Role of glutamate transporters and glutamatergic system in cortical and limbic brain areas in an animal model of depression

Major depressive disorder affects around 5% of the world´s population. Most antidepressant drugs have focused mainly on monoamine neurotransmitters synaptic levels in the brain such as serotonin, noradrenaline and dopamine. However, the delayed latency of the therapeutic actions of antidepressants and the poor efficacy for some subpopulations of patients suggest that mechanisms beyond monoaminergic modulation at synapses may be involved in the antidepressant actions. Several lines of evidence indicate that the pathophysiology of depression is associated with dysregulation of glutamate system and clearance mechanisms in brain regions mediating cognitive-emotional behaviors. Chronic stress results in an increase in extracellular glutamate and dysregulation of the glutamatergic system in cortical and limbic brain areas in patients with MDD and animal models of depression. However, the mechanisms underlying the abnormal glutamatergic transmission in depression are incompletely understood. The excitatory amino acid transporter 3 (EAAT3) - a member of the high-affinity glutamate transporters - which plays an essential role in transporting glutamate across plasma membranes in neurons, and in maintaining extracellular glutamate concentrations below neurotoxic levels, may have a pivotal role in dysregulation of glutamatergic signaling associated to depression. This study aims to evaluate the consequences of unpredictable chronic mild stress (UCMS) on the expression of glutamate transporters and ionotropic receptors in the cortical-limbic brain areas in wild type (WT) mice; and to determine if increased EAAT3 expression in the forebrain in mice can reduce susceptibility to UCMS. On the other hand, we evaluated the consequences of psychosocial stress at the behavioral level in WT mice subjected to chronic social defeat stress (CSDS). WT mice subjected to UCMS and control group were tested to anxiety- and depressive- like behaviors. Moreover, we evaluated long-term memory using object location and recognition tasks. Protein levels of AMPA and NMDA receptors subunits and glutamate transporters were analyzed by western blot. Mice with EAAT3 overexpression driven by CaMKIIα-promoter (EAAT3glo/CMKII) and control (EAAT3glo) littermates were assessed to anxiety- and depressive- like behaviors, and memory tests in baseline and UCMS conditions. Longer immobility time was observed in the tail suspension test in WT mice susceptible to CSDS. However, no significant differences were found between resilient and susceptible individuals in anxiety-like behaviors and anhedonia. Apparently, social defeat in WT mice would only be concerning sociability to an unknown aggressive mouse and the despair behavior. In WT mice, we observed that chronic stress induced anxiety- and depressive-like behaviors and deficits in memory tests, in addition to increased EAAT1, NMDA receptors GluN2A and GluN2B subunits, and AMPA receptors GluA1 subunits protein levels in the hippocampus. Furthermore, in baseline conditions EAAT3glo/CMKII mice showed anxiety-like behavior in the open field test. Interestingly, mice with EAAT3 overexpression driven by CaMKIIα-promoter challenged to UCMS did neither show depressive-like behaviors nor impairment in sociability and memory. Hippocampal glutamatergic system alterations may underlie the depressive-like behaviors. Moreover, we suggest that EAAT3 overexpression in the forebrain in mice may be linked to a resilient phenotype to chronic stress.