Intranasal and Intravenous Oxytocin for Analgesia to Noxious Heat Pain

Oxytocin, 10 International Units (IU) will be administered by IV infusion over 30 minutes and, 30 minutes later, a second 10 IU will be administered over 30 min, for a total dose of 20 IU. The first infusion will be at a rate of 0.125 IU/min for the first 5 min, then at 0.375 IU/min for the remaining 25 min. The second 30-min infusion will be at a fixed rate of 0.333 IU/min.

Saline will be administered as two 30-min IV infusions separated by 30-min, using the same volume of infusion solution and rates of administration as in the intravenous oxytocin intervention.

Oxytocin will be self-administered at the time the first IV infusion begins as a single 0.1 mL spray (12 IU) in each nostril followed in 5 min by another spray in each nostril. The total dose of oxytocin is 48 IU.

Placebo (solution containing the excipients in the oxytocin solution but without oxytocin) will be self-administered at the time the first IV infusion begins as a single 0.1 mL spray in each nostril followed in 5 min by another spray in each nostril.