Protective effects of gonadal hormones on spinal motoneurons following spinal cord injury

Abstract

Spinal cord injury (SCI) results in lesions that destroy tissue and disrupt spinal tracts, producing deficits in locomotor and autonomic function. The majority of treatment strategies after SCI have concentrated on the damaged spinal cord, for example working to reduce lesion size or spread, or encouraging regrowth of severed descending axonal projections through the lesion, hoping to re-establish synaptic connectivity with caudal targets. In our work, we have focused on a novel target for treatment after SCI, surviving spinal motoneurons and their target musculature, with the hope of developing effective treatments to preserve or restore lost function following SCI. We previously demonstrated that motoneurons, and the muscles they innervate, show pronounced atrophy after SCI. Importantly, SCI-induced atrophy of motoneuron dendrites can be attenuated by treatment with gonadal hormones, testosterone and its active metabolites, estradiol and dihydrotestosterone. Similarly, SCI-induced reductions in muscle fiber cross-sectional areas can be prevented by treatment with androgens. Together, these findings suggest that regressive changes in motoneuron and muscle morphology seen after SCI can be ameliorated by treatment with gonadal hormones, further supporting a role for steroid hormones as neurotherapeutic agents in the injured nervous system.

Keywords: atrophy; dendrites; dihydrotestosterone; estradiol; morphology; muscle fibers; neuroprotection; retrograde labeling; steroids; testosterone.

Figure 1

Motoneuron morphology is protected by gonadal hormones following spinal cord injury. Darkfield digital micrographs and matching computer-generated composites of transverse hemisections through the lumbar spinal cords of a sham animal (A, G), an injured animal given a blank implant (SCI; B, H), an estradiol-treated injured animal (SCI + E; C, I), a dihydrotestosterone-treated injured animal (SCI + D; D, J), an injured animal treated with both hormones (SCI + E + D; E, K), and a testosterone-treated injured animal (SCI + T; F, L), after horseradish peroxidase conjugated to the cholera toxin B subunit (BHRP) injection into the left vastus lateralis muscle. Computer-generated composites of BHRP-labeled somata and processes were drawn at 480 μm intervals through the entire rostrocaudal extent of the quadriceps motor pool; these composites were selected because they are representative of their respective group average dendritic lengths. Scale bar: 500 µm. (Images from Byers et al. (2012) and Sengelaub et al. (2018).

Figure 2

Motoneuron dendritic length and distribution is protected by gonadal hormones following spinal cord injury. (A) Dendritic lengths of quadriceps motoneurons of sham animals and injured animals that were either untreated (SCI), or treated with estradiol (SCI + E), dihydrotestosterone (SCI + DHT), estradiol and dihydrotestosterone combined (SCI + E + DHT), or testosterone (SCI + T). Following contusion injury, surviving quadriceps motoneurons lost over 50% of their dendritic length. Treatment with hormones attenuated this dendritic atrophy. (B) Inset: Drawing of spinal gray matter divided into radial sectors for measure of quadriceps motoneuron dendritic distribution. Quadriceps motoneuron dendritic arbors normally display a non-uniform distribution, with the majority of the arbor located between 300° and 120°. Following contusion injury, surviving quadriceps motoneurons in untreated animals (SCI) had reduced dendritic lengths throughout the radial distribution, especially ventromedially (60%, 300° to 360°). Treatment with hormones attenuated these reductions. Bar heights represent the mean ± SEM. *indicates significantly different from sham animals, † indicates significantly different from untreated SCI. (Data from Byers et al. (2012) and Sengelaub et al. (2018).

Figure 3

Muscle fiber area is protected by androgens following spinal cord injury. (A) Cross-section through quadriceps muscle fibers. Scale bar: 100 µm. (B) SCI reduces muscle fiber area; treatment with estradiol (SCI + E) is ineffective, but this reduction is prevented by treatment with dihydrotestosterone (SCI + D), alone or in combination with estradiol (SCI + E + D), or testosterone (SCI + T). Bar heights represent means ± SEM. *significantly different from sham, † significantly different from untreated SCI. (Data from Byers et al. (2012) and Sengelaub et al. (2018).