Researchers recently made 17 women hot…by applying intense heat to their forearms. Women felt less pain when looking at pictures of romantic partners versus when looking at strangers or other objects. Brain scans taken during the heat exposure indicated that parts of the brain associated with safety were more active in the partner condition, whereas areas associated with pain were less active, especially when women were in longer relationships or with particularly supportive partners.
For more details, see the wired.com write-up here if you do not have access to the article/journal.
Eisenberger, N. I., Master, S. L., Inagaki, T. K., Taylor, S. E., Shirinyah, D., Lieberman, M. D., & Naliboff, B. D. (2011). Attachment figures activate a safety signal-related neural region and reduce pain experience. Proceedings Of The National Academy Of Sciences Of The United States Of America, June 27(Early Edition). doi: 10.1073/pnas.1108239108