The transcription factor CREB, but not immediate-early gene encoded proteins, is expressed in activated microglia of lumbar spinal cord following sciatic nerve transection in the rat.

Expression of CREB, JUN, FOS and KROX-24 proteins was investigated in glial cells of the lumbar spinal cord. In untreated rats, CREB, c-JUN and JUN D were present in glial cells of the ventral and dorsal horn. Following sciatic nerve transection, the number of CREB immunoreactive glial cells increased in both the ipsilateral ventral and dorsal horns between 24 h and 48 h, reached a maximum after 5 days and returned to control levels after 20 days. Counterstaining with Cresyl violet, a general stain of cells, revealed that the increase of CREB positive glial cells was congruent with the increase of the number of glial cells. Staining with GFAP, a marker for astrocytes, showed an increase in intensity of labelling but no change in number of GFAP labelled cells. This indicates a constitutive expression of CREB in activated microglia. The number of glial cells labelled by c-JUN and JUN D did not change, and glial cells were not labelled by FOS and KROX-24 proteins following sciatic nerve transection. These findings demonstrate that proliferation and differentiation of glial cells in vivo can occur in absence of JUN, FOS and KROX proteins.