Dexamethasone enhances level of GAP-43 mRNA after nerve injury and facilitates re-projection of the hypoglossal nerve.

Application of dexamethasone was found to induce an enhanced expression level of mRNA encoding the growth associated protein (GAP-43) after peripheral nerve injury. Following hypoglossal nerve axotomy, a dexamethasone releasing pellet (1.5 mg released in 3 weeks) was placed near the transected nerve. GAP-43 mRNA was detected in the hypoglossal nucleus by non-radioactive in situ hybridization histochemistry using an alkaline phosphatase-labeled oligonucleotide probe. A significant elevation of GAP-43 mRNA level was observed 2 weeks after the transection in dexamethasone treated animals. This induction was not observed in the dorsal motor nucleus of vagus which expresses moderately high levels of GAP-43 mRNA even without nerve injury. Although dexamethasone did not alter the maximum level of GAP-43 mRNA in the hypoglossal nucleus after nerve injury, it prolonged the period in which the mRNA expression remained elevated. This may be due to post-transcriptional effect by the glucocorticoid. Dexamethasone treatment also caused a slight facilitation of reprojection. This may be due to the enhancement of GAP-43 mRNA level by the glucocorticoid.