Cellular sources and effects of tumor necrosis factor-? on pituitary cells and in the central nervous system

Cytokine-mediated communication between the immune system and the nervous system has been shown in the past few years. The precise cellular sources of these molecules in the brain is still a controversial issue. We have thus immortalized primary cell cultures from mouse embryonic brains to analyze cloned cells involved in cytokine production. The cell clones obtained were identified as microglial cells and shown to produce several monokines. Among these, TNF? was detected by molecular analysis and cytotoxicity assays and shown to be expressed by microglial cells, after activation with LPS. Surprisingly, the TNF?-mediated cytotoxic activity, which was neutralized by specific antisera, was not detected in the cell supernatants but was mediated through cell-to-cell contact. Using antibodies to TNF? in FACS analysis, specific cell membrane staining on live microglial cells was shown. The results suggest that in the brain the form of TNF? detectable by standard procedures is the cell bound form and not the most common form, secreted TNF?. In addition, the effects of recombinant TNF? in vitro and in vivo were evaluated. In vitro, rTNF? stimulated ?-endorphin, GH, and PRL release from cultured cells prepared from rat anterior pituitary glands. In vivo, the administration of rTNF? to rats was able to modify analgesic responses. The concomitant administration of naloxone, an opiate receptor antagonist, or monoclonal anti-IL-1 antibody decreased the analgesic effects induced by rTNF?. This indicates that the analgesic effect might not be mediated directly by rTNF? but by other mediators, whose action is under the control of TNF?.