Recent evidence has provided support for the incentive- sensitization model of addiction, where repeated stimulation of the brain reward system leads to a long-lasting sensitization of this system and stimulates further consumption of the drug. This phenomenon has been demonstrated with many drugs. In this study the effects of repeated intermittent (every 3-4 days) treatment with THC or methamphetamine to mice were investigated. Later this study was repeated with rats. A robust dose-dependent sensitization was observed by methamphetamine but not by THC. (Source: Varvel SA, et al. Psychopharmacology (Berl). 2007 May 12; [Epub ahead of print]).