The effects of acute and chronic morphine treatment on the process of facial nerve regeneration

Abstract

Prominent cellular responses to axonal interruption include enhanced synthesis of RNA and protein in the neuronal perikaryon, and proliferation of reactive Schwann cells. Since morphine has been shown to significantly depress cellular metabolism, we examined its effect on these and other reparative responses underlying nerve fiber regeneration. Rat facial nerve trunks from saline, acute morphine, and continous morphine-treated animals were examined by light and electron microscopy at 3, 7 and 14 days after crush injury. The number of axonal sprouts/unit area and the diameters of regenerating axons were quantified at each survival interval. Both saline-treated and acute morphine-treated facial nerves demonstrated myelin degradation and Schwann cell hypertrophy (at 3 days post-axotomy), sprout outgrowth (at 7 days) and axon maturation and myelination (at 14 days). In the chronic morphine-treated animals, a retardation of the regenerative process was evident. Axon sprout outgrowth and axonal diameters were reduced at 3 and 7 days post-axotomy. In treated 14-day animals, axon diameters were normal; however, significantly fewer axon profiles/unit area were observed. After chronic morphine exposure, Schwann cell hypertrophy and proliferation, as well as myelin debris removal, were inhibited at all survival periods.