siRNA-mediated suppression of astroglial glutamate transporters GLT-1 and GLAST in infralimbic, but not prelimbic cortex, induces depressive-like behaviors in mice

Abstract

Major depression has been associated with alterations of the monoaminergic systems. However, recent evidence suggests that dysregulations of glutamatergic neurotransmission in the prefrontal cortex (PFC) are involved in the pathophysiology of depression. Previous studies showed that astrocytic glutamate transporter, GLT-1, blockade induces anhedonia, impaired memory and c-fos expression in rat PFC. Given the role of ventral cingulate areas in mood regulation, we hypothesized that reduced expression of GLAST and GLT-1 in the infralimbic cortex (IL) would induce depressive-like behaviors in mice due to dysfunctional glutamatergic neurotransmission. We knocked-down the expression of GLAST and GLT-1 to evoke an increase of excitatory neurotransmission in IL cortex. We microinjected small interfering RNA (siRNA) targeting GLAST or GLT-1 unilaterally into prelimbic (PrL) or IL cortices of mice and examined cellular and behavioral effects. Moreover, the extracellular levels of 5-HT in dorsal raphe nucleus (DRN) were determined by in vivo microdialysis. Local unilateral GLAST-siRNA microinfusion in the mouse IL (4.2 nmol) reduced selectively GLAST mRNA and protein levels to ~80% of control mice. GLAST knockdown mice exhibited depressive-like behaviors in the tail suspension test (TST, 125% of controls) and in the forced swim test (FST, 128% of controls) and anhedonia in the sucrose of preference test (SPT, 50% of controls). Likewise, intra-IL infusion of GLT-1-siRNA (4.2 nmol) reduced GLT-1 expression (~70% of controls), increased immobility time also in the TST (130% of controls) and in FST (118% of controls) and reduced preference for sucrose in SPT (56% of controls). In contrast, reduced GLAST or GLT-1 expression in the PrL cortex did not affect behavioral responses to stress. Moreover, GLAST- and GLT-1-siRNA administered animals showed a reduction in baseline 5-HT extracellular levels in the DRN (4% of controls), even if the responsiveness of the neurons to veratridine was not altered. The present results suggest that astroglial regulation of excitatory neurotransmission in the IL cortex plays a major role in mood control. GLAST and GLT-1 down-regulation impairs the normal clearance of synaptically released glutamate. Given the reciprocal connectivity between the ventral PFC and many cortical and subcortical areas, an increased excitatory function in IL likely translates into downstream alterations of other brain areas involved in major depression, such as the monoamine nuclei. Overall, these findings improve our understanding of the pathophysiology of major depression, and help to identify novel targets in antidepressant drug development.