A cyclic GMP-activated channel in dissociated cells of the chick pineal gland

PHOTOTRANSDUCTION in the vertebrate retina is dependent in part on a cyclic GMP-activated ionic channel in the plasma membrane of rods and cones^1,2. But other vertebrate cells are also photosensitive. Cells of the chick pineal gland have a photosensitive circadian rhythm in melatonin secretion that persists in dissociated cell culture^3,4. Exposure to light causes inhibition of melatonin secretion, and entrainment of the intrinsic circadian oscillator^5,6. Chick pinealocytes express several 'retinal' proteins, including arrestin⁷, transducin⁸ and a protein similar to the visual pigment rhodopsin⁹. Pinealocytes of lower vertebrates display hyperpolarizing responses to brief pulses of light^10,11. Thus it is possible that some of the mechanisms of phototransduction are similar in retinal and pineal photoreceptors. We report here the first recordings of cyclic GMP-activated channels in an extraretinal photoreceptor. Application of GMP, but not cyclic AMP, to excised inside-out patches caused activation of a 15-25 pS cationic channel. These channels may be essential for phototransduction in the chick pineal gland.