This study involves research. Pregabalin is a Food and Drug Administration (FDA) medication approved in the United States for the treatment of nerve pain related to diabetes and post-herpetic neuralgia "shingles", and for seizures in adults. The purpose of this research is to study the effect of oral Pregabalin on spinal neurotransmitters in subjects undergoing Total Knee Replacement Surgery (TKA). TKA is associated with considerable postoperative pain which if unrelieved may result in prolonged hospital stay, inability to participate in rehabilitation programs, poor outcomes, and greater use of health-care resources. This study examines the effect of pregabalin administered for TKA on pain-related neurotransmitter concentrations.