L-dihydroxyphenylalanine that was restored under the influence of endogenous dopamine in DD mice changed mice behavior. They became hypoactive and hypophagic and could carry out complicated tasks that are impossible to implement without both activation and plasticity. "Additional goal of this study was to separate performance factors from cognitive processes as described by Denenberg, Kim and Palmiter (2004)2
It was rather difficult to design the experiments in a proper way because of some things. First, learning, liking and wanting are not alternative behaviors. Second, animals tend to be hypoactive and hyperphagic under the influence of dopamine.
Some hypotheses were posited. The one concerning hedonia assumes that "dopamine mediates the sensory pleasure of rewards, such as food"3 (Wise, 2004). The evidence was caused by the observation that dopamine leads to the growing reward consumption. The hypothesis about learning presumes that if we want to make animals associate rewards with special clues we must intake them dopamine. The last hypothesis in respect to wanting supposes that dopamine does not influence the interconnection liking-reward and learning- reward. But it's responsible for "recognition of motivational conditions" (Salamone, 1996) "converting a neutral stimulus into an attractive wanted stimulus.
Two experiments were conducted. The object of the experiments is genetically engineered dopamine-deficient (DD) mice. Authors tested influence of endogenous dopamine signaling on mice ability for acquisition of an appetitive T-maze. On the basis of two experiments authors came to the conclusion that dopamine had only an impact on the fact whether the mice want reward in the process of goal-directed behavior. However, mice are absolutely indifferent to dopamine influence in regard to liking and learning about rewards. For the experimental design the doze of endogenous dopamine was very important. Too little as well as too much dopamine interferes with reversal learning.
Methods: first experiment had lasted for twenty-three days, the second one - for twenty days with two groups of mice: controlled and under the LD-treated mice. They had to perform their learning, liking and wanting skills by finding rewards in a T-maze, reaching the intersection, making right arm entries, latency to begin consumption and the number of rewards consumed. All these data were thoroughly recorded.
At the experiment one which consisted of two phases at first controlled and DD mice under the LD-influence had to find rewards and remember the way to rewards, then rewards were changed and mice had to find a new way to rewards, studying in such a way their learning ability. The number of pieces consumed in the second phase after switching rewards evidence the degree of liking and the speed with which they began consumption showed the degree of wanting.
Experiment two was conducted on two mice groups that tested saline-LD or caffeine-LD (caffeine was used as a stimulant) and LD-LD 10 times per day for 20 days. In the first part of the experiment mice received one of substances SAL (saline), CAF (caffeine) or LD. During the second half all mice were injected with LD. Some alternatives were for the assumption that if dopamine did not influence learning ability the mice must behave as LD-treated mice in Phase 1. However, if impact exists