Increased dopamine and norepinephrine release in medial prefrontal cortex induced by acute and chronic stress: effects of diazepam

Galanin mediates features of neural and behavioral stress resilience afforded by exercise

Exercise promotes resilience to stress and increases galanin in the locus coeruleus (LC), but the question of whether changes in galanin signaling mediate the stress-buffering effects of exercise has never been addressed. To test the contributions of galanin to stress resilience, male Sprague Dawley rats received intracerebroventricular (ICV) cannulation for drug delivery and frontocortical cannulation for microdialysis, and were housed with or without a running wheel for 21d. Rats were acutely injected with vehicle or the galanin receptor antagonist M40 and exposed to a single session of either footshock or no stress. Other groups received galanin, the galanin receptor antagonist M40, or vehicle chronically for 21d prior to the stress session. Microdialysis sampling occurred during stress exposure and anxiety-related behavior was measured on the following day in the elevated plus maze. Dendritic spines were visualized by Golgi impregnation in medial prefrontal cortex (mPFC) pyramidal neurons and quantified. Exercise increased galanin levels in the LC. Under non-stressed conditions, anxiety-related behavior and dopamine levels were comparable between exercised and sedentary rats. In contrast, exposure to stress reduced open arm exploration in sedentary rats but not in exercise rats or those treated chronically with ICV galanin, indicating improved resilience. Both exercise and chronic, ICV galanin prevented the increased dopamine overflow and loss of dendritic spines observed after stress in sedentary rats. Chronic, but not acute M40 administration blocked the resilience-promoting effects of exercise. The results indicate that increased galanin levels promote features of resilience at both behavioral and neural levels.

The Effects of Stress Exposure on Prefrontal Cortex: Translating Basic Research into Successful Treatments for Post-Traumatic Stress Disorder

Research on the neurobiology of the stress response in animals has led to successful new treatments for Post-Traumatic Stress Disorder (PTSD) in humans. Basic research has found that high levels of catecholamine release during stress rapidly impair the top-down cognitive functions of the prefrontal cortex (PFC), while strengthening the emotional and habitual responses of the amygdala and basal ganglia. Chronic stress exposure leads to dendritic atrophy in PFC, dendritic extension in the amygdala, and strengthening of the noradrenergic (NE) system. High levels of NE release during stress engage low affinity alpha-1 adrenoceptors, (and likely beta-1 adrenoceptors), which rapidly reduce the firing of PFC neurons, but strengthen amygdala function. In contrast, moderate levels of NE release during nonstress conditions engage higher affinity alpha-2A receptors, which strengthen PFC, weaken amygdala, and regulate NE cell firing. Thus, either alpha-1 receptor blockade or alpha-2A receptor stimulation can protect PFC function during stress. Patients with PTSD have signs of PFC dysfunction. Clinical studies have found that blocking alpha-1 receptors with prazosin, or stimulating alpha-2A receptors with guanfacine or clonidine can be useful in reducing the symptoms of PTSD. Placebo-controlled trials have shown that prazosin is helpful in veterans, active duty soldiers and civilians with PTSD, including improvement of PFC symptoms such as impaired concentration and impulse control. Open label studies suggest that guanfacine may be especially helpful in treating children and adolescents who have experienced trauma. Thus, understanding the neurobiology of the stress response has begun to help patients with stress disorders.

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Research in animals has revealed how prefrontal cortex goes “off-line” during stress.

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Prefrontal cortical function is protected by α2A-, but impaired by α1-adrenoceptors.

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Based on this research, α1 blockers and α2A agonists are now in use to treat PTSD.

Differential effects of sertraline in a predator exposure animal model of post-traumatic stress disorder

Serotonin (5-HT), norepinephrine (NE), and other neurotransmitters are modulated in post-traumatic stress disorder (PTSD). In addition, pro-inflammatory cytokines (PIC) are elevated during the progression of the disorder. Currently, the only approved pharmacologic treatments for PTSD are the selective-serotonin reuptake inhibitors (SSRI) sertraline and paroxetine, but their efficacy in treating PTSD is marginal at best. In combat-related PTSD, SSRIs are of limited effectiveness. Thus, this study sought to analyze the effects of the SSRI sertraline on inflammation and neurotransmitter modulation via a predator exposure/psychosocial stress animal model of PTSD. We hypothesized that sertraline would diminish inflammatory components and increase 5-HT but might also affect levels of other neurotransmitters, particularly NE. PTSD-like effects were induced in male Sprague-Dawley rats (n = 6/group × 4 groups). The rats were secured in Plexiglas cylinders and placed in a cage with a cat for 1 h on days 1 and 11 of a 31-day stress regimen. PTSD rats were also subjected to psychosocial stress via daily cage cohort changes. At the conclusion of the stress regimen, treatment group animals were injected intraperitoneally (i.p.) with sertraline HCl at 10 mg/kg for 7 consecutive days, while controls received i.p. vehicle. The animals were subsequently sacrificed on day 8. Sertraline attenuated inflammatory markers and normalized 5-HT levels in the central nervous system (CNS). In contrast, sertraline produced elevations in NE in the CNS and systemic circulation of SSRI treated PTSD and control groups. This increase in NE suggests SSRIs produce a heightened noradrenergic response, which might elevate anxiety in a clinical setting.

Acute effects of restraint, shock and training in the elevated T-Maze on noradrenaline and serotonin systems of the prefrontal cortex

The prefrontal cortex (PFC) participates in cognitive functions and stress regulation. Noradrenaline (NA) and serotonin (5-HT) levels in some regions of the central nervous system are modified by acute stress. The effects depend on the type of stressor and the time elapsed between the presence of the stressor and the assessment. The aims of the present study were to assess the acute effect of different stressors on NA and 5-HT activities in the PFC and its relation with corticosterone levels. Independent groups of male Wistar rats (250-280 g) were submitted to restraint, footshock or training in the elevated T-maze (ETMT). The animals were sacrificed immediately (T0) or one hour (T1) after stress exposure. An untreated group sacrificed concurrently with treated animals was included as control. Samples of the PFC were dissected and the concentration of NA, 5-HT and their metabolites were measured by HPLC. Corticosterone levels were measured in serum. None of the treatments modified NA levels in the PFC. Animals exposed to footshock or ETMT showed significantly higher concentrations of 5-HT at T0. Restraint and footshock treatments were associated with higher corticosterone levels at T0 and T1 after the respective treatment. Taken together the results show that in the PFC, the noradrenergic and serotonergic systems, and the corticosterone levels respond in different ways to different stressors.

