Lithium and melatonin: morphological modifications induced in frog retina pigment screening

Pigment screening (PS) is a phenomenon occurring in the retina of lower vertebrates, which consists in the dispersion, induced by the light, of melatonin granules into processes of the pigment epithelium which extend between photoreceptors, and in the aggregation, induced by the dark, of the same granules within the cell bodies of the pigment epithelium. It has been hypothesized that this phenomenon might be modulated by melatonin, which is present not only in the pineal but also in the retina, and whose retinal content is high in the dark and low in the light. In the present study we demonstrate that a single intraperitoneal injection of lithium chloride affects in a peculiar way PS of light-adapted frogs and of frogs subjected to natural day/night conditions, but not that of dark-adapted frogs. The implications of this findings are briefly discussed, also in view of the reported effect of lithium on the 24-hour rhythm of retinal melatonin, and of the possible relationship between this effect and the altering properties of lithium on other biological rhythms.