Serotonergic 5-HT3 receptors in the central nervous system are involved in pain processing after nerve injury. We are interested in learning if 5-HT3 receptor antagonist ondansetron might be an appropriate drug for treating pain after nerve injury (neuropathic pain), by investigating its bio-distribution in the cerebro-spinal fluid, and the genetic variability that may affect that distribution.

Study procedures will include iv ondansetron administration, serial blood draws, cerebrospinal fluid (CSF) sampling, pregnancy testing, and possible ECG.

Inclusion Criteria:

1. Age between 18 and 70 years old;
2. Patients planned to undergo hip or knee arthroplasty with spinal anesthesia;
3. Ability to provide informed consent

Exclusion Criteria:

1. Not giving consent to participate in the study;
2. Patients with history of or current hepatic or renal insufficiency;
3. Patients with BMI ≥ 33;
4. Patients with heart failure or active arrhythmias;
5. Patients with severe systemic disease that is a constant threat to life;
6. Contraindication or allergy to ondansetron;
7. Pregnancy or lactation.
8. Prisoners

During the infusion of the study drug and up to the surgery start (1 day)