Testosterone supplementation to enhance respiratory function and expression of respiratory neuroplasticity in male rats following cervical spinal cord injury

In addition to loss of sensory and motor function below the level of the lesion, traumatic spinal cord injury (SCI) can reduce circulating steroid hormone production in both males and females for an extended period of time, impeding recovery. In males, who comprise ~80% of new SCIs annually, a significant reduction in circulating testosterone is frequently observed for up to a year post-injury and often remains low relative to age-matched, spinal-intact males for life. Among its many physiological effects, testosterone plays an important role in maintaining normal respiratory function and is necessary for the expression of respiratory neuroplasticity. For example, when testosterone levels are low, young-adult male rats are unable to express phrenic long-term facilitation (pLTF), an inducible form of respiratory neuroplasticity induced by acute, intermittent hypoxia (AIH). Testosterone supplementation is sufficient to restore AIH-induced pLTF in these rats. Complicating the interpretation of these findings, testosterone may exert its influence in three possible ways: 1) directly through androgen receptor (AR) activation, 2) through conversion to dihydrotestosterone (DHT) and then AR activation, or 3) through conversion to 17β-estradiol (E2) and activation of estrogen receptors. Our prior work demonstrated that supplementation of either of these testosterone metabolites (DHT or E2) was sufficient to enhance respiratory function and restore AIH-induced pLTF 2 weeks following C2-hemisection (C2Hx) SCI in male rats, supporting the idea that testosterone supplementation may be sufficient to strengthen respiratory function and restore plasticity. Here, we tested the hypothesis that testosterone supplementation following C2Hx would enhance respiratory function and restore AIH-induced pLTF in young-adult male rats. Rats were randomly assigned to one of five experimental groups: Sham SCI (injury control; C2 laminectomy), SCI placebo (SCI; placebo injections), SCI + low-dose testosterone enanthate (TE; 2.0mg/kg, LD-T), SCI + high-dose TE (7.0mg/kg, HD-T), or a mixed time control group who did not receive AIH during physiological experiments. One week post C2Hx (or sham surgery), rats were given a single dose of TE (or placebo) consistent with prior pre-clinical SCI reports. After one week, respiratory function was assessed using barometric plethysmography in an awake, freely-behaving condition, followed by quantification of AIH-induced pLTF using phrenic nerve recordings in a standard anesthetized, mechanically ventilated preparation. Initial findings indicate that testosterone supplementation has minimal impact on baseline respiratory function two weeks post-C2Hx, with variable effects on hypercapnic respiratory responses. However, pLTF expression results showed a dose-dependent effect of testosterone supplementation. Namely, LD-T rats (n=3) regained the capacity for respiratory plasticity and expressed AIH-induced pLTF at levels similar to Sham SCI rats (n=4; 47% and 48% above BL respectively), while HD-T rats (n=3) showed enhanced pLTF (79% above BL). Collectively, these initial data support the hypothesis that testosterone supplementation may restore AIH-induced pLTF two weeks post cervical SCI. Funding: This project was supported by a grant from the Paralyzed Veterans of America; NIH NHLBI R01HL146477; and the University of Minnesota Division of Physical Therapy and Rehabilitation Science. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.