Electrophysiology of the guinea-pig pineal organ: sympathetically influenced cells responding differently to light and darkness.

Single cell recordings (extracellular) from the posterior part of the guinea-pig pineal organ reveals two groups of sympathetically influenced spontaneously active cells: one group is stimulated by light and inhibited by darkness; the other type of cell whose activity was monitored over a period of 15 h shows activation by darkness and diminished activity after the onset of light. It is suggested that latter type is involved in melatonin synthesis.