Preproglucagon (PPG) neurons innervate neurochemically identified autonomic neurons in the mouse brainstem

Abstract

Preproglucagon (PPG) neurons produce glucagon-like peptide-1 (GLP-1) and occur primarily in the nucleus tractus solitarius (NTS). GLP-1 affects a variety of central autonomic circuits, including those controlling the cardiovascular system, thermogenesis, and most notably energy balance. Our immunohistochemical studies in transgenic mice expressing YFP under the control of the PPG promoter showed that PPG neurons project widely to central autonomic regions, including brainstem nuclei. Functional studies have highlighted the importance of hindbrain receptors for the anorexic effects of GLP-1. In this study, we assessed YFP innervation of neurochemically identified brainstem neurons in transgenic YFP-PPG mice. Immunoreactivity for YFP plus choline acetyltransferase (ChAT), tyrosine hydroxylase (TH) and/or serotonin (5-HT) was visualised with two- or three-colour immunoperoxidase labelling using black (YFP), brown and blue-grey reaction products. In the dorsal motor nucleus of the vagus (DMV), terminals from fine YFP-immunoreactive axons closely apposed a small proportion of ChAT-positive and rare TH-positive/ChAT-positive motor neurons, mostly ventral to AP. YFP-immunoreactive innervation was virtually absent from the compact and loose formations of the nucleus ambiguus. In the NTS, some TH-immunoreactive neurons were closely apposed by YFP-containing axons. In the A1/C1 column in the ventrolateral medulla, close appositions on TH-positive neurons were more common, particularly in the caudal portion of the column. A single YFP-immunoreactive axon usually provided 1-3 close appositions on individual ChAT- or TH-positive neurons. Serotonin-immunoreactive neurons were most heavily innervated, with the majority of raphé pallidus, raphé obscurus and parapyramidal neurons receiving several close appositions from large varicosities of YFP-immunoreactive axons. These results indicate that GLP-1 neurons innervate various populations of brainstem autonomic neurons. These include vagal efferent neurons and catecholamine neurons in areas linked with cardiovascular control. Our data also indicate a synaptic connection between GLP-1 neurons and 5-HT neurons, some of which might contribute to the regulation of appetite.