Peripheral administration of glucagon-like peptide (GLP)-1 reduces food intake in animals and humans, but the sites and mechanism of this effect and its physiological significance are not yet clear. To investigate these issues, we prepared rats with chronic catheters and infused GLP-1 (0.2 ml/min; 2.5 or 5.0 min) during the first spontaneous dark-phase meals. Infusions were remotely triggered 2–3 min after meal onset. Hepatic portal vein (HPV) infusion of 1.0 or 3.0 (but not 0.33) nmol/kg GLP-1 reduced the size of the ongoing meal compared with vehicle without affecting the subsequent intermeal interval, the size of subsequent meals, or cumulative food intake. In double-cannulated rats, HPV and vena cava infusions of 1.0 nmol/kg GLP-1 reduced meal size similarly. HPV GLP-1 infusions of 1.0 nmol/kg GLP-1 also reduced meal size similarly in rats with subdiaphragmatic vagal deafferentations and in sham-operated rats. Finally, HPV and ip infusions of 10 nmol/kg GLP-1 reduced meal size similarly in sham-operated rats, but only HPV GLP-1 reduced meal size in subdiaphragmatic vagal deafferentation rats. These data indicate that peripherally infused GLP-1 acutely and specifically reduces the size of ongoing meals in rats and that the satiating effect of ip, but not iv, GLP-1 requires vagal afferent signaling. The findings suggest that iv GLP-1 infusions do not inhibit eating via hepatic portal or hepatic GLP-1 receptors but may act directly on the brain.

Intrameal hepatic portal and intraperitoneal (IP) infusions of GLP-1 reduce meal size in rats, but only IP GLP-1 requires vagal afferent signaling for this effect.