRs were increased. However, there were no marked changes in the GR levels in the hypothalamus. The changes in the cytosolic GR levels were confirmed at the mRNA level by an in situ hybridization technique. We next examined the suppressive effects of dexamethasone (DEX) infusions into these regions on the circulating corticosterone levels. When DEX was infused into the PFC or hippocampus of the chronically stressed rats, the suppressive response to DEX was abolished, but the response was normal in the hypothalamus. In addition, when DEX was injected systemically to the chronically stressed rats, the suppressive response to DEX was significantly attenuated. These results suggest that the abnormal changes in GRs in the higher centers of the hypothalamo-pituitary-adrenal axis are involved in the chronic stress-induced attenuation of the feedback. Since dysfunction of the PFC or hippocampus is implicated in the pathogenesis of depression, the present findings would help to understand the mechanisms underlying the disrupted feedback system and its relation to brain dysfunction in depression.

Dopamine-receptor stimulation in the prefrontal cortex ameliorates stress-induced rotarod impairment

Exposure to chronic stress is thought to play an important role in the etiology of depression. In this disorder, dopaminergic dysfunction in the prefrontal cortex (PFC) is thought to be involved. Indeed, chronic stress reduces dopaminergic transmission in the rat PFC or induces a behaviorally depressive state. However, a relationship between the reduced dopaminergic activity and the behavior of the chronically stressed rats has not been proven. Here, we examined the effects of local application of a dopamine Type I (D(1)) receptor-specific agonist, SKF 81297, in the PFC on the chronic-stress-induced depressive state using a rotarod test. The chronic stress produced by water immersion and restraint for 4 weeks followed by recovery for 10 days impaired the rotarod performance without changing the traction performance or locomotor activity. Although intra-PFC infusion of 1 or 10 ng of SKF 81297 did not affect this impairment, 100 ng of SKF 81297 significantly ameliorated it. These results suggest that the chronic-stress-induced depressive state is caused by a D(1) receptor-mediated hypodopaminergic mechanism in the PFC. These findings will further understanding of the mechanisms underlying the pathophysiology of depression.

Chronic stress induces impairment of spatial working memory because of prefrontal dopaminergic dysfunction

Although the mechanism responsible for cognitive deficits in stress-related neuropsychiatric disorders has been obscure, prefrontal cortical (PFC) dopaminergic dysfunction is thought to be involved. In animals, the mesoprefrontal dopaminergic system is particularly vulnerable to stress, and chronic stress induces working memory impairment. However, the relation between the working memory impairment and altered dopaminergic activity in chronically stressed rats is unclear. Furthermore, the change of dopaminergic activity in the PFC induced by stress is thought to express as a stress response, not as a disorder of organic function. We have previously reported that chronic stress administered by water immersion and restraint for 4 weeks induces a organic disorder such as hippocampal neuronal degeneration. We therefore examined whether chronically stressed (4 weeks) and recovered (10 d) rats show a working memory impairment caused by reduced dopamine (DA) transmission in the PFC, as suspected in the neuropsychiatric disorders. The stress impaired the spatial working memory evaluated by T-maze task and induced a marked reduction of DA transmission concomitant with an increase in DA D1 receptor density in the PFC. This memory impairment was sufficiently ameliorated by intra-PFC infusion of 10 ng SKF 81297, a D1 receptor-specific agonist. Pretreatment with intraperitoneal injection of 20 microgram/kg SCH 23390, a D1 receptor antagonist, reversed the SKF 81297 response. These results indicate that chronic stress induces working memory impairment through a D1 receptor-mediated hypodopaminergic mechanism in the PFC. These findings provide important information for understanding of the mechanisms underlying PFC dysfunction in stress-related neuropsychiatric disorders.

Social subordination stress, behavior, and central monoaminergic function in female cynomolgus monkeys

BACKGROUND

Social subordination in female cynomolgus monkeys is stressful and activates the hypothalamic-pituitary-adrenal axis. In a previous experiment behavioral depression was observed in a subset of subordinates.

METHODS

In the experiment reported here behavioral and physiological indicators of stress were evaluated in dominant and subordinate female cynomolgus monkeys, and brain dopaminergic activity was assessed, as reflected in the prolactin response to haloperidol, a dopamine2 (D2) receptor antagonist.

