Inflammation-Induced Histamine Impairs the Capacity of Escitalopram to Increase Hippocampal Extracellular Serotonin.

Commonly prescribed selective serotonin reuptake inhibitors (SSRIs) inhibit the serotonin transporter to correct a presumed deficit in extracellular serotonin signaling during depression. These agents bring clinical relief to many who take them; however, a significant and growing number of individuals are resistant to SSRIs. There is emerging evidence that inflammation plays a significant role in the clinical variability of SSRIs, though how SSRIs and inflammation intersect with synaptic serotonin modulation remains unknown. In this work, we use fast in vivo serotonin measurement tools to investigate the nexus between serotonin, inflammation, and SSRIs. Upon acute systemic lipopolysaccharide (LPS) administration in male and female mice, we find robust decreases in extracellular serotonin in the mouse hippocampus. We show that these decreased serotonin levels are supported by increased histamine activity (because of inflammation), acting on inhibitory histamine H3 heteroreceptors on serotonin terminals. Importantly, under LPS-induced histamine increase, the ability of escitalopram to augment extracellular serotonin is impaired because of an off-target action of escitalopram to inhibit histamine reuptake. Finally, we show that a functional decrease in histamine synthesis boosts the ability of escitalopram to increase extracellular serotonin levels following LPS. This work reveals a profound effect of inflammation on brain chemistry, specifically the rapidity of inflammation-induced decreased extracellular serotonin, and points the spotlight at a potentially critical player in the pathology of depression, histamine. The serotonin/histamine homeostasis thus, may be a crucial new avenue in improving serotonin-based treatments for depression.SIGNIFICANCE STATEMENT Acute LPS-induced inflammation (1) increases CNS histamine, (2) decreases CNS serotonin (via inhibitory histamine receptors), and (3) prevents a selective serotonin reuptake inhibitor (SSRI) from effectively increasing extracellular serotonin. A targeted depletion of histamine recovers SSRI-induced increases in extracellular hippocampal serotonin.

Integrating the monoamine and cytokine hypotheses of depression: Is histamine the missing link?

Psychiatric diseases, like depression, largely affect the central nervous system (CNS). While the underlying neuropathology of depressive illness remains to be elucidated, several hypotheses have been proposed as molecular underpinnings for major depressive disorder, including the monoamine hypothesis and the cytokine hypothesis. The monoamine hypothesis has been largely supported by the pharmaceuticals that target monoamine neurotransmitters as a treatment for depression. However, these antidepressants have come under scrutiny due to their limited clinical efficacy, side effects, and delayed onset of action. The more recent, cytokine hypothesis of depression is supported by the ability of immune-active agents to induce "sickness behaviour" akin to that seen with depression. However, treatments that more selectively target inflammation have yielded inconsistent antidepressive results. As such, neither of these hypotheses can fully explain depressive illness pathology, implying that the underlying neuropathological mechanisms may encompass aspects of both theories. The goal of the current review is to integrate these two well-studied hypotheses and to propose a role for histamine as a potential unifying factor that links monoamines to cytokines. Additionally, we will focus on stress-induced depression, to provide an updated perspective of depressive illness research and thereby identify new potential targets for the treatment of major depressive disorder.

A tale of two transmitters: serotonin and histamine as in vivo biomarkers of chronic stress in mice.

