To elucidate a significance of the expression of brain-derived neurotrophic factor (BDNF) in the activated microglia/macrophages of the injured central nervous system. we examined BDNF actions on or BDNF synthesis by macrophages Cultured from the mouse peritoneal cavity. They synthesized BDNF and neurotrophin-3 (NT-3) in addition to expressing high-affinity neutrophin receptors, fulll-length TrkB (FL), truncated TrkB (TK-), and TrkC, thus suggesting an autocrine influence of BDNF Mid NT-3. BDNF. but not NT-3. enhanced phagocytic activity and stimulated synthesis/secretion of interleukin-1 beta in the same manner as lipopolysaccharide ( LPS). Furthermore. there was a significant correlation of the phagocytic activity with the expression of BDNF or TrkB (FL). These results imply that the phagocytic activity of macrophages depends on BDNF synthesis and/or TrkB (FL expression, suggesting that BDNF participates in the activation processes of macrophages by acting in an autocrine manner. (c) 2006 Elsevier Inc. All rights reserved.