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

Stress behaviour and physiology of developing Arctic barnacle goslings ( Branta leucopsis) is affected by legacy trace contaminants

Natural populations are persistently exposed to environmental pollution, which may adversely impact animal physiology and behaviour and even compromise survival. Responding appropriately to any stressor ultimately might tip the scales for survival, as mistimed behaviour and inadequate physiological responses may be detrimental. Yet effects of legacy contamination on immediate physiological and behavioural stress coping abilities during acute stress are virtually unknown. Here, we assessed these effects in barnacle goslings ( Branta leucopsis) at a historical coal mine site in the Arctic. For three weeks we led human-imprinted goslings, collected from nests in unpolluted areas, to feed in an abandoned coal mining area, where they were exposed to trace metals. As control we led their siblings to feed on clean grounds. After submitting both groups to three well-established stress tests (group isolation, individual isolation, on-back restraint), control goslings behaved calmer and excreted lower levels of corticosterone metabolites. Thus, legacy contamination may decisively change stress physiology and behaviour in long-lived vertebrates exposed at a young age.

Dysfunctional neurotransmitter systems in fibromyalgia, their role in central stress circuitry and pharmacological actions on these systems

Fibromyalgia is considered a stress-related disorder, and hypo- as well as hyperactive stress systems (sympathetic nervous system and hypothalamic-pituitary-adrenal axis) have been found. Some observations raise doubts on the view that alterations in these stress systems are solely responsible for fibromyalgia symptoms. Cumulative evidence points at dysfunctional transmitter systems that may underlie the major symptoms of the condition. In addition, all transmitter systems found to be altered in fibromyalgia influence the body's stress systems. Since both transmitter and stress systems change during chronic stress, it is conceivable that both systems change in parallel, interact, and contribute to the phenotype of fibromyalgia. As we outline in this paper, subgroups of patients might exhibit varying degrees and types of transmitter dysfunction, explaining differences in symptomatoloy and contributing to the heterogeneity of fibromyalgia. The finding that not all fibromyalgia patients respond to the same medications, targeting dysfunctional transmitter systems, further supports this hypothesis.

Dopamine D1 and D2 dopamine receptors regulate immobilization stress-induced activation of the hypothalamus-pituitary-adrenal axis

RATIONALE

Whereas the role of most biogenic amines in the control of the hypothalamus-pituitary-adrenal (HPA) response to stress has been extensively studied, the role of dopamine has not.

OBJECTIVES

We studied the effect of different dopamine receptor antagonists on HPA response to a severe stressor (immobilization, IMO) in adult male Sprague-Dawley rats.

RESULTS

Haloperidol administration reduced adrenocorticotropin hormone and corticosterone responses to acute IMO, particularly during the post-IMO period. This effect cannot be explained by a role of dopamine to maintain a sustained activation of the HPA axis as haloperidol did not modify the response to prolonged (up to 6 h) IMO. Administration of more selective D1 and D2 receptor antagonists (SCH23390 and eticlopride, respectively) also resulted in lower and/or shorter lasting HPA response to IMO.

CONCLUSIONS

Dopamine, acting through both D1 and D2 receptors, exerts a stimulatory role on the activation of the HPA axis in response to a severe stressor. The finding that dopamine is involved in the maintenance of post-stress activation of the HPA axis is potentially important because the actual pathological impact of HPA activation is likely to be related to the area under the curve of plasma glucocorticoid levels, which is critically dependent on how long after stress high levels of glucocorticoid are maintained.

Stress as a modulator of motor system function and pathology

Stress is one of the most significant influences on behaviour and performance. The classical account is that stress mainly affects functions of the limbic system, such as learning, memory and emotion. Recent evidence, however, suggests that stress also modulates motor system function and influences the pathology of movement disorders. Most parts of the motor system show the presence of glucocorticoid receptors that render their circuits susceptible to the influence of stress hormones. Stress and glucocorticoids have been shown to modulate temporal and spatial aspects of motor performance. Skilled movements seem to be most prone to stress-induced disturbances, but locomotion and posture can also be affected. Stress can modulate movement through activation of the hypothalamic-pituitary-adrenal axis and via stress-associated emotional changes. The dopaminergic system seems to play a central role in mediating the effects of stress on motor function. This route might also account for the finding that stress influences the pathology of dopamine-related diseases of the motor system, such as Parkinson's disease. Clinical observations have indicated that stress might lead to the onset of Parkinsonian symptoms or accelerate their progression. Glucocorticoids are modulators of neuronal plasticity, thus determining the degree of structural and functional compensation of the damaged motor system. This may particularly affect slowly progressive neurodegenerative diseases, such as Parkinson's disease. That stress represents a significant modulator of motor system function in both the healthy and the damaged brain should be recognized when developing future therapies for neurodegenerative diseases.

