In recent years, it has become apparent that the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signaling pathway is one of the most widespread in the retina (Eldred, 2000). This pathway is distinguished by having NO produced on demand, that NO is not released using synaptic vesicles, and that NO does not bind to specific receptors on the postsynaptic membrane. Basically any synaptic mechanism that can increase intracellular calcium either directly through receptor channels or by release from intracellular stores can potentially activate this pathway. This increased calcium can activate calmodulin which in turn activates either endothelial nitric oxide synthase (eNOS) or neuronal nitric oxide synthase (nNOS) to synthesize NO. NO has been shown to influence the physiology of all neuronal types in the retina and every cell type in the retina has been shown to make NO. The most clearly characterized downstream signaling pathway for NO has been the activation of soluble guanylate cyclase (sGC) to synthesize cGMP. For instance, NO has been shown to increase the gain and extend the voltage range of exocytosis in cone photoreceptors. In bipolar cells, NO donor produces an inward current accompanied by a rise in dim and bright flash response amplitudes, and an increase in membrane conductance. Cyclic GMP has recently been shown to selectively enhance responses to dim, but not bright, stimuli through a purely postsynaptic mechanism that is blocked by inhibitors of cGMP dependent kinase.
Perhaps the most comprehensive studies have examined the role of NO in horizontal cells. Injection of the NO precursor, L arginine, into H1 luminosity type horizontal cells in turtle retina reduces their light responses, dramatically increases their input resistance, decreases their response to a surround, and increases their response to stimulation of their receptive field center. Bath application of L arginine also decreases the kainate responses in H1 cells in a manner similar to cGMP and NO. Thus it is likely that the H1 horizontal cells can serve as their own source and target of NO to negatively modulate the gain at photoreceptor horizontal cell synapses. Finally, Yu and Eldred (2003) have shown that GABAA and GABAC receptor antagonists increase retinal cGMP levels through the activation of nitric oxide synthase (NOS) and that NO stimulates GABA release and inhibits glycine release in retina (Yu and Eldred, 2005). The NO stimulated GABA release from horizontal cells was shown to be due to the reversal of the GABA uptake transporter. In amacrine cells, by working through cGMP, the NO released by light stimulation decreases the rod input and increases the cone input during light adaptation by uncoupling the AII amacrine cells from the cone bipolars. NO also modulates cGMP gated channels by activating a NO sensitive sGC to increase levels of cGMP in photoreceptors, bipolar cells, and ganglion cells. Finally, bath application of L‑arginine or NO donor usually reduces the peak discharge rates of ON responses in ganglion cells by about 40%, and completely blocks the OFF responses in most ganglion cells.
Clearly the NO/cGMP signaling system is critical for many aspects of retinal function and it is important to understand its role in specific retinal cell types. In particular, we are determining which cells contain specific NOS isoforms, what stimuli can activate NO production in identified cells, and what downstream signaling pathways are activated by the NO that is produced. Using NO imaging methods we have shown that although every retinal cell type can make NO, it is not always freely diffusible and is in fact retained within some retinal neurons.