Action spectra of the lateral eyes recorded from mammalian pineal glands

Single neuronal units from the pineal stalk and the pineal body of hamsters, guinea pigs and rats were recorded during photic stimulation of the lateral eyes in order to identify the retinal photoreceptor that mediates the environmental control of the mammalian pineal. Two cell types could be distinguished: one type was characterized by spontaneous spike discharges that were irresponsive to light stimulation of the eyes and the pineal body; the other, also spontaneously active, responded to flash stimulation of the lateral eye with On- and Off-discharges. With increasing light intensity, the spike frequency of the second response type followed a sigmoidal function up to a saturation level. Spectral sensitivity curves of all dark-adapted animals peaked at 500 nm. During light adaptation (18 microW/cm2) action spectra exhibited an additional maximum in the red (560 nm, rats and hamsters) and in the blue (450 nm, guinea pigs) light, respectively. Chromatic adaptation to orange light diminished the sensitivity at longer wavelengths, whereas adaptation to blue-green light enhanced the sensitivities at longer wavelengths. Thus, the spectral sensitivity recorded from pineal units of hamsters, guinea pigs and rats corresponds to those described for retinal ganglion cells, which indicates that both rods and cones contribute to the light-sensitivity of the mammalian pineal gland. Direct illumination of the pineal gland did not influence the activity of pineal units.