How can one test the addictive qualities of certain substances? The most reliable way would be to get some honest (and willing) human subjects to try the substances themselves and describe in perfect detail the psychological and physical results of their use. However, its not easy to obtain willing human subjects (and even if they were willing, there are experimental ethics dilemmas as well) so our scientific community has come up with the alternative solution, albeit a controversial one, animal testing. The lucky animals that receive the brunt of our initial testing focus are mice and rats. However, because we don’t live in a Disney movie, we can’t simply speak with the animals and ask how they’re feeling. The inability of mice and rats to communicate with us directly requires some clever experimental design.
A common way to trust substances for addictive qualities is through the monitoring of conditioned place preference (CPP). An addictive substance will influence a mouse to attach itself to the certain area of the cage where the drug was administered, a portrayal of drug seeking behavior that characterizes so many chemical addictions.
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The above picture displays a common apparatus used for testing CPP. A rat is initially habituated, allowed to free roam the cage until it is shut inside a certain section, and subsequently given a dose of a certain substance/drug. This process is repeated a certain number of times. Finally, the rat is allowed to free roam the cage. If the drug is addictive, the rat often lingers in the place it was administered. If the drug it was given produced undesirable effects, the rat will avoid the area the drug was administered (conditioned place aversion or CPA).
Many commonly consumed drugs by humans are tested on rats in this way. Morphine, heroine, cocaine, opioids (ex. endorphins), and methamphetamine all result in CPP. Some substances, like alcohol and nicotine, can produce CPP in some cases and CPA in other cases. Drugs that inhibit our dopamine release (which causes the “feel-good” feeling or “runner’s high” strongly associated with addiction) produce CPA. By extension, drug treatments can be found by finding substances that can counteract CPP for addictive substances.
Is CPP/CPA tested on rats and mice a reliable model for humans? Do the possible rewards (enhanced drug addiction treatment, knowledge of what drugs can be addicting) outweight the possible drawbacks (getting mice addicted to potentially harmful substances)?
1) http://www.accuscan-usa.com/product/CPP—Conditioned-Place-Preference/1012
2) http://www.sciencedirect.com/science/article/pii/S0301008298000604#sec3.1.1
Mickey wants some morphine: CPP and its ramifications for drug addiction
