Intranasal delivery of a conjugated serotonin transporter siRNA evokes antidepressant responses in stressed mice via internalization into serotonergic neurons

Abstract

Major depression is a widespread debilitating illness with great socio-economic impact. Current antidepressants, which inhibit serotonin transporter (SERT) and/or norepinephrine transporter (NET), display limited efficacy and slow onset of action. In contrast, new findings support that SERT gene silencing by inhibitory RNA (siRNA) produces a more rapid and potent antidepressant response than pharmacological blockade of SERT. Here we examined the effect of RNAi-induced SERT suppression on molecular and cellular changes mediating antidepressant-like responses in an animal model of depression, focusing on signaling cascades involved in synaptic plasticity. We used a SERT-siRNA molecule conjugated with sertraline (C-SERT-siRNA) as previously reported, for specific intracellular delivery to serotonin (5-HT) neuron. Intranasal C-SERT-siRNA administration for 7-day (2.1 nmol/day) to C57Bl/6J mice reduced SERT-mRNA expression (42±4%) after endocytosis in TPH-positive cells. This was accompanied by a selective and widespread reduction of SERT-binding sites (30%-50%) in the most brain regions analyzed. Furthermore, decreased SERT expression and subsequent reduction in 5-HT1A-autoreceptor function markedly increased the extracellular 5-HT concentration in forebrain of C-SERT-siRNA-treated mice. Additionally, C-SERT-siRNA treatment (7-day) increased cell proliferation (Ki-67-positive cells: 151±13% of vehicle-treated mice), neurogenesis (NeuroD-positive and DCX-positive cells: 145±8% and 120±4%, respectively) and dendrite branching in dentate gyrus. Hippocampal neuronal plasticity was associated with an augmented BDNF, VEGF, TRKb, neuritin and PSD95 mRNA expression. Remarkably, these effects occurred much earlier and were of greater magnitude than those evoked by fluoxetine (10 mg/kg/day, i.p., 28-day). Finally, C-SERT-siRNA (7-day), but not fluoxetine, reversed the behavioral dysfunction displayed by a depression mouse model related with the chronic corticosterone consumption: a) it reduced immobility in the tail suspension test, b) it reduced latency to feed in the novelty suppressed feeding test and c) it decreased anhedonia in the preference sucrose test. Our results highlight the critical role of post-transcriptional SERT regulation in stress/depression model and in neuroplasticity-related antidepressant responses. Moreover, intranasal C-SERT-siRNA administration is a useful delivery route with potential therapeutic application. Although many hurdles remain, the present data represent a step forward for the use of RNAi-based medicines to improve current limitations of antidepressant treatments.