β-Endorphin concentrations in brain areas and peritoneal macrophages in rats susceptible and resistant to experimental allergic encephalomyelitis: A possible relationship between tumor necrosis factor α and opioids in the disease

Since the central nervous system and neuropeptides modulate immune functions, we investigated whether the different susceptibility of Lewis and Brown Norway rats to experimental allergic encephalomyelitis could also reflect differences in β-endorphin and substance P concentrations in brain areas and macrophages during the development of the disease. We show that β-endorphin concentrations increase much more in the hypothalamus and macrophages of Lewis rats during the development of the disease, while the increase is much lower or absent in Brown Norway rats. Tumor necrosis factor-α seems to play an important role in this difference. The administration of the opiate receptor antagonist naltrexone worsens the development of the disease, suggesting that the increase of the opioid β-endorphin might represent a mechanism to downregulate the immune response. In both strains, the concentrations of substance P do not change.