Abstract
Abstract: :
Purpose: Cone retinomotor movements (RMM) are controlled by dopamine (DA), which is liberated from dopaminergic interplexiform cells (DA–IPCs). On the other hand, DA–IPCs receive synaptic contacts of efferent fibers from olfactory bulb. Therefore, DA–IPCs act as intermediate between cone RMM and the fish Central Nervous System (CNS). The aim of this work is to find if DA–IPCs' destruction or deafferentation impair the normal elongation of the cone during retinal dark adaptation. Finally, we correlate the amplitude of RMM with habitat conditions of the fish. Materials and Methods: The experiments were performed in four species of teleost fish. Micropterus salmoides, Cyprinus carpio, Roccus americana and Dicentrarchus labrax. The animals were kept in an aquarium under 12h:12h light–dark cycle. All experiments were carried out at 12:00h pm to equalize circadian effects. To destroy DA–IPCs, 6–OHDA was injected intraocularly. After two weeks, the eyes were removed and their eyecups were superfused with exogenous DA in light/dark conditions. Semithin vertical sections of the retinas were obtained, and the cone myoid length was measured, as the distance, in micrometers, between the outer limiting membrane and the base of the cone ellipsoid. Results: In the control eyes, cones were contracted in light–adapted retinas and cones were elongated in dark adapted ones, but whereas Cyprinus carpio shows tiny RMM, the other three species present dramatic movements. In dopamine–depleted retinas adapted to light, the cones shown a significant level of light adaptation. But after dark adaptation, cones remained in a semi–elongated position instead of being elongated. In dopamine–depleted retinas superfused with DA under dark conditions, cones experimented a great contraction. Although semi–elongated position was observed in control dark adapted retinas after isolation. Conclusions: 1) Cones RMM are less extensive in Cyprinus carpio. While Cyprinus carpio is omnivorous, the other three species are predators. Thus, the hypothesis is that the visual system of predator fishes might be, more plastic than omnivorous fishes. 2) DA is not essential for the triggering of contraction, because light could initiate cone contraction independently of DA. Therefore, some other non–dopaminergic system is required for the complete cone elongation in dark adaptation. We suggest that DA–IPCs contain a neuroactive substance "X" which release is impaired after destruction by 6–OHDA, or when retinas are deafferentized from the CNS.
Keywords: photoreceptors • dopamine • plasticity