Retinal growth in carp of the same age: Density and number of dopamine neurons

Abstract

Research has shown that an increase in the size of retina with fish growth is accompanied by a reduction in the density of monoamine neurons along with an increase in their cell number per retina. The age of fish, however, has not been considered in relation to the above. In the present study, juvenile carp (6.4 ± 0.7 cm in body length), hatched 2 months earlier, were reared for 10 months under 2 different sets of conditions; one group was kept in a small aquarium at room temperature (RT, 9–26 °C), and the other was kept in a large aquarium in which the water temperature was constant (CT, 22 °C). The left eye of fish transferred into the RT aquarium was intravitreally injected with 5,7-dihydroxytryptamine (5,7-DHT; 1.0 μg) to destroy indoleamine-accumulating (IA) neurons. The fish grew faster in the CT (16 ± 1.6 cm) than in the RT aquarium (9.3 ± 1.7 cm), resulting in a wide variation among fish of the same age. In fluorescent retinal flatmounts, the density of dopamine (DA) neurons was found to be lower, but their cell number per retina greater in the CT than in the RT group; these differences were statistically significant ( P < 0.001). The data suggest that the above parameters depend on the size of carp but not on their age. In the fish treated with 5,7-DHT and reared in the RT aquarium for 10 months, the growth zone of the retina was determined by circumferential addition of IA cells. Comparisons of the areal size of the original retina with that of the grown retina previously treated with 5,7-DHT revealed that such an expansion accounts for about 77% of the total increase in area; the remaining 23% is due to circumferential cell addition.