Abstract
Purpose :
The axon initial segment (AIS) is a specialized membrane compartment in CNS neurons that is comprised of densely packed voltage-gated sodium (Nav) and potassium channels (Kv), held in place by a network of structural proteins. Outside the retina, variations in AIS length, location, and composition have been shown to have important implications for neuronal physiology, and may be customized to compensate for differences in other morphological features between different cell types. With over 30 different types of retinal ganglion cells (RGCs), it is likely that variations in the AIS similarly help to shape the different spiking patterns across types but this has not been systematically explored. In this study, we investigate the relationship between length, location and composition of the AIS within ON- and OFF-α Sustained (α S) cells of the mouse retina and explore how changes in AIS properties correlate to changes in other morphological features. Previous work has shown that mouse α S RGCs display a systematic gradient of dendritic field size across the nasal-temporal axis. Here, we explored whether AIS properties also varied systematically along this same axis.
Methods :
Retinal whole-mounts were surgically isolated from Thy-1 GFP mice, fixed in 4% PFA solution for 30 minutes, and subsequently immunostained for AnkyrinG and Nav1.6 to label the AIS, and ChAT to identify the specific sublamina within the inner plexiform layer at which the dendrites of targeted cells stratified. Anatomical measurements and image analysis of confocal scans were performed in NIH Image J/FIJI package, and statistical plotting was done in MATLAB.
Results :
Measurements of soma size and dendritic field size revealed that the two scaled linearly in both the ON and OFF sub-populations (p<0.001). Both AIS length and location also increased along the same axis (p<0.001); thus, large cells had long AISs that were far from the soma. Interestingly however, the ratio of individual Nav components within the AIS remained highly consistent for all cells within a given cell type. Physiological testing along with computer modeling suggests that the changes in AIS help to maintain consistency in spiking responses across the population of each cell type.
Conclusions :
Changes in cell size of α S RGCs are accompanied by a scaling of AIS properties; it is likely that such changes help to maintain consistency in the physiological responses across this cell type.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.