Using light-microscopic immunocytochemistry against tyrosine hydroxylase, we have investigated the morphology of dopaminergic cells in 23 species of fishes representing various systematic classes and subclasses and which live in very different habitats. We have, for the first time, observed teleosts with dopaminergic amacrine cells. Thus, in both bony and cartilaginous fishes, dopaminergic cells are differentiated as interplexiform and amacrine cells. The differentiation of dopaminergic cells into amacrine or interplexiform cells in fishes correlates with the absence or presence of cones. In pure-rod retinae, they occur as amacrine cells, and in mixed rod/cone retinae, they occur as interplexiform cells. We conclude therefore that the differentiation of retinal dopaminergic cells in fish does not depend on the evolutionary or systematic classification of a given species. Rather, it is correlated with the occurrence of rods and/or cones, and thus linked more closely to the habitat. We argue that, in fish, the presence of cones and cone-specific horizontal cells may be responsible for inducing dopaminergic cells to differentiate as interplexiform cells. Possible functions of dopamine in all-rod retinae, which may not require adaptation, may include neuromodulation in the inner plexiform layer for the sensitization of the rod pathway, the shaping of biological rhythms, and the control of eye growth.

Teleost fish retinas continue to add neurons throughout life, and evidence from in vitro experiments have implicated insulin-like growth factors (IGFs) in this process. To discover whether these factors are expressed in vivo, we have examined their expression in the cichlid fish, Haplochromis burtoni. Three lines of evidence show that IGFs are present in the fish retina. An IGF-I specific antibody, sm 1.2, binds preferentially to the retinal outer plexiform layer, in areas of cone photoreceptor synaptic endings. Northern blots of mRNA hybridized with riboprobes from trout IGF-I and IGF-II genes revealed transcripts of ~6.5 and 4.9 kb, respectively. The IGF-I probe detected an additional transcript of 1.2 kb in liver but not in retinal mRNA. In situ hybridization with digoxigenin-labeled riboprobes revealed that the IGF gene product is localized in the cone photoreceptors. These results show that cone photoreceptors are the source of IGFs in the fish retina, consistent with the hypothesis that IGFs play a role in regulating production of new neurons in the teleost retina.

The retina of salmonid fishes has two types of cone photoreceptors: single and double cones. At the nuclear level, these cones are distributed in a square mosaic such that the double cones form the sides of the square and the single cones occupy positions at the centre and at the corners of the square. Double cones consist of two members, one having visual pigment protein maximally sensitive to green light (RH2 opsin), the other maximally sensitive to red light (LWS opsin). Single cones can have opsins maximally sensitive to ultraviolet (UV) or blue light (SWS1 and SWS2 opsins, respectively). In Pacific salmonids, all single cones express UV opsin at hatching. Around the time of yolk sac absorption, single cones start switching opsin expression from UV to blue, in an event that proceeds from the ventral to the dorsal retina. This transformation is accompanied by a loss of single corner cones such that the large juvenile shows corner cones and UV opsin expression in the dorsal retina only. Previous research has shown that adult Pacific salmon have corner cones over large areas of retina suggesting that these cones may be regenerated and that they may express UV opsin. Here we used in-situ hybridization with cRNA probes and RT-PCR to show that: (1) all single cones in non-growth zone areas of the retina express blue opsin and (2) double cone opsin expression alternates around the square mosaic unit. Our results indicate that single cone driven UV sensitivity in adult salmon must emanate from stimulation of growth zone areas.