Table of Contents
- GLP-1 Receptors in the Human Gut: What the ENS Map Reveals
- Abstract
- Study at a Glance
- Study Snapshot
- Study Facts Table
- What Researchers Actually Did
- Key Findings: Primary Outcomes
- Key Findings: Neurochemical Subtype Co-localization
- Adverse Events and Safety Profile
- Statistical Approach and Rigor
- Clinical Takeaway
- Why This Matters Clinically
- CED Clinical Relevance
- Clinical Insight
- Fits What We Already Know
- Read This Paper Through Nine Different Lenses
- What is the primary focus of this study?
- Where is GLP-1R most abundantly expressed?
- What are the implications of GLP-1R distribution for GLP-1 receptor agonists?
- Which neuronal subtypes show preferential co-localization with GLP-1R?
- What regions were analyzed in this study?
- How was GLP-1R expression quantified?
- What statistical methods were used for analysis?
- Are there any limitations to the study?
- What are the clinical implications of this research?
- How does this study contribute to the field of neurogastroenterology?
- Read next
GLP-1 Receptors in the Human Gut: What the ENS Map Reveals
Enteric Nervous System
GI Motility
GLP-1 Agonist Side Effects
Colonic Neurochemistry
- GLP-1 receptors are expressed throughout the human enteric nervous system, with significantly higher density in the colon than in the stomach or ileum.
- GLP-1R co-localizes preferentially with inhibitory (nNOS-positive) neurons and CGRP-positive sensory afferents, with sparse overlap on excitatory (ChAT, substance P) neurons.
- The colonic localization pattern offers a plausible anatomical substrate for the GI side effects reported with semaglutide, liraglutide, and tirzepatide.
- This is the first systematic, region-matched, neurochemically coded characterization of GLP-1R across the full-thickness human GI tract.
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.
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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. 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 analyzed 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-localization with neuronal markers (PGP9.5, nNOS, ChAT, Substance P, CGRP, Calretinin, HuC/D) was assessed. Quantification used Mander’s coefficient; 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-localization 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 localization 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.
DOI: https://doi.org/10.1111/jnc.70461
Open Access: Yes (Creative Commons Attribution License)
Study at a Glance
| Design | Descriptive cross-sectional immunohistochemistry study; non-pre-registered |
| Population | 30 patients undergoing GI surgery at Royal London Hospital; 16 female, 14 male; mean age 60 ± 8 years; non-inflamed full-thickness resections |
| Regions analyzed | Antrum, ileum, ascending colon, descending colon, sigmoid colon (n=5 patients per region) |
| Primary endpoint | GLP-1R expression (mean positive pixels/area) and co-localization coefficients (Mander’s M1/M2) across ENS compartments and neuronal subtypes |
| Key finding | GLP-1R expression is significantly greater in the colon than in the stomach or ileum (p<0.05), with preferential localization to nNOS-positive inhibitory and CGRP-positive sensory neurons |
Study Snapshot
| Metric | Finding | Statistic |
|---|---|---|
| Mucosal GLP-1R expression (overall) | Increases from antrum to ascending colon; remains elevated distally | F(4,24)=13.45, p<0.0001 |
| Mucosal GLP-1R/PGP9.5 overlap (M2) | Peak in ileum (0.88) and ascending colon (0.61) | F(4,20)=10.97, p<0.0001 |
| Myenteric plexus GLP-1R (M1 co-loc) | All lower GI regions greater than antrum | F(4,20)=16.69, p<0.001 |
| nNOS vs. ChAT co-localization (M2) | nNOS: 0.365 vs. ChAT: 0.174 (ascending colon) | ~2-fold difference; p<0.05 |
| CGRP co-localization (M2, ascending colon) | Highest of all subtypes: 0.44; 2.75-fold vs. SP neurons | F(4,20)=9.48, p=0.0002 |
| CGRP co-localization (M2, sigmoid colon) | 0.56; highest among all sigmoid markers | F(4,20)=19.58, p<0.0001 |
| Substance P co-localization (M2, ascending colon) | Lowest of all subtypes: 0.16 | Significantly lower than nNOS and CGRP; p<0.05 |
Study Facts Table
| Field | Detail |
|---|---|
| Authors | Kiran Devi Dontamsetti, Rubina Aktar, Madusha Peiris |
| Institution | Wingate Institute of Neurogastroenterology, Queen Mary University of London |