Working memory-related frontal theta activity is decreased under acute stress

Acute stress impairs prefrontal cortex (PFC) function and has detrimental effects on working memory (WM) performance. Converging evidence from electrophysiological studies suggests a close link between WM processes and frontal theta (FT) activity (4-8 Hz). However, the effect of stress on WM-related FT activity has not been investigated yet. To shed light on this topic we acquired EEG data from 31 healthy male subjects who underwent a stressful and a neutral control condition. In both conditions, they performed an n-back WM task at two different difficulty levels. Our results showed that WM-related FT activity was decreased under stress. Behaviorally, we found performance impairments under stress in the difficult task condition that were related to FT decreases. Increased cortisol levels indicated a successful moderate stress induction. These findings indicate that FT is a potential neurobiological marker for intact PFC functioning during WM and further supports the recently made assumption that FT acts in the PFC to optimize performance.

Effects of SCH-23390 in combination with a low dose of 17β-estradiol on anxiety-like behavior in ovariectomized rats

The aim of this study was to explore effects on anxiety-like behavior of D₁ dopamine receptor agonist, SKF-38393, and of D₁ dopamine receptor antagonist, SCH-23390, given alone or in combination with a low dose of 17β-estradiol (17β-E₂) to ovariectomized (OVX) rats. Two weeks after surgery, OVX rats began 14 days of treatment with the vehicle, a low dose of 17β-E₂ (5.0 μg/rat, s.c.), SKF-38393 (0.1 mg/kg, i.p.), SCH-23390 (0.1 mg/kg, i.p.), SKF-38393 plus 17β-E₂, or SCH-23390 plus 17β-E₂. The animals were tested in the black and white model (BWM) and the open field test (OFT). SCH-23390 (0.1 mg/kg, i.p.) alone or in a combination with a low dose of 17β-E₂ (5.0 μg/rat, s.c.) resulted in anxiolytic-like effect in OVX rats in the BWM. Repeated treatment with SCH-23390 and 17β-E₂ profoundly increased anxiolytic-like effect of single substances exerted per se. Coadministration of SCH-23390 with 17β-E₂ increased frequency of rearing and grooming in OVX rats in OFT. SKF-38393 (0.1 mg/kg, i.p.) treatment failed to alter anxiety-like behavior in OVX rats in the BWM. The results of the present study suggest that 17β-E₂ and SCH-23390 interact to exert anxiolytic-like action and that each of these drugs can potentiate effects of each other.

Predator exposure/psychosocial stress animal model of post-traumatic stress disorder modulates neurotransmitters in the rat hippocampus and prefrontal cortex

Post-Traumatic Stress Disorder (PTSD) can develop in response to a traumatic event involving a threat to life. To date, no diagnostic biomarkers have been identified for PTSD. Recent research points toward physiological abnormalities in the hypothalamic-pituitary-adrenal (HPA) axis, sympathoadrenal medullary and immune system that may be implicated in the disorder. The modulation of neurotransmitters is another possible mechanism, but their role in the progression of PTSD is poorly understood. Low serotonin (5-HT) may be a factor, but it may not be the only neurotransmitter affected as modulation affects levels of other neurotransmitters. In this study, we hypothesized the predator exposure/psychosocial stress rodent model of PTSD may alter levels of 5-HT and other neurotransmitters in the rat hippocampus and prefrontal cortex (PFC). Male Sprague-Dawley rats were used in this experiment. We induced PTSD via a predator exposure/psychosocial stress model, whereby rats were placed in a cage with a cat for 1 hour on days 1 and 11 of the 31-day experiment. Rats also received psychosocial stress via daily cage cohort changes. On day 32, the rats were sacrificed and the brains dissected to remove the hippocampus and PFC. Norepinephrine (NE), 5-Hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), dopamine (DA), and 3,4-Dihydroxyphenylacetic acid (DOPAC), and 5-HT levels in the hippocampus and PFC were measured with high-performance liquid chromatography (HPLC). In the hippocampus, 5-HT and HVA were lower, while NE and DOPAC were higher, in the PTSD group vs. controls. In the PFC, only 5-HT was lower, while NE, DA, and DOPAC were higher, in the PTSD group vs. controls. The rate limiting enzymes tyrosine hydroxylase and tryptophan hydroxylase were also examined and confirmed our findings. These results demonstrate that the predator exposure/psychosocial stress model of PTSD produces neurotransmitter changes similar to those seen in human patients and may cause a heightened noradrenergic response.

Early life trauma and attachment: immediate and enduring effects on neurobehavioral and stress axis development

Over half a century of converging clinical and animal research indicates that early life experiences induce enduring neuroplasticity of the HPA-axis and the developing brain. This experience-induced neuroplasticity is due to alterations in the frequency and intensity of stimulation of pups' sensory systems (i.e., olfactory, somatosensory, gustatory) embedded in mother-infant interactions. This stimulation provides "hidden regulators" of pups' behavioral, physiological, and neural responses that have both immediate and enduring consequences, including those involving the stress response. While variation in stimulation can produce individual differences and adaptive behaviors, pathological early life experiences can induce maladaptive behaviors, initiate a pathway to pathology, and increase risk for later-life psychopathologies, such as mood and affective disorders, suggesting that infant-attachment relationships program later-life neurobehavioral function. Recent evidence suggests that the effects of maternal presence or absence during this sensory stimulation provide a major modulatory role in neural and endocrine system responses, which have minimal impact on pups' immediate neurobehavior but a robust impact on neurobehavioral development. This concept is reviewed here using two complementary rodent models of infant trauma within attachment: infant paired-odor-shock conditioning (mimicking maternal odor attachment learning) and rearing with an abusive mother that converge in producing a similar behavioral phenotype in later-life including depressive-like behavior as well as disrupted HPA-axis and amygdala function. The importance of maternal social presence on pups' immediate and enduring brain and behavior suggests unique processing of sensory stimuli in early life that could provide insight into the development of novel strategies for prevention and therapeutic interventions for trauma experienced with the abusive caregiver.

Chronic mild stress-induced changes of risk assessment behaviors in mice are prevented by chronic treatment with fluoxetine but not diazepam

As an important part of risk-related defensive behavior and central element of anxiety, risk assessment in rodents is particularly sensitive to psychosocial stress and may consequently influence the following decision-making and behavioral output. In this study, using a mouse-test battery, we evaluated the possible impacts of chronic mild stress (CMS) on risk assessment behaviors and action selections. For non-stressed control animals, a close relationship between risk assessment and choice behavior was observed in EPM and LDT. For stressed animals, however, 5 weeks of CMS exposure not only increased risk assessment behaviors, but also abolished the correlations between risk assessment and action selection. Pharmacological intervention with GABA-A receptor modulator diazepam (0.25-4 mg/kg) blocked the alterations of conventional spatiotemporal behaviors in response to CMS, but had no effect on the CMS-induced risk assessment behavioral changes. In contrast, 4-weeks of chronic treatment with fluoxetine (4-20mg/kg), a selective serotonin reuptake inhibitor, not only ameliorated the CMS-affected risk-assessment behaviors, but also restored the CMS-impaired correlations between risk assessment and decision making-related action selection. The present findings may shed new light on the better understanding of emotional reactivity and decision making under stressful situations. These results also indicate a differential pharmacological sensitivity in CMS-affected emotional response and risk-assessment behaviors.

