Neural plasticity: consequences of stress and actions of antidepressant treatment

Neural plasticity is emerging as a fundamental and critical mechanism of neuronal function, which allows the brain to receive information and make the appropriate adaptive responses to subsequent related stimuli. Elucidation of the molecular and cellular mechanisms underlying neural plasticity is a major goal of neuroscience research, and significant advances have been made in recent years. These mechanisms include regulation of signal transduction and gene expression, and also structural alterations of neuronal spines and processes, and even the birth of new neurons in the adult brain. Altered plasticity could thereby contribute to psychiatric and neurological disorders. This article revievi/s the literature demonstrating altered plasticity in response to stress, and evidence that chronic antidepressant treatment can reverse or block the effects, and even induce neural piasiicity-iike responses. Continued elucidation of the mechanisms underlying neural plasticity will lead to novel drug targets that could prove to be effective and rapidly acting therapeutic interventions.     

Hostility changes following antidepressant treatment: Relationship to stress and negative thinking

It is unclear whether changes in hostility following treatment are primarily related to improvement in depressive symptoms or are also closely associated with reductions in negative thinking or perceived stress. We evaluated 94 outpatients with major depression before and after eight weeks of fluoxetine treatment by administering the Symptom Questionnaire (SQ) Hostility Scale, the Hamilton Rating Scale for Depression (HAM-D), the Cognitions Questionnaire (CQ) and the Perceived Stress Scale (PSS). We observed significant elevations in scores on these questionnaires in depressed patients as compared to normal controls. Following treatment with fluoxetine, there was a statistically significant reduction in scores on all four questionnaires. We observed that changes in SQ Hostility were significantly positively related to changes in both depression severity and perceived stress, with these relationships remaining significant after adjusting for gender and baseline SQ Hostility. The relationship between SQ Hostility changes and reductions in negative thinking became significant only after adjusting for gender and baseline SQ hostility. Our results suggest that the marked decrease in hostility following antidepressant treatment is related to a reduction in depressive symptoms, stress levels and negative thinking.     

Neural responses to happy facial expressions in major depression following antidepressant treatment

OBJECTIVE: Processing affective facial expressions is an important component of interpersonal relationships. However, depressed patients show impairments in this system. The present study investigated the neural correlates of implicit processing of happy facial expressions in depression and identified regions affected by antidepressant therapy. METHOD: Two groups of subjects participated in a prospective study with functional magnetic resonance imaging (fMRI). The patients were 19 medication-free subjects (mean age, 43.2 years) with major depression, acute depressive episode, unipolar subtype. The comparison group contained 19 matched healthy volunteers (mean age, 42.8 years). Both groups underwent fMRI scans at baseline (week 0) and at 8 weeks. Following the baseline scan, the patients received treatment with fluoxetine, 20 mg daily. The fMRI task was implicit affect recognition with standard facial stimuli morphed to display varying intensities of happiness. The fMRI data were analyzed to estimate the average activation (overall capacity) and differential response to variable intensity (dynamic range) in brain systems involved in processing facial affect. RESULTS: An attenuated dynamic range of response in limbic-subcortical and extrastriate visual regions was evident in the depressed patients, relative to the comparison subjects. The attenuated extrastriate cortical activation at baseline was increased following antidepressant treatment, and symptomatic improvement was associated with greater overall capacity in the hippocampal and extrastriate regions. CONCLUSIONS: Impairments in the neural processing of happy facial expressions in depression were evident in the core regions of affective facial processing, which were reversed following treatment. These data complement the neural effects observed with negative affective stimuli.     

