Preferential neuroprotective effect of tacrolimus (FK506) on unmyelinated axons following traumatic brain injury

Prior investigations of traumatic axonal injury (TAI), and pharmacological treatments of TAI pathology, have focused exclusively on the role of myelinated axons, with no systematic observations directed towards unmyelinated axon pathophysiology. Recent electrophysiological evidence, however, indicates that unmyelinated axons are more vulnerable than myelinated axons in a rodent model of experimental TAI. Given their susceptibility to TAI, the present study examines whether unmyelinated axons also respond differentially to FK506, an immunophilin ligand with well-established neuroprotective efficacy in the myelinated fiber population. Adult rats received 3.0 mg/kg FK506 intravenously at 30 min prior to midline fluid percussion injury. In brain slice electrophysiological recordings, conducted at 24 h postinjury, compound action potentials (CAPs) were evoked in the corpus callosum, and injury effects quantified separately for CAP waveform components generated by myelinated axons (N1 wave) and unmyelinated axons (N2 wave). The amplitudes of both CAP components were suppressed postinjury, although this deficit was 16% greater for the N2 CAP. While FK506 treatment provided significant neuroprotection for both N1 and N2 CAPs, the drug benefit for the N2 CAP amplitude was 122% greater than that for the N1 CAPs, and improved postinjury strength-duration and refractoriness properties only in N2 CAPs. Immunocytochemical observations, of TAI reflected in intra-axonal pooling of amyloid precursor protein, indicated that FK506 reduced the extent of postinjury impairments to axonal transport and subsequent axonal damage. Collectively, these studies further substantiate a distinctive role of unmyelinated axons in TAI, and suggest a highly efficacious neuroprotective strategy to target this axonal population.