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-spe cific glial fibrillary acidic protein as a marker. The effect of the p roximity of axonal cut to cell bodies was also studied by comparing as trocytic reactions elicited by upper cervical and lower thoracic tract otomy. Fast blue was used as a retrograde tracer to identify axotomize d neurons, which were found to concentrate in the caudal part of the c ontralateral red nucleus. The length of reactive astrocytic processes in the dorsomedial and ventrolateral parts of the nucleus was quantifi ed separately since neurons in these two parts project to cervical and lumbar spinal cord, respectively. Those of the ipsilateral nucleus we re also quantified. Sham operation caused a transient increase in reac tive astrocytic processes one day after surgery. An early and a late i ncrease of reactive astrocytic processes was found 2-5 days and 2-8 we eks following both thoracic and cervical tractotomy. Cervical axotomy of lumbar-cord-projecting rubral neurons caused an increase of reactiv e astrocytic processes similar in magnitude to that generated by thora cic axotomy. Following thoracic axotomy, the uninjured dorsomedial are a 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). Surprisin gly, cervical and thoracic tractotomy also elicited a similar increase of reactive astrocytic processes in the ipsilateral nuclei, independe nt of the number of ipsilaterally projected neurons present in each nu cleus. This may be attributed to the retrograde influence from the den ervated spinal target sites which were carried by fibers of the intact rubrospinal tract known to terminate bilaterally. In the lesioned nuc leus, reactive astrocytic processes were often located close to axotom ized cell bodies as early as 3 days following upper cervical and also, to a lesser extent, lower thoracic tractotomy. However, reactive astr ocytic processes in the ipsilateral nucleus usually remained in the ne uropil. These results suggest that axotomy induces two levels of retro grade astrocytic reactions within the soma area of intrinsic central n eurons. Reactive astrocytic processes located proximally to axotomized cell bodies may have a different functional role from those distribut ed in the neuropil.