Dopaminergic mechanisms underlying catalepsy, fear and anxiety: do they interact?

Haloperidol is a dopamine D2 receptor antagonist that induces catalepsy when systemically administered to rodents. The haloperidol-induced catalepsy is a state of akinesia and rigidity very similar to that seen in Parkinson's disease. There exists great interest in knowing whether or not some degree of emotionality underlies catalepsy. If so, what kind of emotional distress would permeate such motor disturbance? This study is an attempt to shed some light on this issue through an analysis of ultrasound vocalizations (USVs) of 22 kHz, open-field test, and contextual conditioned fear in rats with some degree of catalepsy induced by haloperidol. Systemic administration of haloperidol caused catalepsy and decreased exploratory activity in the open-field. There was no difference in the emission of USVs between groups during the catalepsy or the exploratory behavior in the open-field test. In the contextual conditioned fear, when administered before training session, haloperidol did not change the emission of USVs or the freezing response. When administered before testing session, haloperidol enhanced the freezing response and decreased the emission of USVs on the test day. These findings suggest that the involvement of dopaminergic mechanisms in threatening situations depends on the nature of the aversive stimulus. Activation of D2 receptors occurs in the setting up of adaptive responses to conditioned fear stimuli so that these mechanisms seem to be important for the emission of 22 kHz USVs during the testing phase of the contextual conditioned fear, but not during the training session or the open-field test (unconditioned fear stimuli). Catalepsy, on the other hand, is the result of the blockage of D2 receptors in neural circuits associated to motor behavior that appears to be dissociated from those directly linked to dopamine-mediated neural mechanisms associated to fear.

Lactation reduces stress-caused dopaminergic activity and enhances GABAergic activity in the rat medial prefrontal cortex

We investigated the effect of restraint on the release of dopamine, GABA and glutamate in the medial prefrontal cortex (mPFC) of lactating compared with virgin Wistar female rats; besides the expression of D1, neuropeptide Y Y2, GABA receptors and corticotropin-releasing factor (CRF). Results from microdialysis experiments showed that basal dopamine and GABA, but not glutamate, concentrations were higher in lactating rats. In virgin animals, immobilization caused significant increase in dopamine, whereas GABA was unchanged and glutamate reduced. In lactating animals, restrain significantly decreased dopamine concentrations and, in contrast to virgin animals, GABA and glutamate concentrations increased. We found a higher expression of CRF, as well as the D1 and neuropeptide Y Y2 receptors in the left mPFC of virgin stressed rats; also, only stressed lactating animals showed a significant increase in immunopositive cells to GABA in the left cingulate cortex; meanwhile, a significant decrease was measured in virgin rats after stress in the left prelimbic region. The increased inhibition of the mPFC dopamine cells during stress and the down-regulated expression of the neuropeptide Y Y2 receptor may explain the lower CRF and hyporesponse to stress measured in lactating animals. Interestingly, participation of mPFC in stress regulation seems to be lateralized.

Abnormal stress responsivity in a rodent developmental disruption model of schizophrenia

Although numerous studies have implicated stress in the pathophysiology of schizophrenia, less is known about how the effects of stress interact with genetic, developmental, and/or environmental determinants to promote disease progression. In particular, it has been proposed that in humans, stress exposure in adolescence could combine with a predisposition towards increased stress sensitivity, leading to prodromal symptoms and eventually psychosis. However, the neurobiological substrates for this interaction are not fully characterized. Previous work in our lab has demonstrated that rats born to dams administered with the DNA-methylating agent methylazoxymethanol acetate (MAM) at gestational day 17 exhibit as adults behavioral and anatomical abnormalities consistent with those observed in patients with schizophrenia. Here, we examined behavioral and neuroendocrine responses to stress in the MAM model of schizophrenia. MAM-treated male rats were exposed to acute and repeated footshock stress at prepubertal, peripubteral, and adult ages. Ultrasonic vocalizations (USVs), freezing, and corticosterone responses were quantified. We found that juvenile MAM-treated rats emitted significantly more calls, spent more time vocalizing, emitted calls at a higher rate, and showed more freezing in response to acute footshock stress when compared with their saline (SAL) treated counterparts, and that this difference is not present in older animals. In addition, adolescent MAM-treated animals displayed a blunted HPA axis corticosterone response to acute footshock that did not adapt after 10 days of stress exposure. These data demonstrate abnormal stress responsivity in the MAM model of schizophrenia and suggest that these animals are more sensitive to the effects of stress in youth.

The neurobiology of thought: the groundbreaking discoveries of Patricia Goldman-Rakic 1937-2003

Patricia S. Goldman-Rakic (1937–2003) transformed the study of the prefrontal cortex (PFC) and the neural basis of mental representation, the basic building block of abstract thought. Her pioneering research first identified the dorsolateral PFC (dlPFC) region essential for spatial working memory, and the extensive circuits of spatial cognition. She discovered the cellular basis of working memory, illuminating the dlPFC microcircuitry underlying spatially tuned, persistent firing, whereby precise information can be held “in mind”: persistent firing arises from recurrent excitation within glutamatergic pyramidal cell circuits in deep layer III, while tuning arises from GABAergic lateral inhibition. She was the first to discover that dopamine is essential for dlPFC function, particularly through D1 receptor actions. She applied a host of technical approaches, providing a new paradigm for scientific inquiry. Goldman-Rakic's work has allowed the perplexing complexities of mental illness to begun to be understood at the cellular level, including atrophy of the dlPFC microcircuits subserving mental representation. She correctly predicted that impairments in dlPFC working memory activity would contribute to thought disorder, a cardinal symptom of schizophrenia. Ten years following her death, we look back to see how she inspired an entire field, fundamentally changing our view of cognition and cognitive disorders.

