Concomitant release of ventral tegmental acetylcholine and accumbal dopamine by ghrelin in rats

Abstract

Ghrelin, an orexigenic peptide, regulates energy balance specifically via hypothalamic circuits. Growing evidence suggest that ghrelin increases the incentive value of motivated behaviours via activation of the cholinergic-dopaminergic reward link. It encompasses the cholinergic afferent projection from the laterodorsal tegmental area (LDTg) to the dopaminergic cells of the ventral tegmental area (VTA) and the mesolimbic dopamine system projecting from the VTA to nucleus accumbens (N.Acc.). Ghrelin receptors (GHS-R1A) are expressed in these reward nodes and ghrelin administration into the LDTg increases accumbal dopamine, an effect involving nicotinic acetylcholine receptors in the VTA. The present series of experiments were undertaken directly to test this hypothesis. Here we show that ghrelin, administered peripherally or locally into the LDTg concomitantly increases ventral tegmental acetylcholine as well as accumbal dopamine release. A GHS-R1A antagonist blocks this synchronous neurotransmitter release induced by peripheral ghrelin. In addition, local perfusion of the unselective nicotinic antagonist mecamylamine into the VTA blocks the ability of ghrelin (administered into the LDTg) to increase N.Acc.-dopamine, but not VTA-acetylcholine. Collectively our data indicate that ghrelin activates the LDTg causing a release of acetylcholine in the VTA, which in turn activates local nicotinic acetylcholine receptors causing a release of accumbal dopamine. Given that a dysfunction in the cholinergic-dopaminergic reward system is involved in addictive behaviours, including compulsive overeating and alcohol use disorder, and that hyperghrelinemia is associated with such addictive behaviours, ghrelin-responsive circuits may serve as a novel pharmacological target for treatment of alcohol use disorder as well as binge eating.

Figure 1. A ghrelin receptor antagonist attenuates ghrelin-induced ventral tegmental acetylcholine and accumbal dopamine release in rats.

A) Ghrelin (0.33 mg/kg) increased ventral tegmental acetylcholine release relative to vehicle treatment and this increase was attenuated by pre-treatment with JMV2959 (3 mg/kg, i.p.). No difference was observed between vehicle-vehicle and JMV2959-ghrelin treatment and JMV2959 had no effect on acetylcholine release per se. B) Ghrelin (0.33 mg/kg) increased accumbal dopamine release relative to vehicle treatment and this increase was attenuated by pre-treatment with JMV2959. No difference was observed between vehicle-vehicle and JMV2959-ghrelin treatment and JMV2959 had no effect on dopamine release per se (n = 8 for vehicle-vehicle (square), n = 5 for vehicle-ghrelin (circle), n = 6 for JMV2959-vehicle (rhomb), n = 7 for JMV2959-ghrelin (triangle). All values represent mean ± SEM (***P<0.001, **P<0.01 and *P<0.05).

Figure 2. Ghrelin (into the LDTg) increase VTA-acetylcholine and N.Acc.-dopamine and this involves nicotinic acetylcholine receptors in the VTA.

A) Ghrelin (locally administered into the LDTg at a dose of 1 µg in 0.5 µl) increased ventral tegmental acetylcholine release relative to vehicle treatment. Pre-treatment with the unselective nicotinic acetylcholine receptor antagonist, mecamylamine, locally into the VTA (300 µM) did not affect the ability of ghrelin to increase VTA-acetylcholine. Mecamylamine had no effect on acetylcholine release per se. B) Ghrelin (locally administered into the LDTg at a dose of 1 µg in 0.5 µl) increased accumbal dopamine release relative to vehicle treatment. This increase was attenuated by pre-treatment with the unselective nicotinic acetylcholine receptor antagonist, mecamylamine, locally into the VTA (300 µM). Mecamylamine had no effect on dopamine release per se (n = 10 for vehicle-vehicle (square), n = 11 for vehicle-ghrelin (circle), n = 11 for mecamylamine-vehicle (rhomb), n = 11 for mecamylamine-ghrelin (triangle). All values represent mean ± SEM (***P<0.001, **P<0.01 for vehicle-vehicle versus vehicle-ghrelin and (+++P<0.001, ++P<0.01 for vehicle-vehicle versus mecamylamine-ghrelin).