Serotonin receptor and KCC2 gene expression in lumbar flexor and extensor motoneurons posttransection with and without passive cycling

Abstract

Sacrocaudal motoneuron gene expression is altered following a spinal transection. Of interest here is the regulation of serotonin (5-HT) receptors (R), glutamate receptor, metabotropic 1 (mGluR1), and potassium-chloride cotransporter (KCC2), which mediate motoneuron excitability, locomotor recovery, and spasticity posttransection. The examination of these genes in lumbar motoneurons posttransection has not been studied, which is necessary for developing potential pharmacological interventions aimed at restoring locomotion and/or reducing spasticity. Also, if activity is to be used to promote recovery or reduce spasticity postinjury, a further examination of neuromuscular activity on gene expression posttransection is warranted. The purpose of this study was to examine motoneuronal gene expression of 5-HT receptors, KCC2, and mGluR1 at 3 mo following a complete thoracic spinal cord transection, with and without the inclusion of daily passive cycling. Physiological hindlimb extensor and flexor motoneurons were differentially identified with two retrograde fluorescent tracers, allowing for the identification and separate harvesting of extensor and flexor motoneurons with laser capture microdissection and the subsequent examination of mRNA content using quantitative RT-PCR analysis. We demonstrate that posttransection 5-HT1AR, 5-HT2CR, and mGluR1 expression was downregulated, whereas the 5-HT2AR was upregulated. These alterations in gene expression were observed in both flexor and extensor motoneurons, whereas passive cycling influenced gene expression in extensor but not flexor motoneurons. Passive cycling in extensor motoneurons further enhanced 5-HT2AR expression and increased 5-HT7R and KCC2 expression. Our results demonstrate that passive cycling influences serotonin receptor and KCC2 gene expression and that extensor motoneurons compared with flexor motoneurons may be more plastic to activity-based interventions posttransection.

Serotonin controls initiation of locomotion and afferent modulation of coordination via 5‐HT7receptors in adult rats

“…; Chopek et al . ). The reduction in EMG amplitude that we observed is clearly not due to blockade of 5‐HT 7 receptors on motoneurons, however.…”

Altered transcription of glutamatergic and glycinergic receptors in spinal cord dorsal horn following spinal cord transection is minimally affected by passive exercise of the hindlimbs

“…KCC2 has also been implicated in spasticity post‐SCI, as a down‐regulation of KCC2 alters the equilibrium of chloride, increasing the excitability of the cell (Boulenguez et al ., ). In addition, we previously demonstrated a 23% decreases in KCC2 expression in flexor but not extensor motoneurons (Chopek et al ., ) and Cote et al ., showed a 20% decrease in protein expression in motoneurons (Cote et al ., ; Chopek et al ., ). Taken together, these results would suggest that KCC2 likely mediates neuronal excitability in the ventral horn and that although it is linked to nociceptive sensitization in the acute STx model (Huang et al .,), this may not be achieved through alteration in KCC2 gene expression.…”

“…A second part of this investigation was to examine the potential influence ‘passive exercise’ has on gene expression in laminae I‐III following a STx. Exercise in the form of daily passive cycling has been demonstrated to influence gene expression in the motoneuron, dorsal root ganglia and lamina VII following STx (Keeler et al ., ; Chopek et al ., ). Also, post‐STx, daily passive cycling exercise has been shown to attenuate the change in motoneuron biophysical properties (Beaumont et al ., ), normalize spinal reflexes (Cote et al ., ; Chopek et al ., ), and in the contusive SCI model, reduce allodynia and aberrant afferent sprouting (Detloff et al ., ).…”

“…Following spinal cord injury (SCI), gene alteration in the nociceptive, proprioceptive, motor and sensory systems has been shown (Nesic et al, 2005;Ryge et al, 2010;Wienecke et al, 2010;Keeler et al, 2012;Navarrett et al, 2012;Chopek et al, 2015). The consequences of altered gene expression include spasticity due to alteration in motoneuron excitability (Murray et al, 2011) and persistent chronic neuropathic pain, which is present in up 40% of individuals with a SCI (Nesic et al, 2005;Mehta et al, 2013).…”

Spinal circuits for motor learning

“…Locomotor training, which provides instructive input to the spinal cord, can lead to improvement in function [67 ,70,71]. Such training leads to changes in synaptic transmission at specific pathways [72], and in fact has been shown to lead to upregulation of select serotonin receptors and the chloride transporter KCC2 in extensor MNs [68 ]. Thus instructive input can lead to changes in gene expression in spinal neurons.…”