PERINEURONAL MICROGLIAL REACTIVITY FOLLOWING PROXIMAL AND DISTAL AXOTOMY OF RAT RUBROSPINAL NEURONS

Microglial reactivity in the red nucleus of rats was studied following upper cervical and lower thoracic rubrospinal tractotomy using the le ctin binding method. Following axotomy, the contralateral nucleus cont aining the axotomized neurons was identified using the retrograde trac er Fast blue. It was subdivided into dorsomedial (DM) and ventrolatera l (VL) portions known to project to the cervical and lumbar spinal cor d, respectively. Lectin-labeled microglial cells and processes on the contralateral as well as in the ipsilateral nucleus were then quantifi ed. An early and a late increase in microglial reactivity was observed in the nucleus at 2-5 days and 2-8 weeks following thoracic and cervi cal tractotomy with the latter producing a more pronounced reactivity. In rats subjected to thoracic axotomy, a similar microglial increase also occurred in the intact contralateral DM nuclear area suggesting t he possible action of diffusible factor(s) that might have triggered t he microglial activation from the axotomized VL nuclear area. The unin jured ipsilateral nucleus also exhibited a similar pattern of microgli al reactivity irrespective of the number of ipsilaterally projecting n eurons following both cervical and thoracic axotomy. This could have b een elicited by the retrograde influence from the denervated targets c arried by the intact rubrospinal fibers of the opposite side since man y of them in fact terminate bilaterally (Antal, M. et al., J. Comp. Ne urol., 325 (1992) 22-37). In all the axotomized or intact nucleus, mic roglial processes did not appear to surround neuronal cell bodies. The characteristic responses of microglial cells in the red nucleus may b e related to the failure of rubrospinal neurons to regenerate followin g the severance of their axons.