Involvement of the dorsal hippocampal dopamine D2 receptors in histamine-induced anxiogenic-like effects in mice

Anxiety-related behaviors increase histamine and dopamine release in the brain. On the other hand, central histamine counteracts reward and reinforcement processes mediated by the mesolimbic dopamine system. We investigated the effects of the histaminergic system and dopamine D2 receptors agents and their interactions on anxiety-related behaviors using the elevated plus-maze (EPM). The intra-hippocampal (Intra-CA1) microinjection of histamine (10 μg/mouse) decreased the percentage of open arm time (%OAT) and open arm entries (%OAE) but not the locomotor activity, indicating an anxiogenic-like response. Quinpirole (0.5 and 2 μg/mouse) or sulpiride (0.3 and 1 μg/mouse) when injected into the dorsal hippocampus also induced anxiety-like behavior, however, the drugs reversed the anxiogenic response induced by the effective dose of histamine (10 μg/mouse). Taken together and under the present experimental design, our results indicate that activation of the dorsal hippocampal histaminergic receptors causes anxiety-like behaviors altered by dopamine D2 receptor agonist and antagonist. Histamine can decrease dopaminergic tone in the dorsal hippocampus through decreasing the endogenous dopamine release, whereas quinpirole does the same via the postsynaptic DA receptors' activation. Sulpiride however renders the same effect through autoreceptors' blockade and potentiated dopamine transmission. Thus, quinpirole and sulpiride seem to compensate the effects of the intra-CA1 injection of exogenous histamine, and tend to exert anxiolytic effects in the presence of histamine.

Norepinephrine drives persistent activity in prefrontal cortex via synergistic α1 and α2 adrenoceptors

Optimal norepinephrine levels in the prefrontal cortex (PFC) increase delay-related firing and enhance working memory, whereas stress-related or pathologically high levels of norepinephrine are believed to inhibit working memory via α1 adrenoceptors. However, it has been shown that activation of Gq-coupled and phospholipase C-linked receptors can induce persistent firing, a cellular correlate of working memory, in cortical pyramidal neurons. Therefore, despite its importance in stress and cognition, the exact role of norepinephrine in modulating PFC activity remains elusive. Using electrophysiology and optogenetics, we report here that norepinephrine induces persistent firing in pyramidal neurons of the PFC independent of recurrent fast synaptic excitation. This persistent excitatory effect involves presynaptic α1 adrenoceptors facilitating glutamate release and subsequent activation of postsynaptic mGluR5 receptors, and is enhanced by postsynaptic α2 adrenoceptors inhibiting HCN channel activity. Activation of α2 adrenoceptors or inhibition of HCN channels also enhances cholinergic persistent responses in pyramidal neurons, providing a mechanism of crosstalk between noradrenergic and cholinergic inputs. The present study describes a novel cellular basis for the noradrenergic control of cortical information processing and supports a synergistic combination of intrinsic and network mechanisms for the expression of mnemonic properties in pyramidal neurons.

Conditioned fear response is modulated by a combined action of the hypothalamic-pituitary-adrenal axis and dopamine activity in the basolateral amygdala

The present study sought to determine the extent to which the combined activity of the hypothalamic-pituitary-adrenal (HPA) axis and dopaminergic systems is important for the expression of conditioned fear responses. The first experiment examined changes in plasma corticosterone concentration and the conditioned freezing response in rats treated with the dopamine D2 receptor agonist quinpirole (0.25 mg/kg), the dopamine D2 receptor antagonist sulpiride (40 mg/kg), corticosterone (3 or 6 mg/kg), or the corticosterone synthesis blocker metyrapone (30 mg/kg) and subjected to a conditioned fear test. A second experiment assessed the effects of corticosterone (3 or 6 mg/kg) and metyrapone (30 or 60 mg/kg) on fear-potentiated startle. A third experiment assessed the HPA axis modulation of conditioned fear using in vivo microdialysis targeted at dopaminergic neurotransmission in the basolateral amygdala (BLA). Quinpirole and sulpiride decreased conditioned freezing but did not affect plasma corticosterone concentration. Corticosterone and metyrapone did not affect fear-potentiated startle, but metyrapone attenuated conditioned freezing, suggesting that the expression of conditioned freezing requires HPA axis activation. Metyrapone inhibited the increase in dopamine levels in the BLA in response to the conditioned stimulus, whereas corticosterone had no significant effect. These results suggest that HPA axis activation is an initial step in an integrated neuroendocrine-neurochemical-behavioral response when the organism evaluates a threat associated with an environmental stimulus and triggers defense reactions to cope with this situation.

Chronic stress disrupts neural coherence between cortico-limbic structures

Chronic stress impairs cognitive function, namely on tasks that rely on the integrity of cortico-limbic networks. To unravel the functional impact of progressive stress in cortico-limbic networks we measured neural activity and spectral coherences between the ventral hippocampus (vHIP) and the medial prefrontal cortex (mPFC) in rats subjected to short term stress (STS) and chronic unpredictable stress (CUS). CUS exposure consistently disrupted the spectral coherence between both areas for a wide range of frequencies, whereas STS exposure failed to trigger such effect. The chronic stress-induced coherence decrease correlated inversely with the vHIP power spectrum, but not with the mPFC power spectrum, which supports the view that hippocampal dysfunction is the primary event after stress exposure. Importantly, we additionally show that the variations in vHIP-to-mPFC coherence and power spectrum in the vHIP correlated with stress-induced behavioral deficits in a spatial reference memory task. Altogether, these findings result in an innovative readout to measure, and follow, the functional events that underlie the stress-induced reference memory impairments.

Concurrent modulation of extracellular levels of noradrenaline and cAMP during stress and by anxiogenic- or anxiolytic-like neuropeptides in the prefrontal cortex of awake rats

The purpose of this study was to examine the effects of stress and the role of locally infused anxiogenic-like neuropeptides galanin, CCK-8, vasopressin, substance P and neurokinin A, and anxiolytic-like peptides NPY, nociceptin/orphanin FQ, somatostatin and neurotensin, on modulation of noradrenaline (NA) and cAMP efflux monitored simultaneously by microdialysis in the medial prefronatal cortex of awake rats. Concentrations of cAMP were determined by a newly developed method based on derivatization of cAMP with 2-chloroacetaldehyde followed by HPLC with fluorescence detection. Local infusion of forskolin (10 and 30 μM) dose-dependently increased the cAMP levels to 417% and 1050% of the control group, respectively. Similarly, local infusion of NA (10 μM) increased the cAMP to the peak level of 168%. A 5-min tail pinch and a 10-min swim stress rapidly increased the NA and cAMP levels to 167% and 203% (NA) and 141% and 161% (cAMP), respectively. Infusion of galanin and CCK-8 (0.5 nmol, and 1.5 nmol/0.5 μl) dose-dependently increased NA to the peak levels of 191% and 179% and cAMP levels to 174% and 166%, respectively. The peak levels following infusions of vasopressin, substance P and neurokinin A were 91%, 135% and 86% for NA and 131%, 83% and 76% for cAMP, respectively. Infusions of anxiolytic-like peptides at highest concentrations significantly increased (NPY, 136%) or decreased (nociceptin, 71%; somatostatin, 86%) the NA levels, whereas neurotensin had no effect. The cAMP levels decreased to 86% (NPY, neurotensin), 78% (nociceptin), somatostatin infusion was without effect. The present findings confirmed a close correlation between the stress-induced increases in prefrontal cortical NA and cAMP levels, as well as, concurrent changes in NA and cAMP levels following infusions of galanin and CCK-8 (increased levels) and nociceptin/orphanin FQ (decreased levels). Infusions of other neuropeptides showed a more complex pattern of NA and cAMP responses.

