In vivo optophysiology reveals that G-protein activation triggers osmotic swelling and increased light scattering of rod photoreceptors

Proc Natl Acad Sci U S A. 2017 Apr 4;114(14):E2937-E2946. doi: 10.1073/pnas.1620572114. Epub 2017 Mar 20.


The light responses of rod and cone photoreceptors have been studied electrophysiologically for decades, largely with ex vivo approaches that disrupt the photoreceptors' subretinal microenvironment. Here we report the use of optical coherence tomography (OCT) to measure light-driven signals of rod photoreceptors in vivo. Visible light stimulation over a 200-fold intensity range caused correlated rod outer segment (OS) elongation and increased light scattering in wild-type mice, but not in mice lacking the rod G-protein alpha subunit, transducin (Gαt), revealing these responses to be triggered by phototransduction. For stimuli that photoactivated one rhodopsin per Gαt the rod OS swelling response reached a saturated elongation of 10.0 ± 2.1%, at a maximum rate of 0.11% s-1 Analyzing swelling as osmotically driven water influx, we find the H2O membrane permeability of the rod OS to be (2.6 ± 0.4) × 10-5 cm⋅s-1, comparable to that of other cells lacking aquaporin expression. Application of Van't Hoff's law reveals that complete activation of phototransduction generates a potentially harmful 20% increase in OS osmotic pressure. The increased backscattering from the base of the OS is explained by a model combining cytoplasmic swelling, translocation of dissociated G-protein subunits from the disc membranes into the cytoplasm, and a relatively higher H2O permeability of nascent discs in the basal rod OS. Translocation of phototransduction components out of the OS may protect rods from osmotic stress, which could be especially harmful in disease conditions that affect rod OS structural integrity.

Keywords: intrinsic optical signals; optical coherence tomography; osmotic stress; photoreceptor waveguiding; phototransduction.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Aquaporins / metabolism
  • Cell Membrane / chemistry
  • Cell Membrane / metabolism
  • GTP-Binding Protein alpha Subunits / genetics
  • GTP-Binding Protein alpha Subunits / metabolism*
  • Light
  • Light Signal Transduction
  • Mice, Inbred BALB C
  • Mice, Inbred C57BL
  • Mice, Mutant Strains
  • Osmolar Concentration
  • Osmosis
  • Rhodopsin / metabolism*
  • Rod Cell Outer Segment / physiology*
  • Tomography, Optical Coherence
  • Transducin / genetics
  • Transducin / metabolism*


  • Aquaporins
  • GTP-Binding Protein alpha Subunits
  • Gnat1 protein, mouse
  • Rhodopsin
  • Transducin