Individual differences in the effects of chronic prazosin hydrochloride treatment on hippocampal mineralocorticoid and glucocorticoid receptors

The aim of this study was to investigate the noradrenergic regulation of mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs) in high responder (HR) and low responder (LR) male rats, an animal model of individual differences in hypothalamo-pituitary-adrenal axis activity and vulnerability to drugs of abuse. The effects of a chronic treatment with the noradrenergic alpha(1) antagonist (1-[4-amino-6,7-dimethoxy-2-quinazolinyl]-4-[2-furanylcarbonyl] piperazine) hydrochloride (prazosin) (0.5 mg/kg, i.p., 35 days) were assessed on stress-induced corticosterone (CORT) secretion and on hippocampal MRs and GRs in adrenally intact rats. In order to ascertain whether the effects of chronic prazosin treatment on hippocampal MRs and GRs were direct or indirect, through prazosin-induced CORT secretion, we also assessed the effects of the same treatment on adrenalectomized rats with CORT substitutive therapy. When compared with LR rats, HR rats exhibited a delayed return to the basal level of CORT following acute restraint stress; this was associated with a lower binding of MRs and GRs in HR rats than in LR rats. Chronic prazosin treatment had no effect in HR animals but markedly reduced hippocampal MRs and GRs, and increased stress-induced CORT secretion in LR rats. In LR adrenalectomized rats, prazosin reduced hipppocampal MRs but did not change GRs. Our results provide evidence of a differential regulation by noradrenaline of hippocampal MRs and GRs in HR and LR rats. These data could have clinical implications in terms of individual differences in the resistance to antidepressant treatments and individual differences in drug abuse.

Proteasome inhibitor model of Parkinson's disease in mice is confounded by neurotoxicity of the ethanol vehicle

Defects in the ubiquitin-proteasome system have been implicated in Parkinson's Disease (PD). Recently, a rat model of PD was developed using a synthetic proteasome inhibitor (PSI), (Z-lle-Glu(OtBu)-Ala-Leu-al). We attempted to transfer this model to mouse studies, where genetics can be more readily investigated due to the availability of genetically modified mice. We treated C57BL/6 (B6) mice with six intraperitoneal injections of 6 mg/kg PSI in 50 mul of 70% ethanol over a 2-week-period. We found significant decreases in nigrostriatal dopamine in PSI-treated mice compared with saline-treated mice. However, we observed similar decreases in the ethanol-treated vehicle control group. Administration of ethanol alone led to significant long-term alterations in dopamine levels. Ethanol significantly eclipses the effects of PSI in the dopamine system, and therefore is a confounding vehicle for this model.

Modulation of motor function by stress: a novel concept of the effects of stress and corticosterone on behavior

Stress and stress hormones affect a variety of behaviors and cognitive abilities. The influences of stress and glucocorticoids on motor function, however, have not been characterized although the presence of glucocorticoid receptors in the motor system has been documented. Here we demonstrate that stress and the stress hormone corticosterone influence motor system function in rats. Groups of adult female Long-Evans rats underwent either a daily stress-inducing procedure (immobilization or swimming in cold water) or oral corticosterone treatment. While these treatments continued, animals were tested in skilled reaching and skilled walking tasks for a period of 2 weeks. Both acute (day 1) and chronic (day 14) stress and corticosterone treatment reduced skilled movement accuracy in reaching and walking and increased performance speed. Furthermore, both chronic stress and chronic corticosterone treatment altered skilled movement patterns in the reaching task. These findings indicate that stress modulates motor system function and that these effects are partially mediated by glucocorticoids. To examine whether stress-induced changes might also derive from enhanced emotionality, rats were treated with the benzodiazepine diazepam. Based on an inverted U-shaped dose-response relationship, a moderate dose of diazepam significantly improved reaching success while at the same time reducing corticosterone levels. Thus, stress-associated emotional responses such as anxiety might account for diminished movement accuracy. These results suggest that stress affects the motor system both directly via hormonal changes and indirectly via changes in emotionality. These findings are discussed with respect to the role of stress in motor system function and movement disorders.

Serotonin modulates the suppressive effects of corticosterone on proliferating progenitor cells in the dentate gyrus of the hippocampus in the adult rat

This series of experiments explores the interaction between corticosterone and serotonin (5-HT) in the regulation of cell proliferation in the dentate gyrus of the adult rat. Intracerebroventricular 5,7-DHT (5,7-dihydroxytryptamine) (either 200 or 300 microg) resulted in highly significant depletion of 5-HT as measured by high performance liquid chromatography in the frontal cortex but had no effect on the number of proliferating cells in the dentate gyrus by measuring 5-bromo-2'-deoxyuridine (BrdU) and Ki-67 cytochemistry. Treatment with PCPA (p-chlorophenylalanine: a tryptophan hydroxylase inhibitor: 300 mg/kg initially followed by 100 mg/kg/day) resulted in reduced proliferation as measured by Ki-67 after 3 days treatment, but not by BrdU uptake, and not after 14 days treatment by either method. In addition, injection of corticosterone (10-40 mg/kg/day) for 8 days significantly reduced proliferation in the dentate gyrus, as expected, measured by both BrdU uptake and Ki-67 immunostaining. Adrenalectomized (ADX) rats with a replacement subcutaneous pellet of corticosterone showed reduced proliferation when given additional corticosterone (10 mg/kg/day for 8 days), but this was prevented by 5-HT depletion (i.c.v. 5,7-DHT). Finally, a dose-response study showed that progressive doses of corticosterone (0-40 mg/kg/day) in ADX rats resulted in diminished suppression of proliferation in 5-HT-depleted compared with 5-HT-intact rats. These results strongly suggest that 5-HT regulates the sensitivity of proliferating cells in the dentate gyrus to corticosterone.

