Rubral astrocytic reactions to proximal and distal axotomy of rubrospinal neurons in the rat.

Spinal tractotomy-induced perineuronal astrocytic reaction of the rat rubrospinal system was studied using an antiserum to the astrocyte-specific glial fibrillary acidic protein as a marker. The effect of the proximity of axonal cut to cell bodies was also studied by comparing astrocytic reactions elicited by upper cervical and lower thoracic tractotomy. Fast blue was used as a retrograde tracer to identify axotomized neurons, which were found to concentrate in the caudal part of the contralateral red nucleus. The length of reactive astrocytic processes in the dorsomedial and ventrolateral parts of the nucleus was quantified separately since neurons in these two parts project to cervical and lumbar spinal cord, respectively. Those of the ipsilateral nucleus were also quantified. Sham operation caused a transient increase in reactive astrocytic processes one day after surgery. An early and a late increase of reactive astrocytic processes was found 2-5 days and 2-8 weeks following both thoracic and cervical tractotomy. Cervical axotomy of lumbar-cord-projecting rubral neurons caused an increase of reactive astrocytic processes similar in magnitude to that generated by thoracic axotomy. Following thoracic axotomy, the uninjured dorsomedial area of the contralateral nucleus also displayed an increase concomitant with that which occurred within the neighboring, injured ventrolateral nuclear area suggesting the action of diffusible factor(s). Surprisingly, cervical and thoracic tractotomy also elicited a similar increase of reactive astrocytic processes in the ipsilateral nuclei, independent of the number of ipsilaterally projected neurons present in each nucleus. This may be attributed to the retrograde influence from the denervated spinal target sites which were carried by fibers of the intact rubrospinal tract known to terminate bilaterally. In the lesioned nucleus, reactive astrocytic processes were often located close to axotomized cell bodies as early as 3 days following upper cervical and also, to a lesser extent, lower thoracic tractotomy. However, reactive astrocytic processes in the ipsilateral nucleus usually remained in the neuropil. These results suggest that axotomy induces two levels of retrograde astrocytic reactions within the soma area of intrinsic central neurons. Reactive astrocytic processes located proximally to axotomized cell bodies may have a different functional role from those distributed in the neuropil.

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 lectin binding method. Following axotomy, the contralateral nucleus containing the axotomized neurons was identified using the retrograde tracer Fast blue. It was subdivided into dorsomedial (DM) and ventrolateral (VL) portions known to project to the cervical and lumbar spinal cord, respectively. Lectin-labeled microglial cells and processes on the contralateral as well as in the ipsilateral nucleus were then quantified. 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 cervical 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 the possible action of diffusable factor(s) that might have triggered the microglial activation from the axotomized VL nuclear area. The uninjured ipsilateral nucleus also exhibited a similar pattern of microglial reactivity irrespective of the number of ipsilaterally projecting neurons following both cervical and thoracic axotomy. This could have been elicited by the retrograde influence from the denervated targets carried by the intact rubrospinal fibers of the opposite side since many of them in fact terminate bilaterally (Antal, M. et al., J. Comp. Neurol., 325 (1992) 22-37). In all the axotomized or intact nucleus, microglial processes did not appear to surround neuronal cell bodies. The characteristic responses of microglial cells in the red nucleus may be related to the failure of rubrospinal neurons to regenerate following the severance of their axons.