Comparison of sucrose-sucrose to sucrose-ethanol concurrent responding in the rat: reinforcement schedule and fluid concentration effects.

Rats maintained at 80% of their ad lib body weight were trained on a two bar concurrent fixed ratio 8-fixed ratio 8 schedule with sucrose solutions presented at schedule completion. When the solutions were available on the same schedule of reinforcement, rats consistently responded more on the lever associated with the higher sucrose concentration over either less concentrated sucrose solutions or water. However, when a preferred 20% sucrose solution was placed on a high fixed ratio requirement (FR64) and a less preferred 2% sucrose solution remained on the lower ratio requirement (FR8), the rats were observed to increase their responding on the lever associated with presentation of the 2% sucrose solution. Response rates for the low concentrated sucrose solution increased to levels comparable to those seen when that solution was paired with water. These results were compared to prior studies using ethanol and sucrose as the available fluids.