NALOXONE EFFECTS ON SUCROSE-MOTIVATED BEHAVIOR

The opioid system plays an important role in feeding. In general, opio id agonists typically increase feeding and opioid antagonists decrease feeding in nonfood restricted animals. In food restricted animals the effects of these drugs are substantially reduced. Opioid antagonists have shown a marked effectiveness at reducing consumption of sweet foo ds, Explanations for this robust effect have typically focused on drug induced changes in taste, taste perception, or palatability. The curr ent study relates the effects of the opioid antagonist naloxone on mot ivation to obtain different sucrose concentrations to the drug's effec ts on unrestricted sucrose solution consumption. Changes in motivation to respond were assessed under a progressive ratio reinforcement sche dule (PR) which required increased response cost for each successive u nit of sucrose solution. Motivation, as measured by the PR, increased as sucrose concentration increased and naloxone produced a dose-depend ent decrease in motivation to respond for a given sucrose concentratio n. Thus, the effectiveness of naloxone was indirectly related to stren gth of the sucrose concentration. Under unrestricted access to sucrose solutions, naloxone reduced consumption greatest under the higher con centrations. The data suggest at least part of naloxone's effects on s weet tasting food may be mediated through endogenous opioid reward sys tems that are reflected in measures of motivation.