Morphometric analysis of circadian variations in the retinal photoreceptor synaptic terminals of the adult and fetal guinea pig

Abstract

In order to test whether the alterations in photoreceptor synaptic terminal size and shape reported in lower vertebrates occur in a mammalian visual system, adult and fetal guinea pig retinas were exposed to an LD 12:12 lighting cycle, as well as to long-term light (LL) and long-term dark (DD) regimes. Representative random samples from all retinal quadrants, obtained at various times during these lighting regimes, were processed for electron microscopy. The synaptic terminals of all three photoreceptor cell types in this retina (alpha and paranuclear rods, and cones) were analyzed with computer-assisted morphometrics for changes in their area, perimeter, synaptic vesicle density, and the degree of plasmalemmal infolding. The data showed all three types of adult receptor terminals to have increased area and vesicle density, as well as decreased membrane infolding, during the light period, while both types of rods showed increased perimeter measurements in the dark. Results from adults maintained under extended lighting conditions (LL and DD) showed no difference when compared with sample times during a typical LD 12:12 lighting regimen where clear statistical differences existed. Data from fetal retinas showed no significant sustainable pattern in any of the measured variables. These quantitative findings have led to the conclusion that while alterations in perimeter measurements may be explained by using the vesicle recycling hypothesis, observed changes in terminal size and shape may be controlled by a light-initiated or light-enhanced mechanism and effected through an annular configuration of cross-striated fibrils found within these photoreceptor synaptic terminals.

P-Rex2, a Rac-guanine nucleotide exchange factor, is expressed selectively in ribbon synaptic terminals of the mouse retina

“…The terminals of photoreceptors and bipolar cells and their synaptic partners undergo significant anatomical remodeling in response to changes in illumination, including the extension and retraction of processes from the terminal itself and rearrangements associated with post-synaptic processes [46-54]. Plasticity of this nature is best known in the retinas of non-mammalian species [48-53], but adaptive structural changes also occur in mammalian photoreceptor and bipolar cell terminals [46,47,54]. This structural remodeling is dependent at least in part on the actin cytoskeleton as treatment with cytochalaisin D inhibits remodeling [50,52], which would be consistent with a role for P-Rex2 in adaptive remodeling.…”