| Journal / Year | Journal of Neurochemistry, 2026; 170:e70461 |
| Study design | Exploratory, non-pre-registered, cross-sectional immunohistochemistry (IHC) study in human surgical resection tissue |
| N (total) | 30 patients (5 per region); 16 female, 14 male; mean age 60 ± 8 years |
| Tissue regions | Antrum, ileum, ascending colon, descending colon, sigmoid colon |
| Comparator | Between-region comparisons; no drug intervention or control group |
| Primary endpoint | GLP-1R expression density (mean positive pixels/area) and co-localization coefficients (Mander’s M1/M2) with PGP9.5 and neuronal subtype markers |
| Key results | Significantly higher colonic GLP-1R expression vs. antrum and ileum (p<0.05); preferential co-localization with nNOS and CGRP neurons; sparse co-localization with ChAT and SP neurons |
| Adverse events | Not applicable (tissue study) |
| Funding | Medical Research Council [MR/W007045/1] |
| Conflicts of interest | None declared |
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
Read This Paper Through Nine Different Lenses
The same evidence can produce very different conclusions depending on the question being asked. Explore this study through multiple physician-guided interpretive frameworks.
Overview
This study systematically characterizes GLP-1 receptor (GLP-1R) expression across different regions of the human gastrointestinal tract. It reveals that GLP-1R is most abundant in the colon, especially in inhibitory neurons and CGRP-positive sensory afferents.
The findings provide a mechanistic framework for understanding the gastrointestinal side effects associated with GLP-1 receptor agonists, such as nausea, vomiting, and diarrhea.
- GLP-1R expression is significantly higher in the colon compared to the stomach or ileum.
- Preferential co-localization of GLP-1R with nNOS-positive inhibitory neurons and CGRP-positive sensory afferents suggests a role in GI motility regulation.
- The study offers detailed insights into the distribution and functional role of GLP-1R in the human enteric nervous system, particularly in the colon.
Patient Takeaway
This study explains why some patients on GLP-1 receptor agonists experience gastrointestinal side effects like nausea and diarrhea. The high expression of GLP-1R in colonic inhibitory neurons may disrupt normal gut motility.
Understanding these mechanisms can help healthcare providers better manage patient expectations and side effects associated with GLP-1 therapy.
- GLP-1R is most abundant in the colon, particularly in inhibitory neurons.
- This distribution explains why GLP-1 receptor agonists may cause gastrointestinal dysmotility.
- The study provides a mechanistic basis for understanding side effects of GLP-1 therapies.
Clinician’s POV
Clinicians can use this study to better understand the mechanisms behind gastrointestinal side effects of GLP-1 receptor agonists. The high expression of GLP-1R in colonic inhibitory neurons suggests that these drugs may disrupt normal gut motility.
This knowledge can help clinicians manage patient expectations and side effects more effectively, potentially improving treatment outcomes.
- GLP-1R is most abundant in the colon, particularly in inhibitory neurons.
- This distribution explains why GLP-1 receptor agonists may cause gastrointestinal dysmotility.
- Understanding these mechanisms can guide better patient management and treatment decisions.
A Skeptical Read
While this study provides valuable insights into GLP-1R distribution, it has limitations such as small sample sizes and lack of pre-registration. These factors may affect the generalizability of the findings.
Future research with larger, more diverse samples could help confirm these results and further elucidate the role of GLP-1R in gastrointestinal function and side effects.
- The study has limitations including small sample sizes and lack of pre-registration.
- Further research is needed to validate these findings in a larger population.
- Understanding these mechanisms can guide better patient management and treatment decisions.
Study Critic
This study has methodological issues, including small sample sizes and lack of blinding in image analysis. These factors may introduce bias and affect the reliability of the results.
Additionally, the statistical approach could be improved with a priori power calculations and outlier exclusion protocols to enhance rigor.
