Signal Transduction in Retinal Photoreceptors

The signal transduction pathway is the mechanism by which the energy of a photon signals a mechanism in the cell that leads to its electrical polarization. This polarization ultimately leads to either the transmittance or inhibition of a neural signal that will be fed to the brain via the optic nerve. The steps in phototransduction that take place in the vertebrate photoreceptors eye, which constitute a signal transduction pathway, are then:

1. The opsin in the outer segment absorbs a photon, changing the configuration of a molecule inside the cell from the less-energetic cis-form to the more-energetic trans-form. (Light is absorbed by rhodopsin or by one of the various photopsins, causing the retinal to change shape.) 2. This results in a series of unstable intermediates, the last of which binds to the G protein in the membrane and activates transducin, a protein inside the cell. This is the first amplification step - each photoactivated rhodopsin triggers activation of about 100 transducins. (The shape change in the opsin activates a G protein called transducin.) 3. Each transducin then activates the enzyme cGMP-specific phosphodiesterase (PDE). (Transducin, in turn, activates the enzyme phosphodiesterase.) 4. PDE then catalyzes the hydrolysis of cGMP. This is the second amplification step because PDE hydrolyses about 1000 cGMP molecules. (The enzyme hydrolyzes the second messenger cGMP to GMP) 5. With the intracellular concentration of cGMP reduced, the net result is closing of ion channels in the photoreceptor membrane because cGMP was keeping the channels open. (Because cGMP acts to keep Na+ ion channels open, the conversion of cGMP to GMP closes the channels.) 6. As a result, sodium ions can no longer enter the cell, and the photoreceptor hyperpolarizes (its charge inside the membrane becomes more negative). (The closing of Na+ channels hyperpolarizes the cell.) 7. This hyperpolarization means that less glutamate is released to the bipolar cell than before (see below). (The hyperpolarization of the cell slows the release of the neurotransmitter glutamate, which can either excite or inhibit the postsynaptic bipolar cells.) 8. Reduction in the release of glutamate means one population of bipolar cells will be depolarized and a separate population of bipolar cells will be hyperpolarized, depending on the nature of receptors (ionotropic or metabotropic) in the postsynaptic terminal (see receptive field).

Thus, a photoreceptor actually releases less neurotransmitter when stimulated by light, because in the dark, the photoreceptor is at -40 mV, and photons, through a chemical process, hyperpolarize the cell.

ATP provided by the inner segment powers the sodium-potassium pump. This pump is necessary to reset the initial state of the outer segment by taking the sodium ions that are entering the cell and pumping them back out.

Although photoreceptors are neurons, they do not conduct action potentials.