Subjects were adult (approximately 90 days old at the start of the experiment) female selectively bred P rats from the 61st generation. The rats were single-housed on a reverse 12-h/12-h dark-light cycle (lights off at 9:00 AM). Animals had ad libitum access to food and water. The animals used in these experiments were maintained in facilities fully accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care (AAALAC). All research protocols were approved by the institutional animal care and use committee and are in accordance with the guidelines of the Institutional Care and Use Committee of the National Institute on Drug Abuse, National Institutes of Health, and the Guide for the Care and Use of Laboratory Animals (Institute of Laboratory Animal Resources, Commission on Life Sciences, National Research Council 1996).
