Ethanol
Genetic knockouts of the dopamine D2 receptor and vesicular monoamine transport 2 (VMAT2) exhibited a lack of conditioned place preference. Mice lacking the mu opioid receptor exhibited a lack of conditioned place preference. Knockouts of CB1 cannabinoid receptor demonstrated a lack of conditioned place preference. Ethanol appears to have a widespread action on the brain through the many different mechanisms of the drug.
Read more about this topic: Conditioned Place Preference, Knockout Mice