Inhibitory interneurons show early dysfunction in a SOD1 mouse model of amyotrophic lateral sclerosis

Abstract

Few studies in amyotrophic lateral sclerosis (ALS) measure effects of the disease on inhibitory interneurons synapsing onto motoneurons (MNs). However, inhibitory interneurons could contribute to dysfunction, particularly if altered before MN neuropathology, and establish a long‐term imbalance of inhibition/excitation. We directly assessed excitability and morphology of glycinergic (GlyT2 expressing) ventral lumbar interneurons from SOD1G93AGlyT2eGFP (SOD1) and wild‐type GlyT2eGFP (WT) mice on postnatal days 6–10. Patch clamp revealed dampened excitability in SOD1 interneurons, including depolarized persistent inward currents (PICs), increased voltage and current threshold for firing action potentials, along with a marginal decrease in afterhyperpolarization duration. Primary neurites of ventral SOD1 inhibitory interneurons were larger in volume and surface area than WT. GlyT2 interneurons were then divided into three subgroups based on location: (1) interneurons within 100 μm of the ventral white matter, where Renshaw cells (RCs) are located, (2) interneurons interspersed with MNs in lamina IX, and (3) interneurons in the intermediate ventral area including laminae VII and VIII. Ventral interneurons in the RC area were the most profoundly affected, exhibiting more depolarized PICs and larger primary neurites. Interneurons in lamina IX had depolarized PIC onset. In lamina VII–VIII, interneurons were least affected. In summary, inhibitory interneurons show very early region‐specific perturbations poised to impact excitatory/inhibitory balance of MNs, modify motor output and provide early biomarkers of ALS. Therapeutics like riluzole that universally reduce CNS excitability could exacerbate the inhibitory dysfunction described here. Key points Spinal inhibitory interneurons could contribute to amyotrophic lateral sclerosis (ALS) pathology, but their excitability has never been directly measured. We studied the excitability and morphology of glycinergic interneurons in early postnatal transgenic mice (SOD1G93AGlyT2eGFP). Interneurons were less excitable and had marginally smaller somas but larger primary neurites in SOD1 mice. GlyT2 interneurons were analysed according to their localization within the ventral spinal cord. Interestingly, the greatest differences were observed in the most ventrally located interneurons. We conclude that inhibitory interneurons show presymptomatic changes that may contribute to excitatory/inhibitory imbalance in ALS.

Early dysfunction of spinal cord interneurons in a mouse model of amyotrophic lateral sclerosis

“…In conclusion, the major contribution of the study by Cavarsan et al (2023) is the description of morphological changes and electrical activity of the inhibitory interneurons at an early stage in the Sod1 G93A model. Although hyperexcitability has been regarded as a key feature of ALS pathogenesis, Cavarsan et al (2023) provided new evidence showing early dysfunction of selective spinal cord interneurons in Sod1 G93A mice.…”

“…study published in The Journal of Physiology, Cavarsan et al (2023) examined the morphology and excitability of glycinergic inhibitory spinal interneurons in Sod1 G93A mice in three different regions of the spinal cord. Cavarsan et al (2023) hypothesized that reduced activity of inhibitory spinal interneurons could contribute to MN hyperexcitability, an important feature of ALS pathogenesis.…”

“…study published in The Journal of Physiology, Cavarsan et al (2023) examined the morphology and excitability of glycinergic inhibitory spinal interneurons in Sod1 G93A mice in three different regions of the spinal cord. Cavarsan et al (2023) hypothesized that reduced activity of inhibitory spinal interneurons could contribute to MN hyperexcitability, an important feature of ALS pathogenesis. They bred Sod1 G93A mice with mice expressing the enhanced green fluorescent protein under the control of glycine transporter 2 promoter (GlyT2-eGFP) to visualize and record glycinergic interneurons in acute spinal cord slices.…”

“…Furthermore, ventrally located GlyT2 interneurons are a heterogeneous neuronal population. Cavarsan et al (2023) also explored three different subregions of the ventral horn: the Renshaw cell (RC) region, including GlyT2 interneurons within 100 µm of the ventral white matter; lamina IX, with scattered interneurons; and a few MNs and the intermediate region, covering lamina VII and VIII. The investigations demonstrated that Sod1 G93A interneurons in the RC subregion presented the most significant electrical and morphological alterations, including depolarized PIC onset and peak, increased threshold voltage, and longer and larger primary neurites.…”

“…These findings reveal early MN-interneuron dysfunction in an ALS model and reveal precisely which neuron subset might be mostly affected through an electrophysiological characterization of spinal cord inhibitory interneurons. Cavarsan et al (2023) have reported the earliest alterations of Sod1 G93A spinal cord interneurons to date, before MN defects and motor dysfunction. This suggests that GlyT2 interneurons might be an early biomarker of ALS.…”