Naltrexone suppresses the rejection of cardiac tissue transplantation

The present study demonstrates the following: 1. Transplantation of cardiac tissue induces an inflammatory response that ultimately leads to the rejection of the tissue by the host within 9 days; 2. Treatment with the opiate antagonist, naltrexone, significantly increased the survival of the transplanted cardiac tissue to 13 days, suggesting the involvement of opioid signaling molecules in tissue rejection; 3. In further experiments it was demonstrated that in mixed lymphocyte populations from different mice, the DNA synthesis inhibitor, mitomycin C, reduced the lymphocyte proliferative response as did naltrexone; 4. Mice injected with naltrexone for 10 days and given concanavalin A exhibited a suppressed spleen lymphocyte proliferative response compared to controls. Taken together, these data suggest that endogenous opioid signals not only activate immunocytes, but also stimulate DNA synthesis.