The involvement of noradrenergic mechanisms in the suppressive effects of diazepam on the hypothalamic-pituitary-adrenal axis activity in female rats

Aim

To elucidate the involvement of noradrenergic system in the mechanism by which diazepam suppresses basal hypothalamic-pituitary-adrenal (HPA) axis activity.

Methods

Plasma corticosterone and adrenocorticotropic hormone (ACTH) levels were determined in female rats treated with diazepam alone, as well as with diazepam in combination with clonidine (α₂-adrenoreceptor agonist), yohimbine (α₂-adrenoreceptor antagonist), alpha-methyl-p-tyrosine (α-MPT, an inhibitor of catecholamine synthesis), or reserpine (a catecholamine depleting drug) and yohimbine.

Results

Diazepam administered in a dose of 2.0 mg/kg suppressed basal HPA axis activity, ie, decreased plasma corticosterone and ACTH levels. Pretreatment with clonidine or yohimbine failed to affect basal plasma corticosterone and ACTH concentrations, but abolished diazepam-induced inhibition of the HPA axis activity. Pretreatment with α-MPT, or with a combination of reserpine and yohimbine, increased plasma corticosterone and ACTH levels and prevented diazepam-induced inhibition of the HPA axis activity.

Conclusion

The results suggest that α₂-adrenoreceptors activity, as well as intact presynaptic noradrenergic function, are required for the suppressive effect of diazepam on the HPA axis activity.

Neuromodulation of thought: flexibilities and vulnerabilities in prefrontal cortical network synapses

This review describes unique neuromodulatory influences on working memory prefrontal cortical (PFC) circuits that coordinate cognitive strength with arousal state. Working memory arises from recurrent excitation within layer III PFC pyramidal cell NMDA circuits, which are afflicted in aging and schizophrenia. Neuromodulators rapidly and flexibly alter the efficacy of these synaptic connections, while leaving the synaptic architecture unchanged, a process called dynamic network connectivity (DNC). Increases in calcium-cAMP signaling open ion channels in long, thin spines, gating network connections. Inhibition of calcium-cAMP signaling by stimulating α2A-adrenoceptors on spines strengthens synaptic efficacy and increases network firing, whereas optimal stimulation of dopamine D1 receptors sculpts network inputs to refine mental representation. Generalized increases in calcium-cAMP signaling during fatigue or stress disengage dlPFC recurrent circuits, reduce firing and impair top-down cognition. Impaired DNC regulation contributes to age-related cognitive decline, while genetic insults to DNC proteins are commonly linked to schizophrenia.

COMT Val158Met polymorphism interacts with stressful life events and parental warmth to influence decision making

Both genetic and environmental factors have been shown to influence decision making, but their relative contributions and interactions are not well understood. The present study aimed to reveal possible gene-environment interactions on decision making in a large healthy sample. Specifically, we examined how the frequently studied COMT Val¹⁵⁸Met polymorphism interacted with an environmental risk factor (i.e., stressful life events) and a protective factor (i.e., parental warmth) to influence affective decision making as measured by the Iowa Gambling Task. We found that stressful life events acted as a risk factor for poor IGT performance (i.e., high reward sensitivity) among Met carriers, whereas parental warmth acted as a protective factor for good IGT performance (i.e., higher IGT score) among Val/Val homozygotes. These results shed some new light on gene-environment interactions in decision making, which could potentially help us understand the underlying etiology of several psychiatric disorders associated with decision making impairment.

Stress-induced impairment of a working memory task: role of spiking rate and spiking history predicted discharge

Stress, pervasive in society, contributes to over half of all work place accidents a year and over time can contribute to a variety of psychiatric disorders including depression, schizophrenia, and post-traumatic stress disorder. Stress impairs higher cognitive processes, dependent on the prefrontal cortex (PFC) and that involve maintenance and integration of information over extended periods, including working memory and attention. Substantial evidence has demonstrated a relationship between patterns of PFC neuron spiking activity (action-potential discharge) and components of delayed-response tasks used to probe PFC-dependent cognitive function in rats and monkeys. During delay periods of these tasks, persistent spiking activity is posited to be essential for the maintenance of information for working memory and attention. However, the degree to which stress-induced impairment in PFC-dependent cognition involves changes in task-related spiking rates or the ability for PFC neurons to retain information over time remains unknown. In the current study, spiking activity was recorded from the medial PFC of rats performing a delayed-response task of working memory during acute noise stress (93 db). Spike history-predicted discharge (SHPD) for PFC neurons was quantified as a measure of the degree to which ongoing neuronal discharge can be predicted by past spiking activity and reflects the degree to which past information is retained by these neurons over time. We found that PFC neuron discharge is predicted by their past spiking patterns for nearly one second. Acute stress impaired SHPD, selectively during delay intervals of the task, and simultaneously impaired task performance. Despite the reduction in delay-related SHPD, stress increased delay-related spiking rates. These findings suggest that neural codes utilizing SHPD within PFC networks likely reflects an additional important neurophysiological mechanism for maintenance of past information over time. Stress-related impairment of this mechanism is posited to contribute to the cognition-impairing actions of stress.

Acute stress induces selective alterations in cost/benefit decision-making

Acute stress can exert beneficial or detrimental effects on different forms of cognition. In the present study, we assessed the effects of acute restraint stress on different forms of cost/benefit decision-making, and some of the hormonal and neurochemical mechanisms that may underlie these effects. Effort-based decision-making was assessed where rats chose between a low effort/reward (1 press=2 pellets) or high effort/reward option (4 pellets), with the effort requirement increasing over 4 blocks of trials (2, 5, 10, and 20 lever presses). Restraint stress for 1 h decreased preference for the more costly reward and induced longer choice latencies. Control experiments revealed that the effects on decision-making were not mediated by general reductions in motivation or preference for larger rewards. In contrast, acute stress did not affect delay-discounting, when rats chose between a small/immediate vs larger/delayed reward. The effects of stress on decision-making were not mimicked by treatment with physiological doses of corticosterone (1-3 mg/kg). Blockade of dopamine receptors with flupenthixol (0.25 mg/kg) before restraint did not attenuate stress-induced effects on effort-related choice, but abolished effects on choice latencies. These data suggest that acute stress interferes somewhat selectively with cost/benefit evaluations concerning effort costs. These effects do not appear to be mediated solely by enhanced glucocorticoid activity, whereas dopaminergic activation may contribute to increased deliberation times induced by stress. These findings may provide insight into impairments in decision-making and anergia associated with stress-related disorders, such as depression.

