Ž . Brain Research 813 1998 241–253 Research report The microgliarmacrophage response in the neonatal rat facial nucleus following axotomy Manuel B. Graeber a , Fernando Lopez-Redondo b , Etsuko Ikoma b , Masahiro Ishikawa b , ´ Yoshinori Imai b , Kazuyuki Nakajima b , Georg W. Kreutzberg a , Shinichi Kohsaka b, ) a Department of Neuromorphology, Max-Planck-Institute for Psychiatry, Martinsried 82152, Germany b Department of Neurochemistry, National Institute of Neuroscience, Tokyo 187-8502, Japan Accepted 18 August 1998 Abstract Microglia represent a population of brain macrophage precursor cells which are intrinsic to the CNS parenchyma. Transection of the facial nerve in the newborn rat causes death of the affected motor neurons which is accompanied by massive activation of local microglia. Many of these cells develop into macrophages as can be shown by immunocytochemistry for OX-42 and ED1. Using the new polyclonal microglial marker ionized calcium binding adapter molecule 1, iba1, in combination with immunocytochemical double-labeling for the Ž . w 3 x proliferating cell nuclear antigen PCNA , or H thymidine autoradiography, and confocal microscopy, qualitative as well as quantitative differences can be demonstrated between the newborn and the adult axotomized rat facial nucleus. While microglial cells are the only cell population which responds to axotomy by cell division in the adult facial nucleus, GFAP positive reactive astrocytes can be shown to undergo mitosis following axotomy in the newborn rat. Furthermore, ED1 immunoreactivity, early expression of MHC class II molecules and morphological transformation of microglia into macrophages can only be observed under conditions of neuronal degeneration, i.e., in the neonatal rat facial nucleus. Thus, the combination of cellular markers described here should be useful for studies employing the neonatal rat facial nucleus as an in vivo assay system to test the efficacy of neurotrophic factors. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Brain macrophage; ED1; Ionized calcium binding adapter molecule 1; MHC class II antigen; Motor neuron degeneration; Proliferating cell nuclear antigen; Retrograde change 1. Introduction Axotomy of a peripheral motor nerve leads to retro- grade changes in its central nucleus of origin. These Ž alterations not only affect the injured neuron ‘axon reac- . w x tion’ but also its neighboring glial cells 18,21 . In the adult rat facial nucleus, the retrograde changes have been studied in great detail. In fact, the rat facial nucleus has become the most intensely studied model of microglial activation and of the glial changes which accompany w x motor neuron regeneration 8,10,12,18 . It is not surpris- ing, therefore, that this experimental paradigm has also emerged as one of the most widely used systems for testing the in vivo effects of neurotrophic factors which ) Corresponding author. Fax: q81-423-46-1751; E-mail: kohsaka@ncnp.go.jp may prevent lesion-induced motor neuron cell death w x 4,6,22,29,34 . Specifically, brain derived neurotrophic fac- Ž . Ž . tor BDNF , ciliary neurotrophic factor CNTF , glial cell Ž . line derived neurotrophic factor GDNF , and acidic fi- Ž . broblast growth factor aFGF have been shown to rescue axotomized motor neurons of the newborn facial nerve w x 3,16,30,31,36,37 . Yet, estimation of cell death is rather cumbersome as it relies on counting of individual motor neurons in serial sections or the detection of DNA frag- w x mentation 28 . The present study was undertaken to pro- vide a basis for future exploitation of the rat facial nerve model as a system to test factors which influence the survival of motor neurons in vivo. The markers used here, including iba1, ED1 and OX-42, allow rapid evaluation of microglial phagocytic activity in the neonatal rat facial nucleus and provide a qualitative means for the detection of motor neuron cell death in situ. 0006-8993r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0006-8993 98 00859-2