GLIAL REACTION AFTER PYRAMIDOTOMY IN MICE AND RATS

Adult mice and rats were sacrificed by perfusion between 2 and 90 days after right pyramidotomy to study the microglial and astroglial respo nse in the brain and spinal cord. The microglia were detected immunohi stochemically with OX-42, OX-18 and OX-6 to assess respectively the ex pression of complement type 3 receptor, and major histocompatibility c lass I and class II antigens. Cell counting was also carried out in so me animals to determine the possible proliferation of glial cells in t he corticospinal tract and around layer V neurons in the cerebral cort ex. Some operated animals were given rhodamine B isothiocyanate inject ion to investigate whether macrophages/monocytes could have migrated f rom the blood stream to the reactive area. The glial response around t he cell bodies of layer V neurons in the ipsilateral cerebral cortex d id not display any noticeable difference compared with that of the con tralateral side and of the cerebral cortex of the sham-operated and no rmal control animals. In the cervical and lumbar cord segments of the operated animals, reactive microglial cells in the contralateral corti cospinal tract appeared as early as 2 days post pyramidotomy (PP) in r ats and 4 days PP in mice. Activation of microglial cells lasted up ti ll 35 days PP, showing gradual increase in immunoreactive staining and hypertrophy After that, the microglial immunoreactivity subsided and the cells assumed normal appearance by 90 days PP. Quantitative analys is showed a marked increase in the number of microglial cells in the c ontralateral CST up till 60 days PP. In mice, at 6 days PP, astroglial cells were hypertrophic and more intensely stained but showed no incr ease in number. No noticeable changes were noted in the astroglia of t he rats throughout the period studied. Rhodamine-labelled cells were f ound at the lesion site, but not in layer V of the cerebral cortex, no r in the corticospinal tract. Though different glial reactions in the degenerating corticospinal tract were noted in mice and rats, there wa s the same apparent lack of glial reaction around the cell bodies of l ayer V neurons in the two animal species. Such lack of significant gli al response is different from the vigorous glial response around the c ell bodies of peripherally projecting neurons demonstrated in previous work. The possible mechanisms for such difference and the implication of the difference in axonal regeneration were discussed. (C) 1995 Aca demic Press Limited.