All mice were at 8 weeks of age upon arrival from Jackson Labs. Mice were group-housed (n = 4/cage) in tecniplast cages (12″ × 6.5″ × 7″) until 2 weeks prior to the initiation of self-administration procedures. At all times, mice had with ad libitum access to food (Purina Isopro 3000) and water, the temperature of the vivarium was maintained at 21 ± 1 °C, humidity was maintained at 40 ± 2%, and the lights were on a reverse 12L:12D cycle (lights on at 0800). All experiments were approved by the Institutional Care and Use Committee at the University of North Carolina-Chapel Hill and were conducted in accordance with the NIH Guide for the Care and Use of Laboratory Animals (NIH Publication 80–23).
