Objective: 

PPG neurons produce GLP-1, a satiety peptide, and occur primarily in the nucleus tractus solitarius. Our immunohistochemical studies in transgenic mice expressing YFP under PPG promoter control showed that PPG neurons project widely to central autonomic regions, including brainstem nuclei. Functional studies have highlighted the importance of hindbrain receptors for GLP-1's anorexic effects.

Methods: 

Here, 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) were visualised with two- or three-colour immunoperoxidase labelling using black (YFP), brown and blue-grey reaction products.

Results: 

In the dorsal vagal nucleus (DMV), YFP-containing axons closely apposed only a few ChAT neurons, mostly lying rostral to area postrema (AP) and in dorsal and lateral DMV beneath the AP. Within the nucleus ambiguus, some ChAT neurons in the loose formation received appositions, but innervation was mostly absent from the compact formation. In the vagal complex, few TH neurons were closely apposed by YFP axons. In the A1/C1 column in the ventrolateral medulla, close appositions on TH neurons were more common. A single YFP-immunoreactive axon usually provided 1–3 close appositions on individual ChAT- or TH-positive neurons. Serotonin neurons were most heavily innervated, with many raphé pallidus, raphé obscurus and parapyramidal neurons receiving several close appositions from YFP axons.

Conclusions: 

These results indicate that GLP-1 neurons innervate brainstem autonomic neurons, including some vagal efferent neurons. Our data also demonstrate a link between GLP-1 neurons and 5-HT neurons, which are involved in appetite regulation.