Corticotropin-releasing hormone receptor blockade fails to alter stress-evoked catecholamine release in prefrontal cortex of control or chronically stressed rats

Serotonin and Dopamine Show Different Response Profiles to Acute Stress in the Nucleus Accumbens and Medial Prefrontal Cortex of Rats with Neuropathic Pain

The ability to adaptively guide behaviour requires the integration of external information with internal motivational factors. Decision-making capabilities can be impaired by acute stress and is often exacerbated by chronic pain. Chronic neuropathic pain patients often present with cognitive dysfunction, including impaired decision-making. The mechanisms underlying these changes are not well understood but may include altered monoaminergic transmission in the brain. In this study we investigated the relationships between dopamine, serotonin, and their metabolites in key brain regions that regulate motivated behaviour and decision-making. The neurochemical profiles of the medial prefrontal cortex, orbital prefrontal cortex, and nucleus accumbens were analysed using HPLC in rats that received a chronic constriction injury (CCI) of the right sciatic nerve and an acute stress (15-min restraint), prior to an outcome devaluation task. CCI alone significantly decreased dopamine but not serotonin concentrations in the medial prefrontal cortex. By contrast, restraint stress acutely increased dopamine in the medial prefrontal cortex, and the nucleus accumbens; and increased serotonin in the medial prefrontal cortex 2 h later. The sustained dopaminergic and serotonergic responses to acute stress highlight the importance of an animal's ability to mount an effective coping response. In addition, these data suggest that the impact of nerve injury and acute stress on outcome-devaluation occurs independently of dopaminergic and serotonergic transmission in the medial prefrontal cortex, orbital prefrontal cortex and nucleus accumbens of rats.

Time-dependent effects of acute stress on working memory performance: A systematic review and hypothesis

Laboratory procedures such as the Trier Social Stress Test or the (Socially Evaluated) Cold Pressor Test have been used to investigate working memory performance under stress. Researchers so far have reported a diverse spectrum of stress effects (including the lack thereof) on working memory tasks. We conducted a systematic review of the effect acute stress on working memory performance in standardized laboratory procedures. An overview of the existing literature suggests that acute stress affects working memory in a time-dependent manner, presumably due to the differing time scales of the main stress-reactive hormones involved. Based on the empirical evidence, we hypothesize that the immediate stress-induced release of noradrenaline decreases working memory performance within the first 10 min post stress. In addition, rapid cortisol effects impair working memory at a later time-interval beginning about 25 min post stress. We outline future research directions which could further explore the implications of our insights, as for example combined pharmacological and naturalistic stressor interventions.

Ventral tegmental area dopamine revisited: effects of acute and repeated stress

Aversive events rapidly and potently excite certain dopamine neurons in the ventral tegmental area (VTA), promoting phasic increases in the medial prefrontal cortex and nucleus accumbens. This is in apparent contradiction to a wealth of literature demonstrating that most VTA dopamine neurons are strongly activated by reward and reward-predictive cues while inhibited by aversive stimuli. How can these divergent processes both be mediated by VTA dopamine neurons? The answer may lie within the functional and anatomical heterogeneity of the VTA. We focus on VTA heterogeneity in anatomy, neurochemistry, electrophysiology, and afferent/efferent connectivity. Second, recent evidence for a critical role of VTA dopamine neurons in response to both acute and repeated stress will be discussed. Understanding which dopamine neurons are activated by stress, the neural mechanisms driving the activation, and where these neurons project will provide valuable insight into how stress can promote psychiatric disorders associated with the dopamine system, such as addiction and depression.

Coping with chronic social stress in mice: hypothalamic-pituitary-adrenal/ sympathetic-adrenal-medullary axis activity, behavioral changes and effects of antalarmin treatment: implications for the study of stress-related psychopathologies