RESULTS

Subordinates were aggressed more, spent more time in fearful scanning of the social environment, spent less time as the recipients of the active affiliative behavior of being groomed, had more variable heart rates in response to a novel environment, and were hypercortisolemic compared to dominants. Prolactin responses to haloperidol challenge were lower in subordinates than dominants, an observation consistent with the hypothesis that subordinate females have decreased D2 receptor function.

CONCLUSIONS

These observations suggest that social subordination is stressful and may alter brain dopaminergic function in primates. The neurophysiological characteristics of social subordinates may contribute to their susceptibility to depression.

Glucocorticoids as a biological substrate of reward: physiological and pathophysiological implications

The observations presented in this review suggest that glucocorticoids are one of the biological substrates of reward. These hormones are secreted in response to rewarding stimuli, such as food, a receptive sexual partner or drugs of abuse. Furthermore, manipulations of the secretion of glucocorticoids modify reward-related behaviours, and administration of these hormones, in the range of physiological stress levels, has positive reinforcing effects. The rewarding effects of glucocorticoids are probably mediated by a glucocorticoid-induced stimulation of the mesencephalic dopaminergic transmission, one of the principal neural substrates of reward. It is proposed that the rewarding effects of glucocorticoids play the role of counteracting the aversive effects of external aggressions, allowing a better coping with threatening situations. However, a sustained increase in the secretion of these hormones, or an hypersensitivity to their rewarding effects, could determine reward-related pathologies, such as a predisposed state to develop drug-abuse. In conclusion, through their reward-related effects, glucocorticoids may play a key role in tuning adaptation to stress and in determining reward-related behavioral pathologies.

Is there a role for corticosterone in expression of abnormal behaviour in restricted-fed fowls?

Growing parent stock (breeders) of meat-type chickens (broilers), subjected routinely to chronic food restriction, show increased pacing before a single daily meal and increased drinking and pecking at non-food objects (oral stereotypies) afterwards. Expression of these activities is correlated positively with the level of restriction imposed, and is thought to be controlled mainly by central dopaminergic mechanisms. There is published evidence that glucocorticoids can amplify dopamine mediated behaviours, and this paper describes four experiments examining the relationship between corticosterone and behaviour in individually caged broiler breeders. In Experiment 1 (with 3 levels of food restriction), plasma corticosterone concentration was correlated positively with the level of restriction imposed when blood samples were taken in the morning, but not when they were taken in the afternoon. This may be because corticosterone level declines from morning to afternoon with mild but not severe restriction. In Experiment 2 (severe restriction only), plasma corticosterone level did not change significantly with time of day, and mean values of individual birds were not correlated with their observed times spent in oral stereotypies. In Experiment 3 (severe restriction), object pecking increased in a dose-related way after systemic injection of 1-4 mg/kg corticosterone (significant) and 7.5-30 IU/kg ACTH (not significant), and 10-40 mg/kg metyrapone (corticosterone synthesis inhibitor) had no effect. In Experiment 4 (severe restriction), 180 mg/day metyrapone administered in food for 5 days reduced the plasma corticosterone response to injection of 15 IU/kg ACTH on the fourth day, but otherwise had no effect. Significant increases in drinker directed activity after injection of ACTH on the fourth day and 4 mg/kg corticosterone on the fifth day coincided with greatly elevated plasma levels of corticosterone. It is concluded that the oral stereotypies of restricted-fed broiler breeders do respond to induced increases in plasma corticosterone concentration that are supra-physiological, but there may be only a weak association between behaviour and corticosterone within the physiological range.

Neonatal ACTH administration elicits long-term changes in forebrain monoamine innervation. Subsequent disruptions in hypothalamic-pituitary-adrenal and gonadal function

The findings from this study demonstrated that the manipulation of the HPA system resulting from ACTH administration during neonatal development produces long-term, differential effects, not only on adrenocortical activity, but also on the activity and integrity of the forebrain monoamine systems. Increased concentrations of the monoamines within the forebrain regions studied at days 7 and 15, suggest a hastened maturation of these neural systems in animals neonatally treated with ACTH. The observed neurochemical alterations in these animals at one year are suggestive of an accelerated aging in the monoamine systems. A further consequence of these disturbances during development is an altered functioning of the HPG axis, as demonstrated by a delayed onset of puberty as previously reported, as well as significantly decreased proestrus plasma estradiol. Although deficits in sexual behavior also existed, it seems probable that these behavioral changes are a manifestation of altered neural systems regulating the ability to cope with a novel stimulus or situation, rather than a disruption of the "feminization" of the brain during sexual differentiation. This is in contrast to the male rat which exhibits permanent deficits in male typical sexual behavior following developmental ACTH treatment. The clinical relevance of these findings may be extensive. Perinatal exposure to events or agents that markedly increase ACTH and the corticosteroids may cause significant immediate and long-term changes in central monoamine functioning. These changes may constitute some of the most deleterious effects of stress exposure in infants and children. The alterations may be especially devastating in individuals with predispositions to stress-sensitive disorders such as anxiety, depression, and Tourette's syndrome. Finally, the use of ACTH in the treatment of infantile spasms may need to be reassessed in light of the possible long-term effects of ACTH on central monoamine functioning.

