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

• Published in: Molecular metabolism (Germany) 2017-08, Vol.6 (8), p.909-921
• Main Authors: Holt, Marie K, Llewellyn-Smith, Ida J, Reimann, Frank, Gribble, Fiona M, Trapp, Stefan
• Format: Article
• Language: English
• Subjects: Animals; Calcium - metabolism; Dendritic calcium; Endocrinology & Metabolism; Female; GCaMP; Glucagon-Like Peptide 1 - metabolism; Male; Mice; Neurons - drug effects; Neurons - metabolism; Neurons - physiology; NTS; Original; Preproglucagon; Serotonin; Serotonin - pharmacology; Serotonin Receptor Agonists - pharmacology; Solitary Nucleus - cytology; Solitary Nucleus - metabolism; Solitary Nucleus - physiology
• ISSN: 2212-8778; 2212-8778
• DOI: 10.1016/j.molmet.2017.06.002

Abstract Objective Glucagon-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. Methods We 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. Results Close 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. Conclusions PPG 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.