PHOTORESPONSES AND LIGHT ADAPTATION OF PINEAL PHOTORECEPTORS IN THE TROUT

The photoresponses of 116 photoreceptors in the excised perfused pineal body of adult trout were intracellularly recorded. Brief flashes produced slower, but otherwise comparable, responses to retinal receptor cells. The absolute sensitivity, as well as the light-induced membrane conductance changes, were also similar to those of retinal cone cells in the same species. Steady illumination decreased the cell sensitivity to superimposed flashes, in accordance with the Weber-Fechner relation, indicating the presence of light adaptation. This conclusion is supported by the non-exponential nature of the amplitude-intensity relation and by the conspicuous shortening of the time-to-peak with increasing flash intensity. The time-dependent response decay typical of retinal photoreceptors was not found, however. The fact that, during constant illumination, the pineal photoresponses maintain the same amplitude set at the initial peak perhaps represents an original mechanism of light adaptation which allows these cells to maintain their voltage-dependent synthesis and secretion of indolamines as a constant relation with daily illumination.