Mineralocorticoid and glucocorticoid receptors in the brain. Implications for ion permeability and transmitter systems

In this review we have argued that corticosteroid hormones represent an endocrine signal that can influence neuronal communication. The steroids bind to intracellular receptors in the brain, resulting in slow effects that involve gene transcription, but they may also evoke rapid effects via membrane receptors. The signal carried by the corticosteroids is therefore divergent with respect to the dimension of space and time. Within the rat brain, at least two intracellular receptor subtypes, i.e. MRs and GRs, bind corticosterone. The affinity, density and localization of the MRs is different from the GRs, although the actual properties may vary somewhat depending on the condition of the animal. In general, due to the difference in affinity, low corticosteroid levels result in a predominant MR occupation, while higher steroid levels additionally occupy GRs. Recent studies indicate that predominant MR occupation is important for the maintenance of ongoing transmission in certain brain regions and for neuroprotection. By contrast, additional GR occupation (for a limited period of time) results in an attenuation of local excitability; yet, prolonged exposure to high steroid levels may become an endangering condition for neurons. Since predominant MR occupation on the one hand and additional GR occupation on the other hand induce different cellular actions, the ratio of MR/GR occupation is an important factor determining the net effect of corticosteroid hormones in the brain. How coordinated MR- and GR-mediated effects control neuronal communication under various physiological and pathological conditions will be a challenge for future research.

Social isolation-induced enhancement of the psychomotor effects of morphine depends on corticosterone secretion

Short-term social isolation has been shown to increase individual reactivity to addictive drugs, although the biological factors involved in this effect are largely unknown. In this study, we investigated the influence of corticosterone secretion on the effects of social isolation on the response to opioids. The effects of social isolation on morphine-induced locomotor activity were compared in: (i) animals with an intact hypothalamo-pituitary-adrenal (HPA) axis; (ii) animals in which stress-induced corticosterone secretion was blocked by adrenalectomy. The animals in the latter group were implanted with subcutaneous corticosterone pellets (50 mg), which slowly release corticosterone, producing stable plasma levels within the physiological range. Social isolation increased the locomotor response to morphine (2 mg/kg s.c.) in animals with an intact HPA axis, but not in animals in which corticosterone secretion was blocked. These results suggest that corticosterone secretion is required for the expression of the enhanced locomotor response to opioids induced by isolation. Since an enhanced locomotor reactivity to addictive drugs has been found to be frequently associated with an enhanced vulnerability to drug self-administration, these findings suggest a role for glucocorticoids in the vulnerability to the reinforcing effects of opioids.

Rats orally self-administer corticosterone

Corticosterone, the major glucocorticoid in the rat, may modulate the reinforcing properties of addictive drugs as well as act as a positive reinforcer for intravenous self-administration. Since glucocorticoids are generally administered to humans via the oral route, we examined the ability of corticosterone to induce oral self-administration in the rat. In a first experiment, animals with free access to food could choose between a corticosterone solution and water. Three doses (25, 50 and 100 micrograms/ml) were tested. The group receiving the 100 micrograms/ml dose was also submitted to an extinction followed by a reversal test. In a second experiment, we examined whether the reinforcing properties of corticosterone could induce drinking independently of food intake. In the pre-test phase rats had access to food only during a fixed period of the day (11.00 h to 14.00 h). Corticosterone solution (200 micrograms/ml) or tap water were available during this period, with free access to tap water for the rest of the day. During the test period, access to food was shifted forward in time, while the availability of the corticosterone solution remained the same. The first experiment showed that rats preferred a corticosterone solution to tap water, developing self-administration in a dose-dependent manner. This preference could be extinguished, but was regained during the reversal phase. In the second experiment, animals that had access to the corticosterone solution drank more than rats that had access to water in the absence of food. These results indicate that corticosterone has reinforcing properties after oral administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Basal and stress-induced corticosterone secretion is decreased by lesion of mesencephalic dopaminergic neurons

