INTAKE OF HIGH-FAT FOOD IS SELECTIVELY ENHANCED BY MU-OPIOID RECEPTORSTIMULATION WITHIN THE NUCLEUS-ACCUMBENS

The present study was designed to further investigate the nature of fe eding induced by opioid stimulation of the nucleus accumbens through a n examination of the effects of intraaccumbens (ACB) opioids on macron utrient selection. In 3-hr tests of free-feeding (satiated) rats, intr a-ACB administration of the mu receptor agonist D-Ala(2),N,Me-Phe(4),G ly-ol(5)-enkephalin (DAMGO; 0, 0.025, 0.25 and 2.5 mu g bilaterally) m arkedly enhanced the intake of fat or carbohydrate when the diets were presented individually (although the effect on fat intake was much gr eater in magnitude). Intra-ACB injections of DAMGO, however, produced potent preferential stimulatory effects on fat ingestion with no effec t on carbohydrate ingestion when both fat and carbohydrate diets were present simultaneously. Moreover, this selective stimulation of fat in take was independent of base-line diet preference and could be blocked by systemic injection of naltrexone (5 mg/kg). We also examined the e ffect of 24-hr food deprivation on the pattern of macronutrient intake in rats with access to both carbohydrate and fat. In contrast to the DAMGO-induced selective enhancement of fat intake, food deprivation si gnificantly increased the intake of both diets to the same extent; how ever, in this case, only the stimulated fat intake was blocked by syst emic naltrexone. Intra-ACB administration of DAMGO in hungry rats prod uced an effect similar to that observed in free-feeding rats; preferen ce was strongly shifted to fat intake. Similarly, the opioid antagonis t naltrexone (20 mu g) infused directly into ACB preferentially decrea sed fat intake in hungry rats. These findings suggest that endogenous opioids within the ventral striatum may participate in the mechanisms governing preferences for highly palatable foods, especially those ric h in fat.