Daily morphological variations in the viscacha (Lagostomus maximus maximus) retina. Probable local modulatory action of melatonin

Abstract

Given that the local melatonin levels exhibit rhythmic daily changes in the retina of the viscacha, we considered it important to study the likely daily variations in morphology and specific 2-[ 125 I]-iodomelatonin binding in retinas from this rodent and to correlate these putative changes with local indole levels. Adult animals of both sexes were captured in their habitat and were kept under a natural photoperiod. For light and electron microscopic studies the viscachas were sacrificed by decapitation at 08:00, 16:00, and 24:00 hr. A computer-assisted image analysis system was used to measure the thickness of the complete retina, the photoreceptor layer, the rod outer and inner segments, and the outer nuclear layer. The daily variation in 2-[ 125 I]-iodomelatonin binding sites was followed during a 24-hr light-dark cycle, the animals being sacrificed at six time points. The parameters studied showed significant variations throughout the 24-hr period. Maximal specific binding, lysosomal content in the pigment epithelium, and photoreceptor layer outer segment thicknesses were observed at 24:00 hr. Close contact between photoreceptor membranes and microvilli of the pigment epithelium was observed at 08:00 and 16:00 hr. Moreover, the minimal outer segment thickness at 16:00 hr was accompanied by a scarcity of dense bodies, such as lysosomes, a maximum dispersion of melanin pigment granules, and a minimum density of radioligand binding sites. Therefore, in the retina of the viscacha, we suggest that the interaction between melatonin and specific sites could be one of the factors or causes that participate in the regulation of the daily morphological changes observed in viscacha.

Histological Changes in the Retina Provoked by Lithium Treatment in a Nocturnal Rodent <b><i>(Lagostomus maximus maximus)</i></b>

“…In the control viscacha retinas of the present study, we observed ultrastructural changes that demonstrate a daily rhythm of retinal photoreceptor renewal. These results are similar to those previously reported by us for viscachas in their normal habitat [Calderón et al, 2002]. On the contrary, the daily variations in the thickness of the photoreceptor layer, and specifically the outer segments, and ultrastructural variations disappeared completely as a result of the action of lithium, unlike the results reported by Remé et al [1990].…”

“…This band lies approximately 3.5 mm above the optic nerve, containing the longest photoreceptors and pigment epithelial cells [Lascano et al, 1999]. The retina of the viscacha exhibits a close temporal relation between melatonin levels, 2[ 125 I]-iodomelatonin binding, and daily morphological changes, as previously demonstrated in our works [Calderón et al, 2001[Calderón et al, , 2002.…”

“…Previous research carried out in our laboratory has demonstrated daily morphological variations in the retinas of viscachas captured in their habitat. A notable dispersion of melanin pigment granules in the retinal pigment epithelium was observed at 16: 00 h, whereas at 08: 00 and 24: 00 h aggregated melanin pigment granules were found [Calderón et al, 2002]. In lithium-treated viscachas, the retinal levels of melatonin did not show significant differences at any point during the light-dark cycle, indicating that the ion abolished the indole rhythm [Calderon et al, 2001].…”

Multivesicular bodies in neurons: Distribution, protein content, and trafficking functions

“…MVBs in terminals may contribute to slow turnover of membrane components of the synaptic terminal (Wittich et al ., 1994). Ultrastructural studies consistently report MVBs in shedding photoreceptor cells (Calderon et al ., 2002; Calman and Chamberlain, 1982; Williams and Blest, 1980; Schmied and Holtzman, 1987). MVB numbers can fluctuate in parallel with circadian shedding and synthesis of the photoreceptor membrane (Williams and Blest, 1980; Calderon et al ., 2002; Chamberlain and Barlow, 1984; LaVail, 1976).…”

Cornea, retina, and lens morphology in five Soricidae species (Soricomorpha: Mammalia)

“…Melanin pigment granule distribution in the RPE varies individually during the light-dark cycle (Pang and Yew 1979;Calderon et al 2002), so that granules are dispersed throughout the cytoplasm during the day (maximum dispersion at 1600 hours) and aggregated in the apical part of the cells at night (maximum aggregation at 2400 hours; Calderon et al 2002). All our specimens were killed early in the morning (0800-0900 hours) but, in contrast to observations by Calderon et al (2002), the stage of melanin dispersion was not the same in all epithelial cells. Indeed, we observed that pigment granules were dispersed near the ora serrata and congregated near the posterior pole of the eye.…”