Dark and light adaptation of pineal photoreceptors

Dark and light adaptation of pineal photoreceptors was studied in the isolated pineal organ of the rainbow trout, Salmo gairdneri. After intracellular recording, the photoreceptors were iontophoretically injected with Lucifer yellow CH or with horseradish peroxidase for morphological characterization. Pineal photoreceptor cells responded to light with a hyperpolarization whose amplitude was graded with intensity. Following a 30-60 s bleach, receptor responsiveness was greatly reduced with a gradual recovery in the dark. Recovery of membrane potential was complete within 2-4 min in the dark. In response to flashes the hyperpolarizing response increased in darkness in amplitude and duration over a period of more than 30 min and the voltage-intensity curves continuously shifted to lower intensities. After exposure to strong light the time-course of dark adaptation, determined with a threshold criterion, was monophasic and receptor sensitivity increased by at least 5-6 log units. The results show that pineal photoreceptors exhibited the full characteristics of dark adaptation processes previously ascribed to cells proximal to the receptors, i.e. to ganglion cells. Exposure to steady illumination of different intensities induced graded and sustained hyperpolarizations of the receptor membrane potential. The incremental voltage range of responses to test flashes superimposed on the backgrounds was reduced. Voltage-intensity curves were shifted to higher intensities with increasing background illumination indicating that adaptation occurred over a range of about 2.5 log units before the receptors saturated.