Effects of acupuncture on stress-induced relapse to cocaine-seeking in rats

RATIONALE

Cocaine addiction is associated with high rates of relapse, and stress has been identified as a major risk factor. We have previously demonstrated that acupuncture reduces drug self-administration and dopamine release in the nucleus accumbens (NAc), a brain structure implicated in stress-induced reinstatement of drug-seeking behavior.

OBJECTIVE

This study was conducted to investigate the effects of acupuncture on footshock-induced reinstatement of cocaine-seeking and the expression of c-Fos and the transcription factor cAMP response element-binding protein (CREB) in the NAc, used as markers of neuronal activation in conditions of stress-induced reinstatement to cocaine.

METHODS

Male Sprague-Dawley rats were trained to self-administer cocaine (1.0 mg/kg) for 14 days, followed by extinction and then footshock stress. Acupuncture was applied at bilateral Shenmen (HT7) points for 1 min after footshock stress.

RESULTS AND CONCLUSIONS

Acute footshock stress reinstated cocaine-seeking behavior and enhanced c-Fos expression and phosphorylated CREB (pCREB) activation in the NAc shell in cocaine pre-exposed rats. On the other hand, acupuncture at HT7, but not at control point (LI5), markedly reduced reinstatement of cocaine-seeking (86.5 % inhibition vs. control value), c-Fos expression (81.7% inhibition), and pCREB activation (79.3% inhibition) in the NAc shell. These results suggest that acupuncture attenuates stress-induced relapse by regulating neuronal activation in the NAc shell.

Quetiapine ameliorates stress-induced cognitive inflexibility in rats

The antidepressant action of quetiapine has been demonstrated in clinical and preclinical studies. Nevertheless, little is known about its effectiveness in the treatment of frontal-like cognitive disturbances that may be associated with stress-related disorder. Therefore, the aim of the present study was to investigate whether quetiapine would prevent and/or reverse stress-induced cognitive impairments in a rat model of prefrontal cortex (PFC)-dependent attentional set-shifting task (ASST). Because quetiapine augmentation to selective serotonin reuptake inhibitors (SSRIs) has recently been proven to be beneficial in neuropsychiatric disorders, a separate experiment was designed to assess the impact of combined administration of inactive doses of quetiapine and escitalopram on ASST performance in rats. According to our previous studies, 1 h daily exposure to restraint stress for 7 days significantly and specifically impaired extra-dimensional (ED) set-shifting ability of rats. Quetiapine (2.5 mg/kg, PO) given to rats prior to the restraint sessions completely prevented this stress-induced cognitive inflexibility. Similar effect was demonstrated after pretreatment with the α1-adrenoceptor antagonist, prazosin (1 mg/kg, IP). Moreover, acute administration of quetiapine before the test reversed set-shifting deficits in stressed rats (0.63, 1.25 and 2.5 mg/kg, PO) and improved ED performance of cognitively unimpaired control animals (1.25 and 2.5 mg/kg, PO). Finally, the combined administration of inactive doses of quetiapine (0.63 and 0.3 mg/kg in control and stressed rats, respectively) and escitalopram (0.3 mg/kg, IP) facilitated set-shifting performance in either control or stressed rats. In conclusion, quetiapine administration either prevented or reversed stress-induced cognitive inflexibility in rats. In addition to promoting of set-shifting by itself, quetiapine also enhanced the procognitive efficacy of escitalopram. The potential contribution of the antagonism at α1-adrenoceptors to the mechanisms underlying the protective action of quetiapine requires further evaluation. These findings may have therapeutic implications for the treatment of frontal-like disturbances, particularly cognitive inflexibility, in stress-related psychiatric disorders. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Prefrontal/accumbal catecholamine system processes high motivational salience

Motivational salience regulates the strength of goal seeking, the amount of risk taken, and the energy invested from mild to extreme. Highly motivational experiences promote highly persistent memories. Although this phenomenon is adaptive in normal conditions, experiences with extremely high levels of motivational salience can promote development of memories that can be re-experienced intrusively for long time resulting in maladaptive outcomes. Neural mechanisms mediating motivational salience attribution are, therefore, very important for individual and species survival and for well-being. However, these neural mechanisms could be implicated in attribution of abnormal motivational salience to different stimuli leading to maladaptive compulsive seeking or avoidance. We have offered the first evidence that prefrontal cortical norepinephrine (NE) transmission is a necessary condition for motivational salience attribution to highly salient stimuli, through modulation of dopamine (DA) in the nucleus accumbens (NAc), a brain area involved in all motivated behaviors. Moreover, we have shown that prefrontal-accumbal catecholamine (CA) system determines approach or avoidance responses to both reward- and aversion-related stimuli only when the salience of the unconditioned stimulus (UCS) is high enough to induce sustained CA activation, thus affirming that this system processes motivational salience attribution selectively to highly salient events.

Postnatal aversive experience impairs sensitivity to natural rewards and increases susceptibility to negative events in adult life

Evidence shows that maternal care and postnatal traumatic events can exert powerful effects on brain circuitry development but little is known about the impact of early postnatal experiences on processing of rewarding and aversive stimuli related to the medial prefrontal cortex (mpFC) function in adult life. In this study, the unstable maternal environment induced by repeated cross-fostering (RCF) impaired palatable food conditioned place preference and disrupted the natural preference for sweetened fluids in the saccharin preference test. By contrast, RCF increased sensitivity to conditioned place aversion (CPA) and enhanced immobility in the forced swimming test. Intracerebral microdialysis data showed that the RCF prevents mpFC dopamine (DA) outflow regardless of exposure to rewarding or aversive stimuli, whereas it induces a strong and sustained prefrontal norepinephrine (NE) release in response to different aversive experiences. Moreover, the selective mpFC NE depletion abolished CPA, thus indicating that prefrontal NE is required for motivational salience attribution to aversion-related stimuli. These findings demonstrate that an unstable maternal environment impairs the natural propensity to seek pleasurable sources of reward, enhances sensitivity to negative events in adult life, blunts prefrontal DA outflow, and modulates NE release in the reverse manner depending on the exposure to rewarding or aversive stimuli.