The aim of this study was to analyze the individual differences that lead to the development of psychopathological changes in response to chronic social stress. We also assessed the ability of an antagonist of the corticotrophin-releasing hormone (CRH) receptors to reverse the effects of stress. Male adult mice were exposed to repeated defeat experiences for 21 days using a sensorial contact model. After 18 days of defeat, two groups of subjects were established (active and passive), according to their behaviors during social confrontation. Antalarmin treatment was given for 4 and 6 days. The results corroborated previous data indicating that subjects who adopted a passive coping strategy had higher corticosterone levels after 21 days of defeat and decreased resting levels 3 days later. Moreover, they showed higher resting expression levels of hypothalamic CRH than their active counterparts. On day 24, the experimental animals were subjected to another social defeat to determine whether the stress response remained. The increase in corticosterone and hypothalamic CRH levels was similar for all of the stressed subjects, but the passive subjects also had a greater CRH response in the amygdala. Passive subjects had decreased levels of adrenal dopamine β-hydroxylase, tyrosine hydroxylase and plasma adrenaline compared to the active subjects, and lower plasma noradrenaline levels than manipulated controls. The passive profile of physiological changes in both the hypothalamic-pituitary-adrenal and sympathetic-adrenal-medullary (SAM) axes has been associated with changes related to mood disorders, such as posttraumatic stress disorder and depression. The active coping profile is characterized by similar corticosterone resting levels to controls and increased SAM activity. Both profiles showed alterations in the novel palatable and forced swimming tests, with the passive profile being the most vulnerable to the effects of stress in this last test. Pharmacological treatment with antalarmin failed to reverse the effects of stress.

A neurotensin analog, NT69L, attenuates intravenous nicotine self-administration in rats

NT69L is a neurotensin analog that blocks nicotine-induced locomotor activity and has sustained efficacy in a rat model of nicotine-induced sensitization when administered peripherally. Additionally, NT69L attenuates food-reinforcement in rats. The present study tested the effect of acute administration of NT69L on nicotine self-infusion in Sprague-Dawley rats. Rats were trained to self-infuse nicotine intravenously (0.03mg/kg per infusion) following operant training. Once the rats acquired stable responding to nicotine self-infusion they were pretreated with NT69L (1mg/kg, i.p.) or saline 30min before being assessed for nicotine self-infusion. Pretreatment with NT69L significantly attenuated nicotine self-infusion under FR1 (fixed ratio of 1) and FR5 schedule of reinforcement as compared to saline pretreatment. Control rats that were response-independent "yoked" as well as rats that self-infused saline or NT69L showed minimal responses, indicating that nicotine served as a reinforcer. Additionally, NT69L modulated serum corticosterone; brain norepinephrine serotonin; and dopamine receptors mRNA levels altered in the nicotine self-infused rats after a 24h withdrawal period. Pretreatment with NT69L significantly decreased the nicotine-induced increase in serum corticosterone levels and striatal norepinephrine and increased the nicotine-induced reduction in serotonin in both the striatum and the prefrontal cortex (PFC). NT69L might modulate dopamine neurotransmission implicated in the reinforcing effects of nicotine by modulating tyrosine hydroxylase and dopamine receptor mRNA levels in the PFC and striatum. These data support further study of the effects of NT analogs on attenuating the reinforcing effects of psychostimulants.

On the origin of cortical dopamine: is it a co-transmitter in noradrenergic neurons?

Dopamine (DA) and noradrenaline (NA) in the prefrontal cortex (PFC) modulate superior cognitive functions, and are involved in the aetiology of depressive and psychotic symptoms. Moreover, microdialysis studies in rats have shown how pharmacological treatments that induce modifications of extracellular NA in the medial PFC (mPFC), also produce parallel changes in extracellular DA.To explain the coupling of NA and DA changes, this article reviews the evidence supporting the hypothesis that extracellular DA in the cerebral cortex originates not only from dopaminergic terminals but also from noradrenergic ones, where it acts both as precursor for NA and as a co-transmitter.Accordingly, extracellular DA concentration in the occipital, parietal and cerebellar cortex was found to be much higher than expected in view of the scarce dopaminergic innervation in these areas.Systemic administration or intra-cortical perfusion of alpha(2)-adrenoceptor agonists and antagonists, consistent with their action on noradrenergic neuronal activity, produced concomitant changes not only in extracellular NA but also in DA in the mPFC, occipital and parietal cortex.Chemical modulation of the locus coeruleus by locally applied carbachol, kainate, NMDA or clonidine modified both NA and DA in the mPFC.Electrical stimulation of the locus coeruleus led to an increased efflux of both NA and DA in mPFC, parietal and occipital cortex, while in the striatum, NA efflux alone was enhanced.Atypical antipsychotics, such as clozapine and olanzapine, or antidepressants, including mirtazapine and mianserine, have been found to increase both NA and DA throughout the cerebral cortex, likely through blockade of alpha(2)-adrenoceptors. On the other hand, drugs selectively acting on dopaminergic transmission produced modest changes in extracellular DA in mPFC, and had no effect on the occipital or parietal cortex.Acute administration of morphine did not increase DA levels in the PFC (where NA is diminished), in contrast with augmented dopaminergic neuronal activity; moreover, during morphine withdrawal both DA and NA levels increased, in spite of a diminished dopaminergic activity, both increases being antagonised by clonidine but not quinpirole administration.Extensive 6-hydroxy dopamine lesion of the ventral tegmental area (VTA) decreases below 95% of control both intra- and extracellular DA and DOPAC in the nucleus accumbens, but only partially or not significantly in the mPFC and parietal cortex.The above evidence points to a common origin for NA and DA in the cerebral cortex and suggests the possible utility of noradrenergic system modulation as a target for drugs with potential clinical efficacy on cognitive functions.

