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The enteric nervous system (ENS) is a semi-autonomous neural network of approximately 500 million neurons embedded within the walls of the gastrointestinal tract – from oesophagus to rectum – organised into two continuous plexuses that regulate gut motility, secretion, blood flow, and local immune function largely independently of the central nervous system. It is often described as the “second brain,” not metaphorically but structurally: the ENS contains more neurons than the spinal cord, uses the same neurotransmitters as the brain, and maintains gastrointestinal function after complete vagal transection. The gut-brain axis is the bidirectional communication system connecting the ENS to the CNS – via vagal afferent and efferent fibres, spinal afferents, and circulating hormones including GLP-1, ghrelin, leptin, PYY, and CCK. GLP-1 receptors are expressed directly on enteric neurons, vagal afferent terminals, and in the brainstem and hypothalamus – making GLP-1 one of the primary signalling molecules operating across the entire gut-brain axis, simultaneously modulating gut motility, hypothalamic satiety circuits, and mesolimbic reward pathways. Understanding this axis clarifies why appetite regulation is not a simple hormonal process and why gut-derived signals have consequences well beyond the gastrointestinal tract.

Appetite regulation is frequently presented as a relatively straightforward hormonal system: ghrelin goes up, you eat; GLP-1 and PYY go up, you stop. This framing is useful but incomplete. The signals that regulate hunger, satiety, and food reward do not travel directly from gut to brain – they pass through a complex neural architecture that processes, amplifies, and integrates them before they reach conscious awareness. The ENS and the gut-brain axis are that architecture.

The Enteric Nervous System: Architecture

The ENS consists of two main neural plexuses embedded in the gut wall: ^[2]

The myenteric plexus (Auerbach’s plexus): Located between the longitudinal and circular muscle layers of the gut wall, running continuously from oesophagus to internal anal sphincter. The myenteric plexus is primarily a motor plexus – it coordinates the peristaltic reflex and controls the rhythmic smooth muscle contractions that propel gut contents distally. It contains both excitatory (acetylcholine, substance P) and inhibitory (nitric oxide, VIP) motor neurons that act on the muscle layers above and below it.

The submucosal plexus (Meissner’s plexus): Located in the submucosa, between the circular muscle layer and the mucosa. The submucosal plexus is primarily a secretomotor and sensory plexus – it regulates glandular secretion, mucosal blood flow, and epithelial function, and contains sensory neurons that monitor luminal chemical composition, mechanical distension, and osmolarity.

Together these plexuses contain intrinsic primary afferent neurons (IPANs), interneurons, and motor neurons that constitute a complete reflex arc – the ENS can detect luminal contents, process that information, and mount a coordinated motor and secretory response entirely independently of CNS input. This autonomy is what distinguishes the ENS from other peripheral nervous system components: it is not merely a relay station for central commands but a distributed processor capable of independent decision-making about gut function.

The ENS shares its neurotransmitter repertoire extensively with the CNS – serotonin, dopamine, acetylcholine, noradrenaline, nitric oxide, GABA, and numerous neuropeptides are all present in enteric neurons. Approximately 95% of the body’s total serotonin is produced and stored in the gut – primarily in enterochromaffin cells in the mucosa – where it activates intrinsic sensory neurons and coordinates peristalsis. This serotonin is largely separate from central serotonin pools but has bidirectional signalling consequences through vagal afferents.

GLP-1 in the ENS

GLP-1 receptors (GLP-1R) are expressed directly on enteric neurons in the myenteric plexus – specifically on inhibitory neurons that release nitric oxide and VIP. GLP-1R activation on these neurons inhibits excitatory cholinergic neurotransmission, reducing smooth muscle contractility and slowing gastric emptying. This is one of the primary peripheral mechanisms by which both endogenous GLP-1 (secreted by L-cells in response to nutrient contact) and pharmacological GLP-1R agonists slow gastric emptying – a therapeutically relevant effect for post-meal glycaemic control but also the mechanistic basis for the nausea, bloating, and GI discomfort that are the most common side effects of GLP-1 RA medications at initiation. ^[1]

The ENS GLP-1R mechanism also interacts with the vagal afferent pathway: ENS neurons project onto vagal afferent terminals in the gut wall, which carry signals to the nucleus tractus solitarius (NTS) in the brainstem. GLP-1R activation on vagal afferents – in addition to direct ENS activation – contributes to the gastric emptying delay through this vagal-mediated central pathway. The relative contributions of direct ENS activation versus vagal afferent signalling to the overall GLP-1 effect on gastric motility is an area of active research, with the January 2026 JCI review noting that both pathways contribute and that their relative weight may differ between endogenous pulsatile GLP-1 and sustained pharmacological GLP-1R activation. jci

The Gut-Brain Axis: Bidirectional Communication

The gut-brain axis describes the continuous bidirectional communication between the gastrointestinal tract and the central nervous system, operating through four overlapping channels: ^[3]

Vagal afferent signalling: The vagus nerve is the primary fast communication route from gut to brain. Approximately 80–90% of vagal fibres are afferent – carrying information from the gut to the brainstem – rather than efferent. Vagal afferent terminals in the gut wall express receptors for GLP-1, CCK, PYY, leptin, and serotonin, amongst others. Activation of these terminals generates rapid satiety signals that reach the NTS within seconds of meal ingestion – well before blood-borne hormone levels have risen significantly. The vagal pathway is the mechanism by which meal composition information reaches the brain at the speed of neural rather than endocrine signalling.

