Light response Visual transduction Direct transduction Rod / Cone

Instructions:

Light response: Photoreceptors respond to light by changing their glutamate release levels. Click on the Light flash button once. You should be able to observe a brief light increase and a concurrent change in glutamate release from the top and bottom plots on the right. The glutamate signal changes only so slightly in response to light. Below the buttons you will find 4 gain controllers (see below). Try adjusting their values to obtain a pronounced response. What is the direction of the response?

Next we will familiarize ourselves with the visual conditions in the tutorial. Set the light level on top of the photoreceptor to an intermediate value. Click on the buttons and observe the ensuing changes in light.

Visual transduction:Light transduces the visual pigment via the following enzyme cascade: photons – rhodopsin – activated rhodopsin (metarhodopsin II) – a GTP binding protein (transducin) – an enzyme hydrolyzing cGMP (cGMP-phosphodiesterase) – closes a membrane bound cGMP-gated cation channel. Why does light activate this complex metabolic pathway instead of opsins acting directly on the cGMP-gated channels?

Simulate direct Opsin-cGMP activation by selecting direct visual transduction . Can you see light responses? Increase the number of Rhodopsin molecules excited by light. Is there a light response? What happens with more pulses?

In vertebrates, the visual scene is samples with two types of photoreceptors. Rods are extremely sensitive, and can be triggered by a single photon. Cones require significantly brighter light in order to produce a signal.
What is the advantage of having two different photoreceptor types?
To find out we will start with simulating a Rod response in the dark. Set the light level to darkness and change the gains to get a good change in glutamate release for a light flash (remember to select metabolic visual transduction). Now increase the ambient light level. What happens? What gain changes are required to maintain a good response in the light? Can the photoreceptor respond to single photons now?

Notes:

The photoreceptor uses a visual pigment embedded in the bi-lipid membranous discs that make up the outer segment to detect light. The photoreceptor consists of 1) an outer segment, filled with stacks of membranes (like a stack of poker chips) containing the visual pigment molecules such as rhodopsins, 2) an inner segment containing mitochondria, ribosomes and membranes where opsin molecules are assembled and passed to be part of the outer segment discs, 3) a cell body containing the nucleus of the photoreceptor cell and 4) a synaptic terminal where neurotransmission to second order neurons occurs.

Rhodopsin consists of two components, a protein (opsin) and a covalently-bound cofactor called retinal. Opsin is a light-sensitive G protein coupled receptor. Thousands of opsin molecules are found in each outer segment disc of the host cell. Retinal is produced in the retina from vitamin A, from dietary beta-carotene. Isomerization of 11-cis-retinal into all-trans-retinal by light sets off a series of conformational changes ('bleaching') in the opsin, eventually leading it to a form called metarhodopsin II (Meta II), which activates an associated G protein, transducin. This is the first amplification step – each photoactivated rhodopsin triggers activation of about 100 transducins. Transducin activates cGMP phosphodiesterase, PDE then catalyzes the hydrolysis of cGMP to 5' GMP. This is the second amplification step, where a single PDE hydrolyses about 1000 cGMP molecules.

Rhodopsin pigment must be regenerated for further phototransduction to occur. This means replacing all-trans-retinal with 11-cis-retinal and the decay of Meta II to Meta III. In the rod outer segment, Meta III decays into separate all-trans-retinal and opsin.

Similar to other cells, K+ channels maintain the resting membrane potential of the cell. The outward potassium current tends to hyperpolarize the photoreceptor at around -60 mV. cGMP molecules gate activation of an inward sodium current. This so-called 'dark current' depolarizes the cell to about -30 mV. A high density of Na+-K+ pumps enables the photoreceptor to maintain a steady intracellular concentration of Na+ and K+.

In the dark cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current. This dark current keeps the cell depolarized around -30 mV, leading to glutamate release.

Light leads to reduced cGMP levels and closure of cGMP-gated sodium channels. Stopping the influx of Na+ ions effectively switches off the dark current. Reducing this dark current causes the photoreceptor to hyperpolarise, which reduces glutamate release.

https://en.wikipedia.org/wiki/Visual_phototransduction

https://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/photoreceptors/

Composed by Alon Poleg-Polsky, 2019
alon.poleg-polsky@ucdenver.edu