Role of mesolimbic ghrelin in the acquisition of cocaine reward.

The present study examined the ability of ghrelin administration into either the ventral tegmental area (VTA) or nucleus accumbens (NAc) to potentiate cocaine-induced conditioned place preference (CPP). Additionally, we examined the impact of co-injection of the ghrelin 1a antagonist JMV 2959 with ghrelin in order to evaluate the potential attenuation of ghrelin's effects on cocaine-induced CPP. Adult male Sprague-Dawley rats were allowed simultaneous access to either side of the CPP apparatus to establish baseline chamber preferences. The rats were then restricted to either their non-preferred or preferred side over the course of conditioning which lasted for a total of 8 consecutive days. On days in which rats were restricted to their non-preferred side, systemic cocaine (0.5-5.0 mg/kg IP) followed by central ghrelin (300 pmol), or co-administration of ghrelin (300 pmol) with JMV 2959 (10 µg), was administered either into the VTA or NAc immediately prior to the conditioning period. On alternate days rats were treated with vehicle then placed into what was initially determined to be their preferred side. CPP was calculated as the difference in the percentage of total time spent in the treatment-paired compartment during the post-conditioning session and the pre-conditioning session. Our results indicated that ghrelin potentiated cocaine-induced CPP and that this effect was attenuated by JMV 2959. Overall, these findings provide further evidence that mesolimbic ghrelin signaling is indeed critically involved in mediating the rewarding effects of cocaine.

Exendin-4 antagonizes the metabolic action of acylated ghrelinergic signaling in the hypothalamic paraventricular nucleus.

In the current study we investigated the interaction of hypothalamic paraventricular nucleus (PVN) glucagon-like peptide-1 (GLP-1) and ghrelin signaling in the control of metabolic function. We first demonstrated that acylated ghrelin injected directly into the PVN reliably altered the respiratory exchange ratio (RER) of adult male Sprague Dawley rats. All testing was carried out during the initial 2 h of the nocturnal cycle using an indirect open circuit calorimeter. Results indicated that acylated ghrelin induced a robust increase in RER representing a shift toward enhanced carbohydrate oxidation and reduced lipid utilization. In contrast, treatment with comparable dosing of des-acyl ghrelin failed to significantly impact metabolic activity. In separate groups of rats we subsequently investigated the ability of exendin-4 (Ex-4), a GLP-1 analogue, to alter acylated ghrelin's metabolic effects. Rodents were treated with either systemic or direct PVN Ex-4 followed by acyl ghrelin microinjection. While our results showed that both systemic and PVN administration of Ex-4 significantly reduced RER, importantly, Ex-4 pretreatment itself reliably inhibited the impact of ghrelin on RER. Overall, these findings provide increasingly compelling evidence that GLP-1 and ghrelin signaling interact in the neural control of metabolic function within the PVN.