Properties of liver-related PVN-DMV neurons and their role in facilitating the effects of central insulin

Abstract

This proposal is complementary to the report already sent of my PhD project. One major life-threatening condition driving an organism's evolution is limited food availability. This food availability must include glucose since glucose is a major source of fuel for most organisms, including all mammals. Blood glucose homeostasis must also be tightly regulated since both extreme hyper - or hypoglycemia is harmful. Glucose-metabolism related pathologies are consequence of a sedentary lifestyle and bad eating behavior, but the mechanisms involved on this imbalance are still not totally understood. The insulin, a pancreatic peptide that is essential to the absorption of glucose by the majority of the cells, is able to develop peripherical action is response to increase of glucose plasma level. The insulin also exerts its action on central nervous system that affects the eating behavior, inhibition of hepatic glucose production and modulation of autonomic nervous system. Our previous study also identified the influence of central insulin on hepatic glucose release occurred through activation of parasympathetic motor output. Parasympathetic motor output to the liver is regulated by preganglionic cholinergic neurons located in the brainstem's dorsal motor nucleus of the vagus (DMV). We have shown that DMV-related neurons, retrograde labeled with fluorogold, do not respond to the direct action of insulin even its neurons have insulin receptors, indicating that other nucleus would be participating on the brain-liver communication after insulin action. Another key brain region in the integration of metabolic and autonomic regulation, then, is the paraventricular nucleus of hypothalamus (PVN), a heterogeneous nucleus with known autonomic-related projections to the brainstem, including the DMV. Using icv insulin microinjection combined with retrograde labelling from the DMV, we confirmed that central insulin injection increased the number of DMV projecting PVN neurons (PVN’DMV) expressing a marker for neuronal activity (FOS protein) compared to vehicle injection. In a recent published article with data from my PhD project, we have shown that oxytocin (OT) can evoke a depolarization the membrane of the DMV-liver related neurons and can increase the firing frequency of them. Taken together, these findings suggest OT (possibly of PVN origin) can increase the activity of liver-related DMV neurons. Therefore, the present proposal will determine if liver-related OT-PVN neurons serve as insulin sensors to influence DMV motor output. We hypothesize that OT liver-related PVN neurons are second order neurons to the DMV and that these OT-PVN neurons are intrinsically sensitive to insulin. Thus, we aim to identify the phenotype of PVN neurons that make up the hypothalamic-DMV-liver axis and whether these neurons serve as central insulin sensors by using temporal analysis of pseudorabies virus (PRV) retrograde viral transduction to establish a liver-related PVN to DMV circuit and to evaluate whether OT liver-related PVN neurons are sensitive to insulin using advanced electrophysiological experiments paired with retrograde PRV tracing and post-hoc immunohistochemistry reconstruction. (AU)