Abstract
The mammalian retina is considered as the most accessible part of the central nervous system. Visual stimuli are detected by photoreceptors in the outer retina, and then transmitted to the inner retinal neurons including bipolar cells, amacrine cells, and retinal ganglion cells. The first synapses are between the photoreceptors and bipolar cells at the outer plexiform layer, where the visual signals are diverged into parallel pathways via diverse bipolar cell types. While many studies have focused on the effects of spontaneous retinal activities, such as retinal waves, on the development of visual system, fewer efforts have been put to characterize the functional changes of inner retinal neurons in the absence of outer retinal activities. In this dissertation, I examined two most important amacrine cell types – AII amacrine cells (AII-ACs) in developing rabbit retina and starburst amacrine cells (SACs) in adult mouse retina under the conditions of pharmacologically and genetically altered signal transmission from the outer retina to the inner retina, respectively. These two independent studies are described separately here, and they represent a coherent effort in elucidating the cellular mechanisms of activity dependent circuitry adaptation in mammalian retinas. The rod photoreceptor signaling pathways in adult retina have been extensively investigated and characterized in terms of the distinctive components and light-adaptive properties. Gap junctions are composed of connexin 36 (Cx36) and play critical roles in most of these pathways, despite little is known about the contribution and regulation of gap junctions to the development of the AII-AC mediated primary rod pathway. Using immunohistochemistry and microinjection, the first study demonstrates a steady increase in relative Cx36 protein expression in both plexiform layers of the rabbit retina at around the time of eye opening. However, immediately after eye opening, most Cx36 immunoreactive AII-ACs show no gap junction coupling pattern to neighboring cells and it is not until the third postnatal week that AII cells begin to exhibit an adult-like tracer coupling pattern. Moreover, studies using dark-rearing and AMPA receptor blockade during postnatal development both revealed that relative levels of Cx36 immunoreactivity in AII-ACs were increased when neural activity was inhibited. These findings suggest that Cx36 expression in the AII-mediated rod pathway is activity dependent in the developing rabbit retina. It has been shown in rd1 and rd10 models of photoreceptor degeneration (PD) that inner retinal neurons display spontaneous and rhythmic activities. An autosomal dominant PD model called rhoΔCTA, whose rods overexpress a C-terminally truncated mutant rhodopsin and degenerate with a rate similar to that of rd1, was used to investigate the generality and mechanisms of heightened inner retinal activity following PD. Excitatory postsynaptic current (EPSC) oscillations and non-rhythmic inhibitory postsynaptic currents (IPSCs) were observed in both ON- and OFF-SACs. Similar to reported RGC oscillation in rd1 mice, EPSC oscillation was synaptically driven by glutamate and sensitive to blockades of NaV channels and gap junctions, suggesting that akin to rd1 mice, AII-AC is a prominent oscillator in rhoΔCTA mice. However, weakening the AII-AC gap junction network by activating retinal dopamine receptors abolished oscillations in ON-SACs but not in OFF-SACs. The latter persisted in the presence of flupirtine, an M-type potassium channel activator recently reported to dampen the intrinsic AII-AC bursting. These data suggest the existence of a novel oscillation mechanism in mice with PD. In conclusion, disruption of outer retinal activities in either developing rabbit retina or mature mouse retina results in changes of protein expression and synaptic transmission in inner retinal neurons, respectively. These two studies demonstrate a vertically modulated circuitry adaptation in mammalian retinas.