Evidence for the involvement of corticotrophin-releasing hormone in the pathogenesis of traumatic brain injury

The role of the stress system in recovery after traumatic brain injury: A tribute to Bruce S. McEwen

Traumatic brain injury (TBI) represents a major public health concern. Although the majority of individuals that suffer mild-moderate TBI recover relatively quickly, a substantial subset of individuals experiences prolonged and debilitating symptoms. An exacerbated response to physiological and psychological stressors after TBI may mediate poor functional recovery. Individuals with TBI can suffer from poor stress tolerance, impairments in the ability to evaluate stressors, and poor initiation (and cessation) of neuroendocrine stress responses, all of which can exacerbate TBI-mediated dysfunction. Here, we pay tribute to the pioneering neuroendocrinologist Dr. Bruce McEwen by discussing the ways in which his work on stress physiology and allostatic loading impacts the TBI patient population both before and after their injuries. Specifically, we will discuss the modulatory role of hypothalamic-pituitary-adrenal axis responses immediately after TBI and later in recovery. We will also consider the impact of stressors and stress responses in promoting post-concussive syndrome and post-traumatic stress disorders, two common sequelae of TBI. Finally, we will explore the role of early life stressors, prior to brain injuries, as modulators of injury outcomes.

Role of CRF and the hypothalamic-pituitary-adrenal axis in stroke: revisiting temporal considerations and targeting a new generation of therapeutics

Ischaemic neurovascular stroke represents a leading cause of death in the developed world. Preclinical and human epidemiological evidence implicates the corticotropin-releasing factor (CRF) family of neuropeptides as mediators of acute neurovascular injury pathology. Preclinical investigations of the role of CRF, CRF receptors and CRF-dependent activation of the hypothalamic-pituitary-adrenal (HPA) axis have pointed toward a tissue-specific and temporal relationship between activation of these pathways and physiological outcomes. Based on the literature, the major phases of ischaemic stroke aetiology may be separated into an acute phase in which CRF and anti-inflammatory stress signalling are beneficial and a chronic phase in which these contribute to neural degeneration, toxicity and apoptotic signalling. Significant gaps in knowledge remain regarding the pathway, temporality and systemic impact of CRF signalling and stress biology in neurovascular injury progression. Heterogeneity among experimental designs poses a challenge to defining the apparent reciprocal relationship between neurological injury and stress metabolism. Despite these challenges, it is our opinion that the elucidated temporality may be best matched with an antibody against CRF with a half-life of days to weeks as opposed to minutes to hours as with small-molecule CRF receptor antagonists. This state-of-the-art review will take a multipronged approach to explore the expected potential benefit of a CRF antibody by modulating CRF and corticotropin-releasing factor receptor 1 signalling, glucocorticoids and autonomic nervous system activity. Additionally, this review compares the modulation of CRF and HPA axis activity in neuropsychiatric diseases and their counterpart outcomes post-stroke and assess lessons learned from antibody therapies in neurodegenerative diseases.

Traumatic Injury to the Developing Brain: Emerging Relationship to Early Life Stress

Despite the high incidence of brain injuries in children, we have yet to fully understand the unique vulnerability of a young brain to an injury and key determinants of long-term recovery. Here we consider how early life stress may influence recovery after an early age brain injury. Studies of early life stress alone reveal persistent structural and functional impairments at adulthood. We consider the interacting pathologies imposed by early life stress and subsequent brain injuries during early brain development as well as at adulthood. This review outlines how early life stress primes the immune cells of the brain and periphery to elicit a heightened response to injury. While the focus of this review is on early age traumatic brain injuries, there is also a consideration of preclinical models of neonatal hypoxia and stroke, as each further speaks to the vulnerability of the brain and reinforces those characteristics that are common across each of these injuries. Lastly, we identify a common mechanistic trend; namely, early life stress worsens outcomes independent of its temporal proximity to a brain injury.

Early Life Stress Preceding Mild Pediatric Traumatic Brain Injury Increases Neuroinflammation but Does Not Exacerbate Impairment of Cognitive Flexibility during Adolescence

Early life stress (ELS) followed by pediatric mild traumatic brain injury (mTBI) negatively impacts spatial learning and memory and increases microglial activation in adolescent rats, but whether the same paradigm negatively affects higher order executive function is not known. Hence, we utilized the attentional set-shifting test (AST) to evaluate executive function (cognitive flexibility) and to determine its relationship with neuroinflammation and hypothalamic-pituitary-adrenal (HPA) axis activity after pediatric mTBI in male rats. ELS was induced via maternal separation for 180 min per day (MS180) during the first 21 post-natal (P) days, while controls (CONT) were undisturbed. At P21, fully anesthetized rats received a mild controlled cortical impact (2.2 mm tissue deformation at 4 m/sec) or sham injury. AST was evaluated during adolescence on P35-P40 and cytokine expression and HPA activity were analyzed on P42. The data indicate that pediatric mTBI produced a significant reversal learning deficit on the AST versus sham (p < 0.05), but that the impairment was not exacerbated further by MS180. Additionally, ELS produced an overall elevation in set-loss errors on the AST, and increased hippocampal interleukin (IL)-1β expression after TBI. A significant correlation was observed in executive dysfunction and IL-1β expression in the ipsilateral pre-frontal cortex and hippocampus. Although the combination of ELS and pediatric mTBI did not worsen executive function beyond that of mTBI alone (p > 0.05), it did result in increased hippocampal neuroinflammation relative to mTBI (p < 0.05). These findings provide important insight into the susceptibility to incur alterations in cognitive and neuroimmune functioning after stress exposure and TBI during early life.

Sex-Dependent Pathology in the HPA Axis at a Sub-acute Period After Experimental Traumatic Brain Injury

Over 2.8 million traumatic brain injuries (TBIs) are reported in the United States annually, of which, over 75% are mild TBIs with diffuse axonal injury (DAI) as the primary pathology. TBI instigates a stress response that stimulates the hypothalamic-pituitary-adrenal (HPA) axis concurrently with DAI in brain regions responsible for feedback regulation. While the incidence of affective symptoms is high in both men and women, presentation is more prevalent and severe in women. Few studies have longitudinally evaluated the etiology underlying late-onset affective symptoms after mild TBI and even fewer have included females in the experimental design. In the experimental TBI model employed in this study, evidence of chronic HPA dysregulation has been reported at 2 months post-injury in male rats, with peak neuropathology in other regions of the brain at 7 days post-injury (DPI). We predicted that mechanisms leading to dysregulation of the HPA axis in male and female rats would be most evident at 7 DPI, the sub-acute time point. Young adult age-matched male and naturally cycling female Sprague Dawley rats were subjected to midline fluid percussion injury (mFPI) or sham surgery. Corticotropin releasing hormone, gliosis, and glucocorticoid receptor (GR) levels were evaluated in the hypothalamus and hippocampus, along with baseline plasma adrenocorticotropic hormone (ACTH) and adrenal gland weights. Microglial response in the paraventricular nucleus of the hypothalamus indicated mild neuroinflammation in males compared to sex-matched shams, but not females. Evidence of microglia activation in the dentate gyrus of the hippocampus was robust in both sexes compared with uninjured shams and there was evidence of a significant interaction between sex and injury regarding microglial cell count. GFAP intensity and astrocyte numbers increased as a function of injury, indicative of astrocytosis. GR protein levels were elevated 30% in the hippocampus of females in comparison to sex-matched shams. These data indicate sex-differences in sub-acute pathophysiology following DAI that precede late-onset HPA axis dysregulation. Further understanding of the etiology leading up to late-onset HPA axis dysregulation following DAI could identify targets to stabilize feedback, attenuate symptoms, and improve efficacy of rehabilitation and overall recovery.