Chronic treatment with imipramine reverses immobility behaviour, hippocampal corticosteroid receptors and cortical 5-HT1A receptor mRNA in prenatally stressed rats

Prenatal stress in the rat induces enhanced reactivity of the hypothalamus-pituitary-adrenal (HPA) axis, disturbances in a variety of circadian rhythms and increased anxiety-like behaviour. Such abnormalities parallel those found in human depressed patients. Prenatally stressed (PS) rats could represent, therefore, an interesting animal model for the evaluation of the efficacy of pharmacotherapeutic intervention in psychiatric disorders that has often been addressed using control animals. In the present study, PS and non-stressed rats were chronically treated with the tricyclic antidepressant imipramine (10 mg/kg i.p. for 21 days) and assessed in the forced swim test. Glucocorticoid receptor binding sites in the hippocampus were measured and 5-HT(1A) receptor mRNA levels in the frontal cortex were also assessed. PS rats were characterised by increased immobility in the forced swim test, reduced hippocampal corticosteroid receptor binding and increased levels of cortical 5-HT(1A) mRNA. All these parameters were significantly reversed by chronic imipramine treatment. Conversely, no significant effects were observed for non-stressed rats. All these effects are consistent with the expected pharmacotherapy of depression-like abnormalities in PS rats. These results further indicate that PS rats are a relevant animal model of depression.

Influence of early postnatal rearing conditions on mesocorticolimbic dopamine and behavioural responses to psychostimulants and stressors in adult rats

While many experiment with drugs, relatively few individuals develop a true addiction. We hypothesized that, in rats, such individual differences in the actions of addictive drugs might be determined by postnatal rearing conditions. To test this idea, we investigated whether stimulant- and stress-induced activation of nucleus accumbens dopamine transmission and dopamine-dependent behaviours might differ among adults rats that had been either repeatedly subjected to prolonged maternal separation or a brief handling procedure or left undisturbed (non-handled) during the first 14 days of life. We found that, in comparison with their handled counterparts, maternally separated and non-handled animals are hyperactive when placed in a novel setting, display a dose-dependent higher sensitivity to cocaine-induced locomotor activity and respond to a mild stressor (tail-pinch) with significantly greater increases in nucleus accumbens dopamine levels. In addition, maternally separated animals were found to sensitize to the locomotor stimulant action of amphetamine when repeatedly stressed under conditions that failed to sensitize handled and non-handled animals. Finally, quantitative receptor autoradiography revealed a lower density of nucleus accumbens-core and striatal dopamine transporter sites in maternally separated animals. Interestingly, we also found greatly reduced D(3) dopamine receptor binding and mRNA levels in the nucleus accumbens-shell of handled animals. Together, these findings provide compelling evidence that disruptions in early postnatal rearing conditions can lead to profound and lasting changes in the responsiveness of mesocorticolimbic dopamine neurons to stress and psychostimulants, and suggest a neurobiological basis for individual differences in vulnerability to compulsive drug taking.

Handling, genetic and housing effects on the mouse stress system, dopamine function, and behavior

This research was designed to examine how early stimulation (i.e., handling), subsequent housing conditions and genetic factors interact to produce adult differences in stress regulation. High-aggressive (NC900) and low-aggressive (NC100) mice were handled for 3 weeks potspartum and were subsequently isolated or grouped until observed as adults in an open field or a dyadic test. In NC100, handling abolished the temporal variations seen in open-field activity among the nonhandled subjects and reduced corticosterone (CORT) activation. In NC900, these two measures were unaffected by handling. Only among handled NC100 did subsequent group rearing further reduce CORT activation. By contrast, handling caused an up-regulation of D1 dopamine receptors in both lines, and, in NC100, this effect was increased by group rearing. In a dyadic encounter with another male mouse, subjects of both lines showed handling effects. NC100 froze less rapidly and NC900 attacked more rapidly. This multifactorial design showed that the systemic effects of handling are modulated by genetic background, and that measures of these effects are affected by experience beyond infancy. Our findings also showed that the effects of handling vary when assessed across different physiological systems and across social and nonsocial testing conditions.

Prolactin release during exercise in normal and adrenodemedullated untrained rats submitted to central cholinergic blockade with atropine

To study the role of the central cholinergic system in pituitary prolactin (PRL) release during exercise we injected atropine (5 x 10(-7) mol) into the lateral cerebral ventricle of intact or adrenodemedullated (ADM) untrained rats, at rest or submitted to exercise on a treadmill (18 m x min(-1), 5% grade) until exhaustion. The rats were implanted with chronic jugular catheters for blood sampling and with unilateral intracerebroventricular (icv) cannulas placed in the right lateral ventricle. Blood prolactin concentrations were measured before and every 10 min after the start of exercise for a period of 60 min. After the animals started running, plasma prolactin levels rose rapidly in both normal and ADM rats, reaching near maximum at 10 min. Close to exhaustion (19.8 +/- 2.9 min for intact rats and 23.5 +/- 4.1 min for ADM) they were still high, remained increased until 30 min, and returned to preexercise levels at 40 min. Icv injections of atropine decreased the time to exhaustion by 67% in intact rats and by 96.2% in ADM and also reduced the exercise-induced PRL release in both intact (50%) and ADM rats (90%). The results showed that prolactin release induced by exercise was dependent on the exercise workload and could be observed as early as after 10 min of running, remaining increased until 30 min. These data indicate that adrenodemedullation does not affect prolactin secretion induced by exercise, although adrenodemedullated rats proved to be more sensitive to the reducing effect of central cholinergic blockade on their maximal capacity for exercise.

