Innovive rat cages

Thirty‐six adult male Sprague‐Dawley rats (200‐250 g at the start of these studies; Charles River Laboratories, Wilmington, MA) were used and were pair‐housed (2 rats per cage) in individually ventilated Innovive rat cages (San Diego, CA). Six rats were used for drug discrimination experiments, and 30 rats were used for tolerance and physical dependence studies. All animals were maintained in a temperature and humidity controlled colony on a 12‐hours light/dark cycle (lights on at 0700 and off at 1900). All the rats were food restricted for the duration of all studies, and their weights were maintained at approximately 320 g with supplemental feedings after daily behavioral sessions. All behavioral studies were conducted in accordance with the Guide for the Care and Use of Laboratory Animals as adopted by the National Institutes of Health and procedures were approved by the University of Arkansas for Medical Sciences Institutional Animal Care and Use Committee (IACUC). The health of the animals was assessed daily by laboratory technicians and animal care staff.