MORPHINE INDUCES APOPTOSIS IN MURINE THYMOCYTES IN-VIVO BUT NOT IN-VITRO - INVOLVEMENT OF BOTH OPIATE AND GLUCOCORTICOID RECEPTORS

We tested the hypothesis that the previously observed loss of thymic l ymphocytes in mice after treatment with time-release morphine pellets was occurring through the process of apoptosis. Apoptosis is a form of cell death, distinct from necrosis, which involves a specific endonuc lease that fragments the cell's own DNA. Forty-eight hours after impla ntation of a time-release morphine pellet in B6C3F1 mice, thymus weigh t and cellularity was reduced to 30% of that observed in placebo-treat ed mice. Thymocytes from morphine pellet-treated mice were found to ha ve a significantly greater percentage of their DNA fragmented than did thymocytes from either placebo pellet-implanted or naive control mice . The peak level of DNA fragmentation was found to occur approximately 12 hr postpellet implant. When separated on agarose gels, the sizes o f the DNA fragments observed corresponded to the multiples of 180 base pairs which are characteristic of apoptosis. In vivo, the use of eith er the opiate receptor antagonist naloxone, or the glucocorticoid rece ptor antagonist RU-38486, was able to block completely the morphine me diated increase in thymocyte apoptosis. In vitro experiments in which thymocytes were cultured with morphine concentrations as high as 10(-4 ) M showed no evidence of an increased rate of DNA fragmentation. Thes e data indicate that both opiate and glucocorticoid receptors are invo lved in morphine-induced apoptosis and that the opiate receptor respon sible is not located on the thymic lymphocytes.