Circadian rhythm of drinking and running-wheel activity in rats with 6-hydroxydopamine lesions of the ventral tegmental area

Circadian rhythms in drinking and running-wheel (locomotor) activity of rats with 6-hydroxydopamine (6-OHDA, 4 microg/2 microl per rat)-induced lesions in the ventral tegmental area (VTA) were examined under a light-dark (LD) cycle and constant dim light (5 lux). Under the LD cycle, the length of the locomotor activity period was decreased during the dark, and increased during the light period in the lesioned rats. Under the constant dim light conditions, the free-running circadian period (tau) of drinking and activity rhythm was longer in lesioned rats than in sham-operated controls. The elongation of the circadian period was accompanied by decrements in activity. These observations suggest that the mesolimbic dopaminergic system modulates rhythms in circadian drinking and locomotor activity.

Bromocriptine-induced dissociation of hyperglycemia and prolactin response to restraint

The present study investigated the effects of immobilization (restraint stress) on rat chronically treated with a D(2) receptor agonist (bromocriptine, 0.4 mg/100 g body weight, injected daily intraperitoneally (ip) for 2 weeks) on plasma glucose, prolactin, and insulin levels. During restraint, the plasma prolactin of vehicle-treated (VEH) rats increased rapidly, reaching a peak at 10 min (57.9 +/- 8.1 ng/ml, P < .01). In contrast, restraint failed to induce any significant change in the plasma prolactin levels of bromocriptine-treated (BR) rats. The hyperglycemic response to immobilization was 97% higher (P < .05) in BR rats than in VEH rats. Our data demonstrate that prolactin secretion and hyperglycemia in response to restraint can be dissociated by chronic treatment with BR, which also increased the hyperglycemic response to immobilization probably due to central D(2) dopaminergic activity.

Effects of beta-phenylethylamine on the hypothalamo-pituitary-adrenal axis in the male rat

beta-Phenylethylamine (PEA) is a trace neuroactive amine implicated in the regulation of the hypothalamic-pituitary-adrenal (HPA) response to stress. To test this hypothesis, effects of subchronic levels of PEA (50 mg/kg/day treatment for 10 days) on the corticotroph function were studied. PEA treatment induces: (i) a significant increase of corticotrophin releasing hormone (CRH) immunoreactivity in the median eminence (ME), as measured by semi-quantitative immunofluorescence labeling techniques, (ii) a significant increase in CRH mRNA levels in paraventricular nuclei, as detected by in situ hybridization, and (iii) an increase in plasma adreno-corticotrophin hormone (ACTH) and corticosterone levels in responses to stress. PEA treatment has no effect on the number of binding sites and on the dissociation constant of the glucocorticoid receptors in any structure studied. Results of the dexamethasone suppression test were similar in PEA- and saline-treated rats. Taken together, these results suggest that PEA treatment stimulated the HPA axis activity levels directly via the CRH hypothalamic neurons, without altering the negative feed back control exerted by the glucocorticoids.

Enhanced nucleus accumbens dopamine and plasma corticosterone stress responses in adult rats with neonatal excitotoxic lesions to the medial prefrontal cortex

The medial prefrontal cortex modulates the nucleus accumbens dopamine response to stress and has been implicated in feedback regulation of hypothalamic-pituitary-adrenal axis activation by stress. Here we report on the effects of bilateral neonatal (postnatal day 7) ibotenate-induced lesions to the medial prefrontal cortex on nucleus accumbens dopamine and neuroendocrine function in adult rats. Voltammetry was used to monitor the dopamine response to each of five, once-daily exposures to tail-pinch stress whereas alterations in neuroendocrine function were determined from the plasma corticosterone response to a single 20-min episode of restraint stress. Potential lesion-induced deficits in sensory-motor gating were assessed by measuring prepulse inhibition of the acoustic startle response before and after repeated stress. Our data show that each daily stress episode elicited larger and longer-lasting dopamine increases in prefrontal cortex-lesioned animals than in sham-lesioned controls. Furthermore, greater stress-induced elevations in plasma corticosterone were seen in lesioned animals than in their sham-lesioned counterparts. However, while repeated stress potentiated startle responses in animals of both groups, there was no effect of lesion on the amplitude or on prepulse inhibition of the startle response.Together, these findings indicate that neonatal prefrontal cortex damage can lead to changes in mesolimbic dopamine and neuroendocrine function during adulthood. They also add to a growing body of experimental and clinical evidence implicating abnormal prefrontal cortex neuronal development in the pathophysiology of schizophrenia and other disorders linked to central dopamine dysfunction.

Dopamine function in obsessive compulsive disorder: cortisol response to acute apomorphine stimulation

Central dopaminergic dysfunction has been suggested to be involved in the pathogenesis of obsessive compulsive disorder (OCD). In 15 patients with OCD and in 15 age-sex matched controls we evaluated the dopamine (DA) function by measuring the cortisol (CORT) responses to stimulation with the DA agonist apomorphine (APO). The CORT response to acute saline administration was also measured, to exclude the existence of a pathology of the circadian secretion of the hormone which could obscure the significance of the CORT response to APO stimulation. Basal levels of CORT were the same in patients and controls, but the values after saline administration were significantly higher in patients than in controls. APO stimulation-induced CORT rises were significantly higher in patients than in controls, but when the data after APO were corrected for those after saline, there were no significant difference between the two groups of subjects. Our data suggest that there are no alterations of the central dopaminergic function connected with the regulation of the hypothalamo-pituitary-adrenal axis in OCD.

