Systematic Characterisation of GLP-1R in Human Enteric Nervous System: Implications for GLP-1 as a Key Regulator of Colonic Activity.

Glucagon-like peptide-1 (GLP-1) regulates glucose homeostasis, satiety and gastrointestinal (GI) motility through interaction with its receptor (GLP-1R). While central GLP-1 pathways are well studied, the distribution and functional role of GLP-1R within the human enteric nervous system (ENS) remain unclear. We characterised GLP-1R expression across the human GI tract, examining its anatomical localisation among distinct enteric neuronal subtypes. Non-inflamed, full-thickness human GI tissues (antrum, ileum, ascending, descending and sigmoid colon; N = 30, 5 different human samples per region) were obtained from surgical resections and analysed using immunohistochemistry, epifluorescence and confocal microscopy. Mean number of positive pixels of GLP-1R and PGP9.5 was quantified in mucosal varicosities, muscle, submucosal and myenteric plexuses. GLP-1R co-localisation with neuronal markers (PGP9.5, nNOS, ChAT, Substance P, CGRP, Calretinin, HuC/D) was assessed. Quantification used Mander's coefficient, and statistical analysis used one-way ANOVA with Tukey's post hoc test. GLP-1R was expressed abundantly in ENS structures, including mucosal varicosities, muscle, submucosal plexus and myenteric neurons throughout the lower GI tract, with significantly higher expression in the colon compared to the stomach and ileum (p < 0.05). Co-localisation analyses revealed preferential GLP-1R expression in nNOS-expressing inhibitory neurons and CGRP-expressing varicosities, moderate expression in calretinin-expressing neurons, and sparse expression in ChAT-expressing and substance P-immunoreactive excitatory neurons. Whole-mount confocal imaging confirmed GLP-1R localisation to HuC/D-immunoreactive neuronal cell bodies with punctate, membrane-associated staining. GLP-1R is differentially expressed across the human ENS, with increased expression in inhibitory neurons and putative extrinsic sensory afferents, particularly in the distal colon. This suggests that peripherally released GLP-1 acts locally within the ENS to regulate motility and sensory signalling, underpinning both its physiological functions and the gastrointestinal side effects of GLP-1-based therapies.