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.

Accumbal ghrelin and glucagon-like peptide 1 signaling in alcohol reward in female rats.

The present study investigated the relationship between accumbal ghrelin and glucagon-like peptide 1 (GLP-1) signaling in alcohol reward in female rats. Animals with guide cannulae targeting the nucleus accumbens core (NAcC) and shell (NAcS) were habituated to alcohol for 12 weeks through a two-bottle intermittent access paradigm. JMV2959, a ghrelin antagonist, and exendin-4 (Ex-4), a GLP-1 agonist, were microinjected at the onset of the nocturnal cycle. Alcohol, food, water, and total fluid intake were measured 2, 6, and 24 h postinjection. Results indicated that JMV2959 reduced alcohol consumption when injected into both the NAcC and NAcS. Ex-4 administration as well as combined JMV2959 and Ex-4 treatment reduced intake when injected into the NAcS, but not the NAcC. These effects were time-dependent. JMV2959 had no effect on food intake when administered into either the NAcC or the NAcS, whereas Ex-4 decreased food intake when injected separately into both structures. The combination of JMV2959 and Ex-4 decreased food intake when administered only into the NAcC. These effects were also time-dependent. No estrous-related effects on alcohol or food intake were found. However, water and total fluid intake were increased during the metestrus and diestruses phases of the estrous cycle compared with the proestrus and estrus phases. Overall, these findings demonstrate the importance of accumbal ghrelin and GLP-1 signaling in alcohol reward and appetitive motivation.

Ghrelin enhances food intake and carbohydrate oxidation in a nitric oxide dependent manner.

In the present study we sought to investigate interactions between hypothalamic nitric oxide (NO) and ghrelin signaling on food intake and energy substrate utilization as measured by the respiratory exchange ratio (RER). Guide cannulae were unilaterally implanted in either the arcuate (ArcN) or paraventricular (PVN) nuclei of male Sprague-Dawley rats. Animals were pretreated with subcutaneous (2.5-10mg/kg/ml) or central (0-100pmol) N-nitro-l-Arginine methyl ester (l-NAME) followed by 50pmol of ghrelin administered into either the ArcN or PVN. Both l-NAME and ghrelin were microinjected at the onset of the active cycle and food intake and RER were assessed 2h postinjection. RER was measured as the ratio of the volume of carbon dioxide expelled relative to the volume of oxygen consumed (VCO2/VO2) using an open-circuit indirect calorimeter. Our results demonstrated that peripheral and central l-NAME pretreatment dose-dependently attenuated ghrelin induced increases in food intake and RER in either the ArcN or PVN. In fact the 100pmol dose largely reversed the metabolic effects of ghrelin in both anatomical regions. These findings suggest that ghrelin enhancement of food intake and carbohydrate oxidation in the rat ArcN and PVN is NO-dependent.

The glucagon-like peptide-1 analog exendin-4 antagonizes the effect of acyl ghrelin on the respiratory exchange ratio.

The present study investigated the interaction of hypothalamic arcuate nucleus (ArcN) ghrelin and glucagon-like peptide-1 (GLP-1) signaling on metabolic function. Using indirect calorimetry, we first showed that acylated ghrelin, administered into the ArcN, significantly increased the respiratory exchange ratio (RER) in male Sprague-Dawley rats, representing a shift in fuel utilization toward enhanced carbohydrate oxidation and reduced lipid utilization. In contrast, treatment with similar doses of des-acyl ghrelin failed to induce reliable changes in RER. We then examined the ability of exendin-4 (Ex4) to alter acyl ghrelin's energetic effects. Ex4 is a GLP-1 agonist and has been reported previously to suppress food intake in rodent models. Rats were treated with either systemic or direct ArcN Ex4, followed by acyl ghrelin. Our results indicated that both systemic and central injections of Ex4 alone significantly reduced RER and, importantly, Ex4 pretreatment reliably attenuated the impact of ghrelin on RER. Overall, these findings provide compelling evidence that ghrelin and GLP-1 signaling interact in the hypothalamic control of metabolic function.