BDNF overexpression in the ventral hippocampus promotes antidepressant- and anxiolytic-like activity in serotonin transporter knockout rats

Abstract

Brain-derived neurotrophic factor is one of the most studied proteins playing a pivotal role in neuroplasticity events and vulnerability and resilience to stress-related disorders. Most importantly, BDNF is decreased in depressive patients, and increased after antidepressant treatment. Additionally, BDNF was found to be reduced in a genetic subset of depression susceptible patients carrying the human polymorphism in the serotonin transporter promoter region (5-HTTLPR). The serotonin knockout rat (SERT-/-) is one of the animal models used to investigate the underlying molecular mechanisms behind the genetic susceptibility to depression in humans. SERT-/- rats present decreased BDNF levels, especially BDNF exon IV, in the prefrontal cortex (PFC) and ventral hippocampus (vHIP), and display anxiety- and depression-like behavior. To investigate whether upregulating BDNF in the vHIP would meliorate the phenotype of SERT-/- rats, we overexpressed BDNF locally into the rat brain by means of stereotaxic surgery and submitted the animals to behavioral challenges, including the sucrose consumption, the open field, and forced swim tests. Additionally, we measured hypothalamus-pituitary-adrenal (HPA)-axis reactivity. The results showed that lentivirus-induced BDNF IV overexpression in the vHIP of SERT-/- rats promoted higher sucrose preference and sucrose intake, on the first day of the sucrose consumption test, indicative for decreased anhedonia-like behavior. Moreover, it decreased immobility time in the forced swim test, suggesting adaptive passive coping. Additionally, BDNF upregulation increased the time spent in the center of a novel environment, implying decreased novel-induced anxiety-like behavior. Finally, it promoted a stronger decrease in plasma corticosterone levels 60 minutes after restraint stress. In conclusion, modulation of BDNF IV levels in the vHIP of SERT-/- rats led to a positive behavioral outcome placing BDNF upregulation in the vHIP as a potential candidate for the development new therapeutic approaches targeting the improvement of depressive symptoms.