The extent of screening-pigment (SP) migration in the intact octopus retina and the amplitude of the early receptor potential (ERP) correspond with the degree of adaptation to light or darkness. The light-adapted retina has SP granules concentrated in an apical layer, at the tips of the photoreceptor rhabdoms and supporting cells, and the ERP is barely detectable. In the fully dark-adapted retina, the SP granules are mostly at the base of the rhabdoms, and the ERP is at its maximum. Retinae at intermediate stages, between the fully dark- and light-adapted states, show corresponding intermediate stages of SP migration and ERP amplitude. A series of experiments demonstrates the effects on SP migration of the efferent nerves, which form a subset of fibres in the optic nerves. When the optic nerves to one half of the retina have been severed, there is a dramatic difference in the distribution of SP in areas of the retina (of the dark-adapted eye) connected with severed or intact nerves: apical versus basal, respectively. On incubation of a light- adapted retina with 5 micromolar dopamine, but not with other catecholamines or other putative neurotransmitter substances, SP migrates basally and the ERP is significantly larger than for controls. In octopuses treated with reserpine, SP stays in an apical location and the ERP remains very small, regardless of the state of adaptation and of whether the optic nerves are intact. It is concluded that dopaminergic efferents from the optic lobes effect dark-adaptational SP migration in the cephalopod retina. The arrival in the retina of efferent signals that effect adaptational changes through the mediation of dopamine is a remarkable analogue of the vertebrate system.

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