Studying Nitric Oxide in the Developing Retina: Neuromodulatory Functions and Signaling Mechanisms

Abstract

The retina is a highly organized structure responsible for transducing light stimulation into electrical responses. Retinal organization is conserved in all vertebrate species and the generation of retinal cell types is chronologically deter- mined and fairly documented from amphibians to humans. Furthermore, the chick retina is a well-established experimen- tal paradigm for neurochemical and developmental studies regarding the nervous system. Among many signaling mole- cules regulating retinal physiology, nitric oxide (NO) is likely to play a prominent role within the retina. NO is a gaseous signaling transmitter, which regulates a plethora of physiological functions within an organism, including high-order sig- naling events both in the developing and mature nervous system. In this review we focus on different aspects of NO sig- naling in regulating retinal cell neurochemistry, focusing mainly on developing chick retina as a prevalent experimental model. Based on literature and data gathered from our group we conclude that NO is a major atypical neurotransmitter in the retina, regulating signaling events associated with the development of embryonic retinal neurons and glial cells. The retina is a specialized tissue of the central nervous system (CNS), which is responsible for the reception and transduction of light stimuli derived from the outside envi- ronment. Within this tissue, the first processing of visual input takes place, which will be further processed in higher brain structures such as the optic tectum, thalamus and visual cortex. The cell types present in the retina are very well known and comprise the photoreceptors (rods and cones), horizontal, bipolar, amacrine and ganglion cells, as well as the Muller glial cells, and in some species interplexiform cells and the microglia. Most, if not all, neurotransmitter and neuromodulator molecules present in other areas of the CNS are also present in the retina, such as glutamate, dopamine, GABA, acetylcholine, adenosine, etc. The avian retina, especially from Gallus gallus, is a very convenient model for neurochemical studies of the CNS be- cause it is very easy to isolate during most of the embryonic period of development. Moreover, the neurogenesis in the chick retina is well known and the cells from the early de- veloping tissue can be dissociated to prepare cultures where many of the neurochemical properties are maintained as in the intact tissue.