Major depression is the leading cause of disability among Americans between the ages of 15 and 44 and is one of the top ten causes of morbidity and mortality worldwide. Current treatments are well tolerated but can lead to less than 50% remission rates among patients. Due to the fact that most antidepressant drugs act by increasing extracellular levels of serotonin (5-HT), it has been hypothesized that decreased levels of 5-HT might underlie major depressive illness. However, this theory has been difficult to test in a preclinical setting due to a lack of hyposerotonergic mouse models. The tryptophan hydroxylase 2 R439H knock-in mouse (Tph2KI) provides a novel system in which the consequences of chronic 5-HT deficiency can be examined. This mouse exhibits an 80% reduction in brain 5-HT as the result of a mutation in Tph2, the enzyme responsible for brain 5-HT synthesis. In addition, these mice recapitulate several physiological features often associated with depression, including inhibited prolactin secretion and hypothermic responses to serotonergic activation, and decreased levels of the 5-HT metabolite, 5-HIAA. Because of these physiologic parallels to the human condition, Tph2KI mice represent a highly relevant, naturalistic rodent model for studying the impact of low levels of 5-HT on cell biological processes and behaviors related to depression and antidepressant action. Work from the past decade has demonstrated that elevation of 5-HT promotes neurogenesis and has revealed a requirement for neurogenesis in several antidepressant-like effects in mice. Elevation of 5-HT using antidepressant drugs can also prevent the inhibition of neurogenesis and the depressive-like behaviors induced by exposure of rodents to chronic stress. However, whether decreased levels of 5-HT lead to impaired neurogenesis, altered antidepressant-like responses, or increased vulnerability to stress has not been determined. The proposed research will test the hypothesis that chronic 5-HT deficiency will lead to inhibition of baseline adult hippocampal neurogenesis, diminished responses to antidepressant drugs, and decreased resilience to chronic stress. This hypothesis will be evaluated through the performance of three specific aims: 1) To examine the effects of hyposerotonergia on baseline hippocampal neurogenesis. 2) To determine the neurogenic and behavioral responses of 5-HT deficient mice to antidepressants. 3) To study the neurogenic and behavioral consequences of chronic stress in Tph2KI mice. These experiments will provide some of the first experimental evidence regarding the impact of chronic hyposerotonergia on adult neurogenesis, antidepressant responses, and vulnerability to stress. The data obtained from the proposed study will address several prominent depression theories, including the Serotonin Deficiency, the Neurogenesis Deficiency, and the Diathesis-Stress Hypotheses of depression and may shed light on the pathophysiology underlying depressive-like states.

Public Health Relevance

Drugs that enhance serotonin neurotransmission are known to stimulate neurogenesis and are the primary pharmacotherapy in the treatment of major depression (MD), which is the leading cause of disability among Americans between the ages of 15 and 44. However, it has not been definitively proven whether serotoninergic or neurogenic dysfunctions exist in MD, or whether these deficiencies are sufficient to cause MD. The proposed research will evaluate the behavioral and cell biological (i.e. neurogenic) consequences of hyposerotonergia in mice and will enhance our understanding of the mechanisms underlying depressive-like behaviors and antidepressant-like responses.

National Institute of Health (NIH)
National Institute of Mental Health (NIMH)
Postdoctoral Individual National Research Service Award (F32)
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Special Emphasis Panel (ZRG1-F01-L (20))
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Li, Ingrid Y
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Duke University
Anatomy/Cell Biology
Schools of Medicine
United States
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Sachs, Benjamin D; Ni, Jason R; Caron, Marc G (2015) Brain 5-HT deficiency increases stress vulnerability and impairs antidepressant responses following psychosocial stress. Proc Natl Acad Sci U S A 112:2557-62
Sachs, Benjamin D; Rodriguiz, Ramona M; Tran, Ha L et al. (2015) Serotonin deficiency alters susceptibility to the long-term consequences of adverse early life experience. Psychoneuroendocrinology 53:69-81
Sachs, Benjamin D; Salahi, A Ayten; Caron, Marc G (2014) Congenital brain serotonin deficiency leads to reduced ethanol sensitivity and increased ethanol consumption in mice. Neuropharmacology 77:177-84
Sachs, Benjamin D; Caron, Marc G (2014) Chronic fluoxetine increases extra-hippocampal neurogenesis in adult mice. Int J Neuropsychopharmacol 18:
Jacobsen, Jacob P R; Plenge, Per; Sachs, Benjamin D et al. (2014) The interaction of escitalopram and R-citalopram at the human serotonin transporter investigated in the mouse. Psychopharmacology (Berl) 231:4527-40
Sachs, Benjamin D; Ni, Jason R; Caron, Marc G (2014) Sex differences in response to chronic mild stress and congenital serotonin deficiency. Psychoneuroendocrinology 40:123-9
Hara, Makoto R; Sachs, Benjamin D; Caron, Marc G et al. (2013) Pharmacological blockade of a ?(2)AR-?-arrestin-1 signaling cascade prevents the accumulation of DNA damage in a behavioral stress model. Cell Cycle 12:219-24
Sachs, B D; Jacobsen, J P R; Thomas, T L et al. (2013) The effects of congenital brain serotonin deficiency on responses to chronic fluoxetine. Transl Psychiatry 3:e291
Sachs, Benjamin D; Rodriguiz, Ramona M; Siesser, William B et al. (2013) The effects of brain serotonin deficiency on behavioural disinhibition and anxiety-like behaviour following mild early life stress. Int J Neuropsychopharmacol 16:2081-94
Dzirasa, Kafui; Kumar, Sunil; Sachs, Benjamin D et al. (2013) Cortical-amygdalar circuit dysfunction in a genetic mouse model of serotonin deficiency. J Neurosci 33:4505-13

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