Abstract
Purpose :
During the development of the mouse optic nerve, myelination by oligodendrocytes and nerve growth are considered complete by 5 weeks of age. While morphological changes beyond this period are minimal, age-related changes in optic nerve function remain unknown. We used extracellular recordings and histological assessments of wildtype optic nerves to test the hypothesis that the pattern of myelination is associated with functional trends post-development.
Methods :
Axonal conduction in the wildtype mouse optic nerve (aged 4-12 weeks) was measured using an ex vivo extracellular recording method where isolated nerves were inserted into stimulating and recording suction electrodes. Orthodromic electrical stimulation induces simultaneous firing of action potentials in axons, generating compound action potentials (CAPs). The CAP waveforms, fitted to Gaussian functions, provide a relative measure of the total number of responsive neurons (CAP area) and distinguish axon populations by conduction speed (given by peak latency and area). Pooled peak responses across ages were grouped by speed of conduction using a K-means cluster analysis. Within-nerve controls used tetrodotoxin to eliminate the stimulus artifact. Scanning electron microscopy (EM) was used to evaluate axon diameter and g-ratios (relative myelination per axon diameter) at the midpoint of wildtype optic nerves at 5 and 12 wks.
Results :
When compared across age groups (n>7), there was a significant increase in average CAP area between 4 and 5 wks (p=0.00769) and between 6 and 8 wks (p=0.02013). These two time points corresponded with recruitment of cluster-defined groups of axon populations. The slowest axon populations were present at ages 4-6 wks and lost at older ages (8 and 12 wks), when additional fast populations were recruited. EM of optic nerves revealed that, at 12 wks, large diameter axons (>0.61 um) had smaller g-ratios than at 5 wks, indicating thicker myelin around these axons.
Conclusions :
Here, dynamic shifts in total nerve function correspond to the gain and loss of functional axon populations which reflects a period of maturation where axon conduction is refined. These changes coincide with trends observed in histological analyses, where myelin is thicker around large axons, thus increasing conduction speed. These data support the hypothesis that myelination is associated with shifts in optic nerve function post-development.
This is a 2020 ARVO Annual Meeting abstract.