Photoreceptor cell-specific receptor

Kobayashi, M., Takezawa, S., Hara, K., Yu, R.T., Umesono, Y., Agata, V., Taniwaki, M., Yasuda, K. and Umesono, K. (1999) Identification of a photoreceptor cell-specific nuclear receptor. Proceedings of the National Academy of Sciences of the United States of America, 96, 4814-4819. 

K. Umesono, Identification of a photoreceptor cell-specific nuclear receptor, Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 4814-4819. 

The Na" channel has a receptor site for cyclic GMP when cyclic GMP is bound, the channel is closed. This leads to a decrease in the intracellular Na ion concentration, resulting in hyperpolarisation of the cell membrane. This decreases the release of the neurotransmitter glutamate into the synapse that connects the photoreceptor cell to the bipolar neurones. In this specific case, a decrease in the neurotransmitter concentration in the synapse is a signal that results in depolarisation of the bipolar cell. The action potential in the bipolar cells communicate with ganglion cells, the axons of which form the optic nerve. Thus action potentials are generated in the axons which are... 

All photoreceptors have in common photoexcitable pigments associated with the receptor membranes. These photopigment molecules, which are the primary sites of photoreception, are altered by the absorption of light in such a way to change the conductance of photoreceptor cell membranes to a specific ion. Thus, the membrane potential of the receptor cell will be altered, i.e., a receptor potential will occur. The vertebrates and the invertebrates have evolved different mechanisms by which the primary transduction process alters the membrane conductance. We will review here primarily the mechanism of photoreceptors in the vertebrates. 

The primary events of GDP/GTP exchange, catalysed by R, are thought to be very similar for invertebrate photoreceptors. However, the cascade initiated by the liberation from transducin of the subunit with bound GTP differs from that observed for vertebrates. Thea-unit activates the integral membrane enzyme phospholipase C to hydrolyse phosphotidy-linositol the product of this reaction, in turn, activates calcium release. In invertebrates the sodium channels on plasma membranes are closed in the resting state and open on excitation, with the consequent depolarization of the cell, in contrast to the hyperpolarization of vertebrate photoreceptors. It is postulated that the increase in current (as distinct from a decrease in vertebrates) results from Ca " release. The direct effects of Ca " on the channels and indirect effects on the enzymes controlling cGMP concentration, have yet to be clarified. The specific properties of invertebrate photo-receptors and their variations have been described by Nagy (1991). 

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