Melatonin and LSD induce similar retinal changes in the frog

This work is based on histological examination with the light microscope of Nissl-stained frogs’ retinae following melatonin and lysergic acid die~ylamide (LSD) administ~tion. In a previous paper (Kemali et al. 1983) we have demonstrated with the light microscope that administration of LSD induced in the frog retina a modification of the pigment screening (PS) pattern through some unknown mechanism. PS is a phenomenon encountered in the retina of low vertebrates and involves the migration of melanin granules from the retinal pigment epithelium in response to illumination changes. In the light, melanin granules disperse along the pigment epithelium processes, which protrude among the outer segments of the photoreceptors. In the dark, on the other hand, melanin granules aggregate within the cell body of the pigment epithelium (for review see Nguyen-Legros 1978). In another earlier paper (Kemali et al. 1984) we have demons~ated that the adminis~ation of dopaminergic agents to the frog did not induce any substantial modifications of this PS pattern, suggesting that dopamine is probably not directly involved in this retinomotor phenomenon, The aim of the present investigation was to