Serotonergic modulation of the activity of GLP-1 producing neurons in the nucleus of the solitary tract in mouse

Marie K Holt,Ida J Llewellyn-Smith,Frank Reimann,Fiona M Gribble,Stefan Trapp

doi:10.1016/j.molmet.2017.06.002

Marie K Holt, Ida J Llewellyn-Smith + Show 3 more

Open Access

https://doi.org/10.1016/j.molmet.2017.06.002

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Abstract

ObjectiveGlucagon-like peptide-1 (GLP-1) and 5-HT are potent regulators of food intake within the brain. GLP-1 is expressed by preproglucagon (PPG) neurons in the nucleus tractus solitarius (NTS). We have previously shown that PPG neurons innervate 5-HT neurons in the ventral brainstem. Here, we investigate whether PPG neurons receive serotonergic input and respond to 5-HT. MethodsWe employed immunohistochemistry to reveal serotonergic innervation of PPG neurons. We investigated the responsiveness of PPG neurons to 5-HT using in vitro Ca2+ imaging in brainstem slices from transgenic mice expressing the Ca2+ indicator, GCaMP3, in PPG neurons, and cell-attached patch-clamp recordings. ResultsClose appositions from 5-HT-immunoreactive axons occurred on many PPG neurons. Application of 20 μM 5-HT produced robust Ca2+ responses in NTS PPG dendrites but little change in somata. Dendritic Ca2+ spikes were concentration-dependent (2, 20, and 200 μM) and unaffected by blockade of glutamatergic transmission, suggesting 5-HT receptors on PPG neurons. Neither activation nor blockade of 5-HT3 receptors affected [Ca2+]i. In contrast, inhibition of 5-HT2 receptors attenuated increases in intracellular Ca2+ and 5-HT2C receptor activation produced Ca2+ spikes. Patch-clamp recordings revealed that 44% of cells decreased their firing rate under 5-HT, an effect blocked by 5-HT1A receptor antagonism. ConclusionsPPG neurons respond directly to 5-HT with a 5-HT2C receptor-dependent increase in dendritic [Ca2+]i. Electrical responses to 5-HT revealed additional inhibitory effects due to somatic 5-HT1A receptors. Reciprocal innervation between 5-HT and PPG neurons suggests that the coordinated activity of these brainstem neurons may play a role in the regulation of food intake.

Highlights

Glucagon-like peptide-1 (GLP-1) is an incretin best known for its role in glucose homeostasis and appetite regulation [1]
PPG neurons in the nucleus of the solitary tract (NTS), the intermediate reticular nucleus (IRT) and the midline ventral to the hypoglossal nucleus showed intense yellow fluorescent protein (YFP) staining throughout cell bodies and dendrites
The distribution of YFPexpressing neurons in the medulla of YFP-PPG mice was the same as reported before [9,33], with NTS PPG neurons forming the largest subpopulation and the midline neurons being the smallest. 5-HT-immunoreactive axons occurred throughout the caudal NTS and many PPG neurons in the NTS received close appositions from 5-HT axons on their somata and/or proximal dendrites (Figure 1AeD)

Summary

Introduction

Glucagon-like peptide-1 (GLP-1) is an incretin best known for its role in glucose homeostasis and appetite regulation [1]. PPG neurons project from the nucleus of the solitary tract (NTS) and intermediate reticular nucleus (IRT) to autonomic control centers throughout the central nervous system, including sympathetic preganglionic neurons in the spinal cord [9,16]. Many of these nuclei have been shown to express GLP-1 receptors [10,17], and injection of GLP-1 or its analogs into brain tissue or ventricles produces effects on food intake, thermogenesis, and cardiovascular function [18e20]. It seems likely that PPG neurons are the source of GLP-1 released in the brain, the precise physiological role and cellular regulation of the activity of PPG neurons is still under intense discussion [21]

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