BBE/CNS 150 Lecture 13 Wednesday, October 29, 2014 Vision 1: Phototransduction and the Retina

1. BBE/CNS 150 Lecture 13 Wednesday, October 29, 2014 Vision 1: Phototransduction and the Retina Bruce Cohen Kandel Chapter 26

2. Organization of the retina • Glutamate is the major transmitter • Some neurons make dopamine & acetylcholine (amacrine cells) • Inhibitory neurons release GABA. • . Rod Cone Synapses of outer plexiform layer Horizontal cells Bipolar cells Synapses of inner plexiform layer Ganglion cell is unique in firing impulses optic nerve Like Fig. 26-2

3. Photoreception • Photoreceptor organs have evolved independently at least 40 times, each time responding to the visible spectrum and near-UV. • How do we explain the use of a limited part of the spectrum? • Infrared light is not sufficiently energetic to drive photochemical reactions such as the cis-trans isomerization of retinal. • Shorter-wavelength ultraviolet light is too energetic and would destroy organic molecules.

4. Outer segment of photoreceptors contains the visual pigment Rhodopsin Free-floating discs Rhodopsin hn hn Like Figs. 26-5, 26-7

5. Absorption spectra of cone pigments There are 3 types of opsin in human color-sensitive cones. Each opsin interacts distinctly with retinal, producing a distinct absorption spectrum. Blue- green-red- absorbing Mutations that change the spectrum Like Fig. 26-8, 26-9

6. Rods dominate the visual response at low light levels

7. Detection of light by retinal bound to opsin Enzymes From Darnell et al., Mol. Cell Biology Like Fig. 26-8

8. cytosol intracellular messenger Ca2+ cAMP cGMP membrane receptor G protein i q s t effector channel enzyme channel The GPCR pathway in photoreceptors

9. Phototransduction • Starts with photon absorption by rhodopsin • Transducin binds to activated rhodopsin , exchanges GTP for GDP • Activated transducin dissociates into  and subunits • The  subunit binds to, and activates, phosphodiesterase • Intracellular cGMP concentration decreases • Reduction in cGMP closes cGMP-gated cation channels in the plasma membrane • Membrane potential hyperpolarizes • Closing of cGMP-gated channel reduces intracellular calcium • Reduced calcium counteracts the effects of light absorption

10. receptor G protein i q intracellular messenger +cGMP* Ca2+ cAMP cGMP closed +cGMP* open like a previous Lecture Rods and Cones have cGMP-activated Na+/Ca2+ Channels s t effector channel enzyme Excised “inside-out” patch allows access to the inside surface of the membrane channel no channel openings no cGMP

11. The “ribbon synapse” facilitates the tonic high rate of transmitter release Photoreceptor to horizontal cell synapse

12. The Phototransduction Cascade: 1. Amplification 2. Adaptation 1. When fully dark-adapted, many species can detect ~1 photon per photoreceptor cell 2. When fully light-adapted, many species can accurately analyze light at intensities ~1010 fold brighter

13. The Phototransduction Cascade: 1. Amplification 1a. When the rod is dark adapted, the activated Receptor (O*) can activate 500 transducin proteins. 1b. The phosphodiesterase has a turnover number of 4200/sec, near the diffusion limit for catalysis. 1c. Each millisecond that the cGMP-dependent cation channel in the rod outer segment plasma membrane is open,10,000 ions flow through it.

14. The Phototransduction Cascade: (1. Amplification) 2. Adaptive mechanisms 2a. Transducin hydrolyses GTP to GDP and thus inactivates itself. • 2b. The activated receptor (O* or R*) must also be deactivated. • (1) Rhodopsin kinase phosphorylates the carboxyl tail of the receptor • (2) The phosphorylation permits binding of the inhibitory protein, arrestin 3c. Guanylate cyclase must synthesize new cGMP from GTP (1) Guanylate cyclase is partially inhibited by [Ca2+] > ~75 nM. (2) Ca2+ influx through the tonically open cation channel sets the cytosolic level of Ca2+ to ~ 500 nM. (3) When the cation channel closes upon light stimulation, Ca2+ continues to be pumped out via the usual processes, lowering cytosolic Ca2+ to ~50 nM and activating guanylate cyclase

15. Lowered cytosolic Ca2+ Increased cyclic GMP synthesis Cyclic GMP hydrolysis Channel Closure Channel opening Dark State Light Visual excitation is followed by Recovery and Adaptation The role of Ca2+ in adaptation also appears to be important, but this process is not understood in molecular detail yet.

16. Visual information flows from photoreceptors to ganglion cells through the bipolar neurons • There are two types of bipolar cells, “on” and “off” • On bipolar cells are depolarized by light at the center of their receptive field • Off bipolar cells are hyperpolarized by light at the center of their field • Off bipolars have glutamate-gated ion channels that shut off when the photoreceptor stops releasing glutamate • On bipolars have glutamate GPCRs that activate phosphodiesterase and inhibit cGMP-gated channels from opening in the dark • When the photoreceptor stops releasing glutamate, the cGMP channels open and depolarize the on bipolar cell Information flow in the retina

17. Retinal ganglion cells have center-surround receptive fields

18. BBE/CNS 150 End of Lecture 13