A unique hormonal and behavioral hyporesponsivity to both forced novelty and d-amphetamine in periadolescent mice

The identification of critical ontogenetic periods of increased vulnerability to the effects of drugs of abuse could have a great psychobiological and clinical-therapeutical importance. Potential age-related differences in the response of the hypothalamic-pituitary-adrenal (HPA) axis to both stress and psychostimulants has been tested here in an animal model of adolescence. Periadolescent (PND 33-43) and Adult (PND>60) mice of both sexes were injected with d-amphetamine (AMPH, 0, 2, or 10 mg/kg i.p.) and immediately faced with a mild psychological stress experience, i.e. placement in a novel environment. A detailed time-course analysis of both hormonal and behavioral profiles was performed, with animals being sacrificed for trunk-blood collection at different time-points during the test (before the injection, NT group; 15, 30, or 120 min after the injection). Basal corticosterone (CORT) levels (NT group) were consistently higher in periadolescents than in adults. As a whole, a marked increment of blood CORT levels was found in mice of both ages exposed to forced novelty. However, important age-related differences were also observed, with Saline-injected periadolescents still exhibiting elevated levels of locomotion at the end of the 120-min test session and failing to show the increasing profile of CORT release over the baseline that was typical of adults. Upon an AMPH 2 administration, periadolescents exhibited a much lower profile of locomotor hyperactivity than adults, and also failed to show an increase across the course of the session in CORT release, that was observed in adults. When treated with the high AMPH 10 dose, a marked locomotor hyperactivity was found in periadolescents, which however showed much lower levels of the stereotyped licking and gnawing behavior, that was typical of adults. The present results suggest a unique profile of integrated behavioral and physiological hyporesponsivity in mice during periadolescence. The latter also represents a very useful model for the study of the issue of psychobiological risk factors involved in vulnerability to drugs of abuse in human adolescents.

Psychobiological risk factors for vulnerability to psychostimulants in human adolescents and animal models

Adolescence is associated with an increased risk of developing drug abuse/dependence. During this ontogenetic phase, brain and hormonal systems are still undergoing crucial maturational rearrangements, which take place together with significant modifications in psychosocial development. However, the neurohormonal and behavioral facets of adolescence have been poorly investigated in relation to the vulnerability to psychostimulants such as MDMA ("Ecstasy") and amphetamine (AMPH). Novelty-seeking, a temperamental/behavioral trait that is typical of this age period, might substantially contribute to both psychological and psychobiological vulnerability. In humans, an elevated score of novelty-sensation seeking and a derangement of monoaminergic function were both associated with late adolescence MDMA users compared to controls. In animal models of periadolescence, the search for novel stimuli and sensations actually shares a common neurobiological substrate (the reward-related brain mesolimbic pathways) with psychostimulants. The present review summarises recent work in mice, which indicates that periadolescent subjects are characterized by an unbalanced and "extremes-oriented" behavior and by elevated novelty-seeking compared to adults. Repeated and intermittent administration of cocaine or AMPH was associated with the development of a prominent locomotor sensitization in periadolescents, which failed to exhibit the marked sensitization of the stereotyped behavioral syndrome--possibly associated with poor welfare--that was typical of adults. A unique profile of integrated behavioral and physiological hyporesponsivity to both forced novelty and acute AMPH administration during periadolescence was also found. As a whole, these results, together with previous work on this topic, suggest that periadolescents may be more "protected" from AMPH-related aversive properties, and perhaps more vulnerable to the experience of internal states of reward, than older animals. Thus, the present animal model of adolescence seems to represent a reliable and useful method for the investigation of vulnerability to a variety of habit-forming agents or emotional experiences whose positive reinforcing properties may rely on common neurobiological substrates. A deeper understanding of psychostimulant effects during adolescence on the complex interaction between genetic, neurobiologic, psychosocial, and environmental factors will lead to earlier and more effective prevention and treatment.

Elevation of striatal interleukin-6 and serum corticosterone contents in MPTP-treated mice

1. Changes in the content of striatal interleukins (IL-1 beta and IL-6) and serum corticosterone in relation to deterioration of the dopaminergic system induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; a dopaminergic neurotoxin; 20 mg/kg i.p., four administrations/12 h) in C57BL/6J mice were investigated. 2. Striatal dopamine, IL-1 beta, IL-6 and serum corticosterone were measured on days 1 and 7 post-MPTP. 3. Dopamine depletion was more severe on day 7 than on day 1 post-treatment. 4. Increases in IL-6 were observed on days 1 and 7 post-MPTP. The increase in striatal IL-6 content varied with the extent of dopamine depletion, although the IL-1 beta concentration remained unchanged compared with control values on days 1 and 7 post-treatment. 5. Serum corticosterone was not different from control on day 1 post-MPTP. However, marked increases in the serum corticosterone were observed on day 7 post-treatment. 6. These results suggest that changes in striatal IL-6 and serum corticosterone are closely associated with the severity of MPTP-induced dopaminergic degeneration.

