Meehan, C. F. | bioRxiv Science

bioRxiv2020

Increased axon initial segment length results in increased Na+ currents in spinal motoneurones at symptom onset in the G127X SOD1 mouse model of Amyotrophic Lateral Sclerosis.

Amyotrophic lateral sclerosis is a neurodegenerative disease preferentially affecting motoneurones. Transgenic mouse models have been used to investigate the role of abnormal motoneurone excitability in this disease. Whilst an increased excitability has repeatedly been demonstrated in vitro in neonatal and embryonic preparations from SOD1 mouse models, the results from the only studies to record in vivo from spinal motoneurones in adult SOD1 models have produced conflicting findings. Deficits in repetitive firing have been reported in G93A SOD1 mice but not in presymptomatic G127X SOD1 mice despite shorter motoneurone axon initial segments (AISs) in these mice. These discrepancies may be due to the earlier disease onset and prolonged disease progression in G93A SOD1 mice with recordings potentially performed at a later sub-clinical stage of the disease in this mouse. To test this, and to explore how the evolution of excitability changes with symptom onset we performed in vivo intracellular recording and AIS labelling in G127X SOD1 mice immediately after symptom onset. No reductions in repetitive firing were observed showing that this is not a common feature across all ALS models. Immunohistochemistry for the Na+ channel Nav1.6 showed that motoneurone AISs increase in length in G127X SOD1 mice at symptom onset. Consistent with this, the rate of rise of AIS components of antidromic action potentials were significantly faster confirming that this increase in length represents an increase in AIS Na+ channels occurring at symptom onset in this model. HighightsO_LIIn vivo electrophysiological recordings were made in symptomatic G127X SOD1 mice. C_LIO_LIThere were no deficits in repetitive firing in motoneurones in G127X mice. C_LIO_LIIncreased persistent inward currents were still present in the symptomatic mice. C_LIO_LIResults suggest increases in Na+ currents at axon initial segments (AISs). C_LIO_LIImmunohistochemistry showed that motoneurone AISs were longer and thinner. C_LI

neuroscience↗

bioRxiv2020

Spinal motoneurones are intrinsically more responsive in the adult G93A SOD1 mouse model of Amyotrophic Lateral Sclerosis

In vitro studies from transgenic Amyotrophic Lateral Sclerosis models have suggested an increased excitability of spinal motoneurones. However, in vivo intracellular recordings from adult ALS mice models have produced conflicting findings. Previous publications using barbiturate anaesthetised G93A SOD1 mice suggested that some motoneurones are hypo-excitable, defined by deficits in repetitive firing. Our own previous recordings in G127X SOD1 mice using different anaesthesia, however, showed no repetitive firing deficits, and increased persistent inward currents at symptom onset. These discrepancies may be due to differences between models, symptomatic stage, anaesthesia or technical differences. To investigate this, we repeated our original experiments, but in adult male G93A mice at both presymptomatic and symptomatic stages, under barbiturate anaesthesia. In vivo intracellular recordings from antidromically identified spinal motoneurones revealed no significant differences in the ability to fire repetitively in the G93A SOD1 mice. Motoneurones in G93A SOD1 mice fired significantly more spontaneous action potentials. Rheobase was significantly lower and the input resistance and input-output gain were significantly higher in both presymptomatic and symptomatic G93A SOD1 mice. This was despite a significant increase in the duration of the post-spike after-hyperpolarisation (AHP) in both presymptomatic and symptomatic G93A SOD1 mice. Finally, evidence of increased activation of persistent inward currents was seen in both presymptomatic and symptomatic G93A SOD1 mice. Our results do not confirm previous reports of hypo-excitability of spinal motoneurones in the G93A SOD1 mouse and demonstrate that the motoneurones do in fact show an increased response to inputs. Key Point SummaryO_LIAlthough in vitro recordings using neonatal preparations from mouse models of Amyotrophic Lateral Sclerosis (ALS) suggest increased motoneurone excitability, in vivo recordings in adult ALS mouse models have been conflicting. C_LIO_LIIn adult G93A SOD1 models, spinal motoneurones have previously been shown to have deficits in repetitive firing, in contrast to the G127X SOD1 mouse model. C_LIO_LIOur in vivo intracellular recordings in barbiturate-anaesthetised adult male G93A SOD1 mice reveal no deficits in repetitive firing either prior to or after symptom onset. C_LIO_LIWe show that deficits in repetitive firing ability can be a consequence of experimental protocol and should not be used alone to classify otherwise normal motoneurones as hypo-excitable. C_LIO_LIMotoneurones in the G93A SOD1 mice showed an increased response to inputs, with lower rheobase, higher input-output gains and increased activation of persistent inward currents. C_LI

neuroscience↗