BACKGROUND: Stress-induced mental illnesses (mediated by neuroinflammation) pose one of the world's most urgent public health challenges. A reliable in vivo chemical biomarker of stress would significantly improve the clinical communities' diagnostic and therapeutic approaches to illnesses, such as depression. METHODS: Male and female C57BL/6J mice underwent a chronic stress paradigm. We paired innovative in vivo serotonin and histamine voltammetric measurement technologies, behavioral testing, and cutting-edge mathematical methods to correlate chemistry to stress and behavior. RESULTS: Inflammation-induced increases in hypothalamic histamine were co-measured with decreased in vivo extracellular hippocampal serotonin in mice that underwent a chronic stress paradigm, regardless of behavioral phenotype. In animals with depression phenotypes, correlations were found between serotonin and the extent of behavioral indices of depression. We created a high accuracy algorithm that could predict whether animals had been exposed to stress or not based solely on the serotonin measurement. We next developed a model of serotonin and histamine modulation, which predicted that stress-induced neuroinflammation increases histaminergic activity, serving to inhibit serotonin. Finally, we created a mathematical index of stress, Si and predicted that during chronic stress, where Si is high, simultaneously increasing serotonin and decreasing histamine is the most effective chemical strategy to restoring serotonin to pre-stress levels. When we pursued this idea pharmacologically, our experiments were nearly identical to the model's predictions. CONCLUSIONS: This work shines the light on two biomarkers of chronic stress, histamine and serotonin, and implies that both may be important in our future investigations of the pathology and treatment of inflammation-induced depression.

High-fat diet induces neuroinflammation and reduces the serotonergic response to escitalopram in the hippocampus of obese rats.

Clinical studies indicate that obese individuals have an increased risk of developing co-morbid depressive illness and that these patients have reduced responses to antidepressant therapy, including selective serotonin reuptake inhibitors (SSRIs). Obesity, a condition of chronic mild inflammation including obesity-induced neuroinflammation, is proposed to contribute to decreases in synaptic concentrations of neurotransmitters like serotonin (5HT) by decreasing 5HT synthesis in the dorsal raphe nucleus (DRN) and/or affecting 5HT reuptake in DRN target regions like the hippocampus. In view of these observations, the goal of the current study was to examine inflammatory markers and serotonergic dynamics in co-morbid obesity and depression. Biochemical and behavioral assays revealed that high-fat diet produced an obesity and depressive-like phenotype in one cohort of rats and that these changes were marked by increases in key pro-inflammatory cytokines in the hippocampus. In real time using fast scan cyclic voltammetry (FSCV), we observed no changes in basal levels of hippocampal 5HT; however responses to escitalopram were significantly impaired in the hippocampus of obese rats compared to diet resistant rats and control rats. Further studies revealed that these neurochemical observations could be explained by increases in serotonin transporter (SERT) expression in the hippocampus driven by elevated neuroinflammation. Collectively, these results demonstrate that obesity-induced increases in neuroinflammation adversely affect SERT expression in the hippocampus of obese rats, thereby providing a potential synaptic mechanism for reduced SSRI responsiveness in obese subjects with co-morbid depressive illness.

Hippocampal insulin resistance and cognitive dysfunction.

Clinical studies suggest a link between type 2 diabetes mellitus (T2DM) and insulin resistance (IR) and cognitive dysfunction, but there are significant gaps in our knowledge of the mechanisms underlying this relationship. Animal models of IR help to bridge these gaps and point to hippocampal IR as a potential mediator of cognitive dysfunction in T2DM, as well as in Alzheimer disease (AD). This Review highlights these observations and discusses intervention studies which suggest that the restoration of insulin activity in the hippocampus may be an effective strategy to alleviate the cognitive decline associated with T2DM and AD.

Chronic stress from adolescence to aging in the prefrontal cortex: A neuroimmune perspective.

The development of the organism is a critical variable which influences the magnitude, duration, and reversibility of the effects of chronic stress. Such factors are relevant to the prefrontal cortex (PFC), as this brain region is the last to mature, the first to decline, and is highly stress-sensitive. Therefore, this review will examine the intersection between the nervous system and immune system at glutamatergic synapses in the PFC across three developmental periods: adolescence, adulthood, and aging. Glutamatergic synapses are tightly juxtaposed with microglia and astrocytes, and each of these cell types exhibits their own developmental trajectory. Not only does chronic stress differentially impact each of these cell types across development, but chronic stress also alters intercellular communication within this quad-partite synapse. These observations suggest that developmental shifts in both neural and immune function across neurons, microglia, and astrocytes mediate shifting effects of chronic stress on glutamatergic transmission.

