Acute and chronic effects of GLP-1R agonism on NPY/AgRP neuronal activity

Glucagon-like peptide 1 (GLP-1)-based therapeutics have profound effects on body weight and blood glucose management. GLP-1 cells are located in both the periphery and the caudal medulla, specifically within the nucleus tractus solitarius (NTS). However, the effects of GLP-1 or long-acting GLP-1 receptor agonists (GLP1Rags) on synaptic/cellular properties in the brain and their contribution to metabolic changes are not entirely understood.

Based on pilot data, we hypothesize that a DMH GLP-1R → NPY/AgRP neuron circuit is a target for brain-derived GLP-1 neurons and GLP-1Rags. Our objective is to determine if DMH GLP-1R neurons are required for the NTS GLP-1-induced effects on NPY/AgRP activity and metabolism. The project will use

chemogenetics, electrophysiology, and in-vivo calcium imaging to investigate these questions. These experiments will potentially bridge our understanding of the regulation and physiological roles of the GLP-1system in the brain and in the treatment of metabolic disease.

Glucagon-like-peptide-1 receptor agonists (GLP-1Rags) have profound anti-diabetic and antiobesity effects, but the neural systems responsible for mediating these effects are not fully understood. The endogenous function of brain-derived GLP-1, located in the Nucleus Tractus Solitarius (NTS), may provide insight into the effects of GLP-1Rags in the brain.

In particular, NTS GLP-1 neurons target multiple brain regions and reduce food intake and glucose production upon activation, which is similar to the effects of GLP-1Rags. While NTS GLP-1 neurons do not express GLP-1receptors, GLP-1Rags may activate target nuclei downstream of NTS GLP-1 neurons that do express GLP-1Rs, thereby mimicking the effects of stimulating NTS GLP-1 neurons.

We hypothesize one component of the beneficial effects of both NTS GLP-1 and GLP-1Rags on metabolism involves an NTS GLP-1 neuron → dorsal medial nucleus of the hypothalamus (DMH) GABAergic neuron → Arcuate nucleus circuit that ultimately reduces Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neuron activity. This study aims to define this NTS → DMH → arcuate circuit and determine if GLP-1Rags and central GLP-1 converge on the DMH to inhibit NPY/AgRP neurons and improve metabolism.

Previous work and preliminary data demonstrate that: 1) GLP1-Rags decrease the excitability of NPY/AgRP neurons, 2) a GABAergic neuron that expresses GLP-1Rs and leptin receptors (LepRs) resides in the DMH, 3) DMH GLP-1R+ and LepR+ neurons are activated by GLP-1Rags, food presentation/intake, and/or elevated glucose levels, and 4) there is synchrony in the regulation of activity of these neurons. Specifically, when NTS GLP-1 or DMH LepR+/GLP-1R+ neurons are activated, NPY/AgRP neurons are inhibited.

These findings suggest the NTS GLP-1 → DMH GLP-1R/LepR → NPY/AgRP circuit functions as a convergence point for satiety signals. We aim to investigate the role of DMH GLP-1R+ neurons in mediating the effects of GLP-1Rags and NTS GLP-1 neuron activity, as well as explore how this circuit affects NPY/AgRP neuron activity and metabolism. The proposed model (Figure 1) predicts that activation of DMH GLP-1R neurons by GLP-1Rags or by NTS GLP-1 neuronal projections (e.g. in response to a meal or in response to exercise) leads to increased GABA release from DMH GLP-1R neurons and inhibition of NPY/AgRP activity.

This, in turn, mediates various beneficial metabolic effects of central GLP-1 and GLP-1/Rags. The studies outlined below are designed to directly test the predictions.

Specific Aim – To determine if increased activity of NTS GLP-1 neurons alters DMH and arcuate NPY/AgRP neuron physiology and metabolism. We hypothesize that NTS GLP-1 neuron activation leads to inhibition of arcuate NPY/AgRP neurons through a GABAergic DMH interneuron that expresses GLP-1Rs and/or LepRs. We will utilize optogenetic and chemogenetic approaches to activate NTS GLP-1 neurons while simultaneously monitoring DMH GLP-1R+ or LepR+ neurons as well as arcuate NPY/AgRP neuron activity exvivo (electrophysiology) and in-vivo (calcium imaging).

Summary: The proposed studies link the GLP-1R agonist class of anti-obesity/anti-diabetes medications to a novel NTS GLP-1 → DMH GLP-1R/LepR → NPY/AgRP circuit. We will examine the requirement of GLP-1 neurotransmission and GLP-1R signaling in the plasticity of this circuit using patch clamp electrophysiology, invivo calcium imaging, and metabolic phenotyping with unique mouse models already in hand