Long-term effects of prenatal stress and postnatal handling on age-related glucocorticoid secretion and cognitive performance: a longitudinal study in the rat

There is growing evidence that stress during prenatal and postnatal periods of life can modify adaptive capacities in adulthood. The hypothalamo-pituitary-adrenal axis may mediate an animal's responses to perinatal stressful events and thus serve as a neurobiological substrate of the behavioural consequences of these early events. However, little is known about the long-term effects of prenatal stressors throughout the entire life of the animals. The focus of the present study was to examine the long-term influences of a prenatal and postnatal stress on glucocorticoid secretion and cognitive performance. Prenatal stress of rat dams during the last week of pregnancy and postnatal daily handling of rat pups during the first 3 weeks of life were used as stressors. The long-term effects of these manipulations were analysed using a longitudinal approach throughout the entire life of the animals, and were repeatedly tested in adulthood (4-7 months), middle age (13-16 months) and in later life (20-24 months). The study demonstrated that prenatal stress and postnatal handling induced opposite effects on both glucocorticoid secretion and cognitive performance. Prenatal stress accelerated the age-related hypothalamo-pituitary-adrenal axis dysfunctions; indeed, circulating glucocorticoids levels of prenatally stressed middle-aged animals are similar to old control ones, and also induced cognitive impairments. In contrast, postnatal handling protected from the age-related neuroendocrine and behavioural alterations. These results show that the altered glucocorticoid secretion induced by early environmental manipulations is primary to the cognitive alterations observed only later in life and could be one cause of age-related memory deficits.

Engagement in a non-escape (displacement) behavior elicits a selective and lateralized suppression of frontal cortical dopaminergic utilization in stress

Although the preferential activation of the prefrontal cortical (PFC) dopaminergic system is generally observed in stress, limited exceptions to this have been observed. Certain non-escape behaviors have been demonstrated to attenuate physiological indices of stress (e.g., coping or displacement responses). One well-characterized non-escape behavior observed in stress is chewing, or gnawing, of inedible objects. Engagement in this behavior attenuates stress-related activation of the hypothalamopituitary-adrenal axis, in a variety of species. We examined the degree to which engagement in this non-escape behavior modulates stressor-induced activation of the PFC dopamine (DA) system. Rats and mice were exposed to a brightly lit novel environment (novelty stress) in the presence or absence of inedible objects. Following novelty exposure, various dopaminergic terminal fields were collected and dopamine and its major catabolite, DOPAC, were measured using HPLC with electrochemical detection. DOPAC/DA ratios were calculated as an index of DA utilization. In some cases serotonin (5-HT) and its major catabolite, 5-HIAA, were also measured. In animals that did not chew, novelty exposure elicited significant increases in DOPAC/DA levels within PFC, nucleus accumbens (shell and core subdivisions), and striatum (relative to quiet-controls). DOPAC/DA responses were greater in the right PFC than in the left PFC. Animals that chewed displayed significantly lower DOPAC/DA responses in PFC, but not other dopaminergic terminal fields. This effect of chewing was always observed in the right PFC and less consistently in the left PFC. Chewing did not alter novelty-induced increases in PFC 5-HIAA/5-HT responses. Thus, engagement in this non-escape behavior elicits a neuroanatomically and neurochemically specific attenuation of the PFC DA response in stress. Given the pivotal role of the PFC in certain cognitive and affective processes, behavioral regulation of PFC DA utilization may modulate cognitive and/or affective function in stress.

Microinjections of dopamine agonists and cocaine elevate plasma corticosterone: dissociation effects among the ventral and dorsal striatum and medial prefrontal cortex

There is evidence suggesting the involvement of central dopamine (DA) systems in the regulation of plasma corticosterone (CORT). We examined whether or not microinjections of DA agonists and cocaine into three DA-rich terminal regions, the medial prefrontal cortex (MPC), ventral striatum (VStr) and dorsal striatum (DStr), would increase plasma CORT in Sprague-Dawley rats. The highest dose tested (18 nmol) of a mixture of the dopamine D1 receptor agonist SKF 38393 and the D2 agonist quinpirole (SKF/Quin) increased plasma CORT when injected into each of the three brain regions. Microinjections of the medium dose (i.e., 3 nmol) of SKF/Quin into the VStr also increased plasma CORT, while the injections into the MPC and DStr did not. Systemic pretreatment with haloperidol attenuated the elevated CORT induced by intra-VStr injections of SKF/Quin. Cocaine (25, 50, and 100 microg) also increased CORT when injected into the VStr, but not into the MPC or DStr. Microinjections of local anesthetics, lidocaine (100 microg) and procaine (100 microg), which have similar chemical structures to cocaine, into the VStr did not increase CORT. These results suggest that the VStr plays an important role in mediating the elevated plasma CORT induced by DA agonists and cocaine administration.

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.

Relationships between stress-induced increases in medial prefrontal cortical dopamine and plasma corticosterone levels in rats: role of cerebral laterality

In the present study, in vivo voltammetry was used to monitor changes in dopamine levels in the left and right medial prefrontal cortex of rats exposed to mild physical and psychological stress. These were 2 min of tail-pinch and 15 min exposure to cat odour, respectively. Fourteen male Long Evans rats with bilateral carbon fibre recording electrodes were tested on four consecutive days, and records obtained in each medial prefrontal cortex for each stressor. A week later, animals underwent a 20 min restraint stress, with plasma samples taken at 0, 20 and 80 min to determine stress-induced corticosterone responses. It was found that dopamine responses to tail-pinch were significantly longer-lasting in the left hemisphere than in the right, while this asymmetry was not present for the dopamine response to cat odour. Stress-induced dopamine increases elicited by the two stressors were significantly correlated only in the right medial prefrontal cortex. Restraint stress-induced increases in plasma corticosterone were positively correlated with dopaminergic responses to tail-pinch, but were only related to dopamine cat odour responses when individual asymmetries favoured the right medial prefrontal cortex. The data suggest that asymmetric mesocortical dopamine activation depends on the type of stress, and that regulation of dopamine responses to both types of stress is most tightly coupled in the right hemisphere. While neuroendocrine and dopaminergic stress responses are positively linked, this relationship is only asymmetrical for the psychological stressor, suggesting a specialized role for right cortical mechanisms in the integration of emotional and physiological responses to stressful situations. A preliminary report of this work was presented at the Society for Neuroscience meeting in Washington DC, November, 1996.