Exposing the latent phenotype of Gulf War Illness: examination of the mechanistic mediators of cognitive dysfunction.

Though it has been over 30 years since the 1990-1991 Gulf War (GW), the pathophysiology of Gulf War Illness (GWI), the complex, progressive illness affecting approximately 30% of GW Veterans, has not been fully characterized. While the symptomology of GWI is broad, many symptoms can be attributed to immune and endocrine dysfunction as these critical responses appear to be dysregulated in many GWI patients. Since such dysregulation emerges in response to immune threats or stressful situations, it is unsurprising that clinical studies suggest that GWI may present with a latent phenotype. This is most often observed in studies that include an exercise challenge during which many GWI patients experience an exacerbation of symptoms. Unfortunately, very few preclinical studies include such physiological stressors when assessing their experimental models of GWI, which creates variable results that hinder the elucidation of the mechanisms mediating GWI. Thus, the purpose of this review is to highlight the clinical and preclinical findings that investigate the inflammatory component of GWI and support the concept that GWI may be characterized as having a latent phenotype. We will mainly focus on studies assessing the progressive cognitive impairments associated with GWI and emphasize the need for physiological stressors in future work to create a more unified hypothesis that can identify potential therapeutics for this patient population.

Voluntary wheel running as a promising strategy to promote autonomic resilience to social stress in females: Vagal tone lies at the heart of the matter.

Social stress is a major risk factor for comorbid conditions including cardiovascular disease and depression. While women exhibit 2-3× the risk for these stress-related disorders compared to men, the mechanisms underlying heightened stress susceptibility among females remain largely unknown. Due to a lack in understanding of the pathophysiology underlying stress-induced comorbidities among women, there has been a significant challenge in developing effective therapeutics. Recently, a causal role for inflammation has been established in the onset and progression of comorbid cardiovascular disease/depression, with women exhibiting increased sensitivity to stress-induced immune signaling. Importantly, reduced vagal tone is also implicated in stress susceptibility, through a reduction in the vagus nerve's well-recognized anti-inflammatory properties. Thus, examining therapeutic strategies that stabilize vagal tone during stress may shed light on novel targets for promoting stress resilience among women. Recently, accumulating evidence has demonstrated that physical activity exerts cardio- and neuro-protective effects by enhancing vagal tone. Based on this evidence, this mini review provides an overview of comorbid cardiovascular and behavioral dysfunction in females, the role of inflammation in these disorders, how stress may impart its negative effects on the vagus nerve, and how exercise may act as a preventative. Further, we highlight a critical gap in the literature with regard to the study of females in this field. This review also presents novel data that are the first to demonstrate a protective role for voluntary wheel running over vagal tone and biomarkers of cardiac dysfunction in the face of social stress exposure in female rats.

Blood-borne angiotensin II acts in the brain to influence behavioral and endocrine responses to psychogenic stress.

This study elucidates the neural circuits by which circulating angiotensin II (ANGII) acts in the brain to influence humoral and behavioral responses to psychological stressors. To test the hypothesis that systemic ANGII mediates stress responding via the subfornical organ (SFO), we first found that the timing of increased systemic ANGII in response to 60 min restraint coincides with increased c-fos mRNA expression in the SFO. Next, we administered an anterograde neuronal tract tracer into the SFO and found that fibers originating there make appositions onto neurons in the paraventricular nucleus of the hypothalamus that are also c-fos positive following restraint stress. To determine whether circulating ANGII stimulates the release of stress hormones via activation of angiotensin type 1 receptors (AT1R) within the SFO, we delivered lentivirus to knockdown AT1R expression locally in the SFO. Inhibition of AT1R specifically within the SFO blunted the release of adrenocorticotrophin-releasing hormone and corticosterone in response to restraint stress and caused rats to spend more time in the open arms of an elevated-plus maze than controls, indicating that inhibition of AT1R within the SFO is anxiolytic. Collectively, these results suggest that circulating ANGII acts on AT1R in the SFO to influence responding to psychological stressors.

