GLP-1 Receptors in the Human Gut: What the ENS Map Reveals

TL;DR: GLP-1 receptors are abundantly expressed on inhibitory and sensory neurons throughout the human colonic enteric nervous system, providing the first human anatomical framework for why GLP-1 receptor agonists produce dose-dependent GI dysmotility.

Abstract

Study at a Glance

Study Snapshot

Study Facts Table

What Researchers Actually Did

Dontamsetti and colleagues obtained full-thickness, macroscopically non-inflamed surgical resection specimens from 30 patients at Royal London Hospital. Tissue was collected from five anatomical regions: the gastric antrum, ileum, and ascending, descending, and sigmoid colon (n=5 patients per region). Cryostat sections (10 µm) and, separately, wholemount myenteric plexus preparations from sigmoid colon were processed using immunohistochemistry with a validated rabbit anti-GLP-1R antibody (Alamone AGR-021) combined with markers for pan-neuronal tissue (PGP9.5, HuC/D) and defined neuronal subtypes: nNOS (inhibitory), ChAT (excitatory), Substance P (excitatory), CGRP (sensory afferents), and calretinin. Epifluorescence imaging was performed at 40x and wholemount preparations were imaged by Zeiss LSM 880 confocal microscopy with Z-stack acquisition at 0.341 µm intervals.

Quantification relied on Otsu thresholding applied to 8-bit greyscale-converted images in FIJI, with mean positive pixel counts per area of interest as the primary measure of receptor density. Co-localization was quantified via Mander’s overlap coefficients (M1 and M2), calculated with the JaCoP plugin. One-way ANOVA followed by Tukey’s post hoc test was used for all between-region and between-subtype comparisons. The authors explicitly note that sample sizes were not calculated a priori, the study was not pre-registered, and image analysis was not blinded.

Key Findings: Primary Outcomes

• Mucosal expression: GLP-1R positive pixel count increased significantly from the antrum through the ileum, peaked in the ascending colon, and remained elevated in the descending and sigmoid colon (one-way ANOVA: F(4,24)=13.45, p<0.0001). In the ileum, staining spanned the crypt-villus axis; in the colon, expression was concentrated on neuronal fibres at the crypt base.
• Mucosal co-localization (M2): GLP-1R overlap with PGP9.5-positive neuronal fibres peaked in the ileum (M2=0.88) and ascending colon (M2=0.61) (F(4,20)=10.97, p<0.0001).
• Submucosal plexus: Positive GLP-1R staining was present in submucosal ganglia of the ileum and all colonic regions but absent from the surrounding submucosal connective tissue. Not all PGP9.5-positive neurons co-expressed GLP-1R. Descending and sigmoid colon showed significantly higher M1 co-localization than the ileum (F(3,16)=15.77, p<0.0001), despite lower absolute pixel counts at these distal sites.
• Muscle layer expression: GLP-1R increased from antrum to ileum, then decreased at the colon in absolute pixel terms, but co-localization with PGP9.5-positive nerve endings innervating the muscle increased distally (M1: F(4,20)=17.66, p<0.0001; M2: F(4,20)=21.44, p<0.0001).
• Myenteric plexus expression: GLP-1R was present in all myenteric plexus regions examined. Expression was restricted to myenteric neurons with no detectable staining in adjacent smooth muscle cells. All lower GI regions showed significantly greater co-localization than the antrum (M1: F(4,20)=16.69, p<0.001; M2: F(4,20)=15.85, p<0.0001).
• HuC/D wholemount: GLP-1R was confirmed on a subset of HuC/D-positive neuronal cell bodies in sigmoid colon wholemount preparations, with membrane-associated, punctate staining and clear varicosity labelling outside the cell body.

Key Findings: Neurochemical Subtype Co-localization

• CGRP neurons (ascending colon): Highest GLP-1R co-localization among all subtypes assessed. M1=0.34, M2=0.44; 2.75-fold greater than SP-positive neurons. Staining was dense and punctate throughout the plexus (F(4,20)=9.48, p=0.0002).
• nNOS neurons (ascending colon): Second highest co-localization; M2=0.365; approximately 2-fold greater than ChAT-positive neurons (M2=0.174). Staining pattern was punctate.
• Calretinin neurons: Intermediate co-localization; M1=0.38, M2=0.46 (ascending colon). Distribution was clustered rather than uniform across the plexus.
• ChAT neurons: Low co-localization (M2=0.174, ascending colon). Staining was punctate rather than cytoplasmic or membranous.
• Substance P neurons: Lowest co-localization of all subtypes; M1=0.186, M2=0.16 (ascending colon). Scattered, punctate, variable intensity. Significantly lower than nNOS (p<0.05) and CGRP (p<0.01).
• Sigmoid colon neurochemical profile: Consistent with ascending colon findings. CGRP again showed the highest co-localization (M1=0.45, M2=0.56; F(4,20)=19.58, p<0.0001). GLP-1R staining was qualitatively more abundant in the longitudinal muscle layer than the circular muscle. The nNOS-over-ChAT predominance did not reach statistical significance in the sigmoid colon.