Behavioral sensitization to ethanol: genetics and the effects of stress

Some aspects of drug abuse syndromes may be influenced by sensitization to some drug effects. This enhancement of drug effect has been associated with prior drug exposure and with exposure to stressful stimuli. It has been postulated that sensitization to psychomotor stimulant drug effects influences sensitivity to drug reward. The drugs of abuse best characterized for sensitization phenomena include cocaine, amphetamine, and morphine. In general, ethanol's molecular mechanisms of action have been difficult to define relative to drugs with known receptor or transporter binding sites and, likewise, ethanol sensitization has been less thoroughly examined. Evidence supporting the existence of behavioral sensitization to ethanol, for genetic differences in the occurrence of ethanol sensitization, and for the influence of corticosterone on the development of ethanol sensitization is reviewed herein. There appear to be different genetic determinants of acute drug sensitivity and sensitization. Cross-sensitization between stress and ethanol suggest a potential role for hypothalamic-pituitary-adrenal (HPA) axis associated changes in ethanol sensitization, consistent with mechanisms likely contributing to sensitization to other abused drugs. Furthermore, glucocorticoid receptors appear to mediate both ethanol- and stress-induced sensitization to ethanol. A biological link between drug reward and drug sensitization involving HPA axis hormones may exist and, thus, study of the sensitization process may elucidate mechanisms relevant to drug abuse.

Nullification of a positive correlation between urinary levels of alpha 1-microglobulin and ulinastatin by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice

The relationships between urinary levels of alpha 1-microglobulin (alpha 1M) and ulinastatin (UT) were investigated in C57BL/6J mice, a species which reportedly possesses the gene similar to that of humans for synthesizing the precursor protein of alpha 1M and UT. A positive correlation was established in normal mice. However, repetitive administrations (20 mg/kg, IP, four administrations/12 h) of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) nullified the positive correlation. A similar phenomenon was induced by ICV-administered MPTP (18 and 36 micrograms) in the animals. Furthermore, L-dopa administration (50 mg/kg, IV) in MPTP-treated (1 week after the final IP administration of MPTP) mice reversed the tendency of MPTP, although the agent alone did not affect the positive correlation in normal mice. These results suggest that nullification of the positive correlation probably was induced by the central effects of MPTP. We have found previously that the lack of a positive correlation between urinary levels of alpha 1M and UT distinguishes Parkinson's disease from other neuropsychiatric diseases such as dementia (Alzheimer-type and vascular dementia), schizophrenia and mood disorders. Our present results displayed a phenomenon that the lack of correlation between urinary levels of alpha 1M and UT in patients with Parkinson's disease is reproducible in MPTP-treated mice.

A neuroendocrine role in cocaine reinforcement

Cocaine stimulates the secretion of corticosterone and ACTH, probably through a CRF-related mechanism, indicating that the drug activates the HPA axis. Indeed, cocaine has been reported to produce anxiety and to precipitate episodes of panic attack during chronic use and withdrawal in humans and to induce anxiogenic behavior in animals. Cocaine also alters benzodiazepine receptor binding in discrete regions of the rat brain. Some of these changes in binding are obviously related to the convulsions and seizures which are often observed in an acute cocaine overdose. However, data from behavioral studies have suggested that some of these effects may be related directly to cocaine reinforcement since receptor changes also were observed when binding in the brains of rats that self-administered cocaine was compared with that from animals that had received identical yoked, but non-contingent infusions of the drug. In this regard, pretreatment with the benzodiazepine receptor agonists chlordiazepoxide and alprazolam decreased cocaine self-administration without decreasing food-reinforced responding, suggesting that these effects were specific for cocaine. Since this attenuation of self-administration was reversed by increasing the unit dose of cocaine, it is likely that these drugs were decreasing cocaine reinforcement. In contrast, exposure to stress increases vulnerability to self-administer psychostimulants. In these experiments, low-dose cocaine self-administration was related directly to stress-induced increases in plasma corticosterone, such that plasma corticosterone was always greater than 150 ng/ml for rats which subsequently self-administered cocaine at doses of 0.125 mg/kg/infusion or lower, suggesting a threshold for the hormone in cocaine reinforcement. In other experiments, bilateral adrenalectomy completely abolished the acquisition of intravenous cocaine self-administration in naive rats, while metyrapone decreased ongoing self-administration. In addition, ketoconazole pretreatment resulted in patterns of self-administration that were virtually indistinguishable from that observed during saline extinction, suggesting that plasma corticosterone is not only important, but may even be necessary for cocaine reinforcement. The mechanisms through which adrenocorticosteroids alter cocaine reinforcement remain to be determined, but there is increasing evidence that the mesocorticolimbic dopaminergic system is involved. In particular, the medial prefrontal cortex appears to be at least one brain region where dopamine and adrenocorticosteroids may interact to affect cocaine reinforcement.

