|Ph.D Student||Taha Reem|
|Subject||The Role of Nitric Oxide (NO) in Turtle Retinal Neuronal|
Adaptation to Changes in Conditions of
|Department||Department of Medicine||Supervisor||Professor Emeritus Ido Perlman|
|Full Thesis text|
Background: Adaptation to changing conditions of illumination is one of the unique properties of the vertebrate visual system. A human observer with normal vision can function visually from starlight to very bright day-light. Visual adaptation mechanisms were attributed to the retina, and were divided into two categories; receptoral and neuronal (post-receptoral) adaptation. While receptoral adaptation is well understood, little attempts have been directed into the understanding of the mechanisms underlying neuronal adaptation. Nitric Oxide (NO) was suggested to act as a neuromodulator in the vertebrate retina that is synthesized by Nitric Oxide Synthase (NOS) in the retina as the background illumination is increased. This study tested the working hypothesis that NO plays a role in neuronal adaptation.
Methods: Electroretinogram (ERG) responses were recorded from turtle eyecups under different conditions of background illumination while raising retinal NO level with L-arginine (NOS substrate) or lowering it with L-NAME (a competitive NOS inhibitor). Small-amplitude ERG responses were used to calculate sensitivity to light. P-III and P-II components of the ERG were isolated pharmacologically and mathematically in order to assess the function of photoreceptors and ON-center bipolar cells respectively. Similar experiments were conducted on horizontal cells by intracellular recordings. Ganglion cells' activity was recorded extracellularly from the isolated turtle retina using multi-electrode array (MEA).
Results: With bright light stimuli, eliciting large-amplitude ERGs, raising NO increased the amplitude of P-III and P-II while lowering NO reduced these responses. In contrast, raising/lowering NO increased/decreased light sensitivity of photoreceptors (P-III), and decreased/increased that of ON-center bipolar cells (P-II) respectively. When we measured light sensitivity for dark-adapted state and different background lights, we found that background desensitization of ON-center bipolar cells started at lower background levels compared to photoreceptors. This is the manifestation of neuronal adaptation. When NO level was raised, this difference between ON-center bipolar cells and photoreceptors was eliminated, while lowering NO made the difference more pronounced, indicating that neuronal adaptation is affected by NO. NO effects upon horizontal cells light sensitivity were similar to those of photoreceptors. An interesting and surprising finding emerged when the effects of NO upon ganglion cells' responsiveness to light stimuli were tested. Either increasing or decreasing retinal NO level caused a reduction in the peak response in all ganglion cells that were tested.
Conclusions: We show that NO probably plays a role in neuronal adaptation. The opposing effects of NO upon light sensitivity of photoreceptors and ON-center bipolar cells are attributed to NO-dependent increase in guanylate cyclase activity in the photoreceptors and additional effect of NO upon post-synaptic signal transduction pathway in ON-center bipolar cells. This latter NO effect needs further studies, but may involve S-nitrosylation, and not activation of guanylate cyclase. Moreover, further studies of NO effects on ganglion cells are needed to investigate its mechanism of action at the level of ganglion cells.