Structural and functional alterations to rat medial prefrontal cortex following chronic restraint stress and recovery

Chronic stress has been shown in animal models to result in altered dendritic morphology of pyramidal neurons of the medial prefrontal cortex (mPFC). It has been hypothesized that the stress-induced dendritic retractions and spine loss lead to disrupted connectivity that results in stress-induced functional impairment of mPFC. While these alterations were initially viewed as a neurodegenerative event, it has recently been established that stress induced dendritic alterations are reversible if animals are given time to recover from chronic stress. However, whether spine growth accompanies dendritic extension remains to be demonstrated. It is also not known if recovery-phase dendritic extension allows for re-establishment of functional capacity. The goal of this study, therefore, was to characterize the structural and functional effects of chronic stress and recovery on the infralimbic (IL) region of the rat mPFC. We compared neuronal morphology of IL layer V pyramidal neurons from male Sprague-Dawley rats subjected to 21 days of chronic restraint stress (CRS) to those that experienced CRS followed by a 21 day recovery period. Layer V pyramidal cell functional capacity was assessed by intra-IL long-term potentiation (LTP) both in the absence and presence of SKF38393, a dopamine receptor partial agonist and a known PFC LTP modulator. We found that stress-induced IL apical dendritic retraction and spine loss co-occur with receptor-mediated impairments to catecholaminergic facilitation of synaptic plasticity. We also found that while post-stress recovery did not reverse distal dendritic retraction, it did result in over extension of proximal dendritic arbors and spine growth as well as a full reversal of CRS-induced impairments to catecholaminergic-mediated synaptic plasticity. Our results support the hypothesis that disease-related PFC dysfunction is a consequence of network disruption secondary to altered structural and functional plasticity and that circuitry reestablishment may underlie elements of recovery. Accordingly, we believe that pharmacological treatments targeted at preventing dendritic retraction and spine loss or encouraging circuitry re-establishment and stabilization may be advantageous in the prevention and treatment of mood and anxiety disorders.

Rats with high or low sociability are differently affected by chronic variable stress

Depression is strongly related to social behavior. We have previously shown that social behavior of rats is individually stable. The purpose of the present study was to compare the sensitivity of animals with different sociability to chronic variable stress (CVS). Four social interaction tests were performed with 60 single-housed male Sprague-Dawley rats. Twenty rats with the lowest and 20 with the highest average social activity time were selected as low sociability (LS) and high sociability (HS) rats, respectively. Both groups were further divided into control and stress groups with equal average body weight. The CVS procedure lasted for 3 weeks. The stressors applied were cold water and wet bedding, imitation of injection, stroboscopic light, movement restriction in a small cage, tail pinch with a clothespin, and strong illumination during the predicted dark phase. In HS-rats, but not in LS-rats, CVS reduced sucrose intake compared with baseline after 3 weeks, suggesting that HS-rats are more vulnerable to anhedonia elicited by CVS. LS-animals were more anxious in the social interaction and open field tests, but stress eliminated differences with HS-animals in the social interaction test and increased their activity in the forced swimming test. In LS-rats stress increased ex vivo dopamine levels and reduced 5-HT levels in the frontal cortex, suggesting that the increased behavioral activity after stress may be related to increased impulsivity. This study thus revealed that animals with high sociability trait are more vulnerable to anhedonia elicited by chronic stress in conditions of single housing.

