Effect of mTOR silencing in mouse infralimbic cortex on depressive-like behaviour

Background: Over the last years, there has been an increase in the number of scientific publications analyzing the role of different cellular pathways implicated in neural proliferation and plasticity, as well as the brain areas involved in the pathophysiology and treatment of Major Depressive Disorder (MDD). The modulation of neuronal activity in the infralimbic prefrontal cortex (IL) underlies the antidepressant and anxiolytic actions of the N-Methyl-D-aspartate (NMDA) antagonist ketamine, indicating the critical role of this area in the disease [1]. Regarding the glutamatergic system, ketamine, demonstrated a rapid onset of action due to an increased activation of the mammalian target of rapamycin (mTOR) pathway in the medial prefrontal cortex (mPFCx) [2], revolutionizing the treatment perspective of this psychiatric disorder. However, little is known about the role of mTOR in the neurobiology of this pathology.

Aim of the study: We hypothesized that a reduced expression of mTOR in the mPFCx would induce a depressive-like behaviour in mice. In this study, we have silenced mTOR using selective small interfering RNA molecules (siRNAs) administered in the infralimbic (IL) and prelimbic (PL) mPFCx.

Methods: We microinjected siRNAs specifically designed against mTOR (mTOR-siRNA) and artificial cerebrospinal fluid (aCSF) as control unilaterally into infralimbic (IL) or prelimbic (PL) cortices of C57BL/6J mice. We examined the behavioural and molecular effects using in situ hybridization, in vivo microdialysis in dorsal raphe nucleus (DRN), and behavioural tests (forced swimming test, FST, and tail suspension test, TST). Groups were statistically compared by one-way or two-way ANOVA, followed by Tukey's posthoc analysis.

Results: Unilateral infusion of mTOR-siRNA (40 μg/μl) into the IL cortex, induced a depressive-like behaviour 24 h post-administration in the FST (mTOR-siRNA 173.0 ± 6.2 s vs control 149.2 ± 5.2 s, p ˂ 0.05, n = 8–9), and 48 h post-administration in the TST (mTOR-siRNA 187.7 ± 7.4 s vs aCSF 155.2 ± 7.4 s, p ˂ 0.05, n = 7–9). On contrast, the infusion into the PL cortex did not reflect significant changes in the animals’ behaviour, in any of the performed paradigms.
mTOR-siRNA infusion into IL cortex reduced mTOR mRNA levels in the IL (19% vs controls, p ˂ 0.01, n = 5) and PL cortices (15% vs controls, p ˂ 0.05, n = 5), while the infusion into PL cortex decreased the levels of mRNA only in the PL area (16% vs controls; p ˂ 0.05, n = 5). Interestingly, in both IL and PL cortices, the mTOR siRNA administration, reduces the BDNF mRNA levels (7%, p ˂ 0.05 and 12%; p ˂ 0.01, respectively).
The mTOR-siRNA infusion into the IL cortex induced changes in neurotransmitter release in the dorsal raphe nucleus (DRN). After the administration of bicuculline (100 μM), the animals with mTOR silenced in IL cortex displayed an increase of 5-HT release in the DRN (p ˂ 0.05 vs controls, n = 4–5), suggesting a higher inhibitory activity in this brain area.

Conclusions: The present study suggests that mTOR in IL cortex (rodent equivalent of the ventral anterior cingulate cortex (Brodmann 25 area), but not in PL cortex, plays a major role in the depressive-like behaviour. This allows a better understanding of the biological basis of MDD and new approaches for antidepressant drug discovery.