Circuit-Specific Early Impairment of Proprioceptive Sensory Neurons in the SOD1G93AMouse Model for ALS

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease in which motor neurons degenerate resulting in muscle atrophy, paralysis and fatality. Studies using mouse models of ALS indicate a protracted period of disease development with progressive motor neuron pathology, evident as early as embryonic and postnatal stages. Key missing information includes concomitant alterations in the sensorimotor circuit essential for normal development and function of the neuromuscular system. Leveraging unique brainstem circuitry, we showin vitroevidence for reflex circuit-specific postnatal abnormalities in the jaw proprioceptive sensory neurons in the well-studied SOD1G93Amouse. These include impaired and arrhythmic action potential burst discharge associated with a deficit in Nav1.6 Na+channels. However, the mechanoreceptive and nociceptive trigeminal ganglion neurons and the visual sensory retinal ganglion neurons were resistant to excitability changes in age matched SOD1G93Amice. Computational modeling of the observed disruption in sensory patterns predicted asynchronous self-sustained motor neuron discharge suggestive of imminent reflexive defects such as muscle fasciculations in ALS. These results demonstrate a novel reflex circuit-specific proprioceptive sensory abnormality in ALS.Significance StatementNeurodegenerative diseases have prolonged periods of disease development and progression. Identifying early markers of vulnerability can therefore help devise better diagnostic and treatment strategies. In this study, we examined postnatal abnormalities in the electrical excitability of muscle spindle afferent proprioceptive neurons in the well-studied SOD1G93Amouse model for neurodegenerative motor neuron disease, ALS. Our findings suggest that these proprioceptive sensory neurons are exclusively afflicted early in the disease process relative to sensory neurons of other modalities. Moreover, they presented Nav1.6 Na+channel deficiency which contributed to arrhythmic burst discharge. Such sensory arrhythmia could initiate reflexive defects such as muscle fasciculations in ALS as suggested by our computational model.

Growth restriction induced by chronic prenatal hypoxia affects breathing rhythm and its pontine catecholaminergic modulation

Impaired transplacental supply of oxygen leads to intrauterine growth restriction, one of the most important causes of perinatal mortality and respiratory morbidity. Breathing rhythm depends on the central respiratory network modulated by catecholamines. We investigated the impact of growth restriction, using prenatal hypoxia, on respiratory frequency, on central respiratory-like rhythm, and on its catecholaminergic modulation after birth. At birth, respiratory frequency was increased and confirmed in en bloc medullary preparations, where the frequency of the fourth cervical (C4) ventral root discharge was increased, and in slice preparations containing the pre-Bötzinger complex with an increased inspiratory rhythm. The inhibition of C4 burst discharge observed in pontomedullary preparations was stronger in the growth-restricted group. These results cannot be directly linked by the tyrosine hydroxylase activity increase of A1/C1and A2/C2cell groups in the medulla since blockade of α1- and α2-adrenergic receptors did not abolish the difference between both groups. However, in pontomedullary preparations, the stronger inhibition of C4 burst discharge is probably supported by an increased inhibition of A5, a respiratory rhythm inhibitor pontine group of neurons displaying increased tyrosine hydroxylase activity, because blockade of α2-adrenergic receptors abolished the difference between the two groups. Taken together, these results indicate that growth restriction leads to a perturbation of the breathing frequency, which finds, at least in part, its origin in the modification of catecholaminergic modulation of the central breathing network.

Automatic System for Identification and Analysis of Burst

Although there are complex patterns of discharges exist in neurons, which are often clustered in bursts, the pattern of the burst discharge can substantially change with the alternation of the living environment of the neurons. Furthermore, there is a big difference in the physiological and pharmacological characteristics of various types of burst discharges. So it is necessary to analyze the information contained in various types of burst discharges. Here, an automatic system for identification and analysis of the neuron discharge based on plotting histograms of the logarithm of the interspike interval was designed. The system consists of neuron discharge collecting unit, neuron discharge processing unit, battery monitoring, real-time charging unit and bursts processing software. The system can identify the burst discharge from the neuron discharge without any omission and make statistical analysis. By using this device, the electrophysiological experiment that the spontaneous and evoked discharges of wide dynamic range neurons in spinal dorsal horn of rats were recorded was smoothly completed. The result of statistical analysis indicated that the device can give the corresponding interspike interval aimed at various types of burst discharges and respectively identify the burst discharges in the different amplitude spikes, which provide a tool for further research on the biosensor and the neural communication.

Persistent Na+ Current Modifies Burst Discharge By Regulating Conditional Backpropagation of Dendritic Spikes

The estimation and detection of stimuli by sensory neurons is affected by factors that govern a transition from tonic to burst mode and the frequency chracteristics of burst output. Pyramidal cells in the electrosensory lobe of weakly electric fish generate spike bursts for the purpose of stimulus detection. Spike bursts are generated during repetitive discharge when a frequency-dependent broadening of dendritic spikes increases current flow from dendrite to soma to potentiate a somatic depolarizing afterpotential (DAP). The DAP eventually triggers a somatic spike doublet with an interspike interval that falls inside the dendritic refractory period, blocking spike backpropagiation and the DAP. Repetition of this process gives rise to a rhythmic dendritic spike failure, termed conditional backpropagation, that converts cell output from tonic to burst discharge. Through in vitrorecordings and compartmental modeling we show that burst frequency is regulated by the rate of DAP potentiation during a burst, which determines the time required to discharge the spike doublet that blocks backpropagation. DAP potentiation is maginfied through a postitve feedback process when an increase in dendritic spike duration activates persistent sodium current ( I NaP). I NaP further promotes a slow depolarization that induces a shift from tonic to burst discharge over time. The results are consistent with a dynamical systems analysis that shows that the threshold separating tonic and burst discharge can be represented as a saddle-node bifurcation. The interaction between dendritic K+ current and I NaP provides a physiological explanation for a variable time scale of bursting dynamics characteristic of such a bifurcation.