Conference Agenda

Stress is a key risk factor for neuropsychiatric disorders, including depression. Accordingly, the Chronic Mild Stress (CMS) rodent model of depression is widely used in the preclinical setting to study both etiopathogenetic and antidepressant mechanisms. The individual response to stressful stimuli may induce adaptive or maladaptive changes, respectively leading to stress resilience or vulnerability.

Ketamine has recently emerged as the first rapid-acting antidepressant drug effective in patients with treatment-resistant depression. However, the molecular mechanisms underlying ketamine response/non-response are still largely unknown.

In this study, we used the CMS model to unravel ketamine-induced transcriptional changes in the hippocampus that were associated with behavioral response to the drug.

CMS was applied for 5 weeks on male rats, sucrose preference test was used to evaluate the anhedonic phenotype, and acute subanesthetic ketamine (10 mg/kg) was intraperitoneally injected to stress vulnerable animals 24 h before sacrifice. The sucrose preference test allowed to classify the rats as stress resilient or vulnerable and ketamine responder or non-responder. The hippocampus was collected to obtain RNA and transcriptional changes were evaluated by RNA-seq analysis. Transcriptomic analysis revealed specific changes in the different experimental groups. Enrichment analysis showed differences between control and vulnerable rats and between ketamine responders and non-responders in the expression of genes involved in pathways such as glutamatergic synapse, synaptic signaling and organization, modulation of neuronal and dendritic spine morphology.

The integrated results of this study promise to better elucidate ketamine rapid mechanism of action.

Stress is a major risk factor in the onset of mood disorders. Stress-based animal models of depression and clinical studies have shown that depressive phenotypes are associated with synaptic dysfunction and dendritic simplification in cortico-limbic glutamatergic areas. On the other hand, ketamine rapid antidepressant effect is accompanied by the rescue of the changes induced by chronic stress in the hippocampus and prefrontal cortex of animal models of depression. MicroRNAs are regulators of complex patterns of gene/protein expression changes in the brain and, by fine-tuning their target genes, they take part in processes such as stress-response and neuroplasticity.

We used the Chronic Mild Stress (CMS) animal model of depression and in vitro primary neuronal cultures to study whether mechanisms of stress response and ketamine action involved changes in dendrite remodeling of neurons, together with alteration of microRNAs. We focused on miR-9-5p and miR-135 due to their known involvement in stress response and dendritic spine remodeling, as well as in antidepressant effects.

We found that miR-9-5p levels were selectively reduced in the hippocampus of CMS-vulnerable rats and in primary hippocampal neurons incubated with the stress hormone corticosterone. In both models, decreased miR-9-5p levels were associated with dendritic simplification. On the other hand, the expression of miR-135 was reduced in the PFC of vulnerable animals and the downregulation of miR-135 in primary neurons reduced the density of dendritic spines while its overexpression exerted an opposite effect. Bioinformatically predicted targets with a possible impact on neuronal remodelling were validated. Intriguingly, acute subanesthetic ketamine rescued both dendritic retraction and miR9-5p levels in the hippocampus of CMS rats. Similar results were obtained in hippocampal cultures incubated with corticosterone and exposed to acute ketamine.

miR-9 and miR-135 play important roles in the regulation of the stress response in corticolimbic areas with mechanisms promoting neuronal remodeling.

Major Depressive Disorder (MDD) patients develop treatment resistant depression (TRD) in approximately 30% of the patients. Among the different causes that make TRD so challenging in both clinical and research contexts, major roles are played by the inadequate understanding of MDD pathophysiology and the limitations of current pharmacological treatments. Nevertheless, the field of psychiatry is facing exciting times. Combined with recent advances in genome editing techniques, human induced pluripotent stem cell (hiPSC) technology is offering novel and unique opportunities in both disease modeling and drug discovery. Moreover, the field is approaching the advent of (es)ketamine, a glutamate N-methyl-d-aspartate (NMDA) receptor antagonist, claimed as one of the first and exemplary agents with rapid (in hours) antidepressant effects, even in TRD patients. Although ketamine seems poised to transform the treatment of depression, its exact mechanisms of action are still unclear but greatly demanded, as the resulting knowledge may provide a model to understand the mechanisms behind rapid-acting antidepressants, which may lead to the discovery of novel compounds for the treatment of depression. Thus, to better clarify the mechanism of action of ketamine, we used iPSCs derived neurons that have been characterized for transcriptomic and functional changes induced by acute treatment with ketamine. Our results aim to better understand the activity of ketamine on human neurons in order to develop new therapeutic approaches for patients with TRD.

Exposure to stress can be differently perceived by individuals depending on their level of stress resilience or vulnerability. Vulnerability to stress increases the risk of developing several neuropsychiatric disorders, including post-traumatic stress disorder and depression. Thus, exploring the neurobiology of the resilient and vulnerable response to acute stress is critical for the treatment and prevention of stress-related disorders and represents a currently unmet need. Using a model of acute inescapable footshock stress, we demonstrated the selective presence of anhedonic behavior, glial reactivity, and neuronal changes in the prefrontal cortex of vulnerable but not resilient rats, both classified according to their anhedonic-like behavior. Resilient rats instead showed a neurotrophic response, characterized by increased protein expression of brain-derived neurotrophic factor (BDNF) and microtubule-associated protein (MAP)2. Considering its anti-inflammatory and neuroprotective properties, we studied the effect of a single subanesthetic i.p. administration of ketamine (10 mg/kg) on the observed maladaptive changes. Ketamine selectively blocked the excessive release of astrocytic S100B, microglial reactivity, activation of NF-kB, and increased expression of interleukin-18 and tumor necrosis factor-α found in vulnerable animals, together with restoring the elevated protein levels of astrocytic connexin 43, linked to excessive permeability of the hemichannel. At the neuronal level, ketamine counteracted the decreased expression of MAP2 and the increased levels of glial-derived neurotrophic factor (GDNF) that were selectively detected in susceptible rats. Collectively, these results suggest glial reactivity, alteration of brain factors, and the resulting neuronal damage as critical factors characterizing vulnerability to acute traumatic stress and propose ketamine as a pro-resilience agent able to potentially protect against the development of stress-induced psychiatric disorders.