ABSTRACT
OBJETIVO: O objetivo deste estudo é descrever o traçado eletrorretinográfico no gambá sul-americano (Didelphis aurita) obtido com estímulo cromático de comprimento de onda seletivo. O eletrorretinograma é o registro das variações de voltagem nas células retinianas, desencadeadas por estímulo luminoso. O eletrorretinograma representa a atividade elétrica combinada de diferentes células, e sofre variações dependendo da fisiologia retiniana e do método de exame. MÉTODOS: Foram registrados os eletrorretinogramas de seis animais em adaptação ao escuro utilizando filtros cromáticos Kodak Wratten®, e registrada a sensibilidade espectral para comprimentos de onda específicos nas faixas de cores do azul, verde, amarelo, laranja e vermelho. RESULTADOS: Os resultados eletrorretinográficos mais consistentes foram obtidos quando o animal foi estimulado por faixas espectrais seletivas, ao invés de luz branca; e são consistentes com a curva de absorbância das opsinas descritas em fotorreceptores de marsupiais. Estudos prévios sugeriram a tricromacia dos marsupiais por microespectrofotometria de opsinas e imuno-histoquímica de retina. Esse fundamento morfológico não tinha demonstração fisiológica eletrorretinográfica, até este estudo. CONCLUSÃO: O gambá sul-americano tem se mostrado interessante como animal experimental no estudo comparativo da fisiologia visual em mamíferos, especialmente no estudo filogenético da visão cromática. Os marsupiais apresentam um modelo retiniano que superpõe os sistemas fotópico e escotópico; e o gênero Didelphis conserva características encontradas em fósseis do período pleoceno. Portanto, o sistema visual de um marsupial resgata características dos primórdios da evolução dos mamíferos, até o desenvolvimento dos padrões retinianos modernos.
PURPOSE: To describe the electroretinogram of the South-American opossum (Didelphis aurita) obtained by chromatic stimulus of specific wavelengths. The electroretinogram records voltage variations of retinal cells triggered by light stimulation. The electroretinogram represents the combination of electric activity of many different cells and varies according to retinal physiology and examination methods. METHODS: We recorded the electroretinogram of six animals in dark adaptation using chromatic Kodak Wratten® filters, and recorded the spectral sensitivity to specific wavelengths in the spectrum of blue, green, yellow, orange and red light bands. RESULTS: The most consistent electrorretinographic results were obtained when the animals were stimulated by selective spectral bands instead of white light. These results are consistent with the absorbance curve of the opsins described in marsupial photoreceptors. Previous studies using microspectrophotometry of opsins and retinal immunohistochemistry suggested marsupial trichromacy. This morphologic knowledge has not before been physiologically demonstrated by electroretinographic methods. CONCLUSION: The South-American opossum has proven to be an interesting experimental animal for comparative visual physiology studies among other mammals, especially studies on phylogenetic of chromatic vision. The opossum represents a retinal model that superimposes both the photopic and scotopic systems; and the Didelphis genus shows few changes when compared to the fossils of the Pleocene period. Therefore the marsupial's visual system retrieves characteristics from ancient mammal evolution to the retinal patterns found in modern mammals.
Subject(s)
Animals , Male , Female , Color , Color Perception/physiology , Retinal Cone Photoreceptor Cells/physiology , Marsupialia/physiology , Photic Stimulation , Vision, Ocular/physiology , Analysis of Variance , Biological Evolution , Dark Adaptation , Electroretinography , Light , Mammals , Models, Animal , Sensory Thresholds/physiologyABSTRACT
The topography and spectral characteristics of mammalian photoreceptors correlate with both, the present ecological demands and the evolutionary history. The South American Opossum is a marsupial mammal with unspecialized habitus and crepuscular lifestyle. A sparse population of cones (max. = 3000/mm2) can be differentiated into four subtypes by morphological, topographical and immunocytochemical criteria. In spite of this unusual diversity the cone types can be split into two functional groups: The population of single cones labeled by antibody OS-2 for short wavelenght sensitive pigments was ubiquitous but at very low densities (200/mm2). The single cones labeled by antibody (COS-1) against long wavelength sensitive pigments constitute the dominant population in the area centralis (2300/mm2). These two single cone types correlate with the pair typically present in placental mammals. Discrimination of spatial and color contrast may be provided by this "modern" set. The COS-1 labeled double and single cones bearing an oil droplet, display a different pattern by being restricted to the inferior (non-tapetal) half of the retina (max = 800/mm2). This additional set of cones with oil droplets and long wavelength pigments is a conservative feature of the opossum retina and other marsupials. As an accessory cone system it is possibly providing enhanced sensitivity at mesopic conditions. During the early evolution of nocturnal mammals with its prominent expansion of rod vision these cone types were conserved but then were lost in placental mammals. Thus the unique features of mammalian are the result of two evolutionary steps: first a reduction of cone based vision, followed by a secondary differentiation of photopic vision and behaviour relying on the remaining set of cones.