Discovering electrophysiology in photobiology: A brief overview of several photobiological processes with an emphasis on electrophysiology

Abstract

The mini-review gives special attention to holistic approach and mechanisms of processes. The physical and chemical frames and background for visual perception and signaling are discussed. Perception of photons by retinal rod cells is described in more detail starting from photon absorption and culminating in ion currents. Dark noise and temperature-dependence of photocurrents in photoreceptor cells are analyzed. Perception of polarized light, its effects and informational importance are discussed based on underlying mechanisms and specialized morphological structures of biological organisms. Role of statistics of photons in photoreception is questioned. The review also pinpoints new and developing directions and raises questions for future research.

Keywords: Photobiology; dark noise; electrophysiology; optogenetics; polarized light; quantum dots; rod cells; statistics of photons.

Figure 1. Channelrhodopsins are light-gated ion channels from green algae. After a flash of illumination the protein molecule of ion channel temporarily changes conformation due to isomerisation of molecule of retinal bound to lysine of the protein (compare with rhodopsin, see below). It allows selective passage of cations according to electrochemical gradient (Na⁺ selectivity is depicted); within a short time (milliseconds to seconds depending on the protein structure) the channel returns to the initial conformation and ion current stops.

Figure 2. Typical recording of membrane potential in cells of the emerged blade of the growing leaf 3 of barley, and the response of membrane potential to changes in illumination and further addition of NaCl (100 mM) to the root medium. Several phases of responses with different kinetics were observed upon changes in illumination (18 experiments with 9 plants). Light was supplied using fiber optics from the cold light source at the background of dim illumination in the electrophysiological rig. The figure is from with permission from the Oxford University Press, extra information about the recordings is provided by V.Volkov.

Figure 3. Microscope image of isolated Xenopus laevis retinal cells (A) and a rod cell from the preparation (B). Scale bar is 120 μm (A) and 30 μm (B).

Figure 4. Signal transduction chain in a vertebrate retinal rod starting from a photon hν and leading to cyclic nucleotide gated channels. Closure of the ion channels after drop in cGMP results in membrane hyperpolarisation and stops inward ion current of sodium and calcium.

Figure 5. Kinetics of retinal rod responses of Xenopus laevis to pulses of illumination. The color legend indicates the corresponding number of impinging photons. Reproduced with the permission of The Optical Society of America from.

Figure 6. Light-activated ion channel channelrhodopsin-2 can restore basic visual function in photoreceptor-deficient mice with 6–9 orders of magnitude lower sensitivity (according to^44,45).

Figure 7. Basic principle for perception of polarized light in biological organisms. Ordered orientation of pigment molecules in photoreceptor cell allows detection of electric vector E of light (B is the vector of magnetic field of electromagnetic wave). Molecules of photosensitive pigments are depicted in the form of blue ovals.

Figure 8. Underwater world with several photobiological phenomena and examples of perception. Light is reflected from the water surface and becoming partially polarized; light is also passing to water and getting partially linearly polarized; polarized light in water is sensed by fishes; fishes have multilayer reflecting skin with unusual properties; blonde, thorntail and undulate rays (Raja brachyura, Raja clavata, Raja undulate; size of specimens is around 1 m) possess electric sense. The picture is taken with the permission of staff of SEA LIFE London Aquarium.