Expression of the pro-opiomelanocortin gene in dorsal root ganglia, spinal cord and sciatic nerve after sciatic nerve crush in the rat

Neuropeptides related to α-melanocyte-stimulating hormone (α-MSH) stimulate nerve outgrowth following peripheral nerve injury and may play an important physiological role in peripheral nerve regeration. The mechanism of action underlying the neurotrophic effect of pharmacologically administered α-MSH is unknown. Here we investigate the hypothesis that reexpression of the proopiomelanocortin (POMC) gene, the prohormone of α-MSH/adrenocorticotropic hormone (ACTH)-like peptides, is part of the endogenous repertoire of peripheral nerve responses following injury. The effect of sciatic nerve crush on the expression of POMC mRNA between 0.5 h and 14 days after crush was investigated using polymerase chain reaction (PCR) and Northern blot analysis. The presence of a POMC transcript in dorsal root ganglia (DRG), spinal cord and in the sciatic nerve at the crush site could be demonstrated in both control and lesioned animals by PCR using primers located in exon 1 and 3 of the POMC gene. Minute quantities of two POMC transcripts (1200 nt and 800 nt) could be detected by Northern blot analysis of total RNA prepared from DRG, spinal cord and the sciatic nerve of control animals and of animals subjected to nerve crush. POMC mRNA expression was, however, not increased following nerve crush. Probes specific for exons 1 and 2 or specific for exon 3 of the POMC gene were employed to demonstrate that the 800 nt transcript represents the truncated POMC mRNA previously shown to be present in extra-pituitary tissue. The larger 1200 nt transcript comigrates with the full length POMC mRNA expressed in the pituitary gland. The present results demonstrate the expression of small amounts of POMC mRNA in all compartments of the sciatic nerve. The absence of an induction of POMC expression in response to nerve crush suggests that the stimulating effect of exogenously applied α-MSH does not mimic a POMC derived neurotrophic peptide induced in the nerve following nerve injury