1. Behavioral Neuroscience Section, Behavioral Neuroscience Research Branch, National Institute on Drug Abuse Intramural Research Program, National Institutes of Health, Baltimore, MD 21224, USA.
2. Department of Neurosciences, Medical University of South Carolina, Charleston, SC 29425, USA.
3. Electrophysiology Research Section, Cellular Neurobiology Branch, National Institute on Drug Abuse Intramural Research Program, National Institutes of Health, Baltimore, MD 21224, USA.