Neurogenic Plasticity in a Serotonin Transporter Knockout Model: Effects of Maternal and Offspring Genotype

Major depressive disorder (MDD) is debilitating mental disorder that is increasing in prevalence. Many theories have tried to explain the aetiology of depression including the classic monoamine deficiency hypothesis and the newer neurogenic hypothesis. The finding that selective serotonin transporter inhibitors (SSRIs) work by increasing extracellular serotonin levels in the brain and have antidepressant effects has formed the basis of the most widely accepted theory of depression, the monoamine hypothesis. However, a genetic reduction in human and animal serotonin reuptake transporters, which also increases extracellular serotonin, is associated with depressive symptomology. This paradox is not explained by the monoamine hypothesis. The key difference between these two scenarios is that genetically induced increases in serotonin occur from development onward, while SSRIs increase serotonin only in adulthood. Furthermore, SSRIs typically take several weeks to confer a therapeutic effect. This finding has led to the hypothesis that, rather than acute monoamine-increasing effects, it is the downstream effects of such increases on neurogenesis and neural plasticity which confer antidepressant effects. To further elucidate the neurobiology of depression, this study sought to examine the effects of genetically increasing serotonin on early postnatal neurogenesis in a serotonin knockout rat model using BrdU immunohistochemistry. We examined both the offspring and maternal genotype effects. We found that SERT-/- offspring had the highest levels of neurogenesis compared with SERT+/- and SERT+/+ at postnatal day 7. In addition we found a maternal genotype effect with SERT+/+ offspring born and reared by SERT+/- mothers having lower neurogenesis compared to SERT+/+ offspring from SERT+/+ mothers. The potential effects of maternal caregiving, neuroplasticity in altered mood and stress responses and the role of 5-HT receptors are discussed.