There is evidence that certain psychopathological conditions are accompanied by a dysfunction in both the hypothalamo-pituitary-adrenal axis and dopaminergic systems, although the relationship between these two systems is as yet unclear. In the present study we investigated the effect of a specific lesion of dopamine mesencephalic neurons (Ventral Tegmental Area) on basal and stress-induced corticosterone secretion. Three weeks after injection of 6-OHDA, there was a depletion in dopamine in the frontal cortex and in the ventral and dorsal striatum, whereas norepinephrine and serotonin levels were unchanged. The dopamine-lesioned rats exhibited a lower basal and stress-induced corticosterone secretion than the sham-lesioned animals. The results indicate that the dopaminergic system may have a stimulatory influence on the hypothalamo-pituitary-adrenal axis.

Sensitization to the psychomotor effects of amphetamine and morphine induced by food restriction depends on corticosterone secretion

Food restriction has been shown to enhance the behavioral sensitivity to addictive drugs. The biological factors involved in this effect are largely unknown. Since food restriction, among other factors, increases corticosterone secretion, the role of this hormone in the effects of food restriction on the response to psychostimulants and opioids was investigated. The effects of food restriction on amphetamine- and morphine-induced locomotor activity were compared in: (i) animals with an intact hypothalamo-pituitary-adrenal axis; (ii) animals in which food restriction-induced corticosterone secretion was suppressed by adrenalectomy, but which received exogenous corticosterone from a subcutaneous implant, which slowly releases corticosterone, producing a stable plasma level within the normal physiological range over a period of 20 days. It was found that food restriction enhanced sensitivity to the psychomotor effects of amphetamine (1 mg/kg i.p.) and morphine (1 mg/kg i.p.) in animals with an intact hypothalamo-pituitary-adrenal axis, but not in animals in which endogenous corticosterone secretion was eliminated. These results suggest that corticosterone secretion may be one of the mechanisms by which food restriction amplifies the behavioral responses to amphetamine and morphine. Since an enhanced locomotor reactivity to addictive drugs has been found to be frequently associated with an enhanced vulnerability to drug self-administration, these findings point to a role for glucocorticoids in the susceptibility to drug abuse.

Higher and longer stress-induced increase in dopamine concentrations in the nucleus accumbens of animals predisposed to amphetamine self-administration. A microdialysis study

Individual vulnerability to the reinforcing effects of drugs appears to be a crucial factor in the development of addiction in humans. In the rat, individuals at risk for psychostimulant self-administration (SA) may be identified from their locomotor reactivity to a stress situation such as exposure to a novel environment. Animals with high locomotor responses to novelty (high responders, HR) acquire amphetamine SA, while animals with low responses (low responders, LR) do not. In this study we examined by microdialysis whether stress-induced extracellular dopamine (DA) concentrations in the nucleus accumbens differed between these two groups of animals. This neurotransmitter was studied because it is thought to be involved in the reinforcing effects of psychostimulants. Furthermore, previous studies have shown that HR animals have a higher basal DOPAC/DA ratio in the nucleus accumbens and higher extracellular concentrations of dopamine in this structure in response to cocaine. The stress procedure used in this experiment consisted of a 10 min tail-pinch. HR animals displayed a higher and longer stress-induced changes in DA concentrations than the LR group. Regression analysis showed that stress-induced changes in DA levels accounted for 75% of the variance observed in the locomotor response to a novel environment. Since higher DA activity in the nucleus accumbens has been reported in animals in which the propensity to psychostimulant SA is induced by brain lesions or life events, this biochemical modification may be one neurobiological substrate of the predisposition to acquire psychostimulant self-administration.