The ethics of elective psychopharmacology

Pharmacological cognitive enhancers (PCEs) are used to improve cognitive functions, such as attention, learning, memory and planning in patients with impairments in cognition resulting from traumatic brain injury (TBI) or from neuropsychiatric disorders such as Alzheimer's disease (AD), mild cognitive impairment, schizophrenia, and attention deficit hyperactivity disorder (ADHD). Moreover, PCEs have been shown to improve cognition in healthy volunteers with no psychiatric disorders. This article describes the rationale behind the need for their use in neuropsychiatric patients and illustrates how PCEs can ameliorate cognitive impairments, improve quality of life and wellbeing, and therefore reduce the economic burden associated with these disorders. We also describe evidence that PCEs are being used as cognitive enhancers by healthy people. Crucially, as the lifestyle use of these drugs becomes very popular in the healthy population, a final aim is to present an overview of the current and future neuroethical considerations of enhancing the healthy brain. As information regarding their actual use, benefits and harms in various healthy populations is currently lacking, we propose research that aims to obtain relevant empirical data, monitor the short- and long-term effectiveness and side-effects, and initiate accurate surveys to determine current patterns and quantity of usage of PCE drugs by healthy people. Furthermore, in order to instigate a dialogue between neuroethics and neuropsychopharmacology, we urge scientists to explore and communicate the social and ethical implications of their research to the public. Finally, we discuss and highlight other means of enhancing cognition in both patients and healthy adults, including education and physical exercise.

Aircraft noise exposure affects rat behavior, plasma norepinephrine levels, and cell morphology of the temporal lobe

In order to investigate the physiological effects of airport noise exposure on organisms, in this study, we exposed Sprague-Dawley rats in soundproof chambers to previously recorded aircraft-related noise for 65 d. For comparison, we also used unexposed control rats. Noise was arranged according to aircraft flight schedules and was adjusted to its weighted equivalent continuous perceived noise levels (L(WECPN)) of 75 and 80 dB for the two experimental groups. We examined rat behaviors through an open field test and measured the concentrations of plasma norepinephrine (NE) by high performance liquid chromatography-fluorimetric detection (HPLC-FLD). We also examined the morphologies of neurons and synapses in the temporal lobe by transmission electron microscopy (TEM). Our results showed that rats exposed to airport noise of 80 dB had significantly lower line crossing number (P<0.05) and significantly longer center area duration (P<0.05) than control animals. After 29 d of airport noise exposure, the concentration of plasma NE of exposed rats was significantly higher than that of the control group (P<0.05). We also determined that the neuron and synapsis of the temporal lobe of rats showed signs of damage after aircraft noise of 80 dB exposure for 65 d. In conclusion, exposing rats to long-term aircraft noise affects their behaviors, plasma NE levels, and cell morphology of the temporal lobe.

Different stressors produce excitation or inhibition of mesolimbic dopamine neuron activity: response alteration by stress pre-exposure

Stressors can exert a wide variety of responses, ranging from adaptive responses to pathological changes; moreover, recent studies suggest that mild stressors can attenuate the response of a system to major stressful events. We have previously shown that 2-week exposure to cold, a comparatively mild inescapable stressor, induced a pronounced reduction in ventral tegmental area (VTA) dopamine (DA) neuron activity, whereas restraint stress increases DA neuron activity. However, it is not known if these stressors differentially impact the VTA in a region-specific manner, if they differentially impact behavioral responses, or whether the effects of such different stressors are additive or antagonistic with regard to their impact on DA neuron firing. To address these questions, single-unit extracellular recordings were performed in anesthetized control rats and rats exposed to chronic cold, and tested after delivery of a 2-h restraint session. Chronic cold stress strongly attenuated the number of DA neurons firing in the VTA, and this effect occurred primarily in the medial and central VTA regions that preferentially project to reward-related ventral striatal regions. Chronic cold exposure also prevented the pronounced increase in DA neuron population activity without affecting the behavioral sensitization to amphetamine produced by restraint stress. Taken together, these data show that a prolonged inescapable mild stressor can induce plastic changes that attenuate the DA system response to acute stress.

Stress, neurotransmitters, corticosterone and body-brain integration

Stress can be defined as a brain-body reaction towards stimuli arising from the environment or from internal cues that are interpreted as a disruption of homeostasis. The organization of the response to a stressful situation involves not only the activity of different types of neurotransmitter systems in several areas of the limbic system, but also the response of neurons in these areas to several other chemicals and hormones, chiefly glucocorticoids, released from peripheral organs and glands. Thus, stress is probably the process through which body-brain integration plays a major role. Here we review first the responses to an acute stress in terms of neurotransmitters such as dopamine, acetylcholine, glutamate and GABA in areas of the brain involved in the regulation of stress responses. These areas include the prefrontal cortex, amygdala, hippocampus and nucleus accumbens and the interaction among those areas. Then, we consider the role of glucocorticoids and review some recent data about the interaction of these steroids with several neurotransmitters in those same areas of the brain. Also the actions of other substances (neuromodulators) released from peripheral organs such as the pancreas, liver or gonads (insulin, IGF-1, estrogens) are reviewed. The role of an environmental enrichment on these same responses is also discussed. Finally a section is devoted to put into perspective all these environmental-brain-body-brain interactions during stress and their consequences on aging. It is concluded that the integrative perspective framed in this review is relevant for better understanding of how the organism responds to stressful challenges and how this can be modified through different environmental conditions during the process of aging. This article is part of a Special Issue entitled: Brain Integration.

Stress induces glucocorticoid-mediated apoptosis of rat Leydig cells in vivo

Stress can disrupt endocrine signalling in the male reproductive axis through high concentrations of glucocorticoids, the hallmark of stress. Our previous work revealed that a stress level of exogenous glucocorticoids could induce apoptosis of rat Leydig cells, which are the primary source of testosterone. The aim of this study was to investigate whether stress can induce apoptosis in rat Leydig cells in vivo and, if so, whether the process is the result of a direct effect of glucocorticoids. In a chronically stressed rat model, serum corticosterone concentration was increased significantly whereas serum testosterone was decreased. The frequency of apoptotic Leydig cells in stressed rats was also increased. Adrenalectomised rats subjected to chronic stress showed an elevated serum testosterone, while the apoptotic frequency of Leydig cells was not increased. It was established that glucocorticoid-induced Leydig cell apoptosis is mediated by glucocorticoid receptors (GRs), which translocate from cytoplasm to nucleus. Adenovirus microRNA-induced downregulation of GR expression in vitro alleviated the corticosterone-induced increase in apoptosis of Leydig cells. These results indicate that the stress-induced increase in corticosterone secretion resulted in apoptosis in rat Leydig cells in vivo, and thereby decreased testosterone synthesis.

Memory modulation

Our memories are not all created equally strong: Some experiences are well remembered while others are remembered poorly, if at all. Research on memory modulation investigates the neurobiological processes and systems that contribute to such differences in the strength of our memories. Extensive evidence from both animal and human research indicates that emotionally significant experiences activate hormonal and brain systems that regulate the consolidation of newly acquired memories. These effects are integrated through noradrenergic activation of the basolateral amygdala that regulates memory consolidation via interactions with many other brain regions involved in consolidating memories of recent experiences. Modulatory systems not only influence neurobiological processes underlying the consolidation of new information, but also affect other mnemonic processes, including memory extinction, memory recall, and working memory. In contrast to their enhancing effects on consolidation, adrenal stress hormones impair memory retrieval and working memory. Such effects, as with memory consolidation, require noradrenergic activation of the basolateral amygdala and interactions with other brain regions.

