A precise pattern of connections between the retina and central visual nuclei in the brain is established during development. Activity-dependent presynaptic mechanisms and NMDA receptor-mediated postsynaptic mechanisms are thought to play important roles in this developmental process. A model proposed for production of the newly described neurotransmitter, nitric oxide, involves presynaptic activity and activation of postsynaptic NMDA receptors. If present in the developing visual system, nitric oxide could represent a form of retrograde communication from postsynaptic to presynaptic cells that mediates the formation of the proper pattern of connections. This study used the diaphorase histochemical technique to detect the presence of nitric oxide synthase (NOS), the enzyme responsible for the production of nitric oxide, in the developing chick optic tecturn. Results from this study showed that NOS is present in the developing tectum and that its expression coincides temporally with innervation by retinal axons. NOS expression reaches a peak at the time that refinement of the initial pattern of connections is occurring. WGA/HRP labeling of retinal axons confirmed that processes of NOS-positive cells in the tectum extend well into the area of the ingrowing retinal axons. Histochemical results from eyeless chick embryos indicate that NOS expression is dependent on the presence of retinal axons, which suggests that retinal axons synapse on cells that express nitric oxide. Northern blot analysis using a cDNA probe to NOS from rat brain verified the histochemical results. These results are consistent with nitric oxide having a role in development of the proper pattern of connections in the chick retinotectal system.
Changes in the morphology of the early optic cup were observed in embryos of two distantly-related vertebrate species, a teleost fish, northern pike (Esox lucius), and chicken (Gallus gallus). A similar morphological pattern was noted to appear in both species shortly after the involution of the optic vesicle and the formation of the inner retinal layer. At a gross level, three notches were observed in the retinal margin at approximately nasal, dorsal, and temporal positions, while in histological sections a sharp constriction was found in the thickness of the dorsal retinal layer. In both species, this dorsal constriction appeared to be continuous with the central or dorsal notch. The time of appearance and configuration of this morphological pattern is intriguingly similar to the specification and polarity of retinal positional markers, and suggest a segmentation hypothesis for the origin of retinal polarity.
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