Stress and ageing interactions: A paradox in the context of shared etiological and physiopathological processes

Gerontology has made considerable progress in the understanding of the mechanisms underlying the ageing process and age-related neurodegenerative disorders. However, ways to improve quality of life in the elderly remain to be elucidated. It is now clear that stress and the ageing process share a number of underlying mechanisms bound in a very close, if not indissociable, relationship. The ageing process is regulated by the factors underlying the ability to adjust to stress, whilst stress has an influence on the life span and the quality of ageing. In addition, the ability to cope with stress in adulthood predicts life expectancy and quality of life at senescence. The ageing process and stress also share several common mechanisms, particularly in relation to the energy factor. Stress consumes energy and ageing may be considered as a cost of the energy expended to deal with the stressors to which the body is exposed throughout its lifetime. This suggests that the ageing process is associated with and/or a consequence of a long-lasting activation of the major stress responsive systems. However, despite common features, the interaction between stress and the ageing process gives rise to some paradoxes. Stress can either diminish or exacerbate the ageing process just as the ageing process can worsen or counter the effects of stress. There has been little attempt to understand how ageing and stress might interact to promote "successful" or pathological ageing. A key factor in this respect is the individual's ability to adapt to stress. Viewed from this angle, the quality of life of aged subjects may be improved through therapy designed to improve the tolerance to stress.

Lack of interaction between etifoxine and CRF1 and CRF2 receptors in rodents

Hyperactivity of the corticotropin-releasing factor (CRF) system occurs in some patients with anxiety disorders and depression. Blockade of CRF1 and CRF2 receptors can underlie the anxiolytic effects of drugs. In the present investigation, in vivo and in vitro studies were designed to determine whether the anxiolytic drug etifoxine, known to enhance GABAergic synaptic transmission, behaves also as a CRF1 and CRF2 receptor antagonist. A drug exerting multiple actions may be of clinical interest in the treatment of various different forms of mood disorders. Using two animal models, it was found that etifoxine reversed the excess CRF-induced grooming but not the hypo-locomotion of the rat placed in an open field. Etifoxine attenuated the CRF-induced gastric emptying delay in the mouse. On the other hand, in vitro, binding of etifoxine to CRF1 and CRF2 receptors on rat brain membranes was negligible and functionally, etifoxine did not block the CRF1 and CRF2 activation-induced cAMP production in presence of CRF in human neuroblastoma SH-SY5Y cells. The selective anxiolytic properties of etifoxine appear unrelated to an antagonist activity at the CRF1 and CRF2 receptors. The decrease in CRF activity produced by etifoxine may be related to its GABAergic properties.

Stimulation of the locus coeruleus elicits noradrenaline and dopamine release in the medial prefrontal and parietal cortex

Our previous studies have suggested that dopamine and noradrenaline may be coreleased from noradrenergic nerve terminals in the cerebral cortex. To further clarify this issue, the effect of electrical stimulation of the locus coeruleus on extracellular noradrenaline, dopamine and DOPAC in the medial prefrontal cortex, parietal cortex and caudate nucleus was analysed by microdialysis in freely moving rats. Stimulation of the locus coeruleus for 20 min with evenly spaced pulses at 1 Hz failed to modify cortical catecholamines and DOPAC levels. Stimulation with bursts of pulses at 12 and 24 Hz increased, in a frequency-related manner, not only noradrenaline but also dopamine and DOPAC in the two cortices. In both cortices noradrenaline returned to baseline within 20 min of stimulation, irrespective of the stimulation frequency, whereas dopamine returned to normal within 20 and 60 min in the medial prefrontal cortex and within 60 and 80 min in the parietal cortex after 12 and 24 Hz stimulation, respectively. DOPAC remained elevated throughout the experimental period. Phasic stimulation of the locus coeruleus at 12 Hz increased noradrenaline in the caudate nucleus as in the cerebral cortices but was totally ineffective on dopamine and DOPAC. Tetrodotoxin perfusion into the medial prefrontal cortex dramatically reduced noradrenaline and dopamine levels and suppressed the effect of electrical stimulation. These results indicate that electrical stimulation-induced increase of dopamine is a nerve impulse exocytotic process and suggest that cortical dopamine and noradrenaline may be coreleased from noradrenergic terminals.

Co-release of noradrenaline and dopamine in the cerebral cortex elicited by single train and repeated train stimulation of the locus coeruleus

Background

Previous studies by our group suggest that extracellular dopamine (DA) and noradrenaline (NA) may be co-released from noradrenergic nerve terminals in the cerebral cortex. We recently demonstrated that the concomitant release of DA and NA could be elicited in the cerebral cortex by electrical stimulation of the locus coeruleus (LC). This study analyses the effect of both single train and repeated electrical stimulation of LC on NA and DA release in the medial prefrontal cortex (mPFC), occipital cortex (Occ), and caudate nucleus. To rule out possible stressful effects of electrical stimulation, experiments were performed on chloral hydrate anaesthetised rats.

