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In recent years, the term serotonergic vulnerability (SV) has been used in scientific literature, but so far it has not been explicitly defined. This review article attempts to elucidate the SV concept. SV can be defined as increased sensitivity to natural or experimental alterations of the serotonergic (5-HTergic) system. Several factors that may disrupt the 5-HTergic system and hence contribute to SV are discussed, including genetic factors, female gender, personality characteristics, several types of stress and drug use. It is explained that SV can be demonstrated by means of manipulations of the 5-HTergic system, such as 5-HT challenges or acute tryptophan depletion (ATD). Results of 5-HT challenge studies and ATD studies are discussed in terms of their implications for the concept of SV. A model is proposed in which a combination of various factors that may compromise 5-HT functioning in one person can result in depression or other 5-HT-related pathology. By manipulating 5-HT levels, in particular with ATD, vulnerable subjects may be identified before pathology initiates, providing the opportunity to take preventive action. Although it is not likely that this model applies to all cases of depression, or is able to identify all vulnerable subjects, the strength of the model is that it may enable identification of vulnerable subjects before the 5-HT related pathology occurs. Molecular Psychiatry ( Keywords: serotonin; mood disorders; stress; genetics; sex; tryptophan Involvement of serotonin in depressionMood disorders are one of the most prevalent forms of mental illness. 1 According to the DSM-IV, the lifetime risk for major depressive disorder is between 10 and 25% for women and between 5 and 12% for men. 2 Depression has been studied intensively during the last few decades, but the psychological and neurobiological determinants of depression have not yet been precisely defined. Although several factors are known to contribute to the etiology of depression, it is not clear how these factors cause depression, and why some subjects become depressed while others remain unaffected. In terms of biological factors in the etiology of depression, there are several hypotheses. These include neurotransmitter dysfunctions (serotonin, 5-HT; dopamine, DA; norepinephrine, NE); dysregulations in hypothalamic-pituitary-adrenal (HPA) axis activity; and immune system imbalance. The aim of this article is not to discuss all of these hypotheses in detail, but to evaluate the role that serotonin may play within these hypothesized mechanisms.It is widely accepted that diminished serotonergic (5-HTergic) function is involved in the onset and course of depression. 3,4 The 5-HTergic system is a large and complex system. Although nearly all cell bodies of 5-HTergic neurons are located in the raphe nuclei in the brain stem, the axons of these neurons innervate almost the entire brain. The actions of 5-HT are mediated by 16 types and subtypes of receptors. 5 As the 5-HTergic system is assumed to be a modulatory system, the exact func...

Rationale One of the major complaints most people face during aging is an impairment in cognitive functioning. This has a negative impact on the quality of daily life and is even more prominent in patients suffering from neurodegenerative and psychiatric disorders including Alzheimer's disease, schizophrenia, and depression. So far, the majority of cognition enhancers are generally targeting one particular neurotransmitter system. However, recently phosphodiesterases (PDEs) have gained increased attention as a potential new target for cognition enhancement. Inhibition of PDEs increases the intracellular availability of the second messengers cGMP and/or cAMP. Objective The aim of this review was to provide an overview of the effects of phosphodiesterase inhibitors (PDE-Is) on cognition, the possible underlying mechanisms, and the relationship to current theories about memory formation. Materials and methodsStudies of the effects of inhibitors of different PDE families (2, 4, 5, 9, and 10) on cognition were reviewed. In addition, studies related to PDE-Is and blood flow, emotional arousal, and long-term potentiation (LTP) were described. Results PDE-Is have a positive effect on several aspects of cognition, including information processing, attention, memory, and executive functioning. At present, these data are likely to be explained in terms of an LTP-related mechanism of action. Conclusion PDE-Is are a promising target for cognition enhancement; the most suitable candidates appear to be PDE2-Is or PDE9-Is. The future for PDE-Is as cognition enhancers lies in the development of isoform-specific PDE-Is that have limited aversive side effects.