Development of an opioid-specific action of morphine in modifying recovery of neonatally-damaged noradrenergic fibers in rat brain.

In order to determine whether the recovery from damage of central noradrenergic fibers could be modified by an opioid specific action of morphine, animals treated with the neurotoxin, 6-hydroxydopa (6-OHDOPA), were co-treated with morphine sulfate (3.33 mg/kg i.p.). After a period of 6 weeks it was found that morphine treatment in the 6-OHDOPA-group was associated with a recovery of norepinephrine (NE) levels in the cerebellum, and a 2-fold elevation of NE in the pons-medulla, when compared to the group treated with 6-OHDOPA alone. Histofluorometrically, noradrenergic fiber number corresponded to changes in NE content. These actions of morphine were antagonized by the opioid receptor antagonist, naloxone. Therefore, these findings indicate that morphine, by a specific opioid action, is able to enhance the recovery of damaged noradrenergic fibers in the brain.