Gut peptide GLP-1 and its analogue, Exendin-4, decrease alcohol intake and reward

Abstract

Glucagon-like-peptide-1 (GLP-1) is a gut- and neuro-peptide with an important role in the regulation of food intake and glucose metabolism. Interestingly, GLP-1 receptors (GLP-1R) are expressed in key mesolimbic reward areas (including the ventral tegmental area, VTA), innervated by hindbrain GLP-1 neurons. Recently GLP-1 has emerged as a potential regulator of food reward behavior, an effect driven by the mesolimbic GLP-1Rs. Its role in other reward behaviors remains largely unexplored. Since a considerable overlap has been suggested for circuitry controlling reward behavior derived from food and alcohol we hypothesized that GLP-1 and GLP-1Rs could regulate alcohol intake and alcohol reward. We sought to determine whether GLP-1 or its clinically safe stable analogue, Exendin-4, reduce alcohol intake and reward. To determine the potential role of the endogenous GLP-1 in alcohol intake we evaluated whether GLP-1R antagonist, Exendin 9-39, can increase alcohol intake. Furthermore, we set out to evaluate whether VTA GLP-1R activation is sufficient to reduce alcohol intake. Male Wistar rats injected peripherally with GLP-1 or Exendin-4 reduced their alcohol intake in an intermittent access two bottle free choice drinking model. Importantly, a contribution of endogenously released GLP-1 is highlighted by our observation that blockade of GLP-1 receptors alone resulted in an increased alcohol intake. Furthermore, GLP-1 injection reduced alcohol reward in the alcohol conditioned place preference test in mice. To evaluate the neuroanatomical substrate linking GLP-1 with alcohol intake/reward, we selectively microinjected GLP-1 or Exendin 4 into the VTA. This direct stimulation of the VTA GLP-1 receptors potently reduced alcohol intake. Our findings implicate GLP-1R signaling as a novel modulator of alcohol intake and reward. We show for the first time that VTA GLP-1R stimulation leads to reduced alcohol intake. Considering that GLP-1 analogues are already approved for clinical use, this places the GLP system as an exciting new potential therapeutic target for alcohol use disorders.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Peripheral administration of GLP-1 or EX4 reduces voluntary alcohol intake. In an intermittent-access 20% ethanol drinking paradigm, Wistar rats peripherally injected with GLP-1 (0.1 mg/kg) drank less alcohol than those injected with vehicle at 1 h (n = 12 per treatment group (A)).

The reduced alcohol drinking response was primarily exhibited by high alcohol consuming rats (HIGH C, top 30% consumers) and was not detected in low alcohol consuming rats (LOW C; bottom 30%) (B). Rats that received an IP injection of EX4 at a dose of either 0.3 µg (C) or 1.0 µg/kg (D) reduced their 20% ethanol intake at 1 h after alcohol exposure n = 13−25. All values represent mean ± SEM. VEH, vehicle for GLP-1 (glucagon-like-peptide-1); EtOH, ethanol. *p

Figure 2. Peripheral administration of GLP-1 reduces alcohol reward.

Mice treated with vehicle on the testing day spent significantly more time in the compartment previously (during the conditioning sessions) paired with alcohol as compared to the compartment paired with saline. In contrast, mice treated with 0.02 mg of GLP-1 spent an equal amount of time in both the saline- and alcohol-paired compartments (A). Alcohol induced a significant preference for the compartment it was paired with over the compartment paired with saline during the conditioning sessions in NMRI mice injected with vehicle (n = 48) but not those treated with GLP-1 (n = 31) (B). %CPP was determined with the following formula ((test-pretest)/(total time- pretest))×100 to indicate the % preference above a neutral response (i.e. equal preference for each compartment). All values represent mean ± SEM. *p

Figure 3. Blockade of GLP-1Rs increases voluntary alcohol intake.

Rats (n = 12−13) peripherally injected with 0.1 mg/kg EX9, a selective GLP-1R antagonist, displayed a tendency for increased consumption of 20% ethanol during the first hour of alcohol exposure (A) that reached significance at the 24 h measurement time-point (B). All values represent mean ± SEM. #p<0.1, *p<0.05.

Figure 4. Identification of the mesolimbic VTA as the neuroanatomical substrate for GLP-1R-linked effects on alcohol consumption.

VTA-selective unilateral microinjections of GLP-1 (vehicle n = 11; GLP-1 1 µg n = 7, A−B) and EX4 (vehicle n = 9; EX4 0.1 µg n = 9, C−D) reduced 20% ethanol consumption during a 16 h drinking session. A diagram based on Paxinos and Watson at the level of bregma −5.40 mm shows a representative VTA injection site (E). Additionally, schematics illustrate the injection site for each rat from the GLP-1 (F) and the EX4 (G) study. Black circles represent vehicle-injected rats, grey drug-injected and white missed placement. All values represent mean ± SEM. Aq; aqueduct, SNR; substantia nigra pars reticulata. #p<0.1,*p<0.05, ***p<0.005.