Results

Twenty min electrical stimulation of the LC, with burst type pattern of pulses, increased NA and DA both in the mPFC and in the Occ. NA in both cortices and DA in the mPFC returned to baseline within 20 min after the end of the stimulation period, while DA in the Occ reached a maximum increase during 20 min post-stimulation and remained higher than baseline values at 220 min post-stimulation. Local perfusion with tetrodotoxin (TTX, 10 μM) markedly reduced baseline NA and DA in the mPFC and Occ and totally suppressed the effect of electrical stimulation in both areas.

A sequence of five 20 min stimulations at 20 min intervals were delivered to the LC. Each stimulus increased NA to the same extent and duration as the first stimulus, whereas DA remained elevated at the time next stimulus was delivered, so that baseline DA progressively increased in the mPFC and Occ to reach about 130 and 200% the initial level, respectively.

In the presence of the NA transport (NAT) blocker desipramine (DMI, 100 μM), multiple LC stimulation still increased extracellular NA and DA levels.

Electrical stimulation of the LC increased NA levels in the homolateral caudate nucleus, but failed to modify DA level.

Conclusion

The results confirm and extend that LC stimulation induces a concomitant release of DA and NA in the mPFC and Occ.

The different time-course of LC-induced elevation of DA and NA suggests that their co-release may be differentially controlled.

Medial prefrontal cortical integration of psychological stress in rats

The present study aimed to determine whether the medial prefrontal cortex (mPFC) (prelimbic and infralimbic regions) is implicated in the integration of a stress response. Sprague-Dawely rats were implanted with telemetry probes and guide cannulae so that either muscimol or vehicle could be administered locally within the mPFC or dorsomedial hypothalamus (DMH). The heart rate and blood pressure of rats was continuously recorded as either muscimol or vehicle was administered centrally and rats were either exposed to restraint stress or left alone in their home cages. After the stress challenge, or equivalent period, rats that had received intra-mPFC injections were processed for immunohistochemical detection of Fos throughout the neuraxis. Bilateral microinjection of muscimol into the mPFC had no effect upon either baseline cardiovascular parameters or restraint stress-induced tachycardia or pressor responses whereas, in the DMH, pretreatment with muscimol attenuated the cardiovascular stress response. Analysis of Fos expression throughout the CNS of nonstressed rats showed no effect of muscimol injections into the mPFC on baseline expression in the nuclei examined. In contrast, rats that had received muscimol injections into their mPFC and were subsequently restrained exhibited an increase in the number of Fos-positive cells in the DMH, medial amygdala, and medial nucleus tractus solitarius as compared to vehicle-injected rats that experienced restraint stress. These results indicate that, during acute psychological stress, the mPFC does not modulate the cardiovascular system in rats but does inhibit specific subcortical nuclei to exert control over aspects of an integrated response to a stressor.

Alpha2-adrenoceptor mediated co-release of dopamine and noradrenaline from noradrenergic neurons in the cerebral cortex

Previous results suggest that extracellular dopamine (DA) in the rat cerebral cortex originates from dopaminergic and noradrenergic terminals. To further clarify this issue, dialysate DA, dihydroxyphenylacetic acid (DOPAC) and noradrenaline (NA) were measured both in the medial prefrontal cortex (mPFC) and in the occipital cortex (OCC), with dense and scarce dopaminergic projections, respectively. Moreover, the effect of the alpha2-adrenoceptor antagonist RS 79948 and the D2-receptor antagonist haloperidol on extracellular DA, DOPAC and NA was investigated. Extracellular DA and DOPAC concentrations in the OCC were 43% and 9%, respectively, those in the mPFC. Haloperidol (0.1 mg/kg i.p.) increased DA and DOPAC (by 35% and 150%, respectively) in the mPFC, but was ineffective in the OCC. In contrast, RS 79948 (1.5 mg/kg i.p.) increased NA, DA and DOPAC, both in the mPFC (by approximately 50%, 60% and 130%, respectively) and the OCC (by approximately 50%, 80% and 200%, respectively). Locally perfused, the DA transporter blocker GBR 12909 (10 micro m) was ineffective in either cortex, whereas desipramine (DMI, 100 micro m) markedly increased extracellular NA and DA in both cortices. The weak haloperidol effect on DA efflux was not enhanced after DA- and NA-transporter blockade, whereas after DMI, RS 79948 markedly increased extracellular NA, and especially DA and DOPAC in both cortices. The results support the hypothesis that most extracellular DA in the cortex is co-released with NA from noradrenergic terminals, such co-release being primarily controlled by alpha2-adrenoceptors.