Twenty five male Sprague-Dawley rats (Charles River Laboratories), weighing approximately 375 grams were used in these studies. Animals were maintained in hanging Plexiglas® cages (28 × 28 × 18 cm) with ad libitum access to food and water, and in environments with controlled temperature (22°C), and a 12h : 12h light:dark cycle (lights on at 8 am). In the first 10 days, animals were housed in pairs and trained in the operant chambers. After surgeries, they were housed in individual cages. Throughout the experiment rats were maintained according to the “Principles of Laboratory Animal Care”. Experimental procedures were approved by the Institutional Animal Care and Use Committee of the Oregon Health & Science University. Experiments were performed during the dark phase for most animals (N = 17), with the exception of 8 animals (as described below).
