1. Department of Biomedical Sciences, University of Modena, 41100 Modena, Italy, Behavioural Neuroscience Branch, National Institute on Drug Abuse, IRP, NIH, DHHS, Baltimore, Maryland 21224, Department of Biochemistry and Molecular Biology, University of Barcelona, 08028 Barcelona, Spain and Department of Neuroscience, Karolinska Institute, 17177 Stockholm, Sweden