Sex as a Biological Variable in Preclinical Modeling of Blast-Related Traumatic Brain Injury

Approaches to furthering our understanding of the bioeffects, behavioral changes, and treatment options following exposure to blast are a worldwide priority. Of particular need is a more concerted effort to employ animal models to determine possible sex differences, which have been reported in the clinical literature. In this review, clinical and preclinical reports concerning blast injury effects are summarized in relation to sex as a biological variable (SABV). The review outlines approaches that explore the pertinent role of sex chromosomes and gonadal steroids for delineating sex as a biological independent variable. Next, underlying biological factors that need exploration for blast effects in light of SABV are outlined, including pituitary, autonomic, vascular, and inflammation factors that all have evidence as having important SABV relevance. A major second consideration for the study of SABV and preclinical blast effects is the notable lack of consistent model design—a wide range of devices have been employed with questionable relevance to real-life scenarios—as well as poor standardization for reporting of blast parameters. Hence, the review also provides current views regarding optimal design of shock tubes for approaching the problem of primary blast effects and sex differences and outlines a plan for the regularization of reporting. Standardization and clear description of blast parameters will provide greater comparability across models, as well as unify consensus for important sex difference bioeffects.

Acute Downregulation of Novel Hypothalamic Protein Sushi Repeat-Containing Protein X-Linked 2 after Experimental Traumatic Brain Injury

Traumatic brain injury (TBI) causes damage to the hypothalamo-hypophyseal axis, leading to endocrine dysregulation in up to 40% of TBI patients. Hence, there is an urgent need to identify non-invasive biomarkers for TBI-associated hypothalamo-hypophyseal pathology. Sushi repeat-containing protein X-linked 2 (SRPX2) is a novel hypothalamic protein expressed in both rat and human brain. Our objective was to investigate the effect of acquired brain injury on plasma SRPX2 protein levels and SRPX2 expression in the brain. We induced severe lateral fluid-percussion injury in adult male rats and investigated changes in SRPX2 expression at 2 h, 6 h, 24 h, 48 h, 72 h, 5 days, 7 days, 14 days, 1 month, and 3 months post-injury. The plasma SRPX2 level was assessed by Western blot analysis. Hypothalamic SRPX2-immunoreactive neuronal numbers were estimated from immunostained preparations. At 2 h post-TBI, plasma SRPX2 levels were markedly decreased compared with the naïve group (area under the curve = 1.00, p < 0.05). Severe TBI caused a reduction in the number of hypothalamic SRPX2-immunoreactive neurons bilaterally at 2 h post-TBI compared with naïve group (5032 ± 527 vs. 9440 ± 351, p < 0.05). At 1 month after severe TBI, however, the brain and plasma SRPX2 levels were comparable between the TBI and naïve groups (p > 0.05). Unsupervised hierarchical clustering using SRPX2 expression differentiated animals into injured and uninjured clusters. Our findings indicate that TBI leads to an acute reduction in SRPX2 protein expression and reduced plasma SRPX2 level may serve as a candidate biomarker of hypothalamic injury.

The blockade of corticotropin-releasing factor 1 receptor attenuates anxiety-related symptoms and hypothalamus-pituitary-adrenal axis reactivity in mice with mild traumatic brain injury

Recent studies have shown that mild traumatic brain injury (mTBI) is associated with higher risk for anxiety-related disorders. Dysregulation in the hypothalamus-pituitary-adrenal (HPA) axis following mTBI has been proposed to be involved in the development of neurobehavioral abnormalities; however, the underlying mechanisms are largely unknown. The aim of this study was to determine whether the corticotropin-releasing-factor-1 (CRF-1) receptor is involved in the regulation of anxiety-related symptoms in a mouse model of mTBI. Animals with or without mTBI received intracerebroventricular injections of a CRF-1 receptor agonist (CRF; 0.01 nmol/mouse) or antagonist (antalarmin; 1 µg/mouse) for 5 days, and then the animals were subjected to anxiety tests (light-dark box and zero maze). The levels of adrenocorticotropic hormone and corticosterone, the most important markers of HPA axis, were also measured after behavioral tests. Our results indicated that mTBI-induced anxiety-related symptoms in mice through increased levels of adrenocorticotropic hormone and corticosterone, showing HPA axis hyperactivity. Interestingly, activation of CRF receptor by a subthreshold dose of CRF resulted in significant increases in anxiety-like behaviors and HPA axis response to stress, whereas blockade of CRF receptors by a subthreshold dose of antalarmin decreased anxiety-related symptoms and HPA axis response to stress in mTBI-induced mice. Collectively, these findings suggest that the CRF-1 receptor plays an important role in the regulation of anxiety-related behaviors following mTBI induction in mice and support the hypothesis that blockade of the CRF-1 receptor may be a promising therapeutic target for anxiety-related disorders in patients with TBI.

Stress reactivity after traumatic brain injury: implications for comorbid post-traumatic stress disorder

Most people have or will experience traumatic stress at some time over the lifespan, but only a subset of traumatized individuals develop post-traumatic stress disorder (PTSD). Clinical research supports high rates of traumatic brain injury (TBI)-PTSD comorbidity and demonstrates TBI as a significant predictor of the development of PTSD. Biological factors impacted following brain injury that may contribute to increased PTSD risk are unknown. Heightened stress reactivity and dysregulated hypothalamic-pituitary-adrenal (HPA) axis function are common to both TBI and PTSD, and affect amygdalar structure and function, which is implicated in PTSD. In this review, we summarize a growing body of literature that shows HPA axis dysregulation, as well as enhanced fear and amygdalar function after TBI. We present the hypothesis that altered stress reactivity as a result of brain injury impacts the amygdala and defense systems to be vulnerable to increased fear and PTSD development from traumatic stress. Identifying biological mechanisms that underlie this vulnerability, such as dysregulated HPA axis function, may lead to better targeted treatments and preventive measures to support psychological health after TBI.

Traumatic brain injury and resultant pituitary dysfunction: insights from experimental animal models

PURPOSE

Traumatic brain injury (TBI) is a major worldwide cause of disability, often burdening young people with serious lifelong health problems. A frequent clinical complication is post-traumatic hypopituitarism (PTHP) manifesting in several hypothalamus-pituitary axes. The head trauma-induced mechanisms underlying PTHP remain largely unknown. Several hypotheses have been proposed including direct damage to the pituitary gland and hypothalamus, vascular events and autoimmunity. This review aims to provide a summary of the currently limited number of studies exploring hypothalamus-pituitary dysfunction in experimental animal TBI models.

RESULTS

Although the impact of different forms of TBI on a number of hypothalamus-pituitary axes has been investigated, consequences for pituitary tissue and function have only scarcely been described. Moreover, mechanisms underlying the endocrine dysfunctions remain under explored.

CONCLUSIONS

Studies on TBI-induced pituitary dysfunction are still scarce. More research is needed to acquire mechanistic insights into the pathophysiology of PTHP which may eventually open up the horizon toward better treatments, including pituitary-regenerative approaches.

A Tilted Axis: Maladaptive Inflammation and HPA Axis Dysfunction Contribute to Consequences of TBI

Each year approximately 1.7 million people sustain a traumatic brain injury (TBI) in the US alone. Associated with these head injuries is a high prevalence of neuropsychiatric symptoms including irritability, depression, and anxiety. Neuroinflammation, due in part to microglia, can worsen or even cause neuropsychiatric disorders after TBI. For example, mounting evidence demonstrates that microglia become “primed” or hyper-reactive with an exaggerated pro-inflammatory phenotype following multiple immune challenges. Microglial priming occurs after experimental TBI and correlates with the emergence of depressive-like behavior as well as cognitive dysfunction. Critically, immune challenges are various and include illness, aging, and stress. The collective influence of any combination of these immune challenges shapes the neuroimmune environment and the response to TBI. For example, stress reliably induces inflammation and could therefore be a gateway to altered neuropathology and behavioral decline following TBI. Given the increasing incidence of stress-related psychiatric disorders after TBI, the degree in which stress affects outcome is of particular interest. This review aims to highlight the role of the hypothalamic-pituitary-adrenal (HPA) axis as a key mediator of stress-immune pathway communication following TBI. We will first describe maladaptive neuroinflammation after TBI and how stress contributes to inflammation through both anti- and pro-inflammatory mechanisms. Clinical and experimental data describing HPA-axis dysfunction and consequences of altered stress responses after TBI will be discussed. Lastly, we will review common stress models used after TBI that could better elucidate the relationship between HPA axis dysfunction and maladaptive inflammation following TBI. Together, the studies described in this review suggest that HPA axis dysfunction after brain injury is prevalent and contributes to the dynamic nature of the neuroinflammatory response to brain injury. Experimental stressors that directly engage the HPA axis represent important areas for future research to better define the role of stress-immune pathways in mediating outcome following TBI.

