Requirement of central ghrelin signaling for alcohol reward

Abstract

The stomach-derived hormone ghrelin interacts with key CNS circuits regulating energy balance and body weight. Here we provide evidence that the central ghrelin signaling system is required for alcohol reward. Central ghrelin administration (to brain ventricles or to tegmental areas involved in reward) increased alcohol intake in a 2-bottle (alcohol/water) free choice limited access paradigm in mice. By contrast, central or peripheral administration of ghrelin receptor (GHS-R1A) antagonists suppressed alcohol intake in this model. Alcohol-induced locomotor stimulation, accumbal dopamine release and conditioned place preference were abolished in models of suppressed central ghrelin signaling: GHS-R1A knockout mice and mice treated with 2 different GHS-R1A antagonists. Thus, central ghrelin signaling, via GHS-R1A, not only stimulates the reward system, but is also required for stimulation of that system by alcohol. Our data suggest that central ghrelin signaling constitutes a potential target for treatment of alcohol-related disorders.

Fig. 1.

Ghrelin administration into brain ventricles (i.c.v.) or into specific tegmental areas increased whereas a GHS-R1A antagonist (i.p.) decreased alcohol intake in C57BL/6 mice. In a 2-bottle (alcohol/water) free choice limited access paradigm in C57BL/6 mice ghrelin increased alcohol intake (g/kg/90 min) relative to the vehicle treatment when administered into (A) the third ventricle (n = 23 in both groups; **, P < 0.01, paired t test), (B) bilaterally into the VTA (n = 5 for vehicle and n = 7 for ghrelin; **, P < 0.01, unpaired t test), or (C) bilaterally into the LDTg (n = 6 for vehicle and n = 5 for ghrelin; *, P < 0.05, unpaired t test). (D) Peripheral injection of a GHS-R1A antagonist, JMV2959, decreased alcohol consumption compared to vehicle in this paradigm in C57BL/6J mice [F (1, 28) = 15.68, P = 0.001] (n = 15 in each group; *, P < 0.001, Bonferroni post hoc test). All values represent mean ± SEM.

Fig. 2.

Suppressed ghrelin signaling, by either ghrelin receptor (GHS-R1A) antagonist (JMV2959) or GHS-R1A knockout mice, attenuates alcohol (Alc) reward. (A) Alcohol-induced locomotor stimulation was attenuated by a single i.p. injection of JMV2959 [but not by vehicle (Veh) injection] to NMRI mice [F (3, 28) = 12.86, P = 0.001] (n = 8 in each group; ***, P < 0.001, #, P = n.s. for Veh–Veh vs. JMV–Alc, Bonferroni post hoc test). (B) The alcohol-induced increase in accumbal dopamine release was absent in GHS-R1A antagonist (JMV2959, i.p.)-mice but not in vehicle-treated NMRI mice (n = 8 in each group). In B we first demonstrated a significant effect of alcohol to increase dopamine release in comparison to vehicle treatment [treatment F (1, 14) = 0.01, P = 0.65; time F (10, 140) = 4.34, P = 0.001; treatment × time interaction F (10, 140) = 0.79, P = 0.64]. Second, we showed that pretreatment with JMV2959 (i.p.) attenuated the alcohol-induced increase in dopamine release compared to vehicle pretreatment [treatment F (1, 14) = 14.12, P = 0.002; time F (5, 70) = 2.38, P = 0.05; treatment × time interaction F (5, 70) = 1.13, P = 0.35]. This difference (P < 0.001) was evident at time intervals 200, 220, 240, and 260 min (*, P < 0.05; **, P < 0.01; ***, P < 0.001, Bonferroni post hoc test). (C) The acute locomotor stimulation of alcohol observed in wild-type mice was abolished in both heterozygous and homozygous GHS-R1A knockout mice [F (5, 56) = 4.08, P = 0.003]. No difference was found in the locomotor response between any of the genotypes treated with vehicle, indicating that deletion of GHS-R1A does not affect activity per se (wt/wt: n = 8 for Veh and n = 6 for Alc; wt/−: n = 17 for Veh and n = 16 for Alc; −/−: n = 8 for Veh and n = 7 for Alc; *, P < 0.05; Tukey/Kramer). (D) The alcohol-induced increase in extracellular accumbal dopamine release was attenuated in both heterozygous (n = 13) and homozygous GHS-R1A knockout mice (n = 11) compared to wild-type mice (n = 8) [treatment F (2, 29) = 9.55, P = 0.001; time F (14, 406) = 4.65, P = 0.001; treatment × time interaction F (28, 406) = 2.79, P = 0.001]. This difference (P < 0.001) was observed at the time intervals 80, 100, 120, 140, 160, and 180 min (**, P < 0.01; ***, P < 0.001; Bonferroni post hoc test). (E) The alcohol-induced CPP was attenuated by an acute single i.p. injection of the GHS-R1A antagonist, JMV2959, in NMRI mice [F (3, 27) = 4.96, P = 0.01]. An alcohol-induced CPP in mice pretreated with vehicle (n = 7) was obtained and pretreatment with JMV2959 (n = 8) blocked this stimulation. No significant difference was observed between vehicle–vehicle (n = 8) and JMV2959-alcohol treatment (**, P < 0.01; #, P = n.s. for Veh–Veh vs. JMV–Alc, Bonferroni post hoc test). No effect of JMV2959 per se was observed (n = 8). (F) The alcohol-induced CPP was attenuated by an acute central administration of the GHS-R1A antagonist, BIM28163 (i.c.v.), in NMRI mice [F (3, 23) = 4.98, P = 0.01]. An alcohol-induced CPP in mice pretreated with vehicle (n = 7) was obtained and pretreatment with BIM28163 (n = 6) blocked this stimulation. No significant difference was observed between vehicle–vehicle (n = 6) and BIM28163-alcohol treatment (**, P < 0.01; #, P = n.s. for Veh–Veh vs. BIM–Alc, Bonferroni post hoc test). No effect of BIM28163 per se was observed (n = 8). (G) Alcohol induces a significant CPP in wild-type mice (n = 8), but not in GHS-R1A knockout mice (n = 5) [F (1, 11) = 5.15, P = 0.04]. A significant difference between CPP in wild type compared to GHS-R1A knockout mice was obtained (**, P < 0.01, Bonferroni post hoc test). All values represent mean ± SEM.