Link Found Between Pain, Immune, And Reproductive Systems

INDIANAPOLIS-- For the first time a link has been established between the pain system, the immune system and the reproductive system. These findings go far beyond the known pain relief role of pain receptors.

A team of scientists at the Indiana University School of Medicine led by Lei Yu, Ph.D., professor of medical and molecular genetics and a principal investigator at the Walther Oncology Center, has been studying the mu opioid receptor, the body's most significant pain gateway. The mu receptor, named after morphine's initial letter, is the body's biological switch that mediates both the pain-relieving and the euphoric effects produced by morphine and heroin. Dr. Yu's team was responsible for cracking the genetic code for this receptor in 1993. And now, they have made new discoveries about the mu receptor, by studying genetically modified mice in which the function of the mu receptor has been disabled.

The new findings, published in the April 21, 1997, issue of the Journal of Experimental Medicine, suggest a novel role for the mu opioid receptor in both hematopoiesis (the creation and proliferation of cells in the body's blood and immune system), and the reproductive system. "These results indicate that the mu receptor is involved in a range of diverse biological processes in addition to its known involvement in pain relief," said Dr. Yu.

The observation of increased proliferation of "mother" (hematopoietic progenitor) cells which defend the body against infection came as a complete surprise to Dr. Yu and his colleagues. "This is the first time that the mu receptor has been linked to hematopoiesis," said Dr. Yu. Understanding how the link is made ma, lead to the development of new treatments, such as enhancing blood cell production in cancer patients who receive bone marrow transplant, or modulating the immune system in fighting HIV infection.

The observed link between the mu receptor and the sexual function was equally surprising. I.U. researchers noticed that the mutant male mice showed less interest in copulating with receptive females, as well as decreases in sperm count/motility and smaller offspring litter size. Dr. Yu and his colleagues speculate that this discovery may suggest new treatments for impotence and reproductive dysfunction in males.

"These observations in the mouse model correlate with anecdotal accounts about opioid consumption in humans influencing one's immune functions and sex drive, and thus may serve as a model to further study the underlying biology," said Dr. Yu.

In addition to Dr. Yu, other Indiana University researchers on the study were Mingting Tian, Hal E. Broxmeyer, Yi Fan, Zhennan Lai, Shengwen 7hang, Susan Aronica, Scott Cooper, Robert M. Bigsby, Rosemary Steinmetz, Sandra J. Engle, Anton Mestek, Jonathan D. Pollock (now at the National Institute on Drug Abuse), and Jay A. Tischfield. Michael N. Lehman, Heiko T. Jansen, Moyin Ying, and Peter J. Stambrook of University of Cincinnati College of Medicine also participated the study.

Yu's research was funded by the National Institute on Drug Abuse (NIDA), part of the National Institutes of Health. NIDA is the primary federal agency for the conduct and support of research to increase knowledge and develop strategies to deal with health problems and issues associated with drug abuse and addiction.

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