Effects of surgical and pharmacological adrenalectomy on the initiation and maintenance of intravenous cocaine self-administration in rats

Previous research has suggested the potential involvement of the hypothalamic-pituitary-adrenal (HPA) axis in psychostimulant reinforcement. In particular, we have found significant correlations between electric footshock-induced increases in plasma corticosterone and the acquisition, or lack thereof, of intravenous cocaine self-administration in rats. The experiments presented here were designed to further determine the role for corticosterone in cocaine reinforcement in rats by decreasing plasma levels of the hormone with surgical and pharmacological adrenalectomies. Bilateral adrenalectomy completely abolished the acquisition of intravenous cocaine self-administration over a wide range of doses (0.03125 to 1.0 mg/kg/infusion) without affecting food maintained responding. This suppression of self-administration was partially reversed by adding corticosterone (100 micrograms/ml) to the rats' drinking water. In another group of rats, pretreatment with metyrapone, which blocks the synthesis of corticosterone, resulted in dose-related decreases in ongoing cocaine self-administration. These data suggest that corticosterone is not only important, but may also be necessary for both the acquisition and maintenance of cocaine reinforcement in rats.

Sensitization of stress-induced feeding in rats repeatedly exposed to brief restraint: the role of corticosterone

Groups of male Wistar rats lived in cages capable of monitoring feeding and drinking continuously at 0.1-s intervals, 24 h per day. Intact animals were subjected to 20 min of restraint stress or to brief handling (Brief Pick-Up), daily or every third day, 6 h after the beginning of the 12-h light period. In both studies, food-intake increased in the first hour after restraint, peaking between 15 and 45 min. Smaller increases were seen following Brief Pick-Up. More interestingly, the amount of food eaten increased across test sessions, indicating sensitization of the response to stress. Drinking also increased following stress, occurring before feeding and diminishing after the first 15 min. In adrenalectomized animals implanted with slow-release pellets to replace basal diurnal levels of corticosterone (ADX animals), sensitization of the feeding response to restraint stress developed across test sessions, although in these animals, the acute increase in food-intake following restraint stress was attenuated. ADX animals subjected only to Brief Pick-Up showed no increases in food-intake. Daily injections of 3.0 mg/kg corticosterone given to such ADX animals were unable to mimic the effects of restraint on either food-intake or drinking, nor did they augment the effects of restraint in ADX animals. We conclude that sensitization to the effects of brief restraint stress on food-intake can occur independently of a stress-induced rise in plasma corticosterone.

Corticosteroid feedback resistance in rats genetically selected for increased dopamine responsiveness

Pharmacogenetically selected Wistar rat lines were used to investigate the implication of either high or low responsiveness of the dopamine system for the activity of the hypothalamus-pituitary-adrenal (HPA) axis. As selection criterion the gnawing response induced by the dopamine agonist apomorphine was used. This criterion allows to distinguish apomorphine susceptible (apo-sus) rats which show a vigorous gnawing response from apomorphine unsusceptible (apo-unsus) rats. The present study, using male animals of the 9-12th generation of the two rat lines, revealed the following characteristics of the stress response system: (i) in apo-sus rats under basal conditions corticotrophin-releasing hormone (CRH) mRNA level in the paraventricular nucleus (PVN) and plasma adrenocorticotropin (ACTH) concentration were significantly higher; total corticosterone (B) plasma level was similar but free B level was lower; (ii) exposure to a novel environment resulted in a higher and prolonged plasma ACTH and total B response in the apo-sus rats. Moreover, the elevated free B level was also prolonged; (iii) apo-sus rats had increased CRH-induced pituitary ACTH release and B secretion was also increased, but not as prolonged as during novelty. (iv) In dexamethasone-pretreated rats an intravenous ACTH1-24 injection resulted in a similar plasma B response in rats of both lines; (v) In vitro, ACTH1-24 produced a significantly higher B secretion by adrenocortical cells of apo-sus rats reflecting the higher in vivo ACTH priming of the adrenal glands in these animals. (vi) apo-sus rats had higher body and thymic weight.(ABSTRACT TRUNCATED AT 250 WORDS)

The D1 dopamine agonist SKF 38393, but not the D2 agonist LY 171555, decreases the affinity of type II corticosteroid receptors in rat hippocampus and ventral striatum

Type I and type II brain corticosteroid receptors are regulated by adrenal hormones as well as being under neural control. Recent studies have indicated that neurotransmitters such as serotonin and noradrenaline are also involved in the regulation of corticosteroid receptors. In a previous study, we showed that dopamine also modulates activity of the corticosteroid receptor system. In the present study, we examined the roles of the dopamine D1 and D2 receptor subtypes in the regulation of corticosteroid receptors. Adrenalectomized rats whose corticosterone levels were maintained within normal limits by corticosterone replacement implants, were injected intraperitoneally with the D1 agonist SKF 38393 or the D2 agonist LY 171555. Corticosteroid receptors were assayed in the ventral striatum and hippocampus. We have shown that the D1 agonist SKF 38393 decreased type II receptor affinity in both regions, whereas the D2 agonist LY 171555 had no effects. The results show that the influence of the dopaminergic system on corticosteroid receptors appears to be mediated by D1 receptors.