Peripheral versus central insulin and leptin resistance: Role in metabolic disorders, cognition, and neuropsychiatric diseases.

Insulin and leptin are classically regarded as peptide hormones that play key roles in metabolism. In actuality, they serve several functions in both the periphery and central nervous system (CNS). Likewise, insulin and leptin resistance can occur both peripherally and centrally. Metabolic disorders such as diabetes and obesity share several key features including insulin and leptin resistance. While the peripheral effects of these disorders are well-known (i.e. cardiovascular disease, hypertension, stroke, dyslipidemia, etc.), the CNS complications of leptin and insulin resistance have come into sharper focus. Both preclinical and clinical findings have indicated that insulin and leptin resistance are associated with cognitive deficits and neuropsychiatric diseases such as depression. Importantly, these studies also suggest that these deficits in neuroplasticity can be reversed by restoration of insulin and leptin sensitivity. In view of these observations, this review will describe, in detail, the peripheral and central functions of insulin and leptin and explain the role of insulin and leptin resistance in various metabolic disorders, cognition, and neuropsychiatric diseases.

Intranasal insulin and orexins to treat age-related cognitive decline.

The intranasal (IN) administration of neuropeptides, such as insulin and orexins, has been suggested as a treatment strategy for age-related cognitive decline (ARCD). Because dysfunctional neuropeptide signaling is an observed characteristic of ARCD, it has been suggested that IN delivery of insulin and/or orexins may restore endogenous peptide signaling and thereby preserve cognition. IN administration is particularly alluring as it is a relatively non-invasive method that directly targets peptides to the brain. Several laboratories have examined the behavioral effects of IN insulin in young, aged, and cognitively impaired rodents and humans. These studies demonstrated improved performance on various cognitive tasks following IN insulin administration. Fewer laboratories have assessed the effects of IN orexins; however, this peptide also holds promise as an effective treatment for ARCD through the activation of the cholinergic system and/or the reduction of neuroinflammation. Here, we provide a brief overview of the advantages of IN administration and the delivery pathway, then summarize the current literature on IN insulin and orexins. Additional preclinical studies will be useful to ultimately uncover the mechanisms underlying the pro-cognitive effects of IN insulin and orexins, whereas future clinical studies will aid in the determination of the most efficacious dose and dosing paradigm. Eventually, IN insulin and/or orexin administration may be a widely used treatment strategy in the clinic for ARCD.

A look inside the diabetic brain: Contributors to diabetes-induced brain aging.

Central nervous system (CNS) complications resulting from diabetes is a problem that is gaining more acceptance and attention. Recent evidence suggests morphological, electrophysiological and cognitive changes, often observed in the hippocampus, in diabetic individuals. Many of the CNS changes observed in diabetic patients and animal models of diabetes are reminiscent of the changes seen in normal aging. The central commonalities between diabetes-induced and age-related CNS changes have led to the theory of advanced brain aging in diabetic patients. This review summarizes the findings of the literature as they relate to the relationship between diabetes and dementia and discusses some of the potential contributors to diabetes-induced CNS impairments.

Elevating Insulin Signaling Using a Constitutively Active Insulin Receptor Increases Glucose Metabolism and Expression of GLUT3 in Hippocampal Neurons.

