Stress- and non-stress-mediated mechanisms are involved in pain-induced apoptosis in hippocampus and dorsal lumbar spinal cord in rats

Chronic pain has been reported to induce apoptosis. Both chronic excitation of neural pathways involved in pain transmission and control and the stress of pain may be potentially involved in apoptosis induced by pain. Here, we have investigated their possible role in pain-induced apoptosis. Inflammatory pain was induced by injection of formalin in intact and adrenalectomized (ADX) rats. Following exposure to repeated injections of 5% formalin, we detected Bax, Bcl-2, pro-caspase and activated caspase-3 proteins using immunoblotting. The results were compared with those obtained from animals suffered from chronic immobilization stress (IMO). These results showed an increased ratio of Bax/Bcl-2 and activated caspase-3 in hippocampus and dorsal lumbar spinal cord of animals treated with pain and IMO stress; these effects were reduced in ADX animals. On the other hand, the remaining apoptotic effect of pain in adrenalectomized rats was also significant. We surmise that both chronic neural activation and the stress induced by pain are involved in pain-induced apoptosis.