RESUMO
The ongoing research on the roles of the gas nitric oxide (NO) in the nervous system has demonstrated its involvement in neurotransmission, synaptic plasticity, learning, excitotoxicity, neurodegenerative diseases and regulation of the cerebral blood flow. Thus, this molecule has been currently considered an important neuromodulator in CNS. Studies carried out in the visual system, particularly in the retinotectal component, have contributed to this current concept about NO. In the present work, we reviewed critically current data about nitric oxide synthase (NOS) expression in the superior colliculus/optic tectum, as well as the roles of NO in the formation of the retinotopic map and in synaptic plasticity. Several vertebrate species have been used in studies about the NOS expression in the retinotectal system and most of the available results are in agreement with the involvement of NO in the developmental refinement of the retinotectal projections, and its role as a neuromodulator of synaptic function during the processing of visual information. However, the few studies about the functional linkage between NOS expression/NO synthesis and retinotectal topographic refinement/tectal synaptic plasticity are not conclusive and/or sometimes inconsistent, indicating that more experimental data are necessary to improve the understanding about NO functions in this visual subsystem. Predictive models for the involvement of NO as a retrograde messenger in the developmental retinotectal refinement are discussed.
Assuntos
Óxido Nítrico Sintase/biossíntese , Óxido Nítrico/fisiologia , Retina/fisiologia , Colículos Superiores/fisiologia , Animais , Humanos , Plasticidade Neuronal/fisiologia , Óxido Nítrico Sintase Tipo I/biossíntese , Retina/enzimologia , Colículos Superiores/enzimologiaRESUMO
The superficial layers of the rat superior colliculus (sSC) receive innervation from the retina and include nitrergic neurons. We have shown previously that in sSC, eye enucleation reduces NADPH diaphorase staining considerably in all but the most proximal dendrites of nitrergic neurons. We have used immunocytochemistry for neuronal nitric oxide synthase (nNOS) at light and electron microscopic levels and bilateral eye enucleation with varied survival times to determine the regulatory changes imposed by the direct and indirect loss of retinal input on apparent nNOS amount and subcellular distribution. In addition, we have used SDS-PAGE and immunoblotting to test alternatively spliced isoforms in normal and deafferented animals. Our results show that unambiguously identified retinal terminals contact nitrergic neurons. In normal dendrites, nNOS immunoreactivity was distributed almost completely within the cytoplasm of the dendrite and along the postsynaptic membrane at synaptic junctions, in association with endoplasmic reticulum, ribosomes and external mitochondrial membranes. In contrast, nNOS labeling was greatly reduced in sSC deprived of retinal projections, and could only be observed in association with mitochondrial membranes and postsynaptic densities. Immunoblots of the soluble fraction from sSC revealed a surprisingly high proportion of the beta isoform with respect to the alpha counterpart in normal colliculi, suggesting an increase in isoform proportion after enucleation, or at least maintenance of the same proportion. It is suggested that ultrastructural alterations observed in sSC cells of enucleated animals may be consequent to plastic reactions of the sSC cells in response to the removal of retinal afferents.
Assuntos
Dendritos/enzimologia , Neurônios Nitrérgicos/enzimologia , Óxido Nítrico Sintase/análise , Retina/enzimologia , Colículos Superiores/enzimologia , Vias Aferentes/enzimologia , Vias Aferentes/ultraestrutura , Animais , Dendritos/ultraestrutura , Isoenzimas/análise , Isoenzimas/biossíntese , Neurônios Nitrérgicos/ultraestrutura , Óxido Nítrico Sintase/biossíntese , Óxido Nítrico Sintase Tipo I , Ratos , Retina/ultraestrutura , Colículos Superiores/ultraestruturaRESUMO
Nitric oxide (NO) is synthesized in cells of both the central and peripheral nervous system and has been implicated in several forms of synaptic plasticity. The enzyme that produces NO, nitric oxide synthase (NOS), can be visualized in the brain by the reduced nicotinamide adenine dinucleotide phosphate diaphorase histochemistry technique (NADPH-d). We have used NADPH-d activity to detect the presence of NOS-positive cells in the developing rat superior colliculus. Our results showed that NOS is present in cells and neuropil in the developing and adult rat superior colliculus. The first NOS-positive cells appeared at postnatal day 7 and were weakly stained. The number and intensity of the NOS-positive cells increased progressively during the following days reaching a maximum at postnatal day 15. By the end of the third postnatal week, both the number and intensity of stained cells showed an adult-like pattern. The NOS-positive cells showed a Golgi-like mosphology and we have found that all cell types present in the superior colliculus express the enzyme. The expression of NOS by tectal cells parallels the functional development of the retino-collicular and cortico-tectal projections and suggest that nitric oxide synthase-positive cells might be involved in this process. In this review we highlighted some of the recent descriptions of the expression of NOS in the mammalian visual system with emphasis in the superior colliculus and correlate these findings with several developmental events taking place in this structure.