Human corticotrophin releasing factor inhibits cell proliferation and promotes apoptosis through upregulation of tumor protein p53 in human glioma

Corticotropin-releasing factor (CRF) and its receptors have been detected in numerous tumors and have an important role in tumorigenesis and proliferation. However, the role of these peptides has not been established in human glioma and malignant glioma cell lines. The present study evaluated for the first time, the expression of CRF receptor 1 (CRFR1) in 35 human glioma samples, 13 normal brain tissues and human U87 glioma cells using immunohistochemistry, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blot analysis. Levels of CRFR1 were identified to be significantly increased in human glioma and U87 cells and higher levels of CRFR1 were observed in glioma tissues of higher grade. The biological functions of human CRF (hCRF) on U87 cells glioma cells were investigated by cell counting, a bromodeoxyuridine assay and flow cytometry. The U87 cells under hCRF treatment exhibited reduced proliferation, increased apoptosis and a cell cycle arrest in S and G2/M phase. The tumor protein p53 (p53) gene may participate in the activation of hCRF via CRFR1 in U87 cells, therefore p53 mRNA and protein were evaluated using RT-qPCR and western blot analysis. Finally, the present results suggest that hCRF inhibits proliferation and induces cell-cycle arrest and apoptosis in U87 cells via the CRFR1-mediated p53 signaling pathway. Therefore, the present study also suggests that hCRF may be used therapeutically, and CRFR1 may be a putative therapeutic target for human glioma.

Elucidating opportunities and pitfalls in the treatment of experimental traumatic brain injury to optimize and facilitate clinical translation

The aim of this review is to discuss the research presented in a symposium entitled "Current progress in characterizing therapeutic strategies and challenges in experimental CNS injury" which was presented at the 2016 International Behavioral Neuroscience Society annual meeting. Herein we discuss diffuse and focal traumatic brain injury (TBI) and ensuing chronic behavioral deficits as well as potential rehabilitative approaches. We also discuss the effects of stress on executive function after TBI as well as the response of the endocrine system and regulatory feedback mechanisms. The role of the endocannabinoids after CNS injury is also discussed. Finally, we conclude with a discussion of antipsychotic and antiepileptic drugs, which are provided to control TBI-induced agitation and seizures, respectively. The review consists predominantly of published data.

Early and Persistent Dendritic Hypertrophy in the Basolateral Amygdala following Experimental Diffuse Traumatic Brain Injury

In the pathophysiology of traumatic brain injury (TBI), the amygdala remains understudied, despite involvement in processing emotional and stressful stimuli associated with anxiety disorders, such as post-traumatic stress disorder (PTSD). Because the basolateral amygdala (BLA) integrates inputs from sensory and other limbic structures coordinating emotional learning and memory, injury-induced changes in circuitry may contribute to psychiatric sequelae of TBI. This study quantified temporal changes in dendritic complexity of BLA neurons after experimental diffuse TBI, modeled by midline fluid percussion injury. At post-injury days (PIDs) 1, 7, and 28, brain tissue from sham and brain-injured adult, male rats was processed for Golgi, glial fibrillary acidic protein (GFAP), or silver stain and analyzed to quantify BLA dendritic branch intersections, activated astrocytes, and regional neuropathology, respectively. Compared to sham, brain-injured rats at all PIDs showed enhanced dendritic branch intersections in both pyramidal and stellate BLA neuronal types, as evidenced by Sholl analysis. GFAP staining in the BLA was significantly increased at PID1 and 7 in comparison to sham. However, the BLA was relatively spared from neuropathology, demonstrated by an absence of argyrophilic accumulation over time, in contrast to other brain regions. These data suggest an early and persistent enhancement of dendritic complexity within the BLA after a single diffuse TBI. Increased dendritic complexity would alter information processing into and through the amygdala, contributing to emotional symptoms post-TBI, including PTSD.

Evidence of lasting dysregulation of neuroendocrine and HPA axis function following global cerebral ischemia in male rats and the effect of Antalarmin on plasma corticosterone level

Abnormal function of the neuroendocrine stress system has been implicated in the behavioral impairments observed following brain ischemia. The current study examined long-term changes in stress signal regulation 30days following global cerebral ischemia. Experiment 1 investigated changes in the expression of corticotropin releasing hormone (CRH) and its subtype 1 receptor (CRHR1), glucocorticoid receptors (GR) in the paraventricular nucleus of the hypothalamus (PVN), the central nucleus of the amygdala (CeA), and the CA1 subfield of the hippocampus. Tyrosine hydroxylase (TH) was determined at the locus coeruleus (LC). Experiment 2 investigated the role of central CRHR1 activation on corticosterone (CORT) secretion at multiple time intervals following global ischemia after exposure to an acute stressor. Findings from Experiment 1 demonstrated a persistent increase in GR, CRH and CRHR1 immunoreactivity (ir) at the PVN, reduced GR and CRHR1 expression in pyramidal CA1 neurons, and increased LC TH expression in ischemic rats displaying working memory errors in the radial arm Maze. Findings from Experiment 2 revealed increased CORT secretion up to 7 days, but no longer present 14 and 21 days post ischemia. However upon an acute restraint stress induced 27 days following reperfusion, ischemic rats had increased plasma CORT secretions compared to sham-operated animals, suggesting HPA axis hypersensitivity. Antalarmin (2 μg/2 μl) pretreatment significantly attenuated post ischemic elevation of basal and stress-induced CORT secretion. These findings support persistent neuroendocrine dysfunctions following brain ischemia likely to contribute to emotional and cognitive impairments observed in survivors of cardiac arrest and stroke.

Restoration of neuroendocrine stress response by glucocorticoid receptor or GABA(A) receptor antagonists after experimental traumatic brain injury

We previously reported that traumatic brain injury (TBI) produced by moderate controlled cortical impact (CCI) attenuates the stress response of the hypothalamic-pituitary-adrenal (HPA) axis between 21 and 70 days postinjury and enhances the sensitivity of the stress response to glucocorticoid negative feedback. In the current study, we investigated two possible mechanisms for the CCI-induced attenuation of the HPA stress response-i.e, glucocorticoid receptor (GR) and GABA-mediated inhibition of the HPA axis, with the GR antagonist, mifepristone (RU486), or the GABA(A)-receptor antagonist, bicuculline. In addition, we examined the effect of moderate CCI on GR and inhibitory neurons histologically in subfields of the hippocampus, medial prefrontal cortex, and amygdala. We show that at 30-min after onset of restraint stress, GR as well as GABA antagonism with MIFE or BIC, respectively, reversed the attenuating effects of moderate CCI on the stress-induced HPA response. Our histological results demonstrate that moderate CCI led to a loss of glutamic acid decarboxylase 67 or parvalbumin-positive inhibitory neurons within regions of the hippocampus and amygdala but did not lead to significant increases in GR in these regions. These findings indicate that suppression of the stress-induced HPA response after moderate CCI is mediated by the inhibitory actions of both GR and GABA, with a corresponding loss of inhibitory neurons within brain regions with neural pathways affecting limbic stress-integrative pathways.

The pathophysiology of concussions in youth

Mild traumatic brain injury, especially sport-related concussion, is common among young persons. Consequences of transient pathophysiologic dysfunction must be considered in the context of a developing or immature brain, as must the potential for an accumulation of damage with repeated exposure. This review summarizes the underlying neurometabolic cascade of concussion, with emphasis on the young brain in terms of acute pathophysiology, vulnerability, alterations in plasticity and activation, axonal injury, and cumulative risk from chronic, repetitive damage, and discusses their implications in the context of clinical care for the concussed youth, highlighting areas for future investigation.

