Motor Neuron Size‐Dependent Differences in m <scp>RNA</scp> Expression of Tropomyosin‐Related Kinase Receptor B in Adult Rats

Motor neuron plasticity and survival depend on several trophic factors, with signaling via the tropomyosin receptor kinase B (TrkB) receptor being particularly important beyond development and across the lifespan. We hypothesized that differences in motor neuron properties may reflect differential trophic influences across motor units within a single motor unit pool. Based on extensive previous characterization of rat phrenic motor neurons (PhMN), somal surface area was used as an indicator of motor unit type (smaller neurons innervating slow-twitch, fatigue-resistant units versus larger neurons innervating fast-twitch, fatigable units). To examine whether TrkB expression reflects these size-related properties, TrkB mRNA transcripts were studied in individual PhMNs labeled retrogradely with Alexa Fluor 488-conjugated cholera toxin subunit β from five adult male Sprague-Dawley rats. Longitudinal sections of the cervical C3-C5 spinal cord (10 μm thick) were processed using fluorescence in situ hybridization (RNAscope) and imaged using confocal microscopy to measure individual PhMN size and full-length TrkB mRNA transcripts in nuclear and cytoplasmic compartments. Across 287 PhMNs, significant size variation existed both within and between animals. Across all animals, somal surface area averaged 4385 ± 1627 μm² with mean values per animal ranging from 3544 to 5461 μm². PhMNs were classified into tertiles within each animal based on somal surface area. Individual PhMNs averaged 1070 ± 685 TrkB mRNA transcripts per motor neuron. Larger PhMNs in the upper tertile by size expressed a greater total number of TrkB mRNA transcripts per motor neuron, with values on average threefold higher compared to smaller motor neurons in the lower tertile (p < 0.001). Cytoplasmic and nuclear TrkB mRNA density (transcripts/volume) displayed no significant effect of PhMN somal surface area tertile. Overall, the greater TrkB mRNA levels in larger motor units responsible for higher force, expulsive motor behaviors may underlie their functional adaptations and resilience to injury or disease.