Serotonin receptor and KCC2 gene expression in lumbar flexor and extensor motoneurons

19 Sacrocaudal motoneuron gene expression is altered following a spinal transection. Of interest 20 here is the regulation of serotonin (5-HT) receptors, mGluR1 and KCC2 which mediate 21 motoneuron excitability, locomotor recovery and spasticity post-transection. The examination of 22 these genes in lumbar motoneurons post-transection has not been studied, which is necessary for 23 developing potential pharmacological interventions aimed at restoring locomotion and or 24 reducing spasticity. Also, if activity is to be used to promote recovery or reduce spasticity post25 injury, a further examination of neuromuscular activity on gene expression post-transection is 26 warranted. The purpose of this study was to examine motoneuronal gene expression of 5-HT 27 receptors, KCC2 and mGluR1 at three months following a complete thoracic spinal cord 28 transection, with and without the inclusion of daily passive cycling. Physiological hindlimb 29 extensor and flexor motoneurons were differentially identified with two retrograde fluorescent 30 tracers, allowing for the identification and separate harvesting of extensor and flexor 31 motoneurons with laser capture microdissection, and the subsequent examination of mRNA 32 content using qRT-PCR analysis. We demonstrate that post-transection, 5-HT1AR, 5-HT2CR, and 33 mGluR1 expression was down-regulated, whereas the 5-HT2AR was up-regulated. These 34 alterations in gene expression were observed in both flexor and extensor motoneurons, whereas 35 passive cycling influenced gene expression in extensor but not flexor motoneurons. Passive 36 cycling in extensor motoneurons further enhanced 5-HT2AR expression and increased 5-HT7R 37 and KCC2 expression. Our results demonstrate that passive cycling influences serotonin 38 receptor and KCC2 gene expression and that extensor motoneurons compared to flexor 39 motoneurons may be more plastic to activity based interventions post-transection. 40