Ovarian hormones influence corticotropin releasing factor receptor colocalization with delta opioid receptors in CA1 pyramidal cell dendrites

Stress interacts with addictive processes to increase drug use, drug seeking, and relapse. The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect and likely plays a critical role in the interaction between stress and drug addiction. Our prior studies demonstrate that the stress-related neuropeptide corticotropin-releasing factor (CRF) and the delta-opioid receptor (DOR) colocalize in interneuron populations in the hilus of the dentate gyrus and stratum oriens of CA1 and CA3. While independent ultrastructural studies of DORs and CRF receptors suggest that each receptor is found in CA1 pyramidal cell dendrites and dendritic spines, whether DORs and CRF receptors colocalize in CA1 neuronal profiles has not been investigated. Here, hippocampal sections of adult male and proestrus female Sprague-Dawley rats were processed for dual label pre-embedding immunoelectron microscopy using well-characterized antisera directed against the DOR for immunoperoxidase and against the CRF receptor for immunogold. DOR-immunoreactivity (-ir) was found presynaptically in axons and axon terminals as well as postsynaptically in somata, dendrites and dendritic spines in stratum radiatum of CA1. In contrast, CRF receptor-ir was predominantly found postsynaptically in CA1 somata, dendrites, and dendritic spines. CRF receptor-ir frequently was observed in DOR-labeled dendritic profiles and primarily was found in the cytoplasm rather than at or near the plasma membrane. Quantitative analysis of CRF receptor-ir colocalization with DOR-ir in pyramidal cell dendrites revealed that proestrus females and males show comparable levels of CRF receptor-ir per dendrite and similar cytoplasmic density of CRF receptor-ir. In contrast, proestrus females display an increased number of dual-labeled dendritic profiles and an increased membrane density of CRF receptor-ir in comparison to males. We further examined the functional consequences of CRF receptor-ir colocalization with DOR-ir in the same neuron using the hormone responsive neuronal cell line NG108-15, which endogenously expresses DORs, and assayed intracellular cAMP production in response to CRF receptor and DOR agonists. Results demonstrated that short-term application of DOR agonist SNC80 inhibited CRF-induced cAMP accumulation in NG108-15 cells transfected with the CRF receptor. These studies provide new insights on opioid-stress system interaction in the hippocampus of both males and females and establish potential mechanisms through which DOR activation may influence CRF receptor activity.

Heightening of the stress response during the first weeks after a mild traumatic brain injury

The effects of a mild traumatic brain injury range from white matter disruption to affective disorders. We set out to determine the response to restraint-induced stress after a mild fluid-percussion injury (FPI), an experimental model for brain injury. Hypothalamic-pituitary-adrenal (HPA) axis regulation of corticosterone (CORT) and adrenocorticotropic hormone (ACTH) was determined during the first post-injury weeks, which corresponds to the same time period when rehabilitative exercise has been shown to be ineffective after a mild FPI. Adult male rats underwent either an FPI or sham injury. Additional rats were only exposed to anesthesia. HPA regulation was evaluated by measuring the effects of dexamethasone (DEX) treatment on CORT and ACTH. Tail vein blood was collected following 30-min restraint stress, at post-injury days (PID) 1, 7 and 14, prior to (0 min) and at 30, 60, 90 and 120 min after stress onset. Results from these studies indicate that the stress response was significantly more pronounced after FPI in that CORT and ACTH restraint-induced increases were more pronounced and longer lasting compared to controls. DEX suppression of CORT and ACTH was observed in all groups, suggesting that stress hyper-responsiveness after mild FPI is not attributable to reduced sensitivity of CORT feedback regulation. The increased sensitivity to stressful events in the first two post-injury weeks after a mild FPI may have a negative impact on early rehabilitative therapies.

Injury severity differentially alters sensitivity to dexamethasone after traumatic brain injury

We have reported differential short- and long-term dysregulation of the neuroendocrine stress response after traumatic brain injury (TBI) produced by controlled cortical impact (CCI). We have now investigated three possible mechanisms for this TBI-induced dysregulation: (1) effects on the sensitivity of negative-feedback systems to glucocorticoids; (2) effects on the sensitivity of pituitary corticotrophs to corticotropin-releasing hormone (CRH); and (3) effects on neuronal loss in the hilar region of the dentate gyrus and in the CA3b layer of the dorsal hippocampus. TBI was induced to the left parietal cortex in adult male rats with a pneumatic piston, at two different impact velocities and compression depths, to produce either moderate or mild CCI. At 7 and 35 days after surgery, the rats were injected SC with the synthetic glucocorticoid analog dexamethasone (DEX; 0.01, 0.10, or 1.00 mg/kg) or saline, and 2 h later were subjected to 30 min of restraint stress and tail vein blood collection. Whereas all doses of DEX suppressed corticosterone (CORT) and adrenocorticotropic hormone (ACTH) responses to stress on both days, CORT and ACTH were significantly more suppressed after 0.01 mg/kg DEX in the moderate TBI group than in the mild TBI or sham groups. At both 7 and 35 days post-TBI, CRH (1.0 and 10.0 microg/kg IP) stimulated CORT and ACTH in all rats, regardless of injury condition. Hippocampal cell loss was greatest at 48 days after moderate TBI. Enhanced sensitivity to glucocorticoid negative feedback and greater hippocampal cell loss, but not altered pituitary responses to CRH, contribute to the short- and long-term attenuation of the neuroendocrine stress response following moderate TBI. The role of TBI-induced alterations in glucocorticoid receptors in limbic system sites in enhanced glucocorticoid feedback sensitivity requires further investigation.

Metabolic and histologic effects of sodium pyruvate treatment in the rat after cortical contusion injury

This study determined the effects of intraperitoneal sodium pyruvate (SP) treatment on the levels of circulating fuels and on cerebral microdialysis levels of glucose (MD(glc)), lactate (MD(lac)), and pyruvate (MD(pyr)), and the effects of SP treatment on neuropathology after left cortical contusion injury (CCI) in rats. SP injection (1000 mg/kg) 5 min after sham injury (Sham-SP) or CCI (CCI-SP) significantly increased arterial pyruvate (p < 0.005) and lactate (p < 0.001) compared to that of saline-treated rats with CCI (CCI-Sal). Serum glucose also increased significantly in CCI-SP compared to that in CCI-Sal rats (p < 0.05), but not in Sham-SP rats. MD(pyr) was not altered after CCI-Sal, whereas MD(lac) levels within the cerebral cortex significantly increased bilaterally (p < 0.05) and those for MD(glc) decreased bilaterally (p < 0.05). MD(pyr) levels increased significantly in both Sham-SP and CCI-SP rats (p < 0.05 vs. CCI-Sal) and were higher in left/injured cortex of the CCI-SP group (p < 0.05 vs. sham-SP). In CCI-SP rats the contralateral MD(lac) decreased below CCI-Sal levels (p < 0.05) and the ipsilateral MD(glc) levels exceeded those of CCI-Sal rats (p < 0.05). Rats with a single low (500 mg/kg) or high dose (1000 mg/kg) SP treatment had fewer damaged cortical cells 6 h post-CCI than did saline-treated rats (p < 0.05), but three hourly injections of SP (1000 mg/kg) were needed to significantly reduce contusion volume 2 weeks after CCI. Thus, a single intraperitoneal SP treatment increases circulating levels of three potential brain fuels, attenuates a CCI-induced reduction in extracellular glucose while increasing extracellular levels of pyruvate, but not lactate, and can attenuate cortical cell damage occurring within 6 h of injury. Enduring (2 week) neuronal protection was achieved only with multiple SP treatments within the first 2 h post-CCI, perhaps reflecting the need for additional fuel throughout the acute period of increased metabolic demands induced by CCI.

