Effects of opioid receptor ligands in rats trained to discriminate 22 from 2 hours of food deprivation suggest a lack of opioid involvement in eating for hunger

Abstract

It is well accepted that opioids promote feeding for reward. Some studies suggest a potential involvement in hunger-driven intake, but they suffer from the scarcity of methodologies differentiating between factors that intersect eating for pleasure versus energy. Here, we used a unique food deprivation discrimination paradigm to test a hypothesis that, since opioids appear to control feeding reward, injection of opioid agonists would not produce effects akin to 22 h of food deprivation. We trained rats to discriminate between 22 h and 2 h food deprivation in a two-lever, operant discrimination procedure. We tested whether opioid agonists at orexigenic doses produce discriminative stimulus effects similar to 22 h deprivation. We injected DAMGO, DSLET, or orphanin FQ in the paraventricular hypothalamic nucleus (PVN), a site regulating hunger/satiety, and butorphanol subcutaneously (to produce maximum consumption). We assessed the ability of the opioid antagonist, naltrexone, to reduce the discriminative stimulus effects of 22 h deprivation and of the 22 h deprivation-like discriminative stimulus effects of PVN-injected hunger mediator, neuropeptide Y (NPY). In contrast to PVN NPY, centrally or peripherally injected opioid agonists failed to induce discriminative stimuli similar to those of 22 h deprivation. In line with that, naltrexone did not reduce the hunger discriminative stimuli induced by either 22 h deprivation or NPY administration in 2 h food-restricted subjects, even though doses used therein were sufficient to decrease deprivation-induced feeding in a non-operant setting in animals familiar with consequences of 2 h and 22 h deprivation. We conclude that opioids promote feeding for reward rather than in order to replenish lacking energy.