EXPRESSION OF FOS IN THE SPINAL MOTONEURONS LABELED BY HORSERADISH-PEROXIDASE FOLLOWING MIDDLE CEREBRAL-ARTERY OCCLUSION IN RAT

The study was aimed at the investigation of the rat corticospinal syst em both functionally and anatomically using as a functional marker the immediate early gene c-fos, combined with retrograde tracing with hor seradish peroxidase (HRP). This was achieved by mapping c-fos inductio n immunocytochemically in the spinal cord as a result of occlusion of the middle cerebral artery (MCA). Following left-sided MCA occlusion, Fos-like immunoreactivity (Fos-LI) was localized in both the dorsal an d ventral horn neurons at the lumbar segment of the spinal cord. Label ling was often bilateral but was generally more substantial ipsilatera lly. In the ventral horn, some of the Fos-positive neurons were confir med to be somatic motoneurons innervating the tibialis anterior muscle of the lower extremity contralateral to MCA occlusion, as shown by th eir retrograde labelling with horseradish peroxidase injected into the muscle. Fos-LI was absent in the ventral horn of the spinal cord at c ervical, thoracic, and sacral segments in both experimental and sham-o perated rats. These findings suggest that the expression of c-fos may be used as a sensitive transneuronal marker for the study of neuronal activity in the spinal cord elicited by brain damage, viz. focal cereb ral ischaemia, and when coupled with injection of HRP as a retrograde tracer, the method may prove to be useful for the study of transneuron al effect of the damage of the corticospinal motor system. While the e xpression of c-fos in the spinal motoneurons was most probably attribu table to transneuronal effect following MCA occlusion, the possibility of that c-fos can be induced by altered hindlimb activity after the c erebral ischaemic insult cannot be excluded. (C) 1998 Elsevier Science Inc.