This is a study of participants that will receive intravenous (IV) infusions of oxytocin (naturally occurring hormone that is made by the brain).

In this study healthy participants will be recruited for the study. There will be 3 study related visits: 1) screening and informed consent 2) study medication infusions and thermal heat testing 3) study medication infusions and thermal heat testing. During study visits 2 and 3 an IV catheter will be placed and a dose of oxytocin or placebo (inactive solution) will be given over a 10 minute period; 1 hour after the first infusion, a second 10 minute infusion of oxytocin will be administered.

Investigators will perform some tests to evaluate how oxytocin changes perceptions of the skin. Investigators will study a painful perception by placing a probe on the skin of the side of lower leg and heating it up to 113-117 degrees Fahrenheit (F) (45-47 degrees Celsius (C)) for 5 minutes. Each participant will score any pain that is experienced on a 0 to 10 scale for each minute during the heating period. Most people find that pain rises during the 5 minutes, but remains mild; usually less than 5 on the 0 to 10 pain scale. The temperature of the 5 minute heating will be determined according to the participants pain rating during the screening visit.

The primary objective of the study is to determine the dose response of IV oxytocin for analgesia (pain relief) to experimental heat pain.

Equal numbers of adult men and women; ages 18-75 will be recruited. Participants will report to the research unit for three visits, 1) screening and informed consent 2) study medication infusions and thermal heat testing 3) study medication infusions and thermal heat testing separated by at least 72 hours.

For study infusion visits, study participants will come to the Clinical Research Unit and one intravenous (IV) catheter will be inserted in the forearm for oxytocin infusions. Participants in this study will receive a dose of oxytocin or placebo at a steady rate for 10 minutes; one hour later, a second dose of oxytocin will be administered during one visit. During the second study infusion visit, a lower dose of oxytocin will be given by a 10-minute IV infusion and one hour later a second infusion of oxytocin at a higher dose will be given. On one study day the doses will by 0 and 1.3 micrograms (µg) and on the other study day the doses will be 0.3 and 7 µg oxytocin. The order of study days will be randomized and double-blinded. As a result, each participant will receive oxytocin doses of 0, 0.3, 1.3, and 7 µg in this double blind design.

Inclusion Criteria:

1. Male or female > 18 and < 75 years of age, Body Mass Index (BMI) <40.
2. Generally in good health as determined by the Principal Investigator based on prior medical history, American Society of Anesthesiologists physical status 1, 2, or 3.
3. For healthy volunteers, normal blood pressure (systolic 90-140 mmHg; diastolic 50-90 mmHg) resting heart rate 45-100 beats per minute) without medication. For knee arthritis subjects, normal blood pressure or, for those with hypertension, pressure controlled with anti-hypertensives and with a resting heart rate 45-100 beats per minute.
4. Female subjects of child-bearing potential and those < 1 year post-menopausal, must be practicing highly effective methods of birth control such as hormonal methods (e.g., combined oral, implantable, injectable, or transdermal contraceptives), double barrier methods (e.g., condoms, sponge, diaphragm, or vaginal ring plus spermicidal jellies or cream), or total abstinence from heterosexual intercourse for a minimum of 1 full cycle before study drug administration.

Exclusion Criteria:

1. Hypersensitivity, allergy, or significant reaction to any ingredient of Pitocin®
2. Any disease, diagnosis, or condition (medical or surgical) that, in the opinion of the Principal Investigator, would place the subject at increased risk (active gynecologic disease in which increased tone would be detrimental e.g., uterine fibroids with ongoing bleeding), compromise the subject's compliance with study procedures, or compromise the quality of the data
3. Women who are pregnant (positive result for serum pregnancy test at screening visit), women who are currently nursing or lactating, women that have been pregnant within 2 years
4. Subjects with neuropathy, chronic pain, diabetes mellitus, or taking benzodiazepines or pain medications on a daily basis.
5. Subjects with current or history of ventricular tachycardia, atrial fibrillation or prolonged QT interval.
6. Subjects with past or current history of hyponatremia or at risk for hyponatremia; anyone taking thiazide diuretics, loop diuretics, combination diuretics, lithium, carbamazepine, enalapril, Ramipril, celecoxib, temazepam, gliclazide, glimepiride, glibenclamide, glipizide, omeprazole, pantoprazole, desmopressin, SSRI's, MAOI, or the recreational drug ecstasy.
7. Subjects with a known latex allergy.
8. Subjects with a pain score rating of 1 or less during the initial training session to a 5 minute heating of 45°C- 47 °C to the lower calf.

As noted in Participant Flow, there were 14 participants in Arm 1 and 11 participants in Arm 2. One subject in Arm 1 experienced near syncope during intravenous catheter insertion. This rapidly resolved spontaneously, but the subject did not further participate and did not receive study drug or undergo baseline testing.

One week after each oxytocin infusion study day

Peripheral oxyhemoglobin saturation, BP and HR will be measured non invasively before and 15, 30, and 60 min after each IV infusion. Participants were contacted daily for one week after each infusion study day and queried for any side effects.

The original study design called for equal recruitment of subjects across three age ranges: young (18-39-year-old) middle (40 - 59-year-old) and older (60 - 75-year-old) with separate randomization in each group. The protocol was subsequently amended to remove this requirement.