Symptomatic and Disease-Modifying Effects of GABAA Receptor Positive Allosteric Modulation in a Mouse Model of Chronic Stress

Chronic stress is a major risk factor for developing depressive disorders and animal models of stress recapitulate behavioral, cellular and molecular changes that are observed in human depression. Individuals exposed to chronic stress, or patients with MDD experience mood and cognitive dysfunctions. This is in part due to neuronal shrinkage in brain regions involved in several cognitive functions such as the prefrontal cortex (PFC) and the hippocampus (HPC). Also in the context of depression and chronic stress, expression levels and function of the main inhibitory neurotransmitter GABA are reduced. Thus far, drugs targeting this GABA deficit have failed to produce beneficial effects due to broad activity at various GABA receptor subunits, including the 1-subunit, resulting in broad side effects. However, refined and selective activity at the 2/3/5-subunit is hypothesized to exert beneficial effect, devoid of side effects. Here, we show that GL-II-73 and GL-I-54 exert positive allosteric modulation at the 5, and 2/3/5-contianing GABAA receptors respectively, and that they are effective both independently and in combination. Using unpredictable chronic mild stress (UCMS) experiments in male and female C57BL/6 mice (n=12 per group), we showed that acute and chronic administration of a GL-II-73/GL-I-54 racemic mixture (termed "GL-RM") reduced anxiety-like phenotypes and reversed a working memory deficit in UCMS exposed mice. Brains from animals receiving chronic treatment were collected and stained using a Golgi staining technique. Using stereological approaches, neuronal morphology was reconstructed and dendritic length, spine count and spine density were assessed in pyramidal neurons of the PFC and hippocampus. Chronic GL-RM rescued spine density depletions caused by UCMS at apical and basal dendrites (PFC and CA1). Interestingly, spine densities in both brain regions were correlated to cognitive performance, confirming ameliorative benefits of GL-RM. Together, results support the value of selectively targeting GABAA receptors, excluding the 1-subunit, to overcome chronic stress-induced mood symptoms and cognitive deficits, as well as detriments in neuronal morphology. This study confirm results that were observed in old mice, using a 5-selective positive allosteric modulator, and reinforce the concept that the 2/3/5-containing GABAA receptor are suitable targets for the treatment of stress-induced disorders.