Spinal afferents: A separate population of spinal afferent neurons carries pain, discomfort, and distension signals from the gut to the spinal cord and brainstem. These are the fibres responsible for visceral pain and the discomfort associated with gut dysmotility – including the bloating and cramping that can accompany GLP-1 RA initiation when gastric emptying is slowed beyond the comfortable range.

Circulating hormones: GLP-1, ghrelin, PYY, CCK, and leptin – secreted from gut enteroendocrine cells and adipose tissue – enter systemic circulation and act on receptors in the arcuate nucleus of the hypothalamus, the NTS, and the area postrema (a circumventricular organ with reduced blood-brain barrier that monitors circulating signals directly). This is the endocrine arm of the gut-brain axis – slower than vagal signalling but sustained over longer post-meal periods. ^[2]

The ENS-CNS projection: Enteric neurons project to prevertebral ganglia, which in turn project to the spinal cord and brainstem. This pathway provides a third, distinct route for gut-derived neural signals to influence central processing – separate from both vagal and spinal afferents.

Satiety Integration: Hypothalamus and Brainstem

The hypothalamus – particularly the arcuate nucleus – and the brainstem NTS are the primary central integration points for gut-brain axis signals. diabeticstudies

The NTS receives vagal afferent input directly and projects to the hypothalamus, parabrachial nucleus, and limbic system. It is the first central processing station for meal-related gut signals – the point at which the speed of neural signalling from vagal afferents translates into the early satiety sensation that begins during a meal rather than after it. CCK acting on vagal afferents, for example, produces a satiety signal within minutes of duodenal fat exposure – long before caloric intake could be detected through blood glucose or insulin changes.

The arcuate nucleus integrates the circulating hormone signals – GLP-1, PYY, leptin, ghrelin – with the NTS input and with local nutrient sensing (glucose, amino acids) to produce the hypothalamic output that regulates meal termination and inter-meal hunger. Critically, the arcuate nucleus also receives input from the mesolimbic dopamine system – meaning that reward-driven food motivation from the VTA can override hypothalamic satiety signalling when the hedonic drive is sufficiently strong. This is the neural architecture behind eating past fullness in the presence of highly palatable food: the homeostatic satiety circuit and the hedonic motivation circuit are anatomically connected but can be driven in opposite directions simultaneously. ^[2]

The Gut-Brain Axis and GLP-1 Pharmacology

Understanding the gut-brain axis clarifies several otherwise puzzling features of GLP-1 RA pharmacology: jci

Why GLP-1 RAs suppress appetite beyond simple gastric slowing: Pharmacological GLP-1R activation reaches not only ENS and vagal afferent receptors but – because synthetic GLP-1 RAs are DPP-4 resistant and circulate at sustained concentrations – also directly activates GLP-1R in the arcuate nucleus, NTS, and VTA. The appetite suppression from GLP-1 RA medications is therefore simultaneously peripheral (gastric emptying, ENS motility inhibition) and central (hypothalamic satiety signalling, mesolimbic reward suppression). Endogenous GLP-1, which is rapidly degraded by DPP-4 and produces only a pulsatile post-meal signal, cannot reach these central receptors at meaningful concentrations under normal conditions.

Why GI side effects are dose-dependent and often transient: ENS adaptation to sustained GLP-1R activation involves receptor internalisation and downstream signalling desensitisation – the same mechanisms that produce tolerance in other receptor systems. The initial motility inhibition and nausea that many patients experience at GLP-1 RA initiation or dose escalation reflect the ENS’s response to a level of GLP-1R activation it has not previously encountered; desensitisation over two to four weeks explains why GI side effects commonly reduce with continued use at a stable dose.

Why food noise returns after discontinuation: The VTA/GABA mechanism established in the GLP-1 Medication Discontinuation entity – approximately 90% of VTA GLP-1 receptors sitting on inhibitory GABA neurons, with pharmacological GLP-1R activation turning down mesolimbic dopamine signalling volume – is the central arm of the gut-brain axis in operation. When medication is discontinued, this central suppression is withdrawn alongside the peripheral ENS and vagal afferent effects. The return of food noise is not a separate phenomenon from the return of hunger; it is the same gut-brain axis operating without pharmacological override, now re-expressing both its homeostatic hunger drive and its reward-amplifying function simultaneously.

The Gut-Brain Axis and Mental Health

The gut-brain axis has bidirectional consequences for mood and cognition that extend beyond appetite regulation. The ENS-CNS connection means that gut dysfunction – dysmotility, dysbiosis, increased intestinal permeability – can generate ascending neural signals and circulating inflammatory mediators that influence CNS function. The association between irritable bowel syndrome and anxiety/depression, the effect of probiotic interventions on mood outcomes, and the ENS serotonin system’s relationship to central serotonin pathways are all manifestations of this bidirectionality. ^[4]

For us at Creative Touch, the most relevant downstream is the gut-skin axis – the pathway by which gut microbiome dysbiosis and ENS dysfunction influence inflammatory skin conditions via circulating LPS, short-chain fatty acid depletion, and immune dysregulation. This is developed in the Gut Microbiome entity; the ENS and gut-brain axis are the neural signalling complement to the microbiome’s chemical signalling arm in that pathway.