Behavioral analysis: Subjects
All mice were four to six weeks old at the beginning of the experiments. Subjects were maintained on a 12-h light-dark cycle in isolated cages in a temperature and humidity-controlled animal facility. All behavioral training was conducted during daytime. During the training period, animals were kept on free food but on a water-restriction schedule designed to keep them at > 85% of their free food body weight. Continuous water restriction was never longer than 12 h. All animal care and procedures were in accordance with the animal ethics guidelines of the Max-Planck Society.