Chemical modification of class II G protein-coupled receptor ligands: frontiers in the development of peptide analogs as neuroendocrine pharmacological therapies

Recent research and clinical data have begun to demonstrate the huge potential therapeutic importance of ligands that modulate the activity of the secretin-like, Class II, G protein-coupled receptors (GPCRs). Ligands that can modulate the activity of these Class II GPCRs may have important clinical roles in the treatment of a wide variety of conditions such as osteoporosis, diabetes, amyotrophic lateral sclerosis and autism spectrum disorders. While these receptors present important new therapeutic targets, the large glycoprotein nature of their cognate ligands poses many problems with respect to therapeutic peptidergic drug design. These native peptides often exhibit poor bioavailability, metabolic instability, poor receptor selectivity and resultant low potencies in vivo. Recently, increased attention has been paid to the structural modification of these peptides to enhance their therapeutic efficacy. Successful modification strategies have included d-amino acid substitutions, selective truncation, and fatty acid acylation of the peptide. Through these and other processes, these novel peptide ligand analogs can demonstrate enhanced receptor subtype selectivity, directed signal transduction pathway activation, resistance to proteolytic degradation, and improved systemic bioavailability. In the future, it is likely, through additional modification strategies such as addition of circulation-stabilizing transferrin moieties, that the therapeutic pharmacopeia of drugs targeted towards Class II secretin-like receptors may rival that of the Class I rhodopsin-like receptors that currently provide the majority of clinically used GPCR-based therapeutics. Currently, Class II-based drugs include synthesized analogs of vasoactive intestinal peptide for type 2 diabetes or parathyroid hormone for osteoporosis.

Protection conferred by Corticotropin-releasing hormone in rat primary cortical neurons against chemical ischemia involves opioid receptor activation

Different studies have supported neuroprotective effects of Corticotropin-releasing hormone (CRH) against various excitotoxic and oxidative insults in vitro. However, the physiological mechanisms involved in this protection remain largely unknown. The present study was undertaken to determine the impact of CRH administration (at concentrations ranging from 200 fmol to 2 nmol) before and at delayed time intervals following potassium cyanide (KCN)-induced insult in rat primary cortical neurons. A second objective aimed to determine whether kappa and delta opioid receptor (KOR and DOR) blockade, using nor-binaltorphimine and naltrindole respectively (10 microM), could alter CRH-induced cellular protection. Our findings revealed that CRH treatments before or 3 and 8 h following KCN insult conferred significant protection against cortical injury, an effect blocked in cultures treated with alpha-helical CRH (9-41) prior to KCN administration. In addition, KOR and DOR blockade significantly reduced CRH-induced neuronal protection observed 3 but not 8 h post-KCN insult. Using western blotting, we demonstrated increased dynorphin, enkephalin, DOR and KOR protein expression in CRH-treated primary cortical neurons, and immunocytochemistry revealed the presence of opioid peptides and receptors in cortical neurons. These findings suggest protective effects of CRH against KCN-induced neuronal damage, and the contribution of the opioid system in modulating CRH actions.

Lasting Neuroendocrine-Immune Effects of Traumatic Brain Injury in Rats

Traumatic brain injury (TBI) is a principal cause of long-term physical, cognitive, behavioral, and social deficits in young adults, which frequently coexist with a high incidence of substance abuse disorders. However, few studies have examined the long-term effects of TBI on the neuroendocrine-immune system. TBI was induced in adult male rats under isoflurane anesthesia by cortical contusion injury with a pneumatic piston positioned stereotaxically over the left parietal cortex. Controls underwent sham surgery without injury. At 4 weeks post-injury, the plasma corticosterone response to 30-min restraint stress was significantly blunted in TBI rats compared to the sham controls. One week later, transmitters were implanted for continuous biotelemetric recording of body temperature and spontaneous locomotor activity. At 6 weeks post-injury, the febrile response to i.p. injection of the bacterial endotoxin, lipopolysaccharide (LPS; 50 microg/kg), was significantly lower in TBI than in sham rats. At 8 weeks, swimming in the forced swim test was significantly less in TBI than sham rats. At 9 weeks, rats were rendered ethanol (EtOH) dependent by feeding an EtOH-containing liquid diet for 14 days. Cosine rhythmometry analysis of circadian body temperature Midline Estimating Statistic of Rhythm (MESOR), amplitudes, and acrophases indicated differential effects of EtOH and withdrawal in the two groups. Light- and dark-phase activity analysis indicated that TBI rats were significantly more active than the sham group, and that EtOH and withdrawal differentially affected their activity. Given the extensive interactions of the neuroendocrine-immune systems, these results demonstrate that TBI produces lasting dysregulation amidst the central substrates for allostasis and circadian rhythmicity.

Induction of microglial apoptosis by corticotropin-releasing hormone

Neuropeptides are short-chain peptides found in brain tissue, some of which function as neurotransmitters and others as hormones. Neuropeptides may directly or indirectly modulate glial functions in the CNS. In the present study, effects of various neuropeptides on the viability and inflammatory activation of cultured microglia were investigated. Vasoactive intestinal peptide, substance P, cholecystokinin and neuropeptide Y did not affect microglial cell viability, whereas corticotropin-releasing hormone (CRH) induced a classical apoptosis of mouse microglia in culture as shown by nuclear condensation and fragmentation, terminal deoxynucleotidyl transferase dUTP nick-end labeling, and cleavage of caspase 3 and poly(ADP-ribose) polymerase protein. CRH, however, did not influence nitric oxide production or expression of inflammatory genes including those encoding cytokines and chemokines, indicating that CRH did not affect the inflammatory activation of microglia. The CRH-induced microglial apoptosis appeared to involve a mitochondrial pathway and reactive oxygen species, based on the mitochondrial membrane potential change, caspase 9 activation and sensitivity to antioxidants. Taken together, our results indicate that the stress neuropeptide CRH may regulate neuroinflammation by inducing the apoptosis of microglia, the major cellular source of inflammatory mediators in the CNS.

The CRH1 antagonist CP154,526 failed to alter ischemia-induced neurodegeneration and spatial memory deficits in rats but inhibited behavioral activity in the novel open field

Corticotropin-releasing hormone (CRH) has been implicated in ischemia-induced neurotoxicity, due in part to excitatory effects at the hippocampus, and the demonstrated neuroprotective effects of centrally administered, non-specific CRH antagonists. However, a number of issues remain to be clarified from these studies, including the relative contribution of CRH receptor subtypes, and the efficacy of these compounds to alter ischemia-induced behavioral impairments. In the current study, a highly selective, systemically administered CRH1 antagonist (CP154,526) failed to reverse global ischemia-induced cell death in hippocampal CA1 neurons or spatial memory impairments as assessed in the radial arm maze. Similarly, central administration of alpha-helical CRH failed to confer protection against ischemic damage. Interestingly, CRH1 antagonism reversed ischemia-induced hyperactivity in a novel open field, suggesting that modulation of this behavior is independent of effects on hippocampal CA1 cell loss. Failure of the current study to demonstrate neuroprotective effects of either the selective or non-selective CRH antagonists tested challenges the proposed neurotoxic role of CRH in global ischemia. These findings are discussed in relationship to recent findings reconsidering the participation of CRH in excitotoxic-mediated cellular damage.

Class II G protein-coupled receptors and their ligands in neuronal function and protection

G protein-coupled receptors (GPCRs) play pivotal roles in regulating the function and plasticity of neuronal circuits in the nervous system. Among the myriad of GPCRs expressed in neural cells, class II GPCRs which couples predominantly to the Gs-adenylate cyclase-cAMP signaling pathway, have recently received considerable attention for their involvement in regulating neuronal survival. Neuropeptides that activate class II GPCRs include secretin, glucagon-like peptides (GLP-1 and GLP-2), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating peptide (PACAP), corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), parathyroid hormone (PTH), and calcitonin-related peptides. Studies of patients and animal and cell culture models, have revealed possible roles for class II GPCRs signaling in the pathogenesis of several prominent neurodegenerative conditions including stroke, Alzheimer's, Parkinson's, and Huntington's diseases. Many of the peptides that activate class II GPCRs promote neuron survival by increasing the resistance of the cells to oxidative, metabolic, and excitotoxic injury. A better understanding of the cellular and molecular mechanisms by which class II GPCRs signaling modulates neuronal survival and plasticity will likely lead to novel therapeutic interventions for neurodegenerative disorders.

