Book Description

Major Depressive Disorder (MDD) is a debilitating condition that affects 6.7% of American adults and poses a serious public health concern. Meta analyses of clinical studies have identified chronic, low-grade inflammation as a predisposing factor in the development of MDD. Obesity, along with a poor diet, contributes to inflammation, which is especially concerning given the high prevalence of obesity in the United States. Longitudinal studies have established a link between obesity and depression, indicating comorbidity among these two disorders. Unhealthy diets with an excessive caloric content, common in modern Western nations, are a risk factor in the development of obesity and may be the source of obesitys increasing pervasiveness. To better study obesity-induced inflammation in the context of mood disorders, high-fat diet (HFD) treatment provides a reliable method for generating animal models of obesity, particularly using rodents. These HFD models, compared to control diet (CD) treated mice, also display the characteristic phenotypes expected of a model of obesity, including chronic metabolic inflammation and insulin resistance.Neurobiological studies investigating the effects of HFD on the brain have found that rodents treated with HFD exhibit behavioral deficits in anxiety-like behavior, anhedonia, and memory. To further investigate these neurological effects of HFD, and their interactions with genetic and neurophysiological factors, the behavioral consequences of HFD treatment were examined in two different mutant mouse models with an altered ratio of neuronal excitation and inhibition (the E:I ratio). Altered E:I ratio is thought to underlie MDD, and imbalance in the E:I ratio has been shown to produce anxiety- and depression-related behavioral phenotypes in mice. The first model (2+/-) involves mice that are globally heterozygous for the 2 subunit of GABAA receptors. In this model, GABAergic inhibition is reduced, leading to an increased E:I ratio. The 2+/- model has consistently produced an anxious-depressive-like phenotype in behavioral assessments. This phenotype can be diminished by treatment with antidepressants, thus implicating increased E:I ratio in the onset of mood disorders. A second mouse model (SSTCre:2f/f) involves mice in which somatostatin-positive (SST+) interneurons were disinhibited by selectively deleting the 2 subunit of GABAA receptors in these cells. In this model, inhibitory transmission onto hippocampal pyramidal cells was increased by disinhibiting SST+ interneurons, thereby reducing the overall E:I ratio in these mice. Behaviorally, these mice exhibit robust anxiolytic and antidepressant-like phenotypes. Together, results from studies involving 2+/- and SSTCre:2f/f models suggest that an impaired E:I ratio underlies anxiety- and depression-related behavior. The hypothesis examined in the present study, which aims to investigate the interactions between HFD and altered E:I ratio on behavior, is that increasing the E:I ratio (2+/- model) will exacerbate HFD-induced anxiety- and depression-like behavior, including defects in locomotion, grooming, and memory, while reducing the E:I ratio (SSTCre:2f/f model) will reduce HFD-induced anxiety- and depression-like behavior, including defects in locomotion, grooming, and memory. The results show that HFD treatment led to reductions in locomotion and grooming behavior of both the WT control and 2+/- mutant mice in the OFT and SSPT, respectively. However, the effects of HFD on 2+/- mice were not larger than in WT mice, so only an overall diet effect was present. Results from tests assessing short-term working (Y-maze) and recognition (NOR) memory indicate that the effects of diet and genotype are nonadditive, and that HFD treatment and altered E:I ratio impair behavior via different mechanisms. SSTCre:2f/f mutant mice presented an anxiolytic phenotype and increased locomotion in the EPM, consistent with studies in the literature. Additionally, HFD treatment of SSTCre:2f/f mice reduced locomotion in the OFT and EPM, and also decreased grooming behavior in the SSPT. Overall, the behavioral results observed for CD-treated animals in these experiments did not reproduce all of the phenotypes of 2+/- and SSTCre:2f/f mice published in the literature, most likely because the genetic backgrounds of the mice analyzed here (C57BL/6J and mixed 129X1/SvJ/C57BL/6J, respectively) were different from the mice described in earlier studies. Due to these unexpected results, additional behavioral experiments will be needed to further assess possible interactions between HFD treatment and altered E:I balance on anxiety- and depression-related behavior.