Insulin signaling is an integral component of healthy brain function, with evidence of positive insulin-mediated alterations in synaptic integrity, cerebral blood flow, inflammation, and memory. However, the specific pathways targeted by this peptide remain unclear. Previously, our lab used a molecular approach to characterize the impact of insulin signaling on voltage-gated calcium channels and has also shown that acute insulin administration reduces calcium-induced calcium release in hippocampal neurons. Here, we explore the relationship between insulin receptor signaling and glucose metabolism using similar methods. Mixed, primary hippocampal cultures were infected with either a control lentivirus or one containing a constitutively active human insulin receptor (IRß). 2-NBDG imaging was used to obtain indirect measures of glucose uptake and utilization. Other outcome measures include Western immunoblots of GLUT3 and GLUT4 on total membrane and cytosolic subcellular fractions. Glucose imaging data indicate that neurons expressing IRß show significant elevations in uptake and rates of utilization compared to controls. As expected, astrocytes did not respond to the IRß treatment. Quantification of Western immunoblots show that IRß is associated with significant elevations in GLUT3 expression, particularly in the total membrane subcellular fraction, but did not alter GLUT4 expression in either fraction. Our work suggests that insulin plays a significant role in mediating neuronal glucose metabolism, potentially through an upregulation in the expression of GLUT3. This provides further evidence for a potential therapeutic mechanism underlying the beneficial impact of intranasal insulin in the clinic.

Insulin resistance and hippocampal dysfunction: Disentangling peripheral and brain causes from consequences.

In the periphery insulin plays a critical role in the regulation of metabolic homeostasis by stimulating glucose uptake into peripheral organs. In the central nervous system (CNS), insulin plays a critical role in the formation of neural circuits and synaptic connections from the earliest stages of development and facilitates and promotes neuroplasticity in the adult brain. Beyond these physiological roles of insulin, a shared feature between the periphery and CNS is that decreases in insulin receptor activity and signaling (i.e. insulin resistance) contributes to the pathological consequences of type 2 diabetes (T2DM) and obesity. Indeed, clinical and preclinical studies illustrate that CNS insulin resistance elicits neuroplasticity deficits that lead to decreases in cognitive function and increased risk of neuropsychiatric disorders. The goals of this review are to provide an overview of the literature that have identified the neuroplasticity deficits observed in T2DM and obesity, as well as to discuss the potential causes and consequences of insulin resistance in the CNS, with a particular focus on how insulin resistance impacts hippocampal neuroplasticity. Interestingly, studies that have examined the effects of hippocampal-specific insulin resistance illustrate that brain insulin resistance may impair neuroplasticity independent of peripheral insulin resistance, thereby supporting the concept that restoration of brain insulin activity is an attractive therapeutic strategy to ameliorate or reverse cognitive decline observed in patients with CNS insulin resistance such as T2DM and Alzheimer's Disease.

Impaired Glucose Tolerance and Reduced Plasma Insulin Precede Decreased AKT Phosphorylation and GLUT3 Translocation in the Hippocampus of Old 3xTg-AD Mice.

Several studies have demonstrated that mouse models of Alzheimer's disease (AD) can exhibit impaired peripheral glucose tolerance. Further, in the APP/PS1 mouse model, this is observed prior to the appearance of AD-related neuropathology (e.g., amyloid-ß plaques; Aß) or cognitive impairment. In the current study, we examined whether impaired glucose tolerance also preceded AD-like changes in the triple transgenic model of AD (3xTg-AD). Glucose tolerance testing (GTT), insulin ELISAs, and insulin tolerance testing (ITT) were performed at ages prior to (1-3 months and 6-8 months old) and post-pathology (16-18 months old). Additionally, we examined for altered insulin signaling in the hippocampus. Western blots were used to evaluate the two-primary insulin signaling pathways: PI3K/AKT and MAPK/ERK. Since the PI3K/AKT pathway affects several downstream targets associated with metabolism (e.g., GSK3, glucose transporters), western blots were used to examine possible alterations in the expression, translocation, or activation of these targets. We found that 3xTg-AD mice display impaired glucose tolerance as early as 1 month of age, concomitant with a decrease in plasma insulin levels well prior to the detection of plaques (∼14 months old), aggregates of hyperphosphorylated tau (∼18 months old), and cognitive decline (≥18 months old). These alterations in peripheral metabolism were seen at all time points examined. In comparison, PI3K/AKT, but not MAPK/ERK, signaling was altered in the hippocampus only in 18-20-month-old 3xTg-AD mice, a time point at which there was a reduction in GLUT3 translocation to the plasma membrane. Taken together, our results provide further evidence that disruptions in energy metabolism may represent a foundational step in the development of AD.

