Low-dose FK506 after contralateral C7 transfer to the musculocutaneous nerve using two different tubes: A study in rats

The purpose of this project was to study the effect of a subimmunosuppressive dose of FK506 (0.7 mg/kg per day) on nerve regeneration along a long nerve gap (4 cm), using the contralateral C7 nerve root transfer model for musculocutaneous nerve neurotization. Two types of tubes were applied to the nerve gap: a polycaprolactone tube and a collagen tube.Twenty adult male Sprague-Dawley rats were divided into 4 groups (n = 5). A polycaprolactone was used in groups 1 and 3 and a collagen tube in groups 2 and 4. Groups 1 and 2 were daily administered a subimmunosuppressive dose of FK506. Animals were euthanized on day 30. Evaluation consisted of behavioral assessment, needle electromyography studies, biceps muscle weight measurements, and qualitative and quantitative morphometry.Groups 1 and 2 showed higher mean values for fiber counts, axon diameters, myelin thickness and myelin area in C7, better functional evaluation results, and higher biceps weight left to right ratio than groups 3 and 4. There was no evidence of reinnervation potentials, and there were no axons detectable inside the tube lumen in any of the study groups.The present study demonstrated that there was nonsignificant improvement of the functional recovery, after systemic administration of a low dose of FK506. This was attributed to 3 factors: length of nerve gap; duration of follow up; and dose of FK506. However, FK506-treated animals tended to be in a more advanced stage of nerve regeneration compared with the control groups.