Sucrose Bingeing Reduces the Reward Value of Sucrose

Binge eating occurs primarily on highly palatable food (PF) suggesting that the reward value of food has an important role in this behaviour. Bingeing also leads to reward dysfunction in rats and humans. The rewarding effect of binge eating may involve opioid mechanisms as opioid antagonists reduce PF consumption in animals that binge eat and binge eating produces neuroadaptations of opioid receptors in rodents. We tested this hypothesis by using the conditioned place preference (CPP) paradigm. First we established a sucrose CPP in male and female Long-Evans rats (n=8 for each group) using 1%, 5%, 15%, or 30% sucrose solution. Next, rats underwent the sucrose bingeing model in which separate groups of rats (n=8 for each group) received 12hr and 24hr access to 10% sucrose solution and chow, 12hr access to 0.1% saccharin solution and chow, or 12hr access to chow only every day for 28 days. Immediately following these sessions, rats were conditioned and tested in the CPP paradigm using a 15% sucrose solution. Finally, we examined whether the sucrose bingeing model altered morphine reward in female rats. Rats (n=8 for each group) received 12hr and 24hr access to 10% sucrose solution and chow every day for 28 days. Immediately following this access period, rats were conditioned to morphine (6mL/kg) or saline solution in the CPP paradigm and tested for a CPP. In all experiments, rats drank more sucrose solution than water during conditioning sessions. Male rats did not develop a CPP to any concentration of sucrose solution and females developed a CPP to 15% sucrose solution only. Following the sucrose bingeing protocol, sucrose CPP was attenuated in male rats that binged on sucrose and in all female rats. Sucrose bingeing in females did not affect the development of a CPP to morphine. These results suggest that sucrose consumption and sucrose CPP are measures of different psychological components of reward. Furthermore, sucrose bingeing reduces the rewarding effect of sucrose, but not morphine, suggesting that opioid reward is still intact.

Role of Dopamine D2 Receptors in the Consolidation of Amphetamine-Cue Memory in Conditioned Activity in Rats

The neurotransmitter dopamine (DA) plays an essential role in reward-related incentive learning, whereby neutral stimuli gain the ability to elicit approach and other responses. In an incentive learning paradigm called conditioned activity, animals receive a stimulant drug in a specific environment over the course of several days. When then placed in that environment drug-free, they generally display a conditioned hyperactive response. Modulating DA transmission at different time points during the paradigm has been shown to disrupt or enhance conditioning effects. For instance, blocking DA D2 receptors before sessions generally impedes the acquisition of conditioned activity. To date, no studies have examined the role of D2 receptors in the consolidation phase of conditioned activity; this phase occurs immediately after acquisition and involves the stabilization of memories for long-term storage. To investigate this possible role, I trained Wistar rats (N = 108) in the conditioned activity paradigm produced by amphetamine (2.0 mg/kg, intraperitoneally) to examine the effects of the D2 antagonist haloperidol (doses 0.10, 0.25, 0.50, 0.75, 1.0, & 2.0 mg/kg, intraperitoneally) administered 5 min after conditioning sessions. Two positive control groups received haloperidol 1 h before conditioning sessions (doses 1.0 mg/kg and 2.0 mg/kg). The results revealed that post-session haloperidol at all doses tested did not disrupt the consolidation of conditioned activity, while pre-session haloperidol at 2.0 mg/kg prevented acquisition, with the 1.0 mg/kg group trending toward a block. Additionally, post-session haloperidol did not diminish activity during conditioning days, unlike pre-session haloperidol. One possible reason for these findings is that the consolidation phase may have begun earlier than when haloperidol was administered, since the conditioned activity paradigm uses longer learning sessions than those generally used in consolidation studies. Future studies may test if conditioned activity can be achieved with shorter sessions; if so, haloperidol would then be re-tested at an earlier time point. D2 receptor second messenger systems may also be investigated in consolidation. Since drug-related incentive stimuli can evoke cravings in those with drug addiction, a better understanding of the mechanisms of incentive learning may lead to the development of solutions for these individuals.