GLP-1 antagonism with exendin (9-39) fails to increase spontaneous meal size in rats

Intravenous and intraperitoneal (IP) administration of glucagon like peptide-1 (7-36)-amide (GLP-1) inhibits eating, but the physiological relevance of this satiating effect is not yet clear. We addressed this issue by testing the effects of the GLP-1 receptor antagonist exendin 9-39 (Ex (9-39)) on spontaneous eating and on the satiating effect of exogenous GLP-1. Adult, male Sprague-Dawley rats were equipped with chronic IP catheters and received intrameal infusions (0.2 ml/min, 2.5 min) that were remotely triggered 2-3 min after the onset of the first or the second spontaneous nocturnal meal. Infusions of 10, but not 5 or 2.5 nmol/kg body weight (BW) GLP-1 significantly reduced the size of the first spontaneous nocturnal meal compared to vehicle. The first intermeal interval, subsequent meal sizes and cumulative food intake were unchanged by 10 nmol/kg GLP-1. Infusions of 10 or 30 nmol/kg BW Ex (9-39) during the second spontaneous nocturnal meal did not affect the size of that meal and decreased rather than increased meal duration. Co-infusion of 30 nmol/kg BW Ex (9-39) prevented the satiating effect of 10 nmol/kg BW GLP-1 during the first spontaneous nocturnal meal, but again did not increase meal size by itself. That a dose of Ex (9-39) that is sufficient to block the satiating effect of exogenous GLP-1 failed to increase meal size when administered alone under comparable conditions suggests that endogenous intestinal GLP-1 is not required for the control of spontaneous meal size in rats under our conditions. The situations in which GLP-1 is of physiological relevance for satiation require further research.