- The study has methodological issues including small sample sizes and lack of blinding.
- Statistical analysis could be improved with a priori power calculations and outlier exclusion.
- Further research should address these limitations for more robust findings.
Compared to Past Research
This study builds on previous research that has explored GLP-1 pathways in the central nervous system. However, the distribution and functional role of GLP-1R within the human enteric nervous system remained unclear until this systematic characterization.
Understanding these mechanisms provides a more comprehensive view of how GLP-1 receptor agonists affect gastrointestinal function and side effects.
- Previous research focused on central GLP-1 pathways.
- This study fills gaps in understanding GLP-1R distribution in the enteric nervous system.
- It provides a more comprehensive view of GLP-1 receptor agonist effects.
Practical Considerations
The findings from this study have practical implications for clinical practice. Understanding the distribution of GLP-1R in the enteric nervous system can help clinicians better manage gastrointestinal side effects associated with GLP-1 receptor agonists.
This knowledge can guide patient care and improve treatment outcomes by allowing healthcare providers to anticipate and address potential side effects more effectively.
- Understanding GLP-1R distribution aids in managing gastrointestinal side effects.
- This knowledge can guide better patient care and treatment decisions.
- It helps clinicians anticipate and address potential side effects of GLP-1 receptor agonists.
Future Directions
This study opens up several avenues for future research. Larger studies with more diverse populations could validate these findings and further explore the role of GLP-1R in gastrointestinal function.
Additionally, clinical trials incorporating this knowledge could help develop strategies to mitigate gastrointestinal side effects associated with GLP-1 receptor agonists.
- Future research should include larger, more diverse populations.
- Clinical trials can incorporate this knowledge to mitigate side effects.
- It opens up opportunities for further exploration of GLP-1R’s role in gastrointestinal function.
Misreadings & Bad-Faith Takes
One common misunderstanding is that GLP-1R expression is uniformly high across the entire gastrointestinal tract. In fact, it is significantly higher in the colon compared to the stomach or ileum.
Another misinterpretation is that GLP-1R affects only excitatory neurons. The study shows preferential co-localization with inhibitory neurons and sensory afferents, not excitatory ones.
- GLP-1R expression is significantly higher in the colon compared to other regions.
- GLP-1R primarily affects inhibitory neurons and sensory afferents, not excitatory ones.
- Accurate interpretation of these findings is crucial for understanding GLP-1 receptor agonist effects.
Have thoughts on this? Share it:
What is the primary focus of this study?
The study focuses on the systematic characterization of GLP-1 receptor (GLP-1R) expression in the human enteric nervous system across different regions of the gastrointestinal tract.
Where is GLP-1R most abundantly expressed?
GLP-1R is most abundantly expressed in the colon, particularly in inhibitory neurons and CGRP-positive sensory afferents.
What are the implications of GLP-1R distribution for GLP-1 receptor agonists?
The high expression of GLP-1R in colonic inhibitory neurons may explain the dose-dependent gastrointestinal dysmotility observed with GLP-1 receptor agonists.
Which neuronal subtypes show preferential co-localization with GLP-1R?
GLP-1R shows preferential co-localization with nNOS-positive inhibitory neurons and CGRP-positive sensory afferents.
What regions were analyzed in this study?
The study analyzed the gastric antrum, ileum, ascending colon, descending colon, and sigmoid colon from surgical resection specimens.
How was GLP-1R expression quantified?
GLP-1R expression was quantified using mean positive pixel counts per area of interest in immunohistochemical images.
What statistical methods were used for analysis?
The primary statistical framework was one-way ANOVA with Tukey’s post hoc correction for multiple comparisons.
Are there any limitations to the study?
Limitations include small sample sizes, lack of pre-registration and blinding in image analysis, and no outlier exclusion protocol.
What are the clinical implications of this research?
This research provides a mechanistic substrate for understanding the gastrointestinal side effects associated with GLP-1 receptor agonists.
How does this study contribute to the field of neurogastroenterology?
The study offers detailed insights into the distribution and functional role of GLP-1R in the human enteric nervous system, particularly in the colon.