Neural plasticity and stress induced changes in defense in the rat

We investigated the effects of predator stress on behavior and amygdala afferent and efferent neural transmission in rats. Pathways studied were: ventral angular bundle input to the basolateral amygdala; central and basolateral amygdala output to the periaqueductal gray (PAG). Predator stress was ‘anxiogenic’ in elevated plus maze, light/dark box and acoustic startle tests one week after stress. Lasting changes were also observed in neural transmission. Predator stress appeared to potentiate right and depotentiate left hemisphere afferent amygdala transmission. In contrast, predator stress potentiated amygdala efferent transmission to right and left PAG, depending on the amygdala nucleus stimulated. Paired pulse and intensity series analysis suggests that transmission changes may be postsynaptic or presynaptic, depending on the pathway. Path analysis relating brain and behavioral changes suggests that potentiation and depotentiation in both hemispheres participate jointly in effecting some, but not all, of the behavioral changes produced by predator stress. Potentiation in left hemisphere amygdala afferents and efferents predicts anxiolytic-like effects, while potentiation in the right hemisphere amygdala afferents predicts anxiogenic-like effects. Path analysis also supports the view that changes in different neural systems mediate changes in different behaviors. These findings have their parallel in studies in the cat, but there are species differences.     

Regulation of neurogenesis and gliogenesis by stress and antidepressant treatment

Structural and morphological changes in limbic brain regions are associated with depression, chronic stress and antidepressant treatment, and increasing evidence supports the hypothesis that dysregulation of cell proliferation contributes to these effects. We review the morphological alterations observed in two brain regions implicated in mood disorders, the prefrontal cortex and hippocampus, and discuss the similarities and differences of the cellular consequences of chronic stress. We briefly discuss the proposed mechanisms implicated in neuroplasticity impairments that result from stress and that contribute to mood disorders, with a particular interest in adult neurogenesis and gliogenesis. This information has contributed to novel antidepressant medication development that utilizes adult neurogenesis and gliogenesis as preclinical cellular markers for predicting antidepressant properties of novel compounds.     

Adherence to antidepressant treatment

Depression is a common disorder with painful symptoms and, frequently, social impairment and decreased quality of life. The disorder has a tendency to be long lasting, often with frequent recurrence of symptoms. The risk of relapse and the severity of the symptoms may be reduced by correct antidepressant medication. However, the medication is often insufficient, both in respect to dosage and length of time. The reasons for incorrect medication are many, with lack of adherence to treatment being the most important. Although some patients taking antidepressant medication experience side effects, this may not be the most frequent reason for immature discontinuation of treatment. Other reasons for decreased adherence have been investigated in recent years. The patient's beliefs about the disorder and beliefs about antidepressants, including lack of conviction that the medication is needed and fear of dependence of antidepressant medicine, have a great influence on adherence to treatment.     

Hippocampal neurogenesis: opposing effects of stress and antidepressant treatment

The hippocampus is one of several limbic brain structures implicated in the pathophysiology and treatment of mood disorders. Preclinical and clinical studies demonstrate that stress and depression lead to reductions of the total volume of this structure and atrophy and loss of neurons in the adult hippocampus. One of the cellular mechanisms that could account for alterations of hippocampal structure as well as function is the regulation of adult neurogenesis. Stress exerts a profound effect on neurogenesis, leading to a rapid and prolonged decrease in the rate of cell proliferation in the adult hippocampus. In contrast, chronic antidepressant treatment up-regulates hippocampal neurogenesis, and could thereby block or reverse the atrophy and damage caused by stress. Recent studies also demonstrate that neurogenesis is required for the actions of antidepressants in behavioral models of depression. This review discusses the literature that has lead to a neurogenic hypothesis of depression and antidepressant action, as well as the molecular and cellular mechanisms that underlie the regulation of adult neurogenesis by stress and antidepressant treatment.     

Gender differences in depression and response to antidepressant treatment.

Throughout their childbearing years, women are twice as likely as men to experience an episode of major depression. The exact etiology of this difference is unclear, but psychosocial and neurobiologic factors likely contribute. Clinicians should consider gender differences in the phenomenology of depression and response to antidepressant treatment when screening for depressive illness, selecting appropriate treatment and assessing therapeutic response. Treatment considerations when selecting an antidepressant should include the patient's gender, age, and, in women, menopausal status, including the use of concomitant HRT. Further research is needed to refine and extend the existing knowledge base regarding the effect of gender on treatment of depression, including the role of endogenous and exogenous gonadal hormones in response to antidepressant treatment.     

