Antidepressant treatment differentially affects the phenotype of high and low stress reactive mice

Stress-Related Dysfunction of Adult Hippocampal Neurogenesis-An Attempt for Understanding Resilience?

Newborn neurons in the adult hippocampus are regulated by many intrinsic and extrinsic cues. It is well accepted that elevated glucocorticoid levels lead to downregulation of adult neurogenesis, which this review discusses as one reason why psychiatric diseases, such as major depression, develop after long-term stress exposure. In reverse, adult neurogenesis has been suggested to protect against stress-induced major depression, and hence, could serve as a resilience mechanism. In this review, we will summarize current knowledge about the functional relation of adult neurogenesis and stress in health and disease. A special focus will lie on the mechanisms underlying the cascades of events from prolonged high glucocorticoid concentrations to reduced numbers of newborn neurons. In addition to neurotransmitter and neurotrophic factor dysregulation, these mechanisms include immunomodulatory pathways, as well as microbiota changes influencing the gut-brain axis. Finally, we discuss recent findings delineating the role of adult neurogenesis in stress resilience.

Turning strains into strengths for understanding psychiatric disorders

There is a paucity in the development of new mechanistic insights and therapeutic approaches for treating psychiatric disease. One of the major challenges is reflected in the growing consensus that risk for these diseases is not determined by a single gene, but rather is polygenic, arising from the action and interaction of multiple genes. Canonically, experimental models in mice have been designed to ascertain the relative contribution of a single gene to a disease by systematic manipulation (e.g., mutation or deletion) of a known candidate gene. Because these studies have been largely carried out using inbred isogenic mouse strains, in which there is no (or very little) genetic diversity among subjects, it is difficult to identify unique allelic variants, gene modifiers, and epigenetic factors that strongly affect the nature and severity of these diseases. Here, we review various methods that take advantage of existing genetic diversity or that increase genetic variance in mouse models to (1) strengthen conclusions of single-gene function; (2) model diversity among human populations; and (3) dissect complex phenotypes that arise from the actions of multiple genes.

Animal models of major depression: drawbacks and challenges

Major depression is a leading contributor to the global burden of disease. This situation is mainly related to the chronicity and/or recurrence of the disorder, and to poor response to antidepressant therapy. Progress in this area requires valid animal models. Current models are based either on manipulating the environment to which rodents are exposed (during the developmental period or adulthood) or biological underpinnings (i.e. gene deletion or overexpression of candidate genes, targeted lesions of brain areas, optogenetic control of specific neuronal populations, etc.). These manipulations can alter specific behavioural and biological outcomes that can be related to different symptomatic and pathophysiological dimensions of major depression. However, animal models of major depression display substantial shortcomings that contribute to the lack of innovative pharmacological approaches in recent decades and which hamper our capabilities to investigate treatment-resistant depression. Here, we discuss the validity of these models, review putative models of treatment-resistant depression, major depression subtypes and recurrent depression. Furthermore, we identify future challenges regarding new paradigms such as those proposing dimensional rather than categorical approaches to depression.

Maternal exercise increases but concurrent maternal fluoxetine prevents the increase in hippocampal neurogenesis of adult offspring

Treating postpartum depression (PPD) with pharmacological antidepressants like fluoxetine (FLX) is complicated because these drugs can remain active in breast milk and potentially affect infant development. Alternatively, non-pharmacological treatments such as exercise are associated with beneficial effects on infant development but its potential ability to counter the effects of PPD are largely unknown. To investigate this, we treated dams with corticosterone (CORT) or vehicle (sesame oil) from postpartum days 2-25 to model PPD. Within oil and CORT treatments, dams were also assigned to one of these treatments: 1) exercise (voluntary running wheel) + FLX (10 mg/kg, i.p.), 2) exercise + saline (vehicle for FLX), 3) no exercise + FLX, 4) no exercise + saline. Both male and female offspring were analyzed, and this generated a total of 16 experimental groups for this study. Adult male and female offspring (125 d old) of these dams were tested for anxiety-like behavior in the novelty suppressed feeding test and stress reactivity in the dexamethasone suppression test. Hippocampal tissue was processed for doublecortin, a protein expressed in immature neurons. Regardless of sex, maternal exercise increased neurogenesis in the dorsal hippocampus of adult offspring, but concurrent exposure to maternal fluoxetine prevented this effect. Exposure to either maternal exercise or maternal FLX facilitated HPA negative feedback in adult males but not females. Maternal postpartum CORT also facilitated HPA feedback in adult offspring of both sexes. Collectively, these data indicate that maternal exercise increased dorsal hippocampal neurogenesis in both sexes but differentially affected offspring HPA axis based on sex. Alternatively, maternal postpartum FLX facilitated HPA axis negative feedback only in males. These findings indicate that different types of maternal interventions bear long-term effects on offspring outcome with implications for treating PPD.