Morris water maze performance deficit produced by intermittent swim stress is partially mediated by norepinephrine

Intermittent swim stress (ISS) exposes a rat to cold water and the effects of the procedure produce detrimental results on activity measures 24h later. The ISS model can be used with the Morris water maze (MWM) to investigate the impact of stress on a spatial learning and memory task, known to involve the hippocampus. We investigated if the ISS model produced performance deficits in the MWM (experiments 1 and 2). We also investigated the role of norepinephrine by using an alpha-2 adrenergic agonist (i.e., clonidine) to exacerbate ISS-induced deficits (experiment 3), and using antidepressants (i.e., desipramine and reboxetine) that enhance the synaptic availability of norepinephrine to reduce ISS-induced deficits (experiments 4 and 5). Results indicated a main effect for stress in all experiments, with the exception of experiment 2, as ISS did induce performance deficits in the MWM. Clonidine enhanced ISS-induced deficits only in the learning trials, while desipramine and reboxetine reduced ISS-induced deficits in the learning trials. Additionally, only reboxetine reduced memory deficits in the MWM. These findings provide evidence that norepinephrine may act as a partial mediator of ISS-induced deficits in MWM performance.

Molecular modulation of prefrontal cortex: rational development of treatments for psychiatric disorders

Dysfunction of the prefrontal cortex (PFC) is a central feature of many psychiatric disorders, such as attention deficit hyperactivity disorder (ADHD), posttraumatic stress disorder (PTSD), schizophrenia, and bipolar disorder. Thus, understanding molecular influences on PFC function through basic research in animals is essential to rational drug development. In this review, we discuss the molecular signaling events initiated by norepinephrine and dopamine that strengthen working memory function mediated by the dorsolateral PFC under optimal conditions, and weaken working memory function during uncontrollable stress. We also discuss how these intracellular mechanisms can be compromised in psychiatric disorders, and how novel treatments based on these findings may restore a molecular environment conducive to PFC regulation of behavior, thought and emotion. Examples of successful translation from animals to humans include guanfacine for the treatment of ADHD and related PFC disorders, and prazosin for the treatment of PTSD.

Cannabinoid receptor involvement in stress-induced cocaine reinstatement: potential interaction with noradrenergic pathways

This study examined the role of endocannabinoid signaling in stress-induced reinstatement of cocaine seeking and explored the interaction between noradrenergic and endocannabinergic systems in the process. A well-validated preclinical model for human relapse, the rodent conditioned place preference assay, was used. Cocaine-induced place preference was established in C57BL/6 mice using injections of 15 mg/kg cocaine. Following extinction of preference for the cocaine-paired environment, reinstatement of place preference was determined following 6 min of swim stress or cocaine injection (15 mg/kg, i.p.). The role of endocannabinoid signaling was studied using the cannabinoid antagonist AM-251 (3 mg/kg, i.p.). Another cohort of mice was tested for reinstatement following administration of the cannabinoid agonist CP 55,940 (10, 20, or 40 μg/kg, i.p.). The alpha-2 adrenergic antagonist BRL-44408 (5 mg/kg, i.p.) with or without CP 55,940 (20 μg/kg) was administered to a third group of mice. We found that: (1) AM-251 blocked forced swim-induced, but not cocaine-induced, reinstatement of cocaine-seeking behavior; (2) the cannabinoid agonist CP 55,940 did not reinstate cocaine-seeking behavior when administered alone but did synergize with a non-reinstating dose of the alpha-2 adrenergic antagonist BRL-44408 to cause reinstatement. These results are consistent with the hypothesis that stress exposure triggers the endogenous activation of CB1 receptors and that activation of the endocannabinoid system is required for the stress-induced relapse of the mice to cocaine seeking. Further, the data suggest that the endocannabinoid system interacts with noradrenergic mechanisms to influence stress-induced reinstatement of cocaine-seeking behavior.

Catecholamine influences on prefrontal cortical function: relevance to treatment of attention deficit/hyperactivity disorder and related disorders

The primary symptoms of attention deficit/hyperactivity disorder (ADHD) include poor impulse control and impaired regulation of attention. Research has shown that the prefrontal cortex (PFC) is essential for the "top-down" regulation of attention, behavior, and emotion, and that this brain region is underactive in many patients with ADHD. The PFC is known to be especially sensitive to its neurochemical environment; relatively small changes in the levels of norepinephrine and dopamine can produce significant changes in its function. Therefore, alterations in the pathways mediating catecholamine transmission can impair PFC function, while medications that optimize catecholamine actions can improve PFC regulation of attention, behavior, and emotion. This article reviews studies in animals showing that norepinephrine and dopamine enhance PFC function through actions at postsynaptic α(2A)-adrenoceptors and dopamine D1-receptors, respectively. Stimulant medications and atomoxetine appear to enhance PFC function through increasing endogenous adrenergic and dopaminergic stimulation of α(2A)-receptors and D1-receptors. In contrast, guanfacine mimics the enhancing effects of norepinephrine at postsynaptic α(2A)-receptors in the PFC, strengthening network connectivity. Stronger PFC regulation of attention, behavior, and emotion likely contributes to the therapeutic effects of these medications for the treatment of ADHD.

Catecholamine influences on dorsolateral prefrontal cortical networks

The symptoms of attention-deficit/hyperactivity disorder (ADHD) involve impairments in prefrontal cortical top-down regulation of attention and behavior. All current pharmacological treatments for ADHD facilitate catecholamine transmission, and basic research suggests that these compounds have prominent actions in the prefrontal cortex (PFC). The dorsolateral PFC is especially sensitive to levels of norepinephrine and dopamine, whereby either too little or too much markedly impairs PFC function. Recent physiological studies have shown that norepinephrine strengthens PFC network connectivity and maintains persistent firing during a working memory task through stimulation of postsynaptic α(2A)-adrenoceptors on PFC neurons. Conversely, dopamine acts at D1 receptors to narrow spatial tuning, sculpting network inputs to decrease noise (i.e., stabilization of the representation). The stimulant medications and atomoxetine appear to enhance PFC function by indirectly increasing these catecholamine actions through blockade of norepinephrine and/or dopamine transporters. In contrast, guanfacine mimics the enhancing effects of norepinephrine at postsynaptic α(2A)-receptors in the PFC, strengthening network connectivity. Stronger PFC regulation of attention, behavior, and emotion likely contributes to the therapeutic effects of these medications for the treatment of ADHD.