The GLP-1 agonist exendin-4 attenuates self-administration of sweetened fat on fixed and progressive ratio schedules of reinforcement in rats

Abstract

GLP-1 agonists such as exendin-4 (EX4) are used in the treatment of type-2 diabetes and have the additional benefit of promoting weight loss. GLP-1 agonists decrease feeding through peripheral effects, but recent evidence suggests they may also influence sweet or high fat preference, as well as motivation to obtain these tastants. Yet it remains unclear how GLP-1-induced alterations in food preference influences decreases in overall feeding. The current study sought to determine if EX4 affects the reinforcing strength and consumption of a highly palatable sweet/fat reinforcer. Rats were trained to self-administer sweetened vegetable shortening (SVS) under fixed (FR) and progressive ratio (PR) schedules of reinforcement. EX4 (0.3–2.4μg/kg, i.p.) administered one hour prior to operant sessions significantly reduced responses for SVS under both FR and PR schedules, although the lowest active dose (0.6μg/kg) significantly suppressed FR responding only. EX4 also dose dependently decreased locomotor activity (0.6–2.4μg/kg doses), but did not enhance acute kaolin intake, suggesting that nausea did not influence the self-administration results. Analysis of ED50 values show that EX4 is more effective at inhibiting FR responding versus PR, indicating that EX4 may have more potent effects on amount consumed versus motivation for SVS. Although EX4 caused generalized locomotor suppression, these results do not fully explain the decreases in operant responding. For example, a dose of EX4 (0.6μg/kg) that significantly suppressed locomotor activity did not affect the mean total number of lever presses during PR sessions (59±15), although it did significantly reduce lever presses during FR sessions (21±3). In addition, the pattern of intake was constant at the beginning of the sessions in both PR and FR schedules, regardless of the dose. Together these data suggest that EX4 inhibits consumption of a palatable high sweet/high fat reinforcer potentially through altering satiety.