GLP-1 neurons form a local synaptic circuit within the rodent nucleus of the solitary tract.

J. Patrick Card,Ida J. Llewellyn‐Smith,Linda Rinaman,Daniel I. Brierley,Aaron L. Johnson,Stefan Trapp,Rishi Anand,Huiyuan Zheng

doi:10.1002/cne.24482

Abstract

Glutamatergic neurons that express pre‐proglucagon (PPG) and are immunopositive (+) for glucagon‐like peptide‐1 (i.e., GLP‐1+ neurons) are located within the caudal nucleus of the solitary tract (cNTS) and medullary reticular formation in rats and mice. GLP‐1 neurons give rise to an extensive central network in which GLP‐1 receptor (GLP‐1R) signaling suppresses food intake, attenuates rewarding, increases avoidance, and stimulates stress responses, partly via GLP‐1R signaling within the cNTS. In mice, noradrenergic (A2) cNTS neurons express GLP‐1R, whereas PPG neurons do not. In this study, confocal microscopy in rats confirmed that prolactin‐releasing peptide (PrRP)+ A2 neurons are closely apposed by GLP‐1+ axonal varicosities. Surprisingly, GLP‐1+ appositions were also observed on dendrites of PPG/GLP‐1+ neurons in both species, and electron microscopy in rats revealed that GLP‐1+ boutons form asymmetric synaptic contacts with GLP‐1+ dendrites. However, RNAscope confirmed that rat GLP‐1 neurons do not express GLP‐1R mRNA. Similarly, Ca2+ imaging of somatic and dendritic responses in mouse ex vivo slices confirmed that PPG neurons do not respond directly to GLP‐1, and a mouse crossbreeding strategy revealed that <1% of PPG neurons co‐express GLP‐1R. Collectively, these data suggest that GLP‐1R signaling pathways modulate the activity of PrRP+ A2 neurons, and also reveal a local “feed‐forward” synaptic network among GLP‐1 neurons that apparently does not use GLP‐1R signaling. This local GLP‐1 network may instead use glutamatergic signaling to facilitate dynamic and potentially selective recruitment of GLP‐1 neural populations that shape behavioral and physiological responses to internal and external challenges.

Highlights

Neurons synthesizing pre-proglucagon (PPG), the protein precursor for glucagon-like peptide-1 (GLP-1), reside within the caudal nucleus of the solitary tract and adjacent medullary reticular formation
While similar localization studies have not been conducted in rats, central (i.c.v.) administration of a GLP-1 receptor (GLP-1R) antagonist reduced the ability of restraint stress to activate cFos expression by A2 neurons that co-express prolactin-releasing peptide (PrRP) (Maniscalco, Zheng, Gordon, & Rinaman, 2015)
Central GLP-1R antagonism likely alters neural responses to stress across many brain regions, one potential explanation is that PrRP+ A2 neurons express GLP-1R and receive axonal input from GLP-1+ neurons in rats, which would be consistent with reports that a subset of catecholaminergic NTS neurons receive close appositions from the axons of PPG neurons and express GLP-1R in mice (Cork et al, 2015; Llewellyn-Smith et al, 2013; Richard et al, 2015)

Summary

| INTRODUCTION

Neurons synthesizing pre-proglucagon (PPG), the protein precursor for glucagon-like peptide-1 (GLP-1), reside within the caudal nucleus of the solitary tract (cNTS) and adjacent medullary reticular formation. Central GLP-1R antagonism likely alters neural responses to stress across many brain regions, one potential explanation is that PrRP+ A2 neurons express GLP-1R and receive axonal input from GLP-1+ neurons in rats, which would be consistent with reports that a subset of catecholaminergic NTS neurons receive close appositions from the axons of PPG neurons and express GLP-1R in mice (Cork et al, 2015; Llewellyn-Smith et al, 2013; Richard et al, 2015). Ca2+ imaging was performed in ex vivo slices from Glu-Cre/ Rosa26-GCaMP3 mice to assess both somatic and dendritic responses of PPG neurons to GLP-1, and a new transgenic mouse crossbreeding strategy was used to investigate potential colocalization of fluorescent reporter genes identifying PPG- and GLP-1R-expressing neurons

| MATERIALS AND METHODS

| RESULTS

| DISCUSSION