Males and females respond differently to controllability and antidepressant treatment.

BACKGROUND: Women are much more likely to suffer from stress-related mental illness than men; yet few, if any, animal models for such sex differences exist. Previously, we reported that exposure to an acute stressor enhances learning in male rats yet severely impairs learning in female rats. Here, we tested whether these opposite effects in males versus females could be prevented by establishing control over the stressor or by antidepressant treatment. METHODS: Learning was assessed using the hippocampal-dependent task of trace eyeblink conditioning. In the first experiment, groups of male and female rats were exposed to controllable or uncontrollable stress and trained. In a second experiment, they were exposed to an uncontrollable stressor after chronic treatment with the antidepressant fluoxetine (Prozac). In a final experiment, females were exposed to uncontrollable stress after acute treatment with fluoxetine. RESULTS: Establishing control over the stressful experience eliminated the detrimental effect of stress on learning in females as well as the enhancing effect of stress in males. Moreover, chronic but not acute treatment with fluoxetine prevented the learning deficit in females after exposure to stress. Treatment with fluoxetine did not alter the male response to stress. CONCLUSIONS: These data indicate that males and females not only respond in opposite directions to the same stressful event but also respond differently to controllability and antidepressant treatments.     

Rethinking psychopharmacotherapy: The role of treatment context and brain plasticity in antidepressant and antipsychotic interventions

Emerging evidence indicates that treatment context profoundly affects psychopharmacological interventions. We review the evidence for the interaction between drug application and the context in which the drug is given both in human and animal research. We found evidence for this interaction in the placebo response in clinical trials, in our evolving knowledge of pharmacological and environmental effects on neural plasticity, and in animal studies analyzing environmental influences on psychotropic drug effects. Experimental placebo research has revealed neurobiological trajectories of mechanisms such as patients’ treatment expectations and prior treatment experiences. Animal research confirmed that “enriched environments” support positive drug effects, while unfavorable environments (low sensory stimulation, low rates of social contacts) can even reverse the intended treatment outcome. Finally we provide recommendations for context conditions under which psychotropic drugs should be applied. Drug action should be steered by positive expectations, physical activity, and helpful social and physical environmental stimulation. Future drug trials should focus on fully controlling and optimizing such drug × environment interactions to improve trial sensitivity and treatment outcome.     

Response of emotional unawareness after stroke to antidepressant treatment.

OBJECTIVE: Unawareness of impairment (anosognosia) is a phenomenon associated with right hemisphere lesions. Unawareness of emotion has rarely been studied. METHODS: Patients (N = 50) with poststroke major depression were administered the Toronto, Ontario, Canada, Alexithymia Scale to assess impairment in identifying feelings (F1), describing feelings (F2), and externally oriented thinking (F3). After eight weeks of treatment with sertraline or fluoxetine, patients were reassessed. RESULTS: Alexithymia was significantly associated with right hemisphere lesions. Patients with alexithymia had a significant improvement in identifying and describing feelings, but not in externally oriented thinking. In addition, cognitive functions improved after antidepressant treatment in patients without alexithymia with left lesions only. On the contrary, functional activities of daily living and depressive symptoms improved both in patients with alexithymia and those without alexithymia. CONCLUSIONS: The unawareness of emotions is a common impairment after right hemisphere stroke. This disorder may be significantly improved by antidepressant treatment.     

Neural plasticity in schizophrenia

No current biological hypothesis can assimilate the genetic, environmental, and clinical features of schizophrenia. If, as some authors contend, environmental factors have important effects on the course of schizophrenia, then a fruitful research concern may be the adaptation of neuronal circuitry to environmental changes. The plasticity of neuronal connections has been studied by subjecting animals to neurosurgical lesions, brain electro-stimulation, and a variety of rearing environments. The present article approaches the schizophrenia research literature from a theoretical perspective which takes into account the plasticity of neuronal connections. In a speculative manner, it demonstrates how neural plasticity concepts can be invoked to explain the following seemingly disparate features of schizophrenia: the pharmacological support for the dopamine hypothesis, the delayed onset and offset of neuroleptic antipsychotic action, genetic and environmental influences in schizophrenia, the regional alterations in brain structure and function seen in chronic schizophrenic patients, and the various types of behavioral symptoms exhibited by schizophrenic patients. In view of the explanatory potential of neural plasticity concepts, a research program that focuses on these concepts seems warranted.     

