What GLP-1 Is
GLP-1 stands for Glucagon-Like Peptide-1. It is a 30-amino-acid incretin hormone secreted by intestinal L-cells in the distal small intestine and colon in response to nutrient intake — particularly carbohydrates and fats. The active circulating form is GLP-1(7-36) amide; an extended form GLP-1(7-37) exists in smaller proportions with similar activity.
The native hormone has an extremely short half-life of approximately 1–2 minutes. It is rapidly degraded by the enzyme dipeptidyl peptidase-4 (DPP-4), which cleaves the N-terminal His-Ala dipeptide and inactivates it. This rapid degradation is the primary reason synthetic GLP-1 receptor agonists exist: pharmacologically useful research compounds must resist DPP-4 cleavage and persist in circulation long enough to engage the receptor meaningfully.
Why "glucagon-like"? The peptide is encoded by the same gene as glucagon — the proglucagon gene — but processed differently in different tissues. In pancreatic alpha cells, the gene yields glucagon. In intestinal L-cells, the gene yields GLP-1 (and GLP-2). The shared precursor explains the name, not shared function.
The GLP-1 Receptor (GLP-1R) and Where It Is Expressed
GLP-1R is a Class B G-protein-coupled receptor expressed across multiple tissues. Receptor density and downstream signalling differ by tissue, which is why GLP-1 agonism produces a coordinated multi-system response rather than a single isolated effect:
| Tissue | GLP-1R role | Observed effect when receptor is engaged |
|---|---|---|
| Pancreatic beta cells | Glucose-dependent insulin secretion | Insulin released only when glucose elevated — minimal hypoglycaemia risk |
| Pancreatic alpha cells | Glucagon suppression | Reduced fasting and post-meal glucose |
| Hypothalamus (arcuate nucleus) | Satiety signalling | Reduced appetite, reduced energy intake |
| Brainstem (NTS, area postrema) | Nausea and aversion signalling | The GI adverse event profile common to the class |
| Gastric smooth muscle | Delayed gastric emptying | Prolonged postprandial satiety, slower glucose absorption |
| Cardiovascular tissue | Endothelial signalling, atheroma stabilisation | MACE reduction observed in SUSTAIN-6, SELECT, LEADER |
| Liver | Indirect (no direct GLP-1R), via insulin/glucagon balance | Reduced hepatic glucose output |
The central nervous system effects — appetite suppression and the nausea/aversion responses — are the most clinically prominent in research populations. The peripheral GI receptors (gastric, intestinal) drive the slowed gastric emptying that underpins both satiety and the GI adverse event burden characteristic of the class.
The Synthetic Peptide Class That Targets GLP-1R
The native GLP-1 hormone's 1–2 minute half-life makes it impractical for research protocols. The synthetic peptide class engineered around the GLP-1R target has progressively extended half-life through structural modifications:
| Compound | Receptors targeted | Half-life | Structural strategy |
|---|---|---|---|
| Native GLP-1(7-36) | GLP-1R | ~1–2 min | Unmodified hormone — rapidly cleared by DPP-4 |
| Liraglutide | GLP-1R | ~13 hr | C16 fatty acid acylation → albumin binding |
| Semaglutide | GLP-1R | ~7 days | C18 fatty diacid + Aib substitution → DPP-4 resistance + strong albumin binding |
| Tirzepatide | GLP-1R + GIPR | ~5 days | C20 fatty diacid + dual-receptor backbone |
| Retatrutide | GLP-1R + GIPR + GcgR | ~6 days | Triple-agonist backbone with fatty acid modification |
All three compounds currently in active research use — Semaglutide, Tirzepatide, and Retatrutide — bind GLP-1R as a primary target. They differ in whether they also bind other incretin receptors. Tirzepatide adds GIPR (see GIP Receptor Explained); Retatrutide adds both GIPR and GcgR (see Glucagon Receptor Explained).
For a side-by-side comparison of the three compounds, see the GLP-1 Peptides Comparison Guide.
GLP-1 in the Brain: The Appetite Mechanism
The central-nervous-system arm of GLP-1 signalling is responsible for the large weight-loss outcomes observed in clinical research. GLP-1R is expressed in the arcuate nucleus of the hypothalamus (POMC neurons) and in the nucleus tractus solitarius and area postrema of the brainstem. Receptor engagement at these sites produces a coordinated reduction in food reward salience, prolonged meal-induced satiety, and altered preference for energy-dense foods.
This mechanism is distinct from the peripheral metabolic effects (insulin secretion, glucagon suppression). It is why GLP-1 receptor agonists produce body weight reductions far in excess of what their glycaemic effects alone would predict, and why STEP 4 maintenance data showed weight regain after discontinuation: removing the central signal restores baseline appetite drive.
For a detailed treatment of the brain-side mechanism including the appetite-vs-aversion distinction, see How GLP-1 Peptides Work in the Brain.
GLP-1 Research in Australia: Current Compounds
For Australian researchers working with GLP-1 receptor agonist peptides, the three primary compounds in current research use are research-grade Semaglutide, Tirzepatide, and Retatrutide. Each represents a different point on the receptor-target spectrum:
- Single-receptor (GLP-1R only): Semaglutide. Most extensively studied — STEP, SUSTAIN, SELECT programmes. Benchmark compound for comparative research.
- Dual-receptor (GLP-1R + GIPR): Tirzepatide. SURMOUNT programme complete; the established "twincretin" reference compound.
- Triple-receptor (GLP-1R + GIPR + GcgR): Retatrutide. Phase 2 published; Phase 3 (TRIUMPH) ongoing. The compound with the highest Phase data weight-reduction outcome.
RetaLABS supplies all three as research-grade lyophilised peptides for Australian laboratory research. None are approved as therapeutics by the TGA. See the individual Semaglutide, Tirzepatide, and Retatrutide research guides for full per-compound detail.