Lateral fluid percussion brain injury: a 15-year review and evaluation

This article comprehensively reviews the lateral fluid percussion (LFP) model of traumatic brain injury (TBI) in small animal species with particular emphasis on its validity, clinical relevance and reliability. The LFP model, initially described in 1989, has become the most extensively utilized animal model of TBI (to date, 232 PubMed citations), producing both focal and diffuse (mixed) brain injury. Despite subtle variations in injury parameters between laboratories, universal findings are evident across studies, including histological, physiological, metabolic, and behavioral changes that serve to increase the reliability of the model. Moreover, demonstrable histological damage and severity-dependent behavioral deficits, which partially recover over time, validate LFP as a clinically-relevant model of human TBI. The LFP model, also has been used extensively to evaluate potential therapeutic interventions, including resuscitation, pharmacologic therapies, transplantation, and other neuroprotective and neuroregenerative strategies. Although a number of positive studies have identified promising therapies for moderate TBI, the predictive validity of the model may be compromised when findings are translated to severely injured patients. Recently, the clinical relevance of LFP has been enhanced by combining the injury with secondary insults, as well as broadening studies to incorporate issues of gender and age to better approximate the range of human TBI within study design. We conclude that the LFP brain injury model is an appropriate tool to study the cellular and mechanistic aspects of human TBI that cannot be addressed in the clinical setting, as well as for the development and characterization of novel therapeutic interventions. Continued translation of pre-clinical findings to human TBI will enhance the predictive validity of the LFP model, and allow novel neuroprotective and neuroregenerative treatment strategies developed in the laboratory to reach the appropriate TBI patients.

Regulation of IL-18 expression by CRH in mouse microglial cells

Corticotropin releasing hormone (CRH) is a major regulator of the stress response. This study examined whether CRH regulates interleukin-18 expression on microglia, BV2. Our data show that CRH enhanced IL-18 expression and significantly induced the secretion of functional IL-18 protein. Furthermore, CRH induced IL-18 production could be blocked by N-acetyl-L-cystein (NAC), which suggests that reactive oxygen intermediates (ROI) may be involved in regulating IL-18. Indeed, it was also found that CRH increased the generation of ROI. Taken together, these results indicate that CRH is an important mediator that regulates IL-18 expression in the brain during stress.

Corticotropin-releasing hormone potentiates neural injury induced by oxygen-glucose deprivation: a possible involvement of microglia

While corticotropin-releasing hormone (CRH) has been implicated in a variety of brain disorders such as ischemic injury, the molecular mechanism by which CRH elicits its activities is largely unclear. In the present study, we have determined the effect of CRH on oxygen-glucose deprivation (OGD) induced apoptosis in fetal hippocampal neurons. CRH alone at concentrations of 10-200 nM had no effect on neuronal apoptosis. However, when neurons were co-cultured with microglia, CRH alone at concentrations greater than 100 nM induced neuronal apoptosis and CRH potentiated significant neuronal apoptosis following exposure to OGD. The effect of CRH on neuronal apoptosis was inhibited in the presence of the CRH antagonist astressin. Real-time RT-PCR revealed an increase in mRNA levels of Fas ligand (Fas-L), a membrane protein related to the TNF family, in cultured microglia following OGD exposure. In the presence of CRH, OGD-induced Fas-L expression was significantly increased. The effect of CRH on Fas-L expression was inhibited by specific inhibitors of the extracellular signal-regulated protein kinase (PD98059) and p38 mitogen-activated protein kinase (SB203580). These results suggest that CRH potentiates neuronal apoptosis induced by OGD in the presence of microglia and that this effect may be mediated through the induction of proinflammatory mediators in microglia.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Time-dependent changes in CRH concentrations and release in discrete brain regions following global ischemia: effects of MK-801 pretreatment

The excitatory actions of corticotropin-releasing hormone (CRH) in the brain and the neuroprotective effects of CRH antagonists in models of ischemia suggest a role for this peptide in the cascade of events leading to cellular damage. The present study aimed to characterize endogenous activation of CRH in discrete brain regions following global ischemia. Time-dependent changes in CRH concentrations were assessed in 10 brain regions including hippocampal, parahippocampal, and hypothalamic regions as well as the amygdala and the frontal cortex at three post-ischemic intervals: 4, 24, and 72 h (Experiment 1). The impact of pretreatment with a neuroprotective dose of the NMDA antagonist (5R,10S)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; hydrogen maleate) on 24-h ischemia-induced CRH concentrations in the 10 brain regions was also determined (Experiment 2). In vivo microdialysis was used to assess dynamic fluctuations in CRH release at the dorsal hippocampus (CA1 pyramidal layer) and central nucleus of the amygdala (CeA; Experiment 3). Our findings revealed a rapid elevation of CRH concentrations at the piriform cortex (Pir) and hypothalamic nuclei following global ischemia. This was followed by decreased CRH concentrations at the amygdala, the frontal cortex (FC), the CA3, and the hypothalamus 24-h post-ischemia. MK-801 reversed the decreases in the hypothalamic nuclei but not in the other brain regions. Seventy-two hours post-ischemia, CRH levels returned to control values in all regions except the dentate gyrus (DG) where elevated CRH levels were observed. In vivo, a significant increase in CRH release in response to global ischemia was found at the CeA with no alterations at the CA1. These findings support brain region-specific ischemia-induced CRH alterations and suggest that CRH actions to mediate neuronal damage at the hippocampal CA1 layer may be indirect.

Detrimental role of corticotropin-releasing factor on the decrease of CA1 field potential induced by in vitro ischemia in rat hippocampal slices

This experiment was designed to test the hypothesis that endogenous corticotropin-releasing factor (CRF) contributes to the neurodegenerative process following an ischemic insult. To test this hypothesis, the effects of chronic intracerebroventricular administration of CRF or astressin, a CRF-receptor antagonist, on the decrease in the Schaffer collateral-CA1 field potential induced by hypoxia/hypoglycemia (ischemia), were tested in rat hippocampal slices. The chronic treatment with CRF had a significant exacerbating effect on the 10-min ischemia, a condition that did not affect the evoked synaptic response in the hippocampal CA1 area, as compared to vehicle-treated rats. On the other hand, astressin had a significant ameliorative effect on the 15-min ischemia-induced reduction of the evoked synaptic response in the hippocampal CA1 area. These findings suggest that CRF accelerates hippocampal ischemic vulnerability induced by hypoxia and hypoglycemia.

Localization of corticoliberin receptors in the rat brain

In situ hybridization was used to study the distribution of corticoliberin receptors of subtypes 1 and 2 (CL-R1 and CL-R2 respectively) in different structures of the rat brain. Levels of CL-R1 mRNA in the brain were significantly greater than levels of CL-R2 mRNA, and the most intense expression of the CL-R1 gene was seen in forebrain structures, especially various neocortical, archicortical, and paleocortical regions in the cerebellar cortex. In addition, significant levels of CL-R1 mRNA expression were noted in the red nucleus and the reticular nucleus of the tegmentum. Intense expression of CL-R2 mRNA was observed in structures of the olfactory system, corticomedial parts of the amygdala, fields CA1-CA4 of the hippocampus, the ventromedial hypothalamus, and several brain stem nuclei. Moderate levels of CL-R2 mRNA were seen in the dorsolateral neostriatum. These results provide evidence that corticoliberin receptors of both subtypes are widespread in the brain. The different patterns of expression of CL-R1 and CL-R2 in the brain probably provide the basis for the functional specificity of action of corticoliberin in brain structures.