Neural plasticity and memory: towards an integrated view.

The search for the neural determinants of learning and memory has recently focused on phenomena of neural plasticity. The present paper will try to review and relate the functional significance of such specific plastic changes within neural elements to the process of learning in several species and cell types. Pioneer work proposed in simple organisms and recent emerging evidence on the role of the NMDA receptor complex and calcium-related events will be analyzed in the light of experimental findings suggesting that common biochemical events may underlie different forms of experience-dependent neural adaptation.     

Mitochondrial plasticity of the hippocampus in a genetic rat model of depression after antidepressant treatment

Depressive disorders and the treatment thereof have been associated with a number of neuroplastic events, such as neurogenesis and synaptic remodeling in discrete areas of the brain. The associations of these events in changes regarding the energy supply have not been investigated. Here, we investigated the changes in mitochondrial plasticity and its correlation to morphological alterations of neuroplasticity in the hippocampus, both associated with a depressive phenotype, and after treatment, with antidepressant imipramine. Design‐based stereological methods were used to estimate the number and volume of mitochondria in CA1 of the hippocampus in two different strains of rats, the Sprague–Dawley (SD) and Flinders rats, which display a genetic susceptibility to depressive behavior, the Flinders‐sensitive line (FSL) and their corresponding controls, the Flinders‐resistant line (FRL). Results showed a significantly reduced number of mitochondria in CA1, which was significantly smaller in the untreated FSL saline group compared to the FRL group. However, the mean volume of mitochondria was significantly larger in the FSL saline group compared to the FRL saline group. Following treatment, the FSL imipramine group showed a significant increase in the number of mitochondria compared to the FSL saline group. Treatment with imipramine in the SD rats did not induce significant differences in the number of mitochondria. Our results indicate that depression may be related to impairments of mitochondrial plasticity in the hippocampus and antidepressant treatment may counteract with the structural impairments. Moreover, the changes in mitochondrial morphology and number are a consistent feature of neuroplasticity. Synapse, 2013. © 2012 Wiley Periodicals, Inc.     

Neural circuits mediating stress.

Stress has been linked to the pathophysiology and pathogenesis of mood and anxiety disorders. Over the past few years, our understanding of the brain and neuroendocrine circuits that are linked to the stress response have increased dramatically. This article reviews a series of animal and human studies aimed at understanding what are the pathways by which stress is perceived, processed, and transduced into a neuroendocrine response. We focus on the classic stress circuit: the limbic-hypothalamic-pituitary-adrenal (LHPA) axis. These studies indicate that the LHPA stress circuit is a complex system with multiple control mechanisms and that these mechanisms are altered in pathological states, such as chronic stress and depression. These studies also suggest that the interactions between the LHPA and other neurotransmitters, such as serotonin, may provide the neurobiological substrate by which stress may affect mood.     

Neural plasticity and treatment across the lifespan for motor deficits in cerebral palsy

The past decade of research in neuroscience and stroke rehabilitation has demonstrated that the adult brain is capable of recovery through physiological processes (often called 'plasticity'). Some of the recovery is spontaneous and some is a result of experience, including interventions such as physical therapy, which probably enhance or activate changes in brain structure and function. There is virtually no literature on physiological changes in the brains of children or adults with cerebral palsy (CP) after an intervention. It is unclear whether the principles of plasticity that have been deduced from animal models of stroke might also apply to children and adults with CP. But children with CP should have the potential to respond to experience in a similar way to adults, with the additional potential of regulation of neuronal development in response to injury. This article describes mechanisms of plasticity and a rehabilitation strategy to preserve the substrates for motor control in CP and then to apply later therapies for more refinement of motor control.