The As and Ds of stress: metabolic, morphological and behavioral consequences.

Unlike responses to acute stressful events that are protective and adaptive in nature, chronic stress elicits neurochemical, neuroanatomical and cellular changes that may have deleterious consequences upon higher brain functioning. For example, while exposure to acute stress facilitates memory formation and consolidation, chronic stress or chronic exposure to stress levels of glucocorticoids impairs cognitive performance. Chronic stress or glucocorticoid exposure, as well as impairments in hypothalamic-pituitary-adrenal (HPA) axis function are proposed to participate in the etiology and progression of neurological disorders such as depressive illness, anxiety disorders and post-traumatic stress disorder (PTSD). HPA axis dysfunction, impaired stress responses and elevated basal levels of glucocorticoids are also hallmark features of experimental models of type 1 and type 2 diabetes, as well as diabetic subjects in poor glycemic control. Such results suggest that stress and glucocorticoids contribute to the neurological complications observed in diabetes patients. Interestingly, many of the hyperglycemia mediated changes in the brain are similar to those observed in depressive illness patients and in experimental models of chronic stress. Such results suggest that common mechanisms may be involved in the development of the neurological complications associated with Anxiety, Depressive illness and Diabetes: the As and Ds of stress. The aim of the current review will be to discuss the mechanisms through which limbic structures such as the hippocampus and amygdala respond and adapt to the deleterious consequences of chronic stress and hyperglycemia.

Insulin signaling effects on memory and mood.

The escalating obesity/diabetes epidemic is an important health-care issue that has crucial socio-economic ramifications. The complications of diabetes/obesity phenotypes extend to the central nervous system (CNS), including the hippocampus, a brain region that is particularly vulnerable to hyperglycemia and insulin resistance. Deficits in hippocampal synaptic plasticity observed in diabetes ultimately have deleterious consequences upon cognitive function. For example, recent studies using brain imaging technologies have identified cerebral atrophy in diabetic patients, suggesting that the neuroanatomical changes observed in experimental models of diabetes may accurately reflect what is occurring in the clinical setting. Deficits in insulin receptor (IR) signaling and impairments in hypothalamic-pituitary-adrenal (HPA) axis function also contribute to the neurological complications of diabetes phenotypes. The pathophysiological similarities between diabetes and stress-related mood disorders suggest that common mechanistic mediators may be involved in the etiology and progression of the neurological complications of these disorders. When combined with the accumulating evidence from pre-clinical models, these data support the hypothesis that a long-term consequence of diabetes/obesity phenotypes is accelerated brain aging that results in neuropsychological deficits and increased vulnerability to co-morbidities such as depressive illness.

Insulin-mediated synaptic plasticity in the CNS: Anatomical, functional and temporal contexts.

For decades the brain was erroneously considered an insulin insensitive organ. Although gaps in our knowledge base remain, conceptual frameworks are starting to emerge to provide insight into the mechanisms through which insulin facilitates critical brain functions like metabolism, cognition, and motivated behaviors. These diverse physiological and behavioral activities highlight the region-specific activities of insulin in the CNS; that is, there is an anatomical context to the activities of insulin in the CNS. Similarly, there is also a temporal context to the activities of insulin in the CNS. Indeed, brain insulin receptor activity can be conceptualized as a continuum in which insulin promotes neuroplasticity from development into adulthood where it is an integral part of healthy brain function. Unfortunately, brain insulin resistance likely contributes to neuroplasticity deficits in obesity and type 2 diabetes mellitus (T2DM). This neuroplasticity continuum can be conceptualized by the mechanisms through which insulin promotes cognitive function through its actions in brain regions like the hippocampus, as well as the ability of insulin to modulate motivated behaviors through actions in brain regions like the nucleus accumbens and the ventral tegmental area. Thus, the goals of this review are to highlight these anatomical, temporal, and functional contexts of insulin activity in these brain regions, and to identify potentially critical time points along this continuum where the transition from enhancement of neuroplasticity to impairment may take place. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