Stress-induced sensitization to amphetamine and morphine psychomotor effects depend on stress-induced corticosterone secretion

Repeated exposure to stressful situations has been shown to increase individual reactivity to addictive drugs. However, the biological factors involved in such stress-induced changes are largely unknown. In this study, we investigated the role of corticosterone in the effects of restraint stress on the response to psychostimulants and opioids. The effects of repeated stress on amphetamine- and morphine-induced locomotor activity were compared in: (i) animals with an intact hypothalamo-pituitary-adrenal (HPA) axis; (ii) animals in which stress-induced corticosterone secretion was blocked by adrenalectomy, but who received exogenous corticosterone from a subcutaneous implant. The implanted pellets (50 mg) slowly release corticosterone producing a stable plasma level within the normal physiological range over a period of 20 days. Restraint stress increased the locomotor response to both amphetamine (1.5 mg/kg i.p.) and morphine (2 mg/kg s.c.) in animals with an intact HPA axis, but not in animals in which stress-induced corticosterone secretion was suppressed. These results suggest that corticosterone secretion may be one of the mechanisms by which repeated stress amplifies behavioral responses to amphetamine and morphine. Since an enhanced locomotor reactivity to addictive drugs has been found to be frequently associated with an enhanced vulnerability to drug self-administration, these findings point to a role for glucocorticoids in the susceptibility to drug abuse.

Repeated corticosterone administration sensitizes the locomotor response to amphetamine

Repeated exposures to stressful situations has been shown to increase individual reactivity to psychostimulants, although the biological factors involved in such stress-induced changes are still poorly understood. In this study, we investigated the role of corticosterone in the effects of stress on the response to psychostimulants. We found that repeated corticosterone administration (both 1.5 mg/kg, intraperitoneally and 50 micrograms/ml in drinking water, once per day for 15 days) increased the locomotor response to amphetamine (1.15 mg/kg, i.p.). At the doses used in these experiments, corticosterone administration induced similar increases in plasma levels of the hormone to those induced by stress. These results suggest that corticosterone secretion may be one of the mechanisms by which repeated stress increases the behavioral responses to amphetamine. Since an enhanced reactivity to psychostimulants has been found to be an index of a propensity for drug self-administration and a model of certain psychopathological conditions, these findings point to a role for glucocorticoids in such abnormal states.

Possible interaction of the adrenal-gonadal systems on brain catecholamines of adult male rats

Studies from this laboratory showed that gonadectomy (GDX) alters biogenic amines concentrations in diencephalon during the first 40 days. While the GDX females maintain the differences at day 60, the differences are eliminated in males at that time. In the present work, we have studied in three cerebral regions the adrenal involvement in the mechanism responsible for this normalization of catecholamine concentration in long-term castrated adult male rats. A hypersecretion of adrenal steroids seems to compensate for the lack of gonadal effect when the orchidectomized rats reach adulthood only for diencephalic dopamine.

The effects of adrenalectomy on the ontogenesis of brain: noradrenaline and dopamine content

The effect of adrenalectomy on catecholamine content in the diencephalon and the rest of the brain of male and female rats during the post-natal period was studied. Seven days after adrenalectomy, there is no change in noradrenaline or dopamine content. However, the dopamine levels of both the diencephalon and the rest of the brain decrease with age between days 45 and 60, while noradrenaline content in the diencephalon and the rest of the brain remained unchanged. Thus adrenalectomy significantly affected the developmental pattern of brain dopamine.

Anatomy and physiology of the neuroendocrine arcuate nucleus

The arcuate nucleus surrounds the ventral part of the third ventricle and contains densely packed small neurons with 1-3 dendrites. At least fifteen transmitters and neuropeptides have been found in perikarya of arcuate neurons. Each transmitter and neuropeptide have a characteristic distribution. In many cases distributions overlap (for example, dopamine and somatostatin, dopamine and neurotensin, neuropeptide Y and somatostatin) and alpha-MSH and beta-endorphin seem to have identical distributions but there are also distinctive neuronal populations containing only one of the described transmitters or neuropeptides (neuropeptide Y and alpha-MSH). Studies show extensive colocalization of dopamine and neurotensin and sparse colocalization of dopamine and GABA, neuropeptide Y and FMRF-NH2 and neuropeptide Y and somatostatin. Colocalization does not seem to be the rule in the arcuate, however, it is possible that colocalization may vary with the physiological state or sex of the animal. It also should be noted that our techniques may not be sensitive enough. To study efferent projections as a possible organizing principle within the arcuate, retrograde fluorescent tracing was combined with transmitter and neuropeptide immunohistochemistry. Mainly NPY and alpha-MSH neurons were studied and both peptides are present in projections to the preoptic area as well as to the midbrain periaqueductal gray. Some arcuate neurons were found to have collateral axons to both these areas. The arcuate communicates primarily with the pituitary gland, hypothalamus, limbic system, midbrain periaqueductal gray and autonomic nuclei of the brain stem. In this way, the arcuate may be involved in integrating emotional, sensory, vegetative homeostatic and autonomic functions with endocrine functions.