Consuming sugar-sweetened beverages contributes to weight gain, and diets high in sugar alter motivational circuits to promote over-eating and obesity. Preclinical studies often assess motivation for food using progressive ratio procedures, where the amount of work (lever pressing) to obtain food increases within a session. Motivation is then determined by the point at which responding stops (i.e., break point). While useful, this only provides one simplified measure of motivation, and no information about the preferred level of consumption. To address this, we developed a demand procedure for liquid sucrose based on behavioral economics. This approach simultaneously provides information about the preferred level of consumption, and the sensitivity of consumption and effort to alterations in price. Using male Sprague Dawley rats (N=12), we first established and validated our procedure. Lowering the sucrose concentration or inducing satiety reduced the preferred level of consumption and the effort exerted to obtain sucrose. In a separate cohort, we established baseline demand for 1M and 0.25M sucrose. Next, some rats were given 4 weeks of continuous sucrose access (24 hrs/day; n=14), intermittent sucrose access (24 hrs access every third day for 28 days total; n=14) or remained on water (n=13). Across the sucrose access period, rats with intermittent access tended to escalate their sucrose intake, while intake in continuous access rats remained stable. After the access period, consumption and motivation for 0.25M sucrose were unchanged from baseline in all groups. However, motivation for 1M sucrose was reduced only in the continuous access groups compared to their own baseline and water-maintained controls. This was evident in a reduction in maximum price paid, and in effort (i.e., lever presses) as price increased. Intermittent rats also received fewer bottle presentations. Despite these reductions in motivation, the preferred level of consumption when price was low was similar across all groups. These findings suggest that continuous access to 1M sucrose selectively disrupts the motivation to work for 1M sucrose without altering the preference for or willingness to freely consume the solution. Notably, rats with continuous access consumed~ twice as much sucrose across the access period compared to intermittent rats. This could suggest that effects on motivation are dependent upon the total amount of sucrose consumed, rather than the pattern of consumption. These rats are currently undergoing 2 weeks of deprivation after which demand will be re-tested to determine persistence of effects. That motivation for, and consumption of 0.25 M sucrose and preferred level of consumption for 1 M sucrose were similar across all groups suggests that there are not unilateral shifts in motivation or liking of sucrose. But rather, that continuous access to 1M sucrose in the home cage may have caused specific devaluation of 1M sucrose. If this is the case, one would expect that removal of sucrose for 2 weeks will result in a return to baseline demand in the continuous access group. Overall, this work will shed light on effects of prolonged sucrose consumption on motivation and form a basis for examination of underlying mechanisms.
NIH: R01-DK106188 and R01-DK115526 to CRF, and T32-DK101357 to JEF.
