Abstract
Purpose :
Electrical gap junctions are a ubiquitous form of bidirectional synaptic connectivity whose role in visual processing remains underappreciated. Their typical configuration is lateral connectivity between cells in the same neural layer (including the photoreceptor layer of the retina), as opposed to the unidirectional feedforward and feedback connectivity of neurotransmitter-mediated chemical synapses. Although electrical gap junctions are expected to have a rapid response, evidence from studies with gap junction blockers show that their response speed can in fact be slower than for chemical synapses (Cadetti et al., 2005).
Methods :
Methods. Full-field spectral ERGs were recorded with a RETeval device for 6 narrowband chromatic stimuli (magenta, red, yellow, green, cyan, blue) plus white, using 250 ms light pulses at 2 Hz for 30 sec (60 full cycles) at 5 intensities from 3 to 300 cd/m2.
Results :
The spectral ERGs exhibit three components: 1) Rod-pathway responses (up to ~10 cd/m2) whose a/b-wave complex has a scotopic spectral sensitivity with a b-wave peak time of ~60 ms and typical rod-like saturation attributable to transmitter depletion at the rod/bipolar chemical synapse. 2) Cone pathway responses with a photopic spectral sensitivity and a b-wave peak time of ~40 ms. 3) An unexpected additional response component consisting of a slow scotopic/mesopic response with a b-wave peak time of ~90 ms. Its spectral sensitivity peaks in the yellow-green range, and is measurable up to about 50 cd/m2, well beyond the level of the classic rod response.
Conclusions :
The slow component revealed in our spectral ERG protocol appears to reflect activity in a distinct signal processing pathway. It has several paradoxical features: a b-wave even slower than the rod-mediated b-waves, but with a spectral tuning peaking in the yellow-green; light sensitivity comparable to rod-pathway responses but lacking rod-like saturation at high intensities. A hypothetical mechanism for this component could be gap junctions from the M-cones to the rod pathway to account for the yellow-green tuning, with the slower response of the gap junctions accounting for the slower time courses. The lack of saturation could reflect a wide dynamic range of gap junction electrical conductivity.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.