Analysis of Variance 2 Animals 2 Autoradiography 2 Baclofen 2 Choice Behavior 2 Cocaine 2 Conditioning, Operant 2 Dihydroxyphenylalanine 2 Dopamine 2 Dopamine Agonists 2 Dopamine Antagonists 2 Dopamine Uptake Inhibitors 2 Drug Interactions 2 Enzyme Inhibitors 2 Exploratory Behavior 2 Food 2 GABA-B Receptor Agonists 2 Hydrazines 2 Hyperkinesis 2 Maze Learning 2 Membrane Potentials 2 Mesencephalon 2 Mice 2 Mice, Knockout 2 Motivation 2 Motor Activity 2 Neurons 2 Quinpirole 2 Receptors, Dopamine D2 2 Reinforcement Schedule 2