Prior exposure to low or high fat milk enhances naloxone anorexia in rats

Opioid peptides have been implicated in reward-related components of eating, especially with fatty and sweetened foods. This study examined the effect of pre-exposure of rats to milk on the subsequent anorexic effect of naloxone in a separate daily mash intake test. Rats were given constant access to either skimmed or whole milk for 15 days, in addition to normal diet. A third (control) group received only the normal diet. Subsequently, all groups received 0, 0.125, 0.5 and 1.0 mg/kg naloxone prior to their daily mash intake test. Naloxone caused dose-dependent reductions in mash intake in all groups, but this effect was significantly greater in the group with whole milk than in controls, with the group with skimmed milk intermediate. Daily intakes of milk were similar with skimmed and whole milk, and milk intake in separate two-bottle choice tests provided no evidence for overall preferences for either solution. These results suggest that prior exposure to milk enhances opioid involvement in feeding, and possible mechanisms for this are discussed.