Upregulation of the pro-opiomelanocortin gene in motoneurons after nerve section in mice.

In the normal adult mouse a few of the ventral horn motoneurones were slightly immunoreactive for the pro-opiomelanocortin (POMC)-derived peptides beta-endorphin (beta-EP) and alpha-melanotropin (alpha-MSH). After unilateral section of the sciatic nerve the incidence of immunoreactive cells and the intensity of staining increased in both the ipsilateral and the contralateral ventral horns. The proportion of cells which expressed the peptides reached a maximum at approximately 24 h after nerve section, and thereafter declined, but the proportion was still higher than normal at 7 days after the section. Using histochemical in situ hybridization with cDNA oligonucleotide probes for POMC transcript regions which encode for adrenocorticotropin (ACTH) 4-11 and beta-endorphin 1-8, POMC mRNA was detected in very few motoneurones in unoperated or sham operated mice. After unilateral sciatic nerve section increases in the proportion of cells which expressed the POMC mRNA were seen in both the ipsilateral and contralateral ventral horns. The increase, like that seen for peptide immunoreactivity, was maximum at around 24 h and thereafter declined but at 7 days after the section the proportion of POMC mRNA-positive cells had returned to normal. Thus injury to the motoneurones is accompanied by synthesis of POMC-derived peptides via upregulation of the POMC gene in the motoneurone.