The pharmacology of CP-154,526, a non-peptide antagonist of the CRH1 receptor: a review

Since CRH has been shown to mediate stress-induced physiological and behavioral changes, it has been hypothesized that CRH receptor antagonists may have therapeutic potential in disorders that involve excessive CRH activity. CP-154,526 and its close analog antalarmin are potent, brain-penetrable, selective nonpeptide CRH1 receptor antagonists that were discovered in an effort to develop compounds with efficacy in CNS disorders precipitated by stress. Since its discovery many investigators have used CP-154,526 as a tool to study the pharmacology of CRH and its receptors and to evaluate its therapeutic potential in a variety of CNS and peripheral disorders. Systemically-administered CP-154,526 has been demonstrated to antagonize CRH- and stress-induced neuroendocrine, neurochemical, electrophysiological, and behavioral effects. These findings support the hypothesis that CRH1 receptor antagonists may have therapeutic utility in a number of neuropsychiatric disorders. CP-154,526, as well as other CRH1 receptor antagonists that have since been discovered, have also shown activity in several preclinical models of anxiety, depression, and substance abuse, while having little effect on locomotor activity and motor function. Although these effects are on occasion inconsistent among different laboratories, clinical evaluation of CRH1 antagonists appears justified on the basis of these and clinical data implicating the involvement of CRH in several CNS disorders. The effects of CRH1 antagonists on cognition, neurodegeneration, inflammation, and the gastrointestinal system have not been as extensively characterized and additional studies will be necessary to evaluate their therapeutic potential in these areas.

Traumatic brain injury-induced acute gene expression changes in rat cerebral cortex identified by GeneChip analysis

Proper CNS function depends on concerted expression of thousands of genes in a controlled and timely manner. Traumatic brain injury (TBI) in mammals results in neuronal death and neurological dysfunction, which might be mediated by altered expression of several genes. By employing a CNS-specific GeneChip and real-time polymerase chain reaction (PCR), the present study analyzed the gene expression changes in adult rat cerebral cortex in the first 24 hr after a controlled cortical impact injury. Many functional families of genes not previously implicated in TBI-induced brain damage are altered in the injured cortex. These include up-regulated transcription factors (SOCS-3, JAK-2, STAT-3, CREM, IRF-1, SMN, silencer factor-B, ANIA-3, ANIA-4, and HES-1) and signal transduction pathways (cpg21, Narp, and CRBP) and down-regulated transmitter release mechanisms (CITRON, synaptojanin II, ras-related rab3, neurexin-1beta, and SNAP25A and -B), kinases (IP-3-kinase, Pak1, Ca(2+)/CaM-dependent protein kinases), and ion channels (K(+) channels TWIK, RK5, X62839, and Na(+) channel I). In addition, several genes previously shown to play a role in TBI pathophysiology, including proinflammatory genes, proapoptotic genes, heat shock proteins, immediate early genes, neuropeptides, and glutamate receptor subunits, were also observed to be altered in the injured cortex. Real-time PCR analysis confirmed the GeneChip data for many of these transcripts. The novel physiologically relevant gene expression changes observed here might explain some of the molecular mechanisms of TBI-induced neuronal damage.

Corticotropin-releasing hormone induces proliferation and TNF-alpha release in cultured rat microglia via MAP kinase signalling pathways

We have previously demonstrated that corticotropin-releasing hormone (CRH) receptor 1 (CRH-R1) is functionally expressed in rat microglia. In the present study, we show that CRH, acting on CRH-R1, promoted cell proliferation and tumour necrosis factor-alpha (TNF-alpha) release in cultured rat microglia. Exogenous CRH resulted in an increase in BrdU incorporation compared with control cells, which was observed in a range of concentrations of CRH between 10 and 500 nm, with a maximal response at 50 nm. The effect of CRH on BrdU incorporation was inhibited by a CRH antagonist astressin but not by a cAMP-dependent protein kinase inhibitor H89. Exposure of microglial cells to CRH resulted in a transient and rapid increase in TNF-alpha release in a dose-dependent manner. In the presence of astressin, the effects of CRH on TNF-alpha release were attenuated. CRH effects on TNF-alpha release were also inhibited by specific inhibitors of MEK, the upstream kinase of the extracellular signal-regulated protein kinase (ERK) (PD98059) or p38 mitogen-activated protein kinase (SB203580), but not by H89. Furthermore, CRH induced rapid phosphorylation of ERK and p38 kinases. Astressin, PD98059, and SB230580 were able to inhibit CRH-induced kinase phosphorylation. These results suggest that CRH induces cell proliferation and TNF-alpha release in cultured microglia via MAP kinase signalling pathways, thereby providing insight into the interactions between CRH and inflammatory mediators.

Functional characteristics of CRH receptors and potential clinical applications of CRH-receptor antagonists

Corticotropin-releasing hormone (CRH) plays a major role in coordinating the behavioral, endocrine, autonomic and immune responses to stress. CRH and CRH-related peptides and their receptors are present in the central nervous system and in a wide variety of peripheral tissues, including the immune, cardiovascular and reproductive systems, and have been associated with the pathophysiology of many disease states. These observations have led to the development of several CRH receptor type-selective antagonists, which have been used experimentally to elucidate the role of CRH and related peptides in physiological and disease processes, such as anxiety and depression, sleep disorders, addictive behavior, inflammatory and allergic disorders, neurological diseases and pre-term labor. Because of the complex network of multiple CRH receptor subtypes and their tissue- and agonist-specific signaling diversity, antagonists need to be developed that can target specific CRH receptor isoform-driven signaling pathways.

Acute ethanol intake attenuates inflammatory cytokines after brain injury in rats: a possible role for corticosterone

It has been reported that acute ethanol intoxication exerts dose-dependent effects, both beneficial and detrimental, on the outcome of traumatic brain injury (TBI), although the mechanism(s) has not been determined. Given that pro-inflammatory cytokines are either neuroprotective or neurotoxic, depending on their tissue levels, ethanol-induced alterations in brain cytokine production may be involved in determining the recovery after TBI. The present study was undertaken to examine the effect of acute ethanol pretreatments (producing blood alcohol concentrations of 100+/-16 mg/dL, and 220+/-10 mg/dL, considered low and intoxicating doses, respectively) on interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) levels in discrete brain regions. In addition, serum corticosterone levels were also examined because the hormone is a modulator of cytokine production, its secretion is stimulated by ethanol, and it has been associated with the severity of post-injury neurologic dysfunction. The data presented in this report demonstrate that moderate cortical impact brain injury elicits a marked increase in IL-1beta and TNF-alpha in the injured cortex as well as in the hippocampus ipsilateral to the injury. Ethanol pretreatment lowered cytokine levels in the cortex, hippocampus and hypothalamus in a dose-dependent manner after TBI compared to the untreated injured rats. Serum corticosterone levels were markedly increased in the injured rats, and were further augmented in the ethanol-pretreated injured animals in a dose-dependent manner. Our findings suggest that ethanol-induced decrease in pro-inflammatory cytokine production may be linked to increased circulating corticosterone, both of which may contribute to the outcome of brain injury.

The hypothalamo-pituitary-adrenal axis response to experimental traumatic brain injury

Alterations in the hypothalamo-pituitary-adrenal (HPA) axis following traumatic brain injury have not been documented in detail. We used fluid percussion injury (FPI) to evaluate the early changes in components of the HPA axis following experimental traumatic brain injury. Wistar rats were sacrificed at 2 or 4 h following sham or FPI surgery. In situ hybridization histochemistry was used to determine the expression of mRNAs of corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) in the hypothalamus and pro-opiomelanocortin (POMC) in the pituitary. A group of animals undergoing no surgery were used as control. Repeated blood sampling from an indwelling catheter demonstrated that plasma corticosterone (CORT) levels peaked 30 min following surgery in sham and FPI animals but there was no significant difference in CORT concentration between these groups at any time. Pituitary POMC expression was increased following sham and FPI surgery (compared with control non-operated animals) but with no significant difference between the two groups undergoing surgery. Hypothalamic CRH mRNA expression was significantly higher in animals undergoing FPI compared with sham surgery. Hypothalamic AVP mRNA expression was not significantly increased when compared with control nonoperated animals. These data indicate that the anaesthesia and/or surgery associated with FPI or sham surgery induces a generalised activation of the HPA axis. The selective increase in CRH mRNA in animals undergoing FPI may be due to specific effects of traumatic brain injury rather than a general stress response and may suggest an additional neurotransmitter role for CRH following head injury. The absence of an AVP response suggests that the effects of FPI may be mediated through the CRH-alone-containing subpopulation of neurons.

