Pro-opiomelanocortin-derived peptides in transected and contralateral motor nerves of the rat.

An immunocytochemical method was used to reveal beta-endorphin and alpha-melanotropin peptides in nerve profiles at the neuromuscular junctions in the soleus and extensor digitorum longus muscles of the adult rat. After unilateral section of the sciatic nerve, the proportion of endplates with immunoreactive nerve profiles increased in the denervated muscles even after the axonal material in the distal nerve segments had degenerated. When the nerve was sectioned in the mid-thigh region the proportion of immunoreactive endplates increased with time up to 24-36 h after the nerve section and then eventually declined. The incidence of immunoreactive nerve profiles also increased in the proximal stump. After unilateral sciatic nerve section there was also an increase in the incidence of immunoreactive endplates in the contralateral muscles. The increases in the contralateral muscles could be detected at an earlier time if the nerve was sectioned closer to the spinal cord. The possibility is discussed that the expression of pro-opiomelanocortin peptides in the motor nerves is tonically suppressed in the normal adult but the inhibition is released after section of the nerve. The findings indicate that a signal is transferred from the site of injury in the transected nerves to the motor nerves of the contralateral limb via a transneuronal mechanism through the spinal cord.