GLIAL REACTION AFTER FACIAL-NERVE COMPRESSION IN THE FACIAL CANAL OF THE ALBINO-RAT

The facial nerve of the albino rat was compressed by inserting a thin nylon thread in the facial canal. After a survival period of 1-60 days , the animal was perfused with Ringer's solution followed by a 2% peri odate-lysine-paraformaldehyde fixative. Frozen sections from the brain stem containing the facial motor nucleus (FMN) were obtained and stai ned for OX-42 (against CR3 antigen), OX-18 (against Class I MHC antige n), OX-6 (against Class II MHC antigen) and GFAP (anti-glial fibrillar y acidic protein). The first glial reaction in the FMN occurred in the microglia which showed a significant increase in the CR3 immunoreacti vity within 24 h after compression. Upregulation of the GFAP was not n oticed until 2 days after compression. In each case, the staining reac tion was initially light, but increased with time and appeared to peak at 3-4 days for OX-42 and 4-5 days for GFAP. The activated microglia first assumed a perineuronal position but were later displaced by the activated astrocytes. The number of stained microglia was noticeably i ncreased and was most likely the result of proliferation of the reside nt microglia rather than invasion from the blood stream. The increase in the number of GFAP positive cells was most likely the result of mor e resident astrocytes being activated, as previous studies have shown the absence of mitotic activity of astrocytes after lesion of the faci al nerve. In addition to increased CR3 and GFAP activity there was als o an upregulation of Class I MHC antigen in the microglia, as revealed by increased immunostaining against OX-18. This began to appear at da y 1 postoperation and increased with time till it peaked at about 5-6 days postoperation. Many of them like OX-42 positive cells, occupied a perineuronal position. No OX-6 positive cells were detected in the FM N, indicating that there was no detectable change in the class II MHC activity.