Neuroprotective effects of the CRF1 antagonist R121920 after permanent focal ischemia in the rat

The neuroprotective effects of a systemically active, highly selective, corticotropin-releasing factor-1 (CRF1) receptor antagonist, R121920 ((7-(dipropylamino)-2,5-dimethyl-3- [2-(dimethylamino)-5-pyridyl] pyrazolo [1,5-a] pyrimidine), was assessed in two rat models of permanent focal cerebral ischemia, where the middle cerebral artery (MCA) was occluded either through the subtemporal approach or using the intraluminal suture technique. R121920 rapidly crossed the blood-brain barrier after intravenous (IV) bolus administration (10 mg/kg), with peak brain concentrations at 5 minutes (2.26 +/- 0.40 microg/mL), which were approximately 2-fold greater than those in plasma (0.98 +/- 0.24 microg/mL). Treatment with R121920 (10 mg/kg IV followed by 5 mg/kg subcutaneously at hourly intervals for 4 hours) significantly (P < 0.001) reduced total (by 40%) and cortical (by 37%) infarct volume at 24 hours after subtemporal MCA occlusion (MCAO). In the intraluminal suture MCAO model, IV administration of R121920 (10 mg/kg) at the time of ischemia onset (and at multiple times thereafter) reduced both hemispheric infarct volume (by 34%, P < 0.001) and brain swelling (by 50%, P < 0.001) when assessed at 24 hours. In this model of focal ischemia, significant reduction (P < 0.05) in both outcome measures was obtained when R121920 administration was delayed up to 1 hour after MCAO. These results further define the antiischemic properties of selective CRF 1 antagonists in two experimental models of permanent focal cerebral ischemia.

Cytokines and acute neurodegeneration

Cytokines have been implicated as mediators and inhibitors of diverse forms of neurodegeneration. They are induced in response to brain injury and have diverse actions that can cause, exacerbate, mediate and/or inhibit cellular injury and repair. Here we review evidence for the contribution of cytokines to acute neurodegeneration, focusing primarily on interleukin 1 (IL-1), tumour necrosis factor-alpha (TNFalpha) and transforming growth factor-beta (TGFbeta). TGFbeta seems to exert primarily neuroprotective actions, whereas TNFalpha might contribute to neuronal injury and exert protective effects. IL-1 mediates ischaemic, excitotoxic and traumatic brain injury, probably through multiple actions on glia, neurons and the vasculature. Understanding cytokine action in acute neurodegeneration could lead to novel and effective therapeutic strategies, some of which are already in clinical trials.

Influence of corticotrophin releasing factor on neuronal cell death in vitro and in vivo

Several studies have demonstrated that antagonists of the corticotrophin releasing factor (CRF) receptor markedly inhibit experimentally induced excitotoxic, ischaemic and traumatic brain injury in the rat, and that CRF expression is elevated in response to experimentally induced stroke or traumatic brain injury. CRF is also induced by the pro-inflammatory cytokine interleukin 1 (IL-1), which participates in various forms of neurodegeneration. The aim of this study was to test the hypothesis that CRF is toxic directly in vivo or in vitro. In primary cultures of rat cortical neurons, exposure to CRF (10 pM-100 nM) for 24 h failed to cause cell death directly, or to modify the neurotoxic effects of N-methyl-D-aspartate (NMDA). Similarly, infusion of CRF (0.3-5 microg) into specific brain regions of the rat did not induce cell death and did not significantly alter the neuronal damage produced by infusion of excitatory amino acids. These data demonstrate that CRF is not directly neurotoxic, and suggest that either CRF mediates neuronal damage by indirect actions (e.g. on the vasculature) and/or that CRF is not the endogenous ligand which contributes to neurodegeneration through activation of CRF receptors.

Sublethal hypoxia up-regulates corticotropin releasing factor receptor type 1 in fetal hippocampal neurons

We analysed the influence of oxygen-glucose deprivation (OGD) on the expression of corticotropin releasing factor (CRF) receptors (CRF-R1 and CRF-R2) in fetal hippocampal neurons in vitro. A 2 h exposure of neurons to OGD resulted in death of 18+/-2.8% cells at 24 h following exposure, which was considered sublethal hypoxia. Expression of both receptors was quantitated by competitive reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting. Levels of mRNA for CRF-R1 were increased 3.2-fold compared to control neurons, while CRF-R2 mRNA levels remained unchanged. The increase of CRF-R1 mRNA levels was observed at 6 h and peaked at 24 h. CRF-R1 protein levels were also increased by 2.4-fold and 1.7-fold at 24 h and 48 h, respectively. These data suggest that the effects of CRF on neuronal survival are mediated in part through the induction and expression of CRF-R1 following a hypoxic/ischemic insult.

The corticosterone synthesis inhibitor metyrapone prevents hypoxia/ischemia-induced loss of synaptic function in the rat hippocampus

BACKGROUND AND PURPOSE

Ischemia is accompanied by abundant corticosterone secretion, which could potentially exacerbate brain damage via activation of glucocorticoid receptors. We addressed whether manipulating steroid levels during ischemia affects hippocampal synaptic function along with neuronal structure. Moreover, we established whether pretreatment with the glucocorticoid receptor antagonist RU38486 is as effective in preventing deleterious effects after ischemia as is the steroid synthesis inhibitor metyrapone.

METHODS

Rats underwent 20 minutes of unilateral hypoxia/ischemia (HI). Convulsions were monitored after HI, and 24 hours later, field potentials were recorded in vitro in the hippocampal CA1 area in response to stimulation of the Schaffer collateral/commissural fibers. Morphological alterations were determined in brain slices from the same animals. Data were correlated with steroid treatment before HI.

RESULTS

Metyrapone suppressed plasma corticosteroid levels during HI, whereas corticosterone treatment significantly elevated plasma steroid levels. These treatments affected the incidence of visible seizures after HI: corticosterone treatment resulted in the highest incidence, whereas metyrapone attenuated the occurrence of seizures. Moreover, the HI-induced impairment in synaptic transmission in the CA1 area in vitro was exacerbated by concomitant corticosteroid treatment and alleviated by pretreatment with metyrapone. In parallel, degenerative changes in the hippocampus after HI were most pronounced after corticosterone treatment, whereas metyrapone reduced these alterations. RU38486 was effective only in reducing the incidence of seizures shortly after ischemia.

CONCLUSIONS

We tentatively conclude that synaptic function along with cellular integrity is preserved after HI by preventing the ischemia-evoked rise in corticosteroid levels rather than blocking the glucocorticoid receptor.

The rationale for corticotropin-releasing hormone receptor (CRH-R) antagonists to treat depression and anxiety

Neuroendocrine studies strongly suggest that dysregulation of the hypothalamic pituitary-adrenocortical (HPA) system plays a causal role in the development and course of depression. Whereas the initial mechanism resulting in HPA hyperdrive remains to be elucidated, evidence has emerged that corticosteroid receptor function is impaired in many patients with depression and in many healthy individuals at increased genetic risk for an depressive disorder. Assuming such impaired receptor function, then central secretion of CRH would be enhanced in many brain areas, which would account for a variety of depressive symptoms. As shown in rats and also in transgenic mice with impaired glucocorticoid receptor function, antidepressants enhance the signaling through corticosteroid receptors. This mechanism of action can be amplified through blocking central mechanisms that drive the HPA system. Animal experiments using antisense oligodeoxynucleotides directed against the mRNA of both CRH receptor subtypes identified the CRH1 receptor as the mediator of the anxiogenic effects of CRH. Studies in mouse mutants in which this receptor subtype had been deleted extended these findings as the animals were less anxious than wild-type mice when experimentally stressed. Thus, patients with clinical conditions that are causally related to HPA hyperactivity may profit from treatment with a CRH1 receptor antagonist.

Annual review prize lecture cytokines - killers in the brain?

Given at the Meeting of the Physiological Society held at the University of Southampton on 10 September 1998