The diet-microbiome tango: how nutrients lead the gut brain axis.

Nutrients and the microbiome have a profound impact on the brain by influencing its development and function in health and disease. The mechanisms by which they shape brain function have only started to be uncovered. Here we propose that the interaction of diet with the microbiome is at the core of most mechanisms by which gut microbes affect host brain function. The microbiome acts on the host by altering the nutrients in the diet and by using them as precursors for synthetizing psychoactive metabolites. Diet is also a major modulator of gut microbiome composition making this another key mechanism by which they affect the host brain. Nutrient-microbiome-host interactions therefore provide an overarching framework to understand the function of the gut-brain axis.

Nutrient homeostasis - translating internal states to behavior.

Behavioral neuroscience aims to describe a causal relationship between neuronal processes and behavior. Animals' ever-changing physiological needs alter their internal states. Internal states then alter neuronal processes to adapt the behavior of the animal enabling it to meet its needs. Here, we describe nutrient-specific appetites as an attractive framework to study how internal states shape complex neuronal processes and resulting behavioral outcomes. Understanding how neurons detect nutrient states and how these are integrated at the level of neuronal circuits will provide a multilevel description of the mechanisms underlying complex feeding and foraging decisions.

Cerebellar Learning Properties Are Modulated by the CRF Receptor.

Corticotropin-releasing factor (CRF) and its type 1 receptor (CRFR1) play an important role in the responses to stressful challenges. Despite the well established expression of CRFR1 in granular cells (GrCs), its role in procedural motor performance and memory formation remains elusive. To investigate the role of CRFR1 expression in cerebellar GrCs, we used a mouse model depleted of CRFR1 in these cells. We detected changes in the cellular learning mechanisms in GrCs depleted of CRFR1 in that they showed changes in intrinsic excitability and long-term synaptic plasticity. Analysis of cerebella transcriptome obtained from KO and control mice detected prominent alterations in the expression of calcium signaling pathways components. Moreover, male mice depleted of CRFR1 specifically in GrCs showed accelerated Pavlovian associative eye-blink conditioning, but no differences in baseline motor performance, locomotion, or fear and anxiety-related behaviors. Our findings shed light on the interplay between stress-related central mechanisms and cerebellar motor conditioning, highlighting the role of the CRF system in regulating particular forms of cerebellar learning.SIGNIFICANCE STATEMENT Although it is known that the corticotropin-releasing factor type 1 receptor (CRFR1) is highly expressed in the cerebellum, little attention has been given to its role in cerebellar functions in the behaving animal. Moreover, most of the attention was directed at the effect of CRF on Purkinje cells at the cellular level and, to this date, almost no data exist on the role of this stress-related receptor in other cerebellar structures. Here, we explored the behavioral and cellular effect of granular cell-specific ablation of CRFR1 We found a profound effect on learning both at the cellular and behavioral levels without an effect on baseline motor skills.

Inferior olive CRF plays a role in motor performance under challenging conditions.

A well-coordinated stress response is pivotal for an organisms' survival. Corticotropin-releasing factor (CRF) is an essential component of the emotional and neuroendocrine stress response, however its role in cerebellar functions is poorly understood. Here, we explore the role of CRF in the inferior olive (IO) nucleus, which is a major source of input to the cerebellum. Using a CRF reporter line, in situ hybridization and immunohistochemistry, we demonstrate very high levels of the CRF neuropeptide expression throughout the IO sub-regions. By generating and characterizing IO-specific CRF knockdown and partial IO-CRF knockout, we demonstrate that reduction in IO-CRF levels is sufficient to induce motor deficiency under challenging conditions, irrespective of basal locomotion or anxiety-like behavior. Furthermore, we show that chronic social defeat stress induces a persistent decrease in IO-CRF levels, and that IO-CRF mRNA is upregulated shortly following stressful situations that demand a complex motor response. Taken together our results indicate a role for IO-CRF in challenge-induced motor responses.

MicroRNA as repressors of stress-induced anxiety: the case of amygdalar miR-34.

The etiology and pathophysiology of anxiety and mood disorders is linked to inappropriate regulation of the central stress response. To determine whether microRNAs have a functional role in the regulation of the stress response, we inactivated microRNA processing by a lentiviral-induced local ablation of the Dicer gene in the central amygdala (CeA) of adult mice. CeA Dicer ablation induced a robust increase in anxiety-like behavior, whereas manipulated neurons survive and appear to exhibit normal gross morphology in the time period examined. We also observed that acute stress in wild-type mice induced a differential expression profile of microRNAs in the amygdala. Bioinformatic analysis identified putative gene targets for these stress-responsive microRNAs, some of which are known to be associated with stress. One of the prominent stress-induced microRNAs found in this screen, miR-34c, was further confirmed to be upregulated after acute and chronic stressful challenge and downregulated in Dicer ablated cells. Lentivirally mediated overexpression of miR34c specifically within the adult CeA induced anxiolytic behavior after challenge. Of particular interest, one of the miR-34c targets is the stress-related corticotropin releasing factor receptor type 1 (CRFR1) mRNA, regulated via a single evolutionary conserved seed complementary site on its 3' UTR. Additional in vitro studies demonstrated that miR-34c reduces the responsiveness of cells to CRF in neuronal cells endogenously expressing CRFR1. Our results suggest a physiological role for microRNAs in regulating the central stress response and position them as potential targets for treatment of stress-related disorders.

Resilience to social stress coincides with functional DNA methylation of the Crf gene in adult mice.

DNA methylation regulates gene transcription and has been suggested to encode psychopathologies derived from early life stress. We found that methylation regulated the expression of the Crf (also known as Crh) gene and that chronic social stress in adult mice induced long-term demethylation of this genomic region. Demethylation was observed only in the subset of defeated mice that displayed social avoidance and site-specific knockdown of Crf attenuated the stress-induced social avoidance.