Essential Role of Ovarian Hormones in Susceptibility to the Consequences of Witnessing Social Defeat in Female Rats.

BACKGROUND: Women are at greater risk than men of developing depression and comorbid disorders such as cardiovascular disease. This enhanced risk begins at puberty and ends following menopause, suggesting a role for ovarian hormones in this sensitivity. Here we used a model of psychosocial witness stress in female rats to determine the stress-induced neurobiological adaptations that underlie stress susceptibility in an ovarian hormone-dependent manner. METHODS: Intact or ovariectomized (OVX) female rats were exposed to five daily 15-minute witness-stress exposures. Witness-stress-evoked burying, behavioral despair, and anhedonia were measured. Cardiovascular telemetry was combined with plasma measurements of inflammation, epinephrine, and corticosterone as indices of cardiovascular dysfunction. Finally, levels of interleukin-1ß and corticotropin-releasing factor were assessed in the central amygdala. RESULTS: Witness stress produced anxiety-like burying, depressive-like anhedonia, and behavioral despair selectively in intact female rats, which was associated with enhanced sympathetic responses during stress, including increased blood pressure, heart rate, and arrhythmias. Moreover, intact female rats exhibited increases in 12-hour resting systolic pressure and heart rate and reductions in heart rate variability. Notably, OVX female rats remained resilient. Moreover, intact, but not OVX, female rats exposed to witness stress exhibited a sensitized cytokine and epinephrine response to stress and distinct increases in levels of corticotropin-releasing factor and interleukin-1ß in the central amygdala. CONCLUSIONS: Together these data suggest that ovarian hormones play a critical role in the behavioral, inflammatory, and cardiovascular susceptibility to social stress in female rats and reveal putative systems that are sensitized to stress in an ovarian hormone-dependent manner.

Tianeptine increases brain-derived neurotrophic factor expression in the rat amygdala.

Chronic restraint stress affects hippocampal and amygdalar synaptic plasticity as determined by electrophysiological, morphological and behavioral measures, changes that are inhibited by some but not all antidepressants. The efficacy of some classes of antidepressants is proposed to involve increased phosphorylation of cAMP response element binding protein (CREB), leading to increased expression of neurotrophic factors, such as brain-derived neurotrophic factor (BDNF). Conversely, some studies suggest that acute and chronic stress downregulate BDNF expression and activity. Accordingly, the aim of the current study was to examine total and phosphorylated CREB (pCREB), as well as BDNF mRNA and protein levels in the hippocampus and amygdala of rats subjected to chronic restraint stress in the presence and absence of the antidepressant tianeptine. In the hippocampus, chronic restraint stress increased pCREB levels without affecting BDNF mRNA or protein expression. Tianeptine administration had no effect upon these measures in the hippocampus. In the amygdala, BDNF mRNA expression was not modulated in chronic restraint stress rats given saline in spite of increased pCREB levels. Conversely, BDNF mRNA levels were increased in the amygdala of chronic restraint stress/tianeptine rats in the absence of changes in pCREB levels when compared to non-stressed controls. Amygdalar BDNF protein increased while pCREB levels decreased in tianeptine-treated rats irrespective of stress conditions. Collectively, these results demonstrate that tianeptine concomitantly decreases pCREB while increasing BDNF expression in the rat amygdala, increases in neurotrophic factor expression that may participate in the enhancement of amygdalar synaptic plasticity mediated by tianeptine.