Effects of triiodothyronine on the normal and regenerating facial nerve of the rat

Summary The left facial nerve was crushed in 50 adult male rats. The animals were allocated to 5 groups: controls received daily subcutaneous injections of slightly alkaline water; experimental animals received triiodothyronine sodium (T3), 1.0 g per kg for 3, 7 or 28 days or 5.0 g per kg for 28 days. Functional recovery, due to axonal regeneration, was detected by observing the time required for return of the corneal reflex on the left side. Treatment with T3 did not significantly accelerate this recovery.The rats were killed on the 28th post-operative day. Axonal diameters and myelin sheath thicknesses were measured in transverse sections of the temporal branches of the facial nerves. Treatment with T3 (5.0 g per kg, 28 days) did not affect the diameters of axons in the unoperated nerves, but resulted in their myelin sheaths becoming thicker than in the controls. A positive correlation between the two parameters, absent in control animals, appeared after treatment with T3.In the regenerated nerves, the axonal diameters were smaller in T3-treated than in control animals, but the myelin thicknesses were greater. These changes were more pronounced in rats injected with T3 for 28 days than for shorter times. It is suggested that under the influence of exogenous T3, regenerating neurons synthesize axolemmal components more rapidly than axoplasmic material and that the hormone stimulates myelination of both normal and regenerating axons by an action upon the Schwann cells.