Recovery of peripheral nerve function after axotomy: Effect of triiodothyronine

Abstract

The rate of recovery of motor function after crushing the sciatic nerve was measured in rats treated with daily intraperitoneal injections of 3,3′,5-triiodo- l-thyronine, 25 μg/kg. The nerve was crushed at one of four levels, and recovery of motor function was indicated by the return of toe spreading elicited as part of the vestibular placing response. The rate of recovery was calculated from the linear regression of the time to recovery on the distance from the lesion to the entry of the peroneal nerve into the anterior tibial muscle. The recovery rate of the triiodothyronine-treated group was 4.08 ± 0.22 ( se) mm/day, an increase of 22% from the rate of 3.35 ± 0.14 mm/day ( P = 0.006) in the vehicle-injected control rats. The latent periods for the two groups were not significantly different. The outgrowth of the most rapidly growing sensory axons was measured using the “pinch test” on the 7th or 9th day after sciatic crush in animals treated with triiodothyronine in dosages from 1 to 400 μg/kg/day and compared with control animals. No significant difference in distance regenerated or rate of regeneration was found. Control nerves regenerated at a rate of 4.5 ± 0.15 mm/day after a delay of 2.0 ± 0.2 days. A significant degree of hyperthyroidism was documented by measurements of plasma triiodothyronine concentrations taken 4 and 18 h after the injection of the hormone, 25 μg/kg. At the same dosage animals gained weight normally and showed no signs of thyrotoxicity. These results are interpreted as showing that the accelerated recovery of motor function in the hyperthyroid rat is mediated via an effect on the maturation of the regenerated axon rather than on axonal outgrowth. A triiodothyronine-related increase in the rate of myelination would be consistent with the observed results.