1. Method
1.1. Subjects
The experiment used 104 male Sprague-Dawley rats, ranging
between the ages of 60 and 80 days at the start of the experiment.
Animals were group housed in a temperature-controlled room
with a 12 h light/dark cycle. Food and water were provided ad
libitum. All protocols were approved by the local Institutional
Animal Care and Use Committee of the University of Texas at
Arlington and adhered to the guidelines of the Committee for
Research and Ethical Issues of the International Association for the
Study of Pain (Zimmermann, 1983).