Adverse Events and Safety Profile

This is a human tissue characterization study; no drug was administered and no clinical adverse event data were generated. The findings are relevant to the adverse event profile of GLP-1 receptor agonists administered clinically. The paper cites published data indicating that 25 to 60% of patients on semaglutide or tirzepatide report nausea, 5 to 15% experience vomiting, and 10 to 20% report diarrhea. The anatomical data presented here are proposed as a mechanistic substrate for those observations, not as primary safety data.

Statistical Approach and Rigor

The primary statistical framework was one-way ANOVA with Tukey’s post hoc correction for multiple comparisons, applied to mean positive pixel counts and Mander’s co-localization coefficients. Normality was assessed with Shapiro-Wilk testing before parametric analysis. The analytic approach is appropriate for the experimental design. Three meaningful limitations in statistical rigor deserve attention: sample sizes (n=5 patients per region, yielding five data points per group) were not determined by formal a priori power calculation; the study was neither pre-registered nor blinded at image analysis; and no outlier exclusion protocol was defined. The small per-group n makes the ANOVA susceptible to being underpowered for detecting modest between-region differences, while simultaneously being potentially unstable if one high-outlier value drives significance. Mander’s co-localization coefficient, while widely used, is sensitive to threshold selection and image bit-depth conversion, both of which were standardized (Otsu threshold, 8-bit conversion) but not independently validated in this study.

Clinical Takeaway

For clinicians prescribing GLP-1 receptor agonists, this study offers the first systematic human tissue map showing that GLP-1Rs are concentrated in colonic inhibitory motor neurons (nNOS-positive) and CGRP-expressing sensory afferents. The abundance of receptor in the distal colon, co-localized to neurons that modulate motility and visceral sensation, provides anatomical grounding for the GI side effects routinely encountered in clinical practice. When patients on semaglutide or liraglutide report constipation, nausea, or bloating, those symptoms are not inexplicable off-target effects; they reflect activation of a receptor that is densely and specifically expressed exactly where those symptoms originate. This does not change prescribing decisions today, but it should calibrate expectations, counseling, and future dose-optimization strategies.

Clinical Bottom Line: GLP-1 receptors are enriched in human colonic inhibitory and sensory neurons, mapping the anatomical basis for GI side effects seen with GLP-1 receptor agonists.

Why This Matters Clinically

The global prescription volume of GLP-1 receptor agonists has expanded dramatically beyond type 2 diabetes into obesity management, cardiovascular risk reduction, and emerging indications including metabolic-associated steatohepatitis and addiction medicine. Yet the GI tolerability burden remains the primary driver of treatment discontinuation. Prior mechanistic understanding of that burden came almost exclusively from rodent studies, with limited and inconsistent human histological data. This study fills a critical human anatomical gap. By demonstrating that GLP-1R expression is highest in the distal colon and is preferentially concentrated on inhibitory nitrergic neurons and CGRP-positive sensory afferents, it generates testable hypotheses about which enteric circuits are most vulnerable to pharmacological overstimulation. The finding that excitatory (ChAT, SP) neurons express comparatively little GLP-1R is equally informative: it suggests GLP-1’s primary colonic action is to suppress inhibitory tone and amplify sensory signaling rather than directly drive excitatory contraction. Understanding this asymmetry has direct implications for developing GLP-1R agonists or co-agonists with reduced GI liability.

CED Clinical Relevance

At CED Clinic, patients using cannabinoids alongside GLP-1 receptor agonists represent a growing and clinically complex population. Cannabinoids act on CB1 receptors expressed on enteric neurons, including inhibitory motor neurons and sensory afferents, overlapping significantly with the neuronal populations shown here to express GLP-1R. The concentration of GLP-1R on nNOS-positive inhibitory neurons in the colon is precisely the circuit through which cannabinoids modulate colonic motility and visceral sensation. This anatomical overlap raises the plausibility of pharmacodynamic interactions at the ENS level: concurrent GLP-1R agonism and cannabinoid receptor modulation could produce additive or complex effects on colonic transit, secretion, and pain thresholds. These are questions that cannot be answered by this paper alone, but the receptor map it provides is the necessary starting point for framing those clinical and investigational questions in human biology rather than rodent extrapolation.

Clinical Insight

One actionable point: When GI side effects emerge in patients on GLP-1 receptor agonists, the distal colon is the most receptor-dense site, and the inhibitory motor and sensory afferent circuits are the most heavily targeted. Dose titration strategies should account for the fact that the receptor density gradient favors the colon: slower up-titration and split dosing regimens that reduce peak ENS receptor occupancy in the distal gut may have a rational anatomical basis that warrants formal investigation.

Fits What We Already Know

Prior literature, as cited in this paper, had established GLP-1R expression in murine enteric neurons, including both cholinergic and nitrergic subtypes (Amato et al., 2010), and functional studies in mice demonstrated GLP-1-mediated inhibition of cholinergic contractions through a nitrergic mechanism (Amato et al., 2010, 2014). GLP-1R had also been identified in human gastric glands and in CGRP-positive nerve fibres in inflammatory bowel disease tissue (Broide et al., 2013; Anand et al., 2018). The present study extends

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